OCS Study 

MMS 2009-039 

 

Northern Gulf of Mexico Continental Slope 

Habitats and Benthic Ecology Study 

Final Report 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 

OCS Study 

MMS 2009-039 

Northern Gulf of Mexico Continental Slope 

Habitats and Benthic Ecology Study 

Final Report 

Authors 

Gilbert T. Rowe 

Mahlon C. Kennicutt II 

Prepared under MMS Contract 

1435-01-99-CT-30991 (M99PC00001) 

by 

Texas A&M University 

Texas Engineering Experiment Station 

College Station, Texas 77843 

Published by 

New Orleans

July 2009 
iii 

DISCLAIMER 

This report was prepared under contract between the Minerals Management Service (MMS) and 

the Texas A&M Engineering Extension Service. This report has been technically reviewed by 

the MMS, and it has been approved for publication. Approval does not signify that the contents 

necessarily reflect the views and policies of the MMS, nor does mention of trade names or 

commercial products constitute endorsement or recommendation for use. It is, however, exempt 

from review and compliance with the MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office (Mail Stop 5034) 

at the following address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

 New Orleans, Louisiana 70123-2394 

Telephone (504) 736-2519 or 

 1-800-200-GULF 

CITATION 

Rowe, G.T. and M.C. Kennicutt II, eds. 2009. Northern Gulf of Mexico continental slope 

habitats and benthic ecology study: Final report. U.S. Dept. of the Interior, Minerals 

Management. Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 

2009-039. 456 pp. 

ABOUT THE COVER 

Sites sampled during the Deep Gulf of Mexico Benthos (DGoMB) project.
v 

ACKNOWLEDGMENTS 

The group of investigators who participated in the Deep Gulf of Mexico Benthos (DGoMB) 

for the Minerals Management Service (MMS) wish first of all to thank Mr. Gregory Boland and 

Dr. Thomas Ahlfeld of MMS for their support and assistance throughout the project. The Texas 

Engineering Experiment Station of the Texas A&M University System deserves appreciation for 

their support of cost-shared equipment and graduate student support during the initial stages of 

the project. A total of 57 undergraduate and graduate students participated in sample collection 

and analysis over the entire course of the project, many as volunteers, and they have been 

appreciated. The R/V GYRE, its crew and officers, in particular Capt. Dana O. Dyer, provided 

service that was competent and professional, for which we are exceedingly grateful. The 

Department of Oceanography (OCNG), the Department of Biology (BIOL), the Department of 

Wildlife and Fisheries Sciences (WFSC), the Department of Marine Science (MARS) and the 

Geochemical and Environmental Research Group, all of Texas A&M University, provided space, 

staff and financial support throughout various phases of the project, for which we are grateful. 

Other institutions whose contributions should be acknowledged are the University of Texas 

(UT), the University of Washington (UW), the University of South Carolina (USC), the Woods 

Hole Oceanographic Institution (WHOI), the University of Southern Mississippi (USM), the 

Universidad Autonoma de Mexico (UNAM), and the Australian Museum of Natural History. 

The Scientific Review Board was composed of Drs. Will Schroeder, K. L. Smith, Jr., and 

Michael Rex. Their advice throughout the project consistently improved each stage of the 

program. The time and effort they devoted were appreciated by all participants. 

Two close colleagues, whose dedication to the deep Gulf of Mexico should not go 

unacknowledged, were lost during the course of the study. Dr. Robert Avent at MMS nurtured 

the idea of and need for the study within MMS long before it was even initiated. Following its 

inception, he kept close tabs on all activities, even participating on one trip on the GYRE. It is 

unfortunate that he has not been able to see what has been generated out of his dedication. Dr. 

Guin Fain Hubbard passed away in 2006, just as the final report was being prepared. Fain had 

immeasurable influence on the cadre of graduate students tasked to sort and identify the 

macrofauna from the box cores. He probably had more direct influence on their education as 

biologists than any other single individual. 
vii 

EXECUTIVE SUMMARY 

The Deep Gulf of Mexico Benthos (DGoMB) project was initiated for the Minerals 

Management Service of the U.S. Department of the Interior in 1999 to investigate the structure 

and function of the biota associated with the sea floor in the deep water of the northern Gulf of 

Mexico. The purpose of the study has been to determine how living resources inhabiting deepwater habitats might be impacted by oil and gas exploration and exploitation. The strategy of the 

field work was to survey a broad region during an initial stage from which community structure 

could be determined, accompanied by extensive sampling of physico-chemical variables within 

both the water column and the sediments of the sea floor. Eight hypotheses were tested on the 

basis of this survey, all related to how the environmental variables control or affect community 

biomass, standing stocks, diversity and recurrent assemblages of species. The variables tested 

were depth, location within small (5 to 15 km diameter) basins, canyons, geographic location in 

the east vs. western half of the northern gulf basin, location below steep escarpments, proximity 

to high vs. low surface water productivity, and association with or next to methane seeps. The 

components of the seafloor community sampled included sediment bacteria, small protists and 

metazoans (meiofauna, >63 micrometer mesh sieves), small metazoan invertebrates 

(macrofauna, >300 mesh sieves), large bottom-dwelling invertebrates (megafauna, > ca. 1 cm 

diameter), and demersal fishes. Sampling devices utilized to capture these organisms were a 0.2 

m2

 box corer (along with subsampling from within the cores); 40 foot otter trawl, with 2.5 cm 

stretch mesh net; and sea floor photography. Profiles of water column variables were determined 

using a Conductivity-Temperature-Depth profiling sensors (CTD), and a rosette of sampling 

bottles for measurement of standard water-column chemical variables (temperature, salinity, 

oxygen, suspended organic matter, inorganic nutrients and plant pigments). The sampling 

crossed the continental margin from depths of 200 m on the upper continental slope out to 3,750 

m on the Sigsbee Abyssal Plain and extended geographically from the Mexican border on the 

west over to north Florida (Figures 5-1 to 5-3). 

A subset of six locations was chosen from the data generated by the survey to conduct more 

intensive sampling from which diagnostic properties of sea floor community function could be 

measured. The strategy was to compare community function at sites with extremes in high and 

low standing stocks. At these locations, an autonomous benthic lander was used to set incubation 

chambers on the sea floor to measure sediment community oxygen consumption (respiration) 

(SCOC). Subcores taken from box cores were incubated in the ship’s laboratory. Microelectrodes 

made profiles of oxygen concentration in the pore water of the surface sediments in subsamples 

from cores to determine oxygen penetration depth and consumption. A suite of radionuclides was 

used to estimate sediment mixing rates, burial and consumption of organic carbon within the 

sediments. 

The biota, in terms of standing stocks, biodiversity and species composition, was shown to be 

related to water depth, location in the Mississippi and De Soto canyons, co-location with 

methane seeps, sediment organic matter content, association with high surface water primary 

production and an east-west geographic location. Data do not indicate that the biota was affected 

by location in small basins, sediment grain size, oxygen concentration, temperature, salinity or 

chemical contaminants within the sediments. The data base generated at the species level by 

taxonomists associated with the project allows extensive detailed maps to be drawn of a broad 

suite of biological characteristics. 

Stock size and sediment community oxygen consumption (SCOC) decreased exponentially 

with depth, and this is inferred to be controlled by decreasing food supplies from the surface 
viii 

water or exported offshore to deeper water from the upper depths of the continental slope. 

Groups of recurrent species occurred in four primary depth zones that lined the basin between 

isobaths. These zones appear to be related to specific levels of food supply in the form of 

sedimenting particulate organic matter. Diversity among most groups reached a maximum on the 

upper continental slope between 1.2 and 1.6 km depth, from which it declined to the abyssal 

plain. While alpha diversity was lower in the Mississippi Canyon within individual sampling 

sites, the total number of species was higher in the canyon compared to adjacent transects, due, it 

is thought, to the canyon’s higher physical complexity. 

The SCOC declined rapidly as depth increased. It was approximately three to ten times 

lower on the upper slope (500 to 2,000-m depth) than on the adjacent continental shelf (10 to 200 

m depth), but the upper slope was approximately ten times higher than out on the abyssal plain (2 

to 7 mg C m-2 d-1). The turnover time of the living carbon pool (the total standing stock of the 

biota), calculated by dividing the total biomass by the SCOC, was on the order of days to 

months, whereas the detrital organic carbon turnover time was in years. 

The standing stock information from each group has been assembled into a set of food web 

budgets for each of the six sites where community function was measured. Rates of respiration, 

growth and predation have been estimated in terms of carbon cycling (mg C m-2 d-1) at presumed 

steady state for each stock. These data illustrate that the importance of large animals (the 

megafauna, the fishes and the macrofauna) decreases more rapidly with depth than the small size 

groups (meiofauna and bacteria). The detrital carbon is the largest pool in all sediments, but its 

relative importance increases with depth, suggesting that much of it is unreactive. All the 

metazoan stocks declined with water depth in a statistically significant pattern, but the bacteria 

did not, suggesting that the role of the bacteria in community function is increasing as the 

organic matter becomes less reactive. 

The completion of DGoMB was accomplished through cooperation of ten different 

institutions in three countries. Fiftyseven students at the undergraduate and graduate level 

participated in either the field work, sample processing or data analysis. All the data generated 

have been deposited in the archives of the National Oceanographic Data Center (NODC) and the 

faunal listings are included in the Ocean Biological Information System (OBIS). Individual 

species lists and the environmental data are available from the MMS, the Program Manager or 

the individuals who generated the separate data by category, with appropriate restrictions. 

It can be concluded that potential interactions between the biotic communities and O&G 

industry activities, including potentially delaterious faunal alterations of the resident biotic 

communities, could be substantially greater on the upper continental slope (down to 2,000-m 

depth) simply because the standing stocks are so much higher on the upper slope. On the other 

hand, the “upper slope” biota may be better adapted to respond to and thus survive alterations in 

available substrate and organic loading, because of the greater variability at those depths than in 

deeper water. Alternatively, the carbon-based food web models suggest that the introduction of 

biodegradable organics, whatever the source, could stimulate biomass production at all locations 

in the deep GoM where the biota is food limited. The general principle has emerged that the 

biota, in the rather quiescent and benign environment of the deep ocean, is controlled by various 

sources of organic nourishment, including those introduced by mankind’s activities. This might 

cause a shift in the location or depths of the biotic zones described by enhancing concentrations 

of organic material in association with exploration or production at a well head. It remains to be 

seen if and when such a hypothetical model prediction might be valid. 
ix 

TABLE OF CONTENTS 

Page

LIST OF FIGURES ...................................................................................................................... xv 

LIST OF TABLES.................................................................................................................... xxvii 

1. INTRODUCTION..................................................................................................................... 1 

1.1 Program Objectives ........................................................................................................ 3 

1.2 The Program................................................................................................................... 5 

2. PRESENT STATE OF KNOWLEDGE.................................................................................... 7 

2.1 General Deep-Sea Ecology ............................................................................................ 7 

2.2 Deep-Sea GoM—Geologic Setting.............................................................................. 10 

2.3 Geochemistry of Deep-Sea Sediments ......................................................................... 11 

2.4 Physical Oceanography of the GoM ............................................................................ 12 

2.5 Deep-Sea Ecology of the GoM .................................................................................... 14 

3. A CONCEPTUAL MODEL OF THE DEEP-SEA................................................................. 17 

3.1 Biotic Variables............................................................................................................ 19 

3.2 Community Structure ................................................................................................... 21 

3.3 Community Function.................................................................................................... 22 

3.4 Sediment Properties...................................................................................................... 22 

3.5 Water Column Properties............................................................................................. 24 

3.6 Community “Health”.................................................................................................... 24 

4. SAMPLING DESIGN............................................................................................................. 27 

4.1 General Design Considerations.................................................................................... 27 

4.2 Working Hypotheses and Station Selection ................................................................. 28 

4.3 Dependent and Independent Variables......................................................................... 35 

4.4 Statistical Analyses....................................................................................................... 36 

4.4.1 Univariate Analyses........................................................................................ 38 

4.4.2 Power Analysis............................................................................................... 41 

4.4.3 Multivariate Analyses ..................................................................................... 41 

5. THE FIELD PROGRAM ........................................................................................................ 43 

5.1 The Ship—R/V GYRE................................................................................................. 43 

5.2 Cruise I—Survey.......................................................................................................... 44 

5.3 Cruise II—Processes .................................................................................................... 44 

5.4 Third-Year Field Activities .......................................................................................... 47 

6. FIELD METHODS ................................................................................................................. 51 

6.1 Standard Sampling Techniques.................................................................................... 51 

6.1.1 Boxcoring........................................................................................................ 51 

6.1.2 Trawling.......................................................................................................... 51 

6.1.3 Photo-Apparatus ............................................................................................. 53 

6.1.4 Water Column Profiles ................................................................................... 53 
x 

6.1.5 On-site Sampling Mapping............................................................................. 55 

6.2 Community Structure ................................................................................................... 55 

6.2.1 Bacteria ........................................................................................................... 55 

6.2.2 Protozoa (Foraminifera).................................................................................. 55 

6.2.3 Meiofauna ....................................................................................................... 56 

6.2.4 Macrofauna ..................................................................................................... 57 

6.2.5 Megafauna and Fishes..................................................................................... 59 

6.2.5.1 Trawl Samples................................................................................. 59 

6.2.5.2 Photosurveys ................................................................................... 59 

6.3 Community Function.................................................................................................... 59 

6.3.1 Microbial Metabolism..................................................................................... 59 

6.3.2 Sediment Community Oxygen Demand ......................................................... 59 

6.3.3 Foodweb Studies—Stable Isotopes................................................................. 61 

6.4 Sediment Properties...................................................................................................... 61 

6.5 Chemical Contaminants ............................................................................................... 61 

6.6 Geochemistry................................................................................................................ 61 

7. LABORATORY METHODS ................................................................................................. 63 

7.1 Taxonomy..................................................................................................................... 63 

7.2 Community Structure ................................................................................................... 65 

7.2.1 Bacteria ........................................................................................................... 65 

7.2.2 Foraminifera.................................................................................................... 65 

7.2.3 Meiofauna ....................................................................................................... 65 

7.2.4 Macrofauna ..................................................................................................... 66 

7.2.5 Megafauna and Fishes..................................................................................... 67 

7.3 Community Function (Process Studies)....................................................................... 67 

7.3.1 Bacterial Metabolism...................................................................................... 67 

7.3.2 Meiofaunal Feeding Rates .............................................................................. 68 

7.3.3 Sediment Community Respiration.................................................................. 68 

7.3.4 Foodweb Analyses—Stable Isotopes.............................................................. 68 

7.4 Sediment Properties...................................................................................................... 68 

7.4.1 Grain Size........................................................................................................ 68 

7.4.2 Geotechnical Properties.................................................................................. 69 

7.4.3 Total Organic and Inorganic Carbon .............................................................. 69 

7.5 Chemical Contaminants ............................................................................................... 69 

7.5.1 Hydrocarbons.................................................................................................. 69 

7.5.2 Trace Metals.................................................................................................... 70 

7.6 Geochemical Properties................................................................................................ 70 

7.7 Water Column Profiles................................................................................................. 73 

7.8 Archival of Specimens ................................................................................................. 73 

7.9 Statistical Analyses....................................................................................................... 74 

7.9.1 Bathymetric Zonation of Recurrent Groups of Related Organisms................ 74 

7.9.2 Uni-variate Analysis....................................................................................... 74 

7.9.3 Multivariate Analysis...................................................................................... 75 

7.9.4 Depth-Related Changes in Species Composition and Organism 

Densities.......................................................................................................... 75 

7.9.5 Statistical Packages Used................................................................................ 75 
xi 

8. PROGRAMMATIC RESULTS AND DISCUSSION............................................................ 77 

8.1 Physical Oceanography ................................................................................................ 77 

8.1.1 Physical Oceanographic Forcings................................................................... 77 

8.1.1.1 Sea Surface Height Fields ............................................................... 77 

8.1.2 Gulf of Mexico Winds During Summer ......................................................... 84 

8.1.3 Influence of River Discharge .......................................................................... 88 

8.2 Physical Oceanography ................................................................................................ 89 

8.2.1 Water Property Patterns.................................................................................. 89 

8.2.2 Circulation Patterns from ADCP Measurements.......................................... 104 

8.2.2.1 Moored ADCP measurements ...................................................... 104 

8.2.3 Shipboard ADCP Measurements .................................................................. 107 

8.3 Ocean Color Climatology........................................................................................... 115 

8.4 Geology ...................................................................................................................... 128 

8.4.1 Sediment Texture—Grain Size Distributions ............................................... 128 

8.4.2 Sediment Physical Properties........................................................................ 132 

8.4.3 Indications of Bioturbation ........................................................................... 132 

8.5 Biogeochemistry......................................................................................................... 140 

8.5.1 Bulk Sediment Organic Carbon and Nitrogen.............................................. 140 

8.5.2 Sediment Profiles of Organic Carbon........................................................... 141 

8.5.3 Dissolved Organic Carbon (DOC)................................................................ 145 

8.5.4 Dissolved Inorganic Carbon (including δ13C) .............................................. 147 

8.5.5 Stable Isotope Ratios from Sediment-dwelling Bacteria.............................. 150 

8.5.6 Dissolved Oxygen......................................................................................... 150 

8.5.7 Dissolved Manganese and Sulfate ................................................................ 152 

8.5.8 Sulfate Reduction Rates................................................................................ 153 

8.5.9 Sediment Community Oxygen Consumption (SCOC)................................. 154 

8.5.10 Radiochemistry of the Sediments ................................................................. 156 

8.6 Contaminant Chemistry.............................................................................................. 161 

8.6.1 Metals............................................................................................................ 161 

8.6.2 Contaminant Organic Compounds................................................................ 165 

8.7 Large Plant Detritus and Human-Generated Refuse .................................................. 172 

8.8 Benthic Biology.......................................................................................................... 177 

8.8.1 Microbiota..................................................................................................... 177 

8.8.1.1 Benthic Bacterial Abundance and Biomass .................................. 177 

8.8.1.2 Benthic Bacterial Production and Respiration.............................. 183 

8.8.2 Benthic Foraminifera .................................................................................... 188 

8.8.3 Meiofauna ..................................................................................................... 193 

8.8.3.1 Quantitative Distribution of Meiofauna........................................ 193 

8.8.3.1.1 Core Size Comparison, Sediment Sampling 

Depth, and Box Core Comparison............................... 193 

8.8.3.1.2 General Results............................................................ 194 

8.8.3.1.3 Hypothesis Testing ...................................................... 200 

8.8.3.2 Harpacticoida (Copepoda) Community Structure......................... 201 

8.8.3.2.1 Hypothesis Testing ...................................................... 208 

8.8.3.2.2 Spatial and Bathymetric Species Zonation .................. 210 

8.8.3.2.3 Regional and Global Species Estimates ...................... 210 
xii 

8.8.3.3 Meiofauna Biomass....................................................................... 212 

8.8.3.4 Allometric Respiration Estimates and Community Carbon 

Transfer ......................................................................................... 212 

8.8.4 Macrofauna ................................................................................................... 219 

8.8.4.1 Quantitative Distribution of Macrofauna...................................... 219 

8.8.4.2 Dominant Macrofauna Taxa ......................................................... 224 

8.8.4.2.1 Isopod Crustaceans (Geo.”Buz” Wilson, Aust. 

Nat. Mus.) .................................................................... 226 

8.8.4.2.2 Bivalve Molluscs (Min Chen, Mary Wicksten and 

Roe Davenport, TAMU).............................................. 231 

8.8.4.2.2.1 Abundance .............................................. 231 

8.8.4.2.2.2 Diversity.................................................. 236 

8.8.4.2.2.3 Zonation .................................................. 240 

8.8.4.3 Polychaete Annelids (G. Fain Hubbard, TAMUG, and 

Yuning Wang, Texas Tech) .......................................................... 240 

8.8.4.3.1 Polychaete Density ...................................................... 240 

8.8.4.3.2 Polychaete Species Richness and Diversity ................ 240 

8.8.4.3.3 Bathymetric Zonation of Recurrent Groups of 

Polychaetes .................................................................. 252 

8.8.4.4 Amphipod Crustaceans (John Foster, MSU, and Yousra 

Soliman, TAMU) .......................................................................... 256 

8.8.4.5 “Total” Macrofauna Community Structure (Chihlin Wei and 

G. Rowe, TAMU and TAMUG)................................................... 256 

8.8.5 Benthic Megafaunal Invertebrates................................................................ 263 

8.8.5.1 Comparison of Catches by Type of Gear...................................... 263 

8.8.5.2 Species Richness, α Diversity, Biomass and Abundance............. 264 

8.8.5.3 Comparisons by Transects ............................................................ 266 

8.8.5.4 Comparisons by Taxonomic Groups............................................. 266 

8.8.5.5 Diversity Gradients....................................................................... 272 

8.8.5.6 Zonation of the Megafaunal Invertebrates.................................... 276 

8.8.5.7 Megafauna Assessment Using Seafloor Photography 

(Matthew Ziegler, UNM).............................................................. 277 

8.8.6 Demersal Fishes (John McEachran, TAMU, and Richard Haedrich, 

MUN)............................................................................................................ 316 

8.8.7 Near-Bottom and Pelagic Fauna ................................................................... 329 

8.8.7.1 ADCP Detection of Mobile Near-Bottom Fauna.......................... 329 

8.8.7.2 Trap Caught Organisms ................................................................ 343 

8.8.8 Effects of Hydrocarbon Seep Sites............................................................... 344 

9. DISCUSSION AND SYNTHESIS ....................................................................................... 349 

9.1 Animal Distributions and Community Structure........................................................ 349 

9.1.1 Mapping Distributions.................................................................................. 349 

9.1.2 Standing Stocks and Biomass Implications .................................................. 349 

9.1.3 Diversity and Species Richness Generalizations.......................................... 351 

9.1.4 Bathymetric Zonation—Variations and Putative Causes ............................. 352 

9.1.5 Implications of and Conclusions Based on the Hypothesis Test Results ..... 354 

9.1.6 Variations In and Between Size Categories.................................................. 360 
xiii 

9.1.7 Comparison with Other Continental Margins............................................... 362 

9.2 Principal Components Analysis ................................................................................. 362 

9.3 Biogeochemical Processes—Community Function ................................................... 365 

9.3.1 Comparison of Methods................................................................................ 365 

9.3.2 Comparison of Total Community Metabolism with Other Ocean 

Margins ......................................................................................................... 366 

9.4 Community Function and Structure Relationships .................................................... 367 

9.4.1 Carbon Budgets for Process Sites................................................................. 367 

9.4.2 Metabolism as a Function of Biomass.......................................................... 367 

9.4.3 Total Metabolism and Its Relation to Species Composition......................... 368 

9.4.4 Food Web Carbon Budget ............................................................................ 369 

9.4.5 Sequestration of Conservative Chemical Properties (Bioturbational 

Mixing) ......................................................................................................... 375 

10. UNKNOWNS AND RECOMMENDATIONS .................................................................... 377 

10.1 Community Structure ................................................................................................. 377 

10.2 Community Function.................................................................................................. 378 

10.3 Taxonomy................................................................................................................... 379 

10.4 Relationships between Function and Structure .......................................................... 379 

10.5 The Unknowns and the Unknowable ......................................................................... 380 

10.6 Summary of Unknowns and Recommendations for Further Study ........................... 380 

LITERATURE CITED ............................................................................................................... 383 
xv 

LIST OF FIGURES 

Page

Figure 3-1. Preliminary conceptual model of the deep-sea Gulf of Mexico. ...........................18 

Figure 3-2. Macrofauna submodel of system carbon food chain model. .................................19 

Figure 3-3. Sediment submodel of functional bacterial groups as defined by utilization 

of different terminal electron acceptors. ................................................................25 

Figure 4-1. Benthic survey stations in the far northwest GoM.................................................29 

Figure 4-2. Benthic survey stations across the Florida Escarpment.........................................30 

Figure 4-3. Sampling sites along the Mississippi Trough (MT1-MT6). ..................................31 

Figure 4-4. Physiographic comparisons within (B) and outside (NB) mesoscale basins 

in the Central GoM. ...............................................................................................33 

Figure 5-1. Benthic survey stations for Cruise I, May and June 2000. ....................................45 

Figure 5-2. Sampling sites during Cruise II, the first “processes” cruise, June 1-19, 

2001........................................................................................................................47 

Figure 5-3. Sampling sites on Leg 1 and 2, DGoMB cruise 3. ................................................48 

Figure 6-1. Schematic of quantitative boxcore station activities and sampling 

protocols.................................................................................................................52 

Figure 6-2. Schematic of trawl station activities and sampling protocols................................54 

Figure 6-3. Vertical distribution of meiofauna taxa for Cruise I. Average of 5 

replicates. ...............................................................................................................58 

Figure 6-4. Benthic lander. .......................................................................................................60 

Figure 8-1. Sea surface height field for 13 May 2000 with ADCP cruise track and 

CTD stations for Leg 1 of DGoMB cruise 1 overlain. ..........................................78 

Figure 8-2. Sea surface height field for 10 June 2000 with ADCP cruise track and 

CTD stations for Leg 2 of DGoMB cruise 1 overlain. ..........................................79 

Figure 8-3. Sea surface height field for 10 June 2001 with ADCP cruise track, CTD 

stations, and moored ADCP locations for the DGoMB cruise 1 overlain.............80 

Figure 8-4. Sea surface height field for 8 June 2002 with ADCP cruise track, CTD 

stations, and moored ADCP locations for DGoMB cruise 1 overlain...................81 

Figure 8-5. Sea surface height field for 8 August 2002 with the ADCP cruise track for 

DGoMB cruise 1 overlain......................................................................................82 

Figure 8-6. Salinity at approximately 3.5-m depth from thermosalinograph 

observations on NEGoM cruise N9 in July-August 2000. ....................................85 

Figure 8-7. COADS enhanced mean wind field and principal component ellipses for 

June. Data were averaged over the period 1960-1997...........................................86 

Figure 8-8. Wind speed from National Data Buoy Center meteorological buoys 42041 

(2000) and 42001 (2001 and 2002) for May through August during the 

DGoMB field program...........................................................................................87 

Figure 8-9. Daily discharge for 2000-2002 from the Mississippi River, measured at 

the Tarbert Landing, Louisiana, gauge. .................................................................89 

Figure 8-10. Selected station data showing salinity versus density (top) and depth 

(bottom)..................................................................................................................90 

Figure 8-11. Selected station data showing potential temperature versus density (top) 

and depth (bottom).................................................................................................91 
xvi 

Figure 8-12. Locations of selected CTD/bottle stations on the DGoMB cruises in 

summers 2000-2002...............................................................................................92 

Figure 8-13. Density anomaly (σθ in kg·m-3) on western-most transect of DGoMB 

cruise 1, 4 May-17 June 2000................................................................................95 

Figure 8-14. Salinity from the CTD on western-most transect of DGoMB cruise 1, 4 

May-17 June 2000..................................................................................................96 

Figure 8-15. Nitrate (μM) on the western-most transect of DGoMB cruise 1, 4 May-17 

June 2000. ..............................................................................................................97 

Figure 8-16. Dissolved oxygen (mL/L) on the western-most transect of DGoMB cruise 

1, 4 May-17 June 2000..........................................................................................98 

Figure 8-17. Water properties versus depth for DGoMB cruises in 2000 and 2001................100 

Figure 8-18. Water properties versus density for DGoMB cruises in 2000 and 2001. ............101 

Figure 8-19. Water properties versus density for the eastern GoM..........................................102 

Figure 8-20. Water properties versus density for the western GoM.........................................103 

Figure 8-21. Record-length mean speed (solid) and plus and minus one standard 

deviation (dashed) of currents measured using a bottom-moored 

downward-looking 300-kHz ADCP during four deployments of Cruise 3 

in 2001. ................................................................................................................105 

Figure 8-22. Record-length mean speed (solid) and plus and minus one standard 

deviation (dashed) of currents measured using a bottom-moored 

downward-looking 300-kHz ADCP during two deployments during Cruise 

4 in 2002. .............................................................................................................106 

Figure 8-23. Contours of sea-surface height from satellite altimeter data and nearsurface velocity vectors from shipboard ADCP during Cruise 1, leg 1...............108 

Figure 8-24. Vertical section of current speed based on shipboard 150-kHz ADCP data 

collected along a section in the western GoM during Cruise 1, leg 1, in 

June 2000. ............................................................................................................109 

Figure 8-25. Contours of sea-surface height from satellite altimeter data and nearsurface velocity vectors from shipboard ADCP during Cruise 2. SSH data 

are from 15 June 2001 and are courtesy of R. Leben (CU). ................................110 

Figure 8-26. Vertical section of current speed based on shipboard ADCP data collected 

along a section in the north-central GoM during Cruise 2 in June 2001. ............111 

Figure 8-27. Contours of sea-surface height from satellite altimeter data and nearsurface velocity vectors from shipboard ADCP during Cruise 3a.......................112 

Figure 8-28. Contours of sea-surface height from satellite altimeter data and nearsurface velocity vectors from shipboard ADCP during Cruise 3b. .....................113 

Figure 8-29. Vertical section of current speed based on shipboard ADCP data collected 

along a section in the southeastern GoM during Cruise 3b in August 2002........114 

Figure 8-30. Three-year climatology of biweekly averaged SeaWiFS ocean color data 

1998-2000 at the nine DGoMB stations in water depth > 2,000 m in the 

western GoM........................................................................................................121 

Figure 8-31. Average annual cycle of chlorophyll at these deepwater stations in the 

western GoM, illustrating the typical annual range from summertime lows 

of about 0.1 mg m-3 to November-February highs of about 0.3 mg m-3..............121 
xvii 

Figure 8-32. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the six DGoMB stations along the Far West (RW) 

transect in the western GoM. ...............................................................................122 

Figure 8-33. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the six DGoMB stations along the Western (W) 

transect in the GoM. Bottom panel: The average annual cycle of 

chlorophyll at four of the Western stations..........................................................123 

Figure 8-34. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the nine WC, NB, and B DGoMB stations located 

between 93W and 91W over the Louisiana continental slope in the 

western GoM. Bottom panel: The average annual cycle of chlorophyll at 

the seven deepest of these Louisiana slope stations.............................................124 

Figure 8-35. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the six DGoMB stations along the Easternmost 

(S) transect in the eastern GoM. Bottom panel: The annual cycle of 

chlorophyll at S39, the deepest of the Eastern transect stations. .........................125 

Figure 8-36. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at five DGoMB stations along the Mississippi 

Trough (MT) transect in the eastern GoM. Bottom panel: The annual cycle 

of chlorophyll at MT6, the deepest of these Mississippi Trough transect 

stations. ................................................................................................................126 

Figure 8-37. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the five DGoMB stations along the central (C) 

transect in the eastern GoM. Bottom panel: Three-year climatology of 

biweekly averaged SeaWiFS ocean color data 1998-2000 at the high 

productivity (Hi-Pro) station in the eastern GoM................................................127 

Figure 8-38. Sea-surface height (SSH) maps compiled by the University of Colorado, 

showing the location of warm slope eddies (positive SSH anomalies) in 

the eastern GoM in summers 1998-2000.............................................................129 

Figure 8-39. Percent sand, silt and clay at each site. ................................................................130 

Figure 8-40. Relative amounts of percent sand at each site. ....................................................131 

Figure 8-41. The sediment geotechnical properties of cores (5 replicates) taken at the 

NB3 location. .......................................................................................................133 

Figure 8-42. The sediment geotechnical properties of cores (5 replicates) taken at the 

MT6 location........................................................................................................134 

Figure 8-43. Map showing the locations of the stations that have highly variable 

geotechnical properties. .......................................................................................135 

Figure 8-44. Boxcore 134 at station W4 illustrating sediment that has almost no 

sediment bioturbation...........................................................................................136 

Figure 8-45. Boxcore 12 at station MT5 illustrating sediment that has undergone a high 

degee of bioturbation. ..........................................................................................137 

Figure 8-46. Boxcore 111 at station RW5 illustrating a low degree of bioturbation. ..............138 

Figure 8-47. Map showing the degree of sediment bioturbation present at the survey 

sites. .....................................................................................................................139 

Figure 8-48. Percent organic carbon data for survey stations. .................................................140 
xviii 

Figure 8-49. Sediment organic carbon and nitrogen. Top: three sets of organic carbon 

data from, UNAM (abyssal plain) and TAMU (continental margin); 

bottom: two sets of total sedimentary nitrogen data, analyzed at UNAM 

(yellow) and TAMU (red)....................................................................................141 

Figure 8-50. Profiles of organic carbon....................................................................................142 

Figure 8-51. Profiles of organic carbon MT6...........................................................................143 

Figure 8-52. Profiles of organic carbon S36.............................................................................143 

Figure 8-53. Profiles of organic carbon JSSD1........................................................................144 

Figure 8-54. Profiles of organic carbon JSSD4........................................................................144 

Figure 8-55. Profiles of organic carbon S42.............................................................................145 

Figure 8-56. Top: Site means and standard deviations (2σ) for dissolved organic carbon 

(DOC) in the top 5 cm of sediment. Bottom: Pore water depth variations 

for DOC from experimental stations....................................................................146 

Figure 8-57. Site means and standard deviations (2σ) versus station water depth for 

dissolved organic carbon (DOC; top), dissolved inorganic carbon (DIC; 

middle) and stable carbon isotope ratio of DIC (δ13C-DIC bottom) in the 

top 5 cm of sediment............................................................................................147 

Figure 8-58. Top: Site means and standard deviations (2σ) for dissolved inorganic 

carbon (DIC) in the top 5 cm of sediment. Bottom: Pore water depth 

variations for DIC from experimental stations. ...................................................148 

Figure 8-59. Top: Site means and standard deviations (2σ) for stable carbon isotope 

ratios of dissolved inorganic carbon (δ13C-DIC) in the top 5 cm of 

sediment. Bottom: Pore water depth variations for δ13C-DIC from 

experimental stations. ..........................................................................................149 

Figure 8-60. Stable carbon isotope ratio (δ13C) of sedimentary organic matter, 

phospholipid fatty acid (PLFA) 16:0 and i&a15:0 at stations MT1, MT2, 

and MT3...............................................................................................................151 

Figure 8-61. Dissolved oxygen distribution with depth in stations MT3 (solid circle; 

note all values below detection limit), MT6 (open square), S36 (solid 

diamond), and S42 (X).........................................................................................152 

Figure 8-62. Dissolved Mn2+ at station MT3............................................................................153 

Figure 8-63. Sediment Community Oxygen Consumption (SCOC) expressed as carbon 

(see text for conversion details), plotted as a function of water depth. ...............154 

Figure 8-64. Sediment Community Oxygen Consumption (SCOC) as a function of 

depth.....................................................................................................................155 

Figure 8-65. 234Thxs ([234Th(total)] – [238U]) penetration into surface sediments at each 

station, allowing determination of particle reworking (bioturbation) rate 

(Db) and depth (zm), units are mBq/g. Db was calculated from the slope of 

ln[234Thxs(z)] = ln[234Thxs(0)] – (λ/Db)

1/2z............................................................158 

Figure 8-66. 210Pbxs ([210Pb (total)] – [210Pb (supported) = 226Ra]) profiles for all 

stations, all 210Pbxs data below the 234Thxs penetration depth is plotted...............160 

Figure 8-67. Variations in total sediment calcium (A), chromium (B), zinc (C), and 

vanadium (D) concentrations as a function of changes in sediment 

mineralogy (as indicated by sediment aluminum levels) measured by 

inductively coupled plasma spectrometer (ICP)..................................................163 
xix 

Figure 8-68. Variations in total sediment cadmium (A), copper (B), nickel (C), and lead 

(D) concentrations as a function of changes in sediment mineralogy (as 

indicated by sediment aluminum levels)..............................................................164 

Figure 8-69. The concentration of perylene in sediments. .......................................................168 

Figure 8-70. The concentration of total polycyclic aromatic hydrocarbons (PAH) 

without perylene in sediments. ............................................................................169 

Figure 8-71. The total PAH concentration without perylene versus barium

concentration........................................................................................................170 

Figure 8-72. Frequency distribution of total PAH and total PAH without perylene 

concentrations versus cumulative percentage......................................................171 

Figure 8-73. Trash occurrence in trawls...................................................................................174 

Figure 8-74. Plant type occurrence in trawls............................................................................175 

Figure 8-75. Sidwinder missle recovered on the Florida slope. ...............................................176 

Figure 8-76. Longline fishing line, hooks removed, measuring ca. 1 m by 3 m......................176 

Figure 8-77. Global data set for depth-integrated, benthic bacterial abundance versus 

station depth (without 10% adjustment to shelf-slope stations). .........................180 

Figure 8-78. Global data set for depth-integrated, biovolume-based, benthic bacterial 

biomass versus station depth (without 10% adjustment to shelf-slope 

stations)................................................................................................................180 

Figure 8-79. Depth-integrated, biovolume-based, benthic bacterial biomass versus 

station depth for transect stations only.................................................................182 

Figure 8-80. Significant difference (P = 0.020; Table 8-7) between biovolume-based, 

benthic bacterial biomass on upper slope (S42-44) versus lower slope/rise 

stations (S39-41) of the Sigsbee Escarpment.......................................................183 

Figure 8-81. Downcore profiles of 14C-amino acid-based, benthic bacterial respiration.........187 

Figure 8-82. Kinetics of 14C-amino acid-based, benthic bacterial respiration under in 

situ pressure and temperature, except for station S4 where the incubation 

temperature was ~10°C........................................................................................188 

Figure 8-83. Pelosina sp., large agglutinated arborescent foraminifer. A. Specimen in 

situ. B. Specimen removed from sediment to show sub-surface “root” 

structure. C. Two additional specimens, one broken...........................................190 

Figure 8-84. Scatter plot comparing foraminiferal abundance in the surface 1 cm to that 

of meiofaunal metazoans. ....................................................................................192 

Figure 8-85. Scatter plot comparing foraminiferal abundance in the top 3 cm to that of 

meiofaunal metazoans..........................................................................................192 

Figure 8-86. Scatter plot comparing foraminiferal biomass in the surface 1 cm to that of 

meiofaunal copepods and nematodes combined..................................................193 

Figure 8-87. Scatter plot comparing foraminiferal biomass in the top 3 cm to that of 

meiofaunal copepods and nematodes combined..................................................193 

Figure 8-88. Vertical distribution of meiofauna taxa from sediment cores collected at 

station W2 during the shakedown cruise. ............................................................195 

Figure 8-89. Log (x+1) transformed meiofauna abundance (N m-2) versus water depth 

(m) for all stations sampled during DGoMB project...........................................197 

Figure 8-90. Spatial analysis of meiofauna abundance at all DGoMB stations. ......................198 

Figure 8-91. A) Expected number of taxa per 1,000 individuals [ES(1000)] as a 

function of depth. B) Expected number of taxa per 20 individuals [ES(20)] 
xx 

for non-dominant meiofauna taxa (excluding nematodes, harpacticoid 

copepods, harpacticoid nauplii, and unknowns)..................................................199 

Figure 8-92. SeaWIFS chl-a (μg/L) biweekly average (November 1999 through April 

2000). Chl-a concentration was adjusted for remineralization with depth. 

A) Inverse first-order relationship of adjusted chl-a with water depth. Chla concentration decreases with depth reflecting rapid remineralization in 

surface waters. B) Meiofauna abundance versus adjusted chl-a.........................202 

Figure 8-93. Meiofauna abundance (Nm-2) as a function of sediment particulate organic 

carbon (POC). ......................................................................................................203 

Figure 8-94. Harpacticoid copepod abundance (N) and species richness (S), adjusted to 

the number per 10 cm2

, as a function of depth. ...................................................206 

Figure 8-95. Shannon-Wiener diversity index (H’) as a function of depth for pooled 

replicate core samples of harpacticoid copepods.................................................207 

Figure 8-96. Expected number of harpacticoid species per 30 individuals [ES(30)], for 

pooled replicate core samples of harpacticoid copepods.....................................207 

Figure 8-97. A) Average taxonomic diversity (Δ) and B) average phylogenetic diversity 

(Φ+) as a function of depth for the 43 original survey stations. ..........................209 

Figure 8-98. Geographic location of recurrent groups of harpacticoid copepods, based 

on percent similarity (see text).............................................................................211 

Figure 8-99. Species accumulation curves used to estimate regional Harpacticoida 

species abundance in the Gulf of Mexico by extrapolation.................................211 

Figure 8-100. Meiofauna biomass (μg wet wt/m2) versus water depth at all DGoMB 

stations. ................................................................................................................214 

Figure 8-101. Average wet mass (mg) per individual A) Nematode, and B) Harpacticoid, 

as a function of depth...........................................................................................215 

Figure 8-102. Spatial interpolation of meiofaunal biomass (mg C m-2) in the northern 

Gulf of Mexico deep sea, using the inverse-distance weighted model................216 

Figure 8-103. A) Meiofauna mass-dependent respiration rate (d-1) and B) meiofauna 

community respiration (mg C m-2 d-1) at each of the 51 DGoMB stations 

in the northern Gulf of Mexico deep-sea. ............................................................217 

Figure 8-104. Spatial interpolation of the meiofaunal commun ity daily carbon 

requirement (mg C m-2 d-1), based on allometric estimations of community 

respiration. ...........................................................................................................218 

Figure 8-105. a) Total macrofaunal densities as a function of depth (top); b) bottom 

excludes ‘east,’ Mississippi Canyon and abyssal sites. .......................................223 

Figure 8-106. Total macrofaunal densities by depth plotting all sorted samples from the 

Mississippi Trough (MT).....................................................................................224 

Figure 8-107. Total macrofaunal densities by depth plotting all sorted samples from the 

eastern Gulf of Mexico (S). .................................................................................225 

Figure 8-108. Densities of total macrofauna (all fauna >300 micron sieve) at NGoM 

sampling sites.......................................................................................................225 

Figure 8-109. Distribution of macrofauna greater than >300 micron sieve size, excluding 

the large Nematoda. .............................................................................................226 

Figure 8-110. Sampling pattern of the LGL study. ....................................................................227 

Figure 8-111. Sampling pattern of the DGoMB study. ..............................................................227 
xxi 

Figure 8-112. Relationship between isopod species and individuals in individual 

replicates. .............................................................................................................228 

Figure 8-113. Patterns of species richness, using E(S15), in the Gulf of Mexico. Isobaths 

from top to bottom are 200 m, 1,000 m, 2,000 m, 3,000 m.................................228 

Figure 8-114. Species richness, using ES15, and species density (species per core) by 

depth in the Gulf of Mexico.................................................................................229 

Figure 8-115. Species richness and longitude. ...........................................................................230 

Figure 8-116. Depth ranges and longitudinal ranges of Isopoda in the Gulf of Mexico............230 

Figure 8-117. DGoMB and LGL isopod macrofauna species richness compared.....................231 

Figure 8-118. Map of density distribution of bivalves (ind. m-2) in the GoM............................234 

Figure 8-119. Linear regression plot of bivalve log density against depth. ...............................235 

Figure 8-120. Map of diversity (Shannon-Wiener Species Index) distribution of bivalves 

in the GoM. ..........................................................................................................237 

Figure 8-121. Curvilinear regression plot of bivalve diversity against depth. ...........................238 

Figure 8-122. Dendrogram based on percent similarity of bivalve mollusk species..................241 

Figure 8-123. Bathymetric zonation of bivalves based on percent similarity. ...........................242 

Figure 8-124. Abundance of polychaete annelid worms in the northern GoM..........................251 

Figure 8-125. Diversity (H(s’)) of polychaetes as a function of depth.......................................251 

Figure 8-126. ES(50) of polychaetes as a function of depth. Note that the pattern is the 

same as in 8.125, but spread out. .........................................................................252 

Figure 8-127. Map of nine recurrent groups based on percent similarity (polycheate). ............253 

Figure 8-128. Dendrogram of percent similarities of polychaetes in the northern Gulf of 

Mexico. Nine groups are delimited at >20%. .....................................................254 

Figure 8-129. Species richness (γ diversity) of amphipods........................................................255 

Figure 8-130. Species diversity [ES(30)] of amphipods. ...........................................................255 

Figure 8-131. Dendrogram of amhipod species % similarity between sites, with 8 groups 

identified at greater than 20% similarity..............................................................257 

Figure 8-132. Bathymetric zonation of amphipods, based on the dendrogram in the 

previous Figure. ...................................................................................................258 

Figure 8-133. Multidimensional scaling analysis of zones of amphipods based on Bray 

Curtis % similarities between sites. .....................................................................259 

Figure 8-134. Macrofauna zonation based on 973 species.........................................................260 

Figure 8-135. The cumulative number of species renewal (empty triangle), appearance 

(solid rectangular), and disappearance (empty square) plotted against 

depth.....................................................................................................................260 

Figure 8-136. Dendrogram for DGoMB station.........................................................................261 

Figure 8-137. Mean density, biomass, and ES(100), and 1-λ’ as a function of depth................262 

Figure 8-138. The standardized ratio of mean biomass to density as a function of depth. ........262 

Figure 8-139. June 10, 2000. Dr. Mary Wicksten with the large squid, Pholidoteuthis 

adami, caught from the trawl at S42....................................................................265 

Figure 8-140. June 13, 2000. Deep-sea shrimp, Plesiopenaeus armatus, collected from 

the trawl at S37. ...................................................................................................267 

Figure 8-141. June 10, 2000. Giant isopod, Bathynomus giganteus, collected with other 

trawl samples at S42. ...........................................................................................268 

Figure 8-142. June 15, 2000. Deep-sea holothuroid, Elasipodid A, collected from the 

trawl at S38. .........................................................................................................270 
xxii 

Figure 8-143. June 5, 2000. Deep-sea holothuroid, Elasipodid A present on the sea floor 

of Mississippi trough (MT6). Image was taken during the first cruise................271 

Figure 8-144. May 7, 2000.Sea pen, Umbellula sp., collected from the trawl at B1. ................273 

Figure 8-145. June 19, 2000. Flytrap anemones, Actinoscyphia sp., collected from the 

trawl at B2............................................................................................................274 

Figure 8-146. Comparison of diversity gradients (H(s)) at 100, 50 and 25 species of 

megafauna in the trawl samples at the sites sampled...........................................275 

Figure 8-147. Dendrogram of megafaunal groups, based on Bray Curtis similarity of the 

4th root transformed abundance of each species in the trawls samples at 

each site sampled. ................................................................................................276 

Figure 8-148. Distribution of megafaunal zones in the NW GoM.............................................277 

Figure 8-149. Amphipod tubes present in great numbers cover this site at the head of the 

Mississippi trough (MT1). ...................................................................................278 

Figure 8-150. DGoMB photographic sites; rw-real west, ac-Alaminos Canyon, W-west, 

WC-west central, B-basin, NB-non basin, C-central, MT-Mississippi 

Trough, S-east, GKT-top of Green Knoll, JSSD-Joint Studies of the 

Sigsbee Deep........................................................................................................280 

Figure 8-151. NGoMCSS photographic sites; W-west, WC-west central, C-central, Eeast. ......................................................................................................................281 

Figure 8-152. DGoMB total megafaunal density (individuals/ha) by site. ................................284 

Figure 8-153. DGoMB total megafaunal density (individuals/ha) and total area (m2) 

surveyed by transect.............................................................................................285 

Figure 8-154. DGoMB total megafaunal density (individuals/ha) by taxon..............................286 

Figure 8-155. NGoMCSS total megafaunal density (individuals/ha) by taxon..........................287 

Figure 8-156. Density (individuals/ha) of the holothuroid Peniagone sp. seen in large 

numbers in the central GoM during NGoMCSS cruise 3....................................289 

Figure 8-157. GoM photographic sites.......................................................................................290 

Figure 8-158. DGoMB (open triangles) and NGoMCSS (closed triangles) Sargassum 

density (individual clumps/ha) with depth...........................................................291 

Figure 8-159. Total GoM Sargassum density (individual clumps/ha) by transect for all 

sites combined......................................................................................................292 

Figure 8-160. GoM photographic sites.......................................................................................293 

Figure 8-161. DGoMB photographic sites. ................................................................................294 

Figure 8-162. June 5, 2000. Dr. Rowe sifted through iron stones for specimens.......................295 

Figure 8-163. DGoMB trawl and photographic sites where Ironstone was sampled.................296 

Figure 8-164. Bioturbation levels from DGoMB year 1 sediment cores. ..................................297 

Figure 8-165. Bioturbation levels seen in photographs during DGoMB....................................298 

Figure 8-166. Megafaunal densities (individuals/ha) in photographs at similar depts in 

DGoMB (open triangles) and NGoMCSS (closed triangles). .............................299 

Figure 8-167. Megafaunal densities (individuals/ha) for all sites. .............................................301 

Figure 8-168. West-central transect megafauna density (individuals/ha) as a function of 

depth for both DGoMB and NGoMCSS sites......................................................302 

Figure 8-169. DGoMB western (open triangles) and eastern (closed triangles) 

megafaunal densities (individuals/ha) as functions of depth. ..............................304 

Figure 8-170. DGoMB total megafaunal abundances by taxon for both basin and nonbasin sites.............................................................................................................305 
xxiii 

Figure 8-171. DGoMB Mississippi Canyon (MT1-MT6), Mississippi Canyon adjacent 

(C1, C4, C7, and C12), DeSoto Canyon (S35-S37), and DeSoto Canyon 

adjacent (S38-S44) sites. Lines represent the 200, 1,000, 2000, and 3000-

meter isobaths. .....................................................................................................306 

Figure 8-172. DGoMB total megafaunal abundances (number per hectare) of the eight 

most abundant taxa in the Mississippi and DeSoto Canyons and the areas 

adjacent to these canyons.....................................................................................307 

Figure 8-173. Total megafaunal densities (individuals/ha) from the northern GoM 

continental slope (open diamonds), the slope off east Greenland (open 

squares), and Suruga Bay, Japan (open triangles). ..............................................309 

Figure 8-174. Decapoda density (individuals/ha) from both DGoMB and NGoMCSS 

photos as a function of depth. ..............................................................................311 

Figure 8-175. Demersal fish density (individuals/ha) from both DGoMB and NGoMCSS 

photos as a function of depth. ..............................................................................312 

Figure 8-176. Scaphopoda density (individuals/ha) from both DGoMB and NGoMCSS 

photos as a function of depth. ..............................................................................313 

Figure 8-177. Sargassum density (individuals/ha) from both DGoMB and NGoMCSS 

photos as a function of depth. ..............................................................................314 

Figure 8-178. Eastern and western continental slope megafaunal densities 

(individuals/ha) and area surveyed (m2

). .............................................................315 

Figure 8-179. DGoMB stations in the northern Gulf of Mexico that yielded fish 

specimens on the 2000 survey. ............................................................................317 

Figure 8-180. A cluster analysis of the DGoMB stations at which a minimum of five fish 

species were captured using the Bray-Curtis Dissimilarity Coefficient. .............317 

Figure 8-181. June 11, 2000. Boar fishes, Antigonia capros, collected from the trawl at 

S44. ......................................................................................................................319 

Figure 8-182. June 11, 2000. Armored searobins, Peristedion miniatum, collected from 

the trawl at S44. ...................................................................................................320 

Figure 8-183. June 17, 2000. Caelorinchus caribbaeus collected from the trawl at MT1.........321 

Figure 8-184. Abundance (catch per unit effort) of fishes at each of the four depth. ................322 

Figure 8-185. Abundance, scaled as the log of number/hour, of the fishes at each 

DGoMB trawl station...........................................................................................322 

Figure 8-186. Species richness of the fishes represented as number of species at each of 

the four depth zones.............................................................................................323 

Figure 8-187. Species richness of fishes scaled as number of species at each DGoMB 

station...................................................................................................................323 

Figure 8-188. Species Diversity (H’) of the fishes in each of the four depth zones...................324 

Figure 8-189. Process stations for the 2001 field year of DGoMB............................................329 

Figure 8-190. Sampling stations for the 2002 field year of DGoMB including station 

MT-1 (depth = 300 m) where the ADCP mooring was deployed for 39 

hours in April 2002. .............................................................................................330 

Figure 8-191. Location of the 14 June-12 July 2002 ADCP mooring in water depth of 

880 m. ..................................................................................................................330 

Figure 8-192a. Time series plots of acoustic backscatter intensity for summer 2001 

stations 935 (top) and 755 (bottom) m deep. .......................................................333 
xxiv 

Figure 8-192b. Time series plots of acoustic backscatter intensity for summer 2001 

stations 1,823 (top) and 2,740 (bottom) m deep. .................................................334 

Figure 8-193. Time series plot of acoustic backscatter intensity from the April 2002 

deployment in water depth 300 m........................................................................337 

Figure 8-194. Time series plots of unfiltered (top) and 40-hour low-pass filtered 

(bottom) ABI from the June-July 2002 deployment in water depth 880 m.........338 

Figure 8-195. Variance spectra of raw (top) and 40-hour low-pass filtered (bottom) ABI 

data from 48 m above bottom in the June-July 2002 month-long 

deployment...........................................................................................................340 

Figure 8-196. Variance spectra of raw (top) and 40-hour low-pass filtered (bottom) ABI 

data from 7 m above bottom in the June-July 2002 month-long 

deployment...........................................................................................................341 

Figure 8-197. Percent light transmittance from CTD casts made during deployment 

(top), on 22 June (middle), and recovery (bottom) of the June-July 2002 

near-bottom-moored ADCP.................................................................................342 

Figure 8-198. Northern GoM continental slope displaying DGoMB study sites with 

green stars and the two seep sites with orange triangles......................................345 

Figure 8-199. Macrofauna means for DGoMB and SSETIE sites in the Northen Gulf of 

Mexico are plotted against depth; the two seeps GB 425 and GC 234 are 

denoted by arrows with their mean macrofauna abundance in number m-2. ......345 

Figure 8-200. DGoMB and SSETIE sediment community oxygen consumption (SCOC) 

rates (mg-carbon m-2 d-1) are plotted versus depth. .............................................347 

Figure 9-1. Mean macrofauna individual size (mg C per individual) as a function of 

depth.....................................................................................................................350 

Figure 9-2. Non-metric MDS plots for 4th root species abundance data of all 

macrofauna, based on Bray-Curtis similarity. .....................................................352 

Figure 9-3. Mean density, biomass, E(S100), and E(S50) in each macrofaunal zone...............353 

Figure 9-4. Number of ‘zones’ (recurrent groups of species) in each group..........................354 

Figure 9-5. Schematic illustration of pathways by which POC is transferred from 

surface waters to the benthos. ..............................................................................358 

Figure 9-6. Log total community biomass as a function of log detrital organic carbon 

in the sediments, both integrated to a depth of 15 cm. ........................................361 

Figure 9-7. Principal components analysis of environmental variables, A) variable 

loading scores for PC1 versus PC2, B) variable loading scores for PC2 

versus PC3. ..........................................................................................................364 

Figure 9-8. Composite profiles of temperature, dissolved oxygen and percent organic 

carbon in the sediments in the northern Gulf of Mexico. ....................................365 

Figure 9-9. Relationship between sediment community oxygen consumption (SCOC) 

and total community biomass at the process sites, as indicated. .........................368 

Figure 9-10. Log SCOC (sediment community oxygen consumption) (mg C/m2

 day) as 

a function of water depth (km) for process stations.............................................369 

Figure 9-11. Log SCOC (mg C/m 2 day) as a function of water depth (km) for western 

transects, with SCOC estimated from macrofauna biomass................................370 

Figure 9-12. Carbon budget in the food web at MT1, at the Head of the Mississippi 

Canyon. ................................................................................................................371 
xxv 

Figure 9-13. Carbon budget in the food web at steady state for the Sigsbee Abyssal 

Plain. ....................................................................................................................371 

Figure 9-14. Sediment Community Oxygen Consumption (mg C/m^2-day) as a 

function of the macrofauna biomass at the process sites. ....................................373 
xxvii 

LIST OF TABLES 

Page

Table 1-1. Program Team—Key Personnel ..............................................................................4 

Table 2-1. GoM Water Masses with Associated Property Extrema and Potential 

Densities.................................................................................................................13 

Table 4-1. Summary of Benthic Survey Experimental Design: Null Hypotheses, 

Station Selection Criteria, Number (No.) of Stations (Sta.), and Number 

Samples..................................................................................................................35 

Table 4-2. Independent Variables Fixed by the Sampling Plan Design..................................36 

Table 4-3. Dependent Variables To Be Measured ..................................................................37 

Table 4-4. Ancillary Variables To Be Measured at Survey and Experimental Stations.........38 

Table 5-1. Summary of Sampling Conducted During Cruise I...............................................46 

Table 6-1. Effect of Core Tubes Size (Inner Diameter) on Meiofauna Counts and 

Average Density.....................................................................................................57 

Table 7-1. Taxonomists Involved in the Analysis of the Macrofauna and 

Corresponding Taxonomists Working on UNAM Mexican National 

Collections .............................................................................................................64 

Table 7-2. Summary of Parameters To Be Analyzed and the Methods To Be Used..............71 

Table 7-3. Chemical Parameters To Be Analyzed ..................................................................71 

Table 8-1. Summary of Mean CHL at DGoMB Stations for Biweekly Time Series of 

2 Years Versus 3 Years........................................................................................116 

Table 8-2. Variability of StdDev/Mean Ratios for Time Series of Biweekly Mean 

CHL with Different Pixel Resolution and Grid Areas.........................................119 

Table 8-3. Summary Data and Modeling Results for All Stations........................................157 

Table 8-4. DGoMB Replicate Total PAHs (ng/g).................................................................166 

Table 8-5. Benthic Bacterial Biomass at Selected DGoMB Stations1 and from the 

Literature..............................................................................................................178 

Table 8-6. R2 Values From Linear Regression Analyses of Benthic Bacterial Data on 

Abundance and Biovolume-Based Biomass as a Function of Station Depth ......181 

Table 8-7. Hypothesis Testing (Single Factor ANOVA, α = 0.05) for Factors that 

May Determine Benthic Bacterial Abundance or Biomass on the ShelfSlope Regions of the Study Area.........................................................................182 

Table 8-8. Samples from Which Benthic Foraminifera Were Extracted for ATP................184 

Table 8-9. Bacterial Production Rates in Surface Sediments (0–1 cm), Based on 3HThymidine Incubations at in Situ Temperature and Both in Situ Pressure 

and 1 Atm.............................................................................................................185 

Table 8-10. Depth-Integrated (0–15 cm) Benthic Bacterial Respiration Rates Based on 

14C-Amino Acid Incubations at in Situ Temperature and Both in Situ 

Pressure and 1 Atm ..............................................................................................187 

Table 8-11. Benthic Foraminiferal Biomass Data (mgATP/m2

), Abundance (#live/m2) 

and Average Foraminiferal Volume (avg Vol) for Pushcores Collected by 

Alvin in October 2000 ..........................................................................................189 

Table 8-12. Benthic Foraminiferal Abundances (#/m2

) and Biomass (mgC/m2) for the 

Surface cm (0-1 cm) and Integrated over the Top 3 cm (0-3 cm) .......................189 
xxviii 

Table 8-13. Comparison between Foraminiferal and Metazoan Meiofaunal Density 

(#/m2) and Biomass (mgC/m2) for the Surface Centimeter .................................191 

Table 8-14. Comparison between Foraminiferal and Metazoan Meiofaunal Density 

(#/M2

) and Biomass (Mgc/M2) for the Top 3 Centimeters..................................191 

Table 8-15. Effect of Core Tubes Size (Inner Diameter) on Meiofauna Counts and 

Average Density...................................................................................................192 

Table 8-16. DGoMB station Locations, Depth, and Average Meiofaunal Abundance 

(Five Replicate Cores) for Pooled Taxonomic Groups........................................196 

Table 8-17. Average Abundance (AA), Percent Contribution (Contrib.%), and 

Cumulative Percent Contribution (T%) of Meiofauna Major Taxa Per 

Core (5.5 cm i.d.) .................................................................................................198 

Table 8-18. ANOVA Results, Tests for Differences in Meiofauna Abundance ...................200 

Table 8-19. Results of SIMPER Analysis (Primer 5.0) Indicating Family Percent 

Contributions to Total Harpacticoid Abundance .................................................204 

Table 8-20. Total Species (S) and Total Individuals (N) Per Five Pooled Replicates 

Cores (= 118.8 cm2

) ............................................................................................204 

Table 8-21. ANOVA Results of Test for Differences in Harpacticoida Diversity ...............210 

Table 8-22. Biomass Contribution of the Major Taxonomic Groups to Total 

Meiofaunal Biomass (mg C m-2) at Each DGoMB Station .................................213 

Table 8-23. Allometric Estimations of the Mass-Dependent Meiofauna Respiration 

Rate (R, in d-1 Units) and Meiofauna Community Respiration (CO2, m

-2

d-1) and Total Organic Carbon Demand (OrgC, mg C m-2 d-1)............................220 

Table 8-24. Mean Meiofaunal (MB) and Bacterial (BB) Biomass for Pooled Replicates 

and Pooled Taxonomic Groups at Four of the Process Stations..........................222 

Table 8-25. Comparison Of Whole Community Respiration (CR, mg C m-2 d-1) to 

Meiofauna Allometric Respiration Estimates (MR, mg C m-2 d-1) ....................222 

Table 8-26. Depth (m), Total Individuals Per Site, Number of Individuals m-2, Total 

Number of Species, Species Richness, Pielou’s Evenness (J’), Expected 

Number of Species (50), and Shannon-Wiener Species Index (H’) for 

Bivalves................................................................................................................232 

Table 8-27. List of Station Groups Based on Percent Similarity with Respect to 

Species Composition............................................................................................239 

Table 8-28. The 50 Most Abundant Polychaete Species by Number .....................................243 

Table 8-29. Western Transects (W1-6, RW1-6, and AC1) Produced 2,141 Polychaetes 

Representing 172 Taxa and 14 Replicates...........................................................244 

Table 8-30. Characteristic Species of Western Transect Stations...........................................244 

Table 8-31. B and NB Stations Intended to Test Basin Vs Non-Basin Communities 

[B1-3, NB2-5, Additionally Including WC5, WC12, and BH (Bush Hill)] 

Produced 1,930 Polychaetes Representing 150 Taxa and 15 Replicates.............245 

Table 8-32. Characteristic Species of the Basin/Non-Basin Transect Stations.......................245 

Table 8-33. Central Transect Stations [C1, C4, C7, C12, C14, GKF (Green Knoll 

Furrows) and S5 (Sigsbee Abyssal Plain)] Produced 2,445 Polychaetes 

Representing 185 Taxa and 17 Replicates...........................................................246 

Table 8-34. Characteristic Species of the Central Transect Stations ......................................246 

Table 8-35. Mississippi Trough Transect Stations (MT1-6) Produced 6,868 

Polychaetes Representing 333 Taxa and 25 Replicates.......................................247 
xxix 

Table 8-36. Characteristic Species of the Mississippi Trough Transect Stations ...................248 

Table 8-37. Eastern Transect Stations [S35-44 and HP (High Productivity)] Produced 

4,337 Polychaetes Representing 207 Taxa from 21 Replicates...........................249 

Table 8-38. Characteristic Species of the Eastern Transect Stations......................................250 

Table 8-39. Sigsbee Abyssal Plain Stations (S1-5) Produced 267 Polychaetes 

Representing 48 Taxa and 6 Replicates...............................................................250 

Table 8-40. General Results by Site for DGoMB ...................................................................283 

Table 8-41. Fish Faunal Data for All 31 Stations by Depth Zones.........................................324 

Table 8-42. Faunal Comparisons Among Four Groups of Outer Continental Shelf and 

Upper Slope Stations............................................................................................325 

Table 8-43. Faunal Comparisons Among Three Groups of Mid-Slope and Deep 

Stations.................................................................................................................325 

Table 8-44. Faunal Comparisons between Basins and Non-Basins........................................326 

Table 8-45. Summary Statistics for the Six Near-Bottom ADCP Deployments in 2001 

and 2002...............................................................................................................332 

Table 8-46. Correlation Coefficients (r Values) Calculated between Each Depth Bin 

for the Two Shallowest Stations of the Summer 2001 ADCP 

Deployments .......................................................................................................335 

Table 8-47. Correlation Coefficients (r Values) Calculated between Each Depth Bin 

for the Two Deepest Stations of the Summer 2001 ADCP Deployments ...........336 

Table 8-48. Mean Values of Macrofauna Abundance, Higher Taxa, H’(s) (ShannonWeiner), and Evenness for DGoMB Study Stations (Non-Seep) and 

SETTIE Stations (Seep).......................................................................................346 

Table 9-1. Determination of the Mean Size of Macrofaunal Individuals (from 

Selected Samples) ................................................................................................350 

Table 9-2. Process Station Macrofauna Biomass, Based on Densities and Individual 

Weight Estimates, in mg C/Individual.................................................................350 

Table 9-3. Inventories of Organic Matter (mg C m-2-15 cm) in the Detrital and Living 

Fractions at the Process Sites...............................................................................361 

Table 9-4. Eigenvalues of the Correlation Matrix for the Environmental PCA, 

Proportion of Variance Explained by Each Principal Component, and 

Cumulative Variance ...........................................................................................363 

Table 9-5. Variable Loads for the Rotated (Varimax) Factor Pattern of the 

Environmental PCA.............................................................................................363 

Table 9-6. Relationship Between Site, Depth, Total Living and Total Detrital Carbon, 

Sediment Community Oxygen Consumption (SCOC, mg C m-2-day) and 

Carbon Turnover Time in Days (Living Carbon) and Years (Detrital 

Carbon), Where ‘Time’ Equals the Stock Divided by the Rate...........................367 
1 

1. INTRODUCTION 

The topography, geology, geophysics, currents, hydrography, chemistry and biota of the 

continental slope are much less well known than those of the continental shelf. The earliest 

information on the deep-sea biota of the Gulf of Mexico comes from studies that used benthic 

trawling and photography (TerEco 1976, 1983). The largest study, The Northern Gulf of Mexico 

Continental Slope Study (NGoMCSS), concentrated on knowledge of geologic features, water 

circulation, chemistry, and biological communities (LGL and TAMU 1988). New scientific 

findings in the area, including chemosynthetic communities and a better understanding of the 

geological complexity of the region, have significantly altered our view of the northern GoM 

deep-sea (MacDonald 1998; MacDonald et al. 1996). Rapid expansion of energy industry 

activities into the deep-sea has occurred over the past decade. It is expected that this trend will 

continue and that deep-sea regions of the northern GoM will be the site of energy exploration 

and development activities for decades to come. 

An MMS workshop report on the deepwater GoM concluded that there was a need for 

additional information on the composition and structure of benthic communities, the associated 

biogeochemical processes, habitat heterogeneity and physiography, trophic interactions, and the 

biological “health” of the region (Carney 1998). The deep-sea is a setting within which benthic 

communities survive and propagate in the northern GoM. The deep-sea is characterized by total 

darkness, low temperature, nearly featureless mud, sparse food resources, predictable biomass 

patterns, unusual diversity patterns, and poorly defined couplings to topography, 

biogeochemistry, and currents. 

The present program, referred to as Deep Gulf of Mexico Benthos or DGoMB, was designed 

to provide a better understanding of: 

1) the present condition of biological communities in the study area, 

2) the distribution patterns of deep-sea biota, 

3) the biological and physical processes that control the environmental setting, 

and 

4) the effects that these processes have on the character of benthic and 

benthopelagic communities. 

The program has emphasized understanding the make-up and variability of soft-bottom 

biological communities with a secondary effort to characterize the important biological and 

abiotic processes that sustain or change the observed patterns. The study has included: 

1) the composition and structure of slope biological communities, 

2) inferred the relationship between these communities and local conditions and 

forcing factors, 

3) characterized the “health” and functioning of deep-sea communities, and 

4) compared and contrasted the GoM region with similar oceanic basins. 

The DGoMB program design was developed based on historical knowledge of deep-sea 

communities in the GoM. The interdisciplinary nature of the scientific objectives was recognized 

and the study design balances the benthic survey aspects of the program with experimental (or 

“process”) oriented studies needed to understand the deep-sea community’s structure and 
2 

function. A careful analysis of previous information was used to focus the study on the most 

relevant areas and those areas most likely to provide the greatest likelihood of establishing 

generalities about the structure and function of deep-sea benthic communities. The results now 

provide a predictive capability for areas not directly sampled or observed. This predictive 

capability is a framework for ascertaining the potential for, and the most likely impact from, 

fossil fuel exploration and exploitation in the deep-sea. 

Each work element is nested in an integrated design that links data collection to a coherent 

framework that provides maximum complementary information based on a detailed foodweb 

model of the interacting biological components of the system. Survey station results were used to 

choose experimental stations. Experimental stations chosen were a subset of reoccupied survey 

stations tightly linking all observations and data in space and time. Each measurement was taken 

to provide quantitative estimates of unknown elements of the model, to improve the quantitative 

accuracy of the elements of the models, and/or to test and refine the underlying assumptions of 

the model. 

A review of previous studies, particularly the NGoMCSS slope study, provided valuable 

insights for designing the new study (LGL and TAMU 1988). The program design incorporated 

the following strategies: 

1) An equal sampling effort was concentrated at different water depths to 

determine if “zones” exist. Site selection emphasized equally spaced depth 

intervals and replication at the treatment level (sites that are mostly similar 

except for depth) recognizing that gradients in ecosystem properties are more 

likely than “zones.” 

2) Depth dependence in community structure and composition was evaluated in 

the context of other recently recognized confounding factors such as seafloor 

topography, and currents. 

3) Sampling sites were chosen based on characteristics believed to be important 

in establishing biological patterns such as detrital flux to the seafloor as 

influenced by near surface patterns of primary productivity. 

4) Proximity to the Mississippi River and Fan was an important spatial 

consideration. 

5) Benthic communities were nestled in different topographic and 

sedimentologic settings that are influenced by slope failure and varying inputs 

of terrigenous materials. 

6) Sampling sites were selected to compare benthic communities associated with 

known physical features such as the thermohalocline, the oxygen minimum, 

and high current regimes. 

7) The program was designed to provide flexibility by revising and rethinking 

the underlying strategy and design on a regular basis in response to collection 

of new data and re-evaluation of historical data. 

8) Temporal variations in community structure and composition may be 

important and were considered by reoccupying historical sites and visiting a 

subset of sites during each of the three planned field programs. 
3 

9) An improved understanding of community structure and composition required 

better and more thorough measurements of biomass for input into the 

conceptual model. 

10) Improved estimates of the megafaunal role in the ecosystem are important and 

included a three pronged approach to surveys: trawling (with improved 

quantitative methodologies), photosurveys, and baited traps. 

11) The number of sites sampled, the overall geographic area and the intensity of 

sampling were all increased over previous studies. 

12) Replication and the size of the sample were increased over the previous 

studies to ensure that the total number of animals collected adequately 

represented the diversity of the communities being studied. 

13) The data have been managed meticulously so that they are readily available 

for future reassessment as our understanding of the deep-sea evolves. 

The methods and approaches adopted for this program are the same, or compatible with, the 

methods of other corollary programs, thus allowing for seamless integration of new results with 

the results of other programs in the final synthesis. This will result in a holistic evaluation of the 

GoM deep-sea benthos and the linkages between abiotic and biotic processes that control the 

complex ecological patterns of the deep-sea benthos. 

1.1 Program Objectives 

The structure and function of deep-sea benthic communities is the end-result of complex 

interactions between and among the biota and the topography, geology, currents, hydrography, 

chemistry, and physical setting. The NGoMCSS study was an attempt to describe the geology, 

water circulation, chemistry, and biologic communities on the vast northern GoM continental 

slope region (LGL and TAMU 1988). The current project is intended to build on, improve, and 

supplement this pioneering study (Table 1-1). A reassessment of continental slope ecosystems, in 

the context of intensive oil and gas exploration and exploitation in the region, is considered 

essential for the management and protection of biological resources in the deep-sea. 

The overarching goals of DGoMB were to: 

• determine in greater detail the composition and structure of slope bottom 

biological communities and to infer relationships between biological patterns 

and major controlling processes and; 

• characterize the area as to its present “health” and function and compare and 

contrast the region with similar oceanic regions. 
4 

Table 1-1 

 

Program Team—Key Personnel* 

Name Discipline Role Institution 

Gilbert T. Rowe Deep-sea Benthic 

Ecology 

Project Manager, Principal 

Scientist, Chief Scientist, 

Group Leader-Deep-sea 

Ecology 

Texas A&M University 

Galveston 

Mahlon C. Kennicutt II Environmental 

Chemist 

Deputy Program Manager, 

Contaminant Chemist 

Texas A&M University 

Gary A. Wolff Data Management Data Manager Texas A&M University 

Jody Deming Microbiology Co-PI Ecology University of 

Washington 

Paul Montagna Benthic Ecologist Co-PI Ecology, Study 

Design 

University of Texas 

Richard Haedrich Bottom Fishes Co-PI Ecology Memorial University 

 Megafauna Data Analysis 

Richard Heard Macrofauna 

Taxonomy 

Co-PI Ecology University of Southern 

Mississippi 

John Morse Inorganic Geochemistry Texas A&M University 

 Geochemist Group Leader 

William Bryant Geological Geology Texas A&M University 

 Oceanographer Group Leader 

Worth Nowlin Physical Oceanography Texas A&M University 

 Oceanography Group Leader 

Joan Bernhardt Foraminifera University of South 

Carolina 

Norman Guinasso Physical 

Oceanography Field 

Program 

Texas A&M University 

Bob J. Presley Metal Contaminants Texas A&M University 

Terry L. Wade Organic 

Contaminants 

 Texas A&M University 

Steven DiMarco Physical 

Oceanography 

 Texas A&M University 

Michael Rex Community Structure Science Review Board University of 

Massachusetts - 

Boston 

Kenneth L. Smith, Jr. Community 

Dynamics 

Science Review Board Scripps Institution of 

Oceanography 

William W. Schroeder Gulf of Mexico 

Ecology 

Science Review Board Dauphin Island Marine 

Laboratory, 

University of 

Alabama 

*Does not include taxonomists. 

Specific DGoMB objectives were to: 

• improve the conceptual model that serves as the guide for the design and 

overall conduct of the study and to test specific hypotheses related to the 

models; 
5 

• compile and synthesize data from existing databases and on-going programs 

to interpret new results; 

• conduct field collections to describe the distribution and structure of benthic 

communities on the continental slope of the GoM and to elucidate the 

functional interactions among them in known environmental settings; 

• characterize the hydrographic structure and measure the dissolved and 

particulate water column nutrient concentrations, primary productivity, and 

chlorophyll a at the study sites; 

• characterize the sediments at the study sites including grain size and 

hydrocarbon, metal, carbonate, and organic carbon concentrations; 

• characterize the basic attributes of the benthic microbiota and biomass at the 

study sites; 

• characterize the soft-bottom macro- and megafauna at the study sites; 

• relate variations in benthic biota patterns to sedimentary processes and to the 

chemical and physical setting; 

• define basic levels of animal and bacterial activity and production and 

describe interactions between and among benthic biota, the several 

ecological/biological compartments, and the abiotic environment; and 

• compare and contrast the GoM benthic marine environment and communities 

with those in other basins of similar depth ranges and oceanic settings. 

The program was conducted over a 65-month period of performance. Three major 

oceanographic cruises were conducted, one in each of the first three years of the program. The 

fourth year of the program was dedicated to completing sample analyses begun in the first three 

years; the next year was devoted to work by taxonomists on organisms taken in the field 

program; the 5th year has been dedicated to data management and interpretation, model 

refinement, and production of the Synthesis Report. 

1.2 The Program 

The program consisted of four tasks: 

TASK 1—Re-examination of Existing Data and Field Study Design 

All available scientific records and databases have been identified, collected, and reexamined. Previous studies of particular importance are the TerEco Corporation synthesis (1976 

and 1983), the MMS NGoMCSS (LGL and TAMU 1988), and chemosynthetic ecosystem 

studies (MacDonald 1998 and MacDonald et al. 1996). Industry data, MMS leasing history and 

production data, U.S. Geological Survey (USGS) Gloria data, National Oceanographic Data 

Center (NODC) data, governmental holdings of results of various MMS physical oceanography 

programs (Texas-Louisiana Shelf circulation and Transport Process Study [LATEX], NEGOM 

Deepwater), and other information has been integrated into the new findings. These data formed 

the basis for final recommendation of study sites to the Contracting Officer Technical 

Representative (COTR) and the Scientific Review Board (SRB). This information was used to 

describe the study sites, how each site contributed to the overall program design and verification 

of important features of the conceptual model. 
6 

TASK 2—Field Sampling 

The R/V Gyre was used to conduct all three cruises. Conventional sampling methodologies 

were used for the community structure survey and innovative experimental approaches were 

used for process studies. Year II and III cruises occupied those sites selected on the basis of 

information produced in Year I and II, respectively. The goal of the sampling program was to 

describe the benthic communities in distinct and identifiable settings in the study area in time 

and space. A phased-in approach concentrated the benthic survey portion of the work in Year I 

allowing ample time to process samples and analyze data. Year II and III were a mix of infilling 

of survey stations based on programmatic results. Experimental stations were added to address 

key questions related to processes and forcing factors during cruises in years two and three. The 

experimental stations were a subset of survey stations providing for a close integration of all data 

collected. 

TASK 3—Sample and Data Processing and Analysis 

All samples and data were processed as specified in Sections 7 and 8. The quality of data and 

samples was ensured by a comprehensive data management plan (i.e., the Program Management 

Plan). Chain-of-custody and sample tracking activities guaranteed the integrity and quality of the 

samples and data from shipboard collection to final synthesis. All parameters that can be 

reasonably measured onboard the ship (nutrients, salinity, oxygen, etc.) were measured using 

standard protocols, unless otherwise noted in the Results. 

TASK 4—Data Interpretation, Synthesis and Reporting 

This Final Report contains an assessment of historic information, the data collected, 

descriptions of methods and analyses, interpretations of the analyzed information, and the results 

and discussions of the findings. Models have been revised in light of new information. This 

report presents charts, maps, or schematics that portray faunal and habitat variability and the 

major forcing factors related to community structure and function in the deep-sea GoM, based on 

the new data available. In addition to this final report, publications in the open peer-refereed 

literature are being written, a special issue of Deep-Sea Research II is in the planning stage, and 

a growing set of graduate student theses and dissertations are being based on the results accrued 

by the study at the collaborating research and graduate educational institutions. 
7 

2. PRESENT STATE OF KNOWLEDGE 

A comprehensive review of deep-sea biology is beyond the scope of this report. However, a 

brief review of the salient features of deep-sea ecology in general, with a focus on the GoM, 

provides a context for the DGoMB program. References to more detailed discussions are 

provided. 

2.1 General Deep-Sea Ecology 

The biota of the deep-sea has been sampled for almost two centuries. An age of exploration, 

utilizing mostly rather simple qualitative gear (trawls and dredges), continued up until the middle 

of the 20th century. Quantitative sampling developed for fisheries research in shallow waters 

began finding wide use in the deep-sea shortly after World War II. Many of the early qualitative 

and quantitative studies resulted in global generalizations about the “community structure” of the 

deep benthos (Menzies et al. 1973). Benthic metazoans have been subdivided into meiofauna, 

macrofauna, and megafauna, and these categories are further defined on the basis of size or 

major taxa. Near-bottom or demersal fishes are often lumped with the megafauna. Microbiota 

include heterotrophic protists such as ciliates, flagellates and forams and a suite of functional 

groups of bacteria. All the size groups listed, decline markedly in biomass and abundance as 

water depth increases although some decline at faster rates than others. A similar decline in 

abundance was correlated with distance from shore and surface seawater productivity (Rowe 

1971). Most species, it was concluded, exhibit “zonation” with depth (Rowe and Menzies 1969) 

although this is more accentuated in some groups than others. A number of authors have 

generated elaborate schemes of “zones” based on the occurrence of faunal groups. Although the 

deepest parts of the ocean are thought of as “deserts,” in terms of biomass, the diversity of the 

fauna is high compared to shallow water depth (Hessler and Sanders 1967). Many genera appear 

to be cosmopolitan, but in fact, a great many species are endemic to specific oceanic basins or 

subsets of basins. In general, Menzies et al. (1973) supported the argument that the “oldest” 

forms, on geologic time scales, are found at intermediate depth (slope depths) rather than at the 

greatest depths. The deeper living forms tend to be more specialized and hence “younger” in a 

geologic sense. 

The summary of Menzies et al. (1973) marked a significant turning point in deep-sea biology 

from studies of community structure to a recognition and inclusion of studies of the dynamic 

aspects of the biota. These studies have been summarized in a number of articles over the years. 

A decade after Menzies et al. (1973), a series of papers on deep-sea community structure and 

function was published (Rowe 1983). Gage and Tyler (1991) review community structure, 

including an extensive natural history of the different functional, taxonomic and size groups of 

organisms. The latter work is most lucid in its description of animal growth rates. It is pointed 

out that many animals spawn in a seasonal manner, most likely in response to seasonal inputs of 

particulate organic carbon. The deep-sea is the ultimate sink for detrital particulate carbon (Rowe 

and Pariente 1991). 

Recent investigations of “community function” were made possible by new observational 

capabilities such as deep submersibles, the most important being the Deep Submergence Vessel 

(DSV) Alvin, and moorings and landers that operate remotely on the sea floor without 

attachments to a mother ship. Reviews have documented measurements of sediment community 

respiration (Smith and Teal 1973); time series studies of the rapid destruction of wood by deepsea boring organisms (Turner 1973); baited traps to attract large scavengers that rapidly consume 
8 

carcasses (Hessler et al. 1978); measurement of the slow rain of particulate matter to the deep 

ocean and its seasonality (Deuser and Ross 1980); and the seemingly slow deterioration of 

organic matter enclosed in DSV Alvin when it unintentionally was lost for a brief period on the 

sea floor. The reviews imply that low biomass, low rates of metabolic processes, and low growth 

rates (Gage and Tyler 1991) are a result of the minimal input of photic-zone organic carbon 

(Rowe and Gardner 1978). However, some taxonomically narrow groups of organisms can 

rapidly consume allocthonous organic matter (scavengers and wood borers, for example). 

The decade of the 70’s ended with a pivotal biological discovery in the deep-sea: extensive 

biological communities supported by reduced inorganic chemicals at hydrothermal vents 

(Hessler 1981). The spreading centers between the plates of the earth’s crust are often 

characterized by fluids rich in sulfides. Symbiotic bacteria, living in the tissues of large 

invertebrates, oxidize the sulfide and the energy gained is used to synthesize organic matter for 

both the symbionts and the host. Numerous vents around the world are now known to support a 

wide variety of unusual but locally abundant organisms (Tunnicliffe et al. 1998). The latter 

review also notes that similar faunas occur near dead whales and petroleum seeps, including 

those in the GoM (Kennicutt et al. 1985). MMS has sponsored two programs to study GoM seep 

communities (MacDonald 1998; MacDonald et al. 1996) and a third study is planned in the near 

future. In this report, no further discussion of seeps or vents is provided, except related to the 

potential effects of seeps on non-seep communities. 

Contemporary studies of deep-sea benthic communities continue to address a series of longstanding questions. To a large degree, these studies are designed to explore the relationships 

between the structure and function of deep-sea communities. The consensus appears to be that 

biomass and animal abundance decline at log-normal rates with increasing water depth in many 

of the ocean’s basins. However, meiofauna and bacteria decline less rapidly than macrofauna, 

creating an increase in the relative importance of these taxa as depth increases (Rowe et al. 

1991). This has been assumed to reflect a decline in the reactivity of detrital organic matter that 

requires a bacterial recycling loop at the base of the food web (Richardson and Young 1987). 

The Joint Global Ocean Flux (JGOFS) program has included benthic components in the Indian 

Ocean, the east Atlantic off NW Africa, upwelling ecosystems, polar or subpolar environments 

and the equatorial Pacific, all of which demonstrate the direct response of biomass, sediment 

mixing rate, organism growth and respiration to input of particulate organic matter (POC) from 

the water column. Most of the larger studies have been summarized in dedicated issues of marine 

science journals. 

The reasons for variations in diversity remain elusive. Most new ideas are variations on 

Sander’s original stability-time hypothesis (reviewed in Gage and Tyler 1991). Rex (1983) has 

pointed out that a monotonic increase in diversity does not exist in the deep-sea. To the contrary, 

Rex found that diversity indices peak between 2 to 3 km. Further work by others recently 

confirmed this pattern exists in many oceanic basins (Levin and Gage 1998). The idea that “a 

temporal and spatial mosaic” of physical and chemical variability enhances or preserves diversity 

has been tested by emplacement of sediment trays on the sea floor followed by harvesting of 

organisms after various deployment times. Snelgrove et al. (1996), for example, suggest that 

aged patches of Sargassum promote high diversity by providing fauna with a series of microhabitats in time and space. However, the numbers of “expected species” in such experiments are 

lowest in the trays with added organic matter, leading to the possibility that competitive 

exclusion occurs in response to enrichment. 
9 

Some progress has been made in allocating food resources to various size or functional 

groups (stocks) in the deep-sea. It has been suggested for example that among the suite of 

organisms living within the sediments, the bacteria consume approximately 30% of the available 

organic matter at some locations (Rowe and Deming 1985). In the benthic boundary layer above 

the sea floor, organic detritus is parceled among a wide variety of epibenthic holothuroids, 

benthopelagic crustacea and migrating fishes, but most of the total organic matter flux, according 

to Smith’s carbon budget, is consumed by sedimentary organisms (Smith 1992). Another 

consensus is that the rain of pelagic detrital organic carbon is tightly coupled to water column 

processes. Although it has been proposed that the lateral transport of detrital organic carbon is 

important, in the deep Pacific there appears to be a long-term imbalance between the input of 

organic matter and its remineralization (Jahnke et al. 1990; Smith and Kaufman 1999). 

A growing body of information suggests that the flux of organic matter to the seabottom has 

considerable control over both the structure and the dynamics of benthic communities. The basic 

structure of the living components of the near-bottom ecosystem have been identified in terms of 

functional groups (Rowe 1981). Information is mounting on the relative importance of some taxa 

in carbon cycling and energy flow (Rowe 1983; Smith 1986a; Deming and Baross 1993; Smith 

1992). Using biomass spectra to estimate production, Haedrich and Merrett (1991) suggested that 

a number of characteristics of community energy or carbon flow among the fishes can be 

inferred. While fish densities drop markedly over the slope, biomass is relatively constant to 

depths of almost 2,600 m, suggesting that mean size per fish increases with depth over the slope. 

