Deepwater Gulf of Mexico

Environmental and Socioeconomic

Data Search and Literature Synthesis

Volume I: Narrative Report

Texas

Mississippi

Deepwater

Environment

U.S. Exclusive Economic Zone

Yucatan

U.S. DeDartment of the Interior

Minerals Management Service Gulf of Mexico OCS Region

Alabama Georgia

Florida

Gulf of Mexico - Cuba

OCS Study

MMS 2000-049
Deepwater Gulf of Mexico

Environmental and Socioeconomic

Data Search and Literature Synthesis

Volume I: Narrative Report

Author

Continental Shelf Associates, Inc.

Prepared under MMS contract

1435-01 -98-CT-3091 6

by

Continental Shelf Associates, Inc.

759 Parkway Street

Jupiter, Florida 33477-9596

Published by

U.S. Department of the Interior Minerals Management Service Gulf of Mexico OCS Region

OCS Study

MMS 2000-049

New Orleans

July 2000
Disclaimer

This report was prepared under contract between the Minerals Management Service

(MMS) and Continental Shelf Associates, Inc. This report has been technically reviewed

by the MMS, and has been approved for publication. Approval does not signifr that the

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

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

however, exempt from review and compliance with the MMS editorial standards.

Report Availability

Extra copies of this report may be obtained from the Public Information Office at the

following address:

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Park Boulevard

New Orleans, LA 70 123-2394

Telephone: (504) 736-2519 or

1 -800-200GULF

Suggested Citation

Continental Shelf Associates, Inc. 2000. Deepwater Gulf of Mexico Environmental and

Socioeconomic Data Search and Literature Synthesis. Volume I: Narrative

Report. U.S. Department of the Interior, Minerals Management Service, Gulf of

Mexico OCS Region, New Orleans, LA. OCS Study MMS 2000-049. 340 pp.

111
Acknowledgments

This synthesis report was prepared by Continental Shelf Associates, Inc. (CSA) with the

assistance of technical experts (primarily from Texas A&M University) serving as

independent consultants. The CSA Program Manager was David A. Gettleson and the

report editor was Neal W. Phillips. The Annotated Bibliography was compiled by David

B. Snyder with the assistance of Kristen L. Metzger and Marilyn Morris, librarians at

Harbor Branch Oceanographic Institution Inc. Melody B. Powell was the technical

editor. Suzanne R. Short prepared the final graphics and Deborah V. Raffel was

responsible for word processing. Clerical support was provided by Deborah M. Cannon,

Karen I. Stokesbury, and Caroline M. Simpson.

CSA especially wants to acknowledge the chapter authors for their excellent

contributions to this report:

E.G. Ward1 Deepwater Technology

William R. Bryant1 and Jia Y. Liu' Geology

Worth D. Nowlin', Ann E. Jochens', Steven F. DiMarco',

and Robert 0. Reid1 Physical Oceanography

Mahion C. Kennicutt II' Chemical Oceanography

Douglas C. Biggs' and Patrick H. Ressler' Water Column Biology

Gilbert T. Rowe' Non-Seep Benthos

Ian R. MacDonald1 Seep Communities

Stephen T. Viada2 Protected Species

David B. Snyder2 Fishes and Fisheries

Linda A. Tobin3 Socioeconomics

'Texas A&M University

2Continental Shelf Associates, Inc.

3lndependent consultant

V
Contents

Volume I: Narrative Report

Page

Chapter 1: Introduction 1

Background

Objectives 1 Methods 2

Report Organization 2

Literature Cited 4

Chapter 2: Deepwater Technology 5

Introduction

Shallow Water Production from Fixed Platforms 5

Deepwater Production to Date 6

Deepwater Development Activities to Date 7

Deepwater Production Systems 7

Floating Production Systems 9

Subsea Production Systems 14

Transportation Systems 15

Future Deepwater Development Activities 16

Future Deepwater Production 16

Floating Production Structures 16

Sub sea Production Systems 18

Transportation Systems 19

Future Technology Challenges 20

Literature Cited 21

Chapter 3: Geology 25

Basin Histoty 25

Study History 26

Seafloor Morphology 26

Structures 29

Salt 29

Faulting within the Continental Slope 36

Sedimentation in the Northern Gulf of Mexico 37

Sediment Sources and Drainage Patterns 38

Depocenters 40

Continental Slope Basins 42

Salt Induced Physiography of the Upper and Middle Continental Slope 43

Salt Induced Physiography of the Lower Continental Slope 46

Geohazards 49

Literature Cited 54

vii
Contents

(continued)

Page

Chapter 4: Physical Oceanography 61

Introduction 61

Classical Hydrography 64

Low-Frequency, Local Wind Forcing 72

Surface Wind Waves 74

Forcing By Energetic, Episodic Wind Events 77

Loop Current Forcing and Eddy Activity 85

Sea Level and Tides 96

Currents in Deep Water 98

Deep, Barotropic Currents 99

High-speed, Subsurfaceintensified Currents 102

Currents Responsible for Furrows 102

Effects of Shelf/Slope Canyons and Other Rough Topography 106

Numerical Modeling 107

Types of Models and Studies 107

Eddy Shedding Aspects 110

Other Skill Assessment 111

Status of Nowcast/Forecasts 112

Literature Cited 113

Chapter 5: Chemical Oceanography 123

Introduction 123

Dissolved Gases 124

Dissolved Solids 125

Nutrients 125

Dissolved Organic Matter 127

Dissolved Metals 129

Particulate Matter 130

Particulate Organic Matter 131

Particulate Inorganic Matter 132

The Chemistry of Natural Seepage 133

Brine Seepage 133

Hydrocarbon Seepage 134

Conclusions 135

Literature Cited 135
Contents

(continued)

Page

Chapter 6: Water Column Biology 141

Prirnaiy Productivity and Chlorophyll Standing Stocks 141

Introduction 141

Mean Condition 142

Seasonal Changes 145

Deepwater "Hot Spots" from Entrainment of Freshwater 145

Deepwater "Hot Spots" from Cross Isopycnal Mixing 146

Deepwater "Hot Spots" from Mesoscale Divergence 153

Deepwater Zooplankton, Micronekton, and Ichthyoplankton 154

Introduction 154

The Available Database 155

Previous Reviews 155

Ecology, Biology, and Systematics Studies 159

Biornass and Abundance 160

Spatial and Temporal Variability 164

Acoustic Sampling of Zooplankton and Micronekton Biomass 166

Optical Sampling of Zooplankton and Micronekton Biornass 167

A Combined Approach to Biomass Surveys 167

Literature Cited 169

Appendix 6A: Deepwater Water Column Biology 181

Chapter 7: Non-Seep Benthos 189

Introduction 189

Community Description 193

Microbiota 193

Meiofauna 193

Macrofauna 193

Megafauna 197

Motile Scavengers 199

Community Metabolism 199

Community Function - The Cycling of Organic Carbon 199

Comparison of the Gulf Benthos with Other Oceans 202

Conclusions 203

Literature Cited 204

ix
Contents

(continued)

Page

Chapter 8: Seep Communities 209

Introduction 209

Background Biology 210

Microbiology of Chemosynthesis 210

Symbiosis with Invertebrates 210

Discoveries of Chemosynthetic Communities 211

Hydrothermal Vents 211

The Florida Escarpment 211

The Northern Gulf ofMexico Continental Slope 211

The Gulf of Mexico Seep Fauna 212

Vestimentiferan Tube Worms 212

Seep Mussels 214

Vesicomyid Clams 216

Lucinid Clams 216

Polychaete "Ice Worms" 216

Beggiatoa Mats and Other Protozoans 218

Heterotrophic Fauna 218

Types of Seep Communities in the Northern Gulf of Mexico 219

Bush Hill 220

Brine Pool NR1 221

Garden Banks 386 221

Alaminos Canyon 222

Distribution of Seep Communities in the Northern Gulf of Mexico 222

Literature Cited 224

Chapter 9: Protected Species 231

Introduction 231

Cetaceans 232

Listed Species Accounts 232

Nonlisted Species Accounts 235

Distribution and Abundance in the Oceanic Northern Gulf of Mexico 238

Sea Turtles 243

Species Accounts 244

Distribution and Abundance in the Oceanic Northern Gulf of Mexico 246

Sensitivity to Oil and Gas Exploration and Development 247

Literature Cited 250

x
Contents

(continued)

Page

Chapter 10: Fishes and Fisheries 255

Deepwater Fishes 255

Introduction 255

History of Deepwater Fish Sampling in the Gulf of Mexico 257

Epipelagic Fishes 258

Mesopelagic Fishes 260

Bathypelagic Fishes 264

Demersal Fishes 264

Interactions among Fishes of Different Assemblages 270

Deepwater Fisheries 270

Introduction 270

History of Deepwater Fisheries in the Gulf of Mexico 271

Key Species 272

Fishing Gear Types 272

Landings Trends 275

Literature Cited 278

Chapter 11: Socioeconomics 285

Introduction 285

The Importance of Time and Space 286

History of Oil and Gas Development in the Gulf Coast States from 1900s Forward 287

A Natural Extension of Growth 290

The Past Thirty Years 292

Socioeconomic Effects of Oil and Gas Activity across Space 293

Community Impacts 294

Port Development 297

Impact of Oil and Gas Development on the Organization of Work 298

Labor Markets 298

Industrial and Business Linkages 300

Industrial Restructuring 301

Relationships to Physical Environment 302

Agriculture 303

Recreational Uses of the Coast and Oil 303

Industrial Development and Infrastructure 304

Port Fourchon 306

Oil Spills and Flazardous Waste 307

Impacts on Families and Individuals 308

Oil and Louisiana Cajuns 309

Economic Impacts on Individuals and Families 310

Social Problems 312

Concentrated Work Scheduling 312

Impacts of Oil on Education, Migration, and Population 313

Conclusions 3 14

Literature Cited 315

xi
Contents

(continued)

Page

Chapter 12: Synthesis 321

Introduction 321

Operational Contrasts aid Implications 325

High Production Rates 325

Shift in Types of Structures 325

Changes in Drilling and Production 326

Transportation Challenges 327

Socioeconomic Implications 327

Environmental Contrasts and Implications 328

Physical/Chemcial Environment 328

Water Column Ecosystems 330

Benthic Ecosystems 333

Data Gaps and Information Needs 334

Deepwater Teclrnology 334

Geology 334

Physical Oceanography 335

Chemical Oceanography 336

Water Column Biology 336

Benthic Communities 337

Protected Species 337

Fishes and Fisheries 338

Socioeconornics 338

Literature Cited 339

Volume II: Annotated Bibliography

Preface 1

Annotated Bibliography 3

xii
Figures

Figure Page

1.1 Deepwater study area 3

2.1 Deepwater production in the Gulf of Mexico (thousands of barrels

oil equivalent per day, MBOEPD) 6

2.2 Deepwater production systems in the Gulf of Mexico 7

2.3 Schematic showing major components of a TLP 12

2.4 Schematic showing major components of a spar 12

2.5 Subsea production system configuration 14

2.6 Subsea production system components 14

2.7 Schematic of tanker-based FPSO offloading to a shuttle tanker 18

3.1 Bathymetric map of the northern Gulf of Mexico deepwater area 27

3.2 Salt and fault distributions in the northern Gulf of Mexico (Adapted

from: Diegel et al. 1995) 30

3.3 Major salt basins in the northern Gulf of Mexico (Adapted from:

Simmons 1992) 33

3.4 Seismic line across the lower continental slope (Adapted from:

Simmons 1992) 34

3.5 Seismic line across the Sigsbee Escarpment (Adapted from:

Diegel et al. 1995) 35

3.6 Bathymetry derived drainage patterns in the northern Gulf of Mexico

deepwater environment 39

3.7 The location of depocenters through Cenozoic in the northern Gulf

of Mexico (Adapted from: Woodbury et al. 1973) 41

3.8 Seismic line across the Pigmy Basin (Adapted from: Simmons 1992). 44

3.9 Bathymetric contour map of the Pigmy Basin 45

3.10 Perspective view of the seafloor relief of the middle and lower

continental slope, northern Gulf of Mexico 47
Figures

(continued)

Figure Page

3.11 Bathymetric contour map of the Vaca Basin 48

4.1 Geopotential anomaly (dynamic rn) of the sea surface relative to the

1,000-db surface, constructed from Hidalgo 62-H-3 data collected

February-March 1962 62

4.2 Temperature vs. salinity, temperature vs. depth, and salinity vs.

depth based on all data collected during Hidalgo cruise 62-H-3,

February-March 1962 69

4.3 Temperature vs. salinity, temperature vs. depth, and salinity vs.

depth for two stations made in early May 1993 over the continental

slope off Texas 70

4.4 Annual time series of monthly averaged temperatures at six depths

for four regions in the northern Gulf of Mexico (From: Robinson

1973) 71

4.5 Contours of mean dynamic height (dyn cm) of surface relative to

450 db, constructed by averaging the first empirical orthogonal

modes (EOF) obtained from an EOF analysis of the geopotential

anomaly for 13 cruises covering the area (From: Vázquez 1993) 73

4.6 Horizontal current vectors (hourly values from 3-hr low-passed

records) during late August 1992 from two locations off Louisiana at

approximately 90.5°W on the shelf edge and upper slope 78

4.7 Locations of current measurements or model output used in figures

within this review 79

4.8 Components (u positive to the east and v positive to the north) of

hourly currents measured at indicated depths on moorings S

(26°N, 96° lOW) and C (55 km north of S) 81

4.9 Eastward (u) and northward (v) components of currents (hourly

values from 3-hr low-passed records) from moorings located off

Louisiana at approximately 90.5°W 83

4.10 Eastward (u) and northward (v) components of currents from 3- to

40-hr band-passed records made in July and December 1992 at

mooring 10 located off Louisiana at 27.94°N, 92.75°W in water

depth of 200m 84

xiv
Figures

(continued)

Figure Page

4.11 (a) An example of the development of an intrusion of the Loop

Current into the eastern Gulf of Mexico is schematically depicted

with contours of the 22°C isotherm at 1 00-rn depth from

hydrographic cruises conducted in August 1972 through September

1973 (Adapted from: MauI 1977). (b) Frequency of occurrence (%)

of warm Loop Current water in the eastern Gulf of Mexico from

monthly frontal analyses of AVHRR data from 1976 through 1985

(Adapted from: Science Applications International Corporation

1989) 87

4.12 GEK surface current observations (left) and surface dynamic

topography relative to 1,350 db (right) from Alarninos cruise 67A4

in June 1967 (Adapted from: Nowlin and Hubertz 1972) 88

4.13 Sea surface height anomaly (cm) from satellite altimeter data for

9 May 1993 90

4.14 Components of velocity (cIms') normal to a section extending from

approximately 27.4°N, 90.6°W (station 64) to 24.8°N, 89.4°W

(station 78) 91

4.15 Components (u positive to the east and v positive to the north) of

40hr, low-passed currents measured at indicated depths on moorings

S (26N, 96°1OTW) and C (55 km north of S) 93

4.16 Horizontal current vectors (hourly values from 3-hr low-passed

records) from moorings located approximately equidistant along the

200-rn isobath at the edge of the Texas continental shelf 95

4.17 Schematic showing design currents in deepwater Gulf of Mexico for

three classes of phenomena: topographic Rossby waves, Loop

Current eddies, and subsurface-intensified, high-speed jets (shown

by grey domain) 100

4.18 Current vectors (40-hr low-passed; north directed upward) from

Science Applications International Corporation mooring G located in

3,200 m water depth at 25°36.21N, 85°29.8TW off the southern West

Florida Shelf 101

4.19 Average current profiles before (thin line), during (thick line), and

after (dashed line) a subsurface jet event in Mississippi Canyon (data

courtesy of Chevron) 103

xv
Figures

(continued)

Figure Page

4.20 Acoustic Doppler current profiler current speed in Green Canyon

Block 200 from 29 April through 30 April 1994, showing 50 cims

current event propagating upwards in the water column beginning

about 30 April 104

4.21 Time series of vertical profiles of current speeds from the Kantha

model 105

6.1 Deepwater locations of recent measurements of primary production,

1987-1997, on oceaiographic cruises by Texas A&M University

(TAMU) and the Universidad Nacional Autonoma de Mexico

(TJNAM) 144

6.2 A: Dynamic topography (cm, 0 m relative to 800 m) of the

deepwater GulfCet H focal area, as determined from 107

hydrographic stations made on R/V Gyre cruise 97G08. B: Gridded

upper layer geostrophic velocity (0 m relative to 800 m) computed

from the dynamic topography data in A. C: Sea surface salinity map,

superimposed on ship track lines of 97G08. D: Sea surface

chlorophyll map, superimposed on ship track lines 147

6.3 Top panel: Sea surface height anomaly for water depths >200 m

from satellite altimeter data for the NEGOM study area for 29 July

1998 (hindcast data). Middle panel: Salinity at 3 ii' from

thermosalinograph observations on NEGOM cruise N3, 26 July -

6 August 1998. Bottom panel: Chlorophyll (igm3) at -3 m

calculated from flow-through fluorescence on NEGOM cruise N3 148

6.4 SeaWiFS ocean color image for 16 January 1998, processed by the

Department of Marine Science, University of South Florida. 149

6.5 Top panel: Cruise track and station locations for LATEX

hydrographic survey 1105, April-May 1993. Bottom panel: Vertical

contours of bottle nitrate (imolL') along 200-m isobath during

cruise H05 150

6.6 Primary productivity (g Cm2d1) of the LATEX continental margin

in May, August, and November of 1993 and 1994 (From: GonzalezRodas 1999) 151

6.7 Top panel: Deepwater locations of SEAMAP plankton surveys,

1982-1998. Bottom panel: Deepwater locations of plankton

collection stations (excluding SEAMAP), 1958-1999 163

xvi
Figures

(continued)

Figure Page

6.8 Grayscale running plot of S.. collected with an ADCP along a

northsouth transect line from the deepwater off the Mississippi

River, through a cyclone, and into a Loop Current Eddy (LCE-C)

during October 1996 (Adapted from: Wormuth et al. 1999) 168

7.1 Station locations for the MMS-supported Northern Gulf of Mexico

Continental Slope NGOCMS) Study conducted by LGL

Environmental (Gallaway 1988) 192

7.2 Density of macrofauna in the Gulf of Mexico (From: Rowe et al.

1974) 195

7.3 Biomass of the macrofauna in the Gulf of Mexico (From: Rowe et al.

1974) 195

7.4 Comparison of polychaete densities at two different time periods on

the upper slope of the northern Gulf of Mexico (Adapted from:

Hubbard 1995) 198

7.5 Preliminary carbon flow model for the deepsea benthos of the

northern Gulf of Mexico (From: Rowe et l. 2000) 200

8.1 Photographs taken with vertically-mounted 35 mm camera (Figure 3

from MacDonald etal. 1989) 213

8.2 Bathyn2odiolus heckerae, B. broolcsi, B. childressi, Tanmfisheri, Idas

macdonaldi (Figure 11 from Gustafson et a!, 1998) 215

8.3 Photographs of the two types of trails observed in the aggregations of

vesicornyids (Figure 3 from Rosman et al. 1987) 217

9.1 U.S. oceanic northern Gulf of Mexico as defined in the GulfCet II

study (Davis et al. 2000) 239

9.2 Minimum abundance estimates for species sighted within oceanic

waters during two survey periods (From: Davis et al. 2000) 241

10.1 Diagram of the environmental framework used to describe deepwater

fishes of the Gulf of Mexico (Adapted from: Helfman et al. 1997) 256

10.2 Opposing views of expected megafaunal zonation that might be

observed on the Gulf ofMexico continental slope (From: Gallaway

etal. 1988) 269

xvii
Figures

(continued)

Figure Page

10.3 Monthly lengths (a) and numbers (b) of surface longline sets made in

the Gulf ofMexico from 1994 to 1998 273

10.4 Spatial distribution of surface longline sets made in the Gulf of

Mexico during 1998 (From: NMFS 1999c) 274
Tables

Table Page

2.1 Production systems for current deepwater projects in the Gulf of Mexico 8

3.1 Engineering constraints and possible geohazards of intraslope basins

and canyons 53

4.1 Water masses of the Caribbean Sea and Gulf of Mexico 66

4.2 Location, water depth, and period of records, used for computations

given in Table 4.3, for marine buoys in the Gtilf of Mexico 74

4.3 Monthly and annual mean significant wave height (m) and extremes

based on hourly observations from marine buoys (data from the

National Buoy Data Center) 75

4.4 Loop Current eddy properties 92

4.5 Summary of volume computations and mean response of water level

(Reid and Whitaker 1981) and observed tidal constituents measured at a

deep pelagic site located at 24.767°N, 89.648°W by Mofjeld and

Wimbtish (1977) 97

4.6 Percent of observed variance in sea level attributed to various forcing

factors by the model (Reid and Whitaker 1981) 97

4.7 Comparison of relevant properties of several ocean circulation models

applied to the Gulf of Mexico 109

6.1 Previous reviews of the zooplankton, micronekton, and ichthyoplankton

of the Gulf of Mexico (GOM) 156

6.2 Estimates of plankton standing stock in the deepwater Gulf of Mexico

(GOM) (bulk biornass as mL wet displacement volume per 100 m3) 161

7.1 Standing stocks of various size categories of benthic biota on the

northern Sigsbee Abyssal Plain of the Gulf of Mexico 194

8.1 Summary of taxonomic designations of seep mussels from the northern

Gulf of Mexico taken from original descriptions of Gustafson et al.

(1998) 214

xix
Tables

(continued)

Table Page

8.2 Sites where chemosynthetic megafauna have been collected by trawl

(Tn) or submarine (Sub), or definitively photographed by submarine,

remotely operated vehicle (ROy), or photosled (Photosl) 223

9.1 Cetaceans of the Gulf of Mexico 233

9.2 Sea turtles of the Gulf of Mexico 244

10.1 Orders and families of epipelagic (0 to 200 m) fishes known from the

Gulf of Mexico, arranged in phylogenetic order 259

10.2 Orders and families of mesopelagic and bathypelagic fishes known from

the Gulf of Mexico, arranged in phylogenetic order (following

McEachran and Fechhelm 1998) 261

10.3 Deepwater (>200 in) demersal fishes known from the Gulf of Mexico,

arranged in phylogenetic order 265

10.4 Comparison of most abundant species of fish between the Galiaway et al.

(1988) and Pequegnat (1983) studies 266

10.5 Weight (metric tons) of deepwater species landed in Gulf coastal states

in 1998 (From: National Marine Fisheries Service 1999b) 276

10.6 Value (dollars) of deepwater species landed in Gulf coastal states in 1998

(From: National Marine Fisheries Service 1999b) 277

12.1 Operational and environmental contrasts between deepwater and shelf

settings, and their implications 323

xx
ADCP

AXBT

bbl

I3GN

BOPD

BT

CASE

CTD

CZCS

DCM

DOC

DOM

DSDP

EEZ

EJIP

EPA

FAD

FPSO

GMFMC

GUM

lAS

IES

IKMT

LATEX

LC

LCE

LNG

LOOP

MBOPD

MMBOE

MMS

MMSCFPD

MOCNESS

MODUs

NECOP

NGOMCS

NMFS

NOAA

NOS

NPZ

OCS

OPEC

PAH

Acronyms and Abbreviations

acoustic Doppler current profiler

air-dropped expendable bathythermograph

ban-el

Board of Geographic Names

barrels of oil per day

bathythermograph

Consortium for Analysis Studies of Eddies

conductivity, temperature, depth

coastal zone color scanner

deep chlorophyll maximum

dissolved organic carbon

dissolved organic matter

Deep Sea Drilling Program

Exclusive Economic Zone

Eddy Joint Industry Program

Environmental Protection Agency

fish attracting device

floating production system with storage and offloading

Gulf of Mexico Fishery Management Council

Gulf of Mexico

Intra-Americas Sea

inverted echo sounder

Tsaacs-Kidd Midwater Trawl

TexasLouisiana Shelf Circulation and Transport Processes Study

Loop Current

Loop Current eddy

liquefied natural gas

Louisiana Offshore Oil Port

thousand barrels of oil per day

million barrels oil equivalent

Minerals Management Service

million standard cubic feet per day

Multiple Opening/Closing Net Environmental Sensing System

mobile offshore drilling units

nutrient enriched coastal ocean productivity

Northern Gulf of Mexico Continental Slope

National Marine Fisheries Service

National Oceanic and Atmospheric Administration

National Ocean Service

nutrients-phytoplankton-zooplankton

outer continental shelf

Organization of Petroleum Exporting Countries

polynuclear aromatic hydrocarbon

xxi
Acronyms and Abbreviations

(continued)

POC particulate organic carbon

POM Princeton Ocean Model

ROV remotely operated vehicle

RQ respiratory quotient

SAIC Science Applications International Corporation

SSHA sea surface height anomaly

SST sea surface temperature

TABS Texas Automated Buoy System

TLP tension leg platform

TLWP tension leg work platform

TSM total suspended matter

UNAM Universidad Nacional Autonomia de Mexico

USFWS U.S. Fish and Wildlife Service

VLH volatile liquid hydrocarbons

VOC volatile organic compounds

VPR video plankton recorder

WDV wet displacement volume

XBT expendable bathythermograph

XCP expendable current profiler

XCTD expendable current-temperature-depth
Chapter 1: Introduction

Background

Recent years have seen a great increase in oil and gas industry interest in the deepwater

Gulf of Mexico. Contributing factors have included significant hydrocarbon discoveries,

passage of the Deepwater Royalty Relief Act, the availability of infrastructure, arid the

emergence of innovative technologies to find, develop, and produce oil and gas in the

deepwater environment (Cranswick and Regg 1997). This deepwater developmentposes

a number of environmental, socioeconomic, and technological issues. Some are new, and

others are similar to those associated with oil and gas activities on the shelf. However, as

Carney (1998) noted, "deepwater oil and gas development is a reality at the same time

that deepwater environmental research is at a low."

The Minerals Management Service (MMS) is the primary Federal agency responsible for

ensuring that oil and gas related activities on the Outer Continental Shelf (OCS) are

conducted in a safe and clean manner. The MMS has a strong environmental mandate

and conducts an Environmental Studies Program to obtain information for regulatory and

leasing decisions (MMS 1999). As part of that program, an MMS-sponsored "Workshop

on Environmental Issues Surrounding Deepwater Oil and Gas Development" was held in

1997. Workshop participants identified synthesis of existing environmental and

socioeconomic data as an important priority (Carney 1998).

This synthesis report addresses the need for a comprehensive search and integration of

environmental and socioeconomic data for the deepwater Gulf of Mexico. As such, it is

one of several ongoing and planned MMS studies pertinent to deepwater oil and gas

activities. These include two other "desktop" studiesa reanalysis and synthesis of

deepwater physical oceanographic data and a literature review of environmental risks of

chemical products used in deepwater operationsas well as new field studies of the

continental slope benthos, synthetic drilling fluids, and benthic effects at selected

continental slope sites.

Objectives

The general purpose of this program was to gather environmental and socioeconomic

information related to the deepwater environment, in order to

describe the ecosystem;

understand the environmental processes that drive the system; and

understand processes potentially sensitive to anthropogenic activities, particuiarly oil

and gas operations.

1
Introduction

For the purposes of this program, the deepwater environment is defined geographically as

extending from a depth of 305 m (1,000 fi) to the border of the U.S. Exclusive Economic

Zone (Figure 1.1). This includes the deepwater portion of all three Gulf of Mexico OCS

planning areas (Western, Central, and Eastern).

Specific objectives were to develop (1) a synthesis report that summarizes available

information by topic and synthesizes it in a framework to describe the deepwater Gulf

ecosystem, dominant environmental processes, biologically sensitive pathways, and

socioeconomic activities in the area; and (2) a computer-searchable database (annotated

bibliography) incorporating existing literature, relevant data, and ongoing research

pertaining to geological, physical, chemical, and biological processes of the study area,

social and economic data and literature, and deepwater teclmology.

Methods

The synthesis effort was organized by discipline, with specialists in each discipline

serving as chapter authors. Computer searches were conducted using the DIALOG

information retrieval system to identifr published literature, unpublished reports, data

sets, and ongoing or planned studies. Chapter authors helped to select databases and

keywords for the searches. Preliminary bibliographies were circulated to the authors,

each of whom was familiar with the literature in their respective area of expertise.

Additional references were provided by the chapter authors to complete the bibliographic

listing. Subsequently, annotations were downloaded using DIALOG and imported into

Pro-Cite, a bibliographic computer program. Further details on the annotated

bibliography are provided in Volume II.

After the completion of the computer searches, a synthesis plaiming meeting was held

during April 1999. Chapter authors presented preliminary overviews of their respective

topic areas, and discussed approaches for producing the synthesis report.

Report Organization

The report is divided into chapters for the following topic areas:

2

2-Deepwater Technology 7-Non-Seep Benthos

3-Geology 8-Seep Communities

4-Physical Oceanography 9-Protected Species

5-Chemical Oceanography 10-Fishes and Fisheries

6Water Column biology 1 1-Socioeconomics
Figure 11. Deepwater study area. Louisiana Gulf of Mexico l$SIS$!I w-er vironment U. S. Exclusive Economic Zone I . I 

88° 86° 84° 

Alabama Georgia 

o 25 50 75 o 50 100 150 
Introduction

A final Synthesis Chapter draws together infonnation from preceding chapters to identify

significant findings, key issues associated with oil and gas related activities in the

deepwater environment, and critical information needs.

A table of contents is provided at the beginning of each chapter to orient the reader.

References cited in each chapter are listed at the end of that chapter. The Annotated

Bibliography, provided as a separate Volume II, includes both references cited in the

report and other relevant citations.

Literature Cited

Carney, R.S. 1998. Workshop on environmental issues surrounding deepwater oil and gas

development: Final report. OCS Study MMS 98-0022. U.S. Department of the Interior,

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 163 pp.

Cranswick, D. and J. Regg. 1997. Deepwater in the Gulf of Mexico: America's new frontier.

OCS Study MMS 97-0004. U.S. Department of the Interior, Minerals Management

Service, Gulf of Mexico OCS Region, New Orleans, LA. 43 pp.

Minerals Management Service. 1999. Environmental Studies Program website.

http://www.mms.gov/eppd/studies/index.htm.

4
Chapter 2: Deepwater Technology

E. G. Ward

Introduction 5

Shallow Water Production from Fixed Platforms 5

Deepwater Production to Date 6

Deepwater Development Activities to Date 7

Deepwater Production Systems 7

Floating Production Systems 9

Subsea Production Systems 14

Transportation Systems 15

Future Deepwater Development Activities 16

Future Deepwater Production 16

Floating Production Structures 16

Subsea Production Systems 18

Transportation Systems 19

Future Technology Challenges 20

Literature Cited 20

Introduction

In many respects, offshore oil and gas production has its roots in the Gulf of Mexico. Oil

and gas production in the Gulf of Mexico started in 1947 in about 5 m water depth.

Much of the technology and experience developed for the Gulf established standards and

practices for subsequent offshore oil and gas developments throughout the world.

Shaflow Water Production from Fixed Platforms

The first offshore platform was installed in the Gulf in 1947 in about 5 m of water. There

has been a steady march toward ever-increasing depths since that time. In the 1960's,

depths had just exceeded 100 m. In 1978, the Cognac platform was installed in 300 m.

The Bullwinkle platform was installed in 413 m of water in 1989, and is the deepest fixed

platform. During this 40-year span, some 7,000 platforms have been installed and have

served the industry and public well in providing a reliable and economical means of

developing and producing oil and gas. About 4,000 platforms are currently in place and

producing.

Exploratoiy drilling precedes the installation of a production platform. Mobile Offshore

Drilling Units (MODUs) are used to drill exploratory wells. Following a discovery of

hydrocarbons, one or more delineation wells may be drilled to confirm the commercial

significance of the discovery. A production platform is installed once a discovery is

judged to be commercially viable.

The function of these platforms is to provide a workspace that can be used to support the

development drilling of production wells and production equipment. A drilling rig on the

platform is used to drill and complete the production wells. The well system and drilling

operations are functionally similar to that used onshore. A number of wells are drilled

from each platform. Flow rates for wells completed in reservoirs typical of shallow water

locations have typically been in the range of up to 2,500 barrels of oil per day (BOPD).

5
Ward: Dee pwater Technology

The production equipment separates the produced oil, gas, arid water, and the oil and gas

are transported to shore through separate pipelines laid on the seafloor. Unboard pumps

and compressors provide the necessary.energy to transport the oil and gas to shore. The

produced water is passed through an oil-water separator and discharged into the ocean.

These bottom-founded platforms are fixed to the seafloor by a foundation consisting of

pilings driven deep into the ocean floor. The decks upon which the equipment is placed

are located at an elevation high enough above sea level to avoid being inundated by

severe waves. The strength of the platform arid its pile foundation are designed to resist

the lateral forces and overturning moments caused by severe hurricane waves, currents,

and winds. These platforms tend to be quite stiff and the response to loads is mostly

static.

Deepwater Production to Date

As reservoirs have been discovered and produced in shallower waters, the search for new

hydrocarbon reserves has turned to ever-increasing depths. For the purposes of this

report, we will adopt a depth of 305 m (1,000 ft) as the delineation between "deep" and

"shallow" water. Lease sales in the 1980's began to focus on deepwater acreage, and

exploration drilling soon followed. Discoveries were made, but development activities

lagged a bit due to the downturn in oil prices in the mid-l980's and the time to develop

and implement the technology needed to produce oil and gas in water depths of several

thousand feet.

Deepwater discoveries and production have increased very rapidly during the 1990's as

shown in the following Figure 2.1. Production statistics are shown for 1994-1998

(Congdon and Fagot 1999). The production rate shown for 1999 is an extrapolation, but

agrees with an earlier estimate (Morrison 1997). The estimated 1999 production

represents a five-fold increase in 5 years.

w0

1000

900 -

800 -

700 -

600 -

500 -

400 -

300 -

200

100 - 0

+

1994 1995 1996 1997 1998 1999 2000

Figure 2.1. Deepwater production in the Gulf of Mexico (thousands of

barrels oil equivalent per day, MBOEPD).

6
Subsea System Fixed

(300-2,150 m) Platform

(to 500 m)

Compliant

Tower

(500-900m)

Semisubmersible-Based

Floating Production

System

(500 1,200 m)

Tension Leg Spar PlatForm (600-3,000 m)

(500-2,150 m)

7

Ward: Deepwater Technology

Deepwater Development Activities to Date

Deepwater oil and gas development has evolved and built upon the tecimology and

operational experience that have been successfully employed in shallower waters in the

Gulf of Mexico. Production systems have now been installed in water depths

approaching 1,500 m. Lessons learned include the following:

Large, productive reservoirs exist in the Gulf. A well at Ursa recently produced over

50,000 BOPD (Smith 1999). Such daily production rates in the Gulf were unheard of

a few years ago arid are due to the characteristics of deepwater reservoirs and large

advances in well technology. The largest estimated reserves are 1,000 million barrels

oil equivalent (MMBOE) for a prospect named Crazy Horse in about 1,800 m depth

(DeLuca 1999b).

The industry has proved that it can develop reserves in a safe, environmentally sound,

and an economic manner in depths approaching 1,800 m.

In the following section, we will discuss the teelmology involved in these developments

and its implementation.

Deepwater Production Systems

A number of different systems have been developed for deepwater production. These

include bottom-founded structures (fixed platforms and compliant towers), floating

systems (tension leg platforms [TLPs], spars, semisubmersibles), and subsea systems.

These systems are shown in Figure 2.2 (Cranswick and Regg 1997). The systems

installed on specific projects and the water depths are listed in Table 2.1 (Cranswick and

Regg 1997; Offshore Magazine, various articles 1997-1999).

Figure 2.2. Deepwater production systems in the Gulf of Mexico.
Ward: Deepwater Technology

Table 2.1. Production systems for current deepwater projects in the Gulf of Mexico

8

Project Water Depth (m) Production System Type First Production

Baldpate 611 Compliant Tower 1998

Petronius 536 Compliant Tower 2000

Amberjack 315 Fixed platform 1991

Bullwinkle 414 Fixed platform 1989

Cognac 313 Fixed platform 1979

Pompano I 394 Fixed platform 1994

Lena 311 GuyedTower 1983

Cooper 670 Semisubmersible 1995

Genesis 794 Spar 1999

Hoover 1,468 Spar (w/Diana) 2000

King 1,005 Subsea 2000

Oyster 367 Subsea 1997

Shasta 3 18 Subsea 1995

Tahoe 459 Subsea 1994

Tahoe II 459 Subsea 1996

VK862 319 Subsea 1995

Zinc 452 Subsea 1993

Popeye 612 Subsea (shelf) 1996

Macaroni 1,101 Subsea (Auger) 1999

Angus 612 Subsea (Bullwinkle) 1999

Rocky 546 Subsea (Buliwinkle) 1997

Troika 832 Subsea (Buliwinkle) 1998

Diana 1,346 Subsea (Hoover) 2000

Europa 1,189 Subsea (Mars) 2000

Pompano II 570 Subsea (Pompano 1) 1995

Mensa 1,644 Subsea (shelf) 1997

Auger 875 TLP 1994

Brutus 880 TLP 2001

Jolliet 526 TLP 1989

Marlin 990 TLP 1999

Mars 899 TLP 1996

Ram-Powell 995 TLP 1997

Ursa 1,230 TLP 1999

Allegheny 974 TLP (Seastar) 1999

Morpeth 498 TLP (Seastar) 1998
Ward: Dee pwater Technology

Fixed platforms have been installed in depths to 413 rn. While it is technically feasible to

extend the fixed platform concept to the 500 to 750 m water depth range, the large capital

costs for these platforms result in projects that are not economically feasible. The

compliant tower, another bottom-founded structure that has some similarities to a fixed

platform, has also been developed and implemented. Compliant towers can be a Cost

effective alternative in water depths in the range of 500 to 750 m. However, the number

of applications appears to be limited by the limited acreage in the Gulf in this water depth

range. Because of these water depth limitations, the major emphases have been on

various floating production systems and subsea production systems. Floating production

systems and subsea systems have ushered in the development of deepwater production in

the Gulf. Floating production systems in the Gulf include TLPs, spars, and

sernisubmersibles. These systems provided the flexibility of being cost effective over the

range of water depths being developed in the 1990's, and can be extended to even deeper

water. We will next review the deepwater technology that has been developed and

implemented to date.

Floating Production Systems

General Attributes of Floating Systems

Floating production systems generally have a number of functionally similar components

or elements. Major components include the hull, deck, topside equipment, well system,

mooring system, and transportation system for the produced oil and gas. These are

described below to provide a context for understanding, comparing, and contrasting the

specific systems.

Hull and Deck. The hull provides the necessary buoyancy to support deck, topsides,

mooring system, and well risers. The deck (actually several stacked or multilevel decks)

sits atop the hull and supports the topsides, drilling equipment (if any), and personnel

quarters.

Topsides. The topsides includes production facilities and equipment to separate produced

oil, gas, and water; water treatment facilities to clean the produced water so that it can be

discharged overboard; and control and safety systems to monitor, control, and meter

production. The topsides also include power generation equipment and the pumping and

compression equipment needed to send the production to shore. The topsides may also

include drilling equipment. For floating production systems without drilling equipment,

wells are pre-drilled by MODUs, and MODUs are required to return for well

maintenance and recompletions. Such wells are completed as subsea wells that are tied

back to the floating production system by flowlines and risers.

Well Systems. There are two types of well systems used on floating production

systemssurface wells and subsea wells. Surface well systems have a dry wellhead and

tree that is located on the deck of the floating system. A vertical rigid production riser

connects the surface wellhead and tree to the well at the seafloor. Relative movement

between the riser and the floating production system is accommodated by a riser

9
Ward: Deepwater Technology

tensioning system that provides a constant top tension through either mechanical means

or buoyancy. Surface well systems on floating production systems are a direct extension

of the well systems used on platforms, with the main difference being the complexity and

dynamic response of the riser. Surface well systems are limited to floating systems with

little vertical motion and thus little relative motion between the floater and the bottomfixed riser. Advantages of surface well systems include the relative ease of drilling,

maintaining, and servicing wells from a drilling or workover rig on the floating

production system, simpler well monitoring and control equipment, and the ability to

receive the produced oil and gas directly onboard for immediate processing without the

produced fluids having to flow unprocessed through a flowline.

Subsea wells have a wet wellhead and tree that is located on the seafloor. The wells are

offset from the floating production system typically by at least several miles. A subsea

well is connected back to the floating production system by seafloor flowlines and a

compliant flexible production riser. The relative motion between the fixed subsea well

and the floating production system can be accommodated by the flexibility that is

provided by the riser length and shape (e.g., catenary or lazy wave) and/or the riser

material. No riser tensioning system is used. The inherent flexibility of such riser

systems allows subsea wells to be used on floating production systems with significant

vertical motion. Subsea wells have to be drilled, maintained, and serviced by a MODU.

This decouples the drilling and well servicing equipment and operations from the floating

production system. This can result in simpler floating production system and less impact

of simultaneous drilling and production operations. Also, the wells can be spread out and

distributed to penetrate the reservoir where needed resulting in simpler, shorter, more

direct wells which are cheaper to drill. (Compare this to having to drill deviated wells

from a common surface location that have to include significant horizontal segments to

reach different parts of the reservoir.)

A major challenge is ensuring continuous flow of unprocessed oil and gas from subsea

wells through the flowlines on the seafloor. These flowlines lie on the seafloor and are

thus subjected to the cold temperatures at the ocean bottom. The produced oil and gas

are cooled as they flow through the flowliries. Paraffin and asphaltene compounds in

produced oil and hydrates in produced gas, which are normally in solution at reservoir

temperatures and pressures, can be deposited as they cool and eventually plug the

flowlines. Once they are plugged, it can be very difficult and costly to clean out the

flowlines and reestablish production. Several techniques are used to assure flow. Flow

assurance measures include the injection of chemicals at the well head to inhibit the

formation of paraffin, asphaltene, or hydrate plugs; insulated flowlines to maintain a

higher temperature; and periodic cleaning by pumping mechanical devices through the

flowlines. These flow assurance measures add significant capital and operational costs to

the subsea wells, often negating savings due to other advantages. Other disadvantages

include more complex well monitoring and control equipment, and the need for an

umbilical to provide power, control, and chemicals to the subsea wells.

10
Ward: Deepwater Technology

Mooring Systems. Mooring systems connect the floating production system to piled

foundation systems or anchors on the seafloor. The mooring system provides the lateral

restoring forces to resist the horizontal wave, wind, and current forces trying to offset the

floating production system. The significant offset motions are due to wind, current, and

wave drift, and generally have periods of several minutes. Mooring systems are

generally designed to restrict the lateral offset of the floating production system to about

10% of the water depth, e.g. about 30 m in 300 m of water. Both top-tensioned risers and

catenary risers can be designed to accommodate such motions. The mooring system can

be a tendon system, a catenary mooring system, or a taut mooring system.

A tendon system utilizes a number of vertical tendon members. Several tendons are

attached to each corner of the system and are affixed to pile foundations at the seafloor.

The tendons are made of heavy pipe. The tendons are installed in a manner that leaves

them under a very large pre-tension. As the production system is offset, a restoring force

is developed due to the horizontal component of the tendon tensions. (The effect is like

an inverted pendulum.) The seafloor footprint generated by this mooring system is

small.no larger than the structure itself However, the vertical load on the structure due

to the pretension and the weight of the tendons is quite large and increases the buoyancy

requirements for the floating system.

A catenary mooring system uses a number of legs made up of steel wires and chain

segments. Several legs are attached to each corner of the system in a symmetrical

pattern. Each leg is made up of a segment wire line, which is attached to the floating

production system, and a heavy chain segment lies along the bottom and attached to an

anchor of pile at its end. The length of each leg is on the order of 1.5 or more times the

water depth, and drapes down from the floating system in a catenary shape to a point near

the chain touch down point, and then lies on the seafloor and stretches to the end attached

to the anchor or the pile. As the production system is offset, a portion of the chain is

pulled off of the bottom and its hanging weight increases the tension in the upstream legs

and thus provides a restoring force. This is a relatively simple mooring system that has

been used for decades on semisubmersible MODUs. Disadvantages include the weight

of the mooring system and the congestion on the seafloor caused by the large horizontal

footprint of the mooring system.

Tension Leg Platforms

TLPs are the dominant deepwater floating production system in the Gulf to date. Six

TLPs have been installed in depths ranging from 525 to 1,230 m (Chianis and Poll 1997).

The hulls have four columns (up to 26 m diameter) that are connected by pontoons

(Figure 2.3). The first TLP in the Gu1f, Jolliet, was a small TLP with no onboard drilling

capability and a modest production facility. Auger (Bourgeois 1995) and subsequent

TLPs have been much larger and include drilling and extensive production facilities. The

decks are large (up to 90 in square) and can support extensive topside facilities (Zimmer

et al. 1999). The large size of the decks provides flexibility, and the facilities on some

TLPs have been expanded or modified to increase the production capacity (Judd and

Wallace 1996). The largest capacity facility is on Ursa and is rated at 150 thousand

barrels of oil per day (MBOPD) and 400 million standard cubic feet of gas per day

(MMSCFPD) (Jefferis et al. 1999; Smith et al. 1999).

11
Ward: Deepwater Technology

Piles

Eiport Risers

12

Figure 2.3. Schematic showing Figure 2.4. Schematic showing

major components of a TLP. major components of a spar.

The mooring system is made up of three or four tendons attached to each corner. The

tendons are made of large, high strength, thick-walled pipe that is about 1 m in diameter

and 25 mm thick. These are connected to foundation elements that are affixed to the

seafloor by long piles driven into the seabed. These TLPs have little vertical motion due

to the low natural frequencies of vertical motions resulting from the high tendon tensions.

The high tension is produced by excess buoyancy designed into the hull. The small

vertical motions facilitate the use of surface well systems with top-tensioned risers.

TLPs in the Gulf to date have utilized a surface well system. All TLPs except for Joiliet

are equipped with au onboard drilling rig for drilling and servicing production wells.

Subsea wells located at some kilometers away have been tied back to TLPs (DeLuca

1 999a).
Ward: Deepwater Technology

Two mini-TLPs of the Seastar design been installed in depths of 500 and 975 m (Kibbee

1996; Kibbee et al. 1999; Furlow 1999a). These smaller systems are designed to produce

smaller fields than would be economically feasible with larger and more costly TLPs.

These mini-TLPs receive production from a few nearby subsea wells that were drilled by

MODUs. These mini-TLPs support full, but sinai!, production processing facilities.

The hull on these mini-TLPs is a shallow draft column. The mooring system has tendons

that are attached to a cruciform structure at the bottom of the hull. These mini-TLPs have

the attractive motion characteristics of the larger TLPs, which facilitates the use of

catenary risers for both the production risers from the wells and the export risers.

Spars

Two spars have been recently installed in the Gulf of Mexico (Meyer et al. 1998; Krieger

et al. 1999), and a third is being installed in late 1999early 2,000 (Furlow 1999c).

Depths range from 590 to 1,468 m. The spar hulls are a large diameter, deep-draft

column (Figure 2.4), with diameters ranging up to about 40 m and lengths of 215 in. The

deep draft naturally provides small vertical motion so that surface well systems can be

used. Deck sizes range up to about 90 m square. The Hoover-Diana spar has the largest

topside facilities and can process 100 MBOPD and 325 MMSCFPD.

Surface well systems are used on the larger spars, Genesis and Hoover-Diana, which also

have onboard drilling capability. The hulls have a central opening or moon pool through

which the production risers pass. Subsea wells can also be tied back to the spars (Meyer

et al. 1998).

The mooring systems are a multi-leg catenary system. Each of the legs is made up of

wire rope and chain segments, which are affixed to the seafloor with driven piles or

suction caissons. The length of each mooring leg is typically about 1.5 times the water

depth.

Semisubmersible-Based Floating Production Systems

One sei'nisubmersiblebased floating production system is currently active in the Gulf.

Another was installed, but removed when the project was abandoned due to poor

reservoir performance. Semisubmersibles have a long and successful history as

successful drilling rigs for exploratory drilling, and new rigs have been recently built that

can drill in depths over 3,000 m (DeLuca 1999d). Semisubmersible-based floating

production systems were thought to be an attractive option for deepwater a few years ago.

Particularly, the conversion of an existing semisubinersible drilling rig to a floating

production system was seen to have the advantages of building on proven technology and

saving time and money. However, the demand for drilling rigs increased with the need to

explore all the newly leased acreage, and the availability of used, capable rigs that could

be purchased and converted at a reasonable costs diminished drastically. Further,

semisubmersibles have larger vertical motions in storms. This is not a problem during

exploratory drilling since the drilling riser can be disconnected and the well can be

temporarily abandoned for a storm. However, production and export catenary risers must

remain attached during storms, and the large vertical motions create significant technical

and costly design challenges (Hays 1996). Thus, other production systems have been

favored to date.

13
Ward: Deepwater Technology

Subsea Production Systems

A Subsea Production System is simply one or more subsea wells and related equipment

that produce oil and/or gas to a host facility. Subsea well systems have been discussed

previously in the Well Systems section since any Floating Production System can host

one or more subsea wells. Other equipment in a Subsea Production System can include a

subsea manifold, a template, jumpers, flowlines and production risers, umbilical, and

pipelines (Kirkland et al. 1996). The complexity and amount of equipment is determined

proportional to the number of wells in the system. Wells may be individually tied back to

a host, or several wells may be connected to a manifold by short jumper lines, with the

manifold being connected by a flowline to the host.

Technology is not presently available for subsea processing of oil and gas produced from

subsea wells. Thus, the oil and gas must flow unprocessed through the flowlines to the

manifold and the host. As discussed previously, waxes in oils and hydrates in gas can

cause flow restrictions or complete plugging of flowlines and pipelines. Wax and hydrate

inhibitors are pumped from the host through umbilical lines and injected into the

flowlines at the wells or manifold. The umbilical lines also provide power and control

functions from the host to the wells and manifold.

There are many variations in the way the components can be configured for specific

project needs (Beckmann et al. 1996; Prichard et al. 1996; Mason and Upchurch 1996;

McLaughlin and Alford 1996). Figure 2.5 shows a schematic for a subsea system with

three wells and a manifold that is connected to a fixed platform host. Figure 2.6 shows

an artist's rendition of the components of a two well system with a manifold. The

equipment is large: weliheads and trees can be 5 m high, and a manifold can be 10 m

high. Sometimes the equipment and weliheads for a subsea system are integrated into a

single template structure placed on the seafloor. The deepest subsea system in the Gulf is

Mensa in 1,644 iii of water (McLaughlin 1998).

14

* !I

Figure 2.5. Subsea production Figure 2.6. Subsea production system

system configuration. components.
Ward: Dee pwater Technology

Transportation Systems

Processed oil and gas are transported from the floating production systems to shore in

separate pipelines. The pipeline transportation system for deepwater production is

functionally very similar to that which had been developed and used for production from

shallow water platforms for decades. The pipelines either go directly to shore and

connect to other pipelines there, or go to a shallow water platform that is connected to the

existing pipeline infrastructure that goes to shore. The oil and gas is processed

sufficiently to be of sales quality as it leaves the floating production system and is

commingled in the existing pipeline infrastructure. Pumps and compressors add energy

to the oil and gas, respectively, to ensure that it will flow to its destination or to the next

pumping or compression station. Chemicals to inhibit corrosion and to improve the flow

properties may be added as the production leaves the platform. The capability to pump

"pigs" through the pipelines to mechanically scrape and clean the pipe interior is

provided.

However, the sizes and lengths of deepwater pipelines and the seafloor terrain upon

which they rest have created technical arid economic challenges. Iii deepwater, the sizes

of the equipment and pipelines tend to be larger to handle the high production rates from

deepwater production systems, and thicker walled pipe is necessary to resist the higher

water pressures and operating pressures. Thus, the pipelines have become very heavy,

and different installation methods, such as J-lay and significant changes to the S-lay

system originally developed for shallow water, had to be developed. These heavy and

thickwalled pipes can buckle easily during installation as the pipe is laid on the seafloor,

and the laying process has to be carefully controlled. The thick-walled pipe also has

required the development of new welding techniques. The seafloor terrain in deepwater

is much rougher than relatively smooth, uniform, and gradually sloping seafloor

characteristic of the shelf. Hills, valleys, and ridges along with areas with relatively soft,

unstable bottoms and other areas with hard bottoms characterize the topography. The

features can cause instabilities and movement in installed pipelines. Pipeline routes must

be selected to avoid such areas, and the pipelaying process must be carefully controlled to

ensure that the pipeline avoids the features. Deepwater pipelines have been successfully

installed to water depths of about 1,800 m in the Gulf.

Deepwater pipelines are very expensive, and have been estimated to cost more than

$1 million per mile (DeLuca 1 999d). Thus, pipeline costs can be a significant factor in

the overall cost of a deepwater development, particularly when the development is a long

way from the existing infrastructure of pipelines.

15
Ward: Dee pwater Technology

Filture Deepwater Development Activities

Future Deepwater Production

Referring back to Figure 2.1, deepwater production during 2000 should easily exceed

I million barrels per day (MMBOEPD). If the linear trend continues, production could

increase six times the 1994 value, a six-fold increase in just 6 years.

Lease acquisitions, exploration drilling, and discoveries all suggest a continuing strong

increase in deepwatei- production from ever-increasing water depths (Ray 1998). The

Minerals Management Service (MMS) has estimated that total Gulf oil production in

2002 could reach about 2,000 MBOPD arid that roughly 70% would of the total oil and

gas production would be from deepwater. Deepwater leasing has been strong since 1994

and set record highs in 1997. Deepwater drilling has been active in 1999 and there have

been 12 new deepwater discoveries in the Gulf as of September (DeLuca l999b).

Further, there are 26 new deepwater drilling rigs expected to be deliveied by the end of

2000, and 10 of these will begin in the Gulf (DeLuca 1999c).

Estimates of the potential volumes of recoverable deepwater reserves range from 8 to

15 billion barrels of oil (Morrison 1997). The MMS 's most recent (1995) forecast of

Gulf of Mexico deepwater oil and gas potential was about 13 billion BOE (Bacigalupi et

al. 1995). As noted at a recent MMS Information Transfer Meeting (Oyne, December

1999), 370 deepwater wells have been drilled since 1995, with a number of significant

discoveries. The MMS announced the initiation of a new study to revise the estimated

potential in light of recent activities and experience, and it is expected that the estimated

potential will increase substantially. (Note that the estimated potential includes no

reserves from the relatively untested MMS Eastern Planning Area in the northeastern

Gulf).

The above indicates that the deepwater Gulf of Mexico will continue to be an important

oil and gas area. The ever-increasing water depths and the increasing distance from

infrastructure will continue to provide both technical and economic challenges. We next

examine some of the expected impacts on deepwater production systems.

Floating Production Structures

Spars

Spars have been studied for applications to deeper depths, and compared to TLPs (Huang

et al. 1999). Although the number of applications and experience is limited as of this

date, spars appear to be quite flexible and show a low cost sensitivity to increases in

water depth. Spars are significantly more cost effective than large TLPs at depths beyond

2,000 m, and thus are presently the favored concept for a production system with surface

wells for deeper water projects. The applications of a taut moorillg system using

lightweight synthetic (for example, polyester) rope and/or the use of a truss spar may

further reduce the cost and increase the effectiveness of spars.

16
Ward: Dee pwater Technology

Tension Leg Platforms (TLPs)

Large TLPs can be extended beyond the present deepest depth of 1,230 m. It is

theoretically possible to extend them to 3,000 m with the present tendon systems, but

spars are likely more cost effective in depths beyond 2,000 m (1-luang et al. 1999). To

maintain the small vertical motions that are a major TLP advantage, the tendon area and

weight must increase as depth increases to maintain a high vertical stiffness. The tendon

wall thickness must also increase to resist collapse in deeper water. Increases in the

weight of the conventional tendon system require a larger TLP hull to provide the

additional buoyancy, and the hull and tendon weights (and the TLP costs) increase

rapidly beyond 2,000 m. Several ideas for having lighter tendons (stepped diameter steel

tendons, composite tendons) for deepwater TLPs are being studied (Offshore Engineer

1999; Chianis and Poll 1997) and have the potential to significantly increase the depth at

which TLPs are cost effective choices.

The median discovery volume is about 100 MMBOE for the deepwater Gulf (Ray 1998).

Smaller reservoirs in about 1,000 m depth cannot be economically developed with large

TLPs, and the economics get worse with increasing depths. Several mini-TLP concepts

are being pursued to provide cost effective TLPs for these smaller reservoir volumes.

The Seastar concept has already been deployed in depths to 1,000 m, and the concept is

being studied for projects in depths to 1,850 rn (Matten et al. 1999; Furlow 1999a).

Mini-TLPs with three columns have been studied (Huang et al. 1999).

Floating Production Systems with Storage and Offloading (FPSO)

As the push to deeper water leases continues, new discoveries will be made that are

farther and farther from the existing pipeline infrastructure. With deepwater pipelines

costing $1 million per mile (DeLuca 1 999d), transportation costs for the produced oil and

gas can have a significant negative impact on project economics and may prevent certain

discoveries from being developed. Oil can be 'produced to an FPSO, stored, and then

offloaded onto a shuttle and shipped to shore. This transportation option can save the

cost of expensive deepwater oil pipelines. Thus, the FPSO can be an attractive deepwater

production system option for the Gulf (D'Souza 1999a,b).

FPSOs are often a ship-shaped vessel, but other large volume hulls that can accommodate

large storage tanks, e.g., a spar, could be used. Tankers are most commonly used because

of their large tanks, and tanker-based FPSOs can be either a conversion of a trading

tanker or purpose built. A large turret is built into the tanker, and the ship can rotate

relative to the tanker. The mooring system is attached to this geostationary turret. The

tanker is free to rotate around the turret, and weathervanes with the prevailing wind,

waves, and currents. The tanker-based FPSO cannot accommodate a drilling rig. A

MODU is used to drill subsea wells that are tied back to the FPSO through flowlines with

compliant risers, which are attached to the geostationary turret. The turret includes

provisions for containing the flow 'from the risers to the tanker. Associated gas produced

with the oil must be separated and either transported to shore by a gas pipeline, or

reinjected. The export riser for a gas pipeline would be also attached to the turret.

Figure 2.7 shows a schematic of a tanker-based FPSO.

17
Ward: Deepwater Technology

FPSO

with i Sh Topside Turret uttle Facilities Tanker A' Swivel

-. A Mooring Line/

'

Exp.oft...

Riser

"P'ro'd uction\\

Risers.

...

Figure 2.7. Schematic of tanker-based FPSO offloading to a shuttle tanker.

Today there are 66 FPSOs in service or under construction (McNeely et al. 1999). None

have been permitted in the Gulf to date. The MMS is conducting a study to prepare an

Environmental Impact Statement for the use of tanker-based FPSOs in the Gulf (George

et al. 1999). The MMS is also sponsoring a comparative risk analysis that will compare

the risks of an FPSO operating in the Gulf against the risks of other deepwater production

systems currently operating in the Gulf (Gilbert and Ward 2000). Both studies should be

completed in late 2000, and will be used by the MMS in their decisions to permit FPSOs

in the Gulf (Furlow 1 999d).

Subsea Production Systems

Subsea production systems will continue to be pursued as a means of lowering the

development costs for remote deepwater oil and gas fields through reducing or

completely eliminating the need for a floating production system and increasing the

recovery from subsea oil wells. The focus will be on subsea pressure boosting and flow

assurance.

Subsea pumps add energy so that produced fluids from subsea wells can flow to a distant

transportation infrastructure point, such as a shallow water platform or a floating

production system for processing and/or further pressure boosting. They also reduce the

backpressure on wells so that more oil can be produced from the reservoir as it is being

depleted and its pressure drops. Various solutions and systems are being developed.

Subsea multiphase pumps can pump a mixture of oil, gas, and produced water (Chiesa et

al. 1998). The produced fluids can also be separated using subsea separators prior to the pipeline (Radicioni et al. 1998). Gas can be separated from the produced liquids (oil and

18
Ward: Deepwater Technology

water), and the oil can be subsequently separated from the water. The oil can be pumped

into a pipeline using a single-phase pump, and the water can be reinjected into the

formation by another pump (Bringedal et al. 1999). These various pieces of separators

and pumps can be used as building blocks or components, and configured and integrated

into systems that fit the particular reservoir and development requirements (Pourier arid

Alary 1998). Other system components include system and well controls, motors, and

equipment to convert and control power, monitor well performance, and inject chemicals

for flow assurance. The integrated systems become complex. Components and

equipment has been prototyped and tested, and there have been limited pilot trials and

actual applications of integrated systems.

Control functions, power, and any chemicals are provided through an umbilical that is

connected to a host production system or transportation interface point. Umbilicals are

complex and costly, and their cost and installation difficulty increases with depth.

System maintenance is done using Remotely Operated Vehicles (ROVs) deployed from

surface vessels. MODUs are used to undertake well workovers and recompletions.

Flow assurance will continue to be an important consideration in the expanded use of

subsea well and production systems. In addition to the continuing development of

inhibitors, other techniques are being actively developed to prevent wax and hydrate

formation in flowlines. Flowline heating (Bass and Langner 1998), pipe-in-pipe (I-loose

et al. 1996), and insulated pipelines (Chin et al. 1999) are being developed and deployed

as a means to prevent heat loss leading to wax and/or hydrate formation. Trenching

pipelines into the seafloor is also being considered as an option for insulating deepwater

pipelines (Lokay 1999).

Further experience and developments will increase both the economic and technical

confidence and use of subsea wells and development systems in present water depths.

Currently offsets (distance from subsea wells to surface host system) of 20 to 25 miles

are considered to be "at the leading edge" (Furlow 1 999b). Offsets will increase with

experience and confidence. Additional development will also be needed as water depths

increase.

Transportation Systems

Oil and gas pipeline and flowline costs will continue to increase with increasing depth

and distance from infrastructure. Additional developments will focus on the design,

construction, and installation to reduce the costs of extrapolating current technology

(Coutarel 1998; Heerema 1998; Bonnell et al. 1999; DeLuca 1999b).

As discussed above, FPSOs provide an alternative to oil pipelines. However, there is

often a considerable amount of associated gas with oil production in the Gulf (Furlow

1 999e). The present options for handling gas include reinjection in the reservoir and

transporting to shore via a deepwater pipeline (Curole et al. 1997). As the opportunities

for reinj ecting gas are limited, most deepwater developments will include an expensive

19
Ward: Deep water Technology

deepwater pipeline. An alternative is to process the gas onboard the FPSO and convert it

to product that can be transported via a ship in a similar fashion as the use of shuttle

tanicers to transport oil. Several processes to convert gas to different liquid products are

being investigated (Verghese 1998; Agee 1999; Grimmer 1999; Rodvelt et al. 1999).

Though the fundamental idea is attractive and sound, the technical and economic

challenges of successfully deploying these processes on an FPSO are formidable. In

addition, liquefied natural gas (LNG) is being considered, but the gas volumes from a

deepwater development system needed to justify the costs of the special cryogenic

transportation vessels are quite large.

Future Technology Challenges

Experience to date has indicated the oil industry can, with evolving technology,

successfully develop deepwater production in the Gulf of Mexico in depths approaching

1,800 n-i in a safe, environmentally responsible, and economic manner. New technology

will be needed to continue this success and overcome the technical and economic

challenges of producing oil and gas in depths to 3,000 in. High rate, high ultimate

recovery wells, reduced project cycle time, and reductions in capital costs are important

to the economic success of deepwater developments, and will become even more critical

as depths approach 3,000 m.

Many technical challenges have been discussed in the previous section. There are other

technical challenges and needs that were not mentioned. Some of the more significant

topics include drilling wells in areas prone to shallow water flows (Jefferis et al. 1999),

deepwater risers and moorings (Kavanagh and O'Sullivan 1999; Ward et al. 1999), and

geotechnical properties of the deep ocean bottom (Dutt et al. 1997). Research and

development activities are underway, and future needs will continue to be addressed

through projects conducted and/or sponsored by individual companies, through joint

industry funded projects, and through broad industry sponsored programs such as

DeepStar (Verret and Hays 1999) and the Offshore Technology Research Center

(1988-1 999a,b). Teclmology assessment and research and development in support of

deepwater development and regulations are also being sponsored by the MMS through

their Technology Assessment and Research Program.

The worldwide interest in developing deepwater oil and gas has also led other research

activities overseas. These activities provide another source of ideas and technology for

meeting deepwater challenges in the Gulf of Mexico.

20
21

Ward: Deepwater Technology

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Bass, R.M. and C.G. Langner. 1998. Direct electric heating of pipelines. Proc. lO Deep

Offshore Technology Conference, New Orleans, LA

Beckmann, M.M., M.L. Byrd, J. bit, J.W. Riley, C.K. Snell, C. Tyer, and D. Brewster. 1996.

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Bonnell, S.A., M.S. Blackmore, and C.K.W. Tam. 1999. Pipeline routing and engineering for

ultradeepwater developments. Proc. 3 i' Annual Offshore Technology Conference,

Houston, TX. OTC Paper 10708.

Bourgeois, T.M. 1995. Auger tension-leg platform: conquering the deepwater Gulf of Mexico.

Journal of Petroleum Technology 47(4), Apr. 4, 1995.

Bringedal, B., T. Ingebretsen, and K. Haugen. 1999. Subsea separation and reinjection of

produced water. Proc. 3 1 " Annual Offshore Technology Conference, Houston, TX.

OTC Paper 10967.

Chianis, J. and P. Poll. 1997. Studies clear TLP cost, depth limit misconceptions. Offshore

Magazine, July.

Chiesa, G., L. Ursini, and G.F. Aggradi. 1998. A deepwater multiphase boosting system to

support the economical exploitation of deepwater fields. Proc. Deep Offshore

Technology Conference, New Orleans, LA.

Chin, Y.D., J.G. Bomba, and K.R.J. Brown. 1999. Structural and thermal optimization of cased

insulated flowlines. Proc. 3 l Annual Offshore Technology Conference, Houston, TX.

OTC Paper 11042.

Congdon, B. and C. Fagot. 1999. Gulf of Mexico deep water oil and gas production rises

dramatically. Special Information Release, Minerals Management Service.

Coutarel, A. 1998. MJ Lay, a modular deepwater pipelay system. Proc. 30th Annual Offshore

Technology Conference, Houston, TX. OTC Paper 8713.

Cranswick, D. and J. Regg. 1997. Deepwater in the Gulf of Mexico: America's new frontier.

US. Department of the Interior, Minerals Management Service, New Orleans, LA. OCS

Report 97-0004.

Curole, M.A., W.D. Grimes, C.B. Wallace, T.R. Judd, and R.W. Chin. 1997. Deepwater gas

processing challenges. Proc. 1997 76th Annual Convention of the Gas Processors

Association, San Antonio, TX.

D'Souza, R. 1999a. FPSO deployment promising for US Gulf of Mexico. Offshore Magazine

59(5).

D'Souza, R. 1999b. Major technical and regulatory issues for monohull floating production

systems in the Gulf of Mexico. Proc. 3 Annual Offshore Technology, Houston, TX.
Ward: Deepwater Technology

DeLuca, M. 1999. Four quick discoveries boost outlook for US Gulf deepwater, Offshore

Magazine, June 1999.

DeLuca, M. 1999. Record 25 deepwater discoveries worldwide in 1999. Offshore Magazine,

September 1999.

DeLuca, M. 1999. Twenty-six deepwater drilling units set for delivery by year-end 2000.

Offshore Magazine, September 1999.

DeLuca, M. 1999. Product transportation becoming deciding issue in deepwater development.

Offshore Magazine, November 1999.

Duff, RN., W.S. Rainey, T.K. Hamilton, J.H. Pelletier, and Ed-I. Doyle. 1997. Recent advances

in deepwater Gulf of Mexico geotechnical investigations. Proc. 291h Annual Offshore

Technology Conference, Houston, TX.

Furlow, W. 1999a. Mini-TLP may join Spar as workhorse in deepwater production. Offshore

Magazine, July 1999.

Furlow, W. 1999b. What technologies will be required for 100-mile tiebacks? Offshore

Magazine, July 1999.

Furlow, W. 1999c. Record setting deep-draft caisson set for installation. Offshore Magazine,

September 1999.

Furlow, W. 1999d. Turret Location, abandonment issues studied for US Gulf FPSO. Offshore

Magazine, September 1999.

Furlow, W. 1 999e. Is associated natural gas an asset or burden?. Offshore Magazine, November

1999.

George, J.E., W.J. Parker, and D.J. Cranswick. 1999. FPSO Environmental Impact Statement:

What is happening? Proc. 3i Annual Offshore Technology Conference, Houston, TX.

OTC Paper 10705.

Gilbert, R.M. and E.G. Ward. 2000. Planned approach for comparative risk analysis of

deepwater production systems in the Gulf of Mexico. Proceedings of OMAE 2000, 19111

International Conference on Offshore Mechanics and Arctic Engineering , New Orleans,

LA.

Grimmer, P. 1999. GTL: Perspectives of an Oil Company as a Technology Developer. Proc.

3 l Annual Offshore Technology Conference, Houston, TX. OTC Paper 10760.

Hays, P.R. 1996. Steel catenary risers for semisubmersible based floating production systems.

Proc. 28111 Annual Offshore Technology Conference, Houston, TX.

Heerema, E.P. 1998. Major deepwater pipelay vessel starts work in North Sea. Oil and Gas

Journal 96(18), May 4, 1998.

Hoose, J.W., D.R. Schneider, and E.L. Cook. 1996. Rocky Flowline project - the Gulf of

Mexico's first reeled pipe-in-pipe. Proc. 28h1 Annual Offshore Technology Conference,

Houston, TX.

Huang, E., Y. Luo, and J. Zou. 1999. Concept evaluation of deepwater surface tree floating

systems. Proc. 8l SNAME Offshore Symposium, Houston, TX.

