Science for Solutions 

NOAA COASTAL OCEAN PROGRAM 

Decision Analysis Series No. 14 

NUTRIENT ENHANCED COASTAL OCEAN 

PRODUCTIVITY 

IN THE NORTHERN GULF OF MEXICO 

UNDERSTANDING THE EFFECTS OF NUTRIENTS 

ON A COASTAL ECOSYSTEM 

Edited by 

Wiilliam J. Wiseman Jr. Nancy N. Rabalais 

Michael J. Dagg Terry E. Whitledge 

APRIL 1999 

US. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Ocean Service 

Center for Sponsored Coastal Research 
DECISION ANALYSIS SERIES 

The Decision Analysis Series has been established 

by NOAAYs Coastal Ocean Program (COP) to 

present documents that contain analytical 

treatments of major issues or topics for coastal 

resource decision makers. The issues, topics, and 

principal investigators have been selected through 

an extensive peer process. To learn more about the 

COP or the Decision Analysis Series, please write: 

NOAA 

National Centers for Coastal Ocean Science 

Coastal Ocean Program (NCOP) 

13 15 East West Highway, Room 9700 

Silver Spring, MD 20910-3282 

Phone: 301-713-3338 

Fax: 301 -7 13-4044 

Web site: http://www.cop.noaa.gov 

Cover image: frequency of bottom water hypoxia (dissolved oxygen less than 2 mgll) for rnidsummer 1985-1999 (based on data of Rabalais, Turner and Wiseman). 
Science for Solutions 

NOAA COASTAL OCEAN PROGRAM 

Decision Analysis Series No. 14 

NUTRIENT ENHANCED COASTAL OCEAN 

PRODUCTIVITY 

IN THE NORTHERN GULF OF MEXICO 

UNDERSTANDING THE EFFECTS OF NUTRIENTS 

ON A COASTAL ECOSYSTEM 

Edited by 

Wiilliam J. Wiseman, Jr. 

Nancy N. Rabalais 

Michael J. Dagg 

Terry E. Whitledge 

APRIL 1999 

U.S. DEPARTMENT OF COMMERCE 

William M. Daley, Secretary 

National Oceanic and Atmospheric Administration 

D. James Baker, Under Secretary for Oceans and Atmosphere 

National Ocean Service 

Nancy Foster, Assistant Administrator 

National Centers for Coastal Ocean Science 

Donald Scavia, Director 

Center for Sponsored Coastal Ocean Research---Coastal Ocean Program 

David Johnson, Director 
This ~ublication should be cited as: 

Wiseman, William J. Jr., Nancy N. Rabalais, Michael J. Dagg, and Terry E. Whitledge (eds.) 1999. Nutrient 

Enhanced Coastal Ocean Productivity in the Northern Gulf of Mexico - - Understanding the Effects of Nutrients on 

a Coastal Ecosystem. NOAA Coastal Ocean Program Decision Analysis Series No. 14. NOAA Coastal Ocean 

Program, Silver Spring, MD. 156 pp. 

This publication does not constitute an endorsement of any commercial 

product or intend to be an opinion beyond scientific or other results obtained 

by the National Oceanic and Atmospheric Administration (NOAA). No 

reference shall be made to NOAA, or this publicaton furnished by NOAA, in 

any advertising or sales promotion which would indicate or imply that NOAA 

recommends or endorses any proprietary product mentioned herein, or which 

has as its purpose an interest to cause directly or indirectly the advertised I product to be used or purchased because of this publication. 1 
TABLE OF CONTENTS 

... Note to Readers ................................................................................................................... 111 

Acknowledgments ................................................................................................................. v 

. . Contributors ..................................................................................................................... v11 

Preface ................................................................................................................................. ix 

List of Figures and Tables .................................................................................................. xi 

Glossary ............................................................................................................................... xv 

Executive Summary .............................................................................................................. 1 

Chapter 1: Introduction 

Background of NECOP .............................................................................................. -5 

Objectives of NECOP .................................................................................................. 5 

NECOP and LATEX .................................................................................................. -7 

Sampling Program ....................................................................................................... 8 

Significance of NECOP ............................................................................................. 11 

References ................................................................................................................ 13 

Chapter 2: Physical Oceanography and Sediment Dynamics 

Introduction .............................................................................................................. 17 

The Ambient Environment .................................................................................... 18 

Plumes ..................................................................................................................... -19 

The Louisiana Coastal Current .................................................................................. 23 

Benthic Sediment Processes ...................................................................................... 27 

Long-term Changes ................................................................................................... 30 

Summary ................................................................................................................... 32 

References ................................................................................................................ -33 

Chapter 3: Long-Term Watershed and Water Quality Changes 

in the Mississippi River System 

Introduction .............................................................................................................. -37 

Landscape Alterations and Anthropogenic Nutrients ................................................. 38 

Changes in Freshwater Discharge .............................................................................. 40 

Changes in Suspended Sediment ............................................................................... 41 

Changes in Nutrients and Nutrient Ratios .................................................................. 41 

Dams and Algal Blooms ............................................................................................ 47 

References ................................................................................................................. 48 
Chapter 4: Nutrient-Enhanced Coastal Productivity 

and Ecosystem Responses 

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

Scales of Variability .................................................................................................. 53 

Factors Controlling Productivity ................................................................................ 57 

Fate of Fixed Carbon ................................................................................................. 59 

Evidence for Long-term Marine Ecosystem Changes ................................................. 64 

Consequences ........................................................................................................... -69 

References ................................................................................................................ -70 

Chapter 5: Hypoxia 

Introduction ............................................................................................................... 79 

Historical Occurrence and Geographic Extent ............................................................ 80 

Data Documenting Hypoxia on the Louisiana-Texas Continental Shelf ..................... 81 

Dimensions and Variability of Hypoxia ..................................................................... 81 

Linkage of Hypoxia with Freshwater and Nutrients from the Mississippi River ......... 88 

Historical Changes in Oxygen Stress ......................................................................... 92 

Effects of Hypoxia on Living Resources .................................................................... 95 

References ................................................................................................................. 97 

Chapter 6: Modeling 

Introduction ............................................................................................................ 103 

Models of Estuarine and Coastal Waters .................................................................. 104 

NECOP Modeling Approach .................................................................................. 105 . . Model Application ................................................................................................... 108 

Phytoplankton Dynamics ........................................................................................ 109 

Dissolved Oxygen Dynamics .................................................................................. 111 

Sensitivity Analyses .............................................................................................. 113 

Forecast Simulations ............................................................................................... 116 

Summary ................................................................................................................ 120 

References .............................................................................................................. 121 

Appendices: 

A . NECOP Scientists and Managers ................................................................................... 127 

B . NECOP Projects ........................................................................................................... 131 

C . NECOP Publications ................................................................................................... 137 

D . A Concise History of NECOP .................................................................................... 155 
Note to Readers 

Nutrient Enhanced Coastal Ocean Productivity in the Northern Gulf of Mexico - Understanding 

the Efect ofNutrients on a Coastal Ecosystem, edited by William J. Wiseman Jr. (Louisiana 

State University), Nancy N. Rabalais, Michael J. Dagg (Louisiana Universities Marine 

consortium), and Terry E. Whitledge (University of Alaska at Fairbanks) is a synthesis of the 

findings and the management implications of research sponsored by the National Oceanic and 

Atmospheric Administration's (NOAA) Coastal Ocean Program (COP) through its Nutrient 

Enhanced Coastal Ocean Productivity (NECOP) study. Enhanced nutrient enrichment fkom 

anthropogenic sources is one of the major stresses imposed on the sea. This is particularly true 

in the northern Gulf of Mexico where the nation's largest river discharges into an otherwise 

oligotrophic sea. This report contains six chapters devoted to syntheses of specific topics 

important to understanding the oceanography of the area and the cumulative impact of riverborne nutrients on the coastal ecosystem. 

The COP provides a focal point through which NOAA, together with other organizations with 

responsibilities for the coastal environment and its resources, can make significant strides toward 

finding solutions to critical problems. By working together toward these solutions, we can 

ensure the sustainability of these coastal resources and allow for compatible economic 

development that will enhance the well-being of the nation now and in future generations. The 

goals of the program parallel those of the NOAA Strategic Plan. 

A specific objective of the COP is to provide the highest quality scientific information to coastal 

managers in time for critical decision making and in formats useful for these decisions. To help 

achieve this, COP inaugurated a program of developing documents that would synthesize 

information on issues that were of high priority to coastal managers. As a contribution to the 

Decision Analysis Series, this report provides a critical synthesis and foundation for new 

research and assessment in the northern Gulf of Mexico. A list of available documents in the 

Decision Analysis Series can be found on the inside back cover. 

As with all of its products, the COP is very interested in ascertaining the utility of the Decision 

Analysis Series, particularly in regard to its application to the management decision process. 

Therefore, we encourage you to write, fax, call, or E-mail us with your comments. Please be 

assured that we will appreciate these comments, either positive or negative, and that they will 

help us direct our future efforts. Our address and telephone numbers are on the inside front 

David Johnson, Director I I 

NOAA Center for sponsored Coastal Ocean Research 

Coastal Ocean Program 
ACKNOWLEDGMENTS 

The NECOP scientists would like to thank the officers and crews of the R/Vs Pelican, Longhorn, 

Gyre, and Acadiana and the NOAA ships Malcolm Baldrige and Chapman without whose good 

will and assistance the data collection effort could not have been possible. They would also like 

to thank Donald K. Atwood, William F. Graham, Lt. Michael Gallagher, LCDR, John Novano, 

Lt. Victor Ross, Michael Dowgiallo, John Wickham, and James Hendee of NOAA and Dr. 

Thomas Bright of Texas A&M University without whose administrative and managerial support 

the program would neither have been undertaken nor completed. Finally, we wish to thank Ms. 

Jami Donley for editing this volume. 

Over the years numerous agencies and institutions have supported the research that provided the 

bases for this synthesis: NOAA Coastal Ocean Program through the NECOP Program, NOAA 

Ocean Assessments Division, NOAA National Undersea Research Center, Minerals 

Management Service, Louisiana Board of Regents Education Quality Support Fund, Louisiana 

Sea Grant College Program, Department of Energy Climate Change program, National Science 

Foundation, Environmental Protection Agency Gulf of Mexico Program, Louisiana State 

University and Louisiana Universities Marine Consortium. We have had the help over the years 

of many research assistants and associates, students, visiting scientists, volunteers, divers, data 

managers, ship captains, and ships' crews. 
CONTRIBUTORS 

Victor J. Bierman, Ph.D. 

Limno-Tech, Inc. 

Ann Arbor, Michigan 

Michael J. Dagg, Ph.D. 

Louisiana Universities Marine Consortium 

Chauvin, Louisiana 

Scott P. Dinnel, Ph.D. 

Naval Oceanographic Offiice 

Stennis Space Center, Mississippi 

F. Quay Dortch, Ph.D. 

Louisiana Universities Marine Consortium 

Chauvin, Louisiana 

Brian J. Eadie, Ph.D. 

Great Lakes Environmental Research Laboratory 

National Oceanic and Atmospheric Administration 

Ann Arbor, Michigan 

Gary L. Fahnenstiel, Ph.D. 

Great Lakes Environmental Research Laboratory 

Lake Michigan Field Station 

National Oceanic and Atmospheric Administration 

Muskegon, Michigan 

Scott C. Hiz, M.S. 

Limno-Tech, Inc. 

Ann Arbor, Michigan 

Dubravko Justic', Ph.D. 

Coastal Ecology Institute 

Louisiana State University 

Baton Rouge, Louisiana 

Steven E. Lohrenz, Ph.D. 

Institute of Marine Sciences 

University of Southern Mississippi 

Stennis Space Center, Mississippi 

Brent A. McKee, Ph.D. 

Department of Geology 

Tulane University 

New Orleans, Louisiana 

Kristine A. Nelson, B.S. 

Limno-Tech, Inc. 

Ann Arbor, Michigan 

Terry A. Nelsen, Ph.D. 

Atlantic Oceanographic & Meteorological Lab 

National Oceanic & Atmospheric Administration 

Miami, Florida 

David P. Podber, Ph.D. 

Limno-Tech, Inc. 

Ann Arbor, Michigan 

Naureen A. Qureshi, Ph.D. 

Oceanography and Coastal Sciences 

Louisiana State University 

Baton Rouge, Louisiana 

Nancy N. Rabalais, Ph.D. 

Louisiana Universities Marine Consortium 

Chauvin, Louisiana 

Donald G. Redalje, Ph.D. 

Institute of Marine Sciences 

University of Southern Mississippi 

Stennis Space Center, Mississippi 

Barun K. Sen Gupta, Ph.D. 

Department of Geology and Geophysics 

Louisiana State University 

Baton Rouge, Louisiana 

R. Eugene Turner, Ph.D. 

Coastal Ecology Institute 

Louisiana State University 

Baton Rouge, Louisiana 

Terry E. Whitledge, Ph.D. 

Institute of Marine Science 

University of Alaska at Fairbanks 

Fairbanks, Alaska 

John L. Wickham, M.S. 

Coastal Ocean Program 

National Oceanic & Atmospheric Administration 

Silver Spring, Maryland 

William J. Wiseman, Jr., Ph.D. 

Coastal Studies Institute 

Louisiana State University 

Baton Rouge, Louisiana 

Dong-wei Zhu, M.S. 

Lirnno-Tech, Inc. 

Ann Arbor, Michigan 

vii 
PREFACE 

The National Oceanic and Atmospheric Administration initiated the Nutrient Enhanced 

Coastal Ocean Productivity (NECOP) study in 1990 as the first rrlajor field effort of the 

newly created Coastal Ocean Program. The long-term goal of the NECOP was to 

conduct generic studies of nutrient loading to coastal ecosystems of the United States, 

and, in particular, to develop an improved understanding of the effect of anthropogenic 

nutrient loading in these ecosystems. The shelf waters of Louisiana and Texas, which are 

influenced by the Mississippi River and Atchafalaya River systems, were chosen as the 

study area for NECOP. 

The specific objectives of NECOP were to: 

determine the degree to which coastal primary productivity has been enhanced in 

areas receiving terrestial nutrient inputs; 

' determine the impact of the resultant enhanced productivity on water quality, and; 

determine the fate of fixed carbon in coastal areas and its impact on living 

resources within the affected coastal ecosystems. 

From 1990 to 1996, some 40 federal and academic scientists from approximately 14 

institutions joined in the interdisciplinary NECOP investigation of the continental shelf of 

the northern Gulf of Mexico. 
LIST OF FIGURES AND TABLES 

Figures 

Figure 1.1. 

Figure 1.2. 

Figure 1.3. 

Figure 2.1. 

Figure 2.2. 

Figure 2.3. 

Figure 2.4. 

Figure 2.5. 

Figure 2.6. 

Figure 2.7. 

Figure 2.8. 

Figure 3.1. 

Figure 3.2. 

Figure 3.3. 

Figure 3.4. 

Location map of the NECOP study area ........................................................... 6 

Distribution of stations for LUMCON mid-summer shelfwide survey 

and more frequent sampling along transect C. .... .. . ... .. .. . . .. .. . . . . . . . ... .. . . ... .. 9 

Location of field sampling stations used in model calibration for July 

1990. ............................................................................................................. 10 

Schematic diagram of mixing processes and secondary circulations 

within a bottom-detached effluent plume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Characteristic frontal boundary positions near the Mississippi River 

Delta ...... .. . . .. . .. .. .. . .. . . .. . . . . . . .. .... ..... . . . . . . . . .. ... . . . . .. ...... ... . . .. ..... . .... .. . .. . . . . .. . . .. .. . . . . ... -21 

A 56-month record of sediment and water discharge for the Mississippi 

River at Tarbert Landing. ... . ... ..... .. . . ... . .. .... . .. .. .... . ......... ........... ... . . . . .. . . .... . . . . .... 22 

Contours of Brunt-Vaisala frequency across the Louisiana 

Coastal Current on year-day 178, 1986. ........................................................ 24 

Cross-section of eastward speed through the Louisiana 

Coastal Current during summer ...................................................................... 26 

Low-passed, near-bottom currents at the station C6B ..................................... 26 

Seasonal sediment deposition patterns in the Mississippi River Bight ...... . ..... 29 

Sediment accumulation rates in the Mississippi River Bight, based on 

210Pb geochronologies. . ... . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . -32 

Nitrogen and phosphorus fertilizer use this century in the United States up 

to 1997. ......................................................................................................... 39 

The annual average discharge of the Mississippi River at Vicksburg, 

Mississippi, with a 10-yr moving average superimposed ............................... 40 

The relationship between the concentration of nitrate-N and total nitrogen 

in the lower Mississippi River at St. Francisville, Louisiana. .......................... 43 

Monthly average concentrations of nitrate, silicate and total phosphorus 

in the lower Mississippi River for periods indicated. ..................................... 44 
Figure 3.5. 

Figure 3.6. 

Figure 4.1. 

Figure 4.2. 

Figure 4.3. 

Figure 4.4. 

Figure 4.5. 

Figure 4.6. 

Figure 4.7. 

Figure 4.8. 

Figure 4.9. 

Figure 4.10. 

Figure 4.1 1. 

Figure 5.1. 

Figure 5.2. 

The average annual concentration of nitrate and silicate in the Mississippi 

River at New Orleans. ................................................................................... 45 

Monthly variation in average nutrient ratios in the Mississippi River at St. . . . . Francisville, Louisiana. .................................................................................. 45 

The concentration of chlorophyll a in surface waters at LATEX sampling 

stations, between 89S0W and 97OW and water depths of 10 and 100 m 

for the periods April 1992, October 1992, April 1993, July 1993, 

April 1994, July 1994 .................................................................................... 53 

Water column-integrated primary production and chlorophyll 

concentrations along the salinity gradient of the Mississippi River plume. .... .54 

Relationship of surface chlorophyll a and salinity for six LATEX cruises 

from 89S0W to 97OW. ................................................................................... 54 

Water column-integrated primary production from the outflow region of 

the Mississippi River as a function of month of the year shown in 

relationship to mean river discharge at Tarbert Landing from 1955- 1992. .... ..55 

Left panel: Cross correlation function for Mississippi River nitrate flux 

at Tarbert Landing and net production of the upper water column. Right 

panel: best-fit time-delayed linear model for the regression of net 

production on nitrate load.. ............................................................................ 56 

Station locations within the Mississippi Bight for transect C, mooring 

locations, sediment core stations, and stations of Fucik and 

Ward et al.. .................................................................................................... 56 

Average monthly surface layer phytoplankton major taxa composition at 

station C6A and C6B from 1990-1995. .......................................................... 57 

Relationship between mean primary production for the combined central 

and eastern regions of the Mississippi River bight and riverborne nitrate 

and nitrite flux, nitrate and nitrite concentration at Venice and 

orthophosphate concentrations at Belle Chase. ............................................. 58 

Ranges of water column-integrated primary production and associated 

vertical flux of POC measured with free drifting sediment traps in the 

Mississippi River plume during different months of the year. ......................... 61 

The average concentration of biologically bound silica in sediments 

in each section of three dated sediment cores from stations in the 

Mississippi River Bight in depths of 27 to 50 m. ............................................ 67 

Organic carbon accumulation rates at stations 10 and 2. ................................. 68 

Distribution of bottom water less than 2 mg/l dissolved oxygen 

in mid-summer of the years indicated ............................................................. 82 

Estimated areal extent of bottom water hypoxia for mid-summer cruises 

in 1985-1999 .................................................................................................. 83 

xii 
Figure 5.3 

Figure 5.4. 

Figure 5.5. 

Figure 5.6 

Figure 5.7. 

Figure 5.8. 

Figure 5.9. 

Figure 5.10. 

Figure 5.1 1. 

Figure 6.1. 

Figure 6.2 

Figure 6.3. 

Figure 6.4. 

Figure 6.5. 

Figure 6.6. 

Figure 6.7. 

Figure 6.8. 

Figure 6.9. 

Figure 6-10. 

Distribution of frequency of occurrence of mid-summer hypoxia 

over the 60-to 80-station grid from 1985-1999. .............................................. 83 

Comparison of bottom water oxygen less than 2 mg/l for three cruises 

on the Louisiana-Texas shelf in July 1993. ..................................................... 84 

Cross-shelf distribution of dissolved oxygen less than 2 mg/l 

and less than 1 mg/l for 1992 and 1993. ......................................................... 85 

Time-series plots of near-bottom dissolved oxygen concentration 

at station C6B for 1993. ................................................................................. 86 

Bottom water dissolved oxygen for 1985-1992 average conditions 

compared with 1993 ....................................................................................... 87 

Panels showing oxygen surplus and bottom oxygen deficit ........................... .9 1 

Station locations within the Mississippi River bight for hypoxia 

monitoring on transect C, mooring locations, coring stations. ......................... 93 

Glauconite grain abundance and the Shannon-Wiener Diversity Index 

of ostracods for station 10 compared to the application of fertilizer 

in the United States for the years - 1900-1990. ............................................... 94 

Changes in benthic foraminiferans with stratigraphic depth 

in 2'0Pb-dated sediment cores from the Mississippi River bight. ...................... 95 

Location map of study area for NECOP water quality model. ...................... 105 

Schematic diagram of principal model state variables and processes 

in NECOP water quality model. ................................................................... 106 

Model spatial segmentation grid for the Louisiana Inner Shelf portion 

of the Gulf of Mexico ................................................................................. 107 

Component analysis of phytoplankton growth rates in the calibrated 

model as a funtion of temperature, light and nutrient limitations 

in selected surface offshore segments. .......................................................... 109 

Component analysis of loss processes for gross primary production in the 

calibrated model in selected offshore surface segments. ............................... 110 

Component analysis of loss processes for dissolved oxygen 

in the calibrated model in selected bottom offshore segments ....................... 112 

Results of sensitivity analyses for dissolved oxygen and chlorophyll 

concentrations relative to base model calibration for July 1990. ................... 114 

Forecasted responses of average dissolved oxygen concentrations 

to nitrogen load reductions from the MAR under 1985, 1988 and 1990 . . summer-average conditions.. ....................................................................... 1 17 

Forecasted responses of average chlorophyll concentrations 

to nitrogen load reductions from the MAR under 1985, 1988 and 

1990 summer-average conditions. ............................................................... 1 18 

Forecasted responses of average dissolved oxygen concentrations 

... 

Xlll 
to nitrogen and phosphorus load reductions from the MAR conposited 

for 1985. 1988 and 1990 summer-average conditions ................................... 119 

Table 1.1 . Summary of NECOP research activities ............................................. 12 

Table 3.1. Average nutrient concentrations for four stations 

on the lower Mississippi River for two time periods ...................................... 42 

Table 3.2. Historical changes in concentrations and atomic ratios of nitrogen, 

phosphorus and silica in the lower Mississippi River and the northern Gulf 

of Mexico .................................................................................................... 47 

xiv 
Glossary 

algae 

allochthonous 

ammonium 

ammonium nitrate 

anaerobic 

anoxia 

anthropogenic 

AOML 

benthic organisms 

benthos 

biochemical 

oxygen demand 

bloom 

a group of aquatic plants, many of which are microscopic, that are among 

the principal primary producers in freshwater and marine ecosystems; 

examples of algae are phytoplankton, green and brown "pond scum," and 

large marine seaweeds 

consisting of or formed from transported material originally accumulated 

elsewhere 

NH,'; inorganic nitrogen compound; ion of ammonia; algal nutrient 

NH,NO,; inorganic nitrogen compound; algal nutrient; fertilizer 

component 

without oxygen 

the absence of dissolved oxygen 

caused or produced by the influence of humans 

NOAA Atlantic Oceanographic and Meteorological Laboratory 

organisms living in or on the bottom of aquatic environments, e.g., 

polychaetes, clams, snails 

organisms living on the bottom of aquatic ecosystems 

organic matter in the water column; water column oxygen demand 

an explosive increase in the population of phytoplankton; algal blooms are 

often associated with eutrophic conditions and are frequently composed of 

noxious algae species 
Bsi biogenic silica 

burial sedimentation and sequestering of organic matter at the bottom of the sea 

carbon fixation conversion of inorganic carbon to organic carbon by photosynthesis 

carbonaceous rate of oxygen uptake by aerobic organisms in degrading organic matter 

biochemical oxygen 

demand 

CBOD 

chlorophyll 

see carbonaceous biochemical oxygen demand 

pigments found in plant cells that are active in harnessing energy during 

photosynthesis 

compensation depth depth at which primary production (photosynthesis) equals respiration; 

depth of no net production; compensation point; one percent depth 

COP Coastal Ocean Program 

copepod tiny marinelfreshwater crustaceans; mostly planktonic; occur in vast 

numbers; main food link between phytoplankton and larger animals 

ctenophore phylum of marine gelatinous, jellyfish-like zooplankton; voracious feeders 

of plankton; a comb jelly 

cyanobacteria photosynthetic bacteria important as planktonic primary producers; 

formally called blue-green algae (Cyanophyta); some free-living species 

are capable of nitrogen fixation 

demersal living at or near the bottom but capable of active swimming 

denitrification release of gaseous nitrogen or breakdown of nitrates to nitrites and 

ammonia by microorganisms under low oxygen conditions 

density stratification the characteristic of marine waters whereby their density tends to increase 

with depth; the strength of the stratification indicates how rapidly the 

density increases in the downward direction 

xvi 
diagenesis all physical and chemical processes (e.g., compaction and cementation) 

that occur to sediment after initial deposition 

diatom mostly unicellular algae important as planktonic primary producers with 

two part cell wall (frustule) composed of silica; division Bacillariophyta 

DIN see dissolved inorganic nitrogen 

dinoflagellate single-celled organisms which include part of the phytoplankton; main 

contributors to marine bioluminescence; some toxic species (e.g., red 

tides); algal division Dinophyta or Pyrrophyta; protozoan order 

Dionoflagellida 

dissolved inorganic water soluble inorganic nitrogen compounds (e.g., nitrate, nitrite, 

nitrogen ammonium, ammonia) 

dissolved organic water soluble organic matter (carbon); exuded or lost from organisms; part 

carbon of dissolved organic matter 

dissolved organic dissolved organic matter excreted or lost from organisms; includes 

matter dissolved organic carbonlnitrogen 

dissolved organic dissolved organic nitrogen excreted or lost from organisms (e.g., 

nitrogen amino acids, peptides, proteins, urea) 

DOC see dissolved organic carbon 

doliolid free swimming, planktonic invertebrate filter feeder of the chordate 

subphylum Urochordata with a barrel-shaped body; a salp; class 

Thaliacea; e.g., genus Doliolum 

DOM see dissolved organic matter 

DON see dissolved organic nitrogen 

xvii 
eutrophication an increase in organic production of an ecosystem, usually associated with 

high rates of nutrient supply 

extinction coefficient measure of the extent light intensity is reduced as it passes through the 

water column; affected by amounts of suspended particles and dissolved 

substances in the water 

foraminifera single-celled, planktonic and benthic amoeba-like animal with a 

chambered test, most often consisting of calcium carbonate; commonly 

used as indicator of past geological and environmental conditions from 

sediment cores 

fiscal year; fiscal year of U.S. federal government is October 1st to 

September 30th 

glauconite type of green silicate mineral common in sedimentary environments 

LATEX Louisiana-Texas Shelf Physical Oceanography Program 

LCC Louisiana Coastal Current 

LIS Louisiana Inner Shelf 

LUMCON Louisiana Universities Marine Consortium 

LSU Louisiana State University 

MAR Mississippi-Atchafalaya River system 

metadata descriptions of data 

microbial loop microbially dominated plankton communities where the majority of 

primary production is utilized by microorganisms (e.g., bacteria, small 

flagellates and ciliates) in the water column and little energy reaches 

higher trophic levels 

micron unit of length equal to 1W6 meter (10" millimeter); symbol pm 
MMS Minerals Management Service 

NECOP Nutrient Enhanced Coastal Ocean Productivity program 

NOAA National Oceanic and Atmospheric Administration 

NURC National Undersea Research Center(s); part of the NOAA National 

Undersea Research Program 

nitrate inorganic nitrogen compound (NO,) formed by oxidation of plant and 

animal wastes (nitrification); part of nitrogen cycle; utilized as a nutrient 

by plants; component of fertilizer 

nitrification chemical process in which nitrogen compounds in plant and animal wastes 

(mostly ammonia) are converted in the presence of oxygen to nitrites and 

then to nitrates by bacteria; crucial part of nitrogen cycle 

nitrite 

nitrogen 

inorganic nitrogen compound (NO;) formed by oxidation of plant and 

animal wastes (nitrification); part of nitrogen cycle; utilized as a nutrient 

by plants; component of fertilizer 

element (N) crucial to plant and animal life as structural components of 

proteins and nucleic acids; important forms for aquatic plants are nitrite, 

nitrate, and ammonium ions; availability controls phytoplankton 

abundance 

nitrogen fixation conversion of atmospheric nitrogen (N,) into a form useful to plants, either 

nitrite (NO;) or nitrate (NO,); the incorporation of N from nitrogen gas 

into organic nitrogen; large cyanobacteria are primary nitrogen fixers in 

the ocean (e.g., Trichodesmium) 

organic nitrogen nitrogen as components of organic (carbon) compounds essential for plant 

and animal life (i.e., amino acids, proteins, peptides, enzymes, 

nucleotides) 

orthophosphate common form of phosphate, P02,'or PO(OH),, where the PO and OH are 

in the "ortho" position (i.e., adjacent 1,2 positions) in the molecular 

structure; used in fertilizer 

xix 
ostracods 

oxycline 

small, mainly bottom-dwelling crustaceans (class Ostracoda) commonly 

called mussel or seed shrimps; body enclosed in a rounded, bivalve 

carapace impregnated with calcium carbonate 

a vertical gradient in the water column of marked change in oxygen 

concentration 

oxygen minimum a subsurface depth layer in which the concentration of oxygen is 

layer low or depleted 

particulate organic particulate (i.e., >45 microns) organic carbon contained in a 

carbon sample of particulate organic matter; sinking phytoplankton and 

zooplankton, fecal pellets, body parts, tests, etc. 

phaeopigments degraded, decomposing chlorophyll pigments of dead phytoplankton 

photic zone depth layer of body of water which receives enough sunlight for 

photosynthesis; depth varies with extinction coefficient, angle of incidence 

of sunlight, length of day, cloudiness; euphotic zone 

phosphorus essential nutrient for organisms; important component of enzymes, 

membranes, DNA, energy-canying molecules (e.g., ATP), and tissues 

(e-g., bones and teeth) 

phosphate ionic (oxidized) form of phosphorus (H3P04) occurring in nature; essential 

nutrient for marine organisms; often depleted in water column by 

phytoplankton 

photoinhibition slowing or stopping of photosynthesis at high light intensities (e.g., full 

sunlight); perhaps related to excessive ultraviolet radiation 

phytoplankton planktonic plants (e.g., diatoms and dinoflagellates) and planktonic 

photosynthetic bacteria (eg, Cyanophyta) that reside in the water column 

as primary producers 

principal investigator 
pic0 prefix denoting a unit of 10-12; symbol p 

picocyanobacteria extremely small and abundant planktonic cyanobacteria between 0.2 and 

2.0 microns in diameter 

POC see particulate organic carbon 

PP see primary production 

portunid crab a swimming crab (class Crustacea, order Decapoda, family Portunidae); 

includes the commercially important blue crab (Callinectes) 

pycnocline a vertical gradient in the water column of marked change in density 

primary production synthesis of organic matter from inorganic substances by organisms such 

as bacteria, algae, and vascular plants using energy derived from inorganic 

chemicals or sunlight 

RN research vessel 

Redfield ratio ratio of major nutritional elements (e.g., 0, N, Si, P) in marine 

phytoplankton; N:Si:P=16: 16: 1 

salp 

SEAMAP 

common name of free swimming, planktonic invertebrate filter feeders of 

the chordate subphylum Urochordata (class Thaliacea); includes doliolids 

(Doliolum) and salps (Salpa) 

Southeast Area Monitoring and Assessment Program (in the Gulf of 

Mexico) 

sediment oxygen rate of oxygen uptake by benthic aerobic microorganisms in 

demand degrading organic matter in the sediment surface 

Shannon-Wiener informational index of biodiversity 

Index 

abbreviated value of the density of seawater at a specific salinity and 

temperature at standard atmospheric pressure; symbol o, 

xxi 
silicon 

silica 

silicate 

SOD 

stratification 

stoichiometric 

test 

thermal 

stratification 

USGS 

Si; 2"" most abundant element in earth's crust; material composing diatom 

cell wall; essential (limiting) element for diatom growth 

silicon dioxide (SiO,); quartz, flint, opal; biogenic silica (i.e., diatom 

remains) 

SiO,; rock-forming mineral 

see sediment oxygen demand 

the characteristic of a water property that varies rapidly in the vertical but 

very little in the horizontal direction (see density stratification, thermal 

stratification) 

description of chemical reactions in which the reactants combine and react 

in whole-number ratios based on their atomic weights 

external, encasing hard covering or supporting structure of a 

phytoplankton or an invertebrate animal; shell-like structure 

the characteristic of marine waters whereby their temperature 

changes with depth; the strength of the stratification indicates how rapidly 

the temperature changes in the downward direction 

United States Geological Survey 

xxii 
Executive Summary 

by 

Wiliiam J. Wiseman, Jr. and Nancy N. Rabalais 

The continental shelf adjacent to the Mississippi River is a highly productive system, often 

referred to as the fertile fisheries crescent. This productivity is attributed to the effects of the 

river, especially nutrient delivery. In the later decades of the 2oth century, though, changes in the 

system were becoming evident. Nutrient loads were seen to be increasing and reports of hypoxia 

were becoming more frequent. 

During most recent summers, a broad area (up to 20,000 krn2) of near bottom, inner shelf waters 

immediately west of the Mississippi River delta becomes hypoxic (dissolved oxygen 

concentrations less than 2 mgll). In 1990, the Coastal Ocean Program of the National Oceanic 

and Atmospheric Administration initiated the Nutrient Enhanced Coastal Ocean Productivity 

(NECOP) study of this area to test the hypothesis that anthropogenic nutrient addition to the 

coastal ocean has contributed to coastal eutrophication with a significant impact on water quality. 

Three major goals of the study were to determine the degree to which coastal productivity in the 

region is enhanced by terrestrial nutrient input, to determine the impact of enhanced productivity 

on water quality, and to determine the fate of fixed carbon and its impact on living marine 

resources. The study involved 49 federal and academic scientists from 14 institutions and cost 

$9.7 million. Field work proceeded from 1990 through 1993 and analysis through 1996, 

although some analyses continue to this day. 

The Mississippi River system delivers, on average, 19,000 m3/s of water to the northern Gulf of 

Mexico. The major flood of the river system occurs in spring following snow melt in the upper 

drainage basin. This water reaches the Gulf of Mexico through the Mississippi River birdfoot 

delta and through the delta of the Atchafalaya River. Much of this water flows westward along 

the coast as a highly stratified coastal current, the Louisiana Coastal Current, isolated from the 

bottom by a strong halocline and from mid-shelf waters by a strong salinity front. This 

stratification maintains dissolved and particulate matter from the rivers, as well as recycled 

material, in a well-defined flow over the inner shelf. It also inhibits the downward mixing of 

oxygenated surface waters from the surface layer to the near bottom waters. This highly 

stratified flow is readily identifiable by its surface turbidity, as it carries much of the fine 

material delivered with the river discharge and resuspended by nearshore wave activity. A 

second significant contribution to the turbidity of the surface waters is due to phytoplankton in 

these waters. This turbidity reduces the solar radiation penetrating to depth through the water 

column. These two aspects of the coastal current, isolation of the inner shelf surface waters and 

maintenance of a turbid surface layer, precondition the waters for the development of near 

bottom summer hypoxia. 
2 Executive Summary 

Major anthropogenic changes within the watershed of the Mississippi River system over the past 

two centuries have had dramatic impacts on Mississippi water quality. Engineering works for 

navigational improvement and flood control, as well as the drainage of more than half the 

original wetlands, have significantly altered the hydrology of the system and its ability to process 

nutrients. Changes in drainage to control soil erosion concomitantly reduces phosphorus losses, 

but increases nitrate-N transfers to surface waters. Present Mississippi River export of nitrogen 

from the watershed is estimated to be 2.5- to 7.4-fold higher than during pre-agricultural and preindustrial periods. Changes in sediment load transported by the river to the coast are difficult to 

quantify, as are their effects on nutrient loads. What is clear, though, is that nutrient 

concentrations and loading to the adjacent shelf have changed dramatically this century. 

Concentrations of nitrate have doubled since the mid-1950s, while silicate concentrations have 

been halved. Seasonal patterns of nitrate and silicate concentration have also altered during the 

past century, presumably in response to seasonal agricultural activities. Long-term trends in 

nitrate concentration are directly related to trends in watershed nitrogen fertilizer application, and 

long-term silicate trends are inversely related to phosphorus fertilizer application. The net effect 

of these trends has been to move the atomic ratios of N:Si:P to be very close to the Redfield 

ratio. 

The response of the inner shelf ecosystem to the delivery of fresh water, suspended sediments, 

and dissolved nutrients is high primary production with strong spatial and temporal variability. 

Within the effluent plume of the river, primary production is generally limited by light 

conditions controlled by the suspended sediment concentrations. Both downstream and across 

the plume boundaries, primary production peaks around salinities of 20. At higher salinities, 

nutrient limitation appears to control primary production. (Nutrient concentrations away from 

the river outflows are controlled by recycled nutrients. Nutrient availability is constantly being 

altered by mixing and settling processes.) Thus, productivity is observed to decrease westward 

of the birdfoot delta within the Louisiana Coastal Current with a secondary peak in the vicinity 

of the Atchafalaya River delta. On the southeastern Louisiana shelf, a strong correlation exists 

between time-lagged riverine nitrate flux and net primary production. The organisms 

responsible for this production are predominantly diatoms in the spring and picocyanobacteria in 

the summer and fall. 

The dominant fates of photosynthetically fixed organic carbon are grazing and sedimentation, 

often in the form of fecal pellets. While there is evidence for linkage between primary 

production and fish populations, the strongest relationships between primary producers and 

higher trophic levels is with zooplankton - copepods, larvaceans, salps, and doliolids. These 

have been shown to be capable of consuming significant fractions of the primary production 

taking place in the surface waters. Attempts to quantifL the vertical flux of carbon from the inner 

shelf surface waters have produced mixed results. There are indications of significant temporal 

and spatial variability in the flux with higher rates in spring than in summer. No clear 

relationship between surface productivity and carbon export from the surface layers was 

discernable across the seasons. In fact, estimates of vertical carbon flux rates were higher than 

estimates of primary production, at times. Some of the indeterminacy can be attributed to 

methodological differences between studies, but further efforts are needed to resolve outstanding 

questions. 
Executive Summary 3 

Rapid sedimentation of clastic material in the vicinity of the Mississippi delta allows the 

preservation of associated sedimented particles. Analysis of dated cores from this region have 

shown that deposition of marine carbon has increased in proportion to Mississippi River nitrate 

flux. Furthermore, the rate of deposition of biogenic silica from diatoms, an indicator of surface 

layer production, has also increased in proportion to riverine nitrate flux. Analysis of historical 

plankton data suggests that the reduction of silica delivery from the river has been accompanied 

by a shift in diatom composition toward less silicified species and, in at least one case, to an 

increase in the numbers of a sometime toxic diatom. Thus, while riverine delivery of silica has 

decreased recently, the phytoplankton community has adjusted and actually increased the flux of 

carbon to the sediments. The fact that riverine nutrient concentrations lie near the Redfield ratio, 

with the preceding evidence for community changes, implies that further subtle changes in 

nutrient concentrations may have significant effects on the inner shelf ecosystem. A number of 

different scenarios, though, are equally plausible at this time. 

One notable effect of coastal eutrophication is the occurrence of hypoxia. Along the Louisiana 

inner shelf, hypoxic near bottom conditions occur most summers and cover extensive areas of 

the bottom. As the hypoxia develops, motile organisms migrate out of the area, while those 

incapable of leaving are stressed or suffocate. Although the statistics do not allow determination 

of the economic impact on commercial fisheries, it is likely that these are affected by direct 

mortality, forced migration, reduction in habitat, increased predation, altered life cycles, and 

altered food resources. 

While reports of Louisiana inner shelf hypoxia date from the early 1970s, systematic midsummer monitoring of the extent of this phenomenon began in 1985. The area affected since 

then has ranged from 7,000 krn2 up to 20,000 krn2. In summer of 1988, when the Mississippi 

River was in drought, only a single station was found to exhibit near bottom hypoxia. This 

observation reinforces the earlier statement that strong stratification of the water column, 

resulting from high river discharge, is necessary for the establishment of hypoxia. While the 

variations in hypoxic area are significant between years, there has been an increase in the 

affected area over the 15 years of available data. This may, though, be associated with the 

decadal scale of variability of river discharge and nutrient load. On shorter time scales, the shape 

of the hypoxic region changes in response to wind forcing that alters the stratification and 

currents over the inner shelf. 

The estimates of carbon flux from the surface layers to the bottom are sufficient to account for 

the oxygen respiration rates observed in the hypoxic waters. The high flux rates observed in the 

spring are adequate to deplete quickly the oxygen reserve of the deeper waters. The lack of 

reoxygenation by mixing from above, coupled with continued respiration, allows for the 

maintenance of hypoxic conditions throughout the summer. Time-lagged Mississippi River 

discharge is strongly correlated with surface water oxygen surplus resulting from primary 

production, as well as with near-bottom dissolved oxygen deficit, again implicating the river in 

both processes. On longer time scales, core analyses offer an indication of trends in surrogates 

for oxygen stress and primary production that parallel increases in nutrient loads. Glauconite, a 

diagenetic product of anoxic environments, increases toward the present in dated cores. 

Similarly, assemblages of benthic organisms suggest that oxygen stress has increased in recent 

decades. 
4 Executive Summary 

A steady-state model of the eutrophication dynamics of the Louisiana inner shelf has been 

developed to assist with understanding of the processes important to the development of hypoxia. 

The model was calibrated to different sets of field data and used in a screening analysis mode. 

Numerous caveats limit the ability of the model to predict the state of the system in real time, but 

a number of important conclusions can be drawn from the modeling results. Phytoplankton 

growth rates in the model surface layers, because of the adequate supply of nutrients, are 

dominated by light limitation and temperature dependence. The fate of this fixed carbon in the 

surface layers is split roughly evenly between grazing and a combination of phytoplankton 

respiration and dissolved organic carbon exudation. Dissolved oxygen dynamics in the near 

bottom waters indicate that primary production can be a source of dissolved oxygen below the 

surface layer, especially in the shallower waters near the Texas border. The sinks for dissolved 

oxygen are sediment oxygen demand, photosynthetic respiration and carbonaceous biochemical 

oxygen demand, a surrogate for the microbial loop. 

In an effort to assess the response of the system to reductions in nutrient inputs from the rivers, 

the model, calibrated to different observed summer conditions, was run with various levels of 

reduced nutrient inputs. A reduction in surface layer chlorophyll (biomass) and an increase in 

lower layer dissolved oxygen resulted from a reduction of either phosphorus or nitrogen loading. 

There was, though, a tendency for the responses to nitrogen reduction to be larger than those to 

phosphorus reduction, especially for dissolved oxygen. 
Chapter 1 

Introduction 

by John L. Wickham 

Background of NECOP 

Nutrient enrichment, especially fiom anthropogenic sources, is one of the major stresses imposed 

on coastal ecosystems. This is particularly true in the northern Gulf of Mexico where the 

Mississippi River, the nation's largest river, discharges into a predominantly oligotrophic 

system. To better understand coastal ecosystem response to nutrient enrichment, the Nutrient 

Enhanced Coastal Ocean Productivity (NECOP) study was initiated by the Coastal Ocean 

Program (COP) of the National Oceanic and Atmospheric Administration (NOAA) to address 

the effects of nutrient discharge in the coastal waters of the United States. The program focused 

on the outflows of the Mississippi River and its tributary, the Atchafalaya River, and the impact 

of the outflow on the northern Gulf of Mexico coastal waters. From 1990 to 1996,49 federal 

and academic scientists from approximately 14 institutions joined in an interdisciplinary 

investigation of the continental shelf of the northern Gulf of Mexico at a total funding of $9.7 

million. 

Objectives of NECOP 

The basic hypothesis posed for NECOP was that the addition of anthropogenic nutrients from 

sewage, agriculture, industrial sources, and urban runoff have contributed to the development of 

eutrophication in coastal waters with significant impacts on water quality. Three objectives 

were posed: (1) determine the degree to which coastal primary productivity has been enhanced in 

areas receiving terrestrial nutrient inputs; (2) determine the impact of the resultant enhanced 

productivity on water quality, and; (3) determine the fate of fixed carbon in coastal areas and its 

impact on living resources within the affected coastal ecosystems (NOAA 1992a). The study 

area was chosen to satisfy the presence of four criteria: (1) a clear anthropogenic nutrient signal, 

(2) resultant nutrient-enhanced productivity of significant magnitude, (3) a demonstrable impact 

of enhanced production on coastal environmental quality, and (4) renewable resources of 

significant value (Atwood et al. 1994a). 

Using these criteria, the western Louisiana Shelf was selected as the study area for the NECOP 

Program (Figure 1.1). In contrast to the other shelves of the U.S., the extreme northern Gulf of 

Mexico is a highly river-dominated, anthropogenically influenced, and highly productive coastal 

marine ecosystem. The watershed of the Mississippi River covers more than one-third of the 

continental U.S., including the heavily fertilized Midwest. The river discharges approximately 

30 percent of its total flow through the Atchafalaya Bay with the remaining 70 percent flowing 

through the distributaries of the birdfoot delta (Rabalais et al. 1996). Along with high fisheries 
6 Introduction 

productivity, one of the most significant suspected impacts of the nutrient-enhanced productivity 

on the Louisiana Shelf was the seasonal development of hypoxia in bottom waters. Monitoring 

since the mid-1980s has revealed that summer hypoxia develops on an annual basis over much of 

this shelf ranging in size up to approximately 16,000 km2 to 20,000 krn2 from 1993 to 1999 

(Rabalais et al. 1995, 1997, 1999). This hypoxia has an impact on the living resources of the 

region which are among the most productive in U.S. continental waters. For example, excluding 

Alaska, approximately 30 percent of the U.S. fisheries catch in both landing tonnage and dollar 

value comes from the northern Gulf of Mexico. In addition, one of the highest dollar value 

fisheries (shrimp) and highest tonnage fisheries (Gulf menhaden) in the United States exists in 

the Gulf (NOAA-COP 1998a). 

31 31 

Charles Lafayette 

.................... 

_._*'.- ................................ Gulf of Mexico 

Figure 1.1. Location map of the NECOP study area (from Limno-Tech, Inc. 1995). 

As previous research efforts in this region had proposed the linkage of anthropogenic nutrient 

inputs to the development of hypoxia in the nearshore waters of Louisiana, one of the major 

goals of the NECOP study was to attempt to quantitatively relate the input of riverborne nutrients 

in the Mississippi River discharge to the development of hypoxia on the shelf. Toward this end, 

a series of interdisciplinary projects concentrated on selected physical, geological, chemical, and 

biological aspects of the Louisiana shelf ecosystem. Many of the NECOP investigations dealt 

with the hypoxia, the extent and timing of its occurrence, causal factors, impacts, and history 

through retrospective analysis (Atwood et al. 1995a, 1995b). 
Introduction 

NECOP and LATEX 

Although designed to collect extensive data to understand the ecology of the Mississippi River - 

Atchafalaya River (MAR) plume and Inner Gulf Shelf system, especially the biogeochernical 

regime, the NECOP field sampling program was not capable of collecting the large amounts of 

physical oceanographic data required to thoroughly support a mass balance water quality model. 

To address the lack of physical data for the modeling effort, NECOP planned to utilize data from 

a northern Gulf of Mexico physical oceanographic research and modeling program entitled the 

Louisiana-Texas Shelf Physical Oceanography Program (LATEX) sponsored by the Minerals 

Management Service (MMS) of the U.S. Department of the Interior. One of its three study units, 

the Texas-Louisiana Shelf Circulation and Transport Processes Study was conducted by Texas 

A&M University from April 1992-December 1994. The study area encompassed the TexasLouisiana continental shelf offshore of approximately the 10 m isobath from the Mississippi 

River to the Rio Grande. The major field components were moored current meter measurements, 

hydrography, meteorological buoys, and directional wave instruments and non-directional wave 

instruments (NOAA 1998b). A second study, the Mississippi River Plume Hydrography Study, 

carried out a series of six rapid mapping cruises over the Louisiana-Texas inner shelf, collecting 

hydrography and acoustic Doppler current profiles. 

As the LATEX program sampled some of the same locations as NECOP (e.g., Mississippi River 

plume) as well as areas farther offshore, it was hoped that LATEX would support NECOP 

modeling by providing data on offshore boundary conditions. By agreement, LATEX and 

NECOP principal investigators were able to access NECOP data via a remote bulletin board 

system established at the Atlantic Oceanographic and Meteorological Laboratory (AOML) by 

the NECOP data management program (NOAA 1991). While the LATEX data did prove to be 

useful in filling gaps in the data from other sources during certain years, in retrospect, despite the 

best intentions of all parties involved, the data fields necessary as input to the mass balance 

model remained undersampled. This was due to differences in the survey times and sampling 

grids of the two programs, coupled with the inherent scales of variability in the natural system, 

and to the timing of initiation of the two programs. 

Additionally, the MMS funded an extensive modeling program designed to model circulation 

and hydrography in the Gulf of Mexico with special attention given to the LATEX study region. 

The NECOP external review panel recommended that NECOP's circulation modeling effort be 

terminated and the output from the MMS study be used to drive the NECOP mass balance 

eutrophication model (William Wiseman, Michael Dowgiallo, personal communication). While 

this recommendation was followed, the timing of the two programs was such that validated 

circulation results were not available until well after the bulk of the NECOP modeling was 

complete and differences in the two model grids impacted the ability to transfer results from one 

domain to the other. 

It is clear that the good intentions of the agencies, managers, and scientists in the various 

programs were not sufficient to maximize the synergies between the programs. Although wellintentioned planning efforts were made to coordinate NECOP and LATEX, in retrospect, 
8 Introduction 

collaborative planning at a much earlier stage than occurred in this case is necessary to benefit 

optimally from programs studying the same geographical region. 

Sampling Program 

Major NECOP field research activities were conducted from 1990 through 1993. Prior to 

NECOP, various institutions and agencies (e.g., Louisiana State University [LSU], Louisiana 

Universities Marine Consortium [LUMCON], the State of Louisiana, NOAA National Ocean 

Service) had conducted monitoring and studies of the MAR system through or in conjunction 

with other programs. For example, beginning in 1985 (and continuing to the present), 

LUMCON has monitored parts of the hypoxic zone on a monthly basis and annually mapped its 

mid-summer areal extent. In addition to LUMCON, other agencies and institutions funded or 

conducted activities in the hypoxia area prior to or during the NECOP program (e.g., Texas 

A&M University at Galveston, University of Texas, Texas Institute of Oceanography, LSU, 

Louisiana Sea Grant, Louisiana Board of Regents, Louisiana Department of Wildlife and 

Fisheries, MMS, National Marine Fisheries Service) (Gulf of Mexico Program Office et al. 

1997). 

NECOP field work began in the summer of 1990 (NOAA-COP 1992b). The NOAA ship 

Malcolm Baldrige was utilized for two years (1990-91) of major process study cruises in the 

deeper areas of the MAR plume area coinciding with low water flow (1 7 July - 10 August 1990) 

and higher discharge (1 9 February- 17 March 199 1). The 5.6 m draft of the Malcolm Baldrige 

precluded sampling activities in shallow inshore areas. Activities conducted aboard Malcolm 

Baldrige included retrospective sediment analysis, salinity, temperature, pigment and nutrient 

analyses, phytoplankton size and species, particulate and dissolved organic matter, bacterial 

abundance and production, photosynthesis and irradiance, zooplankton, grazing, sedimentation, 

and sampling, among others. The NECOP sampling program was designed to provide data for 

understanding the three study objectives. To this end, large parts of the sampling program were 

designed and devoted to monitoring and process studies in conjunction with a modeling effort to 

consolidate and assist in the interpretation of the field observations. 