Hence, it is inferred that the relative importance of fishes increases compared to other size and 

functional groups with increasing water depth to 2,600 m. At greater water depths, fish biomass 

precipitously declines. 

Sediment-dwelling organisms appear to respond directly to fresh inputs of organic matter 

(Linke 1992; Deming and Baross 1993). Working at 200 to 500 m depths in the sub-Arctic 

Atlantic Ocean, were able to construct a budget that balances carbon input into sediment traps 

with total community respiration and sediment burial. A model of the functional components 

responsible for the cycling of carbon was then constructed. Evidence that little of the carbon was 

channeled through the bacteria supports the suggestion of others, that bacteria are less important 

at high latitudes. The dependence of the cycling of energy and carbon on input rates has also 

been demonstrated in comparisons of deep sub-oxic environments with oxic counterparts 

(Eldridge and Jackson 1991). Deep-sea sediment biomass, in general, appears to be enhanced by 

the fresh input of organic matter (Rowe 1971; Rowe 1983). The latter work modified and 

simplified the conceptual benthic boundary layer model of Smith (1992), and conducted 

numerical simulations to determine how biomass and respiration were redistributed within a 

“fertilized” system. This model, validated independently by work at Deepwater Dump site #106 

in the northwest Atlantic, implies that it might be possible to predict “bioenhancement” (of 

biomass) in response to new carbon. This does not mean that diversity would increase. To the 

contrary, some models suggest that diversity would actually decrease initially due to competitive 

exclusion, in agreement with data presented in Snelgrove et al. (1996). The rates at which new 

“steady states” are reached might be much slower than in shallow water at higher temperatures. 

The relationship between POC input and bathymetric zonation also remains speculative. 

The deep-sea is seemingly benign, rather than stressful. However some exceptions should be 

noted. Low oxygen zones occur in isolated basins, near large rivers and in eastern tropical areas 

affected by upwelling. Stress from low oxygen in the eastern Pacific for example causes a 

decrease in macrofauna and megafauna (Menzies et al. 1973; Rowe and Deming 1985) and 
10 

diminished diversity and tight zonation perpendicular to the oxygen gradient (Levin and Gage 

1998). Likewise, hydrocarbon seeps are now known to occur widely around the world on 

continental margins. Some have unique communities that utilize petroleum as a source of carbon 

and energy, but the effects of the communities to either enhance or degrade the surrounding 

benthos is unclear (Montagna et al. 1989). Another natural potential “stress” studied in the deepsea is high current velocities (Thistle et al. 1991). Referred to as the high engery benthic 

boundary lager experiment (HEBBLE) project, studies in high current velocities appear to harbor 

unique species. Recent observations have confirmed the occurrence of episodic strong currents at 

the base of the Sigsbee escarpment (Bryant and Nowlin, pers. comm.). An asphalt-like pavement 

on the abyssal plain of the Bay of Campeche is characterized by seep-like organisms, suggesting 

that short-chained hydrocarbons can support unique assemblages, even at abyssal plain depths. 

In summary, three characteristics of community structure are recognized as important, 

quantifiable features of deep-ocean communities, regardless of size category: 

1) variations in biomass and density per unit area, 

2) variations in diversity and 

3) zonation of species and recurrent groups of species with depth or parallel to 

bottom topography. 

The general radical decline in density and biomass with depth is presumed to be related to a 

decline in the input of organic matter from the water column; the parabolic variation of diversity 

as a function of depth, with a mid-depth maximum, recurs persistently, but an explanation for it 

is speculative; and while species and groups of species are known to be replaced rapidly as a 

function of depth, the precise causes of species turnover with depth is speculative. Likewise, the 

continuity of the zones along depth intervals is unknown because most studies do not cover an 

appropriate geographic area. All three structural qualities are presumed to be related to food 

supply, but mechanistic explanations of the cause and effect relationships remain obscure. 

2.2 Deep-Sea GoM—Geologic Setting 

The GoM Basin is a study in physiographic contrasts. The simple topography of the West 

Florida and Campeche continental slope, the Mississippi and Bryant submarine fans and the 

extremely flat Sigsbee Abyssal Plain give way to the complex slump structure of the East 

Mexican Slope and the extremely complex topography of the Texas/Louisiana continental slope. 

The continental slope off Texas, Louisiana, and portions of Mississippi is geologically and 

physiographically the most complex in the world. Over ninety basins and seven submarine 

canyons dissect the continental margin of the northwestern GoM. The halokinesis of 

allochthonous salt is the main mechanism of interslope interlobal and interslope superlobal basin 

creation in mid- to lower-slope areas. Most slope basins are filled with turbidites and debris flow 

deposits overlain by a Holocene hemipelagic sediment cover that varies from 2 to 10 meters 

thick except in areas of active slumping such as the Beaumont Basin. The supralobal basins of 

the lower slope are probably formed by the downbuilding of salt withdrawal under the influence 

of turbidite sands. The termination of the slope, the Sigsbee Escarpment, is the southern most 

extent of a salt nappe that extends from the mid- to lower-slope. The salt nappe can prevent the 

migration of thermogenic gas and oil into overlying sediments. Major portions of the middle and 

lower slope appear to be devoid of gas seeps; thus, gas hydrate and chemosynthetic communities 

are not expected to be present except at the edge of the nappe (Sigsbee Escarpment) and in salt-
11 

free basins. In contrast, the upper slope contains numerous salt diapiric structures such as mud 

and gas mounds, fluid expulsion features, hard-grounds, erosional gullies, and numerous gas 

seeps and gas hydrate deposits. 

Superimposed, and eroded into the complex topography of the Texas/Louisiana slope, are 

four major canyon systems: the Mississippi, Keathley, Bryant, and Alaminos Canyon. The 

Mississippi Canyon is a Pleistocene feature resulting from slumping processes, debris flows, and 

turbidity current flows that were most active during the Wisconsinian. Alaminos, Keathley, and 

Bryant Canyon were created by a combination of coalescing salt canopies and turbidity current 

and debris flow erosional events. Three smaller canyons, Farnella, Green, and Cortez breach the 

escarpment and were formed by mass flow events. Smaller upper- and mid-slope canyons that do 

not extend to the escarpment are common features related to mass flow processes. 

Since the late Pleistocene; Alaminos, Bryant, and Keathley Canyon have undergone 

extensive alteration due to infilling of the canyons by the upward migration of salt. As much as 

800 meters of infill has taken place within the Bryant Canyon system over the last 12000 years. 

This equates to an average vertical salt movement of 6 cm/year. It is unknown if the upward 

motion of salt is continuous or episiodic. Recent seafloor faulting indicates that salt migration is 

ongoing today. 

An unusual form of density flow resulting in extensive bedforms on the seafloor was recently 

found associated with Vaca Basin, a supralobal basin due east of Bryant Canyon. The salt infill 

of Bryant Canyon has resulted in brine eroding vast fields of gullies on the slopes of Vaca Basin 

and adjacent areas. Recent studies indicate that other large bed forms, seafloor furrows, occupy 

an area 15 km wide southward of the base of the Sigsbee Escarpment. These furrows are current 

induced structures and extensive erosion is taking place at the present time. 

The geologic setting in the northern GoM is diverse, heterogeneous on several spatial scales, 

and actively changing within timeframes that might be expected to effect biological communities 

living in and on the sediments. This temporal and spatial heterogeneity of substrate in the deepsea GoM has been fully appreciated only in recent years. This realization has lead to a new 

perspective on how biological communities might be interacting with geological processes. 

2.3 Geochemistry of Deep-Sea Sediments 

The two variables generally considered most important to the geochemistry of deep-sea 

sediments overlain by oxic seawater are the rate of sediment accumulation and the relative 

proportion of biogenic material to detrital minerals (e.g., Berner 1980). Both factors are closely 

related to the extent to which biologically driven reactions occur. Sediment accumulation rate 

controls the speed with which material is buried away from the sediment-water interface 

increasingly insulated from the overlying oxic waters. Consequently, the degree to which 

metabolizable organic matter is decomposed, at or very near the sediment-water interface, 

depends on sediment accumulation rate. If accumulation rates are low, little metabolizable 

organic matter is buried and the sediments may only reach suboxic conditions, whereas at higher 

accumulation rates sulfidic conditions can be reached. Related to these conditions is the second 

major variable, the relative input rate of the particulate organic carbon. Sediments on continental 

margins and beneath highly productive open-ocean waters can have a much higher rate of carbon 

input than those beneath oligotrophic waters. Both of these factors combine to control how far 

down the redox “ladder” (e.g., Stumm and Morgan 1996) reactions will progress for electron 

acceptors associated with organic matter remineralization and the depth spacing between the 

various “rungs” on the redox ladder. 
12 

An important overall general concept that is basic to this program is that physical, chemical 

and biologic processes are intimately intertwined. It is also particularly germane that it is the 

availability of metabolizable organic matter that will largely control benthic biogeochemistry and 

community structure. This is not simply related to the total organic matter content of sediments 

(e.g., Morse and Emeis 1990, 1992; Boudreau 1996). It is, therefore, necessary to look at the 

consumption of electron acceptors (e.g., oxygen, nitrate, manganese and iron oxides, sulfate) and 

the production of reaction products (e.g., carbon dioxide, ammonium, hydrogen sulfide) to 

understand the rates and extents to which heterotrophic activity is occurring on and beneath the 

seafloor. 

Although there are of course many complicating factors leading to exceptions, the very 

general rule is that sediments tend to have lower accumulation rates and hence become less 

reducing with increasing water depth. This also reflects decreasing biologic activity with 

increasing water depth. The geochemistry of GoM sediments has been investigated over much of 

the study area and adjacent regions (e.g., Lin and Morse 1991; Rowe et al. 1990; Morse and 

Rowe 1999) to a water depth of ~2,000 m. A primary indicator of biologic activity is the 

integrated rate of sulfate reduction. This generally decreases with increasing water depth. 

However the rates are about three times higher at a given depth on the slope near the Mississippi 

River than at GoM slope far from the river. A distinct secondary maximum in sulfate reduction 

rate has been observed at ~500 m water depth. These results suggest that deep-sea biological 

community structure in the GoM is not simply related to depth, but may be influenced by factors 

such as down slope transport of sediments and organic matter. Further confounding simple 

relationships between structure and geochemical forcing factors, is the presence of hydrocarbon 

seeps. The MMS chemosynthetic community studies on the Louisiana slope, have documented 

integrated sulfate rates near seeps that exceed those in surrounding sediments by over a factor of 

fifty (Morse, unpublished data). 

2.4 Physical Oceanography of the GoM 

The GoM is a semi-closed basin with both broad and narrow continental shelves surrounding 

a deep abyss reaching 3,600 m. At the boundary in the southwestern GoM, the energetic Loop 

Current intrudes northward into the GoM and exits east through the Straits of Florida. The Loop 

Current is a permanent feature of the circulation and virtually dominates the circulation in the 

eastern GoM and its influence is felt throughout the GoM as anticylonic rings and topographic 

Rossby waves spawned by the Loop Current translate west and south. GoM tides are modest. 

The world’s third largest river, the Mississippi River, and dozens of lessor rivers discharge 

tremendous volumes of fresh water into the GoM affecting stratification over large areas and 

enhancing wind-driven coastal jets along frontal boundaries. Wind driven upwelling occurs with 

regularity along the western and northeastern GoM and along the Yucatan Peninsula. Winter 

cyclones and summer hurricanes are not uncommon events. 

The open waters of the GoM can be divided into two regimes: the eastern GoM with the 

anticyclonic Loop Current and the western GoM with the anticyclonic Loop Current eddies and 

associated cyclones. Approximately every nine months (on average, but with considerable 

variability), it sheds anticyclonic, mesoscale current eddies with average lifetimes longer than 

one year (Elliott 1982). These eddies typically migrate to the western GoM, often decaying away 

in the northwest corner of the GoM. The eddies can spawn cyclonic rings during interaction with 

one another or the continental slope. The Loop Current eddies have characteristic diameters of 

200-400 km when newly detached, decreasing by 45% within 150 days and 70% within 300 days 
13 

(Vukovich and Crissman 1986). The eddies translate westward with an average speed of about 5 

km·d-1 and a range of 1-14 km·d-1. Currents associated with the Loop Current and its eddies 

extend to depths near slightly greater than the sill depth of The Florida Straits (800 m). The 

currents may have surface speeds of 150-200 cm·s-1 or more; speeds of 10 cm·s-1 are not 

uncommon at 500 m (Cooper et al. 1990). Because of their longevity, the effects of the Loop 

Current eddies can persist at one location for weeks or even months. 

Anticyclonic Loop Current eddies interact with one another and with the continental slope 

resulting in their decay. In the process of the interactions, and of the interaction of the Loop 

Current with topography, many cyclonic eddies are generated within the GoM. Some evidence 

exists that anticyclonic eddies may also be formed by ring-ring interactions, but they would not 

have the high salinity core characteristic of Loop Current eddies. Below 1,000 m, strong current 

events have been observed (e.g., Science Applications International Corporation [SAIC] 1989). 

The Loop Current and Loop Current eddies may influence the dynamics to the deepest portions 

of the GoM (~3,500 m; Smith 1986b; Hamilton 1990) through the generation of phenomena such 

as barotrophic Rossby Waves. Likewise strong, episodic atmospheric events (e.g., hurricanes or 

cyclones) can generate currents that extend to depths of 1,000 m or more. Speeds up to 50 cm·s-1

have been observed associated with these classes of deep currents. Deep currents are the main 

interest of the MMS-sponsored inventory and synthesis of physical oceanographic data in the 

deepwater GoM. 

Characteristic relationships of physical and chemical water properties (e.g., temperature, 

salinity, dissolved oxygen, and nutrients) can be used to identify water masses and their sources. 

Surface waters of the GoM are greatly modified by heat and freshwater exchanges through the 

surface, river discharges, and wind mixing, but no subsurface water of consequence is thought to 

be locally formed. Waters from the global ocean enter the GoM only through the Yucatan 

Channel from the Yucatan Basin of the Caribbean Sea. Seawater properties can be used to 

identify the five water masses that enter the GoM (Table 2-1). Source regions for these waters 

are discussed in Morrison and Nowlin (1982). Morrison and Nowlin (1977) described the water 

masses found in the Loop Current of the eastern GoM, and Morrison et al. (1983) described the 

water masses and properties found offshore in the western GoM. As the waters of the Loop 

Current enter the GoM, those along its western boundary are vertically mixed by the interaction 

of the current with bathymetry, resulting in an averaging of properties (Nowlin 1972). Moreover, 

after separation, Loop Current eddies eventually spin down in the GoM. That process entails 

mixing, which likewise lessens property extrema. Below the Yucatan Channel sill depth of about 

2,000 m, the potential temperature, salinity, and dissolved oxygen concentrations show no 

measurable horizontal variation throughout the GoM (Nowlin 1972). 

Table 2-1 

 

GoM Water Masses with Associated Property Extrema and Potential Densities 

Water Mass ~ Depth 

Range (m) 

Identifying Feature(s) Sigma-theta 

(mg cm-3) 

Subtropical Underwater 50-250 salinity maximum 25.40 

18°C Sargasso Sea Water 150-250 oxygen maximum (weak in west) 26.50 

Tropical Atlantic Central Water 250-500 oxygen minimum 27.15 

Antarctic Intermediate Water 500-1000 salinity minimum; nitrate, phosphate, 

and silicate maxima 

27.30-27.50 

Upper North Atlantic Deep Water > 1000 silicate maximum ≥ 27.70 
14 

The earliest attempt at a comprehensive synthesis of the physical oceanography of the GoM 

is found in Capurro and Reid (1970). Since then, and particularly in the last 15 years, the study 

of the physical oceanography of the GoM has greatly accelerated. Much, if not most, of this 

increased study has been funded by the MMS acting in response to intense energy exploration 

and production activities on the shelf, slope, and rise. The first such study, initiated in 1982, was 

a five-year program implemented as a series of regional studies. These studies included field 

work in the west, central, and eastern portions of the northern GoM. The results, detailed in a 

final report, emphasize eddy variability. MMS also sponsored a synthesis of GoM meteorology 

and climatology data during this same period (Florida A&M University 1988). A major MMSfunded physical oceanography study was the LATEX program which surveyed and maintained 

current moorings between 1992-1994 along the Texas-Louisiana shelf from the 10-m isobath to 

the shelf break from Brownsville to 90.5oW. The LATEX program was a joint effort 

accomplished by TAMU, Louisiana State University (LSU), and Science Applications 

International Corporation (SAIC). Other study units sponsored by MMS, but not strictly part of 

LATEX, generated significant amounts of related collateral data during the same time period. 

Taken collectively, this study was the largest and most comprehensive shelf study ever 

conducted. Following LATEX, a new series of programs similar in scope to LATEX but 

organized differently, were initiated in the northeastern GoM. The most significant circulation 

studies include a large (>350 drifters) program conducted by Peter Niiler of Scripps Institute of 

Oceanography and Walter Johnson of MMS between February 1996 and February 1997, a 

moored array and hydrography program conducted by SAIC in DeSoto Canyon from March 

1997 to March 1999, and an ongoing hydrography and water chemistry program over the shelf 

and slope between the Mississippi River and Tampa, Florida from November 1997 to August 

2000. In 1998, a data synthesis study of the physical oceanography of the deep water GoM was 

begun by TAMU under MMS sponsorship. The results of this study, it will include a complete 

database of all available information and the design for a deep mooring array. 

2.5 Deep-Sea Ecology of the GoM 

The earliest studies of the deep GoM biota were by Alexander Agassiz (1888) but his 

voyages never penetrated the western Gulf. The National Marine Fisheries Service has long 

maintained an exploratory fishing fleet in Pascagoula, MS and information is available from 

extensive trawling on the continental slopes of the GoM and the Caribbean. In the mid-1960s 

studies on the biota of the deep GoM were initiated (Pequegnat et al. 1990). A wide range of 

investigations of the deep water ensued, including the first quantitative samples with a Campbell 

grab coupled with a camera (Rowe 1966). Several bottom contact and automatic cameras were 

used and camera lowerings were dispersed more or less evenly around the GoM margin, 

including the Gulf of Campeche near Veracruz, Mexico. A benthic “skimmer” was built to catch 

larger forms and it was outfitted with an odometer to determine bottom distance traveled. These 

samplings included the Caribbean, as well as the GoM. Faunal groups studied from the deep 

GoM included squids, crustaceans (Firth 1971; Pequegnat 1970), echinoderms (Carney 1971), 

molluscs, and fishes (Bright 1968) among others. Assemblages of both the megafauna and the 

macrofauna were also studied. Fish feeding habits were intended to link the two. Photographs of 

an “iron stone bottom” north of the Yucatan Strait suggested deep bottom currents sweep some 

areas free of sediments (Pequegnat 1972). Some taxa displayed remarkable “zonation” by 

species, such as the bivalve molluscs (James 1972) and the asteroid echinoderms. The 

macrofauna appeared to be grouped into assemblages, based on measures of similarity, that were 
15 

distributed within “zones” down the slope and onto the abyssal plain, but no justification was 

found for separating deep-living fauna into zoogeographic provinces by latitude or longitude 

(Kennedy 1976). 

In 1968, semi-quantitative samples were taken across the Sigsbee Abyssal Plain and then in 

1972, quantitative samples were obtained from the lower slope up onto the continental shelf on 

two transects seaward of Galveston and Pascagoula. These quantitative data suggested that the 

deep fauna of the GoM was depauperate in numbers and biomass, and that the mean size of the 

macrofauna was in general smaller than that in the Atlantic at similar depths (Rowe 1971, Rowe 

and Menzel 1971, Rowe et al. 1974). Later studies, such as the NGoMCSS, used the conversion 

factors from numbers and size to estimate biomass. The log-normal relationship between 

biomass and depth has now been confirmed for numerous ocean basins (Rowe 1983), but the 

slope of the line for the GoM is steeper than that in most basins. It has been suggested that this is 

due to the low primary production in the GoM. 

The most extensive sampling of the sediment biota of the northern GoM was the NGoMCSS 

study supported by the MMS in the 1980’s. This consisted of paired GoMEX (not to be confused 

with GoMEX) boxcores (Boland and Rowe 1991), bottom survey camera lowerings and trawl 

samples. The stations studied included three transects down the slope in the western, the central 

and the eastern GoM with a line along the upper slope connecting the west and central transects 

along isobaths. Several reports were written describing the work but the most concise summary 

is a paper by Pequegnat et al. (1990). In the descriptions of the characteristics of the three 

dominant size classes (the meiofauna, the macrofauna and the megafauna) the meiofauna 

appeared to dominate the biomass, a feature that would be odd for shallow water but a 

relationship that is becoming increasingly apparent at deep-sea stations when size groups are 

compared (Rowe et al. 1991). It was also suggested that the fauna were distributed in “zones” 

along isobaths on the upper slope, but less so on the lower slope. As is true for most deep ocean 

basins, the meiofauna were dominated by nematodes, the macrofauna by polychaetes and the 

megafauna by echinoderms. Lesser numbers in each size category were also quite predictably 

encountered in taxa such as the molluscs and crustaceans. Similarity indices suggested that the 

eastern GoM, east of the DeSoto Canyon, was not similar to that in the northwestern GoM at 

similar depths. “Zonation” with some groups was modest, at best. This was true of the 

polychaete annelids. Within this group, no species was encountered below the sill depth of about 

2 km that was not encountered at lesser depths as well. 

An important discovery made during earlier slope studies were communities of large 

organisms associated with petroleum hydrocarbon seeps (MacDonald et al. 1996). Such 

communities, bearing a resemblance in both taxa and function to those associated with 

hydrothermal vents, have now been documented at numerous locations on the continental slope 

of the northwestern GoM in areas where fossil hydrocarbons make their way to the sediment 

surface. Bubbling or seeping gas appears as “wipeout zones” in acoustic surveys of the bottom. 

These communities are the subject of ongoing studies supported by MMS and other agencies and 

will not be described in detail here (MacDonald 1998). Cold seeps supported by hydrogen 

sulfide in “ground water” appears to support communities of large invertebrates, again being 

somewhat similar to those at hydrothermal vents, on the lower reaches of the Florida Escarpment 

(Paull et al. 1984). 

The Instituto Ciencias del Mar y Limnologia of the Universidad Nationale Autonoma de 

Mexico (UNAM) has conducted extensive studies in the southern GoM. Early studies of 

megafauna on the shelf were expanded down slope into the “salt diapir zone” just west of the 
16 

Campeche Bank. These studies were extended into the Sigsbee Abyssal Plain close to the 

exclusive economic zone (EEZ) between the U.S. and Mexico (26oN. Lat.) as well. Pathways of 

carbon and nitrogen in the benthic foodweb were inferred by stable isotopic analysis (Soto and 

Escobar 1995). In deep water studies across the Cordilleras Mexicanas or “Mexican Ridges” of 

the upper continental rise out onto the Sigsbee Abyssal Plain regions that contain enhanced 

biomass have been identified under water masses that have high rates of primary production 

(Escobar-Briones et al. 1999). Polychaetes dominated the infauna and a mid-slope maximum in 

abundance similar to that encountered in the northern GoM has been documented (Pequegnat et 

al. 1990). The fauna could be partitioned into three groups within depth intervals of >3 km, 

between 1.5 and 3 km, and <1.5 km. This contrasts with the view of Pequegnat et al. (1990) that 

five faunal groups can be divided into recurrent “zones.” 

Several studies currently being conducted in the GoM are also important. Studies in the Bay 

of Campeche in the northwestern GoM are producing important findings (TAMU/UNAM). In 

1997, at a station in 3.6 km water depth (25o15’ N. Lat. x 93o26’ W. Long.) on the northern 

Sigsbee Abyssal Plain, the first sediment oxygen consumption measurements in the deep western 

GoM were made. The sediment oxygen consumption was equivalent to 3.8 mg carbon/m2-day. 

Quantitative analysis of samples of the benthic community on the Sigsbee Abyssal Plain confirm 

that benthic biota are extremely sparse. Baited bottom traps in the area attracted deep-sea 

scavenging amphipods. The diversity of polychaetes was not appreciably different from that 

measured in the NGoMCSS study (LGL and TAMU 1988). 

Another important investigation is a study of mollusc shell “taphonomy” which measures 

preferential preservation of material in the fossil record. Ongoing, in situ experiments at about a 

dozen long-term sites within the MMS study area have been established. The strategy is to set 

out shell material and assess diagenetic changes over time frames of years. Bottom chamber 

measures of sediment oxygen consumption and quantitative faunal samples at each site have also 

been collected. The hypothesis being tested is that the chemical dissolution of carbonate is 

accentuated in areas of high oxygen consumption. These sites stretch from the brine pool at the 

Flower Gardens down to ~1,000 m depth. Oxygen consumption measurements have been taken 

at a number of locations in the study area. 
17 

3. A CONCEPTUAL MODEL OF THE DEEP-SEA 

A conceptual model of the deep-sea, it’s living and non-living components, has been 

constructed to represent the interacting stocks of organisms that make-up a typical benthic 

boundary layer community (Figure 3-1). A biological boundary layer is defined as approximately 

1 m deep into the sediments, extending up through the mixed layer of the water column to 

approximately 100 m above bottom. This biological boundary layer thus conforms to that 

previously used to describe the principal interacting components of a deep-sea benthic 

community (Smith 1992). This conceptual model has been drawn in software called STELLA II. 

The software allows flexibility in reformulating the structure and internal relationships within the 

system. The formulation presented is a modification of the model used by Smith (1992) and 

represents a deep-sea food chain. The modifications consist of defining every stock variable, 

living or non-living, as a “box” and every process, such as predation or respiration, as an 

“arrow.” Thus all interactions between the biota and the sources and sinks of organic matter are 

explicit either as “boxes” or “arrows.” On the other hand, “physical” processes are not explicitly 

represented. These implicit factors affect the “arrows” between the “boxes.” Each “box” has a 

“size” in terms of concentration, biomass or abundance that is the sum of the “arrows” entering 

the “box” minus the “arrows” leaving the “box.” At steady-state, each “box” does not change 

with time, and thus the inputs equal the outputs or losses. An important process, respiration, has 

been omitted in this conceptual food chain. Each stock can be taken separately in a submodel 

(Figure 3-2). Macrofaunal biomass, for example, is a function of what it consumes, what 

consumes it, respiration, and feces production. 

The contents of each stock can be expressed as a differential equation. The set of differential 

equations of all the stocks can be used to simulate the behavior of the entire food chain over 

time. The problem with doing this in the deep-sea is that data to quantify the stocks and 

processes are sparse at best. While considerable information exists on the stocks in a few 

locations and a few data exist on the rates of processes in others, the locations where both stocks 

and fluxes are known, even in the most minimal sense, are quite limited. One exception is a 

study of the central North Pacific, but even this study treated the sediment dwelling biota as one 

functional group due to a lack of detailed information on the foodweb (Smith 1992; Smith and 

Kaufmann 1999). 

An advantage of this model is that with adequate data it can be used to simulate how the 

ecosystem will function under different conditions. Previously this approach has been used in an 

oligotrophic upper continental slope environment off eastern Greenland (Rowe et al. 1997). 

Boxcore standing stocks of the components represented were coupled with measures of 

community respiration (using a bottom lander with benthic chambers) and laboratory measures 

of bacterial activity to simulate the variations in biomass over time in response to a single 

seasonal pulse of organic matter related to a short ice-free period. In a purely hypothetical 

situation, the bioenhancement of infauna due to the disposal of organic rich material, such as 

sewage sludge or dredge spoils, was modeled, based on a simplified rendition of the Smith 

(1992) model (Rowe 1998). By adding organic matter in a pulse, the community “shifted up” to 

higher biomass and higher respiration and the alteration predicted by the model was validated by 

actual data at Deepwater Dumpsite 106 on the continental rise in the northwest Atlantic Ocean. 

This feature of “shifting” up or down in response to different input terms will be an important 

tool for interpreting the response of community structure and function in the deep-sea GoM in 

reaction to inputs from oil and gas exploration and production activities. The two most relevant 
18 

Figure 3-1. Preliminary conceptual model of the deep-sea Gulf of Mexico. 

CARCASSES

SEDIMENT ORGANICS

DISSOLVED ORGANICS

MICROBIOTA

MEIOFAUNA

CHEMOTROPHS

HIGH MOLECULAR WEIGHT HYDROCARBONS

MACROFAUNA

MEGAFAUNA

FISHES

Scavenging1

Deadfalls

Scavenging2

Predation3

Predation2

Deposit Feeding3

Deposit Feeding2

Bactivory3

Bactivory1

Assimilation1

Lysis & Dissimilation

Advected Organic Detritus

Deposit Feeding1

Bactivory2

Predation1

Chemotrophy

Seeps of HMWHC

Vertical Rain of POC

= fluxes 

= standing stocks of the state variables 

= losses from and inputs to the system 

Key
19 

Figure 3-2. Macrofauna submodel of system carbon food chain model. 

inputs at platforms are organic enrichment due to a “reef” effect and the introduction of 

contaminants. 

3.1 Biotic Variables 

The details of the conceptual model can be described as a number of biotic and abiotic 

variables (living and non-living). In addition, the model is described in terms of various derived 

variables related to community structure and function. In-the-end, an understanding of the model 

components and their interactions are used to select the set of variables to be quantified that best 

describe the functioning of the system being studied. These inferences are then ground-truthed 

by in-the-field observations and the model is revised as needed. The biotic variables include the 

microbiota, meiofauna, macrofauna, megafauna, and fishes. These groups of biota make up the 

overall stocks of the system which are estimated through conventional quantitative sampling as 

described in Sections 6.0 and 7.0. 

Microbiota. The microbiota are the “bottom” of the food chain. Microbiota are represented 

by the bacteria and protists, including benthic foraminifera. Their principal food source is 

thought to be dissolved organic matter, although particulate material can be directly utilized if 

the biota can produce exoenzymes to mobilize particles. The protists can engulf and assimilate 

particulate material as well. The bacteria generally have a density of about 109

 per mL of wet 

sediment, measure about a micrometer in size, and can have very short turnover rates in the 

presence of reactive organic substrates. The protists can be much larger and occur in far fewer 

numbers than the bacteria. Protists are thought to be important components of ecosystems in 

MACROFAUNA

Deposit Feeding

Bactivory

Predation on Meiofauna

Predation

Respiration

Feces

(see Key in Figure 3.1.) 
20 

areas of reduced oxygen. With the exception of the forams, microbiota have rarely been 

evaluated in the deep-sea. 

Bacteria that are specially adapted to utilizing methane and sulfide are common in areas 

characterized by petroleum seeps (MacDonald et al. 1996; MacDonald 1998). Many of these are 

symbiotic, living in the tissues of large invertebrates. While these specialized associations are 

explicit in this model, they are presumed to be confined to “seep” areas. 

Foraminifera are large shelled protozoans, often the size of metazoan meiofauna. Few studies 

have compared foraminiferal biomass to other benthos in the deep-sea, but biomass can exceed 

the meiofauna (Coull et al. 1977) and abundances commonly exceed the meiofauna plus the 

macrofauna (Snider et al. 1984; Gooday 1986). Although little is known regarding foraminiferal 

energetics, it has been shown that foraminiferal metabolism is markedly increased by organic 

enrichment (Linke 1992). Therefore, accurate deep-sea carbon and energy budgets should 

include the foraminifera. 

Meiofauna. In this model, meiofauna are defined as metazoans that are retained on a 63 

micron sieve. Meiofauna include nematode worms, harpacticoid copepods, and several other 

taxa. In some studies benthic forams are also included, but in this model, forams are considered 

part of the microbiota because they are single-celled organisms, rather than metazoans. Most 

meiofauna feed on small particles consisting of detritus, bacteria, other meiofauna, and small 

protozoa such as ciliates and flagellates. Turnover rates for meiofauna can be as short as a few 

days when temperatures are high and food is plentiful. No reliable generalizations can be made 

about their turnover times or growth rates in the deep-sea, but it is assumed that both are 

substantially slower than in shallow water due to food limitation and cold water temperatures. In 

shallow waters meiofauna biomass is less than that of the macrofauna, but in the deep-sea this 

appears to be reversed. 

The finding that meiofauna biomass is higher than macrofauna biomass in the northern GoM 

(Pequegnat et al. 1990) indicates that meiofauna may be responsible for much of the organic 

matter metabolism in deep-sea sediments. Therefore, a survey of meiofauna community density 

and biomass is needed to characterize this energetically important group. In shallow coastal 

systems, meiofauna remove bacteria at a rate that equals sediment bacterial production 

(Montagna 1995). This indicates meiofauna are most likely responsible for maintaining bacterial 

populations in log-phase growth cycles and are therefore indirectly responsible for maintaining 

rates of nutrient recycling. Despite the apparent importance of meiofauna in deep-sea energetics, 

there is no knowledge of the rates at which these processes occur. Therefore, process studies are 

needed to assess meiofaunal consumption rates. Techniques to measure meiofaunal bacterial 

feeding rates on bacteria have only been used in shallow water (Montagna 1995). 

Macrofauna. Macrofauna, in this model, are the invertebrates retained on a 300 micron sieve. 

The principal organisms are polychaete worms (≈50%), bivalve molluscs, and crustaceans in the 

groups Isopoda, Amphipoda, and Tanaidacea. The production to biomass ratio of the macrofauna 

in shallow water communities is often assumed to be unity, but this can vary widely. In the deepsea, it is assumed the ratio is much lower but there is little evidence for this one way or the other. 

Biomass and densities decline sharply with depth in most ocean basins. Macrofauna consume 

microbiota, meiofauna and organic detritus. Macrofauna are preyed upon by megafauna and 

fishes. 

Megafauna. The megafauna are organisms that are routinely sampled by trawls with 2.5 cm 

stretch mesh or organisms that can be seen easily in bottom photographs, usually about 1 cm or 

so in diameter. They are composed for the most part of decapod crustaceans and echinoderms. 
21 

Cnidaria, such as sea pens, soft corals and anemones, are also common in the megafauna. 

Megafauna can be suspension feeders, predators, scavengers or deposit feeders. For the purpose 

of the model, the swimming scavengers that consume carcasses, such as the large amphipod 

Eurythenes grillus, are included in this group. Megafauna have been extensively observed with 

photographic techniques. 

Fishes. Demersal fishes are defined as those species that live on or near the bottom. Fishes 

are both predators and feed on dead falls, megafauna and macrofauna. 

3.2 Community Structure 

Community structure, in the context of this model, has two interpretations. It is represented 

explicitly in the conceptual model as the standing stocks of the living components of the 

ecosystem as discussed above, and as such it represents the relative and absolute importance of 

the stocks in terms of biomass and rates of processes in the model. Secondly, community 

structure refers to the parameters that quantify the living stocks, as described below. 

Biomass. A measure of the standing stock in some currency of mass per unit area of 

seabottom is biomass. Wet weight is a common measure. It can also be measured as dry weight, 

ash-free dry weight or carbon. The model currency is carbon, so the ideal measure is in terms of 

carbon. Biomass tends to be inversely related to depth, in a log-normal fashion. Within the entire 

community, the highest biomasses are found in the total bacterial counts, both in shallow and 

deep water. All the size fractions in the deep-sea have biomass values that are somewhat lower 

that 1 g C/m2. In shallow water, each fraction can have biomasses of 10’s of g C/m2 in unusually 

fertile conditions. This is not expected in the deep-sea. 

Abundances. A surrogate for biomass, that is often measured in ecosystem studies, is animal 

abundance or density. However, mean sizes can vary. In the GoM, mean size seems to decrease 

with depth. Common abundances for the organismal groups in the model are bacteria, 109/mL 

wet sed.; meiofauna, 0.25 to 1.5 x 106/m2; macrofauna, 102 to 104/m2; megafauna and fishes, 

several hundred to a thousand per hectare. The abundances of each group are hypothesized to be 

a function of the input of carbon and energy to the stock. If the relationship of numbers to 

biomass is known, these abundances can be used in the model. The NGoMCSS studies used 

conversion factors from the studies of Rowe (1971) to calculate biomass from densities. 

Diversity. Measurements of the numbers of different species are expressed as diversity 

values. Diversity has been assessed on macrofauna, megafauna, and fishes. Diversity indices 

attempt to lessen the effects of sample size, to aid comparisons between regions of differing 

animal densities. Common indices of diversity are Sanders Rarefaction, Hurlburt’s expected 

species number E(s) and the information function H’(s). The GoM appears to be somewhat 

different from large ocean basins in that maximum diversity is not found on the deep slope or 

upper rise, but rather on the upper slope or outer shelf. Thus it is similar to other isolated 

“mediterranean” basins where diversity declines with depth down the slope. It is expected that 

intense inputs of organic matter will decrease diversity due to competitive exclusion. Diversity is 

not calculated by the model. 

Zonation. The degree to which individual species and groups of species are isolated across 

isobaths (zonation), between geographic regions, or any other physico-chemical gradient is 

referred to as zonation. Zonation by groups of species as a function of depth has been measured 

by “rates of species change” across depth intervals or measures of percent similarity between 

depths. These can be measured on meiofauna, macrofauna, megafauna, and fishes. Groups of 

species appear to occur in zones, but considerable overlap has been observed as well, with few 
22 

distinct, immutable boundaries. On the other hand, some individual species of megafauna tend to 

have a shallow water boundary that is sharp and severe and a deeper water boundary that is a 

slow decline in numbers with depth. Zonation down slope is hypothesized to be a function of 

competition along a gradient of declining food supplies. Zonation is not calculated by the model 

but can be addressed through hypothesis testing. 

3.3 Community Function 

The processes, or arrows, in the model encompass a wide range of interactions amongst the 

model’s components. 

Microbial Activity. The respiration and the assimilation of organic substrates by the 

microbiota are dependent on inputs of organic matter and temperature. In the model this can be 

either sedimenting POC or hydrocarbons. The sedimenting POC can be derived from several 

sources, as indicated above. A basic assumption is that, in general, smaller organisms are 

consumed by larger organisms because it is more energy efficient. If the organic matter is 

reactive, the food web will compete for the organic matter. If the organic matter is highly 

refractory then it is assumed that a food chain will dominate in which the bacteria remobilize the 

organics in order to make it available to metazoans. Predation is represented as arrows between 

the living components of the model as indicated. It is assumed that large organisms will 

preferentially take large prey rather than small prey because it is more energy efficient. 

Macrofauna are assumed to be deposit feeders. Heterotrophic bacteria are assumed to consume 

sediment organic matter. Scavengers consume carcasses and are included in the megafauna. 

Fishes consume megafauna and carcasses. Respiration is one of the most important measures for 

each organismal group because it dominates the carbon cycle. However, respiration is not 

explicit in the conceptual model. Most carbon that is consumed (50 to 90%) is recycled to 

metabolic carbon dioxide. Respiration is estimated from animal size and temperature for the 

larger organisms based on literature values. 

Growth, Reproduction and Recruitment. The rates of growth, reproduction, and recruitment 

are poorly known in deep-sea organisms. The model can calculate growth by fundamental or size 

group. Reproduction and recruitment are not well known. It is assumed that growth, reproduction 

and recruitment can be seasonal in some species but this kind of information for the GoM slope 

is inferential at best. It should be noted that in the NGoMCSS study’s central transect, it did 

appear that there were about 1.5 times as many macrofauna individuals on the upper slope in the 

spring than in other sampling periods, suggesting that some type of growth, reproduction, and/or 

recruitment had occurred, but the mechanism that gave rise to this observation is unclear. 

3.4 Sediment Properties 

Non-living model variables include the “fuel” for the system and those processes that supply 

carbon and energy to the ecosystem. Inputs of carbon are critical to the functioning of the 

ecosystem, and is often present in limiting amounts in the deep-sea. On the GoM slope, labile 

organic matter is transported to the communities from primary productivity (either directly 

settling to the site or being laterally advected), fossil sources of carbon (oil and gas to support 

chemosynthesis), and potentially from large animal carcasses and sinking Sargassum. 

Sedimentary organic matter includes a suite of natural organic compounds found in deep-sea 

sediments. Organic matter is derived mostly from the slow rain of particulate organic matter 

(POM) originating from dead cells, crustacean molts, and fecal pellets produced by plankton in 

the overlying photic zone. Some POM sinks very slowly but aggregates and pellets can rapidly 
23 

reach the seafloor. The composition of organic matter that reaches the sediment is largely 

unreactive and poorly characterized. POM is extensively reworked in the water column either 

being remineralized or transformed to dissolved organic matter (DOM). The amount of POM 

that reaches the sediment, its ultimate repository, is affected by many factors but, in particular, 

the water depth. POM concentrations in the sediments of the deep-sea are low. Relatively 

refractory terrigenous-sourced organic matter is an increased percentage of the POM close to 

shore and river discharge points. POM is usually inversely proportional to grain size, but does 

not correlate well with the biomass of the living components. In spite of its meager reactivity, it 

is assumed to be the basis of the deep-sea food chain except at hydrocarbon seep sites. 

Three sources of sediment organic matter are possible in the deep-sea and two are explicit in 

the model: vertical transport from the photic zone and lateral export from the continental margin. 

The third source is slumping of material from organic matter-rich areas up-slope and its 

importance is unknown. 

The discovery of chemosynthetic communities with large megafaunal communities 

dependent, through their symbionts, on methane and sulfide derived from natural seeps has 

highlighted a non-photosynthetic source of carbon and energy for deep-sea GoM organisms. 

Hydrocarbons migrate to near-surface sediments from deep subsurface reservoirs of oil and gas. 

The megafauna communities of note utilize the hydrocarbons but are living within the larger 

organisms, not in the sediments or within the hydrocarbons. “Seeps” support high biomass but 

their influence on biota outside of the immediate vicinity of the seeps is largely unknown. Recent 

chemical evidence indicates that the biogeochemical influences of seeps in sediments tend to be 

localized (John Morse, pers. comm.). The oil and gas is often degraded by bacteria inducing 

anoxic conditions in the adjacent sediments producing sulfidic environments. 

A poorly quantified input of organics to the deep-sea are falls of carcasses that die or are 

killed in the water column, usually near the surface. Carcasses can range in size from a few 

centimeters to a whale of several tons. The relative importance of this source of organic matter is 

not well established. It is known that scavengers exist that can take advantage of such sources, if 

and when they are available. Similarly, Sargassum, Thallasia, wood, etc. has been observed on 

the bottom as well. 

There is a range of abiotic variables that has been shown to influence biotic patterns in 

marine environments. One type are those related to the physical texture of the sediment and 

including properties such as grain size, permeability, porosity, and organic and inorganic carbon 

content. As the substrate that supports the benthos, variations in these physical properties are 

important in understanding biotic patterns. In addition, it is known that influxes of organic matter 

and contaminants can cause changes in community structure and abundance. Introductions of 

labile carbon will cause opportunistic animals to flourish and others to do less well. Organic 

matter enhancement also leads to anoxic conditions that produce a range of toxic chemicals such 

as sulfides. Organisms are also known to be selectively sensitive to chemical contamination from 

hydrocarbons and trace metals. Some animals can tolerate exposure better than others causing 

shifts in populations when communities are exposed to pollution. In addition, hydrocarbons, 

particularly aliphatics can be metabolized by microbes and may actually enhance sedimentary 

microbial populations rather than exert a toxic effect, though as mentioned above, oxygen and 

sulfate consumption can produce toxic chemicals. 

Explicit in the model is microbial biomass but functional groups of bacteria are not defined. 

All of the bacteria and protists are assumed to be heterotrophs, as opposed to possible 

chemoautotrophs in the “chemotrophs” stock. However, the terminal electron acceptors 
24 

(oxidants) of heterotrophs can differ. In the oligotrophic central gyre areas of the deep-sea where 

sediments are oxic, all the bacteria are assumed to be aerobes. In sediments on continental 

margins, this is probably not true. Below the sediment-seawater interface the functional bacterial 

groups are defined by the terminal electron acceptor (TEA) they use (Figure 3-3). Few 

comparisons of the importance of the different TEA’s (oxidants) have been made, but in rapidly 

accumulating sediments near the Mississippi delta, oxygen and sulfate were of equal importance. 

However, as oxygen declined and became limiting, sulfate reduction dominated heterotrophic 

metabolism. These processes are measured by examination of the porewater chemistry with 

depth in the sediment. These processes are important in situations where organic loading depletes 

oxygen in significant relative to oxygen consumption. Oxidized metals can also be used as 

oxidants. Comparisons with other oxidants are limited but in some situations this process is 

thought to be important. In sediments containing even modest amounts of reactive organic 

compounds, sulfate reduction can become the dominant respiratory process, even greater than 

oxygen. This is because sulfate is the fourth most abundant ion in seawater, having a 

concentration that is 140 times that of oxygen at saturation. Lin and Morse (1991) have made a 

series of cross slope transects around the GoM in which they derived integrated sulfate reduction 

rates down core. Highest rates were near the Mississippi River. Lowest rates were in oligotrophic 

areas of the southern GoM. Methane can be produced as a breakdown product of metabolism of 

large carbon compounds and it can be formed when carbon dioxide is used as the oxidant. 

Methane can then be used as an energy source by bacteria. This methanotrophy occurs in 

methane seeps near hydrocarbon seeps. Therefore, characterization of the geochemical properties 

at a location is important for ascertaining the basic metabolic processes that are important to the 

base of the food chain. Significant alteration of the inputs to the system, in particular carbon, can 

result in large shifts in bacterial populations and metabolic strategies. In addition, the rate of 

sediment accumulation and the intensity of bioturbation (mixing) are important in establishing 

the sediment redox conditions and provide insight into the reactivity of sedimentary organic 

matter. 

3.5 Water Column Properties 

The currents in the overlying water column are important when considering biotic patterns. 

Near-bottom currents are in part responsible for establishing the sediment properties by transport 

to the site and also winnowing/erosion of sediments. The water column is also the medium 

through which the overlying primary productivity is transported to the sediment. The currents 

serve as a mechanism for transport of larvae and juveniles throughout the system. So any model 

of the deep-sea must take into account the dynamic nature of the physical oceanography, and the 

hydrography of the water masses that overlie and interface with seabottom habitats. 

3.6 Community “Health” 

While widely discussed, the concept of ecosystem “health” remains elusive. Due to the 

complex nature of natural ecosystems an overall assessment of health is difficult to define. 

Specific portions of an ecosystem can be characterized by parasite infestation, pathologies, 

reproductive success, demographics (age distribution), and the presence of measurable responses 

to stress (such as stress proteins and the inducement of P450 detoxification enzymes) to name 

just a few potential indicators of “health.” However, a mechanism or approach to provide an 

overall integrated assessment of the community “health” has yet to be agreed to. For example, in 
25 

DETRITAL ORGANICS

POC Input

Burial

MACROFAUNA

MEIOFAUNA

CHEMOTROPHS

Mn REDUCERS

Fe REDUCERS

SRBS

Deposit Feeding1

Deposit Feeding2

AEROBIC BACTERIA

Assimilation3

Assimilation4

Assimilation6

Predation

DIC

CaCO3

Shell Burial

Aerobic Respiration1

Aerobic Respiration2

Chemosynthesis

Sulfate Reduc tion

Fe Reduction

Mn Reduction

Dissolution

DIC Loss

DENITRIFIERS

Aerobic Respiration3

Denitrific ation

Assimilation1

Bactivory

Prod uc tion

Assimilation2

(see Key in Figure 3.1.) (SRBS=Sulfate Reducing Bacterias) 

Figure 3-3. Sediment submodel of functional bacterial groups as defined by utilization of different 

terminal electron acceptors. 
26 

the past it was thought that simple increases in biomass were positive for a group of organisms, 

but now it is recognized that these biomass increases can be accompanied by significant changes 

in community structure. Is this positive or negative for the community and is the community 

“healthy”? Acute deterioration of a community, such as massive mortalities or disappearance of 

species, is easily recognizable, but over longer time frames it has become clear that some of the 

more intractable issues related to sublethal effects are often difficult to quantify or even 

recognize in the early stages of change. For example, loss of biodiversity is seen as a 

deterioration in ecosystem health, but the natural processes that also effect biodiversity are not 

well understood. 

Ecosystem “health” may be measured by such things as community structure, e.g., the classic 

view that a healthy benthic community is one of high diversity and high productivity. Therefore, 

unhealthy ecosystems share a number of properties—lessened productivity, declining 

biodiversity, dominance by lower trophic levels and others. It has also been suggested that 

systems degraded beyond a certain point cannot recover (Rapport and Whitford 1999; Rapport 

et. al. 1998; Rowe and Haedrich 1979; Haedrich et al. 1980; Haedrich and Maunder 1985; 

Snelgrove and Haedrich 1985; Haedrich and Merrett 1988; Merrett and Haedrich 1997). 

Therefore “health” can be inferred from community composition, abundances, and size 

frequencies. “Health” will be assessed by comparisons of the structure of similar ecosystems 

world-wide that have been subjected to varying degrees and types of disturbance. 

At the individual level, “health” has been inferred from physiological responses such as 

disease incidence, size and condition, and reproductive state. Parasites, and the diseases they 

cause, are often important determinants of population health. Measurable effects include 

mortality, decreased reproductive effort, decreased condition, and reduced or aberrant growth. 

The outgrowth of these effects are increased morbidity, decreased fecundity and even effects on 

predator/prey relationships. 

From another point of view, “health” may be a measure of response to the stresses a 

population experiences due to anthropogenic disturbances. As a first order indication of this 

exposure, inventories of chemicals in sediments and biological tissues are measured. While this 

is at least an indication that the potential for impact is present, simply documenting the presence 

of contaminants is not sufficient to infer biological effect in most instances. However, a range of 

biological effects criteria for sedimentary concentrations of contaminants have been used to infer 

the possibility of effects. Once the potential for exposure has been verified, first-order biological 

responses to contaminant exposure are often important variables to monitor. These include a 

range of responses including production of contaminant metabolites, the induction of 

detoxification enzyme systems, and molecular level indications of genetic damage, to name a 

few indicators of sublethal biological response to contaminant exposure. 
27 

4. SAMPLING DESIGN 

4.1 General Design Considerations 

Several general principles guided the development of the overall sampling plan. First, 

treatments, or in this case similar stations, were chosen that would falsify the null hypotheses. 

Due to resource limitations and the large geographic area to be studied, it was impossible to 

measure everything, everywhere. However, it was important to measure what were judged to be 

the most important variables at the most important sites. 

Second, pseudoreplication was avoided, i.e., replication was at the treatment level. A 

treatment was a factor level, or combination of factor levels, applied to a sampling unit. 

Sampling units were stations or replicate samples within stations where all other variables but the 

variable to be tested were as similar as possible. The generic form of all null hypotheses was that 

the treatment level effect equaled zero, i.e., the stations themselves were not fundamentally 

different by other than the variable being tested. For example, if each treatment was only 

represented by a single station, then one would only know that the stations were different, not 

why the stations were different. 

Third, station locations were optimized to test more than one hypothesis. This was a cost 

reduction technique. For example, stations along a transect to test for depth differences were 

paired with stations in specific habitats to test a second hypothesis. 

Fourth, confounding factors were minimized. A common problem was that more than one 

variable could be changing at a given station. For example, two stations could differ by water 

depth, distance from shore, and distance from the Mississippi River. In the end, it is not known 

which variable (or if an interaction of the variables) is causing the observed differences and thus 

generalities are difficult, if not impossible, to discern. Therefore, stations were chosen where 

comparisons could be based on differences in a single or related set of variables. 

Fifth, balanced sampling designs were used. An uneven distribution of sampling effort can 

cause distortion of sample means when there is a difference in the number of observations 

between the datasets being contrasted. 

Sixth, a design of appropriate power was used. Power is the ability to detect change. The first 

five design considerations primarily protect at the α level against Type I errors (rejecting the null 

hypothesis when it is true). But Type II errors protected at the β level were also considered 

(accepting the null hypothesis when it is false). Replication had to be sufficient to detect the 

amount of change that is expected, given variations in the variable of interest. A large multifactorial design with little replication has many interaction terms that are often significant, thus 

limiting the interpretation and robustness of tests for the most important effects. In this case, 

previous studies suggest that a minimum of five replicate boxcores were needed at each site to 

adequately sample within-station heterogeneity. 

Finally, a meta-analysis (or synthesis) has been created (Section 9). Assembled in its entirety, 

this meta-data set contains information that does not exist within the individual analyses. In the 

end, all of the stations and replicates are only surrogates for the environmental factors that 

regulate biological processes leading to the observed patterns of faunal composition. Therefore, 

measurements have been made synoptically at locations, or subsamples (replicates) have been 

taken within a location, so that the meta-data set could be created for the synthesis and 

integration of the overall study results (Section 9). 
28 

4.2 Working Hypotheses and Station Selection 

The sampling design was developed from a series of testable null hypotheses derived from 

the conceptual model. The set of hypotheses was then used to select sampling sites so that power 

was sufficient to detect differences in the dependent variables being measured (i.e., abundance, 

biomass, diversity, analyte concentrations, etc.). The null hypotheses were based on the current 

understanding of deep-sea benthic community structure and function and prior knowledge of the 

range of habitats that occur in the GoM. 

The following characteristics were utilized to choose sampling sites in order to maximize 

hypothesis testing power: 

1) water depth (multiple transects perpendicular to isobaths); 

2) geographic location—juxtaposition to the Mississippi River (east-west 

transects); distance from shore; 

3) physiographic position – in or out of a mesoscale halokinetic basin; in a 

canyon; below an escarpment; 

4) influx of organic carbon—primary productivity derived carbon, petroleum 

seep and chemosynthetic derived carbon; and 

5) temporal change -time series sampling; location of historical sampling sites. 

The sampling design was NOT simply conducting the traditional geographic survey with 

closely spaced stations. This hypothesis-testing approach was adopted to establish generalities 

about communities in the study area because the area is so large that sampling everywhere was 

cost prohibitive. The hypothesis-testing was adopted because it allows a prediction of when and 

where particular types of communities, both in terms of structure and function, will or will not be 

encountered. Hypothesis testing provides a powerful tool for either increasing or limiting 

sampling intensity in time and space as new data are collected and historical data are jointly reinterpreted. Each hypothesis, as described below, explores mechanisms believed to explain much 

of the variation in community structure and function in the deep GoM. 

Based on the above considerations the following kinds of contrasting environments or 

habitats were sampled for comparisons: 

• Depth. 

- Water depth is probably the single most important gradient in determining 

faunal compositions and forcing factors in the study area (Hypothesis HO1). 

Comparisons were made along a series of transects. 

Stations: RW1-RW6, W1-W6, C1-C12, MT1-MT6, S35-S44 (Figures 4-1 to 4-3) 

• Nutrients (organics) 

- The input of organic nutrients from Mississippi River discharge causes an east 

to west gradient in faunal compositions and forcing factors (Hypothesis HO2). 

Comparisons were made along isobaths at similar distances from shore at 

varying distances from the Mississippi River. 
29 

Figure 4-1. Benthic survey stations in the far northwest GoM. 
30 

Figure 4-2. Benthic survey stations across the Florida Escarpment (for regional context see Figure 5-1). 

N 
31 

Figure 4-3. Sampling sites along the Mississippi Trough (MT1-MT6). Historical sites from the NGoMCSS 

study were also included (C1-C14; for regional context see Figure 5-1). 
32 

Stations: RW1-RW6, C1-C12, S37-S42 (Figures 4-1 to 4-3) 

• Basins. 

- The common mesoscale basins found on the slope, unless influenced by seeps, 

have the same faunal compositions and forcing factors as the “normal” slope 

because the entire slope is draped in a similar Holocene “blanket” of silt and 

clay within which the biological communities live (Hypothesis HO3). 

Comparisons were made within and outside of basins at comparable water 

depths and distances from the Mississippi River and shore. 

Stations: WC12, B1-B3, NB2-NB5 (Figure 4-4) 

• Canyons. 

- Faunal compositions and forcing factors are the same in or out of submarine 

canyons (Hypothesis HO4). Comparisons were made between stations within 

and outside of canyons at comparable depths and distances from the 

Mississippi River and shore. 

Stations: MT1-MT6, W5, W6, RW6 (Figures 4-1, 4-3, and 4-4) 

• Steep Escarpments. 

- Faunal composition and community structure parameters are the same on the 

“normal” slope as they are at the base of the Sigsbee Escarpment (Hypothesis 

HO5). Comparisons were made between sites on the slope and at the base of 

the Sigsbee escarpment at similar water depths. 

Stations: S39-S42 (Figure 4-2) 

• Productivity. 

- Cyclonic and anticyclonic features in the surface waters consistently alter 

surface primary productivity and this results in differing seafloor fauna 

composition and community structure (Hypothesis HO6). Comparisons were 

made between stations that underlie areas of historically documented 

differences in sea surface primary productivity, holding other variables as 

constant as possible. 

Stations: RW1-RW6, S35-S36, DeSoto Canyon [HIPRO] (Figures 4-1 and 4-2) 

• Hydrocarbon Seeps. 

- Hydrocarbon seeps affect energy and carbon supplies contributing to different 

faunal composition and structure (Hypothesis HO7). This is tacitly true since 

chemosynthetic fauna are restricted to seep sites. Rather than a direct test, 

stable isotopes were used to assess inputs of chemosynthetic carbon to the 

foodweb. Data produced from the chemosynthetic community studies was 

compared with equivalent non-seep data. Explicit testing by paired stations 

was not attempted. Questions of “how close is close” are still difficult to 

resolve even in studies concentrated at seep sites. The decision to address this 

question in a limited way recognizes the fact that other studies are already 

being conducted at seep sites and that the primary focus of this program was 

non-seep environments. 
33 

Figure 4-4. Physiographic comparisons within (B) and outside (NB) mesoscale basins in the Central 

GoM. Historical stations from the NGoMCSS study are also included (W1-W6, WC-5, and 

WC-12; for regional context see Figure 5-1). 
34 

• Temporal Changes. 

- Faunal composition and community structure of benthic fauna vary with time 

(Hypothesis HO8). Comparisons were made at a subset of survey stations 

sampled during each of the three planned cruise activities. In addition, to 

provide a longer timeframe for comparisons, historically sampled sites were 

reoccupied. The evaluation of data between studies was explicitly addressed 

to ensure that inter- program comparisons were valid. 

Stations: W1-W6, WC6, C1-C12 (Figures 4-3 and 4-4) 

• The above comparisons have been translated into testable null hypotheses 

(Table 4-1). A critically important part of program design was selection of the 

most appropriate locations to test the hypotheses. This was especially true for 

this program because the area to be characterized is large, the area contains a 

wide range of habitats and there are several important forcing factors that tend 

to confound delineation of cause and effect relationships. As summarized in 

the introductory materials, the northern GoM represents one of the most 

complex geological and oceanographic settings in the world. The challenge 

was to determine if peculiar subregions, as listed in the hypotheses, harbor 

faunas with peculiar quantifiable properties. Additional factors explicitly 

considered during the selection of proposed sampling sites include the 

location of historical sampling (to extend timeseries observations), the current 

and future trends in energy resource exploration and exploitation, and possible 

anthropogenic effects related to proximity to existing production facilities 

(primarily accommodated by screening all locations for contaminants). The 

study sites, to the degree possible, were explicitly tied to the hypothesis 

testing as listed. 

Survey stations initially were chosen to sample at depths between 300 m and 3,000 m within 

the EEZ boundary with Mexico. The site selection spanned the range of known conditions 

providing the best tests of the hypotheses posed. Thus, the first year of sampling occurred along 

a transect just north of the Mexican border (RW1-RW6) over to one transect across the Florida 

escarpment (S37-S42; see Figures 4-2 and 4-3). This inclusive range of conditions enabled us to 

gain an understanding of the whole northern GoM deep-sea benthos, rather than isolated parts. 

The smooth upper slope of the northern Florida slope is contrasted with the heterogeneous 

topography off Texas. A transect down the Mississippi Trough (MT1-MT6) provided an 

evaluation of the effect of particulate and nutrient input from a large river (Figure 4-4). The 

basins on the upper slope were included as a test of how such basins might trap shelf-derived 

organic detritus and resultant effects on the biota (WC12, B1-B3, NB2-NB5; see Figure 4-4). 

Two locations at the upper end of the DeSoto Canyon (S36, S35) were chosen because one is 

frequented by whales and the other is characterized by high nitrate concentrations semipermanently in the euphotic zone (see Figure 4-2). In 2001, HIPRO was added to reinforce the 

relationship between high benthic densities in all size groups and the high primary productivity 

along this strip of the upper continental margin. 
35 

Table 4-1 

 

Summary of Benthic Survey Experimental Design: 

Null Hypotheses, Station Selection Criteria, Number (No.) of Stations (Sta.), and Number Samples 

(The number of samples is based on five replicates per station.) 

Null Hypotheses Design Criteria No. 

Stations 

No. 

Samples

HO1: There is no variation in benthic 

fauna explained by depth 

3 Replicate transects over 7 depths; 

occupy historical stations and others 

21 105 

HO2: Faunas do not exhibit an East 

to West gradient 

Additional transect to HO1; remove 

confounding geological effects and water 

mass effects 

7 35 

HO3: Basin faunas are not different 

from non-basin faunas 

3 salt bottom and 3 salt surrounded 

basins 

6 30 

HO4: Canyon fauna is not different 

from slope fauna 

4 Canyons to be compared to noncanyon and non-basin biota from HO1-

HO3

8 40 

HO5: Fauna below escarpments is 

not different from slope 

Add sta. below escarpment in addition to 

HO1- HO4 in area of furrows 

7 35 

HO6: Surface primary production 

does not explain faunal 

differences 

Add sta. to (HO1-HO2) in “hot spot” 

defined by historical water column data 

7 35 

HO7: Proximity to organic input 

causes no bioenhancement 

Add sta. in proximity to “geochemical 

anomalies” (hydrates, brine pools, 

methane seeps) 

8 40 

HO8: There is no variation in benthic 

fauna over time. 

6 stations (or other elements of the 

design, HO1 -HO7) over 2 years 

12 60 

Experimental, or “process” stations were chosen to reflect the greatest range in community 

dynamics, as inferred from the benthic survey data conducted in 2000. The survey site selection 

was based on inferences about where the greatest variations in community structure might be 

observed based previous studies world-wide. The next step, conducted in late 2000 and early 

2001 after a substantial representative portion of the samples had been processed, was to identify 

the experimental or ‘process’ stations at those survey sites where the greatest ranges in 

community structure were actually observed in the 2000 survey. 

The first interim meeting in February 2001 was used to up-date the hypotheses based on 

Survey Cruise data, select sites to be repeated, add new sites to strengthen comparisons and 

identify sites for process studies. Following considerable open discussion among the 

investigators, the team decided on the following four locations for process studies: MT3, S36, 

S42, and MT6. This initial choice provided the following combination of biotic conditions: high 

densities at modest depth (MT3); high densities at mid depth (S36); modest densities at shallow 

depths (S42); and low densities at great depth (MT6). 

New sites added were the giant sediment furrows below Green Knoll (GKF), on the top of 

Green Knoll (GK), a site between the Mississippi Trough (MT) and the DeSoto Canyon 

(HIPRO), and near (ca. 1 km) the Bush Hill seep (BH). See Section 5.3 for a discussion of the 

new sites. 

4.3 Dependent and Independent Variables 

Based on the various hypotheses described in the previous section, a set of dependent and 

independent variables was chosen that would provide the set of observations at each station to 
36 

test the validity of the null hypotheses (Tables 4-2 to 4-4). These variables also provide the 

stocks and processes for initial input to the model. 

4.4 Statistical Analyses 

Statistical analyses to test for differences among treatment means were performed using 

parametric, general linear models. Prior to analysis, data were transformed, generally by natural 

logarithm, to achieve homogeneity of error variance, normality of residual errors, and additivity 

of effects. A data set of residual errors was created for each model and tested for normality. Both 

untransformed and transformed residuals were computed, and the datasets that were normally 

distributed with means of zero were used for analyses. 

The models that follow describe the relationships among the independent design variables 

only. The measured dependent variables are described in the methods sections. The notation used 

follows conventions described by Kirk (1982). 

Table 4-2 

 

Independent Variables Fixed by the Sampling Plan Design 

Physiographic Characteristics Water Masses 

Loop Current 

Consistently “cool” water between eddies 

Warm eddies 

Geographic Location 

East vs West 

Distance from shore 

Time 

Water Depth 

Basins underlain by salt 

Basins surrounded by salt 

Basins subjected to slumping/erosion 

Basins overlain by an undisturbed drape of 

Holocene silt and clay 

Canyons 

Escarpments 

Proximity to seeps 

Basin slope angle Months 

Years 
37 

Table 4-3 

 

Dependent Variables To Be Measured 

Community Structure 

Bacterial density Macrofaunal species composition 

Bacterial biomass Megafaunal density 

Foraminiferal biomass Megafaunal biomass 

Meiofaunal density Megafaunal diversity 

Meiofaunal biomass Megafaunal species composition 

Meiofaunal composition to major group Fish density 

Macrofaunal density Fish biomass 

Macrofaunal biomass Fish diversity 

Macrofaunal diversity Fish species composition 

Community Function 

Bacteria growth rates Fish respiration1

Bacteria respiration Fish predation on megafauna2

Bacteria response to different substrates Fish scavenging on carcasses2

Foraminiferal respiration1 Nutrient Regeneration 

Foraminiferal feeding rates1 Denitrification rate 

Meiofaunal feeding rates on bacteria Sediment accumulation rate2

Macrofaunal respiration1 Sedimentary community oxygen consumption 

Community Function 

Macrofauna growth rates2 Sulfate reduction rate 

Macrofauna predation rates2 Foodweb studies 

Macrofauna predation rates on meiofauna, bacteria, 

and organic matter 

Megafaunal respiration rates2 

Megafaunal predation rate on megafauna, 

meiofauna, bacteria, and organic matter2

Megafaunal scavenging on carcasses2 1

-calculated based on size and temperature, 2-estimated from sub-model. 
38 

Table 4-4 

 

Ancillary Variables To Be Measured at Survey and Experimental Stations 

Water Column Profiles Sediment Properties 

 

• Depth • Grain Size 

• Temperature • Porosity 

• Salinity • Elemental composition (organic carbon, 

nitrogen, sulfur) 

• Oxygen • Percent inorganic carbon (TIC) 

• Nitrate and Nitrite • Permeability 

• Ammonium • Shear Strength 

• Silicate • Bulk Density 

• Phosphate 

• Particulate Matter (PM) 

• Particulate Organic Carbon (POC) Geochemistry1

• Light 

• Currents • Nutrients 

• Dissolved Organic Carbon (DOC) 

Water Column Profile • SO4=/Cl- 

• Dissolved Inorganic Carbon (DIC) 

• Photosynthetic Pigments • δ13C DIC 

• Primary Production • Sulfate Reduction Rate 

• pH 

• H2S 

mical Contaminants • O2 

• Hydrocarbons • Reactive Fe 

• Metals • Reactive Mn 

• Acid Volatile Sulfide 

 1 Composite sample at survey station, profiles for fluxes at experimental station. 

4.4.1 Univariate Analyses 

Univariate analyses were used to test each hypothesis based on the following models. The 

stations used to test each hypothesis are summarized in Table 4-2. 

The first two hypotheses are as follows: 

HO1: There is no variation in benthic fauna with depth, and 

HO2: There is no difference in fauna along an east to west gradient. 

The effect of depth was tested with sites along transects. Multiple transects were necessary to 

replicate at the treatment level. The effect of longitude was tested with sites on an east - west 

gradient along isobaths, so this design tested two major hypotheses. This was a two-way 

completely random factorial analysis of variance (ANOVA) that is described by the following 

model: 

Yijk = μ + αj + βk + αβjk + ∈i(jk)

where Yijk is the measurement for each individual replicate, μ is the overall sample mean, αj is 

the main effect for transects and j=1-4, βk is the main effect for depths and k=1-7 (300, 750, 
39 

1,200, 1,650, 2,100, 2,550, and 3,000 m), αβjk, is an interaction term, and ∈i(jk) is the random 

error for each replicate measurement and i=1-5. 

The third hypothesis is: 

HO3: There is no difference between basin fauna and non-basin fauna. 

Geological complexity exists in the northern GoM. One expression of this complexity are the 

multiple, mesoscale (5 to 15 km diameter) basins formed by salt halokinesis. Such features may 

alter currents or act as receptacles that concentrate a rain of detrital material. Such detrital POC 

could originate in the overlying water or be exported from the continental shelf. This could affect 

benthos. One site within a basin was paired with two nearby locations already sampled for HO1

and HO2 above to test for basin effects. Station pairing was necessary to control for distance from 

shore and depth. The entire experiment was replicated at 6 different sites, so each location was a 

blocking effect. The experiment was a two-way completely random factorial ANOVA that is 

described by the following model: 

Yijk = μ + αj + βk + αβjk + ∈i(jk)

where Yijk is the measurement for each individual replicate, μ is the overall sample mean, αj is 

the main effect for replicate sites and j=1-6, βk is the main effect for treatments and k=1-3 (basin, 

non-basin same distance from shore, and non-basin same depth), αβjk, is an interaction term, and 

∈i(jk) is the random error for each replicate measurement and i=1-5. Differences between sites 

were not of interest because they replicate the basin effect, so it doesn’t matter if that test is 

significant. The main test of interest is a multiple comparison tests among treatment levels if 

there is a significant difference among treatments. 

The fourth hypothesis is: 

HO4: There is no difference between canyon fauna and non-canyon fauna. 

Another form of geological complexity in the GoM is canyons. Often sediment slumping 

occurs in canyons in addition to alteration in near-bottom current patterns. One station within a 

canyon will be paired with nearby stations at similar depths that are not in a canyon. The entire 

experiment is replicated at three (3) different features: Alaminos Canyon, Mississippi Trough 

and DeSoto Canyon, each with distinctive features. The experiment is a two-way completely 

random factorial ANOVA that is described by the following model: 

Yijk = μ + αj + βk + αβjk + ∈i(jk)

where Yijk is the measurement for each individual replicate, μ is the overall sample mean, αj is 

the main effect for replicate sites and j=1-4, βk is the main effect for treatments and k=1-2 

(canyon and non-canyon), αβjk, is an interaction term, and ∈i(jk) is the random error for each 

replicate measurement and i=1-5. 

The fifth hypothesis is: 

HO5: There is no difference between the fauna located immediately 

below an escarpment and that at sites that are not adjacent to an 

escarpment. 
40 

Another form of geological complexity in the GoM is escarpments. The fauna on the steep 

walls may be different from more gently sloping areas, but this was not sampled because 

conventional (surface ship) approaches cannot sample the hard and complex surface of steep 

slopes adequately. This experiment was intended to determine if detrital buildup at the base of an 

escarpment affects the fauna there, a condition that has been observed in shallow water as well as 

the deep ocean (Sokolova ref.). One sampling site adjacent to the base of an escarpment was 

paired with nearby locations at a similar depth that were not adjacent to an escarpment. The 

entire experiment was replicated at 7 different sites. The sites were chosen based on pairing 

stations with samples already taken to test HO1-HO5. The experiment is a two-way completely 

random factorial ANOVA that is described by the following model: 

Yijk = μ + αj + βk + αβjk + ∈i(jk)

where Yijk is the measurement for each individual replicate, μ is the overall sample mean, αj is 

the main effect for replicate sites and j=1-7, βk is the main effect for treatments and k=1-2 

(escarpment and non-escarpment), αβjk, is an interaction term, and ∈i(jk) is the random error for 

each replicate measurement and i=1-5. 

The sixth hypothesis is: 

HO6: There are no differences in benthos in areas with different 

amounts of water column primary production. 

The sites were chosen based on pairing stations with samples already taken to test HO1-HO5. 

The design is nested as a completely random hierarchical design described by the model: 

Yijk= μ + αj + βk(j)+ ∈i(jk), 

where μ = overall sample mean, αj = main effect of treatments and j=1-3 (high, medium, and low 

productivity), αk(j) = nested effect for replicate stations and k =1-7, and ∈i(jk) is the random error 

for each replicate measurement and i=1-5. The F-test used the mean square error for stations as 

the denominator. A second approach was analysis of covariance, where actual values indicating 

primary production (e.g., measured values or chlorophyll standing stock) were used as 

covariates. 

The seventh hypothesis is: 

HO7: There is no difference in benthic fauna near and far from seeps. 

Another form of geological complexity in the GoM is organic and inorganic inputs in 

geochemically anomalous environments, e.g., hydrocarbon or brine seeps. Eight (8) stations are 

added to pair with existing non-seep stations. The station pairs will be in 4 site regions. The sites 

will be chosen based on pairing stations with samples already taken to test hypotheses HO1-HO6. 

The experiment is a two-way completely random factorial ANOVA that is described by the 

following model: 

Yijk = μ + αj + βk + αβjk + ∈i(jk)
41 

where Yijk is the measurement for each individual replicate, μ is the overall sample mean, αj is 

the main effect for replicate sites and j=1-4, αk is the main effect for treatments and k=1-4 (brine 

seep, hydrocarbon seep, control for distance from shore, control for depth), αβjk, is an interaction 

term, and ∈i(jk) is the random error for each replicate measurement and i=1-5. Differences 

between sites are not of interest because they are replicating seep effects, so it doesn’t matter if 

that test is significant. The main test of interest is a multiple comparison test among treatment 

levels if there is a significant difference among treatments. 

The eighth hypothesis is: 

HO8: There are no differences in benthic fauna among different 

sampling dates. 

Six of the stations sampled for HO1-HO7 were chosen for reoccupation in years 2 and 3 so that 

there will be a time series for at least 3 years. In addition, site C7 was occupied in a previous 

study and thus extended the time series to 5 sampling periods for that site. The experiment is a 

three-way completely random factorial ANOVA that can be described by the following model: 

Yijkl = μ + αj + βk + αβjk + γl + αγjl + βγkl + αβγjkl + ∈i(jkl)

where Yijkl is the measurement for each individual replicate, μ is the overall sample mean, αj is 

the main effect for sampling periods and j=1-3, βk is the main effect for sites and k=1-2, γl is the 

main effect term for depths and l=1-3 (shallow, mid-depth, deep), αβjk, αγjl, βγkl, and αβγjkl are 

interaction terms, and ∈i(jkl) is the random error for each replicate measurement and i=1-5. 

4.4.2 Power Analysis 

Power analysis was performed to determine the detectable change in the population at a 

given power (1−β) and sample size (n). Power is calculated by: 

X

n

2 xSDx)tt( + βα

=Δ

where Δ is the percent change in the population, SD is the pooled standard deviation, tα and tβ are 

tabled values for a two-tailed test assuming a pooled estimate of variance from a large sample 

size, and X is the sample mean. Values of α=0.05, and powers of 0.95, 0.80, 0.50 were used in 

the analysis. 

4.4.3 Multivariate Analyses 

A meta-analysis (or synthesis) was performed at the end of the study. The goal of the 

synthesis was to merge all data sets of response variables to create one large data set. All the 

ANOVA’s listed above were analyzed in multivariate mode (MANOVA) to test the null 

hypothesis that the vector of population means equals zero. That is, we determined if all 

measured variables respond to the dependent variables in the design in a similar fashion. The 

advantage of MANOVA is that multivariate error is controlled. That is, error rates are controlled 

at α across all response variables. 
42 

As stated previously, all stations and replicates are simply surrogates for the environmental 

factors that regulate biological processes at different scales. These scales vary greatly from 

small- (replicate boxcores), meso- (across transects, basins, or nearby stations) to large- (across 

the entire GoM). In addition, there is a temporal scale to the variation in all measurements. 

Multivariate analysis has allowed us to test the meta-data set for correlation or covariation 

among the independent variables that are measured. Two multivariate techniques were 

employed: a parametric method (principal components analysis, PCA), and a non-parametric 

methods (multidimensional scaling, MDS). 

Multidimensional scaling is a non-parametric multivariate technique that was utilized for 

examining similarity or dissimilarity between stations, replicates, or other dependent variables in 

the experimental design. First, a similarity or dissimilarity index was computed for elements of 

the design (e.g., stations) and then a plot of the distance among points was created. The plot 

enabled us to identify unknown variables that affect the similarity or dissimilarity between 

sampling locations. Its utility, and thus its popularity, derives from its use of non-parametric 

tests. 

Principal components analysis (PCA) is a parametric variable-reducing technique that makes 

a new set of uncorrelated variables in order of decreasing variance. Analysis of abiotic variables 

was used to summarize the co-varying environmental influences on different levels of 

replications, i.e., different spatial and temporal scales. Factor loading scores were generated for 

abiotic summaries of observations (rows), which were then used in other analyses. 
43 

5. THE FIELD PROGRAM 

The Field Program was designed to initiate benthic community structure descriptions before 

community function and biogeochemical process investigations, thus allowing the choice of 

process study sites to be based on observed community structure. To accomplish this, the first 

year (2000) consisted of an extensive survey of the standing stocks of all size categories under 

investigation. This allowed the selection of the limited number of process sites (four sites 

initially) to be chosen for years two and three based on extremes in organism abundances. The 

Field Program included seven weeks at sea in 2000 for the extensive survey, three weeks at sea 

in June, 2001, for the initial study of processes, followed by two repeated legs of two weeks each 

in June and August, 2002, in the third year onto the Sigsbee Deep in Mexican waters. 

The first cruise concentrated on accumulating samples to test the eight hypotheses based on 

community structure, followed by two years in which there was a mixture of survey and 

experimental activities. The survey field effort (Cruise I) included boxcoring, trawling, 

photosurveys, and hydrocasts. Experimental stations (Cruises II and III) included a variety of 

specialized sampling efforts designed to identify important processes and forcing factors at four 

sites selected from the survey sites. The four sites were to be repeated in year three, but the plan 

was changed when it was decided to venture onto the greatest depths of the abyssal plain in 

Mexican waters. Ultimately, six full-fledged process sites were investigated, with two additional 

locations where a limited number of process measurements were made. 

In brief, the cruises and their objectives are as follows: 

Cruise I: Ship—R/V Gyre; Chief Scientist—Dr. Gil Rowe; Conducted May 3 to 

June 21, 2000. 

Cruise II: Ship—R/V Gyre; Chief Scientist—Dr. Gil Rowe; Schedule—June, 2001; 

Duration 20 days; Activities—12 survey stations (60 boxcores, 12 

trawls, 12 CTD/hydrocasts, 12 photosurveys) and 4 experimental 

stations (20 boxcores, 4 trawls, 4 CTD/hydrocasts, 4 photosurveys, 4 

ADCP casts, microbiological process studies, 4 benthic lander 

deployments, 4 pore water and solids geochemical studies, 4 stable 

isotope food web collections, and 4 specialized biological/ecological 

studies), Staffing—23 scientists, students and observers. 

Cruise III: Legs one and two: Ship – R/V Gyre, Chief Scientist – Dr. Gil Rowe: leg 

one – June 1-15, 2002; process studies on the abyssal plain at two sites, 

community structure sampling at five sites (including the two above); 

leg two – August 1-14, 2002, trawling and box coring to fill in sites that 

need re-sampling 

The details of the field and laboratory methods are provided in Sections 6 and 7. 

5.1 The Ship—R/V GYRE 

R/V Gyre was owned by Texas A&M University and operated by the Department of 

Oceanography of Texas A&M University. As the homeport of the Gyre was Galveston, all field 

work originated and ended in Galveston. The Gyre is a general-purpose, open-ocean research 
44 

vessel. The deck plans and diagrams of the ship can be accessed at the ships URL: 

http://www.ocean.tamu.edu/Cruise/plns.html

The Gyre, now property of TDI-Brooks Intl., was being retired as of 31 August 2005. 

Inquiries should be directed to Dr. Wilford Gardner, Department of Oceanography, Texas A&M, 

College Station, TX, 77843, wgardner@ocean.tamu.edu. 

5.2 Cruise I—Survey (Figure 5-1; Table 5-1) 

Cruise I was devoted to a survey of deep-sea communities of the northern GoM. Each 

standard survey station consisted of the following activities: 

A. One (1) CTD: The CTD was deployed with the starboard hydrowinch using 

conductor cable (the CTD remained attached to the conducting cable for the 

entire cruise, unless problems were encountered); 

B. Five (5) Boxcores: The boxcore was deployed with the hydraulic winch on the 

non-conducting cable. A pinger was attached to the wire above the boxcore at 

depths of 1 km and greater. Otherwise wire out and tension was adequate to 

determine bottom contact. 

C. One (1) Camera Lowering: The camera system was deployed with the nonconducting cable on the starboard hydrowinch. A pinger was attached to the 

frame to determine bottom contact and distance to the bottom. The camera 

takes up to 50 exposures. 

D. One (1) Bottom Trawl: The otter trawl was deployed with the heavy-duty 

winch on the fantail. 

The shipboard scientific crew operated on watches of 12 hours on and 12 hours off. Between 

stations, the trawl samples were sorted to species and fixed in jars by those assigned to trawling. 

The five sediment sievers on watch fixed and bottled the box core material and then assisted with 

the trawl sorting to species, displacement volume measurements (for biomass), preservation and 

labeling. Shipboard marine technicians analyzed samples for oxygen, nutrients, and salinities 

between stations. 

Each watch was assigned a “watch chief” (Clif Nunnally and Chris Gudeman) who had the 

duty of keeping records, notifying the Chief Scientist when problems arose, notifying the watch 

when the ship was on station, waking the new watch at the appropriate hour, and notifying the 

bridge when station activities were completed. The watch chief maintained a log in the main 

laboratory on the main deck that documented each activity in chronological order. Comments on 

the success or failure of the activity and inventory of all samples taken were kept. The watch 

chief and assistant ensured that the station logs on the bridge agreed with those in the laboratory. 

Each “watch chief” reported directly to the Chief Scientist. The “watch chief” communicated 

between the bridge and scientists on the deck. 

5.3 Cruise II—Processes (Figure 5-2) 

Plans for the first processes cruise were made during the first interim meeting in February 

2001. The plans were made on the basis of findings to date. Extremes of high and low densities 

in bacteria, meio- and macrofauna were used for selection criteria. The sites chosen were MT3, 

S36, S42, and MT6. High densities characterized MT3 and S36 at 1,000 and 1,850 m depth in 
45 

Figure 5-1. Benthic survey stations for Cruise I, May and June 2000.
46 

Table 5-1 

 

Summary of Sampling Conducted During Cruise I 

Station Trawl Boxcore Camera CTD 

RW1 1 5 1 1 

RW2 1 5 1 1 

RW3 0 5 1 1 

RW4 0 5 1 1 

RW5 0 5 1 1 

RW6 1 5 0 1 

AC1 0 5 1 2 

W6 1 5 1 1 

W5 0 5 1 1 

W4 0 5 0 1 

W3 1 5 1 1 

W2 0 5 1 1 

W1 1 5 1 1 

WC5 1 5 1 1 

WC12 0 5 1 1 

B1 1 5 1 1 

NB2 1 5 1 1 

NB3 1 5 1 1 

B2 3 5 1 1 

NB4 1 5 0 2 

B3 1 5 1 0 

NB5 1 5 1 1 

C12 1 4 1 1 

C14 1 5 1 1 

C4 1 5 1 1 

C7 1 5 1 1 

C1 1 5 1 1 

S36 1 5 1 1 

S37 1 5 1 1 

S38 1 5 1 1 

S35 1 5 1 1 

S44 1 5 1 1 

S43 1 5 1 1 

S42 1 5 1 1 

S41 1 5 1 1 

S40 1 5 1 1 

S39 1 5 1 1 

MT6 1 5 1 1 

MT5 1 5 1 1 

MT4 1 5 1 1 

MT3 1 5 1 1 

MT2 1 5 1 1 

MT1 1 5 1 1 
47 

Figure 5-2. Sampling sites during Cruise II, the first “processes” cruise, June 1-19, 2001. Sites 

MT3, S36, S42, and MT6 were designated as experimental sites. 

the Mississippi and DeSoto Caynons, whereas low densities were found at S42 and MT6 at 750 

and 2,750 m depth. 

Other sites chosen for repeating were MT1 (to test Ho4-effects of canyons), C7 (to test Ho8-

time), and S41 (to test Ho5-escarpments). New sites added were “HIPRO” (to test Ho6-surface 

productivity), “furrows” (effects of furrows or furrow-causing currents), “Green Knoll,” “Bush 

Hill” (Ho7-seeps), and “Fe Stone” (effects of iron crust). 

Eleven sites were occupied during Cruise 2 (Processes; Figure 5-2). The lander was used 

successfully at S42 and MT3, providing estimates of total community respiration rates measured 

in situ. Shipboard incubations were made of total community oxygen uptake, sulfate reduction, 

thymidine incorporation into bacteria and bacteria grazing by meiofauna at four sites: MT3, S36, 

S42, and MT6. 

5.4 Third-Year Field Activities 

The original plan called for a single cruise of 20 days in June 2002. The revised plan utilized 

two trips of two weeks each in June and August 2002. These concentrated on the Sigsbee Deep 

abyssal plain at depths of 3,400 to 3,650 m within the Mexican EEZ. The locations were chosen 

to provide broad geographic coverage (Figure 5-3). The head of the Mississippi Trough (MT1) 

was added as a ‘process’ site because of its peculiarly soft clay sediments and tremendous 
48 

Figure 5-3. Sampling sites on Leg 1 and 2, DGoMB cruise 3. Note repeated sampling at S36 in the 

DeSoto Canyon on second leg. 

Leg 1 

Leg 2 
49 

densities of a single species of amphipod crustacean. In retrospect, it was realized that this 

should have been a key station initially, but progress on sample processing had not progressed 

adequately to realize this at the time that the four process sites were identified by the group. 

