OCS Study 

MMS 96-0055 

Ecosystem Analysis of the 

Louisiana Bight and 

Adjacent Shelf Environments 

Volume I : The Fate of Organic Matter and 

Nutrients in the Sediments of the Louisiana Bight 

U.S. Department of the Interior Ang Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 96-0055 

Ecosystem Analysis of the 

Louisiana Bight and 

Adjacent Shelf Environments 

Volume I : The Fate of Organic Matter and 

Nutrients in the Sediments of the Louisiana Bight 

Authors 

Robert R . Twilley 

Brent A. McKee 

Prepared under MMS Cooperative Agreement 

14-35-0001-30696 

by 

University of Southwestern Louisiana 

Department of Biology 

Lafayette, Louisiana 70506 

Published by 

U.S. Department of the Interior 

Minerals Management Service New Orleans 

Gulf of Mexico OCS Region November 1996 
DISCLAIMER 

This report was prepared under a cooperative agreement between the University of 

Southwestern Louisiana (USL) and the Minerals Management Service (MMS). This report has 

been technically reviewed by USL and MMS, and it has been approved for publication. 

Approval does not signify that the contents necessarily reflect the views and policies of the 

Service, nor does mention of trade names or commercial products constitute endorsement or 

recommendation for use. It is, however, exempt from review and compliance with 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 

Attention : Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone: (504) 736-2519 or 

1-800-200-GULF 

CITATION 

Suggested citation : 

Twilley, R.R. and B. McKee. 1996. Ecosystem Analysis of the Louisiana Bight and 

Adjacent Shelf Environments. Volume I: The Fate of Organic Matter and Nutrients in 

the Sediments of the Louisiana Bight. OCS Study MMS 96-0055. U.S . Dept. of the 

Interior, Minerals Mgmt. Service, Gulf of Mexico OCS Region, New Orleans, La. 

129 pp. 

iii 
CONTRIBUTORS 

Greg Booth 

Louisiana Universities Marine Center, Louisiana Universities Marine Consortium 

Chauvin, LA 70344 

John Bourgeois 

University of Southwestern Louisiana, Department of Biology, Lafayette, LA 70504 

Brent McKee 

Louisiana Universities Marine Center, Louisiana Universities Marine Consortium 

Chauvin, LA 70344 

Tina Miller-Way 

University of Southwestern Louisiana, Department of Biology, Lafayette, LA 70504 

Robert R. Twilley 

University of Southwestern Louisiana, Department of Biology, Lafayette, LA 70504 

Peter Swarzenski 

Louisiana Universities Marine Center, Louisiana Universities Marine Consortium 

Chauvin, LA 70344 

Terry Whitledge 

University of Texas at Austin, Marine Science Institute, Port Aransas, TX 78373 

v 
PREFACE 

The interrelationships among rates of material cycling through both benthic and watercolumn communities play an important role in the ultimate fate of inorganic nutrients and 

organic matter within coastal ecosystems (Figure 1) . The coupling of benthic and pelagic 

processes contributes to elevated rates of primary productivity in coastal ecosystems by 

recycling nutrients in the water column and across the sediment-water interface. In the 

Louisiana Bight, high concentrations of suspended sediments and nutrients are introduced from 

the Mississippi River to a quiescent shallow shelf ecosystem and are thus either quickly 

deposited to the seabed in the near proximal zone of the plume region or utilized to support 

highly productive zones at the outer fringes of the front. Thus the seabed of the plume region 

may received high inputs of particulates of either tenigenous origin or from organic production 

in the water column. The fate of this particulate material that reaches the seabed of the 

Louisiana Bight is the focus of this report . 

A research program at the University of Southwestern Louisiana and Louisiana 

Universities Marine Consortium Marine Center (LUMCON) was initiated to study the 

biogeochemistry of the Louisiana Bight. This research effort was initially funded by the 

Louisiana Board of Regents through a program entitled Louisiana Stimulus for Excellence in 

Research (LASER). A grant entitled "Development of a research emphasis in oceanographic 

processes on continental shelves influenced by large rivers" was funded in 1987 to LUMCON, 

USL, and LSU (Grant No. 86-LUM (1)-083-13; Dr. Mike Dagg, Principal Investigator) to 

develop an interdisciplinary study of the Mississippi River plume over the Louisiana Bight 

region. Robert Twilley was funded to study benthic nutrient regeneration in the initial LASER 

study, and when Brent McKee arrived at LUMCON, he was later added to the research effort 

to study sediment dynamics in the seabed of the region. These two individual efforts were 

gradually coordinated through the development of this 5-yr LASER program to study the 

deposition, burial and remineralization of nutrients at the sediment-water interface in the plume 

region. 

The LASER program funded ship time, but had limited resources for analytical support 

for the research efforts to study benthic processes in the plume region of the Louisiana Bight. 

Additional ship time to the LASER program was provided by a grant from the National Science 

Foundation (R11-8820219) allowing a total of six cruises from August 1987 to October 1990. 

However, much of the sediment analyses, particularly the 42-station survey of 

geochronologies and nutrient concentration in the plume region, were not supported by the 

LASER program. This report provides the technical summary of information from the LASER 

program in addition to support by the Mineral Management Service (Cooperative Agreement 

No. 14-35-0001-30696) to complete analyses of sediment and pore-water samples that were 

collected during the LASER cruises. During the MMS grant period, we were also able to 

specifically test some ideas on the importance of selected sediment characteristics on benthic 

nutrient regeneration with funding from the National Science Foundation, Coastal Ocean 

Margin Uranium Study (COMUS), by providing ship time for two additional cruises to the 

study area. Thus, a total of eight cruises are summarized in this report of benthic processes in 

the plume region of the Louisiana Bight. The conceptual model that has guided this research 

program in the benthic biogeochemistry of the Louisiana Bight is described in Figure 1 and is 

provided as an overview of the ecological processes in this river-dominated shelf ecosystem. 

vii 
Burial 

Figure 1 Conceptual diagram of the ecological processes of the Louisiana shelf ecosystem showing sources of new nutrients 

and pathways of regenerated nutrients. Sediments and nutrients from the pelagic zone are deposited to the seabed and 

either regenerated, redistributed, or buried in the sediments. 
ABSTRACT 

The major objective of the LASER and COMUS projects was to determine the role of 

benthic processes in affecting the fate of sediments and nutrients in a river dominated shelf 

ecosystem. This technical report describes studies to understand the coupling of the fifth largest 

river system in the world to coastal margin ecosystems in the plume region of the Louisiana 

Bight. Chapters in this technical summary describe the accumulation and regeneration of 

sediment, carbon, nitrogen, and phosphate from the seabed in the plume region of the 

Louisiana Bight. Strong variation in sedimentation and nutrient accumulation rates in 5417 

km2 plume region of the Louisiana Bight indicate the problem of accurately deternuning reliable 

estimates for these processes in river-dominated shelf ecosystems. The area-weighted 

sedimentation rate was 4,442 g m-2 yr-1, carbon accumulation was 64, nitrogen 7.02 and 

phosphorus 2.89 g m-2 yr-1 . Results reveal that approximately 30% of the particulate organic 

carbon supplied by the Mississippi River to the adjacent shelf is remineralized within 4 months 

and an additional 40% is remineralized on a decadal time scale. Based on these results, 

combined with similar information from the Amazon, Changjiang and Huanghe Rivers, we 

conclude that only 30% of the particulate organic carbon supplied to the ocean by rivers is 

buried . Our area-weighted estimate of organic carbon burial in deltaic-shelf sediments is 75 gC 

m-2 yr-1 with less than 50% of the organic carbon buried being of terrestrial origin . The 

resulting global estimate for terrigenous POC burial is 0.05 PgC yr-1, representing less than 

half the value currently used to constrain global C02 budgets (Sarmiento and Sundquist 1992). 

Seasonal deposition of allochthonous organic matter during spring provides the 

predominant mechanism for sustaining peak rates of benthic regeneration in the plume region 

of the shelf. During April, ammonium regeneration in the plume region (station B50 and C50) 

may exceed 500 gmol m-2 h-1, compared to fluxes of less than 200 gmol m-2 h-1 during low 

river discharge in September and October. Further downfield (stations D50 and E50), 

ammonium regeneration is generally less than 200 gmol m-2 h-1 . Seasonal differences in 

benthic regeneration are most evident nearest the mouth of the Mississippi River, while rates 

are more constant both temporally and spatially in more distant regions. The link between 

sediment deposition and benthic nutrient regeneration is clearly demonstrated in results of 

silicate flux at near and far - field stations on the Louisiana Bight. Fluxes of silicate across the 

sediment-water interface to the water column increase linearly with deposition rates from 0.3 to 

2.5 cm/mo, reaching maximum silicate flux rates of 550 gmol m-2 h-1 . Above a deposition rate 

of 2.5 cm/mo, silicate flux was lower at less than half the maximum rates. However, there 

was no clear pattern in ammonium or phosphate regeneration with bulk deposition of sediments 

in the study area; suggesting that the quality of material must also be evaluated. 

Rates of benthic nutrient flux tended to decrease with distance from the riverine source, 

and exhibited contours of rates similar to patterns of sedimentation and nutrient accumulation . 

Modeling nutrient fluxes from pore-water gradients showed little correlation to the actual 

measured nutrient flux ; however these sediment characteristics do provide explanations for 

outliers in benthic regeneration rates. Benthic fluxes always showed a higher correlation to 

some qualitative index (input of carbon, nitrogen, phosphorus, or chlorophyll a) of deposition 

to the seabed than to the quantitative input of sediment. The combination of geochronologies 

on time scale of months and decades along with repeated measures of benthic nutrient 

regeneration provide spatial and temporal resolution of these benthic processes in this riverdominated continental shelf ecosystem. 

xi 
TABLE OF CONTENTS 

Page 

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv 

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi 

Chapter 1. Inventory of Sedimentation and Nutrient Accumulatin 1 

in the Sediments of the Louisiana Bight 

1 .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1 .2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

1 .3 Results and Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Chapter 2. Burial and Remineralization of Terrigenous Organic 29 

Carbon in Deltaic-Shelf Environments 

2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

2.3 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

2.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Chapter 3. Inventory of Benthic Fluxes in the Louisiana Bight 37 

3 .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

3 .2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 

3 .3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

Chapter 4. Seasonal Investigations of Benthic Fluxes within 63 

the Louisiana Bight 

4 .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

4 .2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

4 .3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 

Chapter 5. Spatial Patterns of Benthic Nutrient Regeneration and 77 

Sediment Characteristics on the Louisiana Continental 

Shelf 

5 .1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 

5 .2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 

5 .3 Site Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 

5.4 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

5.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 

5.6 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 

5.7 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 

xiii 
LIST OF FIGURES 

Page 

Figure 1 . Conceptual diagram of the ecological processes of the Louisiana shelf 

ecosystem showing sources of new nutrients and pathways of regenerated 

nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix 

Figure 1 .1 . Diagram of the global carbon cycle between the atmosphere, biosphere, 

and lithosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Figure 1 .2 . Region of the Louisiana Bight that was the study area for estimates of 

sediment and nutrient accumulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Figure 1 .3 . Diagram of Pb-210 distribution with depth in sediments of station B-50 

showing typical geochronology in high deposition area. . . . . . . . . . . . . . . . . . . . . . 7 

Figure 1 .4 . Contours of sedimentation rates in the study area of the Louisiana 

Bight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Figure 1 .5 . Contours of carbon accumulation rates in the study area of the Louisiana 

Bight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Figure 1 .6 . Contours of nitrogen accumulation rates in the study area of the Louisiana 

Bight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Figure 2.1 . Spatial distribution of organic carbon burial rates (gC m-2 yr-1) for the 

5400 km2 study site in the Mississippi deltaic-shelf environment. . . . . . . . . . . 33 

Figure 2.2 . Temporal relationship between the organic carbon seasonally stored at 

station B-50 (a site proximal to the mouth of the Mississippi River; see 

Figure 2.1 for location) and the particulate riverine organic carbon supplied 

during the same period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

Figure 3 .1 . Location of sampling stations at three depths and four different transects 

each at increasing distance from mouth of Southwest Pass. . . . . . . . . . . . . . . . . . 42 

Figure 3 .2 . Sediment oxygen demand at three depths along transects at increasing 

distance from the mouth of Southwest Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

Figure 3 .3 . Benthic fluxes of phosphate at three depths along transects at increasing 

distance from the mouth of Southwest Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 

Figure 3.4 . Benthic fluxes of silicate at three depths along transects at increasing 

distance from the mouth of Southwest Pass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

Figure 3 .5 . Benthic fluxes of ammonium at three depths along transects at increasing 

distance from the mouth of Southwest Pass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 

Figure 3 .6 . Benthic flux of nitrite at three depths along transects at increasing distance 

from the mouth of Southwest Pass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

xv 
Figure 3 .7 . Benthic flux of nitrate at three depths along transects at increasing distance 

from the mouth of Southwest Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

Figure 3 .8 . Rates of exchange of dissolved oxygen, silicate, phosphate, ammonium, 

nitrate, and nitrite across the sediment-water interface in a Louisiana shelf 

ecosystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

Figure 4.1 . The timing of measuring benthic fluxes in the Louisiana Bight relative to 

the seasonal discharge of the Mississippi River. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

Figure 4.2 . Rates of deposition based on accretion of thorium-234 at stations B-50 

and C-50 during different LASER and COMUS cruises . . . . . . . . . . . . . . . . . . . . . 68 

Figure 4.3 . Concentrations of benthic chlorophyll along the 50-m contour of the 

Louisiana Bight at different distances from the mouth of Southwest 

Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 

Figure 4.4 . Benthic fluxes of phosphate along the 50-m contour of the Louisiana 

Bight at different distances from the mouth of Southwest Pass. . . . . . . . . . . . . . 70 

Figure 4.5 . Benthic fluxes of silicate along the 50-m contour of the Louisiana Bight 

at different distances from the mouth of Southwest Pass. . . . . . . . . . . . . . . . . . . . . 71 

Figure 4 .6 . Benthic fluxes of ammonium along the 50-m contour of the Louisiana 

Bight at different distances from the mouth of Southwest Pass. . . . . . . . . . . . . . 72 

Figure 4 .7 . Benthic fluxes of nitrite along the 50-m contour of the Louisiana Bight 

at different distances from the mouth of Southwest Pass . . . . . . . . . . . . . . . . . . . . . 73 

Figure 4.8 . Benthic fluxes of nitrate along the 50-m contour of the Louisiana Bight 

at different distances from the mouth of Southwest Pass . . . . . . . . . . . . . . . . . . . . . 74 

Figure 4.9 . Benthic fluxes of silicate, ammonium, and phosphate relative to the 

measurement of deposition at stations along the 50-m contour. . . . . . . . . . . . . . . 75 

Figure 4.10 . Benthic fluxes of silicate, ammonium, and phosphate relative to the 

measurement of deposition at all stations in the Louisiana Bight. . . . . . . . . . . . . 76 

Figure 5 .1 . Conceptual model of the fate of organic matter in the plume and 

mid-shelf region of the Louisiana continental shelf. . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 

Figure 5 .2 . Histogram of the stages of the Mississippi River at Tarbert Landing, 

Louisiana (USGS) for COMUS 1 and COMUS 3 in m3 s-1 . . . . . . . . . . . . . . . . . 91 

Figure 5.3 . Sampling stations on the two cruises: COMUS 1 (November 1993) and 

COMUS 3 (June 1994) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 

Figure 5.4 . Core chemostat system used to determine benthic nutrient flux rates 

(Miller-Way and Twilley 1996). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 

Figure 5 .5 . Sampling regime used to analyze each core during and after incubation . . . . 94 

xvi 
Figure 5 .6a. Average flux per station for N03, N02, and NH4 for each cruise 

(with standard error bars), excluding the station at South Pass (SP). . . . . . . . 95 

Figure 5 .6b. Average flux per station for P04 and Si04 for each cruise (with 

standard error bars), excluding the station at South Pass (SP). . . . . . . . . . . . . . . 96 

Figure 5 .7 . Pore-water profiles with depth for NH4, from the overlying water 

(-0.5 cm) to a depth of 3 .5 cm into the sediment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 

Figure 5.8 . Pore-water profiles with depth for P04, from the overlying water 

(-0.5 cm) to a depth of 3 .5 cm into the sediment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 

Figure 5.9 . Pore-water profiles with depth for Si04, from the overlying water 

(-0.5 cm) to a depth of 3 .5 cm into the sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 

Figure 5.10. Sediment bulk density (g/cm3) by station for A) COMUS 1 and B) 

COMUS 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 

Figure 5.11 . Sediment chlorophyll a concentrations (fig/cm3 at lcm depth) by station 

for A) COMUS 1 and B) COMUS 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Figure 5.12 . Mean concentrations of sediment carbon, nitrogen, and phosporus 

with standard error bars by station for both cruises. . . . . . . . . . . . . . . . . . . . . . . . . . . 102 

Figure 5 .13 . Redox profiles (Eh, mV) with depth in sediment (-1 corresponds to 

overlying water Eh, and 0 to the sediment-water interface Eh value) . . . . . . . 103 

Figure 5 .14. Dry weight (gdw), ash-free dry weight (afdw), and number of 

individuals per core of benthic macroinfauna by station for COMUS 1 

and COMUS 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104 

Figure 5 .15 . Linear correlations of mean N03 fluxes to three indices of input to 

the seabed : A) Bulk sediment input, B) Input of the component with 

the highest correlation, and C) Input of the component that was 

hypothesized to have the highest correlation (nitrogen) . . . . . . . . . . . . . . . . . . . . . . 105 

Figure 5 .16. Linear correlations of mean N02 fluxes to three indices of input to the 

seabed: A) Bulk sediment input, B) Input of the component with the 

highest correlation, and C) Input of the component that was 

hypothesized to have the highest correlation (nitrogen). . . . . . . . . . . . . . . . . . . . . . . 106 

Figure 5.17. Linear regressions of mean NH4 fluxes to three indices of input to the 

seabed: A) Bulk sediment input, B) Input of the component with the 

highest correlation, and C) Input of the component that was 

hypothesized to have the highest correlation (nitrogen). . . . . . . . . . . . . . . . . . . . . . . 107 

Figure 5 .18 . Linear regressions of mean P04 fluxes to three indices of input to the 

seabed : A) Bulk sediment input, B) Input of the component with the 

highest correlation, and C) Input of the component that was 

hypothesized to have the highest correlation (phosphorus) . . . . . . . . . . . . . . . . . . . 108 

xvii 
Figure 5 .19. Linear regressions of mean Si04 fluxes to three indices of input to the 

seabed : A) Bulk sediment input, B) Input of the component with the 

highest correlation, and C) Input of the component that was 

hypothesized to have the highest correlation (chlorophyll a) . . . . . . . . . . . . . . . . . 109 

Figure 5 .20. Linear correlations of nutrient flux to the pore-water gradient (from 

overlying water to 1 cm) for N03, NH4, P04, and Si04 . . . . . . . . . . . . . . . . . . . . . 110 

Figure 5 .21 . Average benthic fluxes (± standard error) for each cruise for A) N03, 

B) P04, C) N02, D) NH4, and E) Si04 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ill 

Figure 5 .22. Linear correlations of influent nutrient concentrations and benthic fluxes 

for N03 and P04 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 

Figure 5 .23. Spatial contours for N03 flux (~tmol m-2 h-1) as determined by 

cluster analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 

Figure 5 .24. Spatial contours for N02 flux (gmol m-2 h-1) as determined by 

cluster analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 

Figure 5 .25 . Spatial contours for NH4 flux (gmol m-2 h-1) as determined by 

cluster analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 

Figure 5 .26. Spatial contours for P04 flux (gmol m-2 h-1) as determined by 

cluster analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 

Figure 5.27 . Spatial contours for Si04 flux (Rmol m-2 h-1) as determined by 

cluster analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 

XVlll 
LIST OF TABLES 

Table 1 .1 . Organic carbon burial in the Mississippi deltaic-shelf environment. . . . . . . . . . 11 

Table 1 .2 . Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) 

at respective depths of cores sampled in the Louisiana Bight. . . . . . . . . . . . . . . . . 12 

Table 1 .3 . Nutrient concentrations and ratios for each survey station in the 

Louisiana Bight study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

Table 1 .4 . Accretion, sedimentation and nutrient accumulation rates for each 

survey station in the Louisiana Bight study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

Table 1 .5 . Rates of sedimentation and nutrient accumulation for each of four 

regions of the Louisiana Bight based on area-weighted analysis . . . . . . . . . . . . . 26 

Table 2.1 . Organic carbon burial in the Mississippi deltaic-shelf environment. . . . . . . . . . 35 

Table 2.2 . Organic carbon burial and remineralization in major river deltaic-shelf 

environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

Table 3.1 . Description of bottom waters at each of the benthic flux stations during 

LASER and COMUS cruises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 

Table 3.2. Summary statistics for benthic fluxes at stations on the Louisiana Bight 

during LASER and COMUS cruises. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

Table 3.3 . Sediment characteristics of the benthic stations during the LASER and 

COMUS cruises. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 

Table 5.1 . Latitude and longitude of sampling stations with bottom CTD conditions 

for COMUS 1 and COMUS 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118 

Table 5.2 . Stepwise regressions of nutrient fluxes to all nutrient and sediment 

parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 

Table 5.3 . Stepwise regressions of nutrient fluxes to only the depositional 

parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 

Table 5.4 . t-Test on fluxes between cruises COMUS 1 and COMU5 3 . . . . . . . . . . . . . . . . . 121 

xix 
ACKNOWLEDGMENTS 

We would like to especially thank Ms. Claudia Munoz for her skills in organizing and 

producing this technical report . This project supported John Bourgeois, Washington 

Cardenas, and Jorge Espinoza that provided much of the technical and analytical work on the 

benthic samples. John and Washington both completed Master of Science degrees at USL 

during this project with significant contributions to understanding nutrient dynamics in 

continental margin ecosystems. Tina Miller-Way directed most of the benthic flux studies 

during LASER 4-6 and provided many of the calculations and statistical analyses of these flux 

rates. Terry Whitledge provided analytical support as nutrient assays with an autoanalyzer 

aboard the RV Pelican during all six of the LASER cruises. The National Science Foundation 

COMUS (Coastal Ocean Margin Uranium Study) program (Brent McKee, Principal 

Investigator) supported two additional cruises to the study region . The authors would like to 

thank J. Greg Booth, Laura Lawton, and Peter Schwarzenski for their help in the field and 

laboratory assistance during these projects. Many thanks also go out to the following for 

various degrees of technical and/or analytical support: Robert Bourgeois, Ronghua Chen, 

John Foret, Russell Garrett, Mike Radford, Patricia Rafferty, Victor Rivera-Monroy, and the 

captain and crew of the R/V Pelican. 

Foci 
Chapter 1 

INVENTORY OF SEDIMENTATION AND NUTRIENT ACCUMULATION IN 

THE SEDIMENTS OF THE LOUISIANA BIGHT 

1 .1 Introduction 

The interrelationships among rates of material cycling through both benthic and watercolumn communities play an important role in the ultimate fate of inorganic nutrients and 

organic matter within coastal ecosystems (Nixon 1981, Glibert 1982, Harrison et al . 1983, 

Kemp and Boynton 1984, Aller et al . 1985). The coupling of benthic and pelagic processes 

contributes to elevated rates of primary productivity in land margin ecosystems by sequestering 

nutrients in the coastal zone (Zeitzschel 1980, Boynton et al . 1982, Fisher et al . 1982, Klump 

and Martens 1983, Boynton and Kemp 1985). Rates of in situ nutrient remineralization may 

be equivalent to allochthonous inputs of inorganic and organic nutrients (Fisher et al . 1982, 

Nixon and Pilson 1983, Pennock 1987). Over short time scales, nitrogen remineralization 

from the sediments and water column may provide a significant proportion of the nitrogen 

required to support primary production . Over longer time periods, the balance between 

sediment deposition and remineralization rates determines the burial and loss of organic 

nutrients from the ecosystem (McKee et al . 1983, DeMaster et al . 1985). Quantifying the time 

and spatial scales of these major sediment processes that influence the availability of nitrogen 

and fate of carbon (e .g . mineralization and burial) are therefore essential to understanding the 

utilization of organic matter in the Louisiana shelf ecosystem (Dagg et al . 1991). 

Nutrient concentration and production sediment trap data indicate that nutrient inputs 

stimulate high rates of primary production and that these nutrients are quickly sequestered into 

organic matter in the water-column. Individual studies indicate that the fate of this organic 

matter appears to vary, at times fluxing to the sediments and undergoing remineralization or 

transport, while at other times appearing to be efficiently grazed and remineralized in the water 

column. Previous studies of deposition suggest that the proximal region of the Mississippi 

River plume is a zone of extreme input of allochthonous particulate material to the seabed . The 

fate of this organic matter is largely unknown in the Louisiana Bight west of the Mississippi 

River delta, as well as for other deltaic continental shelves. The objective of this chapter is to 

determine area weighted storage of sediments, carbon, nitrogen, and phosphorus in this region 

of the Gulf of Mexico. 

These sedimentary processes in the Louisiana Bight give insights into the fate of 

carbon, nitrogen, and phosphorus in river-dominated continental shelves. The burial of 

nutrients in continental margins influenced by strong inputs of materials from the land may be 

significant to the global biogeochemistry of primary nutrients such as carbon, nitrogen, and 

phosphorus (Figure 1 .1). The coupling of land and coastal waters is driven by the transport of 

rivers and includes detrital material that may be both inorganic sediments and nutrients (N in 

Figure 1 .1) or consists of an organic matrix including dissolved and particulate matter (Organic 

C in Figure 1 .1). The inorganic constituent may be deposited directly to the seabed (such as 

inorganic sediments) or enhance the production of organic matter in the shallow photic zone 

(such as nitrogen and phosphorus) in the vicinity of the river plume. At the fronts of river 

plumes in the region of the Louisiana Bight west of the Mississippi River delta there occurs 

high concentrations of phytoplankton biomass with rates of organic production exceeding 200 

gC m-2 yr-1 (Dagg et al . 1991). The deposition of material then is a combination of the river 

and coastal ocean processes that potentially contributes large loading of material to sediments of 
the Louisiana Bight (Figure 1 .1). Chapter 1 is an inventory of a survey of sediments and 

nutrients (carbon, nitrogen and phosphorus) in a study area west of the Mississippi River delta 

in a region referred to as the Louisiana Bight (Figure 1 .2). Together with estimates of 

sedimentation rates in this region, the amounts of nutrient sequestration in shelf sediments in 

this region are estimated. In Chapter 2 the carbon burial estimates of this region are compared 

to loading rates and estimates of carbon remineralization and compared to other river-dominated 

shelf ecosystems. 

1.2 Methods 

Box core samples were collected from 42 stations in Mississippi River plume region of 

the Louisiana Bight (Figure 1 .2) during the LASER 3 cruise in April 1989. The location and 

dates of collection are described in Table 1 .1 . Depths of water column ranged from 10 to 110 

m and nearly 90 km from the mouth of Southwest Pass of the Mississippi River delta. Large 

diameter (16.5 cm) subcores were taken from box cores at each sampling station . Each core 

was carefully extruded and simultaneously subsectioned at precise 0.5-1 .0 cm intervals . Yield 

tracers (2ogPo) were added to the dried core samples, then leached with a combination HN03, 

HCl and HC104 solution. 21oPo is spontaneously deposited onto silver planchets and 21oPb is 

measured via the polonium method (Nittrouer et al. 1979) . Sediment subsamples were assayed 

for total carbon and nitrogen with a LECO Elemental Analyzer. Total phosphorus was 

determined by dissolving the ashed samples with HCl and determining P04 concentrations . 

The deposition and accumulation (burial) of carbon, nitrogen and phosphorus in bottom 

sediments was calculated from the following equation (Hatton et al. 1983): A = Cd x R x D x 

104 (g m-2 yr-1); where A is the rate of nutrient deposition or burial, Cd is the dry mass 

nutrient concentration, D is bulk density, and R is the sedimentation rate (determined by 234Th 

or 2toPb). 

1 .3 Results and Discussion 

The range in mean total carbon concentrations among the 42 stations was from 8.30 to 

18 .13 mg/gdm (Table 1 .2 and 1 .3). Total carbon concentrations were <10 mg/gdm at D10 and 

D20, whereas at stations along B, C, D, E, and F concentrations were >12 mg/gdm (Tables 

1 .2 and 1 .3) . The stations far-field from Southwest Pass including those along the G and H 

transect had concentrations <12 mg/gdm. 

The range in mean total nitrogen concentrations in the stations of the study area was 

0.48 to 2.02 mg/gdm (Tables 1 .2 and 1 .3). Total nitrogen concentrations followed a similar 

spatial pattern as total carbon . The near-field stations along the B,C, D, E, and F stations have 

concentrations > 1 .5 mg/gdm, with the exceptions of D 10 and D20 that have concentrations 

<0.8 mg/gdm (Tables 1 .2 and 1 .3) . Total nitrogen concentrations along the G transect are 

<1 .25 mg/gdm and along the H transect are <1 .0 mg/gdm. The highest concentration of total 

nitrogen was 2 .02 mg/gdm and occurred at C20 (Table 1 .2) . 

The range in total phosphorus among the stations of the study area was less than for 

total carbon and total nitrogen at 0.45 to 0.78 mg/gdm (Tables 1 .2 and 1 .3). Total phosphorus 

concentrations are all >0.50 mg/gdm along all seven transects except for the D10 and D20 

stations that have concentrations <0.5 mg/gdm (Tables 1.2 and 1 .3) . The highest 

concentration of total phosphate was 0.78 mg/gdm and recorded at E50. 