In addition to the Malcolm Baldrige cruises, monthly sampling cruises were conducted both in 

the shallower coastal areas and deeper areas offshore to study temperature, salinity, oxygen, 

currents, and sediments. Research vessels (RNs) Acadiana and Pelican (LUMCON) were used 

primarily to monitor transect C on a monthly basis and to conduct the annual mid-shelf July 

hypoxia mapping cruises, respectively. Other vessels involved in NECOP activities at various 

times and locations, especially in 1992 and 1993, were the RNs Gyre (Texas A&M University), 

Longhorn (University of Texas), and NOAA ship Chapman. Most of the cruises were conducted 

consecutively, with some undertaken concurrently, with other cruises in the MAR plume region. 

Some cruises were supported in part with funding from programs other than NECOP. The 

NOAA ship Mt. Mitchell, which was originally scheduled for some 35 sea-days of NECOP 

cruise duty in 1992, was reassigned by NOAA to the Persian Gulf for marine environmental 

assessment studies following the Persian Gulf War. The NOAA ship Chapman was rescheduled 

in place of Mt. Mitchell for one 1992 NECOP cruise and RNs Pelican, Gyre, and Longhorn 

were chartered to fulfill remaining 1992 cruise requirements. Chapman was again used in 1993. 
Introduction 

For the annual July hypoxia mid-shelf monitoring, a standardized sampling grid of 

approximately 15 transects and 73 stations was utilized by LUMCON (Figure 1.2). Transect 

Stations C6A and C6B were sampled monthly during the period when hypoxia was present and 

also were the sites of an instrumented mooring. The locations of other sampling stations varied 

based on the special requirements of each principal investigator's study or vessel institution's 

historical sampling stations. For example, Figure 1.3 shows several station transect locations 

that were used in 1990 for calibrating a mass balance water quality model for the Louisiana Inner 

Shelf. These include the NECOPILUMCON, RN GyrelTexas A&M University cruises and 

National Undersea Research Center (NURC) sponsored cruises. The U. S. Geological Survey 

(USGS) stations provided data for the Mississippi and Acthafalaya Rivers. The various sampling 

locations are best described in the metadata and raw data files found in the NECOP Data 

Management Program (see NECOP website at http://www.aoml.noaa.govlocd~necop), the 

published research of the principal investigators (e.g., see papers collected in Atwood et al. 

1994b, 1995b; NOAA-COP 1992a, Appendix C) and in the individual cruise reports on file at 

the offices of the respective research vessel institutions and as attachments to the "NECOP 

Newsletter" (NOAA 199 1 - 1994). 

Figure 1.2. Distribution of stations for LUMCON mid-summer shelfwide survey and more 

frequent sampling along transect C. Stations C6A and C6B are locations of moored 

instruments (from Rabalais et al. 1999). 
Introduction 

A 

A Gulf of Mexico 

0 NURC 

n GYRE 

[3 NECOP 

Figure 1.3. Location of field sampling stations used in model calibration for July 1990. River 

(U.S. Geological Survey), NURC (National Undersea Research Center sponsored 

cruises), Gyre (Texas A&M University), NECOP (LUMCON cruises) (from LimnoTech, Inc. 1995). 

In the spring of 1993, a special shelf-wide monitoring cruise was undertaken on RN Pelican to 

document the effects of heavy runoff and flows associated with the 1993 Mississippi River 

flooding. Along with the special spring cruise and yearly July shelf-wide hypoxia monitoring 

cruise aboard Pelican, two additional July cruises were conducted on Longhorn to document the 

effects of the Mississippi River flooding. In addition, a NOAA National Undersea Research 
Introduction 11 

Program sponsored flood cruise was conducted aboard Pelican in August of 1993. Results from 

these cruises, along with other sources, contributed to the special NOAA report on the summer 

1993 Mississippi River flood (Dowgiallo 1994). 

A brief summary of the major NECOP research activities (cruise dates, location, activity, vessel) 

are listed in Table 1.1. Consult the NECOP website for metadata and raw data. 

Significance of NECOP 

NECOP investigations increased the understanding of the responses of coastal ecosystems to 

nutrient enrichment. Specifically, NECOP helped document the extent of nutrient input and 

hypoxia in the northern Gulf of Mexico and established a direct link between increasing riverborne dissolved nutrients, hypoxia in the bottom waters and the associated ecological changes. 

In addition, NECOP described the long-term historical record of progressive nutrient enrichment, 

developed the first mass-balance water quality model that predicts water quality responses to 

various nutrient reduction strategies in the Mississippi watershed and helped initiate politicalmanagement decisions (Beardsley 1997, Limno-Tech, Inc. 1995, Ortner and Dagg 1995). 

As development of summertime hypoxia in bottom waters is one of the most pressing water 

quality issues facing the Louisiana continental shelf between the Mississippi River and the upper 

Texas coast, findings from NECOP have helped to bring the northern Gulf of Mexico hypoxia 

problem to the attention of the public and resource managers (Beardsley 1997, Showstack 1997). 

In January 1995, the Sierra Club Legal Defense Fund (now the Earthjustice Legal Defense Fund) 

petitioned the State of Louisiana and the U.S. Environmental Protection Agency to convene a 

management conference under the auspices of the Clean Water Act to begin the process of 

restoring water quality in the region. The result was a meeting, the First Gulf of Mexico 

Hypoxia Management Conference in December, 1995, to begin addressing hypoxia and other 

water quality problems resulting from excess nutrients from the Mississippi River watershed 

(Gulf of Mexico Program Office et al. 1997). Subsequently, in 1997, under the leadership of the 

U.S. Environmental Protection Agency, six federal, seven state, and two tribal senior 

representatives formed the Mississippi RiverIGulf of Mexico Watershed Nutrient Task Force. 

This group convened to address the occurrence of hypoxia in the Gulf of Mexico, but also the 

potential for alleviation of the problem through nutrient and river management practices 

throughout the Mississippi watershed (EPA 1998). These activities continue at this time. 

Additional information concerning NECOP, including raw data and metadata, may be found on 

the NECOP Internet Web Page at http://www.aoml.noaa.gov/ocd/necop. 
Introduction 

Table 1-1. Summary of NECOP research activities. 

DATE 

FY I990 

Apr.-Nov. 

Mar.-Nov. 

Ju1.11-24 

Jul. 17-Au~. 10 

Jul. 23-29 

N 7997 

Feb. 19-Mar. 17 

Feb.-Nov. 

Feb.-Nov. 

Jul. 16- 21 

Jul. 23-29 

Sep. 13-17 

FY 7992 

Feb.-Nov. 

Apr. 1-9 

Apr. 1-9 

Apr. 12-19 

Apr. 30-May 13 

May 1-10 

May 5-1 3 

May 5-17 

May 14-21 

Jul. 24-29 

N 7993 

Oct. 24-Nov. 2 

Oct. 24-Nov. 2 

Oct. 26-30 

Feb.-Nov. 

Mar. 26- Apr.2 

Apr. 3-1 3 

Apr.23-May 2 

May 15-23 

Jul. 1-12 

Jul. 14- 24 

Jul. 24-29 

Jul. 26-Aug.7 

Aug. 16-23 

Aug. 31-Sept. 4 

WORK PERFORMED 

monthly Transect C, hydrographic, biological surveys 

monthly moored instrument package Sta. C6A; physical ops. 

MAR plume region; benthic metabolism, DON, DOC, biomass 

MAR plume region; bio-, geo-, chemical process studies 

shelf-wide hypoxia monitoring; hydrographic, biological surveys 

MAR plume region; bio-, geo-, chemical process studies 

monthly Transect C; hydrographic and biological surveys 

monthly moored instrument package Sta. C6B; physical ops. 

MAR plume region; primary prod., zooplktn., chemical-physical processes 

shelf-wide hypoxia monitoring; hydrographic and biological surveys 

MAR plume region; primary production 

monthly Transect C; hydrographic and biological surveys 

MAR plume region; sediment processes, re~ospective~analysis 

MAR plume region; benthic metabolism, DON, DOC, biomass 

MAR plume region: buoyancy and nutrient exchange 

MAR plume region; fish larvae, zooplankton, chlorophyll 

MAR plume region, TDN, DON, DOC, benthic metabolism, biomass 

MAR plume region; nutrients, chlorophyll, oxygen, microbial 

MAR plume region; hydrography, phyto-zooplankton 

MAR plume region; nutrients, chlorophyll, oxygen, microbial 

shelf-wide hypoxia monitoring; hydrographic and biological surveys 

MAR plume region, TDN, DON, DOC, benthic metabolism, biomass 

MAR plume region; Lagrangian drifters, CTD 

MAR plume region; fish larvae, zooplankton, chlorophyll 

monthly Transect C; hydrographic and biological surveys 

MAR plume region; buoyancy and nutrient exchange 

MAR plume region; hydrography, phyto-zooplankton 

Miss. River flood shelf-wide monitoring; hydro-biological surveys, Si) 

MAR plume region; Lagrangian drifters, CTD 

MAR plume region; buoyancy, nutrients, chlorophyll, oxygen 

MAR plume region; buoyancy, nutrients, chlorophyll, oxygen 

shelf-wide hypoxia monitoring; hydrographic and biological surveys 

MAR plume region; buoyancy, nutrients, chlorophyll, oxygen 

MAR plume region; Lagrangian drifters, CTD 

MAR plume region; fish larvae, zooplankton, chlorophyll 

monthly Transect C; hydrographic and biological surveys 

MAR plume region; nutrients, chlorophyll, oxygen, microbial 

shelf-wide hypoxia monitoring; hydrographic and biological surveys 

MAR plume region; Lagrangian drifters, CTD 

monthly Transect C; hydrographic and biological surveys 

shelf-wide hypoxia monitoring; hydrographic and biological surveys 

monthly Transect C; hydrographic and biological surveys 

shelf-wide hypoxia monitoring; hydrographic and biological surveys 

VESSEL 

Acadiana 

Acadiana 

Gyre 

Malcolm Baldrige 

Pelican 

Malcolm Baldrige 

Acadiana 

Acadiana 

Pelican 

Pelican 

Pelican 

Acadiana 

Pelican 

Gyre 

Pelican 

Chapman 

Gyre 

Longhorn 

Pelican 

Longhorn 

Pelican 

Gyre 

Longhorn 

Chapman 

Adadiana 

Pelican 

Pelican 

Pelican 

Pelican 

Longhorn 

Longhorn 

Pelican 

Longhorn 

Pelican 

Chapman 

Acadiana 

Longhorn 

Pelican 

Pelican 

Acadiana 

Pelican 

Acadiana 

Pelican 
Introduction 13 

References: 

Atwood, D. K., A. Bratkovich, M. Gallagher, and G. L. Hitchcock. 1994a. Introduction to the 

dedicated issue. Estuaries 17(4):729-73 1. 

Atwood, D. K., A. Bratkovich, M. Gallagher, and G. L. Hitchcock (organizers). 1994b. Papers 

from NOAA's Nutrient Enhanced Coastal Ocean Productivity study. Dedicated issue of 

Estuaries. Estuaries 17(4):729-9 1 1. 

Atwood, D. K., W. F. Graham, and C. B. Grimes. 1995a. Introduction and summary, p. iv. In: 

Nutrient Enhanced Coastal Ocean Productivity, Proceedings of a 1994 Synthesis Workshop, 

Louisiana State University. Atwood, D. K., W. F. Graham, and C. B. Grimes (organizers). 

Louisiana Sea Grant College Program, Louisiana State University, Baton Rouge, Louisiana. 

Atwood, D. K., W. F. Graham, and C. B. Grimes (organizers). 1995b. Nutrient Enhanced 

Coastal Ocean Productivity. Proceedings of a 1994 synthesis workshop, Louisiana State 

University. Louisiana Sea Grant College Program, Louisiana State University, Baton Rouge, 

Louisiana. 1 19 pp. 

Beardsley, T. 1997. Death in the deep: "Dead zone" in the Gulf of Mexico challenges 

regulators. Scientific American 277(5): 17,20. 

Dowgiallo, M. J. (ed.). 1994. Coastal oceanographic effects of summer 1993 Mississippi River 

flooding. Special NOAA Report. NOAA, Coastal Ocean Office/National Weather Service, 

Silver Spring, Maryland. 76 pp. 

Environmental Protection Agency. 1998. Meeting summary for the first meeting of the 

Mississippi RiverIGulf of Mexico watershed nutrient task force, December 4, 1997, 

Arlington, Virginia. United States Environmental Protection Agency, Office of Wetlands, 

Oceans, and Watersheds, Washington, D.C. 8 pp. 

Gulf of Mexico Program Office, Louisiana Department of Environmental Quality, Lower 

Mississippi River Conservation Committee, and Mississippi Soil and Water Conservation 

Commission. 1997. Proceedings of the first Gulf of Mexico hypoxia management 

conference, December 5-6, 1995, Kenner, Louisiana. Publ. No. EPA-55-R-97-001. United 

States Environmental Protection Agency, Gulf of Mexico Program Office, Stennis Space 

Center, Mississippi. 198 pp. 

Limno-Tech, Inc. 1995. Estimated responses of water quality on the Louisiana inner shelf to 

nutrient load reductions in the Mississippi and Atchafalaya Rivers. Revised Final Report 

(April 5, 1995). Prepared for Louisiana State University and A&M College Sea Grant 

College Program, Louisiana State University, Baton Rouge, Louisiana. Limno-Tech, Inc., 

Ann Arbor, Michigan, and South Bend, Indiana. 48 pp. Appendices. 
14 Introduction 

NOAA. 199 1. Coastal RBBS and the NECOPJLATEX data sharing agreement, pp 2-3. In: 

NECOP Newsletter. National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory, Ocean Chemistry Division, Miami, Florida. 

Number 3 (Dec.). 

NOAA. 199 1 - 1994. NECOP Newsletter. National Oceanic and Atmospheric Administration, 

Atlantic Oceanographic and Meteorological Laboratory, Ocean Chemistry Division, Miami, 

Florida. Numbers 1- 12 (Jun. 91 - Jun. 94). 

NOAA, Coastal Ocean Program. 1992a. Nutrient Enhanced Coastal Ocean Productivity. 

NECOP workshop proceedings, Louisiana Universities Marine Consortium, October 199 1. 

Publ. No. TAMU-SG-92-109, Texas A&M University Sea Grant Program, Galveston, Texas. 

154 pp. 

NOAA, Coastal Ocean Program. 1 992b. Executive Summary, pp. 1-5. In: Nutrient Enhanced 

Coastal Ocean Productivity. NECOP workshop proceedings, Louisiana Universities Marine 

Consortium, October 199 1. Publ. No. TAMU-SG-92- 109, Texas A&M University Sea Grant 

Program, Galveston, Texas. 

NOAA, 1998a. Fisheries of the United States, 1997. National Oceanic and Atmospheric 

Administration, National Marine Fisheries Service, Fisheries Statistics and Economics 

Division, Silver Spring, Maryland. Current Fishery Statistics No. 9700. 126 pp. 

NOAA, 1998b. LATEX (Louisiana-Texas Shelf Physical Oceanography Program) CD-ROM 

Project, Texas-Louisiana Shelf Circulation and Transport Processes Study. National Oceanic 

and Atmospheric Administration, National Oceanographic Data Center, Silver Spring, 

Maryland. 5 CD-ROM discs. 

Ortner, P. B. and M. J. Dagg. 1995. Nutrient-enhanced coastal ocean productivity explored in 

the Gulf of Mexico. Eos, Transactions of the American Geophysical Union 76(10): 97, 109. 

Rabalais, N. N., R. E. Turner, D. Justic, Q. Dortch, and W. J. Wiseman, Jr. 1999. 

Characterization of Hypoxia: Topic 1 Report for the integrated Assessment on Hypoxia in 

the Gulf of Mexico: NOAA Coastal Ocean Program Decision Analysis Series No. 15. 

NOAA Coastal Ocean Program, Silver Spring, MD. 167 pp. 

Rabalais, N. N., R. E. Turner, D. Justic, Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

1996. Nutrient changes in the Mississippi River and system responses on the adjacent 

continental shelf. Estuaries l9(2B):3 86-407. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1997. Hypoxia in the Northern Gulf of 

Mexico: past, present and fbture, pp. 25-40. In: Proceedings of the First Gulf of Mexico 

Hypoxia Management Conference, December 5-6, 1995, Kenner, Louisiana. Gulf of Mexico 

Program Office, Louisiana Department of Environmental Quality, Lower Mississippi River 

Conservation Committee, and Mississippi Soil and Water Conservation Commission. Publ. 
Introduction 

No. EPA-55-R-97-001, U. S. Environmental Protection Agency, Gulf of Mexico Program 

Office, Stennis Space Center, Mississippi. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman, Jr., D. Justic, Q. Dortch, and B. Sen Gupta. 

1995. Hypoxia on the Louisiana Shelf and system responses to nutrient changes in the 

Mississippi River: A brief synopsis, pp. 106- 1 13. In: Nutrient Enhanced Coastal Ocean 

Productivity, Proceedings of a 1994 Synthesis Workshop, Louisiana State University. 

Atwood, D. K., W. F. Graham, and C. B. Grimes (organizers). Louisiana Sea Grant College 

Program, Louisiana State University, Baton Rouge, Louisiana. 

Showstack, R. 1997. Gulf of Mexico hypoxia zone stirs policy discussions. Eos, Transactions of 

the American Geophysical Union 78(43):478,483. 
Chapter 2 

Physical Oceanography 

and Sediment Dvnamics 

by William J. Wiseman, Jr., Brenf McKee, Nancy N. Rabalais, 

and Scott P. Dinnel 

Introduction 

Regions of significant fresh water influence differ from other coastal environments in that the rate 

at which freshwater is delivered to the coastal ocean is so great that it is not readily dispersed 

seaward by the ambient water motions. This freshwater delivery may occur through a number of 

closely spaced river systems or through a single, large river. The former situation is exemplified 

by the South Alaskan coastal zone, the Norwegian coastal region, and the South Atlantic Bight. 

The latter, more intuitive situation is characteristic of the discharges from the Amazon, the Zaire, 

the Po, the Danube, the Chiang-Jiang, the Huanghe, and, in the Gulf of Mexico, the Mississippi. 

The intense delivery of fresh water carries with it not only mass, but other properties and material 

as well, e.g. buoyancy (because the fresh water is lighter than sea water), pollutants, sediments, 

and nutrients derived from both natural and anthropogenic processes. 

The light, fresh water and associated dissolved and particulate matter float atop and mix with the 

ambient coastal salt water. After the fresh water exits the river mouth, there exist two regions of 

flow within which the water dynamics are different. The nomenclature surrounding these regions 

is somewhat confused. Initially, water enters the shelf as a jet or spreads radially. In either case, a 

bulge of fresh water, or plume, develops near the river mouth. Water tends to flow clockwise 

around the outer edge of this bulge. Some distance downstream (to the right looking seaward in 

the northern hemisphere) of the river mouth the flowing water encounters the coast and turns 

downcoast to form a coastal current often referred to as the extended plume. This coastal current is 

usually highly stratified because of the presence of buoyant fresh water. 

The Mississippi is the largest river in North America and one of the top ten major rivers in the 

world (Milliman 1992). Globally, it ranks seventh in terms of both sediment and water discharge 

to the ocean. The Mississippi River transports 66% of the sediments and 60% of the water that is 

carried by rivers to the ocean from the United States (Trefry et al. 1992). Globally, it ranks second 

in terms of drainage basin area and third in terms of river length. The Mississippi River drains 41% 

of the conterminous U.S. and stretches over 3000 kilometers from Minnesota to the Gulf of 

Mexico (Millirnan and Meade 1983, Milliman 1992). It empties into the Gulf of Mexico through 

two sites: the Mississippi Delta and the Atchafalaya Delta (Figure 1.1). The long-term average 

discharge from the Mississippi-Atchafalaya system to the northern Gulf of Mexico exceeds 19,000 

m3/s. It is a dominant control on the ecology of the region. 

The discharge of the Mississippi-Atchafalaya system is controlled, by the U.S. Army Corps of 

Engineers, such that 30% flows seaward through the Atchafalaya Delta and 70% through the 

Mississippi Delta. The Atchafalaya River flows into Atchafalaya Bay, a broad shallow 
18 Physical Oceanography 

embayment, through two main outlets. The Mississippi River enters the gulf through multiple 

outlets, some in deep water and some in shallow water. The most important effluent plumes are 

those from Southwest Pass, South Pass, and Pass a L'outre. Some water from the Mississippi 

Delta flows eastward onto the Mississippi-Alabama shelf. The amount is unclear. It is certain, 

though, that the combined effects of effluent from the Mississippi and Atchafalaya deltas have a 

major impact on the physical oceanography of the west Louisiana inner shelf. Because of the 

buffering effect of the large catchment basin of the Mississippi, discharge does not experience the 

great temporal extremes seen in many smaller rivers, nor the rapid changes in discharge. In 

general, maximum discharge occurs in spring and minimum discharge in late summer. Significant 

interannual variability is observed. Over time, the Atchafalaya River has captured more of the 

Mississippi River system discharge. Since its completion in 1963, the Old River control structure 

has maintained the flow down the Atchafalaya at 30% of the total system discharge. No long term 

trends in total system discharge have been noted, although the seasonal Kendall Tau test for trends 

indicates that there has been an upward trend in the discharge during the months of September 

through December in recent years. 

Major rivers, like the Mississippi, play a disproportionately important role in transporting terrestrial 

materials to the ocean. Approximately 40% of the fresh water and particulate materials entering the 

ocean are transported by the ten largest rivers, which include the Mississippi (Milliman 1992). 

Sediments are among the most important natural materials entering the northern Gulf of Mexico 

from the Mississippi River. Abundant sediment supply is one of the salient features of the northern 

Gulf of Mexico when compared to other U.S. coastal environments. Sediments that are produced 

from weathering of both igneous and sedimentary rocks are transported by the Mississippi and are 

ultimately discharged into shallow coastal environments of the northern Gulf of Mexico, primarily 

as suspended load. This terrigenous sedimentary debris has built a broad deltaic plain which 

includes the largest expanse of wetlands in the United States. Sediment inputs are crucial to the 

maintenance of these wetlands, countering loss processes resulting from subsidence and sea level 

rise. In addition, sediment particles carry natural components (organic carbon, nutrients, metals) as 

well as anthropogenic substances (organic contaminants and heavy metal pollutants) that can be 

released in the coastal zone. Sediments also have active sorption sites that efficiently remove some 

organic and inorganic components from coastal waters and subsequently sequester them in bottom 

sediments. Equally important, if not more so, for the question of eutrophication is the role of 

suspended sediment in controlling the light field in the water column and, consequently, primary 

production. 

It is the structure and variability of the river plumes, coastal current, and associated sediment loads 

with which we will deal in the following pages. The impact of this structure and variability on the 

pathways and transformations of riverborne nutrients and, subsequently, on the ecology of the 

inner shelf will be the focus of subsequent chapters. 

The Ambient Environment 

The west Louisiana shelf is topographically bland. Isobaths run essentially east-west. The 

Mississippi Delta has prograded to the shelf break, therefore much of the water from this delta 

discharges into deep water and the plumes are separated from the bottom (Wiseman and Garvine 

1995). The Atchafalaya River discharges into a shallow bay that, in turn, empties onto the 

broadest and most shoal portion of the shelf. While tides are small everywhere in the Gulf of 

Mexico, the broad shelf offshore of Atchafalaya Bay ensures that tidal currents are relatively 

amplified in this location. The modest tidal currents and shallow depth promote a bottom-attached 

structure for the initial effluent from the Atchafalaya River (Murray and Jarosz 1997). 
Physical Oceanography 19 

Mean winds over the west Louisiana shelf are from the southeast during most of the year. During 

the winter, the dominant synoptic weather pattern is cold air outbreaks. Preceding a front, the 

southeasterly winds strengthen and long waves affect the inner shelf. As the cold front crosses the 

shore, the winds become dry, cold, and rotate clockwise to blow strongly from the northwest. 

These weather systems affect the region every three to ten days. Because the shallow inner shelf 

waters are, to a first approximation, in frictional balance with the wind, the surface waters tend to 

be blown onshore and downcoast. During cold air outbreaks the waters are blown, on average, 

offshore and upcoast. Summer is an anomalous season. Cold air outbreak activity is at its 

minimum and what fronts do occur are weak. Winds along the south Texas coast are upcoast and 

push water back towards the Louisiana shelf. There is, thus, a tendency for inner shelf waters to 

be pushed eastward along the Louisiana inner shelf at this time of the year. 

Plumes 

Once the rate of discharge and density of the discharging waters are known, many characteristics 

of the receiving environment contribute to determine the ultimate structure of the plume over the 

shelf. Most notable of these are the local bathymetry immediately outside the river mouth, the 

strength of the tidal currents, and strength and direction of the ambient winds and currents over the 

shelf. 

If water discharges into a very shallow environment or one where vertical mixing is very strong, 

the plume will remain in contact with the bottom for a significant distance after leaving the mouth 

of the river. In the case of the Atchafalaya River, the plume may remain in contact with the bottom 

of Atchafalaya Bay for more than 15 km seaward of the river mouth. This contact will allow the 

plume to spread in only two-dimensions, while laterally entraining ambient fluid. It will also 

strongly influence the path that the fluid follows as it is advected across and along the shelf. If the 

bottom slope is greater, so that the buoyant plume lifts off the bottom, then rapid buoyant 

expansion of the fresh water plume occurs once the plume leaves the river mouth. This buoyant 

expansion is accompanied by vertical and lateral entrainment of ambient water. A similar pattern 

occurs if the discharge is from the upper layer of an estuary and has already separated from the 

bottom within the estuary proper. 

In the absence of other sources of momentum, such as ambient winds and currents, the plume will 

reach a size where the flow is strongly influenced by the rotation of the earth. Water will flow 

around contours of salinity much as winds flow around contours of pressure on a weather map. 

Even when there exist ambient winds and currents, these are usually not strong enough to disrupt 

this tendency for flow within a large plume except for brief periods of time. Counter-examples 

include the Amazon plume, where the influence of the earth's rotation is very small because of 

proximity to the equator, and the Columbia River plume during summer, when very strong 

northerly winds reverse the expected flow for significant periods of time. Along the Louisiana 

coast, strong northerly winds often affect the path of the Atchafalaya River plume (Walker and 

Harnrnack in press) and Mississippi River plumes (Rouse and Coleman 1976) for brief periods. 

Less frequently, anomalous winds and offshore current systems can cause longer-lived reversals 

of the normal plume path (Hitchcock et al. 1997, Walker et al. 1994). 

An important, but poorly understood, aspect of plumes is how they mix with the ambient shelf 

waters. This mixing has been modeled as simple lateral diffusion and/or vertical entrainment 

(Takano 1954, Wright and Coleman 1971). Neither is strongly supported by observational 

evidence. Recent observations on a number of small river plumes point to the importance of rollers 

at the lateral boundaries of plumes, e.g. Luketina and Imberger (1989). The fluid entrained into _ . the plume by these rollers would subsequently disperse across the plume through secondary flows 
2 0 Physical Oceanography 

(Figure 2.1). Evidence of such an entrainment mechanism in one of the plumes of the Mississippi 

River Delta has been collected (Hitchcock et al. 1997). Breaking waves at the interface separating 

the fresh water plume from the underlying, saltier water is another mechanism that has been 

suggested for mixing in large plumes (Wright 1970). 

r . /

Figure 2.1. Schematic diagram of mixing processes and secondary circulations within a bottomdetached effluent plume. 

Mapping of the plumes of the Mississippi and Atchafalaya Rivers using modern instruments and 

satellites is limited. The buoyant expansion of the plumes from South Pass and Southwest Pass of 

the Mississippi Delta is well documented (Wright and Coleman 1971, Hitchcock et al. 1997). 

These plumes, on average, leave the river mouth as narrow jets and turn westward (Figure 2.2) 

(Walker 1994). They are clearly responsive to wind and tidal forcing. Speeds within the plumes 

have been measured to exceed 1.0 m/s. The plume thickness ranges from 2 to 10 m depending 

upon discharge and ambient winds. There is also some evidence for the existence of entrainment 

by rollers at the offshore edge of the Southwest Pass plume. Mixing with ambient waters is 

initially rapid and diminishes with distance from the river mouth. The vertical salinity profile is 

nearly homogeneous within the plume. These salinities are highly correlated with distributions of 

other parameters, such as nutrients, within the plume (Hitchcock et al. 1997). Direct 

measurements suggest that water parcels require a few days, at most, to transit from the mouth of 

Southwest Pass to the coast (when the plume assumes its most probable shape). 

As a result of the spatial and temporal variability in sediment inputs, there are inherent difficulties 

in accurately estimating the discharge-weighted flux of sediments from rivers (Meade et al. 1990). 

Estimates of sediment discharge rates derived in past decades are not directly comparable to rates 

calculated more recently using quantitative measurements. Continuous sediment discharge 

measurements have been recorded since 1950 for the Mississippi and since 1973 for the 

Atchafalaya River. The Mississippi River currently transports an average of 210 x lo6 mtlyr of 

suspended sediment to the northern Gulf of Mexico (Meade 1996). 
Physical Oceanography 

-90.5 -90 -89.5 -89 -88.5 

Figure 2.2. Characteristic frontal boundary positions near the Mississippi River Delta (dark lines). 

The boundaries are interpreted from a NOAA AVHRR satellite image obtained on 20 

January, 1992. (Data provided by N. D. Walker.) The fronts separate fresh water 

from ambient shelf water. Fresh water turns clockwise and moves westward from 

South and Southwest Passes. West of 90" W the plume merges into the westwardflowing Louisiana Coastal Current and the inshore front disappears. 

Meade et al. (1990) point out that the wash load (suspended silts and clays) may be relatively 

uniform from surface to bottom within a cross-section of a river, however, quite nonuniform 

laterally across the river. Collecting daily records of sediment discharge is expensive and often 

impractical. One possible way of extending our knowledge of sediment discharge rates is via 

sediment rating curves, empirical relationships between sediment discharge rates and water 

discharge rates that are much better documented. This relationship is often very complex (Mossa 

1996). Sediments stored on the bed during low water are in plentiful supply as waters rise, but 

stored materials are soon resuspended and become depleted as the river reaches maximum water 

discharge (Robbins 1977, Nordin and Beverage 1965, Mossa 1996). The sediment discharge peak 

in the Mississippi River at Tarbert Landing precedes the water discharge peak during high 

discharge years and is coincident with the water discharge peak during average or low years. At 

Belle Chase, water and sediment discharge peaks for the Mississippi River correspond very closely 

in time (Mossa 1996). 

The ratio of sediment discharge during flood events (due to storms, continuous heavy rains and 

hurricanes) to that of average low-discharge conditions is likely to be high for many of the rivers 

that drain into the Gulf of Mexico. For example, during the period from 1963 to 1987 sediment 

discharge for the Red River (which empties into the Atchafalaya River) varied by over four orders 

of magnitude between lowest and highest reported values (Mossa 1996). In contrast, sediment 

discharge for the Lower Mississippi River varies by a factor of seven between lowest and highest 

values (Mossa 1996). The Mississippi River's greatest measured flow (64,800 m3/s) is less than 

seven times the lowest annual recorded flow (9,660 m3/s) because of its large and diverse drainage 

basin (Gunter 1979). A 56-month record of sediment and water discharge for the Mississippi River 

at Tarbert Landing (Figure 2.3) demonstrates the intra-annual and inter-annual variability of this 

system. 
Physical Oceanography 

Sediment Discharge: Mississippi River @ Tarbert Landing 

3 1400 , I 

Water Discharge: Missiippi River @ Tarbert Landing 

35000 , 

1987 1988 1989 1990 1991 

Year 

Figure 2.3. A 56-month record of sediment and water discharge for the Mississippi River at 

Tarbert Landing. 

Approximately 90% of sediment currently being eroded off the land surface of the conterminous 

U.S. is being stored somewhere between the river and the sea (Meade et al. 1990). Understanding 

the delivery of sediments to the continental shelf must include some understanding of this storage 

(and subsequent remobilization) process. 

Seasonal sediment storage and remobilization is well documented for the lower Mississippi River 

(Everett 1971, Wells 1980, Meade and Parker 1985). Because of the extensive levee system, there 

are no inputs or outlets in the lower 500 km stretch of the river from Tarbert Landing (just below 

the control structure and diversion of the Atchafalaya River) to Venice (near the mouth of the 

river). This fact has permitted researchers to make upriver-downriver comparisons and calculate 

mass balances for particulate materials in this portion of the river. 

At low discharge there are large differences in cross-sectional area between upriver and downriver 

locations and the resultant deepening of the channel results in decreased velocities downstream, 
Physical Oceanography 23 

downriver cross sectional area is much smaller and increased current velocities are accompanied by 

steeper downstream surface-water gradients resulting in bottom shear stresses that surpass 

threshold values for resuspension (Mossa 1996). Wells (1980) examined suspended sediment 

concentrations at Head of Passes (near the river mouth) and St. Francisville (700 krn upstream), 

and noted that a threshold discharge rate around 17,000 m3/s separated trends of increasing and 

decreasing sediment concentrations downstream. At a water discharge rate of 10,000 m3/s, 43% 

and 25% of material in suspension at St. Francisville remained in suspension at New Orleans and 

Venice, respectively. From these data, Wells (1980) inferred a net storage (sedimentation) of 

sediments in the lower river during low discharge periods. Kesel(1988) also observed a trend of 

lower suspended sediment concentrations at New Orleans relative to Tarbert Landing and attributed 

these differences to deposition in the lower river. 

The suspended sediment load that accompanies the plumes emanating from the passes of the 

Mississippi Delta is rapidly diluted by entrainment of ambient water (Wright 1970, Hitchcock et al. 

1997), yet the signal is still large enough to be useful in mapping the plume from satellite imagery 

(Walker 1996). Heavy sediment loads, particularly during the spring and early summer, restrict 

levels of chlorophyll (and, presumably, primary production) in the immediate vicinity of the delta. 

As the water column clears, chlorophyll levels increase dramatically. Numerous processes 

contribute to the improvement of the light environment available to phytoplankton. Dilution due to 

entrainment is one such process. Settling directly from suspension is another. The abundance of 

fine sediment in the suspended load of the Mississippi River, indicates that the sediment 

concentrations can be reduced only slowly by this mechanism. Furthermore, the halocline at the 

base of the plume and associated secondary circulation patterns will inhibit settling across this 

interface (Wright 1970). One final process that may prove important during certain seasons is 

biological mitigation. Zooplankton may be very effective in grazing fine sediment from suspension 

and repackaging the inorganic materials in fecal pellets of sufficient size to have rapid settling 

velocities (Dagg et al. 1996). 

The Louisiana Coastal Current 

The plumes from Atchafalaya Bay and Southwest Pass, Mississippi River Delta (and possibly 

other outlets of the Mississippi Delta) tend to flow in a clockwise direction until they encounter the 

Louisiana coast. At this point, they contribute to the Louisiana Coastal Current, a highly stratified 

water mass that flows westward along the Louisiana coast during much of the year. This current is 

sometimes referred to as the extended plume of the Mississippi River. The buoyant, low salinity 

waters are separated from the waters of the mid shelf region by a strong surface-to-bottom frontal 

zone. This frontal zone typically intersects the bottom near the 10-15 m isobath. This depth is 

somewhat deeper than might be expected from theory, probably because the winds are generally 

downwelling favorable in the region and tend to force the lighter water against the coast. 

Cochrane and Kelly (1986) described the seasonal progression of the surface salinity patterns over 

the Texas-Louisiana shelf. In spring, a narrow band of low salinity water extends from the 

Mississippi River Delta to, at least, the Texas-Mexican border. As the winds across the south 

Texas shelf become more upwelling favorable, i.e. more northward, the low salinity water is 

pushed back northward and eastward. A nearshore convergence zone has been hypothesized to 

occur and low salinity waters are thought to flow offshore as a jet (S. P. Murray, pers. comm.). 

This convergence zone is believed to migrate up the coast until mid summer. An alternative 

suggestion, which is supported by observation (see below), is that the upcoast flows moves 

slightly offshore and flows eastward just offshore of the fresher effluent from Atchafalaya Bay. 

The reversed (northward and eastward) flow, thus, may actually extend into Louisiana waters, 

even though the mean winds may remain westward in this region, because of longshore pressure 

gradients that are believed to develop. By late summer, the winds relax and return to a more 
2 4 Physical Oceanography 

southeasterly and easterly flow over the Texas shelf and the low salinity surface band near the 

coast again extends into Texas waters. Temperature-salinity volumetric analyses of the waters on 

this shelf (Ulm 1983) identify the waters of the Louisiana Coastal Current as a low salinity arm on 

the volumetric temperature-salinity chart. Similar data sets have been analyzed in a somewhat 

different manner to identify the seasonal variability of fresh water stored on the shelf (Dinnel and 

Wiseman 1986). These authors also estimated the residence time of riverine discharge on the shelf 

to be of the order of one year or longer. They were unable to identity the processes responsible for 

transporting the freshwater off the shelf. 

The stratification variability within the Louisiana Coastal Current proper has been the focus of two 

recent manuscripts (Wiseman et al. 1997, Murray and Jarosz 1997). Stratification within the 

current system is maximum in late spring and early summer when runoff is highest and solar 

heating adds to the stratification. It is weakest in winter when runoff is weak, heat loss to the 

atmosphere is maximum, and mechanical mixing due to winds and waves is strongest. The 

continual arrival of cold air outbreaks at the coast during the late fall, winter, and early spring 

results in aperiodic vertical homogenization of the water column interspersed with periods during 

which the water column restratifies. This allows ventilation of the deeper waters of the inner shelf 

on a quasi-regular basis. Stronger stratification (an example of typical summer stratification is 

shown in Figure 2.4) and weaker winds during summer preclude this ventilation, although 

occasional upwelling may allow the deep waters to interact with the atmosphere over a restricted 

nearshore zone for brief periods of time (Dagg 1988). 

Figure 2.4. Contours of Brunt-Vaisala frequency (cycles min-') across the Louisiana Coastal 

Current on year-day 178,1986. Brunt-Vaisala frequency is a measure of 

stratif~cation; high Brunt-Vaisala frequencies indicate strong stratification. Note the 

secondary pycnocline near the bottom 

Two aspects of the summer hydrography of the inner shelf are particularly germane to the 

development of mid-summer hypoxia. A broad halocline develops over the entire west Louisiana 

shelf. The extent to which this results from direct advection of low salinity water from the Delta 

and from recirculation from the outer Texas shelf, as opposed to offshore dispersion from the 

Louisiana Coastal Current during upwelling events, is unknown. The other unique feature is the 

presence of a weak pycnocline beneath the main halocline, generally near the bottom, which 

separates a well-mixed layer from the waters above (Figure 2.4). This pycnocline is sometimes a 
Physical Oceanography 25 

halocline, sometimes a thermocline, and sometimes both. Its origin is unclear. It may be due to 

turbulent mixing in the bottom layer or it may be the nearshore expression of a broader, shelf-wide 

pycnocline beneath the lower salinity surface layer. A strong, upper layer halocline is a necessary 

condition for the occurrence of hypoxia, while the weaker, deeper pycnocline defines the 

morphology of the hypoxic waters (Wiseman et al. 1997). 

The seasonal mean circulation patterns within the Louisiana Coastal Current were described by 

Cochrane and Kelly (1986). Downcoast flow occurs throughout most of the year in response to 

both the buoyancy forcing due to the river discharge and the winds that are directed towards Texas. 

During summer months the mean flow can reverse and flow towards the east, at least over the 

western portions of the Louisiana inner shelf. Near surface mean current speeds are of the order of 

10 cds offshore of Atchafalaya Bay in fall, winter and spring, being strongest in winter and 

weakest in spring. They are significantly stronger in Texas waters, often exceeding 20 cds 

(Dinnel et al. 1997). In summer, the flows are somewhat more confused. They are weaker and 

northward in central Texas waters. Off Atchafalaya Bay, there appears to be a weak westward 

mean flow in the very nearshore waters and a stronger eastward mean flow further offshore, 

suggesting significant cross-shelf shear in this region (Dinnel et al. 1997, Murray and Jarosz 

1997). 

Velocity variability occurs within the Louisiana Coastal Current on a variety of time scales. 

Besides the seasonal scale, there is variation associated with the river discharge patterns, 

stratification changes, and wind forcing. As mentioned in relation to Mississippi River effluent 

plumes, slope and oceanic currents may also affect the flow patterns of inner shelf waters. 

Because a broad shelf tends to insulate the inner shelf from oceanic influences, though, these latter 

effects are most notable along the south Texas shelf, where the shelf width narrows abruptly. In 

this region, large rings spun off the Loop Current may impact the entire width of the shelf. 

Pulses of river discharge introduce massive fluxes of fresh water into the coastal zone. The 

associated salinity signals appear to travel westward along the coast at significant speeds (Wiseman 

and Kelly 1994). Tf the highly energetic wind-driven signal is filtered from consideration, the 

cross-shelf dynarnical balance at periods longer than 10 days appears to be geostrophic (Wiseman 

et al. 1997). The swiftest currents obsemed are associated with storm winds. Except when under 

the direct influence of a hurricane, these winds are the result of cold air outbreaks. Current speeds 

can exceed 50 cm/s in the surface layers. During winter, similar speeds are observed in the nearbottom waters (Crout et al. 1984). In the stratified season, long-term averaged near-bottom speeds 

at station C6B (see below) offshore of Cocodrie, LA in 20 m of water suggest a mean speed of 

only 1 cds to the west although instantaneous velocities can be significantly higher (Figures 2.5- 

2.6). 

Some success at modeling these flows has been achieved assuming the wind stress is balanced 

either directly by the bottom stress or, in two-layered models, by bottom and interfacial stress 

(Lewis and Reid 1985). More recent analyses indicate the importance of pressure gradients to the 

local balance of forces (Wiseman et al. 1992, Murray and Jarosz 1997, She et al. in review). 

Unfortunately, the national tide gauge array along the Louisiana coast does not lend itself to a 

careful analysis of this term. At the highest energy-containing frequencies, the tidal and inertial 

frequencies, current speeds are not excessive (DiMarco and Reid 1998). Nevertheless, coherence 

between cross-shelf currents and salinity at tidal periods indicates that tidal straining of the 

stratification may be locally important (Wiseman and Kelly 1994). 
Physical Oceanography 

km from shore 

Figure 2.5. Cross-section of eastward (alongshore) speed through the Louisiana Coastal Current 

during summer (redrawn from Murray and Jarosz 1997). Units are cmls. 

Low-passed [25-hr. running mean] C6B eastward bottom currents - 1994 

I I 1 I 

Low-passed [25-hr. running mean] C6B northward bottom currents - 1994 

I I I I 

10 4 

I I I I I 

50 100 150 200 250 300 

Days 

Figure 2.6. Low-passed, near-bottom currents at the station CGB. Note the reduced current 

activity during summer. 
Physical Oceanography 27 

Numerous attempts have been made to model the currents over the Louisiana inner shelf. 

Statistical models, e.g. Lewis and Reid (1985), have been successful in describing the wind-driven 

alongshelf flow in this region. The friction coefficients required by the models are generally 

outside the bounds typically associated with shelf flows. This suggests that some other force 

should enter into the dynamical balance, e.g. pressure gradient forces, as mentioned above. More 

recently, Current and Reid (1998) have utilized a spectral shelf wave model to describe the shore 

parallel flow along the Texas-Louisiana shelf. When upstream currents were specified and 

accurate wind-forcing provided, their model could reproduce roughly half the variance in the 

subtidal current field. Dynalysis of Princeton (Herring et al. 1999) has also developed a fully 

three-dimensional model of the shelf, with emphasis on the inner shelf. Their model, when forced 

by realistic winds and river discharge, also reproduces roughly half the variance of the observed 

subtidal currents. Thus, there is strong hope that we may be able to predict the coastal currents 

with some reasonable level of success. This, as we shall see in chapter 6, is a necessary step in the 

accurate prediction of water quality along the inner shelf. 

Shallow river-dominated coastal environments are usually very turbid. High turbidity conditions 

are often observed during high flow conditions as a result of large riverine inputs of sediments. 

The Mississippi River Bight is a broad shallow shelf environment (approximately 5400 krn2 in 

area) extending to about 100 km west of the Mississippi River Delta. Two distinctive features of 

this area are (1) shelf waters that are intimately coupled with bottom sediments and (2) the 

prevalance of wind-driven resuspension of bottom sediments. A similar situation prevails offshore 

of Atchafalaya Bay. 

The physical characteristics of the northern Gulf of Mexico define sediment dynamics in these 

waters. Large water surface areas (providing a large fetch for prevailing winds) and the small tidal 

range (c 0.5 m) result in a system where the physical dynamics are dominated by winds rather than 

tidal forcing (Wiseman et al. 1997). High rates of sediment discharge are associated with 

freshwater inputs from the Mississippi and Atchafalaya Rivers. Frequent resuspension of bottom 

sediments results from the shallow depths and the turbulent nature of this environment. During the 

winter season, cold front passages are the most important forcing function for sediment dynamics 

in the central northern Gulf of Mexico (Roberts et al. 1988, Moeller et al. 1993). The importance 

of resuspension by wave-bottom interactions is particularly notable at these times (Walker and 

Harnmack in press). Thus, the ambient light regime in coastal current waters is not controlled 

solely by supply from rivers and sedimentation to the bottom, but by an interplay of these 

processes with recurrent intermittent resuspension events that supply additional particulate matter to 

the water column and, consequently, affect primary production. 

Benthic Sediment Processes 

The hypoxia observed in the near bottom waters of the Louisiana inner shelf results, to a large 

extent, from oxidation of organic matter in the lower water column and from sediment oxygen 

demand. The distinction between these two processes is fuzzy because bottom sediments and 

interstitial waters are periodically mixed into the lower water column and then allowed to resettle to 

the bottom. 

The factors that control the nature, distribution, and persistence of sedimentary deposits on 

continental shelves include 1) rate of sediment supply; 2) type of source material; 3) grain-size 

distribution; and 4) energy conditions at the sediment-water interface. Temporary versus permanent 

storage of particulate material in bottom sediments, and the difference in time scales that separate 

these processes, are important factors in understanding sedimentological and biogeochemical 

_processes. The development of radiochemical techniques has provided tools for identifying areas 
2 8 Physical Oceanography 

of active sedimentation, for examining rates of sedimentary processes, and for establishing 

geochronologies within bedded sediments. The terms used to describe sedimentation on various 

time scales (deposition, accumulation) can be distinguished quantitatively when radiochemical 

techniques are used. Deposition is here defined as temporary emplacement of particulate material 

on a sediment surface during a specified period of time. Accumulation is the sum of deposition and 

removal over a longer time scale (McKee et al. 1983). 

In sediment-dominated environments, particle-reactive radiochemical tracers such as 234Th and 'Be, 

can often be used to provide insight into monthly and seasonal deposition rates and patterns 

(Canuel et al. 1990). For example, much of the sediment from the Atchafalaya River that enters 

Fourleague Bay is deposited in the upper bay during high flow regimes but is redistributed to the 

lower bay after winter storms begin (Da et al. 1995). This was revealed from a comparison of 

deposition rates determined by excess 23$h profiles in upper and lower Fourleague Bay. The 

seasonal rate of sediment deposition is high relative to long term accumulation rates (determined by 

2'!Pb), indicating that some sediments are stored for a short time before being exported from the 

Bay. Thus, a significant portion of the sediment derived from this major distributary of the 

Mississippi River system is not delivered directly to the Louisiana Coastal Current with the water 

that accompanied it down the lower river. 

The spatial distributions of fresh riverborne sediments on the shelf adjacent to the Mississippi River vary greatly on seasonal and annual time scales (Figure 2.7) (McKee et al. 1999). This , 

indicates that, even after leaving the estuaries, sediments are highly mobile on the Louisiana inner 

shelf and suspended sediment concentrations should be equally variable. 

It is difficult to quantify directly or indirectly the magnitude of sediment export from estuaries. 

Direct measurements of sediment exported from estuaries are difficult because inlets are often large 

and the deployment of an adequate number of instruments is prohibitively expensive. The use of 

remote sensing techniques to quantify sediment export from estuaries is an area of active research 

that holds great promise for the future (Walker 1996). 

In some cases, changes in the export flux of sediment from an estuary to the shelf can be evaluated 

via sediment geochronologies. For example, a log raft in the upper Atchafalaya began to grow 

between the 1500s and the late 1700s blocking most of the river's flow (Comeaux 1970). The 

Atchafalaya was virtually bypassed until 1839 when raft removal was first attempted and flow 

increased (Gunter 1979). The Atchafalaya River carried approximately 10% of the Mississippi 

flow in 1858 (Elliot 1932). Removal of the raft was completed in 1861 and was followed by a 

dramatic growth in the Atchafalaya Delta. Eventually the inland lakes filled with sediments and 

fine-gained materials escaped Atchafalaya Bay onto the adjacent inner shelf. This is demonstrated 

in a Vb,, profile from a station on the inner shelf adjacent to Atchafalaya Bay (McKee and 

Baskaran 1999). The profile exhibits a distinct break in slope and therefore in sedimentation rate. 

The recent increase in the sediment accumulation rate (since the early 1970s) suggests that this is 

when fine-grained sediments began to escape Atchafalaya Bay and accumulate on the inner shelf 

and alter the ambient light regime. 
Physical Oceanography 

NECOP 92 (April 1992) 

COMUS I (Nov. 1993) 

.w . 

28.61 Fall I Winter 1993 lh -234 lnwaicr 

28.q I Fall 1993 1 

COMUS m (June 1994) 

. . Summer 1994 

Figure 2.7. Seasonal sediment deposition pattern in the Mississippi River Bight. 

The abundance of fine-grained sediments, and the profound influence that these sediments have on 

shelflestuarine processes is one of the salient features of Gulf of Mexico estuaries and inner shelf. 

However, little is known (quantitatively) about sedimentary processes in these environments. With 

the exception of the Mississippi and Atchafalaya Rivers, riverine sediment inputs are poorly 

characterized for Gulf of Mexico estuaries and inner shelf. The large surface area and shallow 

water depths of these regions result in very dynamic sedimentary processes governed primarily by 

wind forcing. Sediment distributions in the water column and in bed sediments are highly variable 

(temporally and spatially) due to the dominance of wind-driven resuspension. Short-term 

(seasonal) depositional rates can be locally high (cdmo) but net (decadal) sediment accumulation 
3 0 Physical Oceanography 

rates are significantly lower. Winter storms and hurricanes provide sufficient energy to redistribute 

sediments within the system and may be a driving force in exporting sediments to the coastal 

ocean. 

The major challenge to understanding sedimentary processes in these environments is to develop 

instrumentation and techniques adequate to measure processes of short duration, high frequency 

time scales, and in conditions that are physically demanding. Based on what is known from the 

sediment record, the sedimentary processes associated with episodic events (such as storms and 

hurricanes) and their impacts on estuaries are very important. However, little is presently known 

about these event-scale phenomena on a mechanistic level. Because such events are unpredictable 

and hazardous for equipment and personnel, field observations during such episodic events will 

continue to be a challenge. 

Long-term Changes 

The suspended sediment load of the Mississippi River has changed considerably since the 1700's 

due to several anthropogenic factors. These include: 1) deforestation and agriculture; 2) changes in 

the land management strategy; and 3) construction of dams, diversions and levees. The suspended 

sediment load carried by the Mississippi River to the Gulf of Mexico has decreased by 

approximately one-half over the past 200 years (Meade et al. 1990). Most of this decrease has 

occurred relatively recently (since the 1950s) as a result of the construction of dams and reservoirs 

on the Missouri and Arkansas Rivers (Meade and Parker 1985). The source (and composition) of 

particulate material discharged by the Mississippi River has also changed during the past century. 

Increased sediment discharge by the Ohio River (as a result of increased erosion from poor land 

management) has partially compensated for the decreased sediment discharge fiom the western 

tributaries (Keown et al. 1986). 