The first of the two emphasized processes with use of the lander, acoustic doppler current 

profilers (ADCP) mooring, and baited trap array. The usual survey site sampling was conducted 

to supplement and fill in the sampling, the survey activities were emphasized on the August trip, 

including trawling. Likewise, some repeat sampling at previous survey and process sites was 

inserted into the latter field program. The principal reason for the two legs was to accommodate 

all of the different components and the specialties, including the additional Mexican (UNAM) 

participants. The standard sampling included CTD cast, camera lowering, box cores (with subsampling) and a trawl. All routine sampling was attempted on leg 1, with the exception of the 

trawls, which were done on Leg 2. At the two process sites (S1 and S4) additional sampling 

included as many as three or four additional box cores devoted to biogeochemical process 

measurements and living foram biomass estimates; the benthic lander deployment; and the 

ADCP mooring deployment. The Mexican participants requested 3 additional box cores at each 

site for preservation of the macrofauna in ethyl alcohol for molecular genetics characterization. 

Baited bottom-moored traps were deployed during trawling operations. 

Given the additions of the two Sigsbee sites and MT1, the total number of process sites was 

seven (7). This provided maximal depth coverage with the most extreme biotic parameters, in 

terms of biomass and abundance, but unfortunately not all the initially planned process analyses 

could be completed on all seven sites. 
51 

6. FIELD METHODS 

The Field Program was designed to collect a range of observations at survey and 

experimental stations. Survey stations were characterized by measuring a set of biological, 

chemical, sedimentological, hydrographic, and oceanographic variables. Samples were collected 

from the sediments, at the sediment-water interface, and in the overlying water column. At 

experimental sites, all standard survey variables were measured but in addition, sediment oxygen 

fluxes were measured to provide information on rates of total sediment community metabolism. 

6.1 Standard Sampling Techniques 

6.1.1 Boxcoring 

A 0.2 m2

 GOMEX or Gray-O’Hara boxcore (Boland and Rowe 1991) was used for the 

sampling on the Survey Cruise. The boxcore was equipped with a cross bar onto which core 

liners of various diameters and lengths were attached with hose clamps. Routinely the boxcoring 

team attached six (6) such subcores to the cross member. On return to the surface, caps were 

placed on the tops of the core liners to prevent contamination. Surface water overlying the 

sediments but outside the core liners was siphoned off through a 300 micron sieve. Then the 

surface 15 cm of sediment was removed for sieving for macrofauna. Once this surficial 15 cm 

was removed, the individual subcores were capped on the bottom, the hose clamps were opened, 

and the core liners were removed. Excess mud on the liners was washed through the 300 micron 

sieves to prevent loss of macrofauna. 

The six subcores were used for meiofauna, bacteria, geology, geochemistry, and 

contaminants. All six subcores were taken on all five boxcore replicates at each station. The 

original plan called for mixing these into a single sample for analysis of each of the variables. It 

was decided not to mix the samples but to initially analyze a single replicate and to archive the 

other cores. In that way, if odd or interesting values were observed in the single replicate, the 

whole suite of replicates from the other cores at each site would be available for further 

measurements (Figure 6-1). Having these archived samples available has been especially 

valuable for validating or rejecting questionable analyses. 

6.1.2 Trawling 

Megafaunal invertebrates and fishes were sampled with bottom trawling and bottom 

photography. Trawling was conducted using a 40’ otter trawl with 2.5” stretch mesh. These types 

of trawls have been extensively employed during similar studies (Haedrich and Rowe 1977; 

Haedrich et al. 1975, 1980). Steel doors or conventional 7’ x 14’ wooden doors were used on a 

single warp (shrimp trawlers use a double warp or two wires). The trawl was lowered at a rate of 

25 m of wire/minute. When the trawl was on the bottom, the vessel increased speed parallel to 

the isobaths (or into the wind or surface current, if this is required to control the vessel’s position 

relative to the wire and its angle with the surface) up to a speed of approximately three to four 

knots. 
52 

Arrive on Station

Record Station 

Parameters

Water Column 

Sampling

CTD

Niskin

Rosette

Quantitative 

Boxcore

Contaminant 

Chemistry

Relocate

Evaluate 

Acceptability Reject

Accept

Quantitative 

Subcores

Enter Sample 

Information

Biotic 

Samples

Ship to 

Laboratories

Foraminifera

Process 

and Store

Process 

and Store

Process

Process

Enter Sample Information & 

Siever’s ID 

Meiofauna

Sieving Macrofauna

Process 

and Store

Water 

Chemistry

Microbiota

Ship to 

Laboratories

Process 

and Store

Sediment 

Properties

Geochemical 

Properties

Figure 6-1. Schematic of quantitative boxcore station activities and sampling protocols. 
53 

A DGPS fix was made at this point. As the trawl was pulled, the wire was gradually paid out 

until the “wire out” was approximately 1.5 to 2 times the water depth. Steaming continued until 

the total bottom time was equal to 30 minutes per 1,000 m of water depth. That is, at 3,000 m the 

bottom time would be 1.5 hours. At the end of the bottom time, another DGPS fix was taken to 

give over-ground distance traveled by the trawl. The trawl was then retrieved at speeds less than 

50 m/minute. Samples retrieved by trawling were processed at sea for distribution to taxonomic 

specialists or archiving (Figure 6-2). 

6.1.3 Photo-Apparatus 

A Benthos digital camera, with a strobe, was utilized to take up to 50 photographs of the

seafloor. The camera was actuated by a bottom contact switch in order to assure that the same 

area of sea bottom was photographed with each exposure at each site (Rowe and Menzies 1969). 

Bottom contact was determined by placing a 12 kHz pinger 25 m above the camera and 

monitoring its position in relation to the sea floor on the strip chart recorder of the Precision 

Depth Recorder (PDR). The trigger weight distance from the bottom to the camera determined 

the area covered by each shot – approximately 2.0 m2. The bottom-contact approach has the 

advantage that the area covered by each shot was consist throughout each lowering and between 

lowerings, as opposed to taking automatic, timed exposures during which the distance to bottom 

is monitored using the pinger and PDR (Hecker 1990). In the earlier NGoMSCSS study at many 

of the same sites, an automatic film camera was coupled with a calibrated pair of laser spots 

directed at the bottom in each exposure to determine area of each exposure. The advantage of 

the digital camera was that the sea floor could be observed immediately after retrieval of the 

camera back aboard ship. The disadvantage was that far fewer pictures were taken (ca. 40 as 

opposed to several hundred). 

6.1.4 Water Column Profiles 

CTD/rosette casts were taken at all survey and all experimental stations (Figures 6-1 and 6-

2). All sampling was conducted to the same standards and using the same types of equipment 

and personnel as employed on the MMS-sponsored programs LATEX-A and NEGoM. A 

SeaBird Model 911plus CTD and a General Oceanics rosette with Niskin bottles were the 

equipment of choice. An altimeter allowed safe lowering of the package to within a few meters 

of the bottom. In vivo fluorescence was measured by a submersible fluorometer. Light 

penetration was measured as downwelling irradiance with a Biospherical Instruments, Inc. 

Model QSP-200L irradiance profiling sensor. Light transmission was measured with a SeaTech 

25 cm transmissometer. Additionally a D&A Instruments and Engineering optical backscatter 

sensor provided continuous profiles of particle scattering. 

In addition, a downward-looking ADCP was deployed to the seabottom at the beginning of 

each experimental station. The ADCP recorded bottom currents for the duration of each 

experimental station. The equipment of choice is a 300 kHzWorkhorse ADCP from RDI, model 

300S Sentinal rated to 6000 m. The ADCP measures horizontal currents at 0.5 to 2.0 m intervals 

in the lower 50-90 m of the water column to define short-term variability in bottom currents. The 

instrument was equipped with a syntactic foam collar for floatation, weighted with expendable 

anchor material, and released using two Benthos acoustic releases. The backscatter data from the 

bottom-mounted ADCP was used to quantify the temporal variability of the bottom nepheloid 

layer during experimental stations. 
54 

Arrive on Station

Record Station 

Parameters

Benthic Trawl

Relocate

Evaluate 

Acceptability Reject

Accept

Rough Sort

Enter Sample Information & 

Sorters ID

Vertebrate 

Samples

Biomass and Numbers Determined to Lowest 

Possible Taxon

Invertebrate 

Samples

Laboratories &Data 

Management

Label, Fix, and 

Store

Label, Fix, and 

Store

Refuse

Count, Identify, and 

Photograph

Figure 6-2. Schematic of trawl station activities and sampling protocols. 
55 

6.1.5 On-site Sampling Mapping 

The specialized navigation program (GERGNav) was utilized throughout all sampling 

operations to determine precisely where each type of sample was taken in relation to all the other 

sampling. This includes drifting during all operations while sampling is being conducted or 

while an instrument is on the way to the bottom or back to the surface. This detailed mapping 

allows investigators to determine the spatial relationships between samples at scales of meters. 

This is important when sampling the complex topography such as mesoscale basins, canyons or 

escarpments in the GoM. It was especially useful for determining the distance over which the 

trawl was on bottom, the extent of a camera transect, and the location of box cores relative to 

each other. Maps were generated while on station showing the location of all the samples taken 

relative to one another on scales of 10’s of meters. 

6.2 Community Structure 

Community structure measurements were made at all the survey and experimental stations 

(Table 4-2) In order to provide quantitative information for the conceptual model, all important 

taxa were sampled including the microbiota, meiofauna, macrofauna, megafauna, and fishes. 

Each type of assemblage was sampled by different techniques that provide the best chance to 

quantitative sampling of the community. In some cases, multiple techniques were employed as a 

cross-check (e.g., trawling, photosurveys and trap baiting for megafauna) and to provide 

complementary data from the same community. Each type of sample had specific field collection 

and processing techniques that are widely used and accepted as standard protocols. These 

protocols are described below. 

6.2.1 Bacteria 

Immediately upon boxcore retrieval shipboard, the temperature of the overlying seawater and 

the sediments was determined with a metal thermometer inserted in the sediment. Determining 

the temperature is important in evaluating the integrity and utility of the recovered sediment for 

measuring microbial activity under simulated in situ conditions. 

For bacterial counts, the sediment subcores were extruded and sliced, using aseptic 

techniques, at predetermined depth intervals. The subsamples were fixed in a final concentration 

of 2% formaldehyde, prepared in a sterile, particle-free (0.2 um filtered) solution of artificial 

seawater (ASW) and stored (and shipped) in the dark at 4oC for shore-based analysis. 

6.2.2 Protozoa (Foraminifera) 

Five replicate subcores were obtained from the boxcorer at each experimental station to 

measure foraminifera biomass. The cores were vertically sectioned at 0-1 cm, 5-6 cm, and 10-11 

cm for analysis. Foraminifera densities at depths greater than ~10 cm were low and thereby 

precluded analysis in deeper sections. When feasible, individual organisms were sorted on shipboard, so the samples did not need to be preserved. 
56 

6.2.3 Meiofauna 

Two core samples were taken from each boxcore sample, and stored for meiofaunal 

community analysis. Meiofauna were collected by a core tube (5.1 cm i.d.) that was mounted 

inside the boxcore. A mounted corer within a box ensured that meiofauna were collected from an 

undisturbed surface. [Insertion of a core tube after the sample has already been sloshed around 

the deck of the ship is known to create artifacts including loss of organisms. Surface disturbance 

can occur when the boxcore is placed on the ship’s deck. Taking the sample from an inner 

subcore reduces edge effects (Eckman and Thistle 1988).] The bow waves of sampling devices 

in deep water can have an impact on estimates of surface dwelling meiofauna (Hulings and Gray 

1971). 

The two critical sampling issues for meiofauna were core size and sampling depth. A 1.9 cm 

diameter core tube (2.8 cm2) was used during previous MMS projects, e.g., Santa Barbara seeps 

(Montagna et al. 1989), CAMP (Montagna 1991), and GoMEX (Montagna and Harper 1996) 

sampling programs. A 3.5 cm i.d. core was used to collect meiofauna in the NGoMCSS study 

(Pequegnat et al. 1990). Densities in the deep-sea Gulf of Mexico are low (Table 6-1), so a large 

core must be used. About 80% of the meiofauna were in the top 4 cm of sediment. A sampling 

depth between 2 and 6 cm was used in seven of nine deep-sea studies reviewed by Thistle et al. 

(1991). The NGoMCSS study (Pequegnat et al. 1990) sampled to a depth of 5 cm. To resolve 

these two issues, a study was performed during the shakedown cruise (16-18 February 2000) to 

determine the most appropriate core size and vertical sampling depth for meiofauna in the 

current study area. To compare sizes, four cores ranging from 2.2 to 6.7 cm inner diameter (i.d.) 

were used to collect the top 1 cm of sediment, and five replicates were taken. To exam the 

vertical distribution of meiofauna, a 5.1 cm core tube was used. Samples were taken at 1 cm 

intervals down to 20 cm, and five replicates were taken. All samples were taken at station W-2 in 

water depths of about 661 m. 

More organisms were found in progressively larger cores (Table 6-1). But, there was no 

statistically significant differences for abundances of total meiofauna, nematodes, harpacticoids 

and other taxa among different core sizes. Although total abundance in the smallest core was 

about half that found in the three larger cores, it was not statistically different. Because each core 

yielded the same abundance estimate, total counts per core were used to choose the appropriate 

core size. For statistical purposes, it is imperative to obtain > 30 organisms in a taxon per sample. 

Therefore, the 5.1 cm core was chosen for the benthic survey. 

There were no meiofauna found below 13 cm sediment depth, so just the top 13 cm are 

plotted (Figure 6-3). Most organisms were found in the top 3 cm. A total of 87% of total 

meiofauna were in the top 3 cm, and 97% of the harpacticoids. In addition, 77% of the 

harpacticoids were found in the top 1 cm. Because the distribution is so skewed to the surface, it 

was decided to sample the top 3 cm only during the benthic community survey cruise. Because 

harpacticoids were so restricted to the top 1 cm, the core was split into 2 sections: 0-1 cm and 1-3 

cm. 

After the sections were collected, the meiofauna were relaxed in 7% MgCl2 and preserved in 

an equal volume of 10% buffered formalin (yielding a final concentration of 5% formalin) 

(Hulings and Gray 1971). The buffered formalin was made up with seawater that was filtered 

through a 0.042 mm mesh to exclude plankton. Rose bengal was added to the preservative to 

distinguish cytoplasm-containing foraminifera from empty foraminiferal tests (shells). 
57 

Table 6-1 

 

Effect of Core Tubes Size (Inner Diameter) 

on Meiofauna Counts and Average Density 

(Based on Five Replicates Taken at Station W-2. Abundance Is Detransformed 

From ln, So Taxa Averages Do Not Sum to the Total Average.) 

 Counts (n/core) Abundance (n/10 cm2) 

Core Size (cm i.d.) 2.2 3.1 5.1 6.7 2.2 3.1 5.1 6.7

Taxa 

 Nematodes 20.2 64.8 161.0 219.0 31.3 59.4 50.5 54.3

 Harpacticoids 5.4 30.8 74.2 99.4 2.3 26.1 20.8 24.4

 Nauplii 7.2 38.6 72.2 85.6 0.2 29.2 22.9 19.0

 Others 12.2 14.0 27.8 45.8 3.6 14.5 8.5 10.6

 Total 45.0 148.2 335.2 449.8 54.0 135.7 111.3 112.2

6.2.4 Macrofauna 

Macrofauna were sampled from five (5) replicate boxcores per station. Sieve size was 300 

microns. Sieving, a critical stage in macrofauna analysis, was conducted using the gentle 

floatation method developed by Howard Sanders of the Woods Hole Oceanographic Institution 

(WHOI). A set of five (5) sieving stations was constructed, one for each replicate boxcore 

sample. Sievers were limited to experienced biologists. One was assigned to each core. The top 

15 cm of each boxcore surface was copped out and deposited into a 50 gallon plastic trash can. 

Sea water, filtered with an in-line water filter to remove surface water organisms and particulate 

matter, was allowed to flow by hose into the bottom of the can with the sample. The trash cans 

were mounted in an adjustable frame, referred to as the “tipper” constructed of marine-grade 

plywood. The water flowed into the trash can, through the sample at the bottom of the can, and 

out a 20 cm long, 3 cm diameter spout 20 cm from the top of the can. On exiting the spout, the 

sample goes immediately into a sieve, which was held in place by an adjustable sieve holder also 

constructed of marine grade plywood. This process floated the animals out of the sediments and 

onto the sieve with the least possible trauma. This was a critical step because severe losses of 

animals can occur. Each siever signed the boxcoring deck log and entered comments in the 

margin on sample quality at the end of the sieving process. The samples were fixed in 10% 

formalin with sea water that was filtered as above. Rose Bengal was added to aid in rough 

sorting in the laboratory. On return to the laboratory, the formalin-Rose Bengal solution was 

changed to a 70% EtOH solution. At this point, the samples were more or less permanently 

preserved. 
58 

Total Meiofauna (n/section)

0 100 200 300 400

Section Depth (cm)

0

2

4

6

8

10

12

14

Nematoda (n/section)

0 50 100 150 200

Section Depth (cm)

0

2

4

6

8

10

12

14

Harpacticoida (n/section)

0 20 40 60 80 100

Section Depth (cm)

0

2

4

6

8

10

12

14

Figure 6-3. Vertical distribution of meiofauna taxa for Cruise I. Average of 5 replicates. 
59 

6.2.5 Megafauna and Fishes 

6.2.5.1 Trawl Samples 

Once onboard, the trawl sample was removed from the net and placed in large plastic tubs 

(Figure 6-2). Organisms were then sorted to species to the best of sorters’ abilities. Each species 

was counted, its displacement volume determined, and each individual’s length was measured 

and recorded. The samples, sorted by species, were placed in ziplock bags or small plastic jars, 

labeled with indelible solvent proof labels, and preserved in 10% formalin and seawater solution. 

All species were stored together in five gallon plastic containers covered with tight plastic lids. 

For fishes, the body cavity of the larger individuals were opened to enhance fixation. Trash was 

separated out and stored in separate five gallon plastic containers. Larger items were 

photographed and discarded or saved for disposal on land. 

6.2.5.2 Photosurveys 

The purpose of the camera lowerings was to quantify the megafauna. This is important 

because the megafauna are an explicit component of the conceptual model of the ecosystem. If 

the biota are abundant then good estimates can be made with few exposures. For example, it has 

been demonstrated that from the upper slope down to the lower slope, on the order of 30 or so 

exposures, each covering an area of six square meters, is adequate to quantify the dominant 

megafauna (Rowe and Menzies, 1969; Grassle et al. 1975). At lower densities, more exposures 

are required. During the NGoMCSS study, approximately 48,000 photographs were taken, but 

only 25% of them were ultimately analyzed. A digital camera was used so that the images could 

be easily scanned upon system retrieval. The images are also easily electronically transmitted 

and archived on CD ROMs. 

6.3 Community Function 

In addition to the standard set of analyses performed at all survey and experimental stations, 

a series of specialized studies was conducted at experimental stations to develop a better 

understanding of rates of biological processes at the process sites. These studies will be 

conducted on samples provided by the standard set of techniques and equipment, specialized 

sampling protocols, and gear. These methods have been used during Cruises 2 and 3a and 3b. 

6.3.1 Microbial Metabolism 

For microbial activity measurements, subsamples from three sediment depths per boxcore 

(per station) were diluted in artificial seawater to prepare sediment slurries for homogeneous 

amendment with the desired radiolabeled substrates. Separate aliquots, in duplicate, were then 

incubated and sacrificed at each of five time intervals to develop rate measurements of substrate 

respiration and incorporation into biomass. Such sediment slurries provide an upper bound on 

microbial activity due to the oxygenation and disruption of microniches caused by sample 

retrieval. Cross-site comparisons will be important. 

6.3.2 Sediment Community Oxygen Demand 

A key component of the experimental studies was the measurement of total sediment 

community oxygen consumption (SCOC) using the benthic lander and in situ chambers (Figure 
60 

6-4). The GoMEX (for Gulf of Mexico) lander was designed to estimate fluxes of oxygen and 

metabolites into or out of the sediments by incubating the bottom water and sediment surface in 

enclosed, stirred chambers. Thorough descriptions of the GoMEX lander have been published in 

Pomeroy et al. (1991), Rowe et al. (1994), and Miller-Way et al. (1994). 

Figure 6-4. Benthic lander. 

The GoMEX lander, as used in this study, consists of a square aluminum-pipe frame 

measuring 2 m on a side. This frame holds 1) two acrylic incubation chambers, 2) a multifunction electronic timed release system, with “burn wire” release mechanisms, 3) a deep-sea 

multishot camera and strobe system, 4) two disposable anchor weights, and 5) glass floatation 

balls. The two chambers each contain an oxygen sensor and thermistor, a submersible pump, an 
61 

additional stirring motor, and three 60 cc hypodermic syringes. Each chamber encloses a 

sediment surface area of 906 cm2 under a volume of seven liters of bottom water. A stirring bar 

inside the chamber is driven by a magnet attached to an electric motor in an oil-filled water-tight 

case outside the chamber. The camera and strobe are attached to the frame opposite the chambers 

to monitor proper contact with the sediments and syringes. Two disposable anchors (totaling 200 

lbs) composed of cylinders of lead are dropped under the control of the electronic timed release, 

with corrosive links of variable duration as a backup. Relocation on the surface is aided by a 

flag, two radio direction finders and a strobe flasher. The chambers remain on the bottom for 

periods of 8 to 36 hours. 

The sequence of events for this “free vehicle” involves floating to the bottom and then 

undergoing a series of programmed mechanical operations that are controlled by an electronic 

timer that burns release wires on command. The chambers incubate bottom water from which 

metabolic substrates are consumed and into which metabolic by-products are produced. The 

fluxes are calculated as: 

timex area

Vol.x Conc.) in (Change = Flux 

6.3.3 Foodweb Studies—Stable Isotopes 

Organism tissues were selected for stable isotope analysis for foodweb studies. The tissue 

was excised or the whole organism was frozen whole at -4oC. Isotope analyses were conducted 

on selected whole sediment, Sargassum recovered in trawls and floating at the surface, selected 

large invertebrates and amphipods collected in deep baited traps. 

6.4 Sediment Properties 

Subcores for sediment properties were taken from each of the five replicate boxcores. These 

were not pooled but were stored separately. The top two centimeters were sampled using a 

spatula or scoop. Shear strength was measured in the shore-based laboratory. 

6.5 Chemical Contaminants 

Subcores for trace metals and hydrocarbons were taken from each of the five replicate 

boxcores. The subcores were extruded and the upper 2 cm of the sediment was sectioned with a 

teflon-coated spatula. Total organic and inorganic carbon were measured on the hydrocarbon 

sample. Hydrocarbon subsamples were immediately placed into precleaned (combusted at 

425°C) 1⁄2-pint glass jars with teflon-lined lids (~150 g). Trace metal samples were immediately 

placed into pre-cleaned plastic jars (~150 g). All sediment samples for contaminant chemistry 

werestored frozen (-20°C). 

6.6 Geochemistry 

At each survey site, near surface (about 0 to 2 cm) samples from each of the five (5) replicate 

samples were frozen but analytical work was completed at shore-based laboratories on only one 

sample. The other five have been archived frozen. 
63 

7. LABORATORY METHODS 

Once samples and data were collected on the various cruises, extensive shore-based 

laboratory analyses were performed. The samples were uniquely labeled and preserved for 

storage onboard the ship to ensure their integrity during transport to the various PI’s laboratories. 

The methods employed standard techniques widely used for similar programs, ensuring that 

comparisons with historical studies are valid. This data compatibility is important to provide the 

greatest value in the final synthesis. The laboratory methods are described in detail below. 

7.1 Taxonomy 

Taxonomy is a vital component of the GoM deepwater study. Many of the hypotheses will be 

tested at the species level. A team of taxonomists was selected was selected to assist with the 

identifications (Table 7-1). The first step was getting good samples but the next step was 

accurate and thorough sorting of the samples to major group. This was accomplished by graduate 

students who were supervised by experienced professional invertebrate zoologists. 

Rose Bengal-stained invertebrates were separated from sediment samples, after sieving and 

preservation, and sorted to major taxonomic group: polychaetes, oligochaetes, mollusks, 

crustaceans, and others. Sorting efficiency was tested by re-sorting the “waste” sediment 

materials prior to storage. All materials collected during the program were stored until the 

program was complete so that it can be reviewed as needed. Poorly sorted samples discovered 

during resorting were sorted again. The initial sorter was identified and retrained to increase 

sorting efficiency. 

The major groups were sent to the appropriate lead taxonomist. The polychaete taxonomist 

was Dr. Guinn Fain Hubbard (TAMU), who also supervised the macrofauna sorting laboratory. 

The taxonomists then sorted the major group to species, identifying those that had been 

described and giving species designations to those that either could not be identified or had not 

yet been described. Mis-sorts (organisms placed in the wrong major group by the sorting lab) 

were returned to Dr. Hubbard in the sorting lab for redistribution to the proper taxonomist. The 

taxonomists then sent a species list and the identified specimens back to TAMU, retaining those 

specimens on which further work was warranted. The species list was then entered into the data 

base by Dr. Wolff. 

The mollusk identifications were made by Mr. Roe Davenport, with assistance from graduate 

student Min Chen and Prof. Mary Wicksten. Dr. Richard Heard (University of Southern 

Mississippi) was the crustacean taxonomist. Dr. George Wilson (Australian National Museum) 

identified the isopods. While most of the adult echini were taken in the trawl samples, juveniles 

were sometimes encountered in the boxcores. 

Each lead taxonomist was responsible for identifications to the species level. Any 

misidentifications by the sorters that were encountered were returned to the sorting laboratory for 

redirection. 
64 

Table 7-1 

 

Taxonomists Involved in the Analysis of the Macrofauna and Corresponding Taxonomists Working on 

UNAM Mexican National Collections 

Taxon DGoMB UNAM Other 

Fishes John McEachran (TAMU)/R. 

Haedrich(MUN) 

Luis Zambrano 

Amphipoda Richard Heard (USM)/John 

Foster/Yousra Soliman(TAMU) 

Fernando Alvarez 

Aplacophora Amely Scheltema (WHOI) 

Ascidiacea 

Asteroidea Archie Ammons (TAMU) Alfredo Laguarda, Francisco Solis 

Bivalvia Min Chen/R.Davenport/

M.Wicksten 

 

Brachiopoda Mary Wicksten (TAMU) 

Bryozoa Nielsen 

 

Cumacea Heard/Chihlin Wei/L. Petrescu Alvarez 

Decapoda Wicksten Alvarez 

Echinoidea Wicksten, Ammons Laguarda, F. Solis 

Echiura EibyeJacobsen 

Gastropoda Rex/Davenport Garcia Cuba(Conus & Terebra) 

Harpacticoida H. Lee (Montagna) Alvarez 

Holothuroidea Wicksten, Ammons Laguarda, F. Solis 

Hydrozoa 

Isopoda Geo. Wilson(Aust. Nat. Mus.) Alvarez 

Kinorhyncha Rhinehard Kristensen (Danish Mus.) 

Mysidacea Heard Alvarez 

Nematoda N.A. 

Nemertini Jon Norenburg(USNM) 

Oligochaeta 

Ophiuroidea Wicksten, Ammons Laguarda, F. Solis 

Ostracoda Gío Argaez 

Polychaeta G. Fain Hubbard Vivien Solis 

Porifera Patricia Gomez 

Priapulida 

Pycnogonida Allen Child 

Scaphopoda 

Scyphozoa 

Sipunculida 

Tanaidacea Heard/Kim Larsen Alvarez 

Turbellaria 

Anthozoa Daphne Fautin (U.Kansas) 

Copepoda Lee (Montagna) 

Chaetognatha Gerald McLellan (USM) 

Halacaridae Anna Hoffman-Mendizabel 

Crinoidea Laguarda, F. Solis 

Cladocera 
65 

7.2 Community Structure 

Community structure refers to the standing stocks, biomass and species composition of any 

assemblage of organisms. It has been used to test the eight hypotheses in this study. For some 

groups only standing stocks was measured. For others, complete identification down to the 

species level was possible. The standing stocks in all size groups were utilized in the carbon 

budget that integrates all the process measurements. 

7.2.1 Bacteria 

Bacterial abundance in sediments was determined using a dual staining technique (acridine 

orange and DAPI stains) and standard epifluorescence microscopy for marine sediments 

(Relexans et al. 1996 and Schmidt et al. 1998). This procedure does not attempt to remove 

bacteria from the sediment grains prior to counting. Separating the bacteria from the sediment 

inevitably results in an underestimate of abundance, since bacteria tightly bound to the sediments 

are left behind. Instead, the method of choice views the bacteria directly in the presence of the 

grains after treatment with Triton-X detergent and sonication (to loosen the attached or 

aggregated cells). For each sediment core, a down-core profile of bacterial abundance was 

determined at ten (10) closely spaced sampling intervals from 0-15 cm with spot checks to a 

depth of 30 cm. Sediment bacterial abundances were rarely examined at depths greater than 15 

cm. The deeper samples (done to 30 cm) are important in understanding bacterial distributions in 

deep-sea sediments. The potentially unique nature of some of the targeted slope sediments 

suggests that significant variations from typical pelagic marine sediment downcore profiles 

known may be observed (Schmidt et al. 1998). Since ultimately abundance was integrated over 

depth in the sediment to provide benthic bacterial mass estimates at each station for modeling 

and hypothesis testing purposes, unusual abundance profiles in the deeper layers of sediment 

would have been important, if encountered. 

Bacteria were also sized in selected samples by measuring the length and width of 

photographed cells stained with acridine orange and projected onto a flat white surface (Lee and 

Fuhrman 1987) to convert bacterial abundance to biomass. This method was a significant 

improvement in the accuracy of benthic bacterial biomass estimates. 

7.2.2 Foraminifera 

Biomass of foraminifera is difficult to measure because foraminiferal assemblages include a 

combination of live specimens and empty shells. Measurement of only the organic fractions was 

appropriately done by measuring ATP content at the experimental stations. From each sample, 

50 specimens were individually measured for length and subsequently extracted for ATP 

following the procedure of Bernhard (1989). 

7.2.3 Meiofauna 

Meiofauna were extracted from sediment using the Ludox centrifugation technique (deJonge 

and Bouwman 1977). Extraction efficiency has been shown to be 95%–99% of organisms over 

all sediment grain sizes. Samples were then sieved and counted to major metazoan taxonomic 

category. Protozoan, primarily Foraminifera and Ciliata, were enumerated when stained as well 

and reported 
66 

The term “meiobenthos” was first coined by Mare (1942) to describe benthic organisms of 

intermediate size. These are metazoans that are smaller than macrofauna but generally larger 

than the single-celled microbenthos (e.g., bacteria, microalgae, and protozoans). By convention, 

the definition of meiofauna is those animals that pass through a 500 micron sieve but are retained 

on a 63 micron sieve (Hulings and Gray 1971; Coull and Bell 1979; Giere 1993). Because deepsea organisms are small, most meiofaunal ecologists use 42 micron sieves to retain meiofauna 

(eight of nine papers reviewed in Thistle et al. 1991). To conform with other studies of deep-sea 

meiofauna, a 45 micron sieve was used to retain meiofauna. 

Meiofauna were identified to the major taxon level. Meiofaunal communities are composed 

of two groups. Temporary meiofauna are those juveniles of the macrofauna that will eventually 

grow into larger organisms. Permanent meiofauna are those groups where adults are less than 

300 microns, e.g., Nematoda, Copepoda, Gastrotricha, Turbellaria, Acarina, Gnathostomulida, 

Kinoryncha, Tardigrada, Ostracoda, and some Rhyncocoela, Oligochaeta, and Polychaeta. In 

1971, Thiel suggested that the grouping of benthic organisms according to size differences had 

little taxonomic or ecological justification, except convenience of sample processing (quoted 

from Thiel 1975). However, ecological literature since 1971 has shown that meiofauna are 

different from macrofauna and have different roles in marine ecosystems (for reviews see: Coull 

and Bell 1979, Coull and Palmer 1984, Giere 1993). Even where meiofauna share ecological 

properties with macrofauna, the processes operate on much smaller spatial and shorter temporal 

scales for the meiofauna (Bell 1980). 

Meiofauna biomass was measured or calculated (Montagna 1984; Montagna and Li 1997) for 

all samples. There is an extensive literature on biomass techniques. Robinson (1984) compared 

five methods for nematode biomass and recommends calculating volume based on a cylinder the 

length of the nematode and the diameter of the nematode’s area/length ratio. Pearre (1980) 

compared five methods for copepod biomass and recommends calculating volume based on a 

cylinder. Techniques and formulas for other meiofauna are given in Duplisea and Hargrave 

(1996). Volume estimates were converted to wet biomass with the assumption that organic 

matter has a specific gravity of 1.2 It is well known that biomass is underestimated by 30%–40% 

in formalin preserved samples, so biomass was adjusted for this loss (Frithsen et al. 1986). For 

small organisms (e.g., meiofauna) biomass was calculated based on volume measurements and 

conversion factors in the above references. Conversion factors specific to the deep-sea GoM do 

not exist, so they will be empirically determined in 10% of the samples by direct measurements 

using a CHN analyzer and electrobalance (Montagna 1984). There is also an extensive literature 

on how best to calculate the volume of meiofauna based on length and width measurements. 

7.2.4 Macrofauna 

The macrofauna samples were stained with Rose Bengal, buffered and fixed in a 10% 

seawater and formalin solution aboard ship. On return to the laboratory, the samples sieved 

again with fresh water if considerable fine sediment has been retained in the sample. After this 

additional sieving, the stained macrofauna were sorted under dissecting microscopes to major 

taxonomic groups and preserved in 70% ethanol. Sorting efficiency was estimated on all sorted 

samples. All residues continue to be properly preserved and have been retained for the duration 

of the project for review as needed. 

Taxa that are routinely considered meiofauna but were retained on the 300 micron sieve were 

also separated and quantified. The numbers and biomass of these larger animals were included in 

the total biomass and density calculations of the macrofauna, but were not included in the 
67 

zonation or diversity estimates. Most of these taxa were nematodes, harpacticoid copepods, and 

forams. 

Determination of biomass as wet weight was accomplished on a subset of representative 

organisms at the group level prior to distribution to the taxonomists. For larger organisms, this 

was done by weighing the organisms in their alcohol preservative as a group (Rowe 1983). For 

smaller organisms, this was done by measuring them with a calibrated ocular micrometer, 

calculating their displacement volumes, and assuming a density of 1.2 g/cc. Organic carbon 

weight, which is the unit used in the food web budgets (See Section 9), was estimated from the 

conversion factors for each major group (Rowe 1983). 

7.2.5 Megafauna and Fishes 

In the field, the fishes and megafauna recovered by trawl were processed as followed: 1) 

sorting of fishes and megafauna to “tentative” species (along with volume, measurements, 

counts, and length by species; 2) selecting specimens for tissue samples for stable isotopic and 

deoxyribonucleic acid (DNA) analyses; 3) processing, labeling, and storing tissue for shorebased analysis; 4) fixation in 10% neutral buffered formalin, and 5) recording any unusual 

occurrences in the trawl performance that might effect the catch. Upon return to the laboratory, 

tissues for stable isotopic analysis were delivered to the appropriate PI. The fish subsamples 

were soaked in water to remove all traces of formalin and then transferred to 70% ethanol. The 

megafauna and fish identifications were verified and transferred to a single jar per species per 

station. These samples were distributed to the taxonomists listed above (Table 7-1). 

The material from 2002 studies within the Mexican EEZ were transported to the collections 

of the Institute of Zoology at the National University of Mexico (UNAM) in Mexico City 

immediately after the GYRE returned to Galveston. The fishes were identified by Richard 

Haedrich and Jessica Bangma, on a visit to UNAM the following year, in cooperation with 

graduate students at that institution. The data are entered now into both UNAM and DGoMB 

data bases. These specimens have acquisition numbers in the UNAM collections and are 

available for taxonomic study on request, either at Institute or on a loan basis, in accord with 

traditional international museum protocols. 

7.3 Community Function (Process Studies) 

At each experimental station a specialized set of process studies provided important 

information to support those data obtained at the standard survey stations during. These studies 

were performed using techniques and methodologies specific to each programmatic component. 

7.3.1 Bacterial Metabolism 

Bacterial productivity was estimated in sediment samples from the process stations using the 

tritiated thymidine approach, as applied by Alongi (1990) to other deep-sea sediments. For 

comparative purposes, we tested for activity in undiluted and diluted surface sediments and in 

samples incubated at atmospheric pressure and in situ pressures, all at in situ temperature. We 

ran endpoint, time-course and variable substrate concentration experiments to assess isotope 

dilution and other procedural issues. Incubations were terminated shipboard using 80% ethanol. 

Returned samples were processed further to assess thymidine incorporation into DNA, again 

following procedures of Alongi (1990). 
68 

7.3.2 Meiofaunal Feeding Rates 

Meiofaunal feeding rates on benthic bacteria were measured by incubating sediment cores 

with radiolabeled thymidine (3HTdr; Montagna 1984, 1993). Initially, 2 uCi of 3HTdr is injected 

and incubated for 8 h, after which formalin is injected into the cores to stop label uptake. A one 

ml subsample was then withdrawn from the core. This subsample was filtered onto a 0.2 micron 

Millipore filter and rinsed three times with seawater to estimate uptake of 3HTdr by bacteria. 

The subsample was dispersed and suspended in five mL of distilled water and 15 mL insta-Gel 

for dual label liquid scintillation counting. Meiofauna were separated from sediments by diluting 

samples with 2% formalin, swirling to suspend animals, and decanting them and the supernate 

onto 63 micron Nitex screen filters. Meiofauna were then rinsed into jars and kept refrigerated in 

2% formalin until sorting at a shore-based facility. 

The labeled meiofauna collected were sorted under a dissecting microscope. The individual 

taxa were placed in one mL of water in a scintillation vial. The meiofauna were then dried at 

60oC and solubilized in 100 mL Soluene tissue solubilizer for 24 h. Samples were ultimately 

counted by dual-label scintillation spectrophotometry in 15 mL of Insta-gel. 

7.3.3 Sediment Community Respiration 

The changes in the chemical concentrations of oxygen, dissolved inorganic carbon (DIC), 

ammonia, nitrate, nitrite, manganese, iron, silicate, and phosphate in the benthic chambers were 

used to estimate total community respiration by organisms living within the sediments in benthic 

chamber incubations. The oxygen concentrations was determined from a continuously recording 

polarographic oxygen electrode, backed-up by a miniaturized Winkler method (uses 25 mL 

samples). The other chemicals were measured in syringe samples from the beginning, middle, 

and end of the incubations. Conventional wet chemistry was employed as described in the 

geochemistry section. 

7.3.4 Foodweb Analyses—Stable Isotopes 

Selected organismal tissues were sampled and preserved for stable isotope analysis. The 

tissues were analyzed for carbon and nitrogen stable isotopic compositions. Most samples were 

measured with a Finnigan MAT Delta S Elemental Analyzer-Isotope Ratio Mass Spectrometer 

(EA-IRMS). If, for any reason, samples cannot be processed by EA-IRMS, they will be 

measured by standard tube combustion methods similar to those described in Cifuentes et al. 

(1988). Finally, the stable carbon isotope ratio of bacteria in sediments were determined by 

proxy, in phospholipid fatty acids, a biomarker of bacterial biomass (Salata 1999). 

7.4 Sediment Properties 

Sediments retrieved from the boxcores were preserved and returned to shore-based 

laboratories for analysis (see below). 

7.4.1 Grain Size 

Grain size was determined by the standard Folk settling method. Fifteen to twenty grams of 

sediment were treated with hydrogen peroxide for twelve hours to remove organic matter. After 

washing with distilled water, the sediment was dispersed with hexametaphosphate solution and 

poured over the appropriate screen. The material that passes through the screen was then 

subjected to a series of prescribed settling techniques that were based on removing materials at 
69 

set times. The individual fractions that were collected were dried and weighed to provide 

quantitative values for each size fraction as a percent of the total weight of sediment. Per cent 

gravel, sand, silt, and clay were calculated. Duplicates were analyzed with every batch of twenty 

samples to estimate precision. 

7.4.2 Geotechnical Properties 

All boxcore samples were examined and sampled for shear strength, bulk density, water 

content, and grain size. Selected samples were tested for permeability. Shear strength was 

determined by hand held vane shear devices and was used to determine the cohesion of the 

sediment at several depths and locations in a non-contaminating and non-disturbing manner 

within each boxcore. Bulk density and water content was determined by standard geotechnical 

techniques. Porosity was determined from the bulk density measurements. Permeability was the 

capacity of the sediment to transmit fluids without impairment of the structure of the medium. 

Permeability regulates the rate of flow of interstitial waters resulting from the compaction 

process and was important in the transport of fluids and gases within sediments. Permeability 

was determined on selected samples by pressure permeameters and a constant gradient 

consolidometer. Of the above sediment physical properties, grain size is the only one for which a 

large number of correlations have been made with benthic communities. It was reasonable to 

presume however that they may correlate or co-correlate with animal distributions. 

7.4.3 Total Organic and Inorganic Carbon 

Total organic and inorganic carbon was determined by standard LECO combustion 

techniques or by Carlo Erba element analyzer. A freeze-dried and homogenized sample is burned 

in the presence of oxygen and accelerator to CO2 which is swept from the combustion chamber 

into a detector. The detector is calibrated with rings of known carbon content and the sample 

carbon content is calculated from the detector response. Inorganic carbon is determined as the 

difference in carbon content between an untreated and acid-treated sediment sample. 

7.5 Chemical Contaminants 

Each survey and experimental site were screened for the presence of chemical contaminants. 

Hydrocarbon and relevant metals were measured at each site. 

7.5.1 Hydrocarbons 

Sediment samples were analyzed for polynuclear aromatic hydrocarbons (PAHs) using 

NOAA National Status and Trends Methods (Denoux et al. 1998; Qian et al. 1998). The 

proficiency and validity of this method has been established by 14 years of laboratory 

intercomparisons. Briefly, deuterated PAH are added before the extraction and are used to 

calculate analyte concentrations. Sediment samples are mixed with anhydrous sodium sulfate and 

are extracted with methylene chloride/acetone in an Automated Solvent Extractor (ASE). The 

extracts are separated from possible interfering compounds by silica/alumina columns. The 

purified extracts are analyzed on a HP 5890/5970 gas chromatograph with a mass selective 

detector (GC/MS) using a selected ion detection technique. The GC/MS is calibrated with known 

concentrations of analytes at five different concentration levels and the average response factors 

of the analytes are used for PAH concentration determination. Concentrations of PAHs are 

reported as nanogram/gram (ng/g) on dry weight basis for sediment samples. Both non-alkylated 
70 

(parent PAHs) and alkylated PAHs are reported. Each sample batch of 20 samples or less 

includes a procedural blank, a matrix spike, a matrix spike duplicate and a standard reference 

material. These quality assurance samples ensured that the analytical results for each batch are 

valid and of acceptable accuracy and precision.

7.5.2 Trace Metals 

Sediment samples were analyzed for Ba, Cd, Cr, Fe, Hg, Pb, and Zn. These have been shown 

to correlate with drilling and production in previous studies (e.g., Kennicutt, et al. 1996a,b) 

and/or are known to be toxic to organisms. The analyses followed the well-established National 

Status and Trends methods. Methods included atomic absorption spectroscopy (AAS), 

instrumental neutron activation analysis (INAA) and/or inductively coupled plasma spectrometry 

(ICP), depending on the metal and the concentration (e.g., Taylor and Presley 1998). INAA was 

used to determine Ba, Cr and Fe. The precision and accuracy for these elements by INAA is 

excellent regardless of the matrix, including pure drilling mud. A more sensitive method was 

used when needed for other metals to insure accurate and precise values. 

Sample preparation begins with freeze-drying a representative sediment aliquot and grinding 

it to a fine powder. No further treatment is needed for INAA, providing a check on the sample 

dissolution techniques needed for AAS/ICP analysis. For INAA, 0.5 g aliquots of the powdered 

samples is weighed directly into plastic vials and heat sealed. The samples are irradiated for 12 

hours in the 1 megawatt TRIGA reactor. After a 10 day cooling period to allow Na, Cl, and other 

interfering isotopes to decay to low levels, the samples are counted using a hyper-pure 

germanium detector coupled to a Nuclear Data Corp. model 9900 multichannel analyzer 

integrated with a Digital VAX II/GPX graphics workstation. Concentrations were obtained by 

comparing counts for each sample with those for sediment and rock reference materials of 

accurately known elemental composition. Details of this method are given in Boothe and James 

(1985), including information on counting geometry, reference materials, spikes, blanks and 

other aspects of QA/QC. 

The National Status and Trends Program methods (Lauenstein and Cantillo 1998) were used 

in for AAS/ICP analysis. The method for Hg is the standard Environmental Protection Agency 

(EPA) sulfuric acid-permanganate digestion of a dry powdered sample followed by stannous 

chloride reduction to Hg metal and detection by cold vapor AA. For other metals, 200 mg 

aliquots of the powdered sediment samples are weighed into teflon “bombs” and completely 

dissolved in a mixture of nitric, hydrofluoric and boric acids by prolonged exposure of the closed 

bombs to a temperature of 130°C. Various dilutions were made on the clear digests to bring them 

into the working range of the AAS or ICP. 

A Perkin-Elmer Corp. model 3300DV (dual view) ICP was used when element 

concentrations permitted. When concentrations were too low for this instrument, a Perkin-Elmer 

3030Z AA equipped with an HGA-600 graphite furnace and an auto sampler were used. Details 

of furnace programs, matrix modifiers, blanks, spikes, reference materials and other QA/QC 

information can be found in the reference given above. The methods utilized ensured that matrix 

spike recovery for all elements was greater than 90% and that recoveries of certified values for 

reference materials from the National Research Council of Canada were 90% or better as well. 

7.6 Geochemical Properties 

The geochemistry component measured a range of relevant sedimentary properties at both 

survey and experimental sites. Single replicate samples were analyzed at the survey sites and 
71 

detailed profiles were determined at experimental sites to determine important flux rates. Various 

geochemical properties were measured on pore waters and solid phases. Analyses that are best 

performed immediately after sample retrieval were conducted onboard, whereas other analyses 

which require more time consuming methods can only be performed at shore-based facilities 

(Tables 7-2 and 7-3). Pore water was separated from the solid phase by centrifugation or pressure 

filtering. 

Table 7-2 

 

Summary of Parameters To Be Analyzed and the Methods To Be Used 

Type of Component Component Analytical Method 

Pore water O2, H2S, Fe and Mn Microelectrodes 

 Total CO2 (DIC) Coulimetric titration 

Sulfate and chloride Ion chromatograph 

 pH Electrodes 

 Nutrients Auto analyzer 

Dissolved organic C (DOC) DOC analyzer 

Sediment Solids C stable isotopes Extract/mass spectrometer 

Porosity Wt. change on drying 

CHNS Carla Erba analyzer 

Reactive Fe and Mn CD extraction, FAAS 

Carbonate carbon Leco C analyzer 

 Metals NS&T Methods 

 Hydrocarbons NS&T Methods 

Biologic Radioisotopes Counting 

 SO4

2- reduction rate 35S tracer method 

Table 7-3 

 

Chemical Parameters To Be Analyzed 

Parameter Survey Station Experimental Station 

Sediment Properties 

 CHNS L L 

 Carbonate content L L 

Contaminants 

 Metals L L 

 Hydrocarbons L L 

Indicators of Biologic Activity 

 Nutrients L L 

 DOC L L 

 SO4/Cl L L 

 DIC L 

 δC13 DIC L 

 Radioisotopes L 

 Reactive Fe and Mn L 

 Sulfate reduction rate F 

 pH F 

 H2S F 

 O2 F 

 Dissolved Fe F 

 Dissolved Mn F 
72 

The geochemistry studies at experimental stations included detailed depth profiles of 

biogeochemically important parameters, measurement of sulfate reduction rates, and benthic flux 

measurements using incubation chambers, in addition to survey variables. Isotopic studies were 

also carried out at selected stations. In addition to the shore-based analyses described for the 

survey stations, near interfacial profiles of O2, H2S, Fe and Mn were acquired by the 

microelectrode method of Luther et al. (1998). Sulfate reduction rates were determined using the 

35SO4

2- tracer technique in order to measure the approximate rate of hydrogen sulfide generation 

within the sediment (Lin and Morse 1991). Pore waters were extracted from sectioned cores to 

produce depth profiles of nutrients, pH, DIC, dissolved organic carbon (DOC), and 

sulfate/chloride ratios. All of these were analyzed on shore. Carbon stable isotopes were 

determined on the DIC samples. Total dissolved inorganic carbon in pore waters was determined 

by Coulometric titration. Pore water nutrients (nitrate, nitrite, ammonia, urea, phosphate, and 

silicate) were determined by standard auto-analyzer techniques much like those for water column 

nutrients. Dissolved organic carbon in pore waters was determined with a high temperature 

combustion DOC analyzer. Selected measurements of the stable carbon isotope ratio of 

porewater dissolved inorganic carbon will be performed according to Salata et al. (1996). 

The following table lists the chemical parameters analyzed and categorizes them by station 

type: survey or process, and whether the analyses were made in the field (F) or laboratory (L). 

Note that at the survey sites only the surficial sediments were analyzed, whereas at experimental 

stations depth profiles were determined for most parameters. 

Analysis of selected radionuclides has been used to determine sediment accumulation and 

mixing (bioturbation) rates at experimental stations. Samples along a depth profile were gamma 

counted and analyzed for 210Pb and 239,240Pu by wet chemistry followed by alpha counting. 

Gamma counting of samples was carried out on a high purity Germanium (HPGe) Well Detector 

for 210Pb (46 keV), 234TH (63 keV), 226Ra (351 keV), 7

Be (575 keV), and 137Cs (661 keV). 

The radionuclide analyses were designed to: 

• measure the natural 7Be and 234Th activity concentrations in the surface 

sediments by gamma counting to determine short-term, near-surface mixing 

(bioturbation) rates. 

• measure the natural 210Pbxs (210Pb-226Ra) activity concentrations in sediments 

from boxcores to determine the steady-state sediment accumulation rates and 

210Pb inventories for the assessment of radionuclide and sediment focusing 

effects. 

• measure the bomb fallout nuclide 137Cs and 239,240Pu activity concentrations in 

sediments to determine sediment accumulation rates from the 1963 bomb 

fallout peak. [It is necessary to determine both nuclides, because 137Cs can 

show substantial post-depositional mobility in marine sediments.] 

For analysis, core material was dried and homogenized. Sediment samples of about 

10 g size were used for non-destructive gamma counting in a low-background, high-efficiency 

high purity Germanium (HPGe) well detector followed by wet chemical extraction procedures 

and alpha counting for individual radionuclides. Procedures are given in Santschi et al. 1999 and 

references therein. 

For 210Pb determinations, samples of approximately 1 g each were used for by alpha counting 

for complete digestion and chemical separation. Gamma counted samples provided a less precise 

210Pb value used as a cross-check. For 226Ra, 234Th, 137Cs, 7

Be, and 137Cs, samples of 10 g each 
73 

were analyzed by non-destructive gamma counting. For 239,240Pu, samples of 10 g each were 

analyzed by wet chemistry and alpha counting. 

7.7 Water Column Profiles 

As summarized in Section 6.1.4, most water column properties are measured at sea by 

standard techniques: autoanalyzer for nutrients, Winkler titration for oxygen, and salinometer for 

salinity. In addition, total suspended particulate matter is determined by gravimetric analysis of 

filters. Particulate organic carbon is determined by total combustion of an acidified filter sample 

followed by detection of CO2. Phytoplankton pigments are determined by a high performance 

chromatography (HPLC) method with quantitative UV detection. The pigment analysis 

quantitatively determines chlorophylls and carotenoids used to assess the relative importance of 

the various phytoplankton groups. The methods employed duplicate other recent MMS projects 

in the GoM (Jochens and Nowlin 1998). 

Continuous monitoring sensors produce detailed profiles at each of the stations during the 

hydrocasts (see Section 6.1.4). A “quick look” at the data was performed at-sea to uncover 

potential problems in the data at the earliest possible opportunity. A quick-look also provides the 

opportunity to scan for features and to revise sampling plans to more appropriately sample any 

observed features. Data are further validated on return to shore by checking all of the metadata 

(e.g., times of sampling, depths, positions, or calibration checks) against the cruise logs and the 

data file format. For CTD data, pressure versus time is examined to remove ship motions. All 

out-of-range, unrealistic outliers, gaps and discontinuities in the dataset are manually checked. 

The continuous data is then discretized at 0.5 m intervals and written into computer files as space 

delimited columns of plain ASCII text. Characteristic plots are prepared for each station. 

The water column chemical measurements were done aboard with discrete water samples 

collected on the upcast of the rosette sampler. A Guideline Model 8400B salinometer was used 

for salinity analyses. The microWinkler technique was used for measurement of dissolved 

oxygen. Nutrients (nitrate, nitrite, ammonia, urea, phosphate, and silicate are analyzed using a 

Technicon Autoanalyzer and standard colorimetrc techniques. At-sea analysis ensures high 

precision and accurate measurements as opposed to preservation, storage, and shipment to shorebased laboratories. In addition, water is sampled for various particulate matter parameters. Twoliter water samples were vacuum filtered through pore-weighed filters for total suspended 

particulate matter (PM) analysis. Another liter of water was filtered through 25-μm 

precombusted glass fiber filters (GFF) for determination of POC. Another liter of water was 

filtered through 0.45 micron Millipore filters for analysis of the major phytoplankton pigments. 

All filters were carefully placed in combusted aluminum foil or plastic bags and properly stored 

for transport to shore-based laboratories for analysis. 

7.8 Archival of Specimens 

All the biological material in this study that has been identified to species by taxonomists has 

been retained for the duration of the project by the taxonomists, at the sorting lab in the 

Oceanography Department at TAMU, at the sorting lab at the Ft. Crockett facility at the 

Galveston campus, or at the Deep-Sea Systematics Collections of the Texas Cooperative 

Wildlife Collections (TCWC) maintained by the Department of Wildlife and Fisheries Sciences 

(WFSC) in the College of Agriculture and Life Sciences (COALS) at Texas A&M in College 

Station, Mary Wicksten, Deep-sea Invertebrates Curator (Prof., Dept. of Biology, TAMU). At 

the end of this project, invertebrate voucher specimens were archived at the U.S. National 
74 

Museum of the Smithsonian Institution as directed by MMS. These include the megafauna and 

macrofauna but not the meiofauna and bacteria. In addition to the voucher specimens given to 

the Smithsonian, regional working systematic reference collections will also be encouraged to 

access, curate and catalogue voucher specimens for future research and education when such 

specimens are available. This will include the TCWC Deep-Sea Invertebrate Collections and The 

Australian National Museum of Natural History. It does not include the collections of the 

Institute of Biology of UNAM in Mexico City, but material can be studied there or be requested 

on loan following international protocols accepted widely within the taxonomic community. All 

fish and megafauna sampled at S1 through S5 (JSSD1 to JSSD5) within the EEZ of Mexico were 

deposited in the taxonomic collections of the Institute of Biology at UNAM in Mexico City. 

Inquiries should be made directly to the Director of that institute. 

7.9 Statistical Analyses 

The discrete values of abiotic environmental information and the individual faunal groups 

were treated with a comprehensive set of numerical and statistical analyses. The purpose of 

these treatments, along with maps of the data, were carried out in order to put the diverse and 

complex data into forms that were more readily comprehensible. The statistical tests 

concentrated on accepting or rejecting the eight hypotheses on which the study was based. 

7.9.1 Bathymetric Zonation of Recurrent Groups of Related Organisms 

• Cluster analysis: Bray-Curtis similarity and group average linkage clustering 

was used on numbers of individuals of each species at each site to find the 

natural faunal groupings. 

• Non-metric Multidimensional Scaling (MDS): Inter-point distance 

representing the dissimilarity between samples served as a surrogate for the 

cluster analysis. 

• Analysis of Similarity (ANOSIM): Statistical validity of zones was 

established by performing ANOSIM tests to the hypothetical zones. 

• Mapping the faunal zones (statistically significant recurrent groups) across the 

entire sampling area illustrate the geographic relationships between the groups 

of organisms. 

7.9.2 Uni-variate Analysis 

• Analysis of variance (ANOVA): One-way ANOVA was used to test the null 

hypothesis that there were no differences between the faunal zones defined by 

previous cluster analysis. 

• The dependent variables that were tested included biotic and abiotic 

parameters. The factor levels in the tests were the different faunal zones. 

• Biotic parameters: density, biomass, species number, Shannon-Weiners index, 

and expected number of species 

• Abiotic parameters: Depth, temperature, POC, % Organic carbon, % Nitrogen, 

and Grain size 

• Multiple Comparisons were performed to determine the effects of each 

variable on each factor level. 
75 

7.9.3 Multivariate Analysis 

• Principle Component Analysis (PCA) was used for reduction (consolidation) 

of environmental variables. 

• Analysis of Similarity (ANOSIM): ANOSIM was used again to test effects of 

environmental data, based on the hypotheses supported by tests of the faunal 

species-level or density data. 

• Testing Matched Similarity Matrices (RELATE): RELATE was used to test 

the null hypothesis that there were no relationships between biotic information 

and abiotic patterns. 

• Biota and Environment Matching (BIOENV): BIOENV routine was used to 

match the biotic abundance data to environmental patterns to get a 

combination of best-matched environmental variables and correlation 

coefficients. 

• Similarity Percentage-species contributions (SIMPER): SIMPER was used to 

breakdown the average similarity within group and the dissimilarity between 

groups to determine how individual species contributed to the similarity 

values. 

7.9.4 Depth-Related Changes in Species Composition and Organism Densities 

Species Recruitment Curve: The addition and loss of species along the depth gradient was 

plotted graphically and the rate of change calculated. The species turnover (the relationship 

between ‘new’ or added versus ‘lost’ species down the gradient) was used to determine the 

boundaries between zones (defined a levels of maximum change or turnover) (adapted from 

Menzies et al. 1973). 

Rate of faunal composition change: The Bray-Curtis similarity was used to compute rates of 

change (or levels of difference) from reference stations within each zone, versus all sites, to 

examine the zonal interaction and estimate the rate of faunal composition change along the depth 

gradient. 

Regression analyses: quantitative biotic (organism density and biomass per unit area) and 

abiotic (concentrations or percents in terms of grams of sediment or volume of sediment pore 

water) data were used in linear and non-linear regression analyses by transect, grouped by 

geographic areas (canyon vs non canyon, basin vs non-basin, east vs west, etc.), and composite 

(all transects together) to determine effects of depth and depth-correlated variables on 

distributions. Where appropriate, these regressions were conducted as an aid to hypothesis tests 

and served as one approach to separating out depth-related controls from variables that might be 

independent of depth. These regressions were conducted on all size groups of organisms, from 

the bacteria up through the fishes. They were also conducted on a number of the abiotic 

variables (%OC, %N, grain size parameters, contaminants, radionuclide accumulation and 

mixing, and oxygen sediment penetration depth). 

7.9.5 Statistical Packages Used 

• SPSS 12.0 (SPSS Inc., 2003) 

• Primer 5.0 (Primer-E, 2000) 
77 

8. PROGRAMMATIC RESULTS AND DISCUSSION 

Results for each work element are summarized in this Section, including tests of the eight 

hypotheses. The different components of the abiotic environment are considered first, followed 

by subsections on each biological size group, starting with the bacteria and ending with the 

fishes. Each subsection starts with a summary of the basic data which are available in the data 

base. This is followed in each subsection by hypothesis tests using various one-way and multiple 

ANOVAs, regression analysis, or, in some cases, non-parametric tests. Principal Components 

Analysis and Multidimensional Scaling have also been utilized in some instances. In almost all 

subsections, the variables, whether abiotic or a part of the biotic community, are mapped 

quantitatively across the entire northern GoM. 

Following the basic presentation of the data, the statistical tests and the maps are then used in 

each subsection for discussions of how and why the geographic or physico-chemical variables 

control the distributions of the variables in question. These discussions are limited in scope for 

the most part to each section; cross-discipline synthesis is provided in Section 9. 

8.1 Physical Oceanography 

8.1.1 Physical Oceanographic Forcings 

The physical oceanographic data provide information about the physical environment above 

each benthic station. The water column influences primary production and carbon flux to the 

underlying sediments. The major physical forcings responsible for current fields, such as sea 

surface topography, winds and freshwater, are considered in this section. The features discussed 

here include energetic, upper-layer current features observed in sea surface height fields – the 

Loop Current, anticyclonic eddies and cyclones. Focus is on the northern GoM, with no 

discussion of the Bay of Campeche, which was outside the DGoMB study area. Wind stress is a 

forcing function for the upper waters. The climatological wind fields for summertime were 

examined, as well as the energetic wind events during the DGoMB summers. Upper currents, 

forced mainly by a combination of winds and eddies, were measured during each cruise. 

Buoyancy forcing from Mississippi River was expected to be relatively minimal because such 

discharge is concentrated largely over the shelf. 

8.1.1.1 Sea Surface Height Fields 

Sea surface height (SSH) fields were prepared under the direction of Dr. Robert Leben of the 

Colorado Center for Astrodynamic Research (CCAR) at the University of Colorado and posted to the 

publicly available web page, “CCAR Gulf of Mexico Near Real-Time Sea Surface Height Data 

Viewer,” at http://www-ccar.colorado.edu/~realtime/gsfc_gom-real-time_ssh/ Satellite altimeter data 

from TOPEX/POSEIDON, ERS-2, and other available sources were blended together using a 

procedure developed at CCAR (Leben et al. 2002; Sturges and Leben 2000; Lillibridge et al. 1997). 

Daily maps of SSH were created using an objective analysis procedure with spatial and temporal 

decorrelation scales of approximately 100 km and 12 d, respectively. The CCAR Gulf of Mexico 

SSH Data Viewer is a tool that allows the user to obtain a custom SSH plot of selected GoM regions 

and dates. All SSH fields included here were generated by this tool, denoted by the CCAR logo in 

the lower left corner of each plot. DGoMB tracks then were overlain on the SSH fields. Because of 

the temporal decorrelation scale, one plot per cruise is adequate to gain an understanding of the 

forcing by eddies during those cruises. Figures 8-1 through 8.5 present the SSH fields from the middate of each cruise, with the ADCP cruise track and CTD and moored ADCP stations shown. 
78 

Figure 8-1. Sea surface height field for 13 May 2000 with ADCP cruise track and CTD stations for 

Leg 1 of DGoMB cruise 1 overlain. [SSH field courtesy of Dr. Robert Leben, CCAR]. 
79 

Figure 8-2. Sea surface height field for 10 June 2000 with ADCP cruise track and CTD stations for Leg 2 

of DGoMB cruise 1 overlain. [SSH field courtesy of Dr. Robert Leben, CCAR]. 
80 

Figure 8-3. Sea surface height field for 10 June 2001 with ADCP cruise track, CTD stations, and moored 

ADCP locations for the DGoMB cruise 1 overlain. [SSH field courtesy of Dr. Robert Leben, 

CCAR]. 
81 

Figure 8-4. Sea surface height field for 8 June 2002 with ADCP cruise track, CTD stations, and moored 

ADCP locations for DGoMB cruise 1 overlain. [SSH field courtesy of Dr. Robert Leben, 

CCAR]. 
82 

Figure 8-5. Sea surface height field for 8 August 2002 with the ADCP cruise track for DGoMB cruise 1 

overlain. [SSH field courtesy of Dr. Robert Leben, CCAR.] 
83 

Examination of Figures 8-1 through 8-5 reveals a number of characteristics that were 

common in SSH fields of the GoM. The Loop Current was a permanent feature present in all 

years. However, the extent of penetration of its northern boundary into the GoM was variable 

(e.g., Vukovich 1988). This is seen in the five figures, which show the Loop Current as the 

region of high SSH (intense red with maximum > 50 cm) that enters the GoM through the 

Yucatan Channel between Cuba and the Yucatan Peninsula of Mexico and exits through the 

Florida Straits between Cuba and Florida. The variability of the extension of the Loop Current 

into the GoM is seen by comparison of the location of the northern boundary. This boundary 

extended farther into the GoM in summer 2000 and 2001 than in summer 2002. Physical 

oceanographic DGoMB data in the region of the Loop Current consist mainly of ADCP data 

collected only during the summer 2002 cruises, so there is little direct data within this feature. 

Historical data show that nutrients are low in the photic zone (approximately the upper 60-100 

m) of the Loop Current (Morrison and Nowlin 1977). 

The extension of the Loop Current far into the GoM is a precursor to the separation of a Loop 

Current Eddy (LCE; e.g., Vukovich 1988; Sturges and Leben 2000). An LCE is an anticyclonic 

(clockwise) circulation feature, with diameters often of ~200-400 km when newly shed from the 

Loop Current. After separation, the LCEs moved into the western GoM and decay or split into 

smaller anticyclones. Like the Loop Current itself, these eddies tend to be low in nutrients in the 

photic zone (see Section 8.2.2 for additional discussion). Figures 8-1 through 8-5 show 

anticyclonic eddies (closed contours of SSH highs, note particularly the features in yellow and 

orange). In May 2000, a decaying anticyclone, which was a combined remnant of LCEs Indigo 

and Juggernaut, was centered near 26°N 95°W (Figure 8-1). It moved eastward as May and June 

progressed (Figure 8-2). A smaller anticyclone, which was a remnant of LCE Juggernaut that had 

split off in April, was centered near 23.5°N 92°W. In June, it intensified, as indicated by the 

increase in the maximum SSH and the horizontal SSH gradient (compare Figures 8-1 and 8-2). 

This combination of LCE remnants, splitting of eddies into parts, and intensification of eddies in 

the presence of other eddies is typical of the dynamics of LCEs in the GoM. The larger 

anticyclone was sampled with ADCP measurements during the two legs of the 2000 DGoMB 

cruise and with CTD/nutrient/oxygen data on Leg 1. Another common feature in the GoM are 

smaller anticyclones in the slope region. For example, there was a small anticyclone in the 

western DeSoto Canyon region, centered about 29°N 87.5°W (Figure 8-2). 

In June 2001, LCE Millennium had nearly separated from the Loop Current (Figure 8-3). It 

was an intense feature with SSH of ~50 cm centered near 26°N 91°W. The DGoMB cruise did 

not sample in this feature. However, it did circumnavigate the small slope anticyclone, called 

Eddy Nansen, off the Mississippi River Delta (centered about 28°N 88.5°W), and ADCP data 

measured the currents between the LCE Millennium and the small slope eddy (see Section 

8.2.2). There was also a warm-core, anticyclonic remnant centered near 26°N 95°W. 

During summer 2002, an LCE moved into the western GoM, elongated east-west along about 

24°N, and began to separate into two parts (Figures 8-4 and 8-5). The June DGoMB cruise 

sampled through the region where this eddy was separating, and the August cruise repeated 

sampling through that area, although the eddy had split in two (Quick Eddy to the west and QE2 

to the east). 

Of great interest to issues of productivity are the presence of anticyclones near the shelf edge 

and the presence of cyclonic eddies that often spin up in association with the anticyclones. The 

cyclonic (counter-clockwise rotating) eddies are seen as the closed circulation regions of low 

SSH (blues). Note that the region near DeSoto Canyon (e.g., near 29°N 87.5°) had cyclones or 
84 

anticyclones near the shelf edge in all of the summers with DGoMB sampling. Similar 

conditions existed in the northwest GoM, where many of the anticyclonic LCEs decay near the 

shelf edge and spin up adjacent cyclonic eddies. Examination of a monthly time series of SSH 

fields throughout the years indicates these small slope eddies (cyclonic or anticyclonic) are 

typical of the slope area. Time scales of these features can be on the order of months, persistent 

enough to have biological impacts. Cyclones also spin up in association with the Loop Current or 

LCEs in deep water (e.g., see the cyclone in Figures 8-4 and 8-5 centered near 25°N 87°W 

northwest of and adjacent to the Loop Current). Off DeSoto Canyon, these features can move 

low salinity, biologically productive Mississippi river discharge off the shelf into deeper water 

(Figure 8-6), with consequent influence on the productivity of the deep water benthos (Jochens et 

al. 2002). These features also can bring nutrient-rich deep water up into the photic zone when 

anticyclones interact with the topography or cyclones. The centers of cyclones are also regions of 

upwelling. This can increase productivity in the surface waters, with, it is presumed, consequent 

increase in delivery of organic matter to the benthos. 

8.1.2 Gulf of Mexico Winds During Summer 

Wind stress produces currents in the upper waters of the ocean that transport nutrients, 

sediments, phytoplankton, and other biologically important matter from regions of generation or 

discharge, as in the case of river-borne material, to other regions in the GoM. Long-term patterns 

generated by wind stress include an anticyclonic circulation in the western GoM with a westward 

intensified boundary current (Sturges 1993) and a persistent cyclonic circulation in the Bay of 

Campeche (Vázquez de al Cerda et al. 2005; Vázquez de la Cerda 1993). The DGoMB study was 

not designed to identify the biological effects of these long-term wind-induced circulation 

patterns. 

To consider the possible effects of wind-forcing during the DGoMB cruises, the 

climatological wind fields were examined to determine whether there was a dominant pattern in 

summer. Then the details of the May through August winds for the years 2000 through 2002 

were examined. 

Jochens et al. (2002) examined the climatological winds using the COADSPACS Enhanced 

1960-1997 data (Figure 8-7). The Comprehensive Ocean-Atmosphere Data Set (COADS) is an 

extensive collection of monthly summaries of surface marine data on a 1-degree latitude by 1-

degree longitude grid. COADSPACS Enhanced data are provided by the NOAA-CIRES Climate 

Diagnostics Center, Boulder, Colorado, from their Web site at http://www.cdc.noaa.gov (see 

Woodruff et al. 1987). To determine the monthly mean wind climatology for the GoM, the 

average mean wind velocity components were computed for each month based on the total 

values available at each grid location. The mean vector was then assembled from the mean u and 

v components. Winds at grid locations with less than 6 years of data are not shown. Also shown 

are ellipses of the principal axes of variance of monthly means for that month. 

Figure 8-8 shows the time periods when tropical cyclones (hurricanes, tropical storms, and 

tropical depressions) were located in the GoM. There were no hurricanes during May through 

August of 2000-2002. Of the five cyclones shown in Figure 8-8, Tropical Storm Barry in 2001 

passed the closest to the two NDBC buoys. It formed near 26.3°N 84.8°W, moved westward to 

26.7°N 87.9°W, and then moved northward where it made landfall at the western end of the 

Florida Panhandle. The Tropical Storms (TS) and Tropical Depressions (TD) during these 
85

Figure 8-6. Salinity at approximately 3.5-m depth from thermosalinograph observations on NEGoM cruise N9 in 

Dots show sample locations and give the track lines. Note that salinity increases from the shelf edge 

resulting from the movement of Mississippi River water off shelf by a slope eddy. 
86 

Figure 8-7. COADS enhanced mean wind field and principal component ellipses for June. Data were 

averaged over the period 1960-1997. NDBC meteorological buoys 42041 and 42001 are 

denoted by the squares. 
87 

Figure 8-8. Wind speed from National Data Buoy Center meteorological buoys 42041 (2000) and 42001 

(2001 and 2002) for May through August during the DGoMB field program. Periods with 

tropical cyclones in the Gulf are shown by vertical lines (TD is Tropical Depression; TS is 

Tropical Storm). 

summers had no observable effect on the winds at the buoys, likely because neither buoy was in 

the direct path of the cyclones. This is consistent with the findings of Nowlin et al. (2001) who 

compared deepwater current meter data with the tracks of tropical cyclones for the period 1977-

2000. They also found the effects of tropical cyclones on oceanic currents were intense but 

localized and short-lived with diminishing effects observable about 7-10 days after passage (see 

also Nowlin et al. 2000 for discussion of literature). 

The NCDC Daily Weather Maps for May through August in 2000 and 2001 and the 3-hourly 

Surface Analysis of the Hurricane Prediction Center for May through August 2002 were 

examined for the occurrence of the passage of cold fronts through the GoM. In all three years, 

there were weak fronts that skirted the northern GoM coastline while moving eastward, with 

occasional extension into the northeast GoM. Such events occurred mainly in May and June. 

There were no frontal passages crossing the deepwater GoM during these months in 2000. In 

2001, one cold front passed through the GoM on 22-23 May. The time of this event is indicated 

by the dashed vertical line in Figure 8-8. The front passed by the buoy locations, but seems to 

have had no pronounced effect on the winds or, by inference, on the oceanic currents and 

transport of materials thereby. In 2002, there were three fronts that passed near the buoys: two in 

May (13-15 and 18-20) and one in June (15-16) as shown on Figure 8-8. The paucity of frontal 

passages in summertime has been noted by DiMego et al. (1976), Henry (1979), Nowlin et al. 

(1998), and Jochens et al. (2002). The strongest frontal passages are those that evolve into 

extratropical cyclones during the non-summer months, particularly in winter. Nowlin et al. 
88 

(2001) compared the deepwater current meter data with the occurrence of extratropical cyclones 

and found that most of these cyclones had no effect on the currents. In summary, the frontal 

passages and extratropical cyclones had little or no effect on deepwater currents and hence on 

transport of materials, and when there was any effect, it was localized in time and space. 

8.1.3 Influence of River Discharge 

The Mississippi River discharges on the order of 1,000 million m3

·d-1. The 72-year mean 

daily discharge from the Mississippi River, measured at Tarbert Landing, Louisiana, is shown in 

Figure 8-9 (repeated in each of the three years 2000-2002). The mean daily discharge peaks in 

May and is at a minimum in October. The standard deviation, also shown, is largest in spring and 

smallest in fall. The major influence of this discharge is over the shelves adjacent to the points of 

discharge. However, eddy processes can move riverine water off-shelf into deep water. There 

may be areas, such as the slope east of the Mississippi River Delta and associated with the 

western DeSoto Canyon, where such movement is frequent enough to have potential influence 

on the near-surface biology with possible consequent impacts to the benthic biology (e.g., see 

discussion of high faunal densities near DeSoto Canyon, 8.5). DGoMB physical oceanographic 

sampling was insufficient to determine how extensive and consistent this might be from year to 

year. 

The daily discharges for each of the three years of DGoMB field work also are shown in 

Figure 8-9. During 2000, the spring discharge was below average. For the DGoMB cruise in 

May-June 2000, the two stations off the Mississippi River Delta in DeSoto Canyon (S35 and 

S36) have near-surface salinities that are slightly lower (< 34) than adjacent stations (>35), 

suggesting some limited mixing of Mississippi River water with oceanic waters. Note the peak 

discharge in the July time period. This discharge occurred several weeks prior to the cruise for 

which the near-surface salinity, with its clear indication of mixing of Mississippi River water 

with oceanic water, is shown in Figure 8-9. The peak discharge in spring 2001 occurred 

approximately two months prior to the DGoMB cruise; the stations nearest the Mississippi River 

(S36 and HiPro) show no influence from the river water. During spring 2002, the daily river 

discharge was above average, with two peaks exceeding one standard deviation above the mean. 

During the June-July 2002 period immediately after the June DGoMB cruise, low salinity water 

over the slope west of DeSoto Canyon was observed (Jochens, personal communication), 

indicating mixing of riverine with oceanic waters. The DGoMB cruise did not sample in this 

region. The August 2002 DGoMB work did not collect CTD data. However, a cruise for another 

program in August-September 2002 that started shortly after the DGoMB cruise sampled near 

surface salinity along the upper slope around the Mississippi River Delta and DeSoto Canyon. 

Near-surface salinities along the 1,000-m isobath to the southeast and southwest of the 

Mississippi River Delta on that cruise were low, with some values < 30, but the values along the 

slope of De Soto Canyon were not unusually low (Jochens and Biggs 2003). The evidence 

suggests that riverine influences extend to the upper slope between approximately –89.5°W and 

–87°W frequently enough to possibly enhance the productivity in this area. The DGoMB cruises, 

however, were not designed to test this or to determine whether there might be other areas 

susceptible to such export of fresh water from the shelf. 
89 

Figure 8-9. Daily discharge for 2000-2002 from the Mississippi River, measured at the Tarbert Landing, 

Louisiana, gauge. The daily discharges for 2000 through 2002 are shown as the thin line. 

The mean daily discharge (thick line) for a 72-year record is repeated in each of the three 

years. The dashed lines show ± one standard deviation from the mean. DGoMB cruises 

occurred between pairs of vertical lines. 

8.2 Physical Oceanography 

8.2.1 Water Property Patterns 

Temperature and salinity measurements were made on the DGoMB cruises in May/June 

2000, June 2001, and June 2002. Measurements of nitrate, phosphate, silicate, and dissolved 

oxygen concentrations were made on the cruises in 2000 and 2001. The vertical patterns of these 

properties were analyzed. The station spacing did not allow detailed or meaningful examination 

of horizontal patterns. The vertical patterns were related to features in the circulation field shown 

by the sea surface height fields (see Section 8.1.1). The measurements were compared to 

historical data sets. This section describes the results of these analyses. 

Patterns observed in water properties during DGoMB cruises: Vertical profiles in depth and 

density for salinity and potential temperature are shown for seven DGoMB CTD stations 

(Figures 8-10 and 8-11). Station locations are shown in Figure 8-12. These stations were selected 

to represent the variety of differing conditions in the GoM. They show the typical range of 

values that were measured at the DGoMB CTD stations. The full range from all the stations is 

slightly broader in the upper 500 m than shown by the selected stations; profiles below 500 m are 

within the bounds shown in the figures. The selected stations are: S4 (blue) located in the Loop 
90 

Figure 8-10. Selected station data showing salinity versus density (top) and depth (bottom). 

Shown are stations RW6 (2000) in a Loop Current Eddy in the western GoM, 

S39 (2000) in a cyclonic eddy in the northeastern GoM, GK-Furrows (2001) at 

the edge of a Loop Current Eddy in the north-central GoM, S4 in the Loop 

Current, S2 at the edge of a Loop Current Eddy in the south central GoM, and 

S5 at the edge of a cyclone in the eastern GoM. Station locations are shown 

in Figure 8-12. 
91 

Figure 8-11. Selected station data showing potential temperature versus density (top) and 

depth (bottom). Shown are stations RW6 (2000) in a Loop Current Eddy in the 

western GoM, S36 (2000) in a slope anticyclonic eddy in the northeastern 

GoM, S39 (2000) in a cyclonic eddy in the eastern GoM, GK-Furrows (2001) 

at the edge of a Loop Current Eddy in the north central GoM, S4 in the Loop 

Current, S2 at the edge of a Loop Current Eddy in the south central GoM, and 

S5 at the edge of a cyclone in the eastern GoM. Station locations are shown 

in Figure 8-12. 
92 

Figure 8-12. Locations of selected CTD/bottle stations on the DGoMB cruises in summers 2000-2002. 

Bathymetric contours shown are 200, 1,000, 2,000, and 3,000 m. 

Current in the southeastern GoM (see Figure 8-4), RW6 (black) located in a Loop Current Eddy 

(LCE) in the northwest GoM (see Figure 8-1), S36 (red) located in a slope anticyclonic eddy in 

the northeastern GoM (Figure 8-2), S39 (light blue) located in a cyclonic eddy in the 

northeastern GoM (Figure 8-2), GK-Furrows (green) located at the edge of an LCE in the north 

central GoM (Figure 8-3), S2 (yellow) located in a splitting LCE in the deep basin of the central 

GoM (Figure 8-4), and S5 (pink) located at the edge of a cyclonic eddy in the deep basin of the 

eastern GoM (Figure 8-4). 

The Loop Current is the source water for the deep basin of the GoM. One CTD station, S4 in 

June 2002, was taken in the Loop Current at approximately 24.25°N 85.5°W (Figure 8-4). The 

vertical profile of salinity at this station is characterized by a salinity maximum of > 36.6 at a 

depth of approximately 150 m and a density surface of 25.4 kg·m-3 σθ (Figure 8-10, blue). The 

waters with this property are part of the Subtropical Underwater water mass that enters the GoM 

with the Loop Current. The profile of potential temperature for this station (Figure 8-11, blue) is 

characterized by warmer water deeper in the water column compared to other stations in the 

GoM. This is a typical characteristic of anticyclonically circulating features including the Loop 

Current. 

The general salinity pattern for all stations consists of four major characteristics. Most 

notable is lack of horizontal variability in salinity below depths of approximately 1,000 m. This 

is present in all station CTD data. Second is a salinity maximum. This maximum is found to be 

reduced with increasing distance from the source waters. However, this is not only a function of 
93 

distance from the Loop Current. For example, station S36 (2000) was located in the northeastern 

GoM in DeSoto Canyon near 29°N 87.5°W (Figure 8-2). Thus, it was geographically closer to 

the Loop Current than was station RW6 (2000), which was located in the western GoM near 

26°N 95°W (Figure 8-1). Yet, station S36 has a very reduced salinity maximum (Figure 8-2, 

red), whereas station RW6 has a predominant salinity maximum pattern comparable to that of 

the Loop Current station S4 (Figure 8-10, black compared to blue). Station S36 sampled in a 

small slope anticyclone, where waters were subject to mixing due to eddy and bathymetric 

effects and to entraining of shelf waters, all contributing to the consequence erosion of the 

salinity maximum. Station RW6 sampled near the middle of an anticyclonic LCE, which had 

carried Loop Current water into the western GoM. Evidence of erosion of the salinity maximum 

is present in RW6 as the sharp reduction in salinity at σθ of less than 25.6 kg·m-3. 

The third characteristic of the salinity profiles is the salinity minimum at about 600-800 m or 

σθ of 27.3-27.5 kg·m-3. This is associated with the Antarctic Intermediate Water that is brought 

into the GoM by the Loop Current. The depth at which this minimum occurs is variable. It 

appears to be related to the type of eddy feature the station sampled or the proximity to the 

bathymetric effect of the slope. For example, the depths of the minima for S4 in the Loop 

Current and RW6 in a LCE in the western GoM are comparable, whereas those for S36 in a 

weak anticyclone over the northeastern GoM slope and S39 in a cyclone are approximately 200 

m higher in the water column. Distance westward may also be a factor as suggested by 

comparison of stations S2 in the deep western GoM basin and S4 in the eastern GoM. Both were 

relatively distant from bathymetry and both were in anticyclonic features. Yet, the minima at S2 

is located about 100 m higher than that at S4. 

The fourth characteristic is the variability in the near-surface salinity values. Considered are 

the salinities in the upper 50-100 m. These upper waters have lower salinity concentrations than 

at depth. Part of this, of course, is related to the presence of the salinity maximum. However, part 

of the variability is due to the mixing of low salinity water from the Mississippi River with the 

near-surface oceanic waters. Of the selected stations shown in Figure 8-10, the lowest nearsurface salinity values occur at S36, which is located southeast of the Mississippi River. Other 

stations west as well as east of the Mississippi River mouth exhibit low salinity. In the cases 

examined, there were circulation features adjacent to the Mississippi River mouth that likely 

transported shelf water, which had been diluted by mixing with river water, to the slope regions 

adjacent to the river. This situation has been observed in other programs (Belabbassi et al. 2004). 

In addition to the lack of horizontal variability in potential temperature below depths of 

approximately 1,000 m, the temperature profiles in Figure 8-11 show generally cooler water 

from the surface through the main thermocline to about 1,000 m. In the main thermocline, the 

depth at which a particular potential temperature value occurs is related to the circulation feature 

present at the time of sampling. The anticyclonic eddies tend to depress isotherms, whereas 

isotherms in cyclones tend to dome upward. For example, station RW6 is in a strong LCE so its 

isotherms are lower in the water column than those of the other stations except S4, which is in 

the Loop Current. In contrast, station S39 is located in a cyclone and its isotherms are higher in 

the water column. 

Vertical sections of properties were examined where the station locations allowed. Spacing 

of stations was large, being generally 35 km or greater. This spacing allows consideration of 

large-scale features only. In general, the few transects showed similar results. Transects taken 

across strong anticyclonic eddies showed the characteristic pattern of depressed isolines near the 

eddy center. Examples are discussed below. There were no transects with reasonably spaced 
94 

stations taken across strong cyclonic eddies. However, the transects that crossed weak parts of 

cyclones showed a tendency for doming isolines. No examples are presented here. 

The transect selected for discussion here was the western-most line on the first DGoMB 

cruise in May-June 2000. It extended from the shelf edge at about 27.5°N 96°W to 

approximately the middle of an anticyclonic LCE at about 26°N 94.5°W (see Figure 8-12 for 

locations). Figure 8-13 shows the contours of density anomaly (σθ) along this transect. The 

station taken at the shelf edge was RW1 at the left side of the figure, while the station taken near 

the middle of the LCE was RW6 at the right side. With the exception of near-surface waters, 

isopycnals show the stability of the water column by their monotonic increase with depth. The 

isopycnals in the upper 500 m or so tend to slope downward from the shelf edge to station RW6. 

The isopycnals below about 500 m, however, tend to be more bowl-shaped, with the center of 

the bowl being near station RW5. This suggests that the LCE was tilted in the water column, 

which is not unusual as the LCE is approaching the shelf edge and so is interacting with bottom 

topography. The vertical pattern of potential temperature (not shown) is similar, although the 

temperature monotonically decreases with depth. 

Salinity contours, shown in Figure 8-14, follow a similar downward trend in the upper 500 m 

from the shelf edge station and bowl-shaped pattern below as seen in density. There is a salinity 

maximum between 100-200 m depth, which is near the 25.4 kg·m-3 σθ line (shown as a dashed 

line), that corresponds to the signature of the Subtropical Underwater mass that the LCE has 

transported into the western GoM from the Loop Current. The Antarctic Intermediate Water 

mass, with its salinity minimum, is evident in water depths of about 900 m and σθ of 27.5 kg·m-3

(also shown as a dashed line). This water mass also was transported to the western GoM by the 

LCE. 