2 
Carbon:nitrogen ratios (atomic) ranged from 8 .2 to 24.3 along the seven transects 

(Tables 1 .2 and 1 .3). Ratios <9 occurred at E20 to E60, whereas the higher ratios of 24.3 and 

21 .1 occurred at H27 and E110, respectively . Nitrogen:phosphorus ratios (atomic) ranged 

from 2 .0 to 6.6 in the Louisiana Bight study area (Tables 1 .2 and 1 .3) . Ratios were <4 at H10 

to H50, with a ratio of only 2.0 at H27. Ratios were generally >5 along both the B and C 

transects . 

A representative example of the distribution of Pb-210 near the mouth of Southwest 

Pass at B50 demonstrates the method of determining accretion rates in the study area (Figure 

1 .3). The concentrations of Pb-210 were ignored in the top 6-cm section of the core due to 

resuspension and high sediment turnover . This disturbance to Pb-210 geochronology in 

surface sediments was evident in the cores along the B and C transects, with much less 

variance in concentrations with depth at stations farther from the mouth of Southwest Pass. 

Low accretion rates at <0.1 cm/yr occurred at only 3 stations including E 10, E20, and 

G27 (Table 1 .4). High rates >2.0 cm/yr occurred at 8 stations including all of the stations 

along the B and C transects, with the exception of C80. The highest rate recorded in the study 

area was 5.1 cm/yr at D50. The spatial rates in sedimentation in the Louisiana Bight study area 

followed the patterns of accretion (Table 1 .4 and Figure 1 .4). Low rates of <0.2 g cm-2 yr-1 

occurred at E10 and E20 ; whereas highest rates >5 .0 g cm-2 yr-1 occurred at B20, B50, C10, 

C20, C50, D50, D65, and D80. The contour of sedimentation rates for the study area (Figure 

1 .4) describe the location of each sampling station and include five zones: >1 .6 g cm-2 yrl, 

0.8-1 .6 g cm-2 yr-1, 0.4-0.8 g cm-2 yr-1, 0.4-0 .2 g cm-2 yr-I and <02 g cm-2 yr-1 . The high 

deposition area is located in the mid-depth regions of the B and C transect near the mouth of 

Southwest Pass. There are two locations of the low deposition zone, one near the shore in the 

northeast region of the study area and the other region far-field from Southwest Pass and 

including most of transect H and the deeper sections of F and G transects. 

Carbon accumulation among the 42 stations ranged from 493 .6 to 1 .4 gC m-2 yr-1 

(Table 1 .4) and followed patterns of sedimentation in the study area (Figure 1 .5) . The 

implications of this pattern of carbon accumulation to fate of carbon in this river-dominated 

shelf region is discussed in Chapter 2. Total nitrogen accumulation rates ranged from 54.4 to 

0.2 gN m-2 yr-1 (Table 1 .4). The pattern of nitrogen accumulation on the shelf includes a zone 

near the mouth of Southwest Pass with rates >30 gN m-2 yr-1 and two zones of lower rates <5 

gN m-2 yr-1 along the northeast corner of the study area and along the western edge of the 

region (Figure 1 .6). There is a sharp change in nitrogen accumulation rates from >30 to <5 

gN m-2 yr-1 at the 50-80 m depths at the C to E transects. Total phosphate accumulation rates 

ranged from 21 .7 to 0.1 gP m-2 yr-i in the study area (Table 1 .4) . The contours of total 

phosphate burial rates are similar to Figure 1 .6 for nitrogen . 

The such strong variation in burial rates of sediment, carbon, nitrogen and phosphorus 

in the plume region of the Louisiana Bight requires some spatially weighed estimate of average 

accumulation . Single cores vary by several orders of magnitude in rates of sedimentation and 

nutrient accumulation within 40 km of Southwest Pass (Figures 1 .4-1 .6 and Table 1 .4). We 

used five sections of the study area based on the magnitude of sedimentation rates to determine 

an area weighted estimate of total nutrient burial in the study area (Table 1 .5). The section with 

highest rates of sedimentation (section A) had an area of 326 km2 (6 % of total) compared to 

minimum rates that had an area of 2383 km2 (44 % of total) . More than 50% of the total study 

area of 5417 km2 included the two lowest ranges in sedimentation rates (Table 1 .5). The 

average sedimentation rate for the study area was 4,442 g m-2 yr-1 . The average carbon 
accumulation rate was 64 gC m-2 yr-1 (Table 1 .5). The area-weighted average nitrogen 

accumulation rate was 7.02 gN m-2 yr-I and total phosphate was 2.89 gP m-2 yr-I (Table 1 .5). 

Coastal and shelf sediments may be considered large reservoirs of deposited nutrients 

that are not regenerated to the water column. The relative magnitude of burial is controlled by 

the quantity and quality of particulate material supplied to bottom sediments (Zeitzschel 1980; 

Klump and Martens 1983). Annual rates of nitrogen accumulation in the plume region of the 

Louisiana Bight range from 0.16 to 57 gN m-2 yr-1 (Figure 3). Rates are highest along the 50-

m depth contour near the mouth of southwest pass of the Mississippi River. The maximum 

rate of nitrogen burial is 465 gmol m-2 h-1 ; equivalent to higher estimates of ammonium 

regeneration in this region of the shelf (Chapter 3). 

4 
Figure 1 .1 . Diagram of the global carbon cycle between the atmosphere, biosphere, and lithosphere. The biosphere is separated 

into three subcompartments including land, coastal ocean, and deep ocean. Linkages between the land and coastal 

ocean include organic and inorganic carbon, as well as nutrients that stimulate photosynthesis in the sea. 
29. 

29 .2of F 

SPOP 

28 . 

site area= 5416 km2 

28. 

28 

uxm ilkm 22km 33km 44km 

-90.40 -90.20 -90.00 -89.80 -89.60 -89.40 -89.20 -89.00 

Figure 1 .2 . Region of the Louisiana Bight that was the study area for estimates of 

sediment and nutrient accumulation . 

6 
Excess 21OPb (dpm g-1) 

10.0 

U 

3 

3 

4 

4 

5., 

Figure 1 .3 . Diagram of Pb-210 distribution with depth in sediments 

of station B50 showing typical geochronology in high 

deposition area. 

7 

0.1 1 .0 
29.60 

29.40 

29.20 

29.00 

00 

28.80 

28.60 

1 

0 11 22 33 44 

km 

d t . *1 -~~~~_ ~ G 

* `~> ~ 

IFFY N 

0.4 

0.4 

N 

o ~00 ~~. 

28.40+-

90.40 

0.2* 

Gulf of ,, . '" 

Mexico ~~ 

90.20 90.00 89.80 89.60 

Sedimentation 

Rate 

(g cm-2 y-1) 

89 .40 89.20 89.00 

Figure 1 .4. Contours of sedimentation rates in the study area of the Louisiana Bight. 
110 

29.60 

29.40 

29.20 

29.00 

28.80 

28 .60 

.s . 

0 km 44 

z 

.

.N

.. -: W v ~~~' .

...h ~ .v . .., . . . .~ .: . :.. : 

., . . . ., 

~. ; :. : . , . . 

>., 

anl":S ., ?,.. . . T 1 ,~ . . 

;`J 3'~y. F ~ ~t 

".; ., .. 

0 . '~ 

" , 300 

4` 

N 

C 

D 150 

E 

S 

28.40 

90.40 90.20 90.00 89.80 89.60 89.40 89 .20 89.00 

Figure 1 .5 . Contours of carbon accumulation rates in the study area of the Louisiana Bight. 

0 . , 

Gulf of ~ 25 ~ C Burial 

Mexico (gC m-2 yr-1) 
29.60 

29.40 

29.20 

29.00 

0 

28.80 

28.60 

N Burial 

(g m-2 y-1) 

28.40 1 

90.40 9020 90.00 89.80 89.60 89.40 89.20 

Figure 1 .6. Contours of nitrogen accumulation rates in the study area of the Louisiana Bight. 

1 

0 44 

km 

z 

.y 

r~ . 

\ 5 * * 5 

5 

r 

o 

Gulf of 

Mexico 

89 .00 
Table 1 .1 . Organic carbon burial in the Mississippi deltaic-shelf environment. 

Core Latitude Longitude Date Time Core Length 

B20 28 55.60 89 28.18 28 April 89 0700 45 

B50 28 50.62 89 29.06 28 April 89 1400 35 

C lo 29 00.00 89 27.60 17 April 89 0457 37 

C20 28 58.00 89 23.50 28 April 89 0725 39 

C50 28 52.50 89 31 .50 28 April 89 1100 35 

C80 28 49.70 89 32.50 18 April 89 1144 32 

D10 29 06.82 89 32.87 17 April 89 0220 30 

D20 29 01.80 89 36.60 19 April 89 1135 30 

D40 28 58.80 89 36 .50 19 April 89 1430 35 

D50 28 56.00 89 35 .60 19 April 89 0709 41 

D65 28 52.70 89 36 .50 16 April 89 1753 25 

D80 28 49.00 89 37 .20 18 April 89 1515 32 

D110 28 39.70 89 36.50 17 April 89 1800 33 

E10 29 12.50 89 43 .50 19 April 89 1936 21 

E20 29 07.24 89 44.63 20 April 89 0709 27 

E30 29 02.50 89 43 .50 17 April 89 0105 35 

E50 28 56.75 89 43 .55 20 April 89 1201 39 

E60 28 51 .75 89 43 .50 16 April 89 1930 33 

E80 28 46.50 89 43 .50 28 April 89 0710 29 

E110 28 38.50 89 43.50 17 April 89 2246 31 

E200 28 32.00 89 41 .10 17 April 89 1935 36 

F10 29 12.90 89 50.00 19 April 89 2055 31 

F20 29 08.25 89 50.00 19 April 89 2148 27 

F35 29 00.50 89 50 .00 16 April 89 2400 33 

F50 28 52 .50 89 50.00 16 April 89 2245 35 

F60 28 48 .60 89 50.00 28 April 89 2056 31 

F80 28 44.25 89 50.00 16 April 89 2100 31 

G10 29 09.40 90 00.00 19 April 89 2355 23 

G20 29 03 .60 90 00.00 20 April 89 0056 32 

G27 28 57.50 90 00.00 19 April 89 0215 30 

G40 28 50.00 90 00.00 18 April 89 2055 17 

G50 28 43.05 90 01 .97 18 April 89 0105 15 

H10 29 02.85 90 11 .08 15 April 89 2005 17 

H20 28 56.60 90 10 .00 18 April 89 2345 19 

H27 28 51 .65 90 10.00 19 April 89 2240 19 

H32 28 45 .50 90 10.00 18 April 89 0344 18 

H50 28 43 .00 90 10.00 18 April 89 0250 16 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 B20 0-1 B20 0-1 17.40 2.25 0.84 9.0 5.9 

L3 B20 2-3 B20 2-3 16.20 1 .80 0.53 10.5 7.6 

L3 B20 4-5 B20 4-5 21 .25 2.20 1 .03 11 .3 4 .7 

L3 B20 9-10 B20 9-10 15.70 1 .80 0.79 10.2 5 .0 

L3 B20 14-15 B20 14-15 15.85 1 .60 0.69 11 .6 5 .2 

L3 B20 19-20 B20 19-20 16.10 1 .90 0.65 9 .9 6.5 

L3 B20 24-25 B20 24-25 16.45 1 .90 0.64 10.1 6.6 

L3 B20 29-30 B20 29-30 17.40 2.00 0.61 10.2 73 

L3 B20 34-35 B20 34-35 15.60 1 .80 0.73 10.1 5 .5 

L3 B20 39-40 B20 39-40 29.30 2.15 0.78 8 .2 6.1 

Mean 18.13 1 .94 0.73 10.10 6.03 

SD 4 .27 0.21 0.14 0.96 0.95 

L3 B50 0-1 B50 0-1 14.00 1 .90 0.82 8 .6 5 .1 

L3 B50 2-3 B50 2-3 13 .35 1 .75 0.71 8 .9 5 .5 

L3 B50 4-5 B50 4-5 13 .30 1 .60 0.67 9.7 53 

L3 B50 5-6 B50 5-6 13 .60 1 .55 0.55 10.2 6.3 

L3 B50 7-8 B50 7-8 12.90 1 .65 0.69 9.1 5.3 

L3 B50 9-10 B50 9-10 18.60 2.10 0.84 10.3 5.6 

L3 B50 14-15 B50 14-15 13 .35 1 .45 0.85 10.7 3.8 

L3 B50 19-20 B50 19-20 13 .35 1 .80 0.75 8.7 5.3 

L3 B50 29-30 B50 29-30 16.65 1 .15 0.62 16.9 4.1 

L3 B50 34-35 B50 34-35 15.70 1 .35 0.66 13.6 4.6 

Mean 14.48 1 .63 0.72 10 .67 5 .07 

SD 1 .88 0.28 0.10 2 .63 0.74 

L3 C10 0-1 C10 0-1 16.85 1 .90 0.78 10.3 5 .4 

L3 CIO 2-3 CIO 2-3 15.00 1 .90 0.76 9 .2 5 .5 

L3 CIO 5-6 CIO 5-6 15.35 1 .95 0.57 9 .2 7 .6 

L3 CIO 8-9 CIO 8-9 16 .20 1 .75 0.57 10.8 6.8 

L3 CIO 9-10 CIO 9-10 16 .20 1 .45 0.62 13 .0 5 .2 

L3 C10 14-15 C10 14-15 16.20 1 .55 0.56 12.2 6.1 

L3 CIO 19-20 CIO 19-20 15 .80 1 .50 0.55 123 6.1 

L3 CIO 24-25 CIO 24-25 14.90 1 .45 0.62 12.0 5 .2 

L3 C10 29-30 CIO 29-30 14.50 1 .50 0.51 113 6.5 

L3 C10 34-35 C10 34-35 14.20 1 .40 0.49 11 .8 6.3 

Mean 15 .52 1 .64 0.60 11 .22 6.07 

SD 0.86 0.22 0.10 1 .31 0.77 

12 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 C20 0-1 C20 0-1 15.75 1 .60 0.87 11 .5 4 .1 

L3 C20 2-3 C20 2-3 15.65 1 .80 0.87 10.1 4 .6 

L3 C20 3-4 C20 3-4 16.55 2.05 0.97 9.4 4.7 

L3 C20 4-5 C20 4-5 16.70 2.45 0.71 8.0 7 .6 

L3 C20 6-7 C20 6-7 17.75 1 .80 0.71 11 .5 5 .6 

L3 C20 9-10 C20 9-10 16.30 2.90 0.64 6.6 10.1 

L3 C20 14-15 C20 14-15 16 .40 1 .60 0.64 12.0 5 .5 

L3 C20 19-20 C20 19-20 15 .70 2.20 0.60 8 .3 8.1 

L3 C20 24-25 C20 24-25 16.35 2.10 0.64 9 .1 7.3 

L3 C20 29-30 C20 29-30 15 .75 1 .90 0.66 9.7 6.3 

L3 C20 34-35 C20 34-35 16.30 1 .80 0.69 10.6 5 .8 

Mean 16.29 2.02 0.73 9.70 6.34 

SD 0.61 0.39 0.12 1 .67 1 .79 

L3 C50 0-1 C50 0-1 17 .00 2.15 0.79 9.2 6.0 

L3 C50 4-5 C50 4-5 16.50 1 .75 0.71 11 .0 5.5 

L3 C50 7-8 C50 7-8 14.85 1 .60 0.67 10.8 5.3 

L3 C50 9-10 C50 9-10 16.75 1 .70 0.55 11 .5 6.8 

L3 C50 14-15 C50 14-15 16.00 1 .55 0.81 12.0 4.3 

L3 C50 19-20 C50 19-20 12.90 1 .25 0.65 12.0 4.3 

L3 C50 24-25 C50 24-25 16.10 1 .60 0.78 11 .7 4.5 

L3 C50 29-30 C50 29-30 14.00 1 .40 0.61 11 .7 5.1 

L3 C50 34-35 C50 34-35 17.50 2.25 0.68 9.1 7 .3 

Mean 15.73 1 .69 0.69 11 .01 5 .45 

SD 1 .52 032 0.09 1 .13 1 .09 

L3 C80 0-1 C80 0-1 15.90 1 .75 0.76 10.6 5 .1 

L3 C80 4-5 C80 4-5 15.95 1 .50 0.69 12.4 4.8 

L3 C80 9-10 C80 9-10 16.15 1 .60 0.68 11 .8 5 .2 

L3 C80 14-15 C80 14-15 15 .70 1 .50 0.56 12.2 5 .9 

L3 C80 19-20 C80 19-20 15 .35 1 .50 0.58 11 .9 5 .7 

L3 C80 24-25 C80 24-25 14.70 1 .30 0.61 13 .2 4.7 

Mean 15 .63 1 .53 0.65 12.02 5.24 

SD 0.53 0.15 0.08 0.85 0.49 

13 
Table 1 .2 . Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 D10 0-1 D10 0-1 9.30 0.85 0.56 12.8 3 .3 

L3 D10 3-4 D10 3-4 9.50 1 .25 0.51 8.9 5 .4 

L3 D 10 4-5 D 10 4-5 8.70 0.70 0.49 14.5 3 .1 

L3 D 10 5-6 D 10 5-6 7.80 0.70 0.47 13.0 3 .3 

L3 D10 6-7 D 10 6-7 8.75 0.80 0.46 12.8 3 .9 

L3 D 10 8-9 D 10 8-9 8.35 0.70 0.48 13.9 3 .2 

L3 D10 9-10 D10 9-10 8.55 0.65 0.42 15.3 3 .4 

L3 D10 14-15 D10 14-15 9.85 1 .05 0.47 10.9 4.9 

L3 D10 19-20 D10 19-20 10.25 0.85 0 .53 14 .1 3 .6 

L3 D 10 24-25 D10 24-25 9.70 1 .05 0.40 10.8 5 .8 

L3 D10 29-30 D10 29-30 10.95 0.95 0.48 13 .4 4.4 

Mean 9 .25 0.87 0.48 12.76 4.04 

SD 0.92 0.19 0.05 1 .89 0.96 

L3 D20 0-1 D20 0-1 9 .45 0.95 0.44 11 .6 4.8 

L3 D20 1-2 D20 1-2 8 .60 0.85 0.49 11 .8 3 .8 

L3 D20 2-3 D20 2-3 8 .55 0.80 0.42 12.5 4.2 

L3 D20 3-4 D20 3-4 8 .35 0.85 0.45 11 .5 4.2 

L3 D20 5-6 D20 5-6 8 .70 0.80 0.43 12.7 4.1 

L3 D20 6-7 D20 6-7 8 .05 0.70 0.45 13 .4 3 .4 

L3 D20 7-8 D20 7-8 8 .00 0.80 0.53 11 .7 3.3 

L3 D20 8-9 D20 8-9 8 .80 0.70 0.42 14.7 3.7 

L3 D20 9-10 D20 9-10 8 .65 0.75 0.42 13 .5 4.0 

L3 D20 14-15 D20 14-15 835 0.40 0.44 24.4 2.0 

L3 D20 19-20 D20 19-20 830 0.35 0.48 27.7 1 .6 

L3 D20 24-25 D20 24-25 8.05 0.60 0.45 15 .7 3.0 

L3 D20 29-30 D20 29-30 8.00 0.45 0.41 20.7 2.4 

Mean 8.45 0.69 0.45 15.51 3 .42 

SD 0.41 0.19 0.03 5.32 0.93 

L3 D40 0-1 D40 0-1 17.50 1 .80 0.79 11 .3 5.1 

L3 D40 1-2 D40 1-2 14.70 1 .65 0.61 10.4 6.0 

L3 D40 2-3 D40 2-3 14.80 1 .70 0.40 10.2 9.4 

L3 D40 3-4 D40 3-4 15.95 2.00 0.75 9.3 5.9 

L3 D40 4-5 D40 4-5 16.30 1 .65 0.62 11 .5 5 .9 

L3 D40 17-18 D40 17-18 14.40 1 .20 0.51 14.0 5 .3 

L3 D40 34-35 D40 34-35 12.15 0.80 0.51 17 .7 3 .5 

Mean 15 .11 1 .54 0.60 12.06 5 .86 

SD 1 .70 0.41 0.14 2.90 1 .79 

14 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 D50 0-1 D50 0-1 

L3 D50 1-2 D50 1-2 13 .15 1 .80 0.44 8 .5 9.1 

L3 D50 2-3 D50 2-3 13 .85 1 .65 0.78 9.8 4.7 

L3 D50 3-4 D50 3-4 16.05 2 .45 0.72 7 .6 7.6 

L3 D50 4-5 D50 4-5 15 .80 1 .75 0.65 10.5 6.0 

L3 D50 5-6 D50 5-6 13 .65 1 .55 0.57 10.3 6.0 

L3 D50 6-7 D50 6-7 1430 1 .70 0.54 9.8 7.0 

L3 D50 7-8 D50 7-8 14.00 1 .65 0.60 9.9 6.1 

L3 D50 8-9 D50 8-9 14.10 1 .60 0.69 10.3 5 .1 

L3 D50 9-10 D50 9-10 15 .80 1 .40 0.62 13 .2 5 .0 

L3 D50 14-15 D50 14-15 14.00 1.30 0.55 12.6 5 .3 

L3 D50 19-20 D50 19-20 14 .75 1 .45 0.68 11 .9 4.7 

L3 D50 24-25 D50 24-25 15 .45 1 .60 0.57 11 .3 6.2 

L3 D50 29-30 D50 29-30 14 .15 1 .35 0.61 12 .2 4.9 

L3 D50 34-35 D50 34-35 14 .10 1 .40 0.66 11 .8 4.7 

L3 D50 35-36 D50 35-36 12 .55 1 .45 0.68 10.1 4.7 

L3 D50 40-41 D50 40-41 13 .70 1 .25 0.69 12.8 4.0 

Mean 14 .34 1 .58 0.63 10.78 5.69 

SD 0.99 0.28 0.08 1 .55 1 .31 

L3 D65 0-1 D65 0-1 15 .15 1 .65 0.84 10.7 4.3 

L3 D65 2-3 D65 2-3 15 .15 1 .65 0.69 10.7 5 .3 

L3 D65 3-4 D65 3-4 13 .75 1 .50 0.57 10.7 5 .8 

L3 D65 4-5 D65 4-5 15 .40 1.55 0.80 11 .6 4.3 

L3 D65 5-6 D65 5-6 13 .65 1 .50 0.68 10.6 4.9 

L3 D65 7-8 D65 7-8 14.20 1 .50 0.63 11 .0 53 

L3 D65 8-9 D65 8-9 14.70 1 .65 0.77 10.4 4.7 

L3 D65 9-10 D65 9-10 15 .75 1 .80 0.73 10.2 5.4 

L3 D65 14-15 D65 14-15 15 .70 1 .55 0.69 11 .8 4.9 

L3 D65 19-20 D65 19-20 15 .25 1 .35 0.69 132 43 

L3 D65 24-25 D65 24-25 14.80 1 .55 0.62 11 .1 5.5 

Mean 14.86 1 .57 0.70 11 .10 4.98 

SD 0.73 0.12 0.08 0.84 0.53 

15 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 D80 1-2 D80 1-2 15 .95 2.20 0.80 8 .5 6.1 

L3 D80 2-3 D80 2-3 15 .80 2.40 0.82 7 .7 6.5 

L3 D80 3-4 D80 3-4 13 .95 1 .50 0.71 10.9 4.7 

L3 D80 4-5 D80 4-5 14.15 1.35 0.73 12.2 4.1 

L3 D80 5-6 D80 5-6 14.40 1.60 0.83 10.5 4.3 

L3 D80 6-7 D80 6-7 15 .75 2 .25 0.73 8 .2 6.8 

L3 D80 7-8 D80 7-8 15 .00 1 .55 0.94 11 .3 3 .7 

L3 D80 8-9 D80 8-9 14.35 1.50 0.89 11 .2 3 .7 

L3 D80 9-10 D80 9-10 15 .00 2.20 0.64 8 .0 7.7 

L3 D80 14-15 D80 14-15 15 .25 1 .85 0.67 9 .6 6.1 

L3 D80 19-20 D80 19-20 15 .25 1 .95 0.69 9 .1 6.3 

L3 D80 24-25 D80 24-25 15 .05 1 .95 0.60 9 .0 7 .2 

L3 D80 29-30 D80 29-30 14.15 1 .75 0.57 9 .4 6.8 

Mean 14.93 1 .85 0.74 9.65 5.68 

SD 0.68 0.34 0.11 1 .44 1 .40 

L3 D110 0-1 D110 0-1 14.45 2.40 0.56 7 .0 9.4 

L3 D110 1-2 D110 1-2 12.40 1.10 0.62 13 .2 3 .9 

L3 DI 10 2-3 D 110 2-3 14.15 1.65 0.55 10.0 6.7 

L3 DI 10 4-5 DI 10 4-5 1330 1.70 0.48 9 .1 7 .9 

L3 DI 10 5-6 D110 5-6 13 .40 1 .45 0.67 10.8 4.8 

L3 DI 10 6-7 D 110 6-7 14.65 1.55 0.56 11 .0 6.2 

L3 DI 10 8-9 D110 8-9 12.45 1 .50 0.78 9 .7 4.3 

L3 D110 9-10 D110 9-10 12.75 1 .25 0.51 11 .9 5 .4 

L3 D110 14-15 D110 14-15 10.10 1 .30 0.94 9 .1 3 .1 

L3 D110 19-20 D110 19-20 11 .55 1 .55 0.52 8 .7 6.6 

L3 D1 10 24-25 DI 10 24-25 10.60 1 .20 0.67 10.3 4.0 

L3 D 110 29-30 DI 10 29-30 19.15 235 0.49 9.5 10.7 

L3 D110 32-33 D110 32-33 58.45 1 .40 0.50 48.7 6.2 

Mean 16.72 1 .57 0.60 13.00 6.08 

SD 12.74 0.40 0.13 10.84 2.24 

16 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 E10 0-1 E10 0-1 12.25 1 .55 0.57 9.2 6.0 