Historically, major flood events (and efforts to control them) have played a major role in sediment 

and water discharges from the Mississippi River. As early as March 1543, a flood was noted by 

De La Vega (DeSoto's chronicler) which lasted about 80 days with water covering the valley for 

approximately 150 km on either side. Other major floods were noted in 1664 and in 1717-1718 for 

New Orleans. The four largest floods in recent times were in 1828,1882,1927 and 1973 (-50 yrs 

apart). Records of annual peak discharges have been recorded intermittently since 1897 and 

continuously since 1927. The largest flood during that period was May 1927 (64,800 m3/s) 

causing extensive damage along the lower Mississippi River Valley. 

Levee construction along the Mississippi began ca. 1717 with the founding of New Orleans, and 

increased gradually until the 1880s when the building rate was accelerated. In the 1930s the whole 

river levee system was greatly extended and stabilized after the disastrous 1927 flood. Subsequent 

to that flood period, the U.S. Army Corps of Engineers began an extensive flood-control program. 

Channelization (preventing overbank flooding and also eliminating 185 km of river channel), levee 

construction, and construction of storage reservoirs on the tributaries were three flood-control 

measures taken. Storage reservoirs enable the tributaries to store more water during high flow thus 

decreasing the peak discharge. As a result there has been a noticeable decrease in peak discharges 

since the 1920s. Everett (1971) noted that between 1890 and 1970, the number of annual peaks 

has decreased and the accumulated storage of water in the basin has increased. Prior to the 1940s, 

an average of four water discharge maxima were noted each year (taking the mean of 10-year 

intervals). Since 1950 there has been about one discharge maximum per year. Water storage has 

increased dramatically (by a factor of six greater than previous times) since the early 1950s. 
Physical Oceanography 3 1 

The amount of suspended sediment carried by the river depends upon streamflow, turbulence, 

particle size and water temperature. Suspended sediment concentration in the Mississippi River has 

decreased from > 900 mg/liter in the early 1950s to <ZOO mglliterin the 1990s: a decrease of over 

400%. Everett (1971) noted that in 1964 the suspended sediment concentration in the Baton Rouge 

area of the Mississippi exceeded 1500 mghter during peak sediment discharge. During the period 

1949-1964 suspended sediment concentrations were < 100 mg/liter about 7% of the time and > 

1000 mg/liter 8% of the time in the Baton Rouge area of the river. Kesel(1988) also noted an 

historical change in the amount of sand in suspension (comparing Quinn's 1894 data to present) 

for the Mississippi River. The percentage of sand in suspension at Tarbert Landing during 1950- 

1983 was - 50% less than the percentage noted during an earlier period (1879-1893). At Belle 

Chase, this difference was approximately 72% less. 

Bed material composition also exhibits an historical change (1933 vs. post 1965), decreasing in 

grain size from 0.2 to 0.1 mm in the zone 320-160 km upriver and from 0.17 to 0.05 mm in the 

zone 0-160 km upriver (Keown et al. 1986). The grain size of bed sediment exhibits a distinct 

fining below the Old River control structure (Keown et al. 1986, Nordin and Queen 1992). At 

Tarbert Landing, the bed sediments are 1 % silty clay and 96% fine sand; at Belle Chase they are 

70% silty-clay and 30% fine sand; and, at Venice they are 80% silty-clay and 20 % fine sand 

(Nordin and Queen 1992). At intermediate locations, the riverbed may be as much as 98% siltyclay. 

Direct measurements of suspended sediment concentration variations in the offshore region, 

outside the river, are not as good as those inside the river system. Available data sets are just 

approaching a length where we can begin to discuss meaningfully the interannual variability of the 

coastal system. They are certainly long enough to characterize the subtidal variability in the 

weather band. 

Yearly to decadal sediment accumulation rates are commonly determined using a set of particlereactive radionuclides ('lOPb, 2393240~~ and 137C~). The decadal geochronologies constructed using 

"Vb integrate over the 10-120 years prior to core collection, depending on core lengths (- 50 cm) 

and sediment burial rates (McKee et al. 1983). Sedimentation rates in the Mississippi River Bight 

(derived from excess 210Pb sediment profiles at 37 stations) are depicted in Figure 2.8. Sediment 

burial rates decrease with distance from the river mouth, from 3.44 g/cm/yr (> 5 cdyr) to 0.01 

glcm yr (0.02 cdyr). The region of highest sediment accumulation is contained within a 20 

kilometer radius of the river mouth, following the commonly observed patterns in deltaic systems 

of decreasing accumulation rates away from the sediment source. A total of 22 x 10" grams of 

sediment accumulate annually in the study area, about 50% of the annual sediment discharge via 

Southwest Pass (45 x 10" grams) (Milliman and Meade 1983). 

The sedimentation rates presented here are "apparent" rates and should be considered maximum 

values. In areas where physical and/or biological mixing is dominant, a combination of 

radiochemical tracers (e.g., 234~h, 7Be, 'lOpb and 2397240 Pu, and 137Cs isotopes) can be used to 

better understand the mixing of surface sediments. 239*N0P~ and 137C~ are byproducts of 

atmospheric testing of nuclear weapons introduced to the environment around 1952 with maximum 

fallout in 1963. In areas where the physical and/or biological mixing is negligible, the peak fallout 

retained in the sedimentary record corresponds to the year 1963. Any deviation from the expected 

atmospheric fallout in sediments is attributed to mixing and thus the rates of mixing and 

sedimentation can be delineated. A distinctive 2392% profile was observed in a sediment core 

collected within the Mississippi River Bight (Baskaran et al. 1997). Sediment mixing was found to 

be negligible at this site. The apparent sedimentation rate (uncorrected for mixing) derived from the 

'lOPb profile was identical to the net sedimentation rate derived from the 2307240~ profile. 
32 Physical Oceanography 

Gulf of 

Mexico 

Sedimentation I 

I Rate I 

Figure 2.8. Sediment accumulation rates in the Mississippi River Bight, based on 21('Pb 

geochronologies. 

Summary 

Spatial and temporal variability of the hydrography (currents and stratification) and suspended 

sediments over the Louisiana inner shelf are strong on a variety of scales. The variability will 

affect the transport of nutrients and plankton. The stratification variability will alter the isolation of 

bottom waters from reaeration from above. The variability and inferred trends in suspended 

sediment concentration will impact light conditions and, consequently, primary productivity in the 

water column. All will be important to the development of hypoxia on the Louisiana inner shelf in 

response to nutrient loading from the river system. 
Physical Oceanography 33 

References 

Baskaran, M., B. J. Presley, S. Asbill, P. H. Santschi, and R. Taylor. 1997. Reconstruction of 

historical contamination of trace metals in Mississippi River Delta, Tampa Bay and Galveston 

Bay sediments. Environmental Science and Technology (submitted). 

Camel, E., C. Martens and L. Benninger. 1990. Seasonal variations in 7Be activity in the 

sediments of Cape Lookout Bight, North Carolina. Geochimia Cosmochimica Acta 54:237- 

245. 

Cochrane, J. D. and F. J. Kelly. 1986. Low-frequency circulation on the Texas-Louisiana 

continental shelf 3. Journal of Geophysical Research 91: 10645-10659. 

Comeaux, M. L. 1970. The Atchafalaya River raft. Louisiana Studies, Winter, pp. 217-227. 

Crout, R. L., Wm. J. Wiseman, Jr., and W.-S. Chuang. 1984. Variability of wind-driven 

currents, west Louisiana inner continental shelf 1978-1979. Contributions in Marine Science 

27:l-11. 

Current, C. L. and R. 0. Reid. Model simulation of wind-driven circulation on the LouisianaTexas shelf: a comparison of 32 months of current and water level data. Journal of 

Geophysical Research in review. 

Dagg, M. J. 1988. Physical and biological responses to the passage of a winter storm in the 

coastal and inner shelf waters of the northern Gulf of Mexico. Continental Shelf Research 

8(2): 167-178. 

Dagg, M. J., E. P. Green, and P. B. Ortner. 1996. Biological removal of fine-grained lithogenic 

particles from a large river plume. Journal of Marine Research 54(1): 149-160. 

Day, J., Jr, C. Madden, R. Twilley, R. Shaw, B. McKee, M. Dagg, D. Childers, R. Raynie, and 

L. Rouse. 1995. The influence of Atchafalaya River discharge on Fourleague Bay, Louisiana 

(USA). pp.151-160 In: K. Dyer and R. Orth [eds.], Changes in Fluxes in Estuaries. Olsen & 

Olsen., Fredensborg, Denmark. 

DiMarco, S. F. and R. 0. Reid. 1998. Characterization of the principal tidal current constituents 

on the Texas-Louisiana shelf. Journal of Geophysical Research 103(2):3,093-3,109. 

Dinnel, S. P. and Wm. J. Wiseman, Jr. 1986. Fresh water on the Louisiana and Texas shelf. 

Continental Shelf Research 6:765-784. 

Dinnel, S. P., Wm. J. Wiseman, Jr., and L. J. Rouse, Jr. 1997. Coastal currents in the northern 

Gulf of Mexico. OCS Study MMS 97-0005, U. S. Department of Interior, Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 113 pp. 

Elliot, D. C. 1932. The improvement of the lower Mississippi River for flood control and 

navigation. U.S Waterways Experiment Station, U.S. Army Corps of Engineers, War 

Department. 345 pp. 

Everett, D. 1971. Hydrologic and quality characteristics of the lower Mississippi River. Louisiana 

Dept. of Public Works Tech. Report # 5.48 pp. 
3 4 Physical Oceanography 

Gunter, G. 1979. The annual flows of the Mississippi River. Gulf Research Reports 6:283-290. 

Herring, H.J., M. Inoue, G.L. Mellor, C.N.K. Mooers, P.P. Niiler, L.-Y. Oe, R.C. Patchen, 

F.M., Vukovich, and W.J. Wiseman, Jr. 1999. Coastal ocean modeling program for the Gulf 

of Mexico. Final Report to the Minerals Management Service, Dynalysis of Princeton, 

Princeton, N.J. 

Hitchcock, G. L., W. J. Wiseman, Jr., W. C. Boicourt, A. J. Mariano, N. D. Walker, T. E. 

Nelsen, and E. Ryan. 1997. Property fields in an effluent plume of the Mississippi River. 

Journal of Marine Systems 12: 107-125. 

Keown, M. P., E. A. Dardeau, Jr. and E. M. Causey. 1986. Historic trends in the sediment flow 

regime of the Mississippi River. Water Resources Research 22:1555-1564. 

Kesel, R. H. 1988. The decline in the suspended load of the lower Mississippi River and its 

influence on adjacent wetlands. Environmental Geology Water Science 11:271-281. 

Lewis, J. K. and R. 0. Reid. 1985. Local wind-forcing of a coastal sea at subinertial 

frequencies. Journal of Geophysical Research 94:8 163-8178. 

Luketina, D. A. and J. Imberger. 1989. Turbulence and Entrainment in a Buoyant Surface 

Plume. Journal of Geophysical Research 94(C9): 12,619-12,636. 

McKee, B. A., C. A. Nittrouer and D. J. DeMaster. 1983. Concepts of sediment deposition and 

accumulation applied to the continental shelf near the mouth of the Yangtze River. Geology 

11:631-633. 

McKee, B. A. and M. Baskaran. 1999. Sedimentary Processes of Gulf of Mexico Estuaries. Pp. 

63-85 In, Biogeochemistry of Gulf of Mexico Estuaries, T. Bianchi, J. Pennock and R. 

Twilley (eds.), John Wiley and Sons, New York. 

McKee, B. A., J. G. Booth and P. W. Swarzenski. 1999. Sediment deposition, redistribution and 

accumulation in the Mississippi River Bight. Continental Shelf Research (submitted). 

Meade, R. H. 1996. River-sediment inputs to major deltas, pp.63-83. In: J. D. Milliman and B. 

U. Haq [eds.], Sea-Level Rise and Coastal Subsidence. Kluwer Academic. 

Meade, R. and R. Parker. 1985. Sediment in rivers of the United States. In: National Water 

Summary 1984: U.S. Geological Survey Water Supply Paper, 2275:49-60. 

Meade, R., T. Yuzyk and T. Day. 1990. Movement and storage of sediment in rivers of the United 

States and Canada, pp. 255-280. In: W.H. Riggs (ed.), The Geology of North America. 

Geological Society of America. 

Milliman, J. 1992. Flux and fate of fluvial sediment and water in coastal seas. pp. 69-90 In: R. 

Mantoura, J. Martin and R. Wollast (eds.), Ocean Margin Processes in Global Change. Wiley 

and Sons, New York, p. 69-90. 
Physical Oceanography 35 

Milliman, J.D. and R.H. Meade. 1983. World-wide delivery of river sediment to the oceans. 

Journal of Geology 9 1 : 1-2 1. 

Moeller, C. C., 0. K. Huh, H. H. Roberts, L. E. Gumley and W. P. Menzel. 1993. Response of 

Louisiana coastal environments to a cold front passage. Journal of Coastal Research 9:434- 

447. 

Mossa, J. 1996. Sediment dynamics in the lowermost Mississippi River. Engineering Geology 

45:457-479. 

Murray, S. P. and E. Jarosz. 1997. Physical oceanographic observations of the coastal plume, in 

(S.P. Murray, ed.) Final Report: An observational study of the Mississippi-Atchafalaya coastal 

plume. OCS Study MMS 98-0040, U. S. Department of the Interior, Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA. 513 pp. 

Nordin, C. F., Jr., and J. P. Beverage. 1965. Sediment transport in the Rio Grande, New Mexico 

U.S. Geological Survey Professional Paper 462-F, pp.Fl-F35. 

Nordin, C. and B. Queen. 1992. Particle size distributions of bed sediments along the thalweg of 

the Mississippi River, Cairo, Illinois, to Head of Passes, September 1989. U.S. Corps of 

Engineering Potamology Program, Report #7. 

Robbins, L. G. 1977. Suspended sediment and bed material studies on the lower Mississippi 

River: U.S. Arrny Engineer District, Vicksburg, Potarnology Investigations Report, 300-1 29. 

Roberts, H. H., 0. K. Huh, S. A. Hsu, L. J. Rouse and D. Rickrnan. 1988. Impact of cold-front 

passages on geomorphic evolution and sediment dynamics of the complex Louisiana coast. 

pp.1950-1963. In: Coastal Sediments '87, Proceedings of a Specialty Conference, American 

Society of Civil Engineers. 

Rouse, L. J. and J. M. Coleman. 1976. Circulation observations in the Louisiana Bight using 

LANDSAT imagery. Remote Sensing of the Environment 53-66. 

She, J., A. Valle-Levinson, and W.J. Wiseman, Jr. In review. Circulation over the inner TexasLouisiana shelf. Journal of Geophysical Research. 

Takano, K. 1954. On the salinity and the velocity distributions off the mouth of a river. Journal 

of the Oceanography Society of Japan 10(3):92-98. 

Trefry, J., R. Trocine, S. Metz, T. Nelsen and N. Hawley. 1992. Suspended particulate matter on 

the Louisiana Shelf: concentrations, composition and transport pathways. pp. 126-130. In: 

Nutrient Enhanced Coast Ocean Product, NECOP Workshop Proceedings Oct. 1991, Texas 

A&M University Press. 

Ulrn, W. F. 1983. A volumetric temperature-salinity census for the continental shelf of the 

northwestern Gulf of Mexico. Unpublished Masters thesis, Department of Oceanography, 

Texas A&M University, College Station, TX. 56 pp. 

Walker, N. D. 1994. Satellite-based assessment of the Mississippi River discharge plume's 

spatial structure and temporal variability. OCS Study MMS 94-0053, U. S. Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 56 

PP- 
36 Physical Oceanography 

Walker, N. D. 1996. Satellite Assessment of Mississippi River Plume Variability: Causes and 

Predictability. Remote Sensing of the Environment 58:21-35. 

Walker, N. D. and A. B. Harnrnck. Impacts of winter storms on circulation and sediment 

transport: Atchafalaya-Vermilion Bay Region, Louisiana, USA. Journal of Coastal Research 

(in press). 

Walker, N. D., G. S. Fargion, L. J. Rouse, and D. C. Biggs. 1994. The Great Flood of 

Summer 1993: Mississippi River Discharge Studied. Eos, Transactions of the American 

Geophysical Union 75(36):409,414-415. 

Wells, F. C. 1980. Hydrology and water quality of the lower Mississippi River: Louisiana Office 

of Public Works Technical Report 21,83 pp. 

Wiseman, Wm. J., Jr. and F. J. Kelly. 1994. Salinity variability within the Louisiana Coastal 

Current during the 1982 flood season. Estuaries 17:732-739. 

Wiseman, Wm. J., Jr. and R. W. Garvine. 1995. Plumes and coastal currents near large river 

mouths. Estuaries l8(3):509-5 17. 

Wiseman, Wm. J., Jr., V. J. Bierman, Jr., N. N. Rabalais, and R. E. Turner. 1992. Physical 

structure of the Louisiana shelf hypoxic region. pp. 21-26. Nutrient Enhanced Coastal Ocean 

Productivity. TAMU-SG-92-109. Texas A&M University Sea Grant College Program, 

College Station, TX. 

Wiseman, Wm. J., Jr., N. N. Rabalais, R. E. Turner, S. P. Dinnel, and A. MacNaughton. 1997. 

Seasonal and interannual variability within the Louisiana Coastal Current: stratification and 

hypoxia. Journal of Marine Systems 12:237-248. 

Wright, L. D. 1970. Circulation, Effluent Diffusion, and Sediment Transport, Mouth of South 

Pass, Mississippi River Delta. Ph. D. dissertation. Department of Marine Sciences, Louisiana 

State University, Baton Rouge, LA. 56 pp. 

Wright, L. D. and J. M. Coleman. 1971. Effluent expansion and interfacial mixing in the 

presence of a salt wedge, Mississippi River dalta. Journal Geophysical Research 76(36):8649- 

8661. 
CHAPTER 3 

Long-term Watershed 

and Water Quality Changes 

in the Mississippi River System 

bv R. Euaene Turner. Nancv N. Rabalais and Dubravko Justic' 

Introduction 

The Mississippi River is the largest source of freshwater and nutrients to the northern Gulf of 

Mexico. This watershed, like others, has undergone major changes affecting water quality since 

the Native American culture was displaced by mostly European immigrants in the early 1800s. 

Major alterations in the morphology of the main river channel and widespread landuse patterns 

in the watershed, along with anthropogenic additions of nitrogen and phosphorus, have resulted 

in dramatic water quality changes this century (Turner and Rabalais 1991). The influence of 

humans with regard to changes in the coastal ecosystem and in degradation of coastal water 

quality is evident as increased coastal productivity and worsening of oxygen stress in the 

offshore bottom waters. Water quality changes in it are likely to be manifested in ecosystem 

changes in the offshore waters, including oxygen deficiency. This chapter reviews the water 

quality changes in the Mississippi River, especially nitrogen and silica which strongly influence 

the quality and quantity of phytoplankton production on the shelf. 

The Mississippi River watershed, the largest in the United States (41% of the area of the 

contiguous 48 states), has undergone massive transformations in the last 200 years. The river 

has been shortened by 229 lun in an effort to improve navigation, and has a flood-control system 

of earthwork levees, revetments, weirs, and dredged channels for much of its length (Turner and 

Rabalais 1991). These modifications have left adjacent lands drier and more susceptible to 

massive conversion to farmland (Abernethy and Turner 1987). Better than half of the original 

wetlands in the United States have been lost to drainage practices (Zucker and Brown 1998). 

Much of this wetland loss is related to agricultural production in areas that were swampy and too 

wet to farm. 

Water quality in streams, rivers, lakes and coastal waters may change when watersheds are 

modified by alterations in vegetation, sediment balance, conversion of forests and grasslands to 

farms and cities, and increased anthropogenic that accompany increased population density, e.g., 

fertilizer application, sewage disposal or atmospheric deposition (Turner and Rabalais 1991, 

Howarth et al. 1996, Caraco and Cole 1999). The estimate of current river nitrogen export from 

the Mississippi River is 2.5- to 7.4-fold higher than from the watershed during pre-agricultural 

and pre-industrial or "pristine" conditions (Howarth et al. 1996). Fertilizer application accounts 
38 Long-term Watershed and Water Quality Changes 

for roughly two-thirds of the estimated increase and atmospheric deposition the other third for all 

of the nitrogen fluxes to the North Atlantic basin. For the Mississippi River Basin, the 

proportion of fertilizer is much greater and the atmospheric contribution is lower. 

Landscape Alterations and Anthropogenic Nutrients 

Substantial population growth in the Mississippi River watershed began in the 1850s when the 

population density of the native American Indians was relatively low, in part because of diseases 

introduced by the Europeans. The population of the Midwest in the early 1600s is estimated to 

be no more than 106,000 (Ubelaker 1992), or < 0.1 persons/km2. By 1990 it was around 50 

persons/km2. The area of land under cultivation rose with population growth, but was preceded 

by the cutting of trees, often by girdling the trunk. Humphreys and Abbott (1 867) reported 

preliminary data indicating that 15% of the Mississippi River watershed was under cultivation or 

"improvements" by 1860. What these numbers indicate is that the factors that dramatically 

promote soil erosion were just coming into force before the 1850s when the first suspended 

sediment records were being collected in New Orleans. The 1987 U.S. Department of 

Agriculture (USDA) census estimated that 65% and 25% of the land in the 14 states of the 

Mississippi River watershed was farmland and harvestable cropland, respectively. 

Soil erosion is accompanied by nutrient losses, especially the nitrate ion, which is relatively 

mobile compared to inorganic minerals and phosphorus. The early settlers were quite familiar 

with soil exhaustion, and the increased nitrogen release resulting from farming practices could 

yield 100 times more nitrogen than land under natural cover (e-g., Miller and Krusekoff 1932). 

Fertilizer applications in the post-World War I1 era, the increased concentration of a similar 

number of livestock in a smaller area, higher atmospheric deposition and urbanization have all 

contributed to additional nutrient loadings into the watershed in the past 100 years. Some of this 

will obviously leak out of the watershed and into the coastal zone. 

Data on the nitrogen and phosphorus fertilizer use and form in the United States and the world 

are available in annual reports of the United Nations, Annual Statistical Yearbook (year = July 

through June) and the U. S. Department of Agriculture (year = January through December). 

Nitrogen and phosphorus fertilizer use in the United States began in the mid-1930s and climbed 

to a peak in approximately 1980 (Turner and Rabalais 1991). Phosphorus fertilizer use in the 

United States reached a plateau around 1980, whereas nitrogen fertilizer use is still increasing 

(Fig. 3.1). The form of nitrogen and phosphorus fertilizers used has changed during the last 

several decades (Turner and Rabalais 1991). In 1929, nitrogen fertilizer was 48% ammonia, 

19% nitrate and the balance mostly organic nitrogen. By 1949, the proportion was 79.5% 

ammonia and 12% nitrate, and the percent ammonia has remained the same or increased slightly 

since then. Forty-two percent of the nitrogen fertilizer and 37% of the phosphorus fertilizer used 

annually in the United States from 1981 to 1985 was applied in states that are partially or 

completely in the Mississippi River watershed, where it equaled 4.2 million mt of nitrogen (as N) 

and 0.53 million mt of phosphorus (as P). The average N and P loading f?om the Mississippi 

River system to the Gulf of Mexico during the same period was 1.83 million mt N and 0.15 

million mt P. Turner and Rabalais (199 1) estimated that a maximum of 44% of the applied 

nitrogen and 28% of the applied phosphorus may have made its way to the Gulf of Mexico. 

Subtracting a natural loading estimate (riverine fluxes prior to World War 11), they estimated that 
Long-term Watershed and Water Quality Changes 39 

the maximal loading fiom fertilizer sources probably represents no more than 22% of the applied 

fertilizer. 

1920 1940 1960 2000 

Year 

Figure 3.1. Nitrogen (as N) and phosphorus (as phosphate) fertilizer use this century in the 

United States up to 1997. From USDA annual agriculture statistical summaries. 

A large proportion of the agriculture acreage in the Mississippi River system is drained to 

promote agricultural expansion and to increase crop production on farmed land. Drainage is 

accomplished by either surface or subsurface manipulations of water (Zucker and Brown 1998). 

Historically, the main reason for drainage on agricultural land has been to enhance crop 

production, but it also allows for improvements in operation efficiency. Drainage systems may 

have a positive impact on some nonpoint source pollution problems in comparison to agricultural 

land without drainage. For example, under certain conditions artificial drainage lowers soil 

erosion by increasing the movement of water through the soil profile and thus reducing and 

lowering phosphorus losses by up to 45%. Subsurface drainage, however, expedites the 

transport of nitrate-N from the soil zone to surface waters. This management practice, coupled 

with the increase in fertilizer applications (see below), increases nitrate-N transfers fiom 

agricultural fields to the receiving waters of the Mississippi River watershed. 

Zucker and Brown (1998) summarized the history of drainage in the U.S. Two periods of 

drainage development occurred primarily during 1870- 1920 and 1945-1 960. Early settlers 

employed small open ditches to drain wet spots in fields and cleaned out small streams. 

Increasingly the need for large outlets was facilitated by the establishment of drainage districts, 

county drains, or other drainage enterprises organized under state laws. By 1920, more than 53 

million acres out of a total of 956 million acres of U.S. farmland had received some form of 

drainage. By 1960, 86.6 million acres of non-federal, rural land had been drained. This figure 
40 Long-term Watershed and Water Quality Changes 

rose to 109.7 million acres by 1985. Of this total, 75.5 million acres (69%) was used for 

cropland. No comprehensive surveys of agriculture drainage have been conducted since 1985. 

Changes in Freshwater Discharge 

Variability in freshwater discharge on seasonal, annual, decadal and longer scales underlies 

many important physical and biological processes affecting coastal productivity and food webs. 

There is significant interannual variability in the annual discharge (Figure 3.2). The 1900-1992 

average discharge rate (decadal time scale) for the lower Mississippi River is remarkably stable 

at about 14,000 m3/s. There is a decrease in flow during the 1950s and 1960s, and the 1990s 

have been a period of higher discharge. The discharge of the Mississippi River increased from 

1935 to 1995 at 0.3%/yr, or 20%. The stage height, however, did not increase over the same 

period. There is some question as to the existence of a trend in discharge from the system. The 

reported differences are likely attributable to the period of record examined. 

Clearly, the discharge of the Atchafalaya has increased during the course of the existing record 

(1930-1997) (Bratkovich et al. 1994). This trend is statistically significant when tested with a 

seasonal Kendall tau test for trend (Hirsch et al. 1982), is visually obvious when examining plots 

of Atchafalaya discharge, and is associated with the tendency of the river to capture flow from 

the Mississippi River proper. Less obvious is a trend in the Mississippi River discharge as 

measured at Tarbert Landing. This trend is also statistically significant and increasing. It 

appears to be due to a tendency for increasing discharge in the last four months of the calendar 

year, September through December. This period, however, is least important in the timing of 

important biological processes that lead to the development of hypoxia or the physical processes 

important in its maintenance. If a longer period of annual discharge were considered (e.g., fiom 

Turner and Rabalais 199 1 for the early 1800s to present), the trends since the 1950s are obvious 

but are concealed within high interannual variability and no long-term change occurs over a 

century and a half. When considering changes to the system related to physical forcing andlor 

biological processes stimulated by nutrients, it is important to consider the river discharge for the 

particular period of concern. 

 annual 1 0 Year Moving Average OT 

1800 1850 1900 1950 2000 

Year 

Figure 3.2. The annual average discharge of the Mississippi River at Vicksburg, Mississippi, 

(m3/s) with a 10-yr moving average superimposed (data from U.S. Army Corps of 

Engineers). 
Long-term Watershed and Water Quality Changes 

Changes in Suspended Sediments 

The suspended sediment loads carried by the Mississippi River to the Gulf of Mexico have 

decreased by one-half since the 1900s (Kesel 1988). The colonization of the Midwest 

undoubtedly increased the soil losses above "background" as a result of land clearing and 

agricultural practices (e.g., plowing). The increased soil losses would have released nutrients 

stored in vegetation and soil organics, and the result is seen in dated sediment cores taken near 

the Mississippi River delta (Turner and Rabalais 1994a). The remains of diatoms increased in 

the 1850s, at the same time as land clearing occurred, and then dropped off within two decades, 

implying a coincidental increase of nutrients into the coastal zone as land was cleared for 

agriculture. Suspended sediment :loading would have increased at the same time. The decrease 

in concentration of suspended sediments has been quite large since the 1950s when the largest 

natural sources of sediments in the drainage basin were cut off from the Mississippi River by the 

construction of large reservoirs on the Missouri and Arkansas Rivers (Meade 1995). This large 

decrease in sediments from the western tributaries was counterbalanced somewhat by a 5-to 10- 

fold increase in sediment loads in the Oh10 River as a result of deforestation and rowcrop 

farming. A decrease is suspended sediments might affect water clarity in offshore waters, 

silicate flux, and organic nitrogen flux. 

Large, turbid rivers, such as the Mississippi, deliver sediments that in turn release significant or 

major fractions of the total riverine nitrogen delivery (Mayer et al. 1998). Organic nitrogen 

desorbs from sediment particles in the freshwaterlseawater mixing zone and eventually 

contributes to the total nitrogen load of the river to the Gulf that influences phytoplankton 

growth. Surface nutrient concentrations in excess of conservative mixing relationships (Lohrenz 

et al. 1999) may be attributable to remineralization of organic matter from particles or that 

delivered in the dissolved form. A decrease in sediment load could affect both the particulate 

and dissolved organic nitrogen flux. Since Mississippi River suspended sediment load has 

decreased (Meade 1995) and the dissolved inorganic nitrogen pool has increased from 

anthropogenic influences (Turner and Rabalais 199 I), the relative importance of particulate 

versus dissolved nitrogen delivery in the Mississippi River has decreased over recent decades 

(Mayer et al. 1998). 

Changes in Nutrients and Nutrient Ratios 

Turner and Rabalais (1 99 1) compared the mean concentration of nitrate, silicate and total 

phosphorus for four lower Mississippi River stations that are 430, 195, 198 and 18 krn upstream 

from the terminus of the river (Table 3.1). With the exception of total phosphorus, the four 

stations were similar to each other for the period 1975-1987 and considered representative of 

average concentrations. The concentration of total phosphorus at the most downstream station is 

considered the most relevant data to influence nearshore Gulf waters. 

The relationship between the concentration of nitrate and total nitrogen at St. Francisville is 

shown in Fig. 3.3. Both the average nitrate and total nitrogen concentrations have increased in 

recent decades (Turner and Rabalais 1991), but the percent nitrogen as nitrate has varied between 

30% and 60%. This percentage leveled off in the 1980s to 1990s at 60%. The increase in total 

nitrogen is almost entirely due to changes in nitrate concentration. The distribution of the 

- -nitrogen in the river now appears mostly as nitrate and organic nitrogen. From 1977 to 1994, the 
42 Long-term Watershed and Water Quality Changes 

percent of the total nitrogen pool averaged 59% nitrate and 37% organic nitrogen. The 

remaining 4% was ammonia (3%), nitrite (1%) and unidentified dissolved organic nitrogen 

molecules. The importance of dissolved forms other than nitrate and the dissolved organic and 

particulate organic nitrogen component are not being dismissed, but less is known about their 

relative contribution of the total flux over a longer time frame and the related biological 

processes on the adjacent shelf. Studies from the Mississippi River plume support the view of an 

active microbial population capable of utilization of organic nitrogen and rapid rates of 

regeneration of inorganic nutrients (Gardner et al. 1994, 1997, Pakulski et al. 1995, Bode and 

Dortch 1996). 

Table 3.1. Average nutrient concentrations for four stations on the lower Mississippi River for 

two time periods. Data are the means of individual measurements, averaged for the 

entire period (mean If: 1 S.E.); number of samples is in parentheses. (From Turner 

and Rabalais 199 1 .) 

St. Francisville Luling New Orleans Venice 

Chemical/Dates at Carrollton 

Total phosphorus (pM) 

1975-1 980 7.3 + 4.2 (71) 8.9 4 0.3 (72) -- 7.4 4 0.3 (72) 

1981 -1 987 6.6 + 0.4 (76) 9.0 + 0.5 (80) -- 7.4 f 0.4 (78) 

Nitrate-N (pM) 

1975-1 980 84.2 + 3.6 (72) 89.4 4 3.7 (67) 85.0 4 3.8 (58) 88.3 f 3.2 (71) 

1981 -1 987 119.95 6.4(29) 110.6+4.6(70) 113.0+ 8.8(24) 111.1f 4.3(77) 

Silicate-Si (pM) 

1975-1 980 96.9 If: 3.0 (56) -- 101.7 f 4.2 (57) -- 

1981-1987 106.9 +- 3.5 (47) -- 108.2 + 5.0 (24) -- 

Upstream ............................................................... ---- Downstream 

Mississippi River nutrient concentrations and loading to the adjacent continental shelf have 

changed dramatically this century, with an acceleration of these changes since the 1950s (Turner 

and Rabalais 199 1, 1994a, Rabalais et al. 1996). Turner and Rabalais (1 99 1) examined water 

quality data for the four lower Mississippi River stations for dissolved inorganic nitrogen (as 

nitrate), phosphorus (as total phosphorus) and silicon (as silicate). The mean annual 

concentration of nitrate was approximately the same in 1905- 1906 and 1933- 1934 as in the 

1950s, but it has doubled in the last 35 years (Fig. 3.4). The increase in total nitrogen is almost 

entirely due to changes in nitrate concentration. The mean annual concentration of silicate was 

approximately the same in 1905- 1906 as in the early 1 %Os, then it declined by 50%. 

Concentrations of nitrate and silicate appear to have stabilized, but trends are masked by 

increased variability in the 1980s and early 1990s data (Fig. 3.5). They found no substantial 

records of total phosphorus concentrations in the lower Mississippi River before 1972. Although 

the concentration of total phosphorus appears to have increased since 1972, variations among 

years are large. The total annual loads of nitrogen and phosphorus have increased substantially 

since the 1950s (Turner and Rabalais 199 1, Justic' et al. 1995a). 
Long-term Watershed and Water Quality Changes 

Figure 3.3. The relationship between the concentration of nitrate-N and total nitrogen in the 

lower Mississippi River at St. Francisville, Louisiana (from Turner and Rabalais 

1991). 

The concentrations of nitrate-N and silicate currently average near 100 pM on an annual basis, 

with much variability about this average (Figs. 3.4 and 3.5). Total P is 7.4 pM at the mouth of 

the Mississippi River. Nutrient flux closely follows river discharge and large interannual 

variations in river discharge which the nitrogen constituents clearly follow (Alexander et al. 

1996); however, there is some seasonality of nitrate flux independent of river discharge (Justic' 

et al. 1997). The effluent of the Atchafalaya River at Morgan City (0.5 krn fiom the terminus of 

the river) varies somewhat fiom the Mississippi River mainstem (Turner and Rabalais 1991). 

The concentrations of nitrate, silicate and total phosphorus at Morgan City (Atchafalaya River) 

were 69%, 94% and 130%, respectively of that in the Mississippi River at St. Francisville for the 

combined data for the years 1973- 1987. Differences between nutrient concentrations of the two 

discharges are presumably a consequence of differences in the water quality of the Red River 

(which mixes with the Mississippi to form the Atchafalaya) compared with that of the 

Mississippi, inputs within the Atchafalaya basin, and processes of transformation within the 

Atchafalaya basin. Over the period 198 1 - 1987, annual nitrogen and phosphorus loading 

averaged 130.3 x 1 o6 kg atoms N and 3.4 x lo6 kg atoms P (Turner and Rabalais 1991). 

The seasonal patterns in nitrate and silicate concentration have also changed during this century 

(Fig. 3.4). There was no pronounced peak in nitrate concentration earlier this century, whereas 

there was a spring peak fiom 1975 to 1985, presumably related to seasonal agricultural activities, 

timed with long-term peak river flow. A seasonal summer-fall maximum in silicate 

concentration, in contrast, is no longer evident. Consequently the seasonal signal of 

si1ica:nitrogen atomic ratio has also changed (Fig. 3.6) The seasonal shifts in nutrient 

concentrations and ratios become increasingly relevant in light of the close temporal coupling of 

river flow to surface water net productivity (1-mo lag) and subsequent bottom water oxygen 

deficiency (2-mo lag) (Justic' et al. 1993). 
Long-term Watershed and Water Quality Changes 

04 1 

2 4 6 8 10 12All 

Month Months 

Figure 3.4. Monthly average concentrations of nitrate, silicate and total phosphorus ratio in the 

lower Mississippi River for periods indicated (modified and updated fi-om Turner 

and Rabalais 199 1, 1994a). 
Long-term Watershed and Water Quality Changes 

Nitrate 

I 

1950 1960 1970 1980 1990 

Figure 3.5. The average annual concentration (pM + 1 S.E.) of nitrate and silicate in the 

Mississippi River at New Orleans (from Turner et al. 1998). 

0 2 4 6 8 10 12 14 

Month 

Figure 3.6. Monthly variation in average (1975-1985) nutrient ratios in the Mississippi River at 

St. Francisville, Louisiana. Derived from data in Turner and Rabalais (1 99 1). 

The rise in nitrate since 1960 is strongly related (R~ = 0.74) with an increase in nitrogen fertilizer 

application in the watershed that was applied in sufficient amounts to account for the changes in 

water quality (Turner and Rabalais 1991). The change in silicate since 1960 is strongly and 

inversely related to phosphorus fertilizer use (R~ = 0.79). The decrease appears to be a 

consequence of upstream phosphorus additions that stimulated fkeshwater diatom production and 

an eventual burial in freshwater sediments of silica in diatom remains (e.g., Schelske and 

Stoermer 1971, Schelske et al. 1986), thus reducing the annual supply of riverine silicate to 
46 Long-term Watershed and Water Quality Changes 

coastal waters. Reductions in silicate concentrations are possibly affected further by water 

retention time, cropping systems, and landuse practices. 

The si1icate:nitrate ratios have changed as the concentrations varied. The si1icate:nitrate atomic 

ratio was approximately 4: 1 at the beginning of this century, dropped to 3: 1 in 1950 and then 

rose to approximately 4.5: 1 during the next ten years, before plummeting to 1 : 1 in the 1980s. 

The ratio appears table at 1:l through 1997 with little variation (Fig. 3.5). The average atomic 

ratios of N:Si, N:P and Si:P are currently 1.1,15 and 14, respectively, and closely approximate 

those of Redfield (1958) of 16:16:1, N:Si:P (Justic' et al. 1995a,b). 

Justic' et al. (1 995b) compared data for two periods: 1960-1 962 and 198 1-1 987 (Table 3.2, data 

from Turner and Rabalais 1991, Justic' et al. 1995b). Substantial increases in N (300%) and P 

(200%) concentrations occurred over several decades, and Si decreased (50%). mo data on total 

P concentration in the Mississippi River were reported prior to 1973; however, total P in the river 

showed a moderate increase between 1973 and 1987. By applying a linear least-squares 

regression on the 1973 -1 987 data, we estimated (pc0.01) that the total P concentration increased 

two fold between 1960- 1962 and 198 1 - 1987.1 Accordingly, the Si:N ratio decreased from 4.3 to 

0.9, the Si:P ratio decreased from 40 to 14, and the N:P ratio increased from 9 to 15. By 

applying the Redfield ratio as a criterion for stoichiometric nutrient balance, one can distinguish 

between P-deficient, N-deficient, and Si-deficient rivers, and those having a well balanced 

nutrient composition. The nutrient ratios for the Mississippi River (1 98 1-1 987 data base) show 

an almost perfect coincidence with the Redfield ratio (Figure 3.2). The proportions of Si, N and 

P have changed over time in such a way that they now suggest a balanced nutrient composition. 

Despite being balanced on an annual basis, seasonal variations in nutrient inputs can affect 

nutrient availability. In particular, there is nearly a two-fold difference in nitrate supply over the 

course of the year (Turner and Rabalais 1991), but only small annual variations in the silicate 

and total phosphorus supply (Fig. 3.4). Consequently, the nutrient supply ratios vary around the 

Redfield ratios on a seasonal basis, with silicate and phosphorus in the shortest supply during the 

spring and nitrogen more likely to be limiting (based on ratios) during the rest of the year (Fig. 

3.6). The proportion of dissolved silica, nitrate, nitrite, organic nitrogen and phosphorus in the 

Mississippi River during high flow (March-May) is higher than expected in the Atchafalaya (R. 

E. Turner unpublished analysis of USGS data). As a result, the elemental ratio of Si:N 

(dissolved silicate and dissolved nitrate-tnitrite) in the lower Mississippi River more closely 

approximates the Redfield ratio than does the Atchafalaya (1.1 1 vs. 1.74, respectively), and these 

differences are more evident during some periods than others. With nutrient concentrations so 

closely balanced, Justic' et al. (1995b) proposed that any nutrient can become limiting, perhaps 

in response to small differences in nutrient supply ratios such as these, or conversely that no 

single nutrient is more limiting than others. These seasonal differences in nutrient ratios cooccur with seasonal variation in river flow, so that the riverine supply of all nutrients is least in 

low flow periods. Fluctuations in the Si:N ratio within the major riverine effluents and 

differences in Si:N ratios between the effluents of the two rivers are believed to be major 

determinants in estuarine and coastal food web structure on a seasonal and annual basis, with 

major implications to the cycling of oxygen and carbon (Turner et al. 1998). 
Long-term Watershed and Water Quality Changes 47 

Table 3.2. Historical changes in concentratiohs (pM) and atomic ratios of nitrogen (N), 

phosphorus (P) and silica (Si) in the lower Mississippi River and the northern Gulf of 

Mexico; x - mean value, n - number of data, S - standard error, p<0.001 - highly 

significant difference in nutrient concentrations between the two periods, based on a 

two-sample t-test. (Modified from Justic' et al. 1995b.) 

Nutrient concentration (yM): 

X 

N~ n 

S 

Average atomic ratios: 

Si:N 

N:P 

Si:P 

Mississippi River Northern Gulf of Mexico 

a~-~~, for the Mississippi River, dissolved inorganic nitrogen (DIN = NO, +NH4 + NO,) for the northern Gulf of Mexico 

btotal P for the Mississippi River, reactive P for the northern Gulf of Mexico 

Creactive Si 

d~urner and Rabalais 1991, for N and Si, reconstructed for P 

ereconst~cted data 

Dams and Algal Blooms 

Eutrophication in the northwestern coastal regions of the Black Sea has been attributed to the 

damming of the Danube River, sequestering of silicate on sediments, and subsequent changes in 

nutrient ratios that led to eutrophication and harmful algal blooms (Humborg et al. 1997). Their 

analysis, however, did not take into account the anthropogenic loading of nitrogen and 

phosphorus that increased substantially over the last several decades. Also, the decline in silicate 

concentration illustrated in their manuscript closely parallels the gradual decline of silicate 

concentration in the lower Mississippi River documented by Turner and Rabalais (1991) rather 

than a "step" function as described for their data. A longer retention time in freshwater areas 

may exacerbate a silicon depletion scenario. In addition, changes in silicate concentrations may 

be related to changes in landuse patterns, cropping systems, and types of crops. 
48 Long-term Watershed and Water Quality Changes 

References 

Abernethy, Y. and R. E. Turner. 1987. US forested wetlands: 1940-1980. BioScience 37: 721- 

727. 

Alexander, R. B., P.S. Murdoch and R. A. Smith. 1996. Streamflow-induced variations in 

nitrate flux in tributaries to the Atlantic coastal zone. Biogeochemistry 33: 149-177. 

Bode, A. and Q. Dortch. 1996. Uptake and regeneration of inorganic nitrogen in coastal waters 

influenced by the Mississippi River: spatial and seasonal variations. Journal of Plankton 

Research 18: 225 1-2268. 

Bradley, R. S., H. F. Diaz, J. P. Eischeild, P. D. Jones, P. M. Kelly and C. M. Goodess. 1987. 

Precipitation fluctuations over the northern hemisphere land areas since the mid-19~ century. 

Science 237: 171-175. 

Bratkovich, A, S. P. Dinnel and D. A. Goolsby. 1994. Variability and prediction of fi-eshwater 

and nitrate fluxes for the Louisiana-Texas shelf: Mississippi and Atchafalaya River source 

functions. Estuaries 17: 766-778. 

Caraco, N. F. and J. J. Cole. 1999. Human impact on nitrate export: an analysis using major 

world rivers. Ambio 28(2): 167-1 70. 

Gardner, W. S., R. Benner, G. Chin-Leo, J. B. Cotner, Jr., B. J. Eadie, J. F. Cavaletto and M. B. 

Lansing. 1994. Mineralization of organic material and bacterial dynamics in Mississippi 

River plume water. Estuaries 17: 8 16-828. 

Hirsch, R. M., J. R. Slack and R. A. Smith, 1982. Techniques of trend analysis for monthly 

water quality data. Water Resources Research 18: 107- 12 1. 

Howarth, R. W., G. Billen, D. Swaney, A. Townsend, N. Jaworski, K. Lajtha, J. A. Downing, R. 

Elmgren, N. Caraco, T. Jordan, F. Berendse, J. Freney, V. Kudeyarov, P. Murdoch and Z. 

Zhao-Liang. 1996. Regional nitrogen budgets and riverine N & P fluxes for the drainages to 

the North Atlantic Ocean: Natural and human influences. Biogeochemistry 35: 75-79. 

Humborg, C., V. Ittekkot, A. Caciasu and B. v. Bodungen. 1997. Effect of Danube River dam 

on Black Sea Biogeochemistry and ecosystem structure. Nature 386: 385-388. 

Humphreys, A. A. and H. L. Abbot. 186 1. Report upon the Physics and Hydraulics of the 

Mississippi River; upon the Protection of the Alluvial Region against Overflow; and upon the 

Deepening of the Mouths. Professional Papers No. 4, Corps of Topographical Engineers, 

U.S. Army, War Department. Reprinted with additions in1 876. 
Long-term Watershed and Water Quality Changes 49 

Justic', D., N. N. Rabalais, R. E. Turner and W. J. Wiseman, Jr. 1993. Seasonal coupling 

between riverborne nutrients, net productivity, and hypoxia. Marine Pollution Bulletin 26: 

184- 189. 

Justic', D., N. N. Rabalais and R. E. Turner. 1995a. Stoichiometric nutrient balance and origin 

of coastal eutrophication. Marine Pollution Bulletin 30: 41-46. 

Justic', D., N. N. Rabalais, R. E. Turner and Q. Dortch. 1995b. Changes in nutrient structure of 

river-dominated coastal waters: Stoichiometic nutrient balance and its consequences. 

Estuarine, Coastal and Shelf Science 40: 339-356. 

Justic', D., N. N. Rabalais and R. E. Turner. 1997. Impacts of climate change on net 

productivity of coastal waters: Implications for carbon budget and hypoxia. Climate 

Research 8: 225-237. 

Kesel, R. H. 1988. The decline in the suspended load of the lower Mississippi River and its 

influence on adjacent wetlands. Environmental Geology and Water Science 1 1 : 27 1-28 1 

Lohrenz, S. E., G. L. Fahnenstiel, D. G. Redalje, G. A. Lang, M. J. Dagg, T. E. Whitledge and Q. 

Dortch. 1999. The interplay of nutrients, irradiance and mixing as factors regulating 

primary production in coastal waters impacted by the Mississippi River plume. Continental 

Shelf Research 19: 11 13-1 141. 

Mayer, L. M., R. G. Keil, S. A. Macko, S. B. Joye, K. C. Ruttenberg and R. C. Aller. 1998. 

Importance of suspended particulates in riverine delivery of bioavailable nitrogen to coastal 

zones. Global Biogeochemical Cycles 12(4): 573-579. 

Meade, R. H. (ed.). 1995. Contaminants in the Mississippi River, 1987- 1992. U.S. Geological 

Survey Circular 1133, U.S. Dept. of the Interior, U.S. Geological Survey, Denver, Colorado, 

140 pp. 

Miller, M. F. and H. H. Krusekoff. 1932. The influence of systems of cropping and methods of 

culture on surface runoff and soil erosion. Missouri Agricultural Experiment Station 

Research Bulletin 177. 

Nixon, S. W. 1995. Coastal marine eutrophication: a definition, social causes, and future 

concerns. Ophelia 4 1 : 199-2 19. 

Pakulski, J. D., R. Benner, R. Amon, B. Eadie and T. Whitledge, 1995. Community metabolism 

and nutrient cycling in the Mississippi River plume: evidence for intense nitrification at 

intermediate salinities. Marine Ecology Progress Series 1 17: 207-2 18. 

Rabalais, N. N., R. E. Turner, D. Justic', Q. Dortch, W. J. Wiseman, Jr. and B. K. Sen Gupta. 

1996. Nutrient changes in the Mississippi River and system responses on the adjacent 

continental shelf. Estuaries 19: 3 86-407. 
50 Long-term Watershed and Water Quality Changes 

Redfield, A. C. 1958. The biological control of chemical factors in the environment. American 

Scientist 46: 205-222. 

Schelske, C. L. and E. F. Stoermer. 197 1. Eutrophication, silica depletion, and predicted 

changes in algal quality in Lake Michigan. Science 173: 423-424. 

Schelske, C. L., D. J. Conley, E. F. Stoermer, T. L. Newberry and C. D. Campbell. 1986. 

Biogenic silica and phosphorus accumulation in sediments as indices of eutrophication in the 

Laurentian Great Lakes. Hydrobiologia 143: 79-86. 

Turner, R. E. and N. N. Rabalais. 199 1. Changes in Mississippi River water quality this 

century. Implications for coastal food webs. BioScience 4 1 : 140- 148. 

Turner, R. E. and N. N. Rabalais. 1994. Coastal eutrophication near the Mississippi river delta. 

Nature 368: 619-621. 

Turner, R. E., N. Qureshi, N. N. Rabalais, Q. Dortch, D. Justic', R. F. Shaw and J. Cope. 1998. 

Fluctuating si1icate:nitrate ratios and coastal plankton food webs. Proceedings of the 

National Academy of Science (USA) 95: 13048-1305 1. 

United Nations. 195 1 to 1985. Annual Statistical Yearbook. United Nations, New York. 

U. S. Department of Agriculture (USDA). 1950 to 1985. Annual Reports. Agricultural 

Statistics. General Printing Office, Washington, DC. 

Zucker, L. A. and L. C. Brown. 1998. Agriculture Drainage. Water Quality Impacts and 

Subsurface Drainage Studies in the Midwest. Ohio State University Extension Bulletin 87 1, 

The Ohio State University, Columbus, 39 pp. 

Ubelaker, D. H. 1992. The sources and metholody for Mooney's estimates of North American 

Indian populations. Pages 243-288 in W. H. Denevan (ed.), The Native American Population 

of the Americas in 1492. Wisconsin Press, Madison. 
Chapter 4 

Nutrient-Enhanced Coastal Productivity 

and Ecosystem Responses 

by Nancy N. Rabalais, Steven E. Lohrenz, Donald G. Redalje, Quay 

Dortch, Dubravko Justic', R. Eugene Turner, Naureen A. Qureshi, Michael 

J. Dagg, Brian J. Eadie, & Gary L. Fahnensfiel 

Introduction 

The Mississippi River system dominates the flux of freshwater and associated dissolved and 

particulate materials on the central northern Gulf of Mexico shelf. The waters of the continental 

shelf adjacent to the Mississippi River are affected on varying spatial and temporal scales by a 

discharge that integrates runoff from 41% of the lower 48 United States. Unlike many estuaries 

and confined seas, the ecosystem adjacent to the discharges of the Mississippi River system is an 

open coastal system variously affected by meteorological conditions, advective processes and 

regional mesoscale circulation. River plumes are complex features, and the intricacies of the 

interaction of physics and biological systems on the Louisiana shelf are not well known (see 

Chapter 2). There is great daily, weekly and seasonal variability in currents and stratification on 

the shelf and, therefore, no simple description of the couplings between nutrient delivery, carbon 

production in surface waters and delivery and cycling in bottom waters. There is, however, 

evidence to implicate changes in riverine nutrient flux with overall primary and secondary 

production, carbon accumulation at the seabed, and low oxygen conditions on the shelf. 