Vertical contours of nitrate and dissolved oxygen along the transect are shown in Figures 

8-15 and 8-16, respectively. Both exhibit the downward trend in isolines indicative of an 

anticyclonic circulation feature. The nitrate concentrations are below detection limits in the 

upper 100-150 m. The phosphate and silicate (not shown) are also very low, being < 0.5 μM and 

< 1 μM, respectively, in the upper water column. This reflects the consumption of the nutrients 

by phytoplankton in the presence of sunlight. The complete utilization of nitrate, with some 

phosphate and silicate still being detectable, indicates the biological system is nitrate limited. All 

three nutrients increase with depth, reflecting nutrient regeneration as detritus falls through the 

water column transport of nutrient-rich deep waters into the GoM. The Antarctic Intermediate 

Water mass is evident in the nitrate maximum at about 900 m and σθ of 27.5 kg·m-3 (shown as a 

dashed line). The oxygen section shows the presence of the Tropical Atlantic Central Water, 

which is characterized by the oxygen minimum located at about 600 m depth and σθ of 27.15 

kg·m-3 (shown as a dashed line). The effects of atmospheric exchanges with the surface are seen 

in the high values of oxygen (> 4.5 mL·L-1) in the upper 100 m. The deep waters below about 

1,000 m also are relatively high in oxygen; the sources of this oxygen are the highly oxygenated 

deep water masses entering into the GoM through the Yucatan Channel. 
95 

Figure 8-13. Density anomaly (σθ in kg·m-3) on western-most transect of DGoMB cruise 1, 4 May-17 June 

2000. CTD data were spaced every 0.5 m. 
96 

Figure 8-14. Salinity from the CTD on western-most transect of DGoMB cruise 1, 4 May-17 June 2000. 

Dashed lines show sigma-theta density surfaces of the Subtropical Underwater and the 

Antarctic Intermediate Water. Salinity data were spaced every 0.5 m. 
97 

Figure 8-15. Nitrate (μM) on the western-most transect of DGoMB cruise 1, 4 May-17 June 2000. 

Dashed lines show two of the sigma-theta density surfaces associated with the Antarctic 

Intermediate Water. Dots show sampling locations. 
98 

Figure 8-16. Dissolved oxygen (mL/L) on the western-most transect of DGoMB cruise 1, 

4 May-17 June 2000. Dashed line shows the sigma-theta density surface of the Tropical 

Atlantic Central Water. Dots show sampling locations. 
99 

Figures 8-17 and 8-18 show profiles versus depth and density, respectively, of dissolved 

oxygen, nitrate, phosphate, and silicate from all DGoMB 2000 and 2001 data. The profiles 

distinguish between data taken in the eastern GoM (pluses) and those taken in the western GoM 

(dots). Here west is taken to be west of 90°W. Overall patterns are the same in the east and west. 

The differences in distributions in the upper 100-200 m of the water column are likely to be 

related to differences in the number and types of features and the bathymetric regions sampled, 

as well as differences caused by surface processes, such as localized photosynthesis and 

respiration, winds and associated mixing, heating and cooling, and precipitation and evaporation, 

that impact rates of oxygen input and consumption. 

The oxygen profiles (Figures 8-17a and 8-18a) show high concentrations in the upper water 

column, reflecting photosynthesis and exchanges with the atmosphere. Values decrease with 

depth in the upper 500 m, as expected from the vertical oxygen distribution of the source waters, 

particularly the Tropical Atlantic Central Water. Local consumption of oxygen in the nearsurface waters during the decay of detritus falling through the water column also contributes to 

this structure. Below about 500 m, the oxygen concentration then increases with depth. Compare 

the high variability of oxygen concentrations in the upper water column with the small variability 

in the deep water column. This is most clearly seen in the density plots, with the small horizontal 

variability seen for σθ > 27.15 kg·m-3. This difference reflects the variability in processes that 

input or consume oxygen. 

The nutrient profiles (Figures 8-17b-d and 8-18b-d) also reflect the processes that increase or 

decrease the concentrations. The upper water column, where the nutrients have largely been 

consumed by phytoplankton, has low concentrations. Concentrations then increase with depth to 

a maximum near the depth or density surface of the Antarctic Intermediate Water mass 

(indicated by the line). This reflects the distribution of the nutrients in the source water masses, 

although the local decay of detritus regenerates the nutrients and contributes to this distribution. 

Deeper in the water column the concentrations decrease or remain fairly constant, again due 

mainly to the distributions found in the source waters. 

Comparison of DGoMB with historical water property patterns: The DGoMB water 

properties were compared to published historical data sets from the eastern and western GoM 

basins. Morrison and Nowlin (1977) published results from a May 1972 cruise of the R/V 

Alaminos conducted in the eastern GoM with emphasis on the region occupied by the Loop 

Current. Morrison et al. (1983) published results from an April 1978 cruise of the R/V Gyre in 

the western GoM. The historical data from the 1972 cruise were available; so plots show actual 

data. The historical data from the 1978 cruise are not available from any identifiable source; so 

plots show estimates based on scanned figures from the Morrison et al. (1983) paper. 

The dissolved oxygen, nitrate, phosphate, and silicate data from the DGoMB cruises (black 

pluses) are shown with the data from the 1972 and 1978 cruises (red dots) for the eastern (Figure 

8-19) and western (Figure 8-20) GoM. Examination of these figures shows that the basic patterns 

seen in the DGoMB data described above were present in the historical data. There is less 

variability (both deep and shallow) in the DGoBM data than in the historical data. In general, this 

likely is due to improvements in sampling and analysis techniques and skills, rather than a 

climatic change in the patterns. The extreme example for nitrate near σθ = 27.4 in the eastern 

GoM is most likely due to sampling problems reported for these historical nitrate data. 

The oxygen pattern in the east (Figure 8-19a) suggests that the oxygen concentrations for σθ

≤26 kg·m-3 might be greater in the 1972 data than in the DGoMB data. This reflects the sampling 
100 

Figure 8-17. Water properties versus depth for DGoMB cruises in 2000 and 2001. Shown are (a) dissolved ox

phosphate, and (d) silicate. Pluses denote data collected west of -90°W; dots from -90°W and east. 
101 

Figure 8-18. Water properties versus density for DGoMB cruises in 2000 and 2001. Shown are (a) dissolved oxy

phosphate, and (d) silicate. Pluses denote data collected west of -90°W; dots from -90°W and east. 
102 

Figure 8-19. Water properties versus density for the eastern GoM. Shown are (a) dissolved oxygen, (b) nitrate, (c

and (d) silicate. Pluses are from DGoMB cruises in May/June 2000 and June 2001; dots are from t

72A09 cruise in May 1972. A line is drawn at 27.4 sigma-theta, which is one of the isopycnals associ

Antarctic Intermediate Water. The line at 27.15 sigma-theta is associated with the 18°C Sargasso Se
103 

Figure 8-20. Water properties versus density for the western GoM. Shown are (a) dissolved oxygen, (b) 

phosphate, and (d) silicate. Pluses are from DGoMB cruises in May/June 2000 and June 200estimated from Morrison et al. (1983) Figure 3 showing data from 78G03 cruise in April 1978. A line 27.4 sigma-theta, which is one of the isopycnals associated with the Antarctic Intermediate Water27.15 sigma-theta is associated with the 18°C Sargasso Sea Water. 
104 

on the earlier cruise was mainly in the region of the Loop Current, while the DGoMB sampling 

was throughout the eastern GoM. The older data also sampled in the area where the oxygen 

maximum associated with the 18°C Sargasso Sea Water is present, whereas many DGoMB 

stations were located in regions where this water mass had eroded away. Thus, this difference in 

DGoMB versus 1972 oxygen concentrations does not reflect a climatic change, but rather the 

fact that the 1972 sampling occurred directly in the source waters for oxygen in the eastern basin 

and so have not been diluted by mixing from other processes that modify oxygen content. 

8.2.2 Circulation Patterns from ADCP Measurements 

8.2.2.1 Moored ADCP Measurements 

Summer 2001: During the 2001 summer cruise, a 300-kHz ADCP was deployed at each of 

four process stations (MT3, S42, S36, and MT6). The instrument was placed 35 m above the 

bottom in a downward-looking configuration. The deployments were short, ranging from 20-60 

hours. Because of the short deployment period, these observations should be considered as a 

representative snapshot of the physical conditions that existed while each process station was 

occupied. The current observations should not be considered as the climatology of currents 

possible at each location. To determine the current climatology, a much longer current meter 

record would be required. 

The near-bottom currents at the four process stations during summer of 2001 were generally 

weak with record-length mean speeds of 2.5, 7.5, 2.5, and 5 cm/s at stations, MT3, S42, S36, and 

MT6, respectively (Figure 8-21). Mean directions were generally aligned along local bathymetry. 

Vertically, current speeds decreased slightly with closeness to the bottom. Standard deviations of 

speed were nearly uniform for all four deployments at 1-2 cm/s. The sea temperature at the 

ADCP transducer head was nearly constant throughout each deployment with values near 5.0°, 

6.0°, 4.1°, and 4.2°C. Vertical and error velocities (an indicator of the amount of horizontal 

inhomogeneity of currents in the water column) were small (1-2 cm/s) for all deployments.

For three of four deployments, the bottom boundary layer appeared to be just less than 30 m 

thick. At S42, there was more vertical structure as the record-length mean cross-slope velocity 

changed sign at about 20 m above bottom, down-slope below 20 m, and upslope above 20 m. 

The record-length mean along-slope component was nearly zero from 30 m above bottom to the 

bottom. Examination of the time series of both velocity components showed a clear near-diurnal 

oscillation with amplitude of about 7 cm/s and about 6 hours out of phase. Because of the short 

deployment it is not possible to determine whether this oscillation is due to tidal forcing, which 

is believed to be small in the deep GoM, or is of inertial origin. Diurnal oscillations were also 

present at stations MT3, S36, and MT6, although significantly weaker than at S42, i.e., 

amplitudes of 2-3 cm/s 

Summer 2002: The 300-kHz ADCP was deployed twice during summer 2002 at process 

stations in the Sigsbee Basin (station 1) and beneath the Loop Current (station 4). Both of these 

stations were in ultra-deep water (depths greater than 3,000 m). Deployment lengths were 36 and 

48 hours. Data quality was a concern for these deployments as quality generally decreased with 

distance from the instrument, such that data more than 20 m from the instrument, i.e., 40 m 

above bottom, were considered useless. We attribute the data loss to a lack of acoustic scatterers. 

Record-length mean currents were much stronger than the previous year’s observations 

increasing from 20 to 40 cm/s between 55 and 40 m above bottom (Figure 8-22). The standard 

deviation of current also increased considerably in this range from 10 to 30 cm/s as the number 
105 

Figure 8-21. Record-length mean speed (solid) and plus and minus one standard 

deviation (dashed) of currents measured using a bottom-moored 

downward-looking 300-kHz ADCP during four deployments of Cruise 

3 in 2001. Instrument was 35 m above bottom. Deployments sites are: 

MT3 (top left), S42 (top right), S36 (bottom left), and MT6 (bottom 

right). 
106 

Figure 8-22. Record-length mean speed (solid) and plus and minus one standard 

deviation (dashed) of currents measured using a bottom-moored downward-looking 

300-kHz ADCP during two deployments during Cruise 4 in 2002. Deployment sites 

are: Site 1 (top) and Site 4 (bottom). 
107 

of usable data decreased. The increase in speed with depth may likely be an artifact of the 

decrease in data quality with depth. The record-length mean of individual velocity components 

show a mean southwest direction at both stations. Neither deployment shows a detectable tidal 

signal. Mean temperatures are about 4.3°C. Vertical and error velocity components are 

disturbingly large (at times greater than 10 cm/s). 

8.2.3 Shipboard ADCP Measurements 

Shipboard ADCP observations of near-surface currents along the cruise track of each cruise 

were obtained using 150 kHz and 38 kHz ADCPs. The instrumentation and settings used during 

each cruise are detailed in Sections 1.2.2 and 1.3.3. The following paragraphs present summaries 

and interpretations of the observations. 

Summer 2000: The physical oceanography of the GoM during the spring and summer of 

2000 was dominated by the Loop Current in the eastern GoM (east of 90°W) and by the presence 

and interaction of Eddies Indigo and Juggernaut in the western GoM (Section 8.1.1). Eddy 

Indigo was centered near 26.25°N, 94.75°W as evidenced by satellite altimeter observations. 

Figure 8-23 shows the sea surface height field of the northwestern GoM on 13 May 2000. 

Superimposed on the SSH field are near-surface (14-m) current velocity vectors showing the 

magnitude and direction of currents along the cruise track. The cruise track of Cruise 1 passed 

through several mesoscale circulation features during this cruise. Observations were made along 

the eastern edge of Eddy Indigo during a north-south transect near 93.5°W before passing west 

near the eddy center and northwest toward the Texas coast. The ship also passed through a 

cyclonic feature centered near 26.25°N, 92°W. Currents were strongest in Eddy Indigo, 

particularly along the eastern limb of the eddy and in the region between the northwestern edge 

of the eddy and the continental shelf. Maximum near-surface speed exceeded 100 cm/s at both 

the eastern and northwestern edge of the eddy. Vertically, the speed core of Eddy Indigo 

extended to roughly 200 m depth, as indicated by the 60 cm/s isotachs (Figure 8-24). 

The mean currents for the length of the record for the entire cruise show surface 

intensification and exponential decay with depth, in agreement with previous studies (Nowlin et 

al. 2001) of moored current meter records of the deep GoM. For the May 2000 cruise, the recordlength mean surface currents exceeded 30 cm/s in the upper 100 m and decayed to less than 15 

cm/s at 250 m depth. Below 250 m mean currents smoothly decayed from 15 cm/s to 9 cm/s at 

750 m. The standard deviation of currents shows a similar decay pattern with depth with values 

of 25 cm/s in the upper 100 m and about 7 cm/s below 600 m. This pattern has been identified as 

the first baroclinic dynamic mode (Charney and Flierl 1981) based on hydrographic data of the 

deep GoM (Nowlin et al. 2001). 

In June 2000, the position of Eddy Indigo changed little, but it weakened slightly. The 

position of the cyclone moved southward to about 25°N. The near-surface current velocity 

vectors observed along the cruise track of Cruise 1, leg 2, are shown in Figure 8-23. The cruise 

track passed through the center of Eddy Indigo and north of the cyclone. The cruise spent 

considerable time south and southeast of the Mississippi River Delta in the north-central GoM. 

Current speeds there were variable and weak as a result of the lack of strong circulation features. 

As during May, the strongest currents were in the northwestern GoM and associated with Eddy 

Indigo. The eddy may have pushed closer to the continental shelf and intensified currents along 

its northwestern limb. Currents exceeded 120 cm/s near surface and 60 cm/s above 200 m. 

Owning to the longer time spent in the quiescent central GoM, mean speeds for the cruise 
108 

Figure 8-23. Contours of sea-surface height from satellite altimeter data and near-surface velocity 

vectors from shipboard ADCP during Cruise 1, leg 1. SSH data are from 13 May 2000 and 

are courtesy of R. Leben (CU). 
109 

Figure 8-24. Vertical section of current speed based on shipboard 150-kHz ADCP data collected along a 

section in the western GoM during Cruise 1, leg 1, in June 2000. Positions of circulation 

features are indicated above the controus. See Figure 8-23 for section location. 

decayed from only 23 cm/s near surface and were roughly 10 cm/s from 250 m to 750 m depth. 

The vertical structure of currents during this cruise, however, was similar to the structure of the 

first dynamic mode for the GoM as seen on the previous month’s cruise. 

Summer 2001 (Cruise 2): By the summer of 2001, a weak anticyclone, likely the remnant of 

Eddies Indigo and Juggernaut, dominated the western GoM. The central GoM was dominated by 

the presence of a newly detached Loop Current Eddy centered at 25.5°, 91°W and the Loop 

Current in the east GoM was confined south of 27°N. The track for Cruise 2 was limited to the 

north-central GoM, south of the Mississippi River Delta. The track circumscribed a weak slope 

anticyclone centered at 28°N, 88.5°W (Figure 8-25). Observed surface currents were generally 

weak during this cruise with notable exceptions on the west side of the weak anticyclone and in 

the region of interaction between the anticclone and the cyclonic feature centered at 27°N, 

87.5°W (Figure 8-26). There was an indication of intensified currents in this region, as currents 

exceeded 80 cm/s. Further, the currents were strongest about 50 m below the surface, perhaps 

due to the influence of the cyclone, whose phenomenon class is typically intensified below the 

surface. 

Summer 2002 (Cruise 3a and 3b): During the summer of 2002, the tracks of both legs 

extended from the northern GoM to the Loop Current. During Cruise 3a in June 2002, the track 

passed through or near several energetic circulation features, including the Loop Current, a 

detached Loop Current Eddy (24°N, 94°W), a strong cyclone northwest of the Loop Current, and 

other weaker features (Figure 8-27). In general, currents were aligned with altimeter contours of 

sea surface height. The strongest observed currents during this cruise were associated with the 
110 

Figure 8-25. Contours of sea-surface height from satellite altimeter data and near-surface velocity vectors from shipb

Cruise 2. SSH data are from 15 June 2001 and are courtesy of R. Leben (CU). 
111 

Figure 8-26. Vertical section of current speed based on shipboard ADCP data collected along a section in the north-cen

2 in June 2001. Positions of circulation features are indicated above the contours. See Figure 8-25 for sectio
112 

Figure 8-27. Contours of sea-surface height from satellite altimeter data and near-surface velocity 

vectors from shipboard ADCP during Cruise 3a. SSH data are from 9 June 2002 and are 

courtesy of R. Leben (CU). 

Loop Current and the currents associated with anticyclonic features. In the Loop Current, 

currents exceeded 135 cm/s in the upper 100 m and 100 cm/s in the upper 150 m. Useful 

observations were limited to the upper 200 m because of the operation frequency (150-kHz) of 

the ADCP used (the 38-kHz ADCP failed to produce useful data). 

Currents in the weak anticyclone near 27°N, 90°W peaked above 60 cm/s and averaged 

greater than 30 cm/s in the upper 200 m. Similar values were found on the eastern limb of the 

Loop Current Eddy. Mean currents in the upper 200 m of the water column ranged from 38 cm/s 

at 10 m depth to 13 cm/s at 200 m. The standard deviation similarly ranged from 25 cm/s at 10 m 

to 10 cm/s at 200 m. 

During Cruise 3b, August 2002, the central GoM was nearly devoid of energetic circulation 

features, as evidenced by a nearly flat SSH field (Figure 8-28). The dominant feature was the 
113 

Loop Current, which extended slightly further northward than in June 2002. Currents in the Loop 

Current exceed 180 cm/s to 50 m depth and 120 cm/s to 150 m (Figure 8-29). The standard 

deviation of currents while in the Loop Current was nearly constant with depth at about 13 cm/s. 

Record-length mean currents by depth level for this cruise decay from about 30 cm/s at the 

surface to about 10 cm/s at 200 m. 

Figure 8-28. Contours of sea-surface height from satellite altimeter data and near-surface 

velocity vectors from shipboard ADCP during Cruise 3b. SSH data are from 8 

August 2002 and are courtesy of R. Leben (CU). 
114 

Figure 8-29. Vertical section of current speed based on shipboard ADCP data collected along a section in the southeas

3b in August 2002. This section was roughly taken as the cruise track passed through the Loop Current (see 
115 

8.3 Ocean Color Climatology 

Phytoplankton pigment concentration in the first optical depth in the deepwater GoM 

undergoes a well-defined seasonal cycle which is generally synchronous off the shelf, where the 

bottom is deeper than 200 m. Müller-Karger et al. (1991) reviewed monthly climatologies of 

near-surface phytoplankton pigment concentration from multiyear series of coastal zone color 

scanner (CZCS) images for the period 1978-1985. They reported that highest near-surface 

pigment concentration (dominated by chlorophyll concentration, CHL) occurs in offshore waters 

between December and February and lowest values occur between May and July. They also 

reported there is only about a 3-fold variation between the lowest (~0.06 mg m-3) and highest 

(0.2 mg m-3) deepwater near-surface pigment concentration. Model simulations show that the 

single most important factor controlling the seasonal cycle in near-surface CHL concentration is 

the depth of the mixed layer (Walsh et al. 1989). Müller-Karger et al. (1991) concluded that 

because of this dependence, annual cycles of algal biomass are one or more months out of phase 

relative to the seasonal cycle of sea surface temperature. 

Müller-Karger et al. (1991) extracted two multi-year time series of regional mean monthly 

variability in pigment concentration from the CZCS data for a western and an eastern deepwater 

GoM location. Each was averaged for a 200 km x 200 km area. Their western GoM location was 

centered at 25°N, 93°W in water depth > 3 km. Their eastern GoM location was also in deep 

water, centered just north of the Yucatan Channel at 24°N, 86°W and in the path of the Loop 

Current inflow. Because CZCS was an experimental sensor that was only turned on as 

prearranged, for at most a few minutes a day, and this led to sparse coverage in space and time, 

the CZCS time series available to Müller-Karger et al. was discontinuous for the some of the 7 

year period. Nevertheless, the means for any month at both western and eastern areas were 

clearly similar in amplitude and phase. Highest values (to 0.4 mg m-3) occurred in DecemberJanuary and lowest values (to 0.05 mg m-3) occurred in summer months. Subsequent study of 

CZCS imagery by Melo-Gonzalez et al. (2000), who compiled three-month averages from 

additional deepwater regions of the GoM and northern Caribbean Sea, reinforced the idea that 

seaward of the continental shelf, deepwater surface pigment concentration is lowest in springsummer (Julian Days 60-300) and highest November-Februrary. 

As we will show from analysis of the 1998-2000 ocean color data collected during the 

follow-on Sea viewing Wide-Field Sensor (SeaWiFS) mission, the 22 DGoMB stations over the 

continental slope in the northwestern GoM (north of 26°N and west of 91°W) show essentially 

the same annual cycle in magnitude and phase of CHL that was reported from the predecessor 

CZCS mission (Figure 8-30). However, at the 22 DGoMB stations over the continental slope in 

the northeastern GoM (north of 26°N and east of 91°W), there are often large biweekly 

variations in both magnitude and phase of the mean CHL concentration. We will show that these 

variations are correlated with the presence of mesoscale eddies that occur in the northeastern 

GoM, and we will cite results from the Northeastern Gulf of Mexico Chemical Oceanography 

and Hydrography companion MMS-funded program (NEGoM) as well as from Hu et al. (2003) 

that document this is because such eddies can entrain low salinity, high CHL Mississippi River 

water and transport this green water off shelf and into the deepwater of the eastern GoM. 

Table 8-1 summarizes the annual mean CHL at DGoMB stations for the period 1998-2000. 

This summary has been compiled three different ways: a) with quick-look (4-km resolution) data 

for 1998+1999; b) with 1-km resolution data for 1998-1999; and, c) with 1-km resolution data 
116 

Table 8-1 

Summary of Mean CHL at DGoMB Stations for Biweekly Time Series of 2 Years Versus 3 Years 

DGoMB 1998+1999 1998+1999 stdev/mean 1998+1999 1998+1999 stdev/mean 4 km v 1 km 1998-2000 1998-2000 

Station 4 km pixels 4 km pixels ratio column 3 1 km pixels 1 km pixels ratio column 6 ratio column 5 1 km pixels 1 km pixels ra

 5x5 pix mean 5x5 pix stdev to column 2 5x5 pix mean 5x5 pix stdev to column 5 to column 2 5x5 pix mean 5x5 pix stdev 

Western GoM

RW1 0.33 0.21 0.64 0.34 0.23 0.70 1.03 0.31 0.20 RW2 0.23 0.09 0.41 0.24 0.12 0.49 1.05 0.24 0.11 RW3 0.20 0.07 0.34 0.22 0.09 0.44 1.09 0.21 0.10 RW4 0.21 0.10 0.47 0.22 0.12 0.57 1.04 0.19 0.11 RW5 0.18 0.05 0.28 0.19 0.07 0.37 1.04 0.18 0.08 RW6 0.18 0.06 0.36 0.18 0.07 0.38 1.01 0.17 0.07 

AC1 0.18 0.07 0.38 0.18 0.07 0.40 0.97 0.17 0.08 

W6 0.17 0.06 0.33 0.17 0.05 0.30 0.98 0.17 0.05 W5 0.17 0.05 0.28 0.17 0.05 0.27 1.01 0.17 0.05 W4 0.19 0.06 0.31 0.18 0.06 0.32 0.96 0.18 0.06 W3 0.19 0.06 0.32 0.19 0.07 0.35 0.99 0.19 0.07 W2 0.21 0.07 0.35 0.21 0.08 0.38 0.99 0.20 0.08 W1 0.23 0.09 0.38 0.24 0.10 0.42 1.06 0.23 0.10 

WC5 0.23 0.09 0.39 0.23 0.10 0.42 1.00 0.22 0.10 

WC12 0.20 0.07 0.34 0.20 0.08 0.42 1.00 0.19 0.08 

B1 0.19 0.07 0.35 0.19 0.08 0.44 0.99 0.19 0.08 

NB2 0.18 0.06 0.34 0.19 0.07 0.38 1.01 0.18 0.07 NB3 0.18 0.06 0.33 0.18 0.07 0.37 1.00 0.17 0.06 

B2 0.18 0.05 0.31 0.17 0.06 0.32 0.96 0.17 0.06 

NB4 0.17 0.04 0.25 0.17 0.05 0.31 1.00 0.17 0.05 

B3 0.18 0.06 0.33 0.17 0.06 0.36 0.97 0.17 0.06 

NB5 0.18 0.06 0.33 0.18 0.06 0.36 1.00 0.18 0.07 

mean for W 0.20 0.07 0.35 0.20 0.08 0.41 1.01 0.19 0.08 
117 

Table 8-1 Summary of Mean CHL at DGoMB Stations for Biweekly Time Series of 2 Years Versus 3 Years (continued) 

DGoMB 1998+1999 1998+1999 stdev/mean 1998+1999 1998+1999 stdev/mean 4 km v 1 km 1998-2000 1998-2000 

Station 4 km pixels 4 km pixels ratio column 3 1 km pixels 1 km pixels ratio column 6 ratio column 5 1 km pixels 1 km pixels ra

 5x5 pix mean 5x5 pix stdev to column 2 5x5 pix mean 5x5 pix stdev to column 5 to column 2 5x5 pix mean 5x5 pix stdev 

Eastern GoM 

C12 0.18 0.06 0.36 0.19 0.15 0.79 1.07 0.20 0.15 C14 0.20 0.09 0.45 0.20 0.11 0.53 1.02 0.19 0.10 

C4 0.24 0.16 0.65 0.23 0.11 0.47 0.95 0.22 0.11 C7 0.34 0.47 1.39 0.30 0.34 1.11 0.89 0.28 0.29 C1 0.94 1.95 2.08 0.63 1.08 1.71 0.67 0.50 0.90 

S36 0.87 1.37 1.58 0.65 0.73 1.12 0.75 0.56 0.63 S37 0.43 0.42 0.96 0.47 0.49 1.04 1.09 0.42 0.45 S38 0.24 0.10 0.44 0.24 0.12 0.52 1.01 0.25 0.14 S35 0.60 0.70 1.17 0.64 0.76 1.18 1.08 0.57 0.64 S44 0.37 0.28 0.75 0.52 0.58 1.10 1.42 0.46 0.53 S43 0.39 0.44 1.13 0.36 0.39 1.08 0.92 0.32 0.33 S42 0.28 0.25 0.88 0.29 0.25 0.88 1.01 0.28 0.25 S41 0.24 0.15 0.63 0.26 0.22 0.85 1.08 0.30 0.44 S40 0.22 0.08 0.39 0.22 0.10 0.44 1.01 0.22 0.12 S39 0.21 0.07 0.33 0.21 0.11 0.52 0.99 0.21 0.10 

MT6 0.19 0.06 0.31 0.18 0.07 0.36 0.99 0.19 0.08 MT5 0.25 0.19 0.78 0.26 0.23 0.88 1.04 0.24 0.20 MT4 0.62 1.45 2.34 0.44 0.62 1.41 0.71 0.41 0.54 MT3 1.42 2.58 1.82 0.84 1.13 1.34 0.59 0.75 1.03 MT2 3.68 5.42 1.47 2.60 3.29 1.27 0.71 2.13 3.00 MT1 4.91 6.58 1.34 2.80 3.58 1.28 0.57 2.47 3.57 

HiPro 0.85 1.41 1.66 0.76 1.10 1.44 0.89 0.66 1.02 

mean for E 0.80 1.10 1.04 0.60 0.71 1.18 0.75 0.54 0.66 
118 

for 1998-2000. Several patterns are evident: (1) means are quite similar for 4-km resolution data 

and for 1 km resolution data; (2) means are quite similar whether the record length is 2 years 

(1998+1999) or 3 years (1998+1999+2000); (3) means are higher at DGoMB stations east of 

91°W than at stations in the western GoM. 

Specifically, note that (1) Column 8 of Table 8-1 (the ratio of means for 2 years of 1 km 

resolution data divided by 2 years of 4 km resolution data) averages close to 1.0, especially for 

stations in the western GoM (range just 0.96-1.09); (2) the last column of Table 8-1 (the ratio of 

means for 3 years of 1 km resolution data divided by 2 years of 1 km resolution data) also 

averages close to 1.0, especially for stations in the western GoM (range just 0.89-1.01); (3) mean 

CHL at the 22 stations north of 26oN and west of 91oW in the western GoM averaged 0.19 mg 

m-3, but mean CHL averaged 0.54 at the 22 stations north of 26o

N and east of 91oW in the 

eastern GoM. After review of the summary results in Table 8-1, we stopped averaging after just 

3 years of data from DGoMB stations in the western GoM in order to compute the annual mean 

CHL. But we continued to average additional data (Oct 1997 – Feb 2001) for stations in the 

eastern GoM, and also to produce composite monthly average ocean color scenes, to better 

describe the inter-annual variability at DGoMB stations there. 

The coefficient of variation (CV) statistics that are summarized in Columns 4, 7, and 12 of 

Table 8-1 present the ratio of standard deviation to mean CHL for the 2-year or 3-year time 

series constructed from biweekly means, in contrast to the CV statistics for 5x5 or 9x9 pixel 

averages that are given in Table 8-2. Note that CV statistics are generally low (usually <50%) for 

stations in the western GOM but that for stations in the eastern GoM these average two-fold 

higher (about 1.0). In other words, variance is rather evenly distributed about the mean (CV << 

1.0) in the western GoM, but variance is more randomly distributed about the mean (CV ~ 1.0) 

in the eastern GoM. 

Additional details of the seasonal cycle of CHL in western and eastern GoM are summarized 

as Figures 8-30 to 37. Figure 8-30 shows the seasonal cycle of CHL at the nine deepest of the 

“Deepwater Stations” in the western GoM (RW6, AC1, W5, W6, NB5, B1, NB4, B2, and B3). 

Water depth at each of these nine stations was greater than 2 km. The top panel in Figure 8-31 

shows the average biweekly CHL at each of the nine stations for the three years 1998-2000, 

while the bottom panel graphs the average biweekly concentration (averaged for the three years). 

In the top panel, the 3-year time series were fit station-by-station using spline fits. In the bottom 

panel, the annual cycle has been summarized by a quadratic fit to the averages of 3 years of 

biweekly data at the westernmost deepwater station, RW6. Both of these visualizations show the 

pattern previously reported from analysis of the CZCS archives by Muller-Karger et al. (1991) 

and by Melo-Gonzalez et al. (2000): deepwater CHL is lowest in spring-summer (Julian Days 

60-300) and highest November-February. 

Figures 8-32 to 8-34 summarize the annual cycle of CHL at the Far Western stations (RW1-

RW6), Western Stations (W1-W6), and Louisiana Slope Stations. Apart from an occasional 

biweekly higher CHL at the two shallowest stations on RW and W transects, the annual cycle 

was the same as that shown in Figure 8-31: deepwater CHL was lowest in spring-summer (Julian 

Days 60-300) and highest Nov-Feb. From Table 8-1, note the annual grand mean average CHL 

was about 0.2 mg m-3 at the 4 deepest stations on the RW transect (range 0.17-0.21), and at the 4 

deepest stations on the W transect (range 0.17-0.19), and at AC1 and the nine Louisiana Slope 

Stations (range 0.17-0.22). From Figures 8-32 and 8-33, note that periods when biweekly CHL 

locally exceeds 0.4 mg m-3 at RW1, RW2, W1, and W2 do not occur only in NovemberFebruary, but vary from year to year. 
119 

Table 8-2 

 

Variability of StdDev/Mean Ratios for Time Series of Biweekly Mean CHL with 

Different Pixel Resolution and Grid Areas 

4 km resolution 1 km resolution 1 km resolution 

5x5 grids 5x5 grids 9x9 grids 

(287 km^2) (18 km^2) (57 km^2) 

 1998+1999 1998+1999+2000 1998+1999+2000 

22 stations west of 91W 

RW1 0.238 0.086 0.129 

RW2 0.147 0.094 0.108 

RW3 0.143 0.085 0.099 

RW4 0.162 0.086 0.123 

RW5 0.161 0.089 0.143 

RW6 0.158 0.095 0.104 

AC1 0.157 0.108 0.134 

W6 0.192 0.084 0.096 

W5 0.146 0.082 0.093 

W4 0.142 0.089 0.155 

W3 0.145 0.078 0.095 

W2 0.194 0.070 0.094 

W1 0.186 0.093 0.104 

WC5 0.149 0.084 0.090 

WC12 0.157 0.092 0.103 

B1 0.150 0.090 0.101 

NB2 0.142 0.084 0.093 

NB3 0.137 0.082 0.090 

B2 0.157 0.090 0.097 

NB4 0.172 0.083 0.092 

B3 0.186 0.079 0.087 

NB5 0.152 0.084 0.095 

grand mean CV in W 0.162 0.087 0.106 

22 stations east of 91W 

C12 0.170 0.134 0.173 

C14 0.181 0.093 0.113 

C4 0.515 0.098 0.136 

C7 0.445 0.116 0.169 

C1 0.512 0.174 0.315 

S36 0.569 0.224 0.221 

S37 0.344 0.202 0.294 

S38 0.197 0.129 0.137 

S35 0.355 0.267 0.267 

S44 0.190 0.274 0.313 

S43 0.463 0.129 0.170 

S42 0.274 0.177 0.402 

S41 0.161 0.303 0.294 

S40 0.177 0.148 0.166 

S39 0.198 0.130 0.234 

MT6 0.168 0.095 0.136 

MT5 0.242 0.244 0.207 

MT4 1.004 0.259 0.294 

MT3 0.753 0.204 0.343 

MT2 0.502 0.206 0.279 

MT1 0.534 0.137 0.243 

Hi-Pro 0.500 0.319 0.253 

grand mean CV in E 0.379 0.185 0.235 
120 

East of 91°W, the “typical” deepwater annual cycle in CHL was often swamped by unusually 

high CHL, especially during summertime. Figures 8-35, 8-36, and 8-37 show that high surface 

CHL in summertime was evident at the 3 stations along the Eastern Transect (S44, S43, S42), at 

5 of the 6 stations on the Mississippi Trough Transect (MT1-MT5), and at the 3 stations farthest 

upslope along the Central Transect (C1, C7, C4). High summertime CHL is evident, as well, at 3 

of the stations farthest upslope along the DeSoto Canyon Transect (S35, S36, S37). Because of 

this, the annual mean CHL concentration at these eastern GoM stations averaged 0.54 rather than 

0.19 mg m-3 (Table 8-1). 

The annual cycle of CHL at DGoMB stations in the western GoM and at the deepest stations 

(water depth > 2 km) in the eastern GoM shows the high-in-winter, low-in-summer pattern 

previously reported from analysis of the CZCS ocean color data archives by Müller-Karger et al. 

(1991). At DGoMB stations in water depths of 300-1,800 m east of 91°W, though, the typical 

“deepwater” annual cycle in CHL is swamped by unusually high summertime CHL. A 

combination of remote sensing altimetry data and hydrographic data collected from R/V Gyre in 

support of the Northeastern GoM Chemical Oceanography and Hydrography program (NEGoM) 

has provided the explanation why this occurs. The altimetry data show that in summers 1998, 

1999, and 2000, warm slope eddies (WSEs) were centered over the deepwater of the DeSoto 

Canyon (Figure 8-38). In general, anticyclonic eddies with SSH > 25 cm are considered to be 

strong WSEs, while those with SSH of just 10-20 cm are considered to be weaker WSEs. A 

strong WSE that was centered south of 28°N in summer 1999, as well as less strong WSEs 

located farther north on the slope in summers 1998 and 2000, all acted to entrain low salinity, 

high CHL “green water” from the Mississippi River and transport this green water plume 

seaward into deepwater. As a result, high surface CHL in summertime was evident at most of the 

DGoMB stations on the Mississippi Trough Transect (MT1-MT5), and at the three stations 

farthest upslope along the Central Transect (C1, C7, C4), and at the three stations farthest 

upslope along the DeSoto Canyon Transect (S35, S36, S37). High summertime CHL was 

evident, as well, at three of the stations along the Eastern Transect (S44, S43, S42). 

That WSEs in deepwater of the eastern GoM do indeed entrain low salinity Mississippi River 

water and transport it off-margin has been verified by Hu et al. (2003) and by Belabbassi et al (in 

review), from analysis of near-surface salinity data collected during R/V Gyre NEGoM cruises in 

summers 1998, 1999, and 2000. By carrying out high performance liquid chromatography 

(HPLC) analyses of the phytoplankton pigments collected from R/V Gyre in these low salinity 

plumes during these same cruises, Qian et al. (2003) verified this low salinity water had locally 

high CHL standing stocks. 

We caution, though, that the results (CHL climatology, seasonal variations, etc.) presented 

here should be viewed in a relative way only, especially the statistics (standard deviations, CVs), 

for several reasons. First, although the accuracy in the SeaWiFS determined CHL (SeaDAS 

version 2) is relatively high and acceptable (Hu et al. 2003) for deep stations without river plume 

interference, CHL tends to be overestimated in shallow water coastal areas (depth < 30 m) due to 

the presence of terrestrial runoff (particles + dissolved matter) under river influence. Second, the 

statistics were performed on the composite data over periods of two-weeks to one month, not on 

the original daily data, which often experienced significant cloud cover. For example, for a pixel 

with 14 valid data values from the 2-week period, only the average value was used in the 

composite data and its weight in the statistics would be the same as another pixel with only 1 

valid data value from the 2-week period. 
121 

Figure 8-30. Three-year climatology of biweekly averaged SeaWiFS ocean color data 1998-

2000 at the nine DGoMB stations in water depth > 2,000 m in the western GoM. 

0 28 56 84 112 140 168 196 224 252 280 308 336 364

0.0

0.1

0.2

0.3

0.4

0.5

0.6

AVG RW6

AVG AC1

AVG W6

AVG W5

AVG NB5

AVG B1

AVG NB4

AVG B2

AVG B3

DGoMB Deepwater Stations 95W - 91W

Julian Day

Remotely Sensed CHL (mg/m^3)

Quadratic fit to averages of 3 yrs of RW6 biweekly data is:

CHL = 0.32408 - 2.3104e-3x + 6.0552e-6x^2; r^2 = 0.882

Data points are averages of 3 yrs biweekly means, 1998-2000

Figure 8-31. Average annual cycle of chlorophyll at these deepwater stations in the western 

GoM, illustrating the typical annual range from summertime lows of about 0.1 mg 

m-3 to November-February highs of about 0.3 mg m-3. 
122 

3643363082802522241961681401128456280

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Deepwater Station RW6

Julian Day

Remotely Sensed CHL (mg/m^3)

Quadratic fit to means of 3 years of biweekly data is:

0.324 - 2.31e^-3x + 6.06e^-6x^2; r^2 = 0.883.

Filled squares = biweekly means; whiskers = std deviation.

Figure 8-32. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean color 

data 1998-2000 at the six DGoMB stations along the Far West (RW) transect in 

the western GoM. Bottom panel: The annual cycle of chlorophyll at RW-6, the 

deepest of the six Far West stations. 

0 26 52 78 104 130 156 

0.0 

0.5 

1.0 

1.5 

RW1

RW2

RW3

RW4

RW5

RW6

Far West Transect

Three years of SeaWiFS imagery, 1998-2000

(as consecutive biweekly means from 1 Jan 98)

Remotely Sensed CHL (mg/m3

)
123 

Figure 8-33. Top panel: Three-year climatology of biweekly averaged SeaWiFS 

ocean color data 1998-2000 at the six DGoMB stations along the 

Western (W) transect in the GoM. Bottom panel: The average 

annual cycle of chlorophyll at four of the Western stations. 

0 28 56 84 112140168196224252280308336364

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

AVG W6

AVG W5

AVG W4

AVG W3

DGoMB Western Transect

Julian Day

Remotely Sensed CHL (mg/m^3)

Quadratic fit to averages of 3 yrs of W6 biweekly data is:

CHL = 0.25771 - 1.2632e-3x + 3.1461e-6x^2; r^2 = 0.767

Data points are averages of 3 yrs of biweekly means, 1998-2000 
124 

0 28 56 84 112 140 168 196 224 252 280 308 336 364

0.0

0.1

0.2

0.3

0.4

0.5

0.6

AVG WC12

AVG B1

AVG B2

AVG NB3

AVG NB4

AVG B3

AVG NB5

DGoMB Louisiana Slope stations

Julian Day

Remotely Sensed CHL (mg/m^3)

Quadratic fit to averages of 3 yrs of WC12 biweekly data is:

CHL = 0.37146 - 2.3698e-3x + 5.6443e-6x^2; r^2 = 0.795

Data points are averages of 3 yrs of biweekly means, 1998-2000

Figure 8-34. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean color 

data 1998-2000 at the nine WC, NB, and B DGoMB stations located between 

93W and 91W over the Louisiana continental slope in the western GoM. 

Bottom panel: The average annual cycle of chlorophyll at the seven deepest of 

these Louisiana slope stations. 

0 26 52 78 104 130 156 

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

WC5

WC12

NB2

B1

B2

NB3

NB4

B3

NB5

Lousiana Slope Stations

 Three years of SeaWiFS imagery, 1998-2000

(as consecutive biweekly means since 1 Jan 98)

Remotel

y Sensed CHL (m

g/m^3)
125 

0 26 52 78 104 130 156

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

S44

S43

S42

S41

S40

S39

Eastern Transect

 Three years of SeaWiFS imagery, 1998-2000

(as consecutive biweekly means from 1 Jan 98)

Remotely Sensed CHL (mg/m^3)

Figure 8-35. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the six DGoMB stations along the Easternmost (S) 

transect in the eastern GoM. Bottom panel: The annual cycle of chlorophyll 

at S39, the deepest of the Eastern transect stations. 

0 28 3082802522241961681401128456 336 364

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Deepwater Station S-39

Julian Day

Quadratic fit to means of 3 years of biweekly data is: 

CHL = 0.291 - 1.02e^-3x + 2.34e^-6x^2; r^2 = 0.133.

Filled squares = biweekly means; whiskers = std deviation.

Remotely Sensed CHL (mg/m^3)
126 

0 26 52 78 104 130 156

0

2

4

6

8

10

12

14

16

18

20

MT6

MT5

MT4

MT3

MT2

Mississippi Trough Transect (MT)

 Three years of SeaWiFS imagery, 1998-2000

(as consecutive biweekly means from 1 Jan 98)

Remotely Sensed CHL (mg/m^3)

Figure 8-36. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at five DGoMB stations along the Mississippi Trough 

(MT) transect in the eastern GoM. Bottom panel: The annual cycle of 

chlorophyll at MT6, the deepest of these Mississippi Trough transect 

stations. 

363330282522191614118520

0.

0.

0.

0.

0.

0.

0.

0.

0.

Deepwater Station MT-6

Julian Day

Remotely Sensed CHL (mg/m^3)

Quadratic fit to means of 3 years of biweekly data is: 

Filled squares = biweekly means; whiskers = std deviation 

CHL = 0.309 - 1.75e^-3x + 4.55e^-6x^2; r^2 = 0.667. 
127 

0 26 52 78 104 130 156

0

1

2

3

4

5

6

"HiPro" Station

 Three years of SeaWiFS imagery, 1998-2000

(as consecutive biweekly means from 1 Jan 98)

Remotely Sensed CHL (mg/m^3)

Figure 8-37. Top panel: Three-year climatology of biweekly averaged SeaWiFS ocean 

color data 1998-2000 at the five DGoMB stations along the central (C) 

transect in the eastern GoM. Bottom panel: Three-year climatology of 

biweekly averaged SeaWiFS ocean color data 1998-2000 at the high 

productivity (Hi-Pro) station in the eastern GoM. 

0 2 5 7 10 13 15

0.

0.

1.

1.

2.

2.

3.

3.

C1

C7

C4

C14

C12

Central Transect 

(C)

Remotely Sensed CHL 

(mg/m^3)

Three years of SeaWiFS imagery, 1998-2000 

(as consecutive biweekly means from 1 Jan 98)
128 

Colored dissolved organic matter (CDOM) fluorescence and light absorption coefficient were 

also measured along with surface salinity and chlorophyll during the R/V Gyre NEGoM cruises 

conducted in 1998-2000. Strong correlations between the CDOM absorption coefficient (ag443) 

and CDOM fluorescence (330 nm excitation and 450 nm emission filters) were consistently 

observed (Hu et al. 2003; Nababan et al. in review). Relatively high ag443 (≥0.1 m-1) was 

generally observed inshore (inner shelf) near the major river mouths, but this ag443 decreased 

rapidly with distance from shore except when riverine waters were entrained and transported 

offshore by WSEs. Because CDOM as well as CHL contributes to the ocean color signal 

measured by SeaWiFS, we likely over-estimated CHL at some of the shallowest DGoMB 

stations (Hu et al. 2003). Specifically, the CHL spikes > 4 mg m-3 shown in Figure 8-36 for 

stations MT1 and MT2 close off the Mississippi River Delta may be overestimated by perhaps 

50-100%. However, the relative synoptic patterns as well as the relative temporal variation 

patterns should remain valid. 

8.4 Geology 

Two hundred and forty-five cores, 30 to 35 cm in length, were recovered from 43 stations 

during the survey in 2000. Each core was analyzed at 1 cm intervals for bulk density, porosity, 

water content, p-wave velocity, void ratio, and impedance. That analysis resulted in 48,500 

individual measurements of geotechnical properties available in the database. All cores were 

split and photographed and shear strength measurements were attempted but the surficial 

sediments have shear strengths lower than our instruments could measure. Grain size was 

determined from the subsample at the sediment-water interface. 

8.4.1 Sediment Texture—Grain Size Distributions 

The sediments of the deep northern GoM were found to be composed primarily of silt and 

clay, with variable fractions of sand (Figure 8-39 and Figure 8-40), in general agreement with 

previous studies (Balsam and Beeson, 2003). The sand fraction is derived almost entirely of 

planktonic foram tests from the overlying water column and ranges from 0.5 up to 63% by 

weight. The percent sand is reduced in the sediments of the upper continental slope nearshore in 

the western GoM because of the high loading of Mississippi River-derived material. The lowest 

value was at MT1 in the Mississippi Canyon head that is notched into the continental shelf just 

off the river’s debouchment into the Gulf’s open waters. The percent sand was also somewhat 

lower at the abyssal plain sites because of carbonate dissolution due to cold and pressure. The 

highest sand fractions were at intermediate depths (1 km to 3 km) of the continental slope, at 

sites in the eastern GoM (S sites, 29.8%, σ=16.1, n=7), whereas generally low values were 

measured along the entire RW transect in the extreme west of the study area (4.9%, σ=2, n=5). 

The sediments west of DeSoto Canyon can be considered terrigenous lutite with some carbonate 

sand, whereas the sediments east of the axis of DeSoto Canyon can be considered primarily 

foram-derived carbonate composed of more or less equal fractions of sand, silt and clay. This 

east-west pattern agrees with the “weight % carbonate” distribute map of Balsam and and 

Beeson (2003). The central GoM is dominated by the Mississippi Fan or Cone of sediments that 

extends seaward and downslope from the delta region. The four shallow sites sampled along the 

axis all had some of the lowest sand fractions (1.48%, σ=0.95, n=5), but the two deepest were 

some of the highest values (23.4%, σ=17, n=4). The “basins” on the “Texas Louisiana Slope” 

had less sand than non-basin sites (3.1 vs. 9.5%), but, as will be seen in later sections (8.8 on 

“benthic biology” in particular), there was no biological evidence that basins were different from 
129 

Figure 8-38. Sea-surface height (SSH) maps compiled by 

the University of Colorado, showing the 

location of warm slope eddies (positive SSH 

anomalies) in the eastern GoM in summers 

1998-2000. Dots mark the location of 

hydrographic stations occupied on each of 

three summertime NEGoM cruises. Top panel: 

Cruise N3 (25 Jul-9 Aug 1998); Middle panel: 

Cruise N6 (15-28 Aug 1999); Bottom panel: 

Cruise N9 (29 Jul-8 Aug 2000). All from 

Belabbassi et al (in review). 
130 

Figure 8-39. Percent sand, silt and clay at each site. 
131 

Figure 8-40. Relative amounts of percent sand at each site. 
132 

adjacent non-basin habitats. The lone area where material larger than sand-sized was 

encountered was in the “iron stone” region of the mid-to-lower Mississippi Fan (MT 5 and 6), 

where cobble-sized reddish-colored sedimentary rocks were common features in box cores, 

bottom photographs and trawls. 

8.4.2 Sediment Physical Properties 

An example of the physical properties is illustrated in Figure 8-41. All five cores at Location 

NB3 had very similar measurements between the geotechnical properties and depth within the 

cores. In contrast to that uniformity at NB3, the cores recovered at C7 display a wide variation in 

properties with depth within each core (Figure 8-42). The location of sites that have large 

variations in geotechnical properties between cores taken at the same location are shown in 

Figure 8-43. The uniformity in shear strength, porosity and composition suggest that deposition 

has been uniform and continuous with little alteration by exogenous processes. On the other 

hand the wide spread in such properties indicates that deposit over time has been altered or 

interrupted by biological or physical processes over time. Resuspension and redeposition are a 

possibility as is biological reworking, as noted below. 

8.4.3 Indications of Bioturbation 

All cores were examined for the degree of bioturbation that the sediments had experienced. 

Figure 8-44 is a photograph of a split core taken at W4, where the sediment had undergone very 

little to no bioturbation. In contrast, the core from MT5 (Figure 8-45) illustrates a high degree of 

bioturbation. A core from Location RW5 (Figure 8-46) illustrates sediment that has undergone 

slight to moderate bioturbation. Figure 8-47 illustrates the bioturbation encountered at all 

locations in cores from the year 2000 survey. There was a very distinct and significant boundary 

between sediments consisting of slight to no bioturbation and those that were highly bioturbated. 
133 

Figure 8-41. The sediment geotechnical properties of cores (5 replicates) taken at the NB3 location. 

Note the compact arrangement of the various properties with depth in the core that indicate 

uniform physical properties. 
134 

Figure 8-42. The sediment geotechnical properties of cores (5 replicates) taken at the MT6 location. Note 

the dispersed properties with depth that indicate a wide variety of sediment types at location 

MT6. 
135 

Figure 8-43. Map showing the locations of the stations that have highly variable geotechnical properties. 
136 

Figure 8-44. Boxcore 134 at station W4 illustrating 

sediment that has almost no sediment 

bioturbation. 
137 

Figure 8-45. Boxcore 12 at station MT5 illustrating sediment 

that has undergone a high degee of 

bioturbation. 
138 

Figure 8-46. Boxcore 111 at station RW5 illustrating a low 

degree of bioturbation. 
139

Figure 8-47. Map showing the degree of sediment bioturbation present at the survey sites. 
140 

8.5 Biogeochemistry 

The geochemistry component measured a range of relevant sedimentary parameters at both 

survey and experimental sites. Surface sediments were analyzed at the survey sites and detailed 

profiles were measured at experimental sites. Various geochemical properties were measured on 

pore waters and solid phases. Analyses that were best performed immediately after sample 

retrieval were conducted onboard, whereas other analyses, which require more time consuming 

methods, were performed at shore-based facilities.

8.5.1 Bulk Sediment Organic Carbon and Nitrogen 

The percent organic carbon data for survey stations are shown in Figure 8-48. The highest 

values were measured on the Florida slope region, but there were no obvious trends with either 

water depth or across an east-west gradient. Values were in the range reported for oceanic 

sediments in non-upwelling regions. Similarly, the C:N data, which ranged from <7 to 10 (Figure 

8-49), did not display uniform geographical trends. The values measured, however, were in the 

range reported for northern Gulf of Mexico shelf sediments (Bianchi et al., 2002; Gordon and 

Goni, 2003) and suggestive of predominantly marine organic matter. The isotopic data discussed 

for stations MT1, MT2 and MT3 support this observation. 

Figure 8-48. Percent organic carbon data for survey stations. 
141 

Figure 8-49. Sediment organic carbon and nitrogen. Top: three sets of organic carbon data from, UNAM 

(abyssal plain) and TAMU (continental margin); bottom: two sets of total sedimentary 

nitrogen data, analyzed at UNAM (yellow) and TAMU (red). 

8.5.2 Sediment Profiles of Organic Carbon 

Profiles of organic carbon were generated for the process sites (Yaeger et al. 2004), as 

provided in (Figures 8-50 to 8-55). They illustrate that maximum values of approximately 1.5% 

were consistently encountered in the Mississippi Canyon (MT3, 1,000 m depth) surface 

sediments, which declined to ca. 1% at about 10 to 15 cm depth into the sediments. In the 

DeSoto Canyon, in somewhat deeper water (1,850 m), the values were slightly less in the top 

layers (0.8 to1.2%). These too declined with depth into the sediments down to ca. 0.6 to 0.8%. 

The deep aspects of the Mississippi Canyon line (MT6, 2,750 m) presented no vertical pattern, 

with values ranging from 0.4 to 0.6%, and one deep outlier of 1.4%. [Recall in Section 8.4 that 

the deep half of the MT had highly variable physical properties, suggesting physical mixing, 

variable deposition and/or intense bioturbation.] On the abyssal plain at the western most process 

station (JSSD1 or S1) the values ranged from 0.1 up to 0.8%, but all the points together on 

average illustrate a decline from ca. 6% at the surface down to ca. 0.2 to 0.3% at a depth of 15 

cm. At the eastern and southern-most site (JSSD4, S4), the values had much the same range (1.5 

to 5%), but the values did not follow a consistent decline with depth. The maximum of 5% was 
142 

encountered at about 5 cm down into the sediment, with what appeared to consistent declines 

both above and below that. The last panel from the upper Florida slope (S42, 740 m depth) 

indicates a sharp decline from 8% at the surface down to 4.5% at less than 4 cm into the 

sediment, followed by no decline over the next 10 to 15 cm. 

Figure 8-50. Profiles of organic carbon. 

MT3 Organic Carbon

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20 25

Depth (cm)

Series1
143 

MT6 Organic Carbon

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25

Depth (cm)

% Organic Carbon

Series1

Figure 8-51. Profiles of organic carbon MT6. 

S36 Organic Carbon 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25

Depth (cm)

% Organic Carbon

Series1

Figure 8-52. Profiles of organic carbon S36. 
144 

JSSD1 Organic Carbon

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 5 10 15 20

Depth (cm)

% Organic Carbon

Series1

Figure 8-53. Profiles of organic carbon JSSD1. 

JSSD4 Organic Carbon

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25

Depth (cm)

% Organic Carbon

Series1

Figure 8-54. Profiles of organic carbon JSSD4. 
145 

Figure 8-55. Profiles of organic carbon S42. 

8.5.3 Dissolved Organic Carbon (DOC) 

Dissolved organic carbon (DOC) data for centrifuged samples from 10 sites are shown in 

Figure 8-56. Mean values ranged from 5 to 20 mgC L-1, and were well within the range reported 

for porewater concentrations at similar depths in North Carolina continental slope sediments by 

Alperin et al. (1999). The mean DOC concentrations were quite variable among these stations, as 

were the standard deviations. Values appeared to be lower at stations GKF and MT6, which were 

two of the deepest stations. There was, however, no obvious trend with water depth. 

The DOC data for six experimental stations are also shown in Figure 8-56. It must be noted 

that the concentrations measured in surface sediments by whole squeezing at stations MT3, 

MT6, S36, S42 were consistently higher than those measured for centrifuged samples (Figure 8-

56). Alperin et al. (1999) noted higher values for centrifuged samples compared with data 

acquired by sippers. Both comparisons suggest sample handling alters the DOC concentrations 

as has been reported previously (e.g., Martin and McCorkle 1993). Depth profiles for DOC 

(Figure 8-56) showed an increase with depth at all sites. The concentrations measured and trends 

observed with core depth were consistent with those reported for North Carolina continental 

slope sediments by Alperin et al. (1999).

S42

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 5 10 15 20

Depth (cm)

Series1
146 

0

0.5

1

1.5

2

2.5

3

MT1 Hipro S42 S41 S36 MT6 GKF C7 Bush

Hill

DOC (mM)

0

2

4

6

8

10

12

14

16

18

012345

DOC (mM)

Depth (cm)

MT3

S42

S36

MT6

S1

S4

Figure 8-56. Top: Site means and standard deviations (2σ) for dissolved 

organic carbon (DOC) in the top 5 cm of sediment. The 

filled circles are the concentration measured for the top 2 

cm of a sectioned core taken at the same location. Bottom: 

Pore water depth variations for DOC from experimental 

stations. 
147 

8.5.4 Dissolved Inorganic Carbon (including δ13C) 

Dissolved inorganic carbon (DIC) concentration and stable isotope ratios of DIC (δ13C-DIC) 

for centrifuged samples from 10 sites are shown in Figure 8-57 (middle and bottom), 

respectively. DIC concentrations were similar across stations and close to the 2.2 mM value 

expected for seawater. Close inspection suggests a decrease in DIC concentration with depth in 

the water column, but the standard deviations were too large to confirm this trend (Figure 8-58). 

With the exception of station MT1, the δ13C-DIC was in the range of 1‰ with no obvious trend 

with water depth. This value is close to what would be expected in the overlying water column. 

In contrast to the DOC data, only one station, MT3, had significant differences between 

centrifuged and whole squeezed samples. The same was observed for δ13C-DIC. It is possible 

that variability between cores at station MT3 were not due to porewater sampling technique. 

Consistent with what Alperin et al. (1999) reported for North Carolina slope sediments, DIC 

concentrations increased slightly with depth at all stations, excepting MT3. At this station, the 

depth profile was more complex. As DIC is a product of organic matter diagenesis, the trend of 

higher DIC concentration was expected as decomposition is taking place in the sediments. 

Further evidence for remineralization of organic matter was provided by the δ13C -DIC (Figure 

8-59 bottom; the value at station S42 may be an outlier.). Values became 13C-depleted with depth 

in the core as the organic matter (about –21‰) was remineralized. The isotopic data suggest the 

greatest amount of organic matter decomposition occurred at station MT3. 

-3

-2.5

-2

-1.5

-1

-0.5

0

0 0.5 1 1.5 2 2.5 3

Depth (km)

δ 13C-DIC

0.8

1

1.2

1.4

1.6

1.8

2

DIC (mM)

0

0.5

1

1.5

2

2.5

3

DOC (mM)

Figure 8-57. Site means and standard deviations (2σ) versus station 

water depth for dissolved organic carbon (DOC; top), 

dissolved inorganic carbon (DIC; middle) and stable 

carbon isotope ratio of DIC (δ

13C-DIC bottom) in the top 5 

cm of sediment. 
148 

0

0.5

1

1.5

2

2.5

3

MT1 Hipro S42 S41 S36 MT6 GKF C7 Bush

Hill

DIC (mM)

0

2

4

6

8

10

12

14

16

18

12345

DIC (mM)

Depth (cm)

MT3

S42

S36

MT6

S1

S4

Figure 8-58. Top: Site means and standard deviations (2σ) for dissolved 

inorganic carbon (DIC) in the top 5 cm of sediment. The filled 

circles are the concentration measured for the top 2 cm of a 

sectioned core taken at the same location. Bottom: Pore water 

depth variations for DIC from experimental stations. 
149 

0

2

4

6

8

10

12

14

16

18

-20 -15 -10 -5 0

δ 13C-DIC

Depth (cm)

MT3

S42

S36

MT6

S1

S4

-3

-2.5

-2

-1.5

-1

-0.5

0

MT1 Hipro S42 S41 S36 MT6 GKF C7 Bush

Hill

δ 13C-DIC

Figure 8-59. Top: Site means and standard deviations (2σ) for stable 

carbon isotope ratios of dissolved inorganic carbon (δ

13CDIC) in the top 5 cm of sediment. The filled circles are the 

concentration measured for the top 2 cm of a sectioned 

core taken at the same location. Bottom: Pore water depth 

variations for δ

13C-DIC from experimental stations. 
150 

8.5.5 Stable Isotope Ratios from Sediment-dwelling Bacteria 

Gas chromatography/combustion/isotope ratio mass spectrometry analysis of phospholipid 

fatty acids (PLFAs) was used at stations MT1, MT2 and MT3 to link organic matter (OM) 

sources with sedimentary bacteria (Boschker et al. 1999; Jones et al. 2003). Previously, it was 

reported that isotope ratios of PLFA i&a15:0 could be used as biomarker of bacterial isotopic 

ratios, while isotope ratios of PLFA 16:0 were more indicative of all living or recently senescent 

sources of organic matter (e.g., Jones et al. 2003). Correspondence between isotope ratios of 

sedimentary OM and PLFA 16:0 indicate living or recently senescent organic matter are the 

main sources of OM in sediments. In turn, correspondence between isotope ratios of sedimentary 

PLFA 16:0 and PLFA i&a15:0 suggest living or recently senescent organic matter was 

predominantly bacterial or both were using similar sources of organic matter. Finally, isotopic 

correspondence between PLFA i&a15:0 with that of sedimentary OM imply close coupling 

between these two pools. 

The stable carbon isotope ratio (δ13C) of sedimentary OM (Figure 8-60) was remarkably 

constant both among stations and down-core. Values were close to –21‰, which is similar to 

values reported for northern Gulf of Mexico shelf sediments (Bianchi et al. 2002; Gordon and 

Goni 2003). In contrast, the δ13C of PLFA 16:0 were more variable, which is partly indicative of 

the lower precision of the measurement, and significantly enriched in 13C. Moreover, the values 

were more 13C-enriched at the deeper stations. The source of organic matter with that level of 

13C-enrichment is not obvious; however, Ishihi et al. (2001) reported similar δ13C for a species of 

Sargassum. Admittedly, Ishihi et al. analyzed attached Sargassum and not the pelagic species 

that has been observed in deep Gulf of Mexico trawl samples and photographs. 

The trend for the δ13C of PLFA i&a15:0, the biomarker of bacterial biomass, was similar at 

all three MT stations. At the surface values were equivalent to that of PLFA 16:0 at station MT1, 

and in between those of PLFA 16:0 and sedimentary OM at stations MT2 and MT3. With depth 

the isotopic value of PLFA i&a15:0 approached that of sedimentary OM. This change with 

depth implies different food sources to the bacterial community along the sediment depth 

gradient. Until the source and behavior of the biomass contributing to the PLFA 16:0 in these 

sediments is identified, it is difficult to speculate on the behavior of the δ13C of the bacterial 

biomarker. 

8.5.6 Dissolved Oxygen 

Depth profiles for dissolved oxygen in pore waters were made using anodic stripping 

voltammetry microelectrodes. The primary objectives were to determine the oxygen gradient and 

“penetration” depth (OPD). The stations studied were MT3, MT6, S36, S42, and deep water 

stations S1, S2, S4 and S5. At the deep-water sites, heavy ship vibration during measurements 

made the microelectrode analyses very difficult and consequently the data should be viewed with 

caution. 
151 

δ 13C-TOC

δ 13C-i&a15:0

δ 13C-16:0

Depth (cm)

-15 -13 -11 -9 -7

-15 -13 -11 -9 -7

-15 -13 -11 -9 -7

Depth (cm) Depth (cm)

2

4

6

8

10

12

2

4

6

8

10

12

2

4

6

8

10

12

Figure 8-60. Stable carbon isotope ratio (δ

13C) of 

sedimentary organic matter, 

phospholipid fatty acid (PLFA) 16:0 

and i&a15:0 at stations MT1, MT2, and 

MT3. 
152 

Oxygen depth profiles for stations MT3, MT6, S36 and S42, are illustrated in Figure 8-61. At 

station MT3 the OPD was close to 0 mm. This is reasonable, given that dissolved manganese 

was observed to occur to within 1 mm of the sediment-water interface (see next section). The 

S36 and 42 stations had close to same OPD: 45 and 40 mm, respectively. Deep stations S1 and 

S2 had OPDs greater than the intervals measured (100 and 130 mm, respectively), whereas deep 

stations S4 and S5 had relatively shallow OPDs of roughly 30 and 12 mm, respectively. 

0 50 100 150 200

0

50

100

150

Dissolved O

2

 (μM)

Depth (mm)

Figure 8-61. Dissolved oxygen distribution with depth in stations MT3 (solid 

circle; note all values below detection limit), MT6 (open square), 

S36 (solid diamond), and S42 (X). 

8.5.7 Dissolved Manganese and Sulfate 

Dissolved iron and sulfide were not detectable at any of the experimental stations, nor were 

any changes in the dissolved sulfate to chloride ratios observed. 

Dissolved Mn2+ was observed only at station MT3 (Figure 8-62). At this station dissolved 

manganese exhibited a classic (e.g., Berner 1980) broad subsurface maximum from about 30 to 

80 cm, which averaged about 210 μM. This is the zone of major manganese reduction. Above 

this zone diffusive transport of manganese is the primary process and below this zone manganese 

is precipitated primarily as the carbonate mineral pseudokuntnahorite (MnCa(CO3)2; Mucci 

1988) These results indicate that, with the exception of station MT3, only oxygen and nitrate 
153 

were important electron acceptors for organic matter oxidation in the DGoMB study 

experimental sites. (Note data do not exist for station MT1, which is likely to have been at least 

as reducing as station MT3.) 

0 50 100 150 200 250

0

50

100

150

Mn2+(μM)

Depth (mm)

Figure 8-62. Dissolved Mn2+ at station MT3. Note the maximum from about 3 to 

8 cm depth and that dissolved manganese was detected to within 

~2 mm of the sediment-water interface. 

8.5.8 Sulfate Reduction Rates 

Sulfate reduction rates were below detection limits. This is consistent with the finding of no 

detectable decrease in dissolved sulfate at any stations during the 2000 fieldwork. Sulfate 

reduction was not measured at deep abyssal sites in 2002, based on the negative results at 

shallower stations in 2000 and 2001. The deep oxygen penetration depths at these sites indicate 

sulfate reduction is not likely to occur. 

It should be noted that although no detectable sulfate reduction was observed, it could be 

occurring at deeper sediment depths than those sampled (~20 cm). Lin and Morse (1991) 

observed reduced sulfur species on the GoM continental slope, but substantially deeper in the 

sediments. 
154 

8.5.9 Sediment Community Oxygen Consumption (SCOC) 

Sediment community oxygen consumption (SCOC) is generally accepted as the most 

resilient method of determining the total heterotrophic metabolism of the complete sedimentdwelling community. It is considered to be a reasonable estimate of gross input of reactive 

organic matter to any given site in the deep ocean (Jahnke, 1990). Two methods were employed 

in this project to estimate SCOC: in situ incubations in which benthic chambers were implanted 

with a remote lander and laboratory incubations aboard ship of small chambers retrieved from 

the box cores. 

Data from 11 measurements have been included, with two earlier studies added, for a total of 

13 SCOC values. The rates have been transformed from oxygen uptake per square meter per 

hour into units of mg C produced m-2 day-1 to agree with units in the food web carbon budget. 

This transformation assumes that the Respiratory Quotient is 0.85. [Thus, to transform the rates 

given back into O2 demand, divide by 0.85 moles CO2/moles O2 and multiply by 12 mg 

C/millimole CO2.] 

The mean rate at the 13 sites was 34.7 mg C m-2 d-1 (range of 4 to 91) at a mean depth of 1.7 

km (range of 500 to 3,650 m). The mean rate of the in situ measurements was less than that from 

the laboratory studies (21.9 vs. 49.7), partly because the in situ rates were from deeper sites than 

those incubated in the laboratory aboard ship. However, some of the difference in rates may be 

attributable to decompression of the laboratory cores, or some other unidentifiable variable 

altered by sample retrieval or incubation conditions. While rates using both approaches at S36 

were within the generally expected range of variation, the ship-board incubations were 

approximately two times the in situ rates at the two other sites where both approaches were used 

(S42 and MT3). 

All 13 rates have been plotted as a function of depth (Figures 8-63 and 8-64). The decline 

with depth is evident; an exponential rate was observed. 

0 1 2 3 4

0

20

40

60

80

100

Depth (km)

SCOC(mgC/m^2day)

y = 103.1* 10^(-0.365x) R^2 = 0.817

Figure 8-63. Sediment Community Oxygen Consumption 

(SCOC) expressed as carbon (see text for 

conversion details), plotted as a function of water 

depth. 
155 

The rates on the continental slope had a wide range of values, whereas those out on the 

abyssal plain were all in a narrow cluster. This reflects the wide range of community 

heterotrophic metabolism at the shallow sites versus the narrow clustering of very low rates out 

at the greatest depths far from coastlines where the input of organic matter is limited. The 

regression line allows us now to predict the mean rate at any depth, but this prediction is far 

more accurate at the deep sites than at those on the upper slope, using all the data. The precision 

can be improved by eliminating the shipboard incubations at all but the shallowest site, and a 

straight line provides a more reasonable estimate of variations in SCOC as a function of depth 

(Figure 8-64). The linear regression of the constrained data has an origin that is similar in 

magnitude to high rates on the continental shelf and in estuaries, suggesting that this regression is 

a better overall estimate of rates than the line generated from all the data. The regression lines 

and the rates at the individual sites can be used now to constrain the rates in the carbon budget 

and be compared with other methods used in this study to estimate carbon cycling at the process 

sites. 

0 1 2 3 4

0

10

20

30

40

50

60

Depth (km)

Sediment Community Oxygen Consumption (mg C/m^2-day)

y = 47.5 - 12.2x R^2 = 0.82

Figure 8-64. Sediment Community Oxygen Consumption (SCOC) as a function of 

depth. Respiration of oxygen expressed as carbon dioxide-carbon; see 

text for conversion details.All data from in situ incubations, with the 

exception of the shallowest site (MT1 at 500m). 
156 

8.5.10 Radiochemistry of the Sediments 

General: Primary data for all stations, including total 210Pb, 226Ra, 210Pbxs, 234Thxs, 238U, 

239,240Pu, %POC and porosity are given in the data report. 234Thxs data from station S36 are not 

available, as samples were counted too long after collection (t1/2 = 24 days). These data are used 

to calculate particle reworking (bioturbation), sedimentation and accumulation rates, discussed in 

detail below. The fallout radionuclides 137Cs and 7

Be were not resolved at sufficient activities in 

any core to be useful. 

Bioturbation rates: Bioturbation rates can be quantitatively derived from 234Thxs profiles as 

described by Santschi et al. (1980, 1999) and from the analysis of peak shapes of different tracer 

concentration profiles. 234Thxs profiles, which are calculated as the difference between 234Th 

(total) and supported 234Th (234Thsupp = 

238U), were used to determine particle reworking 

(bioturbation) rate coefficients, Db, shown in Figure 8-65 and listed in Table 8-3 along with 

mixing depths, inventories, macrofaunal densities and other station data. Assuming steady state 

conditions, Db is calculated as follows: 

[

234Thxs(z)] = [234Thxs(0)]exp(-(λ/Db)1/2z) (1) 

where λ = decay constant of 234Th (0.0288 day -1), and z = depth. Numerical mixing and 

sedimentation models discussed in Santschi et al. (2001) illustrate that the constant Db model is 

conservative in that it tends to over predict mixing rates at depth. 

234Thxs penetration depths are between 0.5 and 4 cm, and bioturbation coefficient values, Db, 

range from approximately 2 – 30 cm2

/yr. Because particle re-working or bioturbation rates are 

expected to decrease exponentially with depth, coincident with the depth distribution of benthic 

infauna (Stull et al. 1996), it is not likely that the depth of more intensively mixed sediment is 

much higher than that observed by 234Thxs. The longer “memory” of the fallout radionuclides 

210Pbxs and 239,240Pu do indicate deeper penetration depths than 234Thxs as summarized in Table 8-

3 for all but the deepest stations. This is not surprising. While active mixing is certainly 

confined to the upper few centimeters, as reflected by 234Thxs data, the relatively long-lived 

radionuclides 210Pbxs and 239,240Pu (t1/2 = 22, 24 x 103 and 6540 yr., respectively) combined with 

low sedimentation and accumulation rates and non-local episodic mixing events, results in 

penetration depths for these isotopes which are often considerably deeper. In deep water, such as 

for stations JSSD1 and JSSD4 (3,520, 3,560 m, respectively) there was very little 239,240Pu and 

the penetration depths for 234Thxs and 210Pbxs are essentially the same. 

Sedimentation and accumulation rates: According to the constant initial concentration (CIC) 

model, sedimentation rates can be calculated assuming steady state conditions and at relatively 

constant porosity, using:

[

210Pbxs(z)] = [210Pbxs(0)]exp(-αz) (2a) 

α = (λ/S) (2b) 

where [210Pbxs(z)] and [210Pbxs(0)] represent excess 210Pb concentration at depth z and at the 

sediment interface, respectively; S = sedimentation rate (cm/yr.); λ = decay constant of 210Pb 

(0.031 year-1). As sediment porosity was nearly constant over the depth intervals modeled for S, 

sediment accumulation rates were also determined using: 

SA = S [(1-φ)(ρ)] (3) 
157 

Table 8-3 

Summary Data and Modeling Results for All Stations 

(n/d or nd = no data, Db values determined using 234Thxs data. An upper and lower limit for S are listed 

this station due to a high error, resulting from few points available below the 239,240Pu penetration dept

this station.) 

Station Depth 

(m) 

210Pbxs

inventory 

(Bq/cm2) 

239,240Pu 

inventory 

(mBq/cm2) 

POC 

inventory 

(upper 5cm) 

(mg/cm2) 

Bioturbation 

coefficient 

(Db) 

(cm2/yr.) 

Bioturbation 

Depth (cm) 234Th

xs/210Pbx

s

Sedimentation 

rate (cm/yr.) 

S42 767 0.96 ± 0.07 1.66 ± 0.33 14.3 ± 0.5 6.6 ± 0.3 1.75/2 0.44 (0.26 – 

1.55)* 

MT3 985 3.48 ± 0.07 6.83 ± 0.82 21.5 ± 0.2 29.3 ± 8.2 3.25/15 0.07 ± 0.06 

S36 1849 1.81 ± 0.12 5.53 ± 1.1 20.2 ± 0.6 n/d nd/15.50 0.09 ± 0.05 

MT6 2737 0.35 ± 0.01 1.89 ± 0.45 11.8 ± 0.2 30.1 ± 33.7 3.25/12.50 0.10 ± 0.03 

JSSD1 3520 0.97 ± 0.28 0.07 ± 0.02 14.5 ± 1.2 2.7 ± 0.8 1.75/2 0.04 ± 0.01 JSSD4 3560 0.69 ± 0.21 0.02 ± 0.01 11.2 ± 0.5 32.8 ± 6.2 1.75/2 0.08 ± 0.03 
158 

Figure 8-65. 234Thxs ([234Th(total)] – [238U]) penetration into surface sediments at each station, 

allowing determination of particle reworking (bioturbation) rate (Db) and depth 

(zm), units are mBq/g. Db was calculated from the slope of ln[234Thxs(z)] = 

ln[234Thxs(0)] – (λ/Db)

1/2z. Errors in ln 234Thxs data are plotted for each point, but 

are often less than the size of the data symbol. 

where SA = sediment accumulation rate (g cm-2 yr.

-1), φ = porosity, i.e., sediment percent pore 

space and ρ = sediment bulk density (2.5 g cm-3). 

The CIC method also assumes that particle re-working rates are not significant over the depth 

interval of the 210Pb profile, and that fluxes of both sediments and 210Pb have been constant. 

234Thxs data were also used to determine the upper depth limit for particle mixing. All 210Pbxs data 

below this depth were then plotted in semi-log format (Figure 8-66); then only those 210Pbxs data 

below the lower limit mixing depth, as defined by the depth where significant 239,240Pu activities 

cease, were used to determine an upper limit for both sedimentation rates (S) and sediment 

accumulation rates (SA). While 239,240Pu data were not of sufficient resolution to accurately mark 

discrete temporal stratifications within the sediment profiles based on known high fallout years, 
159 

they were sufficient to illustrate depth intervals within the sediment profiles where local or nonlocal mixing was suspected (worm burrows, etc.). Inventories, mixing depths and modeled 

parameters are summarized for each station in Table 8-3. 

The ranges of apparent sedimentation and accumulation rates determined by this approach 

for these stations are 0.04 – 0.44 cm yr.-1 and 0.02 – 0.56 g cm-2

 yr.-1, respectively, showing that 

after approximately 1,000 m water depth, sedimentation and accumulation rates become 

essentially constant at approximately 0.08 cm/yr., and 0.04 g cm-2

 yr.

-1, respectively, within the 

error. Apparent sedimentation and accumulation rates could not be corroborated here through use 

of a second tracer radionuclide (137Cs or 239,240Pu). 239,240Pu profiles were of insufficient 

resolution at all stations to allow for identification of discrete peaks corresponding to high fallout 

years. In addition, the 1963 peak for either 137Cs or 239,240Pu would likely have been obliterated, 

as these apparent sedimentation and accumulation rates combined with the mixing depths evident 

at these stations would have placed those peaks in the strongly bioturbated upper few 

centimeters. No other data exist on sedimentation or accumulation rates at these GoM stations, 

either as part of this research effort or in the literature. In fact, reliable, published sedimentation 

rates for the GoM outside the immediate area of the Mississippi River Delta region are 

essentially non-existent in the literature. The estimates of sedimentation rates and sediment 

accumulation rates presented here do fall in line with rates from similar continental margin 

marine settings, including the northwestern Mediterranean Sea (0.01 – 0.60 cm yr.-1, Zuo et al.

1997), the eastern Arabian Sea (0.06 – 0.66 cm yr.-1, Somayajulu et al. 1999), the Bay of Biscay, 

northeastern Atlantic (0.06 – 0.4 cm yr.-1) and the middle Atlantic Bight (0.03 -0.04 cm yr.-1, 

0.02 – 0.05 g cm-2 yr.

-1 Anderson et al. 1994). 
160 

Figure 8-66. 210Pbxs ([210Pb (total)] – [210Pb (supported) = 226Ra]) profiles for all stations, all 210Pbxs data below the 234Thxs penetration depth is plotted. Sedimentation rates 

(S) were determined from the slope of ln[210Pbxs(z)] = ln[210Pbxs(0)] – (λ/S)z, 

using only those points below significant 239,240Pu activity (open circles). Errors 

in ln 210Pbxs data are plotted for each point, but are often less than the size of 

the data symbol. 
161 

8.6 Contaminant Chemistry 

8.6.1 Metals 

Location maps show that some of the sampling sites are within areas with numerous offshore 

drilling and production platforms. This includes areas around sampling sites C1, MT1, 2, 3 and 

WC5. On the other hand, many of the sampling sites were many kilometers from present-day 

intensive petroleum development. It has been previously shown (Boothe and Presley 1989) that 

significant metal contamination of sediments by off-shore drilling and production is restricted to 

within a few hundreds of meters of platforms. Therefore, it was not expected that sediments 

would be contaminated at the DGoMB stations. 

For this study, elements known to be major constituents of drilling fluids (barium [Ba], 

chromium [Cr]), as well as some of environmental concern, were determined. Metals were 

measured to assess potential effects on the benthos. In addition, aluminum, calcium, iron, and 

silicon were determined to facilitate recognition of sediment trace metal variations caused by 

natural (non-anthropogenic) differences in sediment texture and mineralogy. A total of 20 

elements were determined in all sediment samples. 

The data sets for all sampling periods agree well, in almost all cases, with only a few 

differences of more that 10% between individual data points. This, along with the good spike 

recoveries and good results for analysis of standard reference materials, gives confidence in the 

analytical results. Many of the trace elements were in low concentration in these samples, but all 

elements in almost all samples were above the detection limits of the methods used. Both Al and 

Fe were used to normalize trace metal concentrations, as they exhibited a strong covariance. 

Trace metal concentrations that fall above a best fit line through the data are of interest, 

assuming that most of the data are background values. 

Average concentrations of potential pollutant metals; Ag, Cd, Cu, Hg, Pb, and Zn; in the 

study area samples were similar to average crustal abundances and to average values for areas of 

the northern GoM thought to be low in pollutant metals. Average values may not, however, be 

the best way to evaluate this data because of the relatively small number of samples and the 

considerable variability in the data, some of which is due to the variable CaCO3 content of the 

samples. A better comparison is the slope of a best fit line through the data for each element on 

an element versus Al or Fe scatter plot to that expected for Mississippi River Delta material. The 

Mississippi River is the most likely source of most of the silicate fraction of the sediments at the 

study sites. 

When concentrations of several elements are plotted against Al (Figures 8-67 and 8-68), most 

data points not only lie near a best fit line but the line has almost the same slope as one drawn 

through data for Mississippi River Delta sediment. This is true for Be, Co, Cr, Fe, Si, Tl, V, and 

Zn (plus K and Mg not reported here) which have positive slopes on the metal versus Al plots 

and for Ca and Sr (not reported here) which have negative slopes. Thus, for these metals there is 

no indication of additions from human activity. Rather, the metal concentrations in a particular 

sample are determined by the relative amounts of Mississippi River derived silicate material and 

plankton-derived carbonate in the sediment. The picture is less clear for the other elements. Ni, 

Pb, Cd, As, and especially Cu concentrations, show more scatter on the metal versus Al plots 

than do the elements discussed above. Furthermore, the slopes suggest a general enrichment in 

these elements of 25 to 50% over Mississippi River derived material. 

A few points on each of the metal vs Al plots are also far enough off the general trend line to 

suggest possible human influence. On the other hand, Mn concentrations are more variable and 
162 

much more elevated over Mississippi Delta material than are those of any of the other elements. 

It is very unlikely that this Mn enrichment is due to human influence. Rather, it is due to 

remobilization of Mn from buried reduced sediment. The Mn then diffuses up through the 

sediment column and redeposits in near-surface sediment under oxidizing conditions. This 

phenomenon has been well documented for northern GoM sediments by Trefry and Presley 

(1982) and others. As noted above in the geochemistry section (8.5.7), the profile of Mn in the 

pore water suggests that MnO2 can be a quantitatively significant terminal electron acceptor in 

the remineralization of organic matter just below the sediment – water interface. At this point it 

is not possible to attach a rate to that process however. 

Other metals have been shown to undergo the same diagenetic remobilization process that 

affects Mn (e.g., Presley et al. 1992) and it seems likely that this process is at work in the 

DGoMB area. The fact that there is not a good correlation between Ni, Pb, Cd, and Cu 

concentrations and Mn does not mean that all have been unaffected by similar diagenetic 

processes. The situation is complicated by the intensity of reducing conditions, the relative 

amounts of available sulfide which would precipitate metals, and variable metal sulfide 

solubilities. In short, it seems likely that the somewhat anomalous Ni, Pb, Cd, and Cu 

concentrations in the DGoMB area are due to natural diagenetic and transport processes rather 

than to human activity. In any case, all of the concentrations are well below levels that would be 

expected to have harmful effects on organisms. 

In contrast to the elements discussed above, Barium (Ba) shows a few values that are higher 

than those of average crustal material and average clay-rich sediments but are typical of nearplatform sediments from the northern GoM. In the case of Ba, it seems likely that the 

enrichments, which are up to almost a factor of ten, are due to disposal of oil well drilling mud. 

Up to 90% of the dry weight of drilling muds can be Ba (Boothe and Presley 1985), so small 

amounts of mud could account for the Ba enrichments seen in the sediments. The Ba-enriched 

samples are from the three shallowest sites in the Mississippi Trough (sites MT1, 2 and 3) and 

from site C1 and WC5 just to the west of these. All are in an area of intense petroleum 

exploration and development. Previous studies have found even greater enrichments of Ba near 

some drilling platforms in the northern GoM. Barium concentrations of tens of thousands of ppm 

have been found in many samples (e.g., Boothe and Presley 1989). Even at the most Ba-enriched 

sites in the present study, the concentration is well below that thought to adversely affect 

organisms (e.g., Neff et al. 1989) 

In summary, the samples analyzed for this project were a mixture of carbonate and 

terrigenous silicate materials and thus varied considerably in aluminum, calcium, and iron 

concentrations. Trace metal concentrations also varied considerably, as would be expected with a 

varied mineralogy. Concentrations of the elements Al, Ca, Sr, Na, K, Mg, Be, Co, Hg, Cr, Ti, V, 

and Zn were within the ranges expected for coastal GoM sediments with equivalent Fe 

concentration. Copper, As, Cd, Pb, Ni, and especially Mn concentrations were found to be 

somewhat higher than those in Mississippi Delta and shallow GoM samples but were not 

unexpectedly high considering the deeper water from which the present samples came. The 

enrichments are almost certainly due to complex natural transport and diagenetic processes, not 

to human activity. The strong Ba enrichments of up to almost a factor of ten in a few samples are 

suspected to be due to the presence of residues of oil well drilling muds discharged from the 

many drilling platforms in the area. Based on literature data, none of the metal concentrations in 

the study area sediments is high enough to adversely affect marine organisms. 
163 

Figure 8-67. Variations in total sediment calcium (A), chromium (B), zinc (C), and vanadium (D) concentrations 

changes in sediment mineralogy (as indicated by sediment aluminum levels) measured by inductivel

spectrometer (ICP). 
164 

y = 0.0189x + 0.0996

R2= 0.0762

0.000.100.200.300.400.500.60

0 1 2 34 5 6 7

ICP Aluminum (% dry weight)

Cadmium (ppm dry weight)

y = 2.12x + 15.88

R2= 0.11

05

10152025303540

0 1 34 

ICP Aluminum (% dry weight)

Copper (ppm dry weight)

y = 4.13x + 17.0

R2= 0.24

0

1020304050607080

01 234567

ICP Aluminum (% dry weight)

Nickel (ppm dry weight)

y = 3.99x - 3.21

R2= 0.74

05

1015202530

01234

ICP Aluminum (% dry weight)

Lead (ppm dry weight)

A B

C D

Figure 8-68. Variations in total sediment cadmium (A), copper (B), nickel (C), and lead (D) concentrations as a fun

sediment mineralogy (as indicated by sediment aluminum levels). 
165 

8.6.2 Contaminant Organic Compounds 

Polynuclear aromatic hydrocarbons (PAH) are a major toxic component of petroleum. The 

sources of PAH in offshore sediments include natural petroleum seepage, offshore production 

platform operations, spills, sediment transport and atmospheric deposition. Previous studies 

around production platforms showed that sediments close to platforms (<500 m) contained 

discharged drilling muds and cuttings. Hydrocarbons, including PAH and trace metals (Ag, Ba, 

Cd, Hg, Pb, and Zn) contaminants, were associated with these coarse-grained sediments 

(Kennicutt et al. 1996b). However, contaminant concentrations including PAHs close to 

platforms were below concentrations thought to induce negative biological responses. 