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overcoming adversity in 4000 ft. of water. Proc. 31 Annual Offshore Technology

Conference, Houston, TX. OTC Paper 10754.

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23

Ward: Deepwater Technology

Judd, T.R. and C.B. Wallace. 1996. Auger Platform: Debottlenecking and expansion of fluid

handling facilities. Proc. 1996 SPE Annual Technical Conference and Exhibition,

Denver, Co.

Kavanagh, K. and O'Sullivan. 1999. Deepwater flexible, steel and hybrid risers: performance

and technical challenges. Proc. 8thi SNAME Offshore Symposium, Houston, TX.

Kibbee, S. 1996. TLP Technology SeaStar minimal platform for small deepwater reserves.

Offshore Magazine, June 1996.

Kibbee, S.E., S.J. Leverette, K.B. Davies, and R.B. Mallen. 1999. Morpeth SeaStar Mini-TLP.

Proc. 31h11 Annual Offshore Technology Conference, Houston, TX. OTC Paper 10855.

Kirkland, K.G., E.M. Richardson, and C. Hey. 1996. DeepStar evaluation of subsea trees and

manifold concepts. Proc. 28111 Annual Offshore Technology Conference, Houston, TX.

Krieger, W.F., J.C. Heslop, B.E. Lundvall, and D.T. McDonald. 1999. Genesis Spar Hull and

Mooring System: Project Execution. Proc. 31 11, Annual Offshore Technology

Conference, Houston, TX. OTC Paper 10798.

Lokay, B. 1999. How effective is flowline burial as a thermal insulation method? Offshore

Magazine, September 1999.

Mason, P.G.T. and J.L. Upchurch. 1996. Seastar: Subsea cluster manifold system design and

installation. Proc. 2gth Annual Offshore Technology Conference, Houston, TX.

Matten, R., K. Davies, S. Kibbee, and S. Leverette. 1999. SeaStar-fast track, low cost, proven

the mono-coluinn TLP with many Deepwater Applications. Proc. 8th SNAIVIE Offshore

Symposium, Houston, TX.

McLaughlin, D.C. 1998. Mensa Project: An overview. Proc. 30t1, Annual OTC, Houston, TX.

OTC Paper 8576.

McLaughlin, D.C. and B.J. Alford. 1996. Popeye Project: An overview. Proc. 28th Annual

Offshore Technology Conference, Houston, TX.

McNeely, P., R. Kota, P. Powell, P. Leitch, and R. D'Souza. 1999. FPSO fabrication escalating

as subsea, flow assurance mastered. Offshore Magazine 59(8).

Meyer, E.E., M. Al-Sharif, and J.L. Upchurch. 1998. An overview of the Neptune Phase IV

Subsea Tieback. Proc. lOhhl Deep Offshore Technology Conference, New Orleans, LA.

Morrison, D.G. 1997. Low-cost designs for facilities in shelf and deepwater developments.

OMAE JONC Workshop, Yokohama.

Offshore Engineer. 1999. Lightened tethers give TLPs new life. Offshore Engineer Magazine,

September 1999.

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Offshore Technology Research Center. 1988 - 1999b. Annual report. A report prepared by the

Offshore Technology Research Center, Texas A&M University.

Pourier, N. and V. Alary. 1998. Flow enhancement methods for deep water risers. Proc. 10111

Deep Offshore Technology Conference, New Orleans, LA.
Ward: Deep water Technology

Prichard, R.M., K.P. DeJoim, P. Farrell, C. Baggs, and D. Harris. 1996. Pompano subsea

development: Production control system and umbilicals. Proc. 28 Annual Offshore

Technology Conference, Houston, TX.

Radicioni, A., G. D'Alosio, and A. Baker. 1998. CoSSP: A configurable subsea separation and

pumping system for water removal and disposal. Proc. l0' Deep Offshore Technology

Conference, New Orleans, LA.

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June 1998.

Rodvelt, S., D. Schanke, and P.E. Fosen. 1999. Offshore gas to liquids. Proc. 3lh1 Annual

Offshore Technology Conference, Houston, TX. OTC Paper 10764.

Smith, J. 1999. Ursa A-7 well produces 50,150 boe/d for Shell. Offshore Magazine, November

1999.

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Design, fabrication and transportation. Proc. 3 1111 Annual Offshore Technology

Conference, Houston, TX. OTC Paper 10754.

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commercialisation via floating production systems. Conf. Paper at DOT, New Orleans,

LA.

Verret, A.J. and P.A. Hays. 1999. Deepstar's Program related to FPSO's. Proc. 31t1 Annual

Offshore Technology Conference, Houston, TX.

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24
Chapter 3: Geology

William R. Bryant and Jia Yuh Liu

Basin History 25

Study History 26

Seafloor Morphology 26

Structures 29

Salt 29

Faulting within the Continental Slope 36

Sedimentation in the Northern Gulf of Mexico 37

Sediment Sources and Drainage Patterns 38

Depocenters 40

Continental Slope Basins 42

Salt Induced Physiography of the Upper and Middle Continental Slope 43

Salt Induced Physiography of the Lower Continental Slope 46

Geohazards 49

Literature Cited 54

Basin History

The Gulf of Mexico is an Atlantic-type passive continental margin (Martin 1978).

During the Late Triassic, rifling occurred between the North American plate and the

AfricanlSouth American and Eurasian plates. As the North American plate drifted away

from the Africa! South American and Eurasian plates, the Gulf of Mexico basin was born

in the stretched zone. Seawater flowed intermittently into the basin, depositing over 3 km

of salt during the Late Middle Jurassic (Martin and Bouma 1978). During the Late

Jurassic, carbonate deposition was dominant. During the Middle Cretaceous, slow

subsidence of the carbonate shelves with little elastic input resulted in a reef system being

built (Stuart City/Lower Cretaceous reef trend) that extended from southern Texas

eastward to southern Louisiana to the shelf edge of Florida and to the eastern Campeche

Escarpment. In the Late Cretaceous-Paleocene, due to a Laramide orogeny in the interior

continent, detrital sediments started to flux into the northern and western Gulf of Mexico

(Coleman et al. 1986). The Gulf of Mexico was inactive during the late Eocene; in the

Oligocene, the southern Rockies underwent intensive volcanism; in the early Miocene,

the normal faulting molded the relief we see today; during middle Miocene and Pliocene,

the western U.S. underwent a series of broad uplifts (Winker 1982). During the

Cenozoic, the total thickness of sediments was estimated to be over 15 km in the

northwestern Gulf of Mexico (Martin and Bouma 1978). In the southern and eastern part

of the Gulf of Mexico, carbonate deposition remained active since the Late Jurassic-Early

Cretaceous, with only small amounts of detrital sediments being deposited in the

Quaternary (Coleman et al. 1986).

25
Bryant and Liu: Geology

Study History

In the 19th century, geological work was conducted along the borders of the Gulf of

Mexico. Early investigations were by individual scientists arid later by geological survey

institutions. Our basic understanding of the geology of the Gulf of Mexicowas mainly

attributed to the efforts of the petroleum industry. Before the 1920's, survey methods

were limited to surface observations used to find oil and gas seeps, and torsion balance

refraction seismograph to locate the shallow salt structures. In the 1930's, the invention

of the reflection seismograph not only doubled the oil and gas discovery but also enabled

us to examine the geology of the Gulf to greater depths and detail. The thousands of oil

wells drilled helped reveal detailed stratigraphy of the areas in and around the basin. The

first offshore exploration in the Gulf of Mexico started in 1938, and the first offshore

seismic survey was conducted in 1944 (Salvador 1991). Since then, the study of Gulf of

Mexico geology has varied from large-scale sediment studies in the northern Gulf of

Mexico (e.g., Fisk et al. 1954; Fisk 1956; Moore and Scruton 1957; Shepard 1959;

Scruton 1960), to salt tectonics and seismic stratigraphy (e.g., Ewing and John 1966;

Lehner 1969; Bouma et al. 1978; Doyle et al. 1979; Bouma 1981), to sedimentology and

mineralogy (e.g., Roberts et al. 1986) and the cause and results of erosion and turbidite

deposition (e.g. Martin and Bouma 1982; Roberts et al. 1986), to sequence stratigraphy.

Seafloor Morphology

Figure 3.1 illustrates the physiography of the Northern Gulf of Mexico.

The Early Cretaceous carbonate areas that comprise the Florida Terrace and the

Campeche Terrace, with dips less than 2° and 4°, respectively, are relatively flat. On the

seaward side, these terraces are bound by the Florida Escarpment and the Campeche

Escarpment. The two escarpments represent a depth difference ofup to 2,300 rn, cover a

40 km wide area, and display the steepest slope gradients (up to 40°) of the region. The

West Florida upper continental slope, in water depths of about 500 in, contains a terrace

that was composed of Miocene sediments (Coleman et al. 1986). A series of gullies and

small canyons that crease the upper and middle slope are associated with mass movement

processes that range from creep to massive slides to gravity-induced folds tens of

kilometers long (Doyle and Holmes 1985). The northern part of the Campeche

Escarpment also contains a series of gullies and small canyons, s?aced about 5 km apart,

that cut through the slope. At 27.3°N and 25.7°N, two 4,000 km sized, dual-folded

zones lie parallel to the Florida Escarpment. The Vernon Basin is located between these

two fold zones.

At the northern end of the Florida Escarpment is the NNE-SSW aligned De Soto Canyon,

which separates the carbonate-dominated Florida platform from the

terrigenous-dorninated environment to the west. West of the De Soto Canyon many

NNW-SSE directioned canyons formed during the late Wisconsin sea-level lowstand.

These canyons include Dorsey and Sounder Canyons. Except for a canyon due west of

the De Soto Canyon, that cuts through the shelf break, all other canyons seem to originate

at a water depth of 400 in or deeper and extend more than 60 km basinward. To the west

26
usNAtt

Prection: AFirst StaxrdSe nd StanCerdml Han

Figure 3.1. Bathymetric map of the northern Gulf of Mexico deepwaterarea. Water depths range from about 15Cm in th

in the Sigsbee Plain. This bathymetry is a shaded relief based on multibeam and digitized seismic data. The 

azimuth of 45° and an angle of 450
Bryant and Liii: Geology

of these canyons are a series of pancakeshaped, 5 to 15 km wide salt domes, which

become more scattered and smaller iii size in a basinward direction. The first sub-salt

well drilled in the Gulf of Mexico was by Exxon on one of the pancake domes called

Mickey.

The largest submarine canyon in the Gulf of Mexico is the U-shaped Mississippi Canyon,

which is a late Pleistocene erosional feature that cuts through the shelf break (Coleman et

al. 1986). The canyon has a width of about 30 km. Approximately 120 km fromthe

canyon mouth the Pleistocene Mississippi Fan appears. The caiiyonlfan complex extends

for more than 500 km past the shelf edge onto the Sigsbee Abyssal Plain. The

Mississippi Fan occupies an area of about 145,000 lun2.

The Mississippi fan is a large deepsea fan consisting of a broad arcuate accumulation of

predominantly Pliocene and Pleistocene sediments. The Mississippi Fan is flanked on

the east by the West Florida carbonate platform and on the north and west by the Texas

Louisiana Continental Slope. The deeper parts of the fan merge with the Florida Plain to

the southeast and with the Sigsbee Plain to the southwest. Significant contributions to the

fan have come from sources other than the Mississippi Embossment, particularly from

the Campeche Escarpment, the Florida Escarpment, and the De Soto Canyon (Feeley et

al. 1985). Feeley et al. (1985) suggest that eight seismic sequences comprise the

Plio/Pleistocene section of the Mississippi Fan and these sequences are bounded by basin

wide unconformities. They state that the basic transport/deposition mechanisms are mass

transport (slump, debris flow), turbidity current flow (channelized and unchannelized),

and pelagic deposition. They suggest that mass transport appeared to be the dominant

process for deposition and may account for up to 30% of the total amount of sediment

deposited on the Fan. Channelized lobe development (channel and overbank deposits)

apparently occurred late in the evolution of the majority of sediment sequences observed

and that it may be associated with a rise in sea level.

The Texas-Louisiana Slope is located west of the Mississippi Canyon. It occupies an

area of about 120,000 km2 and contains the widest slope (up to 230 km) and the most

rugged morphology in the northern Gulf of Mexico. There are over 105 domes and

basins that have been named and approved by the U.S. Board of Geographic Names

(BGN) (Bouma and Bryant 1994). Although the average continental slope gradient is

less than 10, a local slope gradient can exceed 40°. The domes and basins in the area

range from 5 to 30 km in diameter. The domes are prominent on the upper slope, they

increase in size to become salt massifs that surround basins on the middle slope, and lie

under basins on the lower slope (Simmons 1992). The basinward termination of the

slope is the Sigsbee Escarpment, which is a surficial expression of the basinward salt

front (Moore et al. 1978). This salt front displays an extruded tongue feature with an

elevation of about 600 m above the continental rise, and comprises gradients ranging

from 100 to 20°. Several canyons break the semi-continuous Sigsbee Escarpment: Green

Canyon, Famella Canyon, Cortez Canyon, Bryant Canyon, Keathley Canyon, and

Alaminos Canyon. There are fan deposits between the Green and Farnella Canyons that

merge with the Mississippi Fan. Some channels are visible on top of these coalesced fans

that radiate outwards in a basinward direction. Southward of Bryant Canyon lays the

28
29

Blyant and Liii: Geology

Bryant Fan, which is about 25,000 km2 in size. This fan reaches to the Cortez Canyon in

the east, Keathley Canyon in the west, and extends about 170 km south from the canyon

mouth. The lack of fan deposits in the mouth of Keathley Canyon suggests that this

canyon is structurally controlled (Lee 1990). Due east of Green Canyon is Green Knoll,

a salt diapir with over 400 m of relief Using a submersible, a brine pooi and active seeps

were found on the east side near the crest of the knoll, although no salt exposure was

observed. The oversaturated brine waters coming out from the seeps on the knoll have

braided drainage pafterns (Roberts et al. 1991). Besides the Green Knoll, brine seeps

were also observed in Orca Basin (Shokes et al. 1977), in the East Flower Garden Bank

area (Brooks et al. 1979), and in the drill hole at Site 66 (Manheim and Bischoff 1969).

In the northwest portion of the Gulf of Mexico, the Rio Grande Slope separates the

Texas-Louisiana Slope from the East Mexico Slope. Similar to offshore Mississippi and

Alabama, the Rio Grande Slope displays more than 15 pancake-shaped domes. In the

area are a series of canyons that cut through the shelf break. The canyons are broad and

tend to be located between domes or banks. Between Price Spur arid Calhoun Dome,

there are many tightly spaced canyons that form a 22 km wide valley with a 7 km wide

bank. There are only a few small canyons, hundreds of meters wide that cut through the

Rio Grande Slope. A number of the canyons observed may be an artifact due to the lack

of detailed multibeam data in the western Gulf At the seaward end of the western

continental slope, the Perdido and Alaminos Canyons appear. The submarine fans that

downdip from these two canyons merge basinward and are much smaller in size than the

Bryant Fan (about 5,000 km2).

To the south of the Rio Grande Slope is the East Mexico Slope. The northern portion of

the East Mexican Slope is mainly controlled by diapiric activity. The southern side of the

East Mexico Slope represents the northern end of the Mexican Ridges (Bryant 1986). At

the basinward limit of the Texas-Louisiana, Rio Grande, and East Mexico Slopes, is the

Western Gulf Rise, which is about 40 km wide. In between the Western Gulf Rise, the

Mississippi Fan, and the Campeche Escarpment is the Sigsbee Plain, which has a water

depth of about 3,700 m. In between the Campeche Escarpment and the Florida

Escarpment is the Florida Plain, with a water depth of about 3,400 m. Except for the

Sigsbee Knolls, which are up to 250 m above the Sigsbee Plain, these two abyssal plains

have an average slope gradient of less than 0.5° and are the flattest mapped regions in the

deepest part of the Gulf of Mexico.

Structures

Salt

Of all the geological events taking place within the northwestern Gulf of Mexico the

action of salt, halokinesis, is the most striking and influential in the modification of the

structure of the continental slope. Figure 3.2 displays the location of salt on the

northwestern Gulf of Mexico Continental Margin.
Figure 3.2. Salt and fault distributions in the northern Gulf of Mexico (Adapted from: Diegel et al. 1995). This map only 

distributions north of 26°N and westof88°W. The light-shaded background is the bathymetry map as show
Bryant and Liw Geology

Bradshaw and Watkins (1995) have depicted salt migration pathways in the northwestern

Gulf of Mexico as a complex interplay of basement architecture with subsequent

depositional loading. They suggest that "the original deposition of Middle Jurassic

Louann Salt was systematically offset right lateral along-strike across a series of

northwest-southwest orientated transfer faults." They suggest that two major phases of

postrifi salt migration are evidenced in the shallow Neogene allochthonous salt sheets

across the Sigsbee slope region, and are remnants of a deeper Paleogene salt canopy.

They state that "A Paleogene canopy was emplaced when salt migrated vertically out of

the original middle Jurassic salt basins due to early gravity spreading and differential

sediment loading down dip of Lower Cretaceous and Paleocene shelf margins." The

geographical distribution of this canopy was essentially controlled by earlier

segmentation of Middle Jurassic salt basins across transfer faults, occurring most

landward to the southwest and systematically farther seaward across each transfer fault.

Generation of the Perdido foldbelt was synchronous with emplacement of the Paleogene

salt canopy due to minor lateral migration of mother salt over deep basinal strata.

Bradshaw and Watkins (1995) state that rapid remobilization of Paleogene allochthonous

salt occurred in the Neogene when shelf margins began to prograde into the offshore

Texas area. They suggest that variations in thickness of the sediment across the margin

created lateral pressure gradients within the underlying salt. The pressure caused the salt

to move from areas of higher pressure to areas of lower pressure, or seaward and upward.

They suggest that in some cases salt migrated seaward for long distances and that the

"remobilization of salt along the Neo gene shelf margins was not synchronous." They

state "that shelf margin loads encountered first one basin then another because of

irregular location of salt basins and the irregular advance of the shelfedge." They figured

that the " loading and spreading combined to displace salt over 100 km onto the Sigsbee

slope and the seaward migration of salt created extensive salt withdrawal basins."

Salt deposited in the Gulf of Mexico during late Triassic and Jurassic contains a group of

minerals that include sodium and potassium chlorides (Nelson 1991). In the Gulf of

Mexico, 90% to 98% of the salt is composed of halite (NaC1), which has a density of

2.16 gcc1 (Halbouty 1979; Kupfer 1989). The most common impure salt is anhydrite,

which has a density of 2.96 gcc'. With 95% of halite and 5% of anhydrite, the bulk

density of the salt would be about 2.2 gcc' (Nelson 1991). On top of the salt diapirs, the

meteoric waters may resolve the halite and keep the less soluble anhydrite (Murray 1966;

Posey and Kyle 1988), which forms cap rock (Murray 1966; Posey and Kyle 1988). As

time progresses and when hydrocarbons are present, the anhydrite lying in between the

salt diapir and cap rock will accumulate arid may convert to calcite arid hydrogen sulfide

(Feely and Kulp 1957; Kyle et al. 1987).

In the northern portion of the Gulf of Mexico basin, salt was deposited in interior basins

and in coastal and offshore basins (Martin 1978, 1980a). The interior basins include

onshore Texas, north Louisiana, Arkansas, Mississippi, Alabama, the Florida panhandle,

and the northeast Gulf of Mexico. The coast and offshore basins include onshore south

Louisiana and southeast Texas, and Louisiana and Texas shelf and slope (Humphris

1979; Martin 1978, 1980). In the south of the Gulf of Mexico, the salt basin includes the

31
Bryant and Liu: Geology

Campeche Escarpment, the Bay of Campeche, and onshore southern Mexico. Figure 3.3

illustrates the location of major salt basins in the Gulf of Mexico (Simmons 1992).

Salt and salt structures in the Gulf of Mexico usually have many shapes. Simmons

(1992) described various salt structures in the Gulf of Mexico as follows. A salt pillow is

a circular or elliptical shaped salt structure with conformable contact with overlying

sediments. A salt stock or salt dome is a cylindrical and usually mushroom-shaped salt

that pierces through the overlying sediments. Salt massifs are moderate-sized salt

structures with irregular shape and large overhangs (Martin 1984). A salt wedge is a

large landward-dipping structure that usually thickens with depth (Ray 1988). A salt

tongue is an asymmetrical lobe that usually spreads downdip. A salt sill is a

subhorizontal intrusion at shallow depths (Nelson and Fairchild 1989). Salt canopies, a

common structure on the upper and middle continental slope off Texas and Louisiana, are

two coalescing salt structures (Jackson and Talbot 1989). A salt diapir is salt that has

penetrated the overlying sediments. Other terms used to describe salt include salt

halokinesiswhich is salt movement under gravitational movement; autochthonous salt

is salt that is still attached to its source; allochthonous salt is salt that has being separated

from the mother salt. In the Gulf of Mexico, the majority of salt is allochthonous salt that

has gone through many stages of mobilization and emplaced at a shallower depth than the

original salt deposit.

Jackson and Talbot (1986) classified the driving forces of salt into buoyancy, differential

loading, gravity spreading, thermal Convection, and halotectonics. The low density of

salt makes it more buoyant compared with their surrounding sediments and often rises as

diapiric structures. In the offshore Texas and Louisiana, assuming a salt average density

of 2.2 gcc1, the depth to achieve a sediment density that is greater than the salt density is

about 1,500 m, while to support a salt diapir to rise from the source layer to the seafloor

requires 3,600 m thick of sediment (Nelson 1991). The size of the source and the purity

of the salt will also influence the density inversion depth (Trusheim 1960). Unlike the

theory of buoyancy, differential loading does not require a deeper depth (Jackson and

Galloway 1984). The differential loading can be due to slope gradient like in a prograded

shelf where there is difference in sediment loading and water colunm. A differential

loading can also occur if there is lateral facies change (Simmons 1992). Gravity

spreading is gravity-induced lateral spreading (Ramberg 1981). Gravity spreading

occurred at or above the neutral buoyancy level and is believed to be the cause of

extensive salt tongues observed in the Gulf of Mexico (Ramberg 1981; Jackson and

Talbot 1986; Nelson and Fairchild 1989). I-ialotectoriics is the influence caused by

tectonics and the relief caused by the growth fault extension and basinward compression

may have the relief that can initiate salt buoyancy and differential loading. Figure 14

illustrates the nature of the salt nappe as it appears on the lower continental slope. The

seismic characteristics of intraslope supralobal basins are also illustrated. Figure 3.5

shows the termination of the salt nappe at the Sigsbee Escarpment.

32
East;Texas, North. ouisiana 8'aItBagin SItin Figure 3.3. Major salt basins in the northern Gulf of Mexico (Adapted from: Simmons 1992). Gray color represents the

Stippled areas represent major salt basins, which contain original Middle Jurassic salt greater than 1,000 m t
17E 

Two-way travel time (sec) 

o 

(0 0) 4 ) 01 4 G) N) 

- 0 
35
Bryant and Liu: Geology

The general belief about salt evolution in the northwestern Gulf of Mexico is that salt has

flowed upward from its mother source and spread laterally basinward. Buried salt can be

remobilized and can pierce through its overburden and spread basinward many times. In

terms of chronological evolution of salt in the northwestern Gulf of Mexico, West (1989)

stated that salt moved seaward from the effects of sediment loading and formed a salt

swell in the seaward portions of the depocenter during the middle Miocene. The salt

swell became diapiric and pierced through the overburden and caused seafloor relief

during the upper Miocene. Growth faults and salt swells blocked sedimentation and

allowed the salt to be pushed further seaward during the upper Miocene-Pliocene times.

Continued sediment deposition helped the salt's upward and outward movement (early

Pleistocene). Salt prograded seaward due to further rapid sediment loading (late

Pleistocene).

Faulting within the Continental Slope

In a continental slope environment, we usually expect extension in the upper slope,

compression in the lower slope, and translation in the middle slope. In the Gulf of

Mexico, even with a regional slope gradient of less than 1°, the extension combined with

rapid subsidence, differential compaction, and gravitational sliding caused normal

faulting in the shelf edge and upper continental slope. These normal faulting usually are

listric-shaped and many are down-to-the-basin faults that die out on top of a decollement

layer, which usually is salt or shale (Nelson 1991). As the progradation of the depocenter

proceeds the growth faults may invert to counter-regional growth faults with

mounded-shaped structure on the downdip side (Seni and Jackson 1989).

In the lower continental slope, the compression is manifested by folding and thrusting.

Examples of these folding and thrusting are the shallow-depth deep-rooted Mississippi

Fan Fold Belt and Perdido Fold Belt (Blickwede and Queffelec 1988; Weirner and

Buffler 1989). Some of these fault planes have a span of 30 m at the seafloor and

provided conduits for gas and fluids (Roberts et al. 1986).

In the translation zone of the middle continental slope of the northwestern Gulf of

Mexico, the structure is a mixture of extension, compression, and translation. In the

intraslope basins, normal faulting has been observed on the top of salt diapirs, in the

upper wall along the basin rims, and on the basin floors. In these extension areas, the

cause is a combination of salt diapiric activity, steep slope gradients, and salt withdrawal.

In the salt diapir case, faults are usually high-angled with a radial pattern, and die down

and away from the diapir. Figure 32 illustrates the distribution of major faults in the

northwestern Gulf of Mexico.

36
37

Bryant and Liv: Geology

Sedimentation in the Northern Gulf of Mexico

The sedimentation patterns in the Gulf of Mexico are mainly influenced by

geornorphology, salt halokinesis, faulting, sea level changes, and sediment input.

The last major sea level lowstand, the Wisconsinan, occurred about 18,000 yr ago, and

during that time sea level dropped about 60 to 120 m (Bloom 1983). During the early

Holocene (14,000 to 11,000 yr BP), sea level rose rapidly. The rapid melting of the ice

sheet created large discharges that carried large amounts of sediments that were deposited

on the outer shelf and upper slope in a short time. The rapid deposition and burial

produced overpressured sediments on steep slopes that caused instability and mass

movements and sediment gravity flows (Prior and Coleman 1978; Prior and Coleman

1980; van den Bold et al. 1987). The Mississippi River during the last lowstand in sea

level, late Wisconsinan, carried more than 13 times its current sediment load (80 x 1011

kg-yf' compared with 6 x 1011 kgyr; Perimutter 1985). When sea level was low,

sediments migrated seaward, which caused rapid build-out in front of the prograding

delta lobes (Coleman et al. 1986). During the Holocene sea level rise, with little

terrigenous sediment input deposited sediments a much slower rate (<10 cm-kyf') (van

den Bold et al. 1987).

During sea-level highstand, the majority of the coarse-grained sediments are trapped

within the shelf province and hemipelagic sediment settling became dominant in the deep

water (Bryant et al. 1995). Sea-level highstand deposits tend to be parallel laminated but

are highly bioturbated and comparatively thin compared to sea-level lowstand deposits.

From 60 piston cores taken in the Gulf of Mexico, Davies (1972) observed the upper 20

to 50 cm layer of Globigerina ooze overlying argillaceous lutite and sand/silt interbeds

that range from 1 mm to 150 cm in thickness. From radiocarbon dating, Davies

concluded the sharp base of the ooze is the 11,000 yr ago Pleistocene-Holocene boundary

(Ewing et al. 1958). The upper clay-sized ooze layer displays parallel lamination and

bedding, and is attributed to the pelagic settling. In the Mississippi Fan, single laminae

may represent seasonal fluctuations, whereas the single bed may represent long-term

(100 yr) fluctuation of climate (Huang and Goodell 1970).

Except for the carbonate platforms offshore Florida and Campeche, the detrital sediments

in the deepwater Gulf of Mexico are mainly transported subaerially by river systems

during sea-level lowstands (Beard et al. 1982). During lowstands, the canyons cut into

the outer shelf and upper slope and became conduits that carried coarser-grained

sediments to the deep water. The transporting mechanisms are gravity-induced mass

movements (Middleton and Hampton 1976). Depending on the interaction of the grains

with the density flow, they may be turbulence-supported turbidity currents or

matrix-strength-supported debris flows. Movement can also result from less defonned

whole body movement along a surface or slides, or from slump deposits.

Turbidity currents are considered the most important transportation mechanism that

carried sand and silt beds thicker than 5 cm in the Gulf of Mexico. Turbidite beds display

erosional bases, graded bedding, and Bouma sequence C-D-E. The bioturbation only

occurs in the uppermost part. The bioturbation is not affected by the size of the sediment,

but rather a factor of the sedimentation rate. In the Bouma sequence, C is current ripple
Bryant and Liu: Geology

lamination; D is parallel lamination; £ is a structureless unit. In the Gulf of Mexico, a

complete Bouma sequence is rare. A (graded bed) and B (parallel lamination) are usually

missing. The absence of A and B iii the sediment of the Gulf of Mexico is due to the

predominance of fine sand, silt, and clay sediment (Davies 1972).

The influence of bottom currents on the distribution of sediments has been controversial

in the deepwater environment. Roberts et al. (1982) suggested that certain bedform

erosions in the Gulf were the results of oceanic currents. Martin and Bouma (1982)

suggested slumping was the cause of bed truncations, and van den Bold et al. (1987)

believed bottom currents were insignificant in a water depth deeper than 200 m in the

Gulf of Mexico. Recent Texas A&M University Deep Tow surveys in the Gulf of

Mexico on the lower continental slope confirmed that there were deepwater processes

which produced 20 rn-spaced, 5 m wide mega-furrows that were sub-parallel to the

bathymetric contour lines southward of the Sigsbee Escarpment. These mega-bedforms

indicate swift bottom currents in water depths of over 3,000 m.

Sediment Sources and Drainage Patterns

Due to the mixing and unidentifiable origin of clay minerals after sediment transport

(Hagerty 1970), Davies (1972) used heavy minerals to trace the origin and source of the

sediments in the Gulf of Mexico and categorized five mineral assemblages in the surficial

sediments. The ma] or sources of sediments from the north are the Rio Grande, Colorado,

Brazos, and Mississippi Rivers. By running a watershed model based on the bathymetry,

Liu and Bryant (1999) also delineated similar drainage paths in the Gulf of Mexico.

13'igure 3.6 illustrates the sediment provinces and drainage patterns of the northern Gulf

of Mexico. Four drainage systems were identified: western, central, northeastern, and

southeastern continental slope areas. All drainage patterns align well to today's major

river systems in the northern Gulf: sediment sources from the west (Rio Grande system),

the northwest (Brazos and Colorado Rivers systems), and the north (Mississippi River

system). The carbonate-dominated platforms in the eastern and southern Gulf of Mexico

show fewer drainage paths and suggest less contribution from the southern rim. These

drainage systems merge basinward and can be up to 500 km long. The western system

includes E-W oriented patterns on the East Mexico Slope and sinuous NW-SE oriented

patterns on the Perdido Slope. In the East Mexico Slope area, the paths coincide with

gully/canyon structures. On the Perdido Slope, the Perdido and Alaminos Canyons are

the major tributaries at the base of the slope. The drainage paths tend to move around salt

domes on the upper slope of the Perdido Canyon. On the upper slope of the Alaminos

Canyon, the paths tend to go through the intraslope intralobal basins.

In the area located offshore Texas and Louisiana, sinuous drainage patterns are mainly in

the N-S directions. The path that crosses Keathley Canyon is straight and follows a

major fault that formed the canyon (Lee 1990). The drainage paths to the east of

Keathley Canyon are highly irregular and are located between salt domes on the upper

slope, and through the intras lope supralobal canyons of the lower continental slope. In

Farnella Canyon, the drainage path cuts through the slope and the canyon, while in other

canyons, such as Bryant, and Green Canyons, the drainage paths originate at the mouth of

the canyons.

38
N Sun Shading: azimuth 45 ee9aIIon 45 26N- W97 W96 W95 W94 W9 W92 W91 W90 wor WW W87 w5 

jrrNA0 Prec*lon: AJ6a

F9i1 StVIOajd 

S9cr4a 9an6a

emreMeriaa

Dala $69169: Se Beam (NG

ty.MoSIrnic

Figure 3.6. Bathymetry derived drainage patterns in the northern Gulf of Mexico deepwater environment. The black lin

drainage meanders and passes in between salt domes on the upper slope. The drainage is straighter and tlower contInental slope. The light-shaded background is the bathymetry map as shown in Figure 3.1
Bryant and Liu: Geology

In the northeastern area, many drainage paths align in a NW-SE direction on the

Louisiana-Mississippi Slope and few paths align in an E-W direction on the West Florida

Slope. In the Mississippi Canyon area, the drainage path goes down the canyon and

swerves to the east at a water depth of about 2,800 rn. To the east of the Mississippi

Canyon, drainage paths tend to go around salt domes and the base of the Florida

Escarpment, and merge at a water depth of about 3,100 m. This merged drainage path

then goes between the Mississippi Fan and a series of topographic highs west of the

Florida Escarpment.

In the southeastern part of the Gulf, either due to the carbonate platforms or to the less

detailed topographical data available, the drainage paths are less sinuous. Generally, they

come down from the Florida Escarpment, the Campeche Escarpment, and the Mississippi

Fan. The paths go around the escarpments, fan, and then move south. There are also a

few paths that come down the Pourtales Escarpment, as well as the Tortugas Tenace.

Depocenters

During the Cenozoic, the maximum deposition occurred in depocenters that migrated

eastward and represented the fastest prograding shelf edge that are generally parallel to

present day's coastline (Murray 1952). In the lower Tertiary, the depocenter was in the

Rio Grande Embayment. In the middle Tertiary, the depocenter was in the Mississippi

Embayment (Shinn 1971; Woodbury et al. 1973). In the Eocene, the depocenter was in

the south Texas (Woodbury et al. 1973). In the Miocene, the depocenter was in the south

Louisiana (Woodbury et al. 1973) and in the Pliocene, in the Vermilion to West Delta

central shelf areas (Woodbury et al. 1973). In the Pleistocene, the depocenter was in the

High Island Area to the South Timbalier outer-shelf and tipper-slope areas (Shinn 1971).

The present shelf edge at a water depth of 200 m is a Pleistocene depositional feature

(Moore and Curray 1963; Lehner 1969; Woodbury et al. 1973). Figure 3.7 illustrates the

location of the depocenters and shelf edge location through time.

In the major depocenters of the northern Gulf, subsidence can be up to 17,000 m (Hardin

1962). But in general the supply of sediment has been greater than the subsidence rate,

and despite the transgression and regression that has occurred in the Cenozoic, the shelf

edge still prograded basinward by as much as 402 km with an average of 5 to 6 hmmy

(Hardin 1962; Woodbury et al. 1973; Coleman et al. 1986). The shelf edge prograded

rapidly from late Miocene to middle Pliocene for about 80 km (Woodbury et al. 1973).

During late Pliocene and Pleistocene, the depocenter shifled over 320 km southward

while the shelf edge prograded 80 km southward (Woodbury et al. 1973).

The average Holocene sediment thickness on the continental slope in the northwestern

Gulf of Mexico is 70 cm, which gives an average sedimentation rate of 4.6 cm1,000 yr4.

Roberts et al. (1986) and Beard (1973) documented a much higher rate at 20 to

30 cml,000 yf1, on the slope and in the deep Gulf.

In the Quaternary, there were about 300 m thick of sediments were deposited on the shelf

(Lehner 1969). In the same period, as much as 3,600 in of sediments were deposited

offshore Texas and Louisiana, especially in the salt withdrawal intraslope basins

40
DLun, NAD2 ProctIon: 

Fmt Stndajd P&811n8u14PGMiddr8

Figure 3.7. The location of depocenters through Cenozoic in the northern Gulf of Mexico (Adapted from: Woodbury et a

background is the bathymetry map as shown in Figure 3.1.
Bryant and Liu: Geology

(Coleman et al. 1986), and up to 3,000 m thick of sediments were deposited in the

vicinity of Mississippi Fan. Out of 983 million tons/year of sediments to the Gulf,

775 million tons are detrital (Moody 1967). The sands and silts cover over 35% of the

Gulf of Mexico (Davies 1972).

Continental Slope Basins

The Gulf of Mexico is unique in the construction and evolution of its northwestern

continental margin and in particular the continental slope off Texas and Louisiana

(Figure 3.1). The processes that determined the physiography of the continental slope

are almost completely dominated by the halokinesis of allochthonous salt. Bathymetric

maps constructed by Texas A&M University (Bryant et al. 1990) and maps produced

from multibeam bathymetric data collected by the National Oceanic and Atmospheric

Administration (NOAA) and the National Ocean Service (NOS), reveal the presence of

over 90 intraslope basins with relief in excess of 150 m. Intraslope-interlobal and

intraslope-supralobal basins occupy the upper/middle and lower continental slope

respectively.

Gealy (1955) originally interpreted the origin of the huminocky topography of the

northwestern Gulf of Mexico as the result of gravity slumping. Later, extensive seismic

surveying and drilling revealed the role that salt plays in controlling the geomorphic and

geological features of the area (Moore and Curray 1963; Lehner 1969; Garrison and

Martin 1973; Buffler et al. 1978; Humphris 1978; Watkins et al. 1978; Martin 1980b;

Buffler 1983; Winker and Edwards 1983; Shaub et al. 1984; Bouma et al. 1986; Worrall

and Snelson 1989).

The continental slope off Texas and Louisiana, the major petroleum province in the Gulf

of Mexico, is 180 to 240 km wide and covers the area from the continental shelf break to

the continental rise south of the Sigsbee Escarpment (Figure 3.1). Water depths in the

area range from approximately 200 to 3,400 m. The regional slope angle of the

continental slope from 100 to 3,000 m water depth is about 10. Surprisingly, the

maximum submarine slope angles are reached on the flanks of the intraslope basins and

not on the Sigsbee Escarpment. The slope basins are the major sediment depocenters of

the slope and most hydrocarbon deposits will be found in the basins and in sub-salt areas.

The average relief of intraslope basins is 309 m, while the average east-west dimension is

12.2 km. The slope angles of submarine flanks of the intraslope basins can obtain angles

in excess of 40°, too steep in most cases for any present day engineered structures. The

average angle of the continental slope off Texas and Louisiana is between 8° to 9°. The

slope of the Sigsbee Escarpment averages 15° to 20° but can reach up to 30° locally.

Of the multitude of intraslope basins on the continental slope, Gyre, Orca and Pigmy

Basins are the most popular researched areas. Orca and Pigmy Basins have been drilled

during Deep Sea Drilling Program (DSDP) Leg 96 (Bouma et al. 1986). The popularity

of Urea Basin, an interlobal basin, is its unique brine pool that occupies the western and

northern portion of the basin. The brine pool is the result of salt leaching to seawater

42
Bryant nd Liu: Geology

from the shallow subbottom exposure of salt near the top of diapirs. The anoxic

environment created by the trapped brine preserves biogenic material and is conducive to

the formation of black shales. Orca Basin was formed between coalescing diapirs, while

Pigmy and Gyre Basins were developed by the coalescing of salt canopies.

Salt Induced Physiography of the

Upper and Middle Continental Slope

Detailed bathymetry proves to be extremely valuable in displaying characteristics ofsalt

structural styles based on multichannel seismic interpretation. In general, bathymetric

highs correspond to shallow salt structures buried less than 1 second (two way travel

time) below the seafloor. Bathymetric lows correspond to thicker sediment-filled

intraslope basins (Bryant et al. 1990). The bathymetry of the upper/middle slope consists

of relatively flat ridges and basin floors separated by intraslope escarpments. The

intraslope escarpments have relief up to 500 m across, slopes between 6° and 12°, and in

some instances in excess of 40°. Ridges correspond to laterally spreading, flat-topped

salt tongues overlain by thin sediments (100 to 500 msec).

Figure 3.8 shows multichannel seismic lines, across the Pigmy Basin area that traverses a

salt tongue spreading over the basin. The salt tongue has an asymmetrical lens shape and

clearly resolved base. Its internal seismic character is reflection-free except for multiple

energy reflecting between the top of salt and the seafloor. The leading edges of salt occur

along the base of the escarpments, typically within 200 msec of the seafloor.

Stratigraphic reflections diverge around these leading edges, presumably at the depth of

lateral intrusion. The deformation front represents the veneer of sediment undercut and

uplifted in the vicinity of the advancing salt front. The implied depths of intrusion (100

to 200 m) are in agreement with estimates by Nelson and Fairchild (1989) based on the

thicknesses of onlapped deformation fronts from buried examples in the Mississippi

Canyon area. Little, if any, onlap onto the deformation fronts is observed in this vicinity,

suggesting salt tongues are actively intruding the shallow subsurface. Figure 3.8 shows

the very thick section of sediment within Pigmy Basin.

Basically, the upper/middle Louisiana Continental Slope consists ofvery large salt domes

and salt ridges separated by trough arid valley-like basins. The salt structures responsible

for the large domes and ridges include deeply rooted massifs, and laterally spreading salt

tongues. Each style of salt emplacement has distinctive bathymetric characteristics.

Deeply-rooted massifs are common on the upper and middle slope. The ridges are

commonly elongated in a NW-SE direction, a prominent trend across the northern Gulf of

Mexico, which appears to correspond with a deep crustal fabric. The upper surfaces of

the ridges are generally uneven or domed at various points along the ridges. The adjacent

interdomal basins tend to have a fairly rounded cross-sectional shape. The sediment fill

within the basins dips away along the ridge flanks, reflecting both doming within the

ridges and salt withdrawal from below. The resulting seafloor physiography is very

complex and is well illustrated by a map of the Pigmy Basin area in Figure 3.9.

43
44
Figure 3.9. Bathymetric contour map of the Pigmy Basin. The contour interval is 50 m. The location of the seismic line 

in black line.
Bryant and Liv: Geology

There are tens of intraslope-interlobal basins within the area covered by Figure 3.10.

The basins are very irregular in shape and have steep sidewalls and generally flat basin

floors. Figure 3.9 is a bathymetric map of Pigmy Basin. The flanks of Pigmy Basin

outline the limit of coalescing salt structures, as show in the seismic profile in Figure 3.8.

A good portion of the basin area consists of fairly high angled sidewalls; the stability

characteristics of the basin walls are totally unknown. The flat nature of the basin floor

would indicate the possibility that density flows played a role in their development.

Certainly, the majority of sediments within the basin are the products of turbidity current

activity during the Cenozoic. Very little is being deposited within the basin at the present

time. The sediments that make up the flanks of the basin are high water content

smectite-rich clays that may be very gassy as was the sediments recovered on

DSDP Leg 96, Hole 619 from the Pigmy Basin floor. Installation of seabed engineering

structures within the basin would necessitate the determination of the role that creep and

general slope stability and instability would play in the generation of density flows

resulting from slope failure.

Salt Induced Physiography of the Lower Continental Slope

As stated before, regional salt sheets cover much of the lower Louisiana and portions of

the Texas Continental Slope. A salt nappe complex apparently accounts for much of the

salt across the lower slope. The Sigsbee Escarpment represents the composite leading

edge of the complex (Figure 3.5). Thickness of the salt nappes locally exceeds 6 km

(Wonall and Snelson 1989) (Figure 3.4). Across other areas of the lower slope the salt

sheets are canopies made up of convergent salt tongues. In both cases the shallow salt,

and likewise the bathymetry, is generally flat and relatively smooth across the top.

Figure 3.10 is a physiographic map of the lower continental slope off Louisiana known

as the Vaca Basin area, that shows the nature of the lower slope.

Salt canopies can be located and are characterized by narrow, steep-faced valleys along

internal suture zones between constituent salt tongues. Bathymetric relief across the

Jower continental slope occurs mainly within isolated, generally elliptical supralobal

basins. These basins are predominantly filled by Plio-Pleistocene sediments subsiding

into the underlying salt sheets. They are generally asymmetrical, and the deepest points

within the basins generally correspond to points of maximum sediment fill. The seismic

profile in Figure 3.11 cuts across Vaca Basin and other lower slope basins and illustrates

the nature of intraslope-supralobal basinal structure.

The supralobal basins generally display continuous, high-amplitude reflections

alternating with low-amplitude facies. Similar observations have been described by

Armentrout (1987), Weimer (1989), and Lee (1990), and have been attributed to a

Plio-Pleistocene glacio-eustatic cyclicity. The high-amplitude reflections tend to drape

the area, and are thought to represent, condensed intervals of pelagic and hemipelagic

sediments within transgressive and highstand systems tracts. The low-amplitude facies

onlap the condensed intervals and expand towards the focus of basin subsidence

46
Figure 3.10. Perspectiveviewoftheseafloor relief of the middle and lower continental sbpe, northern Gulf of Mexico. O

relief is typified by asymmetric and rugged basins, for example, Pigmy Basin. On the lower slope, the relief and smoothed basins, for example, Vaca Basin. The vertical exaggeration is lox. Pigniy Basin®rca Basin Beaumont Basin Vaca Basin Cor

Can

q

/ BryantGanyon
26 18N 26 12N 26 O6'N 

Figure 3.11. Bathymetric contour map of the Vaca Basin. The contour nterval is 50 m. The 'ocation of the seismic line 

in black line. 91. 54W 91 48W 9V 42W
49

Bryant and Liu: Geology

suggesting differential sedimentation within the lower siope depressions by

sedimentgravity or density flows during lowstands of the eustatic cycles.

Basin subsidence is accomplished by evacuation of underlying salt. Subsidence histories

are commonly episodic. This is particularly evident in Vaca Basin where initial basin

subsidence appears to be relatively slow and accompanied by the accumulation of

relatively concordant strata. These older strata flank the basin margins at high angles,

indicating large amounts of post-depositional subsidence. An oulap surface overlying a

series of high-amplitude reflections seems to mark a point where basin subsidence

decelerates. The onlapping strata flank the basin margins at lower angles, principally

filling a deep, pre-existing depression. A possible scenario is that subsidence of the Vaca

Basin was initially controlled by differential loading caused by lateral variations in

sediment thickness, but while the sediments were still relatively buoyant compared to the

salt. Sediment densities increase with depth due to compaction, whereas salt densities are

essentially constant with depth. Sediment thickness increased until a critical thickness

was reached where average sediment densities exceeded that of salt. At this point,

subsidence proceeds rapidly until salt is completely evacuated and the sediment fill

bottoms out against the base of salt. Rapid basin subsidence should cease, but the

depression formed by the rapid subsidence will continue filling up with sediments,

onlapping the basin margins. In order for subsidence to continue, the average density of

the infilling sediments must be larger than that of the salt. The density of high porosity

smectite-rich clays typical of the Gulf of Mexico region have relatively low densities and

are generally underconsolidated, making it difficult to achieve the necessary average

densities to exceed that of salt except at very deep burial depths. It is difficult to envision

such clays initiating the necessary forces of subsidence in the thin sediment fills of high

relief basins such as Vaca Basin. In contrast, sand-rich sediments can reach average

densities exceeding that of salt at fairly shallow depths. Thus, it is suggested that the

intraslope-supralobal basins may contain large amounts of sand at depth within their fill;

an interesting and perhaps an important aspect related to the petroleum reservoir

characteristics of intraslope-supralobal basins. The physiography of the lower slope is

comparatively smooth, such as across the Vaca Basin area. Bathymetric relief exists

mainly as rounded depressions corresponding to supralobal or suprasalt sedimentary

basins subsiding into a regionally extensive salt sheet locally 3 to 4 km in thickness. The

supralobal basins have average slopes generally between 60 and 8°, but locally up to 300.

Figure 3.10 is a bathymetric map of Vaca Basin showing the very bowl shaped

appearance of the basin and the smooth round basin walls. The basin appears, as would a

ball pressed into soft mud.

Geohaza rds

The engineering and geological constraints on the continental slope off Texas and

Louisiana related to hydrocarbon recovery will require both novel geological and

geophysical surveys and engineering methods to economically overcome. Significant

seafloor engineering problems in deep waters include slope instabilities, both short-term

(slump) and long-term (creep), pipeline spanning problems, mass transport from
Bryant and Liu: Geology

unknown causes, and unusual stiffness and strength conditions (Hooper and Dunlap

1989). The geohazards (engineering and geologic constraints) present in and on the

central and western continental slope are many in number and are mainly due to the

activity of salt and rapid sedimentation.

The main geohazards on the slope and resulting effects are as follows:

Faults - sediment tectonics, halokinesis

Slope Stability - slope steepening, slumps, creep, debris flow

Gassy Sediments sediment strength reduction, hydrates, sediment liquefaction

Fluid and Gas Expulsion Features

Diapiric Structures salt, mud, hydrates

Seafloor Depressions - blowouts, pockrnarks, seeps

Seafloor Features - sediment waves, differential channel fill, brine-low channels,

seabed furrows

Shallow Waterfiow

Deep Water High-Velocity Currents - mega-furrows, sea-bed erosion

The near-surface geology and topography (the area of most concern in relationship to

submarine slope stability) of the continental slope off Texas and Louisiana are a function

of the interplay between episodes of rapid shelf edge progradation and contemporaneous

modification of the depositional sequence by diapirism and mass movement processes.

Many slope sediments have been uplifted, folded, fractured, and faulted by diapiric

action. Oversteepening on the basin flanks and resulting mass movements have resulted

in the appearance of highly overconsolidated sediments underlying extremely weak

pelagic sediments. The construction of the Mississippi Canyon is in part a function of

sidewall slumping arid pelagic drape of low shear strength sediments. Incontrast, slope

oversteepening and subsequent mass movement have resulted in high pore pressures in

rapidly deposited debris flows on the upper slope and on basin floors, resulting in

unexpected decreased shear strengths. Biogenic and thex-mogenic gas induces the

accumulation of hydrates and underconsolidated gassy sediments, which are common on

the upper slope. On the middle and lower slope, gassy sediments are not common except

in the basins that do not have a salt base such as Beaumont Basin. The salt nappe

restricts the upward movement of gas from below.

Holocene and Pleistocene sediment cores recovered from the continental slope off Texas

and Louisiana from conventional piston coring and from DSDP activities reveal the

presence of unconsolidated gassy clays, silty clays, sands, and clayey sands, many

containing gas hydrates.

The intraslope intralobal basins located on the upper slope range in water depths from

1,500 to 2,200 m. The bathymetry of the Central and Western areas is shown in

Figure 3.2. The bathymetry of the upper to middle continental slope area consists of

relatively fiat ridges and basin floors separated by intraslope escarpments. The intraslope

basin escarpments have relief up to 700 m and slopes between 50 to 300 and in specific

locations up to 50°. Ridges that rim the basins correspond to late laterally spreading

50
Bryant and Liii: Geology

flat-topped salt tongues overlain by a thin sediment cover (Bryant et al. 1995). The

deeper portions of intraslope intralobal basins are salt free and exhibit a dissected

topography consisting of a multitude of small submarine canyons along the walls. Cores

taken on the wall of some basins indicate that as much as 3 m of sediment has been

removed by slumping. The intraslope-supralobal basin on the lower continental slope

where the physiography is comparatively smooth (Figure 3.1) shows that the relief exists

mainly as a rounded depression. The formation of basins on the lower slope is where

subsidence is accomplished by evacuation of underlying salt (salt withdrawal).

The submarine canyons along the Sigsbee Escarpment, Alaminos, Keathley, Bryant,

Cortez, Farnella and Green Canyon are the result of the coalescing of salt canopies, the

migration of the salt over the abyssal plain and the erosion of the escarpment during

periods of low sea stand (Bryant et al. 1992). The bathymetry of the canyons is

illustrated in Figure 3.1. In addition to the canyons that form along the escarpment,

numerous small submarine canyons and gullies line the escarpment along with large

slumps. Seaward of the canyons submarine fans of various sizes extends out onto the

continental rise. A significant portion of the canyon walls and the escarpment contains

slopes of 50 to 10° and slopes in excess of 15° are not rare. Large slope failures are

present in the Green Canyon area.

The maj or faults on the continental slope are associated with massive accumulation of

sediments and are called growth faults. These growth faults form contemporaneously

and continuously with sediment deposition. The growth faults are found mostly on the

upper continental slope and on the continental shelf where sediment accumulation is the

thickest (see Figure 3.2). The most common types of fault on the middle and lower

continental slope are "groups of geometrically classified fault families and fault welds

that are kinematically and genetically linked to each other and to associated salt bodies

and welds. Linked fault systems can contain extensional, contractional, and strike-slip

components. Extensional fault families are formed by basinward translation, subsidence

into salt, or folding. Those fault families that accommodate basinward translation are

balanced by salt extrusion or contractional fault families" (Rowan et al. 1999). Rowan et

al. related five associations of linked fault systems that are directly related to five types of

salt systems: autochthonous salt (salt in place), stepped counterregiona!, roho, salt-stock

canopy, and salt nappe. Faulting resulting from the fonnation of salt diapirs from

autochthonous salt is the most common type fault on the upper slope while faulting from

salt-stock canopy and salt nappe are most common on the middle and lower continental

slope. Extensive faulting can be found on the rim of most intraslope intralobal and

supralobal basin on the middle and lower continental slope. The faults are extensional

faults caused by the upward movement of salt resulting from pressures created by

sediment accumulation within basins. This type of faulting results in the occurrence of a

large number of small faults in the area of the seafloor under going extension. In some

areas of the slope the upward migration of salt results in the seafloor being totally

fractured (faulted) and continuously displaced.

51
Biyant and Llu: Geology

Portions of some of the submarine canyons, like Bryant Canyon, are being filled with salt

due to the loading of the salt by sediments on the margins of the canyon. The salt

migrates upward, filling the canyon that was created by turbidity current flow active

during times of low-sea stand. The migration of salt into the canyon may occur at the

rate of centimeters per year.

On the middle and lower continental slope, salt may be very close to the seafloor in

certain areas and, on features such as the salt plug called "Green Knoll," salt is exposed at

the seafloor and is being dissolved by seawater, resulting in the collapse of the cap of the

knoll. Iii Orca Basin, an intraslope intralobal basin, salt is exposed at the bottom of the

northern portion of the basin and a famous brine pool has formed within the basin.

Where salt is close to the seafloor, for the emplacement of structures that require

foundation piles, new engineering methods will be necessary to accommodate such

structures on salt.

Water currents can be a problem to structures on the continental slope, but they may be a

major problem to structures such as platforms, bottom assemblies and pipelines at the

base of the Sigsbee Escarpment starting in water depths as shallow as 1,200 in and as

deep as 3,300 in. Recent studies have revealed the presence of large mega-ftLrrows at the

base of the Sigsbee Escarpment. These large bedforms, 20 to 30 in wide and as deep as

10 m, occur along the base of the Sigsbee Escarpment and extend to a distance of 20 km

south of the escarpment. They are the result of high velocity bottom currents occurring

along the base of the escarpment (see Chapter 4). The rnega-fttrrows have been found

extending from 90° to 92.5° W Longitudes, and probably extend westward beyond that

location and possibly as far west as Alaminos Canyon.

Shallow waterfiow, also known as geopressured sands, is the uncontrolled flow of sand

and water that can create significant sediment pile up at the welihead. It is the results of

compaction disequilibrium or differential compaction and usually occurs at 360 to 530 m

below the seafloor. It is more likely to occur on the upper and middle slope and less

likely to occur above the salt nappe, the tabular salt blocking the escape of overpressures

from below.

Table 3.1 summarizes the properties related to geohazards of upper, middle, and lower

continental slopes intraslope basins and lower slope canyons and the Sigsbee Escarpment.

52
Upper to Middle Slope Intraslope Interlobal Basins

Steep sidewalls average 10° to 20°, maximum 50°

Small submarine canyons and gullies dissect basin escarpments

Basin wall sediments may be unstable and undergoing modification by creep and slump

processes

Low shear strength debris flow sediments on basin floor

Basin floor subject to debris flows from side wall slumping

a Stiff sediments on higby faulted ridges between basins

Hydrates, gas sweeps, carbonate bioherms and chemosynthetic organisms may be present

Basins may contain low shear strength gassy anoxic sediments

Isolated basins subject to formation of brine pools

Basin sediments underconsolidated at shallow subbottom depths

Lower Slope Entraslope-Supralobal Basins

Elevated faulted ridges between basins

Elevated ridge along basin rim

Basins are bowl shaped with low angle basin floor

Soft surficial sediments within basin

Structures on basin floor subject to debris flow

Basin sediments underconsolidated at shallow subbottorn depths

Lower Slope Canyons and Escarpments

Side walls average 100 to l5, maximum 30

Small submarine canyons and gullies dissect escarpment and smaller canyon escarpments

Canyons and escarpment structurally active from effects of halokinesis

Very rugged topography

Slump deposits and slope failure common

Small submarine fans on canyon floor foimed from debris flows and turbidity currents

In very deep water

Sediments underconsolidated at shallow subbottom depths

High velocity bottom currents and mega-furrows present at base of Sigsbee Escarpment

53

Bryant and Liu: Geology

Table 3.1. Engineering constraints and possible geohazards of intraslope basins and canyons
Bryant and Liu: Geology

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Scruton, P.C. 1960. Delta building and the deltaic sequence, pp. 82-102. In: F.P. Shepard (ed.),

Recent sediments, northwest Gulf of Mexico. AAPG, Tulsa, OK.

Seni, SJ. and M.P.A. Jackson. 1989. Counter-regional growth faults and salt sheet

emplacement, northern Gulf of Mexico, pp. 116-121. Annual Research Conference, Gulf

Coast Section, Society of Economic Paleontologists and Mineralogists. Program and

Abstracts, Vol. 10.

Shaub, F.J., R.T. Buffler, and J.G. Parsons. 1984. Seismic stratigraphic framework of the deep

central Gulf of Mexico. AAPG Bulletin 68:1,790-1,802.

Shepard, F.P. 1959. Sediment environments of the northwest Gulf of Mexico. Eclogae

Geologicae Helvetiae 5 1:598-608.

Shiun, A.D. 1971. Possible future petroleum potential of upper Miocene and Pliocene, western

Gulf basin. In: I.H. Cram, Future petroleum provinces of the United States-their geology

and potential. AAPG Memoir 15(2):824-835.

Shokes, R.F., P.K. Trabant, and B.J. Presley. 1977. Anoxic, hypersaline basin in the northern

Gulf of Mexico. Science 196:1,443-1,446.

Simmons, G.R. 1992. The regional distribution of salt in the northwestern Gulf of Mexico:

styles of emplacement and implications for early tectonic history. Unpublished Ph.D.

dissertation, Texas A&M University, College Station, TX. 180 pp.

Trusheim, F. 1960. Mechanism of salt migration in northern Germany. AAPG Bulletin

44: 1,5 19-1,540.

van den Bold, M.C., T.F. Moslow, and J.M. Coleman. 1987. Origin and timing of seafloor

erosion on the Louisiana continental slope. Gulf Coast Association of Geological

Societies Transactions 37:487-498.

Watkins, J.S., J.W. Ladd, R.T. Buffler, F.J. Shaub, M.H. Houston, and J.L. Worzel. 1978.

Occurrence and evolution of salt in the deep Gulf of Mexico, pp. 43-65. In: A.H. Bouma,

G.T. Moore, and J.M. Coleman (eds.), Framework, Facies and Oil-trapping

Characteristics of the Upper Continental Margin. AAPG Studies in Geology 7.

Weimer, P. 1989. Sequence stratigraphy, facies geometry, and depositional history of the

Mississippi Fan, Gulf of Mexico. AAPG Bulletin 74:425-453.

58
Bryant and Liu: Geology

Weimer, P. and R.T. Buffler. 1989. Structural geology of the Mississippi Fan Foldbelt, deep

Gulf of Mexico Conference, pp. 146-147. SEPM Gulf Coast Section Tenth Annual

Research Foundation.

West, D.B. 1989. Models for salt deformation. Bulletin of the South Texas Geological Society

29:9.

Winker, C.D. 1982. Cenozoic shelf margins, northwestern Gulf of Mexico. Transactions Gulf

Coast Association of Geological Societies 32:427-448.

Winker, C.D. and M.B. Edwards. 1983. Unstable progradational clastic shelf margins. Special

Publication Society of Economic Paleontologists and Mineralogists 33:139-157.

Woodbury, 1-1.0., I.B. Murray, Jr., P.J. Pickford, and W.H. Akers. 1973. Pliocene and

Pleistocene depocenters, outer continental shelf, Louisiana and Texas. AAPG Bulletin

57:2,4282,439.

Worrall, D.M. and S. Snelson. 1989. Evolution of the northern Gulf of Mexico, with emphasis

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The Geology of North America- An Overview. GSA The Geology of North America

A:97-138.

59
Chapter 4: Physical Oceanography

Worth D. Nowlin, Ann F Jochens, Steven F DiMarco, and Robert 0. Reid

Introduction 61

Classical Hydrography 64

Low-Frequency, Local Wind Forcing 72

Surface Wind Waves 74

Forcing By Energetic, Episodic Wind Events 77

Loop Current Forcing and Eddy Activity 85

Sea Level and Tides 96

Currents in Deep Water 98

Deep, Barotropic Currents 99

High-speed, Subsurface-intensified Currents 102

Currents Responsible for Furrows 102

Effects of Shelf/Slope Canyons and Other Rough Topography 106

Numerical Modeling 107

Types of Models and Studies 107

Eddy Shedding Aspects 110

Other Skill Assessment 111

Status of Nowcast/Forecasts 112

Literature Cited 113

Introduction

Although there have been many hydrographic measurements made in the Gulf of Mexico,

few surveys of the entire Gulf have been over short enough durations so that the data

could be combined to obtain nearly synoptic descriptions. Examples of such surveys are

those of Hidalgo 62-H-3, Geronimo cruises in the 1960's, and Kane 1969. The resulting

patterns are similar and illustrated by Figure 4.1 (after Nowlin 1972) based on the

Hidalgo 1962 cruise. Contours represent the streamlines of the surface geostrophic

surface currents relative to a 1,000-rn reference surface. These currents reflect the

medium to large-scale distributions of temperature and salinity, and thus density. This

pattern also is characteristic of time-averaged outputs from numerical models of the

circulation of the Gulf (e.g., see Huribert and Thompson 1980, 1982). Thus, this pattern

more nearly reflects the time-averaged, or background, circulation and not necessarily the

instantaneous currents at any specific time.

The flow lines entering the Gulf through the Yucatan Channel, turning clockwise, and

then exiting the Gulf into the Straits of Florida represent the Loop Current. That current

is a part of the western boundary current system of the North Atlantic, and it is the

principal current and source of energy for the circulation in the Gulf of Mexico. The

Loop Current may be confined to the southeastern Gulf of Mexico or it may extend well

into the northeastern or north central Gulf, with intrusions of Loop Current water even to

the shelf edge of Louisiana or of the Florida panhandle (see e.g., Huh et al. 1981;

Paluszkiewicz et al. 1983).

61
Now/in et al.: Physical Oceanography

'---c/, I.

\2P ( \(OO - '' VELOCITY DIAGRAM

7 \% FOR 25° N. LATITUDE \ .11 .' ': -' uU)

Figure 4.1. Geopotential anomaly (dynamic m) of the sea surface relative to the 1,000-db surface,

constructed from Hidalgo 62-H-3 data collected February-March 1962. Stations shallower

than 11000 db, and for which extrapolation was used by Nowlin (1972) to estimate the

geopotential anomaly, are indicated by x's.

62
Now/in et al.: Physical Oceanography

Closed rings of clockwise rotating (anticyclonic) water called Loop Current eddies

(LCEs) separate aperiodically from the Loop Current. Studies on the frequency of Loop

Current intrusions into the eastern Gulf and of the frequency of LCE separation (Sturges

1992, 1994; Sturges et al. 1993; Vukovich 1988, 1995) clearly show these to be chaotic

processes.

Currents associated with the Loop Current and its eddies extend at least to depths of

800 in, the sill depth of the Straits of FJorida; geostrophic shear is observed to extend to

the sill depth of Yucatan Channel (2,000 m). These features may have surface speeds of

150 to 200 cm-s1 or more; speeds of 10 cms' are not uncommon at 500 m (Cooper et al.

1990). Anticyclonic eddies separate from the Loop Current with frequencies peaked at

8 to 9 months and at 13 to 14 months (Sturges 1994). Lifetimes of these Loop Current

eddies can be one year or more (Elliot 1982). Therefore their effects can persist at one

location for long periodsweeks or even months (see e.g., Nowlin et al. 1998a,b)if the

eddies remain stationary over such periods.

The Gulf may also be forced remotely by the advection of potential vorticity.

Connections in mesoscale variability between the tropical Atlantic, Caribbean Sea, and

Gulf of Mexico were studied using simulation from three ocean models by Murphy et al.

(1999). They found that (primarily ariticyclonic) eddies formed in the Caribbean, in part

due to the advection of potential vorticity from the Atlantic, pass through the Caribbean

along a well defined corridor and may enter the Gulf via the Yucatan Channel. There is

some evidence that such Caribbean eddies may influence Loop Current eddy shedding or

other mesoscale variability in the Gulf.

In addition to currents associated with the Loop Currents and with mesoscale eddies and

their formation and decay, two significant long-period circulation features are believed to

be generated locally by the wind iii the Gulf. There is an ariticyclonic (clockwise

rotating) feature oriented about ENE-WSW with its western extent near 24°N off Mexico

and a cyclonic gyre centered near 20.8°N, 94.5°W ± 10 in the Bay of Campeche.

Atmospheric forcing also results in a variety of episodic or short-period ocean

phenomena. These include surface wind waves, near-inertial oscillations, diurnal

cycling, and energetic current surges. The forcing events include frontal passages,

extratropical cyclogenesis, tropical storms, and hurricanes.

The astronomical tides do not result in currents within the Gulf of much concern in open

water. Transports of water through the Straits of Florida and Yucatan Channel associated

with some of the principal tidal components are sizable, but not of direct concern to the

deepwater area of interest.

In summary, there are at least five classes of energetic currents in the deepwater portion

of the Gulf of Mexico of potential concern to those involved with offshore oil and gas

production and transportation. These classes are:

(1) currents resulting from energetic, episodic atmospheric events, including cold air

outbreaks, extratropical cyclones, and tropical cyclones such as hurricanes;

63
Now/in et al.: Physical Oceanography

surface-intensified currents arising from the major surface circulation features (the

Loop Current, the anticyclonic LCEs derived therefrom, and both cyclonic and

anticyclonic eddies spun up in the Gulf);

currents extending from about 1,000 rn through the deeper water column with little

depth variation (e.g., those believed to be associated with topographic Rossby waves),

sometimes with bottom intensification;

high-speed, subsurface-intensified currents or jets; and

currents responsible for large, linear furrows discovered along the base of the

continental slope in some locations of the northwestern Gulf.

Data available for study of the hydrography and principal significant currents include

meteorological stations; shipboard estimations of weather, sea state, and ship drift; wave

measurements; oceanographic station data (STD/CTD/bottle measurements);

bathythermograph (BT), expendable bathythermograph (XBT), expendable

current-temperature-depth (XCTD), and air-dropped expendable bathythermograph

(AXBT) measurements; surface velocity estimates and temperature estimates from

satellite-reporting surface drifters; time series of currents, temperature, and salinity at

fixed locations using single-point current meters and acoustic Doppler current profilers

(ADCPs); current measurements in the upper several hundred meters using shipboard

ADCPs; expendable current profilers (XCP); tide gauges; inverted echo sounders (IES);

pumped seawater systems collecting and analyzing samples from underway vessels; and

satellite-derived measurements of sea surface temperature (SST), sea surface height

anomaly (SSHA), and sea surface color. For some of these observations, compilations of

variability for the Gulf of Mexico have been prepared. Examples include the monthly

fields of surface and upper layer subsurface temperatures compiled from BTs and XBTs

by Robinson (1973); the representations of steric sea level height around the Gulf by

Whitaker (1971); compilations of temperature, salinity, oxygen, and phosphate

measurements in the Gulf by Churgin and Halminski (1974); or wind fields over the Gulf

by Rhodes et al. (1985, 1989).

Classical Hydrography

Although surface waters of the Gulf of Mexico are greatly modified by heat and

freshwater exchanges through the surface, river discharges, and wind mixing, no

subsurface water masses of consequence are thought to be formed locally. Instead, water

masses originate mainly in the Atlantic Ocean and are modified as they spread through

the Caribbean Sea into the Gulf. These water masses enter the Gulf through the Yucatan

Channel from the Yucatan Basin.

Sill depths of both the Caribbean and Gulf control which water masses eventually enter

the Gulf. Using property distributions, McLellan and Nowlin (1963) estimated the sill

depth in Yucatan Channel to be between 1,650 and 1,900 in. Bryant and Bryant (1990)

show a Yucatan sill depth greater than 1,750 m but less than 2,000 in. Thus, global ocean

water masses in depths greater than these cannot penetrate into the Gulf.

64
Now/in et aL: Physical Oceanography

In the Caribbean, extrema in properties may be used to label four water masses in the

upper 1,000 m: Subtropical Underwater, 18°C Sargasso Sea Water, Tropical Atlantic

Central Water, and Antarctic Intermediate Water. The source regions for these waters are

discussed in Morrison and Nowlin (1982). Small amounts of upper North Atlantic Deep

Water may enter the Caribbean between 1,100 and 1,600 m through the

Anegada-Jungfern Passage and between Hispaniola and Jamaica. However, they are

mixed with Caribbean Mid-Water (Metcalf 1976) before entering the Gulf. The

characteristic water properties, depths, and isopycnal surfaces for the property extrema of

these water masses are given in Table 4.1.

Morrison arid Nowlin (1977) described the water masses found in the Loop Current of the

eastern Gulf; and Morrison et al. (1983) described the water masses and properties found

offshore in the western Gulf. The characteristic water properties, depths, isopycnal

surfaces, and range of values in the extrema cores are given in Table 4.1. These are

briefly described below, together with a description of surface water and deep basin

water.