Increased anthropogenic inputs of nitrogen, coupled with alterations in the morphology of the 

river channel and changes in land use, have led to a doubling of nitrate concentrations since the 

1950s (Chapter 3 this volume). Phosphorus levels have also increased, while silicate has 

declined. Nutrient input ratios have also changed since the 1950s and now approach Redfield 

values (Si:N:P= 16: 16: 1) (Justic' et al. 1993, 1995). This leads to a well-balanced nutrient 

composition, but also to the possibility that any of these nutrients may become limiting, perhaps 

varying in response to small differences in nutrient supply and turnover ratios. Changes in the 

conditions of nutrient limitation, coupled with changes in nutrient flux, are likely to produce 

alterations in primary production, phytoplankton community structure and associated food web 

dynamics and biogeochemical cycling. Surplus nitrogen has been considered the primary cause 

of coastal eutrophication (Ryther and Dunstan 1971, Boynton et al. 1982), and limitation by 

nitrogen has been identified as the most important factor controlling primary production in shelf 

waters west of the Mississippi River delta (Sklar and Turner 1981, Rabalais et al. 1999). There 

is also evidence in the Mississippi River outflow region for limitation by phosphorus 

(Ammerrnan 1992, Chen 1994, Smith and Hitchcock 1994) and silica (Dortch and Whitledge 

1992, Smith and Hitchcock 1994, Nelson and Dortch 1996) at various locations and times in an 
52 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

annual cycle. Primary production on the Louisiana continental shelf is enhanced by the nutrientrich outflow fiom the Mississippi River (Riley 1937, Thomas and Simmons 1960, Lohrenz et al. 

l99OYl997,l999a, Redalje et al. 1994). There is a strong coherence between fieshwater 

discharge, nutrient flux and net production on the Louisiana shelf (Justic' et al. 1993, 1997), and 

long-term patterns of accumulation of carbon and biologically bound silica in dated sediment 

cores from the Mississippi River delta bight parallel nitrogen load increases (Eadie et al. 1994, 

Turner and Rabalais 1994a). 

In addition to the role of nutrients, factors such as light availability and mixing rates must be 

considered in attempts to understand the temporal and spatial patterns in primary production and 

biomass in river-influenced coastal ecosystems. In the Delaware estuary, conditions over annual 

cycles were found to alternate between light- and nutrient-limitation of phytoplankton production 

(Pennock and Sharp 1994). In waters influenced by the Amazon River, phytoplankton 

photosynthesis was irradiance-limited inshore and nutrient-limited offshore, with a narrow 

region of high primary production in between (Smith and DeMaster 1996). This transition from 

light to nutrient limitation along salinity gradients has been reported in numerous studies of 

estuarine ecosystems and river plumes (Kemp et al. 1982, Cloern et al. 1983, Filardo and 

Dunstan 1985, Pennock and Sharp 1986, Fisher et al. 1988). What is less clear is how such 

gradients in light and nutrients interact with mixing processes at the interface of river plumes and 

adjacent shelf waters. Gradients in nutrients, light and biomass across the interface between 

river plumes and adjacent waters can be substantial, and processes of advection and mixing 

affect phytoplankton by altering growth conditions as well as dissipation or accumulation of 

biomass (e.g., Grimes et al. 1991, Franks 1992, Lucas et al. 1999). 

The National Oceanic and Atmospheric Administration Nutrient-Enhanced Coastal Ocean 

Productivity (NECOP) Program focused on three aspects of the Mississippi River plume in order 

to 1) determine quantitatively the degree to which coastal primary productivity has been 

enhanced in areas receiving terrestrial nutrient inputs, 2) determine the impact on water quality 

(particularly dissolved oxygen demand) of this enhanced productivity, and 3) determine the fate 

of the carbon-fixed in coastal areas of enhanced productivity and its impact on living resources 

within the affected coastal ecosystems. In this chapter, we draw upon the results of many 

NECOP investigators, coupled with relevant data fiom the Louisiana-Texas (LATEX) Physical 

Oceanography program (Murray l998), to address several of the above objectives. We assess 

primary production and phytoplankton dynamics across broad areas influenced by the 

Mississippi River, with an emphasis on scales of variability and controlling factors. In addition, 

we identify the factors/processes most important in regulating productivity at different temporal 

and spatial scales. We examine the fate of the carbon fixed in surface waters. We provide 

evidence for historical changes in coastal phytoplankton and productivity in relationship to 

changes in Mississippi River nutrients. In addition, we examine the fate of the carbon fixed in 

surface waters and provide evidence for historical changes in coastal phytoplankton and 

productivity in relationship to changes in Mississippi River nutrients. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Scales of Variability 

Spatial Variation 

Variability in primary production in the region of the northern Gulf of Mexico impacted by the 

Mississippi River is quite high, due to the dynamic and heterogeneous conditions found in the 

riverlocean mixing zone. Highest primary production is typically observed near the river delta 

(Lohrenz et al. 1999a, b). A gradient of decreasing chlorophyll a biomass was observed across 

the Louisiana shelf in a westerly direction from the Mississippi River delta (Figure 4.1). 

Maximum values of biomass and primary production were typically observed at intermediate 

salinities within the Mississippi River plume and coincided with non-conservative decreases in 

nutrients along the salinity gradient (Figure 4.2, Lohrenz et al. 1999a). Similar relationships of 

chlorophyll a biomass with salinity were observed across the broad region of the Louisiana shelf 

influenced by Mississippi River discharge (Figure 4.3). Rates of primary production along a 

salinity gradient of the Mississippi River plume were constrained by low irradiance and mixing 

in the more turbid, low salinity regions of the plume, and by nutrient limitation outside the 

plume. This pattern of localized maxima in phytoplankton biomass and production attributable 

to declining turbidity in the presence of relatively high nutrient levels has been reported for other 

major rivers including the Amazon (DeMaster et al. 1986, Smith and DeMaster 1996), Huanghe 

(Turner et al. 1990) and Changjiang (Xiuren et al. 1988). 

Longitude (w) 

Figure 4.1. The concentration of chlorophyll a in surface waters at LATEX sampling stations, 

between 89.5OW and 97"W and water depths of 10 and 100 m for the periods April 

1992, October 1992, April 1993, July 1993, April 1994, July 1994 (redrawn from 

Rabalais and Turner 1998). Southwest Pass of the Mississippi River birdfoot delta is 

near 89S0W and the Atchafalaya delta is near 91.5 OW. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Figure 

OI* .* . . , .', , 1 

0 10 20 30 40 

Salinity 

Mar 1991 

-.- 

0 10 20 30.40 

Salinity 

4.2. Water column-integrated primary production (PP) and chlorophyll 

concentrations along the salinity gradient of the Mississippi River plume. 

See Lohrenz et al. (1999a) for additional information. 

0 5 10 15 20 25 30 35 40 

Salinity (ppt) 

Figure 4.3. Relationship of surface chlorophyll a and salinity for six LATEX cruises 

(April 1992, October 1992, April 1993, July 1993, April 1994, July 1994) 

from 89.5"W to 97OW. Maximum value of 209 pgll at 13.7 ppt salinity was 

deleted from the plot (from Rabalais and Turner 1998). 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Temporal Variation 

The high rates of primary production in the inner shelf of the northern Gulf of Mexico can be 

attributed to nutrient loading from the Mississippi and Atchafalaya rivers and buoyancy flux that 

keeps recycled nutrients in the photic zone (Riley 1937, Thomas and Simmons 1960, Sklar and 

Turner 198 1, Lohrenz et al. 1990). The pattern of temporal variability and its relationship to 

river outflow is complicated by the dynamic and heterogeneous nature of physical and chemical 

properties in this region. A compilation of the data from the southeastern Louisiana shelf 

(compiled from several studies listed in Lohrenz et al. 1999b) reveals that highest rates occurred 

during spring and summer months with lower rates in the winter (Figure 4.4). Highest values of 

production (> 10 g C/m2/d) and biomass (> 30 mg chlorophyll a/m3) for studies conducted by 

Lohrenz et al. (1990, 1994, 1997, 1999a) were observed in April 1988, July-August 1990 and 

April-May 1992; values were lower in March and September 1991. Sampling in July-August 

1990 followed a peak discharge of the river in June of that year, rather than the long-term 

average peak in March-May. Lohrenz et aLYs results were consistent with those of Justic' et al. 

(1993, 1997), but the coherence between net production in surface waters and river discharge and 

nitrate flux was much stronger in the latter study (Figure 4.5). Differences in results of the two 

studies can be attributed to location of stations in relation to the location of the plume and the 

variability of the plume influence itself through time and space. 'The Lohrenz et al. (1999a) 

study reported data across a broad area from the Mississippi River bight and southeastern 

Louisiana shelf for five periods between 1988 and 1992; the Justic' et al. (1993, 1997) data were 

collected at a single station off Tenebonne Bay in 20-m (Station C6* in Figure 4.6) on a monthly 

basis between 1985 and 1992. 

Figure 4.4. Water column-integrated primary production from the outflow region of the 

Mississippi River as a function of month of the year shown in relationship to 

mean river discharge at Tarbert Landing from 1955-1992. Dotted lines 

represent 1- one standard deviation. Redrawn from Lohrenz et al. (1999b). 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

L 

-6 -5 

Figure 4.5. 

-4-3-2-1 0 12 3 4 5 6 

-0.51.. . . , , . . . . . . . , . . . .j 

0 1x10' 2x10' 3x10' 4x10' 

Time lag (months) N-NO, load (kg d") 

Lefi panel: Cross correlation function (CCF) for Mississippi River nitrate flux at 

Tarbert Landing and net production of the upper water column (1 -1 0 m at station 

C6* in 20-m depth off Terrebonne Bay, see Figure 4.6). Right panel: best-fit timedelayed linear model for the regression of net production (NP) on nitrate load. The 

model is NP, = -0.34 + 3.93 x 10'~ where t and t-1 denote values for the 

current and preceding months, respectively. Symbols denote monthly averages for 

the period 1985-1 992 (from Justic' et al. 1997). 

Figure 4.6. Station locations within the Mississippi Bight for transect C, mooring locations 

(C6A and C6B), sediment core stations (closed triangles), and stations of Fucik 

(1974) and Ward et al. (1 979) (open squares marked with "C" and "P"). Station C6* 

combines data from C6, C6A and C6B. Stippled intensity corresponds to frequency 

of occurrence of mid-summer hypoxia (1 985- 1987, 1990- 1 993). Station 10 of Eadie 

et al. (1994) is close to E30; station 2 near the 80-m contour south of E5O (modified 

from Rabalais et al. 1996). 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 57 

Phytoplankton in the region (Stations C6A and C6B) can be divided into three groups, 

picocyanobacteria, diatoms, and others (Figure 4.7). Others include mostly small flagellates, 

although dinoflagellate blooms do occur sporadically (Dortch 1998, Dortch et al. 1999). 

Although the small flagellates can be numerically abundant, they are much less abundant than 

picocyanobacteria and so much smaller than most diatoms that they are unlikely to dominate the 

biomass. Further, there is relatively little seasonal variation in flagellate abundance ("other" in 

Figure 4.7). Thus, picocyanobacteria and diatoms are the most important phytoplankton groups 

in the region influenced by the Mississippi River. Picocyanobacteria are most abundant in the 

summer and early fall, whereas diatoms are most abundant in the spring . In terms of estimated 

biomass, diatom biomass usually dominates, especially in the spring. The exception was during 

the flood in July 1993, when all phytoplankton were elevated, but especially picocyanobacteria 

(Dortch 1994, Rabalais et al. 1998). 

A. Total Phytoplankton 1.EtlO B. Diatoms 1 1.Ei101 

12345 678910H12 I23456789101112 

Month Month 

C. Picocyanobactena 

12345678 9101112 

Month 

1234 56789101112 

Month 

Figure 4.7. Average monthly surface layer phytoplankton major taxa composition at station C6A 

and C6B (see Figure 4.6) from 1990-1995 (mean + 1 S.E.) (Q. Dortch, unpublished 

data). Number of samples ranges from 14 to 41 for all months except February (n 

= 5) and December (n = 2). 

Factors Controlling Productivity 

The controlling influence of environmental factors is suggested by the observed spatial and 

temporal variability noted above. In the continental shelf waters of the northern Gulf of Mexico, 

there is direct evidence of a temporal relationship between river-borne nutrient fluxes and 
58 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

primary production. Measured primary production in a 6,800 h2 region (eastern = shelf area 

west of Southwest Pass between 89'30'W and 89'40'W and central = between 89'50'W and 

90" 10'W of Lohrenz et al. 1997) near the river delta and on the southeastern Louisiana shelf was 

found to be significantly correlated with nitrate + nitrite concentrations and fluxes from 1988- 

1994 (Figure 4.8, see also Figure 4.5). An outlier in March 1991 was apparently influenced by 

constraints on primary production due to low available irradiance. Although light limitation was 

probably an important factor during winter months, a positive correlation was demonstrated 

between river inputs of nitrate + nitrite and primary production for data collected from other 

times of the year. Even stronger correlations were observed between the concentration of 

orthophosphate and primary production, but these were not significant (smaller sample size). 

Peak nutrient inputs generally occurred in the spring. 

Figure 4.8. Relationship between mean primary production for the combined central and eastern 

regions (= shelf area west of Southwest Pass from 89"30fW to 90°10'W) of the 

Mississippi River bight and (a) riverborne nitrate and nitrite flux, (b) nitrate and 

nitrite concentration at Venice and (c) orthophosphate concentrations at Belle Chase. 

Historical primary production data fkom Thomas and Simmons (1960) are included 

in (a) and (b) for comparison (0). March 1991 outlier denoted by arrow in (a). Error 

bars are 5 1 S.E. (fiom Lohrenz et al. 1997). 

A comparison of the estimated nitrogen uptake by phytoplankton with the river-borne supply of 

nitrate + nitrite (Lohrenz et al. 1997) revealed that fluvial inorganic nitrogen flux was equivalent 

to a large proportion of photosynthetic nitrogen requirements (mean = 106%, S.D. = 58%, n = 

12, March 1991 excluded). Bode and Dortch (1996) measured 15~-nitrate and '%-ammonium 

uptake and found that nitrate accounted for only 39% (st. dev. = 29%, n = 17) of the N taken up. 

The results of these two studies do not conflict because 1) N uptake calculated fiom 14c uptake 

(Lohrenz et al. 1997) estimates phytoplankton uptake whereas 15~ uptake measures uptake by all 

plankton and 2) the two studies concentrated in different areas with Lohrenz et al. (1 997) 

sampling more focused near the river mouth than Bode and Dortch (1996). In fact the different 

results lead to the hypothesis that riverine nitrate fuels productivity near the river but that 

regenerated N continues the productivity over a wider area. This hypothesis is consistent with 

spatial distributions of nitrate and ammonium availability, chlorophyll biomass, and 

phytoplankton counts and species composition. 

Despite the extremely high nutrient inputs to the shelf, nutrients are depleted to low, and 

sometimes undetectable, concentrations within a short distance of the river mouth (Lohrenz et al. 

1990, 1997, 1999, Bode and Dortch 1996, Dortch and Whitledge 1992, Rabalais et al. 1996, 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 59 

1998, Nelson and Dortch 1996). Patterns of nutrient depletion provide evidence that riverine 

inputs of nutrients and their pattern of regeneration ultimately limit the extent of river-enhanced 

production and biomass. Ambient nutrient concentrations and ratios, bioassay experiments, and 

other indicators of nutrient limitation (Rabalais et al. 1999) all suggest that N, P or Si may be 

limiting at some times and places in the outflow of the Mississippi River. The details of when 

and where particular nutrients are limiting and the severity of limitation are not completely 

known. N limitation occurs most often at higher salinities and during low flow periods. In 

contrast, P limitation occurs mostly at intermediate salinities and during high freshwater input. 

Si limitation, which affects diatom growth, appears to be more spatially and temporally variable 

than P or N limitation but is more prevalent in spring than in summer. Phytoplankton species 

may be differentially susceptible to nutrient limitation, with an obvious example being the 

requirement of diatoms for Si, and there are species differences among diatoms with regard to Si 

limitation. Although limitation by both Si and P does occur, N limitation is more frequent and 

extends over a larger area. Consequently, the rate of nitrogen loading is believed to be a critical 

factor in regulating the overall production of phytoplankton over the broad region influenced by 

the river and affected by bottom-water hypoxia (see Chapter 5). Moreover, nitrogen loading has 

increased since the 1950s. 

Although it is likely that the nitrogen supply rate is a key variable controlling overall 

productivity of the shelf region near the Mississippi River delta, the factors controlling 

variability within the study area are more complex. Interplay among nutrient availability, light 

and mixing appears to be important in regulating primary production in plume waters. Based on 

observed nutrient concentrations, the probability of nutrient limitation in low salinity regions is 

low (Lohrenz et al. 1999a). Rather, accumulation of phytoplankton biomass in these regions is 

probably constrained by a combination of low light conditions, rapid changes in salinity, and 

strong turbulent mixing. The higher biomass concentrations in mid-salinity regions (Figs. 4.2 

and 4.3) require that phytoplankton growth rates must have been rapid relative to transit times. 

Lohrenz et al. (1 990), using a steady-state light limitation model, determined that light levels in 

plume waters were sufficient to support higher biomass levels than were observed, and suggested 

rapid changes in salinity and short transit times of plume waters may have constrained biomass. 

Reported growth rates of phytoplankton in Mississippi River plume waters ranged from 1.3ld 

during March 199 1 to as high as 2.7ld during July-August 1990 (Lohrenz et al. 1994, Fahnenstiel 

et al. 1995, Redalje et al. unpublished data). Transport times for plume waters passing through a 

region comparable to the southeastern shelf study area have been reported to be on the order of 

1-2 d (Lohrenz et al. 1990, Hitchcock et al. 1997), comparable to rates of growth. This supports 

the view that short transit times may have constrained biomass levels in the plume waters. An 

alternative explanation to that of growth within plume surface waters may be that enhanced 

growth or retention of biomass or both occurred in longer residence time waters at the plume 

edge or beneath the surface plume (Lohrenz et al. 1999a). 

Fate of Fixed Carbon 

Although the Mississippi River discharges some organic matter and decomposition of some of 

this organic matter could consume oxygen in the coastal ecosystem, the principal source of 

organic matter reaching the bottom waters of the northern Gulf of Mexico influenced by the 

-Mississippi River is marine in origin (Rabalais et al. 1992, Turner and Rabalais 1994a, Eadie et 
60 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

al. 1994). One major objective of the NECOP program was to determine the fate of the carbonfixed in coastal areas of enhanced productivity. Two major fates of photosynthetically fixed 

organic carbon are grazing and export via sedimentation. Evidence for linkages between primary 

production and higher trophic levels in the shelf area influenced by the Mississippi River 

include: 1) high inputs of new nitrogen (Rabalais et al. 1996), 2) direct relationships between 

dissolved inorganic nitrogen inputs and phytoplankton production (Lohrenz et al. 1997, Figure 

4.8), 3) the biomass dominance of diatoms (Bode and Dortch 1996, Nelson and Dortch 1996, 

Dortch 1998, Figure 4.7), 4) high copepod production, as inferred from high nauplii 

concentrations (Dagg and Whitledge 1991), 5) enhanced fish growth in plume regions (Grimes 

and Kingsford 1996), and 6) the potential for river nutrient enhanced fish recruitment (Govoni 

1997). Increased sedimentation of organic matter can result in a greater likelihood for 

development of hypoxic conditions in benthic environments (Justic' et al. 1993, 1997). Hypoxic 

bottom waters (< 2 mgll dissolved oxygen) on the Louisiana continental shelf can cover an area 

up to 20,000 km2 during summer (see Chapter 5), thus comprising one of the largest hypoxic 

zones in continental shelf waters worldwide. 

Grazing 

In a study by Dagg (1995), consumption of phytoplankton by the copepod community varied 

widely but was generally a significant fraction of the daily production, ranging between 4 and 

74%. Other members of the grazing community consume significant quantities of phytoplankton 

in this region. For example, Fahnenstiel et al. (1995) reported microzooplankton grazing rates 

on cells < 20 pm in the plume region averaged 82% of algal growth rates in the summer. In 

contrast, microzooplankton grazing rates on cells > 20 pm were not significantly different fiom 

zero, indicating that larger diatoms were not subject to the same degree of protozoan grazing 

mortality as smaller cells. Slightly farther to the west away from the "plume" region, 

microzooplankton also consumed significant proportions of phytoplankton, including a 

significant consumption of large diatoms by heterotrophic dinoflagellates. With the exception of 

heterotrophic dinoflagellates, most of these protozoan grazers do not produce fecal pellets of 

sufficient size and mass to contribute significantly to vertical carbon flux unless they are 

incorporated into aggregates. 

Gelatinous zooplankton, primarily larvaceans, salps and doliolids, often form large swarms in 

coastal regions of the northern Gulf of Mexico. High concentrations of the larvacean, Oikopleura 

dioica, indicate it is an important chponent of the grazer community in the vicinity of the 

Mississippi River plume (Dagg 1995, Dagg et al. 1996). For example, during May 1992 0. 

dioica populations filtered a mean of 20 %Id of the upper 5 m at stations within the Mississippi 

River plume (Dagg et al. 1996). 0. dioica feed non-selectively on particles in the size range 

between 0.2 and 20 pm (Bedo et al. 1993), including bacteria, phytoplankton and lithogenic 

materials. They produce compact ellipsoid fecal pellets with high sinking rates and probably 

contribute significantly to vertical flux of carbon. Dagg (1995) estimated that larvacean 

consumption of phytoplankton can equal that of the copepod community. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 61 

Carbon Flux 

Rates of primary production in the surface waters of the Mississippi River-influenced continental 

shelf are high (290 to > 300 g c/m21yr) (Sklar and Turner 1981, Lohrenz et al. 1990). One would 

expect that the vertical export of particulate organic carbon (POC) would also be high and 

correspond to the quantity of carbon fixed in the surface waters. Walsh et al. 1989 estimated that 

export would average 35-50% of water column-integrated primary production in the Mississippi 

River region. Suess (1980), based on studies from throughout the world, estimated that POC 

flux reaching the bottom within the 100-m contour of the continental shelf would be 50% or 

more of surface primary production. Although the overall flux of POC on the continental shelf 

influenced by the Mississippi River is high, the relationship between POC export and primary 

production is quite variable in time and space. 

Studies of particle flux on the Louisiana shelf include floating traps (Redalje et al. 1994) and 

moored traps (Qureshi 1995, Gardner and Richardson 1998). Redalje et al. (1994) examined the 

relationship between primary production and the export of POC from the euphotic zone 

determined with free-floating sediment traps. Productivity and POC exports exhibited similar 

trends in spring and fall, but were uncoupled in summer (Figure 4.9). The lowest ratio of export 

to production coincided with the time when production was greatest, and the highest ratios 

occurred when production was the lowest. Export ranged from low values of 3-9% during JulyAugust 1990 to high values during March 199 1, when export exceeded measured water columnintegrated primary production by a factor of about two. The fraction of production exported out 

of the photic zone was dependent, in part, on phytoplankton species composition and on the 

grazing activities of microzooplankton and mesozooplankton. 

0 2 46 81012 

Month 

0246 81012 

Month 

Shelf 

Figure 4.9. Ranges of water column-integrated primary production (PP) and associated vertical 

flux of POC measured with free drifting sediment traps in the Mississippi River 

plume (shelf area to the west of Southwest Pass and between 89'30'W and 89'40'W 

and adjacent shelf region between 89'50'W and 90'1 O'W) during different months 

of the year. (Data from Redalje et al. 1994.) 

3 ' 

2 -- 

1 --

0' 

+ 

- , 
62 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

In another study of the vertical flux of particulate material, particle traps were deployed on an 

instrument mooring within the zone of recurring hypoxia at depths of 5 m and 15 m in a 20-m 

water column in spring, summer and fall of 1991 and 1992 (see Figure 4.6, station C6B off 

Terrebonne Bay, Qureshi 1995). The material that sank into the traps was composed of fecal 

pellets, directly sinking phytoplankton, and other unidentified carbon including molts, dead 

zooplankton (swimmers and/or dead zooplankton that fell into the traps), marine snow, and 

particles with adsorbed organic carbon. Carbon flux was high, approximately 500 to 600 mg 

c/m21d in 15-m water depth (Qureshi 1995). These values exceeded those estimated by Redalje 

et al. (1994) for shelf waters west of the delta. A rough estimate of the fraction of production 

exported from the surface waters (compared to seasonal primary production data of Sklar and 

Turner (198 1) was highly variable and ranged from 10 to 200% with higher percentages in 

spring (Qureshi 1995), similar to the results of Redalje et al. (1 994). Results from the moored 

traps revealed that a large proportion of the particulate organic carbon flux reached the bottom 

incorporated in zooplankton fecal pellets (55%; Qureshi 1995), but also as individual cells or in 

aggregates. Both phytoplankton and zooplankton carbon flux were greater in the spring and the 

fall than in the summer. Redalje et al. (1994) similarly observed lowest fluxes during the 

summer. The temporal extent of the study of Qureshi (1995) provided a comparison of carbon 

fluxes between 1991 and 1992. The total carbon flux was much lower in 1992 than in 1991. 

One difference in the two years was the timing and discharge of the river. The flow in 1992 was 

much less than in 1991. Significant correlations existed behieen carbon flux in 199 1 with 

indicators of riverine discharge (low salinity, high chlorophyll a biomass), but were not 

significant in 1992. 

The high particulate organic carbon flux to the 15-m moored trap was sufficient to fuel hypoxia 

in the bottom waters below the seasonal pycnocline (Qureshi 1995, Justic' et al. 1996). The 

carbon fluxed via fecal pellets alone was usually sufficient to deplete the bottom water oxygen 

reserve in the spring, thus creating hypoxic conditions that then prevailed through the stratified 

summer period. The flux of organic material in summer, while it sustained hypoxia, was 

incremental to the majority flux of particulates in the spring (Qureshi 1995). Because the moored 

sediment traps were serviced by divers, they were not deployed from late fall through early 

spring when high fluxes might have occurred (based on subsequent analysis of phytoplankton 

biomass and composition during that season, Q. Dortch unpublished data). The time series of 

flux measurements is thus limited to the spring through early fall period and, presently, to the 

two years of data that have been analyzed for the four years of trap collections. 

Differences in these two studies can be attributed to methodology (free-floating versus moored), 

location of stations (several over a broad region versus a single location in the zone of hypoxia), 

depth of trap collections (within the photic zone and near the bottom), and temporal distribution 

of studies (selected periods within a four-year period and continuous collections for spring 

through fall but not winter for two years). It is clear that greater temporal and spatial coverage of 

production and flux experiments will be necessary before any conclusive relationship between 

rates of primary production and POC export can be derived. The amount of POC flux, however, 

especially in the spring, is clearly sufficient to lead to the depletion of oxygen below the seasonal 

pycnocline. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Respiration 

The oxygen consumption rates in near-bottom waters of the seasonally oxygen-deficient 

continental shelf were measured during several spring and summer cruises of multiple years 

(Turner et al. 1998, Turner and Rabalais 1998). Rates varied between 0.0008 to 0.29 mg Oz/l/hr, 

and were sufficient to reduce the in situ oxygen concentration to zero in less than four weeks. 

The rates were inversely related to depth and decreased westward of the Mississippi River delta, 

consistent with the decrease in nutrients, chlorophyll and pigment concentrations, and the 

relative proportion of surface-to-bottom pigments. The amount of phytoplankton biomass in the 

bottom waters across the Louisiana inner and middle continental shelf is high, often exceeding 

30 pg/l, and a high percentage is composed of phaeopigments (Rabalais and Turner 1998). 

Previous studies have shown that the respiration rate is proportional to phytoplankton pigment 

concentration (Turner and Allen 1982), and, thus, higher rates of oxygen consumption would be 

expected where higher flux of material reaches the lower water column and sediments. 

Respiration rates per unit phytoplankton pigment were highest in the spring, in shallower waters, 

and also closest to the Mississippi River delta. 

Composition of Sinking Material and Nutrient Availability 

On the Louisiana shelf in the area influenced by the Mississippi River, the prevalence of Si 

limitation, indicated by nutrient concentrations and ratios, suggests that diatoms may at times be 

Si limited, whereas non-diatoms are not (Q. Dortch unpublished data, Rabalais et al. 1999). 

Several lines of evidence support this contention. (1) The annual average abundance of diatoms 

is inversely proportional to the prevalence of Si limitation in the same year, except in 1992, 

which was unusual for other reasons. (2) Cell count bioassay experiments show that diatoms are 

limited by Si, N or Si + N availability, sometimes when other phytoplankton are limited by 

another factor. (3) A shortage of silicate does not just modify the relative abundance of diatoms 

and non-diatoms. It also influences the diatom species composition so that when Si is limiting, 

diatoms with low Si requirements, as manifested by light silicification of the hstule (Dortch et 

al. 1992b; Fahnenstiel et al. 1995) or lower k, for silicate uptake (Nelson and Dortch 1996), 

predominate. 

Despite the limited carbon flux data, there were some clear links between different sources of 

carbon flux and nutrient availability in the coastal zone (Dortch et al. 1992, in press, Fahnenstiel 

et al. 1995, Rabalais et al. 1999). Because of its effect on diatom abundance, silicate availability 

may also be a factor controlling phytoplankton and fecal pellet carbon flux. Diatoms comprised 

a much higher percentage of the total phytoplankton sinking into the moored sediment traps 

(Qureshi 1995) than they were in the surface water (Dortch unpublished data), suggesting 

selective sinking of diatoms. The other phytoplankton found in sediment traps were primarily 

picocyanobacteria that did not contribute substantially to the carbon flux because of their small 

size. Because the peak in cyanobacterial flux was greater in spring when diatom abundance was 

greatest than it was summer and fall when cyanobacterial abundances were highest, it has been 

proposed that the cyanobacteria sink as part of diatom aggregates (Dortch et al. 1997b, in press). 

The diatom species that did sink represent only part of the total diatom community, including 

heavily to moderately silicified species and excluding lightly silicified species that were present 

in the water column (Dortch et al. 1992, 1997b, in press, Fahnenstiel et al. 1995). Thus, Si must 
64 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

be available for high sinking flux of heavily to moderately silicified diatoms. There is an 

apparent contradiction in these results. The greatest sinking fluxes occur when diatom 

abundances are high, which is also when Si limitation is much more likely to occur. Other data 

suggest that Si limitation triggers sinking in many diatom species (e.g., Bienfang et al. 1982). 

Many zooplankton feed on diatoms, which are the most abundant food in the larger size range in 

this system. Fecal pellet production is often proportional to the availability of food (e.g., Corner 

et al. 1972, Butler and Dam 1994). In 1991 there was a close correspondence between fecal 

pellet flux and diatom abundance, although in 1992 there was not (Rabalais et al. 1999, Dortch et 

al. in press). With carbon flux data for only two years (Qureshi 1995), one year cannot be 

labeled as unusual, but other data suggest that 1992 may have been unusual. Because diatom 

abundance is dependent on silicate availability (Dortch unpublished data, Rabalais et al. 1999), 

this implies that Si availability may be a factor controlling fecal pellet production and flux. 

Turner et al. (1998) combined the sediment trap fecal pellet carbon data (Qureshi 1995), 

chlorophyll data, zooplankton data and seasonal and depth-related respiration rate experiments in 

an analysis of how these relationships varied around a Si:DIN ratio of 1 : 1 as delivered by the 

Mississippi River (lagged three months). They showed that 1) there is a strong vertical, rather 

than horizontal, coupling between oxygen consumption in bottom waters and organic loading 

fi-om surface waters, and 2) higher water-column respiration rates are driven by river-derived 

nutrients stimulating in situ organic production that sinks to the bottom layers. They also 

showed that respiration rates in bottom waters were responsive to zooplankton fecal pellet 

production (as predicted by Qureshi 1995) and to diatom production. The trophodynamics of 

carbon production, zooplankton, carbon flux and respiration are sensitive to the Si:DIN ratio in 

the riverine waters. 

Evidence for Long-Term Marine Ecosystem Changes 

Given the high volume of freshwater and associated nutrients delivered by the Mississippi River 

and the stratified coastal system, one might expect a propensity for the high productivity and 

development of hypoxia. Unfortunately, the long-term data sets that demonstrate changes in 

surface water production and bottom water dissolved oxygen, such as available for the northern 

Adriatic Sea and areas of the Baltic and northwestern European coast, do not exist for the 

northern Gulf of Mexico. Data fi-om oceanographic cruises prior to 1985 with which to test these 

hypotheses are limited. A few data sets fi-om the 1950s to present are suitable for comparisons of 

selected parameters. In addition, biological, mineral or chemical indicators of surface water 

production and hypoxia preserved in sediments that accumulate under the plume of the 

Mississippi River provide clues to prior hydrographic and biological conditions. 

Changes in Phytoplankton Community Composition 

The changes in riverine and coastal nutrient concentrations and ratios over time may be expected 

to be accompanied by observable changes in phytoplankton species composition. Published 

reports of phytoplankton species composition for 1955-1 957 near the delta (Simmons and 

Thomas 1962) and for 1972- 1973 approximately 80 krn west of the delta (Fucik 1974, Ward et 

al. 1979) were compared with recently obtained data (1 990- 1993) from near the delta and at 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 65 

stations C6A and C6B in 20 m water depth off Terrebonne Bay (Figure 4.6, Dortch et al. 

unpublished data compiled in Rabalais et al. 1996). This comparison was qualitative because of 

differences in locations, seasons sampled and methodology. The methodology used in earlier 

reports may have missed common non-diatoms, such as small coccoid cyanobacteria and 

phytoflagellates that now often dominate. Consequently, it was not possible to determine 

whether the dominance of these groups has increased. A conservative approach was taken in this 

comparative analysis. Data were compiled by matching season and location as closely as 

possible and including in the recent data only phytoplankton types that would have been 

observed in the earlier studies. In addition, because of uncertainties of taxonomy, differences 

were only noted where organisms were identified unambiguously. 

Demonstrable changes have occurred in phytoplankton species composition from the 1950s and 

1970s to present. Some heavily silicified diatom species are either not observed at all in recent 

samples or are much less dominant. For example, no Melosira species that appear heavily 

silicified were observed in 1990- 1993, but were present in both 195 5- 1957 and 1972- 1973. 

Asterionella japonica (=Asterionellopsis glacialis, Round et al. 1990) was observed recently at 

low salinities in the spring, but is not a dominant species as it was from 1955-1957. Similarly, 

more lightly silicified diatoms are documented for the 1970s and present, especially at higher 

salinities. RhizosoleniaJFagilissima and Ceratulina pelagica, that are so lightly silicified they 

are sometimes difficult to see, were not reported for 1955-1957. Two species of 

Leptocylindricus spp. were frequently dominant in 1990-1993, but were a minor constituent in 

1955- 1957. During 1972- 1973 lightly silicified diatoms were reported, including Rhizosolenia 

delicatula, Leptocylindricus danicus and Ceratulinapelagica, but a more quantitative analysis 

would be required to determine if their abundance was less than at present. The dominance of 

the diatom genus Pseudo-nitzschia, some of which produce the toxin, domoic acid (Parsons et al. 

1998, 1999), appears to have increased dramatically since the 1950s, and concentrations now 

frequently exceed 1 x 106 cells11 (Dortch et al. 1997a). Overall the data suggest that the shiR in 

dominant diatom composition, towards less silicified species occurred between 1955-1957 and 

the present. Methodological differences preclude conclusions about changes in non-diatoms in 

the past, but if Si limitation becomes more severe or persistent than it now is (see below) then a 

shift from diatoms to non-diatoms may occur. 

The fact that silicate-based production and accumulation of biogenic silica (i.e., diatom remains) 

in sediments have increased with increasing nitrogen loading from the Mississippi River (see 

below) indicates that the availability of Si has thus far been sufficient to support increases in 

diatom production. However, present nutrient concentrations and ratios indicate the potential for 

Si limitation on variable time and space scales. The effect of decreasing diatom silicification on 

C flux and hypoxia could be argued several different ways. If tests are less heavily silicified, 

diatoms, and any fecal pellets comprised of diatoms, should be less dense and sink more slowly 

to the bottom. Whether the effect on density makes a significant difference in transit time in 

shallow water, and thus, the possibility of water column regeneration or consumption, is not 

known. Alternatively, Si limitation of diatom growth has been shown to trigger sinking in some 

diatom species (Bienfang et al. 1982, Harrison et al. 1986). As a result, Si limitation, especially 

if it occurs during the spring, when diatoms bloom in response to increased river flow, could 

increase C flux to the bottom. There is obviously a critical need to understand the effect of Si 

limitation on diatom composition since it may be a factor in determining C flux. Likewise, the 
66 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

effects of other nutrient limitation on picophytoplankton and phytoflagellates is unknown. The 

impact of such changes in phytoplankton community composition for delivery of organic matter 

to sediments and associated rates of oxygen depletion is unknown, but may be significant and 

warrants further study. 

Increased Silicate-Based Productivity 

Although the marine ecosystem influenced by the Mississippi River discharge is now bordering 

on Si limitation, up to the present the overall silicate-based productivity of the ecosystem on the 

southeastern Louisiana shelf influenced by Mississippi River discharge appears to have increased 

in response to the increased nitrogen load. This is evidenced by 1) equal or greater diatom 

community uptake of silica in the mixing zone, compared to the 1950s (Turner and Rabalais 

1994b), and 2) greater accumulation rates of biogenic silica (BSi) in sediments beneath the 

plume (Turner and Rabalais 1994a). 

Bien (1958) first documented the non-conservative uptake of silicate in the vicinity of the 

Mississippi River plume by sampling from the river mouth seaward. Biological uptake can be 

statistically modeled as a deviation from the conservative mixing line, which Turner and 

Rabalais (1994b) did for 3 1 adequately sampled data sets in the Mississippi River delta bight. 

They found that the concentration of silicate at the 20 ppt mixing point declined in the last 

several decades during the winter-spring (Jan-Apr) and summer months (Jun-Aug); however, 

there was no discernible change during the fall-winter months (Oct-Dec). They normalized for 

the effects of varying concentrations in the riverine end-member (e.g., Loder and Reichard 198 1) 

and compared the estimated net silicate uptake at 30 ppt as a function of silicate riverine 

concentration (Figure 4.10). Non-conservative uptake of silicate was indicated in all data 

collections. The net uptake (at 30 ppt) above dilution ranged from 1 to 19% of the intercept 

concentration, and the data groups for before and after 1979 were remarkably similar. Further, 

the net silicate uptake appears to be even higher after 1979, than before 1979; these differences 

were apparent in the summer months during peak hypoxia development (June-August) as well. 

The results from this analysis suggest that net silicate uptake in the dilution gradient from river to 

sea has remained the same, or even increased, as the riverine concentration of silicate decreased. 

Turner and Rabalais (1994a) quantified the silica in the remains of diatoms sequestered as 

biologically bound silica (BSi) in dated sediment cores from the Mississippi River bight. 

Relative changes in the % BSi reflect changes in in situ production (Conley et al. 1993). The 

pattern in % BSi in dated sediment cores parallels the documented increases in nitrogen loading 

in the lower Mississippi River, over the same period that silicate concentrations have been 

decreasing (Turner and Rabalais 1994a) (Figure 4.10). The increased % BSi in Mississippi River 

bight sediments is direct evidence for the increase in flux of diatoms from surface to bottom 

waters beneath the Mississippi River plume. The highest concentrations of BSi were in 

sediments deposited in 25 to 50 m water depth in the middle of the sampling area. The % BSi in 

sediments from deeper waters (1 10 and 200 m) were generally stable through time, 

but rose in the shallower stations (10 and 20 m) around 1900 (not illustrated in Figure 4.10). At 

the intermediate depths (27 to 50 m), where both the % BSi concentration and accumulation rates 

were highest, coincidental changes in the % BSi with time were evident, especially in the 1955 to 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 67 

1965 period (a rise and fall) and a post 1975 (1980?) rise that was sustained to the sampling date 

(1 989) (Turner and Rabalais 1994a) (Figure 4.10). 

The increase in BSi in sediments fiom the mid 1850s to the early 1900s support the hypothesis of 

Mayer et al. (1 998) that organic nitrogen associated with the suspended sediment load may be a 

relatively large proportion of the total nitrogen load in river-dominated coastal regions. The BSi 

peaks and declines around the mid-1 800s and later around 1925 are causally related to the 

expansion of land clearing and land drainage efforts within the Mississippi River basin (Turner 

and Rabalais unpublished data). The trough in the 1930s to 1945 era is coincidental with the 

Great Depression and World War 11, and the accelerated accumulation since the 1950s is 

associated with the tripling in the dissolved inorganic nitrogen flux. Mayer et al. (1998) predicted 

that the relative importance of the organic nitrogen associated with the suspended load would 

become less important in the Mississippi River as the suspended load decreased and as the 

anthropogenic dissolved inorganic nitrogen load increased. 

0 

E30 

t I"' 1 

1925 1 950 1 975 2000 

X BSi L 

0 N Loading x 

P '4 

m R1 

CT 

2d 

CO 

0 

Figure 4.10. The average 

concentration of 

biologically bound silica 

(BSi) in sediments in each 

section of three dated 

sediment cores from 

stations in the Mississippi 

River Bight in depths of 27 

to 50 m (stations in Figure 

4.6). A 3-yr running 

average is plotted by time 

determined from Pb-2 1 0 

dating. The figure for 

station E50 is superimposed 

with a 3-yr average 

nitrogen loading fiom the 

Mississippi River 

(modified fiom Turner and 

Rabalais 1994a). 

The data in Turner and Rabalais (1994a) show continued accumulation of biologically bound Si 

in sediments beneath the plume and similar or higher net silicate uptake by diatoms in the plume 

surface waters, in spite of lower concentrations of Si delivered by the Mississippi River, support 

this hypothesis. Furthermore, others have found the record of BSi to be a good indicator of in 

situ production. Conley et al. (1 993) summarized for freshwater lakes that, in general, 
68 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

accumulation of BSi in sediments mimics overlying water column productivity, and that, the 

more diatoms that are produced by nutrient-enhanced growth, the more BSi will be deposited. 

Further Evidence of increased Primary Production 

The organic accumulation in the middle of the Mississippi River bight during the 1980s was 90 g 

~lm~l~r, based on sedimentation rates and % carbon of the sediments (Turner and Rabalais 

1994a). This is approximately 30% of the estimated annual phytoplankton production (Sklar and 

Turner 1981, Lohrenz et al. 1990). If the assumption is made that the BSi:C ratio at the time of 

deposition remained constant this century, then the increased BSi deposition represents a 

significant change in carbon deposition rates (up to 43% higher in cores dated after 1980 than 

those dated between 1900 to 1960). These results are corroborated by the same rate of increase in 

marine-origin carbon in sediment cores also collected within the Mississippi River bight at 

station 10 (Figure 4.1 1) (Eadie et al. 1994) (near station E30 of Turner and Rabalais 1994a, 

Figure 4.6). Eadie et al. (1994) estimated accumulation rates of about 30 g ~lm~lyr in the 1950s 

to 50-70 g ~lm~l~r at present. The rate of burial was significantly higher at a station within the 

area of chronic hypoxia (approximately 70 g clm2/yr), in comparison with another site at whch 

hypoxia had not been documented (approximately 50 g ~lrn~l~r). The 613c partitioning of 

organic carbon into terrestrial and marine fractions further indicated that the increase in 

accumulation of carbon in both cores was exclusively in the marine fraction (Figure 4.1 1). 

Organic Carbon Accumulation 

Organic Cnrbon Arru~nulauon 

Figure 4.1 1. Organic carbon 

accumulation rates (mg 

~lcm~l~r) at stations 10 

(near E30, Figure 4.6) and 2 

(near D80 LaSER). The 

partitioning of the organic 

matter into terrestrial (black) 

and marine ( ray) fractions 

f3 is based on 6 C. The recent 

increase in organic 

accumulation is much larger 

in the core within the region 

of hypoxia. Accumulation 

of terrestrial organic matter 

is also larger at station 10, 

implying it receives more 

river influence than station 

2 (from Eadie et al. 1994). 

The accumulation of carbon in the sediments at station 10 of Eadie et al. (1 994) was strongly 

correlated with Mississippi River nitrate flux. Sediment cores analyzed for different constituents 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 69 

(Turner and Rabalais 1994a, Eadie et al. 1994a) document eutrophication and increased organic 

sedimentation in bottom waters, with the changes being more apparent in areas of chronic 

hypoxia and coincident with the increasing nitrogen loads from the Mississippi River system. 

Other biological and chemical remnants in dated sediment cores from the Mississippi River bight 

were used to document the history of hypoxia which is consistent with increased diatom 

production, increased carbon accumulation, and increased flux of nitrogen (see Chapter 5). 

Consequences 

Nitrogen is the overall limiting nutrient to production in the area influenced by the Mississippi 

River and subject to hypoxia. It follows and is supported with evidence from long-term data sets 

and the sedimentary record that increases in riverine dissolved inorganic nitrogen loads are 

highly correlated with indicators of increased productivity in the overlying water column, i.e., 

eutrophication of the continental shelf waters. The possibility exists that further increases in 

anthropogenic nutrients in the Mississippi River could lead to higher and more widespread 

primary production, and this may intensify and extend the depletion of oxygen that has already 

been observed in the Louisiana shelf ecosystem. [These predictions are supported by a coupled 

physical-biological model of carbon and oxygen budgets for station C6*, Justic' et al. (1996, 

1997).] Such a prediction is complicated, because relationships between increasing nutrient 

inputs and primary production are unlikely to be linear. Other factors such as light and food web 

interactions will inevitably influence the outcome of changing nutrient inputs. Further, a 

complete understanding of processes intermediate between primary production of organic matter 

and oxygen depletion in bottom waters on the Louisiana shelf is still lacking. The limited time 

scale of NECOP process sldies cannot be used to infer decadal trends in productivity, but 

retrospective analysis of other data and surrogates of productivity in accumulated sediments do 

support such an argument. A comparison of data from Lohrenz et al. (1997) with historical 

measurements of primary production near the Mississippi delta (Thomas and Simmons 1960) in 

relation to corresponding river discharge levels and nutrient inputs supports arguments that 

recent rates of primary production have reached higher levels (Lohrenz et al. 1997). The fact 

that nutrient concentrations and fluxes have increased dramatically in the Mississippi River 

(Turner and Rabalais 199 1, Dinnel and Bratkovich 1993, Bratkovich et al. 1994), together with 

the observed coupling between nutrient inputs and productivity (Lohrenz et al. 1997, 1999, 

Justic' et al. 1997), support arguments of a long-term effect on primary production. 

Evidence of increased nutrient loads in rivers with consequences on receiving estuaries and 

adjacent continental shelves are similar elsewhere in the world (e.g. the Baltic Sea, VoP and 

Struck 1997, Larsson et al. 1985, Andersson and Rydberg 1987, Wulff and Rahm 1988; the 

northern Adriatic Sea, Faganeli et al. 1994, Justic' et al. 1987, Justic' 199 1; Chesapeake Bay, 

Officer et al. 1984, Cooper and Brush 1991, 1993, Cooper 1995, Harding et al. 1997; and many 

other areas in the world's coastal ocean, Diaz and Rosenberg 1995). It is not surprising, 

therefore, that the northern Gulf of Mexico has experienced eutrophication and ecosystem-level 

responses (including worsening hypoxia) due to increased nutrients, changing nutrient ratios and 

carbon flux given the magnitude of changes in the nutrient flux to the Gulf. 

Whether a reduction in the nitrogen load to the northern Gulf of Mexico (as suggested by 

management scenarios to reduce the severity and extent of bottom-water oxygen depletion) 
70 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

would reduce the overall secondary production in the Gulf is not known. A reduction in nitrogen 

flux will likely reduce the overall production of phytoplankton (correlation shown in Figure 4.8) 

but the effect on the pelagic food web is unknown. Data from 36 marine systems show a 

relationship between fisheries yield and primary production (Nixon 1988), but the slope is > 1, 

indicating that fisheries yield per unit productivity increases as productivity increases. Nixon's 

(1988) relationship spans almost two orders of magnitude of primary production and almost 

three orders of magnitude in fisheries yield; it is a log-log relationship with considerable 

variability but has a correlation coefficient of 0.84. On the other hand, Caddy (1993) illustrated 

how an increase in nutrient input results in a continuum of fisheries yield with an increase to a 

maximal point as nutrient load increases, then a decline in various compartments of the fishery as 

seasonal hypoxia and permanent anoxia become features of semi-enclosed seas with eutrophic 

and hypereutrophic conditions. If surface production and carbon flux heled more of a demersal 

and benthic secondary production instead of a pelagic fishery, then increasing the area of suitable 

habitat for those organism could potentially offset production that is currently limited by these 

factors. The ultimate consequences of nutrient overenrichment are certainly more complex than 

a simple increase in productivity or a decrease due to degraded habitat. Rather a combination of 

enhancement and degradation is likely affecting different components of the marine ecosystem 

and overall affecting trophic interactions and trophodynamics. 

References 

Andersson, L. and L. Rydberg. 1987. Trends in nutrient and oxygen conditions within the 

Kattegat: Effects of local nutrient supply. Estuarine, Coastal and Shelf Science 26: 559-579. 

Ammerman, J. W. 1992. Seasonal variation in phosphate turnover in the Mississippi River 

plume and the inner Gulf shelf: rapid summer turnover. Pages 69-75 in Proceedings, 

Nutrient Enhanced Coastal Ocean Productivity Workshop. Publ. No. TAMU-SG-92-109, 

Texas Sea Grant College Program, Texas A&M University, College Station, Texas. 

Bedo, A. W., J. L. Acuna, D. Robins and R. P. Harris. 1993. Grazing in the micron and submicron particle size range: the case of Oikopleura dioica (Appendicularia). Bulletin of 

Marine Science 53: 2-14. 

Bien, G. S., D. E. Contois and W. H. Thomas. 1958. The removal of soluble silica from fiesh 

water entering the sea. Geochimica et Cosmochimica Acta 14: 35-54. 

Bienfang, P. K., P. J. Harrison and L. M. Quarmby. 1982. Sinking rate response to depletion of 

nitrate, phosphate and silicate in four marine diatoms. Marine Biology 67: 295-302. 

Bode, A. and Q. Dortch. 1996. Uptake and regeneration of inorganic nitrogen in coastal waters 

influenced by the Mississippi River: spatial and seasonal variations. Journal of Plankton 

Research 18: 225 1-2268. 

Boynton, W. R, W. M. Kemp, W. M. and C. W. Keefe, C. W. 1982. A comparative analysis of 

nutrients and other factors influencing estuarine phytoplankton production. Pages 93-109 in V. 

S. .Kennedy (ed.), Estuarine Comparisons. Academic Press, New York. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 7 1 

Bratkovich, A., S. P. Dinnel, and D. A. Goolsby. 1994. Variability and prediction of fkeshwater 

and nitrate fluxes for the Louisiana-Texas shelf: Mississippi and Atchafalaya River source 

functions. Estuaries 17:766-778. 

Butler, M. and H. G. Dam. 1994. Production rates and characteristics of fecal pellets of the 

copepod Acartia tonsa under simulated phytoplankton bloom conditions: implications for 

vertical fluxes. Marine Ecology Progress Series 1 14: 8 1-9 1. 

Caddy, J. F. 1993. Toward a comparative evaluation of human impacts on fishery ecosystems of 

enclosed and semi-enclosed seas. Reviews in Fisheries Science 1: 57-95. 

Chen, B. 1994. The effects of growth rate, light, and nutrients on the C/Chl a ratio for 

phytoplankton in the Mississippi River plume. Masters Thesis. University of Southern 

Mississippi, Hattiesburg, Mississippi. 

Cloern, J. E., A. E. Alpine, B. E. Cole, R L. J. Wong, J. F. Arthur and M. D. Ball. 1983. River 

discharge controls phytoplankton dynamics in the northern San Francisco Bay estuary. 

Estuarine and Coastal Shelf Science 16: 415-429. 

Conley, D. J., C. L. Schelske and E. F. Stoermer. 1993. Modification of the biogeochemical 

cycle of silica with eutrophication. Marine Ecology Progress Series 10 1 : 179- 192. 

Cooper, S. R. 1995. Chesapeake Bay watershed historical land use: Impact on water quality and 

diatom communities. Ecological Applications 5 : 703-723. 