Sediment samples collected on the shakedown cruise were analyzed for PAH. A total of five 

sediment samples were collected, four with the GoMEX boxcorer and one with an USNEL spade 

corer. The total PAH concentrations ranged from 47 to 159 ng/g with a mean of 113 ng/g and a 

relative standard deviation of 37%. These total PAH concentrations are low. The PAH 

concentration in samples collected with the USNEL spade corer was 108 ng/g and was close to 

the mean, indicating the sampler type does not bias PAH results. The relative standard deviation 

of 37% is expected for the combined effects of analytical uncertainty (~10%) and the natural 

inhomogeneity of sediments. These samples were collected about 23.5 km (14.5 miles) 

southwest of Station W1 and 35.5 km (22 miles) west of Site W2. Samples collected at Site W2 

and W3 had total PAH concentrations of 53 ng/g and 41 ng/g, respectively, which is within or 

slightly below the lower end of the range of total PAH concentrations from the shakedown 

cruise. 

A total of 67 sediment samples were analyzed for PAH from DGoMB Cruises 1, 2, 3a, and 

3b. These samples represent 50 stations. Concentrations for total PAH ranged from not detected 

(ND) to 1033 ng/g with a mean of 140 ng/g and a median of 92 ng/g. Replicate samples were 

collected from selected sites and analyzed as replicates on the Survey Cruise (2000). Some sites 

were sampled during the next consecutive years (2001 and 2002). The data for these replicate 

analyses are reported in Table 8-4. There were nine (9) sites where samples were analyzed as 

replicates from one cruise, analyzed from two cruises, or both. The mean, standard deviation 

(SD), and relative percent standard deviation (RPSD) are reported for within cruise duplicates 

and between cruise duplicates. Three sites (MT1, B4, and JSSD3) had at least three (3) replicates 

from the same cruise. The RPSD for these sites were 25%, 38%, and 45% (Table 8-4) with a 

mean of 36%. 

These results are similar to the RPSD of 27% found from the shakedown cruise. These 

results indicate that the RPSD for within-cruise replicates are less than 45%. The RPSD for the 

seven (7) sites where two or more samples were analyzed from two cruises ranged from 34 to 

109%, with a mean of 61% (Table 8-4). The between-cruise RPSD, as expected, is generally 

higher than the within-cruise RPSD. The mean concentration for sites with replicates was used 

for the geographical display of the data. 
166 

Table 8-4 

 

DGoMB Replicate Total PAHs (ng/g) 

Site Cruise 1 Cruise 2 Cruise 3 Site Mean Site SD Site RPSD 

C 7 37.7 292.0 165.0 180.0 109% 

 

MT 1 A 733.0 221.0 446.0 230.0 52% 

MT 1 B 306.0 

MT 1 C 524.0 

MT 1 Mean 733.0 350.0 

MT 1 SD 156.0 

MT 1 PRSD 45% 

 

MT 3 A 1033.0 191.0 543.0 438.0 81% 

MT 3 B 404.0 

MT 3 Mean 719.0 191.0 

 

MT 6 59.2 96.5 77.9 26.4 34% 

S 36 80.1 147.0 114.0 47.3 42% 

S 41 A 97.4 76.6 109.0 39.5 36% 

S 41 B 153.0 

S 41 Mean 97.4 115.0 

 

S 42 35.1 110.0 72.6 53.0 73% 

BH A 357.0 

BH B 157.0 

BH C 282.0 

BH Mean 265.0 

BH SD 101.0 

BH RPSD 38% 

 

JSSD3 A 105.0 

JSSD3 B 123.0 

JSSD3 C 97.8 

JSSD3 D 98.8 

JSSD3 E 56.3 

JSSD3 Mean 96.2 

JSSD3 SD 24.5 

JSSD3 RPSD 25% 

The concentration of perylene from all cruises is depicted for the sampling sites in Figure 

8-69. Perylene is a PAH produced by biological processes as well as by combustion. Perylene is 

often detected as the major PAH in sediments from relatively pristine areas. In the sediment 

samples analyzed for this study, the percentage of perylene of total PAH ranged from 0 to 63%. 

Perylene concentrations ranged from ND (<1 ng/g) to 110 ng/g with a mean of 17 ng/g. Perylene 
167 

is also produced from combustion or processing of fossil fuels (e.g., perylene is present in 

creosote made from the heating of coal). Due to its anthropogenic as well as biogenic origins, the 

total PAH data for this report is discussed both with and without perylene. Other PAH are 

predominantly from anthropogenic sources. The geographic pattern of perylene concentrations 

(Figure 8-69) is complex but it does not co-vary with the total PAH distribution. The six (6) 

highest perylene concentrations are at sites S26, MT1, C1, B2, NB4, and RW6. These sites are 

found through the sample area with no discernable geographic pattern. 

The concentration of total PAH without perylene is depicted in Figure 8-70. Sites with 

relatively high concentrations include MT1, MT3, and C1. These sites, based on their PAH 

distribution, can be divided into two types: sites with PAH predominantly from oil and those 

with PAH predominantly from combustion sources. For sites MT1 and MT3, oil appears to be 

the major source of PAH. For C1, combustion sources predominate. Combustion PAH in the 

sampling area may be discharged from ships or platforms (e.g., bilge pumping) or come from 

atmospheric deposition of PAH from onshore industrial areas or sediments transported from 

coastal areas. The ship/platform operations are the most likely sources, as atmospheric deposition 

and would be expected to produce similar PAH concentrations over large regions. The 

geographic distribution of PAH indicates a possible input from PAH in the plume of sediments 

from the Mississippi River, but that cannot be confirmed from the present data. 

The total PAH concentration without perylene is plotted versus barium concentration in 

Figure 8-71. Most of the sites lie near the origin because they have both low PAH and low 

barium concentrations. Some sites have high barium and PAH concentrations (MT1, MT3, and 

C1). Barium is a tracer of drilling muds from platform operations. The elevated barium and PAH 

at the MT1, MT3, and C1 sites are likely due to inputs from platform operations in the vicinity of 

these sites. 

A frequency distribution for total PAH and total PAH without perylene concentrations are 

plotted as cumulative percentages (Figure 8-72). The concentrations, plotted on a log scale, have 

similar “S” shaped distributions. The addition of perylene causes a slight increase in 

concentrations over the entire range. The median concentration for total PAH is 92 ng/g and for 

total PAH without perylene is 76 ng/g. This indicates the relative importance of perylene, 

especially at low concentrations. Similar frequency distributions for total PAH were reported for 

the GoMEX program (Kennicutt et al. 1996a) and the cumulative curves had similar shapes. The 

concentrations of PAH found in close proximity to production platforms ranged from 10 to 6400 

ng/g, with the highest concentrations close to the platforms. However, most concentrations were 

less than 1000 ng/g (Kennicutt et al. 1996a). The frequency distribution for total PAH for the 

GoMEX study (Kennicutt et al. 1996a, b) is similar to the distribution found here, except that 

only one value exceeded 1000 ng/g in this study. When the frequency distributions found in this 

study are compared to those for other nearshore studies (NOAA NS&T and EPA-EMAP-NC), 

the median PAH concentration for this study is lower by about a factor of four. Total sediment 

PAH concentrations greater than 4000 ng/g are expected to elicit a biological effect 10% of the 

time (Long and Morgan 1990). The highest total PAH concentration found in sediments from 

this study (1033 ng/g) is four times less than the “biological effects” level. 
168 

Figure 8-69. The concentration of perylene in sediments. 
169 

Figure 8-70. The concentration of total polycyclic aromatic hydrocarbons (PAH) without perylene in sediments. 
170 

Figure 8-71. The total PAH concentration without perylene versus barium concentration. 
171 

Figure 8-72. Frequency distribution of total PAH and total PAH without perylene concentrations versus cumulativ
172 

In summary, concentrations of PAH measured for the DGoMB project from offshore GoM 

sites were low, as expected. Perylene, a PAH with a biogenic source, was the major PAH 

detected at many locations, indicating a biological origin. Sites that had high PAH concentrations 

(MT3, MT1 and C1) also had high barium concentrations. This indicates that drilling operations 

in the vicinity of these sites was a likely source of PAH. Total sediment PAH concentrations at 

all sites were less than 1033 ng/g. It is unlikely that the PAH concentrations found at any of these 

sites would have an adverse effect on the biota living in these sediments. It is notable too that 

the three sites with the highest PAH levels were in or near the Mississippi Canyon, and thus the 

Mississippi River effluent itself cannot be ruled out as a source of contaminants. 

8.7 Large Plant Detritus and Human-Generated Refuse 

Large fragments of plant detritus, such as Sargassum, Thalassia and Zostera (LPD), and 

human-generated refuse (HGR) have occasionally been observed and documented in the deepsea. This component of the present survey was not included in the original sampling plan 

because it was not expected to be of much significance. As the trawl samples were consistently 

characterized by such material, the categories of “large plant detritus” (LPD) and “human 

generated refuse” (HGR) were added. 

Scraps of wood, bark, macrophytes and fruit that serve as both habitat and nourishment to 

marine organisms (Rowe and Staresinic 1979; Turner 1973; Heezen and Hollister 1971), and 

wood construction material have been anecdotally documented in deep-sea trawl samples off and 

on for at least 3 decades (Wolff 1979; Pawson 1982). No definitive quantitative documentation, 

however, yet allows generalizations to be made about LPD or HGR in deep-sea environments. In 

this study, trawls were recovered from 34 stations spread all across the northern GoM. The 

closed dots in Figure 8-73 represent sites where HGR was retrieved, whereas open dots indicate 

samples without HGR. Closed triangles represent poor or failed trawls. 

Samples were initially categorized into HGR and LPD. HGR was further separated into the 

following categories: wood, aluminum cans, ceramics, clinkers, cotton cloth, discarded fishing 

gear, glass, metal, paint utensils, paper products, plastics, rope, rubber, tar, and miscellaneous 

items. 

Natural LPD had a wide and fairly even geographical distribution within the study area 

(Figure 8-74). Approximately one-half of the sites contained Sargassum sp,, which was inhabited 

in many instances by mollusks, crustaceans and covered with epiphytes. Thalassia testudinum

was recovered from water depths up to 3,000 m. 

Trawls from twenty-five of the thirty-four sites (74%) contained HGR (Figure 8-73) which 

was dominated by plastics, aluminum cans, wood, and fishing gear. Unlike the natural refuse, the 

majority of the HGR was concentrated in the area of the Mississippi River Trough (Figure 8-73). 

Petroleum, metal containers, plastics and paint supplies were found at approximately 70% of the 

sites. Each slowly degrade and remain persistent in the environment. 

The concentration of HGR and in particular Sargassum within and adjacent to the Mississippi 

River Trough suggests that the Mississippi River serves as a point source delivery system that 

may contribute to the localization of refuse in this area. Highly erosive deep-sea storms serve as 

significant transportation of sediments loads along with fauna (Aller 1989). It is reasonable to 

assume that these storms may also transport refuse to depressional areas, which may serve as 

debris traps. As sediment moves downslope, it forms debris flows and turbidity currents (Nardin 

et al. 1979), which may serve as an additional transport mechanism. 
173 

At two of the sites that appear as closed triangles in Figure 8-73, the trawl was either 

dislodged from the bridal, or in one instance, from the main cable. The amount of force required 

for such a feat is quite strikingly large and indicative of a rather large object such as a sunken 

ship. It is possible that these areas may serve as hard substrates for epifaunal communities in an 

otherwise soft sediment environment. 

Notably, more than 80% of the aluminum cans originally contained beer. Furthermore, 

greater than 60% of the glass was beer bottles. Other unique items included a fully intact 

sidewinder missile (Figure 8-75) that was recovered along the Florida Escarpment at a water 

depth of approximately 217 m. A 50-caliber shot casing was recovered seaward of the 

Mississippi River Trough in about 2,400 m water depth. Monofilament fishing line was 

recovered in over 22% of the trawls (Figure 8-76). In one instance, within the Mississippi River 

Trough at approximately 500 m water depth, a 2 meter-by-3 meter mass of longliner 

monofilament was reclaimed (Figure 8-75). The hooks were removed, indicating that it was 

discarded as a tangled mass on purpose and not simply lost while in use fishing. 

The amount of HGR in the deep-sea is poorly known. It may be possible that organic 

substrates are absorbed onto HGR which sink to the deep-sea floor and are incorporated into the 

benthic food chain (Gage and Tyler 1991). Observations made through this survey provide 

evidence that HGR accumulates preferentially in certain areas which can be referred to as debris 

traps. 
174 

Figure 8-73. Trash occurrence in trawls. 
175 

Figure 8-74. Plant type occurrence in trawls. 
176 

Figure 8-75. Sidwinder missle recovered on the Florida slope. 

Figure 8-76. Longline fishing line, hooks removed, measuring ca. 1 m by 

3 m. 
177 

8.8 Benthic Biology 

All size categories of the sediment-associated biota were sampled on Cruises 1 and 2. These 

included 1) microbiota, 2) meiofauna, 3) macrofauna, 4) megafauna, and 5) demersal fishes. 

Each fraction was fixed and labeled aboard ship as described above and distributed to respective 

PI’s immediately at the end of the cruise. These resulted in species lists for each category at each 

site sampled, all of which are now available in the database. In addition, biomass (total) for each 

group at each site was measured. For some groups, average biomass of the individuals have 

been measured. Both biomass and size data can be found in the database as well. The results are 

all presented below in this subsection in order of increasing organism size, with special kinds of 

analyses concluding the section. 

8.8.1 Microbiota 

8.8.1.1 Benthic Bacterial Abundance and Biomass 

The microbiota component of the overall project included collection of an unprecidented set 

of data (n > 1000) on bacterial abundance in the sediments. Abundance was measured at four 

depth horizons (0–1, 4–5, 9–10, 14–15 cm) in triplicate cores at each of 59 survey and process 

stations, ranging in depth from 19 to 3,732 m, during three cruises (DGoMB 2000, 2001, and 

2002). Because the station at 19 m was anomalous for its shallow depth (all others stations were 

at 213 m or much deeper), nearly anoxic state and high bacterial abundance (always plotting offscale), the data obtained there (and provided in the database) have been excluded from all 

subsequent ranges and analyses. With that caveat in mind, the horizon-specific measurements 

ranged over more than an order of magnitude, from 1.00 x 108 to 1.80 x 109 bacteria cc–1

sediment. This range falls within that reported previously by the same methods for surface 

sediments from similar station depths elsewhere in the ocean (Alongi 1990; Deming and Baross 

1993). Subsurface DGoMB sediments were often inhabited by numbers of bacteria equivalent to, 

and on occasion higher than (e.g., at stations NB5 and S35), those observed in surface sediments 

(see database for individual measurements), implying bioturbation. The general pattern of 

bacterial abundance decreasing only moderately if at all with depth into the seabed also implies 

that our depth-integrated estimates of bacterial abundance (or biomass), though reflective of 

conditions to 15 cm, underestimate the total carrying capacity of the seabed (to depths below 15 

cm). 

The size (biovolume) of benthic bacteria was also measured at four depths in triplicate cores 

for selected stations (see Table 8-5) in order to convert seabed abundance estimates to biomass in 

a DGoMB-specific manner, rather than relying solely on previously published conversion values. 

Mean bacterial volume in a given sediment horizon ranged from 0.027 (± 0.008; at 5 cm, station 

MT6) to 0.072 (± 0.033; at 15 cm, station MT3) μm3

. This range falls near the middle of sizeclass ranges considered for marine bacterioplankton (Norland 1993) and largely below that 

reported for intertidal sediments (0.07 to 0.22 μm3

; Kuwae and Hosokawa 1999). That downcore 

profiles of bacterial biovolume were essentially constant (overlapping error bars between 

horizons) may also suggest bioturbation. 

Ultimately, we developed station-based estimates of benthic bacterial abundance and 

biomass by integrating results over the sampled depth interval of 0–15 cm and calculating the 

mean value for triplicate cores. The resulting estimates scaled to square meter of seafloor are the 

focus of remaining analyses in this section. These mean values for benthic bacterial abundance 
178 

ranged from 4.40 x 1013 to 1.75 x 1014 bacteria m–2 across station depths of 213–3,732 m. Before 

evaluating these data and biomass estimates developed from them, we revisit two 

methodological issues. 

Table 8-5 

 

Benthic Bacterial Biomass at Selected DGoMB Stations1

 and from the Literature 

Bacterial biomass (g C m-2, 0–15 cm) 

Cruise or 

reference Station or location 

Station 

depth 

(m) 

Based on biovolume, 

abundance2

, and 

390 fg C um–3

Based on abundance2

and 

10 fg C cell–1

S42 767 1.18 (1.30) 0.66 (0.73) 

MT3 987 2.32 (2.55) 1.25 (1.38) 

S36 1828 1.68 (1.85) 1.10 (1.21) 

DGoMB 2000 

MT6 2700 0.92 (1.01) 0.68 (0.75) 

S4 3410 N.D.2 1.48 

DGoMB 2002 S1 3535 N.D. 1.40 

Bay of Biscay 4100 N.D. 0.10 

Rowe and 4500 N.D. 0.21 

Deming, 1985 

Demerara Basin 

4800 N.D. 0.12 

Mesotrophic site 

off West Africa 3100 N.D. 0.64 Relexans et 

al., 1996 Oligotrophic site 

off West Africa 4500 N.D. 0.18 

1

 Where bacterial biovolume measurements were made. 

2

 Parenthetic values adjusted upwards 10% (see text). 

* N.D. = not determined (no bacterial biovolume measurements made). 

First, procedures for determining bacterial abundance and biomass relied upon established 

methods in epifluorescence microscopy to obtain an internally consistent data set. Nevertheless, 

the laboratory dilution scheme used to prepare microscopic slides of shelf and slope sediments 

(DGoMB 2000 and 2001) differed from that used for the abyssal sediments (DGoMB 2002). The 

different schemes were necessary to obtain countable slides, but in comparative dilution studies, 

we determined that the shelf-slope samples may have been underestimated by 10% as a result 

(see Section 7.2.1). Others using our data as provided in the database may wish to increase the 

reported shelf/slopes values by 10%, as we have done in selected analyses, specified below. This 

slight adjustment may be important to evaluations of the data in an absolute sense (e.g., in 

carbon-based partition models of the benthos) or when comparing the entire bacterial data set 

(shelf-slope plus abyssal stations) against another entire data set (e.g., other variables measured 

in parallel sediment cores). 

Second, in order to apply the biovolume measurements made for a subset of stations to the 

global data set, we developed an interpolation scheme as follows. Bacterial biomass was 

calculated from the measured biovolume and abundance measurements using the conversion 

factor of 390 fg C μm–3 (Bjornsen and Kuparinen 1991). Biomass was also calculated more 
179 

simply from abundance measurements using the deep-sea conversion factor of 10 fg C per 

bacterium, as in previous studies (e.g., Rowe and Deming 1985; Alongi 1990; Relexans et al. 

1996). The biovolume-based estimates, which should represent improved estimates, were always 

higher than their counterparts by a factor of 1.4–1.9 (Table 8-5). The factor difference between 

the two approaches (y), when plotted against station depth in meters (x), yielded the linear 

regression equation: y = 2.040 – 0.0002598 x. The negative slope, and thus convergence towards 

unity of the two methods as station depth increases, implies that the calculation of benthic 

biomass directly from abundance (using the deep-sea conversion factor of 10 fg C cell–1) may 

well be an adequate approach to estimating bacterial biomass in abyssal sediments. Unknown but 

different and likely depth-specific conversion factors are needed for shallower shelf-slope 

depths; otherwise, acquiring biovolume measurements is recommended. 

Using the equation described above, we then calculated the biomass for all remaining 

stations. No comparable biovolume-based biomass estimates exist for other sediments, but a 

comparison of the simpler abundance-based estimates can be made to similarly determined 

estimates in the literature (from depths of 3100–4800 m in the Atlantic Ocean). That comparison 

reveals that bacterial biomass in DGoMB sediments is higher (Table 8-5). The comparison is 

most relevant between abyssal sediments, where the simpler estimates may also be considered as 

accurate as biovolume-based values (and no 10% adjustment issue pertains). The available 

abyssal data sets for this comparison are small and depth ranges do not overlap precisely. 

Nevertheless, the abyssal GoM sediments (at 3,410–3,732 m) appear to support more bacterial 

biomass, by a factor of six, than abyssal sediments elsewhere (mean of 1.48 g C m–2, S.D. = 

0.17, n = 5, versus mean of 0.25 g C m–2, S.D. = 0.22, n = 5). 

Graphical examination of the global DGoMB data set on benthic bacterial abundance (Figure 

8-77) and on biovolume-based biomass (Figure 8-78) reveals the importance of considering the 

latter estimates and thus of obtaining biovolume measurements, rarely done for sediment 

bacteria, given the technical difficulties (Kuwae and Hosokawa 1999). For example, when 

bacterial abundance is considered as a function of station depth, no relationship is detectable by 

linear regression analysis, with or without the 10% adjustment to the shelf-slope stations 

(without, R2 = 0.000, n = 58; Figure 8-77; with, R2 = 0.005, n = 58, Table 8-6). Although earlier 

studies with limited data sets (reviewed by Deming and Yager, 1992; Deming and Baross 1993) 

suggested weak depth-dependencies, the study closest to ours in size (ten stations) and depth 

range (695–4350 m) also failed to detect a significant decrease in benthic bacterial abundance 

with depth (Alongi 1990). Alongi attributed the absence of a depth-dependent relationship to 

densities of potential grazers and geological differences among the sites. Deming and Yager 

(1992) found the supply of new resources or POC flux to the seafloor to be a stronger 

determinant of benthic bacterial abundance than ocean depth or measures of organic carbon 

(TOC) already present in the sediment. The latter finding reflects the relatively rapid responses 

of bacteria to pulses of organic carbon, even under abyssal conditions (Deming 1985; Lochte and 

Turley 1988). Each of these explanations will also pertain to the absence of a bacterial depthdependency in the DGoMB study, but we offer an additional interpretation: biovolume-based 

biomass and not abundance may be the more relevant bacterial variable to consider. Even given 

the vast and varied environments sampled in this study, a detectable (albeit weak) relationship 

emerges when the global set of biovolume-based bacterial biomass estimates is regressed against 

station depth (R2

 = 0.300, n = 58; Figure 8-78; with the 10% adjustment to shelf-slope data, 

R2

 = 0.350, n = 58, Table 8-6). 
180 

4 10 13

6 10 13

8 10 13

1 10 14

1.2 10 14

1.4 10 14

1.6 10 14

1.8 10 14

2 10 14

0 500 1000 1500 2000 2500 3000 3500 4000

Bacterial abundance vs. Depth (2000-2002)

y = 9.656e+13 + 3.557e+07x R 2= 1.6994e-06 

Bacteria m -2 (0-15 cm)

Depth (m)

Figure 8-77. Global data set for depth-integrated, benthic bacterial abundance versus 

station depth (without 10% adjustment to shelf-slope stations). Open 

symbols indicate data from DGoMB 2000; solid black symbols, from DGoMB 

2001, gray symbols, from DGoMB 2002. Error bars indicate standard 

deviation of the mean from triplicate cores. Note that lowest data point at 

863-m depth (station W3) was not replicated (has no error bar). 

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000 2500 3000 3500 4000

y = 2.0259 - 0.00028052x R 2 = 0.29952 

g bacterial C m

-2

 (0-15 cm)

Depth (m)

Bacterial biomass vs. Depth (2000-2002)

Figure 8-78. Global data set for depth-integrated, biovolume-based, benthic bacterial 

biomass versus station depth (without 10% adjustment to shelf-slope 

stations). Open symbols indicate data from DGoMB 2000; solid black 

symbols, from DGoMB 2001, gray symbols, from DGoMB 2002. 
181 

Table 8-6 

 

R2

 Values From Linear Regression Analyses of Benthic Bacterial Data on Abundance and BiovolumeBased Biomass as a Function of Station Depth 

Cruises included Stations included1 Number of 

stations Abundance Biomass 

All (no 10% adjustments) 58 0.000 0.300 

All 58 0.005 0.350 

All but S52 57 0.006 0.390 

All but W33 57 0.012 0.394 

DGoMB 2000, 

2001 and 2002 

All but S5 and W3 56 0.344 0.437 

All 53 0.158 0.479 

Only transects (C, RW, MT) 40 0.217 0.525 

Only transects, no W3 39 0.280 0.591 

DGoMB 2000 

and 2001 

MT transect only 9 0.656 0.768 

1

 Data from Hypox station at 19-m depth always excluded; data from 2000 and 2001 adjusted upwards 

by 10% (see text) except in the first case above (see also Figure 8-56). 2

 Station with highest abundance; anomalous abyssal station for signs of oxygen limitation. 3

 Station with lowest abundance; anomalous for lack of triplication (single-core analysis). 

A minor paring, however, of both the abundance and biomass data sets not only improves the 

biomass depth-dependency, but also reveals a weak one for abundance, as summarized in Table 

8-6. Excluding two of the 58 data points – the lowest and the highest for abundance – improves 

the R2

 value for biomass versus depth from 0.350 to 0.437 and for abundance, from 0.005 to 

0.344. These data exclusions are not based solely on their endmember status. The lowest 

estimate came from the only station (W3) where we lacked triplicate cores (see absence of error 

bar on the lowest point, a depth-integrated value from a single core, at 860-m depth on Figure 

8-77). The highest came from the abyssal station (S5) already anomalous for its signs of anoxia. 

If we consider only the slope data, and subsets thereof, to ask whether or not benthic bacterial 

variables are depth-dependent, then stronger depth-dependencies become evident (Table 8-6). 

For example, if the data from stations located in basins or other specific slope features are 

excluded and only transect data (from the C, RW, and MT stations) are analyzed, the R2

 value 

for a biomass depth-dependency is 0.525 (n = 40, Figure 8-77). It improves to 0.591 if station 

W3 (where we lacked replication) is also excluded (n = 39, Table 8-6). The highest R2 value 

obtained from any of our depth-related analyses is 0.768, derived from the regression analysis of 

the MT transect data against depth. When only these MT transect data are considered, a 

relatively strong depth-dependency with bacterial abundance emerges for the first time (R2

 = 

0.656, n = 9; Table 8-6). The more narrowly defined the sampling region, the more likely a 

depth-dependency, as has also occurred in previous and far less extensive studies of benthic 

bacterial abundance. 

When the benthic bacterial abundance and biovolume-based biomass data were used to test 

hypotheses associated with what influences the quantity and composition of life in the deep 

ocean (Section 4), the issue of depth dependency in the Sigsbee region reflected the general 

results described above (Figures 8-78 and 8-79). Specifically, no significant difference was 

detected between slope and abyssal sites in terms of bacterial abundance (P = 0.151), but the 
182 

difference was significant in terms of biomass (P = 0.020, Table 8-7). Support for other 

hypotheses that we could test with our data set – that benthic bacterial abundance is determined 

by east versus west location in the study region or by high versus low overlying productivity – 

was not obtained, nor did considering biomass instead of abundance change the test result (Table 

8-7). A final hypothesis that we could test with our data alone concerned annual variability in 

seabed measurements at seven stations across the shelf and slope that were sampled in both 2000 

and 2001. A possible trend towards higher benthic biomass in 2001 on the upper slope, seen 

graphically in Figure 8-80 (note solid symbols and their positions above the regression line 

primarily at shallower depths) could not be verified as significant (P = 0.322). 

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000 2500 3000 3500

y = 2.243 - 0.00044196x R 2= 0.52511 

g bacterial C m

-2

 (0-15 cm)

Depth (m)

Bacterial biomass vs. Depth (2000-2001)

(transect stations only)

Figure 8-79. Depth-integrated, biovolume-based, benthic bacterial biomass 

versus station depth for transect stations only. Open symbols 

indicate data from DGoMB 2000; solid symbols, from DGoMB 2001. 

Table 8-7 

 

Hypothesis Testing (Single Factor ANOVA, α = 0.05) for Factors that May Determine Benthic Bacterial 

Abundance or Biomass on the Shelf-Slope Regions of the Study Area 

Factor examined Number P-values 

of 

stations for 

abundance 

for 

biomass 

Interpretation 

Shallow versus deep 

station depth 

(in Sigsbee region) 

8 0.151 0.020 No effect on abundance, 

negative effect on biomass 

East versus West 

location 

(3 tests: east–mid, 

mid–west, east–west) 

10 0.633 

0.229 

0.501 

0.233 

0.264 

0.845 

No effect on abundance 

or biomass 

Low versus high 

primary production 

10 0.075 0.191 No effect on abundance 

or biomass 
183 

0.5

1

1.5

2

2.5

3

Bacterial biomass: Sigsbee Escarpment 

(shelf vs. slope)

y = 2.9075 - 0.9675x R 2= 0.62155 

g bacterial C m -2 (0-15 cm)

Station

S42-44 S39-41

Figure 8-80. Significant difference (P = 0.020; Table 8-7) between biovolume-based, benthic 

bacterial biomass on upper slope (S42-44) versus lower slope/rise stations (S39-

41) of the Sigsbee Escarpment. 

8.8.1.2 Benthic Bacterial Production and Respiration 

In addition to the extensive survey of benthic bacterial abundance and biomass described 

above, the microbiota component of the overall project included experiments to measure rates of 

bacterial activity in sediments from selected stations. Rates of bacterial production were 

determined from sediment incubation experiments at in situ temperature (except at station S4 

where refrigeration problems prevented incubation below ~10°C), with and without in situ 

pressure applied shipboard, using 3

H-thymidine as a tracer for DNA production (see Section 

7.3.1). Only surface sediments (0-1 cm) were examined by this method, since the uptake of 3

Hthymidine by anaerobic bacteria at subsurface depths is unlikely. The method yielded detectable 

rates of bacterial production at all six stations examined, covering a depth range of 767–3,535 m 

(Table 8-8). Application of in situ pressure resulted in higher rates (barophilic responses) only 

for sediments from depths ≥ 2,700 m, including the temperature-compromised station at 3,410 m 

(though not abyssal station S1). 

Our estimated in situ bacterial production rates (range of 5–75 μg C m–2 d–1), however, are 

low when compared to the other published rates obtained by similar methods from similar depths 

(pressures) and temperatures. Alongi (1990) reported rates of 34–7010 μg C m–2 d–1 in sediments 

with bacterial abundances similar to ours over a depth range of 695–4350 m in the Solomon and 

Coral Seas near Australia (Table 8-9). The only methodological differences we can identify 
184 

Table 8-8 

 

Samples from Which Benthic Foraminifera Were Extracted for ATP 

Station Depth Interval (cm) Number Extracted

 Cruise 2 

 Process Stations 

MT3, replicate 1 0-1 53 

 1-3 50 

 14-15 50 

MT3, replicate 2 0-1 50 

 1-3 50 

 9-10 50 

S42 0-1 50 

 1-3 50 

 9-10 50 

S36 0-1 75 

 1-3 50 

 14-15 50 

“tree” foraminifers from other S36 boxcores 6 

MT6 0-1 50 

 1-3 50 

 9-10 50 

 Non-Process Stations 

 

C7 0-1 50 

 1-2 50 

Bush Hill 0-1 50 

 1-2 50 

 

Total for DGoMB Cruise 2 984 

 Cruise 3a 

JSSD1BC5 0-1 50 

 1-3 50 

 7-8 50 

JSSD4BC27 0-2 75 

 2-3 50 

 9-10 25 

JSSD4BC31 (replicate) 0-1 57 

 Non-Process Stations 

JSSD2BC13 0-1 50 

 1-2 60 

JSSD5BC38 0-1 50 

 1-2 57 

Total for DGoMB Cruise 3a 574 
185 

Table 8-8 Samples from which benthic foraminifera were extracted for ATP (continued) 

Station Depth Interval (cm) Number 

Extracted 

Atlantis Cruise (October 2000) 

Farnella Canyon (Alvin Dive 3628 0-1 130 

Green Knoll (Alvin Dive 3629) 

Core 66 0-1 28 

Core 67 0-1 40 

Florida Escarpment (Dive 3634) 0-1 100 

 2-3 26 

Total for Atlantic Cruise 324 

TOTAL All Cruises 1,882 

Table 8-9 

 

Bacterial Production Rates in Surface Sediments (0–1 cm), 

Based on 3H-Thymidine Incubations at in Situ Temperature and Both in Situ Pressure and 1 Atm 

Bacterial production 

(μg C m-2 d-1) 

Cruise or reference Station Depth (m) 

in situ atm 1 atm 

Rate ratio1

(in situ atm/ 

1 atm) 

S42 767 4.6 33.0 0.14 

MT3 987 6.3 35.7 0.18 

S36 1828 14.8 33.6 0.44 

DGoMB 2001 

MT6 2700 52.0 26.6 1.95 

S4 3410 44.62 19.52 2.292 DGoMB 2002 

S1 3535 75.1 109.1 0.69 

PNGS1 695 4680 N.A.4 (> 1)4

CSP9 1454 7010 N.A. (> 1) 

QT5 2256 776 N.A. (> 1) 

WB5 2395 475 N.A. (> 1) 

PNGS2 2426 1740 N.A. (> 1) 

WB6 2775 911 N.A. (> 1) 

WB7 2800 733 N.A. (> 1) 

PNGS3 3264 968 N.A. (> 1) 

CSB13 4008 34 N.A. (> 1) 

Alongi, 1990 

(Solomon and 

Coral Seas) 3

CSB4 4350 141 N.A. (> 1) 

1

 Ratios greater than one indicate barophily. 

2

 Incubation temperature (~10°C) was ~5°C warmer than in situ temperature. 3

 Samples were incubated within 1–2°C of in situ temperature (range of 1.8–7.6°C). 

4

 N.A. = not available, but text reports that pressurized rates were higher than 1-atm rates. 
186 

between these two studies are our sampling of a thicker sediment horizon (0–1 cm versus 0–0.5 

cm) and use of a lower ratio, by a factor of seven, of amount of thymidine injected to sediment 

volume. 

Even if we were to adjust our rates upwards by that factor (requiring more assumptions than 

some would find reasonable), they would still be lower overall. Although the comparative data 

sets are small, DGoMB slope/abyss sediments thus appear to support lower rates of bacterial 

production than the slope sediments of the Solomon and Coral Seas near Australia. At abyssal 

depths, rates from the two studies appear to converge. A cross-disciplinary examination of the 

two study areas would be required to derive possible explanations for the shallow discrepancies 

and abyssal convergence. A cross-disciplinary examination of DGoMB data may help to explain 

the non-intuitive finding that our estimates of bacterial production rates under in situ pressures 

tended to increase with station depth. 

In addition to bacterial production rates using the 3

H-thymidine approach, we also measured 

rates of respiration (conversion to CO2), with and without in situ pressure applied shipboard, of 

14C-labeled amino acids substrates at four depth horizons in sediments from the two abyssal 

stations, S4 and S1. From the resulting downcore profiles (Figure 8-81), depth-integrated 

estimates of bacterial respiration in the seabed were obtained and compared to the few literature 

values obtained using the same methods and mixture of amino-acid substrates (Table 8-10). Our 

rates are considerably higher, by 1–2 orders of magnitude, than those obtained for two abyssal 

sites off West Africa (Relexans et al. 1996). Part of this difference may be attributable to the 

different incubation temperatures used: 5 or 10°C for the DGoMB samples; 1–2°C, for the West 

Africa samples (Relexans et al. 1996). Even considering a maximal 10°C difference (between 

our unavoidably warmed samples from station S4 and the West African samples), however, 

could only account theoretically for a doubling of the rate. Another consideration is the relative 

constancy of activity with depth in the DGoMB sediments (Figure 8-81) compared to the West 

African core profiles. In the latter cases, activity often approached undetectable levels at 15-cm 

depth (Relexans et al. 1996), as also observed in other abyssal Atlantic sediments, albeit when 

14C-glumatic acid was used as substrate (Rowe and Deming 1985). Subsurface bacterial activity 

may thus be more important to overall benthic carbon cycling in the GoM than usually 

considered. Although the comparative data sets are very small, the abyssal seabed of the GoM 

appears to support depth-integrated rates of bacterial respiration that are considerably higher than 

observed elsewhere. 

In the meantime, the kinetics of 14C-amino acid respiration, as measured under simulated in 

situ conditions in surface sediments (Figure 8-82), also suggest substantial rates of organic 

carbon remineralization by benthic bacteria in the abyssal GoM. Although the data from 

temperature-compromised station S4 did not fit Michaelis-Menten kinetics, those from station S1 

yielded an in situ amino-acid turnover time (respiration only) of one day (see y-intercept on 

figure). The modest slope of the regression line for increasing concentration of added amino 

acids suggests that the bacterial community may have been respiring the naturally available 

mixture of amino acids at near-maximal capacity. The results of incubating diluted sediments for 

the purpose of determining the cellular fate of the added 14C-amino acids indicated that the bulk 

of the carbon was being respired, with only 9.3–23% incorporated into biomass. The arrival of 

new organic substrates could thus be expected to shift the community from a largely respiratory 

(cellular maintenance) mode into a more rapid growth phase, incorporating a greater fraction of 

the carbon into new biomass. Measured growth rates (bacterial production rates) in surface 

sediments were relatively low (see above), suggesting the potential for stimulation. In summary, 
187 

the abundant bacteria present in the abyssal sediments of the GoM at the time of our sampling 

appeared to be reproducing at lower rates but respiring carbon more rapidly than in other regions 

examined using comparable methods. 

0 0.02 0.04 0.06 0.08 0.1

0

2

4

6

8

10

12

14

16

S4 1 atm

S4 in situ atm

S1 1 atm

S1 in situ atm

Depth (cm)

Bacterial respiration rate (nCi cc -1 h -1 )

Figure 8-81. Downcore profiles of 14C-amino acid-based, benthic bacterial respiration. 

Table 8-10 

 

Depth-Integrated (0–15 cm) Benthic Bacterial Respiration Rates 

Based on 14C-Amino Acid Incubations at in Situ Temperature and Both in Situ Pressure and 1 Atm 

Bacterial respiration 

(μg C m-2 d-1) 

Cruise or 

Reference 

Station Depth 

(m) 

in situ atm 1 atm 

Rate ratio1

(in situ atm/ 

1 atm) 

S4 3410 75.92 1052 0.722 DGoMB 2002 

S1 3535 242 198 1.22 

Mesotrophic 

site off West 

Africa 

3100 5.7 N.A.4

 (> 1.0)4 Relexans et al., 

19963

Oligotrophic 

site off West 

Africa 

4500 1.8 N.A.4

 (> 1.0)4

1

 Ratios greater than one indicate barophily. 

2

 Incubation temperature (~10°C) was ~5°C warmer than in situ temperature. 3

 Samples were incubated at 1–2°C; rates were integrated to 10-cm depth (from 10–15 cm they 

approached zero); substrate incorporation into biomass at the mesotrophic and oligotrophic sites was 

45–65% and 0–5%, respectively. 

4

 N.A. = not available in text, but pressurized rates were higher than 1-atm rates (J. Deming, 

unpublished). 
188 

0

1

2

3

4

5

6

7

8

S1

S4

12.5 25.0

14 C-amino acids added (nCi cc -3 )

t/f (days)

y = 1.02 + 0.13x r 2 = 0.47

y = 3.69 + 0.03x r 2 = 0.01

2.5

Figure 8-82. Kinetics of 14C-amino acid-based, benthic bacterial respiration under in situ 

pressure and temperature, except for station S4 where the incubation 

temperature was ~10°C. 

8.8.2 Benthic Foraminifera 

A total of 1882 foraminifera were extracted for ATP, and thus biomass determinations (Table 

8-10). Samples were taken at all process stations as well as at an additional two stations during 

DGoMB cruise 2 (C-7 and Bush Hill), two more stations on DGoMB cruise 3a (JSSD2, JSSD5), 

and three other sites sampled by the submersible Alvin (Farnella Canyon, Green Knoll, Florida 

Escarpment during an October 2000 Atlantis cruise, Ian MacDonald, Chief Scientist). Only the 

top one or two cm was processed for the survey site cores and auxiliary cores. At site S36 there 

was an abundant community of arborescent foraminifera (up to 5 cm in height), six of which 

were analyzed for ATP although none occurred in the foraminiferal biomass core. 

Results from cores collected by Alvin were far lower in comparison to other data (Table 8-

11). Because these samples were exposed to long periods of warm temperatures during 

submersible ascent and recovery, their results are omitted from analyses and interpretations. 

When Alvin results are omitted, benthic foraminiferal abundances ranged over an order of 

magnitude (~3,600-44,500 individuals m-2; Table 8-12) and foraminiferal biomass ranged from 

~2-53 mg C m-2. 

Numerous large (up to 5 cm long) “tree-like” agglutinated foraminifers occurred at site S36 

(1,848 m), with densities up to ~130 m-2. These are tentatively identified as Pelosina sp. (Figure 

8-83). Although no specimens were in the subcore for foraminiferal biomass quantification, they 

were observed in most boxcores collected from this site. Pelosina sp. biomass, determined from 

ATP analysis, ranged from ~2.8-13 μg C individual-1, thus comprising a monospecific biomass 
189 

Table 8-11 

Benthic Foraminiferal Biomass Data (mgATP/m2), Abundance (#live/m2) and Average Foraminiferal Volume 

for Pushcores Collected by Alvin in October 2000 (n = number of foraminifers extracted (individually)

Station Date Water Latitude Longitude Water ngATP/cm3 mgC/m2 #live/m2 Av

Depth 

(m) 

Depth (cm) (+

Farnella Cyn 21 Oct 2884 26-26.75N 90-46.50W 0 to 1 0.07 0.2 2407 0.0(dive 3628) Green Knoll 22 Oct (dive 3629) c66 2463 25-55.43N 90-12.87W 0 to 1 0.02 0.1 1070 c67 2457 25-55.43N 90-12.87W 0 to 1 0.04 0.1 535 0.Florida Esc. 27 Oct ~2876 28-02.41N 86-33.05W 0 to 1 0.29 0.9 3477 0.(dive 3634) 2 to 3 0 0 0 

Table 8-12 

Benthic Foraminiferal Abundances (#/m2) and Biomass (mgC/m2) for the Surface cm (0-1 cm) and Integrated over the 

Water Latitude Longitude 0-1 cm 

Site 

Depth (m) 

°N °W 

Density 

(# m-2) 

Biomass 

(mgC m-2) Density (# m-2) 

Bush Hill 548 27°47.8 91°28.2 45,455 47.1 81,819 

S42 2001 768 28°15.3 86°25.6 3,636 2.9 7,272 

MT3 2001 985 28°13.3 89°30.6 12,727 8.8 23,636 

C7 2001 1076 27°43.7 89°58.7 25,455 53.4 41,819 

S36 2001 1848 28°55.4 87°39.0 29,091 52.5 44,546 

MT6 2001 2742 27°00.2 88°00.9 4,545 1.8 9,090 

JSSD5 3316 25°29.5 88°16.2 10,909 20.9 27,273 JSSD4 3410 24°14.5 85°29.1 44,545 8.4 50,000 JSSD1 3520 25°00.6 92°00.7 27,273 3.7 30,909 JSSD2 3732 23°30.0 92°00.2 34,545 8.0 74,545 
190 

of up to ~1.7 mg C m-2. A few additional Pelosina sp. specimens were also observed at site C7 

(1,076 m). 

Foraminiferal abundances were roughly comparable to typical deep-sea foraminiferal 

standing stocks. Abundances did not consistently vary with water depth, latitude or longitude. 

Foraminiferal biomass did vary somewhat with depth but this correlation was not significant 

(p>.05 for both 0-1 cm and 0-3 cm). For example, in the surface centimeter at one ~1,000-m site 

(Site MT3; 985 m), foraminiferal biomass was relatively low (~9 mg C m-2) while another 

~1,000-m site (Site C7; 1,076 m) had the highest foraminiferal biomass (~53 mg C m-2). These 

sites were geographically separated by only ~75 km (Figure 8-83). The sites with the two highest 

estimates of foraminiferal biomass (Bush Hill, C7) are near hydrocarbon seeps. The site with the 

third highest biomass (S36) may be situated beneath a gyre that provides high rates of organic 

carbon input. The site with the lowest foraminiferal biomass is on the Mississippi Fan (MT6; 

2,742 m), where sedimentation rate may negatively affect standing stocks. Although most 

samples from Sigsbee Plain (>3,000 m) had low foraminiferal biomass, one Sigsbee sample had 

>20 mg foraminiferal C m-2 (JSSD5). Down-core (i.e., 1-3 cm) maxima in foraminiferal biomass 

occurred at the three shallowest sites, suggesting that residual organic matter is also utilized by 

these benthic communities. 

Figure 8-83. Pelosina sp., large agglutinated arborescent foraminifer. A. Specimen in situ. B. 

Specimen removed from sediment to show sub-surface “root” structure. C. Two 

additional specimens, one broken. Approximate location of the sediment-water 

interface is indicated by the arrows. All scale bars ~1 cm. 

Foraminiferal density never exceeded metazoan meiofaunal density (Tables 8-13 and 8-14; 

Figures 8-84 and 8-85). Foraminiferal biomass did, however, exceed the pooled biomass of 

nematodes + copepods at half the sites (Tables 8-14 and 8-15; Figures 8-86 and 8-87). Because 

foraminiferal biomass exceeded that of the most common metazoan meiofauna (nematodes, 

copepods), foraminifers comprise a major component of deep-water meiofaunal biomass and are, 

thus, a major component of bathyal and abyssal communities. 
191 

Table 8-13 

 

Comparison between Foraminiferal and Metazoan Meiofaunal Density (#/m2

) 

and Biomass (mgC/m2

) for the Surface Centimeter 

(Note that metazoans biomass is exclusively copepods and nematodes. Shaded cells with bold text 

denote samples where foraminiferal biomass exceeded the pooled biomass of copepods and 

nematodes.) 

 Water 0-1 cm 

Site Depth Density (# m-2) Biomass (mgC m-2) 

 (m) Metazoa Foraminifera Nematode & Copepod Foraminifera 

Bush Hill 548 232,841 45,455 22.2 47.1 

S42 2001 768 123,745 3,636 6.9 2.9 

MT3 2001 985 22,981 12,727 1.6 8.8 

C7 2001 1076 140,581 25,455 11.3 53.4 

S36 2001 1848 64,566 29,091 3.9 52.5 

MT6 2001 2742 67,512 4,545 5.0 1.8 

JSSD5 3316 178,883 10,909 16.8 20.9 

JSSD4 3410 268,745 44,545 20.4 8.4 

JSSD1 3520 34,724 27,273 3.0 3.7 

JSSD2 3732 303,637 34,545 20.4 8.0 

Table 8-14 

 

Comparison between Foraminiferal and Metazoan Meiofaunal Density (#/M2

) 

and Biomass (Mgc/M2

) for the Top 3 Centimeters 

(Note that metazoans biomass is exclusively copepods and nematodes. Shaded cells with bold text 

denote samples where foraminiferal biomass exceeded the pooled biomass of copepods and nematodes) 

 Water 0-3 cm 

Site Depth Density (# m-2) Biomass (mgC m-2) 

 (m) Metazoa Foraminifera Nematode & Copepod Foraminifera 

Bush Hill 548 407,851 81,819 36.8 98.1 

S42 2001 768 276,279 7,272 18.5 6.0 

MT3 2001 985 78,961 23,636 5.7 18.9 

C7 2001 1076 242,355 41,819 18.4 74.9 

S36 2001 1848 192,435 44,546 13.7 62.1 

MT6 2001 2742 122,145 9,090 9.3 3.4 

JSSD5 3316 398,424 27,273 31.9 23.8 

JSSD4 3410 493,716 50,000 33.8 9.8 

JSSD1 3520 55,664 30,909 4.4 6.4 

JSSD2 3732 459,959 74,545 29.3 11.9 
192 

Abundance (0-1 cm)

0

6

12

18

24

0 500 1000 1500 2000 2500 3000 3500 4000

Water Depth (m)

Density (#x10 4 m -2 )

Metazoans

Foraminifera

Figure 8-84. Scatter plot comparing foraminiferal abundance in the surface 

1 cm to that of meiofaunal metazoans. 

Abundance (0-3 cm)

0

10

20

30

40

50

0 500 1000 1500 2000 2500 3000 3500 4000

Water Depth (m)

Density (#x10 4 m-2)

Metazoans

Foraminifera

Figure 8-85. Scatter plot comparing foraminiferal abundance in the top 3 cm 

to that of meiofaunal metazoans. 

Table 8-15 

 

Effect of Core Tubes Size (Inner Diameter) on Meiofauna Counts and Average Density 

(Based on five replicates taken at station W-2. Abundance is detransformed from natural log (ln), 

so taxa averages do not sum to the total average.) 

Counts (n/core) Abundance (n/10 cm2) 

Core Size (cm i.d.) 2.2 3.1 5.5 6.7 2.2 3.1 5.5 6.7

Taxa 

 Nematodes 20.2 64.8 161.0 219.0 31.3 59.4 50.5 54.3

 Harpacticoids 5.4 30.8 74.2 99.4 2.3 26.1 20.8 24.4

 Nauplii 7.2 38.6 72.2 85.6 0.2 29.2 22.9 19.0

 Others 12.2 14.0 27.8 45.8 3.6 14.5 8.5 10.6

 Total 45.0 148.2 335.2 449.8 54.0 135.7 111.3 112.2
193 

Biomass (0-1 cm)

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000

Water Depth (m)

 Biomass (mgC m -2)

Nematodes & Copepods

Foraminifera

Figure 8-86. Scatter plot comparing foraminiferal biomass in the surface 1 

cm to that of meiofaunal copepods and nematodes combined. 

Biomass (0-3 cm)

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500 4000

Water Depth (m)

Biomass (mgC m -2)

Nematodes & Copepods

Foraminifera

Figure 8-87. Scatter plot comparing foraminiferal biomass in the top 3 cm 

to that of meiofaunal copepods and nematodes combined. 

8.8.3 Meiofauna 

8.8.3.1 Quantitative Distribution of Meiofauna 

8.8.3.1.1 Core Size Comparison, Sediment Sampling Depth, and Box Core Comparison 

More organisms were found in progressively larger cores (Table 8-15). But, there were no 

statistically significant differences for abundances of total meiofauna (P = 0.6324), nematodes (P 

= 0.7800), harpacticoids (P = 0.3385) and other taxa (P = 0.8238) among different core sizes. 

Thus, even though total abundance (adjusted for unit area) in the smallest core was about half 

that found in the three larger cores, it was not statistically different. Because each core yielded 

the same abundance estimate, total counts per core were used to choose the appropriate core size. 

For statistical purposes, it is imperative to obtain > 30 organisms per taxa per sample. Therefore, 

the 5.5 cm core was chosen for the benthic survey. 
194 

Nearly all meiofauna were found in surface sediments and no meiofauna were found below 

13 cm sediment depth, so just the top 13 cm are plotted (Figure 8-88). Most organisms were 

found in the top 3 cm. A total of 87% of total meiofauna were in the top 3 cm, and 97% of the 

harpacticoid copepods. In addition, 77% of the harpacticoid copepods were found in the top 1 

cm. Because the distribution is so skewed to the surface, it was decided to sample the top 3 cm 

only during the benthic community survey cruise. Because harpacticoid copepods were so 

restricted to the top 1 cm, the core was split into 2 sections: 0 - 1 cm and 1- 3 cm. 

There were no statistically significant differences for abundances of total meiofauna 

(P = 0.2281), nematodes (P = 0.0632), harpacticoids (P = 0.9999) and other taxa 

(P = 0.1988) among different box core types. Because of convenience, the GoMEX boxcore 

(Boland and Rowe 1991) was used throughout the study. 

8.8.3.1.2 General Results 

A total of 586 samples from 51 stations in the study yielded 1.71x105 individuals from 21 

meiofauna taxa. Samples were collected from a depth range of 200-3,700 meters in the northern 

Gulf of Mexico. Mean abundance (extrapolated to number of individuals per m2

, Nm-2) per 

station was 2.63x105

 Nm-2, with standard deviation of 2.01x105 (calculated from Table 8-16). 

Maximum and minimum meiofauna abundances were found at stations MT1 and JSSD3 with 

values of 9.46x105 and 0.60x105 Nm-2, respectively (Table 8-16). A strong linear relationship 

exists between log abundance (R2 = 0.658, P<0.0001) and depth (Figure 8-89). Spatial variability 

in meiofauna abundance (bubble size representing relative abundance) indicates highest values in 

the shallow northeastern stations (Figure 8-90. Relatively lower abundance was observed in the 

western transects, but the general trend of decreasing abundance was consistent along western 

transects (RW & W), and in the west-central area (WC5, WC12, B1-B3, NB2-NB5). 

Exceptionally high abundance was found at stations MT1, MT3, S35, S36, S42 and C7, all 

located in the northeast region at depths ranging from approximately 450 – 1,900 m. Variation 

from the general pattern of decreasing abundance with depth was observed at these northeastern 

stations.The meiofauna community was composed of individuals from 21 taxonomic groups 

(Table 8-17). Nematoda and Harpacticoida (including nauplii) were the two dominant groups, 

accounting for 65.3 and 25.4% of individuals, respectively. Unknown fauna were the next most 

abundant, comprising 6.6% of individuals. The unknown group likely included representatives 

from various soft-bodied taxa including (but not limited to) various taxa within the Turbellaria 

and representatives of the Protista (e.g., Ciliophora). Soft-bodied taxa, such as these, often 

become unrecognizable during bulk fixation with buffered formaldehyde. The remaining 3.7% of 

the meiofauna community was composed of representatives from various taxa, including: 

Polychaeta, Kinorhyncha, Ostracoda, Cyclopoida, Tardigrada, Tanaidacea, Nemertinea, Acari, 

Isopoda, Bivalvia, Gastrotricha, Anthozoa, Priapulida, Gastropoda, Aplacophora, Rotifera, 

Sipuncula, and Loricifera (Table 8-17). The complete major taxa data set is accessible from the 

DGoMB data base. 

The expected number of major taxa per 1,000 individuals [ES(1000)], followed a quadratic 

pattern, where major taxa diversity was maximized at stations just deeper than 1,000 meters, and 

then decreased with increasing depth (Figure 8-91A). This pattern was also observed when only 

non-dominant taxa are considered (Figure 8-91B). The ES(20) for non-dominant taxa (excluding 

nematodes, harpacticoids, nauplii, and polychaetes) followed a similar quadratic pattern, but 

major taxa diversity was highest at stations around 1,800 meters, decreasing thereafter. 
195 

Total Meiofauna (n/section)

0 100 200 300 400

Section Depth (cm)

0

2

4

6

8

10

12

14

Nematoda (n/section)

0 50 100 150 200

Section Depth (cm)

0

2

4

6

8

10

12

14

Harpacticoida (n/section)

0 20 40 60 80 100

Section Depth (cm)

0

2

4

6

8

10

12

14

Figure 8-88. Vertical distribution of meiofauna taxa from sediment cores collected at station W2 

during the shakedown cruise. Cores were sectioned in 1-cm increments, and 

fauna were enumerated from each 1-cm section. Plotted data is the average of 

five replicates (error bars = standard deviation). 
196 

Table 8-16 

 

DGoMB station Locations, Depth, and Average Meiofaunal Abundance 

(Five Replicate Cores) for Pooled Taxonomic Groups 

Station Latitude Longitude Depth 

(m) 

Abundance 

(N m-2) 

AC1 26.393567 -94.573082 2440 129974 

B1 27.202542 -91.405218 2253 157417 

B2 26.550012 -92.215082 2635 139907 

B3 26.164445 -91.735100 2600 155817 

BH 27.780000 -91.500000 545 407852 

C1 28.059838 -90.249917 336 369129 

C12 26.379730 -89.240298 2924 138792 

C14 26.938238 -89.572505 2495 146578 

C4 27.453150 -89.763083 1463 273585 

C7 27.730437 -89.982033 1066 542119 

GKF 27.000000 -90.250000 2460 84348 

HIPRO 28.550000 -88.580000 1565 343118 

JSSD1 25.000000 -92.000000 3545 87547 

JSSD2 23.500000 -92.000000 3725 87295 

JSSD3 24.750000 -90.750000 3635 60441 

JSSD4 24.250000 -85.500000 3400 63451 

JSSD5 25.500000 -88.250000 3350 135698 

MT1 28.541110 -89.825018 482 945657 

MT2 28.447925 -89.671945 677 535216 

MT3 28.221510 -89.494045 990 885995 

MT4 27.827605 -89.166145 1401 246058 

MT5 27.332838 -88.656065 2267 128964 

MT6 27.001648 -87.999130 2743 155312 

NB2 27.134833 -92.000068 1530 168276 

NB3 26.558033 -91.822550 1875 165245 

NB4 26.246750 -92.392287 2020 148409 

NB5 26.245400 -91.209908 2065 117263 

RW1 27.500142 -96.002847 212 411809 

RW2 27.254027 -95.746807 950 219457 

RW3 27.008356 -95.492362 1340 248752 

RW4 26.751420 -95.250175 1575 232842 

RW5 26.507527 -94.996722 1620 170633 

RW6 25.997303 -94.495578 3000 144453 

S35 29.335152 -87.046363 668 501629 

S36 28.918513 -87.672150 1826 799963 

S37 28.553627 -87.766848 2387 291179 

S38 28.279947 -87.327592 2627 157164 

S39 27.483675 -86.999815 3000 83170 

S40 27.839477 -86.751415 2972 99501 
197 

Table 8-16 DGoMB Station Locations, Depth, and Average Meiofaunal 

Abundance (Five Replicate Cores) for Pooled Taxonomic 

Groups (continued) 

Station Latitude Longitude Depth 

(m) 

Abundance 

(N m-2) 

S41 28.013642 -86.573348 2974 181408 

S42 28.251003 -86.419270 763 492537 

S43 28.502943 -86.076790 362 276279 

S44 28.749993 -85.747703 212 318516 

W1 27.577165 -93.551005 420 387228 

W2 27.413927 -93.340328 625 263315 

W3 27.172397 -93.323293 875 262642 

W4 26.730823 -93.319727 1460 187806 

W5 26.267772 -93.332723 2750 104552 

W6 26.002845 -93.320277 3150 124166 

WC12 27.323242 -91.555810 1175 218447 

WC5 27.775912 -91.765678 348 412061 

Water depth (m)

0 1000 2000 3000 4000

Log meiofauna abundance (N m-2) 4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

6.2

R2 = 0.658

P<0.0001

Figure 8-89. Log (x+1) transformed meiofauna abundance (N m-2) versus water depth (m) for all 

stations sampled during DGoMB project. 
198 

Figure 8-90. Spatial analysis of meiofauna abundance at all DGoMB stations. Buffer size 

equals relative meiofaunal abundance. Highlighted contours are 300 and 3,000 m 

from shallow to deep, respectively. 

Table 8-17 

 

Average Abundance (AA), Percent Contribution 

(Contrib.%), and Cumulative Percent Contribution (T%) 

of Meiofauna Major Taxa Per Core (5.5 cm i.d.) 

(Data summarized for all 51 stations (five replicates per 

station)) 

Taxa AA Contrib.% T% 

Nematoda 415.0 65.3 65.3 

Nauplii 74.8 13.1 78.4 

Harpacticoida 74.0 12.3 90.7 

Unknown 37.3 6.6 97.3 

Polychaeta 12.5 1.5 98.8 

Kinorhyncha 2.7 0.3 99.1 

Ostracoda 2.7 0.3 99.5 

Cyclopoida 2.1 0.2 99.6 

Tardigrada 1.0 0.1 99.8 

Tanaidacea 0.8 0.1 99.9 

Nemertinea 0.6 0.1 99.9 

Acari 0.3 0.0 100.0 

Isopoda 0.3 0.0 100.0 

Bivalvia 0.2 0.0 100.0 

Gastrotricha 0.1 0.0 100.0 

Anthozoa 0.1 0.0 100.0 

Priapulida 0.1 0.0 100.0 

Gastropoda 0.0 0.0 100.0 

Aplacophora 0.0 0.0 100.0 

Rotifera 0.0 0.0 100.0 

Sipuncula 0.1 0.0 100.0 

Loricifera 0.0 0.0 100.0 
199 

ES(1000)

6

7

8

9

10

11

12

13

14

AC1

B1

B2

B3

BH

C1

C12

C14

C7 C4

GKF

HIPRO

JSSD1

JSSD2 JSSD3

JSSD4

JSSD5

MT1

MT2

MT3

MT4 MT5

MT6

NB2

NB3

NB4 NB5

RW1

RW2

RW3 RW4

RW5

RW6

S35

S36

S37

S38 S39

S40

S42 S41

S43

S44

W1

W2

W3

W4

W5

W6

WC12

WC5

R2 = 0.213

P = 0.0032

Water depth (m)

0 1000 2000 3000 4000

ES(20) Non-dominant taxa

2

3

4

5

6

7

8

B1 AC1

B2

B3

BH

C1

C12

C14

C4

C7

GKF

HIPRO

JSSD1

JSSD2JSSD3

JSSD4

JSSD5

MT1

MT2 MT3

MT4

MT5

MT6

NB2 NB3

NB4

NB5

RW1

RW2

RW3 RW4

RW5

RW6

S35 S36

S37

S38

S40 S39

S41

S42

S43

S44

W1

W2

W3 W4

W5

W6

WC12

WC5

R2 = 0.156

P<0.0001

A.

B.

Figure 8-91. A) Expected number of taxa per 1,000 individuals [ES(1000)] as a 

function of depth. B) Expected number of taxa per 20 individuals [ES(20)] for nondominant meiofauna taxa (excluding nematodes, harpacticoid copepods, harpacticoid 

nauplii, and unknowns). A quadratic regression was fit to the data (dashed lines = 95% 

confidence intervals). 
200 

8.8.3.1.3 Hypothesis Testing 

In the test for differences over depth and longitude (H01 & H02), significant main effects for 

longitude and depth were observed (P<0.0001, Table 8-18), as well as a significant longitude by 

depth interaction term (P<0.0001). The two western transects (RW and W) had a gradual (very 

linear) decrease in abundance with depth (Figure 8-90). With increasing proximity to the 

Mississippi River, (transects C and MT) abundance increased greatly at stations between 300 and 

1,000 meters (Figure 8-90). The Florida slope transect (S) had highest abundance at station S36 

(ca. 2,000 meters), which was located in the DeSoto Canyon. Transects became more similar 

with increasing depth, with abundance being very similar at all stations > 2,500 meters water 

depth (Figure 8-90). 

Table 8-18 

 

ANOVA Results, Tests for Differences in Meiofauna Abundance 

(Dependent variable = log10 (N+1). ANOVA abbreviations: DF = degrees of freedom, SS = sum 

of squares, MS = mean square, F = F-test value, P = Pr > F. Factor abbreviations: long. = 

longitude, basin = basin vs. non-basin stations, dfs = distance from shore, escarp. = escarpment 

vs. non-escarpment transects.) 

Source DF SS MS F P 

H01 & H02 Depth/Longitude 

 Long. 4 1.222 0.305 12.07 <.0001 

 Depth 4 5.867 1.467 57.94 <.0001 

 Long.*Depth 16 3.104 0.194 7.67 <.0001 

 Error 100 2.531 0.025 

H03 Basins 

 Basin 1 0.012 0.012 0.38 0.5421 

 DFS 2 0.016 0.008 0.25 0.7773 

 Basin*DFS 2 0.023 0.011 0.37 0.6980 

 Error 56 1.708 0.031 

H04 Canyons 

 Canyon 1 0.167 0.167 8.58 0.0059 

 Depth 4 3.931 0.982 50.41 <.0001 

 Canyon*Depth 4 0.338 0.085 4.34 0.0058 

 Crror 36 0.702 0.019 

H05 Escarpment 

 Escarp. 1 0.001 0.001 0.07 0.7906 

 DFS 5 2.998 0.600 50.51 <.0001 

 Escarp.*DFS 5 0.279 0.056 4.70 0.0015 

 Error 46 0.546 0.012 

In the test for differences between basin and adjacent non-basin stations (H03), no significant 

difference was observed between main effects (P = 0.5421, and P = 0.7773, Table 8-18), and no 

significant interaction was observed (P = 0.6980, Table 8-18). 

In the test for differences between canyon and adjacent non-canyon stations (H04), 

significant treatment (P = 0.0059) and depth zone (P<0.0001) effects were observed (Table 8-

18), but a significant interaction term was also observed (P = 0.0058, Table 8-18). Abundance 

was elevated at the head of the canyon (MT1 & MT3) compared to stations of similar depth in 

the adjacent transect (C1 & C7; Figure 8-90). However, the two transects became increasingly 
201 

similar with depth, and showed no differences at stations greater than 1,500 meters (Figure 8-

90). 

In the test for differences in meiofauna abundance due to the presence of an escarpment 

(H05), no significant escarpment effect was observed in comparison to the reference transect 

(P = 0.791). However, a significant main effect was observed for distance from shore (P<0.0001, 

Table 8-18), and a significant interaction term was observed (P = 0.002, Table 8-18). Abundance 

was dramatically lower below the Florida Escarpment than above (Figure 8-90). Station S41 had 

nearly twice the abundance of stations S40 and S39, which are of similar depth, but further 

offshore. Comparison to the reference transect in the western gulf (stations W1-W6) illustrates 

deviation of the escarpment transect from an expected decrease in abundance along a relatively 

constant slope, with elevated abundance just above and below the escarpment (Figure 8-90). 

The amount of chl-a biomass in the overlying water column that reaches the sea floor 

decreases in a log-log relationship with depth (Figure 8-92A). Meiofauna abundance increased 

with increasing chl-a in the water column (Figure 8-92B). Although considerable variability 

exists in the linear regression (R2 = 0.413), the relationship was significant (P<0.0001). 

Accordingly, meiofauna had a moderate, and significant relationship with sediment POC 

concentration (Figure 8-93, R2 = 0.331). Although null hypothesis six (H06) was not tested with 

ANOVA as above, detailed analysis of the meiofauna community with respect to food 

availability and sediment environmental variables was accomplish with multivariate procedures 

(Section 9). 

8.8.3.2 Harpacticoida (Copepoda) Community Structure 

Harpacticoid copepods were collected from 423 samples at 43 locations on the continental 

slope of the northern Gulf of Mexico. In total 12,480 individuals were collected, of which 3654 

were identifiable adult harpacticoid copepod individuals, 696 species were identified from 22 

families and 175 genera (see Appendices for complete species list, grouped by family). Nine 

families accounted for approximately 93% of all harpacticoida (Table 8-19), and two, Tisbidae 

and Ectinosomatidae, accounted for 46%. Only 182 of these species (27%) have been formally 

described in the literature. 

Average abundance over all stations (from five pooled replicate cores = 118.8 cm2) was 172 

± 94, with maximum and minimum values of 412 and 54, found at stations WC5 and MT6, 

respectively (Table 8-20). The average number of species was 52 ± 19 (from pooled replicate 

cores) with maximum and minimum values of 104 and 23, found at stations WC5 and MT6, 

respectively (Table 8-20). Total abundance (N) and species richness (S) decreased significantly 

(R2

 = 0.466 and 0.495, respectively) with increasing water depth (Figure 8-94A & B). The 

number of species was highly correlated to the number of individuals encountered (r = 0.91). 

However, rate of decrease with water depth was approximately five times greater for abundance 

than species richness (slope = -0.068 and -0.014, respectively). 

Shannon-Wiener diversity (H’

) decreased in a strong relationship with depth 

(R2

 = 0.501, P<0.0001, Figure 8-95). H’ had a mean of 3.73 and standard deviation of 0.32 

(calculated from Table 8-20). Maximum values of H’ were found at stations S35, WC5, MT2 and 

MT1, while minimum values were found at stations NB2, NB4, B1, B2 and MT6 (Table 8-20). 

The expected number of species per 30 harpacticoid individuals [ES(30)] formed a moderate 

non-linear, unimodal relationship with depth (R2

 = 0.312, P = 0.0006, Figure 8-96). Mean 

ES(30) was 22.24 with standard deviation of 2.08 (calculated from Table 8-20). Maximum 
202 

Figure 8-92. SeaWIFS chl-a (μg/L) biweekly average (November 1999 through April 

2000). Chl-a concentration was adjusted for remineralization with depth. A) 

Inverse first-order relationship of adjusted chl-a with water depth. Chl-a 

concentration decreases with depth reflecting rapid remineralization in 

surface waters. B) Meiofauna abundance versus adjusted chl-a. 
203 

Sediment POC (μM)

2468

Meiofauna abundance (N m-2)

0

2e+5

4e+5

6e+5

8e+5

1e+6

R2 = 0.331

P<0.0001

Figure 8-93. Meiofauna abundance (Nm-2) as a function of sediment particulate organic carbon 

(POC). 
204 

Table 8-19 

 

Results of SIMPER Analysis (Primer 5.0) Indicating Family Percent 

Contributions to Total Harpacticoid Abundance 

(AA = Average abundance, Contrib.% = percent contribution of family, 

T% = cumulative percent contribution of families.) 

Family AA Contrib.% T% 

 

Tisbidae 40.19 32.98 32.98 

Ectinosomatidae 24.12 13.27 46.24 

Diosaccidae 19.74 9.84 56.09 

Ameiriidae 15.71 8.24 64.33 

Argestidae 11.00 8.08 72.41 

Paranannopidae 9.15 6.50 78.91 

Canthocamptidae 12.38 6.03 84.95 

Paramesochriidae 6.73 4.15 89.10 

Cletodidae 6.62 3.42 92.52 

Neobradyidae 2.73 1.39 93.91 

Thalestridae 2.34 1.09 95.00 

Normanellidae 2.41 1.09 96.08 

Cerviniidae 2.55 1.05 97.13 

Danielssenidae 3.55 0.93 98.06 

Huntemannidae 1.70 0.93 98.99 

Unid. family 1.79 0.61 99.60 

Ancorabolidae 1.21 0.32 99.93 

Laophontidae 0.42 0.03 99.96 

Canuellidae 0.28 0.03 99.99 

Darcythompsonidae 0.19 0.01 100.0 

Longipedidae 0.16 0.00 100.0 

Euterpinidae 0.05 0.00 100.0 

Table 8-20 

 

Total Species (S) and Total Individuals (N) Per Five Pooled Replicates Cores 

(= 118.8 cm2

) (Species diversity indices: expected species per 30 individuals 

[ES(30)], Shannon-Wiener diversity (H’), average taxonomic diversity (Δ), and 

average phylogenetic diversity (Φ+

) at each of the 43 stations where harpacticoid 

copepods were identified to species.) 

Station Depth S N ES(30) H’ Δ Φ+

AC1 2440 51 114 24.91 3.90 102.14 59.79 

B1 2253 27 74 19.56 3.21 105.25 70.97 

B2 2635 32 90 20.27 3.33 102.51 62.74 

B3 2600 37 108 21.00 3.46 96.59 64.92 

C1 336 51 195 19.70 3.73 96.85 58.63 

C12 2924 34 125 17.34 3.31 95.37 70.14 
205 

Table 8-20 Total Species (S) and Total Individuals (N) Per Five Pooled Replicates 

Cores (= 118.8 cm2

) (Species diversity indices: expected species per 

30 individuals [ES(30)], Shannon-Wiener diversity (H’), average 

taxonomic diversity (Δ), and average phylogenetic diversity (Φ+

) at 

each of the 43 stations where harpacticoid copepods were identified to 

species.)(continued) 

Station Depth S N ES(30) H’ Δ Φ+ 

C14 2495 33 118 17.37 3.31 93.49 66.12 

C4 1463 56 148 23.98 3.91 101.77 63.23 

C7 1066 74 212 24.64 4.14 96.27 54.62 

MT1 482 69 322 22.68 3.95 91.44 57.51 

MT2 677 67 338 21.45 3.82 92.72 53.06 

MT3 990 81 418 21.68 3.92 92.99 55.02 

MT4 1401 53 144 23.80 3.87 101.76 63.93 

MT5 2267 44 110 23.35 3.71 102.31 62.27 

MT6 2743 23 54 18.92 3.04 111.88 79.92 

NB2 1530 40 112 22.14 3.59 99.71 64.78 

NB3 1875 50 120 24.02 3.83 101.57 63.81 

NB4 2020 39 108 21.76 3.54 100.51 63.91 

NB5 2065 36 94 21.90 3.50 101.88 65.86 

RW1 212 66 240 23.54 3.99 95.83 57.38 

RW2 950 57 178 23.41 3.89 97.50 59.27 

RW3 1340 61 174 23.47 3.92 95.49 57.34 

RW4 1575 57 148 24.28 3.94 101.27 65.85 

RW5 1620 43 112 22.56 3.64 101.49 66.40 

RW6 3000 49 130 22.99 3.74 96.16 61.75 

S35 668 89 388 23.02 4.10 93.75 54.72 

S36 1826 74 306 23.14 4.02 95.58 56.98 

S37 2387 70 198 24.49 4.09 99.48 60.17 

S38 2627 44 108 23.36 3.71 100.96 63.09 

S39 3000 38 96 21.85 3.50 102.23 66.89 

S40 2972 30 78 19.94 3.24 105.63 71.18 

S41 2974 31 84 20.69 3.35 104.42 72.10 

S42 763 58 178 23.33 3.89 98.33 57.55 

S43 362 57 178 23.17 3.87 95.31 56.08 

S44 212 41 133 20.14 3.64 99.64 64.76 

W1 420 94 306 25.46 4.37 96.45 56.50 

W2 625 65 214 23.12 3.93 97.02 56.36 

W3 875 65 212 23.55 3.98 96.13 57.16 

W4 1460 52 146 23.79 3.86 100.75 66.58 

W5 2750 35 86 22.10 3.49 101.27 64.93 

W6 3150 33 108 17.39 3.02 90.01 68.15 

WC12 1175 52 180 21.75 3.71 98.55 62.62 

WC5 348 104 412 25.26 4.42 96.28 55.24 
206 

A.

0 1000 2000 3000

N 10cm-2

0

5

10

15

20

25

30

35

40

B.

Water depth (m)

0 1000 2000 3000

S 10cm-2

0

2

4

6

8

10

R2 = 0.466

P<0.0001

R2 = 0.495

P<0.0001

Figure 8-94. Harpacticoid copepod abundance (N) and species richness (S), adjusted 

to the number per 10 cm2

, as a function of depth. Abundance and 

richness are highly correlated (r = 0.91). 
207 

Water depth (m)

0 1000 2000 3000

Shannon-Wiener Diversity (H')

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

AC1

B1

B2

B3

C1

C14 C12

C4

C7

MT1

MT2

MT3

MT4

MT5

MT6

NB2

NB3

NB4 NB5

RW1

RW2 RW3 RW4

RW5

RW6

S35

S36

S37

S38

S39

S40

S41

S42 S43

S44

W1

W2 W3

W4

W5

W6

WC12

WC5

R2 = 0.501

P<0.0001

Figure 8-95. Shannon-Wiener diversity index (H’) as a function of depth for pooled 

replicate core samples of harpacticoid copepods. 

Water depth (m)

0 1000 2000 3000

ES(30)

16

18

20

22

24

26

AC1

B1

B2

B3

C1

C14 C12

C4

C7

MT1

MT2 MT3

MT4

MT5

MT6

NB2

NB3

NB4 NB5

RW1 RW2 RW3

RW4

RW5

S35 RW6 S36

S37

S38

S39

S40

S41

S42 S43

S44

W 1

W2

W3 W4

W5

W6

WC12

WC5

R2 = 0.312

P = 0.0006

Figure 8-96. Expected number of harpacticoid species per 30 individuals [ES(30)], for 

pooled replicate core samples of harpacticoid copepods. 
208 

values of ES(30) were found at stations WC5 and W1, while minimum values were found at 

stations C12, C14, and W6. Although the relationship was moderately significant, ES(30) was 

highly variable at both shallow and deep stations (Figure 8-96). 

Average taxonomic diversity (Δ), the average taxonomic distance apart of any two 

individuals chosen at random within a sample, increased with increasing water depth (R2

 = 

0.185, P = 0.004, Figure 8-97A). Determination of average taxonomic diversity was based on a 

4-level taxonomic scheme, from species to order. Δ had a mean value of 98.76 and standard 

deviation of 4.24 (calculated from Table 8-20). Highest average taxonomic diversity was found 

at station MT6 (111.88), with lowest values at station MT1 (91.44). Although the relationship is 

moderately significant, variance in Δ increased at deep stations (Figure 8-97A). Another index of 

taxonomic relatedness is average phylogenetic diversity (Φ+), which is the cumulative branch 

length of the phylogenetic tree within each sample. As with average taxonomic diversity (above), 

average phylogenetic diversity was calculated with a 4-level taxonomic scheme, from species to 

order. Average phylogenetic diversity followed a strong, and highly signficant, increasing trend 

with depth (R2 = 0.500, P<0.0001, Figure 8-97B). The mean value of average phylogenetic 

diversity was 62.33 with standard deviation of 5.73 (calculated from Table 8-20). Maximum 

average phylogenetic diversity of 79.92 was found at station MT6, while the minimum was 

54.62 at station C7 (Table 8-20). Both Δ and Φ+

 suggest proportionally more higher order taxa 

(genera and families) per individual with increasing depth. 

8.8.3.2.1 Hypothesis Testing 

In the test for differences between depth and longitude (H01 and H02), a highly significant 

depth main effect was observed (P<0.0001, Table 8-21), but a weak significant interaction was 

also observed (P = 0.0203). Overall, harpacticoid Φ+ responded similarly over most of the 

transects with a small peak in Φ+

 at mid depths (approx.1,500 m) followed by relatively constant 

diversity until a second peak at depths greater than 3,000 meters. The eastern stations (S transect) 

had a decrease in Φ+ at shallow depths (S44 to S35), but then a constant increase in Φ+ with 

increasing depth. 

In the test for differences in Φ+ between basin and non-basin stations (H03), no significant 

differences were observed for main effects, and no interaction was observed (Table 8-21). 

Similarly, in the test for canyon effects (H04) no significant difference in Φ+

 was observed 

between canyon and non-canyon transects, but a significant depth effect was observed 

(P<0.0001, Table 8-21). No significant canyon by depth interaction was observed. In the test for 

escarpment effects on Φ+, a weak transect main effect was observed (P = 0.035) but strongly 

significant interaction between transect and distance from first station was observed (P = 

0.0003). There was a peak in diversity at the two deep stations below the Florida Escarpment 

(S40 and S41), and then a decrease in diversity moving away from the escarpment (S39). The 

western (W) transect had a peak in Φ+

 at approximately 1,500 meters (W4), and a second peak at 

the deepest station (W6). 
209 

Avg. taxonomic diversity (Delta)

85

90

95

100

105

110

115

R2 = 0.185

P = 0.004

Water depth (m)

0 1000 2000 3000

Avg. phylogenetic diversity (Phi+)

50

55

60

65

70

75

80

R2 = 0.500

P<0.0001

A.

B.

Figure 8-97. A) Average taxonomic diversity (Δ) and B) average phylogenetic diversity (Φ+) as 

a function of depth for the 43 original survey stations. Diversity indices were 

calculated based on the average species abundance from five replicate core 

samples. 
210 

Table 8-21 

 

ANOVA Results of Test for Differences in Harpacticoida Diversity 

(Dependent variable is average phylogenetic diversity (Φ+). DFS = distance from 

shore; DFFS = distance from first station.) 

Source DF SS MS F Value Pr > F 

H01&02 

 Transect 4 110.02 27.51 0.50 0.7332 

 Depth 4 2198.28 549.57 10.06 <.0001 

 T*Depth 16 1749.36 109.34 2.00 0.0203 

 Error 96 5243.96 54.62 

H03 

 Treatment 1 16.45 16.45 0.18 0.6711 

 DFS 2 109.87 54.93 0.62 0.5479 

 T*DFS 2 57.33 28.67 0.32 0.7277 

 Error 23 2045.74 88.95 

H04 

 Transect 1 2.67 2.67 0.05 0.8312 

 Depth 4 1870.96 467.74 8.08 <.0001 

 T*Depth 4 302.41 75.60 1.31 0.2855 

 Error 37 2141.06 57.87 

H05 

 Transect 1 241.31 241.31 4.73 0.0347 

 DFFS 5 56.15 11.23 0.22 0.9521 

 T*DFFS 5 1450.89 290.18 5.69 0.0003 

 Error 47 2396.81 50.99 

8.8.3.2.2 Spatial and Bathymetric Species Zonation 

The concept of geographic or bathymetric zonation was analyzed using cluster and 

subsequent Geographical Information System (GIS) analysis (Figure 8-98). Cluster analysis was 

performed on the harpacticoid data set implementing Bray-Curtis similarity and group average 

linking (not shown, see Baguley 2004). Minimum Bray-Curtis similarity, i.e. the similarity of all 

43 survey stations, was 8.4%. Groups, or “zones,” were chosen on the basis of >20% similarity. 

A total of 17 zones were determined, with 1-7 stations per zone (Figure 8-98). Stations that did 

not group with at least one other station at the 20% level were designated as unique zones. 

Highest zone similarity was found for stations MT1, MT2 and MT3 (36.8%), and highest 

similarity between any two stations was 48.4%, for stations MT2 and MT3. All other zones, that 

included at least two stations, had similarity values between 20 and 36%.

8.8.3.2.3 Regional and Global Species Estimates 

Two Gulf of Mexico data sets were analyzed in order to extrapolate to regional and global 

scales (Montagna and Harper 1996). Species accumulation curves (Figure 8-99) were 

constructed for each data set. The sigmoidal growth model parameters for the two data sets were 

as follows: NGoM (DGoMB data), a = -16.6, b = 541.1, c = 1739.7, and d = 0.72; NWGoM 

(Montagna and Harper 1996), a = -2.28, b = 166.08, c = 457.06, and d = 0.57. The models for 
211 

Figure 8-98. Geographic location of recurrent groups of harpacticoid copepods, based on 

percent similarity (see text). Highlighted contour lines are at 300 and 3,000 m 

water depth. 

Figure 8-99. Species accumulation curves used to estimate regional Harpacticoida species 

abundance in the Gulf of Mexico by extrapolation. 
212 

both NGoM and NWGoM fitted the data very closely, R2

 = 0.99996 and 0.99980, respectively. 

The model interpolation indicates that asymptotes exist at 1740 and 457 species for deep-sea 

(NGoM) and shallow (NWGoM) regions of the Gulf of Mexico respectively.

Summing the shelf and deep-sea estimates yields the regional biodiversity of the entire Gulf 

of Mexico at approximately 2200 species, assuming there is no overlap between shallow shelf 

and deep-sea species. The approximate area of the Gulf of Mexico is 1.5x106 km2, which is 

about 0.4 % of the world’s oceans. Assuming the rate of increase of new species with area 

remains constant then extrapolation suggests a global species richness of 5.5x105

 for the 

Harpacticoida. 

8.8.3.3 Meiofauna Biomass 

Meiofauna biomass is dominated by the two dominant taxa, Nematoda and Harpacticoida 

(Table 8-22). Mean biomass per station was 273 mg wet weight m-2 (43.3 mg C m-2). Maximum 

and minimum biomass values of 157.1 and 3.5 mg C m-2 were found at stations S42 and JSSD3, 

respectively (Table 8-22). A strong linear relationship exists between log meiofauna biomass (R2

= 0.726, P<0.0001) and water depth (Figure 8-100). A general trend of decreasing biomass per 

individual with increasing water depth was observed for Nematoda (R2 = 0.125, P = 0.0202, 

Figure 8-101A), while Harpacticoida had a general increasing trend of biomass per individual 

with increasing water depth (R2 = 0.167, P = 0.0066, Figure 8-101B). Spatial trends in 

meiofauna biomass (Figure 8-102) closely parallel those observed with abundance (See previous 

subsections). However, highest biomass was observed at station S42 (Table 8-21, Figure 8-102). 

High biomass at station S42 reflects proportionally larger nematode individuals at this location 

(Figure 8-101A). 

The general relationship of decreasing meiofauna biomass with water depth was seen over 

the entire northern GoM continental margin. Variance from this pattern was observed primarily 

in the northeastern GoM (Figure 8-102), where highest biomass was often observed at mid-depth 

stations (S36, S42, MT3). Interpolated biomass estimates over the entire sampling region, and to 

the extent of the available bathymetry, illustrates the general pattern of decreasing biomass with 

depth, and also highlights biomass hot spots (Figure 8-102). Average biomass per 2.8 km2 cell, 

as calculated by the model, was 2.7x105

 ± 1.6x105 μg wet mass m-2. A total of 83,844 cells (each 

2.8 km2

) were created by the model, equaling a total area of 6.57x105 km2. Total meiofaunal 

biomass within the region was found to be 2.1x107

 ± 1.2x107 kg Carbon (wet mass converted to 

carbon using conversion factors of 0.25 dry/wet mass, and 0.48 carbon/dry mass, Baguley et al.

2004). 

8.8.3.4 Allometric Respiration Estimates and Community Carbon Transfer 

The mean mass-dependent respiration rate of meiofauna was estimated using the average 

individual biomass at each DGoMB station (Table 8-22, Figure 8-103A). Mean respiration (d-1) 

increased as a function of depth, in a weak, but significant linear relationship (Figure 8-103A, R2

= 0.256, P<0.0001). Mean respiration (d-1) increased with decreasing average biomass (Table 8-

22). Meiofauna community respiration (mg C m-2 d-1) decreased in a strong linear relationship 

with depth (Figure 8-103B, R2 = 0.598, P<0.0001). Variance in community respiration was 

higher in shallower water (200-1,500 meters). Meiofaunal respiration ranged from a low of 0.3 

mg C m-2 d-1 at stations JSSD2-4 to a high of 6.3 mg C m-2 d-1 at S36 and S42. 
213 

Table 8-22 

 

Biomass Contribution of the Major Taxonomic Groups 

to Total Meiofaunal Biomass (mg C m-2) at Each DGoMB Station 

(NEMA = Nematoda, HARP = Harpacticoida, NAUP = Harpacticoida nauplii, POLY = Polychaeta, OSTR 

= Ostracoda, CYCL = Cyclopoida, TANA = Tanaidacea, ISOP = Isopoda, KINO = Kinorhyncha.) 