Surface salinities in the Gulf generally are between 36.0 and 36.5. In waters bounded by

the Loop Current, surface or near-surface pockets of low salinity (<36.0) water have been

observed and are thought to be derived from low salinity Caribbean surface waters

(Schroeder et al. 1974; Morrison and Nowlin 1977). Low-salinity surface waters also

have been observed intruding over the shelf edges (e.g., <33 off Texas-Louisiana and <35

off Louisiana-Mississippi-Alabama). These are derived mainly from the AtchafalayaMississippi River system (Parr 1935; Schroeder etal. 1974).

The Subtropical Underwater is identified in the Caribbean Sea and in the Loop Current of

the eastern Gulf of Mexico by a subsurface salinity maximum at 150 to 250 m centered

about 25.40 kgm3 in sigma-U. In the Loop Current, maximum salinity values in this

core are 36.7 to 36.8. Most of the water in the western Gulf of Mexico has a less

pronounced upper salinity maximumtypically from 36.4 to 36.5, with even higher

values seen from stations taken in recently-detached anticyclonic Loop Current rings that

moved into the region. As the waters of the Loop Current enter the Gulf, those along its

western boundary are vertically mixed by the interaction of the current with bathymetry,

resulting in a flattening of the pronounced subsurface salinity maximum associated with

the Subtropical Underwater source of those waters (Nowlin 1972). Moreover, after

separation, Loop Current eddies eventually spin down in the Gulf. That process entails

mixing, which likewise reduces the salinity at the maximum and may spread the lesser

maximum over a larger range of depths-0 to 250 m according to Morrison et al. (1983).

65
Table 4.1. Water masses of the Caribbean Sea and Gulf of Mexico - Morrison and Nowlin 1982 2 - Nowlin and McLellan 1967; Morrison and Nowlin 1977 3 - Nowlin and McLellan 1967; Morrison et al. 1983 na data not available for the study SUW-LC = Subtropical Underwater within the Loop Current and Caribbean Sea SUW = Subtropical Underwater in the Gulf but outside the Loop Current 18°C W= 18°C Sargasso Sea Water TACW = Tropical Atlantic Central Water AAIW = Antarctic Intermediate Water UNADW = Upper North Atlantic Deep Water * *MIX = Mixture of low silicate UNADW and very high silicate Caribbean Mid-Water *high SiO3 in AAIW and MIX waters results in broad SiO, maxirnurri approximately from 27.50 to 27.70 Eastern Caribbean' Eastern Gulf of Mexico2 Western G

Water Mass Depth m cr kgm Feature Depth m kgm' Feature Range Depthm kgm SUW-LC 150-250 25A0 S 150-250 25.40 Sm 36.7-36.8 SUW 150250 25.40 Smax 36.4-36.5 0250 25.40 

18°CW 200400 26.50 O2max 200-400 26.50 O2max 3.6-3.8 mLL' TACW 400-700 27.15 02, 400-700 27.15 03 2.85-3.25 mLU' 250400 27.15 

AA1W 600-800 27.30 NO3], na na NO3m, na 500700 27.30 AAIW AAIW 700900 600-900 27.40 27.40 PO4max 700-900 S, 800-1000 27.40 27.50 PO4max 1.8-2.5 tg-atL' 34.86-34.89 600-800 700-800 2740 27.50 

AAIW 8001000 27.50 Sj03max 27.50 SjO3max

* 27.50 

IJNADW 11001600 27.70 Smax at sill 27.75 SiO3mjn at sill 27.75 PO411 at sill 27.75 NO3 mm MIX** 900-1200 27.70 S03max 23-25 .LgatL' 1000-1100 27.70 
Now/in et aL: Physical Oceanography

In the Caribbean Sea and Loop Current, the water mass below Subtropical Underwater is

18°C Sargasso Sea Water. It is found at depths from 200 to 400 m and is identified by an

oxygen maximum centered about 26.5 kgm3 in sigma-O. Oxygen values greater than

4.2 rnL-U1 are found in the Caribbean near entrances of this water from the greater North

Atlantic. Within the Loop Current, oxygen maxima are only between 3.6-3.8 rnLL1.

Remnants of this water can be identified by weak relative oxygen maxima with values

slightly greater than 3 mLL' near sigma-0 of 26.5 kgm3 in some western Gulf stations,

but oxygen is not a useful tracer there.

Tropical Atlantic Central Water is identified by relative minima in vertical profiles of

dissolved oxygen. In the Caribbean Sea and Loop Current, it is found from 400 to 700 iii

and identified by oxygen values below 3 mLL' centered about 27.15 kgm3 in sigma-U.

This water is clearly seen with similar characteristics at western Gulf stations although at

depths closer to 400 m.

Beneath the Tropical Atlantic Central Water and extending over a vertical range of 700 to

1,200 in are remnants of the Antarctic Intermediate Water. This water is identified in the

Caribbean and Loop Current by a nitrate maximiun, a phosphate maximum, and a salinity

minimum found at increasing depths. This water is clearly seen in the western Gulf with

similar extrema, but at shallower depths ranging from 500 to 1,100 m.

The silicate maximum associated with Antarctic Intermediate Water also is found

generally beneath the salinity minimum in the Caribbean. By the time this water mass

reaches the Gulf of Mexico, this silicate maximum has been masked by the presence of

very high silicate waters below it. In both the eastern and western Gulf, silicates are high

(-23 to 25 Lg-atL1) throughout the 27.50 to 27.70 sigma-U range. Those at the 27.50

level can be associated with Antarctic Intermediate Water. Those at deeper levels to

27.70 result from a mixture in the Caribbean Sea of low silicate upper North Atlantic

Deep Water and the very high silicate Caribbean Mid-Water beneath it (Morrison et al.

1983). These high concentrations are attributed to veiy slow renewal of the Caribbean

Sea deep waters and re-solution of silicate from sediments (Richards 1958; Metcalf 1976;

Morrison et al. 1983).

Waters of the deep basin of the Gulf from below the sill depth to the bottom exhibit no

discernible horizontal variations and only small vertical gradients in potential temperature

and salinity (McLellan and Nowlin 1963; Nowlin and McLellan 1967). Using a number

of data sets from different cruises, similar results were found for dissolved oxygen by

Nowlin et al. (1969; they also concluded that the dissolved oxygen data of McLellan and

Nowlin [1963] were faulty). Observations below the sill depth had a mean potential

temperature of about 402°C, a mean salinity of 34.972, and a mean dissolved oxygen of

5.0 mLL1 (McLellan and Nowlin 1963; Nowlin et al. 1969). The lack of gradients

suggests either that basin water has a common source or the residence time is great

enough to erode away any horizontal gradients by exchange processes (Nowlin 1972).

The potential temperature versus salinity relationships from stations on the Gulf and

Caribbean sides of the Yucatan sill depth are very similar, which is consistent with

67
Now/in et al.: Physical Oceanography

possible present-day displacement of deep Gulf waters by Caribbean Sea waters (Nowlin

1972).

Shown in Figure 4.2 are composite plots of temperature vs salinity, temperature vs.

depth, and salinity vs. depth for the winter cruise 62-H-3 that covered the entire Gulf.

Seen in these plots is the large range of near-surface values, especially because sampling

extended over the shelves. Also seen are rather large ranges of depths at which specific

values of temperature or salinity are found in the main pycnocline and the very narrow

ranges at depth. This observed near-uniformity of both salinity and temperature (and thus

density) on depth surfaces implies that the major geostrophic currents in the deep Gulf of

Mexico are expected to be nearly barotropic. Later in this review, that is demonstrated to

be the case. However, there must be some horizontal density gradients, especially near

continental slopes, because deep currents appear intensified near the boundaries.

Figure 4.3 better illustrates upper layer waters with two different distributions.

Caribbean type water with a high maximum salinity marking the core of the Subtropical

Underwater (SUW) is found within the region enclosed by the Loop Current and LCEs,

illustrated in the figure by station 215 which is within an older LCE found in the

northwestern Gulf The second type of distribution is illustrated in the figure by station

165 which was located within a cyclone in the northwestern Gulf; there the salinity

maximum at the SUW core is much reduced by vertical mixing (characteristic of open

Gulf waters outside of the Loop Current and of LCEs) and temperatures and salinities are

found higher in the water column than within the LCEs.

The pronounced subsurface salinity maximum associated with the SUW core together

with the greater depth of isotherms serve as markers by which to easily distinguish LCEs

from cyclonic rings or the background waters of the Gulf Moreover, the salinity at the

SUW core and the depth of upper layer (e.g., 15° to 20°C) isotherms are often plotted as

horizontal distributions to indicate not only the signs and extents but also the intensities

of Gulf eddies.

Robinson (1973) described the seasonal variability of the upper waters of the Gulf in

terms of the monthly mean temperatures of the surface and upper 150 m and the depth to

the top of the thermocline. Contoured fields of temperature at six levels and of the

thermocline depth are pictured. Also shown are time series of temperatures at distinct

levels averaged for each 2.5° x 2.5° square. Robinson's graphics illustrate the oceanic

thermal response of the northern Gulf to the annual cycle of insolation (Figure 4.4). The

surface waters warm in spring and summer, with maxima in July-August, and cool in fall

and winter, with minima in February. Deeper waters of the upper 150 m also show

warming, but with maxima smaller arid later in the year with increasing depth. Minima

are similar in time.

68
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Figure 4.2. Temperature vs. salinity, temperature vs. depth, and salinity vs. depth based on all data collected during Hid

February-March 1962.
Figure 4.3, Temperature vs. salinity, temperature vs. depth, and salinity vs. depth for two stations made in ear'y May 199

slope off Texas. Station 215 (large triangles) was in the remnant of a Loop Current Eddy and station 165 (smcyclone. Station positions relative to sea surface height anomaly are shown in Figure 4.13.

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1973). Locations of regions are shown. 30 - I) 27°N. 96°W SURFACE
Now/in et al.: Physical Oceanography

Low-Frequency, Local Wind Forcing

One permanent circulation feature in the deepwater Gulf thought to be locally wind

driven is an anticyclonic (clockwise rotating) feature oriented about ENE-WSW with its

western extent near 24°N off Mexico. It can be seen centered near 24.5°N, 95°W in

Figure 4.1. This feature has been noted by Behringer et al. (1977), among others. There

has been considerable debate regarding the causal mechanism for this anticyclonic

circulation and the associated western boundary current along the coast of Mexico.

Elliott (1979, 1982) attributed LCEs as the primary source of energy for the feature, but

Sturges and Blaha (1975), Blaha and Sturges (1981), and Sturges (1993) argued that wind

stress curl over the western Gulf is adequate to drive an anticyclonic circulation with a

western boundary current. Sturges (1993) found annual variability in the wind stress curl

corresponding to the strongest observed boundary current in July and the weakest in

October. Based on ship drift data, he showed the maximum northward surface speeds in

the western boundary current were 25 to 30 cm-s1 in July and about 5 cm-s1 in October;

the northward transport in the western limb of the feature was estimated to vary from 2.5

to 7.5 x 106 m3s1. Sturges reasoned that the contribution of LCEs to forcing this

anticyclonic feature must be relatively small. Vidal et al. (1999) attempted to attribute

the presence of a northward flow along the western Gulf boundary to ring-slope-ring

interactions, but their evidence does not seem convincing. The numerical circulation

model of the Gulf reported by Oey (1995) shows a seasonal, wind-driven boundary

current in the western Gulf. It is strongest in July and August some 1 to 2 months after

the largest negative wind stress curl, in good agreement with the observations reported by

Sturges (1993).

Geopotential anomaly, derived from temperature and salinity data, and SSHA, derived

from satellite altimeter data, provide evidence of a permanent cyclonic circulation in the

Bay of Campeche in the southwest Gulf of Mexico (18° to 22°N and 92° to 97°W) as

noted by Vazquez (1975). Analyzing the geopotential anomaly of the surface relative to

450 db from 13 cruises in the southern Gulf of Mexico during the period 1958-1991 with

empirical orthogonal functions, Vázquez (1993) showed the dominant mode represented

93% of the variance and yielded a cyclonic flow pattern. The pattern showed a central

low of 4 ± 1 dyn cm below the basin mean with a mean central location of about 19.7°N,

94.5°W (Figure 4.5, after Vázquez 1993). Fields of SSHA from May 1992 through

December 1998 also show a cyclonic gyre in the Bay of Campeche. Averaging one field

per month yields a mean pattern consisting of a central low of 13 ± 5 dyn cm and a center

located about 20.8°N, 94.5°W. The difference in the mean lows from the geopotential

anomaly and SSHA suggests there is a strong barotropic component in the cyclonic gyre.

The most plausible forcing for the cyclone is the positive wind torque, which is present in

the area throughout the year (Velasco and Winant 1996). The seasonal pattern of SSHA

shows the cyclone is at minimum strength in summer, consistent with the minimum

positive wind torque that occurs in season.

72
Figure 4.5. Contours of mean dynamic height (dyn cm) of surface relative to 450 db, constructed by averaging the first e

modes (EOF) obtained from an EOF analysis of the geopotential anomaly for 13 cruises covering the area (FThe first mode contained 93% of the variance. 23N 21N 2ON 19 N 18 N9rw 9TW 96GW 95W 94GW 93W 92°W 91

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- 94°W 93GW 92GW 91
Now/in et al.: Physical Oceanography

Surface Wind Waves

Oceanic surface gravity waves with periods of 1 to 30 s contain large amounts of energy.

Produced by wind acting at the air-sea interface, they consist of two types: sea (also

referred to as wind waves) has typical periods of 0.2 to 9 s and is generated by local

winds; swell, with periods of 9 to 30 s, are waves that have propagated from another

region in which they were generated.

Because the fetch in the Gulf of Mexico is limited, surface gravity waves having large

amplitudes and long periods (10 s) are rare (McGrail and Cat-ties 1983 and references

therein) and are generally associated with extreme episodic weather events such as

hurricanes (National Data Buoy Center [NDBC] 1990).

The NDBC of the National Oceanic and Atmospheric Administration maintains

meteorological buoys in the Gulf of Mexico. Three of these are located along

approximately 26°N latitude in waters with depths near 3,200 m. Table 4.2 gives the

locations of these buoys, together with water depths and the periods of records ('--'15 to

20 years) used by NDBC to compute the significant wave height and wave period from

the buoys. Wave data are calculated by NDBC by applying spectral analysis to data from

accelerometers or inclinometers that measure the heave acceleration or vertical

displacement of the buoy hull during the wave acquisition time (Steele and Mettlach

1993). The significant wave height is calculated as the average of the highest one-third

of all the wave heights measured during a 20-minute sampling period. The average wave

period is the average period of all waves during the sampling period. The dominant wave

period is the period with the maximum wave energy. Also shown in Table 4.2 is

information for one ND]3C buoy (42020) located over the shelf off south Texas.

Table 4.3 gives the monthly and annual means of significant wave height, average wave

period, and dominant wave period for the four buoys as computed by NDBC (data were

obtained from NDBC Internet site, www.ndbc.noaa.gov).

Table 4.2. Location, water depth, and period of records, used for computations given in

Table 4.3, for marine buoys in the Gulf of Mexico

Throughout the deep water Gulf of Mexico, the patterns of significant wave height and

wave period are similar. The monthly mean significant wave heights average 1.1 m and

range from 0.6 to 1.5 m, and the average wave period ranges flom 4.1 to 5.2 s with a

mean of 4.8 s (Table 4.3). Approximately 92% to 94% of the wave heights were 2 m or

less at the deep water buoys. The patterns at the shallow water buoy were similar, and

92% of the wave heights were 2 m or less.

74

Buoy Period Latitude Longitude Total water depth

Number of Record (°N) (°W) (m)

42001 01/1976-12/1993 25.93 89.65 3,246.0

42002 06/1973 - 12/1993 25.89 93.57 3,200.0

42003 11/1976-12/1993 25.94 85.91 3,164.0

42020 0511990 - 12/1993 26.92 96.70 78.6
Table 4.3. Monthly and annual mean significant wave height (m) and extremes based on hourly observa

buoys (data from the National Buoy Data Center). Buoys 42001 (central Gulf), 42002 (western Gul

(eastern Gulf) are located in depths of approximately 3200 m; buoy 42020 (western Gulf) is lo

over the shelf in water depth of approximately 80 m. Buoy Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov D

Mean Signflcant Wave Height (m) 42001 1.3 1.4 1.4 1.2 1.0 0.8 0.6 0.6 0.9 1.2 1.4 

42002 1.4 1.5 1.5 1.3 1.1 0.9 0.7 0.6 1.0 1.2 1.5 42003 1.3 1.4 1.4 1.1 0.9 0.8 0.6 0.6 0.8 1.1 1.4 42020 1.5 1.3 1.4 1.4 1.2 1.2 1.1 0.7 1.0 1.3 1.6 

Maximum SignfIcant Wave Height (m) 42001 5.3 6.4 9.1 4.7 3.4 2.9 5.1 9.1 6.6 6.8 6.6 5

42002 5.2 7.4 7.8 5.9 3.4 5.1 3.1 6.9 9.7 7.2 8.4 542003 6.1 5.7 9.2 4.9 4.7 3.6 3.3 6.4 8.9 4.4 10.7 442020 5.0 3.8 6.1 3.9 3.3 4.6 2.4 2.7 3.4 3.8 4.9 4

Average Wave Period (seconds) 42001 5.0 5.1 5.1 4.8 4.6 4.4 4.1 4.2 4.6 4.8 5.1 5

42002 5.1 5.2 5.2 5.0 4.8 4.7 4.4 4.3 4.7 5.0 5.2 542003 4.8 4.9 5.0 4.7 4.7 4.8 4.7 4.5 4.4 4.8 5.0 442020 5.1 4.9 5.1 5.1 4.8 4.7 4.5 4.1 4.5 5.0 5.2 5

Dominant Wave Period (seconds) 42001 6.4 6.6 6.6 6.2 5.8 5.5 5.2 5.3 5.7 6.2 6.7 6

42002 6.5 6.7 6.8 6.4 6.2 6.0 5.5 5.3 5.9 6.5 6.8 642003 6.2 6.4 6.5 6.0 5.9 6.1 6.2 5.7 5.6 6.0 6.5 642020 6.7 6.4 6.6 6.7 6.2 6.1 5.8 5.1 5.7 6.6 6.8 6
Now/in et a/,: Physical Oceanography

The higher mean significant wave heights and longer average and dominant wave periods

are seen to occur between November and March. As expected, the lowest heights and

shortest periods occur in summertime, when fewer frontal passages or other storms occur

(Nowlin et al. 1 998a). Although the maxima significant wave heights for the period of

record generally are smallest from April to July, there is a less regular seasonal pattern

for the maximum heights than for the mean heights. This is because the maxima are

associated with energetic, episodic wind events, such as hurricanes, which occur between

June and November, or cyclogenesis events, which occur mainly between November and

May (Nowlin et al. 1 998a). With sufficiently long records, it is likely that the maximum

significant wave height would increase to 9 m or more for all months since strong storms

could occur in any month; but, because extreme events are rare, the mean significant

wave heights likely would remain similar to those shown.

The energetic atmospheric events, which produce the larger waves, are of great concern

in the design of offshore structures. Considerable effort has been spent to estimate these

waves, both directional spectra and kinematics, from meteorological data through the use

of hindcast modeling and to validate such models (e.g., Cardone et al. 1976; Forristall et

al. 1978; Forristall et al. 1980). As part of model validation, Ward et al. (1978)

developed a hurricane climatology covering the period 1900 through 1974 based on wave

data from 48 Gulf of Mexico storms with a central pressure of 980 mb or less. They

found maximum significant wave heights ranging from 4.6 to 15.5 m with an average of

9.7 in; 27 of the 48 storms exceeded 10 m. Using the Cardone et al. (1976) wave

hindcast model validated by Ward et al. (1978), 1-laring and 1-leideman (1978) estimated

rare wave heights associated with 22 severe hurricanes occurring in the Gulf of Mexico

between 1900 and 1977. They found the model results varied little between the three

sectors studied off the coasts of south Texas, east Texas-west Louisiana, and east

Louisiana-Mississippi-Alabama. They found 100-yr significant wave heights of 12 to 13

m in water depths of 70 to 700 in and 11 to 12 m in shallower waters; the dominant

spectraJ wave periods were 14 to 15 s in all water depths studied. Maximum 100-yr wave

heights were estimated to be 20 to 22 m.

Tracy and Cialone (1996) reported that during Hurricane Opal, which was an intense

category 4 hurricane in October 1995, the observed maximum significant wave height

and maximum peak period in the central and east deepwater Gulf of Mexico reached

10 m and 13 s at buoy 42001, near which Opal passed by, and 8 m and 22 s at buoy

42003. In August 1992, Hurricane Andrew, which was within 2 mb of becoming a

category 5 hurricane, generated significant surface waves that were well-documented

over both the deep water and the Texas-Louisiana shelf. Stone et al. (1993) and Breaker

et al. (1994) reported on measurements made at buoys 42003, which was -50 km

southwest of the hurricane at its closest, and buoy 42001, which was -240 km southwest.

Prior to the hurricane, seas at 42003 had wave heights of less than 1 m and wave periods

of 4 to 8 s. As the hurricane passed near the buoy, seas reached a maximum significant

wave height of over 6 m and peak wave periods of 17 s. Measurements from buoy 42001

also showed the influence of Hurricane Andrew with a time lag of approximately 9 hr

(Stone et al., 1993). The significant wave height increased from less than 2 m before the

hurricane to over 4 m, while the wave period increased from 7 s to 17 s. Waves at buoy

76
Now/in et aL: Physical Oceanography

42003 were generated primarily by the hurricane until it passed by; waves at buoy 42001,

however, were primarily swell (Breaker et al. 1994).

Hurricanes, however, are not the only intense storms that can generate significant waves.

Approximately 10 times each year, winter cyclones develop over the Gulf of Mexico in a

process called cyclogenesis (Hsu 1988). On 12 March 1993, an intense extratropical

cyclone, comparable to a category 1 hurricane, developed off the south Texas coast

(Nowliri et al. 1998a). From 0600 to 1800 UTC on 12 March, it moved eastward along

the Texas shelf edge before turning to the northeast. At about 0900 UTC 13 March, it

exited the Gulf over Florida at about 85°W (Schumann et al. 1995). Maxima for

significant wave height, average wave period, and dominant wave period were,

respectively, 9.2 m, 9.7 s, and 14.3 s at eastern buoy 42003, 9.1 m, 9.8 s, and 12.5 s at

central buoy 42001, and 7.8 m, 9.1 s, and 12.5 s at western buoy 42002. The peaks in

height and period progressed in time from the western to eastern buoys. The significant

wave heights were greater the farther east the buoys, reflecting the condition that for

cyclones, the maximum wave heights occur to the right of the path of forward movement

of the storm systems (U.S. Army Corps of Engineers 1984). The buoys in the central and

eastern Gulf were on the right side of the storm for a longer time than was the buoy on

the west. On the shelf off south Texas at buoy 42020 the maximum significant wave

height was 6.1 m. The maximum record-length significant wave heights at buoys 42001

and 42020 were associated with this cyclone.

Forcing By Energetic, Episodic Wind Events

Perhaps the currents of longest concern to oil and gas operators are those resulting from

strong, episodic wind events, such as tropical cyclones (especially hurricanes),

extratropical cyclones, and cold air outbreaks. Such wind events can result in extreme

waves and cause currents with speeds of 100 to 150 cms1 over the continental shelves.

Recent examples for the TexasLouisiana shelf and upper slope are given in Nowlin et al.

(l998a). Others (e.g., Molinari and Mayer 1982; Brooks 1983, 1984) have measured the

effects of such phenomena down to depths of 700 (980) m over the continental slopes in

the northwestern (northeastern) Gulf.

Tropical conditions normally prevail over the Gulf from May or June until October or

November. The nominal hurricane season is 1 June through 30 November. Figure 4.6

shows horizontal current vectors (hourly values from 3-hr low-passed records) during late

August 1992 from two locations off Louisiana at approximately 90.5°W on the shelf edge

and upper slope. Figure 4.7 shows mooring locations discussed in this review.

Moorings 13 and 12 were located in water depths of 200 and 504 m, respectively.

Current meter depths are indicated. The eye of Hurricane Andrew passed on a

northeastward track about 85 km north of mooring 13 at 0000 UTC on 26 August.

Near-surface instruments recorded a large surge of water directed to the left of the stormTs

track just before the passage of the eye; maximum values at 10 m on mooring 13 reached

163 cm-sd. Following the initial surge, an oscillation with near inertial period was set up

that penetrated, with diminished amplitude, to the deepest instrument on mooring 13

approximately 24 hours after the initial surge. Some time delay and considerable

77
M12 50 ETTeterdepth- l2rn /// -50 E Meterdepth-lOOm 0 Qv7///Z/ ----- (10)-50-10- 0: 12: 0: 12: 0: 12: 0: 12: 0: 12: 0: 12: 

50 -50 00C 50 Meerdepth-490mtQll3 M&erdepth- lOni - S Mee depth- 190m Meter depth - 100 ni 

______7jr. 7- /1 L\\\\ IA\ 

-500: 12: 0: 12: 0: 12 /0' 12: 0: 12: 0: 12: 

24 25 26 ' / 27 28 29 3

August 1992 Figure 4.6. Horizontal current vectors (hourly values from 3-hr low-passed records) during late August 1992 from two lo

approximately 90.5°W on the shelf edge and upper slope. The cross-shelf components are oriented up and and along shelf components are oriented across the figure (generally eastward to the right). Moorings 13 an

water depths of 200 and 504 m; current meter depths are indicated. The eye of Hurricane Andrew passed o

approximately 85 km north of mooring 13 at about 0000 UTC on 26 August. 3

24 25 26 27 August 1992 28 29 
Figure 4.7. Locations of current measurements or model output used in figures within this review. Stars are moorings S 

LATEX moorings 6-10, 12, and 13; grey diamonds are locations for CCAR model output; triangles denote m(Molinari and Mayer 1982); an inverted triangle is SAIC mooring G; circles denote moorings in the Mississipp

and TAMU); and a hexagon is the Green Canyon Block 200 mooring. Bathymetry contours are 200, I O0O, 
Nowlin et aL: Physical Oceanography

decrease in amplitude with depth is seen, although the maximum speed at 190 m

exceeded 100 crns* There was a coherent but weak response at 490 m at mooring 12

(note change in velocity scale). The inertial oscillation continued for about a week with

diminishing amplitudes. A review of shallow currents associated with Andrew is

presented by Keen and Glen (1999).

Many studies of hurricane effects on the underlying ocean focus on surface wind waves

or storm surges, but a number consider the effects on currents and thermal (and density)

structures. Among those with a focus on hurricanes in the Gulf of Mexico are Leipper

(1967), O'Brien and Reid (1967), O'Brien (1967), Fonistall (1974), Forristall (1980), and

Cooper and Thompson (1989a, 1989b). The papers by Sanford et al. (1987) and Price et

al. (1994) give results of a study of direct current observations within hurricanes

accompanied by model hindcasting and comparisons; they are summarized here.

Although the hurricanes studied were Norbert (off western Mexico at -20°N in

September 1984) and Josephine and Gloria (off the southeast Atlantic coast at 30°N in

October 1984 and September 1985), the results should be equally applicable to the Gulf

of Mexico for hurricanes of similar characteristics. These authors measured upper ocean

(200 m) currents by deploying aircraft expendable current profilers (AXCP) in a pattern

through each hurricane using a weather reconnaissance plane from which meteorological

observations were taken. The oceanic reaction to a severe storm can be separated into the

initial "forced response" with the arrival of the storm and the "relaxation stage response"

with the passage and retreat of the storm. The AXCP measurements were analyzed to

separate surface mixed layer and surface wave currents. The maximum verticallyuniform currents in the surface mixed layer (--50 m) in the three storms were found to be

73, 110, and 170 cm-s-1, largest in the right quadrant. The vertical shear of the mixed

layer currents seemed on the order of 20 to 30 cmls. The maximum surface currents from

combining both mixed layer and surface wave components were estimated at 133 to

346 cms1. These results are consistent with their model results. Their models could

account for between 35% to 90% (with an average of 85%) of the variance of the mixed

layer currents, the agreement increased with increasing current speed. The mixed layer

currents showed patterns of divergence centered behind the eyes of the storms; these lead

to upwelling at the base of that layer and a lowering of sea level above. The mixed layer

divergence and the associated distortion of the thermal and density fields occur on near

inertial periods, giving rise to inertial waves with wave lengths of several hundred

kilometers and decay scales of 5 to 10 d (e.g., see Brooks 1983 results from Gulf of

Mexico in next paragraph).

Figure 4.8 shows hourly current components (u positive to the east and v positive to the

north) measured at indicated depths from 200 to 700 m on moorings S (26°N, 96° 1 'TW)

and C (55 km north of 5). Both moorings were approximately on the 730-rn isobath.

About 0000 UTC on 10 August 1980, the eye of Hurricane Allen passed some 65 km

WSW of mooring S on a track toward the NNW. The effects of the hurricane passage

were reported by Brooks (1983). Currents were stronger at mooring C than at 5,

although currents at both were affectedeven to within 20 m of the bottom. A strong

southward, along-shelf current occurred with the landward passage of the hurricane;

maximum speeds exceeded 90 cms1 in the thermocline at 200 m and 15 cms1 at 700 m.

80
200 1000 C:575 -100tp -100 C4:70m r Raw dala -200 I I I I I I I I r I F 200 / 100 S3: 700 ni - C3:575ni 04: 700 ni 02: 450 rn Jv Raw data -200 I I I I I I I I I _____I I 

1 2 3 4 5 6 7 8 9 10 11 12 13 1

August 1980 Figure 4.8. Components (u positive to the east and v positive to the north) of hourly currents measured at indicated dep

(26°N, 96°10'W) and C (55 km north of S). Both moorings were approximately on the 730-rn isobath. Thee

on a track toward the NNW, passed about 65 km WSW of mooring S at 0000 UTC on 10 August 1980. 1234 Augusi 1980 5 6 7 8 9 1011 121314151617181920
Now/in et aL: Physical Oceanography

This surge triggered a series of internal waves with near inertial period; these elliptical

motions had maximum speeds along shore which reached a range of 50 cm-s1 within

about 3 days and then lasted for about 5 days with decreasing amplitudes. These

oscillations were coherent over the scale of mooring separation (55 km) and with depth.

From October or November until March or April the Gulf experiences intrusions of cold,

dry continental air masses. These result in the formation of extratropical cyclones and

cold air outbreaks, both of which can cause quite energetic surface currents. On average

about 10 to 12 extratropical cyclones are formed over the northern Gulf per year; the

number of frontal passages varies from 0 to 2 per month in summer to order 10 per month

in winter months. To illustrate the effects of an extreme extratropical cyclone,

Figure 4.9 shows eastward (u) and northward (v) components of currents (hourly values

from 3-hr low-passed records) from two moorings located off Louisiana at approximately

90.5°W. Mooring 13 was in water of depth 200 m; mooring 12 was in water depth of

504 m. On 12 March 1993, a class 5 extratropical cyclone moved from west to east

across the Texas-Louisiana shelf with its center approximately over the 1,500 m isobath.

Initially the flow over the outer shelf and slope was toward the northeast as part of an

induced cyclonic circulation over the Texas-Louisiana shelf. Following the passage of

the storm out of the area, on 13 March, there occurred a surge to the southwest followed

by a period (14 to 17 March) of strong motion toward the northeast, with diurnal

modulation. This was foJlowed by an energetic near-inertial oscillation with decreasing

amplitude lasting for over a week. The maximum observed speeds associated with this

event were: 65, 22, 67, 41, and 35 cms1 at mooring/depth (m) 12/12, 12/100, 13/10,

13/1 00, and 13/1 90, respectively.

During the LATEX program of the early 1990's sponsored by MMS, a class of energetic

surface currents previously unreported in the Gulf of Mexico were found over the Texas

and Louisiana shelves (DiMarco et al. 2000). To illustrate, Figure 4.10 shows eastward

(u) and northward (v) components of currents from 3- to 40-hr band-passed records made

in July and December 1992 at mooring 10 located off Louisiana at 27.94°N, 92.75°W in

water of depth 200 m. The July sequence shows maximum amplitudes of 40 to 60 cms

at depth of 12 m for the situation of light winds. The period of diminished amplitudes

followed an atmospheric frontal passage. These are near-circular, clockwise-rotating

oscillations with period near 24.0 hr. They seem to be an illustration of thermally

induced cycling (Price et al. 1986) in which large amplitude rotary currents can exist in

thin mixed layers typical of summer. By contrast, the December sequence shown in

Figure 4.10 evidences no such behavior. Many examples of such currents, in phase at

distinct locations, exist for the Texas-Louisiana shelf, and by implication they exist

further offshore. Currents at 1m depth have been observed to reach 100 cims*

Clearly episodic wind events can cause maj or currents in the deepwater Gulf. The initial

currents give rise to inertial oscillations lasting for up to about ten days with decreasing

amplitudes, superimposed on longer period signals. The number of direct measurements

of energetic currents forced by episodic wind events is rather limited in the deepwater

region of the Gulf of Mexico. Additional examples over the Texas-Louisiana continental

shelf edge and slope are given in Nowlin et al. (1 998a). The references contain other

82
Figure 49. Eastward (u) and northward (v) components of currents (hourly values from 3-hr low-passed records) from m

Louisiana atapproximately9o.5°W. Mooring 13 was in water depth of 200 m; mooring 12 was in water dept12 March 1993, a class 5 extratropical cyclone moved from west to east across the Texas-Louisiana shelf wiapproximately over the I 500-m isobath. 20050 -150

5 200 150 O0 -150

I I M13190ni M1212n,

I I

I I .-

I ... ............. ...........

.

I

J

I..-.

I

I

I

I

I

.

I . ...........................................................................

I

I

I

I

I

I -..

II

1I

-

I I M310m

I

I

II

I

I I I

I -.,I

I.-.I

I I 

M319Orn M1212rn

I I .............

6 7 March 1990

8 9 10 11 12 10 4 15 15 17 18 19 20 21 22 5 6 7 March 1990

8 9 10 11 12 10 14 15 16 
Now/in et aL: Physical Oceanography

60

40-

20-

-20 -

-40 - U (solid)

V (dashed)

Total water depth = 200 m

I III II III 1234567891011121314151617181920

December 1992

3 to 40 hr band-passed filtered data

LATEX Mooring 10: Velocity components at 12 m

40

20

-20

-40

-60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

July 1992

Figure 4.10. Eastward (u) and northward (v) components of currents from 3-to 40-hr band-passed

records made in July and December 1992 at mooring 10 located off Louisiana at 27.94°N,

92.75°W in water depth of 200 m.

84

60
85

Now/in et aL: Physical Oceanography

examples of measured currents, documentation of the effects on property distributions of

energetic, episodic wind events, and models of such storms (e.g., see Leipper 1967, for

effects of a hurricane passage on density and temperature distributions in the open Gulf

or OBrien 1967 or O'Brien and Reid 1967, for the modeled effects of hurricane

passage).

Loop Current Forcing and Eddy Activity

The phenomena of most concern in the past to deepwater operators in the Gulf of Mexico

are surface-intensified currents associated with the Loop Current (LC), anticyclonic Loop

Current eddies (LCEs) detached from the LC, and other eddies (both anticyclonic and

cyclonic). Currents associated with the LC and LCEs extend into the water colunrn to the

of order 1,000 m.

The Loop Current enters the Gulf through the Yucatan Channel and exits through the

Straits of Florida (Figure 4.1). As it enters (where it is called the Yucatan Current), the

LC is westward intensified (e.g., Cochrane 1963). The surface currents that parallel the

current core are approximately 200 cms1 or more at the core and fall off to 50 cm-s' or

less within order 10 km to the west and order 100 km to the east of the core. They may

reverse direction on the eastern side of the channel to flow from the Gulf into the

Caribbean (Cochrane 1963; Maul et al. 1985). The deep flow at the sill in the Yucatan

Channel is not unidirectional into the Gulf. In addition to tidal flows, there is significant

variability between northward and southward flows at subtidal frequencies. Based on a

3-year current record at 1985 m (145 m above the sill) from an instrument moored

halfway between Mexico and Cuba, Maul et al. (1985) showed an average southward

drift of 1.8 cms1 with significant energy near 19 d and 38 d. Moreover, they observed

four periods of persistent southward flow with average 5 cims1 speeds and bursts to

15 cms1. These were separated by 8 to 10 months and appear to follow shedding of

LCEs (R. 0. Reid, personal communication), so they may be due to mass compensation

as the Loop Current pushes into the Gulf following ring shedding.

As the LC moves into the Gulf, it tends to follow the topography along the Campeche

Bank with meanders of small amplitude (Molinari and Cochranie 1972). Using the

available data sets and an analytical model based on conservation of potential vorticity,

Molinari and Cochrane (1972) noted the LC tends to go unstable north of 23.5°N where

the topography changes rapidly from a north-south to east-west orientation; the current

then may veer east and begin to meander. Reid (1972) found with a simple dynamic

model that the variation of the Coriolis parameter could be responsible for the

anticyclonic turn of the LC after it detaches from the Campeche Bank.

The LC then extends northward into the Gulf as a quasi-stationary meander (Hamilton

1997). The extent of the northward intrusion is variable (see e.g., Maul 1977; Vukovich

et al. 1979; Sturges and Evans 1983; Molinari and Morrison 1988). The northward

development of a LC intrusion was described by Leipper (1970) and by Maul (1977)

using time series of contours of the 22°C isotherm at 100-rn depth, which Leipper
Now/in et aL: Physical Oceanography

showed was a good indicator of the position of the LC. One example is schematically

depicted in Figure 4.11a. The sequence shows the intrusion of the LC front from

approximately 24.75°N in August 1972 northward into the Gulf a year later it had

extended to about 28.75 °N. The contours for June and July show the development of an

LCE associated with this intrusion. Note the July contours show the eddy pinching off at

both the east and west sides. By September 1973, the LCE had detached and the LC

front was at 25.5°N, about 30 latitude south of its position the month before. The percent

of time during which warm LC water was located within various regions in the eastern

Gulf is shown in Figure 4.11b (Science Applications International Corporation [SAIC]

1989). Although early investigations suggested that the northward extension was

characterized by a seasonal cycle with minimum intrusion in winter and maximum in

summer or fall (e.g., Leipper 1970; Maul 1977), subsequent investigations found the

phasing was highly variable (Sturges and Evans 1983; Molinari and Morrison 1988).

Data from 1974-1976, analyzed by Molinari and Morrison (1988), showed that the

noi-thward penetration of the LC into the Gulf was well correlated with the location of the

current on Campeche Bank at the time of its separation. The LC did not intnide far in the

Gulf when it was over the eastern part of Campeche Bank at time of separation; the

degree of penetration into the Gulf increased for separations occurring farther west on the

bank.

The LC influences the circulation in the deepwater region and over the slope of the

eastern Gulf both directly as it intrudes northward and indirectly through the eddies and

frontal features, such as filaments, associated with it (Kelly 1991). Maximum surface

currents in the LC range from 100 to 200 cms1 or more, as seen in Figure 4.12 (Nowlin

and Hubertz 1972). Hamilton (1997) reported on the velocity structure of the LC in the

upper 1,500 in as obtained from air-deployed current profilers on 11 May 1984. The

velocities normal to a nominally east-west transect across the LC at about 25°N showed

strong northward and southward flows at, respectively, the West arid east edges of the LC.

Flows in the current core exceeded 120 cms near-surface and were approximately

50 cm-s at 200 m and 10 cms1 at 700 m. Flows below 800 i-n, which is the sill depth of

the Florida Strait, were -40 cm-s' or less and generally in the opposite direction from the

current core, suggesting the LC penetrates only to the depth of the Florida Strait sill.

Hamilton (1990), however, showed that the LC may strongly influence currents below

1,000 m by the excitation of energetic currents associated with topographic Rossby

waves (discussed later).

Strong current events that are associated with the nearby presence of the LC and its

frontal features have been observed over the slopes and shelves of the eastern Gulf.

Ebbesrneyer et al. (1982) attributed to LC forcing the peak currents of 30 to 50 cms1

lasting 20 to 30 days that were observed at depths of 100 to 200 m over the upper slope

off the Mississippi River delta. Filaments that protrude from the LC also influence

currents over the slope. Huh et al. (1981) described an event in which an LC filament

moved, at speeds of20 cms, up the axis of De Soto Canyon to within a few miles of

the coast. Kelly (1991) and Vastano et al. (1991) describe types and effects of intrusions

of the LC and its filaments onto the Mississippi-Alabama slope and shelves.

86
Now/in et aL: Physical Oceanography

30N

28 N

26N

24N

22N

92W 90W 88W 86W 84W 82W BOW

30N

28N

August 1972

April-May 1973

26N June 1973

July 1973

Septemb& 1973

24N

22N

30N

- 28N

26 N

24N

22N

Figure 4.11. (a) An example of the development of an intrusion of the Loop Current into the eastern

Gulf of Mexico is schematically depicted with contours of the 22°C isotherm at 100-rn

depth from hydrographic cruises conducted in August 1972 through September 1973

(Adapted from: Maul 1977). (b) Frequency of occurrence (%) of warm Loop Current water

in the eastern Gulf of Mexico from monthly frontal analyses of AVHRR data from 1976

through 1985 (Adapted from: Science Applications International Corporation 1989).

Percentage contour intervals are 10%.

87

---J I

,-.-..... -t

.-.

-

&

, 1 ----- :,rj__

- -

LE

5

4

(b)

/

/

\ _t'

92W 90W 88W 86W 84W 82W 80W

92W 90W 88W 86W 84W 82W 80W

92W 90.W 88W 86W 84W 82W 80W

30 N

28 N

26 N

24N

22N
2r 2082- -- i/1 I - i .-- L -- ç / 'I - 6 - I -ii - ' -'/ll" I/i 1/, \. 24: k - -,-- c - / I i' j . \\ \\ // - 4 2r i. / ? I/ 67-A-4 eo 30 - 24 

__) \, 2 \ I,4-_ aa - 92 90 VUC AT 

Figure 412. GEK surface current observations (left) and surface dynamic topography relative to 1,350db (right) from A

June 1967 (Adapted from: Nowlin and Hubertz 1972). ee 

92 90 er 9O Br e
89

Now/in et al.: Physical Oceanography

Paluszkiewicz et al. (1983) describe the intrusion of an LC filament, propagating

southeastward at 30 cms1, onto the west Florida shelf.

When the LC penetrates into the Gulf, a LCE may separate from the LC itself. This

process is complex, and the eddy can detach and reaftach several times before fully

separating from the LC (Vukovich 1995). Cochrane (1972) described a May 1969

separation event. It involved the formation of two cyclonic meanders, one protruding

into the LC from Campeche Bank and the other from the west Florida shelf. The

Campeche Bank meander grew until it joined the west Florida shelf meander, creating a

cyclonic shear zone that separated the LCE from the LC. Vukovich and Maul (1985)

observed, for the period 1978 to 1981, that the separation of large anticyclonic eddies was

preceded by development of cyclonic eddies on the eastern side of the LC, but that

development of such cyclonic eddies did not always result in separation of an LCE.

Periods between LCE detachments vary from 4 to 16 months with a primary mode at 8 to

9 months and a secondary near 13 to 14 months (Sturges 1994; results of an analysis by

Vukovich 1995 are similar). Initially detached rings have diameters greater than 250 km.

with typical values near 350 km, which decrease by 45% within 150 days and 70% within

300 days (Elliott 1982). They may have surface speeds of 150 to 200 cms1 or more;

speeds of 10 cms1 are not uncommon at 500 m (Cooper et al. 1990). For more detail,

see Table 4.4. After their separation from the Loop Current, LCEs move into the western

Gulf, with average translation speeds of 5 km per day (range of 1 to 20 km per day), and

in the process may interact with other eddies or with the continental margins to form

additional eddies. They have typical lifetimes of 350 to 400 days (Elliott 1982), and

decay by interactions with boundaries, ring shedding, and ring-ring interactions (e.g.,

Vidal et al. 1992). The net result is that at almost any given time, the Gulf is populated

with numerous eddies, interacting with one another and with the margins. As an

example, Figure 4.13 shows sea surface height anomaly (cm) relative to a mean sea

surface for 9 May 1993. It is based on satellite altimeter gridded data by Robert R.

Leben, Colorado Center for Astrodynamics Research (CCAR), as described in Biggs et

al. (1996). Clearly seen is the Loop Current, one semi-detached anticyclonic feature, the

remnants of two LCEs, and many cyclonic features of various strength. Many of these

separated anticyclonic and cyclonic features would be expected to have surface currents

exceeding 50 or even 100 cm-s1.

According to model results obtained by Oey (1995), transient shelf edge currents can be

forced by the expansion of LC into the Gulf, the passage of a LCE in deepwater, or the

collision of a LCE with the continental slope.

Although the LC and LCEs have been studied since the early 1960s, details of their

velocity distributions and variability remain virtually unknownonly a few estimates of

three-dimensional velocity fields have been reported (e.g., Cooper et al. 1990; Forristall

et al. 1992). As an example, Figure 4.14 shows components of velocity (cms') normal

to a section extending from approximately 27.4°N, 90.6°W (station 64) to 24.8°N,

89.4°W (station 78). Measurements were made with a lowered Neil Brown acoustic

current meter by Forristall et al. (1992); ship motion was estimated using Loran-C and
98 °W 30°N 28°N 26°N 24N 22°N 20°N 96°W 94-°W 92°W 90°W 88° W 86°W 84°W 

Figure 4.13. Sea surface height anomaly (cm) from satellite altimeter data for 9 May 1993. Based on gridded data from 

Colorado Center for Astrodynamics Research, as described in Biggs etal. (1996). Locations of stations 16(filled triangle) are shown. w __//l)- I / / ///!)J / -- / / /(( ' \' t N2O \.c 0' (// - / //__ \\ \ / -. \ / 1/ / "r/ /\c y tT5L IJ / I ii \: / - - ;II) l .'r contour interval is 2.5 cm - () \, ', I / - 's-(7 T-' 

: <r P ) 1 j / ____/__ 4j. 98 °W 96°W 94°W 92°W 90°W 88 °W 86°W 84° W 
100 200 a) a) 0) 30O 400 U) Co '.0

0 (0(0 Station co o, o IN C U) W - CO a) Co r- r- t.- t.- p- i- t

I I Jill ig/,1flt

I I I I I LI IioljIll ii I I %%I I lit %,tII '

I I I I t I ,iI 1 CD '0 1 ' j 1 o - , It lull

1 % I I ljIl

I i i

i i t ili

I i i i ._iII

I

i i i '' Jo 1' I

x ',.6b11i 1

5 1 1ll

i 1 tI l

I

I I CD it 1 l 1 J I,

a o 1 1 it

I

' jI .

I Figure 4.14. Components of velocity (cms-1) normal to a section extending from approximately 27.4°N, 90.6°W (statio

(station 78). Measurements were made by Forristall et al. (1992) using a lowered Neil Brown acoustic curr

was estimated using Loran-C and motion of the instrument relative to the ship was measured with an ultra

system. This section crossed the LCEcalled Fast Eddy during August 1985, and components are taken to swirl speeds of the eddy. Positive components are directed toward 65°.

0 50 100 150 200 250 300 350 

Distance (km)
Now/in et a!,: Physical Oceanography

Table 4.4. Loop Current eddy properties

Shedding frequency Mean of about 10 months, with variations from 4 to 16 months

(Sturges 1994)

Translation speeds Vary over speeds of 1-14 km per day

Average speed of 5 km per day

Diameters of last Initially usually 250 km; typical value near 350 km

closed circulation Decrease by 45% within 150 days; 70% within 300 days

(Elliot 1982)

Swirl speeds For new rings: 150-250 cms1 at surface; 10 cms1 at 500 m

Depths

Energy

Property distributions and motions extend to >1,000 m for

new rings

Filaments are likely confined to the upper 50-3 00 m

Available potential energy for typical new ring is

1 5x 1 Jm 'Ratio of available potential energy to kinetic

energy 4.5 for new rings (Elliot 1979)

Lifetimes 350 to 400 days (Elliot 1982)

Decay modes Interactions with boundariesfomrntion of filaments

Ring shedding; ring-ring interactions

motion of the instrument relative to the ship was measured with an ultra-short baseline

acoustic system. This section crossed the LCE Fast Eddy during August 1985 and

components are taken to represent azimuthal swirl speeds of the eddy. Positive

components are directed toward 65°. Surface currents exceeded 160 cms1 on one side of

this LCE and 120 cms1 on the other side. Speeds at 200 m reached 100 cms1 in this

young LCE located in the north central Gulf.

To illustrate currents produced by LCEs in the northwestern Gulf, and at somewhat

greater depths, in Figure 4.15 are shown 40-br, low passed current components from the

same moorings C and S discussed in connection with hurricane effects (Figure 4.8). At

mooring S large northward component, accompanied by reversing east-west components,

are seen at the upper instrument commencing in late September and continuing into early

November. During this period, northward speeds at 200-m depth averaged about

50 cms1, with bursts exceeding 70 cms1. This period of high speed flow resulted

because of the presence of a clockwise rotating renmarit of an LCE in the area (Brooks

1984). The vector velocities (not shown) reveal that the current varied in direction from

NW to NE several times during this period; the implication is that the western edge of the

92
28 4 Sep 1980 I I I I 11 18 25 2 Oct I I I I 9 16 23 30 6 13 

Nov Figure 4.15. Components (u positive to the east and v positive to the north) of 40-hr, low-passed currents measured at 

moorings S (26N, 96c lOW) and C (55 km north of S). Both moorings were approximately on the 730-rn ilong period of high northward components, accompanied by reversing east-west components, is seen to coSeptember and continue into early November; this was the result of the presence of an anticyclonic eddy o
Now/in et al.: Physical Oceanography

eddy repeatedly moved north and south over the mooring during the period. Maximum

speeds associated with this LCE at 200 m reached 60 crns1; speeds at 450 m were

considerably less, but nevertheless significant at values in excess of 35 cms1; even near

the bottom at 700 m speeds reached 10 cm-s1. Velocities at mooring C were not

correlated with those at mooring S. A cyclonic eddy (also noted to be in the area by

Brooks 1984) was situated between moorings C and S from about 10 until 20 September,

as seen by northwestward flow at S and southeastward flow at C.

Figure 4.16 shows horizontal current vectors (hourly values from 3-hr low-passed

records) from moorings located approximately equidistant along the 200-rn isobath at the

edge of the Texas continental shelf. Mooring 6 was at 27.71°N, 95.66°W; mooring 9 was

at 2781°N, 93.50°W. The influence of Loop Current Eddy V over the shelf edge is seen

as it moves generally eastward past each mooring, beginning at mooring 6 from 22 July

to 9 August. Maximum speeds at the upper instruments (--10 m) ranged from 50 to

100 crns'; those at mid-depth instruments (100 m) approached or exceeded 50 cm-s1. It

should be noted that this LCE remnant was only the northern portion of an old ring that

had been separated into two parts by interaction with another eddy.

Energetic, high frequency currents have been reported to occur with the passage of LCEs

past structures, but they are not well documented; such currents would be of concern to

offshore operators because they could induce structural fatigue of materials.

Only limited information is available concerning the velocity fields within cyclonic and

ancillary anticyclonic eddies; reports include Vukovich and Maul (1985), Forristall et al.

(1992), Hamilton (1992), and notably Berger et aL (1996). Hamilton (1992) reports the

existence in the central Gulf and over the Louisiana continental shelf of cold cyclones

with upper layer currents of 30 to 50 cms-1, little surface temperature expression, the

largest isotherm displacements in the depth range 200 to 800 rn, diameters of 100 to

150 krn, and long lifetimes.

Berger et al. (1996) reported on the existence over the continental slope and offshore of

numerous eddies smaller than LCEs. Three small types of eddies can be identified from

the results of that study: cyclonic eddies, anticyclonic eddies, and a submesoscale

coherent vortex. The cyclones are probably the same features described over the slope by

Hamilton (1992) and mentioned by Hamilton et al. (1999). These features are seen to

affect the then-nal fields to diameters of 1 50 km, although the observed radii of solid

body rotation seems to extend only to 25 to 50 km. Maximum currents are 30 to 50

cm-s1 and occur somewhat beneath the surface, e.g. around 200 m. Velocities extend

into the water column to depths of 800 to 1,000 m. Isotberms are sometimes observed to

be depressed in the surface waters with the doming found in cyclones beginning around

200 m and increasing with increasing depth down to at least 1,500 m. (This is consonant

with occurrence of maximum culTents near 200 m) The secondary anticyclones

observed over the continental slopes of Texas and Louisiana seemed to have horizontal

extents as small as or smaller than the cyclones. However, they were numerous and often

found over the upper slope. Unlike the cyclones, they have their maximum thermal

expression in the surface layers. There are only minimal measurements of the velocity

94
Figure 4.16. Horizontal current vectors (hourly values from 3-hr low-passed records) from moorings located approximate

200-rn isobath at the edge of the Texas contnental shelf. Along shelf components are oriented up-down (ueastward); cross-shelf components are oriented across the figure (off-shelf to the right). Mooring 6 was at 2mooring 9 was at 27.81 N, 93.50GW.
Now/in at aL: Physical Oceanography

structure in these features, though one can speculate that maximum velocities are

<50 cms-1. On one occasion in October of 1993, Berger et al. (1996) observed over the

slope near 92.5°W an eddy not previously observed in the Gulf of Mexico, with thermal

structure that was raised and lowered, respectively, above and below a subsurface level of

about 350 m.

Sea Level and Tides

Sea level is the elevation of the water surface relative to a fixed datum (Reid 1990). It

fluctuates over many time and space scales. This section focuses on those sea level

fluctuations that are due to seasonal thermal variations and to astronomical forces.

The seasonal cycle of heating and cooling induces thermally-forced seasonal sea level

variability in the Gulf of Mexico. Pattulo et al. (1955) conducted a study of sea level

oscillations over the world oceans, including observations from the Gulf of Mexico.

They defined the component of sea level due to the density distribution as steric sea level.

They noted that in the subtropical latitudes, the variations in steric level were mostly

thermally-induced and quite large. Pattulo (1963) found the large sea level variations in

the subtropics were due principally to variations in oceanic heat storage induced by

seasonal variations in local heating.

Whitaker (1971) focused his study of the seasonal variations of steric sea level

specifically on the Gulf of Mexico. He computed steric levels using available

temperature data together with a constant salinity of 36.30, which he showed resulted in

errors of the same order as those due to sampling and smoothing. His monthly maps of

steric sea level relative to 150 in show a general Gulf-wide increase in steric height

during spring and summer to maxima in September-October and a decrease during fall

and winter to minima in Februaiy-March. The approximate range of steric levels through

the year was 0.275 to 0.625 dyn m. The maps also show a higher steric level signature

from the warmer water temperatures associated with the Loop Current and possible LCEs

in the eastern Gulf and a western high," possibly associated with LCEs, in the western

Gulf.

Astronomically induced sea level variations in the northern Gulf of Mexico are

dominated by the diurnal tidal components, rather than the semidiurnal components,

except along the west Florida coast (Reid and Whitaker 1981; see also Reid 1990, for a

review of tides, tidal forcing, and numerical modeling of tides). Tides in the Gulf are

connected to tides in the Atlantic Ocean through the two ports at the Yucatan Channel

and Straits of Florida. Reid and Whitaker (1981) reviewed tides in the Gulf of Mexico

and developed a numerical tide model that agreed well with the observed K1, O, P1, M2,

and S tidal constituents. Table 4.5 shows their computations of tidally-induced volume

influx from the two ports and the Gulf-wide mean response of water level. For

comparison, Table 4.5 also shows water level response to these five tidal constituents

measured at a deep pelagic site (Mofjeld and Wimbush 1977). The results of Reid and

Whitaker (1981) show diurnal tides have nearly uniform amplitudes and phases over the

96
Now/in et aL: Physical Oceanography

whole Gulf, with the K1 and 0 constituents significantly greater in amplitude than the

semi-diurnal tidal amplitudes, and are driven mainly by in-phase forcing through the

ports. This is in agreement with earlier investigators (Grace 1932, 1933; Maimer 1954;

Zetler and Hansen 1972). The semidiurnal tides, however, are smaller than the diurnal

tides and are driven mainly by the direct tide potential. Reid and Whitaker (1981) note

that the response of the M2 tide is nearly resonant to the direct forcing by the tide

potential. They find the greatest response occurs on the west Florida shelf near Cedar

Key, explaining the anomalously large M, tides along the west Florida coast compared to

the rest of the Gulf. Table 4.6 shows the percent of the total variance in sea level

attributed to in-phase and out-of phase port forcing and to direct forcing by the tide

potential.

Table 4.5. Summary of volume computations and mean response of water level (Reid

and Whitaker 1981) and observed tidal constituents measured at a deep

pelagic site located at 24.767°N, 89.648°W by Mofjeld arid Wimbush (1977).

The influx rate is for both ports combined. The phase lag is given relative to

the time of transit of the maximum tide potential at Greenwich. The rms

values are for the amplitude moduli for 20 observational stations, including

that of Mofjeld and Wimbush (1977), for each given tidal component.

Compare the tidal influx rates with the mean volume flux of30 x 106 m3-s1

into and out of the ports that is associated with the quasi-permanents currents.

Table 4.6. Percent of observed variance in sea level attributed to various

forcing factors by the model (Reid and Whitaker 1981)

97

Tidal Influx rate Mean sea level

response

RMS station

response

Observation at deep

pelagic site

constituent Amplitude Phase Amplitude Phase Amplitude Phase

(106 m3s') (degrees) (cm) (degrees) (cm) (cm) (degrees)

K1 14.40 294.3 12.82 24.3 14.93 14.10 28.5

01 12.73 289.7 12.23 19.7 14.43 14.90 18.9

P1 4.19 295.2 3.75 25.2 4.60 4.76 28.4

M2 5.18 105.7 2.39 195.7 15.29 1.30 225.8

S2 2.19 103.3 0.98 193.3 5,74 1.50 130.0

Forcing factor %for

K1

%for

M2

In-phase port forcing 83 35

Direct forcing 12 55

0utof-phase port forcing 1 1

Residual 4 9
Now/in at al.: Physical Oceanography

Tidal currents are induced by the changes in sea level elevation from astronomical tides.

Schmitz and Richardson (1968) detennined the tidal volume transport amplitudes of the

Florida Current were 3.5 ±1 x 106 m3s1 for the K1, 0, and M2 tides and 1.5 ± 1 x 106

m3-s for the S2 tide, in good agreement with estimates across the Straits of Florida by

Werthiem (1954). Durham (1972) calculated the tidal current transports for the K1 and

O tides across the Yucatan Channel to be 12.6 ± 4 x 106 m3s4 and 11.3 ± 4 x 106 m3-s',

respectively.

Rezak et al. (1985) presented the major and minor axes of the K1 and M2 tidal current

ellipses for the Gulf of Mexico as determined by the Reid and Whitaker (1981) model;

these showed the tidal currents over the deepwater Gulfwere much smaller than those

over the shelves. Using a hydrodynamic model, Mungall et al. (1978) estimated the K1

and M2 tidal currents in the deepwater Gulf to be less than 1 cm s' with higher values

(5 to 20 cms') over the shelves. DiMarco and Reid (1998) conducted a tidal analysis of

81 current meter records over the Texas-Louisiana shelf and shelf break. They examined

the principal diurnal constituents (K1, 0, Pi. and Q ) and semi-diurnal constituents (M2,

52, K2, and N2). They found that, as expected, the tidal currents were larger over the shelf

than in deeper water. For example, K1 and M2 tidal current amplitudes at about 1 0-m

depth were, respectively, 3.2 cm-s' and 1.2 cms' from a mooring in water depth of 50 m

as compared to 1.6 CiThS1 and 0.5 cms' in water depth of 500 m. DiMarco and Reid

(1998) showed the tidal vectors of both the diurnal and semi-diurnal constituents rotated

anticyclonically. They found generally that the major axes of the semi-diurnal tidal

current ellipses were directed across the isobaths and the diurnal tidal current ellipses

were circular. Their results provided qualitative verification of the model results of Reid

and Whitaker (1991).

Removed from Yucatan Channel, Straits of Florida, canyons, or other major bathymetric

features, tidal currents in the offshore Gulf are small. In the upper 100 m, currents

associated with the major tidal components may reach amplitudes of 1 to 2 cm-s* At

greater depths they are much smaller. As an example over the upper slope, amplitudes of

M2, K1, and 01 components were 0.23, 0.32, and 0.26 cms at 490 m on LATEX

mooring 12 located in 500-m depth about 90.5°W off Louisiana. For a mooring in

Atwater Canyon Block 618 currents at 16 levels were analyzed for principal tidal

components. As expected, the principal components (M2, K1, and Oi) were 1 to 2 cm-sd

in the upper 100 m, decreasing to vanishingly small values (0. 1 cms or less than the

level of analysis error) for depths between a few hundred meters and near bottom at

1,945 in.

Currents in Deep Water

Discussed in previous sections were currents associated with energetic wind events and

with the Loop Current and mesoscale eddies. In addition to those currents, several other

classes of energetic currents are now known to occur in the deepwater region of the Gulf.

For the most part they are not yet well documented and even less well understood.

98
Now/in et aL: Physical Oceanography

Figure 4.17 is a schematic of design current speeds in the deepwater Gulf of Mexico for

three classes of phenomena: the Loop Current and surface-intensified eddies;

topographic, barotropic Rossby waves; and subsurface-intensified, high-speed jets

(shown by gray domain). The currents associated with anticyclonic eddies have been

most often measured or estimated; those with cyclonic eddies are less well surveyed, but

assumed to be of same or lesser magnitudes. The phenomena of deep barotropic currents,

perhaps with bottom intensification, have been observed and reported in the open

literature on one occasion, but are substantiated by model results and proprietary

measurements. The subsurface-intensified, high-speed jets have now been documented

in many data sets, but not yet reported in the open literature. The physical mechanisms

responsible for those jets are not yet identified. Recently another class of deep currents

was detected by documenting their effects in producing long, deep, linear furrows in the

bottom sediments near the Sigsbee Escarpment. In this section are discussed deep,

barotropic currents believed due to topographic Rossby waves, subsurface-intensified

energetic jets, and currents responsible for the bottom furrows.

Deep, Barotropic Currents

During the rnidl980s, barotropic (depth independent) currents were observed to extend

from depths near 1,000 in to the bottom. Shown in Figure 4.18 are 40-hr low passed

current vectors from SAIC mooring G maintained in the eastern Gulf as part of an

MMS-sponsored physical oceanography program. As described by Hamilton (1990), the

northern edge (eastward currents) of the Loop Current was affecting the array during

December 1984. Then, from January to March the mooring was influenced by the

southward flow of the eastern limb of the LC as it extended further into the Gulf. Note

that currents above 1,000 m were affected in a coherent, surface-intensified manner, but

currents below 1,000 m were not affected. During the period April-June an eddy

separated from the LC. At that time considerable energy appeared in the lower water

columnvertical velocity coherence with slight near bottom intensification is seen.

Hamilton (1990) concluded that such currents result from barotropic topographic Rossby

waves triggered by the Loop Currentperhaps on separation of LCEsand that their

propagation speeds into the western Gulf are larger (perhaps 9 kimd') than the average

propagation speeds (5 lumd') of the separated LCEs.

Sturges et al. (1993) observed similar phenomena from numerical model results for the

Gulf. Deep circulation patterns distinct from those associated with the surface intensified

eddies also were seen in numerical model studies by Inoue and Welsh (1997).

Proprietary oil company measurements show some such barotropic currents with

maximum speeds near 40 cm-sd and periods of weeks. Moreover, data give some

indication of bottom current intensification. This class of barotropic currents, with

possible bottom intensification, is of the high interest to offshore operators attempting oil

production in water depths of 1,000 m and greater; measurements are ongoing in the

north central Gulf by the MMS and by offshore operators.

99
Now/in et aL: Physica/ Oceanography

1,000

2,000

3,000

Loop Current Eddy

( Topographic Rossby

wave activity

Possible boUom tensification

of Rossby wave

Figure 4.17. Schematic showing design currents in deepwater Gulf of Mexico for three classes of

phenomena: topographic Roasby waves, Loop Current eddies, and subsurface-intensified,

high-speed jets (shown by grey domain).

100

Design speed (crn.s1)

0 50 100 150 200

I I
Figure 4.18. Current vectors (4Ohr low-passed; north directed upward) from Science Applications International Corpora

in 3,200 m water depth at25°36.2'N, 85°29.8'W off the southern West Florida Shelf. Position is shown in F
Now/in et al.: Physical Oceanography

High-speed, Subsurface-intensified Currents

Several deepwater oil and gas operators have observed very high-speed,

subsurface-intensified currents lasting of the order of a day at locations over the upper

continental slopes. Such currents may have vertical extents of less than 100 m, with

maxima observed generally within the depth range of 100 to 300 m. Maximum speeds

exceeding 150 cm-s' have been reported. Figure 4.19 shows time-averaged current

profiles before, during, and after the occurrence of such a subsurface jet in Mississippi

Canyon in 1997. Total water depth at the location is estimated to be 800 to 900 m. The

profiles in the upper 100 in are similar, showing a strong surface current with average

speeds of about 30 cm-s1 at the surface decreasing to roughly 12 cm-s' at 100 m. Below

100 m the profiles for the periods before and after the jet event were essentially

barotropic at 8 to 10 crns. The profile during the event shows a strong current at

mid-depth with maximum averaged speeds greater than 30 cims1 near 300 m. Analyses

of individual profiles shows that peak currents generally occurred between 250 to 300 m

with a maximum speed of 56 cm-sd.

Examining data from locations in depths of 1,200 to 1,500 m, scientists at Texas A&M

University have observed currents with subsurface maximum speeds of 50 cms' lasting

for about 1 day with bursts of speed peaking at more than 100 cms1. The higher speed

currents appear to propagate upward, characteristic of baroclinic waves (either sub- or

super-inertial). An example is shown in Figure 4.20. It seems possible that such

phenomena could be intensified near topography. Causal mechanisms are being sought.

Model results also show short-period, subsurface-intensified currents over the Gulf

slopesbut with maximum speeds approaching only 50 cms1. Examples are shown in

Figure 4.21. More evidence for these phenomena is being sought in observations and in

model results.

Currents Responsible for Furrows

In early 1999, William Bryant of Texas A&M University (personal communication)

discovered and mapped, using a deep towed acoustic system, a previously unexplored

bedform just offshore of the Sigsbee Escarpment in the northwestern Gulf of Mexico.

These are large, long furrows eroded into the Holocene deposits blanketing this region.

These furrows have depths of 5 to 10 m, widths of several 1 Os of meters, are spaced on

the order of 100 m apart, and extend unbroken for distances of 1 Os or more km.

Generally they are oriented nearly along depth contours. Bryant has observed them in the

region of 90°W just off the Sigsbee Escarpment and near the Biyant Fan south of Bryant

Canyon from 9l°Wto 92.5°W. Depths in those regions range from 2,000 to 3,000 m.

More recently, the existence of these features has been corroborated and they have been

mapped more extensively in the area of Green's Knoll by offshore oil and gas operators.

Observations of furrows demonstrate that such bedforms are widespread and important

features in regions with cohesive, fine-grained sediments and directionally stable currents

(Flood 1983).

102
I 200 400 600 0 /1 / 04/1008:11- 04112 20:01 -/ 04/12 19:51 04/15 14:21 K Mississippi Canyon ADCP velocity profile: April 1997 04/07 02:21 - 04/1008:01 \ Figure 4.19. Average current profiles before (thin line), during (thick line), and after (dashed line) a subsurface jet event 

(data courtesy of Chevron). 10 20 30 

Speed (cms1)
Now/in et aL: Physical Oceanography

1000

0

200 m

232 m

264 m

296

328 m

360 m

12: 16:

28

April

1994

Figure 4.20. Acoustic Doppler current profiler current speed in Green Canyon Block 200 from 29 April

through 30 April 1994, showing 50 cms1 current event propagating upwards in the water

column beginning about 30 April. Distances shown are in meters from the instrument,

which was close to the surface. Data provided courtesy of Marathon.

104

500

8m

40m -----

72 m

104 m

136m - ,L.-'- ..JtI 68 m

392 m

-500 47.4 -

456 In

20: 0: 4: 8: 12: 16: 20: 0: 4: 8: 12: 16: 20:

29 30
Nowlin et al.: Physical Oceanography

0

50

100

150

E 200

250

o 300

350

400

450

500

Current Speed at 28.06° N, 90.55° W (KM)

cm-s-1 : nowoast ends at Dcc. 9

Figure 4.21. Time series of vertical profiles of currenpeeds from the Kantha model. Shown are two

Gulf of Mexico locations along 90.5°W in approximately 200 m (upper) and 1,000 m

(lower) water depths (courtesy of CCAR, http://www-ccar.colorado.edu/-jkchoi/

gomforecast. html).

105

50

100

0) C' a

150 I

200

28 6 16 26 6 16 26 8

Oct'98 Nov Dec Jan '99

Current Speed at 27.69N, 90.57° W (SH)

cm-s-1 :nowcast ends at Dcc. 9

28 6 16 26 6 16 26 8

Oct'98 Nov Dec Jan '99

I I I 0 10 20 30 40 50 60
Now/in et aL: Physical Oceanography

It appears that the processes responsible for these furrows are active at present. Based on

the change in character of these features from offshore toward the escarpment, and on the

rather good agreement of that change with changes observed in published laboratory

studies of submarine erosion (e.g, Dzulynski 1965; Allen 1969), the tentative conclusion

is that bottom currents responsible for these features have along-isobath components and

increase in strength toward the escarpment. Such work attributes the furrows to rows of

counter-rotating helical currents generally directed along the furrows with rising parts of

the helixes over the furrows. No direct measurements have been reported. The

laboratory experiments indicate that furrows of different separation, wavelengths, and

fundamental character occur for different flow rates. However, it is difficult to scale

these model results to field conditions. Speculation is that near-bottom speeds of currents

responsible for the inshore furrows might be 50 cm-s' or even iii excess of 100 cm's1.