Cooper, S. R. and G. S. Brush. 199 1. Long-term history of Chesapeake Bay anoxia. Science 

254: 992-996. 

Cooper, S. R. and G. S. Brush. 1993. A 2500 year history of anoxia and eutrophication in 

Chesapeake Bay. Estuaries 16: 6 17-626. 

Corner, E. D., R. N. Head and C. C. Kilvington. 1972. On the nutrition and metabolism of 

zooplankton. 8. The grazing of Biddulphia cells by Calanus helgolandicus. Journal of the 

Marine Biological Association, United Kingdom 52: 847-86 1. 

Dagg, M. J. 1995. Copepod grazing and the fate of phytoplankton in the northern Gulf of 

Mexico. Continental Shelf Research 15 : 1303 - 13 17. 

Dagg, M. J. and T. E. Whitledge.1991. Concentrations of copepod nauplii associated with the 

nutrient-rich plume of the Mississippi River. Continental Shelf Research 11: 1409 - 1423. 

Dagg, M. J., E. P. Green, B. A. McKee and P.B. Ortner. 1996. Biological removal of finegrained lithogenic particles from a large river plume. Journal of Marine Systems 54: 149 - 

160. 
72 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

DeMaster, D. J. and R Pope. 1996. Nutrient dynamics in Amazon shelf waters: results fiom 

AMASSEDS. Continental Shelf Research 16: 263-289. 

DeMaster, D. J., G. B. Knapp and C. A. Nittrouer. 1986. Effect of suspended sediments on 

geochemical processes near the mouth of the Amazon River: examination of biogenic silica 

uptake and the fate of particle-reactive elements. Continental Shelf Research 6: 107-125. 

Diaz, R. J. and R. Rosenberg. 1995. Marine benthic hypoxia: A review of its ecological effects 

and the behavioural responses of benthic macrofauna. Oceanography and Marine Biology: 

an Annual Review 33: 245-303. 

Dinnel, S. P. and A. Bratkovich. 1993. Water discharge, nitrate concentration and nitrate flux in 

the lower Mississippi River. Journal of Marine Systems 4:3 15-326. 

Dortch, Q. 1994. Changes in phytoplankton numbers and species composition, pp. 46-49. Iin: 

M. J. Dowgiallo (ed.), Coastal Oceanographic Effects of Summer 1993 Mississippi River 

Flooding. Special National Oceanic and Atmospheric Administration Report, NOAA 

Coastal Ocean Office/National Weather Service, Silver Spring, Maryland. 

Dortch, Q. 1998. Phytoplankton characteristics, pp. 239-268. In: S. P. Murray (ed.), An 

Observational Study of the Mississippi-Atchafalaya Coastal Plume. Final Report. OCS 

Study MMS 98-0040. U.S. Dept. of Interior, Minerals Management Service, Gulf of Mexico 

OCS Region, New Orleans, Louisiana, 5 13 pp. 

Dortch, Q and T.E. Whitledge. 1992. Does nitrogen or silicon limit phytoplankton production in 

the Mississippi River plume and nearby regions? Continental Shelf Research 12: 1239-1309. 

Dortch, Q., D. Milsted, N. N. Rabalais, S. E. Lohrenz, D. G. Redalje, M. J. Dagg, R. E. Turner, 

and T. E. Whitledge. 1992. Role of silicate availability in phytoplankton species 

composition and the fate of carbon, pp. 76-83. In: Proceedings, Nutrient Enhanced Coastal 

Ocean Productivity Workshop. Publ. No. TAMU-SG-92-109, Texas Sea Grant College 

Program, Texas A&M University, College Station, Texas. 

Dortch, Q., R. Robichaux, S. Pool, D. Milsted, G. Mire, N. N. Rabalais, T. M. Soniat, G. A. 

Fryxell, R. E. Turner, and M. L. Parsons. 1997a. Abundance and vertical flux of Pseudonitzschia in the northern Gulf of Mexico. Marine Ecology Progress Series 146: 249-264. 

Dortch, Q., N. N. Rabalais, R. E. Turner, and D. M. Nelson. 1997b. Louisiana shelf: Silicon 

availability, phytoplankton species composition, vertical flux and hypoxia. Invited, ASLO 

annual meeting, 10- 14 February, 1997, Santa Fe, New Mexico (abstract). 

Dortch, Q., M. L. Parsons, N. N. Rabalais, and R. E. Turner. 1999. What is the threat of Harmful 

Algal Blooms in Louisiana coastal waters? Pages 1-1 1 in L. P. Rozas, J. A. Nyman, C. E. 

Profitt, N. N. Rabalais, D. J. Reed, and R. E. Turner (eds.), Symposium, Recent Research in 

Coastal Louisiana, February 3-5, 1998, Louisiana Sea Grant College Program, Baton Rouge, 

Louisiana. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 73 

Dortch, Q., N. N. Rabalais, R. E. Turner, and N. A. Qureshi. In press. Impacts of Changing 

Si/N Ratios and Phytoplankton Species Composition. In N. N. Rabalais and R. E. Turner 

(eds.), Effects of Hypoxia on Living Resources, with Emphasis on the Northern Gulf of 

Mexico. American Geophysical Union, Coastal and Estuarine Studies, Washington, D.C. 

Eadie, B. J., B. A. McKee, M. B. Lansing, J. A. Robbins, S. Metz, and J. H. Trefry. 1994. 

Records of nutrient-enhanced coastal productivity in sediments from the Louisiana 

continental shelf. Estuaries 17: 754-765. 

Fahnenstiel, G. L., M. J. McCormick, G. A. Lang, D. G. Redalje, S. E. Lohrenz, M. Markowitz, B. 

Wagoner, and H. J. Carrick. 1995. Taxon-specific growth and loss rates for dominant 

phytoplankton populations from the northern Gulf of Mexico. Marine Ecology Progress Series 

117: 229-239. 

Filardo, M. J. and W. N. Dunstan. 1985. Hydrodynamic control of phytoplankton in low salinity 

waters of the James River Estuary, Virginia, U.S.A. Estuarine and Coastal Shelf Science 21: 

653-667. 

Fisher, T. R., L. W. Harding, Jr., D. W. Stanley and L. G. Ward. 1988. Phytoplankton, nutrients, 

and turbidity in the Chesapeake, Delaware, and Hudson estuaries. Estuarine and Coastal Shelf 

Science 27: 61-93. 

Franks, P. J. S. 1992. Sink or swim: accumulation of biomass at fronts. Marine Ecology Progress 

Series 82: 1-12. 

Fucik, K. W. 1974. The effect of petroleum operations on the phytoplankton ecology of the 

Louisiana coastal waters. Masters Thesis, Texas A&M University, College Station, Texas, 

82 PP. 

Gardner, W. D., Y. Zhang, and M. J. Richardson. 1998. Sediment flux. Pages 179-198 in S. P. 

Murray (ed.), An Observational study of the Mississippi-Atchafalaya Coastal Plume. Final 

Report. OCS Study MMS 98-0040. U.S. Dept. of Interior, Minerals Management Service, 

Gulf of Mexico OCS Region, New Orleans, Louisiana, 5 13 pp. 

Govoni, J. J. 1997. The association of the population recruitment of Gulf menhaden, Brevoortia 

patronus, with Mississippi River discharge. Journal of Marine Systems 12: 101 - 108. 

Grimes, C. B. and J. H. Finucane. 1991. Spatial distribution and abundance of larval and 

juvenile fish, chlorophyll, and macrozooplankton around the Mississippi River discharge 

plume, and the role of the plume in fish recruitment. Marine Ecology Progress Series 75: 

109-1 19. 

Grimes, C. B. and M. J. ~in~sford. 1996. How do riverine plumes of different sizes influence 

fish larvae: do the enhance recruitment? Marine and Freshwater Research 47: 191 - 208. 
74 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Harding, Jr., L. W. and E. S. Perry. 1997. Long-term increase of phytoplankton biomass in 

Chesapeake Bay, 1950-1994. Marine Ecology Progress Series 157: 39-52. 

Harrison, P. J., D. H. Turpin, P. K. Bienfang and C. 0. Davis. 1986. Sinking as a factor 

affecting phytoplankton species succession: the use of selective loss semi-continuous 

cultures. Journal of Experimental Marine Biology and Ecology 99: 19-30. 

Hitchcock, G. L., W. J. Wiseman, Jr., W. C. Boicourt, A. J. Mariano, N. Walker, T. A. Nelsen and 

E. Ryan. 1997. Property fields in an effluent plume of the Mississippi River. Journal of. 

Marine Systems 12: 109-126. 

Justic', D. 1991. Hypoxic conditions in the northern Adriatic Sea: historical development and 

ecological significance, pp. 95-105. In: R. V. Tyson and T. H. Pearson, eds., Modern and 

Ancient Continental Shelf Anoxia, Geological Society Special Publication No. 58, The 

Geological Society, London, 470 pp. 

Justic', D., T. Legovic', and L. Rottini-Sandrini. 1987. Trends in oxygen content 191 1-1984 

and occurrence of benthic mortality in the northern Adriatic Sea. Estuarine and Coastal Shelf 

Science 24: 435-445. 

Justic', D., N. N. Rabalais, R E. Turner, R. E. and W. J. Wiseman, Jr. 1993. Seasonal coupling 

between riverborne nutrients, net productivity and hypoxia. Marine Pollution Bulletin 26: 184- 

189. 

Justic', D., N. N. Rabalais, R. E. Turner and Q. Dortch. 1995. Changes in nutrient structure of 

river-dominated coastal waters: Stoichiometric nutrient balance and its consequences. 

Estuarine, Coastal and Shelf Science 40: 339-356. 

Justic', D., N. N. Rabalais, and R. E. Turner. 1996. Effects of climate change on hypoxia in 

coastal waters: A doubled CO, scenario for the northern Gulf of Mexico. Limnology and 

Oceanography 4 1 : 992- 1003. 

Justic', D., N. N. Rabalais and R. E. Turner. 1997. Impacts of climate change on net 

productivity of coastal waters: Implications for carbon budget and hypoxia. Climate 

Research 8: 225-237. 

Kemp, W. M., R L. Wetzel, W. R. Boynton, C. F. D'Elia and J. C. Stevenson. 1982. Nitrogen 

cycling and estuarine interfaces: some current concepts and research directions. Pages 209-230 

in V. S. Kennedy (ed.), Estuarine Comparisons. Academic Press, New York. 

Larsson, U. R., R. Elmgren and F. Wulff. 1985. Eutrophication and the Baltic Sea: Causes and 

consequences. Ambio 14: 9- 14. 

Legendre, L. and J. Michaud. 1998. Flux of biogenic carbon in oceans: size-dependent 

regulation by pelagic food webs. Marine Ecology Progress Series 1 64: 1 - 1 1. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 75 

Loder, T. C. and R. P. Reichard. 198 1. The dynamics of conservative mixing in estuaries. 

Estuaries 4:64-69. 

Lohrenz, S. E., G. L. Fahnenstiel, and D. G. Redalje. 1994. Spatial and temporal variations in 

photosynthesis parameters in relation to environmental conditions in coastal waters of the 

northern Gulf of Mexico. Estuaries 17: 779-795. 

Lohrenz, S. E., G. L. Fahnenstiel, D. G. Redalje, G. A. Lang, X. Chen, and M. J. Dagg. 1997. 

Variations in primary production of northern Gulf of Mexico continental shelf waters linked 

to nutrient inputs from the Mississippi River. Marine Ecology Progress Series 155: 435-454. 

Lohrenz, S. E., G. L. Fahnenstiel, D. G. Redalje, G. A. Lang, M. J. Dagg, T. E. Whitledge, and 

Q. Dortch. 1999a. The interplay of nutrients, irradiance and mixing as factors regulating 

primary production in coastal waters impacted by the Mississippi River plume. Continental 

Shelf Research 19: 11 13-1 141. 

Lohrenz, S. E., D. A. Wiesenburg, R A. Arnone, and X. Chen. 1999'0. What controls primary 

production in the Gulf of Mexico? Pages 15 1 - 170 in K. Sherman, H. Kurnpf and K. Steidinger 

(eds.), The Gulf of Mexico Large Marine Ecosystem, Assessment, Sustainability, and 

Management. Blackwell Science, Malden, Massachusetts, 697 pp. 

Lucas, L. V., J. R. Koseff, S. G. Monismith, J. E. Cloern, and J. K. Thompson. 1999. Processes 

governing phytoplankton blooms in estuaries. 11: The role of horizontal transport. Marine 

Ecology Progress Series 187: 17-30. 

Mayer, L. M., R. G. Keil, S. A. Macko, S. B. Joye, K. C. Ruttenberg, and R. C. Aller. 1998. 

Importance of suspended particulates in riverine delivery of bioavailable nitrogen to coastal 

zones. Global Biogeochemical Cycles l2(4): 573-579. 

Minghui, H., Y. Yiping, X. Chunling, and P. J. Harrison. 1990. Phosphate limitation of 

phytoplankton growth in the Changjiang Estuary. Acta Oceanologica Sinica 9: 405-4 1 1. 

Murray, S. P. (ed.). 1998. An Observational Study of the Mississippi-Atchafalaya Coastal 

Plume. Final Report. OCS Study MMS 98-0040. U.S. Dept. of Interior, Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, Louisiana, 5 13 pp. 

Nelson, D. M. and Q. Dortch. 1996. Silicic acid depletion and silicon limitation in the plume of 

the Mississippi River: evidence fiom kinetic studies in spring and summer. Marine Ecology 

Progress Series 136: 163-178. 

Nixon, S. W. 1988. Physical energy inputs and comparative ecology of lake and marine 

ecosystems. Limnology and Oceanography 33(4, part 2): 1005-1025. 

Officer, C. B., R. B. Biggs, J. L. Taft, L. E. Cronin, M. A. Tyler, and W. R. Boynton. 1984. 

Chesapeake Bay anoxia. Origin, development and significance. Science 223: 22-27. 
76 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Parsons, M. L., Q. Dortch, and G. A. Fryxell. 1998. A multi-year study of the presence of 

potential domoic acid-producing Pseudo-nitzschia species in the coastal and estuarine waters 

of Louisiana, USA. Pages 184-1 87 in B. Reguera, J. Blanco, Ma L. Fernandez and T. Wyatt 

(eds.), Hannful Algae. Xunta de Galicia and Intergovernmental Oceanographic Commission 

of UNESCO, GRAFISANT, Santiago de Compostela, Spain. 

Parsons, M. L., C. Scholin, G. Doucette, G. Fryxell, Q. Dortch, and T. Soniat. 1999. Pseudonitzschia in Louisiana coastal waters: molecular probe field trials and domoic acid testing. 

Journal of Phycology 35:1368-1378. 

Pennock, J. R. and J. H. Sharp. 1986. Phytoplankton production in the Delaware Estuary: 

temporal and spatial variability. Marine Ecology Progress Series 34: 143-155. 

Pennock, J. R. and J. H. Sharp. 1994. Temporal alteration between light- and nutrient-limitation 

of phytoplankton production in a coastal plain estuary. Marine Ecology Progress Series 1 1 1 : 

275-288. 

Qureshi, N. A. 1995. The role of fecal pellets in the flux of carbon to the sea floor on a riverinfluenced continental shelf subject to hypoxia. Ph.D. Dissertation, Department of 

Oceanography & Coastal Sciences, Louisiana State University, Baton Rouge, 255 pp. 

Rabalais, N. N. and R. E. Turner. 1998. Pigment and nutrient distributions, pp. 208-230. In S. 

P. Murray (ed.), An Observational Study of the Mississippi-Atchafalaya Coastal Plume. Final 

Report. OCS Study MMS 98-0040. U.S. Dept. of Interior, Minerals Management Service, 

Gulf of Mexico OCS Region, New Orleans, Louisiana, 5 13 pp. 

Rabalais,'N.N., R. E. Turner, and Q. Dortch. 1992. Louisiana continental shelf sediments: 

Indicators of riverine influence, pp.77-8 1. In: Nutrient Enhanced Coastal Ocean 

Productivity. Publ. No. TAN-SG-92-109, Texas Sea Grant College Program, Galveston, 

Texas. 

Rabalais, N. N., R E. Turner, D. Justic', Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

1996. Nutrient changes in the Mississippi River and system responses on the adjacent 

continental shelf. Estuaries 1 9: 386-407. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman, Jr., and Q. Dortch, 1998. Consequences of the 

1993 Mississippi River flood in the Gulf of Mexico. Regulated Rivers: Research & 

Management 14: 161-177. 

Rabalais, N. N., R. E. Turner, D. Justic, Q. Dortch, and W. J. Wiseman, Jr. 1999. 

Characterization of Hypoxia: Topic 1 Report for the Integrated Assessment of Hypoxia in the 

Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 15. NOAA 

Coastal Ocean Program, Silver Spring, Maryland, 166 pp. 
Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 77 

Redalje, D. G., S. E. Lohrenz, and G. L. Fahnenstiel. 1994. The relationship between primary 

production and the vertical export of particulate organic matter in a river impacted coastal 

ecosystem. Estuaries 17: 829-83 8. 

Riley, G. A. 1937. The significance of the Mississippi River drainage for biological conditions in 

the northern Gulf of Mexico. Journal of Marine Research 1 : 60-74. 

Round, F. E., R. M. Crawford, and D. G. Mann. 1990. The Diatoms. Biology and Morphology 

of the Genera. Cambridge University Press, Cambridge, 747 pp. 

Ryther, J. H. and W. M. Dunstan. 1971. Nitrogen, phosphorus, and eutrophication in the coastal 

marine environment. Science 17 1 : 1008- 10 13. 

Sklar, F. H. and R E. Turner. 198 1. Characteristics of phytoplankton production off Barataria 

Bay in an area influenced by the Mississippi River. Contributions in Marine Science 24: 93- 

106. 

Smith, S. M. and G. L. Hitchcock. 1994. Nutrient enrichments and phytoplankton growth in the 

surface waters of the Louisiana Bight. Estuaries 17: 740-753. 

Smith, W. 0. and D. J. Demaster. 1996. Phytoplankton biomass and productivity in the Amazon 

River plume: correlation with seasonal river discharge. Continental Shelf Research 16: 291- 

3 19. 

Strom, S. L. and M. W. Strom. 1996. Microplankton growth, grazing and community structure 

in the northern Gulf of Mexico. Marine Ecology Progress Series 13: 229 - 240. 

Suess, E. 1980. Particulate organic carbon flux in the oceans: surface productivity and oxygen 

utilization. Nature 288: 260-263. 

Thomas, W. H. and E. G. Simmons. 1960. Phytoplankton production in the Mississippi River 

Delta, p. 103-1 16 in F. P. Shepard (ed.), Recent Sediments, Northwest Gulf of Mexico. 

American Association of Petroleum Geologists, Tulsa, Oklahoma. 

Turner, R. E. and R. L. Allen. 1982. Plankton respiration in the bottom waters of the 

Mississippi River Delta Bight. Contributions in Marine Science 25: 173-179. 

Turner, R E. and N. N. Rabalais. 1991. Changes in the Mississippi River quality in this century. 

BioScience 41 : 140-147. 

Turner, R E. and N. N. Rabalais. 1994a. Coastal eutrophication near the Mississippi River delta. 

Nature 368: 619-621. 
78 Nutrient-Enhanced Coastal Productivity and Ecosystem Responses 

Turner, R. E. and N. N. Rabalais. 1994b. Changes in the Mississippi River nutrient supply and 

offshore silicate-based phytoplankton community responses, pp. 147- 150. In: K. R. Dyer 

and R. J. Orth (eds.), Changes in Fluxes in Estuaries: Implications from Science to 

Management. Proceedings of ECSA22ERF Symposium, International Symposium Series, 

Olsen & Olsen, Fredensborg, Denmark. 

Turner, R. E. and N. N. Rabalais. 1998. Bottom water respiration rates in the hypoxia zone 

within the Louisiana Coastal Current, pp. 354-364. In: S. P. Murray (ed.), An Observational 

Study of the Mississippi-Atchafalaya Coastal Plume. Final Report. OCS Study MMS 98- 

0040. U.S. Dept. of Interior, Minerals Management Service, Gulf of Mexico OCS Region, 

New Orleans, Louisiana, 5 13 pp. 

Turner, R. E., N. N. Rabalais, and Z.-N. Zhang. 1990. Phytoplankton biomass, production and 

growth limitations on the Huanghe (Yellow River) continental shelf. Continental Shelf 

Research 10: 545-571. 

Turner, R. E., N. Qureshi, N. N. Rabalais, Q. Dortch, D. Justic', R. F. Shaw, and J. Cope. 1998. 

Fluctuating si1icate:nitrate ratios and coastal plankton food webs. Proceedings of the 

National Academy of Science, USA 95: 13048-1 305 1. 

Vop, M. and U. Struck. 1997. Stable nitrogen and carbon isotopes as indicator of eutrophication 

of the Oder river (Baltic Sea). Marine Chemistry 59: 35-49. 

Walsh, J. J., D. A. Dieterle, M. B. Meyers, and F. E. Muller-Karger. 1989. Nitrogen exchange at 

the continental margin: a numerical study of the Gulf of Mexico. Progress in Oceanography 

23: 245-301. 

Ward, C. H., M. E. Bender, and D. J. Reish (eds.). 1979. The Offshore Ecology Investigation. 

Effects of oil drilling and production in a coastal environment. Rice University Studies 65: 

1-589. 

Wulff, F. and L. Rahm. 1988. Long-term, seasonal and spatial variations of nitrogen, 

phosphorus silicate in the Baltic: An overview. Marine Environmental Research 26: 19-37. 

Xiuren, N., D. Vaulot, L. Zhensheng, and L. Zilin. 1988. Standing stock and production of 

phytoplankton in the estuary of the Changjiang (Yangtse) River and the adjacent East China 

Sea. Marine Ecology Progress Series 49: 141 - 150. 
Chapter 5 

by Nancy N. Rabalais, R. Eugene Turner, William J. Wiseman, Jr., 

Dubravko Justic: Barun K. Sen Gupta, and Terry A. Nelsen 

Introduction 

Oxygen is necessary to sustain the life of most higher organisms, including the fish and 

invertebrates that live in aquatic habitats. The normal condition is for surface water dissolved 

oxygen to be mixed or diffused down into the lower water column where oxygen has been 

consumed by organisms, particularly the micro-organisms. When the supply of oxygen to the 

bottom is cut off due to stratification or the consumption rate of oxygen during the 

decomposition of organic matter exceeds supply, oxygen concentrations become depleted. This 

condition of low oxygen is known as hypoxia. The operational definition for hypoxia in the 

northern Gulf of Mexico is 5 2 mgll, because trawlers do not capture any shrimp or demersal fish 

in their nets below that value (Pavela et al. 1983, Leming and Stuntz 1984, Renaud 1986). The 

oxygen concentration of surface waters is typically > 8 mgll if they are 100% saturated with 

oxygen at summertime temperature and salinity conditions. 

Hypoxia occurs in many parts of the world's aquatic environments. Hypoxic and anoxic (no 

oxygen) waters have existed through geologic time, but their occurrence in shallow coastal and 

estuarine areas appears to be increasing, most likely accelerated by human activities @iaz and 

Rosenberg 1995). Evidence associates oxygen depletion with changes in landscape use and ' 

nutrient management that result in nutrient enrichment of receiving waters. Increases in nutrient 

inputs to watersheds directly relate to population density in watersheds draining to coastal areas 

(Howarth et al. 1996, Caraco and Cole 1999) and population-driven increases in nutrient loads 

such as increases in fertilizer application (Turner and Rabalais 1991, Goolsby et al. 1999). 

The largest zone of oxygen-depleted coastal waters in the United States, indeed the entire 

western Atlantic Ocean, is in the northern Gulf of Mexico on the Louisiana continental shelf. 

The size of the Gulf of Mexico hypoxic zone was generally 16,000 km2 in mid-summer of 1993- 

1997 (Rabalais et al. 1998), covered 12,480 km2 in mid-summer of 1998 (Rabalais et al. 1999), 

and expanded to 20,000 km2 in 1999 (Rabalais et al. unpublished data). The Gulf zone ranks 

third in areal size behind the northwestern shelf of the Black Sea (20,000 km2, Tolmazin 1985) 

and the Baltic basins (84,000 km2, Rosenberg 1985). The hypoxic zone in the northern Gulf of 

Mexico (average for 1993-1999) is about the size of the state of New Jersey. Its total extent on 

the bottom is twice the total surface area of the whole Chesapeake Bay, and its volume is several 

orders of magnitude greater than the hypoxic water mass of Chesapeake Bay (Rabalais 1998a). 
80 H ypoxia 

The watershed that drains through the Mississippi and Atchafalaya Rivers is also immense. The 

Mississippi River system ranks among the world's top ten rivers in length, freshwater discharge 

and sediment delivery (Milliman and Meade 1983) and drains 41% of the lower forty-eight 

United States. Thus, the dimensions of the problem and the drainage system that affects it are of 

much greater proportion than most nutrient-driven eutrophication problems elsewhere. 

Historical Occurrence and Geographic Extent 

Coastal hypoxia was first reported in the northern Gulf of Mexico in the early 1970s off 

Barataria and TerrebonneITimbalier Bays as part of environmental assessments of oil production 

(Ward et al. 1979) and transportation studies (Hanifen et al. 1997). Following the initial 

discovery of hypoxia in 1972- 1974, Ragan et al. (1978) and Turner and Allen (1982a) surveyed 

the shelf in 1975 and 1976 and found low oxygen in the warmer months over most of the areas 

they studied in Louisiana. Environmental assessments and studies of oil and gas production 

revealed low oxygen conditions in most inner shelf areas of Louisiana and Texas studied in midsummer for the period 1978- 1984 (summarized in Rabalais 1992, Rabalais et al. 1999). 

Accounts of low oxygen from the Gulf of Mexico for the mid- 1930s (Conseil Permanent 

International pour 1'Expoloration de la Mer 1936) were records of the oxygen minimum layer, an 

oceanic feature at 400-700 m depth and not continental shelf hypoxia. There were no values of 

oxygen less than 3.0 mg/l in the data set, and the oceanic layer did not impinge on the location of 

the present-day area of continental shelf hypoxia (Rabalais et al. 1999). It was proposed by 

earlier researchers (cited in Turner and Allen 1982a) that intrusion of the oxygen minimum layer 

from deeper waters onto the continental shelf, especially in the area of the Mississippi Canyon, 

was the source of oxygen-depleted bottom water. Several authors have shown that there is no 

physical connection of the oxygen minimum zone with the hypoxia on the continental shelf and 

that the water masses are distinct with regard to chemistry and respiration rates (Pokryfki and 

Randall 1987, Rabalais et al. 1991, Turner et al. 1997). 

Hypoxia on the upper Texas coast is usually an extension of the larger hypoxic zone off 

Louisiana, although isolated areas may be found farther to the south (e.g., off Galveston and 

Freeport, Texas) (Harper et al. 199 1, Pokry&i and Randall 1987). Isolated areas may be an 

artifact of the sampling, and very few systematic surveys have been conducted in this area with 

the exception of the summer SEAMAP cruises (Gulf States Fisheries Commission 1982 et seq.). 

Mid-summer SEAMAP cruises documented hypoxia on the Texas coast in small, isolated areas 

in 1983, none in 1984- 1985, and again in localized areas in most years between 199 1 - 1997 (K. 

Craig, unpublished data). Most instances of hypoxia along the Texas coast are infrequent, shortlived, and limited in extent (Rabalais 1992). 

Hypoxia has been documented off Mississippi Sound during high stages of the Mississippi River 

and off Mobile Bay in bathymetric low areas (Rabalais 1992). There are usually more reports in 

flood years or when more Mississippi River water moves to the east of the birdfoot delta. This 

was the case in 1993 off Mobile Bay and Mississippi Sound (M. Van Hoose, personal 
Hypoxia 8 1 

communication, Waller 1998). Hypoxia was again documented in 1998 off Mississippi Sound 

under similar oceanographic conditions (R. Waller, personal communication). Hypoxia east of 

the Mississippi River is infrequent, short-lived, and limited in extent (Rabalais 1992). From 

limited data where both sides of the delta were surveyed for hydrographic conditions including 

dissolved oxygen (Turner and Allen 1982a), there was no evidence that the area of low oxygen 

formed a continuous band around the delta. 

Data Documenting Hypoxia on the Louisiana-Texas Continental Shelf 

Annual systematic shelfwide surveys of hypoxia on the Louisiana shelf began in 1985 (Rabalais 

et al. 1991) and became part of the NECOP studies in 1990. Surveys of the mid-summer extent 

of hypoxia over a standard station grid provide a broad-scale view for interannual comparisons. 

A single cruise of this nature per mid-summer, however, provides little information on the 

persistence of the zone over large areas, or the temporal sequence of physical and biological 

processes that preceded the cruise. 

Surveys along transect C on the southeastern shelf off Terrebonne Bay from nearshore to 30 m 

water depth on a monthly or twice monthly basis from 1985-1998 provide better temporal 

resolution over a more limited area of the shelf. Transect C data provide a time series suitable for 

examining monthly- and seasonal-scale differences over a representative area of the Louisiana 

shelf and relationships with variability in Mississippi River discharge and nutrient flux. Station 

C6* (includes data from C6A, C6B and C6) in the core of the hypoxic zone on transect C in 

approx. 20-m water depth is the site of an instrument mooring and more detailed data collection 

and experiments. The temporal resolution for selected parameters is the best for station C6* and 

can be extrapolated for a portion of the Louisiana shelf, but not the entire shelf. 

Dimensions and Variability of Hypoxia 

Mid-Summer Extent and Variability 

The distribution of hypoxia on the Louisiana shelf has been mapped in mid-summer (usually late 

July to early August) over a standard 60- to 80-station grid since 1985 (representative maps in 

Figure 5.1). Hypoxic waters are distributed from shallow depths near shore (4 to 5 m) to as deep 

as 60 m water depth (Rabalais et al. 1991, Rabalais et al. 1998), but more typically between 5 

and 30 m. 

For the period 1985 to 1992, the zone of hypoxia was usually in a configuration of disjunct areas 

to the west of the deltas of the Mississippi and Atchafalaya Rivers, and the bottom area averaged 

7,000 to 9,000 krn2 (1 986 and 1990 are illustrated in Figure 5.1, areas for all years are in Figure 

5.2). Hypoxia in mid-summer 1988 was confined to a single station off Terrebonne Bay. A 

reduced grid was mapped in 1989, and, therefore, not comparable. Bottom water hypoxia was 

continuous across the Louisiana shelf in mid-summer of 1993-1997, and the area (16,000 to 

18,000 kin2) was twice as large as the 1985-1992 average (1996 is illustrated in Figure 5.1). The 
8 2 Hypoxia 

somewhat smaller size of the hypoxic area in July 1997 was likely due to the passage of 

Hurricane Danny that either caused wind mixing and reaeration or forced the hypoxic water mass 

closer to shore. The 1998 hypoxia was concentrated on the eastern and central Louisiana shelf 

from the Mississippi River delta to Marsh Island near Atchafalaya Bay and in deeper water than 

usual. Weather patterns helped shape the distribution of the hypoxic water mass in July 1998. 

Persistent winds from the southwest and west pushed the fresher, nutrient-rich water eastward 

and held it on the central and eastern Louisiana shelf. The largest area of bottom-water hypoxia 

to date (20,000 km2) was mapped in July of 1999 (Rabalais, Turner and Wiseman unpublished 

data). Time or other logistical constraints often prevent the complete mapping of the extent of 

hypoxia, either in the offshore direction or to the west. Thus, the areal extent of bottom-water 

hypoxia generated from these surveys is a minimal estimate. The area estimations vary within a 

summer, and they should not be over-interpreted in making year-to-year comparisons or 

identifying trends. 

Figure 5.1. Distribution of bottom water less than 2 mgll dissolved oxygen in mid-summer of the 

years indicated (fiom Rabalais et al. 199 1, 1999). 
Hypoxia 83 

A compilation of fifteen mid-summer shelfwide surveys (1985-1999) (Figure 5.3) illustrates that 

the frequency of occurrence of hypoxia is higher to the west of the Mississippi and Atchafalaya 

Rivers in a down-current direction from the freshwater discharge and nutrient flux. Other 

gradients in biological parameters and processes are also evident in a decreasing gradient away 

from the river discharges (Rabalais et al. 1996). 

Figure 5.2. Estimated areal extent of bottom water hypoxia (52 mgll) for mid-summer cruises in 

1985-1999 (modified fiom Rabalais et al. 1998, 1999). 

Figure 5.3. Distribution of frequency of occurrence of mid-summer hypoxia over the 60- to 80- 

station grid from 1985-1999 (derived from Rabalais, Turner and Wiseman, various 

published and unpublished data). 

There were extensive areas of hypoxia during multiple July cruises in 1993 and 1994 (1993 is 

illustrated in Figure 5.4). Hypoxia was located further to the west earlier in the month in both 
8 4 Hypoxia 

years. Movement of the hypoxic area eastward along with the physical and biological processes 

that support its development and maintenance was confirmed from wind and ADCP data 

acquired during several of the cruises (Rabalais 1998b, Murray 1998, Wiseman et al. unpublished 

data). The multiple cruises demonstrate that the large area of hypoxia is persistent over two to 

three weeks, at least, although changing in configuration. 

I NECOP, Bratkovich et al., JUG 1 -12 

..... LATEX-B, P93-1, Rabalais, July 13-22 

28 0 , , I 

94 92 90 

I 

......,.. :... ....:::..id. 7t2s:: .... ... > c2.0 (rngll) dissolved oxygen 

NECOP, Rabalais et al., July 24-28 ...,. 

Figure 5.4. Comparison of bottom water oxygen less than 2 mgll for three cruises on the 

Louisiana-Texas shelf in July 1993 (modified fiom Rabalais 1998b). 
Hypoxia 

Temporal Variability 

Critically depressed dissolved oxygen concentrations occur below the pycnocline from as early as 

late February through early October and nearly continuously from mid-May through midSeptember. In March, April and May, hypoxia tends to be patchy and ephemeral; it is most 

widespread, persistent, and severe in June, July and August (Rabalais et al., 1991) (1992 and 

1993 transect C data are illustrated in Figure 5.5). The low oxygen water mass on the bottom 

during peak development in the summer changes configuration in response to winds, currents and 

tidal advection. The persistence of extensive and severe hypoxia into September and October 

depends primarily on the breakdown of the stratification structure by winds from either tropical 

storm activity or passage of cold fronts. Hypoxia is rare in the late fall and winter. 

Transect C 

'0 

- E 

.c g 20 

0 

300 I0 20 30 40 

Distance from shore (km) 

Transect C 

1993 n 

- .- -- Distance from shore (krn) 

Figure 5.5. Cross-shelf (transect C) distribution of dissolved oxygen less than 2 mg/l (stippled) 

and less than 1 mg/l (black) for 1992 and 1993 (from Rabalais et al. unpublished , 

data). 

Hypoxia occurs not only at the bottom near the sediments, but well up into the water column. 

Depending on the depth of the water and the location of the pycnocline(s), hypoxia may 

encompass from 10% to over 80% of the total water column, but normally 20 to 50%. In cases 

of the higher percentages, hypoxia may reach to within 2 m of the surface in a 10-m water 

column, or to within 6 m of the surface in a 20-m water column. Anoxic bottom waters can 

occur, along with the release of toxic hydrogen sulfide from the sediments. 

Continuously recording oxygen meters were deployed near the bottom at a 20-m station off 

Tenebonne Bay (example for 1993 in Figure 5.6). There is variability within the year and 

between years, but the patterns generally depict (1) gradual decline of bottom oxygen 

_ concentrations through the spring with reoxygenation from wind-mixing events, (2) persistent 
8 6 H ypoxia 

hypoxia and often anoxia for extended parts of the record in May-September, (3) isolated windmixing events in mid-summer that reaerate the water column followed by a decline in oxygen 

similar to that seen in the spring, (4) isolated upwelling of higher oxygen content waters from 

deeper water during upwelling favorable wind conditions, then a relaxation of the winds and a 

movement of the low oxygen water mass back across the bottom at the site of the oxygen meter, 

and (5) wind mixing events, either tropical storms or hurricanes or cold fronts in the late summer 

and fall that mix the water column sufficiently to prevent prolonged instances of dissolved 

oxygen concentrations less than 2 mg/l. 

Station C6B 1993 Bottom Oxygen (mg/L) 

Figure 5.6. Time-series plot of near-bottom dissolved oxygen concentration (mg/l in 1-h 

intervals) at station C6B for 1993 (Rabalais et al. unpublished data). 
Hypoxia 87 

Hypoxia in Flood and Drought Conditions 

Conditions during extreme events such as the 1993 flood or the 1988 drought can be compared to 

the long-term average conditions (Rabalais et al. 1991, 1998) to identify factors that are 

important in influencing the distribution of hypoxia. The influence of Mississippi River system 

discharge and flux of nutrients was magnified during the 1993 flood. Above normal freshwater 

inflow and nutrient flux fi-om the Mississippi and Atchafalaya Rivers from late spring into midsummer and early fall (Dowgiallo 1994) were clearly related in time and space to the seasonal 

progression of hypoxic water formation and maintenance and its increased severity and areal 

extent on the Louisiana-Texas shelf in 1993 (Rabalais et al. 1998). Flood conditions resulted in 

higher flux of nutrients to the Gulf of Mexico, higher concentrations of dissolved nutrients in 

surface waters, lower surface water salinity, higher surface water chlorophyll a biomass, 

increased phytoplankton abundance, modeled greater carbon export from the surface waters, 

increased bottom water phaeopigment concentrations (an indicator of fluxed degraded surface 

water chlorophyll a biomass), lower bottom water oxygen concentrations compared to the longtern averages for 1985-1992 (Figure 5.7) and a doubled size of the zone (Dortch 1994, Goolsby 

1994, Justic' et al. 1997, Rabalais et al. 1998). 

Bottom Dissolved Oxygen 

234567891011 

Month 

Bottom Dissolved Oxygen 

K J I H G F ED~C e AK 

west Transect east 

Figure 5.7. Bottom water dissolved oxygen for 1985-1992 average conditions C+ s.e.) compared 

with 1993. Left: station C6*; n for average condition ranges between 10 and 30 in 

March-October and between 2 and 6 in November-February. Right: 20-m depth 

stations on transects from the Mississippi River (transect A') to the Calcasieu 

estuary (transect K) for the long-term average conditions for 1985- 1992 (2 1 S.E.) 

compared with 1993; n for average condition ranges between 6 and 9. n for 1993 is 1 

or 2 (modified fkom Rabalais et al. 1998). 

A 52-yr low river flow of the Mississippi River occurred in 1988. Discharge began at normal 

levels in 1988 and dropped to some of the lowest levels on record during the summer months. In 

early June 1988, hydrographic conditions on the southeastern Louisiana shelf were similar to 

.those observed in previous years, i.e., a stratified water column and some areas of oxygen- 
88 Hypoxia 

deficient bottom waters (Rabalais et al. 1991). By mid-July, few areas of lower surface salinity 

were apparent, there was little density stratification, and low oxygen conditions were virtually 

absent. Reduced summer flows in 1988 also resulted in reduced suspended sediment loads and 

nutrient flux and subsequent increased water clarity across the continental shelf. The critical 

depth for photosynthesis was greater than the depth of the seabed, and there likely was 

photosynthetic production of oxygen in bottom waters (Rabalais et al. 1991). A typical seasonal 

sequence of nutrient-enhanced primary production and flux of organic matter progressed in the 

spring and led to the formation of hypoxia, but with lack of stratification and the addition of 

oxygen in bottom waters via photosynthesis, hypoxia was not maintained. 

The difference in hypoxia in a drought versus a flood year suggests a relationship between river 

discharge and the extent of mid-summer hypoxia. A simple linear regression of mid-summer area 

and mean Mississippi River discharge for the preceding year produced an R2 of 0.934 for nine 

years of data fiom 1985-1993 (Wiseman et al. 1997). This relationship, however, fails to hold 

for the additional years of 1994-1998. Thus, a comparison of mid-summer area (a minimal and 

rough estimate) versus discharge is not entirely satisfactory. There is evidence, for example, that 

carbon burial in 1993 was sufficient to support the extensive 1994 hypoxic zone despite 

"normal" flow conditions (Justic' et al. 1997). There is variability in the mid-summer extent of 

hypoxia due to physical conditions at the time of sampling, for example the reduced mid-summer 

size in 1997 and 1998 due to the passage of a hurricane on the southeastern portion of the study 

area and the current regime, respectively. Much stronger temporal relationships are evident for 

the development and maintenance of hypoxia along transect C versus river discharge and nutrient 

flux (Justic' et al. 1993). 

Linkage of Hypoxia with Freshwater and Nutrients from the Mississippi 

River 

The two principal factors leading to the development and maintenance of hypoxia-a stratified 

water column and decomposition of fluxed organic matter-are complexly inter-related and 

integrated and directly linked with the dynamics of the Mississippi and Atchafalaya River 

discharge. 

Stratification of the water column is a requisite for hypoxia development and maintenance. 

Measures of stratification (either surface-to-bottom difference in sigma t or 6 sigma tldepth) are 

correlated in time and space with the intensity of hypoxia (Rabalais et al. 199 1). This 

relationship does not always hold, and the depth of the main pycnocline does not always track 

the depth of the oxycline (Wiseman et al. 1997). The existence of a strong near-surface 

pycnocline is a necessary condition for the occurrence of hypoxia, while a weaker, seasonal 

pycnocline guides the morphology of the hypoxic domain (Wiseman et al. 1997). The 

stratification goes through a well-defined seasonal cycle that generally exhibits maximum 

stratification during summer and weakest stratification during winter. Short-term variability in 
H ypoxia 89 

stratification is due to winds and vertical mixing (Rabalais et al. 1992a, Wiseman et al. 1992, 

Wiseman et al. 1997). 

Although the Mississippi River discharges organic matter and decomposition of some of this 

organic matter could consume oxygen in the coastal ecosystem, the principal source of organic 

matter reaching the bottom waters of the northern Gulf of Mexico influenced by the Mississippi 

River effluent and characterized by hypoxia is from in situ phytoplankton production (Turner 

and Allen 1982b, Rabalais et al. 1992b, Turner and Rabalais 1994, Eadie et al. 1994, Justic' et al. 

1996, 1997). These results are verified by data from studies of the sedimentary environment, 

retrospective sediment analyses, empirical relationships of nutrient flux to primary production 

and net production, carbon flux studies, linkages of surface water net production with oxygen 

uptake in the lower water column, and modeling experiments. The amount of organic loading in 

the Mississippi River is not large enough to account for the observed decline in oxygen over such 

a large area and volume. Also, the 613c signature of the particulate organic material found in the 

sediments of the Mississippi River bight indicates that terrestrial carbon sources are localized 

near the delta and nearshore sediments and the bulk of carbon in the sediments of the hypoxic 

zone is marine in origin (Eadie et al. 1994, Turner and Rabalais 1994). 

The production of carbon via photosynthesis in the surface waters eventually contributes to the 

flux of carbon to the bottom and decline of oxygen through aerobic respiration in the water 

column and in the sediments. The amount of phytoplankton biomass in the bottom waters 

across the Louisiana inner and middle continental shelf is high, often exceeding 30 pgll, and a high 

percentage is composed of phaeopigments (Rabalais and Turner 1998). The respiration rate is 

proportional to phytoplankton pigment concentration (Turner and Allen 1982b), and, thus, more 

oxygen is taken up where higher flux of materials reaches the lower water column and sediments. 

The measured particulate organic carbon flux in sediment traps moored at station C6* (at 16 m in 

a total 20-m water column) was sufficient to fuel hypoxia in the bottom waters below the 

seasonal pycnocline (Qureshi 1995, Justic' et al. 1996). The carbon fluxed was usually sufficient 

to deplete the bottom water oxygen reserve in the spring, thus creating hypoxic conditions that 

then prevailed through the stratified summer period. The flux of organic material in summer, 

while it sustained hypoxia, was incremental to the majority flux of particulates in the spring 

(Qureshi, 1995). 

The oxygen consumption rates in near-bottom waters of the seasonally oxygen-deficient 

continental shelf were measured during several spring and summer cruises of multiple years 

(Turner et al. 1998, Turner and Rabalais 1998). Rates varied between 0.0008 to 0.29 mg 0211kr, 

and were sufficient to reduce the in situ oxygen concentration to zero in less than four weeks. 

The rates were inversely related to depth and decreased westward of the Mississippi River delta, 

consistent with the decrease in nutrients, chlorophyll and pigment concentrations, and the ratio 

of surface-to-bottom pigments. Respiration rates per unit phytoplankton pigment were highest 

in the spring, in shallower waters, and also closest to the Mississippi River delta. Turner et al. 

_ .(1998) combined the sediment trap fecal pellet carbon data, chlorophyll data, and seasonal and 
90 H ypoxia 

depth-related respiration rate experiments in an analysis of how these relationships varied around 

a Si:DIN ratio of 1 : 1 as delivered by the Mississippi River (lagged three months). They showed 

that (1) there was a strong vertical, rather than horizontal, coupling between oxygen consumption 

in bottom waters and organic loading from surface waters, and (2) higher water-column 

respiration rates were driven by river-derived nutrients stimulating in situ organic production that 

sinks to the bottom layers. They also showed that respiration rates in bottom waters were 

responsive to zooplankton fecal pellet production (as predicted by Qureshi 1995) and to diatom 

production. The trophodynamics of carbon production, zooplankton, carbon flux and respiration 

were sensitive to the Si:DIN ratio in the riverine waters. 

There is a time lag between nutrient delivery and production in the surface waters, and a 

subsequent lag in flux of carbon to the lower water column and oxygen uptake in the lower water 

column and sediments. The spatial and temporal variability in the distribution of hypoxia is, at 

least partially, related to the amplitude and phasing of the Mississippi River discharge and 

nutrient fluxes (Pokry£ki and Randall 1987, Justic' et al. 1993, 1996, 1997, Rabalais et al. 1996, 

Wiseman et al. 1997). Net productivity (a surrogate for excess carbon available for export) of the 

upper water column appears to be an important factor controlling the accumulation of organic 

matter in coastal sediments and development of hypoxia in the lower water column. Seasonal 

dynamics of net productivity in the northern Gulf of Mexico are coherent with the dynamics of 

freshwater discharge (Justic' et al. 1993). The surface layer (0 to 0.5 m at station C6*) shows an 

oxygen surplus relative to the saturation values during February-July; the maximum occurs during 

April and May and coincides with the maximum flow of the Mississippi River (Figure 5.8). The 

bottom layer (approximately 20 m), on the contrary, exhibits an oxygen deficit throughout the 

year, but reaches its highest value in July. Bottom hypoxia in the northern Gulf is most 

pronounced during periods of high water column stability when surface-to-bottom density 

differences are greatest (Rabalais et al. 1991). The correlation between Mississippi River flow 

and surface oxygen surplus peaks at a time-lag of one month, and the highest correlation for 

bottom oxygen deficit is for the time-lag of two months (Justic' et al. 1993) (Figure 5.8). A 

similar cross-correlation analysis verified that the seasonal maximum in net production lags 

riverine nitrate flux by one month (Justic' et al. 1997). These findings suggest that the oxygen 

surplus in the surface layer following high flow depends on nutrients ultimately coming from the 

river but regenerated many times. An oxygen surplus also means that there is an excess of 

organic matter derived from primary production that can be redistributed within the system; 

much of this will eventually reach the sediments. 

Similar relationships with freshwater discharge and oxygen depletion in bottom waters at stations 

in a location west of the Atchafalaya River delta and expected direction of materials and 

freshwater flux were identified by Pokryfki and Randall (1987). Time lags were apparent 

between values of river discharge, bottom dissolved oxygen and salinity. The highest crosscorrelation coefficient between bottom water dissolved oxygen (in the area off the Calcasieu 

estuary) and river discharge (from the Atchafalaya) was -0.5 1 at a lag of two months. These 

results are similar to those of Justic et al. (1993) for station C6* on the southeastern shelf. The 

linear regression model developed by Pokryfki and Randall (1 987) did not include any factors for 
H ypoxia 9 1 

biological processes, and the accuracy would have been improved by "incorporating a biological 

component into the time series." 

A Figure 5.8. Left panel: Seasonal changes in 

z25000 . , . . , . , . , , , m 

E 

l2 river flow, stability, surface oxygen surplus 

- . A and bottom oxygen deficit for station C6*. 

- 10 , The solid lines represent the monthly means 

for 1985-199 1. Surface oxygen surplus and 

. 5 bottom oxygen deficit are 02-02' and -(02- 

027, respectively, where the 02 was the - measured oxygen concentration and 02' was 

- 

Q 

n. 4 the oxygen concentration at 100% saturation. 

.- 10000 - V) t; Right panel: Cross-correlation analysis of the 

data on river flow versus surface oxygen 

.- 

=5000 "'L'""'~' surplus and bottom oxygen deficit. (Modified 

0 

0 2 4 6 8 10 12 from Justic' et al. 1993.) 

-1 1,111.1.1.1.11 

0 2 4 6 8 10 12 

Month 

I rn Surface 4 

6-5-4-3-2-10 1 2 3 4 5 6 

Time-lag (months) 
9 2 Hypoxia 

Historical Changes in Oxygen Stress 

The northern Gulf of Mexico adjacent to the discharge of the Mississippi River is an example of 

a coastal ocean that has undergone eutrophication (Turner and Rabalais 1994, Rabalais et al. 

1 996, Chapter 4 this volume) as a result of increasing nutrients (Turner and Rabalais 199 1, 

Goolsby et al. 1999, Chapter 3 this volume). Given the high volume of fieshwater delivered by 

the Mississippi River, the associated nutrients and the stratified coastal system, one might 

expect a propensity for hypoxia to develop naturally. In other words, has hypoxia always been 

a feature of the northern Gulf of Mexico adjacent to the discharge of the Mississippi River? 

Long-term data sets that demonstrate changes in surface water productivity and bottom water 

dissolved oxygen, such as available for the northern Adriatic Sea and areas of the Baltic and 

northwestern European coast, unfortunately do not exist for the northern Gulf of Mexico. 

Sediments accumulating under the plume of the Mississippi River, however, provide the 

historical information to document long-term ecosystem changes. Surficial sediments, directly 

downstream and beneath the surface riverine-Gulf dilution plume within the Mississippi River 

bight reflect the in situ primary production and subsequent transport of organic carbon fiom 

surface to bottom waters (Rabalais et al. 1992b, Turner and Rabalais 1994, see Chapter 4). 

These sedimentary habitats also represent a range of conditions relative to oxygen stress, from 

low to high frequency of occurrence in mid-summer surveys (Sen Gupta et al. 1996, Rabalais et 

al. 1996) (Figure 5.9). Biological and chemical remnants in dated sediment cores fiom the 

Mississippi River bight were used to document the history of hypoxia. 

The surrogates for oxygen conditions indicate an overall increase in oxygen stress (in intensity or 

duration) in the last 100 years, which seems especially severe since the 1950s and coincident 

with the onset of increases in riverine nitrogen loading. Dominance trends of benthic foraminifera 

and ostracods (single-celled organisms and metazoans, respectively, that produce a calcium 

carbonate shell that remains intact in buried sediments) were used as indicators of reduced oxygen 

levels andlor carbon-enriched sediments (Nelsen et al. 1994, Blackwelder et al. 1996, Sen Gupta 

et al. 1996, Rabalais et al. 1996, Nelsen et al. unpublished data). Glauconite, a sediment mineral 

indicative of reducing environments and geologic anoxic settings, was also used to document 

changes in hypoxia (Nelsen et al. 1994). Sen Gupta et al. (1996) and Rabalais et al. (1996) 

examined several cores in the Mississippi River bight (Figure 5.9). Nelsen et al. (1994) and 

Blackwelder et al. (1996) analyzed a core fiom station 10 near the E30 station of Sen Gupta et al. 

(1996) in the core of the hypoxic zone. Both teams also examined cores fiom outside the present 

day distribution of hypoxia. 