L3 E10 1-2 E10 1-2 10.00 1 .20 0.72 9.7 3 .7 

L3 E 10 2-3 E 10 2-3 8.10 1 .05 0.68 9.0 3 .4 

L3 E10 3-4 E10 3-4 8.75 1 .00 0.48 10.2 4.7 

L3 E 10 4-5 E 10 4-5 9.45 1 .20 0.45 9.2 6.0 

L3 E10 11-12 E10 11-12 9.95 1 .15 0.50 10.1 5.1 

L3 E 10 20-21 E 10 20-21 15 .65 1 .05 0.67 17.4 3 .5 

Mean 10.59 1 .17 0.58 10.69 4.62 

SD 2.58 0.18 0.11 2.99 1 .11 

L3 E20 0-1 E20 0-1 17 .35 2.70 0.64 7 .5 9.3 

L3 E20 1-2 E20 1-2 13 .65 1.95 0.83 8 .2 5 .2 

L3 E20 2-3 E20 2-3 14.00 1 .95 0.74 8 .4 5 .8 

L3 E20 3-4 E20 3-4 15 .10 2.25 0.53 7 .8 9.4 

L3 E20 4-5 E20 4-5 13 .20 1 .85 0.81 8 .3 5 .1 

L3 E20 5-6 E20 5-6 1335 1 .65 0.60 9 .4 6.1 

L3 E20 6-7 E20 6-7 15 .10 1 .90 0.60 9 .3 7 .0 

L3 E20 7-8 E20 7-8 13 .40 1 .55 0.69 10.1 5 .0 

L3 E20 8-9 E20 8-9 14 .65 1 .90 0.54 9 .0 7 .7 

L3 E20 14-15 E20 14-15 11 .25 1 .45 0.48 9 .1 6.7 

L3 E20 19-20 E20 19-20 10.20 1 .35 0.53 8 .8 5 .7 

L3 E20 24-25 E20 24-25 10.25 1 .45 0.46 8 .2 6.9 

Mean 13 .46 1 .83 0.62 8 .68 6.66 

SD 2.09 0.38 0.12 0.74 1 .52 

L3 E30 0-1 E30 0-1 14.95 1 .80 0.84 9 .7 4.7 

L3 E30 2-3 E30 2-3 16.95 2.10 0.69 9 .4 6.7 

L3 E30 3-4 E30 3-4 15 .75 2 .15 0.73 8 .5 6.6 

L3 E30 4-5 E30 4-5 17 .55 1 .85 0.79 11 .1 5 .2 

L3 E30 7-8 E30 7-8 15 .60 1 .90 0.74 9.6 5 .7 

L3 E30 9-10 E30 9-10 15 .25 2.05 0.66 8 .7 6.8 

L3 E30 14-15 E30 14-15 14.40 1 .80 0.80 9.3 5 .0 

L3 E30 19-20 E30 19-20 13 .50 1 .55 0.77 10.2 4.5 

L3 E30 24-25 E30 24-25 13 .10 1 .45 0.69 10.5 4.6 

L3 E30 29-30 E30 29-30 12.20 1 .35 0.70 10.5 4.3 

L3 E30 34-35 E30 34-35 1230 1 .30 0.69 11 .0 4.2 

Mean 14.69 1 .75 0.74 9.87 5.29 

SD 1 .78 0.30 0.06 0.87 0.99 

17 
Table 1 .2 . Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 E50 1-2 E50 1-2 13 .15 1 .85 0.68 8.3 6.0 

L3 E50 2-3 E50 2-3 14.95 1 .80 0.79 9.7 5.0 

L3 E50 3-4 E50 3-4 14.85 1 .70 0.85 10.2 4.4 

L3 E50 4-5 E50 4-5 14.50 1 .80 0.47 9.4 8.4 

L3 E50 7-8 E50 7-8 14.55 2.05 0.83 8.3 5.5 

L3 E50 9-10 E50 9-10 14.75 1 .75 0.74 9.8 5.2 

L3 E50 14-15 E50 14-15 12.70 2.00 0.89 7.4 5.0 

L3 E50 19-20 E50 19-20 13 .80 1 .60 0.77 10.1 4.6 

L3 E50 24-25 E50 24-25 13 .80 1 .65 1 .02 9.8 3 .6 

L3 E50 29-30 E50 29-30 13 .00 1 .45 0.77 10.5 4.2 

L3 E50 34-35 E50 34-35 12.40 1 .45 0.72 10.0 4.4 

Mean 13 .86 1 .74 0.78 9.40 5.12 

SD 0.93 0.20 0.14 0.97 1 .27 

L3 E60 0-1 E60 0-1 14.45 1 .90 0.88 8 .9 4.8 

L3 E60 2-3 E60 2-3 13 .40 1 .85 0.65 8 .5 6.3 

L3 E60 4-5 E60 4-5 12.75 1 .55 0.82 9 .6 4.2 

L3 E60 6-7 E60 6-7 12.80 1 .90 0.80 7 .9 5 .3 

L3 E60 8-9 E60 8-9 12.90 2.05 0.57 7 .3 7 .9 

L3 E60 9-10 E60 9-10 14.30 2.20 1 .02 7 .6 4.8 

L3 E60 14-15 E60 14-15 13 .50 2.15 0.68 7 .3 7 .0 

L3 E60 19-20 E60 19-20 13 .80 2.05 0.41 7 .9 11 .1 

L3 E60 24-25 E60 24-25 12.85 1.80 0.63 8 .3 6.3 

L3 E60 29-30 E60 29-30 12.15 1.50 0.73 9 .5 4.6 

L3 E60 32-33 E60 32-33 11 .75 1.65 0.24 8 .3 15 .2 

Mean 13 .15 1 .87 0.67 8 .27 7.05 

SD 0.84 0.23 0.22 0.78 3 .36 

L3 E80 0-1 E80 0-1 12.90 135 0.51 11 .1 5 .9 

L3 E80 0-1 E80 2-3 14.05 1 .65 0.53 9.9 6.9 

L3 E80 0-1 E80 3-4 14.55 1 .45 0.56 11 .7 5 .7 

L3 E80 0-1 E80 5-6 14.00 1 .85 0.57 8 .8 7.2 

L3 E80 0-1 E80 7-8 14.00 1 .65 0.59 9.9 6.2 

L3 E80 0-1 E80 9-10 14.15 1 .55 0.58 10.7 5 .9 

L3 E80 0-1 E80 14-15 13 .15 1 .35 0.57 11 .4 5 .2 

L3 E80 0-1 E80 19-20 12.10 1 .10 0.52 12.8 4.7 

L3 E80 0-1 E80 24-25 12.00 0.95 0.55 14.7 3 .8 

Mean 12.17 131 0.52 10.19 5.49 

SD 4.08 0.46 0.11 3.70 1 .24 

18 
Table 1 .2 . Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 E110 0-1 E110 0-1 11 .80 0.75 0.49 18.4 3.4 

L3 E110 0-1 E110 2-3 13 .25 0.95 0.58 16.3 3.6 

L3 E110 0-1 E110 3-4 13 .45 1 .00 0.57 15 .7 3.9 

L3 E110 0-1 E110 5-6 13 .75 0.80 0.56 20.1 3.2 

L3 E110 0-1 E110 8-9 14.70 1 .05 0.52 16.3 4.5 

L3 E110 0-1 E110 9-10 15 .15 0.55 0.51 32.1 2.4 

L3 E110 0-1 E110 14-15 13 .30 0.65 0.51 23 .9 2.8 

L3 E110 0-1 E110 19-20 11 .80 0.30 0.49 45 .9 1 .4 

L3 E110 0-1 E110 24-25 11 .10 0.35 0.49 37.0 1 .6 

L3 E110 0-1 E110 29-30 11 .70 0.75 0.46 18.2 3.6 

Mean 13 .00 0.72 0.52 24.38 3.03 

SD 1 .36 0.26 0.04 10.44 1 .00 

L3 F10 F10 0-1 13 .20 0.40 0.69 38.5 13 

L3 F 10 F 10 2-3 14.90 1 .90 0.6 9.1 7.0 

L3 F10 F10 3-4 13 .85 1 .45 0.59 11 .1 5.4 

L3 F10 F10 4-5 14.45 1 .95 0.57 8.6 7.6 

L3 F10 F10 15-16 13 .00 1 .50 0.55 10.1 6.0 

L3 F10 F10 30-31 8.05 0.75 0.5 12.5 3.3 

Mean 12.91 1 .33 0.58 15.01 5.11 

SD 2.49 0.63 0.06 11 .59 2.39 

L3 F20 F20 0-1 16.60 2.00 0.72 9.7 6.2 

L3 F20 F20 2-3 16.40 1 .95 0.63 9.8 6.9 

L3 F20 F20 3-4 16.10 1 .75 0.61 10.7 6.4 

L3 F20 F20 4-5 15 .80 1 .85 0.59 10.0 6.9 

L3 F20 F20 15-16 11 .10 1 .00 0.54 13.0 4.1 

L3 F20 F20 26-27 1030 0.95 0.51 12.6 4.1 

Mean 14.38 1 .58 0.60 10.97 5 .75 

SD 2.88 0.48 0.07 1 .47 1 .31 

19 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 F35 F35 0-1 16 .35 1 .55 0.64 12.3 5 .4 

L3 F35 F35 2-3 14.75 1 .65 0.5 10.4 73 

L3 F35 F35 3-4 15 .00 1 .45 0.54 12.1 5 .9 

L3 F35 F35 4-5 14.35 1.40 0.52 12.0 6.0 

L3 F35 F35 7-8 1530 1.65 0.57 10.8 6.4 

L3 F35 F35 9-10 15 .45 1 .80 0.56 10.0 7.1 

L3 F35 F35 14-15 14.30 1.50 0.52 11 .1 6.4 

L3 F35 F35 19-20 13 .20 1 .15 0.54 13 .4 4.7 

L3 F35 F35 24-25 13 .05 1.15 0.55 13 .2 4.6 

L3 F35 F35 29-30 13 .35 1.20 0.52 13 .0 5 .1 

L3 F35 F35 32-33 12.45 1.05 0.63 13 .8 3 .7 

Mean 14.32 1 .41 0.55 12.01 5 .69 

SD 1 .20 0.25 0.05 1 .28 1 .11 

L3 F50 F50 0-1 12.50 1 .05 0.59 13 .9 3 .9 

L3 F50 F50 2-3 13 .70 1 .50 0.57 10.7 5 .8 

L3 F50 F50 4-5 12.65 1 .40 0.56 10.5 5 .5 

L3 F50 F50 6-7 14.40 1 .85 0.6 9.1 6.8 

L3 F50 F50 9-10 14.50 1 .90 0.59 8 .9 7.1 

L3 F50 F50 14-15 12.45 1 .45 0.51 10.0 6.3 

L3 F50 F50 19-20 11 .85 1 .45 0.53 9.5 6.1 

L3 F50 F50 24-25 10.60 0.95 0.51 13 .0 4.1 

L3 F50 F50 29-30 10.30 0.85 0.51 14.1 3 .7 

L3 F50 F50 34-35 10.20 1 .00 0.52 11 .9 4.3 

Mean 12.32 1 .34 0.55 11 .17 5.37 

SD 1 .59 0.37 0.04 1 .95 1 .27 

L3 F60 F60 0-1 12.40 1 .30 0.54 11 .1 5.3 

L3 F60 F60 2-3 12.40 1 .45 0.55 10.0 5.8 

L3 F60 F60 4-5 12.65 1 .50 0.58 9.8 5.7 

L3 F60 F60 7-8 12.85 1 .50 0.54 10.0 6.2 

L3 F60 F60 9-10 13.65 1 .85 0.57 8.6 7.2 

L3 F60 F60 14-15 11 .65 1 .65 0.52 8.2 7.0 

L3 F60 F60 24-25 11 .05 1 .55 0.55 83 6.2 

L3 F60 F60 29-30 9.70 1 .35 0.58 8.4 5.2 

Mean 12 .04 1 .52 0.55 9 .31 6.08 

SD 1 .22 0.17 0.02 1 .07 0.73 

20 
Table 1 .2. Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 F80 F80 0-1 11 .70 1 .55 0.51 8 .8 6.7 

L3 F80 F80 2-3 12.10 1 .55 9.1 

L3 F80 F80 5-6 12.20 1 .60 0.57 8.9 6.2 

L3 F80 F80 7-8 13 .00 1 .55 0.57 9.8 6.0 

L3 F80 F80 9-10 12.40 1 .50 0.56 9.6 5.9 

L3 F80 F80 14-15 13 .25 1 .70 0.56 9.1 6.7 

L3 F80 F80 19-20 12.45 1 .65 0.52 8.8 7.0 

L3 F80 F80 24-25 11 .00 1 .40 0.55 9.2 5.6 

L3 F80 F80 29-30 10.15 1 .40 0.55 8.5 5.6 

Mean 12.03 1 .54 0.55 9.08 6.24 

SD 0.97 0.10 0.02 0.42 0.53 

L3 G10 G10 0-1 8 .95 0.75 0.65 13 .9 2.6 

L3 G10 G10 2-3 6.00 0.70 0.56 10.0 2.8 

L3 G 10 G 10 4-5 6.90 0.90 0.56 8 .9 3 .6 

L3 G10 G10 11-12 9.70 130 0.5 8 .7 5 .8 

L3 G 10 G 10 22-23 9.95 0.95 0.54 12.2 3 .9 

Mean 8 .30 0.92 0.56 10.76 3 .71 

SD 1 .76 0.24 0.05 2.25 1 .27 

L3 G20 G20 0-1 13 .75 1.75 0.67 9 .2 5 .8 

L3 G20 G20 2-3 11 .35 1 .40 0.6 9 .5 5 .2 

L3 G20 G20 4-5 9 .10 1 .30 0.56 8 .2 5 .1 

L3 G20 G20 7-8 11 .25 1 .35 0.58 9 .7 5 .2 

L3 G20 G20 9-10 10.55 1 .35 0.51 9 .1 5 .9 

L3 G20 G20 14-15 11 .75 1 .20 0.48 11 .4 5 .5 

L3 G20 G20 19-20 12.30 0.95 0.48 15 .1 4.4 

L3 G20 G20 14-25 10.40 1 .15 0.53 10.6 4.8 

L3 G20 G20 29-30 9.50 1 .00 0.53 11 .1 4.2 

Mean 11 .11 1 .27 0.55 10.42 5.11 

SD 1 .43 0.24 0.06 2.04 0.58 

21 
Table 1 .2 . Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 G27 G27 0-1 10.65 1 .30 0.53 9.6 5 .4 

L3 G27 G27 3-4 11 .70 1 .35 0.54 10.1 5 .5 

L3 G27 G27 5-6 10.90 1 .50 0.56 8 .5 5.9 

L3 G27 G27 7-8 10.95 1 .35 0.54 9.5 5 .5 

L3 G27 G27 9-10 11 .00 0.90 0.51 14.3 3 .9 

L3 G27 G27 14-15 9.55 1 .00 0.48 11 .1 4.6 

L3 G27 G27 19-20 9.20 1 .00 0.48 10.7 4.6 

L3 G27 G27 24-25 7 .60 0.60 0.53 14.8 2.5 

L3 G27 G27 29-30 9.90 1 .10 0.53 10.5 4.6 

Mean 10.16 1 .12 0.52 11 .00 4.74 

SD 1 .25 0.28 0.03 2.15 1 .05 

L3 G40 G40 0-1 12.00 1 .45 0.61 9.7 5.3 

L3 G40 G40 2-3 11 .55 1 .10 0.56 12.3 4.3 

L3 G40 G40 4-5 12.15 1 .15 0.55 123 4.6 

L3 G40 G40 7-8 11 .70 1 .25 0.54 10.9 5.1 

L3 G40 G40 9-10 

L3 G40 G40 16-17 12.30 1 .20 0.57 12.0 4.7 

Mean 11 .94 1 .23 0.57 11 .42 4.81 

SD 0.31 0.14 0.03 1 .14 0.38 

L3 G50 G50 0-1 9.75 0.95 0.59 12.0 3.6 

L3 G50 G50 2-3 10.15 1 .05 0.61 11 .3 3.8 

L3 G50 G50 4-5 11 .00 0.95 0.57 13 .5 3.7 

L3 G50 G50 7-8 10.60 1 .00 0.53 12.4 4.2 

L3 G50 G50 9-10 9.55 0.95 0.57 11 .7 3.7 

L3 G50 G50 14-15 10.45 1 .00 0.55 12.2 4.0 

Mean 10.25 0.98 0.57 12.17 3.83 

SD 0.54 0.04 0.03 0.76 0.23 

L3 H2O H2O 6-7 10.85 0.85 0.52 14.9 3.6 

L3 H2O H2O 8-9 10.50 0.65 0.56 18.8 2.6 

L3 H2O H2O 11-12 11 .90 0.75 0.58 18.5 2.9 

L3 H2O H2O 13-14 12.70 0.90 0.61 16.5 3.3 

L3 H2O H2O 15-16 10.95 0.55 0.58 23.2 2.1 

L3 H2O H2O 17-18 11 .65 0.75 0.57 18.1 2.9 

Mean 11 .43 0.74 0.57 1834 2.89 

SD 0.81 0.13 0.03 2 .82 0.53 

22 
Table 1 .2 . Concentrations of total carbon (C), nitrogen (N), and phosphorus (P) at respective 

depths of cores sampled in the Louisiana Bight. 

Sample ID Station Depth Total C Total N Total P C:N N:P 

(cm) mg/gdm mg/gdm mg/gdm atomic atomic 

L3 H27 H27 6-7 9.50 0.40 0.55 27 .7 1 .6 

L3 H27 H27 8-9 10.70 0.50 0.61 25 .0 1 .8 

L3 H27 H27 11-12 9.50 0.75 0.51 14.8 3 .3 

L3 H27 H27 13-14 9.75 0.35 0.5 32.5 1 .6 

L3 H27 H27 15-16 10.70 0.50 0.52 25 .0 2.1 

L3 H27 H27 17-18 9.90 0.40 0.51 28 .9 1 .7 

Mean 10.01 0.48 0.53 25 .63 2.02 

SD 0.56 0.14 0.04 6.01 0.64 

L3 H32 H32 0-1 12.30 1 .00 0.6 14.4 3 .7 

L3 H32 H32 2-3 11 .35 1 .00 0.56 13 .2 4.0 

L3 H32 H32 4-5 11 .20 0.75 0.56 17 .4 3 .0 

L3 H32 H32 7-8 10.60 0.90 0.53 13 .7 3 .8 

L3 H32 H32 9-10 9 .65 1 .00 0.48 11 .3 4.6 

L3 H32 H32 14-15 11 .70 0.85 0.93 16.1 2.0 

L3 H32 H32 17-18 11 .25 0.75 0.9 17 .5 1 .8 

Mean 11 .15 0.89 0.65 14.80 3 .26 

SD 0.84 0.11 0.18 2.31 1 .03 

L3 H50 H50 0-1 11 .45 1 .10 0.87 12 .1 2.8 

L3 H50 H50 2-3 11 .85 0.95 0.89 14.6 2.4 

L3 H50 H50 4-5 11 .40 0.80 0.62 16.6 2.9 

L3 H50 H50 7-8 10.75 0.95 0.54 13 .2 3 .9 

L3 H50 H50 9-10 13 .00 1.15 0.54 13 .2 4.7 

L3 H50 H50 14-15 10.70 0.95 0.56 13 .1 3 .8 

L3 H50 H50 15-16 13 .00 0.85 0.63 17 .8 3 .0 

Mean 11 .74 0.96 0.66 1439 3.34 

SD 0.85 0.13 0.17 1 .58 0.88 

23 
Table 1 .3 . Nutrient concentrations and ratios for each survey station in the Louisiana Bight 

study area. 

Station Depth Latitude Longitude Total C Total N Total P C:N N:P 

ID mg/gdm mg/gdm mg/gdm atomic atomic 

B20 20 28.9267 89.4697 18.13 1 .94 0.73 10.90 5.9 

B50 50 28.8437 89.4843 14.48 1 .63 0.72 1036 5.0 

CIO 10 29.0000 89.4600 15.52 1 .64 0.60 11 .07 6.0 

C20 20 28.9667 89.4800 16.29 2.02 0.73 9.42 6.1 

C50 50 28.8750 89.5250 15.73 1 .69 0.69 10.83 5.4 

C80 80 28.8283 89.5417 15.63 1 .53 0.65 11 .95 5.2 

D 10 10 29.1137 89.5417 9.25 0.87 0.48 12.42 4.0 

D20 20 29.0300 89.6100 8.45 0.69 0.45 14.24 3.4 

D40 40 28.9800 89.6083 15.11 1 .54 0.60 11 .43 5.7 

D50 50 28.9333 89.5933 14.34 1 .58 0.63 10.59 5.6 

D65 65 28.8783 89.6083 14.86 1 .57 0.70 11 .06 5.0 

D80 80 28.8167 89.6200 14.93 1 .85 0.74 9.41 5.5 

D110 110 28.6617 89.6083 16.72 1 .57 0.60 12.43 5.8 

E10 10 29.2083 89.7250 10.59 1 .17 0.58 10.55 4.5 

E20 20 29.1207 89.7438 13.46 1 .83 0.62 8 .58 6.6 

E30 30 29.0417 89.7250 14.69 1 .75 0.74 9.77 5.3 

E50 50 28.9458 89.7258 13.86 1 .74 0.78 9.31 4.9 

E60 60 28.8625 89.7250 13.15 1 .87 0.67 8 .19 6.2 

E80 80 28.7750 89.7250 12.17 1 .31 0.52 10.84 5.6 

E110 110 28.6417 89.7250 13.00 0.72 0.52 21 .06 3.1 

F10 10 29.2150 89.8333 12.91 133 0.58 1132 5.1 

F20 20 29.1375 89.8333 14.38 1 .58 0.60 10.62 5.8 

F35 35 29.0083 89.8333 14.32 1 .41 0.55 11 .85 5.7 

F50 50 28.8750 89.8333 12.32 134 0.55 10.73 5.4 

F60 60 28.8100 89.8333 12.04 1 .52 0.55 9.24 6.1 

F80 80 28.7375 89.8333 12.03 1 .54 0.55 9.11 6.2 

G10 10 29.1567 90.0000 8.30 0.92 0.56 10.53 3.6 

G20 20 29.0600 90.0000 11 .11 1 .27 0.55 10.21 5.1 

G27 27 28.9583 90.0000 10.16 1 .12 0.52 10.58 4.8 

G40 40 28.8333 90.0000 11 .94 1 .23 0.57 11 .33 4.8 

G50 50 28.7175 90.0328 10.25 0.98 0.57 12.20 3.8 

H10 10 29.0475 90.1847 0.58 

H20 20 28.9433 90.1667 11 .43 0.74 0.57 18.02 2.9 

H27 27 28.8608 90.1667 10.01 0.48 0.53 24.33 2.0 

H32 32 28.7583 90.1667 11 .15 0.89 0.65 14.62 3.0 

H50 50 28.7167 90.1667 11 .74 0.96 0.66 14.27 3.2 

24 
Table 1 .4 . Accretion, sedimentation and nutrient accumulation rates for each survey station in the 

Louisiana Bight study area. 

Station ID Depth Latitude Longitude Accretion Sedimentation Rates 

Rate Sediments Carbon Nitrogen Phosphate 

cm/yr g/cm2 yr g m-2 yr-1 g m-2 yr-1 g m-2 yr-1 

B20 20 28 .9267 89.4697 2.60 1 .755 318.09 34.05 12.81 

B50 50 28 .8437 89.4843 2.20 1 .485 215.03 24 .21 10.69 

CIO 10 29.0000 89.4600 2.20 1 .485 230.47 24 .28 8 .91 

C20 20 28 .9667 89.4800 2.60 1 .755 285.91 35.42 12.81 

C50 50 28.8750 89.5250 2.20 1 .485 233.64 25 .16 10.25 

C80 80 28 .8283 89.5417 1 .40 0.945 147.66 14 .41 6.14 

D 10 10 29.1137 89.5417 

D20 20 29.0300 89.6100 0.30 0.203 17.11 1 .40 0.91 

D40 40 28 .9800 89 .6083 0.80 0.540 81 .62 8 .33 3 .24 

D50 50 28 .9333 89 .5933 5 .10 3.443 493.57 54.39 21 .69 

D65 65 28 .8783 89 .6083 3 .70 2.498 371 .22 39.17 17.48 

D80 80 28 .8167 89 .6200 2.90 1 .958 292.19 36.21 14.49 

D110 110 28 .6617 89 .6083 0.30 0.203 33.86 3.18 1 .22 

E 10 10 29 .2083 89 .7250 0.02 0.014 1 .43 0.16 0.08 

E20 20 29 .1207 89 .7438 0.07 0.047 636 0.86 0.29 

E30 30 29 .0417 89 .7250 0.62 0.419 61 .46 7.34 3 .08 

E50 50 28 .9458 89 .7258 0.75 0.506 70.16 8.79 3 .93 

E60 60 28 .8625 89 .7250 0.72 0.486 63.91 9.10 3 .27 

E80 80 28 .7750 89 .7250 0.60 0.405 49.29 531 2.11 

E 110 110 28 .6417 89 .7250 0.15 0.101 13.16 0.73 0.53 

F10 10 29 .2150 89 .8333 0.31 0.209 27.01 2.78 1 .21 

F20 20 29 .1375 89 .8333 0.23 0.155 22.32 2.45 0.93 

F35 35 29 .0083 89 .8333 0.76 0.513 73.46 7.23 2.82 

F50 50 28 .8750 89 .8333 0.33 0.223 27.44 2.98 1 .23 

F60 60 28 .8100 89 .8333 0.45 0.304 36.57 4.62 1 .67 

F80 80 28 .7375 89 .8333 0.24 0.162 19.49 2.49 0.89 

G10 10 29.1567 90.0000 0.00 0.00 0.00 

G20 20 29.0600 90.0000 0.74 0.500 55.49 6 .34 2.75 

G27 27 28 .9583 90.0000 0.10 0.068 6.86 0.76 0.35 

G40 40 28.8333 90.0000 0.23 0.155 18.54 1 .91 0.88 

G50 50 28.7175 90.0328 0.25 0.169 17.30 1 .65 0.96 

H 10 10 29.0475 90.1847 0.23 0.155 0.00 0.00 0.90 

H20 20 28.9433 90.1667 0.16 0.108 12 .34 0.80 0.62 

H27 27 28.8608 90.1667 0.11 0.074 7 .43 0.36 0.39 

H32 32 28.7583 90.1667 0.00 0.00 0.00 

H50 50 28.7167 90.1667 0.21 0.142 16.64 1 .36 0.94 

25 
Table 1 .5 . Rates of sedimentation and nutrient accumulation for each of four regions of the Louisiana Bight based 

on area-weighted analysis. 

N 

Sediment Burial Carbon Accumulation 

Section Study Area Unit Area Study Area Range Average Area Study Area 

(km2) Percent (t km-2 yr-1) (dyr) (t km-2 yr-1) (km2) (t/yr) 

A 326 6.00 21,700 7,074,200 >300 394 217 85,498 

B 487 9.00 13,500 6,574,500 150-300 252 379 95,508 

C 975 18 .00 4,800 4,680,000 50-150 75 1300 97,500 

D 1246 23 .00 2,500 3,115,000 25-50 31 1192 36,952 

E 2383 44.00 1,100 2,621,300 <25 13 2329 30,277 

SUM 5,417 100 24,065,000 5417 345,735 

Burial 4,442 64 

(g m-2 yr-1) 
Table 1 .5 . Rates of sedimentation and nutrient accumulation for each of four regions of the Louisiana Bight based 

on area-weighted analysis. 

N 

J 

Nitrogen Accumulation Phosphorus Accumulation 

Section Range Average Area Study Area Range Average Area Study Area 

(t km-2 yr-1) (km2) (dYr) (t km-2 yr-1) (km2) (dYr) 

A >30 42.5 217 9,223 >15 17.33 217 3,761 

B 15-30 29.1 379 11,029 8-15 11 .43 379 4,332 

C 5-15 8.4 1300 10,920 4-8 3.42 1300 4,446 

D 2-5 3.4 1192 4,053 1-4 1 .33 1192 1,585 

E <2 1 .2 2329 2,795 <1 0.65 2329 1,514 

SUM 5417 38,019 5417 15,638 

Burial 7 .02 2.89 

(g m-2 yr-1) 
Chapter 2 

BURIAL AND REMINERALIZATION OF TERRIGENOUS ORGANIC 

CARBON IN DELTAIC-SHELF ENVIRONMENTS 

2.1 Abstract 

Marine sediments are the predominant long-term reservoir of carbon on earth, and as 

much as 80°Io of modern carbon burial has been attributed to deltaic-shelf environments (Berner 

1982, Romankevich 1984). However, little is known about the fate of particulate organic 

carbon supplied by rivers, and the relative contribution of this terrigenous input to carbon 

burial on continental shelves (Deuser 1988, Berner 1989, Smith and Hollibaugh 1993). Here 

we report seasonal and decadal rates of organic carbon storage in Mississippi delta-shelf 

sediments, and calculate the proportion of this carbon sink that is of terrestrial origin . Results 

reveal that approximately 30°Io of the particulate organic carbon supplied by the Mississippi 

River to the adjacent shelf is remineralized within 4 months and an additional 40% is 

remineralized on a decadal time scale. Based on these results, combined with similar 

information from the Amazon, Changjiang and Huanghe Rivers, we conclude that only 30% of 

the particulate organic carbon supplied to the ocean by rivers is buried . Our area-weighted 

estimate of organic carbon burial in deltaic-shelf sediments is 75 gC m-2 yr-I with less than 

50% of the organic carbon buried being of terrestrial origin. The resulting global estimate for 

terrigenous POC burial is 0.05 PgC yr-1, representing less than half the value currently used to 

constrain global C02 budgets (Sarnuento and Sundquist 1992). 

2.2 Introduction 

Global geochemical budgets of organic carbon are based on the steady-state assumption 

that the flux of C02 out of the ocean is equal to the net organic carbon flux into the oceans from 

rivers (terrigenous organic carbon input minus burial in marine sediments) (Berner 1989, 

Sarmiento and Sundquist 1992) . Thus, the magnitude and nature of carbon storage in shelf 

sediments is an important constraint on the global COZ budget . In a study utilizing molecular 

assays, Ittekkot (1988) estimated that about 65% of riverine POC (0.15 PgC yr-1) is refractory 

and therefore may potentially be buried in oceanic environments . Berner (1982) estimated that 

0.10±0.05 Pg yr-1 of organic carbon is buried in deltaic-shelf environments, assuming that 

17.3 Pg of sediment with a mean organic carbon content of 0.75% is buried annually 

accompanied by a 20% loss due to decomposition subsequent to burial . Berner's estimate of 

bulk organic carbon burial is comparable to the value predicted by Ittekkot (1988) for refractory 

riverine POC components . However, studies using molecular or isotopic indicators report that 

the bulk of organic carbon in shelf sediments is autochthonous (in situ coastal production) in 

origin, even in the proximity of river deltas (Gearing et al . 1977, Hedges and Mann 1979) . It 

therefore appears that previous studies have overestimated the preservation potential of 

allochthonous (riverine input) POC and/or underestimated the rate of total organic carbon burial 

(allochthonous plus autochthonous) in continental shelf environments. In this study, we utilize 

direct seabed measurements in deltaic-shelf environments to determine area-weighted organic 

carbon burial rates and to quantify the diagenetic loss of terrigenous organic carbon within the 

seabed on seasonal and decadal time scales . 

29 
2.3 Methods 

In April 1989, we collected 37 undisturbed box cores from a 5400 km2 shelf area 

adjacent to the Mississippi River (within 100 km of the river mouth) . Subcores from each 

station were carefully extruded, subsampled at 1 cm intervals and analyzed for organic carbon 

and 21oPb (to determine sediment burial rates) . One site (B50) located approximately 5 km 

from the dominant river mouth, Southwest Pass (Figure 2 .1), was reoccupied four times over 

a period of 30 months (April 1988, April 1989, April 1990, October 1990) to examine the 

seasonal fate of riverine material. Cores from B50 were subsampled and analyzed for organic 

carbon and 234Th (to determine sediment deposition rates) . Sediment deposition rates 

determined using 234Th geochronologies integrate over the four-month period preceding core 

collection (Figure 2.2) (McKee et al . 1983, Dukat and Kuehl 1995) . Organic carbon burial 

rates were derived by multiplying the dry bulk sediment accumulation rate (g m-2 yr-I) at each 

of the 37 sites, determined using 21oPb geochronologies by the mean percent organic carbon 

content (dry weight) of sediments in the zone of net accumulation within each core. Station 

B50 (site of the seasonal organic carbon storage study presented in Figure 2 .2) is denoted by 

an asterisk in Figure 2.1 . Rates of seasonal organic carbon storage were determined by 

multiplying the dry bulk sediment deposition rate for each study period, determined using 

234Th, by the mean percent (dry weight) organic carbon content of sediments in the zone of 

recent deposition within each core. The riverine organic carbon supplied to B50 during each 

time period was determined as the percent (dry weight) organic carbon content of suspended 

river material (as measured during each time period) multiplied by the mass dry bulk sediment 

deposited at B50 during the four months preceding core collection (as determined by 234Th 

deposition rates) . The percent loss of riverine organic carbon to remineralization (determined 

as the difference between organic supply and storage for each seasonal period) was 30, 31, 28, 

and 34°Io for January-April 1988, January-April 1989, January-April 1990, and July-October 

1990, respectively . 