These currents might be sporadic or quasi-permanent. These furrows and the currents

responsible for them may also exist over a considerable part of the yet unexplored base of

the continental slope in the Gulf of Mexico.

The implications of these furrows and currents for oil and gas production are manifold.

Thse currents may represent a distinctly different phenomena, and thus problems, than

the other classes of currents observed to date and presently under consideration.

Effects of Shelf/Slope Canyons and Other Rough Topography

Submarine canyons may be conduits for exchanges of shelf and slope waters.

Observations and model studies of canyons outside the Gulf of Mexico suggest that

currents near the bottom tend to align with the canyon topography (e.g.. Han et al. 1980)

and that eddies which induce upwelling can form over the canyons near the shelf (e.g.,

Freeland and Denman 1982). The major canyons in the northern Gulf of Mexico are

De Soto Canyon and Mississippi Canyon. De Soto Canyon is the broader of the two

canyons. It nearly separates the Mississippi-Alabama shelf from the shelf off the Florida

panhandle. The Mississippi Canyon is narrower and is located southwest of the

Mississippi River Delta at about 90°W, Other rough topographical features of interest

are the Sigsbee Escarpment and the many canyons inshore thereof off Texas-Louisiana

and the Florida Escarpment off west Florida. The furrows and possible intense currents

associated with them are located off the Sigsbee Escarpment (see discussion of deep

currents).

Drennan (1968) summarized hydrographic investigations conducted from 1963 through

1965 over the east Louisiana-Mississippi-Alabama-west Florida panhandle shelves in the

northeast Gulf of Mexico. He concluded that the De Soto Canyon bathymetry influences

the horizontal circulation over the vicinity of the shelf edge in spring and early summer.

During that time, a current flowed from the Mississippi Delta toward De Soto Canyon

along the shelf edge. As this current moved over the canyon, it branched into a

component that turned north in response to the bottom topography and a component that

flowed toward the southeast. De Soto Canyon may act as a conduit for intrusion of deep

waters onto the upper slope and shelf, as observed in May 1998 where cool water was

106
Now/in et al.; Physical Oceanography

pushed onto the shelf west of Cape San Bias (Nowlin et al. 1998a), and of energetic

surface currents excited by the Loop Current or associated frontal eddies (e.g.,

Ebbesmeyer et al. 1982; Huh et al. 1981).

In a numerical modeling study of flow over shelf-canyon systems, Howard (1992) found

that quasi-geostrophic flow across a canyon generates small scale eddies through vortex

stretching processes and nonlinearities in the dynamics; these eddies freely propagate

away from the canyon. He also found that wider canyons contain better organized eddy

flows. During MMS-sponsored observations over the northeastern Gulf of Mexico

shelves, both cyclonic and anticyclonic eddies were observed over the head of De Soto

Canyon; similar eddies have been observed over other canyons and in numerical models

(see e.g., Freeland and Denman 1982). A 1996-1999 study of the currents and

hydrography over De Soto Canyon, being conducted for the MMS by SAIC, should

provide added insights into the effect of Dc Soto Canyon on the circulation over the

upper slope.

It was noted in discussing tidal currents that such currents in the deepwater region of the

Gulf are very small. In the vicinity of canyons, however, the tidal currents may be more

energetic. At the Mississippi Canyon mooring in 300 m water depth (indicated by

MCTAMU in Figure 4.7), both the diurnal and semi-diurnal tidal currents were of

amplitude >1 cims1 at 50 m off the bottom as compared to <0.1 cm-s' (in the noise level)

away from the canyon. The orientation of the tidal currents was along the canyon axis at

the mooring location. The diurnal tidal current amplitudes appeared to be bottom

intensified, being >2 cms1 at 5 m off the bottom.

Direct current measurements in Mississippi Canyon are sparse. However, Lo (1999)

conducted a modeling study of the buoyancy-driven flow over the Texas-Louisiana shelf

that included the Mississippi Canyon. He found that a near-surface, offshore westward

current split into two branches as it approached the canyon: one branch flowed across the

canyon toward shore and the other formed a slope current along isobaths. He further

found that an anticyclonic eddy was located near the head of the canyon and induced

upwelling at the head of the canyon as water moved from depth onto the shelf along the

canyon axis, consistent with the linear theory of geostrophic adjustment (see e.g.,

Freeland and Denman 1982; Klinck 1989; Howard 1992) and similar to observations over

Dc Soto Canyon noted above.

Numerical Modeling

Types of Models and Studies

Numerical studies of the three-dimensional (3 -D) circulation in the Gulf of Mexico

(GOM) have employed primarily three generic types of models based upon the primitive

equations of motion (enforcing mass, momentum, and energy conservation). The first

type is the n-layer, quasi-Lagrangian model as in the studies of Hurlburt and Thompson

(1980) and Murphy et al. (1999), and is most useful when thermohaline aspects are of

secondary concern to the dynamics. Fulerian models (those employing a fixed 3-D grid)

can, most easily accommodate both thermodynamic and dynamic aspects. Such models

107
Nowlin et al.: Physical Oceanography

using fixed z-levels are of the Bryan-Cox type (Sturges et al. 1993; Inoue and Welsh

1997) or the Sandia-type (Dietrich 1992; Dietrich and Lin 1994). Finally the most widely

used Eulerian model in GOM applications is the Princeton Ocean Model (POM),

developed by Blurnberg and Mellor (1983) and adapted to a curvilinear grid by Blurnberg

and Herring (1984). The POM employs fixed values of the depth-following "sigma"

coordinate (zlh, where h is water depth). Applications of this model have been used in

GOM process studies (Oey 1995), cliinatological studies (Herring and Patchen 1997;

Herring 1999), and hindcast, nowcast, and forecast studies by (Kantha and Piacsek 1993;

Choi and Kantha 1997; and Herring 1999).

Table 4.7 gives a capsulization, including the horizontal and vertical grid resolution for

the different models iii the GOM applications. One concern regarding the use of the

sigma coordinate is the possible error in rendition of the horizontal pressure gradient and

associated error in current (Haney 1991; Mellor et al. 1994). In order to minimize such

error the vertical resolution over the steep continental slope must be consistent with the

horizontal resolution. Specifically the number of cross-slope intervals on the slope

should be compatible with the number of vertical intervals (as in the stair-step rendition

of the seabed in a z-coordinate model). It is not clear how close this condition is met in

all applications for the Gulf of Mexico, so this could be a source of difference among

models. Another is differences in the parameterization of the sub-grid-scale along- and

cross- isopycnal mixing of properties. The latter has an important impact on the shedding

of eddies from the Loop Current.

The model studies discussed herein differ in their focus and in their domain. The

emphasis in the studies by Oey (1995) using the POM model and Dietrich (1992, 1997)

using the Sandia type model is on processes, including shelf dynamics, eddy-eddy and

eddy-slope interaction. The early focus of the Dynalysis GOM studies was in hindcasting

and climatology, and more recently in forecasting (Herring and Patchen 1997; Herring

1999). The Kantha model applications emphasize nowcasting/forecasting, including

satellite altimeter and MCSST data assimilation (Choi and Kantha 1997). Although all of

these model applications included local wind stress over the GUM, they are primarily

driven by specified inflow to the GUM (or to the northwest region of the Caribbean Sea),

and outflow from the Straits of Florida that is constrained to have the same volume

transport as the inflow. The other studies considered here employ large closed domains

over which the forcing is primarily by winds.

The focus of Sturges et al. (1993) and of Inoue and Welsh (1997), using the Bryan and

Cox type model, was on relation of mesoscale variability in the upper to that in the lower

layers of the GUM. The model domain in those studies includes a subregion of the North

Atlantic and all of the Caribbean Sea, closed by walls at 36N, 49°W, and 9'N. Hence

the forcing was via North Atlantic winds. Finally, the study by Murphy et al. (1999),

employing the NRL n-layer model (Wallcraft 1991), was also wind driven and of global

domain. But its primary focus was on anticyclonic eddies that are spawned by the eastern

retroflection of the North Brazil Current. These eddies grow within the Caribbean Sea,

and some even squeeze through the Yucatan Channel to affect the evolution of the Loop

Current. This raises the question as to real time monitoring of inflow to the GUM for

predictive models. We address this question in a later section.

108
Table 4.7. Comparison of relevant properties of several ocean circulation models applied to the Gul

Model 2-layer Bryan-Cox type Bryan-Cox type Sandia SOMS POM Oey version POM Dynalysis version POM Kantha version N-layer NRL version Reference Hurlburt and Thompson 1980 Sturgesetal. 1993 Inoue and Welsh 1997 Dietrich and Lin 1994 Oey 1995 Herring and Patchen 1997 Kantha and Piacsek 1997 Murphy etal. 1999 Horizontal Resolution 20km 1/40 1/80 20km 20km 4-14km except in SW region 1/50 and 1/12° 1/4° in Gulf and Caribbean Levels or layers 1.5 &2 layers 12 fixed z levels 33 fixedz levels 16 fixed z levels 20 fixed i/h sigma levels 15 fixed sigma levels 21 fixed sigma levels 5.5 & 6 layers Irregular 8 to 137 to 20 for different conditions Order of 10 Not applicable Order of 100 Order of 100 Order of 100 Order of 100 Typica

near2

At rest most rCycloncyclonCycloncyclonCyclon

basin sNot av

Cyclon

basin sNot avNot av

Shedding period (mo) Maximum current (cm 1) 10.8 (327d) 73 6.9 (30 wk) 100 6.9 (30 wk 100 9.9 (300 d) 75
Now/in et al: Physical Oceanography

Eddy Shedding Aspects

Clearly the cycle of intrusion of the Loop Current and the shedding of energetic eddies is

the most important aspect of the GOM circulation which a realistic numerical simulation

should get right. The numerical experiments in the early 1970's with linear andlor

fplane models of the GOM could not pass this test. Huriburt and Thompson (1980) were

the first to simulate the shedding of anticyclonic eddies (i.e., warm core rings) from the

Loop Current, using a nonlinear two-layer numerical model that allowed planetary

vorticity tendency (beta effect. Huriburt and Thompson also demonstrated that the mean

shedding period depends inversely on the square root of the maximum inflow speed and

inversely on the buoyancy parameter (thermal stability). Their experiments showed that

a periodic eddy shedding could occur with the lower layer at rest, even with steady inflow

conditions in Yucatan Channel. Hurlburt and Thompson attributed the mechanism for

shedding of eddies to barotropic instability, while the periodic property they related to the

time for the Loop Current to reach its maximum intrusion after the spawning of an eddy.

However, the shedding can be shut down if the horizontal mixing of momentum is too

large, as in the earliest POM applications for the GOM (Bluniberg and Mellor 1985).

Unlike the characteristics of warm core and cold core rings shed from the Gulf Stream in

the North Atlantic (Chassignet 1992; Sturges et al. 1993), the LCEs are always warm

core and are always shed in about the same place. Recognizing this, Pichevin and Nof

(1997) now attribute the shedding of eddies associated with the retroflection of any major

culTent system (like the Loop Current) to the requirement for a momentum balance for

the system, in a time average sense. An important aspect of their analytic study is that it

predicts the size of the eddies to be significantly larger than that deduced from the

baroclinic radius of deformation. This may explain the greater size of the LCE compared

with the North Atlantic warm core rings (beyond that due to the latitude difference).

All of the GOM model applications given in Table 4.7 produce eddy shedding. The

mean period of shedding varies from one application to another and is tunable to a certain

degree, dependent upon the inflow conditions, the buoyancy parameter, and the lateral

mixing coefficient for momentum. The application by Oey (1995) indicates a desired

chaotic character of the shedding with a period range from 8 to 13 months. The

Dynalysis 12 year climatological run, using a Smagorinsky (lateral mixing) coefficient of

0.05, exhibits a range of shedding from 9 to 23 months (Vukovich 1996). As discussed

in n earlier section, the observed range is from about 4 to 16 months (Sturges et al.

1993). Further adjustment of the Smagorinsky coefficient could bring these POM models

more in agreement with the observations, but even the latter is only about a 25 year

sample, and hence is subject to uncertainty. The encouraging thing is that these two

applications do demonstrate a chaotic character of shedding, similar to the real observed

conditions.

Both the Dynalysis and Oey model domains include the northwestern part of the

Caribbean Sea, while most applications of the Kantha model do not. In the study by Oey

(1995), he found that eddy separations were often followed by a reversal in the flow of

deep water near the Yucatan sill, so as to return water to the Caribbean Sea (Oey,

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Now/in et a/.: Physical Oceanography

personal communication). We speculate that this interaction of the two basins may be

important in the separation process and its chaotic character. It may also be important in

governing the circulation in the layer between 800 and 2,000 in (the sill depths for Straits

of Florida and Yucatan Channel, respectively). The results of the Dynalysis

climatological study show that this layer has a mean cyclonic circulation in the eastern

Gulf, in opposition to the mean anticyclonic circulation of the Loop Current in the upper

800 m (Herring and Patchen 1997). The studies based on the Bryan-Cox type model

(Sturges et al. 1993; Inoue and Welsh 1997) support this result.

Other Skill Assessment

The MMS funded the LATEX measurement and data analysis program for the

northwestern Gulf of Mexico as well as the numerical hindcast and climatology study of

GUM circulation carried out by Dynalysis of Princeton. Skill assessment of the

Dynalysis model results was addressed by an independent team including C. N. K.

Mooers from the University of Miami, P. P. Niiler from Scripps Institution of

Oceanography, M. Inoue and W. J. Wiseman from Louisiana State University, and

F. M. Vukovich from SAIC at Raleigh, NC. They addressed the model skill with respect

to response to synoptic time and space scale wind forcing, including a simulation of

hurricane Andrew, as well as extratropical storms which occurred during the LATEX

measurement time window (April 1992 to December 1994). They also addressed the

monthly and seasonal climatology, including of course the Loop Current and LCE

characteristics. Final reports have presumably been submitted to MMS from the skill

assessment group, but were not available to the authors at the time of writing this

literature survey. 1-lowever, an interim report by Vukovich (1996) on eddy shedding skill

(discussed earlier), and one by Mooers (1996) on near coastal current skill, were

available to one of the authors from a project meeting at Dynalysis in March 1996. Some

important aspects of Mooers' analysis are discussed below.

Included in the many runs of the Dynalysis model were hindcasts using synoptic wind

fields employing point specific buoy data blended with ECMWF wind fields, for a time

window encompassing the LATEX current measurement period (April 1992 to December

1994). The Mooers (1996) interim report provides results of auto- and cross-spectra of

the simulated eastward and northward current components at 100 m depth, for the shelf

break LATEX current meter moorings 7 to 11 (between 92° and 95W along the 200-m

isobath). The spectra are based on a time window spanning the three calendar years

1992 to 1994, and cover the frequency range from about 0.01 to 12 cpd. The autospectra

for all of the simulated mooring positions are very similar for given current component.

They show statistically significant peaks at about 0.2, 1.0, and 5 cpd. Moreover, the

coherence among the five mooring positions is high with westward propagation indicated

for the 0.2 cpd peak.

Visual comparisons between observed between observed and simulated time sequences

of currents at the Texas-Louisiana shelf break indicated low correlation (presumably

related to the influence of the LCE impacting the shelf differently in the model versus the

observations). However, the current spectra deduced from the LATEX observations

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Now/in et aL: Physical Oceanography

(DiMarco et al. 1997) for the middle meter (at 100 m depth) for moorings 7 to 11 do

indicate a peak at about 0.2 cpd. For mooring 9 the observed peak at 0.2 cpd and that

simulated by the Dynalysis model are both about the same magnitude. Although the

coherence among the observed shelf break currents data was low, the studies of Hamilton

(1990) using current meter data on the continental slope support the existence of

westward propagating topographic Rossby waves.

Status of NowcastfForecasts

Two operational nowcast/forecasts of GOM circulation are presently in existence and

available on the world wide web. One is for the coastal region north of about 25N being

carried out by Dynalysis as part of the NOPP Ocean Monitoring Program (Herring 1999).

The other is the Gulf wide nowcast/forecasts carried out by the Kantha, Choi, Leben team

at CCAR (Choi et al. 1995; Choi and Kantha 1997). The lafter effort is supported by the

petroleum industry Consortium for Analysis Studies of Eddies (CASE) because a

knowledge of the position and strengths of LCEs is vital in offshore drilling and recovery

operations for oil and gas.

An important finding of the LATEX program (Nowlin et al. 1998a) was that about 50%

of the variance of subtidal observed currents on the Texas-Louisiana inner shelf (water

depths of 50 rn or less) can be explained by the winds. We would therefore expect that

the coastal nowcasts of currents by the Dynalysis operational model would have a skill

that at least matches this finding for that region. The skill for forecasts, however, is

heavily dependent upon the skill of the wind forecast model used to locally drive that

model. Real time observations of near surface currents at several sites over the Texas

shelf are available from the Texas Automated Buoy System (TABS), which is jointly

supported by the Texas General Land Office and the S of the Department of Interior.

Spot checks of the Dynalysis near-surface current predictions over the Texas shelf

indicates reasonable skill for the daily average current. The observed TABS currents

show large near inertial motion superimposed on the low frequency along shelf flow.

Reversals in flow from upcoast to downcoast or vice versa can follow reversals of local

winds within a day. The model predictions track such changes well as long as the wind

predictions are correct. The wind fields used in the predictions are supplied by the NCEP

mesoscale ETA forecast model.

The CCAR (Choi et al. 1995) data assimilative model is reasonably accurate in nowcasts

of the Loop Current and large energetic eddies. The SSHA deduced from satellite

altimeter data represents a blending over time and space of TOPEX and ERS-2 data, and

is most accurate for large scale features. The method of assimilation of the SSHA and of

SST data is discussed in the application of the Kantha-Piacsek model by Horton et al.

(1997). A primary limitation in the forecasting of changes in the Loop Current and

associated LCE is the present lack of real time monitoring of inflow to the GUM.

Motivated by this need, the Eddy Joint Industry Program (EJIP) together with the NOPP

Ocean Monitoring Program are carrying out a series of aerial and ship surveys along one

of the TOPEX tracks (number 65) across the western Caribbean Sea. The purpose is to

provide the calibration of that track so as to allow real time estimates of the S SHA and

112
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Now/in et a/.: Physical Oceanography

associated geostrophic inflow to the western Caribbean and hence to the GUM. In the

meantime, the CCAR model domain has been extended into the Caribbean, with the

TOPEX track 65 serving as the new open boundary (personal communication with Choi).

By monitoring inflow in real time along this track, forecasts of changes within the GUM

should be improved, since the advection of the boundary information into the Gulf

requires several weeks.

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Chapter 5: Chemical Oceanography

Mahion C. Kennicutt II

Introduction 123

Dissolved Gases 124

Dissolved Solids 125

Nutrients 125

Dissolved Organic Matter 127

Dissolved Metals 129

Particulate Matter 130

Particulate Organic Matter 131

Particulate Inorganic Matter 132

The Chemistry of Natural Seepage 133

Brine Seepage 133

Hydrocarbon Seepage 134

Conclusions 135

Literature Cited 135

Introduction

The Gulf of Mexico is a semi-closed sea that receives the bulk of its water from inflows

from the Caribbean Sea through the Yucatan Channel with outflow through the Straits of

Florida. The origins of the water establish the initial chemical characteristics of water

masses within the basin. After inflow, the chemical composition of these water masses

are then modified by a number of processes including in situ processes such as biological

activity (which transfonns nutrients to biomass and produces oxygen), mixing, and

upwelling. In addition, large amounts of runoff enter from the river systems that ring the

basin. Runoff from approximately two-thirds of the United States arid more than half of

Mexico empties into the Gulf of Mexico. These discharges regularly introduce a range of

dissolved and particulate materials to the basin. Atmospheric transport, through wet and

dry deposition, is another pathway that introduces chemicals to the basin. These

processes can also introduce contaminants to the basin. In addition, activities occurring

within the basin proper (shipping, oil and gas exploration and production, etc.) can

introduce contaminants to the water and sediments.

This review summarizes what is known about the chemistry of the open water Gulf of

Mexico (GUM) between water depths of 300 and 3,000 in. Geologic setting is important

when considering the chemical oceanography of the deep GUM. The basin is

characterized by wide continental shelves effectively isolating the deeper GUM from the

direct influences of many coastal processes. The slope is characterized by steep inclines

and a complex topography. Once deposited, sediments can be redistributed by mass

wasting and slumping. These basinal features are important influences on the chemical

composition of deep sea water and sediments. Another important feature of the

deepwater GUM is that this region is underlain by large accumulations of salt, oil, and

gas that have migrated nearly vertically into surficial sediments. The salt in contact with

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porewaters produces brines that migrate into near-surface sediments. Salt movement

creates the tectonic forces that influence local topography. This same salt movement also

creates subsurface conduits that allow liquids and gases to seep into near-surface

sediments and the overlying water column. The active petroleum system that underlies

the deep northern GUM has an important effect on the chemical characteristics of the

slope environment as well. The leaky nature and immense volumetrics of the petroleum

system allow the introduction of large amounts of petroleum hydrocarbons into the basin

from below. Imprinted over these natural introductions of chemicals are inputs from

anthropogenic activities.

The interactions of sources and sinks mediated by complex biogeochemical processes are

the framework that establishes the spatial and temporal patterns of chemicals in the water

and the sediments of the deepwater northern GUM.

Dissolved Gases

The water overlying the deepwater GUM contains a range of dissolved constituents

whose distributions lend clues to the relative importance of various biochemical

processes in establishing the observed chemical distribution. Dissolved constituents

include gases and solids. The dissolved gases include carbon dioxide (CU2), oxygen,

nitrogen, and other trace gases such as methane and hydrogen. The basic control on the

concentrations of gases in near-surface waters is exchange with the atmosphere.

Secondarily, gases such as oxygen, CU2, and methane have their distributions altered by

biological and/or other processes, i.e., they are non-conservative. The distribution of

inert gases (such as nitrogen) are primarily affected by physical redistribution processes

including mixing and diffusion, i.e., they are conservative.

A complete discussion of the CO2 system is beyond the scope of this review. However,

the salient points of the CO2 system that are operative in the water masses of interest to

this review are briefly discussed. Carbon dioxide concentrations are the end result of the

interaction of biological processes (photosynthetic uptake and respiratory production),

water transport, and surface heating and cooling that drives the air/sea partitioning of CU2

(Riley and Chester 1971). Carbon dioxide concentrations in surface waters vary

temporally on scales from hours to months (seasonal). In the open deepwater GUM, the

equilibrium concentrations of CO2 are predictable since the overall size of the carbon

pooi is large and the concentration of living organisms is relatively low compared to

more isolated and smaller bodies of water where CO2 concentrations and speciation can

be highly variable in space and time. Below the thermocline, CO2 concentrations are

affected only by advection, diffusion, and the biological oxidation of organic matter.

Total CU2 rises sharply below the thermocline and increases to a maximum at a depth of

500 to 1,000 m coinciding with the oxygen minimum

Dissolved oxygen is another biologically active gas and is an important product of

photosynthesis. As for CO2, the initial control on dissolved oxygen concentrations is

exchange with the atmosphere. Near-surface oxygen levels are augmented by

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photosynthesis. Below the surface layer, a layer of oxygen supersaturated water tends to

develop as this is the site of intense biological activity. Below the photic zone,

respiration, which produces CU2 and consumes oxygen, increases to a point where it

exceeds the oxygen production and an oxygen minimum develops. The GUM oxygen

minimum layer develops outside of the GUM and enters the GOM as an established

water mass property through the Yucatan Channel (Wust 1964). This low oxygen layer

spreads across the GOM and can intersect the seabed on the continental slope. Typical

oxygen values are 3.5 to 4.5 mLL1 in the upper 200 m, 2.5 to 3.5 mL'L at 200 to

500 m, and 2.5 to 5.5 mLL' in water depths >500 in (Nowlin 1972).

Trace gases in the deepwater GUM can have an atmospheric and a biological source. In

particular, methane exhibits near surface water maxima similar to dissolved oxygen.

Methane is believed to be biologically produced in situ. An additional source of methane

is water advected from the adjacent shelf where sedimentary porewater methane diffuses

into the overlying water column. Methane concentrations have been shown to be

associated with indicators of phytoplankton biomass (ATP) and with suspended

particulate matter maxima (Brooks et al. 1981). Methane appears to be formed in situ in

the reducing microenvironments associated with suspended particulates, some of which

are advected into deeper water from the shelf (Brooks et al. 1981). Typical methane

values are 50 to 250 nLL1 in the upper 200 m of the water column and 10 to 40 nLL1 in

deeper waters distant from natural gas seeps. Significant amounts of methane and higher

molecular weight hydrocarbons are introduced to the water column from natural seepage.

In deeper waters, methane is stored in gas hydrate and the methane can have a biologic or

thermogenic origin.

Dissolved Solids

In addition to dissolved gases, deepwater GUM water masses contain dissolved solids.

The most obvious is "sea salt," which is primarily sodium chloride with smaller amounts

of a range of cations and anions. Salinity distributions in the study area are discussed

elsewhere. In addition to salt, the water masses contain dissolved nutrients, dissolved

organic carbon, and dissolved metals. The primary nutrients are nitrate, phosphate, and

silicate and are essential elements for the growth of organisms. Dissolved organic matter

(DUM) and metals are derived from several sources. In deepwater areas, the DUM can

be quite old, reflecting the long term accumulation of microbially resistant DOM from

marine and riverine sources. DOM can be both natural and anthropogenic in origin.

Dissolved metals can be derived from riverine inputs and resuspension of sediments. As

for DOM, dissolved metals also can be natural or anthropogenic in origin.

Nutrients

Nutrient concentrations and distributions in the deepwater Gulf of Mexico are controlled

by a combination of biogeochemical and physical processes (Armstrong 1974). The

processes that control nutrient concentrationsriver discharges, coastal currents and

winds, upwelling, biological activity, and rainfallare operative to some degree in the

deepwater GUM. In near-bottom waters, remineralization of organic matter can liberate
Kennicutt: Chemical Oceanography

nutrients into the overlying water column. Excess or enhanced nutrient levels can

contribute to plankton blooms followed by oxygen depletion due to organic matter decay.

On the outer continental shelf and in deeper waters, differences in nutrient concentrations

between surface water and bottom waters are substantial. Nutrient concentrations

increase from below the detection limit in surface waters to maximum values in the

deepwater GUM. Waters as deep as 70 m tend to be nutrient-poor. An abrupt increase in

nutrient concentrations occurs between 70 and 100 m water depth. This vertical structure

develops as a result of the fixation of nutrients into biomass by phytoplarikton in the

euphotic zone and remineralization of organic matter in deeper waters. These processes

occur basin-wide and are key to establishing the chemical characteristics of the water

masses in the deepwater GUM.

Nitrogen is essential for life and provides an important chemical building block for many

organic compounds such as amino acids and proteins. Nitrogen occurs in seawater as

dissolved molecular gas (N2) and inorganic and organic compounds. The principal

inorganic forms of nitrogen in seawater are nitrate (NO3), nitrite (NO2D, and ammonia.

Seawater also contains minute amounts of other inorganic compounds as well as

dissolved and particulate organic nitrogen compounds. The distribution of the maj or

inorganic nitrogen species in the deepwater GUM is controlled by biological processes.

Once incorporated into biomass, nitrogenous materials are redistributed in the water

column due to sinking of the detrital remains of dead organisms and upwelling of deeper

waters to the near surface. These processes, operating in unison, establish a nitrogen

cycle." While phytoplankton normally synthesize proteins from nitrate, nitrite and

ammonia, bacteria preferentially incorporate organic nitrogen. During metabolism,

nitrogenous material, mainly in the form of urea, is excreted by living organisms. The

nitrogen cycle is not a closed system in that nitrogenous material is deposited in

sediments and various forms of nitrogen enter the GOM in river and rain water.

Important processes that control the distribution of nitrogen in the open water GUM

include nitrogen fixation, assimilation of fixed nitrogen, and regeneration of nitrogen.

Most nitrogen assimilation occurs in the euphotic zone whereas regeneration of nitrate

occurs throughout the water column and in the sediments. This offset in the spatial

juxtaposition of these processes establishes the commonly observed vertical

heterogeneity observed in water column profiles of inorganic nitrogen. Because the

distribution of nitrogenous materials is primarily influenced by biological processes,

there is a strong seasonal variation in concentrations. Typical nitrate concentrations are 0

to 20 M in the upper 200 m of the water column and 20 to 30 iM in waters deeper than

200 m.

As another nutritive element required by living organisms, the same input and removal

processes control the distribution of phosphorus in the open waters of the GOM as do

nitrogen. Phosphorus occurs in seawater in a variety of dissolved and particulate forms.

Inorganic phosphorus is predominantly in the form of orthophosphate ions (PU43).

Dissolved phosphorus in seawater also occurs as organic compounds mostly derived from

the decomposition of organismal remains and excretion from living organisms.

Phosphorus occurs in the particulate form in seawater in association with living

organisms and detrital remains from dead organisms. As for nitrogen, a "phosphorus

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cycle" has developed in the open waters of the GUM. Removal mechanisms include

sedimentation and inputs are derived from riverine discharges, rainfall, and regeneration

in the water column and sediments. Phytoplankton usually satisfy their phosphorus needs

by direct assimilation of dissolved orthophosphate followed by metabolic transformation

to organophosphorus compounds (phospholipids, phosphonucleotides, etc.). In the open

water GUM, phosphorus is generally available in amounts exceeding the needs of the

resident organisms. Bacteria satisfy their nutritional needs for phosphorus from detritus

and are an active component, in addition to intracellular enzymes, in the process of

regeneration of phosphorus. The close coupling of phosphorus to biological processes

results in strong temporal variations usually associated with the seasons. Vertical and

spatial heterogeneity in phosphate distributions reflects the dynamic interplay of sources

and removal processes. Differences in the locations where these processes occur

establish the observed spatial gradients and patterns in phosphate concentrations. Typical

phosphate concentrations are 0 to 1.0 LM in the upper 200 m of the water column and 1.0

to 1.8 p.M in waters deeper than 200 m.

Silicon is present in seawater in both dissolved and particulate forms. The dissolved

form is commonly determined as silicate (SiO4). A major source of silicon to the GUM

is the subaerial weathering of rocks followed by riverine transport to the sea. Within the

water column there are many organisms, including diatoms and radiolarians, that have

skeletons composed of hydrated silica-opal. Upon their demise, the siliceous skeletons

slowly dissolve in seawater as they sink to the underlying sediments. High

concentrations of silicon are observed in inshore regions and can account for as much as

60% of the water column particulate matter depending on geographic location. Seawater

is undersaturated with respect to silicon and therefore dissolution is almost always an

ongoing process. The incorporation of silicon into biological structural components is an

efficient mechanism of removing silica from seawater. These materials are also

efficiently transported and deposited to the sediments due to the rapid settling and slow

dissolution rates. The open water GOM contains several groups of plants (diatoms) and

animals that require silicon to maintain their structural integrity. The concentration of

silicon in open surface waters is generally low except in regions of upwelling due to the

uptake by organisms in the euphotic zone. As with the other nutrients, the close coupling

of silicon with biological processes produces spatial and temporal variations in its

distribution and patterns. The "silicon cycle" produces heterogeneous patterns in time

and space that reflect the balance of sources and removal processes. Each process can be

more or less important depending on the local setting producing significant regional

variations in silicon distributions. Typical silicate concentrations are 0 to 10 p.M in the

upper 200 m of the water column and 10 to 24 p.M in waters deeper than 200 in.

Dissolved Organic Matter

DOM is defined as that material which passes through a 0.5 micron filter and includes

truly dissolved material as well as colloidal matter. The primary origin of DOM is

biological organisms either through exudates or decomposition of detrital remains The

dissolved portion of organic matter can include a wide range of anthropogenic

compounds such as hydrocarbons and other man-made chemicals (DDT, etc.). DOM in

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the study area arises from riverine and atmospheric transport, decay of dead organisms,

algal metabolic exudates, and zooplankton and larger animal excretions. Riverine

derived DOM is primarily composed of leachates from humic materials and the

decomposition of vegetable matter. Riverine DOM can also include sewage and

industrial effluents. In open GUM waters, in situ biological activity is an important

source of new DOM. Two processes that produce DOM in the open water GUM are the

lysis of cells and the bacterial degradation of organic matter. Algae are known to

produce appreciable amounts of excretion products. Zooplankton and other marine

animals excrete DOM as well. The excretion products are generally nitrogenous in

nature including urea, purines, and amino acids.

DOM is usually measured as dissolved organic carbon (DOC). Few measures of DUC in

the deepwater GUM have been reported. Deepwater GUM DUC concentrations vary

from 0.5 to >2.0 mgL (Fredericks and Sackett 1970). DOC values are highest on the

shelf, lower in the surface waters of the open Gulf, and lowest in the deep open GOM.

Open GOM surface water (0 to 90 m) concentrations ranged from 0.45 to 1.07 mgCU1

with a mean of 0.79 and deepwater (90 to 3,600 m) DOC concentrations varied from 0.33

to 0.94 mgC-L4 with a mean of 0.52. Other observations at a few slope stations reported

DOC concentrations ianging from 45 to 130 micromolar, with 30% to 60% of the DOC

being high molecular weight depending on location and season (Bianchi et al. 1997). As

in the previous study, DOC generally decreased from shelf to slope waters and from

surface to bottom waters. These and other studies suggest that a large portion of

terrestrially derived DUC is preserved and transported to open GUM waters. DOC

generally exceeds particulate organic carbon (POC) concentrations by a factor of 10 to

20, accounting for most of the organic carbon in a typical water column.

DOM is believed to be primarily complex macromolecules that are resistant to bacterial

degradation thus accounting for the old age of DOM in deep GUM waters. DOM can

contain amino acids, carbohydrates, lipids, and vitamins. In addition, anthropogenic

chemicals can occur in a dissolved state as well. One class of compounds of interest is

the hydrocarbons. Lower molecular weight hydrocarbons are at least sparingly soluble in

seawater. In particular, those compounds referred to as volatile organic compounds

(VOC) will be dissolved if they are present. In the deep sea portions of the GOM, these

hydrocarbons can be derived from ship traffic, tanker discharges, discharges from

offshore platforms (produced waters), spills, and natural seepage. In general, dissolved

VOC and higher molecular weight hydrocarbons are below detection limits other than in

areas close to discharge points. VOC rapidly equilibrate and degas to the atmosphere.

Hydrocarbons are also labile being subject to relatively rapid microbial degradation.

Most studies of dissolved hydrocarbons in the GUM have been restricted to shallow

waters and will not be reviewed here. Early studies by Iliffe and Calder (1974) detected

concentrations of extractable non-polar dissolved hydrocarbons in the GOM ranging from

trace amounts to 75 tgL' with the highest concentrations occurring in the Straits of

Florida. The hydrocarbons were primarily n-alkanes containing 15 to 20 carbons.

Atmospheric fallout and tanker traffic were mentioned as possible sources. It is also

unclear whether these hydrocarbons were truly dissolved. Sauer et al. (1978) and Sauer

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(1980) found open GOM surface waters to contain 60 ngL' or less of volatile liquid

hydrocarbons (VLH) of mostly a biological origin. This contrasts with more polluted

coastal and shelf waters with concentrations of more than 500 ngL'. VLH were

composed of normal alkanes, cycloalkanes, arid aromatic hydrocarbons. Brine

discharges, tanker washings, and the underwater venting of waste gases were identified as

sources of VLH. In general, open water GOM dissolved hydrocarbons are at least an

order of magnitude lower in concentration than nearby shelf waters. No appreciable

levels of dissolved hydrocarbons have been detected in the open water GOM; however,

deepwater areas are not well studied. In areas of natural seepage, hydrocarbon

concentrations can be significantly elevated arid sea slicks are known to form in these

areas as well.

It is generally believed that biological organisms are primarily affected by exposure to

dissolved contaminants. In addition, organisms can ingest particulate contaminants

and/or in the case of hydrocarbons, be externally coated if the levels are high enough

(i.e., spills). As the main mechanism of bioaccurnulation, dissolved contaminants and the

associated organismal body burdens are often used to indicate exposure. A confounding

factor in recognizing organismal exposure to organic contaminants is that many

organisms, such as fish, are efficient at depurating toxic compounds. In particular,

aromatic hydrocarbons are transformed by enzymatic modifications and are excreted in

the bile. Therefore, contaminant body burdens may or may not reflect contaminant

exposure history. There are few studies of the concentrations of hydrocarbons in

organisms from the open GOM. Those organisms obligately associated with natural oil

and gas seeps are discussed elsewhere in this chapter. In one report, the levels of

benzene, toluene, ethylbenzene, phenol, fluorene, benzo(a)pyrene, and

bis(2-ethylhexyhl)phthalate were either not detected or below the detection limit in

tissues from representative fish species at two platform sites in about 300 m of water

(Offshore Operators Committee 1997). While the amount of data in the open waters of

the GOM is limited, it is expected that most if not all organisms will be free of significant

arithropogenic contamination.

Dissolved Metals

Metals have both natural and anthropogenic sources in the deepwater GOM. Metals are

released during the weathering and decomposition of rocks (minerals, soils, etc.) and are

then transported to the basin in dissolved and particulate forms. Human activities can

also introduce metals to the water column from dumping of wastes, oil and gas

exploration and production activities, construction, and shipping. Few studies have

analyzed the concentration of dissolved metals in the deepwater GOM. Dissolved metals

that have been detected in riverine runoff include copper, nickel, chromium, arid

molybdenum. It has been estimated that less than 10% of the metals transported to the

sea are in the dissolved form (Trefry and Presley 1976). Atmospheric transport of metals

can also be a pathway that introduces metals to the basin. Direct anthropogenic

introduction of contaminants to the open water GOM occurred during the 1970's when

approved dumpsites were used to dispose of industrial wastes at one location 90 km

offshore off Southwest Pass in 1,000 m of water and a second site 220 km south of

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Galveston. Exploration and production platforms are also known to discharge cuttings

and produced waters that contain dissolved arid particulate metals to the adjacent waters.

Open GOM surface waters had concentrations of 0.082 ppb for copper, 0.11 ppb for

nickel, and 0.0005 ppb for cadmium (Boyle et al. 1984). These levels were

approximately five times lower than those observed in coastal regions. For a second

sampling of surface waters in the northwestern GUM in water depths from 100 to

1,000 m, similar concentrations of dissolved copper, cadmium, and nickel were observed.

Higher values were indicated nearer to shore and nickel, copper, and cadmium

concentrations increased with water depth as a result of organic matter degradation.

Dissolved cadmium increased sharply with water depth and some deep samples were

10 times higher than near-surface samples. Other metal concentrations increased with

depth but the increases were less regular and were usually not more than by a factor of

two. Dissolved metal data in general suffers from artifacts due to the lack of clean

methods for sample collection because of the very low concentrations encountered. It is

also been noted that pore water dissolved metal concentrations are generally higher than

the overlying water suggesting that difftision of metals into the overlying water may be

an important source of some dissolved metals (Presley and Trefry 1980; Trefry and

Presley 1982). Dissolved metals have not been well studied in the deepwater GOM, but

the few reports suggest that dissolved metals occur at very low (>ppb) levels.

As for organic contaminants, it is generally believed that the most biologically available

component of metals is the dissolved fraction. As can be seen above, the levels of

dissolved metals in the open water GOM are expected to be extremely low in

concentration. As such, metals in orgariismal tissues would be expected to be low as

well. This was the conclusion of a study of trace metals in organism associated with two

platforms in about 300 m water depth in the northern GUM (Offshore Operators

Committee 1997). In representative fish species, collected in close proximity to

platforms, arsenic varied from 2 to 59 tgg', cadmium from not detectable to 0.82 tgg',

and mercury from 0.036 to 0.33 jig-g'. The fishes were collected at sites that were

discharging produced waters and sites that were not discharging produced waters. No

statistical differences could be detected between the sites, suggesting that natural

processes and not produced water discharges were responsible for the metal

concentrations observed in tissues. The concentrations detected were considered natural

background levels for fishes collected throughout the GUM.

Particulate Matter

In addition to the dissolved constituents described above, water masses in the deepwater

GUM also contain significant amounts of particulate matter. Particulate matter in the

water column of the open water GOM can be organic or inorganic, living or dead, and

natural or anthropogenic in origin. Particulate matter is defined as those materials that

are retained on a 0.5 micron filter. Particulate matter is often expressed as the weight of

total suspended matter (TSM). Typical TSM values range from 50 to 200 gL' in the

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upper 200 in of the water column and 20 to 100 gL' in deeper waters (Feely et al.

1971) of the open water northern GOM.

Particulate Organic Matter

In the deepwater GUM, the particulate organic matter (POM) in the upper water column

is primarily detritus and phytoplankton. There are few studies of the distribution of POM

in the open water of the GOM. Phytoplankton are the living portion of POM and

generally account for 25% or less of the total particulate organic matter but this can be

highly variable in the euphotic zone. The living fraction may also contain bacteria, fungi,

and yeasts, with bacteria in some instances being a significant fraction of the bioinass.

With increasing depth below the euphotic zone, the living fraction of the POM decreases

rapidly. POM is usually 10 to 20 times lower in concentration than the DOM. POM is

usually measured as PUG. Typical POC concentrations range from 50 to 100 tgL1 in

the upper 200 m of the water column and 30 to 50 igL in waters deeper than 200 m in

the open water GUM (Feely et al. 1971).

Live phytoplankton are confined to the euphotic zone and the water lying immediately

beneath it (Riley and Chester 1971). The composition of this portion of the POM can

vary widely and the biological communities found in the water column are discussed

elsewhere. The living portion of the POM is composed of a wide anay of biochemicals

including proteins, carbohydrates, fats, and pigments. The non-living POM is also a

complex mixture of chemical compounds that varies with water depth. In the euphotic

zone, biochemical metabolites are a major fraction of the POC. Many of these

biochemicals are subject to rapid degradation as they settle through the water colunm.

The residual POM at depth is materials that are resistant to degradation such as cell walls

and exoskeletons. In the deepwater GOM, most of the water column POM will be

derived from phytoplankton but tenestrially-sourced POM contributes as well.

POC in the euphotic zone is spatially and temporally highly variable due to it primary

origin in biological organisms. POC is high in the upper water column and decreases

with depth due to remineralization as the particulates sink through the water column.

Below a depth of 200 m, POC values become relatively constant. An estimate of the

amount of living POC can be based on chlorophyll a concentrations. Plankton biomass in

the open Gulf of Mexico is considered to be low with values of 100 to 150 n1gm3

compared to regions near the Mississippi River of 200 to 1,000 mgm3 (Khromov 1965;

Bodganov et al. 1968). Profiles of chlorophyll a show low values near the surface

followed by maximum concentrations at 50 to 110 m coinciding with the bottom of the

euphotic zone, the pycnocline, or the nutricline (El-Sayed 1972; Hobson and Lorenzen

1972). Similar distributions of POC and chlorophyll a are expected in the open water

GUM.

Included in the POM are anthropogenic chemicals including hydrocarbons and metals.

The higher molecular weight hydrocarbons are non-polar and often occur primarily

adsorbed onto particulates. Few studies are available on the distribution of seawater

particulate hydrocarbons in the study area. It is expected that the concentrations will be

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lower than those for adjacent coastal and shelf waters. Water colunm particulate

hydrocarbons originate from spills, platform discharges, and shipping activities. Some

small, but unknown, input of atmospherically transported POM might also contribute

terrestrial hydrocarbons (i.e., plant biowaxes). Resuspension of sediments is another

important mechanism for introducing particulates into the water column, especially

hydrocarbon-rich particulates in areas of known oil and gas seepage.

A special class of POM is pelagic tars. A summary of the distribution of pelagic tarballs

in the GUM indicates that they are ubiquitous (Geyer 1980; Brooks 1981; Van Vleet et

al. 1984). Tarballs are found in most surface neuston tows in the GUM (Jeffrey 1973,

1977, 1979; Jeffrey et al. 1973, 1974; Parker et al. 1979). Jeffrey (1979) determined that

the average floating tar concentration in the GOM was 1.35 mgm2, based on 220

neuston tows between 1972 and 1976. No apparent change in tarball concentrations was

observed during this period. The highest tar concentrations were observed in the western

portions of the GUM. Koons and Monaghan (1973) estimated the standing crop of

tarballs in the GUM to be about 2,000 metric tons (using a concentration of 1.0 mgm2).

There is also one report of extensive benthic tars recovered in bottom trawls across the

deepwater regions of the northern GUM (Alcazar et al. 1989).

The principal sources of tarballs in the GOM are natural seepage, tanker transportation

activities, and oil spills (e.g., IXTOC blowout). Jeffrey (1979) found that approximately

30% of the tarballs analyzed were tanker sludge residues, about 2% were identified as

fuel oil residues, and 65% were crude oils from many origins, only 20% of the floating

tar had sulfur content greater than 3% and these were found primarily in the western and

southwestern Gulf. Based on the sulfur content of pelagic tars, Jeffrey et al. (1974)

estimated that possibly as much as 60% of the tars originated from foreign crude oils

spilled during tanker operations.

Particulate Inorganic Matter

Particulate inorganic carbon can be derived from either the inorganic remains of

organisms (carbonate or silicate tests) or from riverine transport of landderived

materials. As mentioned above, resuspension of bottom sediments can also contribute to

water colunrn particulates. There are few studies of the distribution of inorganic

particulate matter in the study area. However, it is suspected that the amount of

particulate matter decreases with increasing water depth and that the majority of the

particulate matter is derived from the remains of biological organisms in the open water

GUM.

One portion of the particulate inorganic matter is particulate metals. As for the dissolved

metals, particulate metals will have their origins in terrestrial materials transported by

rivers and the air as well as a range of potential anthropogenic sources. As mentioned

above, more than 90% of the metals detected in the water column are in the particulate

form (Presley et al. 1980). While little is known about particulate trace metal levels in

the study area itself, the metal content in Mississippi River suspended material was

similar to the original rocks in Fe, Al, V. Cr, Cu, Co, Mn, and Ni content but was

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enriched in Zn, Cd, and Pb content. Enrichments in lead and cadmium were believed to

be the result of anthropogenic influences.

Another source of particulate metals to the open GUM is discharges during oil and gas

exploration and production (Boothe 1979; Boothe and Presley 1979, 1987, 1989). Large

amounts of drill muds are discharged that contain a range of trace metals. In the

deepwater GUM, there are few measurements of water colunm particulate metal

concentrations. In one study, increased concentrations of particulate iron were found in

the nepheloid layer above the sea bottom caused by sediment resuspension (Betzer and

Pilson 1971). A study of the distribution of particulate aluminum in the GUM concluded

that the average concentration of Al was 2.0 tgL" (Feely et al. 1971). Aluminosilicate

accounted for 14% of the surface water particulate matter decreasing to 8% in the middle

of the euphotic zone and then increasing to 30% at depths down to 1,000 m. The authors

concluded that the total suspended matter was 50% organic, 20% aluminosilicate, and

30% other inorganic material.

The Chemistry of Natural Seepage

Several features of the benthic environment are important influences on the fluxes of

chemicals in the northern Gulf of Mexico deep-sea. A review of sediment characteristics

is provided elsewhere. As previously mentioned, one of the unique features of the

deepwater northern Gulf of Mexico is the extensive oil and gas seepage documented in

the last 15 years. These seeps have important effects on the benthos and serve as a

unique source of chemicals to the GUM basin. The area is underlain by a thick layer of

salt that has produced a complex topography due to salt movement. This has also set the

stage for migration of deeply generated thermogenic hydrocarbons to the near-surface

sediments and into the overlying water column by producing conduits for fluid flow. The

most recognized impact of this unique geological setting are the "cold seep"

chemo synthetic communities of mussels and tubeworms (Brooks et al. 1987; MacDonald

1992; MacDonald et al. 1996; see also Chapter 8 of this report). This dynamic setting

also affects the chemistry of the basin as these same processes introduce salt (brine

seepage) and hydrocarbons (gases, liquids and solids) to deepwater regions. At some

seep locations, gas hydrate is known to outcrop on the seafloor, disrupting adjacent

sediments. Barite chimneys identified in some areas can be a natural source of barium in

sediments (as contrasted with an anthropogenic origin in drilling muds). These processes

are important to consider in any description of biogeochemical cycles in the deepwater

northern GUM. It is also clear that the background of hydrocarbons in these areas can be

quite high and these natural inputs need to be considered when attempting to recognize

anthropogenic disturbances in the area.

Brine Seepage

While first discovered on the shallow continental shelf, it is now known that hypersaline

brines often breach the seafloor in the deep sea northern Gulf of Mexico (Shokes et al.

1977; Brooks et al. 1979; Roberts and Carney 1994; Roberts and Carney 1997). If the

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topographic setting is right, brine pools have been known to develop. Elevated salinities

in pore waters are common at hydrocarbon seep sites and often brine seeps are associated

with temperature anomalies (Nagihara 1992; Kaluza et al. 1996). These brines can be

brackish to saturated solutions. In addition to the brine poois discovered along the

mid-slope in water depths from 500 to 800 m (NR-1 Brine Pool), a large brine containing

basin was identified in the 1970's called the Orca Basin (Sackett et al. 1979; Addy and

Bebrens 1980; Ishizaka et al. 1986; Aharon et al. 1992; Aharon et al. 1997; Van

Cappellen et al. 1998). The Orca basin is a 400 kin2 basin at a water depth of 2,400 in in

the central northern Gulf of Mexico. Salinities as high as 238 ppt have been measured.

Hydrocarbon Seepage

Large areas of the northern Gulf of Mexico continental slope are known to contain seeps

of gaseous and liquid hydrocarbons (Brooks et al. 1984, 1985, 1986a,b, 1987; Kennicutt

et al. 1987, 1988). Sediments in the deep-sea regions of the northern Gulf of Mexico

contain a mixture of terrigenous, petroleum, and phytoplariktonic hydrocarbons

(Kennicutt et al. 1987). The relative importance of these inputs varies in space and time

(LGL Ecological Research Associates, Inc. and Texas A&M University 1985, 1986;

Kennicuft et al. 1987; Gallaway 1988). Background sediment hydrocarbon

concentrations in the deepwater GUM are generally lower than those reported for GOM

shelf and coastal sediments. The influence of land-derived materials decreases from the

central to the western to the eastern deepwater GOM. Petroleum inputs are detectable at

many locations. Natural seepage of hydrocarbons is an important source of petroleum

hydrocarbons to slope sediments in localized areas. Background hydrocarbon

concentrations can vary by 1 to 2 orders of magnitude along an isobath due to changes in

sediment texture and hydrocarbon inputs. Variability along an isobath can be as great as

that observed across isobaths from 300 to 3,000 m of water depth. In seep areas,

hydrocarbon levels in near-surface sediments can reach concentrations of several percent

by weight. Background hydrocarbon concentrations in deepwater GOM sediments are on

the order of 50 to 100 ppm by weight. The movement of hydrocarbons from deep seated

reservoirs to the shallow sediments, and into the water column followed by slick and

tarball formation, has been documented in the open water GUM (Kennicutt et al. 1988).

Visual observations from submersibles have confirmed active and on-going gas and

liquid seepage at many locations on the central GUM continental slope. Near-surface

hydrocarbons are often extensively altered due to microbial degradation. Seep locations

are characterized by the presence of extensive microbial mats and extensive hard bottoms

formed from authigenic carbonate precipitation incorporating CO2 derived from the

oxidation of hydrocarbons.

Analysis of organisms from areas of natural seepage has shown that animal tissues

collected close to seeps contain significant amounts of hydrocarbons (Wade et al. 1989).

Polynuclear aromatic hydrocarbon (PAH) concentrations were higher in sedentary

organism than in the more mobile species. Tissue PAH concentrations demonstrated that

these populations are chronically exposed to hydrocarbons of a natural origin.

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Conclusions

In general, the chemistry of the deepwater Gulf of Mexico is not well studied. In many

ways, the deep Gulf is buffered from the direct iniluence of coastal waters and processes.

This is reflected in the low levels of anthropogenic contaminants present in the dissolved

and particulate phases of the water colunm. However, oceanographic mechanisms can

transport low-salinity, nutrient-rich waters across the shelf and into the deepwater

enviromTlent. This has been shown to occur off the Mississippi River mouth, for

example, when a seaward-moving surface flow confluence is created by deepwater

cyclone-anticyclone circulation pairs (Chapter 6). Riverine input of dissolved and

particulate materials is important to the chemistry of the deepwater basin. Secondary

movement of materials downslope is an important process as well. The sparse

information on sediments in the area suggests that the benthic environment is mostly

unaffected by anthropogenic impacts. One important feature of the study area is the

widespread occurrence of natural seepage of oil, gas, and brines in near-surface

sediments and up through the water column. This unusual "natural background" of

hydrocarbons and other disturbances is important to consider when attempting to detect

impacts related to oil and gas exploration and production activities in the area.

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Aharon, P., H.H. Roberts, and R. Snelling. 1992. Submarine venting of brines in the deep Gulf

of Mexico: Observations and geochemistry. Geology 20(6) :483-486.

Aharon, P., H.P. Schwarcz, and RH. Roberts. 1997. Radiometric dating of submarine

hydrocarbon seeps in the Gulf ofMexico. Geological Society of America Bulletin

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Alcazar, F., M.C. Kennicutt II, and J.M. Brooks. 1989. Benthic tars in the Gulf of Mexico:

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Armstrong, D.W. 1974. Some dynamics of carbon, nitrogen, and phosphorous in the marine

shelf environment. MS Thesis. Texas A&M University. College Station, TX.

Betzer, P.R. and M.E.Q. Pilson. 1971. Particulate iron and the nepheloid layer in the western

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Bianchi, T.S., C.D. Lambert, P.H. Santschi, and L. Guo. 1997. Sources and transport of landderived particulate and dissolved organic matter in the Gulf of Mexico (Texas

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Bogdanov, D.V., V. Sokolov, and N.S. Kliromov. 1968. Regions of high biological productivity

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Primary Productivity and Chlorophyll Standing Stocks 141

Introduction 141

Mean Condition 142

Seasonal Changes 145

Deepwater "Hot Spots" from Entrainment of Freshwater 145

Deepwater "Hot Spots" from Cross Isopycnal Mixing 146

Deepwater "Hot Spots" from Mesoscale Divergence 153

Deepwater Zooplankton, Micronekton, and Ichthyoplankton 154

Introduction 154

The Available Database 155

Previous Reviews 155

Ecology, Biology, and Systematics Studies 159

Biornass and Abundance 160

Spatial and Temporal Variability 164

Acoustic Sampling of Zooplankton and Micronekton Biomass 166

Optical Sampling of Zooplankton and Micronekton Biomass 167

A Combined Approach to Biomass Surveys 167

Literature Cited 169

Appendix 6A: Deepwater Water Column Biology 181

Chapter 6: Water Column Biology

Douglas C. Biggs and Patrick H Ressler

Primary Productivity and Chlorophyll Standing Stocks

Introduction

Phytoplankton distribution and abundance in Gulf of Mexico waters has been reviewed at

decadal intervals over the last 30 years, first by Bj ornb erg (1971), then by Iverson (in

Iverson and Hopkins 1981), and most recently by Vargo (in Vargo and Hopkins 1990).

However, most of the reports these reviewers featured focused primarily on the

continental shelf and Vargo, in particular, noted that much of the information for his

review came from studies conducted 10 to 20 years before 1990. In fact, data collected

by expeditions in the 1960's and 1970's remain the basis for the general paradigm that

standing stocks and productivity of piankton are both quite low seaward of the

shelf-slope break in the Gulf of Mexico. The present review will support that description

of the mean state but will also show that research carried out since 1987 indicates "hot

spots" in primary production occur whenlwhere nutrient availability is locally enhanced,

even in deepwater (water depths greater than 300 m). Fundamental

nutrients-phytoplarikton-zooplankton (NPZ) food chain theory forecasts that enhanced

primary production, if sustained, will lead to increased animal (zooplankton and

micronekton) production. In this review we will summarize the available evidence from

the Gulf of Mexico that deepwater "hot spots" that are temporally persistent (even if they

are spatially variable) have higher stocks of zooplankton and micronekton.

141
Biggs and Ross/er: Water Column Biology

Mean Condition

The Gulf of Mexico (GUM) is a subtropical ocean basin located within the circulation

regime generally called the Intra-Americas Sea. The near-surface circulation pattern of

the eastern GOM is dominated by the anticyclonic flow of the Loop Current (LC). East

of 90°W, upper layer flow enters through the Yucatan Channel and leaves through the

Florida Straits. Since this current enters from the Caribbean, it acts as a biological

conveyor belt to maintain the exchange of pelagic species between the Caribbean and the

GUM. This conveyor does not fertilize downstream plant plankton, however, since LC

surface waters are among the most oligotrophic in the world ocean. Nitrate, phosphate,

arid other essential plant nutrients are usually below the analytical detection limit

(<0.05 htML') in LC inflow water from the surface to depths of 80-90 m. The extinction

coefficient, "k", that describes how rapidly irradiance decreases with depth according to

the exponential equation I = * e, is usually <0.05 in LC surface water. As a

consequence, the LC inflow into the GUM is almost swimming pooi clear and therefore

is deep blue in color.

In the central and western deepwater GOM as well, the standing stocks and biological

productivity of the plant and animal communities living in the upper part of the water

column are in general those that might be expected in a nutrient-limited ecosystem. In

the late 1960's, as part of a review of plankton productivity of the world ocean, Soviet

scientists characterized the deepwater GOM as very low in standing plankton biomass

(Bogdanov et al. 1968), with mean primary productivity ofjust 100 to 150 mg Cm2d1

(Koblenz-Mishke et al. 1970). A few years later, extensive surveys of phytoplankton

chlorophyll and primary production that span the period 1964 to 1971 were summarized

by El-Sayed (1972) in atlas format, as averages within 2° squares of latitude and

longitude. These atlas maps show that surface chlorophyll generally ranges from 0.06 to

0.32 mgm3 in deepwater central and western GUM. Subsurface there is usually a "deep

chlorophyll maximum" (DCM) within which concentrations are two- to three-fold higher,

and so the atlas reported that chlorophyll in deepwater could reach 21 mgm2 when

integrated from the surface to the base of the photic zone. Most values, though, ranged

from 5 to 17 mg1n2 when averaged in squares where water depth >2,000 m (El-Sayed

1972). Low values of primary production (<0.25 mg Cm3h1) are typical for surface

waters at the majority of the oceanic stations in this atlas, equivalent to <10 mg Cm2h1

when integrated from the surface to the base of the photic zone. If there are on average

12 hours of sunlight per day, this rate is equivalent to <120 mg Cm2d1 and so is in good

agreement with the characterization by Koblenz-Mishke et al. (1970). Allowing for

primary production to proceed 300 days a year in the GUM because of its subtropical

climate, this rate of primaiy productivity is <36 g Cm2-y'. As a consequence the

deepwater GUM is usually placed at the low end of the estimated range of 50 to 160 g

C-m2-y that is generally accepted for the annual gross primary production in open-ocean

ecosystems (Smith and Hollibaugh 1993).

Size fractionation of chlorophyll and primary production was done on subsequent cruises

to deepwater. Early data were summarized by El-Sayed and Turner (1977). They noted

that the <20 im size fraction accounted for on average 83% of the standing crop and 83%

of the total production. These values emphasize the importance of the nano-plankton size

142
143

Biggs and Ressler: Water Column Biology

fraction in the phytoplankton community and further reinforce the paradigm that low

nutrient surface waters are characteristically dominated by small-size phytoplankton and

by blue-green algae like Trichodesmium. Vargo and Hopkins (1990) emphasized the

importance of this blue-green alga in the deepwater GOM, for when abundant in the top

20 m of the water column, Trichodesmium may have photosynthetic rates of tens of mg

Cm2d' (Carpenter 1983). After the potential importance of phytoplankton even smaller

in size than nanoplankton became widely recognized, subsequent researchers working in

the GUM and elsewhere have size fractionated chlorophyll and primary production into

pico (<2 tm), as well as nano (2 to 20 tm), and net (>20 m) fractions (Al-Abdulkader

1996; Gonzalez-Rodas 1999).

When it became known that even low concentrations of trace metals that may

inadvertently be introduced into sampling bottles or into light-dark incubation bottles can

greatly depress measured rates of gross primary production, biogeochemists advocated

the use of trace-metal clean techniques to remeasure primary production in oceanic

ecosystems (Fitzwater et al. 1982). After about 1980, such "clean techniques" were used

routinely to remeasure primary production in the GUM and elsewhere. In their review,

Vargo and Hopkins (1990) noted three such studies. Ferguson and Sunda (1984) reported

rates of 0.11 mg Cn13h1 for a LC station. Ortner et al. (1984) measured similar values

in the LC and calculated that integrated production rates (0 to 90 m) ranged from 14 mg

Cm2h1 (temperature-stratified conditions) to 62 mg Cm2-h' (after wind-mixing to 110

to 120 ni). Yoder and Mahood (1983), who measured primary production from the shelf

out into deepwater during the Southwest Florida Shelf Ecosystems Study, found that

production averaged 0.1 g C-m2-d1 in deepwater outside an eddy-induced upwelling area.

On average, then, it appeared that remeasurements using clean techniques in the 1980's

yielded results that were comparable to those that were obtained during the more

extensive surveys of the 1970's.

J'ignre 6.1 shows the location of primary production measurements using clean

techniques that have been made in deepwater after the Vargo and Hopkins (1990) review,

by Texas A&M University and by the Universidad Nacional Autonomia de Mexico

(UNAM) during the period 1987-1997. The primary productivity data from UNAM

stations made in summer 1997 have not yet been published although a summary is

available from Dr. Elva Escobar-Briones (escobri@mar.icmyl.unam.mx). The

1987-1988 measurements were discussed by Biggs (1992), and the 1990 deepwater

measurements made in support of the Nutrient Enriched Coastal Ocean Productivity

(NECOP) program were reported by Biggs and Sanchez (1997). Three dozen deepwater

measurements made from 1992-1994 in support of the Texas-Louisiana Shelf Circulation

and Transport Processes Study (LATEX) have recently been reported by Al-Abdulkader

(1996) and Gonzalez-Rodas (1999).

During LATEX, size fractionation of chlorophyll and primary production was done along

cross-margin transects that extended from shallow water to the shelf edge, and also at

sampling sites along and seaward of the 200-in isobath of the western and central GUM.

Ten LATEX cniises fielded from 1992 to 1994 sampled the continental margin in May

(1992, 1993, 1994), August (1992, 1993, 1994), November (1992, 1993, 1994), and
TAMLJ T&R cruises 1987 & 

O NECOP 1990 LATEX 1992&1993 LATEX 1993& 1994 * UNAM Justo Sierra cruise 1

Figure 6.1. Deepwater locations of recent measurements of primary production, 1987-1997, on oceanographic cruises b

(TAM U) and the Universidad Nacional Autonoma de Mexico (UNAM).
145

Biggs and Ressler: Water Column Biology

February (1993 only). Nowlin et al. (1998) summarized the circulation and transport

processes; phytoplankton pigment concentrations and species counts were reported by

Neuhard (1994) and Bontempi (1995), and also by Al-Abdulkader (1996) and

Gonzalez-Rodas (1999). In general, the LATEX results support the findings of El-Sayed

and Turner (1977) in that on most cruises, pico+nano plankton averaged more than 3/4 of

deepwater cell counts and accounted for >2/3 of the primary production. The exception

was the "winter" cruise in February, when diatoms of the genera Leptocylindrus and

Chaetoceros comprised >50% of phytoplankton numbers not just in deepwater but across

the outer, middle, and inner shelf as well.

Seasonal Changes

Even in a subtropical ocean there are seasonal changes. Pigment concentration at the

surface in the deepwater GUM undergoes a well-defined seasonal cycle that is generally

synchronous throughout the region. Muller-Karger et al. (1991) reviewed monthly

climatologies of near-surface phytoplarikton pigment concentration from multiyear series

of coastal zone color scanner (CZCS) images for the period 1978-1985. They reported

that highest surface concentrations of chlorophyll occur between December and February

and lowest values occur between May and July. There is only about three-fold variation

between the lowest (0.06 mgrn3) and highest (0.2 mgnf3) deepwater surface pigment

concentrations, however. Model simulations show that the single most important factor

controlling the seasonal cycle in surface pigment concentration is the depth of the mixed

layer (Walsh et al. 1989). Muller-Karger et al. (1991) concluded because of this

dependence, annual cycles of algal biomass are one or more months out of phase relative

to the seasonal cycle of sea surface temperature.

Deepwater "Hot Spots" from Entrainment of Freshwater

Since essential plant nutrients are limiting, any process that increases the nutrient

concentrations available to phytoplankton in the deepwater GUM will increase their

primary productivity. It is well known that freshwater inputs carry high nutrient loads,

but in the GOM these high nutrient inputs are usually measurable only close-in to rivers

and estuaries (Lohrenz et al. 1990, 1994). An exception occurs, however, when surface

currents set up off-shelf flow that carries the river water seaward past the shelf-slope

break and into deepwater. Biggs and Muller-Karger (1994) combined CZCS data with

ship data to document that high-chlorophyll "plumes" do form in the western GUM when

a seaward-moving surface flow confluence is created by deepwater cyclone-anticyclone

circulation pairs. Analogous to a pair of anticlockwise-rotating and clockwise rotating

gears, these circulations entrain coastal water from the western and central GUM and

draw this offshore when the cyclone (anticlockwise circulation) lies immediately to the

north or east of the anticyclone (clockwise circulation).

Both types of these mesoscale features can be detected by the topography of the 15°C

isotherm; this is domed upward in the cyclones and pushed locally deep within the

anticyclones. Both types of features can now be located with satellite altimetry, as well,

since GUM cold-core eddies (15°C isotherm domed) show up as 10 to 20cm local

depressions in sea surface height, whereas warm-core eddies (15°C isotherm pushed
Biggs and Ressler: Water Column Biology

locally deep) show up as 20 to 70 cm local elevations in sea surface height (Leben et al.

1993). As one recent example, Figure 6.2 shows dynamic topography, gridded upper

layer geostrophic velocity, and surface salinity and surface chlorophyll concentrations

over deepwater of the northeast GOM in mid-summer 1997. Low salinity Mississippi

River water was entrained into the flow confluence created by a gradient of >80 dyn cm

in geopotential anomaly between a cyclone located to the north-northeast of a Loop

Current eddy (LCIE). Note that low salinity patches of river water were wrapped

anti-clockwise around the periphery of the cyclone. A comparison of the salinity and

chlorophyll fields shows that surface chlorophyll concentrations in this river water

reached 2.0 rng-m3, and that especially in the concentration range 0.1 to 0.4 mg-m3, the

patches of highest surface chlorophyll correspond spatially to the patches of lowest

surface salinity.

As a second example, Figure 6.3 shows sea surface height anomaly, surface salinity, and

surface chlorophyll over the same region the next summer, in August 1998. This time,

there is not a well-developed cyclone-anticyclone modon pair. Rather, it is the clockwise

circulation around the periphery of a small anticyclone that was located close off the

Mississippi River delta that has entrained river waters eastward along its edge. In the

periphery of the anticyclone, patches of low salinity, high chlorophyll waters got

transported from the inner shelf eastward across the continental margin to deepwater

depths >500 m. For example, note that the irregular-shaped patch of high chlorophyll

(>1 mgm3) seaward of the 200-rn isobath between 86° and 88°W corresponds, spatially,

to a freshwater patch in the core of which surface salinity is <28.

Deepwater "Hot Spots" from Cross Isopycnal Mixing

Recent fieldwork has shown these mesoscale oceanographic features have additional

impacts upon deepwater plankton and micronekton communities, for locally high

nutrients are also introduced to the surface of deepwater ocean regions at eddy edges

where there is enhanced vertical mixing. In fact, the periphery regions of high velocity

surface currents that surround both the cyclonic and the anticyclonic eddies are zones of

locally high vertical shear. Lee et al. (1991) have shown that meanders and eddies in the

Gulf Stream are often marked by local aggregations of phytoplankton, and elevated fish

stocks appear to concentrate in such areas (Atkinson and Targett 1983). The presence of

multiple cyclonic and anticyclonic features in the Gulf of Mexico can result in strong

frontal gradients between these features.

In the CZCS ocean color climatology from 1978-1985 and in imagery from the current

generation ocean color sensor (the Sea Wide-Field Scaruier, or SeaWiFS, in orbit since

October 1997), the periphery of the LC and of the anticyclonic LCEs of diameter 200 to

300 km that are shed from the LC are often seen to be outlined by surface pigment

concentrations that are two- to three-fold higher than the extremely low concentrations

(0.04 to 0.06 mgm3) in the interior of these circulations. Figure 6.4, which illustrates a

'halo" of locally high chlorophyll standing stock around the LC in mid-January 1998, is

one such example. Two other examples from recent fieldwork are presented as

Figures 6.5 and 6.6.

146
Biggs and Ross/er: Water Column Biology

Longitude

Longitude

LOnuitude

Longitude

inr

Figure 6.2. A: Dynamic topography (cm, 0 m relative to 800 m) of the deepwater GulfCet II focal area,

as determined from 107 hydrographic stations made on RN Gyre cruise 97008. B: Gridded

upper layer geostrophic velocity (0 m relative to 800 m) computed from the dynamic

topography data in A. C: Sea surface salinity map, superimposed on ship track lines of

97G08. D: Sea surface chlorophyll map, superimposed on ship track lines. All four figures

are from Chapter 2 of the GulfCet II Final Report (Davis et at. 2000a).

147

25.5 27.0 28.5 30.8 31.5 330 345 360 37.5 0.00 040 0.80 1.20 1.60 250
Biggs and Ressler: Water Column Biology

31 N

30N

29N

28°N

27°N ' I I

I I 90W 89W 88W 87W 86°W 85 W 84W 83W 82W

31 N

30°N

29N

28N

\)

27N

90°W 89°W 88W 87W 86W 85Vv 84W 83W 82W

2TN ' I

I I I 90 W 89W 88W 8TW 86W 85 W 84°W 83W 82W

Figure 6.3. Top panel: Sea surface height anomaly for water depths > 200 m from satellite

altimeter data for the NEGOM study area for 29 July 1998 (hindcast data).