The average glauconite abundance is -5.8% of the coarse fraction of sediments fiom 1900 (oldest 

date in station 10 core) to a transition point in the early 1940s and is -13.4% afterwards (Figure 

5.10) (Nelsen et al. 1994). These data suggest that hypoxia may have existed at some level before 

the 1940 time horizon (at least to 1900) and that subsequent anthropogenic influences have 

exacerbated the problem. 
Hypoxia 

Figure 5.9. Station locations within the Mississippi River bight for hypoxia monitoring on 

transect C (closed circles), mooring locations (C6A and C6B) (closed squares), coring 

stations (closed triangles, those referred to in text are labeled). Stippling corresponds 

to frequency of occurrence of mid-summer hypoxia at monitoring stations (1985- 

1987, 1990-1993, Rabalais, Turner and Wiseman unpublished data). Station 10 of 

Eadie et al. (1994) and Nelsen et al. (1994) is close to E30 (modified from Rabalais et 

al. 1996). 

Benthic foraminifera are useful indicators of reduced oxygen levels or carbon-enriched sediments 

or both (Sen Gupta et al. 1981, Sen Gupta and Machain-Castillo 1993). Benthic foraminifera1 

density and diversity were generally low in the Mississippi River bight, but a comparison of 

assemblages in surficial sediments from areas differentially affected by oxygen depletion indicated 

that the dominance of Ammoniaparkinsoniana over Elphidium spp. (A-E index) was much more 

pronounced under hypoxia than in well-oxygenated waters. The relative abundance of A. 

parkinsoniana was correlated with % BSi (i.e., a food source indicator) in sediments (Sen Gupta 

et al. 1996). The A-E index also correlated strongly with the percentage of total organic carbon in 

surficial sediments. Thus, the index is affected by seasonal hypoxia produced by phytoplankton 

blooms that are recorded in the sediments in BSi and carbon content. In the context of modern 

hypoxia, species distribution in dated sediment cores revealed stratigraphic trends in the 

Ammonia/EZphidium ratio that indicate an overall increase in oxygen stress (in intensity or 

duration) in the last 100 years (Figure 5.1 1). In particular, the stress seems especially severe 

since the 1950s. It is notable that there is no trend in the A-E index for station G50 outside the 
94 H ypoxia 

zone of persistent hypoxia and that the index in 1988 for station C10 fell off the trend line (i.e., 

no low oxygen during the mid-summer 1988 cruise, see Chapter 3). In the last 100 years, both 

Ammonia and Elphidium become less important components of the assemblage, while Buliminella 

morgani shows an unusual dominance (also see Blackwelder et al. 1996). B. morgani, a hypoxiatolerant species, is known only fiom the Gulf of Mexico and dominates the population (> 50%) 

within the area of chronic seasonal hypoxia (Blackwelder et al. 1996). It increased markedly 

upcore in the sediments analyzed by Blackwelder et al. (1996) and for station G27 of the Sen 

Gupta et al. (1996) study. Quinqueloculina (a significant component of the modem assemblage 

only in well-oxygenated waters) has been absent fiom the record of the G27 core since the early 

1900s, but was a conspicuous element of the fauna in the previous 200 years. The historical 

absence of Quinqueloculina since 1900 at station G27 matches the presence of glauconite at 

station 10 since 1900. The occurrence of Quinqueloculina prior to 1870, however, indicates that 

oxygen stress was not a problem prior to then. 

Glauconite (% of Coarse) Ostracods (SWDI) (1 O6 Tons) 

Figure 5.10. 

C 

Transition Zone 

Glauconite grain abundance (A) and the Shannon-Wiener Diversity Index of 

ostracods (B) for station 10 compared to the application of fertilizer in the United 

States (C) for the years -1900-1990 (modified from Nelsen et al. 1994, Nelsen et al. 

unpublished data). A similar decline in the diversity index for benthic 

foraminiferans was reported for selected sections of core 10 in Nelsen et al. (1 994). 
H ypoxia 

60j 1 Bulirninella 

50 16-1 I G27 / Quinqueloculina 

Figure 5.1 1. Changes in benthic foraminiferans with stratigraphic depth in 210~b-dated sediment 

cores from the Mississippi River bight. A line connecting 3-yr averages is 

superimposed on the data for C10; the 1988 outlier reflects the absence of summer 

hypoxia. Foraminiferans that indicate changes in oxygen stress (Bulirninella 

morgani and Quinqueloculina sp.) are shown for G27. Note: the time scale is 

variable among plots (modified from Rabalais et al. 1996, Sen Gupta et al. 1996). 

Effects of Hypoxia on Living Resources 

Surface water phytoplankton production is stimulated by increased nutrient loads. This 

enhanced productivity further enhances pelagic and demersal populations that depend on either 

the living cells or the detrital material that sinks to the seabed, respectively. When the rate of 

organic supply and subsequent depletion of oxygen affects the ability of organisms to reside 

either at the bottom or within the water column or even their survivability, disruptions occur in 
96 Hypoxia 

benthic, demersal and pelagic communities. Hypoxia affects living resources, biological diversity 

and the capacity of aquatic systems to support biological populations. When oxygen levels fall - 

below critical values, those organisms capable of swimming (e.g., demersal fish, portunid crabs 

and shrimp) evacuate the area. The stress on less motile fauna varies, but they also experience 

stress or die as oxygen concentrations fall to zero. Important fishery resources are variably 

affected by direct mortality, forced migration, reduction in suitable habitat, increased 

susceptibility to predation, changes in food resources and disruption of life cycles. 

Hypoxia may affect the abundance, distribution and composition of zooplankton communities. 

The dominant zooplankton in the northern Gulf of Mexico, copepods, were in low abundance or 

absent from oxygen depleted waters < 1 mg/l (Qureshi 1995) similar to results from the 

Chesapeake Bay (Roman et al. 1993). Otherwise, copepod abundances were highest during the 

day in aerated bottom waters during spring and summer. There are several areas of the world 

where excessive nutrients and phytoplankton production have led to a shift in zooplankton 

communities from copepod-based to gelatinous-zooplankton based (i.e. jelly fish and 

ctenophores) (Zaitsev 1993) with devastating effects on fisheries because of increased predation 

by the gelatinous zooplankton on fish larvae and other zooplankton. Despite the fact that some 

effects of hypoxia have been observed on Gulf of Mexico zooplankton, the overall numbers and 

community composition appear to be similar to or higher than other coastal areas. 

The responses of benthic and demersal communities to oxygen stress vary (Rabalais and Harper 

1992, Rabalais et al. 1995, Rabalais and Harper unpublished data). A fairly predictable pattern in 

responses of components of the community follows a decrease in oxygen concentrations fi-om 2 

mg/l to anoxia. Motile fish and crustaceans (e.g., crabs, shrimp and mantis shrimp) are generally 

absent from bottom habitats when the oxygen falls below 1.5-2 mg/l. Distribution of shrimp and 

localization of shrimp catch are related to the extent of hypoxia, and shrimp migration is 

disrupted (Zimmerman et al. 1997). Less motile invertebrates die at oxygen levels below 1.5 

mg/l. The organisms that live in the sediments display stress behavior below 1.0 mgll. In the 

community that typically lives in the sediments, the smaller worms, snails, bivalves and 

crustaceans, there is a fairly linear decrease in benthic diversity and abundance as oxygen 

concentrations fall from 0.5 mg/1 to anoxia. The long-term secondary productivity of the 

benthos under severe, seasonal hypoxia is not known. When oxygen levels return to normal in 

the fall and winter, the amount of food resources available to the fish and shrimp migrating back 

to the area are diminished substantially. The disruption of the benthic community further 

disrupts the natural processes of those communities in processing organic matter, cycling 

nutrients and pollutants, and in supporting higher levels of the food chain. 

Comparisons of ecosystems along a gradient of increasing nutrient enrichment and eutrophication 

or changes of a specific ecosystem over time through a gradient towards increasing 

eutrophication, provide information on how nutrient enrichment affects coastal communities. 

Work by Caddy (1 993) in semi-enclosed seas demonstrates a continuum of fishery yield in 

response to increasing eutrophication. In waters with low nutrients, the fishery yield is low. As 

the quantity of nutrients increases, the fishery yield increases. As the ecosystem becomes 
Hypoxia 97 

increasingly eutrophied, there is a drop in fishery yield but the decreases are variable. The 

benthos are the first resources to be reduced by increasing frequency of seasonal hypoxia and - 

eventually anoxia; bottom-feeding fishes then decline. Loss of a planktivorous fishery follows as 

eutrophication increases, with eventually a change in the zooplankton community composition. 

Where the current Gulf of Mexico fisheries lie along the continuum of increasing eutrophication is 

not known. 

References 

Blackwelder, P., T. Hood, C. Alvarez-Zarikian, T. A. Nelsen and B. McKee. 1996. Benthic 

foraminifera from the NECOP study area impacted by the Mississippi River plume and 

seasonal hypoxia. Quaternary International 3 1 : 19-36. 

Caddy, J. F. 1993. Toward a comparative evaluation of human impacts on fishery ecosystems of 

enclosed and semi-enclosed seas. Reviews in Fisheries Science 1 : 57-95. 

Caraco, N. F. and J. J. Cole. 1999. Human impact on nitrate export: An analysis using major 

world rivers. Ambio 28: 167-1 70. 

Conseil Permanent International pour 1'Exporation de la Mer. 1936. Bulletin Hdydrographique 

pour 1'Anne'e 193 5, Series B 1. Le Bureau du Conseil, Service Hydrographique, 

Charlottelund Slot, Danemark, 105 pp. 

Diaz, R. J. and R. Rosenberg. 1995. Marine benthic hypoxia: A review of its ecological effects 

and the behavioural responses of benthic macrofauna. Oceanography and Marine Biology: an 

Annual Review 33: 245-303. 

Dortch, Q. 1994. Changes in phytoplankton numbers and species composition. Pages 46-49 in 

M. J. Dowgiallo (ed.), Coastal Oceanographic Effects of Summer 1993 Mississippi River 

Flooding. Special National Oceanic and Atmospheric Administration Report, NOAA Coastal 

Ocean Officernational Weather Service, Silver Spring, Maryland. 

Dowgiallo, M. J. (ed.). 1994. Coastal Oceanographic Effects of Summer 1993 Mississippi River 

Flooding. Special NOAA Report, NOAA Coastal Ocean OfPice/National Weather Service, 

Silver Spring, Maryland. 

Eadie, B. J., B. A. McKee, M. B. Lansing, J. A. Robbins, S. Metz and J. H. Trefry. 1994. 

Records of nutrient-enhanced coastal productivity in sediments from the Louisiana 

continental shelf. Estuaries 17: 754-765. 
98 Hypoxia 

Goolsby, D. A. 1994. Flux of herbicides and nitrate &om the Mississippi River to the Gulf of 

Mexico. Pages 32-35 in M. J. Dowgiallo (ed.), Coastal Oceanographic Effects of Summer - 

1993 Mississippi River Flooding. Special National Oceanic and Atmospheric Administration 

Report, NOAA Coastal Ocean Office/National Weather Service, Silver Spring, Maryland. 

Goolsby, D. A., W. A. Battaglin, G. B. Lawrence, R. S. Artz, B. T. Aulenbach and R. P. Hooper. 

1999. Flux and sources of nutrients in the Mississippi-Atchafalaya Basin: Topic 3 for the 

Integrated Assessment of Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program 

Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD, in 

press. - 

Gulf States Marine Fisheries Commission. 1982 et seq. SEAMAP Environmental and Biological 

Atlas of the Gulf of Mexico, 1982. Gulf States Marine Fisheries Commission, Ocean 

Springs, Mississippi. 

Hanifen, J. G., W. S. Perret, R. P. Allemand and T. L. Romaire. 1997. Potential impacts of 

hypoxia on fisheries: Louisiana's fishery-independent data. Pages 87-94 in Proceedings of 

the First Gulf of Mexico Hypoxia Management Conference, December 1995, New Orleans, 

Louisiana. Publ. No. EPA-5 5-R-97-00 1, U.S. Environmental Protection Agency, Gulf of 

Mexico Program Office, Stennis Space Center, Mississippi, 198 pp. 

Harper, Jr., D. E., L. D. McKinney, J. M. Nance, and R. R. Salzer. 1991. Recovery responses 

of two benthic assemblages following an acute hypoxic event on the Texas continental shelf, 

northwestern Gulf of Mexico pp. 49-64. In: R. V. Tyson and T. H. Pearson (eds.), Modern 

and Ancient Continental Shelf Anoxia. Geological Society Special Publication No. 58, The 

Geological Society, London, 470 pp. 

Howarth, R. W., G. Billen, D. Swaney, A. Townsend, N. Jaworski, K. Lajtha, J. A. Downing, R. 

Elmgren, N. Caraco, T. Jordan, F. Berendse, J. Freney, V. Kudeyarov, P. Murdoch and Z. 

Zhao-Liang. 1996. Regional nitrogen budgets and riverine N & P fluxes for the drainages to 

the North Atlantic Ocean: Natural and human influences. Biogeochemistry 35: 75-79. 

Justic', D., N. N. Rabalais, R. E. Turner, and W. J. Wiseman, Jr. 1993. Seasonal coupling 

between riverborne nutrients, net productivity and hypoxia. Marine Pollution Bulletin 26: 

184-189. 

Justic', D., N. N. Rabalais, and R. E. Turner. 1996. Effects of climate change on hypoxia in 

coastal waters: A doubled CO, scenario for the northern Gulf of Mexico. Limnology and 

Oceanography 41 : 992- 1003. 

Justic', D., N. N. Rabalais, and R. E. Turner. 1997. Impacts of climate change on net 

productivity of coastal waters: Implications for carbon budget and hypoxia. Climate 

Research 8: 225-237. 
H ypoxia 99 

Leming, T. D. and W. E. Stuntz. 1984. Zones of coastal hypoxia revealed by satellite scanning 

have implications for strategic fishing. Nature 3 10: 13 6- 13 8. 

Milliman, J. D. and R. H. Meade. 1983. World-wide delivery of river sediment to the ocean. 

Journal of Geology 9 1 : 1-2 1. 

Murray, S. P. (ed.). 1998. An Observational Study of the Mississippi-Atchafalaya Coastal 

Plume. Final Report. OCS Study MMS 98-0040. U.S. Dept. of Interior, Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, La. 5 13 pp. 

Nelsen, T. A., P. Blackwelder, T. Hood, B. McKee, N. Romer, C. Alvarez-Zarikian and S. Metz. 

1994. Time-based correlation of biogenic, lithogenic and authigenic sediment components 

with anthropogenic inputs in the Gulf of Mexico NECOP study area. Estuaries 17: 873-885. 

Pavela, J. S., J. L. Ross, and M. E. Chittenden. 1983. Sharp reductions in abundance of fishes 

and benthic macroinvertebrates in the Gulf of Mexico off Texas associated with hypoxia. 

Northeast Gulf Science 6: 167-1 73. 

Pokryfki, L. and R. E. Randall. 1987. Nearshore hypoxia in the bottom water of the 

northwestern Gulf of Mexico fiom 198 1 to 1984. Marine Environmental Research 22: 75-90. 

Qureshi, N. A. 1995. The role of fecal pellets in the flux of carbon to the sea floor on a riverinfluenced continental shelf subject to hypoxia. Ph.D. Dissertation, Department of 

Oceanography & Coastal Sciences, Louisiana State University, Baton Rouge, 255 pp. 

Rabalais, N. N. 1992. An Updated Summary of Status and Trends in Indicators of Nutrient 

Enrichment in the Gulf of Mexico. Environmental Protection Agency Publ. No. EPAl800-R92-004, Gulf of Mexico Program, Nutrient Enrichment Subcommittee, Stennis Space Center, 

Mississippi, 42 1 pp. 

Rabalais, N. N. 1998a (on-line). Oxygen Depletion in Coastal Waters. In NOAA's State of the 

Coast Report. National Oceanic and Atmospheric Administration, Silver Spring, Maryland, 

URL: http://state~ofcoast.noaa.govlbulletinsktmlkypYPO9~yp.html. 

Rabalais, N. N. 1998b. Hypoxia. Pages 325-354 in S. P. Murray (ed.), An Observational Study 

of the Mississippi-Atchafalaya Coastal Plume. Final Report. OCS Study MMS 98-0040. 

U.S. Dept. of Interior, Minerals Management Service, Gulf of Mexico OCS Region, New 

Orleans, La. 5 13 pp. 

Rabalais, N. N. and D. E. Harper, Jr. 1992. Studies of benthic biota in areas affected by 

moderate and severe hypoxia, pp. 48-5 1. In: Nutrient Enhanced Coastal Ocean Productivity. 

Publ. No. TAMU-SG-92-109, Texas Sea Grant College Program, Galveston, Texas. 
100 Hypoxia 

Rabalais, N. N. and R. E. Turner. 1998. Pigment and nutrient distributions, pp. 208-230. In: S. 

P. Murray (ed.), An Observational Study of the Mississippi-Atchafalaya Coastal Plume. - 

Final Report. OCS Study MMS 98-0040. U.S. Dept. of Interior, Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, La. 5 13 pp. 

Rabalais, N. N., L. E. Smith, D. E. Harper, Jr., and D. Justic'. 1995. Effects of Bottom Water 

Hypoxia on Benthic Communities of the Southeastern Louisiana Continental Shelf. OCS 

Study MMS 94-0054. U.S. Dept. of the Interior, Minerals Management Service, Gulf of 

Mexico OCS Region, New Orleans, Louisiana, 105 pp. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman, Jr., and D. F. Boesch. 1991. A brief summary of 

hypoxia on the northern Gulf of Mexico continental shelf: 1985-1988, pp. 35-46. In: R. V. 

Tyson and T. H. Pearson (eds.), Modem and Ancient Continental Shelf Anoxia. Geological 

Society Special Publication No. 58, The Geological Society, London, 470 pp. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. l992a. Distribution and characteristics of 

hypoxia on the Louisiana shelf in 1990 and 199 1, pp. 15-20. In: Proceedings, Nutrient 

Enhanced Coastal Ocean Productivity Workshop. Publ. No. TAMU-SG-92-109, Texas Sea 

Grant College Program, Texas A&M University, College Station, Texas. 

Rabalais, N.N., R. E. Turner, and Q. Dortch. 1992b. Louisiana continental shelf sediments: 

Indicators of riverine influence, pp. 77-81. In: Nutrient Enhanced Coastal Ocean 

Productivity. Publ. No. TAMU-SG-92- 109, Texas Sea Grant College Program, Galveston, 

Texas. 

Rabalais, N. N., R. E. Tumer, D. Justic', Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

1996. Nutrient changes in the Mississippi River and system responses on the adjacent 

continental shelf. Estuaries 19: 386-407. 

Rabalais, N. N., R. E. Tumer, W. J. Wiseman, Jr., and Q. Dortch. 1998. Consequences of the 

1993 Mississippi River flood in the Gulf of Mexico. Regulated Rivers: Research & 

Management 14: 161-177. 

Rabalais, N. N., R. E. Turner, D. Justic, Q. Dortch, and W. J. Wiseman, Jr. 1999. 

Characterization of Hypoxia: Topic 1 Report for the Integrated Assessment of Hypoxia in 

the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 15. 

NOAA Coastal Ocean Program, Silver Spring, MD, 167 pp. 

Ragan, J. G., A. H. Harris, and J. H. Green. 1978. Temperature, salinity and oxygen 

measurements of surface and bottom waters on the continental shelf off Louisiana during 

portions of 1975 and 1976. Professional Papers Series (Biology) (Nicholls State Univ., 

Thibodaux, Louisiana) 3: 1-29. 
H ypoxia 101 

Renaud, M. 1986. Hypoxia in Louisiana coastal waters during 1983: implications for fisheries. 

Fishery Bulletin 84: 19-26. 

Roman, M. R., A. L. Gauzens, W. K. Rhinehart ,and J. R. White. 1993. Effectsof low oxygen 

waters on Chesapeake Bay zooplankton. Limnology and Oceanography 38: 1603- 16 14. 

Rosenberg, R. 1985. Eutrophication-the future coastal nuisance? Marine Pollution Bulletin 

16: 227-23 1. 

Sen Gupta, B. K., R. F. Lee, and M. S. May. 1981. Upwelling and an unusual assemblage of 

benthic foraminifera on the northern Florida continental slope. Journal of Paleontology 55: 

853-857. 

Sen Gupta, B. K. and M. L. Machain-Castillo. 1993. Benthic foraminifera in oxygen-poor 

habitats. Marine Micropaleonotology 20: 183-20 1. 

Sen Gupta, B. K., R. E. Turner, and N. N. Rabalais. 1996. Seasonal oxygen depletion in 

continental-shelf waters of Louisiana: Historical record of benthic foraminifers. Geology 24: 

227-230. 

Tolmazin, R. 1985. Changing coastal oceanography of the Black Sea. I. Northwestern shelf. 

Progress in Oceanography 15 : 2 127-276. 

Turner, R. E. and R. L. Allen. 1982a. Bottom water oxygen concentration in the Mississippi 

River delta bight. Contributions in Marine Science 25 : 16 1 - 172. 

Turner, R. E. and R. L. Allen. 1982b. Plankton respiration in the bottom waters of the 

Mississippi River delta bight. Contributions in Marine Science 25: 173-179. 

Turner, R. E. and N. N. Rabalais. 199 1. Changes in Mississippi River water quality this 

century. Implications for coastal food webs. BioScience 41: 140-148. 

Turner, R. E. and N. N. Rabalais. 1994. Coastal eutrophication near the Mississippi river delta. 

Nature 368: 619-62 1. 

Turner, R. E. and N. N. Rabalais. 1998. Bottom water respiration rates in the hypoxia zone 

within the Louisiana Coastal Current, pp. 354-364. In: S. P. Murray (ed.), An Observational 

Study of the Mississippi-Atchafalaya Coastal Plume. Final Report. OCS Study MMS 98- 

0040. U.S. Dept. of Interior, Minerals Management Service, Gulf of Mexico OCS Region, 

New Orleans, La. 5 13 pp. 
1 02 Hypoxia 

Turner, R. E., N. N. Rabalais, Q. Dortch, D. Justic', and B. K. Sen Gupta. 1997. Evidence for 

nutrient limitation and sources causing hypoxia on the Louisiana shelf. Pages 106-1 12 in 

Proceedings of the First Gulf of Mexico Hypoxia Management Conference, December 1995, 

New Orleans, Louisiana. Publ. No. EPA-55-R-97-001, U.S. Environmental Protection 

Agency, Gulf of Mexico Program Office, Stennis Space Center, Mississippi, 198 pp. 

Turner, R. E., N. Qureshi, N. N. Rabalais, Q. Dortch, D. Justic', R. F. Shaw, and J. Cope. 1998. 

Fluctuating si1icate:nitrate ratios and coastal plankton food webs. Proceedings of the National 

Academy of Science, USA 95: 13048-1305 1. 

Waller, R. S. 1998. Extent of hypoxia in areas east of the Mississippi River to Mobile Bay. 

1998. Invited talk, Hypoxia Effects Workshop, March 1 1-13, 1998, Baton Rouge, Louisiana. 

Ward, C. H., M. E. Bender, and D. J. Reish (eds.). 1979. The Offshore Ecology Investigation. 

Effects of oil drilling and production in a coastal environment. Rice University Studies 65: 1- 

589. 

Wiseman, Jr., W. J., N. N. Rabalais, R. E. Turner, S. P. Dinnel, and A. MacNaughton. 1997. 

Seasonal and interannual variability within the Louisiana Coastal Current: Stratification and 

hypoxia. Journal of Marine Systems 12:237-248. 

Wiseman, Jr., W. J., V. J. Bierman, Jr., N. N. Rabalais, and R. E. Turner. 1992. Physical 

structure of the Louisiana shelf hypoxic region, pp. 21-26. In Nutrient Enhanced Coastal 

Ocean Productivity. Publ. No. TAMU-SG-92-109, Texas Sea Grant College Program, 

Galveston, Texas. 

Zaitsev, Y. P. 1993. Impacts of eutrophication on the Black Sea fauna. Paged 64-86 in Fisheries 

and Environment Studies in the Black Sea System. Food and Agricultural Organization, 

United Nations, Rome. 

Zimmerman, R. J., J. M. Nance, and J. Williams. 1997. Trends in shrimp catch in the hypoxic 

area of the northern Gulf of Mexico. Pages 64-75 in Proceedings of the First Gulf of Mexico 

Hypoxia Management Conference, December 1995, New Orleans, Louisiana. Publ. No. 

EPA-55-R-97-00 1, U.S. Environmental Protection Agency, Gulf of Mexico Program Office, 

Stennis Space Center, Mississippi, 198 pp. 
Chapter 6 

Modeling 

by Victor J. Bierman, Jr., Scott C. Hinz, Kristine A. Nelson, David P. Podber, 

Dong-wei Zhu, William J. Wiseman, Jr., Nancy N. Rabalais, 

and R. Eugene Turner 

Introduction 

Seasonal hypoxia C< 2 mg 02/1) occurs in bottom waters on the northern Gulf of Mexico inner 

continental shelf from May through September over extensive areas (up to 20,000 km2) (Rabalais 

et al., unpublished data). The two principal factors leading to the development and maintenance 

of hypoxia are physical stratification of the water column and decomposition of organic material. 

Spatial and temporal variation in the distribution of hypoxic water masses is related, in part, to 

freshwater discharge from the Mississippi-Atchafalaya River (MAR), circulation patterns, 

nutrient loadings and a close coupling with net primary productivity. Significant increases in 

nitrogen and phosphorus loadings, and decreases in silicon loadings, have occurred in the 

Mississippi River this century and have accelerated since 1950 (Turner and Rabalais 1991). 

These changes appear to have caused phytoplankton species shifts offshore and an increase in 

primary production (Rabalais et al. 1996). Justic et al. (1993) showed that MAR nutrient 

inputs, net productivity and hypoxia in the northern Gulf of Mexico are closely correlated. 

Water circulation on the Louisiana-Texas shelf is strongly influenced by wind stress and 

freshwater discharges from the MAR (Wiseman et al. 1997, Walker 1996). Thirty percent of the 

flow from the MAR is delivered through the Atchafalaya River Delta. The remaining 70 percent 

flows through the birdfoot delta and eventually discharges approximately 50 percent to the west 

of the delta and 50 percent to the east (U.S. Army Corps of Engineers 1984). The water flowing 

westward ultimately forms the Louisiana Coastal Current (LCC) (Wiseman and Kelly 1994). 

During much of the year, the LCC flows into Texas and Mexican waters (Cochrane and Kelly 

1986). At these times, the distribution of excess fresh water is largely confined to a nearshore 

band extending from the Mississippi birdfoot delta into Mexican waters @innel and Wiseman 

1986). Under upwelling favorable winds, which blow over the Mexican and south Texas coast 

from late spring through mid- or late summer, a return flow occurs (Cochrane and Kelly 1986). 

The summer halocline associated with the LCC isolates near-bottom waters from direct wind 

forcing and this effect, in conjunction with pressure gradients driving upcoast flow along the 

Texas inner shelf, results in slow-moving bottom waters over the Louisiana Inner Shelf (LIS) 

(Rabalais et al. 1996). This allows biological processes to deplete the oxygen content of the nearbottom waters. Hypoxic waters are most prevalent from late spring through late summer and 
104 Modeling 

mostly in water depths of 5 to 30 meters. Hypoxia occurs mostly in the lower water column but 

encompasses as much as the lower half to two-thirds of the water column. 

Mathematical models can be useful tools for synthesizing environmental data and for providing 

better understanding of cause-effect mechanisms that lie behind experimental observations. They 

can also be useful to forecast possible responses of environmental systems to different 

management scenarios. The Nutrient Enhanced Coastal Ocean Productivity (NECOP) program 

included development of a mathematical model in parallel with an observational program. The 

NECOP model was not used to design the data collection program, nor was it used to interpret 

and synthesize all of the observational results. The principal benefits from the model were a 

better understanding of environmental processes controlling primary productivity and bottom 

water dissolved oxygen on the LIS, and an assessment of how these water quality responses 

might be influenced by changes in nutrient loadings from the MAR system. 

The principal emphasis in the NECOP program was on collection of chemical and biological data. 

Originally, it was envisioned that the Louisiana-Texas (LATEX) shelf program sponsored by the 

Minerals Management Service (MMS) would provide physical data and results from a numerical 

circulation model that could be used to drive the NECOP water quality model. As events 

unfolded, the LATEX program did not produce these results in the anticipated time frame. 

Consequently, water circulation in the NECOP model is largely descriptive and based on 

relatively sparse observational data. In this sense the NECOP water quality model should be 

viewed as a preliminary, screening-level analysis. 

Models of Estuarine and Coastal Waters 

Various modeling approaches have been developed for assessing water quality in estuarine and 

coastal waters. The U.S. Environmental Protection Agency (EPA) (1990) provided extensive 

guidance on mathematical models for assessing relationships between nutrient loading and 

nutrient-related water quality criteria. Hinga et al. (1995) reviewed results of three different 

approaches for investigating relationships between nitrogen availability and phytoplankton 

primary production and abundance in coastal ecosystems. These included controlled experiments 

in marine enclosures, assessing historical changes in coastal ecosystems and cross-system 

comparisons. Wyatt (1998) investigated meteorological and anthropogenic influences on marine 

algal blooms and presented simple population models. Chen et al. (1997) developed a coupled 

biological and physical model to study the influence of MAR discharge on biological variability 

on the Louisiana-Texas continental shelf. Chau and Jin (1 998) developed a two-layer, integrated, 

hydrodynamic-eutrophication model to investigate relationships between density stratification 

and bottom water anoxia in Tolo Harbour, Hong Kong. 

The Water Quality Analysis Simulation Program (WASP) (Ambrose et al. 1993) is a generalized, 

multi-dimensional, mass balance modeling framework that has been used to investigate water 

quality problems in a large number of different marine systems. The CE-QUAL-ICM model 

(Cerco and Cole 1995a) is a comprehensive mass balance model that has been used for complex 
Modeling 105 

problems in several different large water bodies. A version of CE-QUAL-ICM was used in 

conjunction with a three-dimensional hydrodynamic model and a sediment diagenesis submodel - 

to investigate eutrophication and dissolved oxygen in Chesapeake Bay (Cerco 1995b, 1995; 

Cerco and Cole 1993). The Environmental Fluid Dynamics Computer Code (EFDC) (Hamrick 

1996) is a general-purpose, three-dimensional, hydrodynamic and water quality model. The 

EFDC model was used for studies in estuaries of Chesapeake Bay, two sites in Florida, the 

Peconic Bay system in New York, Stephens Passage in Alaska and Nan Wan Bay in Taiwan. 

Several recent modeling approaches involved species succession and dynamics of higher trophic 

levels. Roelke et al. (1997) investigated phytoplankton species succession in the Nueces River 

Estuary, Texas, as influenced by anthropogenic activities. They evaluated and compared results 

from a Phytoplankton Ecology Group (PEG) model and a model based on Equilibrium Resource 

Competition (ERC) theory. Vasconcellos et al. (1997) used the ECOPATH approach to conduct 

simulations of 18 different marine trophic models to explore the behavior of systems impacted at 

intermediate trophic levels. Manickchand-Heileman et al. (1998) developed a trophic mass 

balance model to investigate energy flow in a community of fish and invertebrates in the 

southwestern portion of the Gulf of Mexico. 

NECOP Modeling Approach 

As part of the NECOP program, Bierman et al. (1994a) applied a version of the U.S. EPA Water 

Analysis Simulation Program (WASP) to the LIS portion of the northern Gulf of Mexico (Figure 

6.1). 

_ Figure 6.1. Location map of study area for NECOP water quality model. 
106 Modeling 

The conceptual framework for the modeling approach is shown in Figure 6.2. State variables in 

the model include salinity, phytoplankton carbon, phosphorus (dissolved orthophosphate and - 

organic forms), nitrogen (ammonium, nitrate plus nitrite and organic forms), dissolved oxygen and 

carbonaceous biochemical oxygen demand (CBOD). User-specified external forcing functions 

include constituent mass loadings, advective-dispersive transport, seaward boundary conditions, 

sediment fluxes, water temperature, incident solar radiation, and underwater light attenuation. 

Sediment interactions are represented by user-specified values for net settling rates for particulate 

phase constituents, sediment-water diffusive fluxes for dissolved nutrients and sediment oxygen 

demand (SOD) 

External Source Advection and I I Sediment 

Loads Dispersion Conditions Flux I I Temperature I I Light 

Water Column 3-1 Organic + 

: Settling I 

Figure 6.2. Schematic diagram of principal model state variables and processes in NECOP water 

quality model. 

The spatial domain of the NECOP model is represented by a 21-segment water column grid 

extending from the Mississippi River Delta west to the Louisiana-Texas border, and from the 

shoreline seaward to the 30-60 m bathymetric contours (Figure 6.3). The spatial segmentation 

grid includes one vertical layer nearshore and two vertical layers offshore. All of the spatial 

segments are assumed to be completely mixed. The nearshore segments have an average depth of 

5.6 m. The surface offshore segments are completely mixed in the vertical to an assumed fixed 

pycnocline depth of 10 m. The bottom offshore segments are completely mixed from 10 m to the 

seabed. The thicknesses of these bottom offshore segments range between 6.1 and 20.3 m. 
Modeling 107 

The coarse scale of the model segmentation grid was originally determined by the areal distribution and vertical density of historical water quality data. Salinity was used to identify 

characteristic water masses and to determine the geometric boundaries of the grid. The scale of 

the model segmentation grid has two principal limitations: first, near-field horizontal gradients in 

the vicinity of the Mississippi and Atchafalaya River plumes are not explicitly represented; and 

second, important vertical scale characteristics are not fully-represented, including near-bottom 

hypoxia and "layering" of dissolved oxygen concentrations. Rabalais et al. (1996) have shown 

that the low oxygen water mass in bottom waters can move around and change configuration in 

response to winds, currents and tidal advection. Furthermore, they showed that hypoxia can 

occur not only at the bottom near the sediments, but also well up into the water column. 

Figure 6.3. Model spatial segmentation grid for the Louisiana Inner Shelf portion of the Gulf of 

Mexico. 

The temporal domain of the model represents steady-state, summer-average conditions. 

Consequently, the model represents only a single "snapshot" in time. In reality, there is great 
108 Modeling 

daily and weekly variability in current flow and stratification on the LIS (Rabalais et al. 1996). 

The principal reason for this model limitation is that field measurements are not available to - 

characterize temporal variability at the shelfwide spatial scale. Typically, only a single shelfwide 

monitoring effort is conducted each year during the July-August period to characterize the spatial 

extent of hypoxia. Operationally, model forcing functions were assigned constant values that 

represented summer average conditions. The time-variable model was then run to steady-state 

and model output was compared with available field data. It was assumed that data collected 

during the summer shelfwide monitoring effort were synoptic, and that they were in temporal 

equilibrium with the specified summer-average model forcing functions. 

Physical transport in the model is represented by advective flow and bulk dispersion. Bulk 

dispersion is a lumped parameter that represents transport processes at scales smaller than the 

model spatial segments. These processes include molecular diffusion, turbulent eddy diffusion 

and shear flow dispersion. Because the model balances mass and not momentum, magnitudes and 

directions for advective flows must be externally specified by the user. Dispersive mixing 

coefficients across all horizontal and vertical interfaces are calibration parameters that were 

determined by conducting a mass balance to salinity, a conservative tracer. 

Advective flows in the model are descriptive in nature and were based on relatively sparse 

observational data. It is believed that summer average conditions in the spatial domain of the 

model are typically represented by the LCC which has a net westward drift along the shelf 

bathyrnetry. This representation is supported by current meter measurements from a long-term 

mooring maintained by W.J. Wiseman, Jr., LSU, at a location off Cocodrie (Segment 10) in 20 m 

of water. Typical summer average current speeds are approximately 10 and 1 crnls, respectively, 

in the surface and bottom waters. 

Model Application 

The NECOP model was calibrated to a comprehensive set of field data collected during July 1990 

at over 200 sampling stations in the northern Gulf of Mexico (Bierman et al. 1994a). Reasonable 

comparisons were obtained between computed and observed values for model state variables, 

primary productivity and mass settling fluxes for particulate carbon and nitrogen. The model 

calibration was extended to include earlier historical data collected during 1985-1988 by 

LUMCON (Rabalais et al. 1996). The calibrated NECOP model was used for diagnostic 

analyses, sensitivity analyses and numerical experiments to better understand nutrient and 

phytoplankton dynamics, fate of organic carbon and processes controlling seasonal hypoxia. It 

was also used for forecast simulations designed to investigate whether water quality parameters 

on the LIS are responsive to changes in nutrient loadings from the MAR (Lirnno-Tech, Inc. 

1995). 
Modeling 109 

Phytoplankton Dynamics 

An important question in the NECOP program concerns the relative importance of environmental 

factors controlling primary productivity. Under conditions of optimal temperature, light and 

nutrients, phytoplankton growth rates are limited solely by physiology. Under ambient 

conditions water temperature generally determines maximum possible growth rates. Specific 

growth rates actually realized depend on potentially limiting effects due to underwater light 

attenuation and nutrient limitation. 

The components of phytoplankton growth rates in the calibrated model due to temperature, light 

and nutrient effects are shown in Figure 6.4 for four representative surface offshore model 

segments. All rates shown represent vertical averages in the 10-m deep surface mixed layer. 

Maximum growth rates at ambient temperatures are high (approximately 3.0/day) due to high 

summer water temperature in the Gulf of Mexico. Growth rate limitation due to underwater light 

attenuation is substantially greater than growth rate limitation due to non-optimal nutrient 

concentrations. Actual specific growth rates decrease and the degree of nutrient limitation 

increases with increased distance from the Mississippi Delta. Specific growth rates remain 

relatively high in all segments, ranging between 1 .O/day and 1.2/day. These values are consistent 

with taxon-specific growth rates estimated from 14~-autoradiography experiments conducted in 

the plume and primary hypoxic regions (Segments 8,9 and 10) (Fahnenstiel et al. 1995). 

Louisiana Inner Shelf 

Phyto Growth Rate Components - July 1990 

8 10 12 14 

Model Spatial Segment 

Sp Gr Rate Nut Effect Light Effect 

Figure 6.4. Component analysis of phytoplankton growth rates in the calibrated model as a 

function of temperature, light and nutrient limitations in selected surface offshore 

segments. Sp Gr Rate = resultant specific growth rate, Nut Effect = reduction due to 

nutrient limitation, Light Effect = reduction due to light limitation. 
110 Modeling 

In assessing the significance of these results, a distinction should be made between rate limitation 

and stoichiometric limitation. The former refers to limitation of specific growth rates, while the 

latter refers to limitation of the amount of phytoplankton biomass that can be produced. The 

results in Figure 6.4 indicate that light effects are more important than nutrient effects in 

controlling phytoplankton growth rates. It does not follow, however, that reductions in MAR 

nutrient loadings will not significantly affect phytoplankton concentrations. The reason is that 

phytoplankton growth rates can be maintained only if there are adequate nutrient supply rates. 

Another important question in the NECOP program is the fate pathways for phytoplankton 

production. Individual chemical-biological components of gross primary production losses for 

the same four representative model segments are shown in Figure 6.5. Zooplankton grazing 

(ingestion), phytoplankton respiration, and dissolved organic carbon (DOC) exudation are 

computed to account for approximately 50,25 and 20 percent, respectively, of total chemicalbiological losses in each segment. Non-predatory mortality is very small in all four segments. 

The relative magnitude of losses due to zooplankton grazing is consistent with the highly 

productive fisheries in the northern Gulf of Mexico. 

Louisiana Inner Shelf 

GPP Loss Rates - July 1990 

Model Spatial Segment 

Grazing Resp DOC Death 

Figure 6.5. Component analysis of loss processes for gross primary production in the calibrated 

model in selected offshore surface segments. Grazing = zooplankton ingestion, Resp 

= phytoplankton respiration, DOC = dissolved organic carbon exudation, Death = 

non-predatory mortality. 
Modeling 111 

Concern has been raised (e.g. Turner and Rabalais 1991) because interactions among different - 

nutrients (nitrogen, phosphorus and silicon) and different phytoplankton functional groups 

(diatoms and non-diatoms) are not completely understood. Rabalais et al. (1996) suggested that 

the combination of primary production changes and phytoplankton species shifts could affect 

subsequent carbon utilization, carbon flux pathways, and the areal extent or severity of hypoxia 

on the continental shelf. Officer and Ryther (1980) hypothesized that silicon limitation in highly 

eutrophic estuaries would cause shifts in phytoplankton species composition to non-diatom 

phytoplankton which are not readily grazed, but which would sink out of the euphotic zone and, 

through decomposition on or near the sea floor, lead to large areas of hypoxia or anoxia. Dortch 

and Whitledge (1 992) suggested an alternative hypothesis in which hypoxia on the northern Gulf 

of Mexico inner shelf would result from decomposition of sinking large diatoms which are not 

completely grazed, especially in the spring when production is high (Lohrenz et al. 1990), but 

zooplankton biomass is low (Dagg and Whitledge 199 1). Consequently, decreased silicon loads 

would decrease the dominance of large diatoms and decrease the severity and extent of hypoxia. 

The essential point is that hypoxia is linked to the extent of primary production and the fate of 

organic carbon from this primary production. In turn, the fate of this organic carbon is linked to 

phytoplankton species composition and abundance, and zooplankton grazing preferences. 

Dissolved Oxygen Dynamics 

Still another important question in the NECOP program concerns the principal factors controlling 

dissolved oxygen and seasonal hypoxia on the LIS. A components analysis of dissolved oxygen 

dynamics in the calibrated model (Bierman et al. 1994a) revealed that photosynthesis could be an 

important source of dissolved oxygen in the bottom waters of the LIS. Low underwater light 

attenuation and changes in water column depth appear partly responsible for this phenomenon. 

The 1 percent depth is the compensation depth at which photosynthetic oxygen production 

approximately balances oxygen consumption due to phytoplankton respiration. Below Segments 

11-14, west of the primary hypoxic region (Figure 6.3), the 1 percent depth estimated from light 

extinction coefficients in the calibrated model is greater than the total depth of the water column. 

This implies that phytoplankton are a net source of dissolved oxygen in this region. This model 

result is consistent with independent observations confirming that considerable primary 

production occurs on the sediment surface in the model spatial domain, especially further west 

along the inner shelf, For 12 locations at which measurements were conducted, G.T. Rowe 

(Hendee 1994) has reported that bottom productivity rates averaged approximately 30 percent of 

water column productivity rates. 

Individual components of total oxygen depletion rates for four representative bottom offshore 

model segments are shown in Figure 6.6. Oxidation of CBOD in the water column, 

phytoplankton respiration and sediment oxygen demand (SOD) all contribute significantly to 

total oxygen depletion rates. CBOD is the largest component in each of the four segments, and 

its relative contribution increases with increasing distance from the delta. Nitrification is a 

relatively small component in all four segments. The computed contribution of SOD (areal rate 
112 Modeling 

expressed as a volumetric demand) to total oxygen depletion rates in the calibrated model ranges 

from 22 to 30 percent. 

The result that SOD accounts for 22 to 30 percent of total oxygen demand is remarkably 

consistent with results for Chesapeake Bay and Lake Erie. Kemp et al. (1992) measured benthic 

and planktonic oxygen consumption, planktonic oxygen production, and changes in surface and 

bottom dissolved oxygen pools at two mesohaline sites in Chesapeake Bay that experience 

seasonal hypoxia. The water column depths at these two stations were 9 m and 18 m. They 

found that benthic processes accounted for approximately one-third of the total oxygen demand. 

DiToro et al. (1987) conducted a long-term mass balance modeling study of phytoplankton, 

nutrients and dissolved oxygen in Lake Erie, which experiences seasonal hypoxia in the Central 

Basin (mean depth 24 m). They concluded that SOD accounted for approximately 30 to 40 

percent of the total oxygen demand, depending on the annual climatic conditions and seasonal 

thermocline structure. 

Louisiana Inner Shelf 

Oxygen Depletion Rates - July 1990 

-0.40 

15 17 19 

----A 

2 1 

Model Spatial Segment 

CBOD Ph ~espu Nitrif SOD 

Figure 6.6. Component analysis of loss processes for dissolved oxygen in the calibrated model in 

selected bottom offshore segments. CBOD = carbonaceous biochemical oxygen 

demand in the water column, Ph Res = phytoplankton respiration, Nitrif - 

nitrification, SOD = sediment oxygen demand (areal rate expressed as a volumetric 

demand). 
Modeling 

Sensitivity Analyses 

To address uncertainties due to representation of chemical-biological processes, a series of 

sensitivity analyses was conducted with the calibrated model. All internal model process rates 

and stoichiometric coefficients, and SOD, were varied by plus and minus 30 percent and results 

compared to base calibration results. These sensitivity analyses were not designed to represent 

actual uncertainties in each parameter but rather to determine the relative sensitivity of model 

results to systematic variations across different parameters. 

Results are presented for only the five processes to which model responses were most sensitive 

(Figure 6.7). These include variations in underwater light extinction coefficient, saturation light 

intensity, carbon:chlorophyll (C:Chl) ratio, water column oxygen demand (CBOD decay rate) 

and SOD. Model responses were also sensitive to variations in nitrogen:carbon (N:C) and 

phosphorus:carbon (P:C) stoichiometric ratios for the phytoplankton and nutrient mineralization 

rates. 

Dissolved oxygen concentrations were more sensitive to variations in light extinction coefficients 

and saturation light intensities than to variations in any other process parameters. Chlorophyll 

concentrations were sensitive to variations in light extinction coefficients, saturation light intensities and C:Chl ratios but were not sensitive to variations in CBOD decay rate or SOD. In 

general, model responses were not symmetric about equal plus and minus variations in model 

parameters, especially for variations in light-related parameters. 

The sensitivity of dissolved oxygen and chlorophyll concentrations to variations in light-related 

parameters is consistent with observations that primary production in the northern Gulf of 

Mexico is very sensitive to changes in light intensity (Lohrenz et al. 1994, 1990). Model 

responses were not symmetric about equal plus and minus parameter variations because 

underwater light intensity decreases exponentially with depth, not linearly. These are important 

results because light-related parameters represent complex processes and are difficult to measure 

accurately in the environment. 

There appears to be strong coupling between dissolved oxygen concentrations and primary productivity in bottom waters. This coupling is due to low underwater light attenuation and shallow 

water column depths in the hypoxic region. Total suspended solids concentrations are relatively 

low (2-3 mgll) and total water column depths range between 16.1 and 30.3 m in the offshore 

portion of the model spatial domain. Furthermore, water temperature and nutrient 

concentrations are relatively high during the summer stratified period, thus tending to magnify 

productivity responses. 

Responses for chlorophyll concentrations in the model appear counterintuitive because they 

increase in response to increased light attenuation and saturation light intensity (plus 30 percent 

changes) and decrease in response to decreased light attenuation and saturation light intensity 
114 Modeling 

(minus 30 percent changes) (Figure 6.7). Although this behavior could be an artifact of the 

model, a possible reason could be photoinhibition in surface waters. Phytoplankton growth rate 

in the model increases as a function of increasing light intensity up to a saturating level and then 

decreases (photoinhibition) with further increases in light intensity (Thomann and Mueller 1987). 

For surface waters the computed light saturation depth is approximately 5-7 m. This implies 

that phytoplankton growth rates in the model are limited by photoinhibition in nearshore waters 

(Segments 1-7) and in major portions of surface offshore waters (Segments 8-14). Increases in 

light attenuation and saturation light intensity stimulated surface primary productivity in the 

model because they reduced photoinhibition in the surface waters. 

Dissolved Oxygen Sensitivity Analyses 1990 Conditions 

Extinction Saturation Carbon: Water Sediment 

Coefficient Light Chlorophyll Column Oxygen 

hknsity Ratio Oxygen Demand 

Demand 

C hlomphyll Sensitivity Analyses 1990 Conditions 

Extinction Saturation Carbon: Water Sediment 

Coefficient Light Chlorophyll Column Oxygen 

htensity Ratio Oxygen Demand 

Demand 

Figure 6.7. Results of sensitivity analyses for dissolved oxygen and chlorophyll concentrations 

relative to base model calibration for July 1990. 
Modeling 115 

Chlorophyll concentration responses to variations in C:Chl ratio depend on P:C and N:C 

stoichiometric ratios in the phytoplankton. If C:Chl ratio is increased, then this decreases the - 

amount of chlorophyll that can be produced for given P:C and N:C ratios, assuming no changes in 

nutrient loadings. The opposite is true for a decrease in C:Chl ratio. Dissolved oxygen responses 

in bottom waters are confounded because phytoplankton in the model are represented as carbon 

and chlorophyll is only a display parameter; however, the self-shading term in the submodel for 

underwater light attenuation is a function of computed chlorophyll concentration. Consequently, 

when C:Chl ratio is increased, chlorophyll concentration decreases thus causing an increase in 

underwater light intensity in the water column. The net result is that dissolved oxygen 

concentration increases in response to an increase in the C:Chl ratio. 

Dissolved oxygen responses in the model were sensitive to variations in both SOD and water 

column decay rate (CBOD). Average dissolved oxygen responses were approximately linear 

with variations in SOD. Responses to variations in SOD were greater than responses to 

variations in CBOD decay rate. 

Dortch et al. (1992) have hypothesized that the high productivity observed on the LIS is 

maintained by nitrogen recycling within the water column, thus greatly amplifying the effect of 

high riverine nitrate inputs. Results of numerical experiments in which water column nutrient 

remineralization was "turned off' (Bierman et al. 1994a) indicated that average chlorophyll 

concentrations in surface offshore waters decreased by 64 percent. Dissolved oxygen 

concentrations in bottom waters increased near the delta due to decreases in nitrification and 

decreased west of the Atchafalaya River due to decreases in subsurface primary productivity. 

These results illustrate the complex interactions among phytoplankton, nutrient and dissolved 

oxygen dynamics on the LIS. 

Bierman et al. (1994b) presented results for sensitivity of model responses to changes in 

dispersive mixing across seaward boundaries and vertical dispersion. The principal cross-shelf 

transport component in the model was dispersive mixing because, with the exception of some 

off-shelf advective flow in 1990, there were no cross-shelf advective flows specified in any of the 

model applications. Bottom water dissolved oxygen concentrations were found to be 

approximately proportional to changes in cross-shelf dispersive mixing and relatively insensitive 

to changes in vertical mixing. Corresponding chlorophyll concentration responses were relatively 

insensitive to both of these changes. These results suggest that uncertainties in cross-shelf 

exchanges are important, especially for bottom water dissolved oxygen concentrations. In turn, 

uncertainties in cross-shelf exchanges are due to uncertainties in water circulation and mixing, as 

well as to specification of seaward boundary concentrations. 

Near the end of the NECOP program, results became available from the Dynalysis numerical 

circulation model developed as part of the MMSfLATEX program. Velocity fields from the 

Dynalysis model were spatially and temporally processed to derive advective flow fields for use 

as input to the NECOP water quality model. A time-variable version of the NECOP model was 

applied to the stratified period from March through September of 1993. The different grid - 
116 Modeling 

structures and spatial scales of the two models could not be fully reconciled. Furthermore, the 

inability of the Dynalysis model to represent day-to-day variability in flows near the shelf break 

resulted in the presence of an anomalous eddy in the apparent flow field and attempts to remove 

it were not completely satisfactory. Results from the time-variable NECOP water quality model 

were found to be highly sensitive to seaward boundary concentrations that were poorly resolved 

by the available monitoring data. Consequently it was not possible to constrain computed 

bottom water dissolved oxygen concentrations to observations. 

Forecast Simulations 

It is of interest to investigate whether water quality parameters on the LIS portion of the 

northern Gulf of Mexico are responsive to changes in nutrient loadings from the MAR. An 

answer to this question is important in determining whether reducing MAR nutrient loadings is a 

feasible option for improving present water quality conditions, especially seasonal hypoxia. To 

investigate this question the calibrated NECOP water quality model was run for a series of 

forecast simulations. The purpose of these simulations was not to establish target nutrient 

loading objectives, but to determine the range of nutrient loading reductions that may need to be 

evaluated in future studies. The results presented herein are preliminary results from an ongoing 

research program and should be considered provisional in nature. 