STA NEMA HARP NAUP POLY OSTR CYCL TANA ISOP KINO 

AC1 17.52 6.83 0.18 0.20 0.08 0.09 0.30 0.00 0.01 

B1 23.97 8.60 0.36 0.22 0.31 0.05 0.10 0.07 0.02 

B2 12.50 3.52 0.34 0.25 0.12 0.00 0.20 0.07 0.00 

B3 11.88 5.41 0.55 0.28 0.04 0.00 0.22 0.00 0.01 

BH 23.31 20.80 0.98 1.42 0.48 0.16 0.50 0.40 0.02 

C1 89.37 8.04 0.64 1.27 0.24 0.24 0.38 0.00 0.04 

C12 12.83 7.27 0.54 0.17 0.10 0.04 0.13 0.00 0.01 

C14 14.30 7.45 0.32 0.17 0.12 0.02 0.38 0.00 0.01 

C4 60.26 9.12 0.79 0.36 0.44 0.07 0.30 0.13 0.01 

C7 69.86 10.97 0.68 0.81 0.33 0.09 0.45 0.00 0.02 

GKF 3.20 2.87 0.13 0.10 0.00 0.00 0.00 0.00 0.01 

HIPRO 9.24 8.88 0.44 1.71 0.25 0.37 0.30 0.07 0.00 

JSSD1 1.76 2.32 0.30 0.13 0.00 0.00 0.00 0.00 0.00 

JSSD2 1.90 1.57 0.26 0.14 0.02 0.00 0.00 0.13 0.00 

JSSD3 1.51 1.54 0.13 0.13 0.06 0.00 0.00 0.07 0.00 

JSSD4 1.28 1.87 0.22 0.13 0.07 0.00 0.00 0.00 0.00 

JSSD5 3.51 2.71 0.25 0.13 0.15 0.00 0.20 0.00 0.00 

MT1 110.02 6.44 0.56 1.32 0.74 0.40 0.29 0.05 0.02 

MT2 45.12 8.22 0.82 3.07 0.33 0.94 1.70 0.20 0.05 

MT3 87.39 15.57 0.76 1.43 0.75 0.34 0.60 0.20 0.04 

MT4 33.69 9.46 0.51 0.75 0.21 0.16 0.60 0.07 0.01 

MT5 24.92 4.38 0.33 0.42 0.23 0.09 0.50 0.00 0.01 

MT6 9.99 1.37 0.18 0.17 0.03 0.04 0.05 0.03 0.00 

NB2 15.55 6.24 0.43 0.36 0.17 0.02 0.20 0.35 0.01 

NB3 17.31 8.12 0.91 0.42 0.04 0.00 0.20 0.00 0.01 

NB4 10.23 6.51 0.46 0.21 0.04 0.02 0.30 0.07 0.00 

NB5 10.63 9.68 0.35 0.15 0.15 0.02 0.30 0.07 0.01 

RW1 66.47 9.47 1.06 1.04 0.50 0.39 0.10 0.20 0.03 

RW2 23.33 7.96 0.56 0.53 0.30 0.24 0.56 0.07 0.02 

RW3 36.20 7.45 0.53 0.59 0.38 0.10 0.45 0.00 0.01 

RW4 37.36 7.74 0.53 0.61 0.02 0.19 0.70 0.00 0.03 

RW5 24.12 8.61 0.35 0.29 0.12 0.05 0.10 0.13 0.01 

RW6 16.95 10.46 0.28 0.25 0.06 0.02 0.20 0.00 0.01 

S35 49.83 14.81 1.34 1.59 0.67 0.58 1.00 0.47 0.05 
214 

Table 8-22 Biomass Contribution of the Major Taxonomic Groups to Total Meiofaunal Biomass (mg 

C m-2) at Each DGoMB Station (NEMA = Nematoda, HARP = Harpacticoida, NAUP = 

Harpacticoida nauplii, POLY = Polychaeta, OSTR = Ostracoda, CYCL = Cyclopoida, 

TANA = Tanaidacea, ISOP = Isopoda, KINO = Kinorhyncha (continued )) 

STA NEMA HARP NAUP POLY OSTR CYCL TANA ISOP KINO 

S36 98.41 11.48 0.89 0.83 0.43 0.09 0.70 0.20 0.02 

S37 20.51 10.73 0.71 0.53 0.33 0.16 0.40 0.20 0.01 

S38 9.70 6.46 0.48 0.38 0.15 0.11 0.20 0.00 0.01 

S39 6.37 3.10 0.21 0.25 0.04 0.05 0.20 0.07 0.00 

S40 9.82 5.95 0.31 0.35 0.15 0.19 0.30 0.07 0.00 

S41 23.79 5.33 0.27 0.21 0.10 0.04 0.22 0.04 0.00 

S42 141.54 12.92 0.68 0.72 0.17 0.22 0.30 0.13 0.02 

S43 36.72 6.98 0.37 0.95 0.19 0.30 0.50 0.07 0.03 

S44 29.38 7.48 0.69 1.83 0.38 0.60 0.50 0.00 0.02 

W1 46.80 17.31 1.16 1.19 0.33 0.37 0.50 0.07 0.04 

W2 41.97 10.77 0.62 0.62 0.13 0.32 1.00 0.07 0.01 

W3 58.87 10.97 0.95 0.55 0.33 0.35 0.50 0.07 0.02 

W4 16.81 8.73 0.56 0.55 0.15 0.18 0.40 0.00 0.00 

W5 10.23 4.50 0.19 0.09 0.04 0.00 0.30 0.00 0.01 

W6 15.95 5.94 0.27 0.28 0.02 0.05 0.10 0.07 0.01 

WC12 61.48 14.56 0.57 0.53 0.37 0.02 0.80 0.07 0.00 

WC5 81.55 13.25 1.04 1.00 0.15 0.41 0.80 0.20 0.03 

W a te r d e p th (m )

0 1000 2000 3000 4000

Log meiofauna biomass (ug Wet mass/m2

)

4 .2

4 .4

4 .6

4 .8

5 .0

5 .2

5 .4

5 .6

5 .8

6 .0

6 .2

AC1

B 1

B 2 B 3

B H

C 1

C12 C14

C 4 C 7

GKF

H IP R O

JSSD1 JSSD2 JSSD4JSSD3

JSSD5

MT1

MT2

MT3

MT4

MT5

MT6

NB2 NB3

NB4

NB5

RW 1

RW 2

RW 3 RW 4

RW 5

RW 6

S35

S36

S37

S38

S39

S40

S41

S42

S43 S44

W 1

W 2

W 3

W 4

W 5

W 6

W C12

W C5

R 2 = 0 .7 0 0

P < 0 .0 0 0 1

Figure 8-100. Meiofauna biomass (μg wet wt/m2) versus water depth at all DGoMB stations. 
215 

A. Nematoda

0 1000 2000 3000

Avg. wet weight ind-1 (ug)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

AC1

B1

B3B2

C1

C12 C14

C4

C7

MT1

MT2

MT3

MT4

MT5

MT6

NB2

NB3

NB4

NB5

RW1

RW2

RW3

RW4

RW5

RW6

S35

S36

S37

S38

S39

S40

S41

S42

S43

S44

W1

W2

W3

W4 W5

W6

WC12

WC5

R2 = 0.122

P = 0.022

B. Harpacticoida

Water depth (m)

0 1000 2000 3000

Avg. wet weight ind-1 (ug)

1

2

3

4

5

6

AC1

B1

B2

B3

C1

C12

C14

C4

C7

MT1

MT2

MT3

MT4

MT5

MT6

NB2

NB3

NB4

NB5

RW1

RW2

RW3

RW4

RW5

RW6

S35

S36

S37

S38

S39

S40

S41

S42

S43

S44

W1

W2

W3

W4

W5

W6

WC12

WC5

R2 = 0.167

P = 0.006

Figure 8-101. Average wet mass (mg) per individual A) Nematode, and B) Harpacticoid, as a 

function of depth. Data is for the 43 original survey stations, and based on 

biomass estimates from 12,288 nematode and 13,279 harpacticoid individuals. 
216 

Figure 8-102. Spatial interpolation of meiofaunal biomass (mg C m-2) in the northern Gulf of 

Mexico deep sea, using the inverse-distance weighted model. Highlighted 

contour lines are at 300 and 3,000 m from shallow to deep, respectively. 
217 

Water depth (m)

0 1000 2000 3000 4000

Meiofauna community respiration (mg C m-2 d-1) 0

1

2

3

4

5

6

7

R2 = 0.598

P<0.0001

Water depth (m)

0 1000 2000 3000 4000

Meiofauna mass-dependent respiration rate (d-1) 0.02

0.04

0.06

0.08

0.10

0.12

R2 = 0.256

P<0.0001

Figure 8-103. A) Meiofauna mass-dependent respiration rate (d-1) and B) 

meiofauna community respiration (mg C m-2 d-1) at each of the 51 

DGoMB stations in the northern Gulf of Mexico deep-sea. 
218 

Geographic variation in community organic carbon requirement (assuming respiration = 80% 

of total metabolic requirement) was observed mainly at water depths less than 1,500 meters 

(Figure 8-104). Stations MT1 and MT3 in the Mississippi Trough, station S36 located in the De 

Soto Canyon, and station S42 directly above the Florida Escarpment, had the highest community 

organic carbon requirement. These four stations were located in the northeastern Gulf of Mexico 

and also had highest biomass. Stations in the western Gulf of Mexico had comparably lower 

community respiration and lower biomass (Figure 8-102 and 8-104). The theoretical organic 

carbon requirement over the northern GoM deep sea study area was estimated to be 1.5x106 (± 

8.4x105

) mg C km-2 d-1. The ratio of total estimated organic carbon requirement to total estimated 

Figure 8-104. Spatial interpolation of the meiofaunal commun ity daily carbon requirement 

(mg C m-2 d-1), based on allometric estimations of community respiration. Respiration is 

conservatively assumed to be 80% of total carbon consumption. 

biomass was 0.07 (1.5x106 kg C d-1/2.1x107 kg C) in units of d-1; which equals 7% of the 

community biomass. Thus the turnover time of living carbon in the meiofauna fraction of the 

community was approximately 14 days, on average for the entire continental margin.

As a conservative estimate, respiration was assumed to account for 80% of the total 

metabolic budget. The mean meiofaunal respiration rate increased with increasing water depth, 

reflecting overall decrease in animal size with depth (Figure 8-103A), although harpacticoids 

displayed an opposing trend (Figure 8-101B). Meiofauna community respiration decreased with 

depth (Figure 8-103B), reflecting a decrease in overall biomass. 
219 

Geographic variation in community respiration, and therefore the overall metabolic budget, 

was greatest at depths less than 2,000 meters (Figures. 8-103B and 8-104). Highest respiration 

was found at stations near Mississippi River outflow, and where there is an interaction between 

the Loop Current and canyon (MT1-3, S36) and escarpment (S42) features. A comparison of 

experimentally measured consumption versus allometrically-derived requirements, indicates 

poor agreement by measured grazing, confirming poor results from the grazing study (Table 8-

23). Alternatively, it is possible that there was a lack of trophic linkage between bacteria and 

meiofauna, and that deep-sea meiofauna depend heavily on surfaced-derived detritus. Compared 

to total bacterial biomass, meiofauna only require approximately 0.1 to 0.7% of the bacterial 

standing stock to meet their theoretical metabolic requirements (Table 8-24). But, grazing rates 

were, at most, 10% of the theoretical metabolic requirement. 

In comparison to total benthic community respiration (CO2, converted from sediment 

community oxygen consumption, Section 8.5), meiofauna are responsible for approximately 10-

25 % of the total benthic community CO2 flux (Table 8-25). Total global oxygen utilization in 

the deep sea has been estimated to be 1.2x1014 mol O2 yr

-1, which equals approximately 

1.22x1015 g C yr-1 (converted to units of carbon using a respiratory quotient of 0.85, and 

stoichometric conversion of 12g C per mole O2). If meiofauna are responsible for 10 % of this 

flux (conservative), then they are responsible for processing 1.2x1014 g C yr-1, and are therefore a 

globally significant component of the carbon cycle. 

8.8.4 Macrofauna 

8.8.4.1 Quantitative Distribution of Macrofauna 

A total of 215 GoMEX boxcores were taken during DGoMB Cruise 1 in 2000, supplemented 

by replicate sampling at targeted sites on the two consecutive years, to provide detailed 

information about the macrofauna on the continental slope. The quantitative information on 

macrofauna from the boxcores was analyzed to test the eight hypotheses related to what controls 

the quantity and composition of life in the deep GoM. While more detailed sampling would be 

helpful, there is no reason to expect that the geographically broad patterns presented here will 

change as more data are accrued. Regressions of animal abundance as a function of depth for the 

entire data set indicate that mean density declines from about 10,000 down to about 3000 ind./m2

at the base of the escarpment, with further declines out to less than 1,000 out on the abyssal plain 

(Figure 8-105a, top). The variability of these values is extremely high, however, from estimates 

of more than 30,000 down to less than a 1,000 ind./m2

. Variability seems to decline with depth. 

As the highest densities were found in eastern GoM at the S locations or in the MT sites 

(Mississippi Trough), it is worth noting that if these sites are excluded from the analysis, then the 

extreme highs are eliminated and the mean at the shallow sites goes down to just less than 8,000 

ind./m2

 (Figure 8-105b, bottom). The Mississippi Trough (MT) sites plotted as a function of 

depth down the canyon (Figure 8-106) illustrate that the extreme highs nearshore were found at 

the canyon head. However, the tendency for the canyon to have higher values lessened with 

depth (MT5 and MT6). The mean at the shallow station was approximately 16,000 ind./m2, or 

well above the average mean for the entire set of data. MT5 and 6 were both characterized by 

quantities of iron stone material mixed into the sediments. It was sampled with the box core and 

the trawl and it is visible in bottom photographs. It does not form a pavement, however, as has 

been observed in deeper waters, but appears as a bumpy, red-colored material in irregular 

patches at the sediment surface. 
220 

Table 8-23 

 

Allometric Estimations of the Mass-Dependent Meiofauna Respiration Rate 

(R, in d-1 Units) and Meiofauna Community Respiration (CO2, m-2 d-1) 

and Total Organic Carbon Demand (OrgC, mg C m-2 d-1) 

(Mass-dependent respiration was calculated using an allometric rate law which is 

dependent upon the ratio (W) of biomass (B, mg C m-2 d-1) to abundance (A, N m-2). 

Respiration (CO2, mg C m-2 d-1) is the product of the mass-dependent rate ® and the total 

biomass (B), and total carbon demand (OrgC) was calculated under the assumption that 

respiration equals 80% of the total metabolic budget (see discussion).) 

Sta Lat. Long. Depth B A W R CO2 OrgC 

AC1 26.3936 -94.5731 2440 25.2 129974 1.9E-04 0.06 1.5 1.8 

B1 27.2025 -91.4052 2253 33.8 157417 2.1E-04 0.06 1.9 2.4 

B2 26.5500 -92.2151 2635 17.1 139907 1.2E-04 0.06 1.1 1.4 

B3 26.1644 -91.7351 2600 18.5 155817 1.2E-04 0.06 1.2 1.5 

BH 27.7800 -91.5000 545 48.4 407852 1.2E-04 0.06 3.1 3.9 

C1 28.0598 -90.2499 336 100.4 369129 2.7E-04 0.05 5.3 6.7 

C12 26.3797 -89.2403 2924 21.2 138792 1.5E-04 0.06 1.3 1.6 

C14 26.9382 -89.5725 2495 22.8 146578 1.6E-04 0.06 1.4 1.7 

C4 27.4532 -89.7631 1463 71.6 273585 2.6E-04 0.05 3.8 4.8 

C7 27.7304 -89.9820 1066 83.3 542119 1.5E-04 0.06 5.1 6.3 

GKF 27.0000 -90.2500 2460 6.4 84348 7.6E-05 0.07 0.5 0.6 

HIPRO 28.5500 -88.5800 1565 21.8 343118 6.3E-05 0.08 1.6 2.0 

JSSD1 25.0000 -92.0000 3545 4.5 87547 1.3E-05 0.11 0.5 0.6 

JSSD2 23.5000 -92.0000 3725 4.1 87295 4.6E-05 0.08 0.3 0.4 

JSSD3 24.7500 -90.7500 3635 3.5 60441 4.0E-05 0.08 0.3 0.4 

JSSD4 24.2500 -85.5000 3400 3.6 63451 5.9E-05 0.08 0.3 0.3 

JSSD5 25.5000 -88.2500 3350 7.0 135698 1.1E-04 0.07 0.5 0.6 

MT1 28.5411 -89.8250 482 119.9 945657 8.8E-04 0.04 4.8 6.0 

MT2 28.4479 -89.6719 677 60.9 535216 6.4E-05 0.08 4.6 5.7 

MT3 28.2215 -89.4940 990 107.4 885995 2.0E-04 0.06 6.1 7.7 

MT4 27.8276 -89.1661 1401 45.5 246058 5.1E-05 0.08 3.6 4.5 

MT5 27.3328 -88.6561 2267 31.0 128964 1.3E-04 0.06 2.0 2.5 

MT6 27.0016 -87.9991 2743 12.0 155312 9.3E-05 0.07 0.8 1.0 

NB2 27.1348 -92.0001 1530 23.4 168276 1.5E-04 0.06 1.4 1.8 

NB3 26.5580 -91.8226 1875 27.0 165245 1.6E-04 0.06 1.6 2.0 

NB4 26.2468 -92.3923 2020 17.9 148409 1.1E-04 0.07 1.2 1.5 

NB5 26.2454 -91.2099 2065 21.4 117263 1.4E-04 0.06 1.3 1.7 

RW1 27.5001 -96.0028 212 79.6 411809 6.8E-04 0.04 3.4 4.2 

RW2 27.2540 -95.7468 950 33.7 219457 8.2E-05 0.07 2.4 3.0 

RW3 27.0084 -95.4924 1340 45.9 248752 2.1E-04 0.06 2.6 3.2 

RW4 26.7514 -95.2502 1575 47.4 232842 1.9E-04 0.06 2.7 3.4 

RW5 26.5075 -94.9967 1620 33.8 170633 1.5E-04 0.06 2.1 2.6 

RW6 25.9973 -94.4956 3000 28.3 144453 1.7E-04 0.06 1.7 2.1 
221 

Table8-23 Allometric Estimations of the Mass-Dependent Meiofauna Respiration Rate 

(R, in d-1 Units) and Meiofauna Community Respiration (CO2, m-2 d-1) and 

Total Organic Carbon Demand (OrgC, mg C m-2 d-1) (Mass-dependent 

respiration was calculated using an allometric rate law which is dependent 

upon the ratio (W) of biomass (B, mg C m-2 d-1) to abundance (A, N m-2). 

Respiration (CO2, mg C m-2 d-1) is the product of the mass-dependent rate 

® and the total biomass (B), and total carbon demand (OrgC) was 

calculated under the assumption that respiration equals 80% of the total 

metabolic budget (see discussion).)(continued.) 

Sta Lat. Long. Depth B A W R CO2 OrgC 

S35 29.3352 -87.0464 668 70.5 501629 4.9E-04 0.05 3.3 4.1 

S36 28.9185 -87.6722 1826 113.2 799963 2.3E-04 0.06 6.3 7.9 

S37 28.5536 -87.7668 2387 33.6 291179 4.2E-05 0.08 2.8 3.5 

S38 28.2799 -87.3276 2627 17.5 157164 6.0E-05 0.08 1.3 1.7 

S39 27.4837 -86.9998 3000 10.5 83170 6.7E-05 0.07 0.8 1.0 

S40 27.8395 -86.7514 2972 17.3 99501 2.1E-04 0.06 1.0 1.2 

S41 28.0136 -86.5733 2974 30.2 181408 3.0E-04 0.05 1.6 2.0 

S42 28.2510 -86.4193 763 157.1 492537 8.7E-04 0.04 6.3 7.9 

S43 28.5029 -86.0768 362 46.4 276279 9.4E-05 0.07 3.2 4.0 

S44 28.7500 -85.7477 212 41.1 318516 1.5E-04 0.06 2.5 3.2 

W1 27.5772 -93.5510 420 68.7 387228 2.2E-04 0.06 3.9 4.8 

W2 27.4139 -93.3403 625 55.7 263315 1.4E-04 0.06 3.4 4.3 

W3 27.1724 -93.3233 875 72.7 262642 2.8E-04 0.05 3.8 4.8 

W4 26.7308 -93.3197 1460 27.5 187806 1.0E-04 0.07 1.8 2.3 

W5 26.2678 -93.3327 2750 15.4 104552 8.2E-05 0.07 1.1 1.4 

W6 26.0028 -93.3203 3150 22.7 124166 2.2E-04 0.06 1.3 1.6 

WC12 27.3232 -91.5558 1175 78.5 218447 6.3E-04 0.04 3.4 4.3 

WC5 27.7759 -91.7657 348 98.9 412061 4.5E-04 0.05 4.6 5.8 
222 

Table 8-24 

 

Mean Meiofaunal (MB) and Bacterial (BB) Biomass for Pooled Replicates 

and Pooled Taxonomic Groups at Four of the Process Stations 

(Bacterial biomass from Section 8.8.1.) 

Station Depth 

(m) 

Latitude 

(N) 

Longitude 

(W) 

MB 

(mg C m-2) 

BB 

(mg C m-2) 

S42 768 28.2565 86.4284 157 1180 

MT3 985 28.2170 89.5106 107 2320 

S36 1838 28.9118 87.6773 113 1680 

MT6 2737 26.9956 88.0090 12 920 

Table 8-25 

 

Comparison Of Whole Community Respiration (CR, mg C m-2 d-1) 

to Meiofauna Allometric Respiration Estimates (MR, mg C m-2 d-1) 

(Meiofauna account for 10-25% of whole community respiration. 

Note: whole community respiration (mg C m-2 d-1), converted from 

sediment community oxygen consumption (SCOC) as measured 

by the Benthic Lander (Section 8.5). SCOC (mmol O2 m-2 d-1) was 

converted to carbon using a respiratory quotient of 0.85 and 

stoichiometric conversion factor of 12 mg C/32 mg O2.) 

Station Depth CR MR %MR 

 

S42 763 41 6.3 15.4 

S36 1826 26 6.3 24.2 

MT1 482 40 4.8 12.0 

MT3 990 28 6.1 21.8 

C7 1066 49 5.1 10.4 

JSSD1 3545 4 0.5 12.5 

JSSD4 3400 2.6 0.3 11.5 
223 

a 

0

10,000

20,000

30,000

0 1000 2000 3000 4000

Depth (m)

Number m-2

Macrofaunal Density

y = 8851.8e-0.0007x

R2 = 0.66; N=265

Macrofaunal (infauna and epifauna) density

by depth for all stations.

b 

Figure 8-105. a) Total macrofaunal densities as a function of depth (top); b) bottom excludes ‘east,’ 

Mississippi Canyon and abyssal sites. 

y = -2.19x + 8045

R2

 = 0.46; N = 71

0

10,000

20,000

0 1000 2000 3000 4000

Depth (m)

Number m-2
224 

Figure 8-106. Total macrofaunal densities by depth plotting all sorted samples from the Mississippi 

Trough (MT). 

The sites in the eastern GoM illustrate several features that were peculiar to that area (Figure 

8-107). The samples have been partitioned between those lying east and those lying west of the 

DeSoto Canyon axis. Those to the east form a series of samples across the west Florida slope and 

steep escarpment. Those on the west side lie on the eastern margin of the Mississippi sediment 

cone. While the eastern samples formed a general pattern that declined in density with depth, 

with a mean trend line that more or less mimics that of the entire sample set, the macrofauna at 

sites west of the axis did not follow the typical pattern with depth. All of the macrofaunal 

abundance values were higher than their eastern counterparts and the highest values were at S36 

at a depth of 1,850 meters. The high density of macrofauna at this intermediate depth suggests 

that the site was unique among the sites sampled. The geographic distribution of relative 

densities provides a clear picture of these relationships (Figure 8-108). 

8.8.4.2 Dominant Macrofauna Taxa 

Six major groups of macrofauna were distributed to taxonomists and identified to the lowest 

possible taxonomic level possible, and the species lists are available in the database. However, 

each taxon will not be presented in detail in this report. Three have been chosen for detailed 

description because each is representative of one of important categories of macrofauna: the 

Crustacea (Isopoda), the Mollusca (Bivalvia) and the Annelida (Polychaeta). The detailed 

descriptions of each of these taxons individually will be followed by a composite analysis in 

which all 973 species of macrofauna (from the groups in Figure 8-109) have been analyzed as a 

single unit. This gives the reader a comparison between the individual groups and how they 

compare to all the macrofauna groups lumped together. The principal variables of concern in all 

these analyses have been population densities, biodiversity and bathymetric zonation of recurrent 

groups. 

MT1-3

MT1-2

MT1-1

MT3-1

MT3-3

MT3-2

MT4

MT5

MT2

MT6

y = -7.02x + 19477

R2

 = 0.51; N = 15

0

10,000

20,000

30,000

40,000

0 1000 2000 3000 4000

Depth (m)

Number m-2
225 

Figure 8-107. Total macrofaunal densities by depth plotting all sorted samples from the eastern Gulf 

of Mexico (S). Samples from the western side of DeSoto Canyon are indicated by the 

closed circles ( ); those from east of DeSoto Canyon are indicated by open circles 

( ). 

22

30

98  96  94  92  90  88  86  84 82  80 

1000 fms

100 fms

1000 fms

100 fms

30 

28 

26 

24 

22 

20 

18 

SAP1

SAP2

SAP3

SAP5

SAP4

RW1

2

3

4

5

6

AC

W1

2

3

4

5

6

WC5 BH

WC12

B1

NB2

B2 NB3

NB4 NB5 B3

GKF

C1

7

4

14

12

MT1 2

3

4

5

6

HP

S35

36

37

38

39

40 41

42

43

44

Macrofaunal Macrofaunal

Relative Densities Relative Densities

(number m -2)

Figure 8-108. Densities of total macrofauna (all fauna >300 micron sieve) at NGoM 

sampling sites. Note decline with depth over the entire area, but with 

maximum values near the Mississippi Canyon. The abundance of the 

macrofauna has not been related to taxonomic composition, with one notable 

exception. While the macrofauna of the entire GoM was in general dominated 

by polychaete annelid worms and macrofaunal-sized nematode worms, the 

macrofauna in the shallow head of the Mississippi Trough (MT1) was 

dominated in all the replicates by tube-dwelling amphipod crustaceans. Circle 

size is proportional to total macrofauna density. 

S35

S36

S44 S37

S43

S42 S38 S39, 40, 41

West of DeSoto Canyon

y = -2.17x + 15806

R2 = 0.14; N = 9

East of DeSoto Canyon

y = -2.12x + 8491

R2 = 0.60; N = 14

0

10,000

20,000

30,000

40,000

0 1000 2000 3000 4000

Depth (m)

Number m-2
226 

MACROFAUNA

(infauna and epifauna)

HARPACTICOIDA

15,277

9.13%

SCYPHOZOA

12,349

7.4%

TANAIDACEA

11,024

6.6%

OSTRACODA

6,511

3.9%

BIVALVIA

7,884

4.7%

Others

NEMERTINI

1,523

0.9%

APLACOPHORA

2,654

1.6%

ISOPODA

4,101

2.5%

AMPHIPODA

34,345

20.5%

POLYCHAETA

62,797

37.5%

MACROFAUNA

(infauna and epifauna)

HARPACTICOIDA

15,277

9.13%

SCYPHOZOA

12,349

7.4%

TANAIDACEA

11,024

6.6%

OSTRACODA

6,511

3.9%

BIVALVIA

7,884

4.7%

Others

NEMERTINI

1,523

0.9%

APLACOPHORA

2,654

1.6%

ISOPODA

4,101

2.5%

AMPHIPODA

34,345

20.5%

POLYCHAETA

62,797

37.5%

Figure 8-109. Distribution of macrofauna greater than >300 micron sieve size, 

excluding the large Nematoda (round worms usually lumped with 

the meiofauna; see previous section). 

8.8.4.2.1 Isopod Crustaceans (Geo.”Buz” Wilson, Aust. Nat. Mus.) 

The isopods are a speciose order of crustaceans that are a common component of all deep-sea 

faunas. They are particularly important in this report because they are one of two groups of 

macrofauna which have been identified by the same taxonomist, Dr. George ‘Buz’ Wilson, in 

this study and in an earlier investigationn of the deep GoM. [The other is Dr. G. Fain Hubbard, 

who worked up the polychaetes.] The earlier work is referred to as the NGoMCSS (Northern 

Gulf of Mexico Continental Slope Study) or as the LGL study, since it was conducted by LGL 

Ecological Research Associate, Inc. (Figure 8-110). A comparison has been made of the two and 

it is presented in this report, thus allowing an assessment of the two approaches (Figure 8-111). 

Species richness among the sites ranged from 52 species at the Mississippi Canyon site 3 

(MT3) to 1 species at MT1. The low values are not representative of the overall diversity at these 

sites, but are related to low densities. Individuals and species in a sample were correlated, as 

expected, and both species and individuals from an individual core were related significantly: 

Species = 1.2124inds0.6652; R2

 = 0.8215 (Figure 8-112). Species richness was not distributed 

homogeneously across the GoM. Figure 8-113 estimates the distribution of E(S15). As expected 

the shallowest samples were also the poorest in species, but so were the Sigsbee deep samples. 

Below the Mississippi River delta region, species richness seemed to peak around 2,000 meters, 

but had shallower peaks toward the West, particularly around the “Bush Hill” site. The deepest 

diversity hotspots seemed to occur in the vicinity of the saline basin regions between 2000 and 

3,000 meters. The pattern seen in Figure 8-114, however, was highly sensitive to individually 

diverse samples, so less sensitive analyses are given below. 
227 

Figure 8-110. Sampling pattern of the LGL study (Pequegnat et al., 1990). 

Figure 8-111. Sampling pattern of the DGoMB study. 
228 

y = 1.2124x0.6652

R2 = 0.8215

1.0

10.0

100.0

1.0 10.0 100.0

individuals/core

spp/core

Figure 8-112. Relationship between isopod species and individuals in individual replicates. 

Figure 8-113. Patterns of species richness, using E(S15), in the Gulf of Mexico. Isobaths from 

top to bottom are 200 m, 1,000 m, 2,000 m, 3,000 m. Red crosses indicate 

sample locations. 
229 

R2 = 0.339

R2

 = 0.2395 0.0

5.0

10.0

15.0

20.0

25.0

0 500 1000 1500 2000 2500 3000 3500 4000

depth (metres)

Species - spp/core; ES(15)

spp/core

ES(15)

Poly. (ES(15))

Poly. (spp/core)

Sigsbee Deep 

stns (S1-5) 

combined

Figure 8-114. Species richness, using ES15, and species density (species per core) by depth in the 

Gulf of Mexico. Because the Sigsbee Plain (S1-5) stations showed extremely low 

individual densities, they were combined for this analysis. 

Both species density and species richness (Figure 8-114) show an expected peak at mid 

depths, although the polynomial curves are not well supported by the noisy data. The curves are 

also relatively flat, suggesting that overall site diversity did not change considerably with depth. 

Across the GoM longitudinal scale, species richness does not change significantly (Figure 8-

115). Both of these relationships can be explained by broad depth and geographic ranges in GoM 

(Figure 8-116) for most of the species. Various ordinations and classifications of the 

identification data (analyses not shown) did not show any geographic coherence, suggesting that 

most species are found at most GoM sites. Therefore the GoM regional isopod fauna appears to 

be well mixed at large scales. 

The LGL and the DGoMB studies, when compared using simple rarefaction curves (Figure 

8-117), show a surprising difference that apparently contradicts the last statement. Although the 

two programs had similar numbers of identified individuals, the LGL curve falls distinctly below 

the DGoMB curve, thus highlighting the different species count (species richness) between the 

two programs (114 LGL species vs. 135 DGoMB species). This difference can be explained in a 

general way by comparing areas sampled. The DGoMB transects subsume a polygon of 

approximately 220,428 km2

, while the LGL transects lie within a smaller polygon of 

approximately 105,837 km2

. Therefore the often-observed species–area relationship appears in 

these data as well; larger areas find more species. 

Most rare species, the “short range endemics,” have small distributions, suggesting that 

larger numbers of samples may find more species. Certainly the Sigsbee Plain is substantially 

undersampled, given the density of the isopods in this region, and so higher numbers of samples 

are needed to adequately characterize this region. The GoM appears to have a well-mixed and 
230 

Expected Species vs longitude

y = -0.0362x + 5.8514

R2

 = 0.0026

2

4

6

8

10

12

14

-98.000 -96.000 -94.000 -92.000 -90.000 -88.000 -86.000 -84.000

Longitude (decimal)

ES(15)

W E

Figure 8-115. Species richness and longitude. 

y = 205.38x

R2

 = 0.4732

0

500

1000

1500

2000

2500

3000

3500

0.000 2.000 4.000 6.000 8.000 10.000 12.000

Longitudinal Range (degrees)

Depth Range (m)

Short Range 

Endemics

Regional species

Zonal Species

Longitudinal 

divide arbitrary

Figure 8-116. Depth ranges and longitudinal ranges of Isopoda in the Gulf of Mexico. 
231 

0

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500 3000 3500 4000

Individuals

Expected Species - ES(n)

Pooled

DGOM

LGL

Figure 8-117. DGoMB and LGL isopod macrofauna species richness compared. 

diverse regional isopod fauna that, with more than 60 new species, may be substantially distinct 

from adjacent regions in the North Atlantic or Caribbean. If further sampling programs are 

contemplated, the isopod data suggest that the deeper regions of the Sigsbee Abyssal Plain be 

targeted for detailed sampling to further understand how diversity varies with geography.

8.8.4.2.2 Bivalve Molluscs (Min Chen, Mary Wicksten and Roe Davenport, TAMU) 

There were 144 samples collected at 48 sites in this study which yielded a total of 3615 

individuals and 94 species of bivalves from the depths of 213 m to 3,732 m in the northern GoM 

(Table 8-26). The mean density was 147 bivalves m-2, with a standard deviation of 104. The site 

MT3 had the maximum density of 439, and site S3 had the minimum density of 35 individuals 

m

-2. 

8.8.4.2.2.1 Abundance 

Bivalve abundance was highest at the shallow sites, with particularly high values in the 

Mississippi Canyon, and decreased with depth and distance from the shore (Figure 8-118). A 

linear relationship was found between log10 bivalve density and depth 

(y = -0.17x + 2.40, R2 = 0.41, p<0.01; Figure 8-119). Site S5 had extraordinarily high density 

among the deep stations at which depths exceeded 3 km. Hypothesis 1a, there is no difference in 

benthic bivalve density with depth, and Hypothesis 2a, there is no difference in benthic bivalve 

density along an east to west gradient, were tested using a randomized complete block design 

analysis of variance where the depth was the main factor with 3 levels and transect was the 

blocking factor with 5 levels. The result indicated that the mean values for the bivalve density at 

different depths were not significantly different (F2,8=2.823, p=0.118) and the mean values for 
232 

Table 8-26 

Depth (m), Total Individuals Per Site, Number of Individuals m-2, Total Number of Species, 

Species Richness, Pielou’s Evenness (J’), Expected Number of Species (50), 

and Shannon-Wiener Species Index (H’) for Bivalves 

(The sample area per replicate was 0.1725 m2 and three replicates were collected for each location

Site Depth(m)

Total 

Individuals

(N) 

Number of

Individuals

(m-2) 

Total 

Species 

(S) 

Species 

Richness 

(d) 

Pielou’s 

Evenness

(J’) 

Expected 

Number 

of Species

(50) 

Shannon

Wiener 

Species In

(H) 

C1 335 67 129 23 5.23 0.82 20 2.58 C4 1457 107 207 34 7.06 0.91 24 3.19 C7 1072 112 216 27 5.51 0.88 20 2.91 C12 2922 91 176 25 5.32 0.83 19 2.68 C14 2490 89 172 22 4.68 0.81 17 2.50 NB2 1530 69 133 22 4.96 0.91 20 2.81 NB3 1875 45 87 22 5.52 0.93 22 2.87 NB4 2033 65 126 27 6.23 0.91 23 2.99 NB5 2063 27 52 17 4.85 0.93 17 2.64 RW1 213 104 201 23 4.74 0.85 17 2.66 RW2 950 112 216 33 6.78 0.86 22 3.02 RW3 1327 39 75 18 4.64 0.92 18 2.67 RW4 1575 70 135 21 4.71 0.91 18 2.77 RW5 1620 70 135 23 5.18 0.91 20 2.85 RW6 3010 33 64 14 3.72 0.91 14 2.41 WC5 356 90 174 24 5.11 0.85 18 2.69 WC12 1156 69 133 26 5.90 0.92 23 3.01 S35 664 159 307 28 5.33 0.68 16 2.26 S36 1828 142 274 28 5.45 0.72 15 2.40 S37 2386 79 153 13 2.75 0.89 12 2.29 S38 2635 45 87 11 2.63 0.82 11 1.97 S39 3002 29 56 15 4.16 0.94 15 2.55 S40 2974 27 52 13 3.64 0.93 13 2.38 S41 2974 30 58 15 4.12 0.90 15 2.44 S42 766 68 131 18 4.03 0.85 16 2.45 S43 363 121 234 18 3.54 0.74 12 2.15 S44 213 61 118 15 3.41 0.88 14 2.37 
233 

Table 8-26 Depth (m), Total Individuals Per Site, Number of Individuals m-2, Total Number of Spec

Species Richness, Pielou’s Evenness (J’), Expected Number of Species (50), Shannon-Wiener Species Index (H’) for Bivalves (The sample area per replicate 0.1725 m2 and three replicates were collected for each location.) (continued) 

Site Depth(m)

Total 

Individuals

(N) 

Number of

Individuals

(m-2) 

Total 

Species 

(S) 

Species 

Richness 

(d) 

Pielou’s 

Evenness

(J’) 

Expected 

Number 

of Species

(50) 

Shanno

Wiene

Specie

Index 

(H) 

B1 2256 29 56 15 4.16 0.89 15 2.42 B2 2629 44 85 12 2.91 0.83 12 2.07 B3 2620 79 153 23 5.03 0.77 17 2.40 AC1 2469 30 58 10 2.65 0.87 10 2.00 W1 396 124 240 19 3.73 0.81 14 2.39 W2 625 77 149 16 3.45 0.76 13 2.10 W3 865 46 89 25 6.27 0.91 25 2.93 W4 1447 45 87 22 5.52 0.93 22 2.88 W5 2748 24 46 9 2.52 0.85 9 1.86 W6 3145 55 106 11 2.50 0.78 11 1.88 S1 3525 22 43 4 0.97 0.83 4 1.16 S2 3732 24 46 8 2.20 0.96 8 2.00 S3 3670 18 35 5 1.38 0.97 5 1.56 S4 3410 33 64 11 2.86 0.96 11 2.29 S5 3314 160 309 15 2.76 0.72 11 1.95 MT1 481 226 437 13 2.21 0.23 5 0.59 MT2 677 226 437 21 3.69 0.47 10 1.44 MT3 987 227 439 27 4.79 0.76 15 2.51 MT4 1401 70 135 23 5.18 0.87 20 2.74 MT5 2277 42 81 18 4.55 0.92 18 2.65 MT6 2746 24 46 10 2.83 0.85 10 1.95 
234 

<100

100-<200

200-<400

≥400

2000 m 2000 m

200 m

200 m

Figure 8-118. Map of density distribution of bivalves (ind. m-2) in the GoM. 
235 

Figure 8-119. Linear regression plot of bivalve log density against depth. 

the bivalve density in western, central and eastern transects were not significantly different 

(F4,8=0.561, p=0.698), i.e., there was no difference in bivalve mollusc density among transects 

(Table 8-26). The mean value of bivalve density in 0.3km was 225±129 individuals m-2 whereas 

the mean value of bivalve density in 1.5km was 139±43 individuals m-2. The mean value of 

bivalve density in 3 km is 90±53 individuals m-2. The least squares regression analysis of density 

and depth illustrated a linear correlation between bivalve density and depth. Hypothesis 3a, there 

is no difference in benthic bivalve density between basin and non-basin, was tested using a 

single-factor analysis of variance, which indicated that the mean values for the basin bivalve 

density and non-basin bivalve density were not significantly different (F1,5=.002, p=0.963). The 

mean value of bivalve density in basin stations was 98±50 individuals m-2 and 100±38 

individuals m-2 at non-basin stations. Hypothesis 4a, there is no difference in benthic bivalve 

density between canyon and non-canyon, was tested using a single-factor analysis of variance 

which indicated that the mean values for the canyon and non-canyon bivalve density were 

significantly different (F1, 4=84.6, p=0.001. The mean value of bivalve density in canyon stations 

was 437±1 individuals m-2 while the mean value in non-canyon stations was 184±48 individuals 

m

-2. 

Depth(km)

0.0 .5 1.0 2.01.5 2.5 3.0 3.5 4.0

Log(density) (log (individuals per m2)) 

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

Log(density)(log(individuals per 2

Y = -0.17x+2.40 R2 = 041 
236 

8.8.4.2.2.2 Diversity 

Diversity of bivalves increased slightly from the shallow stations, with especially low values 

in the Mississippi canyon, to a maximum at intermediate depths, and then decreased down to the 

deepest station (Figure 8-119). A parabolic (quadratic) relationship was found between diversity 

and depth. The Shannon-Wiener Species Index (Figure 8-120) displayed the same pattern. The 

three sites that had lowest diversity were MT1, MT2, and S1. Hypothesis 1b, there is no 

difference in benthic bivalve diversity with depth (Figure 8-121), and Hypothesis 2b, there is no 

difference in benthic bivalve diversity along an east to west gradient, were tested using a 

randomized complete block analysis of variance where the depth was the main factor with three 

(3) levels and transect was the blocking factor with five (5) levels. The result indicated that the 

mean values for bivalve diversity at different depths were not significantly different (F2, 

8=2.709, p=0.126) and the mean values for bivalve diversity in western, central and eastern 

transects were not significantly different (F4, 8=2.010, p=0.186), i.e., there was no difference in 

bivalve mollusk diversity among transects. The least squares regression analysis of diversity and 

depth showed there was a quadratic correlation between bivalve diversity and depth. Hypothesis 

3b, there is no difference in benthic bivalve diversity between basin and non-basin, was tested 

using a single-factor analysis of variance which indicated that the mean values for the basin 

bivalve diversity and non-basin bivalve diversity were significantly different (F1,5=17.137, 

p=0.009. The mean value of bivalve diversity in basin stations was 2.30±0.11. The mean value in 

non-basin stations was 2.83±0.07. 

Hypothesis 4b, there is no difference in benthic bivalve diversity between canyon and noncanyon, was tested using a single-factor analysis of variance which indicated that the mean 

values for canyon bivalve diversity (H’) and non-canyon bivalve diversity were not significantly 

different (F1, 4=5.61, p=0.08). The mean value of bivalve diversity in canyon stations was 

1.51±0.96, whereas the mean value in non-canyon stations was 2.89±0.31. Evenness of bivalves 

had no clear tendency in the Gulf of Mexico. However, the dominance in MT1 and MT2 were 

high with evenness of 0.23 and 0.47, respectively.The least squares regression analysis showed 

there was a quadratic relationship between diversity and depth. For linear term the p-value was 

0.002 and the power was 0.906 (Table 8-27). For quadratic term the p-value was less than 0.01 

and the power was 0.971. The diversity of bivalve mollusks increased from shallow continental 

slope depths, with especially low values in the Mississippi Canyon, to a maximum at 

intermediate depths (1-2km), followed by a decrease down to the deepest areas (3.7km) in the 

Gulf of Mexico. However, the ANOVA test showed no difference in diversity of bivalve 

mollusks for different depth. 

A parabolic relationship was found between diversity (H’(S)) of bivalves and depth (y = -

0.24x2

 + 0.79x + 1.99, R2 = 0.29, p-value=0.0003), but the maximum documented was on the 

upper continental slope (1-2km), rather than the upper continental rise (3 km), where a maximum 

has been observed in the northwestern Atlantic (Rex, 1983). 

To reduce the influence of the outlier diversity (H’(S)) of MT1 which had standardized 

residual –4.05, iteratively reweighted least squares was used by applying weights that varied 

inversely with the size of the residual. Outlying case diversity (H’(S)) of MT1 that had largest 

residual was thereby given smallest weight. The iteratively reweighted least squares robust 

regression for diversity (H’(S)) became y = -0.22x2

 + 0.66x + 2.20, R2 = 0.40, p-value<0.01. 
237 

Figure 8-120. Map of diversity (Shannon-Wiener Species Index) distribution of bivalves in the GoM. 

<1 

1-<2 

2-<3 

≥3 

200 m 

2000 m 

200 m

2000 m
238 

Figure 8-121. Curvilinear regression plot of bivalve diversity against depth. 

Reported are: curves created from Shannon-Wiener Species Index 

(H) (y=-0.26x2

+0.84x+2.00 R2=0.31) (a) and for Expected Number 

of Species [ES(50)] (y=-2.84x2

+8.62x+12.28 R2=0.47) (b). 

D epth (km )

0.0 .5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Shannon-Wiener Species Indes (H) 

3.5

3.0

2.5

2.0

1.5

1.0

.5

Depth (km )

0.0 .5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

E(s50) 

3 0

2 0

1 0

0

y=-2.84x 2

+8.62x +12.28

R 2=0.47 

(a)

(b)
239 

Table 8-27 

List of Station Groups Based on Percent Similarity with Respect to Species Composition 

Group Description Stations Depth(m) Group Description Stations 

1 Shallow, Mississippi MT1 481 6 Deep area MT5 

 Canyon MT2 677 RW5  NB4 

2 Shallow, non-canyon RW1 213 NB5 

 WC5 356 B2  B1 

3 Mid-slope, eastern MT3 987 AC1 

 and cental area MT4 1401 W5  S36 1828 RW6 

4 Shallow, eastern and S44 213 7 Deep area S4 

 west-central area S35 664 S2  S43 363 S41  W1 396 S3  MT6 

5 Mid-slope area, most W2 625 S1 

 inclusive RW2 950 S40  C7 1072 S39  W4 1447 

NB3 1875 8 Shallow, central area C1 

 WC12 1156  S42 766 9 Deep area S38  W3 865 S37  RW3 1327 S5  RW4 1575 W6  C4 1457 C14  NB2 1530 B3 

C12 
240 

8.8.4.2.2.3 Zonation 

The stations were separated into nine groups according to percent similarity of species

composition (Figure 8-122). Within each group, the stations shared at least 25% of the species, 

except group 4, which shared at least 20% of the species. The species composition appeared to 

change with depth. Group 1 included MT1and MT2, which ranged from 481 m to 677 m in the 

Mississippi Canyon. Group 2 included RW1 and WC5, which ranged from 213 m to 356 m in 

western GoM. Group 3 included MT3, MT4, and S36, which ranged from 987 m to 1,828 m, 

which is in the eastern and central GoM. Group 4 included S44, S35, S43, and W1, which ranged 

from 213m to 664m in eastern and western GoM. Group 5 included W2, RW2, C7, W4, NB3, 

WC12, S42, W3, RW3, RW4, C4, and NB2, which ranged from 625 m to 1,875 m. Group 6 

included MT5, RW5, NB4, NB5, B2, B1, AC1, W5, and RW6, which ranged from 1,620 m to 

3,010 m. Group 7 included S4, S2, S41, S3, MT6, S1, S40, and S39, which ranged from 2,746 m 

to 3,732 m. Group 8 included central site C1 which is 335m.Group 9 included S38, S37, S5, W6, 

C14, B3, and C12, which ranged from 2,386 m to 3,314 m (Figure 8-123). 

The depth interval of 213 m to 677 m contained four groups: 1, 2, 4 and 8. From 625 m to 

1,875 m, there were two groups: 3 and 5. Three groups occurred in the range of 1,620 m to 3,732 

m. They were group 6, 7, and 9. 

8.8.4.3 Polychaete Annelids (G. Fain Hubbard, TAMUG, and Yuning Wang, Texas 

Tech) 

A total of 17,881 polychaete specimens from 96 replicate samples (with at least one replicate 

from each sampling station) were collected during the three annual cruises conducted between 

2000–2002. These represent 543 taxonomic entities, including 53 families, 226 genera, and 410 

species, many of which are undescribed (Tables 8-28 though 8-39). The extremely small size of 

most of the specimens (300μ lower cut-off in sorting) and their generally poor physical condition 

(many battered beyond recognition or broken into several fragments) result in many samples 

being identifiable only to family or genus level. 

8.8.4.3.1 Polychaete Density 

The abundance of polychaetes declined exponentially with depth (Figure 8-124), as expected 

from previous studies elsewhere. Highest densities were found on the upper slope (3,500 ind. 

m

-2); the lowest were on the abyssal plain. The densities in the central gulf were higher than 

those in the eastern or western extremities of the study area, including at those sites in deep 

water, similar to the total macrofauna. Polychaetes were the most abundant of all the 

macrofauna, sensu strictu, with the exception of the Amphipoda at the head of the Mississippi 

Trough. 

8.8.4.3.2 Polychaete Species Richness and Diversity 

The polychaete worms were the most speciose of all the macrofaunal groups (Table 8-29 

through 8-39), with a total of 410. Shannon Weiner diversity (H’(s)) declined from a mean of 

almost 6 on the upper slope down to 3.5 on the abyssal plain (Figure 8-125). Expected species 

per individual declined from the upper slope down to the lower slope at approximately the same 

rate (Figure 8-126). Based on 50 individuals, the expected number of species was approximately 

25 to 35, whereas based on 100 individuals, the expected number was 50 to 60 species. The low 

value on the abyssal plain was on the order of 10 to 15 species. 
241 

Figure 8-122. Dendrogram based on percent similarity of bivalve mollusk species 
242 

Figure 8-123. Bathymetric zonation of bivalves based on percent similarity. 
243 

Table 8-28 

 

The 50 Most Abundant Polychaete Species by Number 

Family Genus Species N # Reps N m-2

Paraonidae Aricidea suecica 535 58 32.31 

Paraonidae Paraonella monilaris 472 69 28.50 

Spionidae Prionospio cirrifera 471 22 28.44 

Cirratulidae Tharyx marioni 429 71 25.91 

Paraonidae Aedicira sp. 385 39 23.25 

Spionidae Prionospio ehlersi 382 20 23.07 

Spionidae Spiophanes berkeleyorum 369 49 22.28 

Spionidae Prionospio cristata 352 7 21.26 

Pilargidae Litocorsa antennata 343 8 20.71 

Opheliidae Tachytrypane sp. A 317 65 19.14 

Nephtyidae Aglaophamus verrilli 308 12 18.60 

Paraonidae Aricidea simplex 284 52 17.15 

Paraonidae Levinsenia uncinata 282 73 17.03 

Paraonidae Aricidea mirifica 267 15 16.12 

Cossuridae Cossura delta 243 28 14.67 

Acrocirridae Macrochaeta clavicornis 181 46 10.93 

Syllidae Exogone sp. A 177 43 10.69 

Paraonidae Levinsenia oligobranchiata 159 30 9.60 

Trichobranchidae Terebellides distincta 156 40 9.42 

Paraonidae Levinsenia gracilis 142 28 8.57 

Cirratulidae Tharyx annulosus 139 40 8.39 

Maldanidae Micromaldane sp. 136 21 8.21 

Paraonidae Aricidea fragilis 136 38 8.21 

Syllidae Exogone longicirrus 127 24 7.67 

Fauveliopsidae Fauveliopsis sp. A 126 38 7.61 

Lumbrineridae Lumbrinerides dayi 123 38 7.43 

Spionidae Prionospio heterobranchia 118 7 7.13 

Paraonidae Sabidius cornatus 116 27 7.00 

Syllidae Exogone sp. B 114 24 6.88 

Paralacydoniidae Paralacydonia paradoxa 108 33 6.52 

Spionidae Genus B 108 23 6.52 

Lumbrineridae Lumbrineris verrilli 98 23 5.92 

Spionidae Prionospio cirrobranchiata 94 9 5.68 

Pilargidae Sigambra tentaculata 93 35 5.62 

Nereididae Ceratocephale oculata 87 34 5.25 

Syllidae Exogone dispar 83 16 5.01 

Amphinomidae Paramphinome jeffreysii 82 26 4.95 

Paraonidae Aricidea catherinae 82 11 4.95 

Longosomatidae Heterospio longissima 81 20 4.89 

Pilargidae Synelmis klatti 81 36 4.89 

Phyllodocidae Protomystides bidentata 76 35 4.59 

Spionidae Spiophanes sp. D 74 22 4.47 

Syllidae Exogone sp. D 74 30 4.47 

Ampharetidae Melinna maculata 69 5 4.17 

Onuphidae Sarsonuphis hartmanae 69 30 4.17 
244 

Table 8-28 The 50 Most Abundant Polychaete Species by Number (continued) 

Family Genus Species N # Reps N m-2

Paraonidae Paraonella sp. A 69 25 4.17 

Paraonidae Aricidea lopezi lopezi 66 17 3.99 

Sigalionidae Sthenelais sp. A 65 23 3.93 

Ampharetidae Isolda pulchella 63 4 3.80 

Opheliidae Armandia maculata 63 31 3.80 

Table 8-29 

 

Western Transects (W1-6, RW1-6, and AC1) Produced 2,141 Polychaetes 

Representing 172 Taxa and 14 Replicates 

(The most abundant species by numbers (≥ 20).) 

FAMILY GENUS SPECIES N N m-2

Pilargidae Litocorsa antennata 246 101.86

Cirratulidae Tharyx marioni 77 31.88 

Paraonidae Paraonella monilaris 55 22.77 

Paraonidae Levinsenia uncinata 45 18.63 

Opheliidae Tachytrypane sp. A 42 17.39 

Paraonidae Aricidea suecica 40 16.56 

Amphinomidae Paramphinome jeffreysii 34 14.08 

Acrocirridae Macrochaeta clavicornis 30 12.42 

Lumbrineridae Lumbrinerides dayi 27 11.18 

Spionidae Genus B 27 11.18 

Nephtyidae Aglaophamus verrilli 26 10.77 

Paraonidae Aedicira sp. 24 9.94 

Phyllodocidae Protomystides bidentata 22 9.11 

Paraonidae Aricidea fragilis 20 8.28 

Table 8-30 

 

Characteristic Species of Western Transect Stations 

FAMILY GENUS SPECIES N N m-2

Pilargidae Litocorsa antennata 246 101.86

Cirratulidae Tharyx marioni 77 31.88 

Paraonidae Paraonella monilaris 55 22.77 

Paraonidae Levinsenia uncinata 45 18.63 
245 

Table 8-31 

 

B and NB Stations Intended to Test Basin Vs Non-Basin Communities [B1-3, NB2-5, 

Additionally Including WC5, WC12, and BH (Bush Hill)] Produced 1,930 Polychaetes 

Representing 150 Taxa and 15 Replicates. 

(The most abundant species by numbers (≥ 20).) 

FAMILY GENUS SPECIES N N m-2

Spionidae Prionospio cristata 313 120.97

Cirratulidae Tharyx marioni 64 24.73 

Paraonidae Levinsenia uncinata 51 19.71 

Paraonidae Paraonella monilaris 50 19.32 

Paraonidae Aricidea suecica 48 18.55 

Opheliidae Ophelina sp. A 41 15.85 

Opheliidae Tachytrypane sp. A 40 15.46 

Trichobranchidae Terebellides distincta 28 10.82 

Paraonidae Aricidea simplex 27 10.43 

Spionidae Prionospio heterobranchia 26 10.05 

Paraonidae Levinsenia oligobranchiata 25 9.66 

Syllidae Exogone longicirrus 24 9.28 

Pilargidae Litocorsa antennata 23 8.89 

Table 8-32 

 

Characteristic Species of the Basin/Non-Basin Transect Stations 

FAMILY GENUS SPECIES N N m-2

Spionidae Prionospio cristata 313 120.97

Paraonidae Levinsenia uncinata 51 19.71 
246 

Table 8-33 

 

Central Transect Stations [C1, C4, C7, C12, C14, GKF (Green Knoll Furrows) and S5 (Sigsbee 

Abyssal Plain)] Produced 2,445 Polychaetes Representing 185 Taxa and 17 Replicates 

(The most abundant species by numbers (≥ 20). 

FAMILY GENUS SPECIES N N m-2

Paraonidae Paraonella monilaris 140 54.11 

Opheliidae Tachytrypane sp. A 61 23.57 

Paraonidae Levinsenia uncinata 52 20.10 

Cirratulidae Tharyx marioni 48 18.55 

Paraonidae Aricidea suecica 45 17.39 

Syllidae Exogone dispar 40 15.46 

Spionidae Spiophanes berkeleyorum 38 14.69 

Pilargidae Litocorsa antennata 38 14.69 

Fauveliopsidae Fauveliopsis sp. A 37 14.30 

Paraonidae Aedicira sp. 36 13.91 

Syllidae Exogone sp. A 36 13.91 

Spionidae Prionospio cirrifera 35 13.53 

Paraonidae Sabidius cornatus 31 11.98 

Paraonidae Aricidea simplex 28 10.82 

Syllidae Exogone longicirrus 28 10.82 

Amphinomidae Paramphinome jeffreysii 25 9.66 

Pilargidae Synelmis klatti 20 7.73 

Paraonidae Levinsenia brevibranchiata 20 7.73 

Table 8-34 

 

Characteristic Species of the Central Transect Stations 

FAMILY GENUS SPECIES N N m-2

Paraonidae Paraonella monilaris 140 54.11 

Syllidae Exogone dispar 40 15.46 

Fauveliopsidae Fauveliopsis sp. A 37 14.30 
247 

Table 8-35 

 

Mississippi Trough Transect Stations (MT1-6) Produced 6,868 Polychaetes 

Representing 333 Taxa and 25 Replicates 

(The most abundant species by numbers (≥ 20).) 

FAMILY GENUS SPECIES N N m-2

Spionidae Prionospio cirrifera 429 99.48 

Spionidae Prionospio ehlersi 356 82.55 

Paraonidae Aricidea suecica 301 69.80 

Paraonidae Aedicira sp. 284 65.86 

Nephtyidae Aglaophamus verrilli 275 63.77 

Spionidae Spiophanes berkeleyorum 236 54.72 

Paraonidae Aricidea mirifica 197 45.68 

Cossuridae Cossura delta 155 35.94 

Paraonidae Aricidea simplex 152 35.25 

Opheliidae Tachytrypane sp. A 116 26.90 

Cirratulidae Tharyx marioni 112 25.97 

Paraonidae Levinsenia gracilis 93 21.57 

Maldanidae Micromaldane sp. 88 20.41

Spionidae Prionospio heterobranchia 88 20.41 

Paraonidae Levinsenia oligobranchiata 81 18.78 

Spionidae Prionospio cirrobranchiata 75 17.39 

Trichobranchidae Terebellides distincta 75 17.39 

Lumbrineridae Lumbrineris verrilli 71 16.46 

Paraonidae Aricidea catherinae 71 16.46 

Paraonidae Levinsenia uncinata 70 16.23 

Ampharetidae Melinna maculata 69 16.00 

Paraonidae Paraonella monilaris 69 16.00 

Ampharetidae Isolda pulchella 62 14.38 

Longosomatidae Heterospio longissima 58 13.45 

Paraonidae Aricidea fragilis 58 13.45 

Paraonidae Aricidea lopezi lopezi 53 12.29 

Cirratulidae Tharyx annulosus 52 12.06 

Spionidae Prionospio steenstrupi 52 12.06 

Pilargidae Sigambra tentaculata 50 11.59 

Paraonidae Aedicira belgicae 47 10.90 

Paralacydoniidae Paralacydonia paradoxa 40 9.28 

Syllidae Exogone dispar 40 9.28 

Syllidae Exogone sp. A 35 8.12 

Fauveliopsidae Fauveliopsis sp. A 32 7.42 

Spionidae Laonice cirrata 32 7.42 

Paraonidae Paraonella sp. A 31 7.19 

Maldanidae Maldane glebifex 29 6.72 

Syllidae Exogone sp. B 29 6.72 

Capitellidae Genus AA 28 6.49 

Cossuridae Cossura soyeri 28 6.49 

Nephtyidae Micronephthys minuta 27 6.26 

Paraonidae Aricidea cerrutii 24 5.57 

Lumbrineridae Ninoe sp. A 23 5.33 

Paraonidae Sabidius cornatus 23 5.33 

Syllidae Exogone atlantica 23 5.33 

Dorvilleidae Schistomeringos rudolphi 22 5.10 

Oweniidae Myriowenia sp. A 22 5.10 
248 

Table 8-35 Mississippi Trough Transect Stations (MT1-6) Produced 6,868 Polychaetes 

Representing 333 Taxa and 25 Replicates (The most abundant species by 

numbers (≥ 20).) (continued) 

FAMILY GENUS SPECIES N N m-2

Pilargidae Synelmis klatti 22 5.10 

Spionidae Genus B 22 5.10 

Capitellidae Notomastus americanus 21 4.87

Spionidae Spiophanes sp. D 20 4.64 

Dorvilleidae Schistomeringos sp. B 20 4.64 

Table 8-36 

 

Characteristic Species of the Mississippi Trough Transect Stations 

(Melinna maculata, Isolda pulchella and Nephtys squamosa are essentially endemic to the 

Mississippi Trough Stations, while Aricidea catherinae, Aricidea cerrutii, Aricidea lopezi lopezi, 

Aedicira belgicae, Capitellidae Genus AA, Ceratocephale loveni, Cossura soyeri, Exogone 

atlantica, Heterospio longissima, Laonice cirrata, Maldane glebifex, Notomastus americanus, 

Prionospio steenstrupi, Schistomeringos rudolphi, Schistomeringos sp. B, and Terebellides 

atlantis all exhibit maximum numbers there.) 

FAMILY GENUS SPECIES N N m-2

Spionidae Prionospio cirrifera 429 99.48 

Spionidae Prionospio ehlersi 356 82.55 

Paraonidae Aricidea suecica 301 69.80 

Paraonidae Aedicira sp. 284 65.86 

Nephtyidae Aglaophamus verrilli 275 63.77 

Spionidae Spiophanes berkeleyorum 236 54.72 

Paraonidae Aricidea mirifica 197 45.68 

Cossuridae Cossura delta 155 35.94 

Paraonidae Aricidea simplex 152 35.25 

Opheliidae Tachytrypane sp. A 116 26.90 

Cirratulidae Tharyx marioni 112 25.97 

Paraonidae Levinsenia gracilis 93 21.57 

Maldanidae Micromaldane sp. 88 20.41

Spionidae Prionospio heterobranchia 88 20.41 

Paraonidae Levinsenia oligobranchiata 81 18.78 

Spionidae Prionospio cirrobranchiata 75 17.39 

Trichobranchidae Terebellides distincta 75 17.39 

Lumbrineridae Lumbrineris verrilli 71 16.46 

Paraonidae Aricidea catherinae 71 16.46 

Ampharetidae Melinna maculata 69 16.00 

Ampharetidae Isolda pulchella 62 14.38 

Longosomatidae Heterospio longissima 58 13.45 

Paraonidae Aricidea lopezi lopezi 53 12.29 

Spionidae Prionospio steenstrupi 52 12.06 

Pilargidae Sigambra tentaculata 50 11.59 

Paraonidae Aedicira belgicae 47 10.90 

Spionidae Laonice cirrata 32 7.42 

Maldanidae Maldane glebifex 29 6.72 

Capitellidae Genus AA 28 6.49 

Cossuridae Cossura soyeri 28 6.49 

Nephtyidae Micronephthys minuta 27 6.26 

Paraonidae Aricidea cerrutii 24 5.57 

Syllidae Exogone atlantica 23 5.33 
249 

Table 8-36 Characteristic Species of the Mississippi Trough Transect Stations (Melinna 

maculata, Isolda pulchella and Nephtys squamosa are essentially endemic to the 

Mississippi Trough Stations, while Aricidea catherinae, Aricidea cerrutii, Aricidea 

lopezi lopezi, Aedicira belgicae, Capitellidae Genus AA, Ceratocephale loveni, 

Cossura soyeri, Exogone atlantica, Heterospio longissima, Laonice cirrata, 

Maldane glebifex, Notomastus americanus, Prionospio steenstrupi, 

Schistomeringos rudolphi, Schistomeringos sp. B, and Terebellides atlantis all 

exhibit maximum numbers there.) (continued) 

FAMILY GENUS SPECIES N N m-2 

Dorvilleidae Schistomeringos rudolphi 22 5.10 

Capitellidae Notomastus americanus 21 4.87

Dorvilleidae Schistomeringos sp. B 20 4.64 

Table 8-37 

 

Eastern Transect Stations [S35-44 and HP (High Productivity)] Produced 4,337 Polychaetes 

Representing 207 Taxa from 21 Replicates 

(The most abundant species by numbers (≥ 20).) 

FAMILY GENUS SPECIES N N m-2

Paraonidae Paraonella monilaris 125 34.51 

Cirratulidae Tharyx marioni 118 32.57 

Acrocirridae Macrochaeta clavicornis 102 28.16 

Paraonidae Aricidea suecica 101 27.88 

Syllidae Exogone sp. A 73 20.15 

Paraonidae Aricidea mirifica 67 18.50 

Spionidae Spiophanes berkeleyorum 66 18.22 

Paraonidae Levinsenia uncinata 64 17.67 

Syllidae Exogone sp. B 64 17.67 

Paraonidae Aricidea simplex 60 16.56 

Lumbrineridae Lumbrinerides dayi 58 16.01 

Opheliidae Tachytrypane sp. A 53 14.63 

Cossuridae Cossura delta 50 13.80 

Syllidae Exogone longicirrus 48 13.25 

Nereididae Ceratocephale oculata 44 12.15 

Spionidae Genus B 38 10.49 

Paralacydoniidae Paralacydonia paradoxa 36 9.94 

Pilargidae Litocorsa antennata 36 9.94 

Cirratulidae Tharyx annulosus 35 9.66 

Oweniidae Myriochele sp. A 35 9.66 

Fauveliopsidae Fauveliopsis sp. A 34 9.39 

Paraonidae Aedicira sp. 33 9.11 

Paraonidae Cirrophorus lyra 32 8.83 

Paraonidae Aricidea fragilis 31 8.56 

Sigalionidae Sthenelais sp. A 31 8.56 

Phyllodocidae Protomystides bidentata 29 8.01 

Paraonidae Sabidius cornatus 28 7.73 

Paraonidae Levinsenia oligobranchiata 27 7.45 

Syllidae Exogone sp. D 27 7.45 

Trichobranchidae Terebellides distincta 27 7.45 

Flabelligeridae Diplocirrus sp. A 26 7.18 

Maldanidae Maldane sp. A 25 6.90 

Maldanidae Micromaldane sp. 23 6.35 
250 

Table 8-37 Eastern Transect Stations [S35-44 and HP (High Productivity)] Produced 4,337 

Polychaetes Representing 207 Taxa from 21 Replicates (The most abundant 

species by numbers (≥ 20).) (continued) 

FAMILY GENUS SPECIES N N m-2

Opheliidae Armandia maculata 23 6.35 

Capitellidae Barantolla sp. A 21 5.80 

Lumbrineridae Ninoe sp. A 21 5.80 

Flabelligeridae Diplocirrus capensis 20 5.52 

Sigalionidae Pholoe sp. B 20 5.52 

Table 8-38 

 

Characteristic Species of the Eastern Transect Stations 

FAMILY GENUS SPECIES N N m-2

Paraonidae Paraonella monilaris 125 34.51 

Cirratulidae Tharyx marioni 118 32.57 

Acrocirridae Macrochaeta clavicornis 102 28.16 

Syllidae Exogone sp. A 73 20.15 

Lumbrineridae Lumbrinerides dayi 58 16.01 

Nereididae Ceratocephale oculata 44 12.15 

Spionidae Genus B 38 10.49 

Oweniidae Myriochele sp. A 35 9.66 

Sigalionidae Sthenelais sp. A 31 8.56 

Opheliidae Armandia maculata 23 6.35 

Lumbrineridae Ninoe sp. A 21 5.80 

Table 8-39 

 

Sigsbee Abyssal Plain Stations (S1-5) Produced 267 Polychaetes Representing 48 Taxa and 6 

Replicates 

(The most abundant species by numbers (≥ 10). No species was found to occur at all five deep 

stations, although Paraonella monilaris and Sigambra tentaculata were found at four of the 

stations. Greater numbers of specimens and numbers of species were found at station S5 than 

at S1-S4.) 

FAMILY GENUS SPECIES N N m-2

Paraonidae Paraonella monilaris 33 31.86 

Paraonidae Sabidius cornatus 23 22.22 

Pilargidae Synelmis klatti 15 14.49 

Cirratulidae Tharyx marioni 10 9.66 

Fauveliopsidae Fauveliopsis sp. A 10 9.66 

Pilargidae Sigambra tentaculata 10 9.66 
251 

Polychaete Density

y = 3035.8e-0.0009x

R2 = 0.64

0

1000

2000

3000

4000

5000

0 1000 2000 3000 4000

Depth (m)

Number m-2

Figure 8-124. Abundance of polychaete annelid worms in the northern GoM. 

Polychaete Diversity by Depth

H' = -1X10-7x

2

 - 2X10-5x + 4.99

R2

 = 0.48

0

2

4

6

8

0 1000 2000 3000 4000

Depth (m)

Diversity (H' log2)

Figure 8-125. Diversity (H(s’)) of polychaetes as a function of depth. 
252 

Figure 8-126. ES(50) of polychaetes as a function of depth. Note that the pattern is the 

same as in 8.125, but spread out. 

The maximum diversity (H(s’)) was found on the upper slope, at a depth of about 750 meters. 

Low values were encountered at 500 m depth in the central trough (MT1) due to a dominance of 

a few species. Evenness was high, but tended to increase gradually from shallow to deep water. 

The maximum ‘evenness’ possible (one) was approached in the deepest sites primarily because 

almost all individuals encountered were different species. Low values were observed in the 

Mississippi Trough at the 500 m site (MT1) due to dominance by a few species. 

8.8.4.3.3 Bathymetric Zonation of Recurrent Groups of Polychaetes 

Nine groups of species were defined on the basis of percent similarity of species composition 

(Figure 8-127, based on the dendrogram in Figure 8-128). In general the groups were confined 

to specific depth ranges, or to particular physiographic features within depth ranges, but with 

frequent exceptions to this generalization. The groups were 1) an abyssal plain group, 2) a 

central Mississippi Cone group, 3) a Mississippi Trough upper slope group, 4) a mid-slope 

group, 5) a lower slope group, and 6) a combination of upper slope and central groups that occur 

intermittently from east to west in seemingly no predictable manner. 

The diversity declines monotonically as well (Figures 8-129 and 8-130), which makes it 

rather unusual compared to the other groups, in which parabolas exhibited maxima at 1,200 to 

1,600 meters depth. 

Polycheate species richness

y = -2.0475x2

 + 1.1838x + 39.919

R2

 = 0.4371

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5 4

depth(km)

ES(100) 
253 

Hipro

MT3

MT1

C1

MT2

W6

B2

S44

S43

S42

S41

S40

S39

S35 S36

S37

MT4 S38

MT5

MT6

C7

C4

C14

C12

RW1

RW2

RW3

RW4

RW6

AC1

W1

W2

W3

W4

W5

WC5 BH

NB2

B1

NB4

B3

NB5

GKF

S1

S2

S3

S5

S4

WC12

NB3

2000 m 2000 m

200 m

200 m

5

5

9

5

6

1

1

1

8

1

4

2

9

1

4

4

5

9

1

6

5

5

5

5

8

3

3

5

8

3

5

5

8

8 2 9 9

9 9

5 8 7

2

9

3

5

RW5

9

3

4

4

Figure 8-127. Map of nine recurrent groups based on percent similarity (polycheate). 
254 

Figure 8-128. Dendrogram of percent similarities of polychaetes in the northern 

Gulf of Mexico. Nine groups are delimited at >20%. 
255 

y = -4.7535x + 16.884

R2

 = 0.615

0

5

10

15

20

25

0 0.5 1 1.5 2 2.5 3 3.5 4

Depth (km)

Number of Species

Figure 8-129. Species richness (γ diversity) of amphipods. 

y = -2.9876x + 11.99

R2

 = 0.5901

0

2

4

6

8

10

12

14

16

0 0.5 1 1.5 2 2.5 3 3.5 4

Depth (km)

E(s30)

Figure 8-130. Species diversity [ES(30)] of amphipods. 
256 

8.8.4.4 Amphipod Crustaceans (John Foster, MSU, and Yousra Soliman, TAMU) 

The amphipods fell into about 8 groups, based on % similarity of species in common 

between sites (Figure 8-131). However, only 3 (Zones 4, 5 and 6) of the groups spanned more 

than just a few locations. Four sites failed to be aligned with any group and oddly these outliers 

included both the shallow site dominated by the amphipod Ampelisca sp. nov. at MT1 with as 

many as 20,000 individuals per square meter and the deepest most southerly site, S2, with very 

few individuals and species. There was less coherence with depth among the amphipods than 

with the other groups analyzed above. For example, the amphipods at abyssal sites were in 

different groups whereas the previous taxa all had a distinct abyssal zone. Although 1 and 2 

were always found in shallow water, Zones 3 and 6 spanned great depths. Zone 5 was isolated to 

the western transects at mid-slope depths. Zone 4 was just upslope and 6 was downslope from 5, 

but both extended over the complete east to west range of the sampling (Figure 8-132). 

A multidimensional scaling analysis has been added for the amphipods to uncover why they 

were not as coherently grouped as the other taxa (Figure 8-133). The large groups (4 and 6) 

hang together fairly well, but with unpredictable outliers, e.g., MT6 and NB 3. The independent 

sites all lie outside the inner group. Nonetheless, this gives us little additional insight into why 

the relationship to depth is substantially less with the amphipods than the other groups. This 

pattern is more similar to the meiofaunal harpacticoid copepods than the other macrofauna. 

8.8.4.5 “Total” Macrofauna Community Structure (Chihlin Wei and G. Rowe, TAMU 

and TAMUG) 

The above groups were analyzed individually because they are numerically the most 

important macrofauna and are traditionally dealt with individually. As a summary of the 

macrofauna community structure, a total of 973 species have been analyzed together to 

determine overall biodiversity gradients and bathymetric zonation. In addition to the above 

groups, the cumacean crustaceans, the gastropod mollusks, the scaphopod mollusks, and spiney 

mollusks were included in the analyses. It does not include the nematode worms or harpacticoid 

copepods normally considered with the meiofauna component, even though they were large 

enough to be sampled with the macrofauna sieve (>300 microns). 

The macrofauna displayed more or less the same patterns exhibited by the individual groups, as 

might be expected. Five recurrent groups could be identified (Figures 8-134, 8-135, 8-136), 

based on rates of change in composition as a function of depth and percent similarity > 20% 

between sites and groups of sites. The analysis in the 2nd figure of this section was attempted 

with the individual groups but is somewhat similar to that utilized by Menzies et al. (1973), 

which Pequegnat et al. (1990) used on the megafauna of the GoM. This analysis considers the 

number of species added and lost across the bathymetric gradient. The depths at which the 

greatest change occurs is defined as a boundary between bathymetric zones of groups of animals 

in which the change in composition is less, e.g., it is homogeneous. This approach can now be 

compared with the analysis of a site-by-site comparison of the relatedness of the fauna based on 

how similar each site is to another in species. This is more useful than the prior analysis because 

it does not presume a priori that depth is the controlling variable. In general the two approaches 

agree. Zones of related groups aligned themselves into 4 groups with depth but with a boundary 

between 2E and 2W bisecting the middle of the upper slope region. The 3th area dipped down 

into the abyssal plain (4), thus cutting group 4 into two areas. In terms of the hypotheses, one 

can thus say that the east and the west GoM are similar, with the exception of the boundary 

between 2E and 2W. 
257 

S2 3732

S39 3001

GKF 2465

S1 3526

S40 2954

MT1 481

S5 3314

C14 2487

S37 2384

S43 361

W1 405

S44 213

C1 336

RW1 213

S36 1828

WC5 345RW2 950

MT2 678MT3 987

S35 663

W3 863

WC12 1166

BH 542

HiPro 1572

W2 625

C7 1072

S42 767

RW4 1574

C4 1463

RW3 1329

MT4 1401

W4 1452

MT5 2275

MT6 2745

C12 2921

NB3 1875

RW6 3008

B1 2255

W6 3146

NB2 1530

S38 2633S41 2979

W5 2753

NB4 2042NB5 2063

S4 3409B2 2629

AC1 2479

B3 2618

100

80

60

40

20

0

Similarity

8 4 7 3 2 1 5 6

Figure 8-131. Dendrogram of amhipod species % similarity between sites, with 8 groups 

identified at greater than 20% similarity. 
258 

Figure 8-132. Bathymetric zonation of amphipods, based on the dendrogram in the previous 

Figure. 
259 

AC1

B1

B2

B3

BH

C1

C12

C14

C4

C7

GKF

Hipro

MT1

MT2

MT3

MT4

MT5

MT6

NB2

NB3

NB4

NB5

RW1

RW2

RW3

RW4

RW6

S1

S2

S36 S35

S37

S38

S39

S4

S40

S41

S42

S43

S44

S5

W1

W2

W3

W4

W5

W6

WC12

WC5

2D Stress: 0.17

Zones

1

2

3

4

5

6

7

8

INDEP.

Figure 8-133. Multidimensional scaling analysis of zones of amphipods based on Bray 

Curtis % similarities between sites. 
260 

Figure 8-134. Macrofauna zonation based on 973 species. Groups determined as equal 

to or greater than 20% similar. 

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 0.5 1 1.5 2 2.5 3 3.5 4

Depth (km)

Cumulative Number of Specie

s

Figure 8-135. The cumulative number of species renewal (empty triangle), 

appearance (solid rectangular), and disappearance (empty 

square) plotted against depth. 
261 

WC5 345

RW2 950

W3 863

RW3 1329

RW4 1574RW5 1620

WC12 1166

NB2 1530

W4 1452

MT3 987

S35 663

MT4 1401

C4 1463C7 1072

S36 1828

S42 767

W2 625

BH 542

HiPro 1572

MT1 481MT2 678

RW1 213

S44 213

C1 336

S43 361

W1 405

GKF 2465

S2 3732S3 3670S1 3526

S4 3409

S39 3001

S40 2954

NB3 1875NB4 2042

AC1 2479

B2 2629

NB5 2063

RW6 3008

W5 2753

B1 2255B3 2618S5 3314

W6 3146

S37 2384S38 2633

MT5 2275

C12 2921C14 2487

MT6 2745

S41 2979

100

80

60

40

20

0

Similarity

2W 2E 1 4 3

Figure 8-136. Dendrogram for DGoMB station. Group average-linkage cluster analysis for 4th

root transformed species abundance data based on Bray-Curtis similarity 

coefficient. X axis is depth (m) followed by station names. Y axis is Bray-Curtis 

similarity (%). The solid line indicates significant evidence of structure 

(SIMPROF test, P<0.05, available on PRIMER v6). Dotted line indicates no 

evidence of structure (SIMPROF test, P>0.05). Five distinct zones and two 

independent stations (WC5, GKF) were identified by SIMPROF. 

Diversity too reflects the patterns in the individual groups (Figure 8-135 and 8-137). The 

maximum was located on the upper continental slope (Figure 8-137). Lowest values were 

encountered at the head of the Mississippi Canyon in all groups. Diversity then declined slowly 

out to the abyssal plain. This pattern replicates that in all the four individual groups considered 

individually. 

The mean size of macrofauna declined with depth (Figure 8-138), as had been suggested by 

earlier workers for the GoM macrofauna. The variance appears to decline as mean size declines. 

In summary, one could suggest that four parameters of macrofauna community structure are 

all, in some way, a function of depth: 

1) animal density declines sharply, 

2) diversity follows a parabolic relationship, with a maximum on the upper 

slope, 

3) species are added and replaced at an uneven rate such that they are grouped 

into distinct zones, and 
262 

0

5000

10000

15000

20000

25000

0 0.5 1 1.5 2 2.5 3 3.5 4

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5 3 3.5 4

Depth (km)

Density (individual/m2) E(s100)

Biomass (mg C/m2) 1-

λ’

(a)

(c)

(b)

0.6

0.7

0.8

0.9

1.0

1.1

0 0.5 1 1.5 2 2.5 3 3.5 4

(d)

Figure 8-137. Mean density, biomass, and ES(100), and 1-λ’ as a function of depth. 

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3 3.5 4

Depth (km)

Body Size

Figure 8-138. The standardized ratio of mean biomass to density as a 

function of depth. 
263 

4) the mean animal size declines. 

The central area in close proximity to the Mississippi River had highest densities, whereas 

the far western transect had the lowest densities, at any given depth. The central axis of DeSoto 

Canyon also had high densities. The highest densities were located at the Mississippi Canyon 

head and these also had the lowest diversity values. In the DeSoto canyon, however, the high 

densities were accompanied by high rather than low diversity. 

8.8.5 Benthic Megafaunal Invertebrates 

A total of at least 185 species of megafaunal invertebrates (over 10 mm in greatest 

dimension, or attached to objects over 10 mm in size) were collected by trawl or trap during the 

DGoMB study in 2000-2002. The amphipod Eurythenes gryllus was taken only in traps. Species 

richness was greatest in the DeSoto Canyon at station S35, with 38 species. Other stations with 

more than 30 species were C4, MT5, S38 and S41, all in the eastern half of the basin. Stations on 

the Sigsbee Abyssal Plain (S1-5 and JSSD stations) had 20 or fewer species, as did stations of 

the Mississippi Trough (MT1, MT2, MT3 and MT4). 

Species diversity calculated by the Shannon-Wiener Index (H) was highest at station MT6 

(H=5.754). Station MT5 also had high diversity (H=4.425). Lowest diversity was at stations of 

the upper slope (MT1, S42, S43 and S44), basins (B1, B2 and B3) and dominated by small 

sponges (NB4 and S37). Most other stations, regardless of depth or geographic location, had 

diversity values between 3 and 4. 

Biomass was highest at stations of the DeSoto Canyon (S35, S37, S38 and S41) and station 

MT3. Much of the biomass was due to wet weight of holothuroids. All samples with biomass 

greater than 6 kg were in the DeSoto Canyon or Mississippi Trough. The greatest number of 

individuals in any sample was taken was at S37, with 1,612 individuals, most of them small 

sponges. Stations MT1, MT5, MT6, NB5 and S38 had more than 300 individuals per station. 

Many species of echinoderms, sea anemones and crustaceans were widespread in geographic 

distribution. The holothuroid Molpadia blakei was taken at the basin and central transects, but 

not in the canyons. The crab Trichopeltarion nobile and the mussel Amygdalum politum were 

more common in canyons than at other stations. Crustaceans were more diverse and abundant in 

the DeSoto Canyon than in the Mississippi Trough. 

The most common group of invertebrates was the Crustacea, with 58 species. Three of these 

were collected and identified for the first time in the Gulf of Mexico. Thirty-three species of 

echinoderms were collected. Of these, an unidentified holothuroid, probably Benthodytes sp., 

was the most abundant and contributed significantly to the biomass of stations of the lower slope. 

An undescribed species of sea anemone living on shells of hermit crabs was collected. 

In situ photographs, sediment samples and observations of newly-caught animals show that 

many species live on or buried in mud. Feeding types varied according to taxonomic group. The 

starfish Dytaster insignis was found to feed on fallen Sargassum. Small barnacles, zooanthids, 

and crabs were encountered associated with larger crustaceans, sponges and gorgonians. 

Parasitic crustaceans infected galatheid crabs. 

8.8.5.1 Comparison of Catches by Type of Gear 

There were no obvious differences between the faunal composition taken in beam trawls 

versus otter trawls. At the abyssal plain stations in the central Gulf of Mexico, no species was 
264 

taken both in trawls and traps, although Munidopsis bermudezi, taken in a trap at JSSD 3, was 

also trawled at S41 and S42.

Trawl net mesh size affected the catch. At the JSSD sites, small mollusks (families 

Nuculanidae, Cuspidariidae, etc.) were taken. These were not caught in previous trawls. The 

brachiopod Chlidonophora incerta was taken by trawl only once in 2000-2001, but was found in 

all JSSD stations. The epibenthic sled captured mostly small (less than l cm) invertebrates, 

mostly demersal or midwater species rather than benthic species. Isopods, amphipods, ostracods 

and other small crustaceans found in the sled samples were not collected in the trawls. 

There were no apparent differences in the fauna taken by beam trawl at RW2 and RW1 with 

that collected elsewhere with an otter trawl. The lack of specimens in stations such as B2 on 12 

May 2000 and RW2 may be due to problems in trawl operation. Both contained midwater 

species, suggesting that the net was off the bottom for at least part of its supposed bottom time. 

Station C7 was a failure because the trawl “fished water”—it was off the bottom for the entire 

tow. The scanty catch at other stations of the western transects may have been due to similar 

problems. 

8.8.5.2 Species Richness, α Diversity, Biomass and Abundance 

The greatest species richness at any station was at S35, with 38 species. Other stations with 

more than 30 species were C4, MT5, S38 and S41. Of the deep stations of the southern Gulf, 

JSSD5 had the greatest species richness, with 20 species. The least species richness (less than 10 

species) was at stations B2 on 12 May 2000, C14, MT1 in 2001, RW2 and S42 in 2001. Stations 

MT2, 3 and 4 as well as JSSD 1, 2, 3, and 4 had less than 20 species per station. There were no 

apparent patterns of species richness in the other stations, which had between 10 and 20 species. 