Middle panel: Salinity at 3 m from thermosalinograph observations on NEGOM

cruise N3, 26 July - 6 August 1998. Bottom panel: Chlorophyll (tgm3) at 3 m

calculated from flow-through fluorescence on NEGOM cruise N3. All panels are

from the NEGOM Annual Report, Year 2 (Jochens and Nowlin 1999). Shading

indicates patches of low salinity, high chlorophyll river water being entrained

anticyclonically around the eddy centered in deepwater of De Soto Canyon.

148
Figure 6.4. SeaWiFS ocean color image for 16 January 1998, processed by the Department of Marine nce, University 

the 'halo' of locally high pigment concentration (light blue color) along the western and NW periphery of the L

image, the Loop Current (medium blue color) extends north of 27°N between 88.5°W and 85.5°W. Land and 

black. Data courtesy of Orbimage and NASA. 
Biggs and Ressler: Water Column Biology

LW

1' 199 201 203 205 352 305 125 111 91 86 1 79 35 V 36 34 29 96 200 292 188 206 207 209 230 232 17 85 213 323 321 41 37 35 33 -1-. / - 50 \ \

2-

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0 50 100 350 200 250 300 350 400 450 SUO 550

)tI3ce .&IMI9 9lliO3 2ock (kb)

---S

Figure 6.5. Top panel: Cruise track and station locations for LATEX hydrographic survey H05, AprilMay

1993. Bottom panel: Vertical contours of bottle nitrate (.imoFL1) along 2OOm isobath during

cruise H05. Dots indicate sample locations. Both panels are from the LATEX data report

(Jochens et al. 1996).

150

2/

'6

-

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11091109 '-:_'

LOUISIANA S

0111101

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-

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-94 -93 -92 -91 -90 49
,ow 95GW nterated poductMty May I 994 integrated produUvity, AUU. 1 94 kiteated productivity, Nov

Figure 8.6. Primary productivity (g Cm-2d-1) of the LATEX continental margin in May, August, and November of 1993 a

Gonzalez-Rodas 1999). Asterisk symbols show location of primary productivity stations. hte9raed producUwlty, May 1 993 integrated producvfty, Aug. 1 993 ntegrated poductMzy tJov. 
Biggs and Ress/er: Water Column Biology

Figure 6.5 shows a hot spot of anomalously high nitrate concentration in surface waters

between 91° and 92°W along the 200 m isobath that was encountered in May 1993. At

stations 36, 37,and 38 on LATEX hydrographic survey H05, nitrate concentrations

>0.5 M-L1 occurred at the surface, just south of a strong surface front where salinity

increased from 32.0 to 36.3. This hot spot of nitrate apparently arose from strong vertical

shear that developed in this frontal zone, for the surface salinity and silicate data and the

vertical contours shown in Figure 6.5 strongly suggest that it was fueled by

cross-isopycnal mixing from below rather than fTom entrainment of freshwater from the

Atchafalaya Bay or Mississippi River to the north and east. Farther west along the 200 m

isobath, an anticyclone (LCE "V") was interacting with the continental margin. Note as

well from Figure 6.5 that the extremely low nutrient interior of the eddy was apparently

drawn onshore between stations 207 to 210.

Al-Abdulkader (1996) measured chlorophyll stocks and primary productivity at

station 37 within the hot spot of anomalously high surface nitrate, and at station 83 some

140 km to the west along the 200 m isobath, and also further west at station 88 at the

deepwater end of LATEX line 4 which reached the NE periphery of LCE "V". Data

from his table 21 show that near-surface chlorophyll at station 37 ranged from 0.4 to

0.5 mgm3, or three-fold higher than the concentrations of 0.15 to 0.17 mgm3 at

station 83 west of the hot spot. Al-Abdulkader's data show that primary productivity in

the upper 50 m of the hot spot ranged from 0.2 to 0.3 mg Cm3h'; integrated to the 0.2%

L depth and knowing that photosynthesis proceeds 12 h per day in May, this is a

production of 220 mg Cm2d1. This is 1.4 times higher than the measured production

integrated to the same irradiance level at his station 83 (158 mg Cm2-d'). At station 88

in the NE periphery of LCE "V", locally low salinity surface water was present (33.6 to

33.8 in the upper 10 m). This surface water was low in nitrate and near-surface

chlorophyll concentrations in it were similar to those at station 83, but high silicate levels

in the upper 10 m at station 88 indicate this low salinity cap was probably entrained

Mississippi River outflow. Data from Al-Abdulkader's table 20 show that primary

productivity in the low salinity surface water was locally high (0.3 to 0.4 mgm3) and that

even below this low salinity layer productivity averaged 0.16 mgm3 to a depth of 100 m.

When integrated to the 0.2% I depth, this is a production of 226 mg Cm2-d', equivalent

to that in the nitrate "hot spot."

A recent dissertation by Gonzalez-Rodas (1999) summarized primary productivity

measurements that were continued and extended on subsequent LATEX cruises.

Figure 6.6 shows Gonzalez-Rodas' summary of integrated primary productivity for the

LATEX continental margin in May, August, and November of 1993 and 1994. Note that

hot spots in deepwater primary production (>2.5 g C-i2d') were present near 27.5°N

and 92°W in August 1993 and also in November 1994. In summer 1993, the northern

edge of LCE-W was interacting with the continental margin between 91° and 93°W; the

locally high shear there apparently fueled a region of anomalously high deepwater

primary production. This eddy had a diameter of some 250 km and at the location where

the productivity was measured, the geopotential anomaly was about +20 cm and current

speeds were about 60 cms1 (see Table 5 in Gonzalez-Rodas 1999). In fall 1994, the

northern edge of another anticyclone, LCE-Y, was interacting with the continental

152
Biggs and Re.ssler: Water Column Biology

margin again between 910 and 92°W. This eddy was even larger in diameter (320 km)

and presented a geopotential anomaly of +36 cm (from Table 5 in Gonzalez-Rodas 1999).

Deepwater "Hot Spots" from Mesoscale Divergence

Because the interiors of the anticyclones are areas ofconvergence, the upper 100 m or so

of the water column in both LC and LCEs are areas in which surface waters are

infrequently renewed and so they are impoverished in nitrogen and phosphorus nutrients.

The interiors of these regions of convergence are generally regarded as biological "ocean

deserts." Measurements of chlorophyll standing stocks, primary productivity, and

zooplankton standing stocks within an LCE sampled in 1988 are in good agreement with

this premise (Biggs 1992). However, the cyclonic coldcore eddies (local areas of

divergence) that are frequently associated with these ariticyclones represent areas of

higher biological productivity.

Subsurface sampling of these GOM eddies from ships showed there was a highly

predictable negative first order relationship between temperature <22°C and nitrate

concentration. Temperature could thus be used as a proxy for nitrate concentration, and

in particular the depth of the 19°C isotherm was a good estimation of the depth of the

10 htML' nitrate concentration (Biggs et al. 1988). Within one cyclone sampled in

1996, the nitracline was domed 40 to 60 m shallower than within the LCE that was

sampled concurrently (see Figure 6 in Zimmerman and Biggs 1999). Because this

doming facilitated a higher flux of new nitrogen into surface waters in cyclone than in

anticyclone, the DCM was locally shallower and chlorophyll reached higher maximum

concentration in the cyclone than in the LCE. Because this resulted in higher standing

stocks of chlorophyll in the upper 100 m in the cyclone, the cyclones are generally

regarded as biological "oases" while the interior of the LCEs are biological "deserts."

At six conductivity, temperature, and depth (CTD) stations made during a survey of a

mesoscale cyclonic eddy that was centered near 26°N and 94°W in November 1987,

integrated chlorophyll standing stock averaged 38 ± 9 mgm2 (Biggs et al. 1988), or two

to three times greater than the mean for the oceanic GUM. Primary productivity

averaged 12 mg C-m3d' in the upper 10 m and integrated production to the 1% light

level was equal to 250 mg Cm2-d1 (Biggs 1992), or double the mean of 100 to 150 mg

Cni2d' reported by Koblenz-Mishke et al. (1970). Similarly, Yoder and Mahood

(1983) reported that for stations located seaward of the 200-rn isobath off the West

Florida shelf within an area of eddy-induced upwelling, the top of the nitracline domed to

depths of just 40 to 60 m below the surface. They measured the average water column

production there at 0.6 g Cm2-d', whereas for three other stations located outside the

eddy-induced upwelling area, production averaged 0.1 g Cm2d1 (Yoder and Mahood

1983). Thus, Yoder and Mahood concluded that subsurface upwelling may enhance

deepwater phytoplaiikton primary production by as much as six-fold. Subsequent studies

of cyclonic gyre formation off the Southwest Florida shelf found that a cold recirculation,

approximately 200 km in size, develops off the Dry Tortugas when the LC flow

overshoots the entry to the Florida Straits and that this persists over time scales of about

100 days (Lee et al. 1994). The "Tortugas Gyre" formation provides enhanced food

153
Biggs and Ressler: Water Column Biology

supply, retention, and shoreward transports for successftil recruitment of locally spawned

snapper and grouper larvae in the western and lower Florida Keys.

In sun-u-nary, when/where anticyclonic and cyclonic hydrographic features occur over

deepwater in the GUM, it is expected they will play an important role in determining

biogeographic patterns and controlling primary productivity in the Gulf of Mexico.

Deepwater Zooplankton, Micronekton, and Ichthyoplankton

Introduction

Historically, the Gulf of Mexico has been considered an oligotrophic, biologically

impoverished ocean since on average the deepwater standing stocks of plankton there are

lower than those found in temperate and higher latitude regions. Soviet-Cuban fisheries

investigations in the 1960's reported that zooplankton standing stocks were low across

much of the GUM (Bogdanov et al. 1968; Khromov l969a) and subsequent reviewers

have reinforced this perception (Hopkins, writing in Iverson and Hopkins 1981, and in

Vargo and Hopkins 1990). The zooplankton and micronekton fauna of the deepwater

GOM are similar in energy content, taxonomic composition, and food habits to those of

other low-latitude oceans (Stickney and Torres 1989; Hopkins and Gartner 1992;

Hopkins et al. 1994, 1996), and the ichthyoplankton fauna of the GUM have been

grouped along with those of the western tropical Atlantic Ocean and Caribbean Sea

(Richards 1990). Thus in several biologically important ways the GUM resembles other

oligotrophic subtropical oceans.

Relegating secondary production in the GUM to oligotrophic status is nevertheless an

oversimplification, because the generally low standing stock levels are not uniformly so

but are instead punctuated by spatial and temporal variation greater than that found in

most other oligotrophic oceans. This variability may be manifested as spatial "hot spots"

and temporal peaks in biomass. For example, Khromov (1 969a,b) reported that while

zooplankton standing stocks in the tropical oligotrophic Caribbean Sea were almost

always low and did not exceed 10 mL wet displacement volume (WDV) per 100 m3 in

waters offshore of the shelf/slope break, GUM stocks exhibited more seasonal,

interannual, and spatial variability, with biomass levels as high as 35 mL WDV per

100 ni3 (range <5 to 35). Also, Hopkins and Lancraft (1984), who compared integrated

wet weight biomass of zooplankton and micronekton in three tropical-subtropical

locations (Caribbean Sea, Gulf of Mexico, and Pacific Ocean near Hawaii), found that the

GUM was the highest in terms of zooplankton and intermediate in rank (above the

Caribbean) in terms of micronekton, exceeding the next greatest by a factor of two in

both cases. If gelatinous plankton were included in the micronekton biomass

comparison, GOM then ranked highest of all three locations in both categories. Finally,

although studies of GUM biomass do generally reveal low standing stocks, reported

estimates can vary by a factor of 10 or more from the minima within a given study (Biggs

et aT. 1988; Richards et al. 1993; Wormuth et al. 2000).

Richards and McGowan (1989) note the considerable commercial fisheriesresources in

the GUM: this is perhaps the strongest contradiction to the dogma that the GOM has low

secondary production. The northern Gulf supports the largest volume fishery in the

154
Biggs and Ressler Water Column Biology

United States, the Gulf menhaden, Brevoortiapatronus (Dagg et al. 1991). In 1998, 5 of

the top 10 U.S. fishing ports in terms of commercial landings were located in Gulf Coast

States, and the GUM accounted for the highest number of fish caught by recreational

fishers of any U.S. subregion (O'Bannon 1999). The deepwater Gulf of Mexico is also

habitat for substantial populations of marine mammals, sea turtles, and seabirds (Mullin

et al. 1994; Davis et al. 2000a). The presence of sizeable populations of these apex

predators in the deepwater GUM implies a reliable supply of lower trophic level prey

resources (Biggs et al. 1988) and suggests that underlying physical processes allow

"oases" of biological productivity to develop in the mostly oligotrophic deepwater GUM.

In this review, we will show that whenlwhere they occur over deepwater in the GUM,

anticyclonic and cyclonic hydrographic features will play an important role in

determining biogeographic patterns of and controlling secondary productivity. The

fisheries potential of the cyclones and of the frontal zones of both types of eddies is

becoming better understood, now that these have been identified as deepwater

concentrating mechanisms for higher trophic levels and apex predators. Continued study

and assessment of zooplanklon and nekton abundance within these mesoscale circulation

features is warranted, since these organisms ultimately serve as food stocks for higher

trophic levels.

The Available Database

Modern investigations of the overlapping categories of"zooplankton," "micronekton,"

and "ichthyoplankton" of the Gulf of Mexico began in the 1950's. Since then,

knowledge of the composition, standing stock biomass levels, and environmental factors

affecting each has been accumulated. For convenience, information on the deepwater

GUM zooplankton, micronekton, and ichthyoplankton community summarized in this

review is broadly divided into three categories, none of which are mutually exclusive:

previous reviews;

studies of the systematics, biology, or ecology of particular taxonomic groups of

zooplankton, micronekton, and ichthyoplankton; and

studies of the standing stock biomass of zooplankton, micronekton, and

ichthyoplankton over time, depth, and location.

The primary focus of the present review will be upon studies of biomass and abundance

(rather than on studies of specific groups), including an assessment of what is known and

some recommendations for future research.

Previous Reviews

There have been several major reviews of Gulf of Mexico zooplankton, micronekton, and

ichthyop!ankton since the 1950's (Table 6.1). Their focus and content varied, from

catalogues of plankton collections yet to be analyzed to lists of known taxa to

surmnarized results of studies of specific regions. Several of the reviewers note that

significant portions of the research done in the GOM have often not reached the

published literature but instead reside in government or contracting agency technical

reports and documents.

155
Biggs and Ressler: Water Column Biology

Table 6.1. Previous reviews of the zooplankton, micronekton, and iehthyoplankton

of the Gulf of Mexico (GUM).

Year Author

1954 Gaitsoff (ed.)

1970 Pequegnat and

Chace (eds.)

1971 Bjornberg

and Rass

1973 Hopkins

and Briggs

1981 Iverson and

Hopkins

1990 Darnell and

Defenbaugh

1990 Vargo and

Hopkins

156

Descriptor

An edited volume containing reviews of GOM zooplankton

and micronekton; first maj or synthesis

Texas A&M University Oceanographic Studies Volume 1;

contains reviews of some groups of zooplankton and

micronekton; emphasis is upon benthic/demersal rather than

pelagic taxa

Reviews of Caribbean and GUM regions by Bjornberg

(zooplankton) and Rass (deep-sea fish), in TJNESCO-FAO

proceedings of a 1968 meeting in Curacao

Summary of knowledge of the eastern GUM; contains

reviews by Hopkins (zooplankton) and Briggs (nekton)

GOM phytoplanktonlzooplankton review in Environmental

Research Needs in the Gulf of Mexico (GOMEX)

symposium proceedings

GUM environmental overview and history of research, from

a special session on the ecology of the Gulf of Mexico

published in American Zoologist

Hopkins' portion reviewed zooplankton and micronekton +

ichthyoplankton. The area of interest was Florida south of

the Keys and the deepwater Gulf to the west of the Florida

coast in MMS's Eastern Planning Area
Biggs and Ressler: Water Column Biology

The earliest overview was by Gaitsoff (1954), who assembled a volume of reviews

written by leading government and university specialists about GUM zooplankton and

micronekton under the auspices of the U.S. Fish and Wildlife Service. A general review

of the state of knowledge of zooplankton was provided by H.B. Moore, to supplement

specific reviews by other specialists of planktonic foraminifera, enidarians, ctenophores,

salps, chaetognaths, crustaceans, mollusks, and fishes. While some detailed information

was available, the general conclusion was that there was still much to be learned about

the GOM zooplankton!micronekton community. In fact, Moore concluded that on

balance "next to nothing of the zooplankton of the Gulf of Mexico" was known at the

time.

Sixteen years later, Pequegnat and Chace (1970) edited a volume on the biology of the

Gulf of Mexico that contained a historical overview, locations, and discussion of

investigations of water column sampling using midwater trawis and meter nets by Texas

A&M University Department of Oceanography investigators in the 1960's. Althoughthe

emphasis of the volume is upon benthic/demersal rather than pelagic taxa, some chapters

summarized the state of knowledge of particular holoplanktonic groups (penaeid and

caridean shrimp, euphausiids, and heteropods) in the deepwater GUM. Around the same

time, the proceedings of "A Symposium on Investigations and Resources of the

Caribbean Sea and Adjacent Regions" were published, which included two reviews of

interest. Bjornberg (1971) reviewed phytoplankton and zooplankton of the Caribbean

and adjacent regions, including the GUM. The state of the knowledge of various

taxonomic categories of zooplankton and micronekton was given, including protozoa,

medusae, siphonophora, ctenophora, rotifera, polychaeta, nemertinea, mollusca,

copepoda, cladocera, ostracoda, mysidacea, amphipoda, isopoda, euphausiacea,

decapoda, chaetognatha, hemichordata, urochordata, and cephalochordata. Bjornberg

concluded that the copepods and chaetognaths were the most well-studied groups,

remarked that much of the study of GOM zooplankton to date had been concentrated in

coastal waters and the Florida Current, and finally noted the need for large-scale,

coordinated study of the zooplankton in shelf and oceanic waters of the Gulf of Mexico.

In the same volume, Rass (1971) reviewed deep-sea fish fauna (members of the

micronekton community) of what he termed the "American Mediterranean Region."

Rass provided a list of 203 species from the GOM and estimated that deepwater fish

represented about one-third of the total number of fish species in the open Gulf of

Mexico.

In a compendium entitled "A Summary of Knowledge of the Eastern Gulf of Mexico,"

Hopkins (1973) reviewed Gulf of Mexico zooplankton. There had been increasing work

in estuarine and coastal systems, but Hopkins noted there was still little published work

on zooplankton in the oceanic GOM. However, knowledge of eastern GUM physical

oceanography had increased considerably, and its potential biological effects were

pointed out by Hopkins The Loop Current and associated upwelling were cited as the

most important factors affecting plankton production in the oceanic GUM, while in

coastal areas runoff from terrestrial sources and seasonal temperature changes were the

most important. Biornass was known to be low in the oligotrophic open Gulf and thought

to vary seasonally with the movement of the Loop Current. The use of zooplanktonic

157
Biggs and Ress/er: Water Column Biology

indicator species as water mass tracers was mentioned in this review, as well as the

ongoing plankton collections that were taking place as part of the EGMEX (Eastern Gulf

of Mexico) program. Hopkins' own quantitative studies of biornass and taxonomic

composition of zooplankton and micronekton in the eastern central Gulf of Mexico were

mentioned as "in progress." Briggs (1973) reviewed midwater fishes of the GUM in the

same volume, but he noted that the ichthyofauna of waters overlying the continental slope

and abyssal plain was still not well known.

In 1981, a review of Gulf of Mexico phytoplankton and zooplankton by Iverson and

Hopkins was included in the proceedings of a 1979 symposium on "Environmental

Research Needs in the Gulf of Mexico." Hopkins'section on zooplankton reviews work

on the shelf, slope, and in the open Gulf subsequent to previous reviews of GUM

zooplankton, micronekton, and ichthyoplankton. Hopkins noted that except for published

work on zooplankton taxonomy, much of the research done remained in "gray literature"

government reports and theses/dissertations. However, Hopkins featured several maj or

research programs that sampled zooplankton in water depths of 200 m or greater in the

review, including:

Ocean Thermal Energy Conversion (OTEC), a program sponsored by the U.S.

Department of Energy. The zooplankton were studied off Mobile Bay (29°N, 88°W)

and off Tampa Bay (27°38'N, 85°34'W). The investigators were able to observe

taxonomic composition and biomass levels as a function of depth and time, although

the sampling strategy did not allow them to completely resolve diurnal or seasonal

trends.

Hopkins' own National Science Foudnation-funded trophodynamic study of

zooplankton and micronekton in the upper 1,000 m at a station in the eastern central

GUM (27°N, 86°W). Diurnal patterns of zooplankton numbers and biomass were

studied with trawling, net tows, and bottle sampling. Vertical migration was

documented for a "significant portion of the zooplankton arid micronekton in the

east-central Gulf." Hopkins estimated that the zooplankton biomass at this reference

station turned over once every 30 to 90 days, supported by the relatively low primary

production in the oligotrophic open GUM. Some inferences were made about trophic

interactions based on the data collected there, and Hopkins includes a list of important

zooplanktonic and micronektonic predators and prey in the system. (Hopkins and

colleagues at the University of South Florida would later publish many papers in peer

reviewed journals detailing the work on GOM zooplankton and micronekton at this

location.)

From the studies cited in Hopkins' review for the 1979 symposium, the temporally and

spatially patchy nature of the zooplarikton and micronekton had become evident.

However, Hopkins emphasized the general lack of basic physiological data for GUM

zooplarikton, and argued such data were urgently needed to better understand the flow of

energy and/or pollutants through the deepwater ecosystem.

158
159

Biggs and Ressler: Water Column Biology

In 1987, a special session on the ecology of the Gulf of Mexico was held at the annual

meeting of the American Society of Zoologists. In 1990, selected papers from that

session were published in an issue of the journal American Zoologist. Darnell and

Defenbaugh (1990) reviewed the history of environmental research in the GUM, noting

that in the 15 years preceding their review, federal agencies (most notably the

Department of the Interior) had spent more than $75 million in research studies of the

northern GOM. As had previous reviewers of the GUM zooplanktonlmicronekton field

of study, these authors reported that much of the results of GUM research remained

"locked up in the various technical reports submitted to the sponsoring agencies, and only

a small fraction [had] appeared in the professional journal literature." However, although

this review provided a list of early historical investigations of the GUM and of major

interdisciplinary investigations since 1960, the bulk of these studies had been targeted to

the continental shelf and not to deepwater.

In 1990, Vargo and Hopkins provided a review of GUM phytoplankton, zooplankton, and

ichthyoplankton in a report to MMS. The area of interest was South Florida, mostly

south of the Florida Keys but also including the deepwater Gulf to the west of the Florida

coast (in the MMS Eastern Planning Area). Hopkins' portion of the review includes

GUM hydrography and circulation relevant to zooplankton and

rnicronektonlichthyoplanktou populations, as well as tabular data and a discussion

regarding the taxonomic dominarits and seasonal trends in abundance and biornass in

GUM waters deeper than 200 m.

Ecology, Bioiogy, and Systematics Studies

Many studies of the diverse zooplankton, micronekton, and ichthyoplankton of the Gulf

of Mexico have concentrated upon the ecology, biology, or systematics of one particular

species or group of organisms. Appendix 6-A is a chronological list of these, including

author and description of subject. Some of the studies are master's theses or doctoral

dissertations that focus upon a particular group (e.g., a 1971 dissertation about two

species of decapod crustaceans that could be used as a water mass tracer in the eastern

GUM). Uthers are papers that were written about the abundance or distribution of a

species or category of particular importance (e.g., several papers estimating the stock of

larval bluefin tuna during different years in the GUM). However, many are stand-alone

taxonomic technical descriptions of particular species (e.g., 1970's descriptions of

copepods, by Park).

The scope of these individual works may be narrow, but in ensemble they are very

important to an understanding of GUM zoopiankton, micronekton, and ichthyoplankton

communities. Such research provides the means to identify and enumerate specimens

found in collected samples; without knowing 'who' is there, we cannot hope to

understand the GUM as a system. To understand the flow of energy and nutrients

through the deepwater biological system, Hopkins (1982) has argued, knowledge of

taxon-specific trophic interactions is often helpful. Thus, we have included Appendix A

because these works provide the taxon-specific ecological information needed to interpret

studies of biomass and abundance and to allow the identification of species or groups of

particular importance.
Bigg.s and Hess/er: Water Column Biology

In brief, the dominant groups of GOM deepwater zooplankton in terms of biomass are

holoplanktonic calanoid copepods, euphausiids, and chaetognaths; meroplanktonic larvae

are "relatively scarce in the oceanic" zooplankton community but become more numerous

closer to shore (Vargo and Hopkins 1990). In terms of feeding, the zooplankton

community includes herbivorous, detritivorous, and omnivorous members (Hopkins

1982). The top three groups of deepwater micronekton in order of biomass are

scyphomedusae, fishes (myctophids and gonostomids), and crustaceans (decapods and

euphausiids) (Hopkins and Lancraft 1984). Zooplanktonic crustaceans comprise the

greater part of the diet of micronektonic midwater fishes (I-Iopkins and Baird 1977;

I-Iopkins et al. 1996) and crustaceans (Hopkins et al. 1994), and gelatinous carnivores are

also known to be important zooplanktonic predators (Biggs et al. 1984; Vargo and

Hopkins 1990). Further, areas of enriched deepwater zooplankton biomass have been

shown to be correlated with increased abundance of squid paralarvae and myctophid

fishes (Wormuth et al. 2000). The major components of the deepwater ichthyoplankton

community are larval myctophids, gonostomids, mackerels, tunas, and flyingfishes (Vargo

and Hopkins 1990). The presence of increased abundance of larval fish in areas of

enriched zooplankton biomass implies that their diets include zooplankton (Govorii et al.

1989; Lamkin 1997a). However, the available information on the feeding habits of

ichthyoplankton is limited, except as the category overlaps with micronekton and

zooplankton.

Biomass and Abundance

The standing stock biornass of zooplankton, micronekton, and ichthyoplankton in the

GUM has been observed to vary in both space and time, but despite numerous studies on

the ecology and systematics of particular taxonomic groups, a much smaller amount of

work has been done to determine the scales of the variability at the coarse-to-mesoscale

level and how these determine the patterns in biomass over time. Most of the work has

been done using traditional net sampling techniques: a survey of bulk biomass values

from the literature reveals up to 1 0-fold and higher variability in standing stock levels

(see Table 6.2).

Figure 6.7 includes two maps showing the locations of maj or collections of plankton

biomass data. Despite fairly extensive sampling coverage in many deepwater parts of the

GUM over the last 30-odd years, there has been no overall summary of the biomass

results. Arnold (1958) offered only a cursory analysis of plankton biornass; the

deepwater points supplied by Houde et al. (1976) were almost incidental to an

ichthyoplarikton study that concentrated on the west Florida shelf and there has been

little published analysis of the extensive set of bulk biomass data collected by the

SEAMAP program. There have however been numerous publications and analyses of the

amount, composition, and variability of the biomass at particular locations in the

deepwater GUM (Conimins and Home 1979; Flock and Hopkins 1981; Hopkins 1982;

Hopkins and Lancraft 1984) and regions (Houde and Chitty 1976; Houde et al. 1976 and

1979; Biggs et al. 1988; Richards et al. 1989; Grimes and Finucane 1991; Biggs 1992;

Richards et al. 1993; Cummings 1984; Gasca et al. 1995; Biggs et al. 1997; Zimmerman

and Biggs 1999).

160
Year Author Descriptor Biomass

1958 Arnold GUM-wide, upper 10 m, silk mesh in

metal tube, horizontal tows

1958 Arnold GUM-wide, upper 10 m, all-metal

sampler, horizontal tows

*1969a Khromov GUM-wide, vertical hauls, upper

100 m, silk Juday meter nets with 38

meshes per inch (0.5 mm aperture),

'inedible forms removed'

1973 Hopkins in review article, mentions biomass 1 - 10

estimates for the eastern central GUM

that were obtained during EGMEX

(eastern Gulf of Mexico)

investigations

1976 Houde eastern GUM from multiple years and 2.20 - 9.86

et al. seasons, upper 200 m, 61 cm diameter

bongo nets with 333 pm mesh,

oblique hauls

1981 Iverson Tampa OTEC site in eastern GUM, 6.17

and Hopkins upper 200 m, 0.75 m open nets with

202 pm mesh, vertical and oblique

hauls; average value reported here

1988 Biggs western GUM, tipper 100 m, open 4 40

et al. meter nets with 333 jnn mesh, oblique

hauls during two months (April and

November) of the same year

1989 Richards northeast GOM, upper 200 m, 61 cm 2.3 - 12.3

et al. diameter bongo nets with 333 m

mesh, oblique hauls, data from

SEAMAP program

161

Biggs and Ressler: Water Column Biology

Table 6.2. Estimates of plankton standing stock in the deepwater Gulf of Mexico (GUM)

(bulk biomass as rnL wet displacement volume per 100 m3)

5.2 - 5.4

10.9 - 13.5

<5 - 35
Biggs and Ressler: Water Column Biology

Table 6.2. Estimates of plai±ton standing stock in the deepwater Gulf of Mexico

(GUM) (bulk biomass as rnL wet displacement volume per 100 m3)

(Continued)

1991 Grimes front between Mississippi plume and 1 - 12

and Finucane ocean water, neuston tows, 947 im

mesh, horizontal tows

1992 Biggs western GUM, upper 200 m, open 70 4 - 6

cm diameter bongo nets with 333 im

mesh, oblique hauls, range of average

day - night values is reported here

1993 Richards northeast GUM, upper 200 m, 61 cm 1.50 - 33.05

et al. diameter bongo nets with 333 tm

mesh, oblique hauls, data from

SEAMAP program

1997 Biggs western GUM, upper 100 in, open 3.6 - 9.0

et al. meter nets with 333 p.m mesh, oblique

hauls

1997a Larnkin upper 200 m, 61 cm diameter bongo 10 - 13

nets with 333 p.m mesh, oblique hauls,

data from SEAMAP program; range

between averages for the eastern and

western Gulf (respectively) are

reported here

1999 Zimmerman central GUM, various discrete 4,0 - 32.5

and Biggs intervals in the upper 125 m, 1/4 m

mouth area MOCNESS with 333 p.m

mesh nets

2000b Davis et al., northeast GOM during two different <0.1 -33.5

Volume III: years, various discrete intervals in the

Data Appendix upper 400 m, 1 m2 mouth area

MOCNESS with 333 p.m mesh nets

Notes: A comparison of literature biomass values is difficult because of differences in gear, sampling

technique, and measurement methods. The values above are a sampling of those values reported in wet

displacement volume per volume of seawater or similar, with equipment and sampling technique as noted.

Volume units were converted as necessary into mL 100 m3. The implicit assumption is that these bulk

values are useful in describing the general biomass patterns of various sizes and kinds of zooplankton,

micronekton, aild ichthyoplankton in the deepwater GOM. * values in this paper were originally reported

as g/in, but a footnote indicated that they were volume values that had been converted to weights by

assuming a zooplankton 'specific weight' ofl.

162
Biggs and Ressler: Water Column Biology

30

28

26

24-

22

20

18-

SEAMAP plankton surveys in deepwater GOM, 1982-1998

-98 -96 -94 -92 -90 -88 -86

Longitude

+ Arnold 1958

1-loudcctal.1976

OTEC

+ Biggactal. 1988

* Biggs 1992

* Biggsetal. 1997

XZimmerman and Bigga 1999

7N Wormuth et al. in Davia ci aL 1999

Hopkins at a1 Standard Station

Figure 61. lop panel: Deepwater locations of SEAMAP plankton surveys, 1982-1998. Bottom panel:

Deepwater locations of plankton collection stations (excluding SEAMAP), 1958-1999.

163

-90 -88 -86 -84 -82 -80

Longitude
Biggs and Res.sler: Water Column Biology

Spatial and Temporal Variability

The analyses that are available indicate that while overall biornass levels are low, there is

mesoscale spatial variability in biomass across the Gulf. The combined standing stock of

zooplankton, micronekton, and ichthyoplankton generally varies with distance from shore

(shelf areas are generally enriched as opposed to the deepwater areas: Khromov 1 969a;

Iverson and Hopkins 1981), depth in the water column (highest in the upper 200 m and

decreasing with depth: Vargo and Hopkins 1990), and the proximity to riverine input

(emiched areas downstream: Bogdanov et aL 1968; Khromov 1969a). Regions of

upwelling, high current shear, or physical aggregation are "hot spots" that have greater

standing stocks (Wormuth 1982; Vargo and Hopkins 1990; Larnkin 1997a; Wormuth et

al. 2000).

There is also evidence for temporal variability in deepwater stocks, both between years

and within a given year. In general, two- to four-fold increases in zooplankton standing

stock appear to follow closely in time after changes in local forcing factors (Bogdanov et

al. 1968, Khromov 1969a). These forcing factors may range from changes in river

outflow (Dagg et al. 1991) to upwelling due to the passage of deepwater eddies.

Variation in overall plankton biomass may also result from turnover of the deepwater

zoop!ankton standing stock, estimated at 30 to 90 days for zooplankton in the eastern

GOM (Iverson and Hopkins 1981). The biomass in a given depth interval also varies on

the time scale of a day by a factor of two or more because of diel vertical migration.

Although generally a small portion of the total plankton biomass (Richards 1985),

ichthyoplankton distributions are especially variable, for many taxa exhibit pronounced

seasonality and year- toyear variation in abundance. Much of this variation appears tied

to length and time of year of spawning (Houde and Chifty 1976; Ditty et al. 1988; Vargo

and Hopkins 1990).

Details of the OTEC sampling off Mobile and Tampa Bays were reported by various

groups (Lawrence Berkeley Laboratory l980a, b, c; Flock and Hopkins 1981) and

summarized by Commins and Home (1979) as well as by Iverson and Hopkins (1981).

In addition to taxonomic and size frequency data, Commins and Home (1979) reported a

peak in zooplankton abundance in October and a minimum in June 1978 at the Tampa

site, while at the Mobile site abundance was greatest in June and least in August.

Approximately 98% of the zooplankton were found to occur in the upper 200 m of the

water colunm. Die! vertical migration was evident at both sites.

A Very extensive analysis of the zooplankton and micronekton community of the

so-called 'Standard Station' in the Eastern GUM (27°N, 86°W) has been done by T.L.

Hopkins and colleagues. Trends in biomass and abundance over depth and time at this

location were elucidated in addition to the ecological information gathered about groups

of zooplankton and micronekton found there. Biomass results from these studies were

not included in Table 6.2 because they were usually reported in dry-weight units based

on length-weight regressions for particular groups of organisms, rather than in bulk

WDV. However, because spatial variation was not the focus of Hopkins' study, it is

unclear whether conclusions drawn from the data collected at this single location are

generally applicable to the rest of the GUM.

164
Aiggs and Ressler: Water Column Biology

Probably the most complete and systematic sampling of the standing stocks of

zooplankton, micronekton, and ichthyoplankton in the deepwater GUM is being carried

out as part of an ongoing state-federal project administered by the Gulf States Marine

Fisheries Commission. Known as SEAMAP-Qulf of Mexico (Southeast Area

Management and Assessment Program), the primary goal has been to census the

abundance of eggs and ichthyoplankton larvae of commercially important fish stocks.

Figure 6.7A shows the station locations where SEAMAP cruises collected deepwater

plankton, primarily using 333 jim mesh bongo nets and 947 jim mesh neuston tows

according to standard fisheries methods but supplemented with Tucker trawls on more

recent cruises. Samples are collected one to three times per year on a 1/2 x 1/2 degree

grid in different seasons. Although many of the samples collected by SEAMAP have

been from the continental shelf, so far about 2,100 have been tows in water depth

>200 m.

Data reports for the SEAMAP program are produced each year, and aliquots of the

plankton collected (both sorted and unsorted) are available for loan. Even more helpful,

though, are the summaries of sampling locations, biomass values, and the environmental

data collected at each plankton station that are available from the SEAMAP data manager

(for more information on SEAMAP data products, see

http://www.gsmfc.org/searnap.html). So far there has been no summary of the

interannual or decadal variability of these data. However, some published studies have

used SEAMAP collections from particular regions or over certain periods of time.

Grimes and Finucane (1991) attributed increased abundance of larval fish caught in

SEAMAP neuston tows taken in the front between Mississippi River plume and oceanic

waters to enriched primary and secondary production there, as indicated by elevated

chlorophyll a and zooplankton WDV. Richards et al. (1993) reported that both

zooplankton WDV and several taxa of larval fish varied across the LC boundary, being

lower in abundance in LC interior than in the periphery or outside. Recently, Lamkin

(1997a) used 6 years of SEAMAP data 1983-1988, in an investigation of the frontal

zones associated with the northern excursions of the LC. Lamkin found a positive

conelation between the abundance of larval nomeid fish and the location of the northern

edge of the LC. In particular, Cubiceps pauciradiatus has adult spawning grounds and

larval habitats closely related to sharp temperature gradients. Larvae of apex predators

like bluefin and yellowfin tuna seem to be most abundant along LC frontal zones and

within eddy peripheries, where zooplankton biomass and myctophid larvae numbers in

SEAMAP bongo collections were also elevated (Richards et al. 1989). Adult tuna, as

well, can be caught in such frontal zones (Roffer Offshore Fish Finding Service, personal

communication).

Locations of other studies that produced the biomass estimates listed in Table 6.2 are

plotted in Figure 6.7B. Work by Houde and Chilly (1976) and Houde et al. (1976)

included a study of eastern GUM ichthyoplarikton: bulk plankton displacement volumes

were reported, but most of the analyses concentrated upon shelf waters and

ichthyoplankton composition and stock estimates for species of interest, rather than

deepwater biomass. As in most studies, bulk biomass was greater on the shelf than in the

165
Biggs and Ressler: Water Column Biology

deepwater part of the study area. There appeared to be a positive relationship between

bulk displacement volume and egg/larval abundance, although the association was not

always strong. Also notable is the "distinct seasonality" in the data (especially the eggs

and fish larvae, due to seasonal spawning), with highest biomass and numbers of eggs

and larvae during the spring-summer versus fall-winter, but these seasonal fluctuations

were much more apparent on the shelf than in the deepwater part of the study area.

The studies of Biggs et al. (1988) and Biggs (1992) report opportunistic sampling during

cruises to study LC eddies in the deepwater western GUM. The results provide further

evidence that the upper 200 in of LCEs are low in plankton stocks, especially in contrast

to LCE periphery. Using a 1/4 in2 Multiple Opening/Closing Net Environmental Sensing

System, or MOCNESS (for a description of gear see Wiebe et al. 1985), Zimmerman and

Biggs (1999) collected samples in a transit through a cyclone, an LCE, and the LC itself.

This sampling documented higher standing stocks of zooplankton and micronekton in the

cyclone than in the LC or the LCE. Recently, Wormuth et al. (2000) reported on

extensive 1 in2 MOCNESS sampling, which they supplemented with IKMT (Isaacs-Kidd

midwater trawl) collections, as a part of the GulfCet II multidisciplinary study of marine

mammal, sea turtle, and seabird abundance and distribution.. Their trawling carried out in

support of this recently completed research program, which was co-sponsored by the U.S.

Geological Survey and MMS, also documented that cyclones had locally higher standing

stocks of zooplankton and rnicronekton than did LCEs.

Acoustic Sampling of Zooplankton and Micronekton Biomass

Besides traditional net sampling, acoustic methods are also currently recognized as an

important way of studying zooplankton and niicronekton (Greene and Wiebe 1990;

Wiebe et al. 1997; Greene et al. 1998). Under typical open ocean conditions and using

frequencies on the order of 102 kllz, the particles responsible for acoustic volume

backscattering (Sw) are assumed to be zooplankton and micronekton (Stanton et al. 1994;

Medwin and Clay 1998). There are several approaches to making standing stock

measurements of zooplankton and micronekton using acoustics (for a survey, see Hersey

and Backus 1962; Greene and Wiebe 1990; Wiebe and Greene 1994). One of the

simplest is to use a single-frequency echosounder to measure acoustic backscattering

from a volume of water, arid to then relate this measurement to number or bioniass of

sound-scattering organisms in that volume as determined by direct sampling with nets.

To date, there have been few acoustic surveys of deepwater zooplankton, micronekton, or

ichthyoplankton in the Gulf of Mexico. After the early work ofvan Schuyler and Hunger

(1967) and Thompson (1971) on acoustic volume backscattering, no studies using special

purpose acoustics to measure zooplankton, micronekton, or ichthyoplankton in the

deepwater GOM have reached the published literature. However, both moored and

vessel-mounted acoustic doppler current profilers (ADCPs) are routinely used to measure

the velocity of near-surface currents, and recently several volume backscattering studies

using ADCPs have been completed and published (Biggs et al. 1997; Zimmerman 1997;

Ressler et al. 1998; Wormuth et al. 2000; Zimmerman and Biggs 1999). The ADCP

transmits a sound pulse into the water and then awaits the return of sound scattered back

by passively drifting particles in the water colunrn. The Doppler shift of this

166
Biggs and Ressler: Water Column Biology

backscattered sound is then used to estimate current speed and direction. However, the

ADCP also measures the intensity of the backscattered acoustic return, which is

proportional to the number and backscattering cross sections of the particles in a given

ensonified volume of water (Clay and Medwin 1977; Medwin and Clay 1998). Although

the ADCP was not designed as a scientific echosounder (Brierly et al. 1998), ADCPs

have been successfully used to estimate the concentration of sound scatterers (Flagg and

Smith 1989; Ashjian et al. 1994; Zhou et al. 1994; Griffiths and Diaz 1996; Ressler et al.

1998). Some of the studies cited above (Ressler et al. 1998; Wormuth et al. 2000) have

employed 'sea-truth' sampling of zooplankton and rnicronekton using a 1 m2 mouth area,

333 im mesh size MOCNESS. With 1) information about the acoustical properties of

the ADCP and relevant hydrographic data, 2) net sampling of sound scattering

zooplankton and micronekton concurrent with ADCP surveys, and 3) some acoustic

theory to refine the estimate of what is being measured and how different sizes,

abundances, and taxa of zooplankton and rnicronekton are impacting the signal, it is

possible to produce ADCP-derived estimates of standing stock biornass and map

zooplankton and rnicronekton biomass distribution over depth, space, and time

(Figure 6.8).

Optical Sampling of Zooplankton and Micronekton Biomass

Underwater optical techniques, such as the video plankton recorder (VPR), provide

another more recently developed means of surveying zooplankton. Deepwater VPR

studies have not taken place in the GOM, but in other regions such observations have

been used in concert with net and acoustic sampling to study the coarse-scale abundance

and composition of zooplankton populations (Benfield et al. 1996; Davis et al. 1996;

Benfield et al. 1998).

A Combined Approach to Biomass Surveys

Traditional direct sampling and alternative acoustic and optical techniques are

complementary approaches. Net sampling provides taxonomic information that cannot

currently be gathered with acoustical or optical tecirniques; it also provides necessary

'sea-truth' information needed to interpret acoustical and optical data. However,

acoustics and optics can make nearly continuous measurements over various temporal

and spatial scales, providing zooplankton-micronekton-ichthyoplankton data with

sufficient resolution to examine temporal and spatial trends in a manner impossible with

net sampling at single discrete locations. This capacity is also useful given the growing

amount of coarse to mesoscale oceanographic data available from satellites. A

combination of net, acoustical, and optical techniques appears to be the optimum way to

study variations in zooplankton and micronekton standing stock biomass, and such a

unification of technologies will lead to better understanding of the interaction of

hydrography and ecology in the deepwater Gulf of Mexico.

167
Night, Cyclone 104274 106 138 170 202 2210 0010 0610 1210 1810 0010 0610 

Time (CDT) Day, Cyclone Night, Confluence Night, LCE 

-92 -87 -82 -77 -72 -67 -62 S(dBrem'4ir') Figure 6.6. Grayscale running plot c S coUected with an ADCP along a north-south transect line from the deepwater offie 

through a cyclone, and into a Loop Current Eddy (LCE-C) during October1996 (Adapted from: Wormuth etaon the plot indicate higher S,, while dark colors indicate less intense returns. Since local time and location arthe x-axis, such field survey data include temporal variability (higher at night than in the daytime) as well as (higher S in the cyclone than in the anticyclone).
169

Biggs and Ressler: Water Column Biology

Acknowledgments: A number of people helped us with the technical aspects of this

review. Rezneat Darnell (Texas A&M University) and Dennis Emiliani (National Marine

Fisheries Service) were helpful sources of information for the expeditions of the 1950's

and 1960's, and Mark McDuff (SEAMAP Data Manager) provided the location of the

deepwater SEAMAP stations. Gloria Guffy and the microtext and reference staff at

Texas A&M University Sterling C. Evans Library helped with interlibrary loans of

source material, and Joel Ortega made the base maps with bathyrnetry that we have used

in Figs 6.1, 6.2, and 6.7.

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180
:1z,:

1956, 1 957a, Fleminger Thesis and publications about copepods in the Gulf of

1957b, 1957c, Mexico (GUM), primarily using 195 1-1953

1959 near-surface collections from U.S. Bureau of

Commercial Fisheries vessel M/V Alaska

1956 Voss Extensive review cephalopods of the GUM, although it

contains only limited information about the smaller

planktonic or micronektonic species

1957 Gonzalez Thesis about the distribution of copepods off the

Mississippi delta in GOM during 1956

1968

1969 Bright and Paper on deep-sea hatchet fishes of the GOM; known

Pequegnat species are listed along with observation on abundance,

distribution, behavior, and taxonomy

1969 Grice Paper on calanoid copepods of GUM and Caribbean

Sea, mainly taxonomic work

Appendix 6A: Deepwater Water Column Biology

Some studies of the systematics, biology, or ecology of particular

taxonomic groups of zooplankton, micronekton, and ichthyoplankton

.L:2ji. 1L

Jones Paper on planktonic foraminifera of Caribbean and

limited areas of GOM; taxonomy, distribution, standing

crop, use as hydrographic indicator species

Texas A&M thesis on heteropod distribution and

taxonomy in the GUM (first investigation of GUM

heteropod fauna per se)

181

Biggs and Ressler: Water Column Biology

1970, 1975 Lipka Thesis and dissertation about systematics, biology, and

zoogeography of GUM cephalopods

Year Author Descriptor

1969 Taylor
Biggs and Ressler: Water COIL/mn Biology

Appendix 6A: Deepwater Water Column Biology

Some studies of the systematics, biology, or ecology of particular

taxonomic groups of zooplankton, micronekton, and ichthyoplankton

(Continued)

Year Author Descriptor

1970, 1974, Park Taxonomic work regarding calanoid copepods from the

1975a, 1975b, GOM and Caribbean Sea

1975c

1971 Austin Dissertation about the use of pteropods and

foraminifera as indicator organisms of eastern GOM

water masses

1971 Cruise Thesis in which two species of the decapod genus

Lucifer are examined and suggested as hydrographic

indicator species in the eastern GUM

1971 Schroeder Dissertation on the distribution of euphausiids in the

oceanic GOM, Yucatan Strait, and Caribbean; includes

Longhurst-Hardy sampling

1973 Ferrari Dissertation on the distribution of epipelagic cyclopoid

copepods in the GUM and Yucatan Strait; includes

Longhurst-Hardy sampling

1973 Hopkins Paper about the diet of the GUM hatchetfish Sternoptyx

and Baird diaphana

1974 Minello Thesis on the vertical distribution and feeding

relationships of calanoid copepods in the western GOM

1976 Berkowitz Thesis on neuston and near-surface zooplankton in the

northwest GUM, including taxonomic composition

1977 Hopkins Paper on the feeding ecology of midwater fishes; topics

and Baird include diet composition, feeding chronology, daily

food ration, prey choice, resource partitioning, and

bioenergetics

1977 Nafalctitis Large multi-authored volume about groups of fishes

et al. and zoogeographic provinces of the western north

Atlantic, including the GUM; of special interest is the

section on the myctophid fishes

182
Biggs and Ressler: Water Column Biology

Appendix 6A: Deepwater Water Column Biology

Some studies of the systernatics, biology, or ecology of particular

taxonomic groups of zooplankton, micronekton, and ichthyoplankton

(Continued)

Year Author Descriptor

-- 1 980a, b Richards Two papers on distributions of a) bluefin tuna larvae

and Potthoff and b) other scombrid larvae and swordfish in the

GOM during the spring of 1977 and 1978

1981 Flock Paper on zooplarikton collections near an Ocean

and Hopkins Thermal Energy Conversation site off of Tampa Bay,

including taxonomic analysis and abundance of six

particular groups

1981 Heffernan Vertical disribution and feeding ecology of the penaeid

and Hopkins shrimp genera Gennadas and Bentheogennema

1982 Smith Dissertation on the abundance and foraging ability of

physonect siphonophores the subtropical waters in the

western GOM and off the Bahamas

1983 Murdy List of midwater fish collections in the GUM during the

et al. period 1965J 973 by the Rf\TAlaminos, including

locations and species accounts

1984 Cummings Paper describing the vertical structure and taxonomic

composition of calanoid copepods in the western GUM

1984 Michel National Oceanic and Atmospheric Administration

tecimical report on chaetognatha in "Caribbean Sea and

Adjacent Areas," including GUM; includes information

on distribution and identification

1984 Richards Distribution and abundance of nine groups of GOM

et al. ichthyoplankton, from 1982 SEAMAP bongo arid

neuston collections

1 985a Hopkins Paper about trophic ecology of the GUM myctophid

and Baird Lampanyctus alatus

183
Biggs and Ressler: Water Column Biology

Appendix 6A: Deepwater Water Column Biology

Some studies of the systematics, biology, or ecology of particular

taxonomic groups of zooplankton, micronekton, and ichthyoplankton

(Continued)

1 985b Hopkins Paper on the feeding ecology of four species of GUM

and Baird hatchetfish

1986 Kelley Distribution and abundance of 10 groups of GUM

et al. ichthyoplankton, from 1983 SEAMAP bongo and

neuston collections

1986 McGowan Distribution and abundance of bluefin tuna larvae in the

and Richards GUM for 1982 and 1983, and spawning stock biomass

estimates for 1977-1978 and 1981-1983

1987 Gartner Paper about myctophids of the eastern GUM;

et al. information includes vertical distribution, species

composition, and abundance

1987 Shuert Vertical distribution and feeding ecology of the

and Hopkins copepod Euchaeta marina in the eastern GUM

1988 Dagg A study of copepod nauplii in northern GOM, primarily

et aL from the continental shelf, but including some

deepwater stations

1988 Ditty Paper on seasonal and depth distribution of larval fishes

et al. in the northern GOM includes tables of seasonal

occurrence, depth of occurrence, and previous studies

for specific taxa

1988 Gartner Paper about escapement of myctophids from midwater

et al. trawls, including size-selectivity (in terms of catch) of

net mesh size

1988 Lancrafi Paper on the trophic ecology of a GUM stomiid,

et al. Gonostoma elongatum

184

Year Author Descriptor
Appendix 6A: Deepwater Water Column Biology

Some studies of the systematics, biology, or ecology of particular

taxonomic groups of zooplankton, micronekton, and ichthyoplankton

(Continued)

185

Biggs and Ressler: Water Column Biology

1990 Grimes A review of information about larval king mackerel in

et al. the GUM, including information gained from

SEAMAP collections

1990 Passarella Thesis on oceanic cephalopods in the eastern GUM

1990 Kelley Distribution and abundance often groups of GUM

et al. ichthyoplankton, from 1986 SEAMAP bongo and

neuston collections

1990 Richards List and drawings of fishes of the western central

North Atlantic' (which includes GUM) by family.

includes species information and whether eggs,

juvenile, or larval stages are known

1991 Dagg and Paper about concentrations of copepod nauplii in the

Whitledge Mississippi plume, primarily at the edge of the

continental shelf

1991a, 1991b Gartner Life histories, age and growth patterns of three species

of eastern GUM myctophids using analysis of otoliths

1991 Goldman and Distribution and abundance of paralarval squid south of

McGowan Florida keys

Year Author Descriptor

1989 Bemett Paper on the copepod genus Pleuromamma in the

and Hopkins eastern GOM

1989 Hopkins

et al.

Caridean shrimp in the eastern GOM

1989 Ortner Correspondence analysis of zooplankton community

et al. structure and copepod species composition in GUM
Biggs and Ressler: Water COIL/mn Biology

Appendix 6A: Deepwater Water Column Biology

Some studies of the systematics, biology, or ecology of particular

taxonomic groups of zooplankton, micronekton, and ichthyoplankton

(Continued)

1991

1991

Grimes

and Finucane

Passarella

and Hopkins

Includes frequency arid percent frequency of

occurrence of families of ichthyoplankton from

SEAMAP neuston tows across a front between

Mississippi River water and oceanic water

Paper on the species composition and food habits of

rnicronektonic cephalopods in the eastern GUM

Paper on reproductive patterns of myctophids in the

eastern GUM; includes abundance estimates for several

GUM myctophid genera

Distribution and abundance of 10 groups of GUM

ichthyoplankton, from 1984-1985 SEAMAP bongo and

neuston collections

Principal component analyses of larval fish

assemblages across a Loop Current Boundary in the

GUM, using SEAMAP bongo collections

1994 Kinsey Paper about euphausiids in GOM, including cluster

and Hopkins analysis of vertical distribution, diet and morphology

186

Year Author Descriptor

1992 Cheng Paper about the marine insect Halobates in the GUM

and and Caribbean Sea

Wormuth

1992 Flock Paper describing species composition, vertical

arid Hopkins distribution, and food habits of sergestid shrimp in the

eastern GUM

1992 Hopkins Paper about resource partitioning and predation of

and Gartner eastern GUM myctophids

1993 Kelley

et al.

1993 Richards

et al.

1993 Gartner
1996 Hopkins Trophic structure and predation of eastern GUM

et aL midwater fish

1 996a, 1 996b Sutton Papers about stomiid fish assemblage in the eastern

and Hopkins GOM, including species composition, abundance,

vertical distribution, diet

1997a Lamkin Paper about the abundance of the larval fish Cub iceps

pauciradiatus in the Loop Current of the GUM

1997b Lamkin Paper describing larval stages of the stromateoid fish

Ariomma melanum in the GUM

Biggs and Ressler: Water Column Biology

Appendix 6A: Deepwater Water Column Biology

Some studies of the systematics, biology, or ecology of particular

taxonornic groups of zooplankton, micronekton, and ichthyoplankton

(Continued)

Paper about decapod and mysid assemblage in GUM,

including cluster analysis of vertical distribution, diet

and morphology

Distribution of pteropods in the GUM, Florida Current,

and Caribbean Sea; checklist of species in addition to

brief information about the geographical and vertical

distribution, biogeography, and ecology of each

Index for estimating bluefin tuna abundance based on

larval surveys (SEAMAP surveys) in the GUM

Diets and resource partitioning of micronekton

(midwater fishes and shrimps) in the Gulf of Mexico

Included in a survey of zooplankton biomass in the

northeast GUM are ongoing studies of the taxonomy,

distribution, and abundance of cephalopod paralarvae

and myctophid fish

187

Year Author Descriptor

1994 Hopkins

et al.

1997 Scott

and Turner

1998 Hopkins

and Sutton

2000 Wormuth

et al.

1994 Suárez
Chapter 7: Non-Seep Benthos

Gilbert T Rowe

lntroduction 189

Community Description 193

Microb iota 193

Meiofauna 193

Macrofauna 193

Megafauna 197

Motile Scavengers 199

Community Metabolism 199

Community Function The Cycling of Organic Carbon 199

Comparison of the Gulf Benthos with Other Oceans 202

Conclusions 203

Literature Cited 204

Introduction

The study of the deep benthos of the GUM can be divided into three eras. Early studies

prior to the 19th century were exploratory zoogeographic investigations. In the 1960's,

Willis E. Pequegnat initiated modem investigations of the standing stocks of the

megafauna. In the 1980's, the MMS supported an investigation of the continental slope

that encompassed state-of-the-art quantitative sampling of the meiof'auna, macrofauna,

and megafauna. Considerable work also has been initiated in the 1990's, but little of it

has been published yet.

The earliest explorations were concerned mostly with dredging the deep-sea floor. The

most extensive work is summarized by Agassiz (1888). Few generalizations could be

made in these works; they dealt mostly with lists of species and descriptions of new

species. Publications resulted on a broad suite of invertebrate taxa. None of the early

studies penetrated the western Gulf. These early investigations have been reviewed by

Gaitsoff (1954) and Geyer (1970).

The second important period of deep Gulf studies was initiated by Willis E Pequegnat in

the mid 1960's. A diverse series of investigations employed the R'V ALAM1NOS.

Efforts were made to characterize all geographic areas of the deep Gulf and the far

western Caribbean. The composite samples were divided up by taxon or by specialized

ecological problem among the graduate students at Texas A&M University working with

Pequegnat. The individual studies are available at Texas A&M University or at

University Microfilms. Many sampling trips extended well into what is now recognized

as Mexico's Exclusive Economic Zone (EEZ) because at the time, no restrictions had

been placed on scientific sampling of these international waters. A similar geographic

distribution of studies would not be possible today without permission from Mexico.

These studies have been summarized by Pequegnat (1983) in a volume submitted to the

189
Rowe: Non-Seep Benthos

MMS. The work concentrated on samples taken with a sampling device developed by

Pequegnat called a "skimmer" (Pequegnat et al. 1970). It was equipped with an odometer

wheel to estimate distance covered. The anterior "mouth" of the skimmer measured 3 m

wide by 1 m high. The anterior frame of the rigid, hour glass-shaped structure was

covered with 1.25 cm galvanized wire mesh but its bulbous cod end was covered with

0.6 cm mesh. The distances traveled averaged several kilometers. The original meter

wheel data are available in the field notes taken aboard ship at the time of sampling. This

method sampled "megafauna," principally, and not other size categories. It had the

advantage that the samples were protected by the rigid posterior framework, and thus

fragile forms were damaged less than in conventional otter or beam trawls.

Faunal groups studied from the deep Gulf included the crustaceans (L. Pequegnat 1970;

Roberts 1970; Firth 1971), echinoderms (Booker 1971; Carney 1971), mollusks (James

1972), and fishes (Bright 1968), among others. Tabulation of fish gut contents was

intended to link the megafauna to its food source (Rayburn 1975). Kennedy (1976)

compared the species composition of the eastern and western Gulf faunas and concluded

that they were not the same. The macrofauna appeared to be grouped into assemblages

that were distributed within zones down the slope onto the abyssal plain (Kennedy 1976;

Roberts 1977), but no justification was found for separating the abyssal plain fauna into

zoogeographic provinces by latitude or longitude. That is, even though the fauna along

the slope changed with increasing depth and these changing faunas differed in the eastern

and western Gulf, down on the basin floor the fauna showed little in the way of east-west

differences.

The stations occupied by the ALAM1NOS also incorporated bottom multi-shot

photography to quantify areas of the seafloor. The principal rationale for this was to

quantify the densities of the epibenthic megafauna captured by the skimmer and other

miscellaneous trawls and dredge samples. It also allowed the users to count

"lebenspuren" (animal tracks and burrows) and qualitatively categorize the types of

sediments present. Although a 70 mm format camera was used on occasion, a 35 mm

camera built by Alpine Geophysical seems to have produced more good film. It was

rigged to take photographs on bottom contact of a trigger switch. The advantage of such

an approach is that distance between the camera and the bottom is always known and this

allows the operator to calculate the area each photograph covers with a given lens angle.

This technique can be calibrated with a grid in shallow water, if necessary. The records

available indicate that on the order ofup to 50 photographs were taken per lowering and

each photograph covered an area of approximately 1 m2. An appendix in Pequegnat

(1983) features a large number of the photographs.

In addition to the biological studies, photographs of an "iron stone bottom" north of the

Yucatan Channel suggested deep bottom currents can be strong enough in the eastern

Gulf to sweep large areas free of sediments (Pequegnat 1972). The photographic

negatives of the entire collection of lowerings are archived in the Oceanography

Department at Texas A&M University (with the present exception of a station on loan to

the Universidad Nacional Autonomia de Mexico (UNAM).

190
191

Rowe: Non-Seep Benthos

Abyssal plains constitute a significant proportion of the surface of the earth. Although

generally acknowledged as the ultimate sink of detritus from the continents, their biota

has long been considered sparse and depauperate. The abysso-benthic communities of

the Sigsbee Deep in the western Gulf of Mexico, known as one of the flattest surfaces on

the deep ocean floor, is less well known than many other abyssal plains, in spite of its

modest depth (3.6 to 3.8 km) arid proximity to the United States and Mexico. The works

of Pequegnat remain the most extensive on the Sigsbee Deep plain.

The third historical timeframe represents the most extensive sampling of the sediment

biota of the continental slope of the northern Gulf of Mexico to date. The Northern Gulf

of Mexico Continental Slope (NGOMCS) study was conducted in the 1980's by LGL

Ecological Research Associates, Inc. with support from the MMS. This consisted of

paired Gray-O'Hara or GOMEX box cores (Boland and Rowe 1991), bottom survey

camera lowerings, and bottom trawling. The stations studied included three transects

down the continental slope off Texas, Louisiana, and Florida (Figure 7.1). Sampling

stopped at depths just shy of 3 km and therefore did not extend out onto the abyssal plain.

Thus, comparison with shelf and abyssal plain samples was not possible.

The work on deepwater benthos at Texas A&M University beginning in the 1960's up

through the IV]IMS-supported studies of the 1980's has been published in a concise

summary by Pequegnat et al. 1990). The focus was on the northern continental slope.

Other documentation of the studies of the slope include reports to MMS (Gallaway 1988;

Gallaway et al. 1 988a,b) and a dissertation on the polycbaete annelid worms (Hubbard

1995).

Considerable work has been initiated in the 1990's, but little of it has been published yet.

Th Instituto Ciencias del Mar y Limnologia of UNAM initiated extensive studies in the

southern Gulf of Mexico with the acquisition of the deep ocean research vessel J1JSTO

SIERRA. Mexican biologists are conducting studies of megafauna, demersal fishes,

macro fauna, and meiofatma from the continental shelf down across the slope onto the

Sigsbee Abyssal Plain. Stable C and N isotopes have been used to infer pathways

through a benthic food chain (Soto and Escobar 1995). Studies in deep water across the

Cordilleras Mexicanas or "Mexican Ridges" of the upper continental rise out onto the

Sigsbee Abyssal Plain have identified regions that contain enhanced biomass under

surface water masses characterized by accelerated rates of primary production

(Escobar-Briones and Soto 1997; Escobar-Briones et al. 1999). Polychaetes dominated

the infauna; they encountered a mid-slope maximum in abundance similar to that

described in the northern Gulf (Pequegnat et al. 1990). The fauna could be partitioned

into three groups that conformed to depth intervals of >3 km, intermediate depths of 1.5

and 3 kin, and shallow waters <1.5 km. This contrasts with the view of Pequegnat et al.

(1990) that the continental margin is characterized by five zones.
Rowe: Non-Seep Benthos

Figure 7.1. Station locations for the MMS-supported Northern Gulf of Mexico

Continental Slope (NGOCMS) Study conducted by LGL Ecological

Research Associates, Inc. (Gallaway 1988). Log transformed polychaete

densities (individualsm2) are shown from Hubbard (1995). Ranges of

densities from smallest to largest circles are as follows (numbers in

parentheses are numbers of occurrences): 250-630 (8); 630-1,000 (7);

1,000-1,340 (6); 1,340-1,560 (7); 1,560-1,800 (8); 1,800-2,040 (6);

2,040-2,270 (6); 2,270-2,470 (5); 2,470-2,860 (6).

En 1997, a two-ship operation by Texas A&M University and TJNAM was conducted at a

common station (25°15'N, 93°26'W) on the northern Sigsbee Abyssal Plain, at a depth of

3.65 km. The JTJSTO SIERRA traveled up from Tuxpan, sampling along an east-west

line across the Cordillera Tampiquena or Mexican Ridges (Escobar-Briones et al. 1999),

while the GYRE went due south out of Galveston, directly to the site. As this site is well

within the Mexican EEZ, all sampling conducted from the GYRE had to be approved

a priori by a suite of Mexican federal agencies. It is nestled within several nearby

locations that Pequegnat sampled with the skimmer and bottom photography.

Total sediment community respiration was measured using a benthic lander containing a

pair of automatically operated benthic incubation chambers used to measure fluxes of

metabolic gases across the sediment water interface. The lander and its operation have

been described previously by Rowe et al. (1994) and Rowe et al. (1997). Oxygen

consumption by the bottom and its contained biota is calculated from the decline of

oxygen within the chamber over time, the volume of the chamber, and the area of the

seafloor it covers. It was deployed once in the western Sigsbee Deep at a depth of

3.6 km.

192
Rowe: Non-Seep Benthos

Community Description

The standing stocks of the size categories of the biota have been grouped together, in

terms of biomass and numbers per square meter, for comparison (Table 7.1).

Microbiota

The microbiota of the deep Gulf sediments is not well characterized. While direct counts

have been coupled with some in situ and re-pressurized metabolic studies have been

made in other deep ocean sediments (Deming and Baross 1990), none has been made in

the deep Gulf. Direct counts using a fluorescing nuclear stain have been made at several

depths down the slope, thus allowing the bacterial biomass to be estimated from their

densities and sizes (Cruz-Kaegi 1998). Mean biomass was estimated to be 2.37 g C-m

for the shelf and slope (combined), and 0.37 g C-m2 for the abyssal plain. These data

indicated that the bacteria are the most important component of the functional biota in

terms of biomass. Comparisons are not yet possible with other ocean basins. Cruz-Kaegi

(1998) developed a budget for carbon cycling based on her estimates of biomass and

metabolic rates in the literature. Her budget illustrates that on the deep slope of the Gulf,

a large fraction of the organic carbon supplying the benthos with energy is cycling

through the bacteria.

Meio fauna

The extensive sampling by the NGOMCS study discovered that the meiofauna appears to

have a biomass that is higher than that of the macrofauna. The regressions of logio

numbers per square meter and biomass (as micrograms wet weight per square meter)

were the following (from Gallaway et al. 1988b):

Logio density = 5.87 0.00018 (depth in meters), and

Logio biomass 6.4 - 0.0002 (depth in meters).

Unfortunately, no estimates of biomass were made directly. Biomass was estimated from

conversion factors for each group in both the macrofauna and the rneiofauna. The

densities were 2.5 log units higher than macrofauna, which is not unusual. The biomass

was 0.5 log units above macrofauna, which is a reversal with the relationship found in

shallow water environments. If true, this confirms earlier studies in the Atlantic (Thiel

1983; Rowe et al. 1991) that implied that meiofaunal-sized organisms increase in

importance (in terms of biomass) at deep-sea depths relative to the macrofauna.

Cruz-Kaegi (1998) also observed this in her studies. No information is available on the

species composition or diversity of the meiofauna of the deep Gulf.

Macrofauna

Abundance and Biomass. Anchor dredge samples across the Sigsbee Deep and van

Veen grabs from the northern continental slope shelf suggested that deep biomass in the

Gulf was depauperate in numbers and biomass, similar to that in other ocean basins, but

that the mean size of the macrofauna was in general smaller than that in the Atlantic at

similar depths (Rowe 1971, 1983; Rowe and Menzel 1971; Rowe et al. 1974). The

lognormal relationship between abundance and depth (Figure 7.2) and biomass and depth

(Figure 7.3) has been confirmed now for numerous ocean basins (Rowe 1983), but the

slope of the line (biomass as a function of depth) for the Gulf appeared to be steeper than

that in most basins. It was suggested that this is due to the Gulf's low primary

productivity.

193
Rowe: Non-Seep Benthos

Table 7.1. Standing stocks of various size categories of benthic biota on the northern

Sigsbee Abyssal Plain of the Gulf of Mexico. The JTJSTO SIERRA and

GYRE data are from a two-ship operation in 1997 by Texas A&M University

and the Universidad Nationale Autonorna de Mexico at a common station at a

depth of 3.65 km. ALAMIINOS megafaunal numbers are from Pequegnat

(1983); biornass was recently estimated based on preserved specimens from

the "systematics working collection" at Texas A&M University. Values are

means, with standard deviation and number of samples given where available.

194

(125 tm sieve) (macrofaunal taxa)

490 indm2

-I- 147 (n5)

(nematodes)

808 indm2 (total)

(250 tm sieve)

Macrofaunal biomass 32.2 mg Cm2 NA NA

± 15.3 (n=5)

Megafaunal numbers NA NA 10.5 ha'

Dytaster insignis

(seastar)

Benthodytes typica

(sea cucumber)

Megafaunal biomass NA NA 0.15 mg Cm2

Scavengers NA 21 per 24 hour

trap deployment

NA

Eurythenes grillus

(amphipod)

Fishes NA NA NA

Parameter Ship

JIJSTO SIERRA GYRE ALAMThOS

Bacterial numbers NA 6.9x108 cells/mL NA

+ 1.2x108 (n=5)

Bacterial biomass NA 406 mg Cm2 NA

Meiofaunal numbers 288x103m2 NA NA

+ 81x103 (n=25)

Meiofaunal biomass 83 mg Cn12 NA NA

+ 22 (n25)

Macrofaunal numbers 1,281 indm2 318 indni2 NA

+ 635 (n=5) + 159 (n5)
Depth

Figure 72. Density of macrofauna in the Gulf of Mexico

(From: Rowe et at. 1974).

195

Rowe: Non-Seep Benthos

4

3-

2

1-

0

mm

U

D

y 286-

D

0.00032x R2 0,62

D

Lj1

0 1000 2000 3000 4000

Depth

Figure 7.3. Biomass of the macrofauna in the Gulf of Mexico

(From: Rowe et al. 1974).