Results of the forecast simulations in the present report are premised on all of the assumptions 

inherent in the calibrated NECOP model, as well as on additional assumptions contained in the 

forecast simulations themselves. The calibrated model has the following principal assumptions: 

(1) the actual environmental system is fully represented by the conceptual framework of the 

model including model state variables, governing equations, process mechanisms and external 

forcing functions; (2) nitrogen and phosphorus are the only nutrients that potentially limit 

primary productivity; (3) the actual environmental system is represented only at the coarse 

spatial scale of the model segmentation grid; near-field gradients in the vicinity of the Mississippi 

and Atchafalaya River plumes, and hypoxia in near-bottom waters are not explicitly represented; 

and (4) the actual environmental system is represented in terms of a single "snapshot" in time 

corresponding to an assumed summer average, steady-state period; the potential influence of 

meteorological events, shelf-edge upwellings and mesoscale shelf circulation are not explicitly 

represented. 

In addition, the forecast simulations themselves have the following assumptions: (1) all forecast 

results represent estimates of future states of the system and do not contain any information on 

the time frame required for the system to fully respond to imposed changes in nutrient loadings; 

and (2) all forecast results presented assume that seaward and sediment boundary conditions will 

eventually change by the same percentage as the corresponding imposed changes in nutrient 

loadings. 

The principal water quality response parameters investigated were bottom water dissolved 

oxygen concentrations and surface water chlorophyll concentrations. To address uncertainties 
Modeling 117 

due to potential differences in environmental conditions, separate forecast simulations were 

conducted for July 1985, August 1988 and July 1990 conditions for each load reduction. With - 

respect to spatial scale, all results for dissolved oxygen concentration represent the average of 

responses for bottom offshore waters (Segments 15-2 1) and all results for chlorophyll 

concentration represent the average of responses for surface offshore waters (Segments 8-14). In 

all cases, results are expressed in terms of changes relative to base calibration results, not the 

absolute values of the forecasts. 

Forecast results indicate that both dissolved oxygen concentrations (Figure 6.8) and chlorophyll 

concentrations (Figure 6.9) are responsive to reductions in nitrogen loadings from the MAR. 

There is more variability in responses among different years for dissolved oxygen than for 

chlorophyll. As nitrogen loads are reduced from 10 to 70 percent, forecasted chlorophyll 

concentrations for 1985, 1988 and 1990 conditions decrease by amounts ranging from 

approximately 1 to 72 percent. For the same nitrogen load reductions, forecasted dissolved 

oxygen concentrations for 1985 and 1988 conditions increase by amounts ranging from 

approximately 12 to 59 percent. In contrast, forecasted dissolved oxygen concentrations for 

1990 conditions increase by amounts ranging from approximately 28 to 148 percent. 

Average Dissolved Oxygen Responses 

Nitrogen Load Reductions 

Percent Load Reduction 

1985 1988 0 1990 

Figure 6.8. Forecasted responses of average dissolved oxygen concentrations to nitrogen load 

reductions from the M under 1985,1988 and 1990 summer-average conditions. 
Modeling 

Average Chlorophyll Responses 

Nitrogen Load Reductions 

Percent Load Reduction 

1985 1988 r. 1990 

Figure 6.9. Forecasted responses of average chlorophyll concentrations to nitrogen load 

reductions fkom the MAR under 1985, 1988 and 1990 summer-average conditions. 

Differences in responses of dissolved oxygen concentrations for 1990 conditions, as compared to 

1985 and 1988 conditions, are probably due to differences in water circulation patterns. Summer 

average conditions on the LIS are typically represented by the LCC which has a net westward 

drift along the shelf bathymetry. Typical summer average current speeds are approximately 10 

and 1 cmls, respectively, in the surface and bottom waters. During the summer of 1990, net 

eastward drift was observed in both surface and bottom waters at speeds of approximately 2 and 

0.8 cds. It is possible that much smaller flow magnitudes in 1990 affected the relative 

importance of physical transport versus chemical-biological processes for dissolved oxygen. 

Dissolved oxygen may have been affected more than chlorophyll because dissolved oxygen 

responses are more influenced by chemical-biological processes at the sediment-water boundary 

than are chlorophyll concentrations. This result is important because it indicates that estimates 

of water quality responses to changes in MAR nutrient loadings can be strongly influenced by 

hydrometeorological conditions and water circulation on the LIS. 

All of these forecast results represent ultimate steady-state responses. They provide no 

information on the time scales for potential water quality responses to MAR nutrient loading 

reductions. On the basis of statistical correlations, Justic et al. (1993) estimated that there are 

time lags of approximately one and two months, respectively, between changes in MAR inflows 

and responses of surface net primary productivity and bottom water dissolved oxygen 

concentrations on the LIS. Wiseman et al. (1997) observed good correlations between area of 

mid-summer hypoxia and mean discharge for the preceding 11 months of August through June. 
Modeling 119 

They further indicated that, particularly for years with large floods followed by mild winters, the 

organic carbon substrate laid down by a major river flood may fuel the onset of hypoxia for more 

than a single year. 

Average Dissolved Oxygen Responses 

Composite Results for 1985, 1988, 1990 

Percent Load Reduction 

N Reduction P Reduction 

Figure 6.10. Forecasted responses of average dissolved oxygen concentrations to nitrogen and 

phosphorus load reductions fiom the MAR composited for 1985, 1988 and 1990 

summer-average conditions. 

The ultimate response of bottom water dissolved oxygen concentrations depends on the relative 

importance of SOD versus water column processes in controlling bottom water oxygen depletion 

rates. Total SOD consists of aerobic processes in the surficial sediments and anaerobic processes 

in deeper sediment layers. While surface layer processes may respond to loading changes on 

seasonal to annual time scales, processes in deeper sediment layers may take many years to 

respond. For example, forecast results fiom a coupled water-sediment mass balance model for 

Lake Erie (DiToro et al. 1987) indicated that the SOD component of total oxygen depletion rates 

did not reach steady-state until 5- 10 years after changes in external nutrient loadings. Results 

fiom a similar coupled model for Chesapeake Bay (Cerco 1995b) showed that decade-long 

simulations were required to achieve a near-complete response to external loading reductions. 
120 Modeling 

An important management issue is the relative influence of nitrogen versus phosphorus loadings 

in controlling bottom water dissolved oxygen on the LIS. Model forecast results indicated a - 

general tendency for responses to be somewhat greater for nitrogen loading reductions than for 

phosphorus loading reductions (Figure 6.10). Turner and Rabalais (1991) suggested that nitrogen 

appears to be relatively more important than phosphorus in limiting primary productivity on the 

LIS. Lohrenz et al. (1997) found that primary productivity in shelf waters near the Mississippi 

Delta was significantly correlated with nitrate plus nitrite nitrogen concentrations and fluxes over 

a six year period from 1988 to 1994. Fahnenstiel et al. (1995) reported that soluble nitrogen 

concentrations explained over 50 percent of the variability in phytoplankton growth rates in the 

northern Gulf of Mexico. Although nitrogen appears to be relatively more controlling than 

phosphorus, the computed differences are not large and, considering the uncertainties in this 

screening-level modeling analysis, fWther investigation appears warranted. 

Summary 

Mathematical models can be useful tools for synthesizing environmental data and for providing 

better understanding of cause-effect mechanisms that lie behind experimental observations. They 

can also be usehl to forecast possible responses of environmental systems to different 

management scenarios. The NECOP program included development of a mathematical model in 

parallel with an observational program. The principal benefits fiom the model were a better 

understanding of processes controlling primary productivity and bottom water dissolved oxygen 

on the LIS, and an assessment of how these water quality responses might be influenced by 

changes in nutrient loadings from the MAR system. 

The NECOP model was calibrated to shelfwide monitoring data collected during mid-summer in 

1985, 1988 and 1990. The calibrated model was used to conduct diagnostic and sensitivity 

analyses of phytoplankton and dissolved oxygen dynamics, and forecast simulations to 

investigate whether water quality on the LIS might be responsive to changes in nutrient loadings 

from the MAR. The purpose of the forecast simulations was not to establish target nutrient 

loading objectives, but to determine the range of nutrient loading reductions that may need to be 

evaluated in future studies. 

Diagnostic results indicated that underwater light attenuation appears relatively more important 

than nutrient limitation in controlling rates of primary production on the LIS. Of course, primary 

production can be maintained only if there are adequate nutrient supply rates from land-based 

sources, remineralization or sediment-water fluxes. Zooplankton grazing (ingestion), 

phytoplankton respiration and DOC exudation appear to be the principal fate pathways for 

organic carbon from phytoplankton production. Photosynthesis could be an important source of 

dissolved oxygen to bottom waters in the region of the Atchafalaya River discharge and further 

west along the LIS. Oxidation of carbonaceous material in the water column, phytoplankton 

respiration and SOD all appear to contribute significantly to total oxygen depletion rates in 

bottom waters. Nitrification appears to be a relatively small component of water column 
Modeling 121 

depletion rates. The estimated contribution of SOD to total oxygen depletion rates on the LIS 

ranges from 22 to 30 percent. 

Dissolved oxygen concentrations were very sensitive to variations in underwater light extinction 

coefficients and saturation light intensities, relative to base calibration results. Dissolved oxygen 

concentrations were also sensitive to variations in both SOD and water column oxygen demand 

(CBOD decay rate). Responses to variations in SOD were greater than responses to variations in 

CBOD decay rate. Chlorophyll concentrations were sensitive to variations in underwater light 

extinction coeflicients, saturation light intensities and C:Chl ratios, relative to base calibration 

results. With respect to physical transport processes, dissolved oxygen concentrations were 

sensitive to variations in cross-shelf dispersive mixing and relatively insensitive to changes in 

vertical mixing. Average responses were approximately proportional to changes in cross-shelf 

mixing. Chlorophyll concentrations were relatively insensitive to variations in cross-shelf 

dispersive mixing and vertical mixing. 

Much of the primary productivity on the LIS appears to be maintained by nutrient recycling 

within the water column, thus amplifying the effect of MAR nutrient inputs. Results from 

sensitivity analyses in which water column nutrient remineralization rates were "turned off' 

indicated that average chlorophyll concentrations in surface offshore waters decreased by 64 

percent. 

Forecast results indicated that both dissolved oxygen and chlorophyll concentrations are 

responsive to reductions in nitrogen loadings from the MAR. There was more variability in 

responses among different years for dissolved oxygen than for chlorophyll. As nitrogen loads 

were reduced from 10 to 70 percent, forecasted chlorophyll concentrations for 1985, 1988 and 

1990 conditions were estimated to decrease by amounts ranging from approximately 1 to 7 1 

percent. For the same nitrogen loading reductions, forecasted dissolved oxygen concentrations 

for 1985 and 1988 conditions were estimated to increase by amounts ranging from approximately 

12 to 59 percent. In contrast, forecasted dissolved oxygen concentrations for 1990 conditions 

were estimated to increase by amounts ranging from 28 to 148 percent. It appears that water 

quality responses to changes in MAR nutrient loadings are strongly influenced by interannual 

differences in hydrometeorological conditions and water circulation on the LIS. 

Forecast results indicated a general tendency for responses to be somewhat greater for nitrogen 

loading reductions than for phosphorus loading reductions. Although nitrogen appears to be 

relatively more controlling than phosphorus, the estimated differences are not large and, 

considering the uncertainties in the modeling analysis, further investigation appears warranted. 

References 

Ambrose, R. B., Jr., T. A. Wool, and J. L. Martin. 1993. The Water Quality Analysis 

Simulation Program, WASPS. Part A: Model Documentation. U.S. Environmental Protection 
122 Modeling 

Agency, Office of Research and Development, Environmental Research Laboratory, Athens, 

Georgia. 

Bierman, V .J., Jr., S. C. Him, D. Zhu, W. J. Wiseman, Jr., N. N. Rabalais and R. E. Turner. 

1994a. A preliminary mass balance model of primary productivity and dissolved oxygen in 

the Mississippi River plumelinner Gulf shelf region. Estuaries 14(4):886-899. 

Bierman, V. J., Jr., S. C. Him, D. Zhu, W. J. Wiseman, Jr., N. N. Rabalais and R. E. Turner. 

1994b. Primary production and dissolved oxygen in the Mississippi River plumelinner Gulf 

shelf region: Components analysis and sensitivity to changes in physical transport. In: 

NECOP Synthesis Workshop, Baton Rouge, Louisiana, April 26-27, 1994. 

Cerco, C. F. 1995b. Response of Chesapeake Bay to nutrient load reductions. Journal of 

Environmental Engineering 12 1 (8):549-557. 

Cerco, C. F. 1995c. Simulation of long-term trends in Chesapeake Bay eutrophication. Journal 

of Environmental Engineering 12 1 (4):298-3 10. 

Cerco, C. F. and T. Cole. 1993. Three-dimensional eutrophication model of Chesapeake Bay. 

Journal of Environmental Engineering 1 19(6): 1006- 1025. 

Cerco, C. F. and T. Cole. 1995a. User's Guide to the CE-QUAL-ICM. Release Version 1.0. 

Technical Report EL-95-1, U.S. Army Corps of Engineers, Waterways Experiment Station, 

Vicksburg, Mississippi. 

Chau, K. W. and H. Jin. 1998. Eutrophication model for a coastal bay in Hong Kong. Journal of 

Environmental Engineering 124(7):628-63 8. 

Chen, C., D. A. Wiesenburg and L. Xie. 1997. Influences of river discharge on biological 

production in the inner shelf: A coupled biological and physical model of the Louisiana-Texas 

Shelf. Journal of Marine Research 55:293-320. 

Cochrane, J. D. and F. J. Kelly. 1986. "Low-frequency circulation on the Texas-Louisiana 

continental shelf." Journal of Geophysical Research 9 l(C9): 10,645-1 0,659. 

Dagg, M. J. and T. E. Whitledge. 1991. Concentration of copepod nauplii associated with the 

nutrient-rich plume of the Mississippi River. Continental Shelf Research 1 1 : 1409- 1423. 

Dinnel, S. P. and W. J. Wiseman, Jr. 1986. Freshwater on the Louisiana and Texas Shelf. 

Continental Shelf Research 6(6):765-784. 
Modeling 123 

DiToro, D. M., N. A. Thomas, C. E. Herdendorf, R. P. Winfield and J. P. Connolly. 1987. A 

post-audit of a Lake Erie eutrophication model. Journal of Great Lakes Research 13(4):801- - 

825. 

Dortch, Q. and T. E. Whitledge. 1992. Does nitrogen or silicon limit phytoplankton production 

in the Mississippi River plume and nearby regions? Continental Shelf Research 12(11): 1293- 

1309. 

Dortch, Q., A. Bode and R. R. Twilley. 1992. Nitrogen uptake and regeneration in surface 

waters of the Louisiana continental shelf influenced by the Mississippi River, pp. 52-56. In: 

Nutrient Enhanced Coastal Ocean Productivity. Publication No. TAMU-SG-92- 109, Sea 

Grant Program, Texas A&M University, Galveston, Texas. 

Fahnenstiel, G.L., M. J. McCormick, G. A. Lang, D. G. Redalje, S. E. Lohrenz, M. Markowitz, 

B. Wagoner, and H. J. Carrick. 1995. Taxon-specific growth and loss rates for dominant 

phytoplankton populations from the northern Gulf of Mexico. Marine Ecology Progress 

Series 1 17:229-239. 

Hamrick, J. M. 1996. A User's Manual for the Environmental Fluid Dynamics Computer Code 

(EFDC). The College of William and Mary, Virginia Institute of Marine Science. Special 

Report 33 1. Gloucester Point, Virginia. 

Hendee, J. C. 1994. Data management for the Nutrient Enhanced Coastal Ocean Productivity 

Program. Estuaries 17(4):900-903. 

Hinga, K. R., H. Jeon, and N. F. Lewis. 1995. Marine Eutrophication Review. Part 1: 

Quantifying the Effects of Nitrogen Enrichment on Phytoplankton in Coastal Ecosystems and 

Part 2: Bibliography with Abstracts. NOAA Coastal Ocean Program, Decision Analysis 

Series No. 4. U.S. Department of Commerce, NOAA, Coastal Ocean Office, Silver Spring, 

MD. 120 pp. 

Justic, D., N. N. Rabalais, R. E. Turner and W. J. Wiseman, Jr. 1993. Seasonal coupling between 

riverborne nutrients, net productivity and hypoxia. Marine Pollution Bulletin 26(4): 18401 89. 

Kemp, W. M., P. A. Sampou, J. Garber, J. Tuttle, and W. R. Boynton. 1992. Seasonal 

depletion of oxygen from bottom waters of Chesapeake Bay: Roles of benthic and planktonic 

respiration and physical exchange processes. Marine Ecology Progress Series 85 : 137- 152. 

Limno-Tech, Inc. 1995. Estimated Responses of Water Quality on the Louisiana Inner Shelf to 

Nutrient Load Reductions in the Mississippi and Atchafalaya Rivers. US. Environmental 

Protection Agency, Gulf of Mexico Program, Stennis Space Center, Mississippi. 48 pp. plus 

Appendices. 
124 Modeling 

Lohrenz, S. E., M. J. Dagg, and T. E. Whitledge. 1990. Enhanced primary production at the 

plurne/oceanic interface of the Mississippi River. Continental Shelf Research. 10(7):639-664. 

Lohrenz, S. E., G. L. Fahnenstiel, and D. G. Redalje. 1994. Spatial and temporal variations of 

photosynthetic parameters in relation to environmental conditions in coastal waters of the 

northern Gulf of Mexico. Estuaries 17(4):779-795. 

Lohrenz, S. E., G. L. Fahnenstiel, D. G. Redalje, G. A. Lang, X. Chen, and M. J. Dagg. 1997. 

Variations in primary production of northern Gulf of Mexico continental shelf waters linked 

to nutrient inputs from the Mississippi River. Marine Ecology Progress Series 155:45-54. 

Manickchand-Heileman, S., L. A. Soto, and E. Escobar. 1998. A preliminary trophic model of 

the continental shelf, south-western Gulf of Mexico. Estuarine, Coastal and Shelf Science 

46:885-899. 

Officer, C. B. and J. H. Ryther. 1980. The possible importance of silicon in marine 

eutrophication. Marine Ecology Progress Series 3 $3-9 1. 

Rabalais, N. N., R. E. Turner, D. Justic, Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

1996. Nutrient changes in the Mississippi River and system responses on the adjacent 

continental shelf. Estuaries l9(2B):3 86-407. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman, Jr., and Q. Dortch. 1998. Consequences of the 

1993 Mississippi River Flood in the Gulf of Mexico. Regulated Rivers: Research and 

Management 14:161-177. 

Roelke, D. L., L. A. Cifuentes, and P. M. Eldridge. 1997. Nutrient and phytoplankton dynamics 

in a sewage-impacted Gulf Coast estuary: A field test of the PEG-Model and Equilibrium 

Resource Competition theory. Estuaries 20(4):725-742. 

Thomann, R. V. and J. A. Mueller. 1987. Principles of Surface Water Quality Modeling and 

Control. Harper and Row Publishers, New York. 

Turner, R. E. and N. N. Rabalais. 1991. Changes in Mississippi River Water quality this 

century - Implications for coastal food webs. BioScience 4l(3): 140-147. 

U.S. Army Corps of Engineers. 1984. Mississippi River, Baton Rouge to the Gulf, Louisiana, 

Project. Final Environmental Impact Statement, Supplement 11, Appendix E, Water Quality. 

U.S. Army Corps of Engineers, New Orleans District. 

US. Environmental Protection Agency. 1990. Technical Guidance Manual for Performing 

Waste Load Allocations, Book 111: Estuaries, Part I - Estuaries and Waste Load Allocation 

Models. U.S. Environmental Protection Agency, Office of Water, Washington, D.C. 
Modeling 125 

Vasconcellos, M., S. Mackinson, K. Sloman, and D. Pauly. 1997. The stability of trophic massbalance models of marine ecosystems: a comparative analysis. Ecological Modelling 100: 125- 

134. 

Walker, N. D. 1996. Satellite assessment of Mississippi River plume variability: causes and 

predictability. Remote Sensing of the Environment 5 8:2 1-3 5. 

Wiseman, W. J., Jr., N. N. Rabalais, R. E. Turner, S. P. Dinnel, and A. MacNaughton. 1997. 

Seasonal and interannual variability within the Louisiana Coastal Current: Stratification and 

Hypoxia. Journal of Marine Systems 12:237-248. 

Wiseman, W. J., Jr. and F. J. Kelly. 1994. Salinity variability within the Louisiana Coastal 

Current during the 1982 flood season. Estuaries 17(4):732-739. 

Wyatt, T. 1998. Harmful algae, marine blooms, and simple population models. Nature & 

Resources 34(3):40-5 1. 
Appendix A 

NECOP Scientists and Manaaers 

Principal Investigators (1 990-1 996) 

James Ammemzan, Texas A&M University, Department of Oceanography 

Donald K. Atwood, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory 

Ronald Benner, University of Texas at Austin, Marine Science Institute 

Victor J. Bierman, Jr., Lirnno-Tech, Inc. 

Patricia L. Blackwelder, University of Miami, Rosenstiel School of Marine and 

Atmospheric Sciences 

William C. Boicourt, University of Maryland, Horn Point Environmental Laboratory 

Alan Bratkovich, National Oceanic and Atmospheric Administration, Great Lakes 

Environmental Research Laboratory 

Luis A. Cifuentes, Texas A&M University at Galveston, Department of Oceanography 

Richard B. Cofin, U.S. Environmental Protection Agency, Environmental Effects 

Research Laboratory 

James B. Cotner, University at Minnesota, Department of Ecology, Evolution and 

Behavior 

Michael J. Dagg, Louisiana Universities Marine Consortium 

Scott P. Dinnel, University of Southern Mississippi, Center for Marine Science 

Quay Dortch, Louisiana Universities Marine Consortium 

Brian J. Eadie, National Oceanic and Atmospheric Administration, Great Lakes 

Environmental Research Laboratory 

Gary L. Fahnenstiel, National Oceanic and Atmospheric Administration, Great Lakes 

Environmental Research Laboratory 

Wayne S. Gardner, University of Texas at Austin, Marine Science Institute 
128 Appendix A, ScientistsIManagers 

William F. Graham, National Oceanic and Atmospheric Administration, National Sea Grant 

College Program 

Churchill B. Grimes, National Oceanic and Atmospheric Administration, Panama City 

Laboratory 

Donald E. Harper, Jr., Texas A&M University at Galveston, Marine Laboratory 

Nathan Hawley, National Oceanic and Atmospheric Administration, Great Lakes 

Environmental Research Laboratory 

James C. Hendee, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory 

Gary L. Hitchcock, University of Miami, Rosenstiel School of Marine and Atmospheric 

Sciences 

Feenan D. Jennings, Texas A&M University at Galveston, Texas Sea Grant Program 

Burton H. Jones, University of Southern California, Hancock Institute for Marine Studies 

Dubravko Justic', Louisiana State University, Coastal Ecology Institute 

Frances J. Kelly, Jr., Texas A&M University, Department of Oceanography 

George A. Knauer, University of Southern Mississippi, Institute of Marine Sciences 

Thomas D. Leming, National Oceanic and Atmospheric Administration, Mississippi 

Laboratories 

Steven E. Lohrenz, University of Southern Mississippi, Institute of Marine Sciences 

Michael McCormick, National Oceanic and Atmospheric Administration, Great Lakes 

Environmental Research Laboratory 

Brent A. McKee, Tulane University, Department of Geology 

Simone Metz, Florida Institute of Technology, Department of Oceanography 

Richard L. Miller, National Aeronautics and Space Administration, John C. Stennis Space 

Center 

Tina Miller- Way, Marine Environmental Sciences Consortium, Dauphin Island Sea 

Laboratory 

Harold 0. Mofield,: National Oceanic and Atmospheric Administration, Pacific Marine 

Environmental Research Laboratory 

John Morse, Texas A&M University, Department of Oceanography 
Appendix A, ScientistslManagers 

Terry A. Nelsen, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory 

David Nelson, Oregon State University, College of Oceanography 

Peter B. Ortner, National Oceanic and Atmospheric Administration, Atlantic Oceanographic 

and Meteorological Laboratory 

Nancy N. Rabalais, Louisiana universities Marine Consortium 

Donald G. Redalje, University of Southern Mississippi, Center for Marine Science 

John A. Robbins, National Oceanic and Atmospheric Administration, Great Lakes 

Environmental Research Laboratory 

Gilbert T. Rowe, Texas A&M University, Department of Oceanography 

John H. Trefry, Florida Institute of Technology, Department of Oceanography 

R. Eugene Turner, Louisiana State University, Coastal Ecology Institute 

Robert Twilley, University of Southwestern Louisiana, Department of Biology 

Andrew C. Vastano, Texas A&M University, Department of Oceanography 

Terry E. Whitledge, University of Texas at Austin, Marine Science Institute 

William J. Wiseman, Jr., Louisiana State University, Coastal Studies Institute 

Program Management Committee 

Donald K. Atwood, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory, 1990- 1994 

Thomas Bright, Texas A&M University, Texas Sea Grant College Program, 1990-1993 

William F. Graham, National Oceanic and Atmospheric Administration, National Sea Grant 

College Program, 1990- 1996 

Terry A. Nelsen, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory, 1995-1996 

William J. Wiseman, Jr. Louisiana State University, Coastal Studies Institute, 1994-1996 

Executive Director 

LT Michael Gallagher, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory, 1993- 1995 
130 Appendix A, ScientistdManagers 

LCDR John Novaro, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory, 1990- 1993 

LT Victor Ross, National Oceanic and Atmospheric Administration, Atlantic Oceanographic 

and Meteorological Laboratory, 1995-1996 

Data Mananer 

James C. Hendee, National Oceanic and Atmospheric Administration, Atlantic 

Oceanographic and Meteorological Laboratory, 1990- 1996 

Program Coordinator 

Michael J. Dowgiallo, National Oceanic and Atmospheric Administration, Coastal Ocean 

Program, 1990- 1995 

John L. Wickham, National Oceanic and Atmospheric Administration, Coastal Ocean 

Program, 1995- 1996 
Appendix B 

NECOP Proiects 

Proiect Title and Principal Investinators (1 990-1996) 

Benthic Oxygen Demand and Nutrient Regeneration in Sediments 

Gilbert T. Rowe, Texas A&M University at Galveston 

Buoyancy and Nutrient Exchange in the Mississippi River Ouflow Region 

Alan Bratkovich, National Oceanic and Atmospheric Administration 

Scott P. Dinnel, University of Southern Mississippi 

Burton H. Jones, University of Southern California 

Terry E. Whitledge, University of Texas at Austin 

Environmental Control of Primary Production and Fate of Organic Matter in Ou2Jtow Areas 

of the MississippiLAtchafalaya Rivers and Adjacent Shelf Waters 

Scott P. Dinnel, University of Southern Mississippi 

Gary L. Fahnenstiel, National Oceanic and Atmospheric Administration 

Steven E. Lohrenz, University of Southern Mississippi 

Michael McCormick, National Oceanic and Atmospheric Administration 

Donald G. Redalje, University of Southern Mississippi 

Eutrophication Model of the Louisiana Shelf 

Victor J. Bierman, Jr., Limno-Tech, Inc. 

Dubravko Justic', Louisiana State University 

William J. Wiseman, Jr., Louisiana State University 

Fate of Nutrient Enhanced Coastal Primary Production on the Lower Trophic Levels ofthe 

Marine Food Web Around the Mississippi Discharge 

Churchill B. Grimes, National Oceanic and Atmospheric Administration 

Fates and EJj'ects of Riverine and Shelf-Derived Dissolved Organic Matter on Mississippi 

River Plume/Gulf Shelf Processes 

Ronald Benner, University of Texas at Austin 

James B. Cotner, University of Minnesota 

Brian J. Eadie, National Oceanic and Atmospheric Administration 

Wayne S. Gardner, University of Texas at Austin 
132 Appendix 6, Projects 

Hypoxia Hydrographic and Biological Surveys 

Nancy N. Rabalais, Louisiana Universities Marine Consortium 

R. Eugene Turner, Louisiana State University 

William J. Wiseman, Jr., Louisiana State University 

Hypoxia Monitoring and Carbon Process Studies 

Nancy N. Rabalais, Louisiana Universities Marine Consortium 

R. Eugene Turner, Louisiana State University 

William J. Wiseman, Jr., Louisiana State University 

Hypoxia on the Louisiana Inner Shelf: A Modeling Study 

Victor J. Bierman, Jr., Limno-Tech, Inc. 

William J. Wiseman, Jr., Louisiana State University 

Impacts of Hypoxia on Benthic Populations and Fisheries Resources 

Donald E. Harper, Jr., Texas A&M University at Galveston 

Nancy N. Rabalais, Louisiana Universities Marine Consortium 

Investigation of Benthic-Pelagic Coupling 

Brent A. McKee, Tulane University 

Lagrangian Study of the Louisiana Coastal Current 

William C. Boicourt, University of Maryland 

Gary L. Hitchcock, University of Miami 

Terry A. Nelsen, National Oceanic and Atmospheric Administration 

William J. Wiseman, Jr., Louisiana State University 

Large Scale Synoptic Sampling of Sur$ace Chlorophyll and Suspended Sediments 

Michael J. Dagg, Louisiana Universities Marine Consortium 

Thomas D. Leming, National Oceanic and Atmospheric Administration 

Brent A. McKee, Tulane University 

Richard L. Miller, National Aeronautics and Space Administration 

NECOP Data Management 

Donald K. Atwood, National Oceanic and Atmospheric Administration 

James C. Hendee, National Oceanic and Atmospheric Administration 

NECOP Project Management at NOAA Atlantic Oceanographic and Meteorological Lab 

Donald K. Atwood, National Oceanic and Atmospheric Administration 

Terry A. Nelson, National Oceanic and Atmospheric Administration 
Appendix B, Projects 133 

NECOP Project Management at Sea Grant, Publications and Technical Advisory 

Committee Support 

William F. Graham, National Oceanic and Atmospheric Administration 

NECOP Workshop Proceedings, October 1991 

Feenan D. Jennings, Texas A&M University at Galveston 

Nutrient Analysis Support on RN Pelican to LUMCON 

Donald K. Atwood, National Oceanic and Atmospheric Administration 

Nutrient Enhanced Coastal Ocean Productivity (Phase I): The Role of Phosphate 

James Ammerman, Texas A&M University 

Nutrient Enhanced Coastal Ocean Productivity (Phase 2): Benthic Metabolism, Oxygen 

Demand, and Sulfate Reduction 

John Morse, Texas A&M University 

Gilbert T. Rowe, Texas A&M University 

Nutrient Enhanced Coastal Ocean Productivity (Phase 2): The Role of Dissolved Organic 

Nitrogen 

Luis A. Cifuentes, Texas A&M University at Galveston 

Richard B. Coffin, U.S. Environmental Protection Agency 

Nutrient Enhanced Coastal Ocean Productivity (Phase 2): The Role of Phosphate 

James Amrnerman, Texas A&M University at Galveston 

Organic Matter Decomposition, Nitrogen Recycling, and Oxygen Depletion in the 

Mississippi River/Gulf Shelf Region 

Ronald Benner, University of Texas at Austin 

James B. Cotner, University of Minnesota 

Brian J. Eadie, National Oceanic and Atmospheric Administration 

Wayne S. Gardner, University of Texas at Austin 

Phytoplankton Size and Species Composition in Relation to Hypoxia and Carbon Flux on 

the Louisiana Continental Shelf 

Quay Dortch, Louisiana Universities Marine Consortium 

David Nelson, Oregon State University 
134 Appendix B, Projects 

Primary Production and Vertical Flux of Organic Carbonfiom the Photic Zone in the 

Mississippi River Plume 

Gary L. Fahnenstiel, National Oceanic and Atmospheric Administration 

George A. Knauer, University of Southern Mississippi 

Steven E. Lohrenz, University of Southern Mississippi 

Donald G. Redalje, University of Southern Mississippi 

Retrospective Analysis of Nutrient Enhanced Coastal Ocean Productivity via Mississippi 

Delta Sediments: The Time History of Hypoxia Indicators and Carbon Burial on the 

Louisiana Continental Shelf and Slope 

Patricia L. Blackwelder, University of Miami 

Brian J. Eadie, National Oceanic and Atmospheric Administration 

Brent A. McKee, Tulane University 

Simone Metz, Florida Institute of Technology 

Terry A. Nelsen, National Oceanic and Atmospheric Adrninistration 

John A. Robbins, National Oceanic and Atmospheric Administration 

John H. Trefry, Florida Institute of Technology 

Satellite Estimation of &$ace Flow Fields 

Frances J. Kelly, Jr., Texas A&M University at College Station 

Andrew C. Vastano, Texas A&M University at College Station 

Silicon Cycling and Limitation in Relation to Hypoxia and Carbon Flux on the Louisiana 

Shelf System 

David Nelson, Oregon State University 

Spatial and Temporal Scales of Nutrient and Pigment Variability 

Gary L. Hitchcock, National Oceanic and Atmospheric Administration 

Terry E. Whitledge, University of Texas at Austin 

Sediment Processes to Investigate the Fate of Organic Matter in the Louisiana Shelf System 

Brent A. McKee, Tulane University 

Tina Miller-Way, Marine Environmental Sciences Consortium 

Robert Twilley, University of Southwestern Louisiana 

Suspended Sediment on the Louisiana Continental She@ Concentrations, Composition and 

Transport Pathways 

Nathan Hawley, National Oceanic and Atmospheric Administration 

Terry A. Nelsen, National Oceanic and Atmospheric Administration 

John H. Trefry, Florida Institute of Technology 
Appendix €3, Projects 135 

Synoptic Investigations of Benthic-Pelagic Coupling: The Production and Fate of OrganicMatter Within the Louisiana Shelf System 

Brent A. McKee, Tulane University 

Robert Twilley, University of Southwestern Louisiana 

Synthesis of Our Understanding of the Louisiana Inner She8 Results from NECOP 

Michael J. Dagg, Louisiana Universities Marine Consortium 

Terry E. Whitledge, University of Texas at Austin 

William J. Wiseman, Jr., Louisiana State University 

Time Dependent Model of Productivity and Hypoxia in the Mississippi Delta 

Harold 0. Mofjeld, National Oceanic and Atmospheric Administration 

Zooplankton Grazing and the Fate of Fixed Carbon in Louisiana Coastal Waters 

Michael J. Dagg, Louisiana Universities Marine Consortium 

Peter B. Ortner, National Oceanic and Atmospheric Administration 
Appendix C 

NECOP Publications 

Ackerman, A. 1995. An optimized flow field determination for the Louisiana inner shelf using 

conservation of water mass and salinity and its application to a simple dissolved oxygen model. 

Masters thesis, Department of Oceanography & Coastal Sciences, Louisiana State University. 

Baton Rouge, Louisiana. 

Ackerman, D. C., W. J. Wiseman, Jr., V. J. Bierman, Jr., N. N. Rabalais, and R. E. Turner. 

1997. An optimization method for determining flow fields in multidimensional water quality 

models applied to the Louisiana shelf, pp. 527-536. In: Water Environment Federation 70th 

Annual Conference and Exposition, Volume 4 - Parts I and II: Surface Water Quality and 

Ecology, Paper 9759003, Alexandria, Virginia. 

Ammerman, J. W. 1991. Rapid phosphate cycling in the Mississippi River plume region. 

Second scientific meeting of The Oceanography Society, St. Petersburg, Florida. 1991. 

(abstract) 

Ammerman, J. W. 199 1. Rapid phosphate utilization. Annual meeting of the American 

Society for Microbiology, Dallas, Texas. 199 1. (abstract) 

Ammerman, J. W. 1992. Seasonal variation in the phosphate turnover in Mississippi River plume 

and the inner Gulf shelf: Rapid summer turnover, pp. 69-75. In: Nutrient Enhanced Coastal 

Ocean Productivity, NECOP Workshop Proceedings, October 1991, Texas A&M University 

Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Ammerman, J. W. 1994. Enzymology on the run: Continuous underway measurement of 

microbial enzyme activity in aquatic environments. NATO Advanced Study Institute, 

Molecular Ecology of Aquatic Microbes, 1994, Italy. (abstract) 

Ammerrnan, J. W. and W. Glover. 1997. Increased nitrogen loading in the Mississippi River 

may have enhanced phosphorus deficiency in the Gulf of Mexico. Contributed talk presented 

at the American Society of Limnology and Oceanography Aquatic Sciences Meeting, 

February, 1997, Santa Fe, New Mexico. (abstract) 

Ammerman, J. W, W. B. Glover, R. H. Sada Ruvalcaba, and M. J. D. McRae. 1995. 

Continuous underway measurement of microbial enzyme activities in surface waters of the 

Mississippi River plume and the Louisiana shelf, pp. 1-8. In: Nutrient-Enhanced Coastal 

Ocean Productivity, Proceedings of the April 1994 Synthesis Workshop, Baton Rouge, La. 

Louisiana Sea Grant College Program, Louisiana State University, Baton Rouge, Louisiana. 

Arnon, R. M. W. and Ronald Benner. 1994. Rapid cycling of high-molecular-weight dissolved 

organic matter in the ocean. Nature 369:549-552. 
138 Appendix C, Publications 

Amon, R. M. W. and R. Benner. 1996. Bacterial utilization of different size classes of dissolved 

organic matter. Limnology and Oceanography 41 :4 1-5 1. 

Amon, R. M. W. and R. Benner. 1998. Seasonal patterns of bacterial abundance and production 

in the Mississippi River plume and their importance for the fate of enhanced primary 

production. Microbial Ecology 35:289-300. 

Arwood, D. 1993. The influence of environmental variables on portioning of the end products of 

phytoplankton photosynthesis in the Mississippi River plume. M.S. Thesis; Donald Redalje, 

advisor. University of Southern Mississippi, Stennis Space Center, Mississippi. 58 pp. 

Atwood, D. K., A. Bratkovich, M. Gallagher, and G. L. Hitchcock (organizers). 1994. Papers 

from NOAA's Nutrient Enhanced Coastal Ocean Productivity Study. Dedicated issue of 

Estuaries. Estuaries 17(4):729-911. 

Atwood, D. K., A. Bratkovich, M. Gallagher, and G. L. Hitchcock. 1994. Introduction to the 

dedicated issue. Estuaries 17(4):729-73 1. 

Atwood, D. K., W. F. Graham, and C. B. Grimes (organizers). 1995. Nutrient Enhanced 

Coastal Ocean Productivity. Proceedings of a 1994 synthesis workshop, Louisiana State 

University. Louisiana Sea Grant College Program, Louisiana State University, Baton Rouge, 

La. 119 pp. 

Benner, R. 1991. Ultra-filtration for concentration of bacteria, viruses, and dissolved organic 

matter. Marine Particles: Analysis and Characterization, Geophysical Monograph 63, 

American Geophysical Union, pp. 18 1 - 185. 

Benner, R., G. Chin-Leo, W. Gardener, B. Eadie, and J. Cotner. 1992. The fates and effects of 

riverine and shelf-derived DOM on Mississippi River plumeIGulf shelf processes, pp. 84-94. 

In: Nutrient Enhanced Coastal Ocean Productivity, NECOP Workshop Proceedings, October 

1991, Texas A&M University Sea Grant Program TAMU-SG-92-109, Galveston, Texas. 

Benner, R., J. D. Pakulski, M. McCarthy, J. I. Hedges, and P.G. Hatcher. 1992. Bulk chemical 

characteristics of dissolved organic matter in the ocean. Science 225: 1561-1564. 

Benner R. and M. Strom. 1993. A critical evaluation of the analytical blank associated with DOC 

measurements by high temperature catalytic oxidation. Marine Chemistry 41(1-3):153-160. 

Bierman, V. J., Jr. 1997. Estimated responses of water quality on the Louisiana inner shelf to 

nutrient load reduction in the Mississippi and Atchafalaya Rivers, pp. 114-124. In: 

Proceedings of the First Gulf of Mexico Hypoxia Management Conference, December 1995, 

New Orleans, Louisiana. Publ. No. EPA-55-R-97-001, U.S. Environmental Protection 

Agency, Gulf of Mexico Program Office, Stennis Space Center, Mississippi. 

Bierrnan, V. J., Jr., S. C. Hinz, W. J. Wiseman, Jr., N. N. Rabalais, and R. E. Turner. 1992. 

Mass balance modeling of hypoxia and associated water quality parameters in the Mississippi 

River plumelinner Gulf shelf region, pp. 237-248. In: Proceedings: Water Environment 

Federation 65th Annual Conference & Exposition, New Orleans, Louisiana, September 20-24, 

1992, Water Environment Federation, Alexandria, Virginia. 
Appendix C, Publications 139 

Bierman, V. J., Jr., S. C. Hinz, W. J. Wiseman Jr., N. N. Rabalais, and R. E. Turner. 1992. 

Mass balance modeling of water quality constituents in the Mississippi River, pp. 27-36. In: 

Nutrient Enhanced Coastal Ocean Productivity, NECOP Workshop Proceedings, October 

1991, Texas A&M University Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Bierman, V. J., Jr., S. C. Hinz, W. J. Wiseman, Jr., N. N. Rabalais, and R. E. Turner. 1994. 

Mass balance modeling of the impacts of nutrient load reductions in the Mississippi River on 

water quality in the northern Gulf of Mexico, pp. 413-424. In: WEFTEC '94. Proceedings of 

the Water Environment Federation 67th Annual Conference and Exposition, Volume IV: 

Surface Water Quality and Ecology, October 15-19, 1994, Chicago, Illinois, Water 

Environment Federation, Alexandria, Virginia. 

Bierman, V. J., Jr., S. C. Hinz, D.-W. Zhu, W. J. Wiseman, Jr., N. N. Rabalais, and R. E. 

Turner. 1994. A preliminary mass balance model of primary productivity and dissolved 

oxygen in the Mississippi River Plumehner Gulf shelf region. Estuaries 17(4):886-899. 

Bierman, V. J., Jr., S. C. Hinz, D.-W. Zhu, W. J. Wiseman, Jr., N. N. Rabalais, and R. E. 

Turner. 1995. Primary production and dissolved oxygen in the Mississippi River Plumelinner 

Gulf shelf region: Components analysis and sensitivity to changes in physical transport, pp. 9- 

20. In: Nutrient-Enhanced Coastal Ocean Productivity, Proceedings of the April 1994 

Synthesis Workshop, Baton Rouge, La. Louisiana Sea Grant College Program, Baton Rouge, 

Louisiana. 

Blackwelder, P., T. Hood, C. Alvarez-Zarikian, T. A: Nelsen, and B. McKee. 1996. Benthic 

foraminifera from the NECOP study area impacted by the Mississippi River plume and 

seasonal hypoxia. Quaternary International 3 1 : 19-36. 

Bode, A. and Q. Dortch. 1996. Uptake and regeneration of inorganic nitrogen in coastal waters 

influenced by the Mississippi River: Spatial and seasonal variations. Journal of Plankton 

Research l8:225 1-2268. 

Bratkovich, A. and S. P. Dinnel. 1992. Lower Mississippi River historical influx nitrate flux and 

Mississippi River outflow buoyancy flux, pp. 37-42. In: Nutrient Enhanced Coastal Ocean 

Productivity, NECOP Workshop Proceedings, October 1991, Texas A&M University Sea 

Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Bratkovich, A, S. P. Dinnel, and D. A. Goolsby. 1994. Variability and prediction of freshwater 

and nitrate fluxes for the Louisiana-Texas shelf: Mississippi and Atchafalaya River source 

functions. Estuaries 17(4):766-778. 

Chen, B. 1993. The effects of light and nutrient conditions on primary production and chemical 

composition for high density phytoplankton cultures. M. S. Thesis; Donald Redalje, advisor. 

University of Southern Mississippi, Stennis Space Center, Mississippi. 

Chmura, G. L., A. Smirnov, and I. D. Campbell. 1999. Pollen transport through distributaries 

and depositional patterns in coastal waters. Palaeogeography, Palaeoclimatology, 

Palaeoecology 149 (1999): 257-270. 
140 Appendix C, Publications 

Chin-Leo, G. and R. Benner. 1992. Enhanced bacterioplankton production and respiration at 

intermediate salinities in the Mississippi River plume. Marine Ecology Progress Series 87(1- 

2):87-103. 

Cotner, J. B., Jr. and W. S. Gardner. 1993. Heterotrophic bacterial mediation of ammonium and 

dissolved free amino acid fluxes in the Mississippi River plume. Marine Ecology Progress 

Series 93:75-87. 

Cruz-Kaegi, M. 1992. Microbial abundance and biomass in sediments of the Texas-Louisiana 

shelf. Masters thesis, Texas A&M University, College Station, Texas. 

Cruz-Kaegi, M. E. and G. T. Rowe. 1992. Benthic biomass gradients on the Texas-Louisiana 

Shelf, pp. 145-149. In: Nutrient Enhanced Coastal Ocean Productivity, NECOP Workshop 

Proceedings, October 1991, Texas A&M University Sea Grant Program, TAMU-SG-92-109, 

Galveston, Texas. 

Dagg, M. J. 1995. Copepod grazing and the fate of phytoplankton in the northern Gulf of 

Mexico. Continental Shelf Research 15: 1303- 13 17. 

Dagg, M. J. , E. P. Green, B. A. McKee, and P. B. Ortner. 1996. Biological removal of fine 

grain lithogenic particles from a large river plume. Journal of Marine Research 54: 149-160. 

Dagg, M., C. Grimes, S. Lohrenz, B. McKee, R. Twilley, and W. Wiseman, Jr. 1991. 

Continental shelf food chains of the northern Gulf of Mexico, pp. 67- 106. In: Sherman, K., 

L. M. Alexander and B. D. Gold (eds.). Food chains, yields, models and management of 

large marine ecosystems. Westview Press, Boulder, Colorado. 

Dagg, M. J. and P. B. Ortner. 1992. Mesozooplankton grazing and the fate of carbon in the 

northern Gulf of Mexico, pp. 117-121. In: Nutrient Enhanced Coastal Ocean Productivity, 

NECOP Workshop Proceedings, October 1991, Texas A&M University Sea Grant Program, 

TAMU-SG-92- 109, Galveston, Texas. 

Dagg, M. J. and P. B. Ortner. 1995. Zooplankton grazing and the fate of phytoplankton in the 

northern Gulf of Mexico, pp. 21-27. In: Nutrient-Enhanced Coastal Ocean Productivity, 

Proceedings of the April 1994 Synthesis Workshop, Baton Rouge, La. Louisiana Sea Grant 

College Program, Baton Rouge, Louisiana. 

Dagg, M. J. and T. E. Whitledge. 1991. Concentrations of copepod nauplii in the nutrient-rich 

plume of the Mississippi River. Continental Shelf Research 11: 1409-1423. 

Dinnel, S. and A. Bratkovich. 1990. Statistical characterization of riverine fluxes of mass, 

buoyancy, and nutrients for the Mississippi River out flow region. Eos, Transactions, 

American Geophysical Union 7 l(43): 1407 (October 23). (abstract) 

Dinnel, S. and A. Bratkovich. 1991. Freshwater fill-times and seasonal distribution of 

hydrographic data in the Mississippi River outflow region. Eos, Transactions, Ametican 

Geophysical Union 72(5 l):9O (December 17). (abstract) 

Dinnel, S. P. and A. Bratkovich. 1993. The relationship of nitrate flux to water discharge and 

nitrate concentration in the Mississippi River. Journal of Marine Systems 4:315-326. 
Appendix C, Publications 141 

Dinnel, S. P., T. Whitledge, A. Bratkovich, and B. H. Jones. 1995. Buoyancy and nutrient 

exchange in the Mississippi River outflow region, pp. 28-33. In: Nutrient-Enhanced Coastal 

Ocean Productivity, Proceedings of the April 1994 Synthesis Workshop, Baton Rouge, La. 

Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 

Dong, D. Y., A. W. Bratkovich, and S. P. Dinnel. 1993. Nutrient enhanced coastal ocean 

productivity (NECOP): CTD observations from RN Longhorn cruise May 14-21, 1992. 

National Oceanic and Atmospheric Administration, Great Lakes Environmental Research 

Laboratory, Ann Arbor, Mich. NOAA Technical Memorandum ERL GLERL-80. 109 pp. 

Dong, D. and T. Whitledge. 1992. Nutrient Enhanced Coastal Ocean Productivity: CTD 

Observations for the RN Longhorn Cruise May 4-1 1, 1992. University of Texas, Marine 

Science Institute Report, Port Aransas, Texas. 150 pp. 

Dortch, Q. 1994. Changes in phytoplankton numbers and species composition, pp. 46-49. In: 

M. J. Dowgiallo (ed.), Coastal Oceanographic Effects of Summer 1993 Mississippi River 

Flooding, Special NOAA Report, National Oceanic and Atmospheric Administration, Coastal 

Ocean Office/National Weather Service, Silver Spring, Maryland. 

Dortch, Q. 1994. Phytoplankton survey, pp. 124-142. In: S. P. Murray and J. Donley (eds.), 

Mississippi River Plume Hydrography: Annual Report. OCS Study/MMS 94-0028, U. S. 

Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Regional Office, 

New Orleans, Louisiana. 

Dortch, Q., A. Bode and R. R. Twilley. 1992. Nitrogen uptake and regeneration in surface 

waters of the Louisiana continental shelf influenced by the Mississippi River, pp. 52-56. In: 

Proceedings, Nutrient Enhanced Coastal Ocean Productivity Workshop. Publ. No. TAMUSG-92-109, Texas Sea Grant College Program, Texas A&M University, College Station, 

Texas. 

Dortch, Q., D. Milsted, N. N. Rabalais, S. E. Lorenz, D. G. Redalje, M. J. Dagg, R. E. Turner, 

and T. E. Whitledge. 1992. Role of silicate availability in phytoplankton species composition 

and the fate of carbon, pp. 76-83. In: Nutrient Enhanced Coastal Ocean Productivity, NECOP' 

Workshop Proceedings, October 1991, Texas A&M University Sea Grant Program, TAMUSG-92-109, Galveston, Texas. 

Dortch, Q., D. M. Nelson, R. E. Turner, and N. N. Rabalais. 1995. Silicate limitation on the 

Louisiana continental shelf, pp. 34-39. In: Nutrient-Enhanced Coastal Ocean Productivity, 

Proceedings of the April 1994 Synthesis Workshop, Baton Rouge, La., Louisiana Sea Grant 

College Program, Baton Rouge, Louisiana. 

Dortch, Q., M. L. Parsons, N. N. Rabalais, and R. E. Turner. 1999. What is the threat of 

harmful algal blooms in Louisiana coastal waters? Pages 134-144 in L. P. Rozas, J. A. 

Nyman, C. E. Proffitt, N. N. Rabalais, D. J. Reed, and R. E. Turner (eds.), Recent Research 

in Coastal Louisiana: Natural System Function and Response to Human Influences, Louisiana 

Sea Grant College Program, Baton Rouge, Louisiana, 304 pp. 
142 Appendix C, Publications 

Dortch, Q., C. Pham, N. N. Rabalais, and R. E:Turner. 1992. Respiration rates in bottom 

waters of the Louisiana Shelf, pp. 140-144. In: Nutrient Enhanced Coastal Ocean 

Productivity, NECOP Workshop Proceedings, October 1991, Texas A&M University Sea 

Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Dortch, Q., N. N. Rabalais, R. E. Turner, and N. A. Qureshi. accepted. Impacts of changing 

SiIN ratios and phytoplankton species composition. In: Rabalais, R. R. and R. E. Turner 

(eds.). The Effects of Hypoxia on Living Resources, with Emphasis on the Northern Gulf of 

Mexico, American Geophysical Union, Coastal and Estuarine Studies, Washington, D.C. 

Dortch, Q., N. N. Rabalais, R. E. Turner, and G. T. Rowe. 1994. Respiration rates and hypoxia 

on the Louisiana Shelf. Estuaries 17(4):862-872. 

Dortch, Q., R. Robichaux, S. Pool, D. Milsted, G. Mire, N. N. Rabalais, T. M. Soniat, G. A. 

Fryxell, R. E. Turner, and M. L. Parsons. 1997. Abundance and vertical flux of Pseudonitzschia in the northern Gulf of Mexico. Marine Ecology Progress Series 146(1-3):249-264. 