The location with the highest species diversity was MT6 (H=5.754). Most stations, regardless 

of depth or location, had an index between 3 and 4. Stations RW2 and B2 12 May 2000 had low 

indices, but this may have been a result of the trawl being off the bottom for part of the tow. The 

shallowest stations of the upper slope (MT1, S42, S43, S44) were less diverse than those at 

greater depths. Of stations taken on the middle slopes (1,000-2,500 m), the basin stations (B1, B2 

and B3) had lower diversity on average than other stations at comparable depths. The diversity 

indices of non-basin stations (NB) were higher than those of the basin stations, except for NB4, 

where the fauna was dominated by small sponges. Station S37, which also had a low diversity 

index (H=1.256), was also dominated by numerous individuals of a small sponge, as well as two 

species of echinoderm. The highest mid-slope diversity was at MT5 (H=4.4). On the lower slope 

(2,500 m and deeper to the abyssal plains), all stations except MT6 had a diversity index between 

3 and 4. 

Five stations had wet weight samples weighing greater than 10 kg: MT3, S35, S37, S38 and 

S41. Much of this biomass was due to holothuroids, but not always the same species. At station 

S41, sea anemones contributed 5.3 kg; at MT3, 3.3 kg was due to six large crabs. Stations S36 

and S42 also had biomass over 10 kg if one included a single large squid, Pholidoteuthis adami

(Figure 8-139). Each squid weighed approximately 10 kg. 
265 

Figure 8-139. June 10, 2000. Dr. Mary Wicksten with the large squid, Pholidoteuthis adami, caught from t

S42. 
266 

There were 13 stations with a sample weight exceeding 6 kg. Four were in the Mississippi 

Trough: MT1, MT3, MT5 and MT6. Five were located in or near the DeSoto Canyon: S35, S37, 

S38, S40 and S41. The other stations were B3, C4 and C12. Of the deep abyssal plain sites of the 

southern Gulf, the greatest sample mass was at station JSSD5, with 0.82 kg. 

It was difficult to count the number of individuals at some sites because many sponges and 

sea cucumbers were broken or smashed. The greatest number of individuals was at station S37, 

with 1612 individuals. Of these, approximately 600 were small sponges (family Ancorinidae) 

and 650 were the large orange brittle star Ophiocrates heros. Stations MT1, MT5, MT6, NB5 

and S38 had more than 300 individuals per station. Station JSSD5 had the most specimens, 414, 

of the southern stations. Of these, 207 were the white brittle star Ophiosphalma planum. It 

should be noted that JSSD5 was located in the central GoM offshore of the MT transect down 

the Mississippi Trough. The most northerly of the abyssal plain sites investigated in June and 

August, 2002, JSSD5 (=S5) had high abundances in all size categories of benthos compared to 

the other abyssal sites. 

8.8.5.3 Comparisons by Transects 

The fauna with the greatest species richness was taken in the two canyon transects, the 

Mississippi Trough stations (MT) and the DeSoto Canyon stations (S). In both, the crab 

Trichopeltarion nobile and the mussel Amygdalum politum were taken in quantity. However, 

various sea anemones, mussels and the flatback lobster Stereomastis sculpta were more abundant 

in the Mississippi Canyon. In the DeSoto Canyon, the burrowing shrimp Glyphocrangon 

aculeata and G. alispina were more abundant, as was the crustacean fauna in general. 

Of the various transects, the RW and W stations had the least species and biomass. Sampling 

problems may have affected the results. The NB stations were dominated by echinoderms and 

small sponges. The B stations had a fauna characteristic of their depth, dominated by 

echinoderms. The C stations were intermediate in biomass and diversity. There were no obvious 

differences between eastern and western transects except for the holothuroid Molpadia blakei, 

taken at the B, C and NB transects but not farther east. 

Throughout the study area, echinoderms were abundant and widespread, especially 

elasipodids and other holothuroids, and not apparently restricted to any single or set of transects. 

These occurred at depths of 2,000 m or more at almost all stations. Other widespread groups 

included the ophiuroids Bathypectinura heros and Ophiopsalthma planum, the barnacle 

Arcoscalpellum antillarum, the shrimp Nematocarcinus rotundus and Glyphocrangon 

longirostris and the flatback lobster Stereomastis sculpta. 

8.8.5.4 Comparisons by Taxonomic Groups 

The most speciose group in the megafauna was the Crustacea, with 58 species. Of these, nine 

were barnacles, two were isopods and the rest were decapods. The most speciose station for 

crustaceans was S42, with 17 species. Station S35 had the most individuals, 186, but of these, 82 

were the large barnacles Arcoscalpellum regina, attached in clumps. The other 104 belonged to 

12 other species. Other stations with more than 10 decapod species were C4, MT5, S36, S41, 

S42 and W1. Common and widespread species included Benthesicymus bartletti, Glyphocrangon 

aculeata, Nematocarcinus rotundus, Penaeopsis serrata, Plesiopenaeus armatus (Figure 8-140) 

and Stereomastis sculpta. The largest crustaceans were the golden crab Chaceon fenneri (80 g, 

MT5) and giant isopod Bathynomus giganteus (Figure 8-141, about 250 g, @ S42). Sabinea 
267 

Figure 8-140. June 13, 2000. Deep-sea shrimp, Plesiopenaeus armatus, collected from the trawl at S37. 
268 

Figure 8-141. June 10, 2000. Giant isopod, Bathynomus giganteus, collected with other trawl samples at S42
269 

hystrix (@ MT4), Benthesicymus carinatus (@ B2, 19 June 2000) and Neoscalpellum debile (@ 

S38 and S41) represent the first reports of these species in the Gulf of Mexico. 

The next most speciose group was the Echinodermata, with approximately 33 species. By far 

the most abundant group was the Elasipodida, soft-bodied deep-sea holothuroids (sea 

cucumbers). “Elasipodid A” (Figure 8-142, Figure 8-143) contributed 14.0 kg to the biomass of 

station S37 alone. Other locations with over 1 kg of this species were B3, C12, MT5, MT6, S38, 

S40 and S41. These are all on the lower slope, at 2,000 m or deeper, and none was in the western 

half of the GoM basin. The species designation may involve more than one species. Often, 

elasipodids arrive at the surface smashed or entangled in nets. Distinctive features are lost during 

capture, or do not remain intact if the specimen is preserved in formalin. The majority of the 

specimens may be Benthodytes lingua, but specimens of Psychropotes sp. or other species of 

Benthodytes might be included. The large whitesea cucumber Mesothuria lactea was also 

abundant, especially at station MT3. Twenty-seven specimens of this species taken at MT3 had 

an estimated displacement of 13.0 kg. 

Brittle stars (Ophiuroidea) were common inhabitants of the continental slope. Many small 

individuals remain unidentified. The two most abundant species were Ophiocrates heros, 

especially at S37, and Ophiosphalma planum at JSSD5. (In previous reports, O. planum was 

called Ophiomusium planum). The most common sea stars (Asteroidea) were Dytaster insignis, 

Nymphaster arenatus and Litonotaster intermedius. A few heart urchins (Echinoidea) (order 

Spatangoida, Brissopsis sp. and others) were taken, as well as the leathery sea urchin 

Phormosoma placenta. No crinoids were encountered. 

Approximately 40 species of mollusks were collected. No more than six live species of 

mollusks was taken at any station, in the megafaunal fraction. Those with five or six species 

were C1, MT1, JSSD5, and S40. Empty shells were common, as were shells occupied by hermit 

crabs. These are not included in counts of individuals or biomass. The greatest displacement 

volume, estimated at 10 kg, was of a scaled squid, Pholidoteuthis adami, at station S36 and S42. 

The greatest biomass for a group of mollusks was 1.6 kg of the nesting mussel, Amygdalum 

politum, at MT1. At MT6, the scaphopod Antalis occidentalis contributed 90 g. of biomass. The 

most abundant mollusk per station was Amygdalum politum. There were 138 mussels at MT1, 

where they also dominated the biomass for mollusks. Amygdalum politum also was taken at 

MT2, MT4, and W1. Although many gastropods were collected, they were scattered in 

occurrence. Chitons (class Polyplacophora) were taken only at W3. Cephalopods included 

octopuses, squid, and fragments of the shells of Argonauta sp. Finned octopuses, family 

Stauroteuthidae, were taken at stations S35, MT2 and W6-RW6. Approximately 17 species of 

cnidarians were collected. Some of these were very small members of the order Zoanthidea, 

which cannot be identified to species. Others were very small sea anemones that could not be 

removed from the rocks to which they were attached. Of the identifiable species, the sea 

anemones were most abundant in terms of numbers and size. At MT5, the most individuals, 93 

of the large sea anemone Chondrophellia coronata, were collected. These sea anemones had a 

displacement volume equivalent in wet weight to 3.4 kg. At station S41, four individuals of an 

unidentified sea anemone of the family Hormathiidae (Hormathiid “A”) had a displacement 

volume equal to 5.3 kg. 

Sea anemones tended to be the most common cnidarians. Specimens from at least one 

species, the “cloak anemone” covering the shells of hermit crabs is an undescribed species (D. 

Fautin, University of Kansas, pers. comm.) No more than five species of cnidarians was taken 
270 

Figure 8-142. June 15, 2000. Deep-sea holothuroid, Elasipodid A, collected from the trawl at S38. 
271 

Figure 8-143. June 5, 2000. Deep-sea holothuroid, Elasipodid A present on the sea floor of Mississippi trough 

was taken during the first cruise. 
272 

per station. Species-rich stations included C4, C12, MT5 and S42. Station C12 had the most 

individual cnidarians, 110, of which the majority were sea pens (Figure 8-144, Umbellula sp. and 

an unidentified pennatulacean). There were 67 cnidarians at station B1, 61 of them a small 

unidentified sea anemone. Most of the cnidarians seem to be widespread species in the Gulf of 

Mexico. One specimen of the flytrap anemone, Actinoscyphia sp., was collected at station B2, 19 

June 2000. Flytrap anemones (Figure 8-145) have been photographed (but not collected) 

previously in the Gulf of Mexico (I. MacDonald, TAMU-Corpus Christi, pers. comm). 

Eight species of sponges, at least, were collected. At present, the taxonomy of deep-sea 

sponges (classes Hexactinellida and Demospongiae) is in a state of confusion. By morphology 

alone, it is certain that at least five species were present. Five recognizable species were 

collected at S38, the most species-rich station for sponges. The greatest number of individuals, 

approximately 600, was collected at S37. Over 100 sponges also were taken at stations NB5 and 

JSSD5. 

Vermiform (worm-shaped) animals appeared sporadically in trawls. Most were unidentified 

annelids of the class Polychaeta, including species of the family Onuphidae and Serpulidae. Of 

particular interest are the pogonophoran polychaetes taken at B2, 12 May 2000. These animals, 

once assigned to a separate phylum, now are included in the Annelida. They are noted for their 

association with chemosynthetic bacteria, located within the worm’s body. Pogonophorans live 

near gas hydrate seeps. No such seep has been reported previously at or near B2. Station B2 was 

sampled again on 19 June 2000 in hopes of locating more pogonophorans, but none was taken. 

The brachiopod Chlidonophora incerta was taken at S37 during 2000-2001. It was collected 

at all JSSD stations, at which the trawl was outfitted with a fine mesh at the cod end. A few 

ascidians were collected. All were attached to pieces of rock, wood or “clinkers” (coal residue 

from ships). 

8.8.5.5 Diversity Gradients 

Expected Species (E(s)) has been calculated for the composite megafauna (e.g., all groups 

combined together) based on 25, 50 and 100 individuals (Figure 8-146). There was little 

evidence for a depth gradient, although the highest values were located in the center of the 

distribution across the gradient. Low diversity was consistently encountered on the upper slope, 

including the head of the Mississippi Trough, but S42 on the upper Florida slope was particularly 

diverse. At the deep sites on the abyssal plain, the values were consistently low. The qualitative 

narratives above referred to gear type, taxa and transects with the highest species richness, and 

rarely were any sites in the western GoM mentioned. This may reflect a real difference between 

east and west or simply reflect a reduced sampling intensity in the west. 

• Es25: N=19~1588, r2=0.05, F2,33=2.03, P= 0.15, Y=11.88-2.76X+0.69X2

• E(s50): N= 25~1588, r2= -0.06, F2,32= 0.47, P= 0.95, 

Y=13.65-0.51X+0.14X2

• E(s100): N=56~1588, r2= -0.36, F2,30= 0.44, P= 0.64, 

Y= 15.77+1.87X-0.61 X2
273 

Figure 8-144. May 7, 2000.Sea pen, Umbellula sp., collected from the trawl at B1. 
274 

Figure 8-145. June 19, 2000. Flytrap anemones, Actinoscyphia sp., collected from the trawl at B2. 
275 

0

5

10

15

20

25

30

0 0.5 1 1.5 2 2.5 3 3.5 4

Depth (m)

Expected Number of Specie

s

E(s25)

E(s50)

E(s100)

Figure 8-146. Comparison of diversity gradients (H(s)) at 100, 50 and 25 species of 

megafauna in the trawl samples at the sites sampled. 
276 

8.8.5.6 Zonation of the Megafaunal Invertebrates 

The composite megafauna was subjected to a similarity analysis similar to that for the species 

lists of the meiofauna harpacticoid copepods (Section 8.8.3) and the composite macrofauna 

(Section 8.8.4). In this case, seven (7) different groups or bathymetric zones could be identified 

on the dendrogram (Figure 8-147) based on percent similarity in species between sites. A map of 

these zones reveals their affinities to specific areas of the GoM continental margin (Figure 8-

148). Zone 1 on the upper slope was located at both the most extreme east and west sites, 

suggesting that little longitudinal difference existed at the shallow margins of the study. The 

abyssal plain had five sites also extending across the entire GoM that has so far been sampled. 

In this case, S5 in the central GoM retained its affinity to east and west, unlike the macrofauna 

which was penetrated at this central deep location by the next shallowest zone. Zone 7 was 

located at intermediate depths, both above and below the escarpments, as well as on the 

Mississippi Cone. Mapping the longitudinal extent of the zones along the middle slope was 

limited because of sparse sampling along the western transects, unfortunately. The canyons and 

the basins were not particularly unique in terms of their species composition. Zones 2, 3 and 6 

occurred in both the Mississippi and DeSoto canyons, but they also occurred at a few locations at 

similar depths outside the canyon. Only Zones 6, 7 and 8 occurred at more than three sites, 

perhaps reflecting the poor sampling in the Western Gulf. 

R

W2 950

R

W1 213

S44 213

NB2 1530

W3 863S43 361

MT1 481

W1 405

MT2 678

S35 663

WC5 345

C4 1463

MT4 1401

S36 1828

MT3 987

S42 767

S1 3526

S4 3409

S5 3314

S2 3732

S3 3670

S37 2384

MT5 2275

S38 2633

S39 3001

MT6 2745

S40 2954

C12 2921

W6-RW6 3077

NB4 2042

S41 2979

B3 2618

NB5 2063

NB3 1875

B1 2255B2 2629

C1 336

C14 2487

100

80

60

40

20

0

Similari yt

14 2 3 6 8 7 5

Figure 8-147. Dendrogram of megafaunal groups, based on Bray Curtis similarity of the 4th root 

transformed abundance of each species in the trawls samples at each site 

sampled. 
277 

Figure 8-148. Distribution of megafaunal zones in the NW GoM. 

8.8.5.7 Megafauna Assessment Using Seafloor Photography (Matthew Ziegler, UNM) 

The epibenthic megafauna is the most widely studied size group in the deep ocean because 

they are visible to the naked eye. They are comprised of large bottom-dwelling organisms 

dominated by echinoderms, cnidarians (sessile sea anemones, pens and whips), decapod 

crustaceans, and demersal fish. 

The objective of the bottom photography was to provide a general description of the northern 

GoM continental slope megafauna identified to the lowest possible taxonomic level. This 

included comparing DGoMB with NGoMCSS photographic density data to look at megafauna 

density variations with time, examining the relationship between megafaunal density and depth, 

and comparing megafaunal densities of contrasting regions both in and outside the GoM. 

DGoMB images were analyzed for megafauna, area, bioturbation level, and general features 

– current markings, plant material, consolidated material, and water clarity. Area viewed was 

calculated by photographing a grid in water. This measurement was reinforced by comparison 

with the average size of the trawl-caught fauna. Based on this approach, area viewed was 

calculated at 2 m2/image. Images obscured by clouds of resuspended sediment were overlain 

with the photographed grid and the visible area calculated. Only those animals that were readily 

identifiable in each image were counted. While macrofaunal features such as those exhibited by 

amphipods and polychaetes at site MT1 (Figure 8-149) can be numerous in many of the images, 

they were often too small to count accurately, but their presence was noted. Density of animals 
278 

Figure 8-149. Amphipod tubes present in great numbers cover this site at the head of the Mississippi trough (MT1). 

during the second visit to MT1 during the second DGoMB cruise. 
279 

per unit area was standardized to individuals/hectare (equivalent to 10,000 m2

). Trawl 

specimens, with the help of taxonomic experts, aided in identification of the animals in the 

photographs. 

Photographs obtained during NGoMCCS were digitized as described by Gallaway et al. 

(1988a). Subsamples of 100 or 200 frames per site (of the total 800 frames) were analyzed for 

benthic megafauna, including demersal fishes, anthropogenic material, terrigenous material, 

consolidated materials, and lebenspurren. Animal density was standardized to individuals/ha. 

Qualitative maps were made using ESRI ArcInfo software to illustrate large-scale 

distribution patterns of megafauna and other associated features. Two types of parametric tests 

were employed to examine megafauna distribution using the statistical software package SPSS. 

The first, linear regression analysis (p=0.05), considered the relationship between megafauna and 

depth down the slope. The second, analysis of variance (ANOVA; p=0.05), was used to compare 

DGoMB and NGoMCSS and associated regional variations. 

Linear regression analyses (p=0.05) were carried out on animal population density for 

DGoMB and NGoMCSS individually, both studies combined by similar sites, both studies 

combined by all sites, and for individual taxa from both studies combined – alcyonarians, 

asteroideans, decapods, demersal fishes, echinoids, holothuroids, ophiuroids, polychaetes, 

scaphopods, and Sargassum. Megafaunal densities from each site were grouped into 100 m depth 

intervals, the dependent variable in the analyses. Linear regression analyses (p=0.05) were run by 

depth interval for all DGoMB sites, real west transect sites (RW1-RW4, RW6), west transect 

sites (W1-W5), central transect sites (C1, C4, C7, C12), Mississippi Trough transect sites (MT1-

MT6), and east transect sites (S39-S44; Figure 8-150). Linear regression analyses (p=0.05) were 

done by depth interval for all NGoMCSS sites, west transect sites (W1-W5 cruise 2), central 

transect sites (C1-C5 cruise 2 & C1-C12 cruise 3), west-central transect sites (WC1-WC12 cruise 

5), and east transect sites (all of cruise 4 sites; Figure 8-151). Using the density data from similar 

sites for both DGoMB and NGoMCSS – W1, W2, W3, & W5 (DGoMB cruise 1 and NGoMCSS 

cruise 2), C1, C4, C7, C12 (DGoMB cruise 1 and NGoMCSS cruise 3), S43/E1, S42/E3, S41/E5 

(DGoMB cruise 1/NGoMCSS cruise 4), WC5, and WC12 (DGoMB cruise 1 and NGoMCSS 

cruise 5) – linear regression analyses (p=0.05) were performed by depth interval for all similar 

DGoMB and NGoMCSS sites, similar west transect sites, similar central transect sites, similar 

west-central transect sites, and similar east transect sites. Using the combined DGoMB and 

NGoMCSS densities, linear regression analyses were run by depth interval for all DGoMB and 

NGoMCSS sites combined, all west transect sites, all central transect sites, all west-central 

transect sites, and all east transect sites. Casewise diagnostics (±3 standardized residuals) in 

SPSS was used to identify outliers. A post analysis examination of standardized residuals was 

used to verify that they were independent, normally distributed, had constant variances, and 

means near zero. 

ANOVA’s (p=0.05) were run to compare megafaunal densities – grouped into 100 m depth 

intervals – by program (DGoMB and NGoMCSS) and site for both similar sites and all sites, by 

region (eastern and western GoM) for both DGoMB alone and combined with NGoMCSS data, 

and by feature (basins and non-basins or canyons and non-canyons) using only DGoMB 

densities. In the case of examining DGoMB megafaunal densities by region, ANOVA’s were 

also performed on average depth and area viewed numbers. The test was further used to compare 

density numbers from the northern GoM continental slope, the slope off east Greenland, and 

Suruga Bay, Japan. The assumptions for ANOVA were verified using a K-S test for normality 
280 

Figure 8-150. DGoMB photographic sites; rw-real west, ac-Alaminos Canyon, W-west, WC-west central, B-basi

C-central, MT-Mississippi Trough, S-east, GKT-top of Green Knoll, JSSD-Joint Studies of the Sigs

represent the 200, 1,000, 2000, and 3000-meter isobaths. 
281 

Figure 8-151. NGoMCSS photographic sites; W-west, WC-west central, C-central, E-east. Lines represent the 2

and 3000-meter isobaths. 
282 

(p=0.05) and a Levene’s test for homogeneity (p=0.05). Outliers were identified using boxplots 

in SPSS. Data that did not meet the assumptions for ANOVA were transformed. In cases where 

two-factor ANOVA’s were performed, a model was chosen to look only at the main effects 

because a lack of replications prevented examination of interactions between factors. 

During DGoMB, 1,421 images were taken at 45 sites and analyzed for megafauna, 

bioturbation level, current, and other features of interest (Table 8-40). These images encompass a 

total area of 2,559 m2. A total of 273 megafaunal representatives were viewed, constituting a 

total density of 1,067 individuals/hectare (ha). The highest densities were 3,009 burrowing 

anemones/ha (Figure 8-152) at Bush Hill. Sites at the top of Green Knoll and in the west – W3, 

W5, and WC5 – had the lowest megafaunal densities with no individuals represented in 

photographs. The highest megafaunal densities were observed in the Mississippi Trough (MT) 

transect where a total of 54 individuals were seen over an area of 418 m2 for a density of 

1,290/ha (Figure 8-153). The next highest densities occurred on the western transect with 937 

individuals/ha followed by the central transect with 900/ha and the real west (rw) transect with 

895/ha. The transect with the lowest densities was the one in the east (s) where megafaunal 

densities reached 780/ha. A total of 15 major taxonomic groups were identified from 

photographs during DGoMB (Figure 8-154). The taxonomic group most well represented was 

the Holothuroidea (Sea Cucumbers) with densities approaching 189 individuals/ha, followed by 

demersal fish (199/ha), decapods (161/ha), and alcyonarians with 149 individuals/ha. 

During NGoMCSS a total of 8,967 benthic photographs were analyzed from a total of 44,000 

photographs taken at 55 sites (Gallaway et al. 1988b). These photographs contain an area 

equivalent to 24,132 m2

 in which a total of 17,094 animals were tabulated. Total megafaunal 

density for NGoMCSS was 7,084 individuals/ha. The numbers for total abundance and density 

included 11,156 representatives of the holothuroid Peniagone sp. that were encountered at six 

central transect stations during cruise 3. Without the influence of this species, the total was 

reduced to 5,938 individuals for a density of 2,461 individuals/ha. A total of 18 major taxonomic 

groups were identified during NGoMCSS (Figure 8-155). This included four taxa that were not 

viewed during DGoMB – Ascidiacea, Bivalvia, Scleractinia, and Isopoda. The densities of these 

four groups, Isopoda (0.4/ha), Scleractinia (4/ha), Ascidiacea (4.1/ha), and Bivalvia (7.8/ha), 

were among the lowest seen during NGoMCSS. The highest densities were found with the 

actiniarians (707/ha), the decapods (645/ha), and the sponges (404/ha). 
283 

Table 8-40 

 

General Results by Site for DGoMB 

Site Average 

Depth 

Total Area 

Viewed (m2) 

Total 

Abundance 

Total Density 

(#/hectare) 

AC1 2550.0 43.2 7 1620 

B1 2256.0 79.4 2 252 

B2 2611.0 7.8 1 1282 

B3 2649.0 27.6 1 362 

NB2 1535.0 52.6 3 570 

NB 3 1874.0 52.4 1 191 

NB 5 2110.0 50.0 8 1600 

RW1 207.0 87.0 15 1724 

RW 2 945.0 84.0 7 833 

RW 3 1317.5 51.4 1 195 

RW 4 1570.0 65.6 5 762 

RW 5 1628.5 69.4 4 576 

W1 464.5 39.4 8 2030 

W 2 619.5 22.6 1 442 

W 3 875.0 10.8 0 0 

W 5 2742.5 2.0 0 0 

W 6 3125.0 64.0 4 625 

WC5 328.0 2.4 0 0 

WC12 1300.0 3.4 1 2941 

C1 350.5 83.6 2 239 

C4 1445.5 80.8 6 743 

C7 2000 1022.0 80.0 5 625 

C12 2922.0 89.0 17 1910 

MT1 2000 474.0 69.6 13 1868 

MT 2 679.5 54.0 2 370 

MT 3 995.5 87.0 9 1034 

MT 4 1403.0 84.0 13 1548 

MT 5 2294.5 72.0 8 1111 

MT 6 2757.5 52.0 9 1731 

S35 663.5 88.0 9 1023 

S36 1838.5 83.6 3 359 

S37 2382.0 89.6 13 1451 

S38 2647.0 63.4 10 1577 

S39 2998.5 88.0 5 568 

S41 2000 2963.0 78.0 14 1795 

S42 766.0 89.0 2 225 

S43 359.0 39.6 5 1263 

S44 212.5 90.0 4 444 

Bush Hill 530.5 43.2 13 3009 

C7 2001 1083.0 79.4 13 1637 

GKT 1897.5 7.8 0 0 

MT 1 2001 475.5 27.6 8 2899 

S41 2001 3039.5 52.6 14 2662 

JSSD-2 3732.0 21.4 2 935 

JSSD-5 3316.0 51.0 5 980 
284 

Figure 8-152. DGoMB total megafaunal density (individuals/ha) by site. 

ac1

b1

b2

b3

nb2

nb3

nb5

rw1

rw2

rw3

rw4

rw5

w1

w2

w3w5

w6

wc5

wc12

c1

c4

c7

c12mt1

mt2

mt3

mt4

mt5

mt6

s35

s36

s37

s38

s39

s41

s42

s43

s44

Bush Hi

c7 2001

G

0

500

100015002000250030003500

Density (individuals/ha)
285 

895.4

936.6

899.8

1290.0

780.0

357.4

138.8

333.4

418.6

3

0.0

200.0400.0600.0800.0

1000.01200.01400.0

Real West West Central Miss. Trough Eas

Transect

Density (individuals/ha)

Figure 8-153. DGoMB total megafaunal density (individuals/ha) and total area (m2) surveyed by transect. 
286 

Figure 8-154. DGoMB total megafaunal density (individuals/ha) by taxon. 

52

149

28

4 4

161

189

8

16 12

233

52

60

0

50

100150200250

Actiniaria Alcyonaria Asteroidea Cephalopoda Crinoidea Decapoda Demersal Fish Echinoidea Gastropoda Gorgonian Holothuroidea Ophiuroidea PolychaetaPorif

Taxon

Denisty (individuals/ha)
287 

44.8

706.6

4.1 15.9 7.8 3.7

52.4

644.7

204.4

6.2 16.4

109.9

0.4

176.0

30.5

403.8

0

100200300400500600700800

Alcyonaria Actiniaria Ascidiacea AsteroideaBivalvia Cephalapoda Crinoidea Decapoda Demersal Fish Echinoidea Gastropoda HolothuroideaIsopoda Ophiuroidea PolychaetaPorifera Sca

Taxon

Density (individuals/ha)

Figure 8-155. NGoMCSS total megafaunal density (individuals/ha) by taxon. 
288 

During Cruise 3 of NGoMCSS very high densities of the holothuroid Peniagone sp. were 

seen in the photographs at C3, C4, C7, C8, C9, and C10. Peniagone sp. was not viewed at any 

other sites during NGoMCSS or DGoMB. Further, no representatives of this holothurian were 

caught in any of the trawls. The grouping of Peniagone was centered on site C9 at 1,325 m depth 

where densities reached 154,669/ha (Figure 8-156). Around this central site there appears to be a 

gradient of the species both up and down the slope. The photographic depth range for this 

species is 864-1,611 m. 

The Phaeophyceae plant Sargassum was commonly seen in photographs from both studies. 

The combined density distribution of Sargassum from photographs across the slope is shown in 

Figure 8-157. On the whole, DGoMB densities were higher than NGoMCSS densities (Figure 8-

158). From Figure 8-159, it is interesting to note on the apparent lack of Sargassum between the 

far western GoM and the west-central GoM. Further, Sargassum was not seen in the Mississippi 

Trough, the DeSoto Canyon area and many eastern GoM sites. By combining both DGoMB and 

NGoMCSS Sargassum data by location, it becomes apparent that there was a gradient in 

Sargassum density between the western (296/ha) and the eastern GoM (41/ha; Figure 8-159). 

When photographing the benthos there are often many features that commonly recur. During 

DGoMB the characteristics most commonly observed in benthic photographs were trash, 

pteropod tests, ironstone, and markings from current effects. Figure 8-160 shows that items of 

trash occurred at nine stations all centered on the Mississippi Trough with all but one below the 

1,000-m isobath. Occurrences of pteropod tests were also centered around the Trough with shells 

littering the sediments at four sites, all but one of which were below the 2000-m isobath (Figure 

8-161). Ironstone appeared on the bottom in slate-like layers and was primarily composed of an 

iron oxide precipitate (Morse, personal communication). Pequegnat first noted it in benthic 

skimmer samples in the 1960’s. Generally, this “Ironstone” region is thought to be centered on 

the Mississippi Fan, or cone of sediments derived from the trough. The portion of this region that 

was sampled during DGoMB is seen in Figure 8-162 and Figure 8-163. Ironstone occurrences in 

this figure are from both photographs and trawls. Finally, the winnowing and erosion effects of 

currents were common. The DGoMB sites where current effects were visible are all below the 

2000-m isobath. Further, these currents appear to be situated on and below the Sigsbee and 

Florida Escarpments. 

DGoMB sediment cores from cruise 1 were analyzed for bioturbation levels – low, medium, 

and high. The distribution of core bioturbation levels across the slope is seen in Figure 8-164. In 

order to compare the bioturbation levels seen in the cores with what is seen in photographs, the 

bioturbation levels from DGoMB cruise 1 photographs were subjectively determined and are 

shown in Figure 8-165. Photographic bioturbation levels were defined in terms of the percent of 

an image covered by tubes, tracks, or burrows. Low levels had less than 10% coverage, medium 

levels had 10-50% coverage and high levels were greater than 50% coverage. Values based on 

core samples and those based on photographic samples were generally comparable. The cases 

where they were not may be due to the difference in spatial scales between the cores (cms) and 

the photos (meters). The cores are considering evidence within the sediments, whereas the 

photos pictured only the sediment – water interface. The two may not be directly related. 

Two types of comparisons were performed to quantitatively examine the difference in 

megafaunal densities between DGoMB and NGoMCSS. The first compared only the sites that 

DGoMB and NGoMCSS had in common and the second compared all sites for both studies. 

Total megafaunal densities for similar sites (Figure 8-166) were evaluated with a two-way 
289 

Figure 8-156. Density (individuals/ha) of the holothuroid Peniagone sp. seen in large numbers in the cent

NGoMCSS cruise 3. Sizes of the circles represent the density (individuals/ha) of Peniagone sp.. 

the 1,000 and 2000-meter isobaths. 
290 

Figure 8-157. GoM photographic sites. Closed circles represent sites where Sargassum was visible in photogra

circle represents density (individual clumps/ha). Open circles represent sites where Sargassum wa

photographs. Lines represent the 200, 1,000, 2000, and 3000-meter isobaths. 
291 

0

200400600800

1000120014001600

0 500 1000 1500 2000 2500 3000 

Depth (m)

Density (individual clumps/ha)

Figure 8-158. DGoMB (open triangles) and NGoMCSS (closed triangles) Sargassum density (individual clumps/ha
292 

296

144

47 41

0

50

100150200250300350

Real West West Central Central Eas

Transect

Density (individual clum ps/ha)

Figure 8-159. Total GoM Sargassum density (individual clumps/ha) by transect for all sites combined. 
293 

Figure 8-160. GoM photographic sites. Stars represent sites where trash was seen in the images. Open circles 

where trash was not seen. Lines represent the 200, 1,000, 2000, and 3000-m isobaths. 
294 

Figure 8-161. DGoMB photographic sites. Stars represent sites where pteropod tests were seen covering the se

Open circles represent sites where pteropod tests were not seen. Lines represent the 200, 1,000, m isobaths. 
295 

Figure 8-162. June 5, 2000. Dr. Rowe sifted through iron stones for specimens. Image is taken at MT6. 
296 

Figure 8-163. DGoMB trawl and photographic sites where Ironstone was sampled. Lines represent the 200, 1,000

meter isobaths. 
297 

Figure 8-164. Bioturbation levels from DGoMB year 1 sediment cores. Lines represent the 200, 1,000, 200

isobaths. 
298 

Figure 8-165. Bioturbation levels seen in photographs during DGoMB. Lines represent the 200, 1,000, 20

isobaths. 
299 

0

10002000300040005000600070008000

0 500 1000 1500 2000 2500 3000 

Depth (m)

Density (individuals/ha)

Figure 8-166. Megafaunal densities (individuals/ha) in photographs at similar depts in DGoMB (open triangles) 

(closed triangles). 
300 

ANOVA (p=0.05). The assumptions for this parametric test were verified for each program using 

a K-S test for normality (p>0.05) and a Levene’s test for homogeneity (p>0.05). Assumptions for 

site could not be verified because of a lack of replication. Results of the analysis indicate that 

there was no significant difference in megafaunal densities (p>0.05) both between programs and 

between similar sites. 

Total megafaunal densities from all sites in both DGoMB and NGoMCSS (Figure 8-167) 

were compared with a 2-way ANOVA (p=0.05). A total of 10 outliers and three extremes were 

identified using boxplots and removed. The assumptions for a two-way ANOVA were verified 

for each program using a K-S test for normality (p>0.05) and a Levene’s test for homogeneity 

(p>0.05). An attempt was made to verify the assumptions for an ANOVA by Depth Interval, but 

again lack of replication made this difficult. The density within the 400, 500, 800, and 900 m 

depth intervals – the only intervals with adequate replication – were determined to be normally 

distributed (p>0.05) using a K-S test. The ANOVA indicated no significant difference in 

megafaunal densities between Program and Depth Interval for all sites (p>0.05). 

DGoMB density and abundance numbers were examined by themselves to look at the 

significance of their relationship with depth. Linear regression analyses (p=0.05) were run for 

both density and abundance by depth for all DGoMB sites, “real west” transect sites, “west” 

transect sites, “central” transect sites, “Mississippi Trough” transect sites, and “east” transect 

sites. No outliers were identified. The analyses indicated that there was not a significant (p>0.05) 

relationship between depth and megafaunal abundance for all sites and by transect. Assumptions 

for the analyses were verified through a post analyses examination of standardized residual. 

NGoMCSS density and abundance numbers were examined by themselves to look at the 

significance of depth. Linear regression analyses (p=0.05) were performed on density abundance 

by depth for all sites, “west” transect sites, “central” transect sites, “west-central” transect sites, 

and “east” transect sites. Peniagone and the unidentified sponge species found in large numbers 

at C10 were identified as outliers and they were removed from the analyses. The analyses 

indicated that there was not a significant (p>0.05) relationship between depth and megafaunal 

density and abundance for all sites and by transect. Post analyses verification of standardized 

residuals verified the assumptions of the regression. 

DGoMB and NGoMCSS density and abundance numbers were examined to look at the 

significance of their relationship with depth. Linear regression analyses (p=0.05) was done for 

abundance by depth for all similar sites, similar “west” transect sites, similar “central” transect 

sites, similar “west-central” transect sites, and similar “east” transect sites. Peniagone and site c4 

(NGoMCSS cruise 2) were identified as outliers and removed from the analyses. The analyses 

indicated that there was not a significant (p>0.05) relationship between depth and megafaunal 

density and abundance for all sites and by transect. Post analyses verification of standardized 

residuals indicated that they were independent, normally distributed, had constant variances, and 

means near zero. The analyses did indicate that there was a significant relationship (p<0.05, 

R2

=0.065) between WC-transect megafaunal densities and depth interval (Figure 8-168). 

Assumptions for the regression were verified through a post-analysis verification of standardized 

residuals. 
301 

0

100020003000400050006000700080009000

0 500 1000 1500 2000 2500 3000 3500 

Depth (m)

Density (individuals/ha)

Figure 8-167. Megafaunal densities (individuals/ha) for all sites. The open triangles represent DGoMB data; the 

represent NGoMCSS data. 
302 

R2 = 0.3706

0

500

1000150020002500300035004000

0 200 400 600 800 1000 1200 

Depth (m)

Density (individuals/ha)

Figure 8-168. West-central transect megafauna density (individuals/ha) as a function of depth for both DGoMB and 
303 

One-way ANOVA’s (p=0.05) for DGoMB density, area viewed (m2

), and average depth (m) 

were performed to examine the difference between eastern and western benthic populations,. 

Outliers were identified for the density data using boxplots and removed. To meet the 

assumptions of normality and equal variances the density data had to be square root +1 

transformed. This allowed the east and west density data, along with the untransformed area 

viewed, and average depth numbers to meet the assumptions for an ANOVA – K-S test for 

normality (p>0.05), Levene’s test for homogeneity (p>0.05). The population densities in the east 

were different from those in the west (p<0.05, R2=0.289; Figure 8-169). 

One-way ANOVA’s (p=0.05) were performed on the density, area viewed, and average 

depth values for both basin and non-basin sites. Data were normally distributed (K-S test of 

normality p>0.05), and had equal variances (Levene’s test for homogeneity p>0.05) for the 

variables listed. For the “area viewed” values, equal variances (Levene’s test for homogeneity 

p<0.05) could not be assumed. Results of ANOVA (p>0.05) indicated that there was not a 

significant difference between basin and non-basin density, abundance, and area viewed values. 

Average depth however was shown to be significantly different (p<0.05, R2=0.718) between 

basins and non-basins. Examination of the major megafaunal groups present at basin and nonbasin sites show that while they only share one group—Holothuroidea—many more taxonomic 

groups were found at non-basin sites (Figure 8-170). 

It has been shown previously that canyon fauna differs from non-canyon fauna at similar 

depths (Rowe 1971; Ohta 1983) with certain species serving as indicator species for the canyon 

fauna. The idea of canyons being different was examined in the GoM using DGoMB 

photographic data from transects down the Mississippi and DeSoto Canyons (Figure 8-171). 

Canyon transects were compared with transects from areas immediately adjacent to the canyons. 

A univariate ANOVA was performed on megafaunal density and abundance numbers for both 

canyon transects and adjacent transects. Based on boxplots, it appears that density at MT1 2001 

(2,899/ha) along with density at MT2 (two individuals), were outliers and removed from the 

analysis. Levene’s test for homogeneity (p>0.05) and the K-S test of normality (p>0.05) ensured 

that variances were equal and the density and abundance data were normally distributed. 

ANOVA indicated that there was not a discernable difference (p>0.05) in megafaunal densities 

between canyon and non-canyon transects. Examination by taxon at both canyon and noncanyon sites (Figure 8-172) indicates that the four transects shared 63-75% of the eight most 

abundant taxa. 
304 

0

500

10001500200025003000

0 500 1000 1500 2000 2500 3000 

Depth (m)

Density (individuals/ha)

Figure 8-169. DGoMB western (open triangles) and eastern (closed triangles) megafaunal densities (individuals/

of depth. 
305 

Holothuroidea

Decapoda

Decapoda

Actiniaria

Alcyonaria

Demersal Fish

Polychaeta

Porifera

0

0.5

1

1.5

2

2.5

3

3.5

Basin Non-Basin

Abundance

Figure 8-170. DGoMB total megafaunal abundances by taxon for both basin and non-basin sites. 
306 

Figure 8-171. DGoMB Mississippi Canyon (MT1-MT6), Mississippi Canyon adjacent (C1, C4, C7, and C12), 

(S35-S37), and DeSoto Canyon adjacent (S38-S44) sites. Lines represent the 200, 1,000, 2000, 

isobaths. 
307 

Actiniaria

Actiniaria

Actiniaria

Actiniaria

Porifera

Porifera

Porifera

Gorgonian

Porifera

Gorgonian

Gorgonian

Gorgonian

Ophiuroidea

Ophiuroidea

Ophiuroidea

Ophiuroidea

Alcyonaria

Alcyonaria

Alcyonaria

Decapoda

Decapoda

Decapoda

Demersal Fish

Demersal Fish

Demersal Fish

Holothuroidea

Holothuroidea

Holothuroidea

05

10152025

Mississippi Canyon Adj. Miss. Canyon DeSoto Canyon Adj. DeS

Abundance

Figure 8-172. DGoMB total megafaunal abundances (number per hectare) of the eight most abundant taxa in the Mis

Canyons and the areas adjacent to these canyons. 
308 

GoM photographic megafaunal density data were compared with density data from several 

other studies the relative to evaluate richness of the deep GoM. This included in Suruga Bay, 

Japan by Ohta (1983) and the continental slope off east Greenland by Mayer and Piepenburg 

(1996). ANOVA was performed by location after the data were ln+1 transformed. The data were 

normally distributed (K-S test of normality p>0.05), but variances were not equal (Levene’s test 

for homogeneity p<0.05). However, ANOVA is robust with departures from homogeneity. 

Results of ANOVA indicate that megafaunal densities were significantly different (p<0.05) by 

location. Because equal variances could not be assumed Tamhane’s was used for multiple 

comparisons. Results of multiple comparisons indicate that megafaunal density numbers were 

significantly different (p<0.05) between the GoM, Greenland, and Japan. Megafaunal densities 

were up to two orders of magnitude higher than the GoM on the continental slope off Greenland 

and up to an order of magnitude higher in Suruga Bay, Japan (Figure 8-173). 

Megafaunal densities from photographs taken during DGoMB and NGoMCSS were 

compared with one another by site, transect, region, and program. ANOVA results indicate that 

megafaunal density was not statistically different from NGoMCSS for any of these cases. 

Furthermore, the studies share four out of the top six taxa by density. While NGoMCSS results 

list four more groups than DGoMB, these are all groups that are relatively rare with less than 

eight individuals/ha appearing study wide. Therefore, it would seem that the megafaunal 

populations of the northern GoM continental slope have not changed significantly in the past 15 

years in terms of numbers and types of animals. 

Food availability, it has been suggested, is the single most important factor affecting benthic 

megafaunal distribution (Sokolova 1959). This factor manifests itself in abundance and biomass, 

which both typically exhibit negative linear functions of depth and distance from shore. As depth 

increases, there is a greater chance for the remineralization and use of organic matter before 

reaching the benthos. For the megafauna, this means decreased biomass as food decreases with 

depth (Lampitt et al. 1986). As distance from shore increases, surface production decreases 

because of a decrease in primary producers, resulting in a commensurate decrease in both 

benthic abundance and biomass. 

The low benthic biomass seen in equatorial regions is the result of both large nutrient-poor 

mid-oceanic gyres and high surface water temperatures that degrade organic matter at a higher 

rate. In contrast to these depauperate conditions, there are areas of upwelling – typically along 

western continental margins – where cold, nutrient rich waters increase surface production which 

in turn increases benthic abundance and biomass (Pfannkuche et al. 1983). In some instances too 

much surface production can lead to anoxia on the bottom (Thiel 1982). 

Since food is the single most important factor in shaping the benthos (Sokolova 1959), it 

follows that in areas where food is concentrated, abundances will be as well. This is seen with 

chemosynthetic seep communities and might explain the intense concentration of Peniagone sp. 

seen in the central gulf during NGoMCSS cruise 3. In the case of the seep communities, the food 

source is presumed to be constant over some finite period of time, while for Peniagone sp., the 

food source may be more transient or intermittent in both time and space. Photography or trawls 

have not sampled this group on three subsequent visits to the area. Transient food resources are 

likely a forcing factor for many members of the benthic megafauna. Overall it appears that while 

megafauna numbers and composition change on the small scale in response to localized 

conditions, they still remain constant on the whole. 
309 

1.E+001.E+011.E+021.E+031.E+041.E+051.E+06

0 500 1000 1500 2000 2500 3000 3500 4000

Depth (m)

Density (individuals/ha)

Greenland Slope 

Surgura Bay, Japan

Northern GoM Con

Figure 8-173. Total megafaunal densities (individuals/ha) from the northern GoM continental slope (open diamond

east Greenland (open squares), and Suruga Bay, Japan (open triangles). 
310 

Aside from seeps and the Peniagone sp. grouping, there is statistically no significant trend 

for the accumulation of megafauna on a particular area of the northern GoM continental slope. 

ANOVA showed that megafaunal densities remained constant between the eastern and western 

gulf, between large basin features and the adjacent slope, and between canyons and non-canyon 

areas. While there was a statistically significant difference shown in DGoMB eastern and 

western GoM densities, this result might best be explained by sampling effort, an issue that will 

be discussed later. Furthermore, there does not appear to be any relationship between total 

megafauna density (individuals/ha) and depth. Examination of individual taxa relationships with 

depth indicates that some of the more common and widely distributed groups—the decapods and 

demersal fishes—exhibit a negative linear relationship with depth (Figures 8-174 and 8-175). 

Any general trend seen overall in the density data might be attributed to these two groups. The 

less common scaphopods exhibited a positive linear relationship with depth (Figure 8-176). 

Their small size and difficulty in identifying as dead or alive makes conclusions regarding this 

group difficult. 

In general, most activity appears to be centered on the Mississippi Trough area where the 

highest densities for DGoMB were seen. The highest densities for NGoMCSS were seen just to 

the east of this area and the Peniagone sp. grouping was seen just to the west of it. Further, the 

distributions of nearly all other features of interest—the trash, pteropod tests, and ironstone 

(Figures 8-160 to 8-163)—were centered on the trough. Increased activity around the trough is 

evidence that it is an area of enhanced food resources. 

Of all the items seen in the slope photographs, the distribution of Sargassum might hold the 

most interest. Thought to be an important source of energy to the benthos, Sargassum can have a 

long residence time on bottom (Schoener and Rowe 1970). Using combined data from both 

DGoMB and NGoMCSS, linear regression analysis showed that Sargassum exhibited a positive 

linear relationship with depth (Figure 8-177). 

The distribution of Sargassum densities from photographs indicates that there were three 

general areas on the northern GoM continental slope where this plant material is accumulated: 

the eastern slope, the central slope, and the far western slope, supposedly with zones of no 

accumulation in between. Furthermore, there appears to be a gradient in density of Sargassum, 

with the lowest values being found in the east and the highest in the west. At first glance the 

distribution of Sargassum seems counterintuitive. It would appear however that Sargassum was 

accumulating in areas with decreased biological activity—the western and deep GoM. The lower 

densities of Sargassum seen in areas with more activity may reflect higher utilization of the plant 

material by the benthic fauna. 

GoM megafauna in worldwide terms are depauperate when compared with similar 

photographic studies at higher latitudes. There appears to be a latitudinal gradient in megafaunal 

density with the highest values found in Greenland and the lowest values found closer to the 

equator in the GoM (Figure 8-178). Latitudinal gradients in the deep-sea benthos have been 

observed in terms of diversity before (Rex et al. 1993). Differences in surface production, habitat 

variability, and available substrate might all be reasons for the differences in densities observed. 
311 

R2 = 0.0909

-500

0

500

100015002000250030003500

0 500 1000 1500 2000 2500 3000 3500 

Depth (m)

Density (individuals/ha)

Figure 8-174. Decapoda density (individuals/ha) from both DGoMB and NGoMCSS photos as a function of depth. 
312 R2 = 0.1307

0

200400600800

1000120014001600

0 500 1000 1500 2000 2500 3000 

Depth (m)

Density (individuals/ha)

Figure 8-175. Demersal fish density (individuals/ha) from both DGoMB and NGoMCSS photos as a function of dep
313 

R2 = 0.2347

0

50

100150200250300350400450500

0 500 1000 1500 2000 2500 3000 

Depth (m)

Density (individuals/ha)

Figure 8-176. Scaphopoda density (individuals/ha) from both DGoMB and NGoMCSS photos as a function of dept
314 

R2 = 0.2877

0

200400600800

1000120014001600

0 500 1000 1500 2000 2500 3000 

Depth (m)

Density (individuals/ha)

Figure 8-177. Sargassum density (individuals/ha) from both DGoMB and NGoMCSS photos as a function of depth
315 

0

200400600800

10001200

Western GoM Eastern GoM

Density (individuals/ha)Density

Area

Figure 8-178. Eastern and western continental slope megafaunal densities (individuals/ha) and area surveyed (m2). 
316 

8.8.6 Demersal Fishes (John McEachran, TAMU, and Richard Haedrich, MUN) 

Trawling for demersal fishes was conducted during 2000, 2001, and 2003 surveys of the 

Deepwater Program: Northern Gulf of Mexico Study (DGoMB). However, the only 

comprehensive survey occurred in the 2000 survey. During the 2000 survey fishes were captured 

at 31 of the 43 stations representing all of the DGoMB transects (Figure 8-179). During the 2001 

survey trawl collections were taken at only two of the stations (MT1 and S42). In 2003 trawling 

was conducted outside of the survey area. Because of the disparities in coverage among the 

surveys only the 2000 survey is included in this report. This report describes the species 

composition, distribution, relative abundance and diversity over the study area, and to test three 

hypotheses generated by the DGoMB project. A longer version of this report was previously 

published. 

A total of 1065 individual demersal fishes, representing 119 species and 42 families, were 

collected in the 31 trawl collections conducted on the 2000 survey (Figure 8-180). The families 

Macrouridae (grenadiers or rattails), with 21 species; Ophidiidae (cuskeels), with 15 species; and 

Alepocephalidae (slickheads), with eight species, dominated the samples. Families of secondary 

importance were: Halosauridae (halosaurs), with four species, Ipnopidae (tripodfishes), with four 

species, Moridae (morid cods), with two or three species; and Trichiuridae (cutlassfishes) with 

two or three species. Discrepancies in number of species within families were due the inability, 

due to damage, to identify some fishes to species. A total of 34 specimens, representing 11 

species and 10 families of demersal fishes, were captured at two stations (MT1 and S42) on 2001 

survey. Three of the species and two of the families represented on the 2001 survey were not 

captured on the 2000 survey. For the two surveys a total of 121 demersal species representing 44 

families were captured. 

A cluster analysis (Bray-Curtis Dissimilarity Coefficient) was used to group stations by 

species composition and relative abundance. The analysis, based on 19 stations at which a 

minimum of five species were captured, grouped fishes into four nearly discrete depth 

assemblages (Figure 8-180). These assemblages consisted of: an outer continental shelf 

assemblage between 188 and 216 m, an upper slope assemblage between 315 and 785 m, a midslope assemblage between 686 and 1,369 m, and a deep assemblage between 1,533 and 3,075 m. 

The similarity analysis suggested that depth is the major factor determining species distributions 

and that the upper slope and mid-slope assemblages were more similar in species composition 

than either was to the deep assemblage and outer continental shelf assemblage. The outer 

continental shelf assemblage was most distinct. 
317 

Figure 8-179. DGoMB stations in the northern Gulf of Mexico that yielded fish 

specimens on the 2000 survey. Depth contours are 200 m (solid 

line), 500 m (dashed line), 1,000 m (solid line), 2,000 m (dashed 

line), and 3,000 m (solid line). Outer Continental Shelf Stations: 

RW-1, S-44 (188-216 m); Upper Slope Stations: C-1, MT-1, S-42, 

S-43, W-1, W-2 (315-785 m); Mid-Slope Stations: C-4, C-7, MT-2, 

MT-3, MT-4, S-35, W-3, WC-5 (686-1,369 m); Deep Stations: B1, B-2, B-3, C-12, MT-5, MT-6, NB-2, NB-3, NB-5, S-36, S-37, S38, S-40, S-41, W-6 (1,533-3,075 m). 

Figure 8-180. A cluster analysis of the DGoMB stations at which a minimum of 

five fish species were captured using the Bray-Curtis 

Dissimilarity Coefficient. The stations are indicated by their 

codes (Figure 8-179) and depth at the start of the trawl. 
318 

A total of 16 species representing 12 families were captured at the two outer continental shelf 

stations. The most species rich families were Paralichthyidae (summer flounders), with three 

species; and Bothidae (lefteyed flounders), with two species. The remaining families were 

represented by one species each. The most abundant species were Antigonia capros (Figure 8-

181, Caproidae: boar fishes) and Peristedion miniatum (Figure 8-182, Peristediidae: armored 

searobins), making up 52% and 15% of the specimens, respectively. 

Forty-seven species representing 27 families were captured at the six upper slope stations. 

The most species rich family was Macrouridae (grenadiers), with 14 species. The other 26 

families were represented by one or two species each. The most abundant species were 

Caelorinchus caribbaeus (Figure 8-183, Macrouridae) making up 11.6%, Steindachneria 

argentea (Steindachneriidae: luminous hake) making up 8.3%, and Yarella blackfordi 

(Phosichthyidae: lightfishes) making up 7.8% of the specimens, respectively. 

Forty-five species representing 22 families were capture at the eight mid-slope stations. The 

most species rich families were Macrouridae, with 14 species and Alepocephalidae (slickheads), 

with four species. The 20 other families were represented by one or two species each. The most 

abundant species were Nezumia cyrano (Macrouridae), Coryphaenoides zaniophorus

(Macrouridae), and N. aequalis (Macrouridae) making up 15.8%, 13.8%, and 11.1% of the 

specimens, respectively. 

A total of 32 species representing nine families were captured at the deep stations. The most 

species rich families were Ophidiidae (cuskeels), with 12 species; Alepocephalidae, with six 

species; and Ipnopidae (tripod fishes), with three species. The six other families were represented 

by one or two species each. The most abundant species were Dicrelene kanazawai (Ophidiidae) 

and Bassozetus robustus (Ophidiidae) making up 11.8% and 9.9% of the specimens, 

respectively. 

The largest numbers of fishes per unit of effort were obtained from the upper slope and midslope stations (Figure 8-184). The deep stations produced the least number of fishes per unit 

effort. Across the station transects, the greatest numbers of fishes per unit effort were obtained 

from transects along the Mississippi Trough and the DeSoto Canyon (Figure 8-185). The lowest 

abundances occurred at the stations to the west the Mississippi. 

The upper slope and mid-slope zones had the greatest and second greatest species richness, 

respectively, among the four depth zones, and the deep zone had considerably greater species 

richness than the outer continental shelf zone (Figure 8-186). Across the station transects the 

greatest species richness occurred along the Mississippi Trough and DeSoto Canyon transects 

(Figure 8-187). With one exception (W-2), stations to the west of the Mississippi had relatively 

low species richness. 

The upper slope zone had the greatest H’ diversity, followed closely by the deep and the midslope zones (Figure 8-188). The outer shelf zone had the lowest species diversity. 
319 

Figure 8-181. June 11, 2000. Boar fishes, Antigonia capros, collected from the trawl at 

S44. 
320 

Figure 8-182. June 11, 2000. Armored searobins, Peristedion miniatum, collected from the trawl at S44. 
321 

Figure 8-183. June 17, 2000. Caelorinchus caribbaeus collected from the trawl at MT1. 
322 

Figure 8-184. Abundance (catch per unit effort) of fishes at each of the four depth. 

Figure 8-185. Abundance, scaled as the log of number/hour, of the fishes at each 

DGoMB trawl station. Largest symbol equals a catch per unit effort of 290 

fishes/hour. Remaining symbols are scaled accordingly. 
323 

Figure 8-186. Species richness of the fishes represented as number of species at each 

of the four depth zones. 

Figure 8-187. Species richness of fishes scaled as number of species at each DGoMB 

station. The largest symbol equals 18 species. The remaining symbols 

are scales accordingly. 
324 

Figure 8-188. Species Diversity (H’) of the fishes in each of the four depth zones. 

The first hypothesis states that variation in demersal fauna is correlated with depth. The 

highest catch per unit effort (CPUE), greatest species richness, and highest species diversity are 

on the upper slope zone (Table 8-41). The next highest catch per unit effort and species richness 

are at the mid-slope zone. The deep zone had the second highest species diversity. The relatively 

low percent overlap among the zones suggests that they are well defined. With exception of the 

outer shelf zone fish species diversity appears to be strongly inversely correlated with depth. The 

data for the outer shelf zone are based on only two stations and the low values are probably due 

to sampling error. 

Table 8-41 

 

Fish Faunal Data for All 31 Stations by Depth Zones 

Depth Zones Shelf Upper 

slope 

Mid-slope Deep 

CPUE 52.8 152 98 5.2 

Species 16 48 45 32 

H’ Diversity 0.80 1.39 1.29 1.36 

% overlap 3.2 16.8 5.9 

The second hypothesis states that the fauna exhibit an east-to-west gradient. Comparisons 

lumping the outer shelf and upper slope zones revealed that the greatest abundance of fishes 

were obtained at the stations along the Mississippi Trough and the Central transects and the 

greatest species richness and H’ diversity occurred at the stations along the West and East 

transects (Table 8-42). The stations along the Mississippi Trough had the greatest fish abundance 

and the stations along the West transects had greatest species richness and H’ diversity. The 

percent overlap between the four areas varied from 9.9 to 22.8 suggesting that the regions are 
325 

rather distinct from one another. The Mississippi Trough was treated separately from the 

remainder of the Central Area transects because at shallow depths it is well defined feature of the 

bottom of the Central Area. 

Table 8-42 

 

Faunal Comparisons Among Four Groups 

of Outer Continental Shelf and Upper Slope Stations 

(The last % overlap value is between East and West transects.) 

Shelf / Upper Slope (n=10) West Central Trough East 

CPUE 65.3 196.0 234.0 103.0 

Species 36 15 27 34 

H’ Diversity 1.28 0.81 1.14 1.22 

% overlap 9.9 13.4 22.8 16.9 

Comparisons lumping the mid-slope and deep zones revealed that the stations along the East 

transects had the highest abundance off fishes, and the greatest species richness and H’ diversity 

(Table 8-43). The lowest abundance occurred at stations along the West transects, and the lowest 

species richness and H’ diversity occurred at stations along the Central transects. The 

comparisons failed to reveal a consistent east-to-west gradient. The percent overlap among the 

three areas is considerably greater than the percent overlap between the two shallow zones and 

the greatest overlap is between the stations along the East and the West transects. 

Table 8-43 

 

Faunal Comparisons Among Three Groups 

of Mid-Slope and Deep Stations 

(The last % overlap value is between East and West transects.) 

Lower Slope/Rise (n=15) West Central East 

CPUE 3.4 4.0 8.1 

Species 17 10 21 

H’ Diversity 1.13 0.94 1.23 

% overlap 24.1 30.8 51.1 

The third hypothesis states that the basin faunas are distinct from the non-basin faunas. 

Comparisons lumping three stations from oceanographic basins and lumping the three stations 

from non-basin areas suggests that fish abundance, species richness, and H’ diversity were 

higher in non-basins than in basins (Table 8-44). However, the differences are slight and the 

sample sizes are low. Percent overlap in species composition is rather low but again differences 

may reflect the small sample sizes. 
326 

Table 8-44 

 

Faunal Comparisons between 

Basins and Non-Basins 

Lower slope 

(n=6) 

Basin Non-basin 

CPUE 2.3 4.0 

Species 6 8 

H’ Diversity 0.73 0.86 

% overlap 31.3 

The DGoMB survey suggests that demersal fishes of the northern GoM are divided into four 

depth assemblages: outer continental shelf, upper slope, mid-slope, and deep slope. The outer 

continental shelf assemblage extends from 188 to 216 m. This zone yielded the third highest 

abundance and lowest species richness and lowest H’ species diversity among the four depth 

zones. The upper slope assemblage extends from the 315 and 785 m. This zone yielded the 

highest abundance of fishes, the greatest species richness and highest H’ species diversity. The 

mid-slope assemblage extends from the 686 and 1,369 m zone and thus partially overlaps the 

upper slope zone. This zone yielded the second highest abundance, second greatest species 

richness and the third highest H’ diversity of fishes. The deep assemblage extends from the 1533 

and 3,075 m depth zones. This zone yielded the lowest abundance, and second lowest species 

richness and second highest H’ diversity of fishes. The greatest amount of overlap occurred 

between the upper and mid-slopes. The high overlap may partially be due to the fact that the two 

zones overlapped but also to the fact that there are distinct differences between the species and 

families occurring on the upper and mid-slope from those occurring on the upper slope and rise. 

The 2000 DGoMB survey captured a total of 119 demersal fish species representing 42 

families. Pequegnat (1983) captured 206 demersal fish species in the 10 year (1964-1973) survey 

of the northern GoM and the Texas Cooperative Wildlife Collection (TCWC) houses 208 

demersal deep-sea fish species, representing 72 families from the northern GoM. Thus the 2000 

DGoMB survey sampled about 57.2% of the species and 58.3% of the families of fishes thought 

to regularly occur in the northern GoM. McEachran and Fechhelm (1998 and in press), recorded 

233 demersal deep-sea fishes, representing 72 families, from the northern GoM. Thus the 

DGoMB survey sampled 45.5% of the species and 48.6% of the families known from the 

northern GoM. 

The families Macrouridae, with 21 species; Ophidiidae, with 15 species; and 

Alepocephalidae, with eight species, dominated the samples on the 2000 DGoMB survey, and 

the families Halosauridae, with four species, Ipnopidae, with four species, Moridae, with three 

species; and Trichiuridae, with three species were of secondary importance. Pequegnat (1983) 

also found Macrouridae, Ophidiidae, and Alepocephalidae to be the most specious families 

followed by Gadidae (=Phycidae, Merlucciidae, Steindachneriidae, and Moridae), Bothidae 

(=Bothidae, Parilichthydae), and Rajidae. Halosauridae and Trichiuridae were less species rich in 

Pequegnat (1983) than in the present survey and Rajidae (skates) was represented by more 

species in Pequegnat (1983) than in the present study. Ipnopidae was well represented in both 

surveys (listed as Bathypteroidae in Pequegnat 1983). Records of demersal deep-sea fishes in the 

TCWC and in McEachran and Fechhelm (1998 and in press) also suggest that the above families 

are some of the dominate components of the demersal deep-sea fauna of the northern GoM. 

There are 37 species of Macrouridae, 24 species of Ophidiidae, 14 species of Alepocephalidae, 
327 

four species of Halosauridae, eight species of Ipnopidae, four species of Moridae, and six species 

of Trichiuridae in the northern GoM (McEachran and Fechhelm, 1998 and in press). Thus the 

2000 DGoMB survey sampled 56.8% of the Macrouridae, 62.5% of the Ophidiidae, 57.1% of the 

Alepocephalidae, 100% of the Halosauridae, 50% of the Ipnopidae, 75% of the Moridae, and 

50% of the Trichiuridae in the northern GoM. Other families such as Rajidae, Dalatiidae (spiny 

sharks), and Peristediidae (armored searobins) that have relatively high species diversity in the 

northern GoM (McEachran and Fechhelm, 1998 and in press) were poorly represented in the 

present survey. Thus the DGoMB survey sampled the dominant demersal fish families according 

to their presumed relative diversity in the northern GoM. 

The DGoMB survey included two stations on the outer continental shelf at which a total of 

16 species representing 12 families were collected. The most species rich family was 

Paralichyidae and the most abundant species were Antigonia capros and Peristedion miniatum. 

Pequegnat (1983) listed Poeciliopsetta beani (Pleuronectidae), Hymenocephalus italicus 

(Macrouridae), and Pontinus longispinus (Scorpaenidae) as the most abundant species at this 

depth zone. According to the holdings of the TCWC about 139 demersal fish species in 50 

families are normally encountered on the outer continental shelf of the northern GoM, and about 

half of these species also occur on the upper slope. Thus the outer continental shelf was not 

adequately surveyed in this study and the values given for this zone may not reflect the species 

assemblage. 

The present survey found Macrouridae to be the most abundant family on the upper slope, 

with Caelorinchus caribbaeus the most abundant species. Pequegnat (1983) also found 

Macrouridae to be the most abundant family on the upper slope, with Nezumia aequalis, 

Bathygadus melanobranchus, and Corphaenoides colon being the most abundant species in the 

family. All of the macrourids captured on Pequegnat’s study were captured on the present study, 

however, specimens classified as C. colon in Pequegnat (1983) are now identified as C. 

zaniophorus. Caelorinchus caribbaeus was abundant but not dominant in Pequegnat’s study. 

Dibranchus atlanticus (Ogcocephalidae: batfishes) and Symphurus marginatus (Cynoglossidae: 

tonguefishes) were the first and fourth most abundant species on the upper slope in Pequegnat’s 

study but neither was captured on the present study. 

The DGoMB survey found Marcrouridae and Alepocephalidae to be the most specious 

families on the mid-slope, with Nezumia cyrano, Coryphaenoides zaniophorus, and N. aequalis

the most abundant species. Pequegnat (1983) listed Macrouridae, Ophidiidae, and 

Alepocephalidae as the most abundant families in this depth zone, with Gadomus longifilis, 

Dicrolene intronigra, and Synaphrobranchus oregoni/brevidorsalis the most abundant species. 

All of these species were captured on the present study but Coryphaenoides zaniophorus was 

identified as C. colon in Pequegnat’s survey. 

The DGoMB survey found Ophidiidae, Alepocephalidae, and Ipnopidae to be the most 

specious families in the deep zone, with Dicrelene kanazawai and Bassozetus robustus 

(Ophidiidae) the most abundant species. Pequegnat (1983) listed Ophidiidae and 

Alepocephalidae as the most specious families in this depth zone, with Dicrolene kanazawai, 

Lamprogramus, Bassozetus normalis, and Bathyonus pectoralis as the most abundant species. 

Lamprogramus sp. and Bathyonus pectoralis were not captured on the present study. 

Both the DGoMB survey and Pequegnat (1983) found that the demersal fish fauna of the 

northern GoM is subdivided into several depth assemblages. In the present study there are four 

assemblages: the outer continental shelf (188-216 m), the upper slope (315-785 m), the midslope (686-1,369 m), and the deep slope (1,533-3,075 m). The upper slope and mid-slope were 
328 

most similar in species composition and outer continental shelf had the most distinct species 

composition. Pequegnat (1983) described five depth assemblages: shelf/slope transition (150-450 

m), archibenthal zone (475-950 m), upper abyssal zone (975-2,250 m), mesoabyssal zone (2,275-

3,200 m), and lower abyssal zone (3,225-3,850 m). Similarity indices basally grouped the 

shelf/slope and archibenthal zones, and the abyssal zones, with the upper abyssal zone grouped 

with both the mesoabyssal and lower abyssal zones. Thus the present study found that outer shelf 

zone was distinct from the three slope zones while Pequegnat found greater similarity between 

zones either above or below 1,000 m than between zones above or below 200 m. Both studies 

agreed that the demersal fauna is strongly zoned by depths but not in how the four or five zones 

are linked by species composition. Part of the disparities may be due to sampling design and 

sampling intensity. The present study is based on a single year survey of 31 stations. Pequegnat 

was a seven year survey of 121 stations (86 skimmer dredge hauls, seven two meter dredge 

hauls, and 28 trawl hauls). Also Pequegnat sampled a greater number of stations on the outer 

continental shelf (10 versus 2 in the present study) and greater number of stations over a wider 

depth range in the deep slope zone (>1,533 m) (31 versus 15 in the present study). Thus the 

faunal composition of Pequegnat’s study may better typify the outer shelf and deep slope zones 

than the present study. 

Both the present study and Pequegnat (1983) corroborate the first hypothesis but not the 

second or third hypotheses of the DGoMB study. The first hypothesis states: Variation in 

demersal fauna is best correlated with depth. Similarity analyses based on species compositions 

grouped stations by depths in both studies. The faunal overlap between zones was relatively 

small suggesting that zones were distinct and the zones of the two studies showed a fair degree 

of overlap. However, affinities among the zones varied between studies. The present study 

suggested that the outer continental shelf zone was distinct from the three deeper slope zones 

while Pequegnat suggest that the two upper zones were distinct from the two or three deeper 

zones. Because of the differences in sampling intensity, the Pequegnat’s. grouping may better 

present the affinities among the depth zones. 

The second hypothesis states: Faunas exhibit an east-west gradient. Results of the present 

study demonstrate east-west differences in abundance, species richness, and H’ diversity of 

demersal fishes. However, the differences do not suggest an east-west gradient. Comparisons 

among the two shallow zones indicated that the central transects had higher abundance and lower 

species richness and H’ prime diversity values than either the west or east transects. Percent 

overlap was low among the transects but was higher between the east and the trough transects 

and lowest between the central and western transects. Comparisons among the two deeper zones 

indicated that the central transect had a higher abundance than the west transect but a lower 

abundance than the east transect. The central transects had a lower species richness and lower H’ 

diversity than either the west or east transects. Percent overlap was relatively low among the 

transects but was highest between the east and west transects. Thus there is little indication of an 

east-west gradient of the demersal fishes. 

The third hypothesis states: Basin faunas are different from non-basin faunas. Comparisons 

between the three basin and three non basin stations were not definitive possibly because of the 

small sample size. 
329 

8.8.7 Near-Bottom and Pelagic Fauna 

8.8.7.1 ADCP Detection of Mobile Near-Bottom Fauna 

Beginning in the second field year of the DGoMB study, a downward-looking RD 

Instruments Workhorse ADCP was deployed four times, for 27, 20, 55, and 26 hours at four 

stations in the northeastern GoM. In summer 2001 this ADCP looked down from 35 m above 

bottom at DGoMB process stations MT3, S42, S36, and MT6 so that near-bottom current 

velocity and near-bottom acoustic backscatter data could be collected concurrent with benthic 

lander deployments at these stations (Figure 8-189). 

Figure 8-189. Process stations for the 2001 field year of DGoMB. The four ADCP 

deployment sites are marked with red triangles, and water depths for these deployments 

are indicated. 

Additional Workhorse ADCP data were obtained from a deployment of 39 hours in April 

2002 and from a deployment of four weeks duration in June-July 2002 (Figures 8-190 and 8-

191). During the 2002 deployments, the Workhorse ADCP looked down from 60 m above 

bottom to give additional vertical information on how near-bottom current velocity and nearbottom backscatter intensity decreased with height into the water column immediately above the 

bottom. 

It was intended that the ADCP backscatter data gained from these 2001 and 2002 nearbottom deployments would give comparative information about the mean biomass of small 
330 

Figure 8-190. Sampling stations for the 2002 field year of DGoMB including station MT1 (depth = 300 m) where the ADCP mooring was deployed for 39 hours in April 2002. 

Figure 8-191. Location of the 14 June-12 July 2002 ADCP mooring in water depth of 880 m. 

Location of the CTD cast made 22:49 GMT on 14 June 2003 is also shown. 

mobile near-bottom fauna that have previously been difficult to inventory, as well as whether 
331 

this backscatter would vary on a diel cycle. It was not possible to collect any of the organisms 

responsible for this near-bottom backscatter, however, because baited traps were not 

incorporated into the ADCP moorings. 

To analyze the acoustic volume backscatter intensity signal (ABI) obtained by the Workhorse 

ADCP, we first converted the raw data from binary to ASCII format using the software program 

BBLIST (http://www.rdinstruments.com), along with the date and time that they were recorded. 

Programs written for the visual data analysis software program, PV-Wave, that were originally 

created by S.F. DiMarco and R.L. Scott for analysis of ADCP data collected during the DeSoto 

Canyon Eddy Intrusion Study, were used to correct the data for spherical spreading loss and to 

estimate the absolute backscatter coefficient, or Sv, in units of decibels. Table 8-45 summarizes 

the six deployment locations, details of data collection, and ABI averages. 

The comparison of 4 deployments in different water depths in June 2001 showed that mean 

near-bottom backscatter intensity increased as the bottom was approached and was greatest at the 

stations with shallowest water depths (Figures 8-192a and b). The largest intensity and the most 

variability were seen in deployment two (S42), where the water depth was 755 m, and the 

smallest intensity and variability in deployment four (MT6), where the water depth was 2,740 m. 

There was also a higher mean and more vertically structured current velocity at deployment two 

(S42). 

For each deployment, cross-correlation coefficients were calculated for each of the 12 depth 

bins investigated in this study. In general, correlation values decreased for each deployment as 

the vertical separation between the bins increased (Tables 8-46 and 8-47). There was no apparent 

phase change seen in the data collected during the four deployments to indicate diel vertical 

migration of scavengers or zooplankton. However, all stations with the possible exception of S42 

(depth = 755 m) were below depths at which twilight should penetrate to give a day/night cue. 

Each of the deployments was less than 60 hours, so it was difficult to determine whether low 

frequency (multi-day) temporal variability in ABI was in fact present. 

The data collected from the 39 h deployment at MT-1 at 300 m depth in April 2002 also 

showed that ABI increased near-bottom (Figure 8-193). When Bins 7 to 20 m above bottom were 

averaged, the mean ABI was –41 dB, whereas when only those bins 7 to 10 m off bottom were 

averaged, the mean ABI was 3 dB greater (–38 dB). This increase in mean backscatter as the 

bottom was approached was also evident in the four-week 14 June to 12 July 2002 record (Figure 

8-194). The mean backscatter signal in bins averaged from 7 to 10 m above bottom (-74 dB) was 

again approximately 3 dB greater than in bins averaged from 7 to 20 m above bottom (-77 dB). 

As in the 2001 deployments, the correlation coefficients decreased with vertical distance 

between depth bins for both the 2002 records. 

A signal with approximately one cycle per day (CPD) periodicity is present in the monthlong, June-July 2002 record. This had not been seen in the April data or in the previous 

summer’s ADCP deployments. This signal is surprising since, based on an average irradiance 

extinction coefficient of k = 0.05 for the deepwater northern GoM, the day/night light cue 

believed to initiate vertical migration in shallow water should not be present at 880 m. However, 

diurnal current oscillations present in this region may affect the abundance of scatterers at the 

deepwater mooring site. Between 3-4 July, there was a marked increase in the ABI 

measurements. This broad peak in ABI was seen throughout the ensonified column of water 10-

60 m off bottom and represents a positive shift in ABI by about 7 dB (Figure 8-194). This event 
332 

Table 8-45 

 

Summary Statistics for the Six Near-Bottom ADCP Deployments in 2001 and 2002 

Station Date/Time 

(local time) 

Location Water 

Depth 

Average 

Backscatter 

(dB) 

Deployment 1 

Station MT-3 

6/3/01 [noon] to 

6/4/01 [3:00pm] 

Lat 28°13.3N 

Lon 89°30.0W 

935 m -75.3 

(7-29 m above 

bottom) 

Deployment 2 

Station S-42 

6/6/01 [6:15pm] to 

6/7/01 [2:00pm] 

Lat 28°15.0N 

Lon 86°25.0W 

755 m -70.2 

(7-29 m above 

bottom) 

Deployment 3 

Station S-36 

6/9/01 [9:45am] to 

6/11/01 [4:30pm] 

Lat 28°55.2N 

Lon 87°40.1W 

1,823 m -78.3 

(7-29 m above 

bottom) 

Deployment 4 

Station MT-6 

6/12/01 [11:00pm]to 

6/14/01 [00:30am] 

Lat 27°00.0N 

Lon 88°00.0W 

2,740 m -82.0 

(7-29 m above 

bottom) 

Deployment 5 

Station MT-1 

4/7/02 [5:51am] to 

4/8/02 [9:16pm] 

Lat 28°54.0N 

Lon 89°83.0W 

300 m -42.0 

(7-54 m above 

bottom) 

Deployment 6 

Louisiana slope 

6/14/02 [4:30pm] to 

7/12/02 [11:27am] 

Lat 27°51.0N 

Lon 91°71.0W 

880 m -78.0 

(7-55 m above 

bottom) 
333 

Figure 8-192a. Time series plots of acoustic backscatter intensity for summer 2001 stations 935 

(top) and 755 (bottom) m deep. The ABI was averaged from the 4 beams of the ADCP, and 

backscatter intensity is plotted for every other depth bin 9-29 m off bottom. 
334 

Figure 8-192b. Time series plots of acoustic backscatter intensity for summer 2001 

stations 1,823 (top) and 2,740 (bottom) m deep. The ABI was averaged from the 4 

beams of the ADCP, and backscatter intensity is plotted for every other depth bin 9-29 

m off bottom. 
335 

Table 8-46 

 

Correlation Coefficients (r Values) Calculated between Each Depth Bin 

for the Two Shallowest Stations of the Summer 2001 ADCP Deployments 

(Significant correlations (values ≥ 0.53 are significant at the p = 0.05 level) are highlighted.) 

Station S-42 (bins given as depth off bottom) 

 29m 27m 25m 23m 21m 19m 17m 15m 13m 11m 9m 7m 

29m 1.00 0.81 0.71 0.62 0.21 -0.47 -0.61 -0.47 -0.25 0.001 0.32 0.44

27m - 1.00 0.88 0.72 0.36 -0.34 -0.69 -0.62 -0.27 0.04 0.36 0.50

25m - - 1.00 0.86 0.46 -0.26 -0.66 -0.66 -0.34 0.01 0.36 0.48

23m - - - 1.00 0.70 -0.12 -0.59 -0.59 -0.35 -0.05 0.29 0.45

21m - - - - 1.00 0.49 -0.18 -0.42 -0.31 -0.15 0.02 0.18

19m - - - - - 1.00 0.64 0.12 -0.28 -0.45 -0.54 -0.44

17m - - - - - - 1.00 0.70 0.05 -0.36 -0.60 -0.63

15m - - - - - - - 1.00 0.60 0.10 -0.23 -0.44

13m - - - - - - - - 1.00 0.79 0.46 0.16

11m - - - - - - - - - 1.00 0.83 0.55

9m - - - - - - - - - - 1.00 0.83

7m - - - - - - - - - - - 1.00

Station MT-3 (bins given as depth off bottom) 

 29m 27m 25m 23m 21m 19m 17m 15m 13m 11m 9m 7m 

29m 1.00 0.86 0.64 0.54 0.63 0.59 0.58 0.57 0.39 0.28 0.14 0.36

27m - 1.00 0.69 0.53 0.59 0.61 0.52 0.53 0.41 0.27 0.09 0.36

25m - - 1.00 0.62 0.51 0.54 0.44 0.51 0.46 0.16 0.06 0.37

23m - - - 1.00 0.51 0.38 0.39 0.28 0.13 0.02 0.03 0.35

21m - - - - 1.00 0.70 0.43 0.39 0.21 0.22 0.04 0.27

19m - - - - - 1.00 0.60 0.51 0.29 0.27 0.02 0.22

17m - - - - - - 1.00 0.62 0.17 0.12 -0.09 0.23

15m - - - - - - - 1.00 0.51 0.33 0.10 0.29

13m - - - - - - - - 1.00 0.49 0.26 0.27

11m - - - - - - - - - 1.00 0.46 0.05

9m - - - - - - - - - - 1.00 0.36

7m - - - - - - - - - - - 1.00
336 

Table 8-47 

 

Correlation Coefficients (r Values) Calculated between Each Depth Bin 

for the Two Deepest Stations of the Summer 2001 ADCP Deployments 

(Significant correlations (values ≥ 0.53 are significant at the p = 0.05 level) are highlighted.) 

Station S-36 (bins given as depth off bottom) 

 29m 27m 25m 23m 21m 19m 17m 15m 13m 11m 9m 7m 

29m 1.00 0.89 0.79 0.72 0.68 0.65 0.62 0.58 0.60 0.55 0.60 0.59

27m - 1.00 0.85 0.76 0.71 0.66 0.61 0.54 0.59 0.52 0.55 0.53

25m - - 1.00 0.89 0.80 0.75 0.71 0.65 0.66 0.60 0.60 0.58

23m - - - 1.00 0.89 0.76 0.72 0.68 0.72 0.60 0.59 0.57

21m - - - - 1.00 0.83 0.76 0.72 0.75 0.66 0.64 0.62

19m - - - - - 1.00 0.85 0.74 0.74 0.71 0.67 0.66

17m - - - - - - 1.00 0.86 0.78 0.78 0.72 0.71

15m - - - - - - - 1.00 0.86 0.80 0.75 0.72

13m - - - - - - - - 1.00 0.86 0.76 0.72

11m - - - - - - - - - 1.00 0.83 0.77

9m - - - - - - - - - - 1.00 0.83

7m - - - - - - - - - - - 1.00

Station MT-6 (bins given as depth off bottom) 

 29m 27m 25m 23m 21m 19m 17m 15m 13m 11m 9m 7m 

29m 1.00 0.92 0.85 0.84 0.82 0.70 0.70 0.73 0.64 0.63 0.57 0.57

27m - 1.00 0.93 0.91 0.89 0.77 0.75 0.75 0.65 0.65 0.61 0.60

25m - - 1.00 0.94 0.90 0.84 0.76 0.74 0.65 0.64 0.58 0.57

23m - - - 1.00 0.93 0.85 0.80 0.77 0.68 0.65 0.61 0.61

21m - - - - 1.00 0.89 0.84 0.80 0.68 0.66 0.61 0.62

19m - - - - - 1.00 0.84 0.77 0.60 0.59 0.58 0.58

17m - - - - - - 1.00 0.92 0.78 0.78 0.72 0.67

15m - - - - - - - 1.00 0.85 0.82 0.75 0.69

13m - - - - - - - - 1.00 0.91 0.80 0.75

11m - - - - - - - - - 1.00 0.83 0.70

9m - - - - - - - - - - 1.00 0.88

7m - - - - - - - - - - - 1.00
337 

Figure 8-193. Time series plot of acoustic backscatter intensity from the April 2002 

deployment in water depth 300 m. Labels on plot indicate distance 

above bottom. 
338 

Figure 8-194. Time series plots of unfiltered (top) and 40-hour low-pass filtered (bottom) ABI from 

the June-July 2002 deployment in water depth 880 m. Labels on plots indicate distance above 

bottom. Time interval between samples is 5 minutes. 
339 

was likely due to increased near-bottom currents (Figure 8-22) which likely acted to resuspend 

flocculent “marine snow” or similar large-size particulate matter from a local topographic high 

situated NE of the mooring site and carry this material downcurrent into the area where the 

ADCP was moored. 

The four-week deployment was long enough to allow spectral analysis to be performed. This 

analysis was conducted to determine what, if any, variability can be detected in the patterns of 

deepwater scatterers. The spectra show a peak in energy density centered at 0.9-1.2 cycles per 

day (Figures 8-195 and 8-196), although this peak is less evident in the spectra created for 7 m 

depth nearest bottom. Spectral analysis was also performed on the 14 June to 12 July ABI data 

after the application of a 40-hour low-pass filter to remove the noise at higher frequencies as well 

as the peak centered at 0.9-1.2 cycles per day. The spectra of the filtered data confirm that there 

was low frequency variability present in the deepwater data on weekly (0.07-0.14) time scales, as 

well as on 3-4 day (0.25-0.33) time scales. 

Bernal (2001) reported that particulate matter concentrations in bottom nepheloid layers were 

greater during spring and summer than during fall in the northeastern GoM. She also reported 

interannual variability in particulate matter concentrations near bottom. Three casts with a CTD 

with transmissometer attached were taken during the course of the four-week deployment in 

summer 2002 to look for possible nepheloid layers or aggregations of marine snow near the 

deployment site. These CTDs were taken at the mooring site when the ADCP was deployed on 

14 June, during an opportunity on 22 June, and at the mooring site when the ADCP was 

recovered on 12 July. No indication of markedly increased particulate matter near-bottom 

(nepheloid layer) was found in any of the transmissometer data (Figure 8-197), although percent 

transmittance generally decreased about 0.1% to 0.4% from mid-water minimum values as 

bottom was approached in all of these records. 

In writing this chapter we have assumed that the acoustic backscatter intensity signal in the 

ADCP data is due to mobile near-bottom scavengers and/or to relatively large-size resuspended 

flocculent particles, rather than to small-size particles like resuspended silt-clay sediment, or to 

physical factors such as sharp salinity differences. This conjecture is warranted by the 300 kHz 

frequency of the sound emitted by the Workhorse ADCP. Acoustic waves can generally detect 

objects one quarter of their wavelength. If the speed of sound in seawater is approximately 1.5 

km/s, then the wavelength of the emitted signal from the Workhorse ADCP is approximately 5 

mm (1,500 m/s/300 kHz), and thus the size limit of detection of sound reflected directly from 

particles would be about 1.25 mm. 

As a corollary, studies trying to measure suspended or resuspended sediments directly 

generally use frequencies in the mega Hz range, and such studies have optimum signal-to-noise 

returns when concentrations of suspended sediments exceed 104

 ug/L. From the transmissometer 

data collected in June-July 2002, it is unlikely that suspended or resuspended sediments of these 

concentrations were present in what appears to be a very weak nepheloid layer. 

Very little acoustic backscatter should reflect off a salinity gradient unless this exceeds 10 

psu, at a frequency of 300 kHz. In fact, the companion CTD data show no indication of any nearbottom salinity gradient >0.1 psu at the deployment site. Rather, near-bottom salinity there 

varied from 34.9 - 35.0. 