0 1000 2000 3000 4000
Rowe: Non-Seep Benthos

Abundance values are a function of sieve size. The NGOMCS study used 300 micron

sieves and the studies aboard J1JSTO SIERRA used 125 micron sieves. It might be

expected that they both sampled far more individuals per square meter than the earlier

studies by Rowe, who used a 420 micron sieve. The values for the NGOMCS samples,

however, ranged from 518 to 5,369 individualsm2, which is similar to that encountered

by Escobar-Briones et al. (1999) in the southwestern Gulf and by Rowe et al. (1974) in

the northern Gulf.

Regressions of biomass and abundance as a function of depth from the NGOMCS study

(Gallaway et al. 1988b) follow:

Log0 individualsm2 3.52 - 0.000 109 (depth in meters), and

Logjo micrograms wet weightm2= 5.88 0.000227 (depth).

Figure 7.1, which shows polychaete densities at the NGOMCS stations, also illustrates

this decline in abundance with depth (Hubbard 1995).

It has become clear that deep offshore abyssal plains in other oceans may have somewhat

fewer organisms than the Sigsbee Deep, but they are at much greater depth (Glover et al.

in press). The deep Gulf has on the order of 150 to 500 individualsm2, depending on

sieve size, taxa included, and region studied. Escobar-Briones et al. (1999) for example

suggested that the southern Gulf in the region of the Bay of Campeche had fewer

macrofauna than the northern Gulf. They suggested that this was due to higher primary

pro duction that resulted from the interaction of warm eddies and shelf water.

Diversity. Macrofaunal diversity was studied extensively in the NGOMCS study. The

general pattern was a decline in diversity from the upper slope down to the lower slope,

as reviewed by Lohse (1999). She suggested, by comparison with other studies on the

outer shelf, that a "diversity maximum" was located on the upper slope. This contradicts

earlier work in other basins which suggested that a diversity maximum is routinely

encountered at 2 to 3 km depth. Hubbard (1995) also utilized the polychaete annelid

fraction of the NGOMCS study to assess Gulf diversity. The polychaete fraction was

used because it was the most abundant taxon (65%) and he was confident that he could

separate them at the species level.

The studies of diversity used several measures. The diversity index referred to as the

Information Function, or H'(s), was calculated from the polychaete data because it is

simple to use, relatively independent of sample size, and has been used in numerous other

studies, including Gulf of Mexico benthos. Fragments were not included in the analysis.

The equation used follows:

H'(s) - - (ln pi x p1),

where p is the proportion of each species, i, to the total population sampled, i through n.

Also, Hurlbert's (1971) revision of the Sanders "rarefaction" curve, or Expected Species

(per 100 individuals), was calculated and plotted by Lohse, directly from the NGOMCS

data reports, and by Hubbard from his own polychaete data.

196
197

Rowe: Non-Seep Benthos

In general, the Gulf slope macrofauna is veiy speciose. Dominants are few. Rare species

are common. While shallow shelf studies in the Gulf typically encounter numerous

individuals represented by 50 to 60 or so species, the slope data suggest that 100 or so

species would be expected for a similar number of individuals. It has not yet been

possible to compare the Gulf with other similar studies because of differences in

teclmique.

There has been little attempt to uncover any seasonal variation in deep Gulf standing

stocks or processes. Sets of samples along the central transect of the NGOMCS study

were taken in spring and fall, which showed some change in animal abundances on the

upper slope (Figure 7.4). Polychaete abundance was twice as high in spring than in two

fall periods. Species composition did not appear to change, just densities. The best

description of this is in Hubbard (1995).

Megafauna

Zonation. Megafauna in the Gulf appears to occur in zones that are restricted to fairly

predictable depth intervals (Pequegnat 1983; Pequegnat et al. 1990). These were

designated 1) the Shelf/Slope Transition, down to about 500 in; 2) the Archibenthal Zone,

1-lorizon A (500 to 775 m); 3) the Archibenthal Zone, Horizon B (800 to 1,000 m); 4) the

Upper Abyssal Zone (1 to 2.3 kin); 5) Meso-Abyssal Zone (2.3 to 3.2 kin); and 6) Lower

Abyssal Zone (3.2 to 3.8 km). These names were given earlier by Meiizies et al. (1973)

for similar zones encountered along several continental margins of the world oceans. In

Pequegnat et al. (1990), percent similarities were presented of evidence that groups of

fishes and rnegafauna in the region of the De Soto Canyon in the eastern Gulf occur in

recurrent groups that could be considered zones. The rate of change in species

composition of the bottom fishes also suggested that there are depth intervals that can be

considered zones and other intervals that can be regarded as boundaries between zones.

The authors mention the species that dominate each zone. Most of these can be found

archived in the deep-sea systematic working collections at Texas A&M University. The

predictability of zones such as this, regardless of what they are called, may be useful for

understanding the effects of environmental variation, whether it is natural or the result of

some alteration by human activities.

The deep Gulf summary by Pequegnat (1983) suggested that megafauna on the abyssal

plain was substantially reduced in both numbers and species compared to the continental

slope. The megafauna was dominated by the carnivorous sea star Dytaster insigiiis and

the surficial deposit feeding sea cucumber Benthodytes typica. Both of these species had

wide bathymetric distributions that extended well up onto the continental slope. Other,

less abundant megafauna species also were observed with some regularity. This included

the brittle star Ophiomusiurn planum, which reached high densities in isolated locations

when encountered. Others observed were the sea cucumber Psychropotes semperiana

and the penaeid crustacean Benthesicymus cereus/iridescens. A number of other large

crustacean species were observed in these earlier studies, but it is not clear if they were

associated with the bottom or were captured up in the water column (Nematocarcinus

ensfer, for example). There was little evidence that many demersal fishes extended out

onto the abyssal plain. Pequegnat (1983) suggested that the "terminal predator" on the

abyssal plain was the sea star D, insignis; he inferred this because the predatory sea stars

increased in abundance at depth intervals over which the demersal fishes were declining
Rowe: Non-Seep Benthos

500

400 -

300

200 -

100 -

0

Spring

Fall .

C-line, West of Mississippi Canyon

Depth (m)

Figure 7.4. Comparison of polychaete densities at two different time periods on the

upper slope of the northern Gulf of Mexico (Adapted from: Hubbard 1995).

Abundance and Biomass. The abundance of the dominant megafauna was estimated

with two methods in the studies that Pequegnat (1983) has reviewed: multishot bottom

photographic transects and use of the skimmer with an odometer wheel. The sea star

D. insignis, according to Pequegnat, had mean densities of approximately 5 per hectare at

depths of 3.6 km. B. typica reached similar values: from 4 to 7 individuals per hectare.

Similar numbers were observed for the large crustaceans B. cereusliridescens,

Nematocarcinus ensfer, and the sea cucumber P. semperiana. It is not clear, however, if

the crustaceans were caught on the bottom or in the water column; therefore, they have

been left out of the assessment of total benthic standing stocks.

Biomass was not measured in these earlier studies. However, many of the specimens are

archived in a "systematics working collection" originally established by Willis and Linda

Pequegnat on the Texas A&M University campus. This enabled our review to find a

representative number of the dominants, B. lypica and D. insignis, and determine their

mean biomass on preserved specimens approximately 30 years after they were captured.

The D. insignis were dried specimens. They had a mean dry weight of 2.88 g per

individual (G=2.2, n=1 1). Mean disk diameter was 23.3 mm (c=9, n=1 1). The

holothurian B. typica individuals have been preserved in 70% ethanol and they were

measured wet: mean=4.04 grains per individual (c=1.4, n13). They had a mean length

of 6.6 cm (=1.1, n=13) and a diameter of 1.7 cm (c=0.35, n=13). Thus, the sea star had

a mean dry weight of 14.4 g per hectare and the holothurian had a mean wet weight of

22.2 g per hectare. The latter value would be equivalent to approx. 3.3 g per hectare dry

198

0 1000 2000 3000
199

Rowe: NonSeep Benthos

weight (Rowe 1983). The two together would be equivalent to approximately 1 .77x1 0

g dry weightrn2.

Some megafauna are distributed in peculiar patches. The most consistent such animal

patches in the Gulf appear to be the contagious distribution of the holothuroid Peniagone

sp. These were rarely taken in trawls, but occurred as clumps of dozens of individuals in

photographs, in the photographic surveys made in the NGOMCS study. The highest

densities at a lowering were 1.6 x 1 ü per hectare at a depth of 1.25 km. The reasons for

the patchiness are not known. Other patches have been attributed to the proximity to

fossil hydrocarbon seeps. Odd "reefs" of sponges are seen occasionally, but the causes of

these accumulations are not known. Densities estimated with photography were always

far higher than that estimated with the trawls.

Motile Scavengers

Little is known about deep-living scavengers in the GOM. On several occasions, baited

traps have been used to capture organisms. None of the information in these studies has

been published however. In the 1997 Texas A&M University/UNAM studies with

GYRE and JTJSTO SIERRA, a deep baited trap was deployed and a single species was

captured: the cosmopolitan amphipod crustacean Eurythenes grillus. The numbers taken,

however, appeared to be small (21 per 24 hour deployment) compared to similar trap

deployments in other ocean basins. The significance of this group needs further study in

the deep Gulf.

Community Metabolism

Sediment oxygen consumption has been measured at a single site on the deep abyssal

plain in the eastern Gulf, and the rate measured was equivalent to approximately

6.7 mg Cni2d' (Hinga et al. 1979). On the Sigsbee Abyssal Plain, an unpublished value

of 4.1 mg Cm2-d' was measured. At this time it is impossible to know if this difference

is real or simply due to a difference in gear used. In any case, the two values are not

remarkably different from values at similar depths and temperatures elsewhere in the

world's oceans. Values listed by Smith and Hinga (1983) for this depth range go from

2.6 to 25 mg Cm2d', and so our rate falls at the low end of the range.

Community Function - The Cycling of Organic Carbon

The information on the stock sizes and respiration rates of the biota have been put

together into a carbon budget for the slope and abyssal plain (Figure 7.5; Rowe et al.

2000) based in part on previous work by Cruz-Kaegi (1998). A budget of this sort allows

a comparison of how carbon is both stored and cycled within an ecosystem. Values for

the boxes are standing stocks in units of mg Cm2, integrated to a depth of 20 cm,

whereas the arrows are fluxes of carbon between the boxes, with units of mg Cm2d'.

The arrow at the upper left represents inputs of organic carbon to the system whereas the

arrows that do not enter a box are losses of organic carbon from the system. Most of the
Rowe: Non-Seep Benthos

9.7 (7.7to 11.7)

Input of POC

9.5e3

Longterm Burial

0.12

Deposit Feeding

Sediment Organics

and Microbiota

(3200 + 400)

Respiration

4.85

4.7 Meiofauna

(83)

Grazing

0.43

Predation

2.5e

Predation

4.3

Macrofauna

(32.5)

Respiration

Respiration

Megafauna

(0.16)

Respiration,,

3. 6e

0.55

Respiration

Predation 4.0e

\!/ Predation

Figure 7.5. Preliminary carbon flow model for the deepsea benthos of the northern Gulf of Mexico

(From: Rowe et al. 2000). Values in boxes are standing stocks of organic carbon in mg

Cm-2, integrated to a depth of 20 cm. Arrows represent fluxes, with values expressed in

mg Cm2d-i.

200
201

Rowe: Non-Seep Benthos

losses are the remineralization of organic matter to CO2. The input term, in this case, is

estimated from the sum of the respiration loss terms by the community, plus long-term

burial. An independent estimate of input can also be derived from measures of primary

productivity in the overlying water and then applying the "Suess relationship" (Suess

1980) to estimate arrival of particulate matter at the seafloor. We assume the input varied

by a factor of two or so over an annual cycle, with the highest productivity occurring

during the winter when the phytoplankton biomass is highest (Muller-Karger et al. 1991).

Most of the loss terms are respiration. Respiration remineralizes organic carbon to

metabolic CO2. It is the largest consumer of organics in any typical benthic community

and it can be estimated from oxygen fluxes. The sum of the aerobic respiration by the

sediment dwelling organisms (bacteria, meiofauna and macrofauna) is equal to the value

measured with the incubation chambers. This can be converted to carbon by assuming a

respiratory quotient (RQ) of 0.85 (moles of CO2 produced per moles of 02 consumed).

The respiration of the individual groups was estimated by Cruz-Kaegi (1998)

independently from known size and temperature relationships in the literature (Mahaut et

al. 1995). The regression produced by Mahaut et al. (1995) relating size and respiration

rates was used to calculate a first-order respiration coefficient for the mean size of each

size groups.

The estimates of respiration for each size category allow us to further partition the flow

of organic matter through the food web. The sum of the arrows entering each standing

stock must equal the sum of the arrows leaving each stock, for the system to be in steady

state. Given the information generated on the respiration rates of each of the components

above, we can then calculate exchanges between the boxes to maintain steady state. This

is a step-wise analysis that has been utilized previously on benthos over a broad range of

latitudes (Cruz-Kaegi 1998), in addition to the Demerara Abyssal Plain (Rowe and

Deming 1985) and the continental margin off NE Greenland (Rowe et al. 1998). The

resulting solutions for the predator-prey relationships are not always unique. As

indicated, they are based on reasonable inferences of how the size classes are most

probably partitioning their resources.

Cruz-Kaegis (1998) relationships are close to several of the measurements described in

Relexans et al. (1996). A value for bacterial efficiency that is high tends to "conserve"

carbon within the system, rather than burning it off as metabolic CO2. The production of

the bacteria must be held in check by bactivory to maintain steady state and this has been

directed primarily into the meiofauna, with a small fraction into the macrofauna. It was

assumed the meiofauna had a 10% growth efficiency and thus the loss terms are

respiration and predation by macrofauna, as indicated. The total meiofauna demand must

therefore be met by bactivory and direct sediment consumption. Macrofauna fluxes are

calculated in a similar fashion.

The megafauna was assumed to be growing at a very slow rate and this is transferred by

predation to the fishes. Fish growth is assumed to be zero; thus, the megafauna growth

rate is equal to fish respiration.
Rowe: Non-Seep Benthos

The standing stock of the organic matter was calculated from the concentration of organic

carbon (0.6%), the porosity (85%), and the density of the sediment. Thus, organic carbon

in the surface 5 cm of sediment is 3,600 mg C-m2. The burial rate was estimated from

long-term burial rates (William R. Bryant, pers. comm.), as follows. Approximately 1 to

2 m of Holocene pelagic sediments, composed primarily of foram ooze, are spread rather

uniformly over the entire Sigsbee Deep. On the continental margins, the thickness of the

Holocene material is 3 to 4 m deep. Thus, the rate of accumulation on geologic time

scales are Ca. 100 to 200 cm per 13,000 years; at the time scales of our budget this is ca.

0.015 cmy'. Multiplying this by the concentration in a 1 cm thick layer (225 mg Cm2)

gives a burial rate of ca. 3.5 mg Cm2y', or 0.0095 mg C'm2d1, the appropriate units in

the budget. This value is comparable to the low flux rates estimated for the larger sized

groups of organisms (megafauna and fishes).

Comparison of the Gulf Benthos with Other Oceans

The standing stocks of the principal components of the benthic community have been

measured together for the first time at a site in the Sigsbee Deep in the northwest GOM

(Rowe et al. 2000). The relationship of bacteria numbers in abyssal sediments from the

limited data available for the world's oceans has been regressed on depth, organic carbon

concentrations, and particulate organic carbon (POC) fluxes to the seafloor by Deming

arid Baross (1990). They found that the best predictor of logio total bacterial biomass was

log 0POC fluxes. If we assume that our "input" term in our budget, estimated from the

sum of the measured or estimated carbon demand, is equal to the POC fluxes measured in

sediment traps in their study, then we can use this term in their regression. Our value for

biomass (408 mg Cm2-20 cm depth) suggests, based on their regression, that the POC

flux should be on the order of 24 mg Cm2 d*

The abundance and biomass of the macrofauna was somewhat higher than previously

estimated in the southern GOM (Rowe and Menzel 1971). Escobar-Briones et al. (1999)

made comparisons with several similar ocean basins and noted that abundances were

higher in the northern Gulf than in the Bay of Campeche. We suggest that this area

experiences intensified surface primary production that results from the interaction of

warm eddies (Muller-Karger et al. 1991). The differences with the measurements in the

southern Gulf presented by Rowe and Menzel (1971) may be due to gear: the earlier

studies used a semi-quantitative anchor dredge and not a spade or box corer.

The biomass of the macrofauna in the Sigsbee Deep was not statistically separable from a

general mean for the "global" ocean at a depth of 3.7 km, based on more than 700 values

measured with a wide variety of sampling gear (Rowe 1983). A regression of logio

biomass as wet preserved weight as a function of depth in meters suggests that biomass at

3.7 km depth at the deep end of the Gulf regression line would be expected to be

approximately 0.64 grn2.

A fair number of trawls have been taken across extensive areas of the continental margin

in both the northwest Atlantic (Haedrich and Rowe 1977; Haedrich et al. 1 980), and the

202
Rowe: Non-Seep Benthos

northeast Atlantic (Lampitt et al. 1986), thus making it possible to compare densities and

biomass of megafauna and fishes with the Gulf. Lampitt et al. (1986) plotted logio

biomass of total invertebrate megafauna as a function of depth in the northeast Atlantic

near the Porcupine Bight. Their regression line predicts that 0.31 g wet weight-m2

should be encountered at 3.7 km depth. Haedrich et al. (1980) measured a wet preserved

weight of approximately 0.08 gm2 of fishes and 0.05 gm2 of megafaunal invertebrates

(echinoderms and crustaceans) between depths of 3.2 and 37 km, suggesting that the

abundance and biornass of these groups is lower than in the Porcupine Bight of the

northeast Atlantic. The lower value above is higher than we have estimated for similar

depths in the Gulf (0.006 g wet preserved weight m2= approximately 0.155 mg-C m2);

thus, compared to the Atlantic, the deep Gulf megafauna appears to be relatively

depauperate. The extensive data from both the east and west sides of the North Atlantic

had ranges of as much as three orders of magnitude at any given depth.

The diversity of the polychaete fraction confirms that the Gulf of Mexico harbors a

diverse fauna, as is typical for much of the deep-sea floor (Rex 1983). The numbers of

species, 1-I' (s) and the expected number of species per hundred individuals however are

rather low compared to values on the upper continental slope and outer continental shelf

(Lohse 1999). This pattern relative to depth is different from that in other basins:

maximum diversity, regardless of "index" used, is found on the lower slope or the upper

rise. The values we encountered were somewhat lower than that in other basins. Thus, a

preliminary conclusion is that the pattern in the Gulf is different from most basins, with

maximum diversities at the shallower depths of the upper slope and outer shelf (100 to

1,000 rn), rather than at the slope base (2 to 3 km), as described by Rex (1983). The

apparent decline with depth might be due to the limiting sill depth, the variability of the

deepwater environment (seeps from fossil hydrocarbons buried deep in the sediments,

temperature variations due to warm eddies originating in the Loop Current in the eastern

Gulf, nepheloid layers associated originally with the Mississippi River, sediment siumps

and turbidity flows), or inadequate sample size. The latter artifact cannot be discounted

because we continued to add equal numbers of previously unencountered species with

each additional box core, thus suggesting that many more species were present than we

had actually sampled. This question can only be answered by more intense sampling.

The abyssal plain of the Gulf differs from those in the maj or ocean basins because it is

shallower (3.6 to 3.8 kin) and warmer (4.2°C). Thus, one might expect that the

characteristics of communities of organisms might differ from "typical" oceanic abyssal

plains.

Conclusions

The study of the deep benthos of the Gulf of Mexico can be divided into three eras. Early

studies prior to the 19th century were exploratory zoogeographic investigations. In the

1960's Willis Pequegnat initiated modern investigations of the standing stocks of the

megafauna. In the 1980's the MMS supported an investigation of the continental slope

that encompassed state-of-the-art quantitative sampling of the meiofauna, macrofauna,

and megafauna.

203
Rowe: Non-Seep Ben thos

Metazoan biomass is dominated by the meiofauna. The Eastern Gulf is not similar in

species composition to the Western Gulf and this can attributed to a difference in

substrate. The Western Gulf sediments are telTigenous whereas the Eastern Gulf

substrate is carbonate. Fauna! diversity of the macrofauna is high but it is not

characterized by a mid-depth maximum. The low biornass of the macrofauna and the

megafauna suggest that the biota is food limited. Budgets of carbon cycling on the slope

suggest that most of the organic matter is cycled through the smaller animals of the

communities.

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Pequegnat, W., B. Gallaway, and L. Pequegnat. 1990. Aspects of the ecology of the deep-water

fauna of the Gulf of Mexico. American Zoologist 30:45-64.

Rayburn, R. 1975. Food of deep-sea fishes of the northwestern Gulf of Mexico. M.S. Thesis,

Texas A&M University, College Station, TX.

Rex, M. 1983. Geographic patterns of species diversity in the deep-sea benthos, pp. 453-472.

In: G. Rowe (ed.), Deep-Sea Biology, Vol. 8, The Sea. Wiley, Interscience, New York,

NY.

Re1xans, J.-C., J. Deming, A. Dinet, J..-F. Gaillard, and M. Sibuet. 1996. Sedimentary organic

matter and micro-rneiobenthos with relation to trophic conditions in the tropical northeast

Atlantic. Deep-Sea Research 1(43): 1,343-1,368.

Rex, M. 1983. Species diversity in the deep sea. In: G. Rowe (ed.), Deep-Sea Biology, Vol. 8,

The Sea. Wiley, Interscience, New York, NY.

Roberts, T. 1970. A preliminary study of the family Penaeidae and their distribution in the deep

water of the Gulf of Mexico. M.S. Thesis, Texas A&M University, College Station, TX.

Roberts, T. 1977. An analysis of deep-sea benthic communities in the northeast Gulf of Mexico.

Ph.D. Dissertation, Texas A&M University, College Station, TX.

Rowe, G. 1971. Benthic biornass and surface productivity, pp. 441-454. In: J. Costlow (ed.),

Fertility of the Sea, Vol. II. Gordon and Breach, New York, NY.

Rowe, G. 1983. Biornass and production in deep-sea macrobenthos, pp. 97-12 1. In: G. Rowe

(ed.), Vol. 8, The Sea, Deep-Sea Biology. Wiley, Interscience, New York, NY.

Rowe, G. and J. Derning. 1985. The roles of bacteria in the turnover of organic carbon in

deep-sea sediments. Journal of Marine Research 43:925-950.

Rowe, G. and D.W. Menzel. 1971. Quantitative benthic samples from the deep Gulf of Mexico

with some comments on the measurement of deep-sea biomass. Bull. Mar. Sci.

2l(2):556-566.

Rowe, G., P.T. Polloni, and S.G. Hornor. 1974. Benthic biomass estimates from the

northwestern Atlantic Ocean and the Northern Gulf of Mexico. Deep-Sea Res.

21:641-650.

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Rowe, G., M. Sibuet, J. Deming, A. Khripounoff, J. Tietjen, S. Macko, and R. Theroux. 1991.

'Total" sediment biornass and preliminary estimates of organic carbon residence time in

the deep-sea benthos. Marine Ecology Progress Series 79:99-114.

Rowe, G., G. Boland, W. Phoel, R. Anderson, and P. Biscaye. 1994. Deep sea-floor respiration

as an indication of lateral input of biogenic detritus from continental margins. Deep-Sea

Research II(41):657-668.

Rowe, G, G. Boland, E.G. Escobar Briones, M.E. Cruz-Kaegi, A. Newton, D. Piepenburg,

I. Walsh, and J. Denting. 1997. Sediment community biomass and respiration in the

northeast water polynya, Greenland; a numerical simulation of benthic lander and spade

core data. Jour. Mar. Systems 10:497-515.

Rowe, G., G. Boland, B. Escobar Briones, M. Cruz-Kaegi, A. Newton, D. Piepenberg, J. Walsh,

and J. Deming. 1998. Sediment community biomass and respiration in the northeast

water polynya, Greenland: a numerical simulationofbenthic lander and spade core data.

Journal of Marine Systems 14:337-354.

Rowe, G., G. Boland, E. Escobar Briones, and J. Deming. 2000. Community structure on the

Sigsbee Abyssal Plain of the Gulf of Mexico. Abstract, Ocean Sciences Meeting, San

Antonio, TX.

Smith, K.L., Jr. and K. Hinga. 1983. Sediment community respiration in the deep sea, pp.

331-370. In; G. Rowe (ed.), Deep-Sea Biology, The Sea, Vol. VIII. Wiley, Interscience,

New York, NY.

Soto, L. and E. Escobar. 1995. Coupling mechanisms related to benthic production in the

southwestern Gulf of Mexico, pp. 233-242. Jn: A. Eleftheriou, A. Ansell, and C. Smith

(eds.), Proc. 28th Eur. Mar. Biol. Syinp. Olsen and Olsen, Copenhagen.

Suess, E. 1980. Particulate organic carbon flux in the oceans-surface productivity and oxygen

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The Sea, Vol. 8, Deep-Sea Biology. Wiley-Interscience, New York, NY.
Chapter 8: Seep Communities

lati K MacDonald

Introduction 209

Background Biology 210

Microbiology of Chemosynthesis 210

Symbiosis with Invertebrates 210

Discoveries of Chemosynthetic Communities 211

Hydrothermal Vents 211

The Florida Escarpment 211

The Northern Gulf of Mexico Continental Slope 211

The Gulf of Mexico Seep Fauna 212

Vestimentiferan Tube Worms 212

Seep Mussels 214

Vesicomyid Clams 216

Lucinid Clams 216

Polychaete "Ice Worms" 216

Beggiatoa Mats and Other Protozoans 218

Heterotrophic Fauna 218

Types of Seep Communities in the Northern Gulf of Mexico 219

Bush Hill 220

Brine Pool NR1 221

Garden Banks 386 221

Alaminos Canyon 222

Distribution of Seep Communities in the Northern Gulf of Mexico 222

Literature Cited 224

Introduètion

This is a review of publications relevant to the so-called seep fauna of the northern Gulf

of Mexico. The objective is to provide the best citations for the most pertinent facts.

Peer-reviewed articles are given priority where available, unless an important sequence in

the knowledge base is captured by reference to reports or other secondary literature.

Reports and dissertations are cited when nothing has yet been published in peer-review

sources. Personal cormnunications are avoided. An initial discussion of background

biology mentions the seep fauna analogues at hydrothermal vents. This material is

provided without any attempt to give comprehensive citation. The reader needing wider

information on chemosynthetic fauna at hydrothermal vents should consult reviews by

Turmicliffe et al. (1998) for distribution and ecology or Nelson and Fisher (1995) for

physiology and symbiont-host interaction. Nor does this discussion treat the worldwide

occurrence and attributes of seep fauna outside the Gulf of Mexico. A recent review by

Sibuet and Olu (1998) provides a reliable starting point for the cold seep literature.

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MacDonald: Seep Communities

Background Biology

Microbiology of Chemosynthesis

Chemosynthesis is a mode of life practiced by numerous groups of bacteria, which are

able to oxidize simple compounds such as hydrogen sulfide (H2S) and methane (CH4)

(Janriasch 1989). The sulfide-oxidizing forms use energy released by the oxidation

process to drive the cellular machinery of carbon fixation. These bacteria produce

carbohydrates, proteins, and other complex organic compounds starting with the basic

building blocks of nutrients and water. Like photosynthetic plants, chemosynthetic

bacteria are thus able to form new organic compounds at the base of the food chain.

Ecologically, chemosynthetic bacteria differ from green plants because they do not need

light and do require free oxygen. In their free-living forms, chemosynthetic bacteria are

found where a substrate is enriched with H2S or CI-L1. Such conditions often occur in the

anaerobic sediments of marshes or sewage treatment ponds where the decomposition of

organic matter produces these chemically reduced compounds in abundance. Because

they require the means for oxidizing their chemical nutrient source, chemosynthetic

bacteria typically live at the interface between reduced sediments and oxygenated water

(Nelson et al. 1986). A common form is Beggiatoa, which lives in Jong filaments that

form pale-colored mats on sediment surfaces (Larkin et al. 1994; Nikolaus 1995). In

shallow aquatic habitats, chemosynthetic bacteria are one component of complex systems

comprising numerous pathways for producing and recycling organic matter.

Below the photic zonein depths of 300 m or morephotosynthesis is no longer

possible and nutrient limits sharply constrain the possibilities for community structure.

Where seepage of hydrocarbons, venting of hydrothermal fluids, or other geological

processes supply abundant reduced compounds, chemosynthesis becomes the dominant

component of the ecosystem. In the northern Gulf of Mexico, these conditions are met

where oil and gas seep into seafloor sediments at depths of-400 m and greater.

Although chemosynthesis remains an exclusively microbial process at the cellular level,

chemosynthetie communities in the deep sea achieve prominence because of symbiotic

partnership between chemosynthetic bacteria and invertebrate hosts (Fisher 1990).

Symbiosis with Invertebrates

Free-living chemosynthetic bacteria are limited to interfaces because they simultaneously

require oxygen and reduced compounds that would spontaneously oxidize in the presence

of oxygen. Symbiotic partnership with invertebrate hosts greatly extends the possible

habitat for the chernosynthetic mode of life (Tunnicliffe et al. 1998). Specific adaptations

vary, but the basic arrangement is that the bacteria live within specialized cells in the host

organism. The host physiology supplies oxygen and chemosynthetic substrates to the

bacteria, often by means of specialized blood chemistry (Arp et al. 1984, 1987), and

exploits the resulting bacterial productivity (Nelson and Fisher 1995).

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Discoveries of Chemosynthetic Communities

Hydrothermal Vents

Awareness of chemosynthetic communities in the Gulf of Mexico came in a series of

steps. The initial recognition that certain locations in the seafloor might harbor dense

assemblages of invertebrates occuned in 1977, when anomalous concentrations of clam

shells were photographed in the vicinity of hydrothermal vents in the eastern Pacific

Ocean (Lonsdale 1977). The connection between chemical enrichment and this fauna

was not made until Submarine ALVIN visited the area (Corliss et al. 1979) and

disovered assemblages of vestimentiferan tube worms, RfUa pachyptila, which were

unknown to science; the vent mussel, Bathyrnodiolus thermophilis; and a clam,

Calyptogena magnfIca. Interestingly, chemosynthetic tube worms, Lamellibrachia

barharni, had already been described as a new species, but without recognition of their

nutritional mode (Webb 1969). Aceata bullisi, a bivalve commonly associated with Gulf

of Mexico tube worms, had also been described from material undoubtedly collected at

an oil seep (Volkes 1963; Kohl and Vokes 1994). The importance of chemoautotrophic

symbionts for these species was not elucidated until 2 years later (Cavanaugh et al. 1981;

Felbeck et al. 1981).

The Florida Escarpment

Because the Gulf of Mexico is not a tectonically active region and lacks hydrothermal

venting, chemosynthetic fauna were not anticipated here. Another ALVIN cruise, this

time to the base of the Florida Escarpment in the eastern Gulf, unexpectedly found

vestimentiferans and Bathymodiolus-like mussels at so-called cold seeps near the location

26°02'N and 84°55'W and at a water depth of approximately 3,200 m (Paull et al. 1984;

Hecker 1985). This site is a continental margin brine seep, also called a cold seep, where

brines fonned by dissolution of the limestone Florida Platform seep out into hemipelagic

sediments at the base of the Florida Escarpment (Paull and Neumann 1987). The brines

are enriched in sulfides and the stable isotope signature of the source fluid can be seen in

the tissue of the chemosynthetic fauna (Paull et al. 1985). The vestimentiferans and their

symbionts depend upon this sulfide source (Cavanaugh 1985; Cary et al. 1989), but it

would be shown that the mussels possessed methylotrophic symbionts (Cavanaugh et al.

1987) that could be supported by methane dissolved in the brine. Because of its depth,

the Florida Escarpment site can only be sampled with ALVIN, so the site has not been

well sampled and was last visited during two dives in June 1992. The reader can review

summaries of ALVIN dives in the Gulf of Mexico via the URL

http://www.marine.whoi.edu/webpub/divelog/default.htm.

The Northern Gulf of Mexico Continental Slope

Concunent with discoveries on the Florida Escarpment, chemosynthetic fauna were

found at hydrocarbon seeps on the northern Gulf of Mexico continental slope. The initial

discovery came as a result of sampling by otter trawl, which targeted several known oil

seeps (Kennicutt et al. 1985). The trawl recovered vesicomyid clams (Calyptogena

ponderosa and Vesicoinya cordata), a new species of mussel, Bathymodiolus childressi,

and two still undescribed vestimentiferan species Lamellibrachia n. sp. c.f. barhami

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MacDonald: Seep Communities

Webb, and Escarpia n. sp. The first photographic evidence of these communities

(Boland 1986) was obtained in the course of a research program sponsored by MMS

(Gallaway 1988), which also sponsored the first submarine dives to Gulf of Mexico

hydrocarbon seeps. These species and their associated benthic communities will be

discussed in more detail below. Additional findings about hydrocarbon seep

communities that were published during the initial discovery phase included the

following: 1) demonstration that these fauna derived their nutritional requirements from

hydrocarbons (Brooks et al. 1987); 2) demonstration of methanotrophy by the seep

mussel (Chuldress et al. 1986); 3) recognition of the association of the fauna with specific

seep attributes (Kermicutt et al. 1988a); and 4) the first submarine dives to visit a seep

community (MacDonald et al. 1989).

The Gulf of Mexico Seep Fauna

The following four sections provide a synopsis for each of the maj or invertebrate groups

that live at fluid seeps and that host chemoautotrophic symbionts.

Vestimentiferan Tube Worms

These unusual worms lack mouth, gut, and anus and were originally assigned to a new

phylum, Vestimentifera (Jones 1985). Subsequently the vestimentiferan group has been

re-understood as highly adapted polychaetes (Southward 1991). Vestimentiferans

include the first tube worm species discovered at hydrothermal vents, Rftia pachyptila

(Jones 1980), but are related to the Pogonophora, a group of small, thin tube worms that

have a cosmopolitan distribution on reducing sediments (Southward and Southward

1987). All vestimentiferans occupy a tube composed of 3-chitin (Gaul et al. 1992) and

possess blood containing a unique hemoglobin that is capable of simultaneously and

reversibly binding oxygen and sulfide; the latter is normally a powerful toxin (Arp et al.

1987). Vestimentiferan anatomy features a vascularized plume (red in color), which is

the gas-exchange organ, that extends prominently from the anterior tube-end, and can be

retracted in defense (Gardiner and Jones 1993). Additionally, a sac-like organ called the

trophosome extends within the tube and harbors chemoautotrophic bacteria, which live in

specialized host-cells (Nelson and Fisher 1995).

Two species are common in the Gulf of Mexico: Lamellibrachia n. sp. cf. barhami Webb

and Escarpia n. sp. The life-mode of Gulf of Mexico tube worms was first described by

MacDonald et al. (1989), who write that Lamellibrachia is the larger animal and forms

bush-like clusters of several hundred individuals, typically 1 cm in diameter and often

2 rrì long. The tube is often held 1 m or more above the seafloor. Figure 8.1 shows

clusters of tube worms at a mid-slope seep site. Escarpia are smaller in total length and

often live at the base of Lamellibrachia clusters. They are distinguished by a short, flared

tube extended 10 to 15 cm above the sediment. Gulf of Mexico tube worms possess

symbionts that utilize H2S, which the tube worm absorbs from fine root-like structures

that extend below the buried portions of the tubes (Julian et al. 1999). This is clearly an

adaptation to life in a sedimentary environment, where sulfidic fluids are not venting

vigorously, as they do at hydrothermal vents, but are instead dissolved in the pore volume

of the upper sediment column. Buried tube length may be as much as 30% of total

length. These animals grow at rates typically less than 1 cm-y', so that a large adult may

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MacDonald: Seep Communities

Figure 8.1. Photographs taken with vertically-mounted 35 mm camera (Figure 3 from MacDonald et al.

1989). A: Bushlike clusters of Lamellibrachia sp. (t), carbonate boulder (c), and bacterial mat

(b). B: Small tangle of Lamellibrachia sp. with attached Acesta bullisi (a), obturacular plume of

Lamellibrachia sp. (o), and an escarpiid vestimentiferan (e). C: Basket-like cluster of

Lamellibrachia sp. with seep mussels in center, epifaunal sponge (s). D: Transition between a

bed of seep mussels and sparse clusters of Lamellibrachia sp.; note dead mussel shells.

213
MacDonald: Seep Communities

be 200+ years old. A single large cluster marks a location where hydrocarbon seepage

has continued unabated for several hundred years or more (Fisher et al. 1997). Larvae of

Lamellibrachia n. sp. and Escarpia n. sp. have been cultured invitro from fertilized eggs

(Young et al. 1996), and were shown to be strong swimmers during a 2 to 3 week larval

phase. This is interpreted as an adaptation to finding active seep habitats for settlement.

Seep Mussels

Members of the mytilid genus Bathymodiolus harbor symbionts in specialized cells

within their gill linings, but retain functional feeding grooves and a gut (Fisher et al.

1 988b, 1992). The vent mussel, B. thermophilus, maintains symbionts that oxidize

thiosulfate (Nelson and Fisher 1995). A key discovery in the history of the Gulf of

Mexico seeps was that the seep mussel, B. childressi, possesses methanotrophic

symbionts (Childress et al. 1986). Subsequently, other species of seep mussels were

shown to possess both methane-oxydizing and sulfide-oxidizing symbionts (Fisher et al.

1992). An early experiment demonstrated that growth of juvenile B. childressi could be

turned on or off by regulating the supply of gas to experimental specimens, effectively

proving that the animal is able to supply its entire energy and carbon requirements from

methane (Cary et al. 1988). Methane and oxygen are supplied to the symbionts through

the ventilation of the gills (Nix et al. 1995). Because the requirement is for dissolved

CH4, seep mussels are restricted to locations where Cl-I4 concentrations are high, for

example near active gas vents, as first described by MacDonald et al. (1989), but

dissolved CU4 can also be supplied in seafloor brines (MacDonald et al. 1 990a,b). At

such sites, mussels may completely cover the seafloor in mats that are bound together by

byssal threads and extend for tens of meters or more. The maximum length of an adult is

12 to 13 cm. The growth rates are slow, with juveniles requiring possibly 20 years to

reach maturity and large adults frequently surviving 40 years (Nix et al. 1995).

Taxonomy of Gulf of Mexico seep mussels was resolved by Gustafson et al. (1998), who

described five new species. Table 8.1 summarizes the ilames and pertinent aspects of

these species. Figure 8.2 shows outlines and relative sizes of these species.

Table 8.1. Summary of taxonomic designations of seep mussels from the northern Gulf

of Mexico taken from original descriptions in Gustafson et al. (1998).

Bathyinodiolus brooksi

Bathyrnodiolus childressi

Bathymodiolus heckerae

Idas macdonaldi

TarnufIsheri

Alaminos Canyon (2,250 rn

depth)

Mid-slope hydrocarbon seeps

Florida Escarprneiit

Single collection from GB386

Mid-slope hydrocarbon seeps

214

Possesses sulfide-oxidizing

symbionts

Common seep mussel

Methylotrophic symbionts

Found in coarse, oil-soaked

sediment

Possesses sulfide-oxidizing

symbionts

Name Locality Comment
Bath ymodiolus brooksi (Xl) 50mm Figure 8.2. Bathymodiolusheckerae, B. brooks B. chlldress Tamu f/shari, and Idas macdonaidi (Figure II from Gust

of a graded series of shell outlines illustrating change in shape wfth increase in size. Only one specimen of hhiIIustated. Dottend lines connect the relative positions of the anterior edge of the umbones in specimens of size. Note scale bar and magnifications. Bathymodiolus heckerae (Xl) "idas" macdonaldi (X1O) "Bathymodiolus childressi (Xl) Tamu fisheri (X2)
MacDonald: Seep Communities

Vesicomyid Clams

The family Vesicomyidae is found in deep-sea settings worldwide. The species

Calyptogena magntIca was among the specimens collected during the discovery dives at

the Galopágos Rift (Boss and Turner 1980). The two species known from the Gulf of

Mexico are Calyptogena ponderosa and Vesicomya cordata, which were originally

described from material collected in the Gulf of Darien, Caribbean Sea (Boss 1968).

These are surface-dwelling bivalves that plow long, curving furrows across the seafloor

(Rosman et al. 1987). These "trails" and the accumulation of shell debris are distinctive

features marking active seeps on the mid-slope (Figure 8.3). Most of what is known

about vesicomyids comes from study of specimens at hydrothermal vents or cold seeps

(Ohta et al. 1987) in other regions of the world. In the basic feeding mode, the foot is

thrust forward and down into an anoxic substratum while the siphon is extended into the

bottom water with the exposed portion of the shell (Hessler et al. 1985; Fisher et al.

1988a; Hessler and Kaharl 1995). This allows that animal to absorb H2S across the foot

epithelium, from where it is transported to symbiont-lined gills via specialized blood

chemistry (Arp et al. 1984). Adult vesicomyids from the Gulf of Mexico are 75 to 90 cm

long, and have a deep, heavy shell, which is frequently stained by exposure to anoxic

sediment (Fisher 1990). Nothing is known of the growth rates, but deep-sea bivalves are

typically long-lived. Accumulations of dead shells with clusters of live individuals

suggest persistent occupation of active seep sites (Rosman et al. 1987; Callender and

Powell 1997). These aggregations have been found on the flow-fields where expulsion of

oil-rich mud generates shallow anoxic layers (Lee 1995).

Lucinid Clams

These are probably the most widely-distributed chemosynthetic invertebrates in the Gulf

of Mexico (Powell etal. 1998). However, due to an infaunal habit, living adults are

almost never seen in photo-surveys (MacDonald and Schroeder 1993), whereas

accumulations of dead shells are common in and near seeps (Callender and Powell 1992).

As with the vesicomyids, information on life-history comes primarily from work done

outside the Gulf of Mexico. These animals live in deep, U-shaped burrows and

manipulate the oxygen tension in their burrows by moving up and down in the passage to

the surface (Fisher 1990). Common species in the Gulf of Mexico are Lucinorna atlantis

and Thiasira oleophila (Callender and Powell 1997). The chalky shells are subcircular,

shallow, and have a small but distinct beak at the hinge. Symbionts live in enlarged gills

and utilize H2S (Dando et al. 1986). The stable carbon isotope signatures of L. atlantis

are the best data supporting a chemoautotrophic nutritional source (Brooks et al. 1987).

Growth rates and life-spans are unknown, although Powell et al. (1998) believe lucinids

to be slow-growing.

Polychacte "Ice Worms"

This polychaete, Hesiocaeca methanicola, received attention in the press following its

discovery in 1997, but relatively little is published about its life history or ecology

(Desbruyeres and Toulmond 1998). The worm inhabits shallow burrows on the surface

216
217

MacDonald: Seep Communities

Figure 8.3. Photographs of the two types of trails observed in the aggregations of vesicomyids (Figure 3

from Rosman et al. 1987). The upper frame shows a vesicomyid in characteristic plowing

position and its vee-shaped, curving trail. The lower frame shows the faint surface trails

formed by a second trail-making organism that occurred in the aggregations (arrows). Only

trails similar to those in the upper frame were included in the analyses.
MacDonald: Seep Communities

of shallow gas hydrate deposits. It does not possess chernosynthetic symbionts, but the

stable carbon isotope ratios of its tissue are consistent with a diet derived from

chemosynthetic production (Fisher et al., in press).

Beggiatoa Mats and Other Protozoans

Colonies of the free-living autotrophic bacteria Beggiatoa are found on reducing

sediments in freshwater and marine environments (Nelson et al. 1986). The cells are

vacuolated and inhabit large filaments (Larkin and Strobi 1983). Cell vacuoles function

as storage compartments for nitrate; functionally, Beggiatoa migrate vertically between

the sediment-water interface and as deep as 10 cm into anoxic sediments, using nitrate as

an electron acceptor for the oxidation of hydrogen sulfide (McHatton et al. 1996). At

Gulf of Mexico hydrocarbon seeps, Beggiatoa form mat-like colonies that extend for

several meters in some cases (MacDonald et al. 1989; Larkin et al. 1994). Zonation in

the mats is closely linked to sediment geochemistry, with orange and white mats

overlying sediments with high concentrations of hydrocarbon gases and extractable liquid

hydrocarbons (Sassen et al. 1993b, 1994). Nikolaus (1995) found two color phases in the

Beggiatoa mats from Gulf of Mexico seeps. A white form was characterized by high

activities of the enzyme RUBP-carboxylase, an indicator of Calvin cycle functions that

implies autotrophic carbon fixation. A pigmented fonn did not test positive for

RUBP-carboxylase, which Nikolaus interpreted as a possible indication of heterotrophic

function in these colonies. Unusually high abundance of benthic Forarninifera were seen

in samples of sediment from beneath Beggiatoa mats (Sen Gupta and Aharon 1994).

LaRock et al. (1994) report high productivity in bacterioplankton in bottom waters at

hydrocarbon seeps.

ileterotrophic Fauna

A succinct description of the heterotrophic fauna found at a "typical" oil seep can be

found in MacDonald et al. (1989) and is quoted in full below with updated taxonomic

designations indicated in square brackets.

"A diverse assemblage of common slope fauna was recorded in the still

photographs and the video tapes [from Bush Hill]. Bathypelagic organisms

included tunicates, squid and trichiurid fishes. The fish Hoplostethus sp. was

frequently seen hovering over the tube worm clusters. Other fishes (including

Chaunaxpicius, Urophycis cirratus and Peristedion greyae) were frequently

observed swimming near or resting on the bottom. Crustaceans included decapod

crabs (Geryon sp. [Chaceon], Bathyplax typhia and Rochinia crassa), shrimp and

the giant isopod Bathynomus giganteus."

"Epifaunal organisms were observed on the tube worms and mussels. The

bivalve Acesta bullisi (R. D. Turner, pers. comm; Boland 1986) was commonly

attached to the ends of the tubes of Lamellibrachia sp. Examination of

photographs and collected specimens showed that the tubes were inserted through

a concave process in the posterior margin of the A. bullisi shells, and that the

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MacDonald: Seep Communities

obturacular plumes of the Lamellibrachia sp. were extended within the mantle

cavity of the bivalves.... Several photographs showed a galatheid crab,

Munidopsis n. sp. (L. Pequegnat, pers. comm.), clinging to the ends of the tubes

of the escarpiid. Other tubes of both species were encrusted with hydroids and

sponges. Epifauna on the mussels included nerite gastropods {Cataegis cf

merloglypta] (E. N. Powell, pers. comm), an undescribed caridian shrimp,

Alvinocaris n.sp. stactophila] (A. Williams, pers. comm.) and the crab

Benthoschascon schmitti. The hag fish, Eptatretus sp., was observed at rest in

several mussel beds."

Types of Seep Communities in the Northern Gulf of Mexico

Roberts and Carney (1997) distinguish among slowly seeping oil and gas seeps, rapid,

mud-prone expulsion features (mud volcanoes), and quiescent, mineral-prone seeps.

Reilly et al. (1996) categorizes complex communities, which comprise a mixture of tube

worms and seep mussels, and simple communities, which consist of a single species

usually seep mussels or vesicomyid clams. MacDonald (1998a) and MacDonald et al. (in

press) identify brine-pooling and sediment diffusion habitats, noting that so-called simple

and complex communities can occur in close proximity. At slow oil and gas seeps, fluids

migrate to the seafloor from deep (3,000 to 5,000 m subbottom) reservoirs that are

broadly distributed across the continental slope. Near the seafloor is a layer of

unconsolidated hemipelagic sediment that is several hundred meters thick. The upper

sediment column diffuses and retains oil and gas over areas considerably larger than the

fault axis (Reilly et al. 1996). In the upper meter or so of the sediments, microbial

degradation of the labile carbon in the oil and gas depletes available oxygen and reduces

seawater sulfate to H2S. This provides chemosynthetic substrates for invertebrates with

sulfide oxidizing symbionts, notably vestimentiferan tube worms. Increased alkalinity

due to microbial productivity causes extensive precipitation of carbonate. Accumulating

fluid and carbonate produces low mounds with basal diameters of 10 m to over 500 m

and slopes of 10% or greater. At localized vents, methane bubbles through near-bottom

waters and generates sufficient local concentrations to support seep mussels. Gas

hydrates form where free gas is trapped beneath layers of rock or other shallow

obstructions. The result is often a patchwork of tube worm clusters and carbonate

boulders extending over the surface of the seep, with the greatest concentrations along

fault axes.

At mud volcanoes (Neurauter and Roberts 1994), formation of chemosynthetic

communities is controlled by the intensity and frequency of mud discharge (Roberts et al.

1990). Rapid fluid flux often includes abundant hydrocarbons, but burial of

slow-growing fauna will limit community formation at active sites. Because the fluid

flux is associated with shallow salt in most cases, halite dissolution produces

concentrated brines and the increased density of these briny fluids tends to create pools or

channels with distinct, stable interfaces (MacDonald et al. 1 990a, b). Seep mussels can

colonize the stable edges of mud-filled craters or channels. Repeated burial over

thousands of years of activity is indicated by recovery of mussels shells in cores taken at

mudprone sites (MacDonald et al. 1995).

219
MacDonald: Seep Communities

Mineral-prone seeps occur with decreased rates of venting and formation of surface

domes capped with lithified layers. Lithified surface layers and crusts represent the

terminal phase of seeps regardless of whether the active seepage mode was slow, oil and

gas driven, or rapid and mud prone (Lee 1995; Sager et al. 1999). Lithification greatly

reduces sediment porosity and limits seepage to faults and fissures in the crust. Layers of

bivalve shell may remain over large areas for many years after most seepage and all

chemosynthetic production has ceased.

Dependence upon seeping hydrocarbons places Gulf of Mexico chemosynthetic fauna in

a deep-sea locality that may be affected by human activities (MMS 1988, 1992). The

offshore energy industry has experienced several expansive episodes in the past 20 years.

All of these have increased activities at ever greater depths. The amount of seafloor

influenced by seepage is quite small compared to the extent of the subbottom

hydrocarbon system and industry engineers generally strive to avoid the unstable

substrate at seeps (Reilly et al. 1996). Current interest lies in improving the capacity to

predict where seep communities will occur and in understanding processes that contribute

to either stability or change in this environment so that anthropogenic changes could be

distinguished from natural processes. Type cases of "typical" chemosynthetic

communities are given below.

Bush Hill

The "Bush Hill" site (27°47' N, 91°30.4' W) described by MacDonald et al. (1989) was

the first hydrocarbon seep community to be sampled from a submersible. Reilly et al.

(1996) describe it as the type-example of a complex chemosynthetic community. The

Conoco tension leg work platform (TLWP) was installed <2 km west southwest from the

mound and began producing oil in the late 1980's. The major seep area is a 300 m-EW

by 500 m-NS mound with a crest depth of 570 m, rising about 40 m above the

surrounding seafloor, and composed of mud, carbonate, and shallow gas hydrate. The

N-S axis of the mound is situated along the surface trace of a westdipping fault that is

the conduit by which oil and gas reaches the surface from deposits located at

approximately 1,200 m subbottom depth. Surface sediments contain, by weight, up to

10% liquid hydrocarbons, which Kennicutt et al. (1 988b) described as having fingerprints

identical to oil produced by the TLWP wells. However, reservoirs tapped by TLWP

wells were located at subbottom depths of 3,000 m or greater by Cook and D'Onfto

(1991), who suggest that the field comprises a complex of many reservoirs, charged from

a common source, but seeping only from the shallowest strata. Sulfide concentrations in

shallow sediments (<10 cm) associated with tube worm clusters have been measured in

the 100 to 250 iM range with use of micro-electrode technique (Escorcia et al. 1999).

Methane concentrations in near-bottom waters are 30 to 60 jtM in the vicinity of active

gas vents and below 1 iM elsewhere (MacDonald et al. 1989; Nix et al. 1995). Shallow

gas hydrates have been recovered by coring at Bush Hill (Brooks et al. 1986). Layers of

gas hydrate breach the sediment layer near the highest point of the mound (MacDonald et

al. 1 994a). Tube worm clusters extend over much of the mound crest while mussels are

confined to the active gas vents.

220
MacDonald: Seep Communities

Brine Pool NR1

The focus of this chemosynthetic community is a small (190 m2) pool of brine (salinity

121.35) found near 27°43.4'N and 91°16.5'W at a water depth of 650 m (MacDonald et

al. 1 990b). Brine fills a crater at the center of a -1 00 m wide mound. The mound rises

about 6 m above the surrounding seafloor, but the crater and its diatreme extend at least

30 m below the surface. The brine contains microbial methane (S13C -63.8) in

concentrations that are supersaturated at standard temperature and pressure. Streams of

CH4 bubbles emanate continually from the center of the pool. The pooi is ringed by a

large (540 m2) bed of seep mussels (MacDonald and Fisher 1996). Mussels settled on the

"shoreline" of the pool include numerous juveniles, whereas the periphery of the bed

comprised a single settlement class without juveniles. Sulfide levels are below levels of

detection in the pool, but rise sharply in fluids collected beneath the surrounding mussel

bed (Fisher, personal communication). The bed of seep mussels comprises a striking

example of a mono-specific aggregation of chemosynthetic fauna, but numerous other

species of heterotrophic animals are also commonly observed at the site (MacDonald

1992; MacDonald and Fisher 1996). Recent findings challenge the Reilly et al. (1996)

designation that this Brine Pool NR1 is a "simple" community because small, but

significant clusters of vestimentiferans are known to occur to the south of the pool

(MacDonald et al., in press).

Garden Banks 386

Located at 580 m water depths near 27°36.9'N and 92°l5.5'W, this flat-topped mound is

described as a mud volcano by Reilly et al. (1996), but active seepage in the form of gas

venting and mud or fluid discharge does not occur over the mound (Reilly et al. 1996;

Sager et al. 1999). A rubble-strewn crust of authigenic carbonate extends over the entire

600 m wide area of the upper mound. Bivalve shells are common, but no living seep

mussels or clams have been recovered from the site and tube worms are restricted to

stunted individuals lining small fractures in the rocky substratum (MacDonald et al.

1995). Because active mud volcanoes similar of similar morphology are common in the

region, and because of the accumulation of dead bivalve shells, one can surmise that this

site was previously more active in terms of fluid expulsion and biological productivity.

Mineral-prone seeps probably do not generally represent aggregations of biological

activity requiring extensive protection. It would require careful study, however, to

distinguish a mineral-prone, relatively inactive seep and biological assemblage like the

mound in Garden Bauks 386 from more active features.

221
MacDonald: Seep Communities

Alaminos Canyon

Chemosynthetic communities at this site comprise at least six aggregations of seep

mussels (B. brooks'i) and tube worms (Lamellibrachia and Escarpia) distributed along the

northeastern wall of a submarine canyon in water depths of 2,250 m (Brooks 1990;

Brooks et al. 1990). The largest aggregation, dubbed Neptune's Garden, is located near

26° 21 'N and 94° 21' W. The mussels occupy beds as large as 10 m2 on a rocky

substratum (Fisher et aL 1992; MacDonald et al. 1 994b). Elevated salinity observed in

water samples indicates possible brine seepage at the site. Hydrocarbon seepage has been

documented in piston core samples from the region (MacDonald and Schroeder 1993).

Distribution of Seep Communities in the Northern Gulf of Mexico

Seeps and chemosynthetic communities can be detected with seismic survey methods by

looking for migration conduitsalso called seismic wipe-outs--that coincide with

surface mounds and low-angle faults (Reilly et al. 1996; Roberts and Carney 1997).

Side-scan sonar has also shown promise and may be more cost-effective in some

applications (Sager and MacDonald 1998). As the discussion above indicates, the timing

of migration and seepage is not necessarily predicted by structures that are detected with

seismic data. The geochemistry of hydrocarbon seeps has been thoroughly described

(Anderson et al. 1983; Brooks et al. 1984, 1986; Kenriicutt et al. 1987, 1988a) and will

reliably predict regional occurrence of chemosynthetic communities for at least the

so-called lush" communities of most concern for management (MMS 1988). However,

brine-pooling communities like Brine Pool NR1 are not always associated with

thermogenic hydrocarbons in the surface sediments. Submersible and photo-surveys

have been executed haphazardly and with a definite bias toward sites <1,000 m depth,

this due to the cost and depth limitations of available submersibles. Surveys of

chernosynthetic communities need to be evaluated critically, therefore, with an eye to the

underlying limits of the data and the motivating goals of the survey. The following

briefly summarizes different evidence for the regional distribution of chemosynthetic

communities in the Gulf of Mexico.

Sassen et al. (1993 a) demonstrated that, where data permit comparison, most major oil

fields in the deepwater Gulf of Mexico are associated with chemosynthetic communities.

Recent exploration and production have not been thoroughly documented by submersible

observations and there is some question about community development in water depths

between 1,000 and 2,000 m where data are lacking (MacDonald 1 998b). However, these

authors and other authors (Abrams 1996; Kaluza and Doyle 1996) note that salt tectonism

generates migration conduits across the entire Gulf of Mexico slope. All hydrocarbon

fields are therefore highly susceptible to leakage. A map of oil discovery and production

should, therefore predict the general localities where chemosynthetic communities might

be found. First-hand observations are required to confirm community occurrence at

scales of 1 km or less. Table 8.2, reproduced from MacDonald et al. (1996), compiles.

direct observations of chemosynthetic communities in the northern Gulf of Mexico.

222
223

MacDonald:Seep Communities

Table 8.2. Sites where chemosyiithetic megafauna have been collected by trawl (Tn) or

submarine (Sub), or definitively photographed by submarine, remotely operated

vehicle (ROy), or photosled (Photos!). Fauna indicates type of chemosynthetic

megafauna found at site: Vvestimentiferan tube worms, M=seep mytilids,

C=vesicomyid or lucinid clams, PG=pogonophoran tube worms. Lease block

designators follow MMS standard abbreviations. Data sources give precedence to

observations published in open literature.

Fauna Latitude

(North)

Longitude

(West)

MMS

Lease Block

Depth

(m)

Observation

Method

Data

source

V,M 26°21.20' 94°29.80' AC0645 2200 Sub

M 27°23 .50' 94°29.45' EB0602 1111 Tn! 2

P,G 27°27.55' 93°08.60' GBO5 00 734 Trl 2

v,c 27°30.05' 93°02.0 1' GB0458 757 TrI 2

M 27°3 1.50' 92010.50 GB0476 750 Sub 3

M,C 27°33.40' 92°32.40' GB0424 570 Sub 3

V 27°35.00' 92°30.00' GB0425 600 Sub 3

v,c 27°34.50' 92°5 5.95' 080416 580 Sub 3

v,c 27°36.0O' 94°46.00' EBO3 76 776 Sub 3

P,G 27°36.15' 94°35,40' EB0380 793 Trl 2

M,C 27°36.50' 92°28.94' GB0382 570 Sub 3

v,c 27°36.60' 94047 35' EB0375 773 Trl 2

v,c 27°36.82' 92° 15.25' GBO3 86 585 Sub, Tn 2, 3

V,c 27°3 7.15' 92°14.40' GB0387 781 Sub, Tn! 2, 3

V 27°37.75' 91°49.15' GCO31O 780 Tn 2

v'c 27°38.00' 92° 17.50' GB0342 425 Tn 2

C 27°39.15' 94°24.30' EB0339 780 Trl 2

V,c 27°39.60' 90°48.90' GCO287 994 Sub, Tn 2

C 27°40.45' 90°29. 10' GCO293 1042 Trl 2

v,c 27°40.50' 92° 18.00' GB 0297 589 Trl 2

V,M,C 27°40.88' 91°32.10' GCO272 720 Sub, Trl 2,3,4

V,c 27°42.65' 92° 10.45' GBO300 719 Trl 2

V 27°43.l0' 9103 0. 15 GCO229 825 Trl 2

V,M 27°43 .30' 91° 16.3 0' GCO233 650 Sub 5

V,M,C 27°43 .70' 91° 17.55' GCO233 813 Tn 2

V,M 27°44.08' 9 1° 15 .27' GCO234 600 Sub 3, 6

V,M 27°44.30' 91° 19. 10' GCO232 807 Sub 3

V,M 27°44.80' 9 1° 13 .3 0' GCO234 550 Sub 3, 7 v'c 27°45.00' 90°16.3 1' GCO2 10 715 Sub 3

C 27°45.50' 89°58.30' GCO2 16 963 Sub, Photosl 8,2

V,M,C 27°46.33' 90° 15.00' GCO2 10 796 Sub 3

V,M 27°46.65' 91°30.35' GC0184/5 580 Sub, Trl 2,3,9

V,M 27°46.75' 90° 14.70' GCO 166 767 Sub, Trl 2, 3

V,M 27°49. 16' 91°3 1.95' GCO 140 290 Sub 10

V 27°50.00' 90° 19.00' GCO 121 767 Sub 3

V,M 27°53 .56' 90°07.07' GCOO81 682 Photosl 11

V,c 27°54.40' 90°1 1.90' GC0079 685 TrI 2

V,M 27°55.50' 90°27.50' GC003O 504 Sub 3

V,P,G 27°56.65' 89°58.05' GCOO4O 685 Tn 2

C 27°57.10' 89°54.30' MC0969 658 Trl 2

V 27°57.25' 89°57.5 0' EW 1010 597 Sub, Tn 2, 3

V 27°58.70' 90°23 .40' EW 1001 430 Sub, Tn 2, 3

v,c 29°1 1.00' 88°00.00' VK0826 545 Sub, ROV, Trl 3, 4, 12

Data sources: 1-Brooks et al. (1989), 2-Kennicutt et al. (1988a,b), 3-GERG unpubL data, 4-Callender arid Powell

(1992), 5-MacDonald et al. (1990b), 6-MacDonald et al. (1990a), 7-MacDonald et at. (1990a), 8-Rosman et al.

(1987), 9-MacDonald et at, (1989), 10-Roberts et al. (1990), 1 1-Bola.nd (1986), 12-Boss (1968), Gallaway et al.

(1990), Volkes (1963).
MacDonald: Seep Communities

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230
Chapter 9: Protected Species

Stephen T. Viada

Introduction 231

Cetaceans 232

Listed Species Accounts 232

Nonlisted Species Accounts 235

Distribution and Abundance in the Oceanic Northern Gulf of Mexico 238

Sea Turtles 243

Species Accounts 244

Distribution and Abundance in the Oceanic Northern Gulf of Mexico 246

Sensitivity to Oil and Gas Exploration and Development 247

Literature Cited 250

Introduction

Protected species that may occur within oceanic waters of the northern Gulf of Mexico

(defined as the continental slope and abyssal regions) include 28 marine mammals and

5 sea turtles. All marine mammals are protected under the Marine Mammal Protection

Act of 1972. Certain marine mammals and all sea turtles are also "listed species"; that is,

they are classified as endangered or threatened under the Endangered Species Act of

1973.

This chapter discusses only those species of marine mammals and turtles that have been

reported from the deepwater Gulf of Mexico or whose known ranges may include these

waters. Other protected species occur in the Gulf of Mexico, but primarily in coastal or

continental shelf waters, or within adjacent waters of the western Atlantic Ocean or

Caribbean Sea. For example, the West Indian manatee (Trichechus manatus), which

consists of two subspecies, the Florida manatee (T. m. latirostris) and the Antillean

manatee (T. m. manatus), may be found in the northern Gulf of Mexico but inhabits only

coastal marine, brackish, and freshwater habitats (Reeves et al. 1992; Jefferson et al.

1993; O'Shea et al. 1995). Similarly, the Gulf sturgeon (Acipenser oxyrinchus desotol) is

an anadromous fish that also inhabits coastal marine (including inner continental shelf),

brackish, and freshwater habitats of the central and eastern Gulf of Mexico (Gilbert 1992;

Mettee et al. 1996). The roseate tern (Sterna dougallii) is a seabird that commonly

ventures into oceanic waters; however, its western Atlantic population is known to only

approach the far southeastern Gulf to breed in scattered colonies along the Florida Keys.

The Bermuda petrel (Pterodroma cahow) is a pelagic seabird that is limited to oceanic

waters of the northwestern Atlantic and breeds in small colonies on Bermuda. The brown

pelican (Pelicanus occidentalis) is commonly found in the northern Gulf of Mexico but

occurs only within coastal habitats and waters of the inner continental shelf (Clapp et al.

l982a,b; Harrison 1983; Warbam 1990; Olsen and Larsson 1995; Harrison 1996; Olsen

and Larsson 1997; National Geographic Society 1999).

231
V/ada: Protected Spec/es

Cetaceans

Whales and dolphins are members of the mammalian taxonomic order Cetacea, which

includes two suborders: Mysticeti (baleen whales) and Odontoceti (toothed whales, and

dolphins and their allies). Twenty-eight species of cetaceans comprising 7 mysticete

whales and 21 odoritocete whales and dolphins are known to occur in the Gulf of Mexico

(Table 9.1) (Davis et al. 2000; Jefferson et al. 1992).

Listed Species Accounts

Five mysticetes (the northern right, blue, fin, sei, and humpback whales) and one

odontocete (the sperm whale) that occur or have been reported in the Gulf of Mexico are

listed as endangered species. The sperm whale is the only endangered species that is

considered to be common and perhaps a resident species in certain deepwater areas of the

Gulf (Davis et al. 2000). Listed mysticetes do not normally enter the Gulf with any

regularity (Jefferson 1995; Davis and Fargion 1996). Sightings of these species in the

Gulf are rare or uncommon, and are believed to be in most cases representative of

extralimital strays from their normal ranges in the western Atlantic Ocean where they

may inhabit both deepwater habitats and waters of the continental shelf (Wursig et al.

2000). Consequently, there are insufficient sightings records to estimate numbers of

individuals or stocks" of mysticetes in the Gulf (Jefferson 1995). Brief species

accounts of the listed cetaceans are presented below.

Mysticetes

The northern right whale (Eubalaena glacialis) primarily inhabits temperate and

subarctic coastal and continental shelf waters. The western North Atlantic population, or

stock, ranges between the Maritime Provinces of eastern Canada to northeastern Florida.

Right whales forage primarily on subsurface and localized concentrations of zooplankton

such as calanoid copepods (Leatherwood and Reeves 1983; Jefferson et al. 1993; Perry et

al. 1999). Sparse, historical sightings and strandings records (one confirmed sighting

from Florida and one confirmed stranding from Texas) suggest that this species is not a

normal inhabitant of the Gulf of Mexico. The sightings records that do exist are

considered to be those of extralimital strays from their wintering grounds off the

southeastern United States (Jefferson 1995; Jefferson and Schiro 1997; Wilrsig et al.

2000).

The blue whale (Balaenoptera musculus) is an oceanic species that may move into

selected habitats on the continental shelf to feed. Blue whales are distributed from the

equator to polar regions of both hemispheres. Blue whales feed almost exclusively on

localized concentrations of zooplankton, primarily euphausiid crustaceans (k.riJl)

(Yochem and Leatherwood 1985; Jefferson et al. 1993; Perry et al. 1999). Their presence

in the Gulf of Mexico is considered to be very rare, as sighting records consist of two

stranded individuals on the Texas coast and two nonconfirnied sightings (Jefferson and

Schiro 1997; WUrsig et aL 2000).

232
SUBORDER MYSTICETI

Family Balaenidae

Eubalaena glacialis

Family Balaenopteridae

Balaenoptera musculus

Balaenoptera edeni

Balaenopteraphysalus

Megaptera novaeangliae

Balaenoptera acutorostrata

Balaenoptera borealis

SUBORDER ODONTOCETI

Family Physeteridae

Kogia sinms

Kogia breviceps

Physeter niacrocephalus

Family Ziphiidae

Mesoplodon densirostris

Ziphius cavirostris

Mesoplodon europaeus

Mesoplodon bidens

Family Delphinidae

Stenella frontal/s

Tursiops truncatus

Stenella clymene

Pseudorca crass/dens

Lagenodelphis hosei

Orcinus orca

Peponocephala electra

Stenella attenuata

Feresa attenuata

Glob icephala inacrorhynchus

Grampus griseus

Steno bredanensis

Stenella longirostris

Stenella coeruleoalba

BALEEN WHALES

Right whales

Northern right whale

Rorquals

Blue whale

Bryde's whale

Fin whale

Humpback whale

Minke whale

Sei whale

TOOTHED WHALES AND DOLPHINS

Sperm whales

Dwarf sperm whale

Pygmy sperm whale

Sperm whale

Beaked whales

Blainville's beaked whale

CuvierTs beaked whale

Gervais' beaked whale

Sowerby's beaked whale

Dolphins (Delphinids)

Atlantic spotted dolphin

Bottlenose dolphin

Clymene dolphin

False killer whale

Fraser's dolphin

Killer whale

Melon-headed whale

Pantropical spotted dolphin

Pygmy killer whale

Shortfinned pilot whale

RissoTs dolphin

Rough-toothed dolphin

Spinner dolphin

Striped dolphin

a Status: E = endangered under the Endangered Species Act of 1973; S = strategic stock under

the Marine Mammal Protection Act of 1972, as indicated by Waring et al. (1999).

233

Viada: Protected Species

Table 9.1. Cetaceans of the Gulf of Mexico

Scientific Name Common Name Status'

E, S

E, S

None

E, S

E, S

None

E, S

None

None

E, S

S

S

S

S

None

None

None

None

None

None

None

None

None

S

None

None

None

None
Viada: Protected Species

The fin whale (Balaenopteraphysalus) is also an oceanic species of both hemispheres,

and may be found from the tropics to polar zones. They are sighted near the coast in

certain areas where deep water approaches the coast. Fin whales feed on localized

concentrations of zooplankton, fishes, and squid. The predominant prey of fin whales

varies greatly in different geographic locales, based on what is locally abundant

(Leatherwood and Reeves 1983; Jefferson et al. 1993; Perry et al. 1999). There are seven

reliable sightings and four unreliable sighting reports of fin whales from the Gulf of

Medco. From these data, they are not considered to be abundant in the Gulf. Sighted

individuals may be extralimital strays from their western Atlantic population or, less

likely, a small, resident, and relict population (Jefferson and Schiro 1997; WUrsig et al.