Dortch, Q. and T. E. Whitledge. 1992. Nitrogen or silicon limitation in the Mississippi River 

plume and nearby regions. Continental Shelf Research 12:1293-1309. 

Dowgiallo, M. J. (ed.). 1994. Coastal oceanographic effects of summer 1993 Mississippi River 

flooding. Special NOAA Report. NOAA Coastal Ocean OfficeLNational Weather Service, 

Silver Spring, Maryland. 76 pp. 

Downing, J. A. (chair), N. N. Rabalais, R. Diaz, R. Zimmerman, J. L. Baker, and M. D. Durn. 

1999. Gulf of Mexico Hypoxia: Land-Sea Interaction. Council for Agricultural Science and 

Technology, Task Force Report No. 134,40 pp. 

Eadie, B. J., R. Amon, R. Benner, J. Cavaletto, J. B. Cotner, W. S. Gardner, M. Lansing, and 

D. Pakulski. 1995. Organic matter decomposition, nitrogen recycling, and oxygen 

consumption in the Mississippi River plumeIGulf shelf region, pp. 40-55. In: NutrientEnhanced Coastal Ocean Productivity, Proceedings of the April 1994 Synthesis Workshop, 

Baton Rouge, La. Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 

Eadie, B. J., G. L. Bell, J. A. Robbins, and P. A. Meyers. 1992. Carbon flux and 

remineralization in Lake Michigan, 1978-1992. American Geophysical Union, San Francisco, 

California (December 1992). (abstract) 

Eadie, B. J. and M. B. Lansing. 1992. Stable isotope shifts in POM and DOM across the salinity 

gradient off the Mississippi delta. American Society of Limnology and Oceanography, Sante 

Fe (Spring, 1992). (abstract) 

Eadie, B. J., M. B. Lansing, and R. Benner. 1997. Biogeochernical and stable isotope 

transformations in the POM and DOM across the salinity gradient off the Mississippi delta. 

American Society of Lirnnology and Oceanography. Sante Fe, New Mexico.(Feb., 1997). 

(abstract) 

Eadie, B. J., B. A. McKee, M. B. Lansing, J. A. Robbins, S. Metz, and J. H. Trefry. 1994. 

Records of nutrient-enhanced coastal productivity in sediments from the Louisiana continental 

shelf. Estuaries 17(4):754-765. 
Appendix C, Publications 143 

Eadie, B. J., J. A. Robbins, P. Blackwelder, S. Metz, J. H. Trefry, B. McKee, and T. A. 

Nelsen. 1992. A retrospective analysis of nutrient enhanced coastal ocean productivity in 

sediments from the Louisiana Continental Shelf, pp. 7-14. In: Nutrient Enhanced Coastal 

Ocean Productivity, NECOP Workshop Proceedings, October 1991, Texas A&M University 

Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Eadie, B. J. and others. 1994. Records of nutrient enhanced coastal ocean productivity in 

sediments from the Louisiana continental shelf. American Geophysical UnionIAmerican 

Society of Limnology and Oceanography, San Diego, Calif. (Feb, 1994). (abstract) 

Fahnenstiel, G., M. H. Markowitz, M. J. McCormick, S. Lorenz, and D. Redalje. 1992. Taxonspecific production and growth rates of dominant phytoplankton from the northern Gulf of 

Mexico. Journal of Phycology 28: 13. (abstract) 

Fahnensteil, G. L., M. Marcovitz, M. J. McCorrnick, D. G. Redalje, S. E. Lorenz, H. J. Carrick, 

and M. J. Dagg. 1992. High growth and microzooplankton-grazing loss rates for 

phytoplankton populations from the Mississippi River plume region, pp. 1 11-1 16. In: 

Nutrient Enhanced Coastal Ocean Productivity, NECOP Workshop Proceedings, October 

1991, Texas A&M University Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Fahnenstiel, G. L., M. J. McCormick, G. A. Lang, D. G. Redalje, S. E. Lohrenz, M. Markowitz, 

B. Wagoner, and H. J. Carrick. 1995. Taxon-swecific growth and loss rates for dominant 

phyto$ankton populations from the northern ~df of ~ixico. Marine Ecology Progress Series 

117:229-239. 

Fahnenstiel, G., D. Redalje, and S. Lorenz. 1994. Has the importance of photoautotrophic 

picoplankton been overestimated? Limnology and Oceanography 39(2):432-438. 

Gardner, W. S., R. Benner, R. Amon, J. B. Cotner, Jr., J. Cavaletto, and J. Johnson. 1996. 

Effects of high molecular weight dissolved organic matter on nitrogen dynamics in the Gulf of 

Mexico Mississippi River Plume. Marine Ecology Progress Series 133:287-297. 

Gardner, W. S., R. Benner, G. Chin-Leo, J. B. Cotner, Jr., B. J. Eadie, J. F. Cavaletto, and M. 

B. Lansing. 1994. Mineralization of organic material and bacterial dynamics in Mississippi 

River plume water. Estuaries 17(4):8 16-828. 

Gardner, W. S., E. E. Briones, E. Cruz-Kaegi, and G. T. Rowe. 1993. Ammonium excretion 

by benthic invertebrates and sediment-water nitrogen flux in the Gulf of Mexico near the 

Mississippi River outflow. Estuaries 16(4):799-808. 

Gardner. W. S.. J. F. Cavaletto. J. B. Cotner. and J. R. Johnson. 1997. Effects of natural light w 

on itrogen cycling rates in ihe ~ississi~ii River plume. Limnology and Oceanography 

42(2):273-28 1. 

Gardner, W. S., J. B. Cotner, Jr., and L. R. Herche. 1993. Chromatographic measurement of 

nitrogen mineralization rates in marine coastal waters with 15~. Marine Ecology Progress 

Series 93:65-73. 
144 Appendix C, Publications 

Gardner, W. S., L. R. Herche, P. A. St. John, and S. P. Seitzinger. 1991. High performance 

liquid chromatographic determination of 15NH4: [15NH4 + 14NH4] ion ratios in seawater for 

isotope dilution experiments. Analytical Chemistry 63: 1838-1843. 

Gardner, W. S. and P. A. St. John. 1991. High-performance liquid chromatographic method to 

determine ammonium ion and primary mines in seawater. Analytical Chemistry 63537-540. 

Gulf of Mexico Program Office, Louisiana Department of Environmental Quality, Lower 

Mississippi River Conservation Committee, and Mississippi Soil and Water Conservation 

Commission. 1997. Proceedings of the first Gulf of Mexico hypoxia management 

conference, December 5-6, 1995, Kenner, La. Publ. No. EPA-55-R-97-001, United States 

Environmental Protection Agency, Gulf of Mexico Program Office, Stennis Space Center, 

Mississippi. 198 pp. 

Grimes, C. B. 1997. Distribution, abundance, feeding growth and mortality of fish larvae 

associated with the Mississippi River discharge plume, and the potential impacts of hypoxia, 

pp. 76-85. In: Proceedings of the First Gulf of Mexico Hypoxia Management Conference, 

December 1995, New Orleans, Louisiana. Publ. No. EPA-55-R-97-001, U.S. Environmental 

Protection Agency, Gulf of Mexico Program Office, Stennis Space Center, Mississippi. 

Harper, Jr., D. E. and N. N. Rabalais. 19971. Responses of benthonic and nektonic organisms, 

and communities, to severe hypoxia on the inner continental shelf of Louisiana and Texas, pp. 

41-54. In: Proceedings of the First Gulf of Mexico Hypoxia Management Conference, 

December 1995, New Orleans, Louisiana. Publ. No. EPA-55-R-97-001, U.S. Environmental 

Protection Agency, Gulf of Mexico Program Office, Stennis Space Center, Mississippi. 

Hendee, J .C. 1994. Data management for the Nutrient Enhanced Coastal Ocean Productivity 

program. Estuaries 17(4):900-903. 

Hendee, J. C. 1994. Object-oriented database management systems and their application to 

oceanography. Earth System Monitor 4(4):6-9. 

Hitchcock, G. and T. Whitledge. 1992. Nutrient/pigment variability in the Mississippi River 

plume and adjacent waters, pp. 43-5 1. In: Nutrient Enhanced Coastal Ocean Productivity, 

NECOP Workshop Proceedings, October 1991, Texas A & M University Sea Grant Program, 

TAMU-SG-92- 109, College Station, Texas. 

Justic', D., N. N. Rabalais, and R. E. Turner. 1995. Stoichiometric nutrient balance and origin 

of coastal eutrophication. Marine Pollution Bulletin 30(1):41-46. 

Justic', D., N. N. Rabalais, and R. E. Turner. 1996. Effects of climate change on hypoxia in 

coastal waters: A doubled CO, scenario for the northern Gulf of Mexico. Lirnnology and 

Oceanography 4 1 (5):992- 1003. 

Justic', D.. N. N. Rabalais. and R. E. Turner. 1997. Im~acts of climate change on net . , 

productivity of coastal'waters: Implications for carbo; budget and hypoxia.- Climate Research 

8:225-237. 
Appendix C, Publications 145 

Justic', D., N. N. Rabalais, and R. E. Turner. 1994. Riverborne nutrients, hypoxia and coastal ' 

ecosystem evolution: Biological responses to long-term changes in nutrient loads carried by the 

Po and Mississippi Rivers, pp. 161-167. In: K. R. Dyer and R. J. Orth (eds.), Changes in 

Fluxes in Estuaries: Implications from Science to Management. Proc. Joint ECSAJERF 

Conference. International Symposium Series. Olsen & Olsen, Fredensborg, Denmark 

Justic', D., N. N. Rabalais, R. E. Turner, and Q. Dortch. 1995. Changes in nutrient structure of 

river-dominated coastal waters: Stoichiometric nutrient balance and its consequences. 

Estuarine, Coastal and Shelf Science 40:339-356. 

Justic', D., N. N. Rabalais, R. E. Turner, and Q. Dortch. 1997. Stoichiometric nutrient balance 

and eutrophication in the coastal waters influenced by the Mississippi River, pp. 44-46. In: 

Proceedings, Clean Enough? Mississippi River Conference, September 1997, Lake 

Pontchartrain Basin Foundation, Metairie, Louisiana, 124 pp. 

Justic', D., N. N. Rabalais, R. E. Turner, and W. J. Wiseman, Jr. 1993. Seasonal coupling 

between riverborne nutrients, net productivity and hypoxia. Marine Pollution Bulletin 

26(4): 184-1 89. 

Limno-Tech, Inc. 1995. Estimated responses of water quality on the Louisiana Inner Shelf to 

nutrient load reductions in the Mississippi and Atchafalaya Rivers. Revised final report (April 

5, 1995). U.S. Environmental Protection Agency, Gulf of Mexico Program Office, John C. 

Stennis Space Center, Mississippi. Prepared for Louisiana State University and A&M College 

Sea Grant College Program, Louisiana State University, Baton Rouge, La. Limno-Tech, Inc., 

Ann Arbor, Mich., and South Bend, Ind. 48 pp. Appendices. 

Lohrenz, S. E. (ed.). 1993. Proceedings of the 1993 NECOP data workshop. University of 

Southern Mississippi, Gulf Coast Campus, Long Beach, MS. 12-14 January 1994. 

University of Southern Mississippi, Center for Marine Science Technical Report No. 163. 

Stennis Space Center, Mississippi. 89 pp. 

Lohrenz, S. E., M. Dagg, and T. Whitledge. 1990. Enhanced primary production at the 

plume/oceanic interface of the Mississippi River. Continental Shelf Research 10: 639-664. 

Lohrenz, S. E., G. L. Fahnenstiel, and D. G. Redalje. 1994. Spatial and temporal variations of 

photosynthesis parameters in relation to environmental conditions in coastal waters of the 

northern Gulf of Mexico. Estuaries 17(4):779-795. 

Lohrenz, S. E., G. L. Fahnenstiel, D. G. Redalje, G. A. Lang, X. Chen, and M. J. Dagg. 1997. 

Variations in primary production of northern Gulf of Mexico continental shelf waters linked to 

nutrient inputs from the Mississippi River. Marine Ecology Progress Series 155:45-54. 

Lohrenz, S. E., G. L. Fahnenstiel, D. G. Redalje, G. A. Lang, M. J. Dagg, T. E. Whitledge, and 

Q. Dortch. 1999. The interplay of nutrients, irradiance and mixing as factors regulating 

primary production in coastal waters impacted by the Mississippi River plume. Continental 

Shelf Research 19: 11 13-1 141. 
146 Appendix C, Publications 

Lohrenz, S. E., D. G. Redalje, and G. L. Fahnenstiel. 1995. Optical properties of Mississippi 

River plume and adjacent waters during March 1991, pp. 67-74. In: Nutrient-Enhanced 

Coastal Ocean Productivity, Proceedings of the April 1994 Synthesis Workshop, Baton 

Rouge, La. Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 

Lohrenz, S. E., D. G. Redalje, G. L. Fahnensteil, and G. A. Lang. 1992. Regulation and 

distribution of primary production in the northern Gulf of Mexico, pp. 95-104. In: Nutrient 

Enhanced Coastal Ocean Productivity, NECOP Workshop Proceedings, October 199 1, Texas 

A&M University Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Lohrenz, S. E., D. G. Redalje, G. L. Fahnenstiel, M. J. McCormick, G. Lang, K. Prasad, X. 

Chen, D.A. Atwood, and B. Chen. 1995. Phytoplankton rate processes in coastal waters of 

the Northern Gulf of Mexico and relationships to environmental conditions, pp. 56-66. In: 

Nutrient-Enhanced Coastal Ocean Productivity, Proceedings of the April 1994 Synthesis 

Workshop, Baton Rouge, La. Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 

Lopez-Veneroni, D. and L. A. Cifuentes. 1992. Dissolved organic nitrogen distribution and 

transport in the continental shelf of the northwest Gulf of Mexico, pp. 57-68. In: Nutrient 

Enhanced Coastal Ocean Productivity, NECOP Workshop Proceedings, October 199 1, Texas 

A&M University Sea Grant Program, TAMU-SG-92-109, College Station, Texas. 

Lopez-Veneroni, D. and L. A. Cifuentes. 1994. Transport of dissolved organic nitrogen in 

Mississippi River plume and Texas-Louisiana continental shelf near-surface waters. Estuaries 

17(4):796-808. 

Lopez-Veneroni, D., L. A. Cifuentes, and R. B. Coffin. 1995. An isotopically-constrained 

model for denitrification and nitrogen burial in the continental shelf of the NW Gulf of Mexico, 

75-80. In: Nutrient-Enhanced Coastal Ocean Productivity, Proceedings of the April 1994 

Synthesis Workshop, Baton Rouge, La. Louisiana Sea Grant College Program, Baton Rouge, 

Louisiana. 

MacNaughton, A. 1998. The evolution of inshore hydrography of the west Louisiana inner shelf, 

1992. Masters thesis, Department of Oceanography & Coastal Sciences, Louisiana State 

University, Baton Rouge, Louisiana. 

Mallini, L. J. 1992. Development of kinetic-colorimetric flow analysis technique for determining 

dissolved manganese and iron and an assessment of the behavior of dissolved manganese in 

the far-field plume of the Mississippi River. Masters thesis, Center for Marine Science, 

University of Southern Mississippi, Stennis Space Center, Mississippi. 

MacRae, M. J. D., W. Glover, J. W. Ammerman, and R. H. Sada Ruvalcaba. 1994. Seasonal 

phosphorus deficiency in the Mississippi River Plume: Unusually large areal extent during the 

record flood of 1993. AGUJALSO Ocean Sciences Meeting, 1994, San Diego, Calif. 

(abstract) 

McKee, B. A. and M. Baskaran. 1999. Sedimentary processes of Gulf of Mexico estuaries, pp. 

63-85. In: Bianchi, T., J. Pennock, and R. Twilley (eds.). Biogeochernistry of Gulf of 

Mexico Estuaries. John Wiley and Sons, New York. 
Appendix C, Publications 147 

McNeil, C. S. and C. B. Grimes. 1995. Diet and feeding ecology of striped anchovy, Anchoa - 

hepsetus, along environmental gradients associated with the Mississippi River discharge 

plume, pp. 81-89. In: Nutrient-Enhanced Coastal Ocean Productivity, Proceedings of the 

April 1994 Synthesis Workshop, Baton Rouge, La. Louisiana Sea Grant College Program, 

Baton Rouge, La. 

Miller-Way, T., G. S. Boland, G. T. Rowe, and R. R. Twilley. 1994. Sediment oxygen 

consumption and benthic nutrient fluxes on the Louisiana continental shelf: A methodological 

comparison. Estuaries 17(4):809-8 15. 

Morse, J. W. and G. T. Rowe. 1999. Benthic biogeochemistry beneath the Mississippi River 

plume. Estuaries 22(2A):206-214. 

Nelsen, T. A., P. Blackwelder, T. Hood, B. McKee, N. Romer, C. Alvarez-Zarikian, and S. 

Metz. 1994. Time-based correlation of biogenic, lithogenic and authigenic sediment 

components with anthropogenic inputs in the Gulf of Mexico NECOP study area. Estuaries 

17(4):873-885. 

Nelsen, T. A., P. Blackwelder, T. Hood, C. Zarikian, J. H. Trefry, S. Metz, B. Eadie, and B. 

McKee. 1995. Retrospective analysis of NECOP area sediments: biogenic, inorganic and 

organic indicators of anthropogenic influences since the turn of the century, pp. 90-101. In: 

Nutrient-Enhanced Coastal Ocean Productivity, Proceedings of the April 1994 Synthesis 

Workshop, Baton Rouge, La. Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 

Nelson, D. M. and Q. Dortch. 1996. Silicic acid depletion and silicon limitation in the plume of 

the Mississippi River: Evidence from kinetic studies in spring and summer. Marine Ecology 

Progress Series 136(1-3): 163-178. 

NOAA-AOML. 1993. The proceedings of a workshop on future research, monitoring and 

modeling of coastal interactions in the Northern Gulf of Mexico. February 2-3, 1993. 

National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Meteorological 

Laboratory, Ocean Chemistry Division, Miami, FL. 54 pp. 

NOAA Coastal Ocean Program. 1992. Nutrient enhanced coastal ocean productivity. NECOP 

workshop proceedings, Louisiana Universities Marine Consortium, October 1991. Texas 

A&M University Sea Grant Program, Galveston, TX. TAMU-SG-92-109. 154 pp. 

Ortner, P. B. and M. J. Dagg. 1995. Nutrient-enhanced coastal ocean productivity explored in 

the Gulf of Mexico. Eos, Transactions, American Geophysical Union 76(10): 97,109 (March 

7). 

Parsons, M. L., Q. Dortch, and G. A. Fryxell. 1998. A multi-year study of the presence of 

potential domoic acid-producing Pseudo-nitzschia species in the coastal and estuarine waters of 

Louisiana, USA, pp. 184-1 87. In: B. Reguera, J. Blanco, Ma L. Fernandez and T. Wyatt 

(eds.), Harmful Algae. Xunta de Galicia and Intergovernmental Oceanographic Commission 

of UNESCO, GRAFISANT, Santiago de Compostela, Spain. 

Pakulski, J. D., R. Benner, R. Amon, B. J. Eadie, and T. Whitledge. 1995. Community 

metabolism and nutrient cycling in the Mississippi River plume: Evidence for intense 

nitrification at intermediate salinities. Marine Ecology Progress Series 117: 207-218. 
148 Appendix C, Publications 

Pakulski, J. D., R. Benner, T. Whitledge, R. Arnon, B. J. Eadie, L. Cifuentes, and D. Stockman. 

1996. Microbial metabolism and nutrient cycling in the Mississippi and Atchafalaya River 

plumes. American Geophysical Union Ocean Sci. Mtg. San Diego, Calif. Feb, 1996. 

(abstract) 

Powers, S. P. 1997. Recruitment of soft bottom benthos. Ph.D. dissertation, Department of 

Marine Biology, Texas A&M University at Galveston. 

Powers, S. P., D. E. Harper, Jr., and N. N. Rabalais. 1995. Underwater research methods for 

studying larval recruitment of benthic invertebrates, pp. 83-94. In: D. E. Harper, Jr. (ed.), 

Diving for Science ... 1995. Proceedings of the American Academy of Underwater Sciences, 

Fifteenth Annual Scientific Diving Symposium, 4-8 October 1995, La Jolla, California. 

Pullcowski, D. and others. 1994. Nutrient regeneration, carbon remineralization and bacterial 

productivity in the Mississippi River Plume. American Geophysical UnionIAmerican Society 

of Limnology and Oceanography, San Diego, Calif. (Feb, 1994) (abstract) 

Qureshi, N. A. 1995. The role of fecal pellets in the flux of carbon to the sea floor on a riverinfluenced continental shelf subject to hypoxia. Ph.D. dissertation, Department of 

Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana. 

Qureshi, N. A., N. N. Rabalais, and Q. Dortch. 1992. Seasonal differences in the sedimentation 

of zooplankton fecal pellets in the northern Gulf of Mexico, pp. 122-152. In: Nutrient 

Enhanced Coastal Ocean Productivity. NECOP Workshop Proceedings, October 1991, Texas 

A & M University Sea Grant Program, Publ. No. TAMU-SG-92-109, Galveston, Texas. 

Rabalais, N. N. 1998. Testimony to the U.S. Senate Subcommittee on Oceans and Fisheries, 

Senate Committee on Commerce, Science, and Transportation. Hearing on S. 1480, Harmful 

Algal Bloom Research and Control Act of 1997, with specific comments on hypoxia and 

nutrient enrichment, May 20, 1998, 16 pp. 

Rabalais, N. N. 1992. Influence of hypoxia on the interpretation of effects of petroleum 

production activities, pp. 255-259. In: Proceedings, Twelfth Annual Gulf of Mexico 

Information Transfer Meeting. U.S. Minerals Management Service, Gulf of Mexico OCS 

Region, MMS Contract No. 14-35-0001-30499, OCS Study MMS 92-0027, New Orleans, 

La. 

Rabalais, N. N. 1992. Hypoxia component: Mississippi River plume hydrography, LATEX 

physical oceanography program, pp. 479-482. In: Proceedings, Twelfth Annual Gulf of 

Mexico Information Transfer Meeting. U.S. Minerals Management Service, Gulf of Mexico 

OCS Region, MMS Contract No. 14-35-0001-30499, OCS Study MMS 92-0027, New 

Orleans, La. 

Rabalais, N. N. 1994. Nutrient Enhanced Coastal Ocean Productivity (NECOP): Predicting 

hypoxia and effects on benthic populations. In: Proceedings, NOAA 1994 Summer 

Colloquium on Operational Environmental Predication, National Oceanic and Atmospheric 

Administration, NOAA Science Center, Camp Springs, Md. 9 pp. 
Appendix C, Publications 149 

Rabalais, N. N. 1994. Hypoxia studies in the Mississippi River plume area, 1993, pp. 312-314: 

In: Proceedings: Thirteenth Annual Gulf of Mexico Information Transfer Meeting, December 

1993, OCS Study MMS94-0061. US. Dept. of the Interior, Minerals Management Service, 

Gulf of Mexico OCS Region, New Orleans, Louisiana. 

Rabalais, N. N. 1994. Hypoxia vs. production activities: Which influences the benthic 

community?, pp. 157- 16 1. In: Proceedings: Thirteenth Annual Gulf of Mexico Information 

Transfer Meeting, December 1993, OCS Study MMS94-0061. U.S. Dept. of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, Louisiana. 

Rabalais, N. N. 1996. Mississippi River Water Quality: Status, Trends, and Implications, 13 pp. 

In: L. Durant (ed.) Proceedings of the 1996 Environmental State of the State Conference, 

Environmental Research Consortium of Louisiana, Baton Rouge, Louisiana. 

Rabalais, N. N. 1997. "Dead Zone in the Gulf of Mexico Expanding." Living Oceans News, 

Publ. National Audubon Society's Living Oceans Program, Fall 1997:6. 

Rabalais, N. N. 1998. Hypoxia in the Gulf of Mexico, pp.26-29. In: Proceedings, 1998 

Louisiana Environmental State of the State-III, Environmental Research Consortium of 

Louisiana, Baton Rouge, Louisiana, November 1998. 

Rabalais, N. N. 1998. Hypoxia and the Magnuson-Stevens Act for Sustainable Fisheries, pp. 

231-235. In: J. Speir (ed.), Sustainable Fisheries for the 21st Century? A Critical Examination 

of Issued Associated with Implementing the Sustainable Fisheries Act. Tulane Institute for 

Environmental Law and Policy, Tulane University, New Orleans, Louisiana. 

Rabalais, N. N. 1998 (on-line). Oxygen Depletion in Coastal Waters. In NOAA's State of the 

Coast Report. National Oceanic and Atmospheric Administration, Silver Spring, Maryland. 

http://state~of~coast.noaa.gov/bulletinshtmVhyptmVhO9hyp.html. 

Rabalais, N. N. 1999. Hypoxia in the Gulf of Mexico. In: Proceedings, Louisiana Environment99: Law, Science and the Public. Tulane Institute for Environmental Law and Policy, Tulane 

University, New Orleans, Louisiana, February 1999. 

Rabalais, N. N. and L. M. DiPinto. 1993. Nutrient Enhanced Coastal Ocean Productivity 

(NECOP): Predicting hypoxia and effects on benthic populations. In: Proceedings, NOAA 

1993 Summer Colloquium on Operational Environmental Prediction, National Oceanic and 

Atmospheric Administration, Silver Spring, Maryland, 15 pp. 

Rabalais, N. N., R. S. Carney, and E. G. Escobar-Briones. 1999. Overview of continental shelf 

benthic communities of the Gulf of Mexico, pp. 17 1- 195. In: H. Kumpf, K. S teidinger and 

K. Sherman (eds.), The Gulf of Mexico Large Marine Ecosystem, Sustainability and 

Management. Blackwell Science, Malden, MA. 

Rabalais, N. N. and D. E. Harper, Jr. 1991. Studies of the benthic oxygen phenomenon off 

Louisiana, pp. 57-63. In: H.-J. Krock and D. E. Harper, Jr. (eds.), International Pacific 

Scientific Diving. 1991. Proceedings of the American Academy of Underwater Sciences, 

Eleventh Annual Scientific Diving Symposium, 25-30 September 1991, Honolulu, Hawaii. 
150 Appendix C, Publications 

Rabalais, N. N. and D. E. Harper, Jr. 1992. Studies of benthic biota in areas affected by 

moderate and severe hypoxia, pp. 150-153. In: Nutrient Enhanced Coastal Ocean 

Productivity, NECOP Workshop Proceedings, October 1991, Texas A & M University Sea 

Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Rabalais, N. N., L. E. Smith, D. E. Harper, Jr., and D. Justic'. 1995. The effects of bottom 

water hypoxia on benthic communities of the southeastern Louisiana continental shelf. 

MMSIOCS Study 94-0054, U.S. Minerals Management Service, Gulf of Mexico OCS Region, 

New Orleans, Louisiana, 105 pp. 

Rabalais, N. N., L. E. Smith, E. B. Overton, and A. L. Zoeller. 1993. Influence of hypoxia on 

the interpretation of effects of petroleum production activities. OCS StudyIMMS 93-0022. 

U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New 

Orleans, Louisiana, 158 pp. 

Rabalais, N. N., R. E. Turner, and Q. Dortch. 1992. Louisiana continental shelf sediments: 

indicators of riverine influence, pp. 13 1- 135. In: Nutrient Enhanced Coastal Ocean 

Productivity, NECOP Workshop Proceedings, October 1991, Texas A&M University Sea 

Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Rabalais, N. N., R. E. Tuner, D. Justic', Q. Dortch, and W. J. Wiseman, Jr. 1999. 

Characterization of Hypoxia: Topic 1 Report for the Integrated Assessment ofn Hypoxia in the 

Gulf of Mexico: NOAA Coastal Ocean Program DecisionAnalysis Series No. 15. NOAA 

Coastal Ocean Program, Silver Sping, MD. 167 pp. 

Rabalais, N. N., R. E. Turner, D. Justic', Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

1996. Nutrient changes in the Mississippi River and system responses on the adjacent 

continental shelf. Estuaries 19(2B):386-407. 

Rabalais, N. N., R. E. Turner, D. Justic', Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

1998. Mississippi River nutrient changes and consequences to hypoxia on the Louisiana 

shelf, pp. 37-39. In: Proceedings, Clean Enough? Mississippi River Conference, September 

1997, Lake Pontchartrain Basin Foundation, Metairie, Louisiana, 124 pp. 

Rabalais, N. N., R. E. Turner, D. Justic', Q. Dortch, W. J. Wiseman, Jr., and B. K. Sen Gupta. 

in press 1999. Gulf of Mexico biological system responses to nutrient changes in the 

Mississippi River. In: J. E. Hobbie (ed.), Estuarine Synthesis: A Synthetic Approach to 

Research and Practice, Island Press. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1992. Distribution and characteristics of 

hypoxia on the Louisiana Shelf in 1990 and 1991, pp. 15-20. In: Nutrient Enhanced Coastal 

Ocean Productivity, NECOP Workshop Proceedings, October 1991, Texas A&M University 

Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1994. Distribution and characteristics of 

hypoxia on the Louisiana Shelf in 1990 and 1991, pp. 50-54. In: Nutrient Enhanced Coastal 

Ocean Productivity, NECOP Workshop Proceedings, October 1994, Texas A&M University 

Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 
Appendix C, Publications 151 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. in press 1999. Hypoxia in the Gulf of - 

Mexico. Journal of Environmental Quality. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1997.. The hypoxic zone in the Gulf of 

Mexico. In: Proceedings From the Corn Belt to the Gulf ... Agriculture & Hypoxia in the 

Mississippi River Watershed, American Farm Bureau Federation, St. Louis, Missouri, July 

1997. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1997. Hypoxia in the northern Gulf of 

Mexico: Causes and Consequences, pp. 73-84. In: Proceedings, North Central Extension - 

Industry Soil Fertility Conference, Vol. 13, St. Louis, Missouri, November 1997. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1997. Hypoxia in the northern Gulf of 

Mexico: Past, present and future, pp. 25-38. In: Proceedings of the First Gulf of Mexico 

Hypoxia Management Conference, December 1995, New Orleans, Louisiana. Publ. No. 

EPA-55-R-97-001, U.S. Environmental Protection Agency, Gulf of Mexico Program Office, 

Stennis Space Center, Mississippi. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1998. Excess nutrients from the 

Mississippi River degrade water quality in the Gulf of Mexico, pp. 94-102. In: 1998 

Proceedings Illinois Agricultural Pesticides Conference, Champaign, Illinois, January 1998. 

Rabalais, N. N., R. E. Turner, and W. J. Wiseman, Jr. 1999. Hypoxia in the northern Gulf of 

Mexico: Linkages with the Mississippi River, pp. 297-332. In: H. Kumpf, K. Steidinger, 

and K. Sherman (eds.), The Gulf of Mexico, a Large Marine Ecosystem, Sustainability and 

Management, Blackwell Science, Malden, MA. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman Jr., and D. F. Boesch. 1991. A brief summary 

of hypoxia on the northern Gulf of Mexico continental shelf: 1985-1988, pp. 35-47. In: R. V. 

Tyson and T. H. Pearson (eds.), Modern and Ancient Continental Shelf Anoxia. Geological 

Society Special Publication No. 58. The Geological Society, London. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman, Jr., and Q. Dortch. 1998. Consequences of the 

1993 Mississippi River Flood in the Gulf of Mexico. Regulated Rivers: Research & 

Management 14: 161- 177. 

Rabalais, N. N., R. E. Turner, W. J. Wiseman Jr., D. Justic', Q. Dortch, and B. Sen Gupta. 

1995. Hypoxia on the Louisiana Shelf and system responses to nutrient changes on the 

Mississippi River: A brief synopsis, pp. 106-113. In: Nutrient-Enhanced Coastal Ocean 

Productivity, Proceedings of the April 1994 Synthesis Workshop, Baton Rouge, La. Louisiana 

Sea Grant College Program, Baton Rouge, Louisiana. 

Rabalais, N. N., W. J. Wiseman, Jr., and R. E. Turner. 1994. Comparison of continuous 

records of near-bottom dissolved oxygen from the hypoxia zone along the Louisiana coast. 

Estuaries 17(4):850-861. 

Redalje, D. G. 1993. The labeled chlorophyll a technique for determining photoautotrophic 

carbon specific growth rates and carbon biomass, pp. 563-572. In: P. F.Kemp, B. F. Sherr, 

E. B. Sherr, and J. J. Cole (eds.). Handbook of Methods in Aquatic Microbial Ecology, Lewis 

Publishers, Boca Raton, Fla. 
152 Appendix C, Publications 

Redalje, D. G., S. E. Lohrenz, and G. L. Fahnenstiel. 1992. Phytoplankton dynamics and the 

vertical flux of organic carbon in the Mississippi River plume and inner Gulf of Mexico shelf 

region, p. 526. In: P. G. Falkow.ski and A. D. Woodhead (eds.), Primary Productivity and 

Biogeochemical Cycles in the Sea, Plenum Press, New York. (abstract) 

Redalje, D. G., S. E. Lohrenz, and G. L. Fahnenstiel. 1992. Temporal variability in the 

relationship between production and the vertical flux of particulate matter in a river impacted 

coastal ecosystem. Journal of the Mississippi Academy of Sciences 37(1):49. (abstract) 

Redalje, D. G., S. E. Lohrenz, and G. L. Fahnenstiel. 1992. The relationship between primary 

production and the export of POM from the photic zone in the Mississippi River plume and 

inner Gulf of Mexico shelf regions, pp. 105-1 10. In: Nutrient Enhanced Coastal Ocean 

Productivity, NECOP Working Proceedings, October 1991, Texas A&M University Sea Grant 

Program, TAMU-SG-92-109, Galveston, Texas. 

Redalje, D. G., S. E. Lohrenz, and G. L. Fahnenstiel. 1994. The relationship between primary 

production and the vertical export of particulate organic matter in a river-impacted coastal 

ecosystem. Estuaries 17(4):829-838. 

Robichaux, R. J., Q. Dortch, and J. H. Wrenn. 1998. Occurrence of Gymnodinium sanguineurn 

in Louisiana and Texas coastal waters, 1989-1994. NOAA Technical Report NMFS 143: 19- 

25. 

Rowe, G. T., G. S. Boland, and W. C. Phoel. 1992 Benthic community oxygen demand and 

nutrient regeneration in sediments near the Mississippi River plume, pp. 136-139. In: Nutrient 

Enhanced Coastal Ocean Productivity, NECOP Workshop Proceedings October 1991, Texas 

A&M University Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Rowe, G. T., J. Morse, G. S. Boland, and M. L. Curz-Kaegi. 1995. Sediment metabolism and 

heterotrophic biomass associated with the Mississippi River plume, pp. 102-105. In: NutrientEnhanced Coastal Ocean Productivity, Proceedings of the April 1994 Synthesis Workshop, 

Baton Rouge, La., Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 

Rowe, G., J. Morse, B. J. Eadie, L. Cifuentes, and G. Boland. 1993. Oxygen demand 

underestimates sediment heterotrophic activity. American Geophysical Union Ocean Sciences 

Mtg., Seattle, Washington (April, 1993). (abstract) 

Sawyer, T. K., T. A. Nerad, N. N. Rabalais, and S. P. McLaughlin. 1997. Protozoans isolated 

from Louisiana shelf sediments subject to hypoxialanoxia with an emphasis on freshwater 

amoebae and marine flagellates. Bulletin of Marine Science 61(3): 859-867. 

Sen Gupta, B. K., R. E. Turner, and N. N. Rabalais. 1996. Seasonal oxygen depletion in 

continental-shelf waters of Louisiana: Historical record of benthic foraminifers. Geology 

24(3): 227-230. 

Smith, S. M. 1993. Nutrient Regulation of Phytoplankton on the Louisiana Shelf. Master's 

thesis, University of Miami. Miami, Florida. 
Appendix C, Publications 153 

Smith, S. M. and G. L. Hitchcock. 1994. Nutrient enrichments and phytoplankton growth in 

surface waters of the Louisiana Bight. Estuaries 17(4):740-753. 

Stumpf, R. P. and T. D. Leming. 1994. MississippiIAtchafalaya River plume features, pp. 30- 

31. In: Dowgiallo, M. J. (ed.). Coastal oceanographic effects of 1993 Mississippi River 

flooding. Special NOAA Report. National Oceanic and Atmospheric Administration, Coastal 

Ocean OfficeINational Weather Service, Silver Spring, Maryland. 

Trefry, J. H., S. Metz, T. A. Nelsen, R. P. Trocine, and Brian J. Eadie. 1994. Transport of 

particulate organic carbon by the Mississippi River system and its fate in the Gulf of Mexico. 

Estuaries 17(4):839-849. 

Trefry, J. H., R. P. Trocine, S. Metz, T. A. Nelsen, and N. Hawley. 1992. Suspended 

particulate matter on the Louisiana Shelf: Concentrations, composition and transport pathways, 

pp. 126-130. In: Nutrient Enhanced Coastal Ocean Productivity, NECOP Workshop 

Proceedings, October 1991, Texas A&M University Sea Grant Program, TAMU-SG-92-109, 

Galveston, Texas. 

Turner, R. E., N. Qureshi, N. N. Rabalais, Q. Dortch, D. Justic', R. F. Shaw, and J. Cope. 

1998. Fluctuating silicate:nitrate ratios and coastal plankton food webs. Proceedings National 

Academy of Science, USA 95: 13048- 1305 1. 

Turner, R. E. and N. N. Rabalais. 1991. Changes in Mississippi River water quality this century: 

implications for coastal food webs. BioScience 41(3): 140-147. 

Turner, R. E. and N. N. Rabalais. 1994. Changes in the Mississippi River nutrient supply and 

offshore silicate-based phytoplankton community responses, pp.147-150. In: K. R. Dyer and 

R. J. Orth (eds.), Changes in Huxes in Estuaries: Implications from Science to Management. 

Proc. Joint ECSAIERF Conference. International Symposium Series, Olsen & Olsen, 

Fredensborg, Denmark. 

Turner, R. E. and N. N. Rabalais. 1994. Coastal eutrophication near the Mississippi river delta. 

Nature 368:619-621. 

Turner, R. E. and N. N. Rabalais. 1994. Light conditions in the coastal boundary layer of the 

northern Gulf of Mexico, pp. 3 14-3 18. In: Proceedings: Thirteenth Annual Gulf of Mexico 

Information Transfer Meeting, December 1993, OCS Study MMS94-0061. U.S. Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 

Louisiana. 

Turner, R. E., N. N. Rabalais, Q. Dortch, D. Justic', and B. K. Sen Gupta. 1997. Evidence for 

nutrient limitation and sources causing hypoxia on the Louisiana shelf, pp. 106-1 12. In: 

Proceedings of the First Gulf of Mexico Hypoxia Management Conference, December 1995, 

New Orleans, Louisiana. Publ. No. EPA-55-R-97-001, U.S. Environmental Protection 

Agency, Gulf of Mexico Program Office, Stennis Space Center, Mississippi. 

Twilley, R. R., T. Miller-Way, and B. McKee. 1995. Synoptic investigations of benthic-pelagic 

coupling within the Louisiana shelf ecosystem, pp. 114-1 19. In: Nutrient-Enhanced Coastal 

Ocean Productivity, Proceedings of the April 1994 Synthesis Workshop, Baton Rouge, La., 

Louisiana Sea Grant College Program, Baton Rouge, Louisiana. 
154 Appendix C, Publications 

Whitledge, T. E. 1994. Nutrient concentrations at the MississippilAtchafalaya River outflows, 

pp. 36-38. In: M. J. Dowgiallo (ed.), Coastal Oceanographic Effects of Summer 1993 

Mississippi River Flooding, Special NOAA Report, National Oceanic and Atmospheric 

Administration, Coastal Ocean Office/National Weather Service, Silver Spring, Maryland. 

Wiseman, W. J., Jr., V. J. Bierman Jr., N. N. Rabalais, and R. E. Turner. 1992. Physical 

structure of the Louisiana Shelf hypoxic region, pp. 21-26. In: Nutrient Enhanced Coastal 

Ocean Productivity, NECOP Workshop Proceedings, October, 1991, Texas A&M University 

Sea Grant Program, TAMU-SG-92-109, Galveston, Texas. 

Wiseman, W. J., Jr. and R.W. Garvine. 1995. Plumes and coastal currents near large river 

mouths. Estuaries 18(3):509-517. 

Wiseman, W. J., Jr. and F. J. Kelly. 1993. Salinity variability within the Louisiana Coastal 

Current during the 1982 flood season. Estuaries 17(4):732-739. 

Wiseman, W. J., Jr., N. N. Rabalais, R. E. Turner, S. P. Dinnel, and A. MacNaughton. 1997. 

Seasonal and interannual variability within the Louisiana Coastal Current: Stratification and 

hypoxia. Journal of Marine Systems 12:237-248. 

Wiseman, W. J., Jr.,N. N. Rabalais, R. E. Turner, G. Hitchcock, and N. D. Walker. 1997. 

Physical variability in the Louisiana inner shelf hypoxic region, pp. 57-62. In: Proceedings of 

the First Gulf of Mexico Hypoxia Management Conference, December 1995, New Orleans, 

Louisiana. Publ. No. EPA-55-R-97-001, U.S. Environmental Protection Agency, Gulf of 

Mexico Program Office, Stennis Space Center, Mississippi. 

Wiseman, W. J., Jr. and W. Sturges. 1999. Physical oceanography of the Gulf of Mexico: 

Proccesses that regulate its biology, pp. 77-92. In: H. Kumpt, K. Steidinger, and K. 

Sherman (eds.), The Gulf of Mexico, a Large Marine Ecosystem, Sustainability and 

Management, Blackwell Science, Malden, MA. 
Appendix D 

A Concise Historv of NECOP 

Fiscal Year (FY) 1988 - The concept of NECOP is first discussed at the OCEANIC02 Coupling 

Workshop at the NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, Mich. 

FY 1989 - The NECOP Program develops from an embryonic form at the NOAA Coastal Ocean 

Productivity/Nutrient Enhanced Workshop in Baton Rouge, La. A NECOP Program 

Management Committee (PMC) is established. Initial solicitation for proposals for FY 1990- 

199 1 is announced by the PMC. The NOAA Coastal Ocean Program (COP) is formed, and 

NECOP becomes the first field effort under the auspices of the new office. 

FY 1990 - Proposals are selected and NECOP principal investigators (PIS) receive funding for 

two years. An initial Technical Advisory Committee (TAC) is established. The first year of 

NECOP field cruises are conducted aboard research vessels (R/Vs) Pelican and Acadiana 

(Louisiana Universities Marine Consortium), Gyre (Texas A&M University), and NOAA ship 

Malcolm Baldrige. A data management center and program is established at the NOAA Atlantic 

and Meteorological Laboratory (AOML), Miami, Fla., including the availability of a Remote 

Bulletin Board System for data sharing. 

FY 1991 - A major cruise planning workshop of NECOP PIS is held in February at AOML to 

discuss the second year of field activities. The second process study-oriented cruise aboard 

Malcolm Baldrige is conducted. Hypoxia monitoring cruises continue on Pelican and Acadiana. 

Proposals are solicited for third and fourth years of research. 

FY 1992 - An October synthesis workshop is convened for all PIS at LUMCON in Cocodrie, La. 

to review the first two years of the program and implement improvements. An independent 

review panel provides mid-life program review. A successful review and selection of proposals 

for the 2nd two years of NECOP is conducted. A major NECOP planning workshop is held at 

Texas A&M University, College Station, in February for the third year of field work. New TAC 

members are elected. In April, the NECOP project draws favorable reviews from the Nutrient 

Enhanced Productivity Theme Team at the annual COP review meeting in Silver Spring, Md. 

Proceedings of the Cocodrie workshop are published through Texas A&M University Sea Grant 

in June. Field cruises are undertaken on RNs Pelican, Acadiana, Gyre, Longhorn (University of 

Texas) and NOAA Ship Chapman. 

FY 1993 - This is the last year of major field activities in NECOP. The PMC and TAC meet in 

November in Miami to discuss cruises and program improvements. A data management 

workshop is held in January at the University of Southern Mississippi to discuss data synthesis 

and data products. A TAC meeting is convened in Miami, Fla., to define an approach a 
156 Appendix D, History 

philosophy for program synthesis. Record flooding occurs in upper Mississippi River and lower 

Missouri River. Cruises aboard RNs Pelican, Longhorn, and Chapman are undertaken to assess 

effects of flooding. 

FY 1994 - Beginning of a major effort to synthesize NECOP results including continuation of 

water quality model development using 1992 and 1993 NECOP data. A session is dedicated to 

NECOP at the 12th Biennial Meeting of the Estuarine Research Federation (Hilton Head, North 

Carolina, Nov. 14-1 8). Nine abstracts are accepted for the special session and other NECOP 

presentations are made in other sessions. In March, a special NOAA report on coastal 

oceanographic effects of summer 1993 Mississippi River flooding is published jointly by COP 

and the National Weather Service with contributions by NECOP PIS. NECOP PIS, Data and 

Program Managers meet in Baton Rouge, La. (April 26-28) to develop an initial synthesis of the 

program results and to make plans for a final program synthesis. Summer hypoxia monitoring 

continues aboard Pelican. 

FY 1995 - Work continues on improving the accuracy of the NECOP Water Quality Model. The 

model is calibrated using July 1990 cruise data and simulations run for 1985, 1988, and 1990 

summer conditions. A final contract report on the NECOP Water Quality Model is delivered by 

Limno-Tech, Inc. to Louisiana State University and NOAA. As a major data synthesis effort, an 

issue of the journal Estuaries is devoted to NECOP papers. An additional synthesis product is 

provided by publishing the proceedings of the 1994 NECOP Synthesis Workshop in Baton 

Rouge, La. Fourteen papers by NECOP PIS are contained in the report. In addition, a NECOP 

presentation is given at the 1995 annual meting of the American Association for the 

Advancement of Science, a summary article is published in Eos, Transactions of the American 

Geophysical Union (Vol. 76 [lo]: 97, 109), and a NECOP modeling presentation is given to the 

Gulf fo Mexico Program. The NECOP PMC meets in September in Baton, La., to plan for FY 

1996 avtivities. Summer hypoxia monitoring continues aboard Pelican. Web Page created 

(http://www.aoml.noaa.gov/ocd/necop). 

FY 1996 - NECOP continues with final data synthesis and further calibration of the Water 

Quality Model. As a result of the scientific findings from NECOP and public interest in the 

hypoxia zone, the First Gulf of Mexico Hypoxia Management Conference is convened 

December 5-6, 1995, in New Orleans, La. sponsored by the Gulf of Mexico Program. The 

conference is called to address management issues associated with the hypoxia zone. A number 

of NECOP scientists participate in the conference. Final submission of data to the NECOP Data 

Management Program is made and all data forwarded to the NOAA National Oceanographic 

Data Center by the NECOP Data Manager. The accuracy and usefulness of the NECOP Water 

Quality Model is expanded through the incorporation of additional field data into the model and 

with planned fbture transfer to Louisiana State University for operational purposes. Also, 

Louisiana State University is provided with expert assistance for application of the NECOP 

Water Quality Model. Summer hypoxia monitoring continues on the Louisiana Inner Shelf 

aboard Pelican to strengthen the model and map the geographic extent and intensity of hypoxic 

coastal waters. At a September PMC meeting in Silver Spring, Md., a final synthesis of NECOP 

findings is planned for publication through the COP Decision Analysis Series (this report). 
OTHER TITLES IN THE DECISION ANALYSIS SERIES 

No. I 

No. 2 

No. 3 

No. 4 

No. 5 

No. 6 

No. 7 

No. 8 

No. 9 

No. 10 

No. I I 

No. 12 

No. 13 

No. 14. 

No. 15 

No. 16 

No. 17 

No. 18 

No. 19 

No. 20 

Able, Kenneth W., and Susan C. Kaiser. 1994. Synthesis of Summer Flounder Habitat Parameters. 

Matthews, Geoffrey A., and Thomas J. Minello. 1994. Technology and Success in Restoration, Creation 

and Enhancement of Spartina alternijlora Marshes in the Unites States. 2 vols. 

Collins, Elaine V., Maureen Woods, Isobel Sheifer, and Janice Beattie. 1994. Bibliography of Synthesis 

Documents on Selected Coastal Topics. 

Hinga, Kenneth R., Heeseon Jeon, and Noelle F. Lewis. 1995. Marine Eutrophication Review. 

Lipton, Douglas W., Katherine Wellman, Isobel C. Sheifer, and Rodney F. Weiher. 1995. Economic 

Valuation of Natural Resources: A Handbook for Coastal Policymakers. 

Vestal, Barbara, Alison Reiser, et al. 1995. Methodologies and Mechanisms for Management of 

Cumulative Coastal Environmental Impacts. Part I-Synthesis with Annotated Bibliography; Part 

11-Development and Application of a Cumulative Impacts Assessment Protocol. 

Murphy, Michael L. 1995. Forestry Impacts on Freshwater Habitat of Andromous Salmonids in the 

Pacific Northwest and Alaska-Requirements for Protection and Restoration. 

William F. Kier Associates. 1995. Watershed Restoration-A Guide for Citizen Involvement in 

California. 

Valigura, Richard A., Winston T. Like, Richard S. Artz, and Bruce B. Hicks. 1996. Atmospheric Nutrient 

Inputs to Coastal Areas-Reducing the Uncertainties. 

Boesch, Donald F., et al. 1977. Harmful Algal Blooms in Coastal Waters: Options for Prevention, 

Control and Mitigation. 

McMurray, Gregory R., and Robert J. Bailey, editors. 1998. Change in Pacific Northwest Coastal 

Ecosystems. 

Fonseca, Mark S., W. Judson Kenworthy, and Gordon W. Thayer. 1998. Guidelines for the Conservation 

and Restoration of Seagrasses in the United States and Adjacent Waters. 

MacWin, S. Allen, editor. 1998. Bering Sea FOCI (1991-1997)-Final Report. 

Wiseman, William, Nancy N. Rabalais, Michael J. Dagg, and Terry E. Whitledge, editors. 1999. Nutrient 

Enhanced Coastal Ocean Productivity in the Northern Gulf of Mexico. 

Rabalais, N.N., R.E. Turner, D. Justic, Q. Dortch, and W. J. Wiseman, Jr. 1999. Characterization of 

Hypoxia: Topic 1 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. 

Diaz, R.J., and A. Solow. 1999. Ecological and Economic Consequences of Hypoxia: Topic 2 Report for 

the Integrated Assessment on Hypoxia in the Gulf of Mexico. 

Goolsby, D.A., W.A. Battaglin, G.B. Lawrence, R.S. Artz, B.T. Aulenback, R.P Hooper, D.R. Keeney, and 

G.J Stensland. 1999. Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin: Topic 3 

Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. 

Brezonik, P.L., V.J. Bierman, Jr., R. Alexander, J. Anderson, J. Barko, M. Dortch, L. Hatch, G.L. 

Hitchcock, D. Keeney, D. Mulla, V. Smith, C. Walker, T. Whitledge, and W.J. Wiseman, Jr. 1999. Effects 

of Reducing Nutrient Loads, Especially Nitrate-Nitrogen, to Surface Water, Ground Water, and the Gulf of 

Mexico: Topic 4 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. 

Mitsch, W.J., J.W. Day, Jr., J.W. Gilliam, P.M. Groffman, D.L. Hey, G.W. Rangall, and N. Wang. 1999. 

Reducing Nutrient Loads, Especially Nitrate-Nitrogen, to Surface Water, Ground Water and the Gulf of 

Mexico: Topic 5 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. 

Doering, O.C., F. Diaz-Hermelo, C. Howard, R. Heimlich, F. Hitzhusen, R. Kazrnierczak, J. Lee, L. Libby, 

W. Milon, T. Prato, and M. Ribaudo. 1999, Evaluation of the Economic Costs and Benefits of Methods for 

Reducing Nutrient Loads to the Gulf of Mexico: Topic 6 Report for the Integrated Assessment on Hypoxia 

in the Gulf of Mexico. 
U.S. Department of Commerce 

National Oceanic and Atmospheric Administration 

Coastal Ocean Program 

13 15 East-West Highway 

Silver Spring, Maryland 209 10