2.4 Results and Discussion 

During this study, sediment deposition rates at B50 were proportional to Mississippi 

River discharge, ranging from 69 to 436 g m-2 d-1 . The corresponding supply of terrestrial 

POC to this site during each sampling period ranged from 1 .6 to 8.7 gC m-2 d-1, based on 

sediment deposition rates and riverine POC contents (range : 1 .97 to 2.41 %; Figure 2.2). The 

organic carbon content of sediments stored seasonally at B50 (sediments residing in the seabed 

for less than four months based on excess 234Th) ranged from 1 .36 to 1 .61 % and the resulting 

organic carbon deposition rates (1 .1 to 6.3 gC m-2 d-1) were 28-34°Io less than the riverine 

carbon supply (Figure 2.2) . Thus, approximately 30% of the terrigenous carbon was 

remineralized within four months, less than some estimates of carbon decomposition proximal 

to river mouths (Spitzy and Ittekkot 1991), but consistent with studies that have identified the 

labile fraction of riverine sediments based on molecular assays (Ittekkot 1988, Ittekkot et al . 

1985) . 

The decadal geochronologies constructed using 21 oPb integrate over the 10-120 years 

prior to core collection, depending on core lengths (-50 cm) and sediment burial rates (McKee 

et al . 1983, DeMaster et al . 1985, Kuehl et al . 1986). Sediment burial rates decrease within 

100 km of the river mouth, from 3 .44 g cm-2 yr-1 (5.1 cm yr-1) to 0.01 g cm-2 yr-1 (0.02 cm yrI) . A total of 22 x 1012 grams of sediment accumulate annually in the study area, about 50% of 

the annual sediment discharge via Southwest Pass (45 x 1012 grams) (Milliman and Meade 

1983). We use this determination of sediment burial to constrain our estimate of the 

terrigenous organic carbon flux (from the Mississippi River) reaching our study area. The 

30 
mean organic carbon content of buried sediments in the study area is 1 .3°Io (range : 0.6 to 

2.1 %). We have divided the study area into 5 regions based on organic carbon burial (Table 

2.1, Figure 2.1) with rates ranging from >300 gC m-2 yr-1 (maximum: 494 gC m-2 yr-1) to <25 

gC m-2 yr-1 (minimum: 1 gC m-2 yr-I). The region with rates >300 gC m-2 yr-1 is restricted to 

<15 km from the mouth of Southwest Pass and makes up about 4% of the study area, 

compared to 43% of the shelf study area with rates <25 gC m-2 yr-1 (Table 2 .1). Deltaic-shelf 

environments typically exhibit strong spatial gradients for organic carbon burial rates (Figure 

2.2), making assessments of regional carbon burial difficult based on single measurements. 

Therefore, an area-weighted approach was used here (37 rate values from a 5400 km2 area), 

resulting in an organic carbon burial rate of 64 gC m-2 yr-1 for the entire area. 

A number of studies using 13C and biomarkers have partitioned the carbon content of 

shelf sediments, between autochthonous and allochthonous origin, at locations throughout our 

study area (Gearing et al . 1977, Shultz and Calder 1976, Thayer et al . 1983, Eadie et al . 1994, 

Hedges and Parker 1976). We have estimated the percentage of the buried organic carbon that 

is of allochthonous origin within each of the 5 regions, based on these studies and a two 

component mixing model (Newman et al. 1973) (Table 2.1) . Results from this model, using 

stable carbon isotopes, have been shown to agree well with organic biomarker analyses (lipids, 

syringyl and vanillyl compounds) from conjoint cores within the study area (Eadie et al. 1994) . 

In the region of high carbon burial rates, an average of 70% of the POC is of terrestrial origin, 

in contrast to the region of minimum rates where only 5% of the POC is from the river. We 

estimate that only 40% of the total organic carbon preserved in the study area is allochthonous 

in origin, based on area-weighted estimates of percent terrigenous carbon and organic carbon 

burial rates . Thus burial of riverine POC in the study area is 26 gC m-2 yr-1 . If we assume that 

the 22 x 102 g yr-1 of riverine sediment buries in the study area had an initial organic carbon 

content of 2 ± 0.2% (based on historical information, Trefry and Presley 1982, Malcolm and 

Durum 1976), then the river input of POC to the study area over the past century has been 4.4 

x 1011 gC yr-1 or 81 gC m-2 yr-1 . Therefore, 67°Io of the terrigenous POC originally associated 

with the sediments buried in the study area has been remineralized over the past century, with 

the remaining 33% being buried. 

Similarly, data from other major deltaic-shelf environments (sedimentation rate, 13C 

and organic carbon content) can be used to estimate rates of organic carbon burial and the 

relative preservation of riverine carbon (Table 2.2) . Together with the Mississippi, the 

Amazon, Changjiang and Huanghe Rivers account for 15% of global riverine POC supply and 

20°Io of the bulk sediment supply to the oceans. The Amazon, Changjiang and Huanghe 

studies cited include examinations of both proximal and distal portions of river dispersal 

systems (DeMaster et al . 1985, Kuehl et al . 1986, Showers and Angle 1986, Alexander et al. 

1991, Cauwet and Mackenzie 1993, Aller et al . 1985, Tan et al . 1991, Zhang et al . 1992, 

Niino and Emery 1961). In these environments, organic carbon burial rate is 75 gC m-2 yr-1 

(range : 39 to 120 gC m-2 yr-1) and the buried organic carbon composition is 54% 

autochthonous and 46% allochthonous (Table 2.2) . Similar results were noted for shelf 

sediments adjacent to the Columbia River (burial rate : 36 gC m-2 yr-1 ; -50% allochthonous) 

(Carpenter 1987, Prahl and Carpenter 1984). Burial rates in these deltaic-shelf environments 

are considerably higher than those reports for non-deltaic shelves (5-7 gC m-2 yr-1) 

(Christensen 1994, Wollast 1991, Anderson et al . 1994), for highly productive oceanic 

environments (0.3 gC m-2 yr-1) (Berner 1982), and for the open ocean (0.06 gC m-2 yr-1) 

(Stein 1991). 

Bulk sediment accumulation in the four deltaic-shelf areas examined (0.1 Pg yr-1 ; 9.5 x 

lOlo m2) represents 7°Io of the global sediment discharge to the oceans . Based on this 

31 
percentage and a global continental shelf area of 30 x 106 km2, the area of deltaic shelves 

influenced by terrigenous input is estimated to be 1 .4 x 106 km2 . Using an average burial rate 

of 75 gC m-2 yr-1, deltaic shelves are a global sink for an estimated 0.11 PgC yr-1 of organic 

carbon. This agrees well with Berner's estimate for deltaic shelves (Berner 1982, 1989). 

However, two assumptions have been implicit when applying Berner's shelf carbon burial 

estimate to global carbon budgets : (1) all the organic carbon buried in deltaic-shelf sediments is 

terrigenous ; and (2) the bulk of organic carbon buried in continental shelves accumulate in 

deltaic-shelf environments. Our data indicate that only 0.05 PgC yr-1 (less than half) of the 

organic carbon buried in deltaic-shelf environments is of terrestrial origin, inconsistent with the 

first assumption. Furthermore, data from recent studies (Christensen 1994, Wollast 1991, 

Anderson et al . 1994) indicate significant burial of organic carbon in non-deltaic shelf 

environments. This is inconsistent with the second assumption and emphasizes the need for 

more global-scale information regarding carbon burial in non-deltaic shelf sediments. 

Based on our data, autochthonous carbon burial rates average 39 gC m-z yr-1 in deltaicshelf environments, resulting in a global estimate of 0.06 PgC yr-1 . This represents about 20% 

of the average in situ production on continental shelves (200 gC m-2 yr-1) . Therefore, only 

80% of in situ production is remineralized within deltaic-shelf systems, compared to 97-98% 

loss in shelf environments which are not influenced by rivers (Christensen 1994, Wollast 

1991). The enhanced preservation potential of autochthonous POC in deltaic-shelf 

environments, relative to non-deltaic shelves, is consistent with previous observations of more 

efficient organic carbon burial in environments with higher rates of total sediment burial 

(Canfield 1989, Heinrichs and Reeburgh 1987). Some of the COZ released from terrigenous 

organic carbon remineralization in shelf environments may be fixed by in situ production in the 

water column and later buried in shelf sediments with an "autochthonous" carbon signal . 

Therefore, more information concerning carbon burial on continental shelves is needed, 

especially direct measurements of organic carbon content coupled with bulk sediment burial 

rates and multiple tracer techniques that utilize isotopes and molecular biomarkers (Berner 

1989, Prahl et al . 1994). 

The mean burial rate for terrigenous organic carbon in deltaic-shelf environments is 36 

gC m-2 yr-I or about 31 % of allochthonous supply (Table 2.2) . The high percentage of riverine 

organic material remineralized (-2 times higher than previous estimates) (Wollast 1991) may be 

due to a co-oxidation of refractory allochthonous organics with more labile autochthonous 

material, as has been hypothesized for the Amazon Shelf (Aller et al . in press), or a result of 

more efficient remineralization due to the dominant role of metal oxides as oxidants of organic 

matter in these environments . 

Our revised estimate of terrigenous organic carbon burial can be used to help constrain 

oceanic C02 flux models . Sarmiento and Sundquist (1992) revised previous global carbon 

estimates (Houghton et al . 1990, Tans et al . 1990) by using the net oceanic flux of riverine 

organic carbon to constrain estimates of air-sea COZ exchange. Based on our analysis, the 

average burial rate of riverine organic carbon in deltaic-shelf sediments (0.05 PgC yr-1) is less 

than half the value used by Sarnuento and Sundquist (1992) (citing ref. 1 and 2). The steadystate assumption of their model is that the net oceanic carbon flux from rivers (terrigenous input 

minus burial) is balanced by a flux of C02 out of the ocean. Our results indicate that this net 

oceanic flux of organic carbon from rivers is significantly larger than current estimates because 

terrestrial carbon is more poorly preserved (more completely remineralized) in shelf sediments 

than previously thought. 

32 
29.60 

l 

0 km 44 

w 

w 

29.40 

29.20 

29 .00 

28.80 

28.60 

28.40 

90.40 90.20 90.00 89.80 89.60 89.40 89 .20 89.00 

Figure 2.1 . Spatial distribution of organic carbon burial rates (gC m-2 yrl) for the 

5400 km2 study site in the Mississippi deltaic-shelf environment. 

.~.,.::. . j: 

.>t . ~. K

:

. ..~. xs ..~: p \ ~ 

s 

; 

.: . V r :$. . 1 h"9? 0 ::<x:> :: 

`' f. Mr7~ 

" . p .r 

s 0 

0~` 1 C 

D 150 

E 

Sp 

Gulf of ~ 25 C Burial 

Mexico (gC m_2 yr_1~ 
10 

N 

7 

a 6 

0 

U 4 

v . ,.., 

Q 

3 

O 

2 

0 1 

0 

Jan - April Jan - April Jan - April July - Oct 

1988 1989 1990 1990 

'C 

N 

U 

on `r 

bA 

~. 0 

0 

U 

U 

. ., 

ono 

O 

Figure 2.2 . Temporal relationship between the organic carbon seasonally stored 

at station B50 (a site proximal to the mouth of the Mississippi River; 

see Figure 2.1 for location ) and the particulate riverine organic 

carbon supplied during the same period . 

34 
Table 2.1 . Organic carbon burial in the Mississippi deltaic-shelf environment. 

w 

Region Boundaries Mean Area Carbon % Terri,genous Orb. C Burial 

Value Burial tPOC* 

(gC m-2 yr-1) (gC m-2 yr-1) (kM2) (gC m-2 yr-1) (gC Yr-i x lOlo) (gC m-2 yr-1) 

A >300 394 217 8 .5 70 6.0 276 

B 150-300 252 379 9.5 50 4.8 127 

C 50-150 75 1300 9.8 30 2.9 22 

D 25-50 31 1192 3 .7 10 0.4 3 

E <25 13 2329 3 .1 5 0.2 1 

Mean 64 5417 34 .6 41 14 .3 26 

* Estimated percentage of organic carbon buried in each region that is of terrestrial origin, based on samples 

from the study area cited in the literature (Gearing et al . 1977, Shultz and Calder 1976, Thayer et al. 1983, Eadie 

et al . 1994, Hedges and Parker 1976, Newman et al . 1973). 
Table 2.2 . Organic carbon burial and remineralization in major river deltaic-shelf environments . 

Burial Remineralization 

River Riverine Total % Alloch Autoch Alloch % References 

Supply Burial Alloch thonous thonous thonous Alloch 

(gC m-2 yr-1) (gC m-2 yr-1) thonous ---(gC m-2 yr-1)--- (gC m-2 yr-1) thonous 

Mississippi 81 64 40 26 38 55 68 This Study 

Amazon 125 75 70 52 23 73 58 Kuehl et al . 1986 

Aller et al. in press 

Changjiang 202 120 45 54 66 148 73 DeMaster et al . 1985, 

Heinrichs and Reeburgh 1987 

Canfield 1989, Prahl et al . 1994 

Huanghe 49 39 30 12 27 37 76 Alexander et al . 1991, 

Heinrichs and Reeburgh 1987 

Mean 75 46 36 39 78 69 
Chapter 3 

INVENTORY OF BENTHIC FLUXES IN THE LOUISIANA BIGHT 

3.1 Introduction 

The coupling of benthic and pelagic processes apparently accounts for the elevated rates 

of primary productivity that are associated with land margin ecosystems. This is partially 

explained by the high rates of in situ nutrient remineralization that may be several orders of 

magnitude greater than allochthonous inputs . The annual contribution of nitrogen from 

sediments to primary production in shallow marine systems was from 28 to 35°Io in North 

Carolina (Fisher et al . 1982), from 15 to 27% during the summer in upper Chesapeake Bay 

(Boynton and Kemp 1985), 65°Io in the Patuxent River estuary (Boynton et al . 1980), 35% in 

the Potomac River estuary (Callender and Hammond 1982), and 25°Io in Narragansett Bay 

(Nixon 1981). Most conceptual models for shallow marine systems predict that seasonal 

deposition of organic matter during spring, in concert with high rates of both water column and 

benthic remineralization during summer months, provide the predominant mechanisms for 

sustaining peak rates of primary productivity during the summer in land margin ecosystems 

(Boynton et al . 1982). 

The contribution of sediment remineralization in shelf ecosystems is less clear. In the 

North Sea, 75% of the nitrogen requirements are met by benthic regeneration (Billen 1978), 

and in the Kiel Bight this flux may provide 100% of the phytoplankton demand for nutrients 

(Zeitzschel 1980). Wa1sh et al. (1978) estimate that sediments contribute 38% of the total 

nitrogen regeneration in the Mid Atlantic Bight, equivalent to the regeneration by zooplankton 

in the water column. Rowe et al. (1975) originally suggested that sediments in shallow shelf 

ecosystems, such as in the New York Bight, may provide "more than the total required 

nitrogen" based on an ammonium flux of 144 gmol m-2 h-1 . In a later analysis, regenerated 

nitrogen in the Mid Atlantic Bight during the summer of 1980 provided 50-80 °Io of nitrogen 

productivity, yet the contribution by sediments was only 7 % of the total nitrogen regeneration 

(Harrison et al. 1983) . This lower estimate of benthic regeneration was based on an 

ammonium flux of only 15.4 ug at m-2 h-1, although rates of benthic regeneration in Table VI 

in that article average 75 gmol m-2 h-1 . Other direct measurements of benthic ammonium flux 

in Christiansen Basin, and stations in the New Jersey and Long Island shelf regions average 71 

and 105 gmol m-2 h-1, respectively (Rowe 1978) . Ammonium flux from sediments in the 

shelf off the mouth of the Changjiang River, East China Sea, ranged from 108 to 142 gmol m2 h-1 (negative indicates uptake by sediments), with an average of only 29 gmol m-2 h-1 (Aller 

et al. 1985). The variation observed in rates of benthic nitrogen regeneration in shelf 

ecosystems depends on the further oxidation of ammonium to nitrate . It appears that benthic 

regeneration of nitrogen in most shelf ecosystems is about half that amount observed in 

shallow land margin systems such as estuaries . 

The LASER and COMUS cruises provided several opportunities to study benthic 

fluxes in the plume region of the Louisiana Bight (Figure 1 .2). Chapter 3 provides an 

inventory of all these flux studies with information on logistics and ambient sediment and 

bottom water conditions. This information should provide necessary information on sediment 

boundary conditions for modelling efforts of this region of the Louisiana-Texas continental 

shelf. In addition, this chapter describes the spatial patterns of benthic fluxes as described 

from the first three LASER cruises . This spatial coverage is more thoroughly described 

37 
relative to specific sediment characteristics in Chapter 5. The temporal pattern of benthic fluxes 

at selected stations in the study area is described in Chapter 4. The hypothesis of these studies 

is that benthic nutrient regeneration in river-dominated continental shelf is spatially and 

temporally controlled by input of terrigenous materials . The quantity of materials deposited to 

the seabed is specifically linked to discharge and loading rates of sediments and nutrients 

through Southwest Pass to the Louisiana Bight. The quality of material is a function of the 

transformation of these materials to primary productivity and chlorophyll biomass that is 

eventually delivered to the seabed, at a different spatial and temporal pattern as to bulk 

deposition . How these allochthonous and autochthonous materials control the patterns of 

benthic nutrient regeneration is the focus of studies described in Chapters 3, 4, and 5. 

3.2 Methods 

3 .2 .1 Benthic Fluxes 

Intact sediment cores from stations in the shelf were incubated aboard ship under near 

ambient conditions to determine the exchange of inorganic nitrogen (ammonium, nitrate, 

nitrite), phosphate, silicate and dissolved oxygen across the sediment-water interface. Intact 

sediment cores were used to measure ambient rates of nutrient fluxes and sediment oxygen 

demand which are dependent on the chemical and physical properties of the intact sediment 

structure. Undisturbed sediment cores were subsampled from a box core with 9 cm diameter, 

20 cm long acrylic tubes . The bottom of the core was sealed with a rubber stopper, and the 

upper end was sealed with gaskets and plexiglass plates (6 mm thick). The ambient overlying 

water was removed via siphon and replaced with filtered (GFF,1 .1 um pore size) ambient 

bottom water to represent ambient conditions. 

Problems associated with substrate and dissolved oxygen depletion in batch core 

experiments are remedied by a flow-through design in which an autoanalyzer pump (Cole 

Palmer) delivers the experimental solutions from reservoirs at controlled flow rates (typically 4 

mL/min) through influent and effluent lines connected to the cores (Koike and Hattori 1978, 

Miller-Way and Twilley 1996). Miniature immersible water pumps (0.47 L/min capacity at 0.3 

m head, Edmund Scientific) with variable transformer control were used to mix the overlying 

water within the cores without resuspending sediment. Reservoirs of the respective filtered 

water and sediment cores were incubated at ambient seawater temperatures onboard ship. The 

influent and effluent solutions were sampled at approximate 1 .5 h intervals for 8 h for analyses 

of dissolved oxygen and inorganic nutrients (P04, N03, NH4, N02, Si04) . Rates of nutrient 

flux were calculated using influent (I) and effluent (E) concentrations of the respective nutrient 

by the following equation: (I-E)V/A = Flux (gmol m-2 h-1) ; where V is the flow rate (L/h), and 

A is the surface area of the core. 

3 .2 .2 Sediments Analyses 

Flux cores at each station were subsampled with a 2.5 cm2 acrylic core to a depth of 10 

cm and analyzed for total carbon, nitrogen and phosphorus. Similar subsamples were 

centrifuged at 2000 rpm for 20 min and the supernatant (pore-waters) filtered (GFF glass fiber) 

and assayed for dissolved inorganic nitrogen and phosphorus using standard colorimetric 

techniques (Strickland and Parsons 1972) . Sample preparation and reagent additions were 

made in anaerobic environments using nitrogen filled glove bags to prevent precipitation of 

inorganic P in pore-waters. The top 2 cm of each core was homogenized and analyzed for 

size, water content, and bulk density by drying 3 cm3 fresh sediment at 85°C to constant 

weight . These dried samples were ground, and subsamples ashed at 450°C for 3 h. Other 

38 
subsamples were assayed for total carbon and nitrogen with a LECO Elemental Analyzer. 

Total phosphorus was determined by dissolving the ashed samples with HCl and determining 

P04 concentrations . Surface sediment chlorophyll was extracted from 1 cm3 samples with 

DMSO for 1 h and assayed on a Turner fluorometer. 

3.3 Results and Discussion 

3.3 .1 Benthic Flux Inventory 

A complete inventory of benthic nutrient fluxes and sediment oxygen demand of all the 

LASER and COMUS cruises is given in Table 3.2 . There were no sediment respiration 

estimates during the COMUS cruises. This composite of benthic fluxes includes a wide spatial 

array of the study area (Figure 1 .2) and for some stations a temporal measure of processes for 

a118 cruises (such as C-50). The spatial trends in benthic fluxes is described below (Section 

3.3 .2) and temporal variatio in Chapter 4. The COMUS results are analyzed in Chapter 5 . 

3 .3 .2 Spatial Fluxes (LASER 1 - LASER 3) 

The ambient conditions of bottom waters at stations where benthic fluxes were 

measured are based on CTD casts prior to each experiment (Table 3 .1) . These waters are 

usually approximately 1 m above the sediment-water interface . Ambient temperatures of 

bottom waters ranged from 14.9°C at C-80 during LASER 2 to about 27°C that was recorded 

at all of the 20 m stations during LASER 1 (Table 3.1). The lowest dissolved oxygen 

concentration of bottom waters during the first three LASER cruises was 0.25 mg/L and 

occurred at E-20 during the August 1987 cruise. Concentrations of DO <3 .0 mg/L occurred at 

only two other occassions during these three cruises; one of these was again at the E-20 station 

during April 1989 and the other at D-50 also during the April 1989 cruise (Table 3 .1) . 

Concentrations of nitrate recorded during LASER 1-3 ranged from 1 .3 to 60.1 gmoUL 

at E-50 and E-20, respectively ; and E-20 also had the high concentrations of nitrite at 3 .2 

gmoUL (Table 3.1). Peak concentrations of ammonium (7.7 gmol/L, at B-20), phosphate (4.2 

gmol/L, at E-20), and silicate (46.7 gmol/L, at B-20). Thus the shallow water stations 

generally had the higher concentrations of nutrients, either at the near- or far -field locations 

relative to the mouth of Southwest Pass . Lowest concentrations of phosphate and silicate 

occurred at the E-80 station. 

Pore-water and sediment characteristics of the cores used in estimates of benthic ffluxes 

during LASER 1-3 cruises are given in Table 3.3 . 

Rates of sediment oxygen demand (SOD) are sensitive to temperature control and the 

high rates at C-80, E-80 during LASER 1 and E-20 during LASER 3 (Figure 3.2) are 

associated with a 5°C higher water temperatures in the flux cores compared to ambient 

temperatures from CTD casts at each respective station (Table 3 .1) . All the other SOD rates for 

LASER 1-3 (Figure 3 .2) were determined under conditions of water temperature that are within 

2°C of ambient conditions. SOD rates along the 50-m contour decrease with distance from 

Southwest Pass during LASER 1 and 3 from 3 to about 1 .8 g m-2 d-1 (Figure 3.2). Rates 

along the 80 m contour are generally lower than the more shallow stations (ignoring the 

LASER 1 rates) with no clear pattern relative to distance from Southwest Pass. With the 

exception of LASER 1, there is also a spatial pattern of SOD rates among the stations along the 

20 m contour, with slightly lower rates at increase distance from Southwest Pass. 

39 
There was generally a release of phosphate during LASER 1 and 3 at all stations 

measured, whereas phosphate fluxes were negligible at these stations during LASER 2. There 

was no clear spatial pattern in phosphate fluxes relative to either depth or distance from 

Southwest Pass (Figure 3.3). Highest rates of phosphate release from sediment to the water 

column ranged from 40-50 gmol m-2 h-1, particularly during LASER 3 . Highest uptake of 

phosphate was -10 gmol m-2 h-1, measured at E-20 during LASER 1 . 

Rates of silicate release from sediments were higher during LASER 1 and 3 compared 

to lower rates during LASER 2 (Figure 3 .4). Flux rates >450 gmol m-2 h-1 were observed at 

D-20 and C-80 during LASER 1 and C-50 and E-80 during LASER 3; thus no clear spatial 

pattern in peak rates among the cruises. During LASER 2, silicate flux was similar among all 

the stations ranging from 100-200 gmol m-2 h-1 . One of the lowest rates of silicate flux, at 50 

gmol m-2 h-1, was observed at D-50 next to one of the highest rates of benthic silicate flux 

during LASER 3 (Figure 3.4). 

Benthic fluxes of ammonium ranged from uptake of -10 gmol m-2 h-1 to release of 550 

gmol m-2 h-1 during LASER 1 to 3 (Figure 3.5). Rates were generally lower during LASER 1 

and n and higher during LASER 3. There was a general spatial trend of higher ammonium 

flux at stations nearer to Southwest Pass and decreasing with distance to the west, particularly 

along the 50-m contour for all three LASER cruises. This spatial gradient in ammonium flux 

was particularly evident at this depth during LASER 3. Lowest rates of ammonium 

regeneration from sediments were consistently observed at the E station at each of the three 

depths. 

There was generally a release of nitrite from sediments to overlying water during 

LASER 1 and III, whereas there was an uptake of nitrite during LASER 2 (Figure 3 .6) . 

Highest rates of release ranged from 20-40 gmol m-2 h-1, particularly at SO-m transect during 

LASER 1 and 3 . 

Nitrate uptake occurred across the sediment-water interface was observed at most of the 

stations during LASER 1, 2, and 3 (Figure 3 .7) . There were some notable exceptions that 

included rates of nitrate release of >75 gmol m-2 h-1 at stations B-20 during LASER 3, and E80 during LASER 2. Nitrate release also occurred at most of the 20-m stations during LASER 

2. Benthic nitrate uptake was up to 50 gmol m-2 h-1 during LASER 1 and 2; but during 

LASER 3 there was an uptake rate of -150 gmol m-2 h-1 at B-50. The other sites where nitrate 

uptake occurred during LASER 3 also had rates of about 50 gmol m-2 h-1 (Figure 3 .7). For all 

three LASER cruises, 7 stations had nitrate release, 6 had nitrate uptake, and 2 stations had 

negligible rates of nitrate exchange. 

Fluxes of nutrients and dissolved oxygen across the sediment water interface of the 

Louisiana shelf ecosystem are within the upper range of rates for most shelf ecosystems 

(Figure 3 .8) . These remineralization rates are based on measurements at 9 stations in the shelf 

region near the vicinity of the Mississippi River plume, during August 1987, April 1988 and 

1989. Mean shelf rates of ammonium regeneration ranged from 75 to 200 ttmol m-2 h-1 . 

Lohrenz et al . (1990) have estimated phytoplankton demand for nitrogen (2.0 x 107 g N d-1) 

based on models of primary productivity in a 1700 km2 region of the Louisiana shelf in April 

1988. Using a mean flux of 200 gmol m-2 h-1 of ammonium from sediments during this 

period, we calculate that benthic remineralization could contribute about 40% (0.8 x 107 g N d1) of the phytoplankton demand for nitrogen in this ecosystem. The contribution of sediments 

40 
may be even higher in summer when river nutrient input is lower and benthic remineralization 

rates are higher. Thus in the Louisiana Bight, sediments may be significant in sustaining high 

rates of primary productivity during trhe summer when allochthonous input is low. 

41 
Mississippi 

w . " 

River 

~ . 

O 

" - --, 

'" - 

-- 

,~ ~ . / --' 

' 

, 

; ' southwest ~~ - 

Pass ' ., 

I 

'~ '- O -' -- 

" A 

.- NP B 

' ' C D 

E 

~--lOp f 

46 91 -- - 

90° 89° 30' 

Figure 3 .1 . Location of sampling stations at three depths and four different transects 

each at increasing distance from mouth of Southwest Pass. 

20' 

10' 

29° 

50' 

40' 

30' 

42 
N 

0 

A 

O 

0 

N 

O 

A 

O 

LASER I 

IS L L ~ IS l. L ~ 

20 m 50 m 

D l., L r, 

80 m 

LASER II 

IS L L r80 m 

5 

4 

N 

~ 3 

0 

2 

A 

O 1 

0 

ti l: L h t3 l: L E 

20m SO m 

LASER In 

13 C; ll h 13 C: ll h B C: 1) h 

20 m 50 m 80 m 

Figure 3.2 . Sediment oxygen demand at three depths along transects at 

increasing distance from the mouth of Southwest Pass. 

43 
7( 

N 

O 

3( 

w 1( 

a, -1( 

0 

a -3( 

LASER I 

E3 C I) E B C D E 

50 m 

B C D E 

20 m 

N 

O 

K 

a 

0 

a 

70 

N 

50 

O 

30 

K 

w 10 

a -

0 

a 

-JV 

80 m 

B C D E B C D E B C D E 

20m SO m 80m 

Figure 3.3 . Benthic fluxes of phosphate at three depths along transects 

at increasing distance from the mouth of Southwest Pass. 

44 

B C D E B O D E B C D E 

20m SO m 80m 
N 

G 

O 

K 

w 

U 

" rl 

N 

0 

U 

. ., 

ti C: ll h li l: 1J h t3 l: L 

20m SO m 80m 

LASER II 

600 

500 

400 

300 

200 

100 

ti lL ll h t5 l: L 

SO m 80m 

60~ 

~3 l; ll 

20 m 

Sa 

0 40 

30 

20 

41 

. la ., 

LASER III 

13 (: ll h li C: ll h ~ C: 1) h 

20m SO m 80m 

Figure 3.4 . Benthic fluxes of silicate at three depths along transects 

at increasing distance from the mouth of Southwest Pass. 