2000).

The humpback whale Megaptera novaeangliae) feeds and breeds in coastal waters, and

migrates across oceanic waters from breeding areas in the tropics to polar or sub-polar

regions. Humpback whales feed on localized concentrations of zooplankton, fishes, and

squid (Winn and Reichley 1985; Jefferson et al. 1993; Perry et al. 1999). There are seven

reliable sightings and two unreliable sightings reports of generally small, humpback

whales from the Gulf of Mexico. The time of the year of the records suggest that these

individuals may be extralimital strays during their breeding season or during their

migrations, and their small body sizes suggest that they may be inexperienced yearlings

on their first return migration (Jefferson and Schiro 1997; Wtirsig et al. 2000).

The sei whale (Balaenoptera borealis) is an oceanic species that is not commonly sighted

near the coast. They occur from the tropics to polar zones in both hemispheres, but

appear to be more common in mid-latitude temperate zones. Sei whales feed on localized

concentrations of zooplankton, small fishes, and squid (Gambell 1985; Jefferson et al.

1993). Sparse sightings data in the Gulf of Mexico, consisting of one questionable

sighting and three strandings from eastern Louisiana, suggest that their presence there is

rare, or of accidental occurrence (Jefferson and Schiro 1997).

Odontocetes

The sperm whale (Physeter macrocephalus) is the largest toothed whale and is distributed

from the tropics to polar zones in both hemispheres. They are deep diving mammals and

inhabit oceanic waters, although they may come close to shore in certain areas where

deep water approaches the coast. Their distribution varies by gender and age

composition. Mature females, and calves and immature whales of both sexes are found

in social groups in temperate and tropical waters. Mature males are mostly solitary, and

may travel into polar seas as high as 70° N or 700 S (Perry et al. 1999). Sperm whales are

known to feed on cephalopods, demersal fishes, and benthic invertebrates (Rice 1989;

Jefferson et al. 1993; Perry et al. 1999). The sperm whale is the only great whale that is

considered to be common in the Gulf of Mexico (Jefferson 1995). Historical sightings

records suggest a Gulf-wide distribution, primarily on the continental slope. During

GulfCet I and II surveys, however, sperm whales on the slope were sighted almost

entirely in the north-central and northeastern Gulf in loose groups of two to eight

individuals (Davis and Fargion 1996; Davis et al. 1999). Congregations of sperm whales

234
Viada: Protected Species

are also common inhabitants of waters over the shelf edge in the vicinity of the

Mississippi River delta in water depths of 500 to 2,000 m. From these consistent

sightings it is believed that there is a resident population of sperm whales in the Gulf

consisting of adult females, calves, and immature individuals (Brandon and Fargion

1993; Mullin et al. 1994b; Sparks et al. 1996; Jefferson and Schiro 1997). Recent

minimum population estimates of sperm whales within the oceanic northern Gulf of

Mexico totaled 530 individuals from 1991-1994 surveys and 387 individuals from

1996-1997 surveys (Davis et al. 2000).

Nonlisted Species Accounts

Two nonlisted mysticete whales and 20 nonlisted odontocete whales and dolphins are

known to occur in the Gulf. Brief species accounts of these nonlisted cetaceans are

presented below.

Mysticetes

The Bryde's whale (Balaenoptera edeni) is the second smallest baleen whale and is

generally confined to tropical and subtropical waters (between 40°N and 40°S latitudes).

Unlike other baleen whales, it does not have a well-defmed breeding season in most areas

and thus calving may occur throughout the year. Resident populations may be common

in certain areas. Bryde's whales feed on both fishes and invertebrates (Leatherwood and

Reeves 1983; Cummings 1985; Jefferson et al. 1993). The Bryde's whale is represented

by more sightings records than any other species of baleen whale in the Gulf. All

Bryde's whale sightings made during recent surveys in the Gulf of Mexico were from the

continental shelf edge in the vicinity of De Soto Canyon and along the 100 m isobath in

the north-central Gulf. These data suggest that the Gulf may represent at least a portion

of the range of a dispersed, resident population of Bryde's whale (Davis and Fargion

1996; Jefferson and Schiro 1997; Davis et al. 2000; Wursig et al. 2000).

The minke whale (Balaenoptera acutorostrata) is the smallest baleen whale and is widely

distributed from tropical to polar habitats. Minke whales may be found in oceanic

regions but appear to prefer coastal waters and waters of the continental shelf. Their diet

consists of invertebrates and fishes (Leatherwood and Reeves 1983; Stewart and

Leatherwood 1985; Jefferson et al. 1993). There are 10 reliable records and 2

questionable records of minke whales in the Gulf. All of these are stranding records from

winter or spring. These records suggest that minke whales may migrate into Gulf waters

in small numbers during the winter or, more likely, that sighted individuals represent

strays from low-latitude breeding grounds in the western North Atlantic (Davis and

Fargion 1996; Jefferson and Schiro 1997; Davis et al, 2000; Wttrsig et al. 2000).

Odontocetes

Pygmy and Dwarf Sperm Whales. The pygmy spenri whale (Kogia breviceps) and its

congener, the dwarf sperm whale (K simus), are known from deep waters in tropical to

warm temperate zones (Jefferson and Schiro 1997). They appear to be most common on

235
Viada: Protected Species

the continental slope and along the shelf edge (Wflrsig et al. 2000). Little is known of

their natural history. Data collected from stomach contents of stranded individuals

suggest that these species feed on cephalopods, fishes, and crustaceans in deep water

(Leatherwood and Reeves 1983; Jefferson et al. 1993). Kogia spp. has been sighted

throughout the Gulf across a wide range of depths and bottom topographies, though they

may more commonly be associated with water mass fronts along the continental shelf

edge break and upper slope (Baumgartner 1995).

Beaked Whales. Two genera and four species of beaked whales are known to occur in

the Gulf of Mexico. These include three species in the genus Mesoplodon (Sowerby's

beaked whale {M bidens], Blainville's beaked whale [M densirostris], and Gervais'

beaked whale M europaeus]) and Cuvier's beaked whale (Ziphius cavirosiris).

Generally, beaked whales appear to prefer deep water, though little is known of their

respective life histories. Stomach content analyses suggest that these whales feed

primarily on deepwater cephalopods, although they will also take fish and some benthic

invertebrates (Leatherwood and Reeves 1983; Jefferson et al. 1993). In the Gulf, beaked

whales have been sighted at depths between approximately 700 and 2,000 m. Cuvier's

beaked whale is probably the most common beaked whale in the Gulf (Davis and Fargion

1996; Jefferson and Schiro 1997; Davis et al. 2000; Wtirsig et al. 2000).

Deiphinid Whales and Dolphins All remaining species of nonendangered and

nonthreatened cetaceans found in the Gulf are members of the taxonomically diverse

family Delphinidae.

The Atlantic spotted dolphin (Stenellafrontalis) is endemic to the Atlantic within tropical

to temperate waters. They feed on a wide variety of fishes and cephalopods. The

Atlantic spotted dolphin is the only other species of cetacean (other than the bottlenose

dolphin) that commonly occurs on the continental shelf of the Gulf of Mexico (Brandon

and Fargion 1993; Jefferson and Schiro 1997). Current Gulf surveys sighted the Atlantic

spotted dolphin primarily on the continental shelf and shelf edge at depths less than

250 m, although with some individuals sighted along the slope at depths of up to 600 m

(Davis et al. 2000).

The botfienose dolphin (Tursiops truncatus) is a common inhabitant of both continental

shelf and slope. They are opportunistic feeders of a wide array of prey items (Brandon

and Fargion 1993; Jefferson and Schiro 1997). Current data suggest genetically discrete,

inshore and offshore stocks, or populations of bottlenose dolphins. Interaction between

the two populations is thought to be minimal (Wtirsig et al. 2000). Bottlenose dolphins in

the Gulf are corrunonly sighted at depths less than approximately 1,200 m (Mullin et al.

1994b; Jefferson and Schiro 1997; Davis et al. 1999).

The Clymene dolphin (Stenella clymene) is endemic to the Atlantic, and found only in

tropical and subtropical waters. This species appears to feed on fishes and squid

(Leatherwood and Reeves 1983; Jefferson et al. 1993; Mullin et al. 1994a). They are not

considered to be rare in the Gulf, though current sightings have been made almost

236
Viada: Protected Species

exclusively to the west of the Mississippi River at depths of 600 to 3,000 m (Mullin et al.

1994a; Jefferson 1995; Jefferson and Schiro 1997; Wursig et aL 2000).

The false killer whale (Pseudorca crassidens) is found in tropical to warm temperate

zones in deep offshore waters. It feeds on primarily fish and cephalopods, although it has

been known to also feed on cetaceans (Jefferson et al. 1993; Leatherwood and Reeves

1983). In the Gulf, most sightings of false killer whales have occurred along the

continental slope, although some were sighted over the outer continental shelf (OCS) as

well (Davis arid Fargion 1996).

The Fraser's dolphin (Lagenodeiphis hosei) has a pantropical distribution in oceanic

waters and nearshore in areas where deep water approaches the coast. Fraser's dolphins

feed on fish, cephalopods, and crustaceans (Leatherwood and Reeves 1983; Jefferson et

al. 1993; Jefferson and Schiro 1997). In the Gulf, sightings of Fraser's dolphins have

occurred in the western part at depths of around 1,000 m (Davis and Fargion 1996;

Jefferson and Schiro 1997; Davis et al. 2000).

The killer whale (Orcinus orca) is probably the most cosmopolitan of all cetaceans and

can be found in almost any marine region in all oceans. Generally, they appear to prefer

nearshore, cold temperate to subpolar zones. Killer whales feed on marine mammals,

marine birds, fishes, sea turtles, and cephalopods (Leatherwood and Reeves 1983;

Jefferson et al. 1993). In the Gulf, most sightings of killer whales have been along the

continental slope, within a broad area of the north-central Gulf (Jefferson and Schiro

1997; O'Sullivan and Mullin 1997; Wtirsig et al. 2000).

The melon-headed whale (Peponocephala electra) is a deep-water, pantropical species.

It is known to feed on cephalopods and fishes (Leatherwood and Reeves 1983; Jefferson

et al. 1993; Mullin et al. 1994c; Jefferson and Schiro 1997). In the Gulf, the

melon-headed whale has been sighted in continental slope waters, west of the Mississippi

River (Davis and Fargion 1996; Jefferson and Schiro 1997; Davis et al. 2000; Wtirsig et

al. 2000).

The pantropical spotted dolphin (Stenella attenuata) is a tropical species known from the

Atlantic, Pacific, and Indian Oceans. It is known to feed on epipelagic fishes and squid

(Jefferson et al. 1993; Leatherwood arid Reeves 1983). The pantropical spotted dolphin

is the most common and abundant cetacean on the slope and especially outer slope waters

of the Gulf at depths greater than 1,200 m (Brandon and Fargion 1993; Jefferson 1995;

Jefferson and Schiro 1997; Wursig et al. 2000).

The pygmy killer whale (Feresa attenuata) is apparently widely distributed in tropical

waters, though little is known of its biology. Its diet includes cephalopods and fishes,

though reports of attacks on other de!phinids are reported (Leatherwood and Reeves

1983; Jefferson et al. 1993). The pygmy killer whale does not appear to be commonly

found in the Gulf of Mexico. Sightings of this species in the Gulf have been at depths of

500 to 1,000 m (Davis and Fargion 1996; Jefferson and Schiro 1997; Davis et al. 2000;

Witrsig et al. 2000).

237
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The short-finned pilot whale (Globicephala macrorhynchus) is found in warm temperate

to tropical waters of the world. Short-finned pilot whales feed primarily on squid and

fishes. In the Gulf, it is most commonly sighted along the mid- to upper-slope at depths

of 250 to 2,000 m and often in areas of steep bottom topography (Davis and Fargion

1996; Jefferson and Schiro 1997; Davis et al. 2000; Wtirsig et al. 2000).

The Risso's dolphin (Grampus griseus) is a pantropical species that inhabits deep oceanic

and continental slope waters. Risso' s dolphins feed primarily on cephalopods and

secondarily crustaceans (Leatherwood and Reeves 1983; Jefferson et al. 1993;

Baumgartner 1995). In the Gulf, it occurs at depths of 150 to 2,000 m, especially in areas

along the upper continental slope (Baumgartner 1995). It is considered common in these

areas of the Gulf (Davis and Fargion 1996; Jefferson and Schiro 1997; Davis et al. 2000;

Wursig et al. 2000).

The rough-toothed dolphin (Steno bredanensis) is a circumiropical and subtropical

species. This species feeds on cephalopods and fishes (Leatherwood and Reeves 1983;

Jefferson et al. 1993). In the Gulf, they are sighted almost exclusively west of the

Mississippi at depths of 900 to 2,000 m, and occur year-round (Davis and Fargion 1996;

Jefferson and Schiro 1997).

The spinner dolphin (Stenella longirostris) is a pantropical species (Jefferson and Schiro

1997). They commonly associate with pantropical spotted dolphins. Spinner dolphins

appear to feed on fishes and squid. In the Gulf, most sightings of spinner dolphins have

been east of the Mississippi at depths of 500 to 1,800 m (Jefferson and Schiro 1997;

Davis et al. 2000; Wiirsig et al. 2000).

The striped dolphin (Stenella coeruleoalba) is primarily a tropical species, though it may

also range into temperate seas. Striped dolphins are known to feed on squid and fishes.

In the Gulf, they are found at depths of 600 to 2,000 m (Jefferson and Schiro 1997;

Wtirsig et al. 2000).

Distribution and Abundance in the Oceanic Northern Gulf of Mexico

Prior to 1989, most historical study programs designed to assess the distribution and

abundance of cetaceans within the Gulf of Mexico were conducted exclusively within

waters of the continental shelf. The first systematic surveys for cetaceans within

continental slope and abyssa! waters of the Gulf began in 1989. During this period,

systematic aerial surveys were conducted over selected survey blocks off Louisiana and

Mississippi by the National Marine Fisheries Service (NMFS) (Mullin et al. 1991,

1 994b). Subsequent, shipboard surveys for cetaceans within a broad area of the oceanic

northern Gulf were conducted by NMFS in spring of 1990 and 1991 (Jefferson 1995).

These early study efforts provided the first data pertaining to the distribution and

abundance of oceanic cetaceans within the northern Gulf of Mexico but provided limited

information regarding geographic coverage (Mullin et al. 1991, 1994b) or variability in

seasonal distributions of cetacean populations (NMFS shipboard surveys) (Jefferson

238
ucatan Figure 9.1. U.S. oceanic northern Gulf of Mexico as defined in the GulfCet II study (Davis et al. 2000). o-25 50 75 10

.

0 50 ioa 150 2
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1995). Texas A&M University began a series of extensive survey programs in 1992

within the OCS and continental slope waters of the northern Gulf of Mexico in an effort

to determine seasonal and geographic distributions, and habitat associations of cetacean,

sea turtle, and seabird populations. These programs, entitled GulfCet I and II, represent

the most recent and systematic survey efforts within the oceanic northern Gulf of Mexico

(Davis and Fargion 1996; Davis et al. 2000). Systematic shipboard and aerial surveys

were conducted within waters from 100 to 2,000 m west of 87°30.0W during the

GulfCet I program and east of 88°10.0'W during the GulfCet II program. In addition,

Gulf of Mexico waters within the U.S. EEZ greater than 100 m deep were surveyed by

ship during the GulfCet II program (Figure 9.1). From these surveys, at least 17 species

of cetaceans that typically inhabit deep, tropical waters were sighted throughout the

oceanic northern Gulf. Sighting rates of cetaceans within this area were, however,

approximately 80% greater east of 90.0°W longitude. Relative minimum abundance

estimates of cetaceans known to occur in the oceanic northern Gulf were calculated from

survey data collected during two survey periods (1991-1994 and 1996-1997) within the

"oceanic northern Gulf' survey area (see Figure 9.1) and are presented graphically in

Figure 9.2. Bottlenose dolphins were the most common species in terms of total

numbers of sightings in the oceanic northern Gulf. Pantropical and spinner dolphins were

the most abundant species, in terms of total numbers of individuals, and occurred in

fewer but much larger groups than bottlenose dolphins (Davis et al. 2000).

Davis et al. 2000 suggest that the spatial and temporal distribution and abundance of

cetaceans within the northern Gulf of Mexico may be strongly influenced by various

circulation patterns. These patterns are generally driven by river discharge, wind stress,

arid the Loop Current. The maj or river system in this area is the Mississippi-Atchafalaya

River (approximately 30% of the Mississippi discharge is diverted upriver into the

Atchafalaya River). Most of the river discharge into the northern Gulf is transported to

the west arid along the coast. Circulation on the continental shelf is largely wind-driven,

with localized effects from freshwater (i.e., river) discharge. Beyond the shelf, mesoscale

circulation is largely driven by the Loop Current in the eastern Gulf. Meanders of the

Loop Current create warm-core anticyclonic eddies (anticyclones) once or twice annually

that migrate westward. The anticyclones in turn spawn cold-core cyclonic eddies

(cyclones). Together, anticyclones and cyclones govern the circulation of the continental

slope in the central and western Gulf (see Chapter 4). The Loop Current and anticyclones

are dynamic features which transport large quantities of high-salinity, nutrient-poor water

across the near-surface waters of the northern Gulf. Cyclones, in contrast, are dynamic

features which contain high concentrations of nutrients and stimulate localized

production. The combination of input of nutrients into the Gulf from river outflow and

mesoscale circulation features enhances productivity, and thus the abundance of cetacean

prey species such as fish and squid. The dynamics of these oceanographic features in

turn affect the spatial and temporal distribution of prey species and ultimately influence

cetacean diversity, abundance, and distribution (Mullin et al. 1994b; Davis et al. 1999).

Studies conducted during the GulfCet I program demonstrated correlations of the

distributions of cetaceans with certain geomorphic features such as bottom depth or

slope. These studies suggested that bottom depth was the most important variable in

240
1991-1994 - 1996-1997 Species not sighted during these survey periods I 10 100 1,000 10,0

Estimated Minimum Abundance 

Figure 9.2. Minimum abundance estimates for species sighted within oceanic waters during two survey periods (From: D

Northern right whale * Minke whale * Sei whale * Bryde's whale Blue whale * Fin whale * Humpback whale * Pygmy/Dwarf sperm whales Sperm whale I I Mesoplodon spp. beaked whales * Cuvier's beaked whale Pygmy killer whale Short-finned pilot whale I I Risso's dolphin I I 1 Fraser's dolphin Killer whale Melon-headed whale False killer whale I I Pantropical spotted dolphin I I Rough-toothed dolphin I I Clymene dolphin I I Striped dolphin I I Atlantic spotted dolphin I I I - - I I Spinner dolphin Bottlenose dolphin -
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habitat partitioning among cetaceans in the Gulf, and stated that species in the northern

Gulf were differentiated most clearly with bottom depth (Baumgartner 1995; Davis and

Fargion 1996; Davis et al. 1998). For example, GulfCet I and II surveys, along with

other historical surveys and opportunistic sightings of cetaceans within the U.S. Gulf of

Mexico found that only the bottlenose dolphin and Atlantic spotted dolphin were

common inhabitants of the continental shelf. The remaining species of cetaceans known

to regularly occur in the Gulf (with possible exception of the Bryde's whale) were sighted

on the continental shelf edge and slope (Mullin et al. 1 994b; Jefferson 1995).

A major objective of the subsequent GulfCet II program was to correlate a number of

enviroimiental parameters, such as selected hydrographic features, with cetacean

sightings data in an effort to characterize cetacean habitats in the Gulf of Mexico (Davis

et al. 2000). From GulfCet II surveys, sightings of cetaceans along the slope were

generally concentrated in cyclones (cold core rings) where production (in this case,

measured chlorophyll concentration) was elevated. Sightings of these deepwater species

were much less frequent in water depths greater than 2,000 in and in anticyclones (warn

core rings). Sperm whales tended to occur along the mid-to-lower slope, near the mouth

of the Mississippi River and, in some areas, in cyclones and zones of confluence between

cyclones and anticyclones. From these data it was suggested that the greater densities of

cetaceans sighted along the continental slope, rather than abyssal areas, of the northern

Gulf probably result from localized conditions of enhanced productivity, especially along

the upper slope, and as a result of the collisions of mesoscale eddies with the continental

margin (Davis et al. 2000).

In the north-central Gulf, the relatively narrow continental shelf south of the Mississippi

River delta may be an additional factor affecting cetacean distribution, especially in the

case of sperm whales (Davis et al. 2000). Outflow from the Mississippi River mouth

transports large volumes of low salinity, nutrient-rich water southward across the

continental shelf and over the slope. River outflow may also be entrained within the

confluence of a cyclone-anticyclone eddy pair and transported beyond the continental

slope. In either case, this input of nutrient-rich water leads to a localized deep-water

environment with enhanced productivity and may explain the presence of a resident

population of sperm whales within 50 km of the Mississippi River delta in the vicinity of

the Mississippi Canyon.

Temporal variability in the distribution of cetaceans in the northern Gulf of Mexico may

also be primarily dependent upon the extent of river discharge and the presence and

dynamic nature of mesoscale hydrographic features such as cyclones. Consequently, the

distribution of cetacean species will change in response to the movement of prey species

associated with these hydrographic features. GulfCet I and II survey data determined that

most of the cetacean species that were routinely or commonly sighted in the northern

Gulf apparently occur in these waters throughout the year, although seasonal abundances

of certain species or species assemblages in slope waters may vary (Baumgartner 1995;

Davis and Fargion 1996; Davis et al. 1998, 2000). For example, comparisons of summer

and winter abundances of commonly sighted species from slope waters suggested that

dwarf/pygmy sperm whales and pantropical spotted dolphins may be more abundant

242
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during summer, whereas Risso's dolphins may be more abundant during winter. In the

case of sperm whales, sightings data indicated that concentrations of whales would shift

spatially in association with the presence and dynamics of mesoscale features.

Concentrations of sperm whales were sighted near the mouth of the Mississippi River

during periods when this area was strongly cyclonic (late summer 1996). This

concentration of whales subsequently shifted 100 to 200 km due east over the De Soto

Canyon in mid-summer 1997 in response to a similar shift in the relative position of the

cyclone (Davis et al. 2000).

Sea Turtles

Five sea turtle species are known to inhabit the waters of the Gulf of Mexico (Table 9.2)

(Pritchard 1997). These are the loggerhead, green, hawksbill, Kemp's ridley, and

leatherback turtles.

As a group, sea turtles possess elongated, paddlelike forelimbs that are substantially

modified for swimming and shells that are depressed and streamlined (Marquez 1990;

Ernst et al. 1994; Pritchard 1997). They depend on the land only during the reproduction

period, when females emerge to nest on sandy beaches. They are long-lived and

slow-maturing. Generally, their distributions are primarily circumtropical, although the

various species differ widely in their seasonal cycles, geographical ranges, and behavior.

There are also considerable differences in behavior among populations of the same

species (Marquez 1990).

Most sea turtles (except perhaps the leatherback) exhibit differential distributions among

their various life stages (hatchling, juvenile, adult) (Marquez 1990; Hirth 1997; Musick

and Limpus 1997). After reaching the sea, hatchling turtles actively swim directly away

from the nesting beach until they encounter zones of water mass convergence and/or

sargassum rafts, which are rich in prey and provide shelter (NMFS and FWS 1991b;

Hirth 1997). Most then undergo a passive migration, drifting pelagically within

prevailing current systems such as oceanic gyres. After a period of years (the number of

which varies among species), the juveniles actively move into developmental habitats

within shallow waters of the inner continental shelf. When approaching maturity,

subadult juvenile turtles move into adult foraging habitats that, in some populations, are

geographically distinct from their juvenile developmental habitats (Musick and Limpus

1997).

All sea turtle species that inhabit the Gulf are listed as either endangered or threatened

species under the Endangered Species Act of 1973 (Pritchard 1997). It is believed that

human activities are the cause of the collapse of sea turtle numbers. These activities

impact every stage of their life cycle and include the loss of nesting beach and foraging

habitats; harvesting of eggs and adults for consumption; incidental mortalities at sea

through pelagic and ground fishing practices; and harm or mortality from increasing

loads of nonbiodegradable waste and pollutants (Lutcavage et al. 1997).

243
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Table 9.2. Sea turtles of the Gulf of Mexico

Scientific Name Common Name Statusa

Family Cheloniidae

Caretta caretta Loggerhead turtle T

Chelonia mydas Green turtle T!E'

Eretmochelys iinbricata Hawksb ill turtle B

Lepidochelys kempi Kemp's ridley turtle E

Family Dermochelyidae

Dermochelys coriacea Leatherback turtle E

a Status: E endangered, T = threatened under the Endangered Species Act of 1973. b Green sea turtles are threatened except for Florida breeding populations, which are listed as endangered.

Species Accounts

The loggerhead (Caretta caretta) is a large sea turtle that inhabits the continental shelves

and estuaries of temperate and tropical environments of the Atlantic, Pacific, arid Indian

Oceans. Typically, this species wanders widely throughout the marine waters of its range

and is capable of living in varied environments for a relatively long time (Marquez 1990;

NMFS and FWS 1991b; Ernst et al. 1994). They may remain dormant during winter

months, buried in moderately deep, muddy bottoms (Marquez 1990). Loggerheads are

carnivorous and, though considered primarily predators of benthic invertebrates, are

facultative feeders over a wide range of food items (Ernst et al. 1994). Loggerheads are

considered to be the most abundant sea turtle in the Gulf of Mexico (Dodd 1988).

Loggerhead nesting along the Gulf Coast occurs primarily along the Florida panhandle,

although some scattered nesting has been reported along the Texas and Alabama coasts

as well (NMFS and U.S. Fish and Wildlife Service [EJSFWSJ 1991b). The loggerhead is

currently listed as a threatened species.

The green turtle (Chelonia mydas) is the largest hardshell turtle and considered to be a

circumglobal species. They are commonly found throughout the tropics and as stragglers

in a far more extensive area, generally between 40°N and 40°S latitudes (NIV]1FS and

USFWS 1991 a; Hirth 1997). In the continental United States, they are found from Texas

to Massachusetts. Green turtles are onmivorous; adults prefer feeding on plants but

juveniles and hatchlings are more carnivorous (Ernst et al. 1994; Hirth 1997). The adult

feeding habitats are beds or pastures of seagrasses and algae in relatively shallow,

protected waters; juveniles may forage in areas such as coral reefs, emergent rocky

bottom, sargassum mats, and in lagoons and bays. Movements between principal

foraging areas and nesting beaches can be extensive, with some populations regularly

carrying out transoceanic migrations (Hirth 1997; Ernst et al. 1994; NMFS and FWS

1991a). Green turtles occur in some numbers over grass beds along the south Texas coast

and the Florida Gulf coast. Reports of nesting along the Gulf of Mexico coast are

infrequent, and the closest significant nesting aggregations are along the Florida east

coast and the Yucatan Peninsula (NMIFS and USFWS 1991a). The green turtle is

currently listed internationally as a threatened species internationally and as an

endangered species in Florida.

244
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The hawksbill (Eretmochelys imbricata) is a small to medium-sized sea turtle that occurs

in tropical to subtropical seas of the Atlantic, Pacific, and Indian Oceans. In the

continental United States, the hawksbill has been recorded in all the Gulf states and along

the Atlantic coast from Florida to Massachusetts, although sightings north of Florida are

rare. They are considered to be the most tropical of all sea turtles and the least conimonly

reported sea turtle in the Gulf of Mexico (Marquez 1990; Hildebrand 1995). Coral reefs

are generally recognized as the resident foraging habitat for juveniles and adults. Adult

hawksbills feed primarily on sponges and demonstrate a high degree of feeding

selectivity on a relatively limited number of sponge species (primarily demosponges)

(Ernst et al. 1994). Nesting within the continental United States is limited to southeastern

Florida and the Florida Keys. Juvenile hawksbills show evidence of residency on

specific foraging grounds, although some migrations may occur (NMFS and USFWS

1993). Some populations of adult hawksbills undertake reproductive migrations between

foraging grounds and nesting beaches (Marquez 1990; Ernst et al. 1994). The hawksbill

is presently listed as an endangered species.

The Kemp's ridley (Lepidochelys kempi) is the smallest sea turtle. This species occurs

mainly in the Gulf of Mexico and along the northwestern Atlantic coast as far north as

Newf'oundlarid. Juveniles and adults are typically found in shallow areas with sandy or

muddy bottoms, especially in areas of seagrass habitat. Kemp's ridleys are carnivorous

and feed primarily on crabs, though they also feed on a wide variety of other prey items

as well (Marquez 1990; NMFS and USFWS 1992; Ernst et al. 1994). The major Kemp's

ridley nesting area is near Rancho Nuevo, along the northeastern coast of Mexico

(Tamaulipas). Some scattered nesting also has been reported in other areas of Mexico

and Texas, Colombia, Florida, and South Carolina (Ernst et al. 1994). Adult Kemp's

ridleys exhibit extensive internesting movements, but appear to travel relatively near the

coast, especially within shallow waters along the Louisiana coast. The Kemp's ridley is

currently listed as an endangered species.

The leatherback (Dermochelys coriacea) is the largest and most distinctive living sea

turtle. This species possesses a unique skeletal morphology, most evident within its

flexible, ridged carapace, and in cold water maintains a core body temperature several

degrees above ambient. They also have unique deep diving abilities. This species is also

the most pelagic and most wide-ranging sea turtle, undertaking extensive migrations from

the tropics to boreal waters. Though considered pelagic, leatherbacks will occasionally

enter the shallow waters of bays and estuaries. Leatherbacks feed primarily on gelatinous

zooplankton such as jellyfish, siphonophores, and saips, though they may, perhaps

secondarily, ingest some algae and vertebrates (Ernst et al. 1994). Data from analyses of

leatherback stomach contents suggest that they may feed at the surface, nocturnally at

depth within deep scattering layers, or in benthic habitats. Florida is the only site in the

continental U.S. where the leatherback regularly nests (NMFS and USFWS 1992; Ernst

et al. 1994; Meylan et al. 1995). The leatherback is currently listed as an endangered

species.

245
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Distribution and Abundance in the Oceanic Northern Gulf of Mexico

As in the case of cetaceans, most historical surveys on the distribution and abundance of

sea turtles within the Gulf of Mexico were conducted within coastal waters and on the

continental shelf In 1981, aerial surveys were conducted by the USFWS within

relatively small, widely separated blocks which encompassed continental shelf and

limited continental slope waters within the central and western Gulf (Fritts et al.

1 983a,b). In 1989, systematic aerial surveys were initiated within selected survey blocks

over the continental slope off Louisiana and Mississippi by NMFS (Mullin et al. 1991,

1 994b). Subsequent, shipboard surveys within a broad area of the oceanic northern Gulf

were conducted by NMFS in the springs of 1990 and 1991 (Jefferson 1995). These early

study efforts provided the first data pertaining to the distribution and abundance of sea

turtles within the OCS and oceanic waters of the northern Gulf of Mexico but provided

limited information regarding geographic coverage (Fritts et al. 1 983a,b; Mullin et al.

1991, 1 994b) or variability in seasonal distributions of sea turtles (NMFS shipboard

surveys) (Jefferson 1995). Systematic aerial and shipboard surveys conducted by Texas

A&M University during the GulfCet I and II programs represent the most recent

assessments of sea turtle distribution and abundance within the oceanic northern Gulf of

Mexico (Davis and Fargion 1996; Davis et al, 2000).

During the GulfCet I and II surveys, only two species of sea turtles were sighted within

the oceanic northern Gulf: loggerheads and leatherbacks (Davis et al. 2000). Other

species of chelonid sea turtles (all species other than the leatherback turtle) were not

sighted or identified on the slope during these surveys. However, because of the

extensive geographic ranges of these sea turtle species and their known migratory

movements between feeding and reproductive habitats, their absence in these survey data

does not preclude the possibility of their presence in these deepwater areas. In addition,

the presence and movements ofjuvenile turtles, especially Kemp's ridleys, within the

oceanic waters of the Gulf of Mexico remains unknown.

GulfCet I and II surveys found the abundance of sea turtles in the Gulf of Mexico

significantly greater within the eastern Gulf; east of Mobile Bay (Lohoefener et al. 1990;

Davis et al. 2000). Sightings of loggerhead turtles were significantly higher on the

continental shelf than the slope. The overall abundance estimate of loggerheads from

GulfCet II surveys was 147 individuals. Loggerheads over the slope were about 12 times

more abundant in winter than summer. This difference was considered statistically

significant. There were also winter sightings of individual loggerheads over very deep

waters (>1,000 m). The significance of the oceanic Gulf to loggerheads was not

identified from these surveys, though it was suggested that they may traverse these waters

in order to reach distant foraging sites or to seek wanner waters during winter (Davis et

al. 2000). Historic sightings data suggest that leatherback turtles appear to spatially

utiflze both shelf and slope habitats in the Gulf of Mexico (Fritts et al. 1983a,b; Collard

1990; Davis and Fargion 1996). The overall abundance estimate of leatherbacks on the

slope from GulfCet II surveys was 168 individuals. Leatherback densities were about 2.5

times more abundant in summer than winter surveys, though this difference was not

considered statistically significant. Disjunct summer and winter distributions of

leatherbacks suggest that specific areas may be important to this species, either

246
Viada: Protected Species

seasonally or for short periods of time. For example, GulfCet I and II survey data show

that the region from Mississippi Canyon to De Soto Canyon, especially near the shelf

edge, may be an important habitat for this species. In addition, the high variability in the

relative numbers of individual leatherbacks sighted within specific areas, such as

concentrations of leatherbacks sighted during GulfCet I surveys, suggest that their

distributional patterns were irruptive in nature (Davis et al. 2000).

Sensitivity to Oil and Gas Exploration and Development

Maj or impact producing factors from deepwater oil and gas exploration and development

on marine mammals and sea turtles include degradation of water quality resulting from

operational discharges; noise from platforms, driliships, service vessels, helicopters, and

seismic surveys; oil spills and oil spill response activities; discarded debris from OCS

structures and service vessels; and collisions with service vessels. Sea turtles may be

attracted to fixed offshore petroleum platforms and consequently may be at greater risk of

impacts from OCS activities than marine mammals (Lohoefener et al. 1990).

The major operational discharges generated during offshore oil and gas exploration and

development include drilling fluids, cuttings, and produced waters. Many operational

discharges include components or compounds that may be injurious to marine mammals

and sea turtles. However, most waste fluids are treated prior to discharge, and plumes of

released wastes mix rapidly with ambient seawater and are thus diluted. These

discharged fluids may, however, have sublethal effects on marine mammals or sea turtles

either indirectly, as a result of their impacts on prey species (reduction in prey), or

possibly directly through prolonged exposure to the discharge or through the ingestion of

affected prey species (National Research Council 1983; American Petroleum Institute

1989; Kennicutt 1995).

Fixed production facilities and drillships, and OCS-related helicopter and service-vessel

traffic produce a broad array of sounds at frequencies and intensities that may be detected

by marine mammals and sea turtles (Geraci and St. Aubin 1987). These sounds could

directly and adversely affect cetaceans by physically injuring their hearing; producing

behavioral or physiological disturbances which may disrupt normal activities or lead to

short- or long-term displacement from areas which may be important for feeding or

reproduction; or masking their ability to utilize (i.e., receive) sounds produced for

echolocation or communication (Richardson et al. 1995). Sound may also disperse

potential cetacean prey species (National Research Cotmeil 1994). Studies involving

turtle hearing sensitivity or noise-induced stress are limited (Geraci and St. Aubin 1987).

Impacts related to OCS-related sounds on sea turtles may include behavioral disruption,

temporary or permanent displacement from the area near the sound source, and potential

long-term effects such as hearing loss.

Seismic activity may affect the distribution of marine mammals and sea turtles. During

the GulfCet I field surveys, it was noted that groups of sperm whales were displaced from

or moved away from an area for several days subsequent to the onset of a seismic survey

(Mate et al. 1994; Davis and Fargion 1996). These observations suggest that this group

247
V/ada: Protected Species

of sperm whales was temporarily displaced as a result of activities associated with the

seiinic survey. Acoustic surveys conduced during the GulfCet II program measured

pulsed signals from a seismic survey vessel. These data suggest that these low frequency

signals (2.5 kHz with little energy below 1 kHz) were probably audible to sperm whales

or mysticete whales in the area. In addition, acoustic and visual data were examined

from periods when seismic surveys were underway in an effort to detect whether these

seismic signals affected cetacean distribution. Results from these analyses suggested that

there were no significant differences between overall sighting rate (and distribution) and

sound level for cetaceans, including sperm whales (Davis et al. 2000). However, seismic

signals used during this study were measured from distant vessels and the received

seismic pulses were thus relatively low in intensity and frequency. Many cetaceans and

sea turtles habituate to low level background noise and coexist with human activities.

1-Jistorical records show the presence of sperm whales near the mouth of the Mississippi

over a period of many years. During this period, this area has been exposed to a variety

of sources of anthropogenic noise and recently is the site of intense oil and gas industry

activity and heavy seismic exploration. The continued presence of sperm whales near the

mouth of the Mississippi River suggest either tolerance or insensitivity to various sources

of anthropogenic noise in sperm whales (Davis et al. 2000).

OCS service helicopters and surface vessels can affect cetaceans and sea turtles from

machinery noise and/or visual disturbances (Richardson et al. 1995). The degree of

impacts associated with helicopter and service vessel traffic appear to be highly variable,

though transient, and cause short-term behavioral changes such as disruption of activities

or departure from the area of disturbance (Davis and Fargion 1996). Areas with heavy

vessel traffic may be avoided by cetaceans and sea turtles, although generally most

cetaceans exhibit considerable tolerance to ship and aircraft noise.

Spilled oil may affect marine mammals and sea turtles through various pathways: direct

contact; inhalation of oil or related volatile distillates; ingestion of oil (directly, or

indirectly through the consumption of oiled prey species); and impairnient of feeding by

fouling of baleen (in mysticetes) or ingestion of floating tar by turtles (Geraci 1990).

Studies have shown that direct contact of oil with sensitive tissues such as eyes and other

mucous membranes produce irritation and inflammation. Cetacean skin may also

experience irritation when exposed to oil or petroleum products such as fuels in high

concentrations or long exposure (Geraci and St. Aubin 1987). However, under less

extreme exposures (concentrations or durations) or contact, oil does not appear to readily

adhere to or be absorbed through cetacean skin. Cetacean skin may therefore serve

somewhat as a barrier to substances such a allergenic hydrocarbons in the marine

environment (Harvey and Dahiheim 1994). Nevertheless, cetaceans observed in or

within the proximity of surface oil associated with the Exxon Valdez spill showed no

evidence of avoidance or abnormal behavior when swimming near or within oil. Their

lack of response may then subject them to increased levels of exposure to spilled surface

oil (Harvey and Dahlheim 1994). Oil can adhere to turtle skin or shells; however, no

evidence of resultant tissue damage exists. Marine mammals or turtles surfacing within

or near an oil spill may inhale petroleum vapors. Small doses of oil, when aspirated,

have been shown to cause acute fatal pneumonia in mammals. Studies on effects of

248
Viada: Protected Species

petroleum vapors on tenestrial mammals and seals showed (in cases of prolonged

exposures and high concentrations) absorption of hydrocarbons in organs and other

tissues, and damage to the brain and central nervous system. However, short-term

inhalation of petroleum vapors at concentrations similar to those found in oceanic oil

spills may not be necessarily detrimental either in terms of structural tissue damage or

respiratory gas exchange. Sea turtles have shown apneic response when confronted with

disagreeable odors and may thus be able to minimize their exposure to inhaled petroleum

vapors. Ingested oil, particularly the lighter fractions, can be toxic to marine mammals

and sea turtles. Ingested oil can remain within the gastrointestinal tract and be absorbed

into the bloodstream and thus irritate and/or destroy epithelial cells in the stomach and

intestine. Certain constituents of oil, such as aromatic and polycyclic aromatic

hydrocarbons, include some well-known carcinogens. These substances, however, do not

show significant biomagnification in food chains and are readily metabolized by many

organisms. Hatchling and juvenile turtles feed opportunistically at or near the surface in

oceanic waters, and are especially sensitive to spilled oil and oil residues such as floating

tar (Lutz and Lutcavage 1987; Lutcavage et al. 1995). Tar found in the mouths of turtles

may have been selectively eaten or ingested accidentally while feeding on organisms or

vegetation bound by tar (Geraci and St. Aubin 1987; Geraci 1990). Spilled oil may also

foul the baleen fibers of mysticete whales, thereby impairing food-gathering efficiency or

result in the ingestion of oil or oil contaminated prey (Geraci and St. Aubin 1987). Oil

spill response activities that may affect both cetaceans and sea turtles involve the

application of dispersant chemicals to spilled surface oil. These dispersant chemicals

contain toxic constituents that are considered to be low when compared to toxic

constituents of spilled oil (Wells 1989). There are currently little available data regarding

the effects of oil dispersants or coagulants on marine mammals or sea turtles (Tucker and

Associates, Inc. 1990). Certain species or stocks of marine mammals or sea turtles may

be at greater potential risk from spilled oil, based on their relative exposures. These

include those species or stocks that may inhabit or frequent restricted areas such as bays

and estuaries (e.g., coastal bottlenose dolphins or Kemp's ridley turtles), or those with

particular feeding strategies or a dependence on selected localized habitats for feeding,

shelter, or reproduction (e.g., surface-feeding baleen whales, sperm whales off the

Mississippi River mouth, and post-hatchling sea turtles in offshore debris lines

[convergence zones]) (Würsig 1990; B.E. Witherington 2000, personal communication,

Florida Marine Research Institute, Melbourne Beach, FL).

Ingestion of; or entanglement with, solid debris can adversely impact marine mammals

and sea turtles (Can 1987). Ingestion of plastic debris can impact the alimentary canal or

remain within the stomach. Certain species of sea turtles, such as loggerhead and

leatherback turtles, appear to ingest more debris than other species (Lutcavage et al.

1995). Recent survey data suggest that these species are the most commonly sighted

adult sea turtles in the oceanic waters of the Gulf of Mexico (Davis et al. 2000).

Entanglement in plastic debris can result in reduced mobility, drowning, and constriction

of and subsequent damage to limbs (Lutcavage et al. 1995). Currently, the discharge or

disposal of solid debris from both OCS structures and vessels is prohibited by the MMS

to lessees (30 CFR 250.40) and the U.S. Coast Guard (MARPOL, Annex V, Public Law

249
V/ada: Protected Species

1OQ-220 [101 Statute 1458]). Therefore, debris from OCS activities is not expected to

significantly adversely affect marine mammals or sea turtles in the Gulf.

Increased service vessel traffic associated with offshore OCS activities may increase the

probability of collisions between vessels and cetaceans and sea turtles, especially at night

and during periods of reduced visibility. Certain marine mammals, such as deep diving

species that spend extended periods of time at the surface (such as sperm whales), may be

particularly vulnerable to collisions with offshore vessels. Because of their relatively

small size and dark coloration on their dorsal surfaces (especially in the case of rafting

hatchlings and small juveniles), sea turtles resting on the surface or just below the surface

may be difficult to sight from moving vessels. Individual turtles that may be attracted to

offshore platforms are also at greater risk of collision with OCS service vessels.

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254
Deepwater Fishes 255

Introduction 255

History of Deepwater Fish Sampling in the Gulf of Mexico 257

Epipelagic Fishes 258

Mesopelagic Fishes 260

Bathypelagic Fishes 264

Demersal Fishes 264

Interactions among Fishes of Different AssenThiages 270

Deepwater Fisheries 270

Introduction 270

History of Deepwater Fisheries in the Gulf of Mexico 271

Key Species 272

Fishing Gear Types 272

Landings Trends 275

Literature Cited 278

Chapter 10: Fishes and Fisheries

David B. Snyder

Deepwater Fishes

Infroduction

The Gulf of Mexico with its great variety of estuarine, coastal, and shelf habitats supports

a diverse ichthyofaima. Several general reviews, compilations, and taxonomic guides

exist for fishes of the inshore and shelf regions of the Gulf (Rivas 1954; Briggs 1973;

Wails 1975; Darnell and Kleypas 1987; Hoese and Moore 1998). But until recently,

there have been no comprehensive taxonomic or ecological accounts of the Gull's

deepwater fishesthose inhabiting the pelagic and demersal habitats beyond the

continental shelf break. Potts and Ramsey (1987) produced a guidebook for deepwater

fishes of the Gulf of Mexico, and more recently, McEachran and Fechhelm (1998)

published the first of a two volume systematic summary of Gulf of Mexico fishes that

included deepwater species. With this taxonomic foundation now in hand, the stage is set

for an ecological summary of knowledge concerning Gulf of Mexico deepwater fishes.

This section reviews the available information on the taxonomic composition and

ecological characteristics of fish assemblages inhabiting pelagic and demersal

components of the Gulf of Mexico deepwater (>200 m) environment. The pelagic

component may be further subdivided into three depth-related layers: epipelagic (0 to

200 m), mesopelagic (200 to 1,000 m), and bathypelagic (>1,000 m) (Figure 10]).

These subdivisions reflect to some degree concomitant patterns in physical environmental

variables such as light, temperature, and dissolved oxygen. In some cases, the distinction

between mesopelagic and bathypelagic is blurred, so the term midwater is used when

referring to these two zones collectively. The dernersal component simply consists of the

255
DEPTH (in meters) 10o0 -2000 3000... Cantin- I EPIPELAGJCMESOPELAGIC ental CoriIlnerIlIr I Continenlel shelt Slope Rise ..4000 BENTHOPELAGICZONES BE NJTH ICBATH VP E LAG IC =DEMERSAL LIGHT TEMPERA

0 5 O0 Permenerl T

- 1 1 I ABY

Figure 10.1. Diagram of the environmental framework used to describe deepwater fishes of the Gulf of Mexico (Adapte

etal. 1997).
257

Snyder: Fishes and Fisheries

seafloor seaward of the continental shelf break (or about 200 m) including, where

applicable, the continental slope, continental rise, and abyssal plain. This general

pelagic/demersal scheme has been effectively used to frame discussions of deep-sea

fishes in other regions (e.g., Marshall 1979; Haedrich 1997; Merrett and Haedrich 1997).

History of Deepwater 1ish Sampling in the Gulf of Mexico

Biological sainpling in the deep-sea is costly and time-consuming, requiring large vessels

equipped with specialized gear. Despite the onerous nature of deepwater investigations,

numerous sampling programs have been conducted in the Gulf of Mexico over the past

100 years that either directly or indirectly targeted deepwater fishes. During the first

such program, which extended from 1877 to 1880, A. Agassiz aboard the U.S. Coast

Survey steamer BLAKE collected the first samples of deepwater (to 3,658 rn) fishes from

the Gulf of Mexico (Pequegnat 1976). These efforts consisted mainly of bottom trawling

and dredging, but midwater fishes were collected as trawls were retrieved through the

water column. In 1884, the U.S. Fish Commission vessel ALBATROSS also made some

deepwater collections in the Gulf of Mexico. Goode and Bean (1896) later described

many of the fishes collected in Gulf waters by the BLAKE and the ALBATROSS. With

the exception of a few collections made near the Dry Tortugas in the late 193 0's by

W.H. Longley (Longley and Hildebrand 1941), there was a hiatus in sampling of the deep

Gulf until the 1950's. At this time, the Bureau of Commercial Fisheries vessel OREGON

and to a lesser extent the vessels SILVER BAY, COMBAT, and PELICAN began

exploratory fishing in the Gulf of Mexico. Although these vessels mostly sampled shelf

waters, some deepwater stations were sampled with pelagic and boftom tending gear

types, and these efforts continue today with the NMFS vessel OREGON II. Most of the

early efforts were under the direction of H.R. Bullis and S. Springer (Springer and Bullis

1956; Bullis and Thompson 1965). Many of the fishes collected during these surveys

contributed to publications on deepwater demersal fishes (Grey 1956, 1958, 1959; Nelson

and Carpenter 1968; Gutherz 1967; Rohr and Gutherz 1997) and epipelagic fishes (Bullis

1955a,b; Iwarnoto 1965).

The next series of deepwater collections were made from 1964 to 1973 under the

supervision of W.E. Pequegnat of Texas A&M University (Pequegnat 1976, 1983).

Pequegnat's sampling program, conducted aboard the research vessel ALAMINOS,

primarily focused on the demersal environment. Bottom trawls, dredges (including a

skimmer dredge), and benthic photographs were taken at 264 stations in water depth

ranging from 81 to 3,493 m across the northern Gulf of Mexico. Samples collected

during these cruises provided an information base from which several dissertations and

theses were produced, some of which focused directly (Bright 1968; Rayburn 1975;

Martin 1978) or indirectly (Roberts 1977) on fishes. Also during the same period, 38

midwater (0 to 3,600 m) fish collections were made throughout the Gulf of Mexico with

an Isaacs-Kidd midwater trawl (Bright and Pequegnat 1969; Murdy et al. 1983). Other

collections of midwater fishes were made in the 1960's by Russian and Cuban

investigators aboard the ALEXANDER KOVALEVSKTI (Rass 1971) and by Woods

Hole Oceanographic Institution (Backus et al. 1977).
Snyder: Fishes and Fisheries

In 1971, Ti. Hopkins at the University of South Florida began a sampling program that

focused on mesopelagic fishes and invertebrates (micronekton). His program continued

for 20 years with 24 sampling cruises made aboard various research vessels to the same

"standard station" in the eastern Gulf of Mexico (27°N Lat, 86°W Long). Using nets

capable of being closed at discrete depths (Tucker trawis and MOCNESS nets), Hopkins

and his students amassed considerable information on species composition, life histories,

vertical migration, and feeding ecology of mesopelagic fishes (e.g., Hopkins and Lancraft

1984; Gartner et al. 1987; Sutton and I-Iopkins 1996a,b; Hopkins et al. 1997).

Finally, in the early 1980's the MMS funded a series of studies investigating the

continental slope biota of the northern Gulf of Mexico. These studies were conducted by

LGL Ecological Research Associates, Inc and Texas A&M University (LGL and TAMU

1985, 1986; Gallaway 1988; Gallaway et al. 1988). Bottom trawl samples were collected

during five cruises made from 1983 to 1985. In all, 59 stations were sampled along three

downslope transects and along selected depth contours in the western, central, and eastern

Gulf. Information on the density, diversity, distribution, and food habits of demersal

fishes was obtained during the biological sampling portion of these efforts.

Epipelagic Fishes

A diverse assemblage of epipelagic fishes inhabits the euphotic zone, the upper 200 m

layer of the Gulf of Mexico. The most recognizable members of this assemblage are

tunas, billfishes, dolphins, and sharks. Lesser known representatives include driftfishes,

oarfishes, opahs, flyingfishes, and molas. A phylogenetic listing of epipelagic fishes

reported from the Gulf of Mexico is given in Table 10.1. The overall assemblage is

composed of 10 orders with 34 families containing 120 species. The most diverse orders

are Perciformes (perch-like fishes), Beloniformes (flyingfishes and halfbeaks), and

Tetraodontiformes (molas, triggerfishes, and puffers).

As mentioned above, there has been little direct scientific investigation into any aspect of

the epipelagic assemblage. Not surprisingly, much of what is known has been collected

only for the economically important species such as tunas and billfishes. Exploratory

fishing expeditions by the U.S. Bureau of Commercial Fisheries and observations of

commercial longline fisheries have provided information on species composition,

distribution, abundance and behavior of sharks, tunas, and billfishes (Springer 1957;

Iwamoto 1965; Bransetter and McEachran 1983; Anderson 1985, 1990; Russell 1993).

Biological data (size, age, and reproductive condition) on billfishes and tunas have been

gathered by researchers visiting recreational fishing tournaments held along the Gulf

coast (NMFS 1 999a). Recently, J. Franks and colleagues of the Gulf Coast Research

Laboratory have investigated age, growth, reproduction, and stock identity in wahoo

(Acanthocybium solandri) using specimens obtained from recreational fishing

tournaments. Information on spawning of tunas and other species has been inferred from

larval fish collections made in the eastern Gulf (e.g., McGowan and Richards 1989;

Richards et al. 1989).

258
Snyder: Fishes and Fisheries

Table 10.1. Orders and families of epipelagic (0 to 200 m) fishes known from the Gulf of

Mexico, arranged in phylogenetic order

Order Oreetolobiformes

Rhincodontidae (whale shark)

Order Lamniformes

Alopidae (thresher sharks)

Cetorhinidae (basking shark)

Larnnidae (mackerel sharks)

Order Carcharhiniformes

Carcharhinidae (requiem sharks)

Order Myliobatiformes

Dasyatididae (stingrays)

Mobulidae (mantas)

Order Lampridiformes

Lampridae (opah)

Lophotidae (crestfishes)

Regalecidae (oarfishes)

Order Lophiiforrnes

Antennariidae (frogfishes)

Order Beloniformes

Belonidae (needlefishes)

Exocoetidae (flying fishes)

Hemiramphidae (halfbeaks)

The most frequently caught tuna species during exploratory fishing trips were skipjack

(Katsuwonus pelamis), yellowfin (Thunnus albacares), blackfin (T. allanticus), and

bluefin (T thynnus). The highest catches of tunas were made along the 1,000 fm curve

east of the Mississippi delta (Bullis 1955b ; Iwamoto 1965). The most common sharks in

deepwater catches were silky (Carcharhinusfalcformis), spinner (C. brevipinna), and

sandbar (C. plumbeus) (Anderson 1985, 1990; Russell 1993).

The distribution and abundance of epipelagic species is influenced by several interrelated

variables: water temperature, thickness of the isothermic layer, circulation patterns, and

biological productivity (Farm 1968). Fishermen contend that yellowfin tuna aggregate

near sea surface temperature boundaries or water mass convergence zones; however,

Power and May (1991) found no correlation between longline catches of yellowfm tuna

and sea surface temperature in the Gulf of Mexico. A consequence of water mass

convergence zones is the surface accumulation of flotsam and Sargassum (Carr 1986).

This drifting material plays an important ecological role as habitat for many fishes.

Floating seaweed (Sargassum); jellyfishes, siphonophores, and driftwood attract juvenile

and adult epipelagic fishes. Larger predators forage around flotsam. As many as 54 fish

species are closely associated with floating Sargassum at some point in their life cycle,

but only 2 spend their entire lives there: the sargassumfish and the sargassum pipefish

(Adams 1960; Dooley 1972; Bortone et al. 1977). Most fish associated with Sargassum

are temporary residents, such as juveniles of species that reside in shelf or coastal waters

as adults (Dooley 1972; Bortone et al. 1977). However, several larger species of

259

Order Syngnathiformes

Syngnathidae (pipefishes)

Order Perciformes

Echeneidae (shark suckers)

Carangidae (jacks)

Coryphaenidae (dolphins)

Bramidae (pomfTets)

Lobotidae (tripletail)

Kyphosidae (chubs)

Gempylidae (snake mackerels)

Istiophoridae (bilifishes)

Xiphiidae (swordfish)

Scombridae (tunas and mackerels)

Centrolophidae (drififishes)

Nomeidae (man-o-war fish)

Ariommatidae (ariommatids)

Stromateidae (driftfishes)

Order Tetraodontiformes

Monacanthidae (filefishes)

Diodontidae (puffers)

Molidae (molas)
Snyder: Fishes and Fisheries

recreational or commercial importance including doiphinfish, yellowfin tuna, blackfin

tuna, skipjack tuna, Atlantic bonito, little tunny, and wahoo feed on the small fish and

invertebrates attracted to Sargassum (Manooch et al. 1983; Manooch and Mason 1984;

Morgan et al. 1985).

Some epipelagic fishes aJso exhibit another behavioral phenomenon related to flotsam

attraction: affinity for large moored objects. This is often referred to as the FAD (fish

attracting device) effect, and it has been documented in various regions of the world for

tunas, dolphins, and other species (e.g., Holland et al. 1990; Higashi 1994; Relini et al.

1994). In many areas of the world, fishers have taken advantage of this behavior by

deploying FADs to attract tunas and other species. Deepwater oil and gas structures in

the Gulf of Mexico act as large FADs and attract epipelagic fishes of all kinds.

Information on spawning of tunas and other species has been inferred from larval fish

collections made in the eastern Gulf (e.g., McGowan aild Richards 1989; Richards et al.

1989, 1993). Much of the information available on spawning in epipelagic fishes points

to the importance of temperature gradients and mesoscale oceanographic features.

Spawning by bluefin tuna in the Gulf of Mexico appears to be associated with the Loop

Current (McGowan and Richards 1989). Adult bluefin tuna migrate into the Gulf of

Mexico in springtime and spawn. This appears to be the only maj or spawning area for

the western Atlantic bluefin stock (NMFS 1999a). Yellowfin and skipjack tuna also

spawn in the Gulf of Mexico (NMFS 1 999a). Other, noncommercially important species

such as big-eye cigarfish (Cubicepspauciradiatus) also spawn along the edge of the

Loop Current (Richards et al. 1993; Lampkin 1997).

Mesopelagic Fishes

Mesopelagic fishes are generally found from 200 to 1,000 m depths in the water column.

This group is phylogenetically primitive and among the most morphologically specialized

of fishes (Marshall 1979). Their long evolutionary history has presumably allowed for

the adaptive radiation that has occurred in response to the cold, dark midwater

environment (Marshall 1979). Mesopelagic fish assemblages in the Gulf of Mexico are

taxonomically diverse, consisting of eight orders comprising 30 families with 213 species

(Table 10.2). The most specious groups are stomatiids (dragonfishes), myctophids

(lanternfishes), and gonostomatids (bristlemouths). These and all other taxonomic groups

listed in Table 10.2 are entirely restricted to the midwater environment. The Stomiidae

with 73 species are the most diverse family of fishes known for the Gulf of Mexico

(Sutton and Hopkins 1 996a; McEachran and Fechhelm 1998). The second most diverse

group is the myctophids represented by 49 species in the Gulf of Mexico (Backus et al.

1977; Gartner et al. 1987).

T.L. I-Iopkins, his students, and colleagues have contributed most of the information on

mesopelagic fish ecology in the Gulf of Mexico. This group has published data on

species composition, abundance, biomass, vertical migration patterns, reproductive

patterns, and trophic structure for the common families. Common families include

gonostomatids, rnyctophids, sternoptychids (hatchetfish), and stomiids (e.g., Hopkins and

Lancraft 1984; Hopkins and Baird 1985b; Gartner et al. 1987; Gartner 1993; Sutton and

Hopkins 1996a).

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Snyder: Fishes and Fisheries

Table 10.2. Orders and families of mesopelagic and bathypelagic fishes known from the

Gulf of Mexico, arranged in phylogenetic order (following McEachran

and Fechhelrn 1998)

MESOPELAGIC (200-1,000 m)

Order Anguillifoi-mes

Nernichthyidae (snipe eels)

Order Osmeriformes

Microstomatidae (niicrostomatids)

Order Stomiiforrnes

Gonostomatidae (bristlernouths)

Sternoptychidae (marine hatchetfislies)

Phosichthyidae (lightfishes)

Astronestbidae (snaggletooths)

Chauliodontidae (viperfishes)

Idiacanthidae (black dragonfishes)

Malacosteidae (loosejaws)

Melanostomidae (scaleless dragonfishes)

Stomiidae (scaly dragonfishes)

Order Aulopiformes

Giganturidae (giganturids)

Aulopidae (flagfins)

Scopelarchidae (pearleyes)

Alepisauridae (lancetfishes)

Everrnannellidae (sabertooth fishes)

Paralepididae (barracudinas)

Order Myctophiformes

Myctophidae (lanternfishes)

Neoscopelidae (neoscopelids)

Order Lampridiformes

Stylephoridae (tube-eye)

Trachipteridae (ribbonfishes)

MIESOPELAGIC (200-1,000 m) (continued)

Order Stephanoberyciformes

Gibberichthyidae

Melamphaidae (bigscales)

Stephanoberycidae

Barbourisiidae

Cetornirnidae (whalefishes)

Mirapinnidae

Rondeletiidae

Order Berycifonnes

Anoplogasteridae (fangtooth)

Diretmidae (spinyfins)

BATHYPELAGIC (>1,000 m)

Order Osmeriformes

Argentiuidae (argentines)

Bathylagidae (deep-sea smelts)

Opisthoproctidae (spookfishes)

Platytroctidae (tubeshoulders)

Order Aulopiformes

Notosudidae

Ornosudidae

Order Gadiformes

Melanonidae (pelagic cods)

Order Lophiiformes

Ceratiidae (seadevils)

Diceratiidae (deep-sea anglers)

Himatolophidae (football fishes)

Linophrynidae

Melanocetjdae

Oneirodidae

Mesopelagic fishes tend to be small and can be extremely abundant; they are often

responsible for the deep scattering layer in sonar images of the deep sea. Gonostomatids

(bristlernouths) numerically dominated mesopelagic fish collections in the Gulf of

Mexico (Murdy et al. 1983; Hopkins and Lancraft 1984). The genus Cyclothone

contributed 34% of the numbers (ito 2 fishm2) of micronekton in the upper 1,000 m

(I-Iopkins and Lancraft 1984). Another species, Gonostoma elongatuin, exhibited a

density of 0.2 fishm2 and standing stock of 75.6 mg-m2 (dry weight) (Lancraft et al.

1988). The second most abundant group was the myctophids (lanternfishes). The most

abundant myctophid species in the eastern Gulf were Ceratoscopelus warmingii,

Notolychnus valdiviae, Lepidophanes guentheri, Lampanyctus alatus, Diaphanus

dumerilii, Benthosema suborbitale, and Mycrophum affine (Gartner et al. 1987). The

most common sternoptychids (hatchetfishes) collected were Argyropelecus aculeatus,

A. hetnigymnus, Sternoplyx diaphana, arid S. psuedobscura (Hopkins and Baird 1985b).

These four species ranged in abundance from 0.02 1 to 0.053 fishm2 in the upper

261
Snyder: Fishes and Fisheries

1,000 m (Hopkins and Baird 1 985b). For the entire stomiid assemblage sampled in the

upper 1,000 m of the eastern Gulf, minimum biomass and standing stock estimates were

1.86 individualsm2 and 35.3 kg-kuf2 (dry weight). The most abundant stomiid species

were Photostomias guernei, Chauliodus sloani, and Stomias affinis.

A common characteristic of mesopelagic fishes is the phenomenon of die! vertical

migration. Many species will migrate vertically each right from depths of 400 to 800 rn

into epipelagic waters, often reaching the surface. This phenomenon has been studied for

several of the common Gulf of Mexico species and three basic patterns have been found:

synchronous migration, asynchronous migration, and no migration. Synchronous

migration is when all individuals of a species undergo nocturnal vertical migration.

Individuals of Gonostoma elongatum were found to migrate synchronously, moving from

daytime depths of 425 to 725 m to 25 to 325 m depths at night (Lancrafi et al. 1988).

This species migrated as much as 400 m vertically, but never occurred above the 25 m

layer. Myctophids, also synchronous migrators, spend the daytime in depths of 200 to

1,000 m, but migrate vertically at night into near-surface waters. Individual myctophid

species migrated in patterns that formed a complex series of layers of migrating fishes

that clearly separated into 5 day and 5 night groups (Gartner et al. 1987). Four stomiid

species of the genus Astronesthes also migrated synchronously between 400 to 700 m and

0 to 200 m. There was also circumstantial evidence of bathypelagic (below 1,000 m) to

epipelagic (0 to 300 m) migration by two other stomiid species, Echiostomias barbatum

and Leptostomias bilobatus (Sutton and Hopkins 1996a). The most abundant stomiids

(P. guernei, C. sloani, and S. affinis) were asynchronous vertical migrators; about half of

the individuals migrated from 500 to 900 m depths to 20 to 300 m at night while other

half of the individuals remained at daytime depths (Sutton and Hopkins 1 996a).

Hatchetfishes of the genus Sternoplyx were found not to migrate and remained in 500 to

800 m depths throughout the die! cycle (Hopkins and Baird 1985b). Two species of the

genus Argyropelecus did migrate into the epipelagic zone at night to feed upon

zooplankton.

Feeding cycles correspond closely with vertical migratory patterns in mesopelagic fishes,

and hunger may be an important driving force behind die! migrations. Many midwater

species depend upon the food-rich epipelagic layers to fulfill their daily energy

requirements. In general, three feeding guilds are recognized for mesopelagic fishes:

micronekton feeders, zooplankton feeders, and generalists (Gartner et al. 1997).

Representatives from all of these groups occur in the Gulf of Mexico (Hopkins et al.

1997). Myctophids, sternoptychids, and gonostomatids were the primary zooplankton

feeding taxa. They consumed 31%, 27%, and 14% of the planktonic food biomass eaten

daily by the midwater assemblage (Hopkins et al. 1997). The primary fish-consuming

family was the Stomiidae, accounting for 61% of all fish eaten.

Myctophids fed mostly on crustacean zooplankton (copepods) (Hopkins and Baird 1 985a;

Hopkins and Gartner 1992; Hopkins et al. 1997). The hatchetfish A. aculeatus appeared

to migrate vertically into the epipelagic zone where young stages consumed ostracods

and copepods and larger individuals consumed pteropods and euphausiids. All size

classes of A. hemigymnus foraged at 300 to 500 m on ostracods and copepods. Juvenile

262
Snyder: Fishes and Fisheries

S. diaphana ate copepods, ostracods, and amphipods, and adults consumed mostly

amphipods and euphausiids in water depths of 500 to 800 m. S. heniigymnus remained

below 800 m where juveniles fed upon copepods, polychaetes, and euphausiids, and

adults fed upon amphipods and fishes (Hopkins and Baird 1985b). The gonostomatid G.

elongatum fed mainly upon copepods and ostracods as young individuals, and on

euphausiids as adults mostly at night in the epipelagic zone (Lancraft et al. 1988).

Stomiids proved to be an important upper level group of predators in the pelagic food

web (Sutton and Hopkins 1996a). The stomiid assemblage formed four basic categories

with respect to food habits: myctophid predation, zooplanktonlsmall micronekton

predation, penaeidean shrimp predation, and copepod/rnicronekton predation (Sutton and

I-Iopkins 1 996b). The storniids investigated were selective with respect to prey items

ingested. Stomiids inflict the highest predation impact on myctophids in low-latitude

ecosystems, and historic use of predation-avoidance arguments to explain certain

mesopelagic phenomena (e.g., vertical migration, ventral photophores) appears to be

substantiated. The stomiids may be important in the transport of energy from the

mesopelagic to the bathypelagic and benthopelagic.

Hopkins and colleagues (Gartner et al. 1987; Hopkins and Gartner 1992; Hopkins et al.

1997; Hopkins and Sutton 1998) interpreted much of the observed interplay between

vertical migration and feeding mode in the light of ecological competition among fish

and invertebrate species. Considerable dietary overlap was found among the abundant

myctophid species that inhabit the epipelagic zone at night (Hopkins and Gartner 1992).

When both vertical distribution and diet were considered together, little inter- or

intra-specific overlap occurred. This was construed as a prime example of resource

partitioning that evolved in response to competition during the evolution of the midwater

ecosystem. The high degree of overlap in diet allows species packing in an otherwise

structureless water column (the epipelagic zone at night).

The daily consumption of the eastern Gulf midwater assemblage was estimated to be 2.5

to 4.3 kg Cknf2 in the upper 1,000 m (Hopkins et al. 1997). Most (80%) of this was

provided by zooplankton and the remainder was larger prey, mostly fish. The ingestion

rates accounted for only 5% to 10% of the daily production, but 95% of fish daily

production. These dietary findings agree with similar studies performed in other

oligotrophic regions of the oceans. In Chapter 6 of this report, Biggs and Ressler argue

that for areas other than the eastern Gulf, and where productivity "hot spots" occur, the

pelagic environment is more productive, or mesotrophic.

263
Snyder: Fishes and Fisheries

Bathypelagic Fishes

The deeper dwelling bathypelagic fishes inhabit the water column at depths greater than

1,000 m and seldom migrate into shallower waters. This group is composed of bizarre,

little-known species such as gulper eels, slickheads, deep-sea anglers, bigscales, and

whalefishes (McEachran and Fechhelm 1998). There are 4 orders, 13 families, and

49 species known for the Gulf of Mexico. Many of these species also occur above

1,000 m. Like mesopelagic fishes most species are capable of producing and emitting

light (bioluminescence) to aid in communicating in an environment devoid of sunlight.

Like the mesopelagic fishes, these species display some of the most interesting

evolutionary adaptations to the deep-sea environment. Unfortunately, there have been no

studies directed at bathypelagic fishes in the Gulf of Mexico.

Demersal Fishes

Demersal fishes are those that are either in direct contact with the substrate or hover

above it from the shelf slope transition to the abyssal plain. The taxonomic composition

of the demersal ichthyofauna in the Gulf of Mexico includes 27 orders with 70 families

representing 300 species (Table 10.3). The most diverse order is the Gadiformes

(cod-like fishes) with 7 families and 44 species followed by Anguilliformes (eels) with

5 families and 35 species, Ophidiiformes (brotulas and cusk-eels) with 4 families and

33 species; and Perciformes (perch-like fishes) with 10 families and 28 species.

Representatives of these groups were collected during the MMS-sponsored demersal

sampling programs summarized by Pequegnat (1983) and Gallaway et al. (1988). These

two programs have provided the most comprehensive data available on the demersal

assemblage. The latter program collected samples from eastern, central, and western

transects which allowed some spatial comparisons to be made in species composition,

density, diversity, food habits and depth-related zonation patterns.

The five most abundant species collected during Pequegnat's sampling program were

Gadomus longfihis, Dicrolene intronigra, Synaphobranchus ore goni, Dibranchus

atlanticus, and Nezumia aequalis. The top five species collected by Gallaway et al.