Due to the difficulty in collecting samples, very little research has been conducted to study 

zooplankton scatterers in the deep-sea. Deep-sea benthopelagic plankton were studied at depths 

of 1,000 to 4,700 m using an opening-closing net attached to a Deep Tow instrument. Horizontal 

transects were performed near-bottom at an average speed of 1.5 kt, but at this speed, net 
340 

Figure 8-195. Variance spectra of raw (top) and 40-hour low-pass filtered (bottom) ABI 

data from 48 m above bottom in the June-July 2002 month-long deployment. 
341 

Figure 8-196. Variance spectra of raw (top) and 40-hour low-pass filtered (bottom) ABI data 

from 7 m above bottom in the June-July 2002 month-long deployment. 
342 

Figure 8-197. Percent light transmittance from CTD casts made during 

deployment (top), on 22 June (middle), and recovery (bottom) of the 

June-July 2002 near-bottom-moored ADCP. 

xmiss vs. depth

400

450

500

550

600

650

700

750

800

850

900

93 93.5 94 94.5 95

xmiss %

depth (m)

xmiss vs. depth

400

450

500

550

600

650

700

750

800

850

900

93 93.5 94 94.5 95

xmiss %

depth (m)

xmiss vs. depth

400

450

500

550

600

650

700

750

800

850

900

93 93.5 94 94.5 95

xm iss %

depth (m)
343 

avoidance is expected by the faster swimming organisms so that the collected biomass may be an 

underestimate. However, Wishner did find higher benthopelagic biomass in her net samples 

taken 10 m above bottom than in samples taken 100 m above bottom. The larger ABI 

measurements seen near-bottom in this study suggest that there are near-bottom scatterers 

associated with the sea floor. Regrettably, neither baited traps nor near-bottom net tows were 

undertaken in this study to “sea truth” the backscatter data, and so it is impossible to conclude if 

the increased bottom signal was due solely to benthopelagic organisms, or whether most of the 

ABI came from aggregates of marine snow and other large-size particulate matter suspended or 

resuspended near-bottom. 

In conclusion, although this study has collected the first near-bottom ABI data in the deep 

GoM, the data that were collected can only allow us to speculate on the processes affecting the 

distribution and patchiness of deep-sea benthopelagic organisms. More deployments of ADCPs 

in the deep sea are needed to evaluate their ability to measure the abundance of small, mobile 

near-bottom fauna. If ADCP data could be made available from ADCP deployments that are 

done routinely by oil and gas exploration companies in conjunction with their development of 

lease blocks for oil and gas production, this would greatly increase the available ABI database. 

Ressler (2001) has begun the process to match ABI with scatterer size and density, but it is 

still difficult to identify the specific types of scatterers that contribute to a backscatter signal 

measuring using ADCPs. Multi-frequency acoustic techniques offer a promising solution to these 

problems, but we still need to know more about how zooplankton and micronekton patches vary 

both spatially and temporally. Such information could help to better explain the distribution and 

foraging strategies of larger, apex predators. 

8.8.7.2 Trap Caught Organisms 

A total of 133 amphipods of the species Eurythenes gryllus were collected in the abyssal 

plain of the Gulf of Mexico using baited traps at three sites. The specimens were identified based 

on morphological criteria; schemes of diagnostic features were useful on the recognition of 

differences with schemes shown on the literature. The meristic data analysis revealed significant 

differences between Gulf of Mexico and north Atlantic populations (n= 147 individuals). The 

smallest size range recorded in specimens from the Gulf of Mexico (1.4 to 7.9 cm) in relation 

with ranges reported on the literature and with one found on the north Atlantic and was related 

with the oligotrophic condition in the deep Gulf of Mexico. The longitude – biomass allometric 

growth relationship obtained from specimens collected in the Gulf of Mexico was expressed by a 

power curve that fits the data with an equation W = 0.0232L2.8674 (r2

 =0.98). 

A total of 133 amphipods of the species E. gryllus were collected on the abyssal plain of the 

Gulf of Mexico using baited traps at three sites. The specimens were identified based on 

morphological criteria; schemes of diagnostic features were useful on the recognition of 

differences with schemes shown on the literature. The meristic data analysis revealed significant 

differences between Gulf of Mexico and north Atlantic populations. The normal size-frequency 

distribution and the late sexual maturity of the individuals suggest that strong predation is not 

acting over the smallest instars. The shorter size range (1.4 to 7.9 cm) in relation with ranges 

reported on the literature and with one found on the north Atlantic was related with deficient 

food conditions on the deep Gulf of Mexico. The longitude – biomass allometric growth 

relationship obtained from 94 individuals was expressed by a power curve that fits the data with 

an equation W = 0.0232L2.8674 (r2

 =0.98). Microscope photography showed the presence of 

chemosensorial microstructures which are useful to the location of food. The C and N stable 
344 

isotope analysis shown that the amphipod is positioned on a high trophic level with a feeding 

based on detritus and degraded exported materials from the water column. The sequences of the 

16S RNA mitochondrial gene obtained from two locations of the Gulf of Mexico E. gryllus were 

similar to sequences obtained of amphipods from the Atlantic and Pacific, nevertheless the 

difference percentage (3.6 to 9.5 %) between them revealed that the Gulf of Mexico has a an 

independent or isolated population. 

Amphipods of the species E. gryllus were collected in three locations in the abyssal plain of 

the Gulf of Mexico using baited traps with the objective to test isolation of the scavenger 

amphipods in this peripheral sea. The sequences of the mitochondrial gene 16S RNA from E. 

gryllus obtained from two locations of the Gulf of Mexico showed small differences with 

sequences of other Eurythenes amphipods from the Atlantic ocean (3.6 to 3.9 %) and from the 

Pacific ocean (4.0 to 4.1 %). Larger differences (4.2 to 9.5 %) were recorded with specimens 

from shallower sites (<500m), e.g. in seamounts from the Pacific Ocean. The largest difference 

(18 %) was recorded in the region with Eurythenes sp. from the Tongue of the Ocean, sequences 

that suggest the existence of a different species. The differences with the external genus 

Abyssorchomene were of 25 % suggest that the abyssal scavenger amphipods of the E. gryllus

species of the Gulf of Mexico still maintain a continuous genetic flow with the north Atlantic 

amphipod populations. Their predominant swimming behavior allows the populations to move 

horizontally along the abyssal water masses in the region. 

8.8.8 Effects of Hydrocarbon Seep Sites 

Cold seeps in the Gulf of Mexico provide an additional source of carbon to benthic fauna 

communities that are typified by high biomass and low diversity (Brooks et al. 1987, Agard et al 

1993). The larger megafauna that inhabit cold hydrocarbon seeps (tube worms, clams and 

mussels) utilize endosymbiotic bacteria to mediate methane and petroleum (Ahraon 2000, 

Brooks et al., 1987, Sassen et al., 1993). This symbiotic relationship thus only occurs near 

seeping fluids and limits the spatial extent of the community (Aharon 2000, Hecker 1985). 

Bioenhancement however may not be limited to the megafauna and bacteria. The benthic 

infauna may also be affected by the additional food found at cold seeps. 

Seep sites previously studied in 1993, 1995 and 2001 during the Shelf and Slope 

Experimental Taphonomy Initiative Experiment (SSETIE) were used as reference seep sites to 

compare macrofauna community structure and sediment community function with DGoMB sites 

(Figure 8-198). Sediment cores taken from Gulf of Mexico cold seeps were higher in abundance 

of macrofauna and exhibited lower diversity (Figure 8-199) (Nunnally, 2003). The mean 

abundance of macrofauna found at the cold seeps: Garden Banks 425 and Green Canyon 234, 

was 28,774 individuals per square meter (Table 8-48). The mean abundance of DGoMB sites 

within similar depth ranges (200-1,000 meters) was significantly less, at 8,883 individuals m-2

(Table 8-48). The diversity of taxonomic groups at these seeps was also lower than similar 

DGoMB stations, a mean of 10 groups at seeps compared to 21 groups. The mean ShannonWeiner index of diversity (H’) for cold seeps was 1.36, versus 1.79 for DGoMB stations, a 

significant difference. The only community attribute measured in which cold seeps and DGoMB 

stations did not differ significantly was Eveness, where cold seeps had a value of 0.64 and 

DGoMB stations had an average of 0.58 (Table 8-48). 

An interesting comparison is with the other location at the same depth that also had 

extremely high standing stocks: MT1 at the head of the Mississippi Trough (see Section 8.5.4 on 
345 

Figure 8-198. Northern GoM continental slope displaying DGoMB study sites with green stars and the 

two seep sites with orange triangles. 

Figure 8-199. Macrofauna means for DGoMB and SSETIE sites in the Northen Gulf of Mexico are 

plotted against depth; the two seeps GB 425 and GC 234 are denoted by arrows with 

their mean macrofauna abundance in number m-2. 
346 

Table 8-48 

 

Mean Values of Macrofauna Abundance, Higher Taxa, H’(s) (Shannon-Weiner), and Evenness for 

DGoMB Study Stations (Non-Seep) and SETTIE Stations (Seep) 

 TYPE Mean Std. Deviation N 

Major Taxa non-seep 21 4.2 92 

seep 10 0.5 4 

Abundance 

(N/m2

) non-seep 8883 5616 92 

seep 28774 9738 4 

H’(s) non-seep 1.80 .3054 92 

seep 1.35 .0354 4 

Eveness non-seep .584 .0936 92 

seep .635 .0071 4 

macrofauna). Average density there was approximately 21,000 ind. m-2 and diversity was low, 

but the fauna was dominated by a single species of amphipod. In that location it was inferred 

that the high standing stock was a function of the input of particulate matter that was swept off 

the shelf into the canyon, rather than seep-derived hydrocarbons emanating up through the 

sediments. Both types of habitats illustrate the effects that enhanced organic supplies can have 

on community structure: enhance standing stocks but reduce alpha diversity. 

Sampling site termed “Bush Hill” or BH on the map of stations was added during the field 

program to specifically test the hypothesis that organic input by cold seeps affects the fauna at 

some distance from the actual seep. This sampling was thus positioned two kilometers from the 

large cold seep known as Bush Hill, one of the original seeps discovered in the GoM. The 

macrofauna community at BH showed no differences in densities or diversity from the other sites 

tested. The community structure values were 4006 individuals m-2, 19 higher taxonomic groups, 

an H’(s) of 1.89 and an Evenness of 0.64. Although these values do not differ significantly from 

the other DGoMB stations studied. The low abundance and high diversity of taxonomic groups 

indicated that at two kilometers distance from an active seep, the benthos exhibited no 

recognizable bioenhancement within the macrofauna. 

Sediment community oxygen consumption was measured using incubation chambers at the 

two SETTIE locations above. The paired chambers are identical to those utilized with a remote 

lander (as described in Section 8.4), but they were set in place and recovered by the DSRV 

Johnson Sea Link, rather than being deployed by the lander. The rates have been plotted along 

the regression line which describes SCOC as a function of depth in the GoM, as presented in 

Section 8.4. In general, the SCOC rates from all the sites best fit a log-log or log-normal 

relationship with depth (Figure 8-200). Note, however, that the two seep site rates are well 

above the regression line as plotted, indicating that rates were enhanced above what one would 

expect at that depth. The mean of the two seep sites (ca. 110 mg C m-2 d-1 is about three times 

that encountered on the upper slope (see Section 8.4), but is only half the mean rate measured on 

the adjacent continental shelf. This suggests, by subtraction of the non-seep rates from the seep 

rates, that on the order of 70 to 80 mg C m-2 d-1 was being utilized by the community that is 

supplied by the seep alone, in addition to pelagic sources. 
347 

Figure 8-200. DGoMB and SSETIE sediment community oxygen consumption (SCOC) rates (mgcarbon m-2 d-1) are plotted versus depth. Seep sites GB 425 and GC 234 are denoted by 

the blue diamonds. 

Evidence of enhancement of food supplies for the benthos by cold hydrocarbon seeps in the 

Gulf of Mexico appears to be localized on a scale of 5 meters or less from the seepage of gascarrying fluids. This conforms to the deep-sea oasis theory relating chemosynthetic communities 

to oases in the desert (Carney, 1994). Each are dependent on a ‘below-ground’ supply that is 

lacking from the background environment. This supply limits bioenhancement to within the 

periphery of the seeping fluids. 

Cold seeps do concentrate a significantly large standing stock biomass reservoir in the midst 

of a low-biomass habitat. Diversity at such seep sites is diminished, due we presume to the 

competition for a type of resource that is not widely distributed in space. This competition for 

newly-introduced and peculiar form of food in a food-limited environment assures that little of 

the labile carbon ever leaves the vicinity of a cold seep. This reasoning however does not 

eliminate the possibility that carbon and energy are exported from the site as living biomass, 

either in the fishes or the vagile megafauna that feed within the seep-encompassed sediments and 

on the seep bacterial symbiont-supported sessile megafauna. 

Gulf of Mexico SCOC

R2

 = 0.88

0

50

100

150

200

250

0 500 1000 1500 2000 2500 3000 3500 4000

Depth (meters)

SCOC (mg-C/m2/d)
349 

9. DISCUSSION AND SYNTHESIS 

9.1 Animal Distributions and Community Structure 

9.1.1 Mapping Distributions 

For the first time in the study of life in the deep ocean, animal distributions can be mapped 

by species, groups of species or population abundances, over a broad geographic area. The 

species-level maps include most major taxonomic groups, from the harpacticoid copepods up 

through the fishes. In previous studies, single transects across depth gradients were used to 

investigate rates of change with depth (Carney 2005; Rex et al. 2005; Kendall and Haedrich 

2006). Transects were then compared at different latitudes and longitudes to infer geographic 

differences. In the present study, a set of transects extending from the Mexican border to Florida 

now allows each size group, or some quantitative characteristic of the group (diversity, biomass, 

etc.), to be mapped. In those groups where taxonomic identifications were taken to the species 

level, individual species can be portrayed, as illustrated in Section 8, Results. 

9.1.2 Standing Stocks and Biomass Implications 

Densities of large taxonomic groups, such as the macrofauna, meiofauna, etc., can be 

contoured, as can recurrent assemblages that share species in common, within the large 

taxonomic groupings. The relationships between the groupings, whether based on quantitative or 

qualitative features, and large-scale taxonomic features, can thus be visualized. The large 

Mississippi Trough, a canyon-like feature in the central GoM, exhibits greater densities in all 

size groups, principally on the upper slope. While densities of organisms decline with depth in a 

systematic way, as seen in the contours or ‘bubble’ maps for each large size group, the MT sites 

in the trough on the upper continental slope were exceptionally high, suggesting that more 

organic matter is entrapped in the trough than elsewhere. Densities were also higher in the 

DeSoto Canyon, but not in the deep, western Alaminos Canyon, where remarkably low densities 

were sampled. 

The decline in total biomass is a reflection of two parameters: individual animal size (Table 

9-1) and total abundance (Table 9-2), both of which decline with depth. This is illustrated by 

measurements of the mean size of the macrofauna taken from the batch sediment community 

respiration deck-board incubations at the process sites (Figure 9-1). Previous studies have 

observed a marked decline in animal density and biomass as a function of depth on all 

continental margins (Rowe 1983), but the significance of a decline in individual size is, 

according to these data, an important component of this. 
350 

Table 9-1 

 

Determination of the Mean Size of Macrofaunal Individuals (from Selected Samples) 

Station Depth(km) Repl N Wet wt. (mg) OC (mg) Size (mg/ind) Size (mgC/ind) 

C7 1.07 3 189 85.71 3.01 0.453 0.0159 

MT1 0.48 3 334 148.40 6.71 0.444 0.0201 

MT3 0.99 3 266 109.91 4.48 0.413 0.0168 

MT6 2.75 3 52 1.129 0.050 0.022 0.00096 

S36 1.83 2 78 10.87 0.375 0.139 0.00482 

S42 0.77 3 201 85.55 3.749 0.425 0.0187 

Table 9-2 

 

Process Station Macrofauna Biomass, Based on Densities and Individual Weight Estimates, in mg 

C/Individual 

Station Depth Mean Size (mg-C/Indiv) Density Biomass (mg-C/m2) 

MT1 0.481 0.0201 21801 438.6 

MT3 0.987 0.0169 9219 155.5 

S42 0.767 0.0187 4917 91.7 

S36 1.828 0.0048 10859 52.3 

MT6 2.745 0.00096 1485 1.43 

y = 0.057e-1.4209x

R2 = 0.9623

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 0.5 1 1.5 2 2.5 3

Depth (km)

Mean Size (mg-C/N)

Figure 9-1. Mean macrofauna individual size (mg C per individual) as a function of 

depth. 
351 

9.1.3 Diversity and Species Richness Generalizations 

The comprehensive taxonomic analyses in which individuals were identified to species, 

ranging from the meiofaunal harpacticoid copepods up to the fishes, allows comparisons to be 

made of the species richness and diversity between size groups (e.g., fishes, megafauna, 

macrofauna and meiofauna), between prevailing taxa within the different size groups (e.g., 

amphipod macrofauna vs. the polychaete macrofauna, etc.), and in contrasting geographic 

regions with presumably different conditions (canyon vs. non-canyon, etc.). 

In most of the cross-slope representations of species diversity using E(s) as a measure, the 

maximum was encountered on the mid to upper continental slope (1.2 to 1.6 km depth). The 

form and statistical spread of the data varied by group, however. The polychaete worms, the 

most speciose of all the macrofauna, declined rather evenly with depth, without an abrupt midslope maximum, whereas the other major groups displayed pronounced parabolic shapes. 

The lowest diversity values for the macrofauna were in the head of the Mississippi Trough 

(MT1). This low was the result of the dominance by a single amphipod Ampelisca sp. nov. 

(Soliman and Wicksten, submitted), which reached population densities of tens of thousands per 

square meter. This high density and low diversity is inferred to result from the extraordinary 

accumulation of silt and clay from the continental shelf in the pocket-like incision of the trough 

into the shelf in proximity to the Mississippi delta and the principal outflow of the river 

(Southwest Pass) onto the shelf. The trawls from the canyon head (MT1) contained fragments of 

large, aquatic vegetation. The substrate was so soft that all the weight had to be taken off the box 

corer to prevent “over penetration” and loss of material out the top of the corer. Comparisons of 

E(s) in the canyon with adjacent cross-slope transects illustrate that species diversity was 

generally lower in, as opposed to outside (the C line), the upper canyon (MT1through MT3), but 

species richness for the entire down-canyon transect (the MT sites) was higher than that outside 

the canyon. This, we suggest, is a function of greater structural complexity within the canyon. 

The cause of the “intermediate depth species maximum” (IDSM) remains open to 

speculation. The increase in diversity with depth on the upper slope is presumed to be a 

relaxation of competitive exclusion up to the maximum at 1.2 to 1.6 km. But it is cause of the 

decline seaward of the upper slope maximum that is particularly equivocal. Recruitment into the 

GoM derives from the southern Yucatan Strait, with a sill depth between 1.5 and 2 km. Perhaps 

this restricts recruitment into the deeper aspects of the basin. The abyssal plain has been formed 

over geologic time by turbidity flows emanating from the Mississippi Cone or fan of sediments. 

A frequent submersion of the fauna under flows of fine organic-poor clay may retard 

recruitment. Evidence is mounting that the deep GoM is characterized by strong bottom 

boundary currents; this too, if true, could select for a community of low diversity. Note that 

species composition at GKF (Green Knoll Furrows) is different from that in the standard zones 

(Figure 9-2), reminiscent of the studies of communities in the Western Boundary Undercurrent 

of the NW Atlantic (ref. HEBBLE study). 
352 

AC1

B1

B2 B3

BH

C1

C12 C14

C4

C7

GKF

Hipro

MT1

MT2

MT3 MT4

MT5

MT6

NB2

NB3

NB4

NB5

RW1

RW2

RW3

RW4

RW5

RW6

S1

S2

S3

S36 S35

S37

S38

S39

S4

S40

S41

S42

S43

S44

S5

W1

W2 W3

W5 W4

W6

WC12

WC5

2D Stress: 0.17

Figure 9-2. Non-metric MDS plots for 4th root species abundance data of all macrofauna, 

based on Bray-Curtis similarity. Bubble size is directly proportional to depth and 

colors represent different faunal zones. Note that “Green Knoll Furrows” (GKF) 

species composition is distinctly different. 

9.1.4 Bathymetric Zonation—Variations and Putative Causes 

The distribution of recurrent groups of species of macrofauna (Figure 9-2), megafauna, and 

fishes, leads to the conclusion that the entire fauna, by and large, is zoned with depth. In general 

the depth zones for all the macrofauna, megafauna and fishes, grouped together, appeared to be 

1) upper slope, 2) mid-slope, 3) lower slope and 4) abyssal plain assemblages. Figure 9-2 

illustrates how each depth zone of total macrofauna conforms to more or less similar depths, with 

some exceptions. “Hipro” for example is similar to the shallow sites, even though it is in rather 

deep water. We suggest that this submersion is related to high primary productivity in the 

surface water, hence the “Hipro” name. Sub-groupings were also related to physiographic 

features, such as the Mississippi canyon, east vs. west, etc. The macrofauna, the most speciose 

of the large size categories, was divided on the mid-slope into an east and a west sub-group 

(Figure 9-2 green and yellow bubbles), each having its own characteristic biomass and diversity 

(Figure 9-2). GKF was located at the base of Green Knoll in the area of giant “Aggie bus-sized” 

furrows (W. Bryant, personal observation), and the species composition was not similar to any of 

the other groups. Each zone has a characteristic biomass and diversity, as well as species 

composition. The correlation of zonal characters with depth suggests that the declining food 

supply with depth is a possible cause of the zonation. In particular, both organism density and 

biomass decline along the depth gradient and each zone occupies a distinct region along the 

gradient (Figure 9-3). Note that the two mid-slope east and west zones are separated on the basis 

of biomass and density, indicating that less organic matter is supplied to the west area than to the 

east. This supports the underlying hypothesis that food resources control zonation. 
353 

10

20

30

40

50

60

70

10

15

20

25

30

35

40

E(s100)

(c)

E(s50)

(d)

Faunal Zones

1 2W 2E 3 4 1 2W 2E 3 4

2.5

3.0

3.5

4.0

4.5

1.0

1.5

2.0

2.5

3.0

1 2W 2E 3 4 1 2W 2E 3 4

Log Density (individual/m2)

Log Biomass (mg C/m2)

(a) (b)

Figure 9-3. Mean density, biomass, E(S100), and E(S50) in each macrofaunal zone. Note white 

cross indicates the mean values, gray box indicates 95% confidence level, and 

vertical line indicates the range between maximum and minimum value. 

Some individual taxa appeared to occur in more zones than others. The bivalve mollusks for 

example occupied nine depth-dominated zones, whereas the harpacticoid copepods were 

encountered in what appeared to be as many as 17 separate regional clumps, with diameters of 

50 to 150 km, cutting across depth contours (Baguley et al. In press). Such differences may be 

due to larval dispersal in the bivalves versus brooding in the copepods. 

A plot of the number of zones as a function of relative animal size (Figure 9-4) indicates that 

the smaller organisms occurred in more groups, and these appeared as clumps that, on the map, 

were less aligned with depth than the larger groups. We infer that this is a function of motility 

and larval dispersal: larger organisms that are motile tend to be zoned, whereas the smaller 

groups are restricted to small geographic areas and are thus isolated in particular regions. 

Among groups that are about the same general size, the brooders occurred in more groups (e.g., 

the amphipods) than those that do not brood their young (mollusks, polychaetes and isopods). 
354 

0

2

4

6

8

10

12

14

16

18

Harpaticoid

Polychaete

Amphipod

Cumacean

Megafauna

Bivalve

Isopod

Macrofauna

Aplacophran

fish

Number of faunal zones

>3 stations

3 stations

2 stations

1 station

Figure 9-4. Number of ‘zones’ (recurrent groups of species) in each group. Colors 

indicate number of sites in each zone. For example, the Harpacticoids 

had six zones with only one site in a zone (blue), three zones with more 

than three sites in three zones, five zones with three sites and two zones 

with two sites. The blue indicate isolated sites that are not grouped with 

any other site, based on species composition. 

9.1.5 Implications of and Conclusions Based on the Hypothesis Test Results 

An underlying premise of all the hypotheses tested was that the input of food resources was 

the paramount controlling variable of the distribution of deep-ocean organisms. For example, 

the patterns of macrofaunal abundance can be used to infer the locations where the input of 

organic matter is accentuated within the study area. In general, the western GoM was 

characterized by lower densities than the eastern GoM, leading to the assumption that the organic 

matter available to the deep benthos is less in the west than in the eastern GoM. This was 

unexpected since the Mississippi River generally flows to the west and the eastern GoM is 

characterized by the warm relatively unproductive waters of the loop current. The high 

abundance values in the upper reaches of the Mississippi Trough reflect larger inputs of organic 

matter that can be assumed to be related to the Mississippi River. This could either be direct 

input from river-borne material or enhanced primary production that accumulates in the trough 

where it intersects the continental shelf just west of the delta. The differences in the densities on 

the two sides of the DeSoto Canyon suggest that in general Mississippi cone sediments are 

enriched in carbon compared to those off Florida. The cone sediments are primarily terrigenous 

and it might be assumed richer in organics than the predominantly carbonate sediments off west 

Florida. The high numbers at S36 near the canyon axis suggest that perhaps this is a location of 

organic matter accumulation. It is also worth noting that the surface water over and just inshore 

of S36 is thought to be characterized by unusually high productivity for the GoM, probably due 

to the incursion of high nitrate water onto the upper slope and continental rise (See Sections 8.1, 

8.2 and 8.3). 
355 

The hypothesis tests discussed below reflect the degree to which the geographic or seafloor 

physiographic features control animal distributions because they affect the delivery of organics 

to the sea floor. 

Standing Stocks Decline Sharply but Predictably with Increasing Depth 

[HO1: Standing stocks are independent of depth] 

The testing of null hypotheses was utilized in this study because it was recognized that a 

complete geographic coverage would be impossible (see Section 4, Table 4-1). Each null 

hypothesis, if properly tested, would allow the elimination or inclusion of particular 

environmental features in terms of their significance to animal distributions in the GoM, even 

though total geographic coverage would never be attainable. 

Depth has long been recognized as a dominating independent variable that correlates strongly 

with deep-sea distributions, although the specific underlying mechanisms for the correlations are 

obscure and remain ill-defined. Because total stocks and rates of processes are known from 

numerous previous studies to decline sharply with depth, it has been presumed that depth is 

inversely related to food supply. Thus, depth was the primary variable to be considered in this 

study. 

The principal statistical tool utilized for investigating the relationship of the standing stocks 

with depth was regression analysis. The statistical significance of a regression is tested by 

calculating the probability that the slope of the regression line is different from zero. The 

differences in the height of one regression line from another above the abscissa can be tested, as 

can the differences in the slopes of the lines. These tests have utilized extensively on the 

sediment characteristics and on each group of organism in Section 8. 

Without doubt, many of the features did, as expected, correlate with depth. Animal densities 

and depth were inversely correlated, as has been demonstrated by countless studies, including 

earlier studies in the GoM (Rowe 1983). Among the different size groups, however, the smaller 

the organisms, the less profound was the decline with depth. The result was that the relative 

importance of the bacteria and the meiofauna increased with depth because their declines were 

less pronounced. The declines with depth, in all size categories, are thought to reflect a general 

overall decline in the input of organic matter – the lower the input of organic carbon, the lower 

the total biomass. This apparently affects the larger organisms to a greater degree than the 

smaller size groups. Pequegnat et al. (1990) had previously recognized the increase in the 

relative importance of the meiofauna. 

An unexpected surprise was the relative increase in the bacterial densities on the abyssal 

plain relative to the slope. This is counter to all the conclusions about the metazoans. That this 

was a sampling or analytical artifact needs to be explored in future studies. There is no 

reasonable explanation, at this time, for an increase in bacterial densities on the abyssal plain 

when all other indicators show a continued decline in organic matter with increased depth and 

distance from land. In conclusion, depth does affect organism densities, with the possible 

exception of the bacteria.

Diversity too correlates with depth, but this varies with the group being studied. In general, 

an intermediate depth maximum is observed, but this was absent or reduced in some groups. In 

this pattern, diversity first increases with depth to a maximum at intermediate depths. Then it 

declines gradually to a low on the abyssal plain, regardless of the diversity index used. The 

increase is due, it is thought, to a relaxation in competitive exclusion, followed by a general 

decrease in organic matter with depth. There is no real evidence in support of this however; it is 

only conjecture. Nonetheless, it is a repeatable correlation with a characteristic shape. It is 
356 

repeatable with each group in which the taxonomy has been taken to lowest possible level 

(species). 

There are several important exceptions to the general abundance-depth pattern. Highest 

densities in most groups were NOT found in the shallowest samples (200 to 300 m), but rather in 

the head of the Mississippi Trough (500 to 1,000 m). Likewise, densities on the abyssal plain 

were highest on the large sediment cone that extends south from the continental slope off the 

Mississippi River to the area where the Mississippi Trough appears to debouch onto the northern 

extremity of the Sigsbee Abyssal Plain (SAP), that is at Site S5. The species composition of the 

infauna at that site was more akin to shallower sites than to that at the slightly deeper SAP sites 

(S1 through S4). It can be inferred that both exceptions to the more general abundance-depth 

pattern are the result of the entrapment and channeling of organic detritus by the Mississippi 

Trough. 

The Biota Associated with the Sea Floor in the Eastern GOM Has Some Demonstrable 

Differences from that in the Western GOM, but these Difference Are Limited. 

[HO2: the eastern GoM fauna is not different from the western GoM fauna] 

The transect with the lowest standing stocks was that in the far west (RW), whereas all the 

sites with the highest densities of all groups considered were located in the eastern half of the 

GoM, from the C line eastward. However, the zones of recurrent groups of macrofauna lined the 

basin from east to west, with the exception of the upper slope zone, which was divided down the 

middle (referred to as Zone 2E and 2W). This suggests that quantitatively the east and the west 

may differ, but qualitatively for the most part they are rather similar. The quantitative difference 

may be related to surface water primary production [see HO4 discussion below]. 

Canyons* Increase Standing Stocks, Lower Diversity and Increase Species Richness 

[HO3: Submarine canyons have no discernable effect on standing stocks, species 

composition, diversity and zonation of the deep GoM benthos] 

The second most convincing hypothesis to be accepted (as posed in Section 4, Table 4-1) was 

the significance of the Mississippi Trough on the biota. This canyon head between 500 and 1,000 

m contained a fauna which grouped together, being dominated by a single species of amphipod 

crustacean. This dominance was most pronounced at the shallowest site. This reflects, again, the 

input from shallow water, of detrital material building up at a rapid rate. The material is thought 

to be a combination of surface productivity, shelf sediment export and fluvial plant detritus, 

based on the seasonal surveys of surface-water pigments, PAHs in the sediments, organic matter, 

and large plant material in the trawl samples. Stable isotopes suggest that pelagic Sargassum

detritus may be important too. Animal densities were extremely high, but not of polychaete 

indicator species (e.g., capitellids) and nematodes expected adjacent to or impacted by 

eutrophication or oil contamination, but of amphipod crustaceans, supposedly quite sensitive to 

stressful conditions. The megafauna and the fish fauna were also encountered in high abundance 

 *

 Two other canyon-like features were sampled. The Alaminos Canyon, just to the east of the deep end of the 

RW transect, was characterized by some of the lowest animal densities sampled on the deep slope. It is assumed that 

it is no longer active but rather a remnant of the Holocene Transgression. The DeSoto Canyon, that area between the 

eastern Mississippi Cone and the Florida Escarpment, was characterized by high animal densities. However, it is not 

a typical submarine canyon formed as an incision in the continental slope but rather a geophysical feature. The high 

animal densities there are assumed to be a combination of high surface primary production, as reflected in the ocean 

color analyses (see Section 8.3), and detrital material trickling down the eastern flank of the slope off the Mississippi 

River mouth, eventually being focused and thus building up in the tight constraints of the canyon-like valley, as 

sampled at S35 to S39. 
357 

in the canyon head, which may reflect the widely-recognized preference of small crustaceans as 

prey items. 

Standing Stocks are Directly Proportional to Surface Primary Production 

[HO4: Geographic variation in rates of surface water primary production have no discernable 

effects on the standing stocks of the deep-living biota of the Gulf of Mexico] 

Surface primary productivity was indirectly assessed by mapping surface water pigment 

concentrations seasonally over the entire DGoMB area (see Section 8.3). The effects of 

variations in surface-water primary production on the deep benthos is difficult to distinguish 

from other variables, especially depth. However, a comparison of animal densities along the 

upper slope suggests that proximity to the high surface pigment regions near and east of the 

Mississippi River clearly implicates surface primary productivity as at least a secondary control 

on bottom faunal densities. Noting that high densities were encountered in both the Mississippi 

and the DeSoto canyons, the program added a “HiPro” site at equivalent depths between the two, 

but out of canyon-like influences. The high densities at the “HiPro” site add credence to the 

importance of surface sources of organic matter as having an effect on animal densities. As noted 

above (Figure 9-2), the “HiPro” site is in relatively deep water (ca. 2 km) but has a species 

composition associated with the upper slope, suggesting that some process allows shallow 

species to exist in slightly deeper water than normal; it is tempting to suggest that this 

submersion is due to a higher than normal delivery of POC. This supports the strong link 

between surface water primary production and deep ocean biomass. 

Earlier studies comparing the GoM with other regions suggested that the GoM was 

characterized by low benthic biomass because in general the productivity of surface water is low 

compared to the NWAtlantic or the coast of Peru, both of which are known to be characterized 

by primary productivity that far exceeds that in the GoM (Rowe 1983). The present study 

attempts to distinguish the effects within the GoM basin, not between geographically distant 

basins, a substantially more difficult task. There is little justification however for rejecting the 

idea that the higher biomasses that characterize the eastern GoM at all depths on the upper slope 

are not, at least in part, a function of the higher surface productivities there compared to the 

western margin of the basin. 

How much phytoplankton production of these surface waters reaches the seabed? Figure 9-5 

summarizes a 1980s paradigm that away from land only a slow vertical “rain” of primary 

production escapes the surface waters, although there are occasional foodfalls of animal 

carcasses that may also be important. While this may be what happens in the western and central 

deepwater GoM, the SeaWiFS ocean color climatology data indicate the paradigm should be 

revised to include the role of eddies and eddy-like features (see discussion of WSEs in 8.3) that 

entrain green water and transport it off the continental margin into deepwater in the easterm 

GoM. 

In the western and central deepwater GoM, the standing stocks and biological productivity of 

the plant and animal communities living in the upper part of the water column are also, in 

general, those that might be expected in a nutrient-limited ecosystem. In the late 1960s, as part of 

a review of plankton productivity of the world ocean, Soviet scientists characterized the 

deepwater GoM as very low in standing plankton biomass, with mean productivity of just 100-

150 mg C m-2 d-1 (Koblenz-Mishke et al. 1970). A few years later, extensive ship surveys of 

phytoplankton chlorophyll and primary production that span the period 1964-1971 were 

summarized by El-Sayed (1972) in atlas format, as averages within 2o squares of latitude and 

longitude. These atlas maps showed that surface CHL generally ranges 0.06 - 0.32 mg m-3 in 
358 

deepwater central and western GoM. There is usually a subsurface “deep chlorophyll maximum” 

(DCM) within which concentrations are 2-3 fold higher, and the atlas reported that CHL in 

deepwater could reach 21 mg m-2 when integrated from the surface to the base of the photic zone. 

Most values for CHL, though, ranged 5-17 mg m-2 where water depth was > 2 km (El-Sayed 

1972). 

During an algal bloom, the C:CHL ratio for living phytoplankton cells ranges from 23 to 79 

(Antia et al., 1963). From mesocosm experiments, Antia et al. (1963) reported that vigorously 

growing phytoplankton with measurable nitrate in the water had C:CHL ratios of about 25, while 

senescent phytoplankton in nitrate-depleted water had C:CHL ratios of 60 or more. Using a 

C:CHL ratio of 25, a CHL standing stock of 0.1 mg m-3 is equivalent to a POC concentration of 

about 2.5 mg m-3. From 14C uptake experiments conducted in the GoM, El-Sayed (1972) has 

shown that primary production at “blue water” locations increases the phytoplankton POC pool 

by on average 0.25 mg C m-3 h-1, which is equivalent to an increase of about 10% per hour. Such 

low values of primary production are typical for surface waters at the majority of the oceanic 

stations in the El-Sayed (1972) atlas, equivalent to only about 10 mg C m-2 h-1 when integrated 

from the surface to the base of the photic zone. If there are on average 12 hours of sunlight per 

day, this rate is equivalent to 120 mg C m-2 d-1 and so is in good agreement with the 

characterization by Koblenz-Mishke et al. (1970). Allowing for primary production to proceed 

365 days a year in the GoM because of its subtropical climate, this rate of primary productivity is 

about 44 g C m-2 y

--1. As a consequence, the deepwater GoM is usually placed at the low end of 

the estimated range of 50-160 g C m-2

 y

--1 that is generally accepted for the annual gross primary 

production in open-ocean ecosystems (Smith and Hollibaugh 1993). 

Figure 9-5. Schematic illustration of pathways by which POC is transferred from surface waters to 

the benthos. From Lalli and Parsons (1997) after Nybakken (1988). 
359 

If on average 10% of the primary production sinks out of the photic zone, and if 3-10% of 

this flux, in turn, reaches the seabed, then at a typical deepwater location just 100-360 mg C m-2

y

-1 might reach the benthos. However, productivity in “hot spots” of locally higher nutrient 

concentrations over the continental slope can be more than an order of magnitude higher than the 

typical 100-150 mg C m-2 d-1 (Biggs and Ressler 2001). For example, Gonzalez-Rodas (1999) 

documented 14C uptake of > 2 g C m-2 d-1 in the northern margins of two deepwater eddies 

interacting with the continental slope of the central GoM. When and where such eddy 

interactions with the slope are common, as they appear to be in the eastern GoM during 

summertime, we hypothesize that POC flux to the seabed is likely to be several times greater 

than the deepwater average of 100-360 mg C m-2 y

-1. Based on the difference between grand 

means for remotely-sensed CHL between the 22 DGoMB stations north of 26oN and west of 

91o

W in the western Gulf (0.19 mg CHL m-3) and the 22 DGoMB stations north of 26oN and east 

of 91oW in the eastern Gulf (0.54 g CHL m-3), we hypothesize that most of the stations along 

central and eastern DGoMB transects C, MT, and S receive at least 0.54/0.19 = 2.8 times greater 

annual POC input than do stations located west of 91oW. 

These estimated rates of input to the seafloor (0.3 to1.0 mg C m-2 d-1) are slightly less than 

those measured in situ using a remote benthic lander (see Section 8), in which rates ranged from 

ca. 2 to 7.5 mg C m-2 d-1. Rates on the middle and upper continental slope were substantially 

higher, by more than a factor of 10 to 20, than those estimated above. The difference may be 

attributable to input from the margin, which could be accentuated in the GoM by the fluvial input 

of sediments, nutrients and organic matter. 

Enhanced Standing Stocks and Accelerated Sediment Community Metabolism within 

Close Proximity to Methane Seep Sites 

[HO5: Methane seeps have no effect on the biota of the deep Gulf of Mexico] 

Methane seeps appear to have a profound effect of animal densities, possibly species 

composition and rates of sediment community oxygen consumption. However, there is little 

evidence so far that the effects go beyond the immediate proximity of a seep community. 

Effects of Deep, Steep Escarpments Remain Unknown 

[HO6: Steep escarpments have no effect on the biota] 

The general decline in abundance as a function of water depth was slightly altered at the base 

of steep escarpments, giving some credence to the positive effects of proximity to escarpments. 

On transects W and S the densities at approximately 3 km depth just below the Sigsbee and the 

Florida Escarpments, respectively, were slightly above the general regression line, suggesting 

that material moving down the escarpment enhances organic input at the escarpment base. This 

is very meager evidence, however, and would need other evidence before it is accepted. The 

cause, if the increases are real, may be due to other processes, such as seep activity associated 

with the slope-rise junction or other local seep effects that are encountered along the steep 

escarpment. 

Mesoscale Basins as Peculiar Habitats 

[H07: Mesoscale basins on the continental slope have no effect on the biota] 

Three of the numerous small basins dotting the continental slope were sampled in and 

adjacent to the basins. The basin, it was thought, could trap sediments that could enhance 

seafloor densities and rates of metabolic processes. There was no evidence that the basins 

differed in any way from non-basin environments. This hypothesis was thus accepted, based on 

the data to date, for the faunal groups, bacteria and sediment properties (Section 8). It should be 
360 

pointed out however that the basins visited, as far as can be determined, were not characterized 

by active methane seeps. 

Potential Effects Associated with Seasonal or Inter-annual Variations were Not Discernable 

[H08: Variations in the biota are independent of time over the periods in which the study has 

taken samples] 

The study always sampled in May or June over a three-year period. Thus, no seasonal effects 

would be discerned. It was suggested initially that Site C7, a site repeated from earlier MMSsupported studies, might be characterized by seasonal variations in animal density or 

composition. However, further scrutiny of the earlier data indicated that that location was close 

to a methane seep and the intermittent high densities of macrofauna observed were more likely a 

result of input of organics from the seep than to seasonality. It should be noted that the 

macrofaunal densities encountered during the initial survey (2000) were almost all significantly 

higher than those encountered at the same sites the following year (2001) (See Section 8.4.4). An 

explanation for this eludes us to date. 

9.1.6 Variations In and Between Size Categories 

Within most shallow benthic communities, with normal levels of organic matter available for 

food and no obvious physiological stressors, the biomass is dominated by the larger categories of 

organisms (mega- and macrofauna), with lesser total biomass in smaller sized organisms 

(meiofauna, microfauna and bacteria). This distribution is accentuated at high latitudes. Under 

stressful conditions such as low oxygen, the reverse occurs: the smaller organisms dominate the 

biomass. It has been suggested that in the deep ocean, the upside-down biomass distribution also 

occurs, with domination by bacteria and the meiofauna (Rowe et al. 1991). In most locations in 

the deep GoM this appears to be the case as well: most of the biomass is contained in the smaller 

organisms – bacteria, forams and meiofauna – rather than in the larger forms (Table 9-3). The 

pattern for the meiofauna and macrofauna was suggested by earlier work in the GoM (Pequegnat 

et al. 1990), and the present study, which has included bacteria and foraminiferans in addition to 

metazoan meiofauna, confirms it. The largest size category, the megafauna, is far below densities 

of similar groups in other ocean basins, based on photography and bottom trawling. The most 

palatable explanation is that the overall productivity of the surface water is low compared to 

other basins where similar studies have been undertaken. Additionally, the warmer temperatures 

in the surface water column suggest that remineralization of sinking organic matter would be 

higher, resulting in proportionately less reaching the sea floor. This has long been an explanation 

for higher biomass at high latitudes (Rowe 1983). Also, the sea floor temperature (4o C) is higher 

in the GoM than at similar depths in both the Atlantic and Pacific basins, which would result in 

slightly higher metabolic rates per unit biomass, on average, which may also contribute to the 

general dominance of small organisms. Working under similar depth/temperature conditions in 

the Caribbean, Richardson and Young (1987) have suggested that the bacteria must function to 

remobilize refractory or relatively unreactive organic detritus, thus requiring them to maintain a 

relatively high concentration to support grazing meiofauna, principally nematodes. The relative 

distribution of biomass among the size groups in the deep GoM supports this contention as well. 

The first step in understanding food web relationships is quantifying the standing stocks of 

the components. Each process site can be characterized by the biomass of organic carbon (per 

square meter, integrated down to a depth of 15 cm) of each size category (Table 9-3). This 

summary is constructed from information in Section 8, Results. This includes the detrital 
361 

Table 9-3 

 

Inventories of Organic Matter (mg C m-2-15 cm) in the Detrital and Living Fractions at the Process Sites 

Depth Site Sed. Org.C Bacteria 

Meio

fauna

Macro

fauna

Mega

fauna Fishes Total Living Living /[OC] Bacti/Living

500 MT1 760000 2350 208 816 30.6 35.9 3441 0.0045 0.68 

1000 MT3 702000 2550 194 34 59.5 35.4 2873 0.0041 0.89 

750 S42 375000 1300 197 47.3 7.9 24.7 1577 0.0042 0.82 

1850 S36 672000 1850 272 19.6 9.43 7.4 2158 0.0032 0.86 

2750 MT6 222000 1010 27 1.56 6.67 1.8 1047 0.0047 0.96 

3450 S4 321000 1480 10.5 2.1 0.75 0 1493 0.0046 0.99 

3600 S1 287000 1400 13.8 8.3 0.19 0.4 1423 0.005 0.98 

organic carbon for comparison. Of the total organic matter, the living fraction appears to be only 

.03 to .05%, with no predictable pattern over this range. The highest detrital carbon and the 

highest living carbon are at the shallow sites (Figure 9-6), whereas the opposite is true – the 

lowest of both detritus and biota are at MT6. It is somewhat surprising however that the lowest 

values are at MT6 at 2,750 m depth rather than out on the abyssal plain much further offshore at 

about 1 km greater depth. The large size groups (macrofauna, megafauna and fishes) declined 

Figure 9-6. Log total community biomass as a function of log detrital organic carbon in the sediments, 

both integrated to a depth of 15 cm. 

2.90

3.00

3.10

3.20

3.30

3.40

3.50

3.60

5.30 5.40 5.50 5.60 5.70 5.80 5.90 6.00

Log Organic Carbon (mg C/m^2-15 cm)

Log Total Biota (mg C/m^2-15 cm)
362 

by about two orders of magnitude over the depth range of the process sites, whereas the 

meiofauna declined by a factor of 10 to 15 and the bacteria declined by a factor of 1.6 to 2.0 

from the highest to the lowest biomass. This suggests that the smaller organisms are better 

adapted to surviving and maintaining their population densities at great depths than the larger 

organisms. Contemporary ecological theory would suggest that this could be related to declining 

organic energy available at the depths involved (Banaver 2002). If total detrital organics and 

total living carbon are a direct reflection of organic material delivery to a particular habitat, it is 

interesting that MT6, on the lower continental slope, is the lowest of the process sites in both 

criteria, but it is not located on the abyssal plain. 

9.1.7 Comparison with Other Continental Margins 

Biomass declines with depth, but its mean is lower and the slope is steeper than in most other 

basins, within the macrofauna and the megafauna; this is not true for the bacteria or the 

meiofauna – although they decline with depth in selective sets of data, the slope is less steep and 

well defined for the meiofauna than for larger forms. The regressions are not in general 

statistically significant for the bacteria in deeper water, principally because densities were greater 

on the abyssal plain than on the slope. 

This suggests that GoM productivity is lower and material exported vertically to the sea floor 

is less reactive, making the smaller forms (the bacteria–nematode link) necessarily more 

important in the food web. The depauperate “typical” soft sediment fauna, interspersed by a 

mosaic of seeps with their luxuriant biota, suggests that rapid burial of relatively slowly reacting 

material is sequestered or “saved” by rapid burial for deeply-buried reactions that lead to oil and 

gas deposits. This differs from typical continental margins where OM is not buried rapidly but 

consumed by the biota almost completely on the sediment surface. 

9.2 Principal Components Analysis 

Principal Components Analysis (PCA) was used to search for groups of environmental 

variables, independent of the biotic analysis, which would add insight into the causes of the 

faunal distributions. In the PCA, the first three principal components accounted for 61.5 percent 

of the total variance in the data set (Table 9-4). However, four PCs out of 15 had eigenvalues 

greater than one, which means the first four were significant. The sign of variable loads (negative 

or positive) indicates gradients in concentrations. Variables that load negatively will have highest 

concentrations for negative PC loads with decreasing concentrations moving in the positive 

direction, and vice versa. PC1 accounted for 33.5% of the total variance, had high positive 

loadings by clay, total PAH’s, tin, chromium, and high negative loadings by sand, strontium and 

calcium (Table 9-5; Figure 9-7). PC1 is interpreted as the sediment properties, with high silt, 

clay, organic (PAH) and metal (Cr and Sn) contaminants near the Mississippi River, and higher 

sand and natural background metals (Ca and Sr) with increasing distance from the Mississippi 

River. 

PC2 accounted for 16.7% of the total variance and highly positive loadings by chl-a and 

POC, weak positive loadings by OrgN and NH4, and weak negative loadings by NO3- and urea 

(Table 9-5, Figure 9-7A). PC2 is interpreted as particulate organic matter (POM) flux. PC3 

accounted for 11.3% of the total variance and had highly positive loadings by DOC, and highly 

negative loadings by urea (Figure 9-7B). PC4 accounted for 10.2 % of the total variance and had 

moderate positive loadings by silt, NH4, NO3, and PAH. However, PC3 and PC4 did not have 
363 

Table 9-4 

 

Eigenvalues of the Correlation Matrix for the Environmental PCA, 

Proportion of Variance Explained by Each Principal Component, and 

Cumulative Variance 

 Eigenvalue Difference Proportion Cumulative 

1 5.019 2.504 0.335 0.335 

2 2.515 0.818 0.168 0.502 

3 1.697 0.162 0.113 0.615 

4 1.535 0.606 0.102 0.718 

5 0.929 0.077 0.062 0.780 

6 0.851 0.217 0.057 0.837 

7 0.635 0.044 0.042 0.879 

8 0.591 0.060 0.039 0.918 

9 0.530 0.218 0.035 0.954 

10 0.312 0.111 0.021 0.974 

11 0.201 0.066 0.013 0.988 

12 0.135 0.105 0.009 0.997 

13 0.030 0.017 0.002 0.999 

14 0.014 0.008 0.001 1.000 

15 0.005 0.000 1.000 1.000 

Table 9-5 

 

Variable Loads for the Rotated (Varimax) Factor Pattern of the Environmental PCA 

Factor1 Factor2 Factor3 Factor4 Factor5 

Chla 0.11096 0.90356 -0.00047 0.08619 0.03993 

Sand -0.76437 0.21025 0.20489 -0.19623 -0.48221 

Silt 0.43118 -0.03573 0.05246 0.65701 0.25268 

Clay 0.65541 -0.28278 -0.27347 -0.09372 0.52084 

NH4 -0.26511 0.35890 0.18627 0.66324 0.12841 

POC -0.04478 0.81893 0.06908 0.05581 0.08796 

UREA 0.01605 -0.14464 -0.80871 -0.09793 0.09566 

NO3 -0.19939 -0.49356 -0.21657 0.53803 -0.07637 

DOC -0.09904 -0.03622 0.87375 -0.00081 -0.06273 

OrgN -0.00531 0.27374 -0.10288 0.11126 0.88406 

TPAHWP 0.55797 0.14311 0.14306 0.55440 -0.19339 

Ca -0.87212 -0.30214 0.09420 -0.23840 0.02223 

Sr -0.85733 -0.25022 0.14018 -0.20549 0.05741 

Cr 0.86572 -0.12582 -0.06502 -0.16531 0.09411 

Sn 0.83863 -0.00196 0.14965 -0.17825 -0.01304 
364 

A.

-1 0 1

PC1

-1

0

1

Chla

Sand

Silt

Clay

NH4

POC

UREA

NO3

DOC

OrgN

TPAHWP

CaSr

Cr Sn

B.

PC2

-1 0 1

PC3

-1

0

1

Chla

Sand

Silt

Clay

NH4

POC

UREA

NO3

DOC

OrgN

TPAHWP CaSr

Cr

Sn

Figure 9-7. Principal components analysis of environmental variables, A) variable 

loading scores for PC1 versus PC2, B) variable loading scores for PC2 

versus PC3. 
365 

obvious interpretations. PCs 1-4 were regressed against abundance to determine if they were 

significantly related to the biotic community. PC1 had a weak, but significant, positive 

relationship with meiofauna abundance, but accounted for only 22% of the variance in the biotic 

data set (Figure 9-7A, R2

 = 0.215). PC2 had a moderate, and significant, relationship with 

meiofauna abundance (Figure 9-7B, R2

 = 0.303). PC3 and PC4 did not have significant 

relationships with abundance. The ranges of values in deep water are remarkably narrow 

compared to shallow water environments, which probably accounts for the limited number of 

relationships that the PCA was able to identify with obvious interpretations. In this data set for 

example organic carbon, which normally is thought to control biochemical reactions, was above 

1% at only a couple of sites. Oxygen, temperature and salinity were essentially invariable below 

1 km depth (Figure 9-8). 

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15

Depth (km)

Temp(C)

DO(ml/l)

%OC

Figure 9-8. Composite profiles of temperature, dissolved oxygen and percent organic 

carbon in the sediments in the northern Gulf of Mexico. 

9.3 Biogeochemical Processes—Community Function 

9.3.1 Comparison of Methods 

Four approaches have been utilized to estimate benthic community heterotrophic metabolism 

(= organic matter remineralization): 

1) the food-web organic carbon model, 
366 

2) the organic carbon gradient/sediment accumulation rate model derived from 

radionuclide accumulation and age dating, 

3) the vertical pore water profiles of the oxidation states of metabolites (O2, Mn, 

NO3, Fe and S), and 

4) benthic chamber incubations conducted in situ with a benthic lander or in the 

laboratory with chambers recovered from box cores. 

These can be ranked in the following order of descending rates of total carbon turnover: 

food-web model > chamber incubations > pore water profiles of metabolites > organic carbon 

gradient model. Resolving the disparities between these approaches is difficult, but some 

important issues are readily evident. The food-web model is based on information from the 

literature for estimating respiration rates as a function of animal size and temperature, with only 

limited direct measurements in the laboratory during this study. That this would lead to 

overestimation of rates, rather than simple stochastic inaccuracies, is not known. The pore water 

profiles and the organic carbon profiles may underestimate rates because they do not incorporate 

highly reactive material being reworked right at the sediment – water interface. A similar 

distinction has been observed over large areas of the deep Atlantic by, who refer to the chamber 

incubation-based rate the ‘total flux’ (of oxygen) and the rate based on the oxygen gradient as the 

‘diffusive flux’, but they did not elaborate on the theoretical explanation of the differences. 

9.3.2 Comparison of Total Community Metabolism with Other Ocean Margins 

There is little reason based on the results of this study to suggest that the sea floor 

community metabolism differs appreciably from that measured at similar depths in other ocean 

basins. The abyssal plain sites had by far the lowest values in terms of carbon respiration rates 

(2.6 to 7.5 mg C m-2d-1), but these are not different, based on the data so far, from Pacific and 

Atlantic measurements on the middle continental rise at equivalent depths. The upper continental 

slope values also are somewhat less than what has been measured in the NW Atlantic continental 

margin organic carbon “depocenter” (Anderson et al. 1994). At similar depths in the Pacific such 

comparisons are difficult because at those depths the intense oxygen minimum impinges on the 

bottom and aerobic metabolism is inhibited (Devol and Christensen 1993). 

An abrupt decline of known values of SCOC from the continental shelf adjacent to the slope 

sites was also observed. The highest values on the shelf have been measured near the Mississippi 

Delta in the area that becomes hypoxic or anoxic during stratified periods. The maximum rates 

there are on the order of 500 mg C m-2 d-1, but that includes the high sulfate reduction that is 

known to occur near the delta but which is not incorporated into the oxygen uptake rates during 

low oxygen periods. In general, average shelf rates, according to the above authors, is on the 

order of 100 to 250 mg C m-2 d-1, plus or minus 50%, over an annual cycle. Thus, the rates on the 

upper slope are on the order of 1⁄2 to 1/10 of the average shelf rates, but decline with depth to the 

abyssal plain by a factor of 10. This decline is typical of most continental margins. 
367 

9.4 Community Function and Structure Relationships 

9.4.1 Carbon Budgets for Process Sites 

The rate at which the carbon stock turns over can be calculated by dividing the stock at each 

site by the SCOC, both from Section 8, Results (Table 9-6). The biota turns over on the order of 

months at the upper to middle continental slope sites but this goes up to nearly a year on the 

abyssal plain. 

Table 9-6 

 

Relationship Between Site, Depth, Total Living and Total Detrital Carbon, Sediment Community Oxygen 

Consumption (SCOC, mg C m-2-day) and Carbon Turnover Time in Days (Living Carbon) and Years 

(Detrital Carbon), Where ‘Time’ Equals the Stock Divided by the Rate 

Depth Site Sed.Org.C Total Living SCOC Biota (t=d) [OC] (t=y) 

500 MT1 7.60E+05 3441 35.7 96.4 58.3 

1000 MT3 7.02E+05 2873 28 102.6 68.7 

750 S42 3.75E+05 1577 56 28.2 18.3 

1850 S36 6.72E+05 2158 22.4 96.3 82.2 

2750 MT6 2.22E+05 1047 

3450 S4 3.21E+05 1493 4.8 311 183.2 

3600 S1 2.87E+05 1423 4 356 196.6 

On the other hand, the detrital carbon turns over much more slowly, but the general trend is 

the same as that for the biota. At the shallower sites, the turnover time is on the order of tens of 

years, whereas on the abyssal plain the turn over approaches 200 years. A comparison of similar 

calculations for the western North Atlantic suggests that the turnover time in the Gulf of Mexico 

at similar depths is somewhat longer for the living and the detrital carbon (Rowe et al. 1991). 

This possibly reflects the low concentrations of organic matter, and its diminished reactivity, in 

the Gulf compared to the N. Atlantic. It might have been suspected that the opposite would be 

the case because the temperature at depth in the GoM is higher than that in either the Atlantic or 

Pacific. The continental shelf off Louisiana has much faster carbon recycling time, which 

indicates that the delivery of organic matter to continental shelf benthos is much higher than the 

adjacent deep sea. 

9.4.2 Metabolism as a Function of Biomass 

SCOC declines as a function of total biomass, when all the infaunal organism biomass is 

summed up (Figure 9-9). It has been suggested that bacterial biomass is a function of POC input, 

rather than depth (Deming and Baross 1993), and SCOC is regarded as a the best surrogate for 

POC delivery to the sediment-water interface; thus it is logical that biomass of all the sediment 

organisms correlates with SCOC. Likewise, as the food-web model illustrates, the respiration of 

each group is parameterized as a function of the biomass of that group. Summing up those rates 

for those organisms living within the sediments is thus equivalent to the measured SCOC. As 

indicated in the introduction, a basic assumption of the approach used has been that THE 

underlying cause of variations in both structure and function is the amount and quality of organic 

matter entering the benthic boundary layer ecosystem. 
368 

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000

Community Biomass (mg C/m^2)

SCOC (mg C/m^2-day)

S42

Abyssal Plain

MS Canyon Head

DeSoto Canyon

Upper Florida Slope

MT1

MT3

S36

S1 & S4

Figure 9-9. Relationship between sediment community oxygen consumption (SCOC) and total 

community biomass at the process sites, as indicated. Data from Section 8 – Results. 

9.4.3 Total Metabolism and Its Relation to Species Composition 

The species composition of the macrofauna, megafauna and fish groups (and meiofauna?) 

differed at the “process sites,” reflecting, it is suspected, substantial differences in the intensity, 

and to some degree the quality, of organic matter input. For example, MT1, characterized by 

intense deposition of material originating from within the Mississippi River plume, was 

dominated by the amphipod Ampelisca sp. nov. and the megafauna, dominated by organisms that 

feed on suspended particulate matter (the anemone Actinauge longicornis and the bivalve 

mollusk Amygdalum politum). The dominance by crustaceans at the other sites on the upper 

continental slope changes to a dominance by echinoderms at depths greater than 1.5 km. This 

“echinoderm boundary” has been observed elsewhere, but never adequately explained (Rowe 

and Menzies 1969; Pequegnat et al. 1990). Bottom-dwelling demersal fish have all but 

disappeared by the abyssal plain. The boundaries between the “zones” of recurrent species of 

macrofauna, we contend, reflect regions of similar POC input and sediment type. Because these 

zones are each characterized by different densities of organisms, aligned along isobaths, we infer 

that each is characterized by different rates of POC input that decline downslope. Thus there is 

an upper and lower boundary of POC input rates which govern the distribution of each zone. 

Unfortunately, we have yet to quantify these controlling rates using conventional methods. 

It is possible to define these boundaries indirectly however. By plotting the upper and lower 

depth boundaries of each zone over the SCOC – depth plot, the “net POC input” can be 

estimated for the presumptive upper and lower POC boundaries for each zone (Figure 9-10), 

making the assumption that SCOC is a good reflection of POC input. The weakness of this 

approach with this data is that the SCOC was measured at a limited number of sites, and defining 

this relationship was not the intention of the study. Note that this does not include sites in the 

western GoM. To generate a similar relationship for the western transects, the SCOC has been 
369 

Figure 9-10. Log SCOC (sediment community oxygen consumption) (mg C/m2 day) as a 

function of water depth (km) for process stations. 

estimated from the macrofauna biomass (Figure 9-11). Note that the mid-slope zone (Zone 2) is 

bounded by a rate that is below the rate in the zone at the same depth interval (Zone 3), more 

inferential evidence that it is the input of POC that controls the zones. 

9.4.4 Food Web Carbon Budget 

The standing stocks, in terms of organic carbon (mg C m-2-15 cm), have been assembled for 

each of the process sites (Table 9-3) for comparison. A summary of measured carbon fluxes 

would also include the total measured sediment community oxygen consumption (SCOC, Table 

9-6) and the rate at which organic matter is buried (OMB), the latter calculated from percent 

organic carbon profiles and mean rate of sediment accumulation at each site estimated by the 

rates of the fluxes are in units of mg C m-2 d-1. The variation in stocks along the rows (Table 9-3) 

allows a comparison of the carbon stocks at each site, as mean organism size increases, whereas 

the variation in the columns provides a comparison of differences in stock size at the different 

experimental sites, from shallow to deep water. The last three columns and rows in Table 9-6 

allow comparison of net measured rates with the total stocks, either by site or across the depth 

gradient. The sum of the burial and the community respiration rates can be assumed to be more 

or less equal to net organic carbon flux to the sea floor at that site. In general, the net 

accumulation is so small relative to the respiration (Rowe et al. 1991) that many investigators 

ignore it in food web carbon budgets (Smith and Hinga 1983, Smith 1992). In general, however, 

there is considerably more detrital organic matter than living material at all the sites, which is not 

unusual. There is universally more organic matter, both living and detrital, at the shallow sites 

compared to the deep ones; that too is typical of most continental-margin depth gradients. At all 
370 

Figure 9-11. Log SCOC (mg C/m 2 day) as a function of water depth (km) for western 

transects, with SCOC estimated from macrofauna biomass. 

sites, regardless of depth, most of the organic matter was found in the smaller size groups. The 

relative decline in total biomass from small- to large-sized organisms was greater in deep than in 

shallow water, which is counter to the “bigger – deeper” rule, which simply illustrates that for 

the GoM the larger organisms are of less significance in deep water. The organic detritus and the 

bacteria declined by factors of two or less with depth, whereas the larger organisms declined by 

one to two orders of magnitude over the same depth interval. This implies that the quality of the 

organic matter in the sediments and the portion of the total bacteria that are active are both 

declining with depth. 

The standing stock and rate data from two sites have been put into two, oversimplified, 

contrasting food-web carbon budgets to illustrate how the shallow, upper-slope sites with 

relatively high biomass differ from the deep, abyssal plain sites with low biomass, meager input 

of organic matter and low SCOC. The shallow budget is based on MT1 at the head of the 

Mississippi Trough at a depth of 500 m (Figure 9-12), whereas the deep site is JSSD 1 on the 

Sigsbee Abyssal Plain at a depth of 3,650 m (Figure 9-13). The boxes represent each stock, 

whereas the arrows represent fluxes between the stocks. Each living stock is represented by a 

loss to respiration. This was estimated from the literature, based on animal size and temperature, 

for the animals. The bacteria respiration stock was estimated by subtracting the sum of the 

meiofauna and macrofauna respiration rate estimates from the chamber-determined “whole” 

sediment community rate, SCOC, in Table 9-6. 

Steady state has been assumed and therefore the sum of the fluxes into each stock must equal 

to the sum of the negative fluxes out of each box. The fluxes between the stocks, mostly 

predator-prey relationships, have been based on presumed secondary production (growth) and 
371 

PELAGIC INPUT

44.3 31 REWORKED AS FECES

EXPORT DOWNSLOPE 8 CO2

16 FECES

LONGTERM

BURIAL

11

CO2

11

MEIOFAUNA

[209]

BACTERIA

[2,350]

ORGANIC

DETRITUS

[760E3]

MACROFAUNA

[816]

MEGAFAUNA

[30.6]

FISHES

[35.9]

14 CO2

0.5 CO2

4

0.5 2P

+ EXPORT

1

14

FECES

CO2

0.1

0.5

FECES

0.4

1

7

4

21

16

0.4 FECES

MT1

Figure 9-12. Carbon budget in the food web at MT1, at the Head of the 

Mississippi Canyon. Stocks (boxes) are mg C/m^2-15 cm and 

fluxes (arrows) are mg C/m^2-day. Data from Section 8, 

calculated as indicated in text or by algebraic substitution, at 

steady state. 

JSSD1

PELAGIC INPUT

6.2

0.9 REWORKED AS FECES

3.2 CO2

0.16 FECES

LONGTERM

BURIAL

2.2

CO2

0.7

MEIOFAUNA

[13.5]

BACTERIA

[1,343]

ORGANIC

DETRITUS

[287E3]

MACROFAUNA

[8.3]

MEGAFAUNA

[0.19]

FISHES

[0.4]

0.14 CO2

? CO2

0.06

? 2P

+ EXPORT

0.01

0.8

FECES

CO2

?

0.005

FECES

2P

0.01

0.06

0.6

0.2

4.0

0.005

FECES

0.007

0.9

? ?

CARCASSES

? ?

Figure 9-13. Carbon budget in the food web at steady state for the 

Sigsbee Abyssal Plain. Units are mg C/m^2-15 cm for the 

stocks (boxes) and mg C/m^2-day for the fluxes (arrows). 

Data from Section 8, calculated as indicated in the text, or 

calculated by algebraic substitution. 
372 

assimilation efficiencies. The growth to respiration efficiency for the bacteria is assumed to be 

50%. The respiration to growth ratio of the megafauna and the fishes was assumed to be 10%. 

Growth rates for the fishes were derived from growth-curve constants available in “FishBase,” 

but were transformed into first-order (linear) rates for simplicity. Fluxes between stocks have 

been rounded off to one significant digit. Assimilation efficiency (assimilated food/total food 

ingested x 100) has been assumed to be 50% for invertebrates and 75% for the fishes. 

Each of the two budgets or stock/flux models is represented by the same six stocks in a 

similar order, with organic detritus on the upper left, terminating with the fishes on the lower 

right. The input terms in both extremes are defined as a rain of sedimenting particulate matter 

(POC) as the principal supply of carbon to the system, but the deep site also includes an input 

from carcasses in the lower left, the value of which is unknown. At both sites, the bacteria feed 

only on organic detritus, the meiofauna feed on both the bacteria and the organic detritus, the 

macrofauna feed on the latter three components, the megafauna feed on the latter four 

components, but the fishes feed only on the megafauna and the macrofauna. Although the 

relative proportions of the food sources to each feeding stock are unknown, it is presumed that 

each stock will prefer to feed on the largest size available because this is the most energetically 

efficient pathway. The secondary production of each stock at steady state is divided up to meet 

the needs of the next level of the food web. The production to biomass ratio for the macrofauna 

is assumed to be one (P/B=1) and the meiofauna are assumed to produce a cohort every month. 

The carbon-based food web budgets, calculated on the basis of biomass per individual, total 

biomass and temperature for each experimental site, provide an integrated picture of how carbon 

is cycled within a food web. By summing up all the respiration values, one can estimate total 

community oxygen consumption (SCOC). At a site in the deep NW GoM investigated the rate 

calculated by the model was several times that measured with a single benthic chamber in situ on 

the lander. Nonetheless, the general trends between the two are in agreement: directly measured 

rates with chambers and indirectly estimated rates based on biomass follow parallel, linear 

patterns that also correspond to depth or unique location, e.g., the head of the Mississippi 

Trough. The pore water profiles followed the same pattern too, but with some exceptions on the 

upper slope. Biomass, we can conclude, is a better determinant of SCOC than is depth (Figure 9-

14), a relationship pointed out previously. 

Several generalizations can be stated about these two budgets, both in terms of how they are 

similar and how they differ. Most of the carbon is in the detritus box, followed by the bacteria, at 

both the shallow and the deep sites. The “dead” carbon is several orders of magnitude higher 

than the living bacteria. The animal stocks are also orders of magnitude lower than the detritus 

and bacteria components, but the ratio between the dead and small to the larger boxes 

((bacteria+detritus carbon) / (meiofauna+ macrofauna+megafauna+fishes)) is much larger at the 

deep site. At both sites, a large fraction of the detritus is recycled back into the detritus box as 

feces, but the ratio of the recycled POC to the “fresh” POC is greater at the deep site. This transit 

of “mud” through the guts of animals results in mixing of surface material and results in the 

bioturbation redistribution of radionuclide tracers investigated [See Section 8.5]. The food-web 

carbon budgets confirm the radionuclide profiles that suggest that the mixing is proportional to 

how many animals are present. That is, the total turned over or recycled through the guts of 

organisms is considerably greater in shallow water than out on the abyssal plain. At both sites, 

most of the cycling occurs within the three small components living in and on the sediments, 

rather than through the larger organisms (megafauna and fishes). 
373 

y = 0.7717x - 0.3415

R2 = 0.7558

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

1 1.5 2 2.5 3

Log Macrofauna Biomass

Log SCOC

Figure 9-14. Sediment Community Oxygen Consumption (mg C/m^2-day) as a 

function of the macrofauna biomass at the process sites. The high 

SCOC is S42 on the west Florida upper slope (740 m depth), whereas 

the high biomass is MT1 at the head of the Mississippi Canyon. The 

two low values are on the Sigsbee Abyssal Plain. 

The relative contributions of each category of biomass contribute to the total suggest that as 

input declines the larger forms are eliminated first, as suggested previously (Thiel 1975). Species 

composition may also be important at this “reductionist” level of analysis. In some cases 

“keystone species” are seemingly important at some process sites. Can we possibly characterize 

each site by these species? For example, S42 (on the upper Florida escarpment; 750 m) shares 

dominance with two big crustaceans and about 10 other large crustaceans (decapods), but not a 

single species that is functionally dominant at structuring the assemblage of smaller organisms. 

This may simply be a function of S42 being some sort of faunal transition zone, as suggested in 

the literature (Menzies et al. 1973). 

The clear dominance of the bacteria at the abyssal plain and lower slope sites (JSSD1, JSSD4 

and MT6) suggests that bacteria have greater importance in deep water and support the idea that 

they are necessary for the remobilization of organics. But new thoughts on bacterial role below 

The sharing of total biomass between the mid-sized and larger metazoans at upper slope sites 

with the highest biomass values suggests, it follows, that they are not dependent on the bacteria 

and size groups more or less share newly arrived detritus that is energy-rich and reactive. In 

essence, the different size categories not only share this material, but compete for it. All these 

forms support the limited numbers of predators that, from the top down, affect community 

structure. 

In general, the megafaunal invertebrates collected during the current study followed much 

the same pattern of depth distribution reported previously by Pequegnat (1983) and Pequegnat et 

al. (1990), who reported that overall species diversity decreased with depth. However, in this 

study, diversity tended to be lowest at the shallowest stations, in basins, and in areas dominated 

by small sponges. The greatest species diversity was at the deepest stations of the Mississippi 

Trough. On the upper and middle slopes, diversity tended to be slightly more variable from 

station to station than on the lower slopes and abyssal plain, where, except for the high value at 
374 

station MT6, diversity indices were comparable throughout the sampling area. At the deepest 

stations, echinoderms tended to dominate the megafauna. 

Soto et al. (1999) concluded that diversity and biomass of decapod crustaceans in the 

southwestern Gulf of Mexico were associated with substrate heterogeneity and seasonal pulses of 

organic material from rivers. The abundance and species diversity of megafaunal invertebrates in 

the Mississippi Trough and DeSoto Canyon would seem to support this relationship in the 

northern Gulf as well. The great biomass of echinoderms at depths of 2,500 m or more may be 

misleading, because much of this wet weight is due to fluid in the coelomic cavities of the 

animals. 

On the upper slopes (1,000 m or less), many animals were associated with soft, silty bottoms. 

The crabs Acanthocarpus alexandri, Lyreidus bairdii, and Bathyplax typhla were characteristic 

of such areas. Live specimens of the first two species burrowed into mud when kept in aquaria. 

Species of Nephropsis have been photographed in burrows in the mud (Pequegnat, 1983). 

Studies on the related Nephrops norvegicus indicate that groups of lobsters build tunnels in mud. 

On the other hand, Soto (1985) reported that the crab Rochinia hystrix favored hard substrates. 

The nested mussel Amygdalum politum was particularly common in the area of the 

Mississippi Trough. This mussel is able to attach to conspecifics, forming a mass that will not 

sink into loose sediment. Its abundance in the Mississippi Trough may be related to the flow of 

organic food particles in the area. Stalked barnacles (Arcoscalpellum and Neoscalpellum spp.) 

did not show any relationship to any large-scale environmental feature, but often were attached 

to “clinkers” or sponge spicules. The sponges Hyalonema sp. and Euplectella sp. possess anchor 

spicules that raise the feeding parts of the sponge above the substrate. These sponges usually 

were collected in clay. 

The data and the model of the small-size categories suggest that we might re-consider the 

role of bacteria in the food webs of the deep GoM. POC rains down from surface waters to 

support metabolic processes and growth of all the organisms. It contains with it an ill-defined 

fraction of bacteria on the particulate matter but it can probably be presumed that these have 

been inactivated by the cold and pressure of the environment – the attached surface bacteria too 

have become inert POC particles while sinking into an environment alien to them. Once on the 

sea floor they contribute to the total organic matter at a level that is proportional to detrital 

organic matter and the inorganic clay particles, including siliceous or calcarious biogenic 

material. These inert cells are introduced into a living bacterial assemblage composed of 

psychrophilic, barotolerant (or barophilic) populations that are presumed to be consuming 

detrital organics in proportion to that available to them. 

Within shallow environments it is presumed that most of the bacterial activity is supported by 

DOM rather than POM. This would imply that the sinking POM must be remobilized into DOM 

before it is available to potentially active bacterial populations, either through “messy feeding” 

or released with the feces. In this scenario, much of the POM would be consumed first by 

invertebrates who would digest some fraction of it and release a companion fraction in the form 

of DOM to be consumed by the bacteria (Jumars et al. ref.). The model for these pathways is 

implied for the food-web models representing our composite “process sites” but we make no 

distinction in the detritus box between DOM and POM. Our model does include fecal production 

that re-enters the detrital pool (Figure 9-10 and 9-11), but again no distinction is made between 

POC and DOC. A model in which a distinction between POC and DOC illustrates that bacteria 

consume DOC primarily and this repackaging conserves carbon and energy; it suggests that the 

bacteria are dependent on the invertebrates but diminishes the role of the bacteria energetically as 
375 

an intermediary between organic detritus (POC) and the invertebrates. Such a configuration takes 

DOM and re-packages it into living organisms that can then be consumed by the invertebrates in 

a food web reminiscent of the “microbial loop” hypothesized for surface waters. The two 

budgets illustrate that the relative roles of the different size groups within the sediment are about 

equal, and implies that the bacteria, for the most part, are not actively growing. The relative 

proportion of them that are inactive increases with increasing depth, probably due to the hostile 

environment of low temperature, high pressure and meager amounts of reactive organic matter 

available. 

9.4.5 Sequestration of Conservative Chemical Properties (Bioturbational Mixing) 

The evidence suggests that biological reworking and mixing carries non-conservative 

chemicals, including contaminants, one might presume, to depth in the sediment, thus isolating 

them from further interaction with the biota of the Benthic Boundary Layer (BBL). This 

phenomenon has previously been noted for the continental shelf of the NW Atlantic, again using 

210Pb as a tracer. In the case of GoM slope sediments, it might be suggested that the grain size 

gradient, from sand-sized foraminiferal ooze down to clay-sized material, with depth as the 

CaCO3 material dissolves, enhances sediment binding capacities. 
377 

10. UNKNOWNS AND RECOMMENDATIONS 

10.1 Community Structure 

It is presumed in contemporary literature [See the works of Gray in the references for a 

summary of these issues in the marine environment.] that community structure (the diversity of 

assemblages) is controlled by a suite of ecological parameters in concert with the given 

biogeographical history of a habitat within an ecosystem. For example, 

Species Evenness = f(food input, variability, stress, competition, predation) 

Species diversity, defined either as the alpha (relationship of numbers of species to 

individuals or area), beta (turnover or change in species composition along a gradient), or gamma 

diversity (total number of species), are each controlled, to a greater or lesser extent, by the 

following parameters: E(s) = f(controls of evenness, plus historical contributions), Gamma 

(species richness) = f(geological history, sources of species available for recruitment, structural 

complexity, all the other parameters listed above that affect evenness and alpha diversity 

measures). These processes work together, on each taxonomic group in question in a different 

way and at different rates, toward success (survival) or failure (extinction) of each species. 

While considerable information on each of these measures of structure has been developed, 

NONE of the variables listed has been measured directly. In fact, these variables cannot be 

measured directly without applying the scientific method to direct experiments that test 

hypotheses. This study has been based on the correlations accrued from the extensive sampling 

grid, accompanied by considerable sampling replication. The conclusions drawn are based on 

correlations, and thus are no more than that. They are not based on experiments that manipulate 

the fauna and the conditions directly. 

We are left with a set of statistical models that are purported to describe how structure of 

communities is related to variables in the deep ocean. However, that is all they are: statistical 

models. They have not been validated experimentally. 

We are left, therefore, with a set of questions related to community structure: 

• How do physical variables in concert affect community structure, if at all? 

• Is structure a function of OM input alone, or to what degree is it just OM or a 

set of physico-chemical constraints? Can this be tested experimentally? 

• How long does it take a set of community structural traits to be established? 

• Is this a function of ocean basin age (10^x years) or ecological conditions 

(months to years)? and; 

• What biological variables, which are characteristic of particular species and 

larger taxa, are important in this (growth rate, mode of reproduction, mean 

and maximum age, feeding strategies, dispersal vs brooding of eggs and 

juveniles, etc.)? 

The present study attempted to be sequential, moving from a survey, relating community 

structure to the environment, into an experimental mode at a select set of “process” sites. What 

actually happened was that the suite of information at the process sites was enlarged to include 

some rates of processes, but no experiments were conducted in the classic sense. The result was 

that we know in better detail what is happening at the process sites, but the relationships between 
378 

the environment and community structure were not studied using experiments to test hypotheses, 

in the classic sense. We must conclude therefore that this study added evidence to what we 

believe controls structure, but simply allowed the investigators to revise their hypotheses without 

actually conducting experiments. 

In the context of the responsibilities of the Minerals Management Service, this study has 

provided considerable information on what organisms might be affected by a broad suite of 

impacts from O&G activities, but it does not give definitive insights into what the responses 

might be. 

This leads to one major conclusion concerning continued studies: direct experimentation 

should be designed and conducted to test hypotheses related to effects of oil and gas (O&G) 

activity on the deep ocean. 

10.2 Community Function 

An underlying theme throughout this study was that organic matter supplies are the ultimate 

control of both community structure (see above) and function, the latter defined as rates of 

biological and biogeochemical processes. The broad geographic sampling pattern comparing 

different habitats, followed by intensive rate measurements at a small set of contrasting habitats 

with characteristic communities, has allowed food web models to be constructed that contrast 

extremes encountered. The models implicate food supplies directly as the underlying or 

supporting component on which the food web relies. These appear to be very convincing, even 

though it is conceded that the specific estimates of the individual fluxes may be subject to large 

variations. 

It must be conceded that most of the individual predator-prey transfers between the stocks in 

the food web have not been validated. The boundary conditions are subject to validation by 

direct sampling: more and better community respiration measurements, sediment trap 

measurements, and better, more extensive geochemical dating. However, these approaches will 

refine the boundaries, not the internal workings within the system. Attempts to measure some of 

these were tried, but their success is questionable because the rates derived do not match indirect 

measurements or predicted values of rates based on known shallow water work or deep-sea 

studies conducted elsewhere. 

The next step in understanding biological community function ought to include direct 

measurement of rates of processes on the sea floor using a second generation of instruments 

capable of conducting experiments associated with the bottom boundary layer and the 

community of organisms living there. This would allow the validation of the models that have 

been developed from the present study. Without this validation, the results of this study will 

remain only hypothetical models. 

A specific set of additional questions would include the following: 

1) What fraction of the OM delivered to the sea floor from the water column is 

refractory and what is reactive? 

2) Are there processes, within the food web, that transform refractory matter into 

reactive? 

3) Are the bacteria inactive and just accumulating with the sediments, sort of like 

a conservative chemical parameter? 
379 

4) Even if bacteria are not active, are they eaten by the deposit feeders? 

5) What happens when organic matter is added to the system? Does this confirm 

the model predictions? 

The next step in understanding the deep ocean’s responses to O&G activity is direct 

experimentation on the sea floor to test hypotheses related to potential O&G activity. For 

example, what is the effect of added hard substrate associated with exploratory drilling, 

processing on the sea floor, production and transport of product from site to site along the sea 

floor? What is the effect of added materials that are lost from or are byproducts of the processes 

and additions related to O&G activity. Reasonable hypotheses could be proposed for many of 

the most salient effects, and these need to be tested. Drilling mud for example has many different 

compositions. Each may have a different specific effect. However, the degree to which each 

contributes reactive organic matter may be rather general, and determining this might be very 

important for predicting the effects of O&G activity. These issues could and should be 

approached using hypothesis-based experiments in situ with state-of-the-art instrumentation 

available in the O&G service industry and government. 

In summary, the next step in making progress toward rational decision-making in the 

environmental effects of O&G activity in the deep ocean would be to initiate experimental 

studies to test hypotheses that are related directly to O&G activity. 

10.3 Taxonomy 

The taxonomy of the larger forms sampled in this study is fairly well known, with few 

undescribed or poorly-described species encountered (the fishes and megafauna), but this is not 

true for the macrofauna and the meiofauna. On the order of 50% or more of all the groups of 

macrofauna studied were considered species that are either new to science (undescribed in the 

taxonomic literature) or so poorly described that their specific designation could not be 

determined. In these cases the individuals of specific morpho-types were lumped into single 

“species” categories and designated in the data base as “species a, b. ..., etc.” or “species 1, 2, 

..., etc.” throughout. While this does no harm to the analyses of community structure in this 

report, the limited descriptions in the literature will remain a serious impediment to the 

usefulness of future work. Without good descriptions, an investigator will need to see the actual 

preserved “type” organisms themselves to which each designation refers in order to determine if 

a particular species observed in DGoMB is the same as that in both previous and future 

investigations. The reason for this problem is the serious lack of trained, committed taxonomists 

working on the invertebrate groups composing the bulk of the macrofauna and the meiofauna. 

10.4 Relationships between Function and Structure 

The goal of correlating structure and function met with considerable success. The broad 

sampling pattern allowed the degree to which zonation (beta diversity) extends continuously 

along isobaths to be tested and, ultimately, to be mapped for each size group and for 

taxonomically separate groups. The zonation’s correlations with biomass and SCOC implicate 

the decline in the input of organic matter as a function of depth as a principal control of beta 

diversity or zonation. However, no direct experimental evidence is available to confirm these 

revised hypotheses. A direct step in such a question would deploy a set of sediment traps across 

and down the depth gradient. In this study, SCOC has been used as a surrogate for POC input 

(net). As each so-called zone has a characteristic biomass and species composition, it is 
380 

reasonable to presume that the input of supporting organic matter also controls those biological 

variables. However, as above, this has not been tested with direct experiments. 

10.5 The Unknowns and the Unknowable 

Following the general philosophical approach of the Census of Marine Life (CoML), deep 

ocean ecology must recognize that a distinction ought to be made between the “unknown,” 

which can be understood, quantified, described, etc., through continued and improved 

observations and experiments, and those things that are, for one reason or another, categorized as 

the “unknowable.” A goal of CoML in a variety of marine ecosystems at all depths is the 

tabulation of every known species, accompanied by the locations at which each has been 

observed. It is recognized however that descriptions of “all” species in the deep ocean will, for 

all practical purposes, never be accomplished. While much can and will be gained by future 

work that iinvolves more sampling from the surface with conventional gear and hypotheses 

concerning structure and function can be tested using direct experiments, some parts of the 

models and a complete listing of species for any given location will never be realized. The 

unknowns are listed in this Section, but the unknowables are not. 

10.6 Summary of Unknowns and Recommendations for Further Study 

This section is a list of specific unanswered questions which derive from the individual 

subsections in Section 8 and 9 of the final report. 

Relationships between Physical Processes (Deep Bottom Currents) and the Biota 

The fauna at the base of Green Knoll was not similar to any of the 5 zones described. This 

suggests that the fauna may be controlled by the conditions at that site, perhaps a deep, bottomfollowing current. The area is characterized by large bus-sized channels and it is presumed that 

these were or are caused by currents flowing around the base of the knoll. This is all conjecture, 

however. It is not known what caused this community to differ from others in the study. 

Time-Dependent Variations in the Biota and Rates of Biological Processes 

The study was conducted over a 3 year period, but always in the May or June, with the 

exception of two weeks in August, 2002. This sampling did not allow any variations in time on 

the scale of seasons to be determined. It has been observed elsewhere that the production of 

phytoplankton on the surface varies seasonally and this results in a seasonal pulse of organic to 

the sea floor. The fauna may respond to this pulse, but further study will require seasonal 

sampling at specified sites to determine this. 

Direct Responses of the Biota in Terms of Species Composition and Rates to Pulsed Inputs 

of Organic Matter to the Sea Floor 

While the premise behind the hypotheses tested was that the input of organics controls much 

of what is observed in the biota, this has not been tested directly. Experiments should be 

conducted to determine the effects of organic pulses on the sea floor. This issue is particularly 

important to MMS because loss of product or drilling mud onto the sea floor could, from a 

theoretical point of view, increase total biomass and amplify rates of processes on the sea floor 

measurably. 
381 

Distributions of Standing Stocks 

While the standing stocks of all the major size groups represented in the bottom boundary 

layer food web conformed to more or less predictable declines with increasing depth, the bacteria 

densities did not. The bacteria declined in a smooth regression down to the base of the 

continental slope, but then increased out on the abyssal plain. This needs to be investigated 

further. 

Rates of Metabolic Processes and Organic Carbon Turnover 

The cycling of organic matter on and in the sea floor was estimated by a number of methods. 

The general trends in these rates were fairly well correlated from site to site, but the absolute 

values did not agree. It is recommended that a small number (2 to 3) of easily accessible, 

contrasting locations be chosen to conduct controlled sampling and experiments on rates of 

processes in order to resolve the disagreements. The most secure approach would be to conduct 

experiments in situ with a Remotely Operated Vehicle or a manned research submersible to 

eliminate issues that arise from decompression and temperature changes. 
383 

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Appendices 

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Appendices.htm. 

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