45 
N 

0 

a 

. .. 

d 

N 

O 

K 

w 

0 

d 

N 

iE 

0 

" rl 

0 

Q 

550 

450 

350 

250 

150 

Si 

-51 

LASER I 

B C D E 

20 m 

B C D E 

50 m 

B C D E 

80 m 

550 

450 

350 

250 

150 

50 

-50 

B C D E B C D E B C D E 

20m SO m 80m 

Figure 3.5 . Benthic fluxes of ammonium at three depths along transects 

at increasing distance from the mouth of Southwest Pass. 

LASER III 

46 

B C D E B C D E B C D E 

20m SO m 80m 
fn 

N 

iE 

O 

z 

N 

iE 

z 

N 

O 

N 

. .. 4. 

"rV 

z 

45 LASER II 

35 

25 

1d ' 

5 

-s 

-15 

B C D E 

20 m 

B C D E 

50 m 

B C D E 

80 m 

B C D E B C D E B C D E 

20m SO m 80m 

Figure 3.6 . Benthic flux of nitrite at three depths along transects at 

increasing distance from the mouth of Southwest Pass. 

47 

"' B C D E B C D E B C D E 

20m SO m 80m 
N 

O 

N 

cC Lr V 

z 

1JV 

100 

B C D E 

20 m 

LASER II 

50 

N 

O 

-50 

. ., 

-100 

-150 

100 

B C D E B O D E 

50 m 80 m 

B O D E B O D E B C D E 

20m SO m 80m 

50 

N 

O 

G 

-50 

-ioo 

-150 ' - ' 

B C D E B C D E B C D E 

20m SO m 80m 

Figure 3.7 . Benthic flux of nitrate at three depths along transects at 

increasing distance from the mouth of Southwest Pass . 

48 

LASER III 
80 

40 

N i 

-40 

w 

-so 

-120 

*100 *10 *10 

~ Aug 87 

Apr 88 

D Apr 89 

Resp Si04 P04 NH4 N03 N02 

Figure 3.8 . Rates of exchange of dissolved oxygen, silicate, phosphate, ammonium, nitrate 

and nitrite across the sediment-water interface in a Louisiana shelf ecosystem. 
Table 3.1 . Description of bottom waters at each of the benthic flux stations during LASER and COMUS cruises. 

0 

Station Cruise CTD Latitude Longitude Date Time Station Bottom Water 

ID Station Salinity Temp DO N03 N02 NH4 P04 SI04 

(psu) (C) (mg/L) (umoUL) (umoUL) (umoUL) (umoUL) (umoUL) 

B-20 L2 2062 28 55.60 89 28.18 4/22/88 1900 33.39 19.62 5.86 4.13 0.42 3.20 46.65 

B-20 L3 3035 28 55.53 89 28.18 4/16/89 0701 31 .15 20.80 3.17 11.38 1 .17 7 .68 0.80 6.76 

B-50 L3 3036 2850.63 8929.02 4/16/89 1351 3639 19.92 3.14 14.81 1 .08 4.11 1 .18 7.10 

B-50 L4 4023 2850.43 8929.50 9/23/89 0121 3632 25.08 4.82 

B-50 L5 5007 2850.82 8929.22 4/23/90 2100 35.66 20.93 5.95 

B-50 L6 

B-50 C 1 2550.47 8929.07 36.49 20.00 2.90 

B-50 C3 2850.48 8929.19 36.25 19.80 5.34 

C-20 L1 1104 2858.00 8928 .50 7/31/87 0230 34.84 27.27 1 .21 11 .78 1 .45 0.61 2.10 19.95 

C-20 L2 2032 2858.00 8928 .61 4/19/88 1717 35.71 18.87 3.22 15.56 1 .18 5.44 0.71 12.67 

C-20 L3 3037 2857.97 8928 .59 4/17/89 0720 33 .71 20.87 2.69 11 .67 128 0.99 0.58 5.60 

C-50 L1 1075 2852.50 8931 .50 7/23/87 2025 5.76 0.93 2.21 ns 11 .68 

C-50 L2 2006 2852.46 8931 .34 4/16/88 2155 3628 18 .20 4.31 1728 0.06 0.10 1 .19 7.47 

C-50 L3 3038 2852.43 8931 .77 4/17/89 1146 36.45 20.20 3.19 12.74 0.66 1 .99 0.79 13.07 

C-50 L5 5022 2852.48 8931 .67 4/25/90 1515 36.04 21 .35 5.89 

C-50 L6 6034 2852.42 8931 .09 

C-50 C 1 2825.48 8931 .68 36.75 20.00 3 .00 

C-50 C3 2852.51 8931 .52 36.15 20.50 6.10 

C-80 L1 1028 2849.70 8932.50 7/23/87 1528 36.24 21 .91 3 .46 3.39 0.50 1 .78 0.62 6.29 

C-80 L2 2013 2849.70 8932.59 4/17/88 2025 35.91 14.94 3 .61 25.94 0.11 1 .56 1 .50 9.71 
Table 3.1 . Description of bottom waters at each of the benthic flux stations during LASER and COMUS cruises. 

Station Cruise CTD Latitude Longitude Date Time Station Bottom Water 

ID Station Salinity Temp DO N03 N02 NH4 P04 SI04 

(psu) (C) (mg/L) (umoUL) (umoUL) (umoUL) (umoUL) (umoUL) 

C-80 L3 3041 28 49.70 89 32.86 4/18/89 1140 36.40 19.69 2.99 6.39 1 .05 0.52 0.68 23.38 

D-20 L1 1074 2901 .80 8936.60 7/28/87 2357 35.37 26.72 4.94 2.97 2.98 2.74 ns 15.87 

D-20 L2 2023 2901 .71 8936.69 4/18/88 1905 33.74 19.50 6.79 

D-50 L1 1066 2856.00 8935 .60 7/27/87 2139 36.25 21 .10 4.17 5.73 1 .01 1 .74 0.83 12.44 

D-50 L2 2044 2856.02 8935 .63 4/20/88 2344 36.16 17.57 3.63 20.04 0.67 1 .24 11 .20 

D-50 L3 3043 2855.88 8935 .66 4/19/89 0704 36.41 19.71 2.29 13 .28 034 1 .74 2.01 15.66 

D-50 L4 4036 2856.02 8935 .50 9/26/89 1831 33.76 28.01 5.42 

D-50 L5 5023 2855.82 8935 .82 4/25/90 1852 36.00 21.47 6.05 

D-50 L6 

D-50 C 1 2856.03 8935.58 nd 20.00 4.00 

D-50 C3 2856.01 8935.60 35 .98 18 .75 3.86 

E-20 L1 1001 2908 .45 8943.26 7/21/87 1026 34.37 27.42 0.25 60.12 3 .17 2.96 4.15 42.26 

E-20 L2 2033 2908 .41 8943.40 4/19/88 2200 33.82 19.60 8 .70 5.00 0.88 3.80 0.65 7.78 

E-20 L3 3047 2907.24 8944.63 4/19/89 1139 35.93 20.60 2.40 3.28 1 .39 1 .16 1 .31 7 .22 

E-20 L6 6032 2907.25 8844.52 

E-20 C3 2907.25 8944.56 35.84 21 .99 2.21 

E-30 C 1 2902.44 8943 .60 3420 26.00 3.50 

E-50 L1 1051 2856.74 8943 .55 7/26/87 2310 36.20 22.38 3.57 5.41 1 .35 2.33 0.93 11 .76 

E-50 L2 2043 2856.83 8943 .60 4/20/88 1955 3620 17.88 3.59 16.40 0.55 1 .28 12.25 

E-50 L3 3048 2856.75 8943.55 4/20/89 1220 36.44 20.18 3.49 1 .29 0.71 0.58 0.42 7.91 
Table 3.1 . Description of bottom waters at each of the benthic flux stations during LASER and COMUS cruises. 

Station Cruise CTD Latitude Longitude Date Time Station Bottom Water 

ID Station Salinity Temp DO N03 N02 NH4 P04 SI04 

(psu) (C) (mg/L) (umoUL) (umoUL) (umoUL) (umoUL) (umoUL) 

E-50 L5 5042 2856.73 89 43.50 4/27/90 1936 35.33 21 .00 5.61 

E-50 L6 6033 2856.73 8943.62 

E-50 C1 2856.88 8943.53 34.40 20.00 4.00 

E-50 C3 2856.69 8943.54 30.70 20.00 5.20 

E-60 C 1 2851 .65 8943.52 nd 2035 4.35 

E-80 L 1 1039 2846.50 8943.50 7/24/87 2250 36.27 20.91 3.34 3.13 0.50 0.88 0.51 7.63 

E-80 L2 2052 2846.66 8943.60 4/21/88 1940 36.46 18.31 4.90 9.57 0.12 2.15 3.51 

E-80 L3 3040 2846.49 8943.53 4/18/89 0710 36.43 19.50 4.55 2.56 0.31 0.52 0.20 4.13 

F-20 C 1 2908.26 8950.03 32.53 26.80 2.59 

F-50 C 1 2852.51 8950.08 nd 21 .00 4.00 

F-50 C3 2852.49 8950.03 34.70 19.27 3 .10 

G-50 C 1 2843.19 9001.97 35.18 24.50 5 .92 

G-50 C3 2842.79 9002.00 nd 20.40 6.50 

H-50 C 1 2842.97 9010.08 33.46 25.00 5.34 

H-50 C3 2842.80 9010.22 nd 20.80 5.80 

SP C3 2900.36 8909.28 0.00 24.90 5.20 
Table 3 .2 . Summary statistics for benthic fluxes at stations on the Louisiana Bight during LASER and COMUS cruises. 

Station CRUISE CTD Latitude Longitude Date Respiration P04 FLUX Si04 FLUX 

ID Station X SE X SE X SE 

(g m-2 d-1) (ug-at m-2 h-1) (ug-at m-2 h-1) 

B-20 L2 2062 2855.60 8928.18 4/22/88 0.74 0.03 127 .9 13 .2 

B-20 L3 3035 2855.53 8928.18 4/16/89 1 .25 0.03 29.4 10.2 185 .2 20.5 

B-50 L3 3036 2850.63 8929.02 4/16/89 1 .19 0.04 47.2 13 .2 221 .1 18.7 

B-50 L4 4023 2850.43 8929.50 9/23/89 19.1 18 .6 

B-50 L5 5007 2850.82 8929.22 4/23/90 0.69 0.22 -24.9 42.4 259 .4 175.4 

B-50 L6 0.89 0.22 18.3 10.9 189.6 52.1 

B-50 C 1 2550.47 8929.07 39 .4 4.5 291 .4 29.6 

B-50 C3 2850.48 8929.19 40.7 3 .0 5063 61 .6 

C-20 L1 1104 2858.00 8928.50 7/31/87 039 0.04 19 .8 3 .2 283 .5 16.8 

C-20 L2 2032 2858.00 8928.61 4/19/88 0.73 0.04 1 .2 0.8 168.9 17.4 

C-20 L3 3037 2857.97 8928.59 4/17/89 0.86 0.04 44.2 10.8 320.9 34.5 

C-50 L 1 1075 2852.50 8931 .50 7/23/87 1 .03 0.15 22.1 8.7 312.8 30.6 

C-50 L2 2006 2852.46 8931 .34 4/16/88 0.73 0.04 29.6 8.2 162.0 19.5 

C-50 L3 3038 2852.43 8931 .77 4/17/89 1 .07 0.07 40.8 29.0 446.5 32.2 

C-50 L5 5022 2852.48 8931.67 4/25/90 1 .33 0.22 6.5 5 .2 570.8 168.0 

C-50 L6 6034 2852.42 8931.09 33 .1 24.8 298.4 71 .6 

C-50 C 1 2825 .48 8931 .68 41 .3 8.4 313.5 45 .9 

C-50 C3 2852.51 8931 .52 22.3 2.7 461 .9 41 .8 

C-80 L1 1028 2849.70 8932.50 7/23/87 0.93 0.05 42.4 6.4 476.5 17 .3 

C-80 L2 2013 2849.70 8932.59 4/17/88 0.54 0.02 8.9 1 .3 183.6 12.3 

C-80 L3 3041 2849.70 8932.86 4/18/89 0.54 0.05 1 .3 17.1 188 .3 47 .8 

D-20 L1 1074 2901 .80 8936.60 7/28/87 1 .10 0.10 21 .5 10.1 438 .3 48 .2 

D-20 L2 2023 2901 .71 8936.69 4/18/88 0.64 0.04 0.2 1 .2 168 .9 15 .3 
Table 3 .2 . Summary statistics for benthic fluxes at stations on the Louisiana Bight during LASER and COMUS cruises. 

Station CRUISE 

ID 

CTD 

Station 

Latitude Longitude Date Respiration 

X SE 

(g m-2 d-1) 

P04 FLUX 

X SE 

(ug-at m-2 h-1) 

Si04 FLUX 

X SE 

(ug-at m-2 h-1) 

D-50 L1 1066 2856.00 8935 .60 7/27/87 0.71 0.04 22.0 42 207 .8 15.9 

D-50 L2 2044 2856.02 8935 .63 4120/88 0.57 0.03 168 .1 10.4 

D-50 L3 3043 2855 .88 8935 .66 4/19/89 0.85 0.04 9.2 3 .3 50.5 65.7 

D-50 L4 4036 2856.02 8935 .50 9/26/89 10.9 20.8 280.3 150.5 

D-50 L5 5023 2855 .82 8935 .82 4/25/90 1 .03 0.28 24.9 233 595 .4 197.2 

D-50 L6 0.71 0.18 9.3 9.2 229.9 46.6 

D-50 C1 2856.03 8935 .58 43.4 5 .3 339.2 58.3 

D-50 C3 2856.01 8935 .60 16.6 2.4 481 .7 20.6 

E-20 Ll 1001 2908.45 8943.26 7/21/87 1 .11 0.06 -152 93 370.1 39.2 

E-20 L2 2033 2908 41 8943.40 4/19/88 0.38 0.02 2.0 0.7 113 .2 7.0 

E-20 L3 3047 

. 

2907.24 8944.63 4/19/89 1 .19 0.07 16.3 4.9 146.8 16.6 

E-20 L6 6032 2907.25 8844.52 0.73 0.11 4.7 5 .6 242.1 45.5 

E-20 C3 2907.25 8944.56 2.6 2.5 195 .6 32.4 

E-30 C 1 2902.44 8943.60 23.5 4.8 483 .7 48.0 

E-50 L1 1051 2856.74 8943.55 7/26/87 0.49 0.05 10.1 4.5 266.4 25.2 

E-50 L2 2043 2856.83 8943.60 4/20/88 0.77 0.04 158.1 8.7 

E-50 L3 3048 2856.75 8943.55 4/20/89 0.67 0.13 55.0 15 .3 120.6 7.8 

E-50 L5 5042 2856.73 8943.50 4/27/90 1 .14 0.34 36.7 40.8 747.9 217.4 

E-50 L6 6033 2856.73 8943.62 1 .02 0.35 -8.4 16.6 206.8 127.0 

E-50 C1 2856.88 8943.53 58.8 4.9 516.9 28.3 

E-50 C3 2856.69 8943 .54 14.7 2.0 441 .5 16 .3 

E-60 C1 2851 .65 8943 .52 11 .6 2.0 314.4 17 .6 
Table 3 .2 . Summary statistics for benthic fluxes at stations on the Louisiana Bight during LASER and COMUS cruises. 

Station CRUISE CTD Latitude Longitude Date Respiration P04 FLUX Si04 FLUX 

ID Station X SE X SE X SE 

(g m-2 d-1) (ug-at m-2 h-1) (ug-at m-2 h-1) 

E-80 L 1 1039 28 46.50 89 43.50 7/24/87 1 .64 0.06 5 .6 2.4 359.7 13.7 

E-80 L2 2052 2846.66 8943.60 4/21/88 0.71 0.03 88.9 23.0 

E-80 L3 3040 2846.49 8943.53 4/18/89 0.59 0.08 14.9 9.6 461 .5 64.8 

F-20 C 1 2908.26 8950.03 12.9 3 .9 479.6 613 

F-50 C 1 2852 .51 8950.08 11 .0 1 .7 276.3 29.2 

F-50 C3 2852 .49 8950.03 -0.1 2.5 229.2 19.1 

G-50 C1 2843 .19 9001 .97 83 0.8 189.7 15.2 

G-50 C3 2842.79 9002.00 1 .2 1 .0 101 .0 7.6 

H-50 C 1 2842.97 9010.08 11 .0 1 .6 190.8 28.2 

H-50 C3 2842.80 9010.22 4.1 1 .2 135.2 19.1 

SP C3 2900.36 8909.28 -19. 9 5.1 -246 .4 233.8 
Table 3.2 . Summary statistics for benthic fluxes at stations on the Louisiana Bight during LASER and COMUS cruises. 

Station CRUISE CTD Latitude Longitude Date NH4 FT,UX N02 FLUX N03 FLUX 

ID Station X SE X SE X SE 

(ug-at m-2 h-1) (ug-at m-2 h-1) (ug-at m-2 h-1) 

B-20 L2 2062 2855 .60 8928.18 4/22/88 30.2 8.7 22.6 1 .6 37 .7 3 .1 

B-20 L3 3035 2855 .53 8928.18 4/16/89 253.8 26.8 21 .9 4 .2 93 .6 72.3 

B-50 L3 3036 2850.63 8929.02 4/16/89 573.6 40.1 42.7 18 .5 -145 .7 26.4 

B-50 L4 4023 2850.43 8929.50 9/23/89 22.4 46.4 

B-50 L5 5007 2850.82 8929.22 4/23/90 287.8 124.4 -0.8 6.4 -14.0 19.1 

B-50 L6 108.1 99.1 10.2 4.5 -17.9 9.6 

B-50 C 1 2550.47 8929.07 210.0 43.7 7.8 0.7 -3 .9 7.7 

B-50 C3 2850.48 8929.19 550.5 48.6 3 .8 0.7 -33 .4 3.8 

C-20 L1 1104 2858.00 8928.50 7/31/87 79.5 11 .8 19.3 1 .6 -56.9 7.0 

C-20 L2 2032 2858.00 8928.61 4/19/88 37.6 10.6 2.0 0.8 54.1 32.2 

C-20 L3 3037 2857.97 8928.59 4/17/89 179.3 20.4 143 4.9 -3.2 7.6 

C-50 L1 1075 2852.50 8931 .50 7/23/87 120.4 15.7 20.6 2.0 -8.4 4.5 

C-50 L2 2006 2852.46 8931 .34 4/16/88 154.9 17.8 4.1 0.4 -39.2 9.5 

C-50 L3 3038 2852.43 8931 .77 4/17/89 449.0 38 .4 31 .8 13 .4 -13.0 39.2 

C-50 L5 5022 2852.48 8931 .67 4/25/90 408.0 114 .0 8.8 4.4 88.7 64.6 

C-50 L6 6034 2852.42 8931 .09 50.2 264 .9 -2.5 6.7 -15.0 5.6 

C-50 C1 2825.48 8931 .68 165 .4 46 .7 5.2 0.5 -12.2 5 .1 

C-50 C3 2852.51 8931 .52 381 .4 32.5 3.2 2.7 -32.0 12 .5 

C-80 L 1 1028 2849.70 8932.50 7/23/87 71 .7 10.9 17.5 1 .9 10.4 2.9 

C-80 L2 2013 2849.70 8932.59 4/17/88 82.0 12.9 3.4 0.0 -64.9 14.5 

C-80 L3 3041 2849.70 8932.86 4/18/89 75 .4 14.1 -5 .9 4.4 -38.5 22.1 

D-20 L1 1074 2901.80 8936.60 7/28/87 63 .1 21 .8 7.2 11 .4 -25 .8 6.7 

D-20 L2 2023 2901 .71 8936.69 4/18/88 40.4 18.7 -0.6 0.1 -34.2 13 .8 
Table 3.2 . Summary statistics for benthic fluxes at stations on the Louisiana Bight during LASER and COMUS cruises. 

Station CRUISE 

ID 

CTD 

Station 

Latitude Longitude Date NH4 FLUX 

X SE 

(ug-at m-2 h-1) 

N02 FLUX 

X SE 

(ug-at m-2 h-1) 

N03 FLUX 

X SE 

(ug-at m-2 h-1) 

D-50 L 1 1066 2856.00 8935.60 7/27/87 65 .5 17.0 10.8 5 .0 -25.1 5.7 

D-50 L2 2044 2856.02 8935.63 4/20/88 35 .8 5 .1 -0.7 0.7 -55.1 11 .8 

D-50 L3 3043 2855.88 8935.66 4/19/89 159.6 13 .5 6.2 2.0 2.2 6.8 

D-50 L4 4036 2856.02 8935.50 9/26/89 3 .1 49.5 -5.1 2.4 -3 .5 18.4 

D-50 L5 5023 2855.82 8935.82 4/25/90 168.8 143 .2 17.4 10.5 30.7 33.2 

D-50 L6 73 .3 159.1 10.8 9.8 -17.9 9.0 

D-50 C1 2856.03 8935.58 175 .3 49.3 2.4 0.3 -24.1 12.7 

D-50 C3 2856.01 8935.60 153 .7 13 .1 4.3 1 .2 -87.2 48.9 

E-20 L1 1001 2908.45 8943.26 7/21/87 15 .8 30.0 -11 .3 2.4 -35.3 7.5 

E-20 L2 2033 2908.41 8943.40 4/19/88 29.3 9.5 1 .9 0.7 55 .1 19.3 

E-20 L3 3047 2907.24 8944.63 4/19/89 0.9 16.0 43 2.0 -34.4 7.2 

E-20 L6 6032 2907.25 8844.52 77.4 101 .2 -5.8 3 .9 -2.2 9.8 

E-20 C3 2907.25 8944.56 29.8 8.4 1 .1 1 .7 -9.7 12.5 

E-30 C 1 2902.44 8943 .60 100.2 10.9 0.6 1 .6 1 .6 3.2 

E-50 Ll 1051 2856.74 8943 .55 7/26/87 40.7 8.9 12.8 4.1 -17.1 5.1 

E-50 L2 2043 2856.83 8943 .60 4/20/88 5.1 3.0 0.7 0.7 -41 .3 19.2 

E-50 L3 3048 2856.75 8943 .55 4/20/89 121 .4 20.7 27.8 8.3 -46.6 7.4 

E-50 L5 5042 2856.73 8943 .50 4/27/90 240.7 162.9 7.4 5 .1 27.6 14.5 

E-50 L6 6033 2856.73 8943 .62 157.5 149.4 4.0 5 .3 -49.7 32.7 

E-50 C1 2856.88 8943 .53 194.8 17.2 3.2 2.4 27.4 8.8 

E-50 C3 2856.69 8943 .54 76.7 23.2 9.0 4.4 -43.3 19.1 

E-60 C 1 2851 .65 8943 .52 100.0 27.5 2.7 1 .4 -5.6 7.5 
Table 3 .2 . Summary statistics for benthic fluxes at stations on the Louisiana Bight during LASER and COMUS cruises. 

Station CRUISE CTD Latitude Longitude Date NH4 FLUX N02 FLUX N03 FLUX 

ID Station X SE X SE X SE 

(ug-at m-2 h-1) (ug-at m-2 h-1) (ug-at m-2 h-1) 

E-80 L1 1039 28 46.50 89 43.50 7/24/87 36.9 6.0 13.8 1 .6 7.5 2.8 

E-80 L2 2052 2846.66 8943.60 4/21/88 -8.3 7.8 -0.9 0.4 89.6 35 .4 

E-80 L3 3040 2846.49 8943.53 4/18/89 26.1 17.2 0.8 5 .0 -73.5 38 .4 

F-20 C1 2908.26 8950.03 243.6 37.7 -1 .2 0.6 -33.6 3 .0 

F-50 C1 2852.51 8950.08 197.4 23.0 0.7 0.4 -4.7 8 .4 

F-50 C3 2852.49 8950.03 31 .0 9.8 2.5 2.4 -42.4 29 .1 

G-50 C 1 2843.19 9001.97 37.4 14.2 -3.0 1 .0 10.8 3 .1 

G-50 C3 2842.79 9002.00 -0.9 8.0 -0.4 0.8 -24.5 1 .8 00 

H-50 C 1 2842.97 9010.08 66.8 14.0 -5.1 2.6 -2.6 6.1 

H-50 C3 2842.80 9010.22 47.2 13.2 -1 .9 3.7 -8.4 3 .0 

SP C3 2900.36 8909.28 1461 .8 286.6 29.1 4.7 -454.3 276.2 
Table 3.3 . Sediment charactertistics of the benthic stations during the LASER and COMUS cruises . 

Station CRUISE CTD Latitude Longitude Date Sediment POREWAT'ER TOTAL SEDIMENT C:N 

ID Station Chlorophyll NH4 SI04 P04 Carbon Nitrogen Ratio 

(ug/cm3) (umoUL) (umoUL) (umoUL) (mg/gdm) (mg/gdm) 

B-20 L2 2062 2855 .60 8928.18 4/22/88 2.5 

B-20 L3 3035 2855 .53 8928.18 4/16/89 0.40 17.4 2.3 9.0 

B-50 L3 3036 2850.63 8929.02 4/16/89 0.56 14.0 1 .9 8.6 

B-50 L4 4023 2850.43 8929.50 9/23/89 0.54 166 194 19.1 

B-50 L5 5007 2850.82 8929.22 4/23/90 0.46 

B-50 L6 0.87 1065 200 5.4 

B-50 C1 2550.47 8929.07 0.44 100 97 7.7 19.5 2.4 9.7 

B-50 C3 2850.48 8929.19 0.30 182 26 5.1 16.2 1 .6 12.2 

C-20 L1 1104 2858 .00 8928.50 7/31/87 96 254 53.0 17.5 2.5 8.2 

C-20 L2 2032 2858 .00 8928.61 4/19/88 0.40 19.0 2.7 8.2 

C-20 L3 3037 2857.97 8928.59 4/17/89 0.34 215 691 17.0 16.3 1 .5 12.7 

C-50 L1 1075 2852.50 8931 .50 7/23/87 337 1491 43.0 17.8 1 .8 11 .5 

C-50 L2 2006 2852.46 8931 .34 4/16/88 0.48 17.1 2.3 8.7 

C-50 L3 3038 2852.43 8931 .77 4/17/89 0.87 488 502 71 .0 15.8 1 .8 10.2 

C-50 L5 5022 2852.48 8931 .67 4/25/90 0.42 

C-50 L6 6034 2852.42 8931 .09 

C-50 C1 2825.48 8931 .68 0.40 105 102 6.3 17.8 2.2 9.7 

C-50 C3 2852.51 8931 .52 0.50 231 30 9.5 17.2 1 .9 10.5 

C-80 L1 1028 2849.70 8932.50 7/23/87 16.0 2.0 9.3 

C-80 L2 2013 2849.70 8932.59 4/17/88 0.30 16.5 2.4 8.0 

C-80 L3 3041 2849.70 8932.86 4/18/89 0.63 5 105 12.0 15 .5 1 .4 12.9 
Table 3.3 . Sediment charactertistics of the benthic stations during the LASER and COWS cruises . 

Station CRUISE CTD Latitude Longitude Date Sediment POREWATER TOTAL SEDIMENT C:N 

ID Station Chlorophyll NH4 SI04 P04 Carbon Nitrogen Ratio 

(ug/cm3) (umoUL) (umoUL) (umoUL) (mg/gdm) (mg/gdm) 

D-20 L1 1074 2901 .80 8936.60 7/28/87 159 426 17.0 83 1 .2 8 .1 

D-20 L2 2023 2901 .71 8936.69 4/18/88 0.25 7.5 0.9 9.7 

D-50 L1 1066 2856.00 8935.60 7/27/87 285 1151 21 .0 16.6 2.2 8 .8 

D-50 L2 2044 2856.02 8935.63 4/20/88 0.32 16.9 2.6 7.6 

D-50 L3 3043 2855 .88 8935.66 4/19/89 0.33 64 74 55.0 13.2 1 .8 8 .5 

D-50 L4 4036 2856.02 8935.50 9/26/89 0.11 241 227 26.2 

D-50 L5 5023 2855 .82 8935.82 4/25/90 0.33 

D-50 L6 0.98 657 184 4.9 

D-50 C1 2856.03 8935.58 0.44 78 75 11 .1 19.2 2.5 9.0 

o D-50 C3 2856.01 8935.60 0.30 76 29 6.1 16.9 2.2 9.0 

E-20 L1 1001 2908.45 8943.26 7/21/87 442 587 13.0 13 .5 1 .3 12.1 

E-20 L2 2033 2908.41 8943.40 4/19/88 0.26 13 .1 2.0 7.6 

E-20 L3 3047 2907.24 8944.63 4/19/89 028 99 72 15 .2 2.1 8.4 

E-20 L6 6032 2907.25 8844.52 0.51 1018 272 21 .7 

E-20 C3 2907.25 8944.56 0.17 39 28 7.3 14.9 1 .7 10.5 

E-30 C 1 2902.44 8943.60 0.30 113 110 18.5 20.0 2.9 8.2 

E-50 L1 1051 2856.74 8943.55 7/26/87 225 1068 26.0 16.9 2.2 9.0 

E-50 L2 2043 2856.83 8943 .60 4/20/88 0.28 17.5 2.9 7.0 

E-50 L3 3048 2856.75 8943 .55 4/20/89 0.31 24 66 17.0 3.2 6.2 

E-50 L5 5042 2856.73 8943 .50 4/27/90 

E-50 L6 6033 28 56.73 89 43 .62 
Table 3.3 . Sediment charactertistics of the benthic stations during the LASER and COMLJS cruises . 

H-50 C1 28 42.97 90 10.08 0.34 65 62 0.7 13.6 1 .1 14.4 

H-50 C3 28 42.80 90 10.22 0.23 38 23 2.8 11 .4 1 .0 13 .9 

SP C3 29 00.36 89 09.28 0.80 328 34 0.3 18.8 2.2 9.9 

Station CRUISE CTD Latitude Longitude Date Sediment POREWATER TOTAL SEDIMENT C:N 

ID, Station Chlorophyll NH4 SI04 P04 Carbon Nitrogen Ratio 

(ug/cm3) (umoUL) (umoUL) (umoUL) (mg/gdm) (mg/gdm) 

E-50 C1 28 56.88 89 43.53 1 .08 127 124 32.8 20.0 3 .3 7.1 

E-50 C3 2856.69 8943.54 0.30 94 52 16.5 17.9 2.5 8.5 

E-60 C 1 2851.65 8943.52 0.40 90 87 1 .6 18.0 2.4 8.8 

E-80 L1 1039 2846.50 8943.50 7/24/87 34 897 16.0 14.7 1 .4 12.3 

E-80 L2 2052 2846.66 8943.60 4/21/88 0.11 15 .9 2.5 7.4 

E-80 L3 3040 2846.49 8943.53 4/18/89 0.04 12 40 19.0 15.3 2.8 6.4 

F-20 C1 2908.26 8950.03 0.48 119 115 14.5 18.5 2.4 9.0 

F-50 C 1 2852.51 8950.08 0.27 215 212 13.2 17.1 2.2 9.0 

F-50 C3 2852.49 8950.03 0.30 52 34 6.8 16.8 2.2 8.9 

G-50 C 1 2843.19 9001 .97 0.47 51 48 1 .3 11 .6 1 .2 11 .7 

G-50 C3 2842.79 9002.00 030 28 22 2.8 10.7 1 .0 12.4 
Chapter 4 

SEASONAL INVESTIGATIONS OF BENTHIC FLUXES 

WITHIN THE LOUISIANA BIGHT 

4.1 Introduction 

The rate and efficiency of material cycling through both benthic and water-column 

communities are central to the ultimate fate of inorganic nutrients and organic matter within the 

Louisiana Bight region, and land-margin ecosystems in general. Of particular importance to 

understanding ecosystem response to nutrient enhancement is an understanding of those 

processes directly concerned with the input, remineralization, and loss of nutrients . The 

coupling of these processes is central to understanding the linkages of nutrient enrichment to 

the development of hypoxia in this region of the Louisiana shelf ecosystem. Previous studies 

of deposition suggest that the proximal region of the Mississippi River plume is a zone of 

extreme input of allochthonous particulate material to the seabed. Much of this material appears 

to be remineralized or transported out of the region. An important focus of this report is to 

determine the relative contributions of deposition and regeneration (redistribution) to the fate of 

sediments and nutrients in the Louisiana shelf ecosystem. 

The plume region of river-dominated land margin ecosystems are sites of high 

sedimentation of allochthonous carbon, nitrogen and phosphorus. However, nutrient retention 

may vary within the plume region, particularly during high flow, depending on differences in 

depth and circulation . A common observation in river-dominated environments is that most of 

the sediment discharge is initially deposited near the river mouth; sedimentation rates decrease 

with increasing distance from the river (DeMaster et al. 1985; Nittrouer et al. 1987) . Chapters 

1, 2, and 3 show distinct spatial patterns in sedimentation, nutrient accumulation and benthic 

nutrient regeneration in the plume region of the Louisiana Bight. The spatial pattern in benthic 

nutrient regeneration is a combination of both the delivery of allochthonous materials to the 

seabed, with particularly high rates near Southwest Pass. There are also regions of nutrient 

regeneration in sediments that are not directly related to zones of terrigenous sedimentation, but 

may be more influenced by deposition of organic matter produced in the water column. 

Specific temporal patterns of benthic fluxes in response to river flow and water column 

processes should give more insights as to the coupling of benthic and pelagic processes in the 

plume region of the Louisiana Bight. This chapter describes patterns of deposition and benthic 

nutrient regeneration at selected stations along a 50-m contour west of Southwest Pass during 

times of high and low river discharge. These repeated measures of deposition and fluxes are 

compiled from LASER and COMUS cruises since 1987 (Figure 4.1). 

4.2 Methods 

The temporal sequence of benthic flux measurements in the Louisiana Bight relative to 

discharge of Mississippi River are shown in Figure 4.1 . The LASER (L1-L6) and COMUS 

(C1 and C3) cruises are discussed in this chapter since they focus on the temporal benthic 

fluxes along the 50-m contour of the Louisiana Bight study area (Figure 2.1). The NECOP 

(N1-N2) cruises focussed on the study of the effects of hypoxia on benthic fluxes at selected 

sites, and are not included in this report . Cruises during high flow regimes include L2, L3, 

L5, compared to cruises during low discharge that included L1, L4, and L6. Both the C1 and 

C3 cruises occurred at the end of a high flow period . 

63 
4.2 .1 Deposition 

Box core samples were collected from stations in Louisiana Bight study area along the 

50-m contour at transects B, C, D, and E during LASER and COMUS cruises periodically 

from 1987 to 1994 (Figure 4.1) . Cores were subsampled from box cores within this grid after 

high and low river discharge periods for measurements of 234Th, which is a naturally 

occurring (234Th) radionuclide used to determine rates of deposition . These radionuclides are 

very particle-reactive (i .e ., rapidly sorbed onto particle surfaces) and have proven to be very 

useful as particle tracers (Broecker et al . 1973; Santschi et al . 1980) . Because of natural 

radioactive decay, this radionuclide is particularly useful in examining rates of sedimentary 

processes at time scales of days to months (e .g ., sediment deposition related to flood and storm 

events) . 

Large diameter (16.5 cm) subcores were taken from box cores at each sampling station. 

Each core was carefully extruded and simultaneously subsectioned at precise 0.5-1 .0 cm 

intervals . Yield tracers (232U/22gTh) were added to the dried core samples, then leached with a 

combination HN03, HCl and HC104 solution. Thorium and uranium are isolated and purified 

via ion exchange methods, plated onto stainless steel planchets and counted on a lowbackground beta detection system (McKee et al . 1986) . The deposition of materials in bottom 

sediments was calculated from the following equation (Hatton et al . 1983): A = Cd x R x D x 

104 (g m-2 yr-1) ; where A is the rate of nutrient deposition, Cd is the dry mass nutrient 

concentration, D is bulk density, and R is the deposition rate (determined by 234Th) . 

4.2 .2 Benthic Regeneration 

Cores were subsampled from each of the box cores at stations along the 50-m contour 

in the study area to measure rates of benthic nutrient regeneration . Estimates of benthic flux 

followed the procedures of Miller-Way and Twilley (1996) as described in Section 3 .2 .1 (also 

see Chapter 5, Figure 5.4). Sediment analyses associated with these cores were as described 

in section 3.2 .2 . 

4.3 Results and Discussion 

Deposition rates calculated from samples collected in April of three consecutive years 

range from 69 to 436 gdw m-2 d-1 near the mouth (station B50) of southwest pass . This 

interannual variation reflects differences in cumulative discharge during the 3-4 months prior to 

sampling. Rates of deposition for similar discharge periods were generally less at downfield 

stations . During the low flow period, river discharge decreases by about 85°Io, resulting in 

reduced riverine input to the shelf. 

Deposition rates (100-day time scale) within the plume region of the Mississippi River 

(Figure 1) are 5-10 times greater than the sediment accumulation rates (100-year time scale) 

determined using 21oPb. The contrast in fate of materials deposited to the seabed based on 

relative rates of deposition and burial indicate that a substantial portion of these materials may 

be redistributed. Redistribution may occur from the shelf to shelf slope, or to more distal parts 

of the dispersal system along the shelf. Redistribution of materials delivered during high river 

flow leads to a more uniform distribution of particulates throughout the dispersal system. 

The effect of high and low river discharge on the delivery of sediment to the Louisiana 

Bight study area, as measured by 234Th deposition, for the 8 cruises are depicted in Figure 

64 
4.2 . There are no data for L1 and L4 The most complete temporal data set of deposition exists 

for the B-50 and C-50 stations . Highest input rates of the 6 events measured at B-50 was 

LASER 5 during April 1990 at 3.8 cm/mo and the lowest was during LASER 2 in April 1988 

at 0.2 cm/mo (Figure 4.2) . At C-50 the April 1990 event is not as high as measured at B-50, 

with a rate of 2.5 cm/mo, nearly a cm/mo less than B-50. Generally rates of deposition during 

each cruise was higher at B-50 than C-50, except for April 1988 and April 1989 (Figure 4.2) . 

The best record for sediment chlorophyll is at B-50 and D-50 that depict the temporal 

quality of material in the top 1 cm depth of sediment (Figure 4.3). The highest chl-a 

concentration at D-50 and B-50 occurred in October 1990 at about 1 ug/cm3 (for dates that 

measurements exists). The lowest concentration at D-50 occurred in September 1989, whereas 

at B-50 the lowest concentration was in June 1994. The low concentration at D-50 was about 

0.1 ug/cm3. The high concentrations of sediment chl-a (Figure 4.3) did not occur during 

events of highest deposition (Figure 4.3) . There was not particular spatial pattern relative to 

distance from Southwest Pass as indicated by value of nearly 1 ug/cm3 also measured at E-50 

during November 1993. 

Generally there was a release of phosphate among the stations for most of the cruises 

(Figure 4.4) . Rates >50 gmol m-2 h-1 occurred at E-50 during two cruises including April 

1989 and November 1993. These two dates were also the highest rates of phosphate release 

for the other stations along the 50-m contour, with the exceptio of D-50 during the April 1989 

cruise. Phosphate uptake was recorded only once at B-50 during the April 1990 cruise. 

However, negligible benthic exchange of phosphate was observed on three occasions at D-50, 

twice at E-50 and once at C-50. Rates of phosphate flux were generally lower at D-50 during 

the cruises. 

There was always a release of silicate from sediments at all stations along the 50-m 

contour for all cruises (Figure 4.5). Benthic fluxes of silicate >575 gmol m-2 h-1 occurred 

during April 1990 at C-50, D-50, and E-50; and these rates represented peak fluxes at these 

three locations. During the same cruise, silicate flux at B-50 was one of the lowest rates 

measured during this study. The temporal trend in benthic silicate fluxes at stations D-50 and 

E-50 were very similar among the 8 cruises (fluxes in September 1989 were not measured at E50). 

There was always a release of ammonium from sediments at the stations along the 50-m 

contour for the times measured during this study (Figure 4.6) . Rates were generally higher at 

B-50 and C-50 compared to D-50 and E-50. At B-50 and C-50 peak rates of ammonium flux 

were 400-600 gmol m-2 h-1 and occurred in April 1989, April 1990, and June 1994 at both 

sites . In fact, the temporal trend in benthic ammonium fluxes among the 8 cruises was similar 

for these two stations . Temporal trends at D-50 and E-50 are similar with highest rates in April 

1990 and November 1993 . These peak rates are only about 200 gmol m-2 h-1, much lower 

than the peak rates at B-50 and C-50. 

In most experiments there was a release of nitrite from sediments along the 50-m 

contour of the study area (Figure 4.7). Highest rates of nitrite release ranged from 30 to 40 

gmol m-2 h-1 at B, C, and E stations during the April 1989 cruise . Nitrite release at D-50 

during this cruise was much lower at about 8 gmol m-2 h-1. Nitrite uptake was only measured 

in 4 out of 27 experiments and all rates were < -7 gmol m-2 h-1 . 

65 
There was generally a pattern of nitrate uptake at stations along the SO-m contour 

(Figure 4.8). Nitrate release was observed during only 4 of the 27 measurements and rates in 

3 of these observations were <25 [tmol m-2 h-1 ; an exception was the measurement at C-50 

during April 1990 with a release rate of 75 gmol m-2 h-1. The peak nitrate uptake rate was 

measured at B-50 during the April 1989 and was 145 gmol m-2 h-1 . The bottom water nitrate 

concentration at this station was only 14.8 gmoUL (Table 3.1). There were 9 observations 

where nitrate uptake rates ranged from 25 to 50 gmol m-2 h-1 ; and several times the flux of 

nitrate was measured as negligible . 

The relation of benthic fluxes with rates of sediment deposition (measured as an 

accretion rate) for just the stations along the 50-m contour of the Louisiana Bight study area is 

shown in Figure 4.9 . Ammonium and phosphate fluxes are not particularly correlated to rates 

of deposition. Benthic fluxes of silicate shows some evidence of an increase in flux with 

deposition rate up to 2.5 cm/mo; above that rate of deposition the flux of silicate is lower. This 

metric of deposition is based on quantity of material added; there also needs to be an evaluation 

of the quality of that material. 

Figure 4.10 includes deposition and benthic fluxes for all of the stations in the study 

area of the Louisiana Bight, covering several effects of distance from Southwest Pass and 

water depth. Again, ammonium and phosphate fluxes show no trend with deposition. And 

silicate again shows some evidence of silicate flux increasing with deposition up to 2-2.5 

cm/mo; above which there is a decrease in flux . 

Seasonal deposition of allochthonous organic matter during spring provides the 

predominant mechanism for sustaining peak rates of benthic regeneration in the plume region 

of the shelf. During April, ammonium regeneration in the plume region (station B-50 and C50) may exceed 500 gmol m-2 h-1, compared to fluxes of less than 200 Rmol m-2 h-1 during 

low river discharge in September and October. Further downfield (stations D-50 and E-50), 

ammonium regeneration is generally less than 200 gmol m-2 h-1 . Seasonal differences in 

benthic regeneration are most evident nearest the mouth of the Mississippi River, while rates 

are more constant both temporally and spatially in more distant regions. The link between 

sediment deposition and benthic nutrient regeneration is clearly demonstrated in results of 

silicate flux at near and far-field stations on the Louisiana shelf. Fluxes of silicate across the 

sediment-water interface to the water column increase linearly with deposition rates from 0.3 to 

2.5 cm/mo, reaching maximum silicate flux rates of 550 gmol m-2 h-1 . Above a deposition rate 

of 2.5 cm/mo, silicate flux was lower at less than half the maximum rates. However, this may 

be a station effect ; there was a strong regression between deposition and silicate regeneration 

for station C-50, but silicate fluxes at B-50 were similar disregardless of the sediment 

deposition rate (Figure 4.2). Thus, the response of benthic fluxes to temporal variation of 

particulate input may vary spatially in the Louisiana shelf. 

66 
35 

30 

25 

M 

20 

W 

C7 

x 15 

U 

Q 

10 

5 

0 

Figure 4.1 . The timing of measuring benthic fluxes in the Louisiana Bight relative to the seasonal discharge of 

the Mississippi River. 

1987 1988 1989 1990 1991 1992 1993 1994 
5 

B-50 234Th 

4 

0 

3 

z 0 

2 

O 

a 

w 

A 

1 

0 

Aug-87Apr-88 Apr-89 Sep-89 Apr-90 Oct-90Nov-93 Jun-94 

CRUISE DATES 

5 

G50 234Th 

4 

0 

v 

z 0 

O 

a w Q 

0 

1 

Aug-87Apr-88 Apr-89 Sep-89 Apr-90 Oct-90Nov-93 Jun-94 

CRUISE DATES 

Figure 4.2 . Rates of deposition based on accretion of thorium-234 at stations B50 and C-50 during different LASER and COMLTS cruises . 

68 
1 . : 

0. . 

0.1 

0.~ 

0. : 

1 

B-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

M 

U 

a. 0 

0 

U 

1 .2 

1 

0.8 

0.6 

0.4 

0.2 

0 

1 .2 

1 

0.8 

0.6 

0.4 

02 

0 

C-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

D-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

1 .2 

1 

0.8 

0.6 

0.4 

0.2 

0 

E-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

Figure 4.3 . Concentrations of benthic chlorophyll along the 50-m contour of the 

Louisiana Bight at different distances from the mouth of Southwest Pass. 

69 
N 

i~ 

O 

a 

w 

.a. 

0 

a 

70 

50 

30 

10 

-10 

-30 

70 

50 

30 

10 

-10 

-30 

C-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

E-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

Figure 4.4 . Benthic fluxes of phosphate along the 50-m contour of the Louisiana 

Bight at different distances from the mouth of Southwest Pass. 

70 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 
1000 

800 

600 

400 

200 

n 

N i~ 

O 

w 

U 

100 

80 

6040 

20 

B-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

G50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

100 

80 

60 

40 

24 

100 

80 

60 

40 

20 

D-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

E-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

Figure 4.5 . Benthic fluxes of silicate along the 50-m contour of the Louisiana Bight 

at different distances from the mouth of Southwest Pass. 

71 
800 

600 

400 

200 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

N 

O 

~. 

a 

. ., 

a 0 

Q 

8( 

6( 

4( 

2( 

B-50 

G50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

800 

D-50 

600 - 

400-

200- 

8( 

6( 

4( 

2( 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

E-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

Figure 4.6. Benthic fluxes of ammonium along the 50-m contour of the Louisiana 

Bight at different distances from the mouth of Southwest Pass. 

72 
60 

50 

40 

30 

20 

10 

0 

-10 

N 

G 

O 

a 

a 

w 

z 

6( 

5( 

4( 

3( 

2( 

1( 

-1( 

6^ 

5 

4 

3 

2 

1 

-1 

B-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

C-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

v 

p D-50 

0- 

0

-

0 -

0 - 

0 - 

0 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

60 

50 

40 

30 

20 

10 

0 

-10 

E-50 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

Figure 4.7 . Benthic fluxes of nitrite along the 50-m contour of the Louisiana Bight at 

different distances from the mouth of Southwest Pass. 

73 
-1 

-1 

N 

G 

O 

K 

w 

.~ 

~. 

z 

-1 

-1 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

100 

D-50 

50 - 

0-

-50 -" 

-100 - 

-1 Sn 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

100 - 

50 - E-50 

0 - 

-50- -r T 

-100-

-150 

Figure 4.8 . Benthic fluxes of nitrate along the 50-m contour of the Louisiana Bight at 

different distances from the mouth of Southwest Pass . 

74 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

100 

Aug-87 Apr-88 Apr-89 Sep-89 Apr-90 Oct-90 Nov-93 Jun-94 

l0n 
N 

O 

rS. 

U 

as 

8 

7 

6 

5 

4 

3 

2 

1 

Silicate 

i 

700 

600 

500 

400 

300 

Zoo 

ioo 

0.5 1 1 .5 2 2.5 3 3 .5 4 

Ammonium 

I 

= 1 

7 

I 

0 0.5 1 1 .5 2 2.5 3 3.5 4 

80 

Phosphate 

60 - 

40-

1 20- 

I 

0 - 

-20-

-40 

0 0.5 1 1 .5 2 2.5 3 3 .5 4 

Deposition, cm/mo 

Figure 4.9 . Benthic fluxes of silicate, ammonium, and phosphate relative to the 

measurement of deposition at stations along the 50-m contour. 

75 
81 

71 

61 

51 

41 

31 

21 

11 

Silicate 

1 

41 

0 0.5 1 1 .5 2 2.5 3 3.5 4 

N 

0 

U 

Ammonium 

7 

I 

f 

0 0.5 1 1 .5 2 2.5 3 3 .5 4 

700 

600 

500 

400 

300 

200 

100 

0 

-100 

80 

60 

40 

20 

0 

-zo 

-40 

Phosphate 

i 

S 

0 0.5 1 1 .5 2 2.5 3 3.5 4 

Deposition, cm/mo 

Figure 4.10. Benthic fluxes of silicate, ammonium, and phosphate relative to the 

measurement of deposition at all stations in the Louisiana Bight. 

76 
Chapter 5 

SPATIAL PATTERNS OF BENTHIC NUTRIENT REGENERATION AND 

SEDIMENT CHARACTERISTICS ON THE LOUISIANA CONTINENTAL 

SHELF 

(This Chapter is a revision of a Master of Science thesis by John Bourgeois) 

5.1 Abstract 

The Mississippi River exhibits influences on the coastal processes that are important not 

only to the productivity of the region, but also to the global carbon and nitrogen cycles. Rates 

of benthic nutrient regeneration were measured on the Louisiana continental shelf at a total of 

nineteen stations over two cruises. Ten stations were sampled on the first cruise (COMUS 1, 

November 1993), while nine stations were sampled on the second cruise (COMUS 3, June 

1994). Rates of benthic regeneration were determined by a flow through core chemostat 

system for N03, N02, NH4, P04, and Si04. At each site, samples from the incubated cores 

were also taken for redox, pore-water concentrations, sediment composition (carbon, nitrogen, 

phosphorus, and chlorophyll a), and macroinfauna, as well as coordinate measures of 

234thorium for deposition analysis . Rates of benthic nutrient flux tended to decrease with 

distance from the riverine source, with a significant difference between the two cruises only for 

N03 and P04. Modeling nutrient fluxes from pore-water gradients showed little correlation to 

the actual measured nutrient flux. The sediment characteristics provided some insight for 

outliers in the analysis . Benthic fluxes always showed a higher correlation to some qualitative 

index (input of carbon, nitrogen, phosphorus, or chlorophyll a) of deposition to the benthos 

than to the quantitative input of sediment. 

5.2 Introduction 

The Mississippi River is one of 20 rivers worldwide that, in total, are responsible for 

almost 50% of the terrigenous materials being transported to coastal oceans. The Mississippi 

River ranks third in the world in drainage basin area, sixth in water discharge, and seventh in 

sediment load (Milliman and Meade 1983; Milliman 1991). The riverine influence on the 

coastal processes is not only important in the global cycling of carbon and nitrogen, but is also 

responsible for the high productivity found in these coastal regions. The low energy Louisiana 

continental shelf is therefore the site of high allochthonous nutrient deposition. 

The cycling of material in this land margin ecosystem is dependent on the coupling of 

benthic and water column processes (Figure 5 .1, in Twilley et al . 1994). The linkage of these 

ecological processes is important in determining the productivity of the coastal zone, as nutrient 

regeneration (the release of inorganic nutrients into the water column) rates relative to 

allochthonous input determine the amount of material available to primary producers. If rates 

of input are higher than regeneration, the result is nutrient burial and therefore reduced 

availability of those nutrients to the water column. 

As outlined in Figure 5.1, understanding the fate of materials is linked to the various 

ecological and physical processes that are spatially distributed relative to the river plume . The 

high concentrations of materials introduced by the river corresponds to high rates of 

deposition. These deposition rates are proportional to benthic nutrient regeneration rates with 

77 
respect to proximity to the river mouth. Primary production in this zone is largely inhibited by 

water column turbidity. 

However, in more distal regions of the plume, particulate matter settles out from the 

water column, but dissolved nutrients remain, allowing high rates of autotrophic production . 

Benthic regeneration rates are hypothesized to be high in this zone due to the combination of 

relatively high quantity of deposition to the seabed (although not as high as the "Turbid Zone"), 

in combination with a higher quality (material of marine origin) of this deposited material. 

Continuing along this longitudinal axis away from the plume, primary production becomes 

more dependent on remineralization, particularly in the water column, as allochthonous input is 

now minimal. Benthic regeneration rates in this zone should prove efficient largely because of 

the high quality of material being deposited. Although rates of benthic regeneration are low, 

this zone is receiving little input to the sediments . This zone may also be subject to hypoxia 

resulting from lateral redistribution of benthic sediments into this area. This distal zone 

receives a majority of its sediments via this lateral movement of materials rather than deposition 

from the water column. 

Benthic nutrient regeneration is important in determining the fate of organic matter in 

the Louisiana shelf, and therefore the amount of nutrients available for production . Benthic 

nutrient regeneration has been estimated to supply anywhere from 15% to 65% of the 

phytoplankton demand for reduced nitrogen (Boynton et al . 1980; Fisher et al . 1982). 

Previous studies have been performed on the effects of the quantity and quality of materials 

being deposited, and deposition was found to influence benthic remineralization (Hargrave 

1973). Muffler and Suess (1979) found that sedimentation rates are directly related to amounts 

of organic material produced by autotrophs and deposited to the benthos. This implies that 

benthic regeneration is directly proportional to sedimentation (Billen et al . 1991). 

The reactivity of the sediments being deposited is also important in determining benthic 

nutrient regeneration (Canfield 1989) . Studies on the quality of material in relation to benthic 

nutrient regeneration have shown that organic material of marine origin (plankton-derived) is 

about 5 times as reactive as material of terrestrial origin (Hedges et al . 1988) . The research in 

this thesis analyzes the influence of both the quantity and quality of deposited material on the 

patterns of benthic nutrient regeneration on the Louisiana continental shelf. 

The primary objective of this study was to deternune the spatial and temporal patterns 

of benthic nutrient regeneration as controlled by the benthic boundary layer characteristics ; 

particularly the relationship between pore-water nutrient concentrations and sediment 

deposition to benthic nutrient regeneration . This analysis allowed an evaluation of the benthic 

nutrient regeneration trends in the plume region of the continental shelf, and consequently 

elucidated the significance of this source of nutrients to primary productivity . Benthic nutrient 

fluxes were compared to biological as well as geophysical and chemical characteristics at the 

sediment-water interface. Nutrient exchange was also evaluated in terms of the quantity and 

quality of deposited material. The factors compared to nutrient flux for this study were chosen 

a priori based on two criteria; first, those factors which were thought to influence nutrient flux 

based on previous investigations (Dagg et al . 1991 ; Dr. R.R. Twilley, personal 

communication), and secondly, those factors that we had the means to analyze. Quantity of 

input was evaluated by thorium-234 radiochemical techniques . Quality of input was analyzed 

in terms of chlorophyll a sediment concentration, as well as concentrations of carbon, nitrogen, 

and phosphorus in the sediment, and carbon to nitrogen ratios, to distinguish material of 

marine (high quality) versus terrestrial (low quality) origin . 

78 
Benthic nutrient regeneration has been hypothesized to make a significant contribution 

to primary production . The quantification of the effects of the proposed factors on benthic 

nutrient fluxes can further our basic ecological understanding of shelf processes. This new 

understanding can potentially be used in conjunction with ecosystem modeling to help project 

the response of this system to changes in riverine input. The objectives of this study are listed 

below with the statistical approach used to evaluate the factors that control spatial and temporal 

patterns of benthic nutrient regeneration on the Louisiana shelf. 

Objective 1 : To assess the temporal pattern in benthic nutrient regeneration rates using 

two cruises on the Louisiana continental shelf. This objective was evaluated by compiling 

benthic nutrient regeneration rates for each nutrient by cruise, only for those stations that were 

sampled on both cruises. The values were then subjected to a t-test to determine significant 

differences between cruises for each of the five nutrients. 

Objective 2: To examine spatial trends in benthic nutrient regeneration rates on the 

Louisiana continental shelf. Regeneration rates at all stations from both cruises were combined 

and subjected to cluster analysis. Major divisions were mapped, and the average flux rate for 

each grouping was calculated and included on a map (one for each nutrient). These maps were 

then interpreted based on their degree of agreement with the conceptual model (Figure 5.1) and 

other relevant factors (e .g . macroinfauna). 

Objective 3: Determine the importance of pore-water concentration gradients to rates of 

benthic nutrient regeneration in conjunction with other physical, chemical, and biological 

characteristics . Regressions of benthic nutrient flux with the respective pore-water nutrient 

gradient were plotted. Any points not agreeing with the linear curve fit were identified by 

cruise and station and were examined for any anomalous characteristics that may account for 

the variability. A stepwise regression on all sediment characteristics was used as a basis for the 

examination of those characteristics which most influence the nutrient fluxes. 

Objective 4: Examine the relative influence of quantity versus quality of depositional 

flux to benthic nutrient regeneration rates. A stepwise regression analysis of deposition rates 

versus regeneration rates was employed, comparing the input of material to benthic fluxes . 

Quantity of deposition was quantified in terms of sediment fluxes to the benthos derived from 

thorium-234 analysis. Quality of deposition was determined by taking sediment deposition 

rates and multiplying them by the concentration of carbon, nitrogen, phosphorus, and 

chlorophyll a. 

5.3 Site Description 

Sampling for this research took place on the Louisiana continental shelf, in the dispersal 

area of the Mississippi River's Southwest Pass. The research was accomplished during two 

cruises aboard the R/V Pelican (LUMCON (Louisiana Universities Marine Consortium), 

Cocodrie, LA). The cruises were named COMUS 1 and COMUS 3; an acronym based on the 

title of a companion NSF project (Coastal Ocean Margin Uranium Study, Dr. B.A. McKee, 

Principal Investigator) . COMUS 1 took place from 1 to 12 November 1993, during a typical 

low flow period for the Mississippi River. The mean flow of the river during the cruise was 

10,157 m3 s-1 , ranging initially from 12,712 m3 s-1 and decreased to 9,884 m3 s-1 by the end 

of the cruise (Figure 5 .2) . The lowest discharge level of 1993 was two days later, 14 

November, at 9,604 m3 s-1 . COMUS 3 took place from 1 to 10 June 1994, during typically 

high river flow, with an average flow of 13,098 m3 s-1 for this time period . Initial flow was 

18,788 m3 s-1 and decreased to 11,480 m3 s-1 by the end of the cruise (Figure 5.2). About 

79 
two weeks prior to COMUS 3, on 11 May 1994, the river was at its peak discharge for the 

year at 32,592 m3 s-1 . All river discharge numbers were taken at Tarbert Landing, Louisiana 

by the U.S . Geological Survey (personal communication, Dr. B.A. McKee). 

Nineteen benthic stations were established over the two cruises, seven of these stations 

were repeated during both cruises for a total of twelve independent sampling stations (Figure 

5.3). These repeated stations were all along the 50 m contour. The station labels consisted of 

a letter (from B to H with increased distance from the mouth of Southwest Pass) followed by a 

number (which denotes station depth in m). The latitude and longitude of these stations were 

listed in Table 5 .1 . The one notable exception in the sampling stations was South Pass (SP) as 

it was the only station outside of the Southwest Pass dispersal system. The temperature, 

salinity and oxygen conditions for the bottom waters of the stations were outlined in Table 5 .1 . 

Conspicuously absent was any hypoxia during either cruise . 

5.4 Methods 

5 .4 .1 Field Sampling 

The continuous flow or chemostat system for determining benthic nutrient flux as 

outlined by Miller-Way (1994) and Miller-Way and Twilley (1996) was employed. This 

microcosm system consisted primarily of a water reservoir, the sediment cores, and a peristaltic 

pump (Figure 5 .4). The pump drew water from the reservoir into the cores, and from the 

cores into sampling vials. The system was plumbed with thick-walled (1 .6 mm wall thickness) 

TygonTM tubing (to reduce the slight reaeration found by Miller-Way and Twilley 1996) and 

polypropylene connectors at tubing junctions . Effluent tubing was routed through a series of 

three-way valves and allowed to flow into a common waste line when sampling was not 

occurring. During sampling, the effluent was directed into a series of vials via the appropriate 

three-way valve. Flow rate was controlled by the IsmatecT"' multichannel cartridge pump which 

can maintain a maximum of 16 lines . The flow rate was set and monitored at a range of 6.5 to 

7.0 nil/min, with residence times ranging from 260 to 280 min. Therefore, during a 9 h (540 

min) incubation period, approximately two complete water volume turnovers should occur. 

A sediment core was a LexanT'" tube 15 cm in diameter and 30 cm in height, which was 

sealed off at either end by Plexiglas disks. These disks were held tight by rubber caps on the 

bottom and rubber sleeves on the top, both fastened with screw clamps. The top Plexiglas disk 

had three holes which were sealed with rubber septa to serve as ports for inflow lines, outflow 

lines, and for direct sampling access . The influent and effluent ports consisted of stainless 

steel needles (which have Luer lock" fittings) inserted through a rubber septum into the 

overlying water of the core, and attached to the appropriate lines with polypropylene 

connectors . 

Field sampling consisted of subcoring box cores to a sediment depth of approximately 

20 cm. Intact cores were capped with plexiglas disks and rubber caps for safe transport to the 

incubation chamber. Bottom water was collected at the site by the shipboard CTD system 

(which also recorded ambient temperature and dissolved oxygen) . The water was filtered 

through a 0.45 gm nylon cartridge filter and a 0.20 gm GelmanTM capsule filter in series to 

remove most bacteria and suspended particulate matter. This filtration allowed the assumption 

that any changes in nutrient concentrations between influent and effluent lines can be attributed 

solely to processes at the sediment-water interface in the cores. The filtered water was then 

placed in the reservoirs (a 20 liter rectangular carboy) and the ambient dissolved oxygen 

80 
conditions were maintained by either increasing (an aquarium pump) or decreasing (nitrogen 

gas) oxygen concentrations . 

Cores were placed in the incubation chamber, which consisted of a large NalgeneT" tub, 

with water at ambient temperature controlled by a NeslabT" CFT 25 heat circulator (and ice 

when necessary) . One difference in the present system as compared to the system described by 

Nishio et al . (1982), was the presence of a control line which originated in the reservoir, 

bypassed the cores, and emptied directly into a sampling flask. This line served to correct for 

any tubing-induced changes in concentrations . Benthic flux rates were based on the change in 

concentration of each measured nutrient between the influent (control) and effluent lines. Flux 

rates were calculated according to the following equation : 

(CI - CE) * flow rate / core surface area = flux; 

where CI was the influent concentration (control line) of a nutrient, and CE was its effluent 

concentration. Nutrient regeneration rates were expressed as ~Imol m-2 h-1 . Influent 

concentrations were measured by sampling a single control line for both of the cores from the 

corresponding station. 

A homogeneous overlying water column in the core was maintained by an internally 

mounted NalgeneTM stir bar suspended from the top plexiglas disk. A variable speed stirrer 

(Cole-ParmerTM Micro-V model no. 4805-00) was placed upside down on the top of the core 

and adjusted so that the water is well mixed without disturbing the sediment-water interface. 

Small plexiglas squares were also internally mounted from the top plexiglas disk to prevent any 

possible vortexing that may have been caused by the stirring . 

For each of the two research cruises, sampling consisted of 2 cores per station. Each 

pair of cores was incubated for 9 h, with sampling for ambient nutrient flux every 3 h, giving a 

total of 3 sampling intervals for each set of cores. After the incubation, breakdown of the 

sediment cores consisted of measuring different parameters on different cores, due to the 

destructive nature of some of the techniques (Figure 5.5). One core was sectioned at 1 cm 

intervals to a depth of 4 cm (in a hypoxic chamber (glove box) to prevent precipitation of Fe+3 

and P04 in the pore-water) . The four samples were then centrifuged and the pore-water 

decanted and filtered with GFF 25 mm filters (again in an hypoxic chamber) for nutrient 

analysis. In addition, the first core was used for a redox profile (prior to being cut) . The 

second core was also used for a series of measurements, including a sediment plug for bulk 

density and concentration of carbon, nitrogen, and phosphorus, a smaller plug for chlorophyll 

analysis, and finally, the remaining sediment was sieved for macroinfauna. 

234Thorium profiles in the sediment were examined for excess 234Th during both 

cruises, and deposition rates calculated according to McKee et al . (1983) . Deposition refers to 

the "temporary emplacement of particles on the seabed," (McKee et al . 1983) . 234Thorium is 

rapidly adsorbed onto particles and deposited to the seabed with those particles, as the 

23athorium becomes buried and isolated from new atoms, the radioactive decay can be 

measured. A typical profile with depth shows a logarithmic decrease in radioactivity, and 

deposition can be estimated from the slope of that line (McKee et al . 1983). The 234Th profile 

was from a third core taken at the stations and was done as a part of the COMUS project (Dr. 

B.A. McKee, Principal Investigator) ; only the seven stations along the 50 m contour are 

included in this analysis. This gave a representative seasonal rate of deposition, as the rate is 

integrated over a 100 day period due to the 24 .1 day half-life of 234Th. 

81 
5.4 .2 Analytical Techniques 

Depth profiles of temperature, salinity, and oxygen were made at each station with a 

SeaBird CTD, a Beckman RS-5 salinometer, and an Orbisphere oxygen meter. Nutrient 

regeneration rates were determined for inorganic nitrogen (ammonium, nitrate, and nitrite), 

phosphate, and silicate. Samples for ambient flux and pore-waters were stored in 4 ml sample 

cups, capped, and frozen until analysis on an Alpkem RFA/2 autoanalyzer at the University of 

Southwestern Louisiana following the procedures of Strickland and Parsons (1972), with the 

exception of ammonium, which was determined after Parsons et al . (1984) . 

Redox measurements (Whitfield 1969) were made in the first core at 1 cm intervals to a 

depth of 4 cm, including measurements of the overlying water and at the sediment-water 

interface, using a platinum electrode mounted on a modified micrometer for precise control of 

depth. Bulk density was determined by cutting the point off of a 50 ml syringe and inserting it 

into the core to a depth of 2 cm. The sediment plugs were placed in pre-weighed 250 ml 

bottles and frozen . The samples were later thawed, weighed for wet weight, dried at 60°C for 

72 h, and weighed again for dry weight. The dried plugs were ground and analyzed for total 

phosphorus by dissolving the ashed (480°C for 3 h) sediment in HCl and assaying for P04 

concentrations (Parsons et al . 1984; Aspila et al . 1976). Total carbon and nitrogen were 

determined from the same ground sediment with a LECO Elemental Analyzer. 

Chlorophyll concentrations in a 1 cm3 sediment plug were measured by adding 5 ml of 

DMSO/acetone and incubating for 1 h in the dark. The mixture was centrifuged, the 

supernatant decanted and read on a Turner fluorometer (calibrated at K=237.8) . Macroinfauna 

was extracted by first placing the entire core into a bucket and spraying the core thoroughly 

with sea water. Then, the contents of the bucket were passed through a 1 mm sieve. Once the 

core was sieved, any retained organisms on the sieve were transferred to a 125 ml bottle with 

10% formalin and Rose Bengal, and then frozen. Upon return to the laboratory, the organisms 

were thawed and identified to general taxonomic group, counted for density, dried for 72 h at 

60°C, and weighed. They were then ashed at 500°C for 6 h and re-weighed. 

5 .4 .3 Statistical Analyses 

The statistical methods employed included four models applied to four different 

objectives . The first of these objectives was to determine any temporal difference in nutrient 

flux . A t-test was used to evaluate any difference between the two cruises for each nutrient 

flux . Only the stations sampled during both cruises (i .e . the stations along the 50 m contour) 

were used in this analysis. A cluster analysis was performed to uncover any spatial patterns in 

nutrient flux. All nutrient flux data from both cruises were combined to determine the 

groupings of all the stations . These groupings were then mapped. 

The significance of nutrient pore water concentrations in determining nutrient flux was 

analyzed by plotting the measured fluxes of N03, NH4, P04, and Si04 against pore water 

concentration gradient with depth (from the overlying water to 1 cm). Expected fluxes were 

calculated according to Fick's first law (which is based on concentration gradients, Li and 

Gregory 1974), and the relationship of these calculated fluxes to actual observed fluxes was 

examined. A stepwise regression was run for each nutrient flux against all other factors 

measured. The results of the stepwise regressions were used to attempt to explain any outliers 

from the linear curve fit of observed flux to pore-water gradient. 

82 
The final objective was to ascertain the relative influence of the quantity versus quality 

of sedimentation on benthic nutrient regeneration. A stepwise regression was run for each 

nutrient flux against the quantitative flux of sediments to the benthos, as well as against the 

qualitative indices (carbon, nitrogen, phosphorus, and chlorophyll a deposition). Linear curve 

fitting was used on all analyses, non-linear methods were attempted, but never elucidated 

patterns in the data better than the linear model. All statistics were run on JMP 3 .0 .2 for 

Macintosh (Quadra 650). 

5.5 Results 

5.5 .1 Benthic Fluxes 

Benthic nutrient regeneration rates in gmol m-2 h-1 were calculated for each flux 

measurement, and the average rate (with standard error bars) for each station per cruise was 

plotted in Figure's 5.6a and 5 .6b. The station at South Pass (SP) was removed in these 

Figure's due to differences in scale, but the values for this station can be found in each of the 

respective tables . Nitrate fluxes ranged from 54.9 to -255.9 gmol m-2 h-1, N02 fluxes ranged 

from 30.5 to -9.7 gmol m-2 h-1, NH4 fluxes ranged from 686.1 to -22.4 gmol m-2 h-1, P04 

fluxes ranged from 77 .2 to -9.0 ~i.mol m-2 h-1, and Si04 fluxes ranged from 794.3 to 69.9 

gmol m-2 h-1 for the Southwest Pass dispersal system over both cruises. 

Pore-water concentrations for N03 and N02 were all below 0.3 gmol L-1 at all depths, 

with the exception of the top cm at South Pass (COMUS 3), and thus were excluded from 

graphical analysis. The general trend for NH4, P04, and SiOq was a general increase in 

concentration with depth (Figures . 5.7, 5 .8, 5 .9). Ammonium pore-water concentrations 

ranged from 328.2 to 28 .0 gmol L-1 in the 0-1 cm section and from 527 .9 to 40.3 gmol L-I in 

the 3-4 cm section. Phosphate pore-water concentrations ranged from 32.8 to 0.3 gmol L-1 in 

the 0-1 cm section and from 53.7 to 1 .0 gmol L-1 in the 3-4 cm section. Silicate pore-water 

concentrations ranged from 212 .1 to 21 .5 gmol L-1 in the 0-1 cm section and from 332.4 to 

21 .7 gmol L-1 in the 3-4 cm section. Phosphate and Si04 concentrations in the pore-waters 

both appeared to be lower during COMUS 3 for most stations . 

5.5 .2 Sediment Characteristics 

Bulk density tended to decrease with distance from Southwest Pass until the region of 

maximum deposition (around D-50 and E-50), and then began to increase (Figure 5 .10; values 

ranged from 0.26 to 0.79 g/cm3. Chlorophyll a concentrations did not show a consistent 

trend. There was a sharp peak in November at station E50 (1 .1 gg/cm3), while the rest of the 

values ranged from 0.80 gg/cm3 at SP (COMUS 3) to 0.17 gg/cm3 at E-20 (COMUS 3) 

(Figure 5 .11) . There are spatial differences in total carbon, nitrogen, and phosphorus 

concentrations (% dry weight) in shelf sediments (Figure 5.12) . Carbon values ranged from 

2.02°Io to 1 .04%, nitrogen values ranged from 0.34°Io to 0.09°Io, and phosphorus values 

ranged from 0.090% to 0.040°Io. Stations G-50 and H-50 were noticeably lower in total 

nitrogen for both cruises, but no other patterns were obvious. 

The redox (Eh) profiles with depth, from the overlying water to 4 cm into the sediment, 

showed a rather consistent pattern during COMUS 1, with a sharp change in Eh from the 

surface to lcm (Figure 5.13) . The majority of Eh values in the sediment ranged from 100 to 

200 mV. For COMUS 3, however, the profiles divided into two groups (neither of which 

83 
showed as sharp a change in Eh with depth) . The first of these two groups, which was at a 

lower Eh (between 0 and -100 mV at 4 cm), consisted of the three stations closest to the river 

outlets: B-50, C-50, and SP. The remaining stations for COMUS 3 ranged in Eh values from 

140 to 320 mV at 4 cm, and all were above 300 mV at the sediment surface. 

Benthic macroinfauna was dominated by polychaete worms and some small 

crustaceans, while only one vertebrate, a four inch long worm eel, was found during either 

cruise (Figure 5 .14) . Macroinfaunal density and biomass showed no consistent pattern across 

the shelf; the largest number and biomass occurred at station B-50 during COMUS 3 and 

station F-20 during COMUS 1 . 

5.6 Discussion 

5.6 .1 Factors Controlling Nutrient Regeneration 

Sediment Characteristics : Table 5.2 displays the results of stepwise regressions of each 

nutrient flux to the series of benthic core parameters. For all nutrients except for P04, porewater gradients showed the highest correlation to benthic nutrient regeneration. Phosphate 

fluxes, however, showed the strongest correlation to a redox gradient. Nitrate fluxes showed 

the strongest correlation to a single parameter, which was the gradient in N03 concentration 

between the overlying water and pore-water at 4 cm depth. The r2 value was 0.95, compared 

to only 0.61 for the next highest correlation between flux and first variable for the other 

nutrients . This high correlation reflects variation in the N03 concentration in the overlying 

water, since all N03 pore-water values at 4 cm depth were below accurate detection . 

Therefore, the only difference in the two pore-water gradients for N03 and N02 was the values 

in the 1 cm section for station SP. Macroinfaunal mass and density occurred in the regression 

analysis only for the fluxes of NH4 and P04. They were not significant to the other fluxes 

(Table 5 .2) . 

Deposition versus Regeneration : Analysis of deposition was performed for seven 

stations on the 50 m depth contour, which were sampled during both cruises. Table 5.3 

displays the results of stepwise regressions of each nutrient flux in relation to the deposition of 

sediments and the relative deposition of carbon, nitrogen, phosphorus, and chlorophyll a. The 

first graph in each of the Figs . 5.15 through 5.19 is of the mean nutrient flux versus the flux of 

sediments to the benthos . The second graph in the series is between the nutrient flux and the 

depositional component with the highest correlation to the flux as determined by the stepwise 

regression analysis. Finally, the third graph shows the correlation of nutrient flux to the 

component which most logically would be related to nutrient flux . Since the relative magnitude 

of burial and regeneration of nutrients was determined somewhat by the quantity and quality of 

particulate matter being deposited, the indices graphed in Figs. 5.15 to 5.19 should reflect the 

differential effects of deposition to benthic nutrient regeneration (Zeitzschel 1980). 

For four of the five nutrients, rates of benthic regeneration did not follow the pattern of 

sediment deposition to the seabed . The exception was P04 flux, which was correlated to 

deposition to the seabed . In Figures. 5 .15 to 5 .19, A) represents the correlation of the 

respective nutrient flux to sediment deposition. Both B) and C) represent two indices of 

quality, B) is the index that showed the highest correlation and C) is the index that was 

hypothesized a priori to show the highest correlation. In all cases, some index of quality of 

deposition had the highest correlation to nutrient flux (B), but never was that qualitative index 

the hypothesized index (C). For example, deposition of phosphorus explained Si04 flux better 

84 
than sediment inputs, but chlorophyll a inputs had a lower correlation with fluxes than 

sediment deposition (Figure 5 .19) . The above trend was found throughout the analysis, as 

chlorophyll a deposition best explained N03 flux, carbon deposition best explained N02 and 

P04 flux, and phosphorus deposition best explained NH4 and Si04 flux . It is evident that the 

quality of deposited material, such as chlorophyll a or nutrient content, did not improve the 

correlations with benthic fluxes than the quantitative inputs . 

Pore-Water Gradients versus Regeneration : A gradient (dC/dz) of nutrient 

concentrations between overlying waters and pore waters (to a depth of 1 cm) was calculated 

from the difference in these concentrations and dividing by two (2) cm. Values for overlying 

water were based on concentrations in the control line (CI) of the chemostatic system. This 

pore-water gradient was compared to specific benthic fluxes for N03, NH4, P04, and Si04, 

(Figure 5.20) . Nitrate, and particularly NH4 were the only two nutrients that showed a 

significant r2 value in the correlations between fluxes and pore-water gradients. The difference 

in the r2 value in Figure 5.20 to those in Table 5 .2 (when pore-water gradient from OW to l cm 

was the first step) can be attributed to the fact that station SP was included in the stepwise 

regression analysis, yet was removed for Figure 5 .20. For N03 fluxes, there was a much 

stronger correlation of benthic flux to the OW to 4 cm pore-water gradient (Table 5.2). Neither 

bulk density nor any other sediment characteristic provided insight into the two most 

conspicuous outliers, H-50 and D-50 from COMUS 3 . Nitrate flux has been shown to be a 

function of N03 concentration in the overlying water. This high correlation with pore-water 

gradient can be attributed to the consistently low concentration of N03 in pore-waters, thus the 

overlying water concentrations control N03 benthic flux . 

Ammonium benthic fluxes showed the strongest correlation to pore-water concentration 

gradients (Figure 5.20) . Examination of points that deviated from the curve fit provided some 

insight into what factors caused variation in this the correlation. Stations B-50 (COMUS 3) 

(which showed the highest densities of macroinfauna among stations for either cruise (Figure 

5.14)) and C-50 (COMUS 3) both had decapods present. The larger burrows of the decapods 

(in contrast to the ubiquitous polychaetes) combined with the higher sediment deposition at 

these stations, resulted in higher fluxes and pore-water gradients. Station E-50 (COMUS 1) 

had the highest deposition rate measured on either cruise, and also had a significant flocculent 

layer (personal observation) . Ammonium remineralization occurs primarily in the uppermost 

flocculent layer (at the sediment-water interface), which is not adequately sampled when cores 

are sectioned at 1 cm resolution (Kemp et al . 1982). Neither P04 nor Si04 benthic fluxes were 

correlated with pore-water gradients of respective nutrient concentrations . Examination of the 

other factors for the stations with extreme deviations from the linear model did not reveal any 

anomalies that could account for the high variability. 

Diffusive fluxes of nutrients across the sediment-water interface are influenced, in 

addition to concentration gradients, by sediment porosity (QS) (Berner 1980) . Expected fluxes 

can be calculated using the equation J = -QS D dC/dz, where J is the diffusive flux, 0 is the 

sediment porosity, D is the nutrient specific diffusion coefficient, and dC/dz is the nutrient 

pore-water concentration gradient (Berner 1980; Li and Gregory 1974). When the observed 

fluxes were compared to expected fluxes calculated from the pore-water gradients (Li and 

Gregory 1974), even less correlation was found than when observed fluxes were compared to 

only the pore-water gradients. In most cases, expected fluxes remained at least one order of 

magnitude lower than the actual measured fluxes and the r2 values were even lower than those 

observed in Figure 5.20. The activity of organisms in the benthos is equally as influential in 

determining nutrient fluxes (Berner 1980; McCaffrey et al . 1980) . The macroinfaunal densities 

85 
and biomass of the stations do not account for the incongruence of the observed and expected 

fluxes . More focus is needed on the utility of an advective model to calculate expected benthic 

fluxes that may reflect patterns of observed fluxes . Aller and Aller (1992) also stressed the 

importance of meiofauna, which can increase nutrient flux by 20-40% from porosity-tortuosity 

changes. Berner (1980) noted the difficulty of understanding diagenesis in the zone of 

bioturbation due to the multi-faceted chemical and physical changes caused by the presence of 

burrowing organisms. 

5.6 .2 Temporal Differences 

Significant differences in benthic nutrient fluxes between COMUS 1 and COMUS 3 

occurred for N03 and P04, but not for N02, NH4, and Si04 (Table 5.4, Figure 5 .21) . Nitrate 

uptake is a function of N03 concentration in the overlying water (Figure 5 .22), and therefore 

the significant temporal difference can be explained by the seasonal supply of this nutrient to 

the study area by the river (Turner and Rabalais 1991). Higher river discharge during COWS 

3 supplied greater amounts of N03 to the plume region of the shelf causing significantly greater 

rates of benthic uptake . 

There was also a seasonal difference in benthic P04 regeneration rates, associated with 

differences in river discharge. However, P04 regeneration rates did not show a correlation to 

P04 concentrations in the overlying water (Figure 5 .22) . This lack of correlation to benthic 

P04 flux was also observed by Nixon et al . (1980) for Narragansett Bay. However, P04 

regeneration does show a high correlation to indices of sediment input (Figure 5 .18) . This 

indicates that a difference in riverine input is also driving the difference in P04 flux, since P04 

is adsorbed onto particles that are transported to the seabed . However, there was no significant 

difference in the total phosphorus concentrations in the sediments between the two cruises. In 

addition, P04 fluxes in November were higher than those in June, suggesting that the mean 

bottom temperature differences of 22.4°C (COMUS 1) and 20.7°C (COMUS 3) may have 

influenced rates of P04 regeneration . This is in agreement to results found by Nixon et a1 . 

(1980) in a cold temperate system. 

Another possible explanation for the temporal difference in P04 flux is the flooding in 

the midwestern portion of the Mississippi River drainage basin two months prior to COMUS 

1 . Heavy agricultural runoff could have loaded the shelf sediments with phosphorus, thus 

accounting for the greater P04 fluxes on the first cruise . 

The general lack of any temporal difference in the regeneration rates of NH4 and Si04 

indicates that there is a steady supply of these nutrients to the water column and/or to the pore 

waters from the seabed . Rates of benthic regeneration of NH4 and Si04 were similar during 

both cruises, indicating less dependence on river supply for exchange across the sedimentwater interface. Thus the benthic regeneration rates of 150 and 300 [tmol m-2 h-1 of NH4 and 

Si04 respectively provide a consistent supply of these nutrients to the shelf ecosystem. Many 

researchers have observed that benthic nutrient regeneration serves as a buffer of nutrient 

availability from periodic allochthonous delivery of nutrients (Hartwig 1974; Rowe et al . 1975 ; 

Nixon et al . 1976 ; Rowe et al . 1977; Propp et al . 1980; Flint and Kamykowski 1984). The 

lack of difference in NH4 and Si04 regeneration rates, with respect to the differences in 

riverine input, suggests that benthic regeneration of these nutrients accommodates the seasonal 

demand for nutrients in the water column. 

86 
5 .6 .3 Spatial Trends 

The conceptual model of the Mississippi River plume area presented in Figure 5.1 

shows three distinct areas: 

Turbid Zone- High deposition and high benthic regeneration ; 

Second Zone- Moderate deposition and moderate benthic regeneration ; 

Third Zone- Low deposition and low benthic regeneration . 

Spatial groupings of benthic nutrient fluxes based on cluster analyses were used to test this 

conceptual model (Figures . 523 - 5.27) . Values of benthic flux for stations in each spatial 

group were averaged and included on the maps. The high rates of benthic flux at South Pass 

(SP) resulted in this station being grouped separately from the other stations . Average rates of 

benthic fluxes in the other groups indicate that rates decrease for each respective nutrient with 

increased distance from the mouth of Southwest Pass. Station SP, which was sampled directly 

in the mouth of South Pass (3 m depth), was typical of a Turbid Zone station. The patterns 

established in the dispersal system of Southwest Pass represent trends of the Second and Third 

Zones (with decreased flux rates at increased distance from Southwest Pass) . Thus the spatial 

zonation followed the general pattern suggested in the conceptual model. 

Patterns of benthic N03 uptake (Figure 5 .23) divide into three main regions of the shelf 

based on cluster analysis. Stations SP and D-50 each constituted a unique cluster due to 

extreme uptake rates of -454.3 and -71 .5 gmol m-2 h-1 respectively . The next group has an 

average rate of -25.9 gmol m-2 h-1 and consists of stations that are adjacent to and distant from 

D-50. Although this does not correspond to the linear nature of the conceptual model, N03 

uptake did not show a correlation to deposition, and therefore other factors such as bottom 

currents may have supplied D-50 with higher concentrations of N03 than the surrounding 

stations. Examination of benthic parameters reveals that N03 regeneration rates are driven 

primarily by the N03 concentrations in the overlying water. This is reflected in the correlation 

of fluxes to the nutrient gradient from the overlying water to the pore-waters, since all porewater concentrations were below limits of detection (Table 5.3). Total nitrogen concentrations 

in the sediments do not seem to influence the groupings, as only G-50 and H-50 show 

significantly lower concentrations (Figure 5.12). 

Nitrite (Figure 5 .24) on the other hand, shows a more specific link of nutrient flux to 

distance from Southwest Pass . The first two groups show a decreasing release of N02, while 

the third starts to show slight uptake. Again, the pattern seems to be linked to concentrations 

of N02 in the overlying water, just as they were for N03 (Table 5 .3). Nitrate uptake is linked 

to denitrification in the sediments, and therefore NOZ release occurs at those stations where 

N03 uptake is great, and decreases as N03 uptake decreases. 

The pattern of NH4 fluxes decreases with increased distance from Southwest Pass with 

the exception of one station (Figure 5.25) . F-20 is grouped with stations B-50 and C-50, 

which are near Southwest Pass and the average rate of benthic NH4 regeneration for this group 

is 299.1 gmol m-2 h-1 . The station F-20 also had high biomass (and density) of macroinfauna 

caused by a shallow burrowing worm eel present in core two (2) resulting in resuspension and 

increased NH4 flux . F-20 had the highest macrofauna density and biomass among all stations 

during COMUS 1 (Figure 5.14) . Macroinfaunal density did not make a significant 

contribution to the stepwise regression models with NH4 fluxes, although animal excretions 

and subsequent burrow ventilation have been shown to enhance NH4 fluxes (Aller 1980; 

Pelegri 1994). In agreement with the conceptual model, proximity to the river (and 

87 
consequently, amount of deposition) seems to be directly associated with the magnitude of 

nitrogen flux across the sediment-water interface. 

Phosphate and Si04 fluxes (Figs. 5.26 and 5.27, respectively) both clustered into two 

main groups with higher rates in regions proximal to the riverine source . These results are in 

closer agreement with the conceptual model (Figure 5.1) than the nitrogen flux rates, in that 

regeneration rates decrease with an increase in distance from the riverine source . The stations 

in each cluster were similar for both P04 and Si0413 with the exception of F-20 which was in 

the cluster of higher rates for Si04, but was in the distal cluster of lower rates for P04. This 

can be attributed to the presence of macrofauna at station F20, and their differential effect on 

Si04 compared to P04. Since P04 showed the highest correlation with sediment deposition, 

the stations in the proximal cluster follow the projected pattern of sediment dispersal from 

Southwest Pass (personal communication, Dr. B.A. McKee). 

Benthic regeneration rates at South Pass were unique for all of the nutrients measured. 

Benthic fluxes of N03, N02, and NH4 at SP were higher and consistent with the conceptual 

model. Nitrate uptake rates were higher, while rates of N02 and NH4 release were greater at 

SP. However, P04 and Si04 benthic fluxes were uptake at SP compared to regeneration 

observed in the Southwest Pass dispersal area. Uptake rates at SP can perhaps be attributed to 

the extreme deposition observed in this area (personal communication, Dr. B.A. McKee), the 

high sediment load thus burying P04 and Si04 faster than they can be regenerated. This was 

also reflected in the lower pore-water concentrations for P04 and Si04 at SP. 

Patterns of benthic nutrient regeneration on the Louisiana continental shelf are nutrient 

specific. Phosphate and Si04 fluxes are linked directly to deposition from the river, NH4 

fluxes seem dominated by in situ production, and N03 fluxes are a results of dissolved inputs 

(either from a riverine source or upwelling) . Silicate fluxes varied by one-half between 

clusters, while N03, NH4, and P04 all varied by an order of magnitude from the high to low 

grouping. 

5.6 .4 Deltaic versus Pelagic Systems 

The seabed of deltaic systems receive inputs of terrigenous materials while deposits to 

sediments in pelagic shelves consists primarily from material produced in situ . A comparison 

of these two types of continental shelves provides insight into the relationship of benthic 

nutrient regeneration to the quantity and quality of deposited material (Figure 5.28) . Average 

rates of N03 and NH4 benthic fluxes show little difference among continental shelves and thus 

little direct link to riverine influence. Despite low riverine inputs of organic N to pelagic 

systems, the total inputs of organic N to pelagic systems may not be different from deltaic 

systems due to regional upwelling. 

Phosphate and Si04, however, show greater benthic regeneration rates in deltaic 

systems (Figure 5 .28) . Since P04 is transported to the seabed via adsorption onto particles, 

more effective scavenging and transport of P04 occurs in continental shelves with higher 

suspended solids . The fact that Si04 shows higher regeneration rates in deltaic systems is 

related to the direct riverine input of Si04 from terrestrial sources. The difference observed for 

P04 and Si04 fluxes does not mean that a higher quality of deposition does not increase 

benthic regeneration rates, but rather that there could be a significant difference in the amount 

of P04 and Si04 reaching the benthos in deltaic systems. In agreement with the spatial 

88 
analyses, riverine deposition establishes the patterns of P04 and Si04 regeneration, while N03 

and NH4 benthic fluxes are controlled by other factors such as upwelling and in situ 

production . 

5.7 Summary 

Benthic nutrient regeneration of N03, N02, NH4, P04, and Si04 was examined on the 

Louisiana continental shelf in November 1993 and June 1994. Temporal differences in these 

fluxes between cruises were examined, as well as any spatial trends occurring across the shelf. 

Pore-water gradients for all the nutrients were obtained and compared to the nutrient fluxes ; 

sediment deposition was also compared to nutrient fluxes . Sediment characteristics and 

macroinfaunal densities and biomass were used in conjunction with these objectives to explain 

spatial and temporal variation. 

Modeling of nutrient fluxes from pore-water gradients proved difficult, as correlations 

of regeneration to pore-water gradients were moderate for N03 and NH4, and non-existent for 

P04, and Si04. Quality of deposition does provide a slightly better correlation to regeneration 

than quantity of deposition, but the precise index of quality does not fit our hypothesis . Nitrate 

and P04 were the only two nutrients that showed a significant difference in regeneration with 

respect to time. These differences are a result of different riverine inputs . 

Pore-water modeling of nutrient fluxes and examining the influence of the quantity 

versus quality of deposition on benthic nutrient regeneration are complex issues, encompassing 

more than the preliminary parameters examined here. A more in depth analysis is needed to 

really look at these issues, in particular, data involving greater deposition rates. This research 

provides a starting point for further examination and perhaps experimentation on these benthic 

processes. 

89 
MISSISSIPPI RIVER PLUME/LOUISIANA SHELF ECOSYSTEM 

INPUT 

PLUME REGION / MID-SHELF REGION 

Low Production New Production Regenerated Production 

(0-10 ppt) (10-20 ppt) (20-30 ppt) 

TURBID ZONE 

REGENERATION 

N,P,Si 

DEPOSITION 

O 

02 

SEDIMENT f i 

DENITRIFICATION 

BURIAL 

(C,D Stations) 

REGENERATION 

0 

REGENERATION 

N,P,Si 

DEPOSITION 

REDISTRIBUTION 02 

(E,F,G Stations) 

REGENERATION 

REGENERATION 

DEPOSITION 

REDISTRIBUTION 

REGENERATION 

N,P,Si 

1 

2 

I 

DENTTRIFICATION 44,~j DENTTRIFICATION 

BURIAL BURIAL 

Figure 5 .1 . Conceptual model of the fate of organic matter in the plume and mid-shelf region of the Louisiana 

continental shelf (from Twilley et al. 1994). 
35,000 

30,000 

M 25,000 

20,000 

15,000 

, 

10,000 

5,000 

0 

1 2 3 4 5 6 7 8 9 10 11 12 

November 1993 

z 

M 

0 

v 

1 2 3 4 5 6 7 8 9 10 

June 1994 

Figure 5.2 . Histogram of the discharge of the Mississippi River at Tarbert Landing, Louisiana 

(USGS) for COMUS 1 and COMUS 3 in m3 S-1 . The middle two bars indicate 

the lowest and highest river stages of the year, while their dates show their 

proximity to the respective cruises . 
MISSISSIPPI RIVER BIGHT 

W 

H 

29 .60 

29 .20 

28.80 

28.40 

A ~n v 

Gt F2o E20 

4 

E30 

- - 

SP 

E50" ~ 

0 m 

F50 ~ 0 C50 ~" 

E60 

H50 G50 

Nov '93 only 

June '94 only 

Both cruises 

-90.40 -90.00 -89.60 -89.20 

LONGITUDE 

Figure 5.3 . Sampling stations on the two cruises: COWS 1 (November 1993) and 

COMUS 3 (June 1994) . 

92 
Direction of Flow 

Tubing Control 

Influent 

Effluent 

110 

w 

.~.J-. : . 

. : . p: . : . : . : . ; . . . : . ; . ; . : . : . : . : . 

Gas Tank 

(to control for 

ambient oxygen) 

Water Reservoir 

Peristaltic 

Pump 

Intact 

Sediment 

Core 

Sampling 

Flasks 

Figure 5 .4 . Core chemostat system used to determine benthic nutrient flux rates (Miller-Way and Twilley 1996). 
Two Cores per Station 

Nine hour incubation at simulated ambient conditions 

Flux at 3 hr. 

CORE 1 CORE 2 

Flux at 3 hr. 

Flux at 6 hr, 

Flux at 9 hr. 

Flux at 6 hr. 

Flux at 9 hr. 

After Incubation: 

a. Redox Profile 

b. Porewaters 

a. Sediment Plug 

-bulk density 

-carbon 

-nitrogen 

-phosphorus 

b. Chlorophyll Plug 

c . Macroinfauna 

Figure 5.5 . Sampling regime used to analyze each core during and after incubation. 

94 
110 

C 

fD 

Fo 

a 

a ~° 

o x 

u 

Cp 

n 

o a ., 

z 

O 

o 

a 

o z 

o 

n 

n 

CD 

.. . . 

r* 

NH4 Flux (pmol m-2 h-1) 

N) w ~ cry ~ 

0 0 0 0 o g O o 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

NH4 Flux (Vmol m-2 h-1 

"r t.~ w ~ cn o~ 

°o o °o °o °o °o o° °o 

B50 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

NOZ Flux (Vmol m-2 h-1) 

o ~n o cn O 

C 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

NOZ Flux (Vmol m-2 h-1) 

o in o v~ o v~ 

B50 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

N03 Flux (pmol m-2 

N03 Flux (pmol m -2

. . . 
COMUS 1 

60 

.. 

50 

N i 

40 0 

30 

20 

O 

a 10 

n 

600 

'-' 500 

N 

400 

0 

300 

x 

200 

a 

O 

100 

n 

0 0 0 0 0 0 0 0 0 0 0 

V7 N kn N M v') ~O N ~n ~n V~ 

= U a) W W W W Li . fi.~ C7 

COMUS 3 

60 

.. 

50 

N 

0 40 

30 

a 

20 

O 

a' 10 

0 

600 

500 

N 

400 

0 

300 

K 

200 

Ov 

100 

n 

0 0 0 0 0 0 0 0 0 0 0 

v1 kn tn N cn v~ ~D N tn kn VII 

a1 U A w w w w w w C7 x 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

GU U Ll W W W W fi fi ~ ~ 

kn kn tn " m kn ~o c~ W) 

C7 x as U Q W W W W w w C7 x 

Figure 5 .6b. Average flux per station for P04 and Si04 for each cruise (with 

standard error bars), excluding the station at South Pass (SP). 

96 
COMUS 1 

-0.5 

0.0 -

kj"\ 

0.5 - O 

1 .0 - 

1 .5 -

2.0-

8 

'~ 2.5 O 

3 .0 - 

3 .5 r~ 

4.0 

0 100 200 300 400 

NH4, gmol L-1 

COMUS 3 

-0 .5 

0.0 

0.5 

1 .0 .. 

1 .5 

2.0 

2.5 

3 .0 

3 .5 

4.0 

500 600 

O 

O 

O 

0 100 200 300 400 500 600 

NH4, gmol L-1 

f- B50 

-"- C50 

~- D50 

-f- E50 

-p-- F50 

O G50 

H50 

--p- E30 

--~--- E60 

- F20 

-f B50 

- C50 

~ D50 

-"-- E50 

-{}--- F50 

O G50 

- H50 

-p- E20 

---~e-e-- SP 

Figure 5.7 . Pore-water profiles with depth for NH4, from the overlying water (-0 .5 cm) 

to a depth of 3 .5 cm into the sediment. All values are in ~tmol L-1 . 

See Figure 5.3 . for station locations. 

97 
COMUS 1 

-0.5 

0.0 

0.5 

u 

1 .0 -

.~ 

;6 

1 .5 

2.0-

2.5 

3 .0 

3 .5 

4.0 

0 

COMUS 3 

0.5 

0.0 

0.5 

1 .0 

1 .5 

.~ 

y 2, .0 

"~ 2.5 

3.0 

3.5 

4.0 

0 10 20 30 40 50 60 

P04, ~Lmol L-1 

-At- B50 

-0 - C50 

~- D50 

-"- E50 

.- .p. .,- F50 

O G50 

H50 

-O- E30 

---- E60 

F20 

f B50 

-~- C50 

-~ D50 

-"- E50 

---p-- F50 

O G50 

~- H50 

-p-- E20 

SP 

Figure 5 .8 . Pore-water profiles with depth for P04, from the overlying water (-0.5 cm) 

to a depth of 3 .5 cm into the sediment. All values are in ~tmol L-1 . 

See Figure 5.3 . for station locations. 

98 

10 20 30 40 50 60 

P04, ~tmol L-1 
COMUS 1 

-0 .5 

0.0 

0.5 

1 .0 

?~ 1 .5 

.+ 

y 2.0 

2.5 

3 .0 

3 .5 

4.0 

O 

O 

0 50 100 150 200 

Si04, gmol L-1 

COMUS 3 

-0 .5 

0.0 

0.5 

.. 

1 .0 

v 

~" 1 .5 

., 2.0 

E 

2.5 

3.0 

3 .5 

4.0 

250 300 350 

- B50 

--"- C50 

-,~ D50 

~- E50 

--~- F50 

O G50 

H50 

--p- E30 

--~+-- E60 

F20 

0 50 100 150 200 250 300 350 

Si04, 9mol L- I 

Figure 5 .9 . Pore-water profiles with depth for Si04, from the overlying water (-0.5 cm) 

to a depth of 3 .5 cm into the sediment. All values are in ~tmol L-1 . 

See Figure 5.3 . for station locations. 

f B50 

--f- C50 

~- D50 

-~- E50 

--p- F50 

O G50 

---A- H50 

-p-- E20 

---- SP 

99 
o.s 

0.7 A) COMUS 1 

~^ 0.6 

U 

0.5 

'~ 0.4 

Q 

,x 0.3 

a 

0.2 

n 

0.1 

B50 C50 D50 E50 F50 G50 H50 E30 E60 F20 

0.8 

0.7 B) COMUS 3 

0.6 

U 

0.5 

0.4 

Q 

0.3 

0.2 

0.1 

n 

B50 C50 D50 E50 F50 G50 H50 E20 SP 

Figure 5.10. Sediment bulk density (g/cm 3) by station for A) COMUS 1 

and B) COMUS 3. 

100 
1 .2 

1 .0 

U 

ci 0. 

A) COMUS 1 

0.6 

0 

U 

0.4 

. ., 

0.2 

0.0 

1 .2 

B50 C50 D50 E50 F50 G50 H50 E30 E60 F20 

B) COMUS 3 

1 .0 

an 

0.8 

0 0.6 

0 

U 

0.4 

.r., 

Ci 0.2, 

0.0 

B50 C50 D50 E50 F50 G50 H50 E20 SP 

Figure 5.11 . Sediment chlorophyll a concentrations (Vg/cm 3 at lcm 

depth) by station for A) COMUS 1 and B) COMUS 3 . 

101 
0 

N 

ITI 

CD 

N 

ao 

CD 

o 

M 

F~+

" 

O 

o 

v~ ~n 

a o ... 

D 

O 

n 

o 

n o 

o 

vo 

CD 

a 

0 

0 

B50 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

SP 

Sediment Phosphorus (% dry wt) 

0 0 0 0 0 0 0 0 0 0 0 

O O O O O O O O O O 

Sediment Phosphorus (% dry wt) 

0 0 0 0 0 0 0 0 0 0 0 

O O O O OO O O O O O 

B50 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

SP 

Sediment Nitrogen (% dry wt) 

0 0 0 0 0 0 0 0 

O O ~ ~ N tJ w w 

O LA O LA O LA O U 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

SP 

B50 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

SP 

Sediment Nitrogen (% dry wt) 

0 0 0 0 0 0 0 0 

O O ~ ~ N N w w 

O Cn O vi O V~ O vi 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

SP 

B50 

C50 

D50 

E20 

E30 

E50 

E60 

F20 

F50 

G50 

H50 

SP 

Sedimen

O O 

O ~ 

Sedimen

0 0 

O ~A 
-1 

U 

a 

N 

3 

w -1 

0 

U 

a 

N 

v' 3 

COMUS 1 

0 

0 

4 +-

-100 

-100 

0 100 200 

Eh, mV 

COMUS 3 

300 400 

-F B50 

--f- C50 

f D50 

-"- E30 

-p-- E50 

p E60 

-A- F20 

-O- F50 

-9-; - G50 

---~---- H50 

~- B50 

-"- C50 

-~ D50 

-~- E20 

-p- E50 

p F50 

-~ G50 

-p- H50 

~ SP 

Figure 5 .13. Redox profiles (Eh, mv) with depth in sediment (-1 corresponds to overlying water 

Eh, and 0 to the sediment-water interface Eh value) . 

0 100 200 300 400 

Eh, mV 
COMUS 1 

0.45 1 

0.40 

0.35 ° 

~0 0.30 `oo coq 

0.25 

0.20 i -

0.15 

0.10 

0.05 KNJ 

0.00 ~ 

I~ l~V R~l ._._ . ~ NA 

B50 C50 D50 E30 E50 E60 F20 F50 G50 H50 

0 

3' 

9 

c 0 

w 0 

0 .30 

COMUS 3 

-0.25 

0 

3 0 .20 

0 .15 

0 

0 0 .10 

0.05 

0.00 

f35U C:SU L)-,)U FLU EJU l^JU CiSU HJU 

L 

N 

UZ 

N 

° 1 

a 

C 

w 

O 

V 

25 

B50 C50 D50 E30 E50 E60 F20 F50 G50 H50 B50 C50 D50 E20 E50 F50 G50 H50 

Figure 5.14. Dry weight (gdw), ash-free dry weight (afdw), and number of individuals per 

core of benthic macroinfauna by station for COMUS 1 and COMUS 3. 

d 

UZ 

N~7 

F 

51 

a C 

.C 1 

w 

O 

U 

104 

acv wv uDv nIv cju rev wv n3v 

B50 C50 D50 E30 E50 E60 F20 F50 G50 H50 
r2 = 0.003 

20- 

r 0 - ` 

c~ 

E r 

-20 

3 ~ 

7 -40 

O -60 

z 

-ioo 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .0 

Sediment Deposition (g cm-2 mo"1) 

40 "' 

r2 = 142 

20- 

U . 

L 

Fi 

N~ 

0 - 

- 20- 

7 -40-

-60-

Z 

-SO 

-100 

0.00 0.05 0.10 n 

40 

20 

0 

s 

N 

1 2 -20 

F 

3 -40 

K 

-60 

O 

z 

-SO 

-100 

Chlorophyll a Deposition (mg cm -2 mo 1) 

GO 

0.0 0.5 1 .0 1 .5 2.0 2.5 3.0 3.5 

Nitrogen Deposition (mg cm-2 mo"1) 

Figure 5 .15. Linear correlations of mean N03 fluxes to three indices of input 

to the seabed: A) Bulk sediment input, B) Input of the component 

with the highest correlation, and C) Input of the component that 

was hypothesized to have the highest correlation (nitrogen). 

105 
A) 

s 

0 

E i 

K 

N 

O 

z 

s 

s 

V 

W 

N 

O 

z 

" rz =0.123 

8 - 

6 -

4-

2 ' 

0 

-L 

-4-

-6 1 

Sediment Deposition (g cm -2 mo 1) 

B) 

in 

0 

E 

z 

K 0 

O z 

t2 = ~.1 g5 

8 - 

6-

4-

2 

0-

-2- 

-4 

-6 

0 2 4 6 8 10 12 14 16 18 20 

Carbon Deposition (mg cm -2 mo 1) 

Cl 

10 -

0 /=O, 

8 

6 

4 

L 

0' 0 

_2 

-4 

-6 

0.0 0.5 1 .0 1 .5 2.0 2.5 3.0 3.5 

Nitrogen Deposition (mg cm -2 mo') 

Figure 5.16. Linear correlations of mean N02 fluxes to three indices of input to 

the seabed : A) Bulk sediment input, B) Input of the component 

with the highest correlation, and C) Input of the component that 

was hypothesized to have the highest correlation (nitrogen) . 

106 
A) 

6 

5 

4 

E 

3 

z 

a xl 

z 

r2 = 0.254 

00 - 

00 -

00 -

00 - E 

"~ 

00 - 

01 

00 

--- --- --- --- --- --- --- --- --- --- ---

Sediment Deposition (g cm-2 mo"1) 

6 

5 

M B 

s 4 

V 

3 

Y 

2 

zl 

V r2 = 0.357 

00 - 

00 -

00-

00- 

00- 

0 

00 /' - --- --- --- --- --- --- -- 

Phosphorus Deposition (mg cm-2 mo 1) 

600 

t2 = 0.128 

500- 

:c 400-

0 

E 

300-

=L 

200 

0 

x 100 

z 

o ~ 

-1,ti 

0.0 0.5 1 .0 1.5 2.0 2.5 3.0 3.5 

Nitrogen Deposition (mg cm -2 mo 1) 

Figure 5.17. Linear correlations of mean NH4 fluxes to three indices of input to 

the seabed: A) Bulk sediment input, B) Input of the component 

with the highest correlation, and C) Input of the component that 

was hypothesized to have the highest correlation (nitrogen). 

107 
A) 

N 

c 

0 

8 

3 

x 

a 

d. 

r 

N 

0 

E 

3 

x 0 

ov 

2 

r 

50 -_ 0.604 

40 

30 

20 

10 

0 ~ 

10 

60 

50 

s 40 

N 

c 

G 

0 30 

8 

3 Zo 

to 

0 

in 

Sediment Deposition (g cm-2 mo"1) 

Rl 

2 r 

= 0.721 

0 2 4 6 8 10 12 14 16 18 2C 

Carbon Deposition (mg cm -2 mo"1) 

C) 

"" 2 

r 

50 = 0.585 

40-

30 -

M 20 

10 

0 ~ 

10 

Phosphorus Deposition (mg cm -2 mo 1) 

Figure 5.18 . Linear correlations of mean P04 fluxes to three indices of input to 

the seabed : A) Bulk sediment input, B) Input of the component 

with the highest correlation, and C) Input of the component that 

was hypothesized to have the highest correlation (phosphorus) . 

108 
60 

500 

N 400 

O 

300 

x 0 

200 

a O 

100 

A) 

r 2 = 0 .334 / 

Sediment Deposition (g cm -2 mo 1) 

B) 

r2 = 0.402 

400-

E 

0 

=L 3007 

x 

o 

200] 

100 

0 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 

Phosphorus Deposition (mg cm -2 mo 1) 

C) 

600 

r2 = 0.293 

500 

s 

<v 400 

o 

300 

K 

200 

O 

100 

0 

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 

Chlorophyll a Deposition (mg cm -2 mo 1) 

Figure 5.19. Linear correlations of mean Si04 fluxes to three indices of input 

to the seabed: A) Bulk sediment input, B) Input of the component 

with the highest correlation, and C) Input of the component that 

was hypothesized to have the highest correlation (chlorophyll a) . 

109 
40 

r2 = 0.236 

20- 

0 - 

- C3 H50 

-20- 0 

E 

-40 

a 

-60 

z 

Mir 

t 

"C3 D50 

-100 

-14 -12 -10 -8 -6 -4 -2 0 

N03 Gradient (dC/dz) 

bU 

50 

40 

N 

~ 30 C 1 E30 0 

E 

3 20 

C3 E20 "C1 F50 

10 

M C1 H50 

-10 

0 2 4 6 8 10 12 14 16 

POy Gradient (dGdz) 

600 -

r 2 = 0.468 

500 - 

400-

300 -

0 

C3 B50 

C3 C50 

200-

CI E50 

100 

z o M Em S : ft 

-100 

0 20 40 60 80 100 120 

NH4 Gradient (dC/dz) 

600 

500 

400 

N 

300 

3 

200 

100 

Si04 Gradient (dC/dz) 

r2 = 0.005 

" C3 E20 

C3 H50 

Figure 520. Linear correlations of nutrient flux to the pore-water gradient (from overlying water 

to lcm for N03, NH4, P04, and Si04. 

110 
3. 

s 

E 

E Z' 

3 

t . 

O 

z 

n 

~E 

E 

x 

O 

COMUS 1 COMUS 3 

A) 

0 

-10 - 

a 

-20- 

-30 - 

40 - 

1 

-50 

35 

B) 

30 - 

r 

~E 25 -

O 

15 - 

~y 

20-

,., 

10 

5 

0 

COMUS 1 

C) 

i 

r 

E 

z 

x 

z z 

350 

._^ 300 

r 

E 250 

'o 

5200 

150 

K 

0 

100 o° 

so 

COMUS3 

E) 

r 

I.VMW l WMW 3 COMUS 1 COMUS 3 COMUS 1 COMUS 3 

Figure 521 . Average benthic fluxes (± standard error) for each cruise for A) N03, B) P04, C) N02, D) NH4, and E) Si04. 

Only the seven stations sampled on both cruises were used for this comparison (*indicates significant difference 

at alpha--0 .05) . 
N 

r2 = 0.496 

i 

60 

r2 = 0.164 

50 - 

401 

N 

30 

0 

~ Zo 

x 

10 

O 

a 

0 

-10 

0 5 10 15 20 25 30 0.0 0.2 0.4 0.6 0.8 1 .0 1 .2 1 .4 

Influent N03 (gmol L-1) Influent P04 (gmol L-I) 

Figure 5.22. Linear correlations of influent nutrient concentrations and benthic fluxes for N03 and P04. Both 

graphs exclude station Sp (COMUS 3) . 

40 

20 

0 

N 

-GO 

0 

-40 

-60 

O 

z 

-80 

-100 
MISSISSIPPI RIVER BIGHT 

SPATIAL ANALYSIS FOR N03 FLUX 

29.60 

29.20 

W 

d 

a 

28.80 

28.40 

0 

-25 .9 " :1 .5 

-5.5 -454.3 

Nov '93 only 

June '94 only 

" Both cruises 

-90.40 -90.00 -89.60 -89.20 

LONGITUDE 

Figure 5.23. Spatial contours for N03 flux (9mol m-2 h-1) as determined by cluster 

analysis. Numbers indicate average benthic flux for stations within each 

spatial group. 

113 
MISSISSIPPI RIVER BIGHT 

SPATIAL ANALYSIS FOR N02 FLUX 

29.60 

29.20 

A 

d 

28 .80 

-1 .9 1.5 ~ ~ 

5.3 

29.1 

Q Nov '93 only 

June '94 only 

*Both cruises 

28.40 L 

-90.40 -90 .00 -89.60 -89 .20 

LONGITUDE 

Figure 5.24. Spatial contours for N02 flux (9mol m-2 h-1) as determined by cluster 

analysis. Numbers indicate average benthic flux for stations within each 

spatial group. 

114 
MISSISSIPPI RIVER BIGHT 

SPATIAL ANALYSIS FOR NH4 FLUX 

29 .60 

29.20 

W 

A 

H 

28 .80 

28.40 

op v 

0 m 

299.1 

122.9 

35.0 

" 1461 .8 

C)Nov'93 only 

June '94 only 

Both cruises 

-90.40 -90.00 -89 .60 -89.20 

LONGITUDE 

Figure 5.25. Spatial contours for NH4 flux (gmol m-2 h-1) as determined by cluster 

analysis. Numbers indicate average benthic flux for stations within each 

spatial group. 

115 
MISSISSIPPI RIVER BIGHT 

SPATIAL ANALYSIS FOR P04 FLUX 

29 .60 

29.20 

W 

H 

F-~ 

a 

28.80 

28.40 

v 

32.4 

7.s 

" -19.9 

ONov '93 only 

June '94 only 

Both cruises 

-90.40 -90.00 -89.60 -89 .20 

LONGITUDE 

Figure 5 .26. Spatial contours for P04 flux (9mol m-2 h-1) as determined by cluster 

analysis. Numbers indicate average benthic flux for stations within each 

spatial group. 

116 
MISSISSIPPI RIVER BIGHT 

SPATIAL ANALYSIS FOR Si04 FLUX 

29.60 

29 .20 

W 

Q 

28.80 

28.40 

v 

0 m 

439.9 

214.2 -246.4 

ONov '93 only 

(DJune'94 only 

Both cruises 

-90.40 -90 .00 -89.60 -89 .20 

LONGITUDE 

Figure 5 .27 . Spatial contours for SiOq flux (gmol m-2 h-1) as determined by cluster 

analysis. Numbers indicate avergae benthic flux for stations within each 

spatial group. 

117 
Table 5 .1 . Latitude and longitude of sampling stations with bottom CTD conditions for COMUS 1 

and COMUS 3 . 

Bottom Conditions 

Cruise Station Latitude Longitude Temperature Oxygen (mg/1) Salinity (psu) 

COMUS 1 B50 2850.47 8929.07 20.00 2.90 36.49 

COMUS 1 C50 2825.48 8931 .68 20.00 3.00 36.75 

COMUS 1 D50 2856.03 8935 .58 20.00 4.00 not available 

COMUS 1 E50 2856.88 8943 .53 20.00 4.00 34.40 

COMUS 1 F50 2852.51 8950.08 21 .00 4.00 not available 

COMUS 1 G50 2843.19 9001 .97 24.50 5.92 35.18 

COMUS 1 H50 2842.97 9010.08 25 .00 5.34 33.46 

COMUS 1 E30 2902.44 8943 .60 26.00 3.50 34.20 

00 COMUS 1 E60 2851 .65 8943 .52 20.35 4.35 not available 

COMUS 1 F20 2908.26 8950.03 26.80 2.59 32.53 

COMUS 3 B50 2850.48 8929.19 19.80 5.34 36.25 

COMUS 3 C50 2852.51 8931 .52 20.50 6.10 36.15 

COMUS 3 D50 2856.01 8935.60 18.75 3.86 35.98 

COMUS 3 E50 2856.89 8943 .54 20.00 5.20 30.70 

COMUS 3 F50 2852.49 8950.03 19.27 3.10 34.70 

COMUS 3 G50 2842.79 90.02.00 20.40 6.50 not available 

COMUS 3 H50 2842.80 9010.22 20.80 5.80 not available 

COMUS 3 E20 2907.25 8944.56 21 .99 2.21 35.84 

COMUS 3 SP 2900.36 8909.28 24.90 5.20 0.00 
Table 5.2 . Stepwise regression of nutrient fluxes to all nutrient and sediment parameters . 

Nutrient Flux Parameter r-square 

N03 Pore-water gradient (OW-4cm) 0 .95 

Bulk Density 0 .97 

N02 Pore-water gradient (OW- lcm) 0 .61 

Sediment phosphorus 0 .84 

Pore-water gradient (OW-4cm) 0 .93 

C:N ratio 0 .96 

Sediment chlorophyll a 0.97 

Redox gradient (OW- lcm) 0.98 

Sediment nitrogen 0.99 

Sediment carbon 0.99 

Redox gradient (OW-4cm) 0.99 

Bulk density 1 .00 

NH4 Pore-water gradient (OW- lcm) 0.68 

Bulk Density 0.75 

Redox gradient (OW-4cm) 0.80 

Pore-water surface value 0.86 

Sediment carbon 0.88 

Macroinfauna number 0.91 

Macroinfauna dry weight 0.93 

Redox gradient (OW- lcm) 0.94 

Sediment chlorophyll a 0.96 

C:N ratio 0 .96 

P04 Redox gradient (OW- lcm) 0 .40 

Sediment chlorophyll a 0 .50 

Sediment phosphorus 0 .58 

Pore-water gradient (OW-4cm) 0 .72 

Sediment nitrogen 0 .76 

Macroinfauna asked dry weight 0 .80 

Sediment carbon 0 .85 

C:N ratio 0 .88 

Redox surface value 0 .91 

Si04 Pore-water gradient (OW- lcm) 0.30 

Sediment carbon 0.48 

Pore-water surface value 0.57 

Redox gradient (OW-4cm) 0.67 

Bulk density 0.76 

Pore-water gradient (OW-4cm) 0.79 

119 
Table 5 .3 . Stepwise regression of nutrient fluxes to only the 

depositional parameters . 

Nutrient Flux Parameter r-square 

N03 Chlorophyll deposition 0.14 

Nitrogen deposition 0.30 

N02 Carbon deposition 0.19 

Bulk sediment deposition 0.31 

Nitrogen deposition 0.46 

NH4 Phosphorus deposition 0.36 

Bulk sediment deposition 0.59 

P04 Carbon deposition 0.72 

Bulk sediment deposition 0.79 

Si04 Phosphorus deposition 0.40 

120 
Table 5.4 . t-Test on fluxes between cruises COMUS 1 and COMUS 3. 

Nutrient DF t-Test Prob.>Itl Cruise Mean Std. Error 

N03 Flux 66 3 .256 0.0018* COMUS 1 -0.9 9.1 

COMUS 3 -38 .7 7.2 

N02 Flux 66 0.577 0.5657 COMUS 1 2.0 1 .2 

COMUS 3 2.9 0.9 

NH4 Flux 82 0.787 0.4338 COMUS 1 149.6 24.7 

COMUS 3 177.1 24.7 

P04 Flux 82 4.060 0.0001 * COMUS 1 30.4 2.8 

COMUS 3 14.2 2.8 

Si04 Flux 82 0.995 0.3227 COMUS 1 302.5 24 .3 

COMUS 3 336.7 24.3 

alpha = 0.05 ; * denotes significant difference 

121 
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129 
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