(1988) were Urophycis cirratus, S. oregoni, Coelorhincus caribbaeus, D, atlanticus, and

Bembrops gobiodes. Table 10.4 shows the top species and their depth of maximum

occurrence. The numbers of fish per hectare collected on the three transects were 1,222

(western), 620 (central), and 1,511 (eastern). Densities of demersal fishes were estimated

from photography as 198.5 fishha' during cruises II to V. Densities estimated by

trawling were much less than those estimated by benthic photography; some were as high

as 12.5 times less. Diversity (Shannon's H') declined with depth along all three

transects, but did not differ significantly among the three transects. Some evidence for

differing spatial patterns in species composition was provided by sampling along three

(eastern, central, and western) downslope transects. A basic finding was that the eastern

Gulf supported a more dense and species rich assemblage than the western Gulf

(Gallaway et al. 1988).

264
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Table 10.3. Deepwater (>200 m) demersal fishes known from the

Gulf of Mexico, arranged in phylogenetic order

Order Myxiniformes

Myxinidae (hagfishes)

Order Chimaeriformes

Chimaeridae (ratfishes)

Rhinochimaeridae (longnose raffishes)

Order Orectolobiformes

Odontaspidae (sand tiger sharks)

Order Carcharhiniformes

Scyliorhinidae (catsharks)

Carcharhinidae (requiem sharks)

Triakidae (smoothhound sharks)

Order Hexanchiformes

Hexancbidae (sixgill and sevengill sharks)

Order Squaliformes

Echinorhinidae (bramble sharks)

Squalidae (dogfish sharks)

Order Squatiniformes

Squatinidae (angel sharks)

Order Torpediniformes

Narcinidae (electric rays)

Torpedinidae (torpedo rays)

Order Raj iformes

Rajidae (skates)

Order Myliobatiformes

Dasyatididae (stingrays)

Order Notacanthiformes

Halosatiridae (halosaurs)

Notocanthidae (notacanthid eels)

Order Anguilliformes

Synaphobranchidae (cutthroat eels)

Ophichthidae (snake eels)

Colocongridae

Congridae (conger eels)

Nettastomatidae (duckbill eels)

Serrivornerjdae (sawtooth eels)

Order Osmeriformes

Alepocephalidae (smoothheads)

Order Ateleopodoiformes

Ateleopodidae

Order Aulopiformes

Aulopidae (fiagfins)

Ipnopinae (tripodfishes)

Synodontidae (lizardfishes)

Order Polymixiiformes

Polymixiidae (beardfishes)

Order Ophidiiformes

Carapidae (pearlfishes)

Ophidiidae (cusk-eels)

Aphyonidae

Bythitidae

265

Order Gadiformes

Bregrnacerotidae (codlets)

Phycidae (bakes)

Macrouridae (grenadiers)

Merlucciidae (offshore hakes)

Steindachneriidae (luminous hake)

Moridae (moras)

Order Batrachoidiformes

Batrachoididae (toadfishes)

Order Lophiiforiues

Antennariidae (frogfishes)

Chaunacidae (gapers)

Ogcocephalidae (batfishes)

Thaumatichthyidae

Order Beryciformes

Trachichthyidae (slimeheads)

Berycidae (alfonsinos)

Holocentridae (squirrelfishes)

Order Zeiformes

Grammicocephalidae (diamond dories)

Macrurocyttidae

Parazenidae

Zeidae (dories)

Caproidae (boarfishes)

Order Gasterosteifonnes

Centriscidae (snipefishes)

Syngnathidae (pipefishes)

Order Scorpaeniforrnes

Scorpaenidae (scorpionfishes)

Triglidae (sea robins)

Order Perciformes

Acromatidae

Serranidae (groupers and sea basses)

Epigonidae

Apogonidae (cardinalfishes)

Malacanthidae (tilefishes)

Lutjanidae (snappers)

Zoarcidae (eelpouts)

Uranoscopidae (stargazers)

Percophididae (flatheads)

Callionymidae (dragonets)

Order Pleuronectiformes

Paralichthyidae (flounders)

Bothidae (lefteye flounders)

Cynoglossidae (tonguefishes)

Order Tetraodontiformes

Triacanthodidae (spikefishes)

Monacanthidae (filefishes)
Snyder: Fishes and Fisheries

Table 10.4. Comparison of most abundant species of fish

between the Gallaway et al. (1988) and Pequegnat (1983) studies.

Fish are ranked in order of decreasing abundance

266

Gallaway et al. (1988)

(All Transects)

Pequegnat (1983)

(All Stations)

Species

Depth of

Max. Pop.

(m)

Species

Depth of

Max. Pop.

(m)

Poecilopsetta beani 348 Poecilopsetta beani 250

Bembrops gobiodes 348 Bembrops gobiodes 400

Coelorinchus caribbaeus 348 Coelorinchus caribbaeus 300

Hymenocephalus italicus 348 Hymenocephalus italicus 450

Urophycis cirrata 348 Urophycis cirrata 450

Dibranchus atlanticus 657 Dibranchus atlanticus 650

Nezumia aequalis 657 Nezumia aequalis 900

Synaphobranchus sp. 839 Synaphobranchus sp. 1,000

Gadomus longfihis 1,341 Gadomus 1ongfihis 1,050

Monomitopus sp. 839 Monornitopus sp. 1,050

Dicrolene sp. 1,341 Dicrolene sp. 1,200

Stephanoberyx monae 1,341 Stephanoberyx monae 1,200

Parasudis truculenta 348 Parasudis truculenta 250

Setarchus guentheri 348 Pontinus longispinus 200

Chlorophthalmus agassizif 348 Yarella blackfordi 650

Epigonuspandionis 348 Bathygadus melanobranchus 900

Malacocephalus occidentalis 348 Aidrovandia gracilis 1,450

Peristedion greyae 348 Halosaurus guentheri 900
267

Snyder: Fishes and Fisheries

Despite problems with examining stomach contents of fishes brought to the surface from

great depths (i.e., stomach eversion, regurgitation, rapid decomposition of stomach

contents), several investigators have examined food habits in deepsea fishes (Bright

1970; Rayburn 1975; Gallaway et al. 1988). The specimens examined consumed a wide

range of organisms including fishes, epifauna, infauna, rneiofauna, and planktonic

invertebrates. Fishes were important in the diets of the eel S. oregoni, and greeneye

Chlorophthalmus agassizi. The stomachs of three common species, B. gob iodes,

G. ion gJilis, and N. aequalis contained mostly natant crustaceans (shrimps). The batfish

D. àtianticus consumed pagurids (hermit crabs) and polychaete worms, whereas

Coelorhincus caribbaeus fed mostly upon polychaetes. Rayburn (1975) found that

Nezumia hildebrandi, Halosaurus guntheri, D. intronigra, and Cariburus zaniophorus

fed upon infauna, predominantly polychaetes and amphipods. Bathygadus vailianti

consumed calanoid copepods and infauna. Monomitopus agassizii ate ostracods,

copepods, and amphipods.

A central focus of the dernersal slope studies (Pequegnat 1983; Gallaway et al. 1988) was

the analysis of depth distribution of the megafaunal (larger invertebrates and demersal

fishes) assemblage as a whole. Like others working in similar environments elsewhere in

the world (e.g., Gage and Tyler 1996), the Gulf investigators suggested that species

assemblages form discrete depth-related groups along the continental slope and upper

abyssal plain (Pequegnat 1983; Gallaway and Kennicutt 1988; Pequegnat et al. 1990),

Fishes figured prominently in the formation of depth-related faunal zones, and when

analyzed separately, fish data seemed to agree with the overall megafaunal zonation

schemes. Depth zones were objectively derived using cluster analyses of the megafaunal

data sets. In the earlier study, Pequegnat (1983) recognized five depth zones, based upon

the distribution of benthic megafauna:

Shelf/Slope Transition Zone (118 to 475 m)

Archibenthal Zone-Horizon A (500 to 775 in)

Archibenthal Zone-Horizon B (800 to 975 m)

Upper Abyssal Zone (1,000 to 2,275 m)

Mesoabyssal Zone (2,300 to 3,225 m)

Lower Abyssal Zone (3,250 to 3,850 m)

Gallaway et al. (1988) conducted further sampling both horizontally and vertically along

the depth gradient seeking to test the validity of the earlier scheme. They (LGL

Ecological Research Associates, Inc. and Texas A&M University 1986) initially decided

that four basic zones existed: shelf/slope transition, archibenthal, upper abyssal, and

mesoabyssal. Following similar analysis of their data, Gallaway et al. (1988) revised the

existing scheme as follows:

Shelf/Slope Transition Zone (300 to 500 m)

Upper Archibenthal Zone (500 to 800 m)

Lower Archibenthal Zone (800 to 1,650 m)

Upper Abyssal Zone (1,650 to 2,250 m)

Mesoabyssal Zone (2,250 to 3,000 m)
Snyder: Fishes and Fisheries

Minor differences between these two approaches include a change in terminology; a

deepening of the lower limit of the Lower Archibenthal Zone; and classification of the

Upper Abyssal Zone as largely transitional in nature (Gallaway 1988). Thus, the same

general zonation scheme was upheld.

The fish assemblage found in each zone was characterized by one or several common

species (Pequegnat et al. 1990). In the shelf/slope transition zone B. gobioides,

D. atlanticus, C. caribbaeus, and P. bean! were most common. The upper archibenthal

zone was characterized by two grenadiers (C. coelorhincus and Bathygadus macrops),

whereas the lower archibenthal zone supported high numbers of two additional grenadier

species (N. aequalis and B. melanobranchus). In the upper abyssal zone, the dominant

fish species was G. long/ilis. The mesoabyssal zone supported a depauperate fish

assemblage consisting of only five species including Dicrolene kanazawai and Basozetus

normalis. The lower abyssal zone was depauperate, but supported a unique fish

assemblage represented by Barathronus bicolor and Bathytroctes macrolepis.

That megafauna form discrete bathymetric zones is certainly appealing to managers, for

good reason. Unfortunately, imposing artificial boundaries on continuous

species-specific variation can present problems. Carney et al. (1983) cautioned against

the use of unproven zonation schemes presented for the deep sea in general and presented

an alternative and simpler scheme that described the slope assemblage as undefined and

essentially continuous (Figure 10.2). The interpretation of multivariate data along

environmental gradients can be affected by sampling adequacy, sampling scale, and

analytical technique (Carney et al. 1983). The Pequegnat (1983) original zonation

scheme was based upon 25 trawl hauls and 79 dredge tows made across the depth

gradient at various locations along the continental slope. Fishes were collected primarily

in the 25 trawl hauls, not in the dredges. Of these samples, only 12 were taken in depths

greater than 1,000 m: seven from 1,000 to 2,000 m, two from 2,000 to 3,000 in, and three

from 3,000 to 4,000 m. This may be too few samples, distributed in haphazard fashion,

to adequately represent the structure of the existing demersal assemblage. Another

possible sampling bias is that the areas selected for trawling (level bottom) may

inaccurately represent the average (and untrawlable) topography of the slope (see

Chapter 3, Geology). Gallaway et al. (1988) recognized some of these problems and

attempted to resolve the problem by taking samples at smaller downslope intervals along

their (western) transect. Nevertheless, their study added 58 trawl hauls (17 in water

depths greater than 1,000 m), but used a smaller trawl (9 m opening vs. 24 m opening).

Additional problems may arise when certain analytical techniques such as cluster analysis

are used to form depth-related assemblages (Carney et al. 1983). Given the potential

problems with the basic underpinning of the original zonation schemes, a more

conservative approach for managers may be to accept the Carney et al. (1983) scheme

(i.e., no true zones on the slope) until further data are available to support another

proposed zonation scheme. In a similar analysis of fish depth distribution along the

northeastern Atlantic continental slope, Haedrich and Merrett (1990) claimed that for

benthopelagic fishes of the northeastern Atlantic slope, data do not produce repeatable

patterns among adjacent stations. These authors concluded that the zonation concept, or

any concept involving organized comnmnities, was not valid for deep sea fishes.

268
1000 IIa.. Lu 2000 3000 SHELF-SLOPE TRANSITION DISTINCT S

IN DISTINCT DiSTINCT S

Figure 10.2. Opposing views of expected megafaunal zonation that might be observed on the Gulf of Mexico continenta

Gallaway et al. 1 988). 0 50 100 150 200 DISTANCE OFFSHORE (nmi) (Pequegnat 1983) (Carney 
Snyder: Fishes and Fisheries

This same debate over continuous versus discrete assemblages along gradients has been

waged by plant ecologists and others in the past (see summaries by McIntosh 1985, 1995;

Brown 1995). In these circles, the continuum or individualistic hypothesis usually has

been favored or at least considered as a null hypothesis (Price 1986).

Interactions among Fishes of Different Assemblages

Midwater (mesopelagic and bathypelagic) fish taxocenes have most likely evolved in the

deep sea, whereas demersal and epipelagic assemblages are more derived from shallow

water ancestors and may be secondary invaders into the deepwater environments

(Haedrich 1997). Despite this modular taxonornic nature of fish assemblages inhabiting

the broad environmental subdivisions, considerable ecological interaction occurs among

the groups. Most of the important ecological interactions in deepwater proximately

involve trophic processes, and ultimately (presumably) reproductive success. When the

feeding and ontogenetic migratory patterns are considered there is considerable

exchange among the taxonomically disparate groups of fishes. Mesopelagic fishes feed

in the epipelagic zone, where they are in turn fed upon by epipelagic predators such as

tunas and dolphin. Epipelagic and midwater fishes contribute organic matter in several

forms to the demersal habitat where it will contribute to the productivity of the

invertebrate food base that supports the demersal fish assemblage.

Deepwater Fisheries

Introduction

Commercial fisheries in the Gulf of Mexico are among the most productive in the world.

Species such as menhaden; brown, white, and pink shrimp; oysters; blue crab; and red

snapper drive the landings and dockside values. Gulf coastal states derive considerable

revenues from regional commercial fisheries (Browder et al. 1991; O'Bannon 1999). In

1998, commercial fishery landings in the U.S. Gulf of Mexico, which includes western

Florida, Alabama, Mississippi, Louisiana, and Texas, exceeded 1.5 billion pounds and

were worth over $700 million (O'Bannon 1999). Most of these landings were generated

from fish and invertebrate species caught in estuarmne, coastal, and shelf waters. Species

caught in the deepwater (>200 m) region contribute minimally to the overall regional

landings, but are still very important to a smaller sector of the commercial fishing

population (NMFS 1999a.

Deepwater fisheries of the Gulf of Mexico have not been formally reviewed, but

Mcllwain (1999) briefly described the deepwater fisheries and fishery practices occuning

in the De Soto Canyon region of the eastern Gulf of Mexico. He listed deepwater

trawling, butterfish trawling, golden crab trapping, surface longlining, and bottom

longlining as the major fishing endeavors of that area. Both butterfish trawling and

golden crab trapping appear to be of minimal importance in the overall deepwater fishery

(Josh Bennett, 2000, NMFS, Miami, FL, personal communication). Surface longlining,

270
271

Snyder: Fishes and Fisheries

shrimp trawling, and bottom longlining are the most important deepwater fisheries in

terms of pounds landed. All three of these methods were initially "discovered" through

the exploratory fishing efforts of the Bureau of Commercial Fisheries, which began

around 1950. Species sought by deepwater commercial fishers can be divided into three

basic groups: epipelagic fishes, dernersal fishes, and invertebrates. The following review

examines the history of deepwater commercial fisheries, key species, gear types, and

landings trends associated with the three basic fisheries. Commercial fisheries are the

subject of this review, not because recreational fisheries are unimportant or do not occur

in deepwater, but because there has not been enough information published on them.

History of Deepwater Fisheries in the Gulf of Mexico

The history of deepwater commercial fishing in the Gulf began in 1950 with federally

funded exploratory fishing programs. The program began investigating deepwater

shrimp resources and graduated to tuna fishing in the waters of the open Gulf (Bullis

1955a; Iwamoto 1965). After unsuccessful attempts to catch tunas with purse seines and

live bait fishing, the Bureau's exploratory efforts shifted to longlining for tunas in 1952

(Bullis 1 955a,b,c). About the same time, Japanese fishers began fishing for tunas in the

Gulf, also using longline gear (Iwamoto 1965; Lopez et al. 1979). Japanese vessels

fished in the Gulf of Mexico for the next 30 years from 1952 until 1982, when the

longlining ceased under an international agreement (Honma et al. 1985; Prager and

Browder 1992). In the early 1970's a domestic swordfish fishery became established in

the Gulf (South Atlantic Fishery Management Council 1985). This was a seasonal

fishery, mostly in fall and winter. By about 1983, an increasing demand for tuna caused

many swordfish longliners to switch methods to target them (Taniguchi 1987). The Gulf

tuna longlining fleet reached 350 to 400 vessels in 1988-1989 (Russell 1993), and

continues today with a similar number of vessels (Cramer and Adams 1999). In the

southern Gulf, Mexican fishers have been longlining for yellowfin tuna since about 1980

following the departure of the Japanese fleet (Prager and Browder 1992).

In the 1980's, the NMFS conducted another exploratory fishery program to examine the

efficacy of trawling for gulf butterfish (Peprilus burti). Commercial bufterfish vessels

and captains from the northeastern U.S. were hired to fish in various areas of the eastern

Gulf (Vecchione 1987). Although the results were promising (Vecchione 1987), 110

extensive butterfish fishery has developed (but see Mcllwain 1999).

While the epipelagic fishery was developing, exploratory fishing with bottom trawls

proved productive when in the early 1950's, exploitable quantities of royal red shrimp

(Pleoticus robustus) were found around the Gulf of Mexico in 500 to 900 rn water depths

(Bullis 1956). Commercial fishers did not begin targeting royal red shrimp until the

1960's (Gulf of Mexico Fishery Management Council [GMFMC] 1996). Exploratory

efforts by the Bureau of Commercial Fisheries were also the first to attempt bottom

longline fishing in deepwaters of the Gulf of Mexico (Nelson and Carpenter 1968). The

primary species caught by bottom longlining during this cruise was tilefish (Lopholatilus

chamaeleonticeps), which was of no commercial importance at that time. In the late
Snyder: Fishes and Fisheries

19,0's7 another exploratory fishing program sought to develop a fishery for deep

dwelling golden crab (Otwell et al. 1984).

Key Species

Epipelagic fishes found in the commercial catch include dolphin, sharks (mako, silky,

and thresher), snake mackerels (escolar and oilfish), swordfish, tunas (bigeye, blackfin,

bluefin, and yellowfin), and wahoo. These species are widespread in the oceanic waters

of the Gulf, generally in the upper 200 m of the water column. Sharks, swordfish, and

tunas are the most important fishery species and are currently managed as a unit (Highly

Migratory Species) by the NMFS Office of Sustainable Fisheries, Highly Migratory

Species Division (NMFS 1999a). As such, these species are covered by a management

plan as mandated in the Magnuson Stevens Fishery Management Act of 1976. These

same species are sought by offshore or bluewater fishers.

Demersal fishes caught in deepwater include groupers (snowy, Warsaw, and

yellowedge), snappers (queen and silk), and tilefishes (blueline tilefish, goldface tilefish,

and tilefish). These are often referred to as reef fishes (i.e., GMFMC 1981). However,

this group of deeper dwelling fishes, particularly tilefishes and yellowedge grouper, may

be found on level clayey bottoms in 80 to 450 m water depths rather than on reefs or hard

bottom. Other deepwater snappers and groupers associate with hard bottom outcrops in

water depths ranging from 80 to 600 m.

Deepwater invertebrates important to commercial fisheries in Gulf of Mexico are royal

red shrimp and, to a much lesser extent, golden crab. Royal red shrimp occur over

specific substrata in different areas of the Gulf: blue-black terrigenous silt and silty clay

off the Mississippi Delta and calcareous mud off the Dry Tortugas (Roe 1969; GMFMC

1996). Peak abundance of royal red shrimp in the Gulf of Mexico occurs in the depth

range from 250 to 500 m (Roe 1969). Golden crab occur in a similar depth range as royal

red shrimp but prefer hard bottom and outcroppings such as the Florida Escarpment

(Lindberg and Lockhart 1993).

Fishing Gear Types

Epipelagic fishes are primarily caught with drifting longlines fished in the upper water

column. Longlines consist of a monofilament main line suspended in the water colunm

by regularly spaced buoys. The buoy lines are used to adjust the fishing depth of the

mainline that can range from 15 to 75 rn. Leaders with baited hooks are attached along

the length of the mainline as it is being deployed. Leaders range from 15 to 70 m long

(Russell 1993). From 1994 to 1998 sets by permitted longline vessels in the Gulf of

Mexico ranged from 3 to 103 km and averaged 60 km (NMFS 1999c). The number of

sets per month ranged from 77 to 502 and averaged 322 (Figure 10.3). In 1998 the

number of sets ranged from 77 to 391 and averaged 259. Longlines are often set near

oceanographic features such as fronts or downwellings often with the aid of sophisticated

onboard temperature sensors, depth finders, and positioning equipment. Figure 10.4

depicts the spatial distribution of longline sets in the northern Gulf of Mexico for 1998.

272
273

Snyder: Fishes and Fisheries

Figure 10.3. Monthly lengths (a) and numbers (b) of surface longline sets made in the Gulf of Mexico

from 1994 to 1998. Values shown are mean (doffed horizontal line), median (solid

horizontal line), tenth and ninetieth percentiles (error whiskers), and twenty4ifth and

seventy4ifth percentiles (gray box) (From: NMFS 1 999c).
30' 2g -- 27 - 26 - 24 - 2322 - 9. Q6 B8 86 - . .h4:-... .. I - 'b.Jr. .. N LEGEND - Lease Ares - Longne Set 80 0 80 160 Kilometers I I or Br 86 Figure 10.4. Spatial distribution of surface longline sets made in the Gulf of Mexico during 1998 (From: NMFS 1999c).
275

Snyder Fishes and Fisheries

The primary homeports for longline vessels are Dulac, Louisiana; Venice, Louisiana;

Destin, Florida; Madiera Beach, Florida; and Panama City, Florida (Tanaguchi 1987;

NMFS 1999a).

Deepwater demersaL'reef fishes are caught with bottom longlines, traps, and

hook-and-line. Most of the landings are produced by bottom longlines. The longlines are

similar to the surface longlines except that the lines are anchored to the bottom. Bottom

longlines are much shorter than surface longlines, usually less than 3 km, but can reach

30 km (GMFMC 1981).

Shrimp trawling is one of the most important commercial endeavors in the Gulf of

Mexico. However, most of the shrimp grounds lie well inshore of the 200 m isobath.

Trawling for royal red shrimp occurs in water depths of 400 to 500 m offshore Florida,

Alabama, and Texas (GMFMC 1996). Gear used for royal red shrimp is very similar to

shallow water shrimp trawling gear, but to accommodate the greater depths involved, all

components including winches, trawl doors, lines, and vessels must be heavier.

Golden crab gear consists of rectangular wire mesh traps that are attached in series along

a weighted mainline Most of the fishing for golden crab has been offshore of western

Florida. Very few fishers are involved in the golden crab fishery. In fact, so few that the

landings data cannot be released separately because of confidentiality (Josh Bennett,

NMFS, Miami, personal communication).

Landings Trends

Epipelagic fishes comprised most of the weight arid value of deepwater landings in the

Gulf of Mexico for 1998 (Tables 10.5 and 10.6). Yellowfin tuna represented 55% of the

value and 47% of the weight of all deepwater species landed. Swordfish, dolphin, and

wahoo collectively accounted for another 19% of the value and 21% of the weight.

Louisiana and Florida led Gulf Coastal states in terms of value and weight of epipelagic

species landed, followed by Texas. Alabama and Mississippi reported no landings of

epipelagic fish in 1998.

Demersal fish landings were dominated by yellowedge grouper, which accounted for

9.7% of the value and 8.8% of the weight of deepwater species landed in 1998. Two

tilefish species and snowy grouper contributed another 5% of the value and 7% of the

weight reported for deepwater species in 1998. Most 1998 reef fish catches were landed

in Louisiana and Florida. Alabama and Texas contributed little to the landings, and

Mississippi did not report any deepwater reef fish landings in 1998.

Invertebrate landings were restricted to specific states. A majority of the royal red

shrimp were landed in Alabama where 1998 landings weighed 123.5 mt and were valued

at $586,575. Florida and Texas reported only small fractions of this amount during 1998.

Florida was the only Gulf coastal state to report landings of golden crab in 1998 (133 mt

valued at $205,000).
Snyder: Fishes and Fisheries

Table 10.5. Weight (metric tons) of deepwater species landed in Gulf coastal states in

1998 (From: National Marine Fisheries Service 1 999b)

a No landings of deepwater species were reported by Mississippi.

276

Species

Statea

Alabama Florida Louisiana Texas Grand

Total Percent

EPIPELAGIC FISHES

Tuna, yellowfin 237.6 1,341.8 137.4 1,716.8 47.0

Swordfish 162.1 306.2 41.2 509.5 14.0

Dolphin 157.8 30.7 2.4 190.9 5.2

Wahoo 17 62.6 2 81.6 2.2

Escolar 8.5 51.1 59.6 1.6

Shark, shortfin mako 6.1 35.8 41.9 1.1

Tuna, blackfln 12.4 20.9 2.9 36.2 1.0

Oilfish 4.8 10.1 14.9 0.4

Tuna, bluefin 1.3 12.2 13.5 0.4

Tuna, bigeye 4.4 8.9 133 0.4

Tuna, unc 5.7 5.7 0.2

Tuna, albacore 2.6 2.6 0.1

Shark, silky 2.3 2.3 0.1

Shark, longfin mako 1.4 1.4 0.0

Shark, thresher 1 1 0.0

Tuna, skipjack 0.5 0.5 0.0

REEF FISHES

Grouper, yellowedge 252.7 46.2 20.8 319.7 8.8

Tilefish 104.6 17.1 14.4 136.1 3.7

Grouper, snowy 58.9 3.1 62 1.7

Tilefish, blueline 58.5 58.5 1.6

Snapper, silk 46.8 0.7 47.5 1.3

Grouper, warsaw 13.7 15.1 9.9 38.7 1.1

Speckled bind 22.6 22.6 0.6

Snapper, queen 8.5 5.4 13.9 0.4

Brotula, bearded 5.9 4.7 10.6 0.3

Barrelfish 1.7 1.7 0.0

Hake,Atlantic,red & white 0.9 0.9 0.0

Tilefish, goldface 0.6 0.6 0.0

Bass, longtail 0.2 0.2 0.0

INVERTEBRATES

Shrimp, royal red 123.5 9.7 1.2 134.4 3.7

Crab, deepsea golden 113 -- 113 3.1

GRAND TOTAL 123.5 1,312.4 1,982.6 233.6 3,652.1
aNO landings of deepwater species were reported by Mississippi.

277

Snyder: Fishes and Fisheries

Table 10.6. Value (dollars) of deepwater species landed in Gulf coastal states in 1998

(From: National Marine Fisheries Service 1 999b)

Statea

Species Alabama Florida Louisiana Texas Grand

Total Percent

EPIPELAGIC FISHES

Tuna, yellowfin 1,233,884 7,338,444 691,142 9,263,470 55.4

Swordfish 924,546 1,335,696 166,690 2,426,932 14.5

Dolphin 494,432 71,054 5,595 571,081 3.4

Wahoo 67,432 143,740 4,611 215,783 1.3

Tuna, bluefin 7,316 113,605 -- 120,921 0.7

Escolar -- 21,105 96,643 117,748 0.7

Tuna, bigeye 24,545 71,281 95,826 0.6

Tuna, blackfin 26,374 22,425 5,864 54,663 0.3

Shark, shortfinmako 15,315 32,911 -- 48,226 0.3

OiI'tish 11,855 22,332 34,187 0.2

Tuna, unc -- 23,555 23,555 0.1

Shark, silky 3,177 3,177 0.0

Tuna, albacore -- 3,079 3,079 0.0

Shark, longfin mako -- 2,256 2,256 0.0

Shark, thresher 1,055 -- 1,055 0.0

Tuna, skipjack 466 466 0.0

REEF FISHES

Grouper, yellowedge 1,277,115 254,121 93,160 1,624,396 9.7

Tilefish 269,540 53,062 38,725 361,327 2.2

Grouper, snowy 252,151 14,809 -- 266,960 1.6

Snapper, silk 216,501 3,064 219,565 1.3

Grouper, warsaw 48,663 67,973 35,603 152,239 0.9

Tilefish, blueline 112,586 -- 112,586 0.7

Speckled hind 85,537 85,537 0.5

Snapper, queen -- 37,801 23,181 60,982 0.4

Brotula, bearded 276 14,799 9,570 24,645 0.1

Barrelfish - 6,194 -- -- 6,194 0.0

Hake,Atlantic,red & white -- 1,715 - 1,715 0.0

Tilefish, goldface 1,439 1,439 0.0

Bass, longtail 591 591 0.0

TNVERTEBRATES

Shrimp, royal red 586,575 30,778 -- 5,068 622,421 3.7

Crab, deepsea golden -- 205,440 - 205,440 1.2

GR4ND TOTAL 586,851 5,388,607 9,704,309 1,048,714 16,728,481
Snyder: Fishes and Fisheries

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Chapter 11: Socioeconomics

Linda A. Tobin

Introduction 285

The Importance of Time and Space 286

1-listory of Oil and Gas Development in the Gulf Coast States from 1900's Forward 287

A Natural Extension of Growth 290

The Past Thirty Years 292

Socioeconomic Effects of Oil and Gas Activity across Space 293

Community Impacts 294

Port Development 297

Impact of Oil and Gas Development on the Organization of Work 298

Labor Markets 298

Industrial and Business Linkages 300

Industrial Restructuring 301

Relationships to Physical Environment 302

Agriculture 303

Recreational Uses of the Coast and Oil 303

Industrial Development and Infrastructure 304

Port Fotirchon 306

Oil Spills and Hazardous Waste 307

Impacts on Families and Individuals 308

Oil and Louisiana Cajuns 309

Economic Impacts on Individuals and Families 310

Social Problems 312

Concentrated Work Scheduling 312

Impacts of Oil on Education, Migration, and Population 313

Conclusions 314

Literature Cited 3 15

Introduction

The socioeconomic effects of oil and gas development on communities, families, and

individuals in the Gulf of Mexico region are significant dimensions of the petroleum

industry. For decades, increased outer continental shelf (OCS) oil and gas production has

piqued the interests of researchers wanting to further their understanding of the impacts

this natural resource extraction has on coastal communities. Evolving from simple

land-based exploration and production platforms to technologically sophisticated offshore

structures produced significant social changes and responses over the years within those

regions most affected by oil and gas activities. The industry is dependent on a multitude

of suppliers (McKenzie et al. 1993), creating a vast interdependent network that has

consequently involved and impacted nearly every community within the region.

Recent deepwater Gulf of Mexico operations, in depths of water 300 m or greater, have

prompted sociologists to question whether impacts are comparable to, or different from,

those activities of more shallow offshore oil and gas operations. Determining the specific

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Tobin: Socioeconomic.s

socioeconomic impacts of deepwater development is difficult, if not impossible, because

many of the activities associated with deepwater exploration, drilling, and production are

integrated within regular offshore operations. Therefore, this chapter will include

literature on existing social, economic, and infrastructural conditions in places that are

likely to be affected by deepwater development, including those port and coastal

communities now impacted by offshore oil and gas activities, including deepwater. One

of the disadvantages of research on socioeconomic effects of offshore development

involves timing; too often research begins long after development starts and impacts

occur. 'When researchers are able to study regions, communities, families, and

individuals before these events, they have a more comprehensive framework in which to

make assessments. Therefore, we believe a broad review of socioeconomic effects

already identified in these regions will benefit and provide a better understanding of

potential impacts of deepwater development to communities in the future.

There is a rich variety of sources from which existing literature addresses the

socioeconomic impacts of oil and gas development in the Gulf of Mexico region. Social

impact assessments, funded by various agencies, carried out by social scientists, and

published in numerous scientific journals, serve as important sources of information.

Likewise, economic and community development initiatives provide researchers with

clues concerning internal and external forces of oil industry activities impacting

community sustainability. Technical reports presented at industry symposiums and

articles in trade publications contribute to our knowledge of offshore oil and gas

activities. Lastly, regional and local media reports sifted through to gain a sense of

heartfelt (and therefore sometimes biased) opinions commonly found in communities

where the potential to lose may be as great as the potential to gain complete the literature

review. Choosing to ignore any of this literature results in a picture a little less sharp, a

little less focused, than what is necessary if we are to understand, as completely as

possible, the socioeconomic impacts of oil and gas development on Gulf of Mexico

communities.

The Importance of Time and Space

Key to understanding what oil industry activity has meant to the Gulf Coast region is

knowing that oil and gas activity has developed unevenly in the Gulf of Mexico region,

both temporally and spatially, and with increased activity in deepwater drilling, will

continue to do so. Certain communities fortuitously benefited because of their location

relative to offshore activities and the acceptance by residents of such activities, while

other communities in other coastal regions either consciously chose not to participate, or

location prevented them from so doing. Fifty years of oil industry development in the

Gulf of Mexico has seen significant changes in activities, and technological

advancements and improvements. With time, diffusion and integration of oil industry

networks and activities have resulted in an almost complete envelopment of numerous

coastal communities and areas. This results in sometimes disparate, yet more frequently

often, similar social and economic consequences for communities within the region.

Thus it can be said that uneven economic development is a consequence of oil and gas

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Tobin: Socioeconomics

industry involvement. And as national and worldwide demand for oil and gas grows,

social, cultural, and economic impacts increase in those areas.

However, the market dynamics of the petroleum industry are primarily controlled by the

Organization of Petroleum Exporting Countries (OPEC), a 13-nation cartel of mostly

Arabian Gulf nations created in the early 1970's. At that time, the OPEC nations

controlled almost 75% of the world's proven crude oil reserves (Hodel and Deitz 1993).

The OPEC oil embargo of 1973 strongly encouraged U.S. oil corporations toward

increased national oil exploration and production in an effort to reduce foreign oil

dependency. Ironically, petroleum conservation legislation was soon to follow, and a

nationwide recession in the early 1980's effectively laid the groundwork for a collapse in

the industry, the severity of which had never before been seen. Socioeconomic effects

were far-reaching arid long-lasting nationally, but even more so along the coastal states of

the Gulf of Mexico, which had gained the most economically, although not without cost,

but which also stood to lose the most. State and community leaders seeking

socioeconomic robustness within their areas began to realize that a strong presence of oil

and gas industry-related companies in their midst also meant that they were susceptible to

global forces over which they had no control. Regional economic sustainability, a goal to

which all state and community leaders aspired, appeared elusive.

A brief history of oil and gas development in the coastal states along the Gulf of Mexico

will provide a framework that will help us to understand how and why the industry

developed unevenly. It is interesting to note the differences between states in their

degree of acceptance of oil industry development and activities over the years, arid the

factors that played significant roles in those differences.

History of Oil and Gas Development in the

Gulf Coast States from 1900's Forward

The oil and natural gas industry has operated along the Gulf coast for nearly 100 years,

beginning with the push for overwater drilling at Caddo Lake, southeast of Shreveport,

Louisiana, in 1905. Several technical problems were solved at that location that had

significant implications later for the move offshore into the Gulf of Mexico. First,

downhole pressures encountered were beyond any previously experienced. Because of

inability to deal with the pressures effectively, blowouts and fires became common. This

led to the emergence of increasingly reliable blowout preventers, which would be

required for the high pressure reservoirs along the Gulf coast. Second, it was necessary

to be able to drill and produce without direct contact to the land. Construction engineers

developed pilings that could be driven deep enough into the lake bottom to provide solid

footing, on top of which platforms were constructed, while barges transported drilling

equipment and supplies to the drill site. One final technological innovation occurred, an

underwater pipeline connecting the producing wells to one of four gathering stations on

the lake (Brantly 1971; Cicin-Sain et al. 1992). That pipeline would be a precursor to the

vast network of pipelines that would later bring oil and gas from offshore production

platforms to onshore facilities.

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Tobin: Socloeconomics

Access to coastal Louisiana drilling prospects in the early 1900's was difficult due to the

marshy, swampy environment, and it was not until 1933 with the Texas Company's

introduction of the submersible drilling barge that exploration of marsh and shallow

coastal waters became cost efficient (Lankford 1971). The submersible barge could be

towed to the drilling site and sunk, providing a stable drilling base. After drilling was

completed, the barge was raised and used elsewhere. Consequently, the marine

environment became more accessible to exploration companies (Gramling 1996).

However, another dilemma arose as exploration and production moved into increasingly

isolated areas of coastal marshes and estuaries: the labor force necessary to perform those

activities found it increasingly difficult to corrmmte on a daily basis. Two organizational

responses emerged, significantly impacting the industry's organization of work. First,

because these isolated areas had no housing facilities available for the work crews,

companies built and provided living quarters situated near the work site. Later on, as

offshore oil activities proliferated, living quarters became part of the work site, built on

top of oil platforms situated over open water. Second, concentrated working periods

became common, in which the division of work was frequently broken down by the

week: one week at work and one week off This type of concentrated work scheduling

has become the model for offshore development and has increased in importance as

offshore activities have grown. Oil companies are eager to recoup their investment

expenditures, which are considerable, as quickly as possible (Gramling 1989, 1995,

1996). Therefore, as oil industry activities have progressed farther offshore, work

schedules have also been extended, with today's workers commonly staying 7, 14, or

21 days, with daily shifts of 12 hours on, 12 hours off. Because of this type of

concentrated work scheduling, offshore workers may not necessarily live in the Gulf

Coast region, instead choosing to make infrequent long distance commutes from their

homes to whatever site they leave from for offshore. Gramling and Brabant (1986), in

their analysis of labor survey data conducted in East St. Mary Parish, an area heavily

involved in offshore support activities, found that only 30% of the 381 offshore workers

in the sample lived within 100 miles of where they met to go offshore. This phenomenon

eliminated the necessity for geographic concentration of human settlement in the vicinity

of this type of economic activity (Gramling 1996).

Louisiana's coastal zone incorporates all land and water bottoms within the territorial sea

(about 3 miles offshore) and the adjacent shorelands, encompassing approximately 9.5

million acres in southern Louisiana, of which 5.2 million acres are water. Marshes

constitute 63% of the land area in the coastal zone, while swamps, forests, and

agricultural areas constitute 9% each (Lindstedt et al. 1991). The movement of oil

activities into the Louisiana marsh in the 1920's and 1930Ts was aided by the development

of the highway network along the natural levees, parallel to the waterways crossing the

marsh (Kriiffen 1968). Land use and other economic patterns were significantly modified

by the oil and gas industry. There was a pronoimced shift of population from scattered

rural homesites and abandonment of more isolated areas, to more permanent structured

locations and concentration in long "string town" settlements (Kniffen 1968; Davis and

Place 1983). Settlement density along the highways on the crests of the levees

approached that of urban row housing. Demand for land had a decisive and cumulative

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Tobin: Socioeconomics

impact on the land use and land cover patterns as the region's rural character was being

changed by phases of oil exploration and development. Towns grew on the crest of the

levees while adjacent lower extremes of the levees were cleared for agriculture. Bayou

Lafourche, from Donaidsonville to the Gulf of Mexico was known as the world's

longest street," referring to the importance of the bayou for communication,

transportation, and the focus of economic life (Emmer et al. 1992). These growing urban

areas helped provide a small, but growing local labor supply for expanding oil and gas

activities. Construction and support sites for the offshore industry grew in number as the

industry moved ahead. Activities built to serve oil industry development were

constructed on these higher lands next to navigable waterways and as close as possible to

the offshore platforms they were supporting. Most of these locations were already

occupied by trade centers, agricultural enterprises, and fishing communities. The more

inland parishes and municipalities were the locations for company offices, or the

manufacturing and processing of the oil industry hydrocarbons (Emmer et al. 1992).

Therefore, even small traditional communities were transformed into vital interconnected

pieces of an increasingly global industry network.

Practical solutions often solved immediate problems: extensive networks of canals were

cut through marshes to allow submersible drilling barges to operate (Cicin-Sain et al.

1992). Private landholders and the State of Louisiana allowed almost unlimited access

via barge to drill sites, and later, a network of pipeline corridors was laid through those

same wetlands. The Texas-Louisiana coast in the late 1940's was an ideal site for

production because the proximity of vast refining capacity and pipeline availability made

transportation to market easy. Chemical plants and refineries were built from Freeport,

Texas to Lake Charles, Louisiana and further east (Burleson 1999). By the late 1950's an

average of 100 production platforms a year were being placed offshore, in ever-deeper

water (Gramling and Freudenburg 1995).

The importance of the coastal zone to the oil industry in Louisiana cannot be overstated:

from 1926, when production was first recorded in Louisiana's coastal zone, until 1983,

58% of the state's oil and 47% of its gas was produced in that part of the state. More

than one-half of the state's leasing activity has been in the coastal zone, and state

revenues generated from oil and gas activities in that area totaled $12 billion of $28

billion during that same period. Offshore technology expanded rapidly in the late 1940's,

after a movable submersible marine drilling rig was successfully used in Breton Sound.

Innovations were constantly being developed, including surplus war ships that were used

as floating warehouses, drilling platforms, and crew quarters (Lindstedt 1991). By the

early 1950's huge inland submersibles were capable of drilling to 20,000 ft depths

(Aibright and McLaughlin 1952; McLaughlin 1953; Gramling 1996).

The 1950's and early 1960's were years of intensified exploration and development

activity as large and small operators were continually drilling in the wetlands (Davis and

Place 1983). From the mid 1960's through the early 1970's, the oil and gas fields of

Louisiana's coastal zone were extended. A massive network of manmade waterways or

canals, constructed to meet local needs, became a permanent landscape feature,

significantly enlarging fields and intensifying canals' regional impact (Lindstedt et al.

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Tobin: Socioeconomics

1991). Davis and Place (1983) note that marsh or swamp drainage ditches had been used

as early as the 1700's by farmers and settlers to drain potential crop and pasture land, and

as transportation routes; later, the cypress found in the swamps was accessed and

removed by logging companies through excavation of logging canals. The oil and gas

potential of the coastal zone served as an impetus to the industry to dredge more access

canals throughout the coastal lowlands. Those interconnecting canals served as

transportation waterways to oil and gas activities throughout the coastal zone. Movement

of oil from isolated wells to refineries also necessitated the building of extensive pipeline

networks throughout the coastal zone. As oil and gas production moved further offshore,

so did the pipeline, and by 1951, the first large-diameter pipeline for offshore production

was laid in the Gulf (Clark 1963; Gramling 1996).

Although Davis and Place (1983) found that early exploration in offshore waters was

erratic due to high development costs and legal problems, during the 1950's and 1960's

large fields continued to be discovered and developed. Between 1952 and 1960, 34% of

the wildcatting operations had been successful along Louisiana's coast, resulting in

further expansion of activities with increased socioeconomic effects. The first fabrication

yard solely for construction of fixed drilling platforms was established at Bayou Boeuf,

Louisiana, in 1955. Along with 13 others, this yard has produced most of the platforms

placed in leases off Texas and Louisiana. By 1955, 10 refineries, 14 petrochemical

plants, and 3 carbon black flims had located in southern Louisiana. Population and

employment patterns were changing in the coastal zone; for instance, between 1950 and

1960, the population in seven coastal parishes increased an average of 30% or about

7,800 people per parish (Bobo and Charlton 1974; Davis and Place 1983).

Production in many of the large fields was slowing by the 1970's, not only because new

fields had become harder to find, but discoveries were smaller. The coastal zone's

production peaked at 513 million barrels in 1970, and rapidly declined thereafter. Less

than onefourth of its peak production was being produced by 1983.

A Natural Extension of Growth

Residents of the central and western Gulf have in most instances generally accepted oil

industry activities in their areas. This is because offshore activity in the Gulf, particularly

in Louisiana, occurred as a gradual extension of landbased gas and oil production

through the coastal marshes and into ever deeper offshore waters. Growth was

incremental, and focused on the solution of local problems for local use, largely isolated

from mainstream economy and industrial development. It did not appear suddenly, or

appear as a harm, threatening to the region, but rather as a natural evolution (Cicin-Sain

et al. 1992). Texans, already accustomed to producing oil and gas fields within their

State, saw the development of the offshore oil industry as another facet of a burgeoning

industry.

At least in coastal Louisiana, the economy had always been traditionally based on

extractive activities. At the turn of the century, the mainstay of the local economy

included furs, shrimp, fish, oysters, cypress lumber, moss, waterfowl, and crawfish.

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Tobin: Socioeconomics

Marsh inhabitants consumed marsh products and marketed them to settlements in the

region. Oil production was seen as just one more product from this rich resource-laden

environment that would benefit the people. Comeaux (1972) notes that prior to World

War II, most coastal Louisiana extractive activities occurred in the marsh and swamps.

Not until the 1950's was the current major commercial species, shrimp, known to be

available, in the open Gulf. Temporally, the significance of this was that oil and gas

development arid fisbing/shrimping grew up together. Also, during the 1930's and

1940's, there was no general environmental awareness, no conception of the marsh as a

potentially vulnerable ecosystem (Gramling 1996). The canals and channels carved

through the marshes that solved problems of accessibility and served as transportation

routes were seen as solutions, and not as having future environmental consequences.

Today most would agree that those canals and channels, while beneficial to the

development of the oil industry, have also played a role in the loss of coastal wetlands.

One of the most important issues in the development of the offshore oil industry involved

the dispute over subsurface land ownership, with coastal states and the Federal

government vying for ownership of the petroleum-rich environment, particularly in

waters beyond the 3-mile state limit By the early 1950's, Louisiana and Texas had

already sold over 300 leases in the Gulf beyond 3 miles offshore, and most of that was off

the coast of Louisiana. Regardless, the 1953 Submerged Lands Act gave the states title

to offshore lands that were within approximately 3 miles of their coastline, and

subsurface lands beyond 3 miles fell under the ownership of the federal government. The

first OCS lease sale was held in the Gulf off the coast of Louisiana in 1954, followed

shortly thereafter by a lease sale off Texas, and in 1959, off the coast of Florida. The

Secretary of the Interior leased through competitive bidding those offshore lands beyond

the 3-mile limit to companies for the purpose of developing oil, gas, salt, and sulphur

resources (Gramling 1995). Offshore development was proceeding along the western and

central sections of the Gulf of Mexico, but a different story was developing in the waters

of the eastern Gulf, off the beaches of Florida.

Development and acceptance of offshore oil industry activities along the eastern side of

the Gulf of Mexico, has been elTatic, controversial, and at least in state waters,

nonexistent since 1962. Historically, opportunities for economic activities were more

limited in Florida marshlands than Louisiana because the wetlands in Florida were

different. Agricultural opportunities along the natural levees were constrained by the

absence of rich alluvial soils common to Louisiana, and the ecosystem of the Florida

marshes differed in the types of edible foods found, and building materials available

(Gramling and Freudenburg 1995). Consequently, there was no natural extension of

marsh activities into the Gulf because resource extraction in the region never reached the

magnitude of that in Louisiana. The growth and subsequent economic importance of the

Florida tourism industry, particularly along the coastal waters, was not compatible with

oil industry activities. Most Floridians along the Gulf of Mexico do not welcome nor

encourage those activities in their waters. Today there is some small production in

federal waters in the Eastern Planning Area of the Gulf of Mexico, but that is not off the

coast of Florida, rather Alabama.

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The economic bases of the majority of coastal states (Louisiana, Texas, and to a lesser

extent Alabama and Mississippi) have benefited from oil activities in the Gulf of Mexico.

History of oil industry development has shown not only must the industry find oil

offshore and move it ashore, everything required to operate the over-water facility needs

to be delivered to an onshore support site, and transported to the platform (Davis and

Place 1983). Therefore, a huge support system has developed over time, one that today

essentially keeps all operations on schedule. Fabrication yards, refineries, terminals, boat

docks, airports, helicopter bases, repair facilities, shipyards, and pipe storage areas are all

essential to offshore operations. Communities along the industry's corridor became bases

of operation, attracting numerous types of businesses involved in meeting the needs of

the onshore and offshore operators. Expansion of these support facilities stimulated the

local economies and prompted an increase in residential, commercial, industrial, and

utility land uses, impacting nearly every community in the region (Applied Technology

Research Corporation 1994). In fact, there are those who believed it was difficult to find

one community, at least in Louisiana, that had not profited by the exploration and

development of the hydrocarbon reserves off the state's coast.

However, the oil industry, not unlike other extractive industries, is cyclical in nature.

This means there are oscillations in the price of oil and natural gas, and changes in

demand; it is an international industry characterized by episodic fluctuations of advance

and decline conditions in the market, and eventual exhaustion of the resource (McKenzie

et al. 1993). Decisions that govern oil exploration and production in the Gulf of Mexico

are made by companies that operate within the larger global political economy of energy.

It involves high levels of risk, and has a history of domination by large integrated

companies able to take on those risks, while being regulated by government (Laska et al.

1993). Therefore, economic development and social stability in oil-dependent regions of

the coast depend on the continuation of profitable oil prices.

The Past Thirty Years

The 1970's witnessed a dramatic increase in exploratory drilling in a climate of rising oil

and gas prices. The 1973-1974 oil embargo by the Arab members of the OPEC served as

a tremendous stimulant for further offshore development. Rapid employment growth and

significant income gains were not uncommon for any community involved in oil

activities; it was a time of prosperity for the region. But there were also socioeconomic

problems associated with this period of time. Population expansions of many of the Gulf

of Mexico communities could be traced to the growth in demand for oil and gas

(McKenzie et al. 1993), but these were accompanied by strains on existing transportation

networks, community infrastructures, and the delivery of social services. National

publications brought news of employment opportunities in the Gulf coast region, and a

transient labor force arrived en masse, problematic in itself. The high-paying jobs so

often associated with oil industry activities made it difficult for Gulf coast economies to

diversify, because businesses with lower profit margins could not compete for labor.

Gramling and Freudenburg (1990) demonstrated that in terms of employment, 90% of the

variance in Louisiana coastal parish local employment could be explained by national and

international petroleum indicators, a sure sign of the dependency they had on oil and gas

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activities. By 1980, many offshore operators assumed that $40 per barrel of oil was a

permanent reality and that large amounts of offshore acreage were desirable at nearly any

price (LeBlanc 1980).

This was not to be. In the early 1980's, the economy of the Gulf coast region went into a

tailspin. Falling crude oil prices and reduced demand for petroleum products created a

crude oil glut in 1982 that hurt exploration both onshore and offshore. When there is a

decline in oil and gas prices, oil and gas activities follow suit. Communities involved in

these activities, particularly those whose economic base is primarily connected to the

industry, experience economic downturns that can be difficult to overcome. Across the

region, job growth slowed, and unemployment rates swelled to well above the U.S.

average. Gramling (1996) noted that unemployment levels in some parishes went from a

low of about 4% to a high of 20% by the mid-to-late 1980Ts. The national savings and

loan crisis of the late 1980's gained fuel from the oil price collapse, because banks and

S&Ls with outstanding loans to oil companies, and real estate developers who borrowed

to build during the oil boom, were caught short by the rapidity of economic decline.

Anyone involved in the housing industry was hurt by employment declines and

outinigration (Laska et al. 1993). The Gulf coast was feeling the effects of a regional

recession.

Since that time, the 1990's has seen the oil industry recover from its lows of 1986, and the

Deepwater Royalty Relief Act of 1998, along with technological advancements has

enabled oil exploration and development to become more economically feasible in waters

of 300 m or greater beyond the continental shelf. In 1998, according to MMS, deepwater

production was 36% of the total Gulf of Mexico OCS activity, but this is anticipated to

greatly increase over the years.

In reviewing briefly the history of oil and gas development in the Gulf coast region, it is

obvious that dramatic economic and social changes have occurred as oil exploration

marched inextricably into the Gulf of Mexico. In some communities, as we shall see,

cultural traditions have been irrevocably altered, while concurrently, new community

patterns were being forged in newly created "boomtowns." The socioeconomic benefits

to the Gulf coast region have been beyond the wildest dreams of most people, yet at the

same time, they have levied a price on those communities. In the next pages, we shall

examine more closely the impacts of oil and gas development on the coastal region of the

Gulf of Mexico.

Socioeconomic Effects of Oil and Gas Activity across Space

When we examine the activities of oil development and its socioeconomic effects across

the central and western Gulf of Mexico region, we notice that the impacts within and

between communities vary. Not surprisingly, they appear to be greater in rural coastal

communities that were traditional settlements, and which were transformed into various

types of staging areas and support sites. Of course, few communities within these coastal

areas were left untouched by the development of the oil and gas industry. Louisiana, the
Tobin: Socioeconomics

primary location in the continental U.S. for offshore exploration, has accepted and

welcomed the oil industry as playing a pivotal role in the lives of thousands of

individuals and dozens of communities across the state. The value of Louisiana oil and

natural gas produced in 1954 was about $5.4 million; by 1980 the value had risen to more

than $11.4 billion (Manuel 1984). Like Louisiana, Texas has always actively encouraged

and promoted the development of the offshore oil industry. By 1996, Texas was

producing one-third of the nation's natural gas, the majority of those reserves lying

offshore or along the coastal plain, and one-fourth of the nation's oil, a significant

amount occurring in the coastal zone (U.S. Department of Commerce 1996).

One important point must be made when assessing the socioeconomic impacts of oil

development along the Gulf of Mexico, particularly in the 1980's. McKenzie et al.

(1993) found that the socioeconomic effects of non-OCS (state) activities and those of the

OCS are inextricably mixed due to the high number of businesses and industries that

serve both sectors of the oil and gas industry. That being argued, some of their analyses

showed that non-OCS production was a better predictor of some socioeconomic change

in the 1980's than was OCS production. They further argue that though there is a general

perception that OCS declines severely impacted the U.S. economy in the 1980's, there

was, at the same time, a general decline in the economy as a whole across the nation,

suggesting that factors other than OCS activity, or oil activity in general, may have

contributed to the Gulf Coast decline.

Community Impacts

One of the primary characteristics of any resource extraction, such as oil or natural gas, is

that extractive activities must locate where the resource is. These activities may or may

not take place in a populated area with existing infrastructure and support sectors; if not,

provisions must be made to support the primary activity. Frequently this involves the

rebuilding of physical, economic, and social environments in those locations (Gramling

1996). This rebuilding was particularly evident in Louisiana's traditional communities in

the coastal zone from the very early stages of oil activity. However, there are

consequences associated with rebuilding efforts, and one of those consequences that has

been noted over the years is loss of community flexibility. This means that human and

financial capital are concentrated too narrowly on the primary extractive and support

sectors, and therefore not easily transferable to new diversified activities. Economic

diversification contributes to community sustainability. 'When capital, either human or

financial, is constrained within a specific type of industrial activity, communities become

vulnerable to fluctuations that occur within that industry, including the possibility of

slowdowns. Other rebuilding consequences may be the creation of new support sectors

that use up or destroy local resources; and finally, high-paying j ohs in the extractive

sector create intense labor competition, so that once again, the introduction of alternative

economic activities becomes difficult. When the new extractive industry declines or

ceases to exist, the community frequently finds itself inflexible and overspecialized, a

condition referred to as overadaptation" (Freudenburg and Gramling 1992).

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An example of this could be seen in Morgan City, Louisiana. Located in East St. Mary

Parish, Morgan City was a traditional corrimunity with an extractive economy derived

from basin and marsh resources (cypress lumber, furs, alligators, crawñsh, shrimp, etc).

Because of its location, bounded by the Atchafalaya River, a major distributary of the

Mississippi River, which intersects the Gulf Intracoastal Waterway close by, before

traveling to the Gulf of Mexico 20 miles away, Morgan City was ideal for the

development and support of offshore oil (Freudenburg and Gramling 1994). The first

offshore activities took place just south of East St. Mary Parish, which helped to establish

its importance in the industry in the coming years. It became a staging area for a

multitude of offshore activities, and the demand for land increased as sites were

increasingly needed for fabrication yards and support facilities. To accommodate the

expansion, reclaimed swamp was chosen (Stallings et al. 1977) because it was

conveniently close to water, and cheaper than agricultural land. Between 1955 and 1975,

a total of 373 new businesses was added to the list of firms located in both Morgan City

and Houma, Louisiana, each year (Davis and Place 1983). By 1970 the population of

East St. Mary Parish had increased by 200%, from a stable number of 12,796 in 1940 to

36,227 (U.S. Depatlinent of Commerce 1940, 1970). This was attributable to the

development of the oil industry.

At the time large-scale development of offshore oil began in the 1950's, Morgan City was

the self-proclaimed "shrimp capital of the world." The offshore oil activities brought

tremendous employment opportunities, and jobs, even those requiring few skills, paid

well. If one job didn't work out, there were always others to be had. Consequently,

those who had worked the shrimp boats or participated in the shrimping industry turned

to the oil industry for jobs that were guaranteed to pay well, as opposed to the type of

livelihood they knew so well that entailed hard work but no promise of financial security.

Shrimp boats were converted to crew boats by owners wanting to get a share of the

economic "good times" associated with the development of offshore oil. But by the mid

1980's, when the oil slowdown had hit the area hard, those same residents gave thought to

returning to shrimping as a source of livelihood, only to discover the area no longer had a

resident shrimp fleet or processing facility (Gramling and Brabant 1986). This

phenomenon in Morgan City, East St. Mary Parish, was a classic case of overadaptation.

Morgan City was impacted in other ways besides increases in employment opportunities

and population growth. Housing shortages were acute, public utilities services couldn't

keep up with demand, arid the numbers of medical facilities weren't adequate for the size

of the population. By the mid 1 970Ts, the violent crime rate in Morgan City was two or

three times the average rate for U.S. cities of comparable size (Cicin-Sain et al. 1992).

There could be found labor camps in Morgan City, which housed transients lured to the

area by the promises of guaranteed jobs. The owners of the camps fed them, transported

them to and from the work sites, and exacted exorbitant prices from them for services

provided (Brabant 1993). Oil and gas extraction brought economic prosperity to many in

the Morgan City area, but it was accompanied by significant social and economic

changes as well.

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Tobin: Socioeconomics

Morgan City was just one community of many in a vast region along the Gulf coast that

felt the impacts of oil industry development. A multitude of staging areas and support

sites spread throughout the coastal region as the industry expanded. Gramling (1996)

points out that wherever road or rail met waterways, local docks and staging areas

appeared. What was important was location. Places like Intracoastal City and Venice,

both located in Louisiana, had virtually no notable populations in the 1950's, but because

of their proximity to the Gulf, became important staging areas without ever developing

significant permanent populations. During the 1970's, one could drive down to

Intracoastal City, Louisiana, and view a large number of bustling offshore support

enterprises, but only a handful of permanent housing was seen. Oyster shell parking lots

were filled with vehicles, a testament to the frenetic pace of the oil industry activities.

According to a local business owner, when the slowdown came in the 1980's, Intracoastal

City almost shut down completely, and there was little activity for a long period of time.

Lafayette, Louisiana, located in the heart of the "Oil Patch," and only about 30 miles

from the Gulf, was not always tied to the oil and gas industry, but rather developed first

as a railroad town, and then later functioned as a transportation center for farming

communities in the area. Because of its location, the rapid development of oil activities

created growth in Lafayette, particularly in administrative activities. It offered urban

amenities that smaller communities could not offer, and so became headquarters location

for many petroleum corporations. Between 1940 and 1980, the city of Lafayette grew

from a population of 19,210 to 81,861, an increase of 327%, according to U.S.

Department of Commerce (1940, 1980). The oil industry became so important to the

economy of the city that Lafayette became the focus of much media attention. National

publications targeted conspicuous consumer wealth brought on by oil industry

development. The University of Southwestern Louisiana, in an attempt to accommodate

students working offshore, offered classes that met just every other week, capitulating to

the demands of students with concentrated work schedules. When the slowdown in the

industry came, unemployment in Lafayette Parish rose to 9.6% in 1986. Property values

dropped, business failures were not uncommon, and there was high outmigration

(Gramling and Freudenburg 1990). Over the 1984-1989 period that covered the worst

years of the oil bust, Lafayette Parish lost over one-third of its total revenues.

Many coastal Texas communities were significantly impacted by oil industry

development as well. The discovery of onshore oil east of Houston, Texas in 1901 and

subsequent discoveries closer to the city set the stage for the city to become a major oil

center. By 1919, three-quarters of Gulf coast oil was coming from fields in the Houston

area, and by the mid-1930s, Houstonians were proudly labeling their city the "oil capital

of the world." The East Texas oil field, developed in the 1930's, had 26,000 wells on it

by 1939 (Feagin 1985).

Between the 1940's and 1980's prosperity continued in the greater Houston area. There

was substantial development of support industries linked to oil exploration and

production, including refineries, oil tools and services, and petrochemical plants. In the

early 1980's about 35% of the jobs in the area were connected directly to the oil and gas

industry (Feagin 1990). Houston was chosen over Galveston, and Beaumont, as an oil

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and gas center by oil-related companies, according to Feagin (1990), because it was less

exposed on the coast to hurricanes, and also had the necessaiy inftastructural support. It

was a major port due to having a ship channel deep enough for large ships, and was a

center for 17 railroads. By the 1960!s and 1970's, Houston's role in the oil economy had

become central in research and operations in exploration, production, refining, and

marketing. Thirty-four of the 35 largest oil companies had major office and plant

facilities in the Houston area, and at least 400 other major oil and gas companies also

were located there, along with hundreds of oil-related service companies. In 1984, 60%

of Texas oil industry onshore personnel reported to work at sites in the greater Houston

area (Centaur Associates, Inc. 1986).

As in Louisiana, the economic benefits from the oil industry were substantial in Houston

until the slowdown. Unparalleled prosperity was enjoyed until the 1980's, when the most

serious economic downturn in that city's history occurred. Between 1981 and 1986

Houston lost more than 100,000 energy-related industrial jobs. The unemployment rate

rose to 9.7% in 1983, recovered somewhat, and then late in 1986 the Chamber of

Commerce estimated the unemployment rate had risen to nearly 11%. Besides serious

job losses, Houston experienced housing foreclosures, increased office vacancy rates, and

outmigration in the 1980's (Feagin 1990).

Studies of local economies like Morgan City and Lafayette, Louisiana, and Houston,

Texas provide examples of oil and gas divisions of labor over space. Employment within

the industrial sectors of extraction, manufacturing, or services is associated with different

working conditions, opportunities, and job outcomes (McGranahan 1988; Lobao 1990).

Houston, and Lafayette on a much smaller scale, developed into administrative

headquarters for both major and independent oil companies. Located in different states,

both cities demanded educated labor that could be assured of high wages, which is still

true today. Demand for labor in Morgan City was considerably different. Highly paid

jobs resulted from complex manufacturing needs of oil and gas fabrication yards.

Educational attainment was not important; the ability to become skilled at one's job, was.

Another community in Louisiana, Abbeville, appeared to have a strong presence of

producer services, and still does. A center for oil field logistics and operations, Abbeville

remained reasonably resilient throughout the industry slowdown in the 1980's. These

patterns of industrial organization continue to significantly impact the socioeconomic

conditions of Gulf coast local economies.

Port flevelopment

As the industry grew, communities wanting to attract the fabrication and construction

industries associated with offshore activities and thus produce local jobs approved

long-term bond issues for the construction of the marine equivalent of local industrial

parks, or small ports contiguous to the waterfront near the community, committing local

resources to the continuation of the offshore sector. Conversion of existing facilities,

such as dock space, significantly altered the environment. Port development was and still

is a major goal of most coastal communities. Texas port authorities believe the

development of one or more deepwater ports along the Texas coast, at locations such as

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Houston and Corpus Christi, would have a substantial impact in the area because

deepwater port facilities would reduce both tanker traffic in Texas' bays and estuaries

and the frequency of lightering operations in the Gulf of Mexico. This would reduce the

risk of major and chronic oil spills (U.S. Department of Commerce 1996).

Studying the spatial development of the oil and gas industry across the Gulf of Mexico

coastal region allows us to understand how and why the industry evolved as it did.

Because of this differential spatial development, community and regional socioeconomic

effects have also differed. Subsequently, among the many effects that can be observed is

the impact offshore development has had on the organization of work.

Impact of Oil and Gas Development on the Organization of Work

Because of the long-distance commuting made possible by concentrated work schedules,

a widely dispersed work force in the offshore industry has been created. In a 1980 labor

survey of a Louisiana parish heavily involved in offshore activities, not only was it found

that only about 30% of the 381 offshore workers surveyed lived within 100 miles of

where they met to go offshore, 70% of them had been commuting long distances for over

10 years, while they continued to live in communities far from the staging site (Gramling

1980; Gramling and Brabant 1986). These concentrated work schedules are believed to

be particularly beneficial in geographical regions experiencing high unemployment,

because they effectively spread the unemployment lowering effects of new jobs over a

wider region. As has already been pointed out, from the perspective of a petroleum

corporation, temporal and spatial management strategy of concentrated work scheduling

made the exploitation of remote sites, or resources, more profitable (Gramling 1989,

1996). Later on, as offshore development spread globally, experienced offshore workers

from the Gulf of Mexico were used in those operations, with lengthened work schedules.

Initially, this relieved corporations of having to train inexperienced work crews, until

those countries involved in oil and gas extraction passed legislation that among other

requirements, mandated quotas for number of local residents to be used in oil and gas

development.

Labor Markets

One of the limitations on the expansion of oil industry development is the availability of

a labor force. One of the characteristics of the offshore labor force observed by some in

1979 was that most of the new workers in the entry-level categories received no formal

training before going offshore; rather they received on-the-job training after they had

arrived on the platform. There was a high rate of turnover reported among the personnel

of the drilling contractors, marine service companies, and food service firms. Workers

were quicidy finding out that offshore work was demanding, dangerous, and the

schedules were unlike any they had known. Additionally, being away from family and

friends for extended periods of time convinced some workers that offshore disadvantages

overshadowed financial gains. This created a labor force composed of a high proportion

of new and inexperienced workers at any given time, which was not only costly to

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companies, it also affected issues of safety. Personnel managers complained of labor that

was unskilled and undisciplined. This rapid development of the offshore industry in the

1970's, along with high rate of employee turnover, resulted in a huge demand for workers

in an industry dealing with a chronic shortage of workers (Perez 1979). By the late

1970's and early 1980's, the offshore oil industry supported dozens of small independent

companies wanting to cash in on the rapidly escalating oil prices, but found it difficult to

find enough qualified, experienced people available to fill the positions that were waiting

(Hodel and Deitz 1993).

Interestingly, some of those same observations and complaints are being made today. In

a recently concluded coastal Louisiana oil and gas labor demand study (forthcoming),

on-site managers and personnel administrators recognized and expressed concern about a

number of issues that are similar to those of the 1970's: the availability of skilled or

certified workers, the increasing emphasis on industry safety, the need to keep turnover

rates down, and a growing frustration with new employees wanting high wages for less

work. On-the-job training is still recognized as an important part of the labor process iii

some sectors.

The rapid growth of oil industry activities required an immediate labor force. Yet from

the 1970's until the early 1980's, and then again in the mid 1990's until recently,

observations have been made that a shortage of available workers seems to be a chronic

problem in times of increasing activity. To meet their growing needs, companies

involved in the offshore oil industry had to attract potential workers from afar, rather than

rely solely on local labor. This was the situation regardless of whether they were

offshore workers with concentrated work schedules, or onshore laborers needed for oil

industry activities working regular schedules. It was therefore necessary for a labor force

to migrate to the Gulf Coast region. Historically, economic factors have been shown to

overshadow social factors as the major causes of migration between different areas.

Migration is a function of "push" and "pull" forces: if there is a lack of economic

opportunities in the sending area, there is a push, and the pull is the presence of economic

opportunities in the receiving area (Maruggi and Wartenberg 1996). Activities along the

upper Texas and Louisiana coasts presented economic opportunities in the way of

plentiful jobs at high wages, making that region very attractive. For example, welders

were being paid $18 an hour in Morgan City's fabrication yards in the mid-1970's, and

fabrication companies still couldn't hire enough to meet demand (Freudenburg and

Gramling 1992).

Labor supply problems notwithstanding, by 1981 the offshore oil industry in Louisiana

provided direct employment to 41,781 persons. Additionally, it was indirectly

responsible for the creation of 83,500 other jobs in construction, maintenance and repair,

retail and wholesale trade, business services, and 34 other economic sectors. It was

estimated in this same study that for every new job created in the Louisiana oil and gas

industry, two new jobs were created in other sectors of the state's economy (Scott 1981).

The American Petroleum Institute (1984) estimated that in 1982 Texas employed

292,5222 persons in the oil and natural gas producing segment of the industry, of which a

substantial portion worked in activities related to offshore oil production.

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These types of economic advantages for labor persisted until the severe slowdown started

in the 1980's, when companies associated with domestic oil development began to lay off

significant numbers of workers. McKenzie et al.'s (1993) study of coastal Texas,

Louisiana, Mississippi, and Alabama found that between 1982 and 1986, the number of

mining industry jobs, of which oil and gas were predominant, decreased by 28.63%.

Some who had migrated to the region left the area, and inmigration nearly ceased. A

significant number of those who stayed in the Gulf of Mexico coastal regions eventually

found new employment in other industrial sectors. This employment was to have serious

consequences for the industry when it rebounded in the early 1990's.

When the industry began to turn around in the early 1990's, there was again an increasing

demand for labor. This time, it was more difficult to attract workers. Many of those who

had worked in the oil and gas industry previously, and experienced the slowdown of the

1980's had no desire to reenter an industry that they believed was too volatile.

Respondents in a forthcoming 1997 study of the socioeconomic sustainability of a small

Louisiana coastal community in a parish significantly involved in the oil and gas industry

frequently reported economic or financial responses during the slowdown of the 1980's as

involving diversification of economic activities, or obtaining employment in an industry

other than oil and gas. Regardless of economic opportunities, there appeared to be a

reluctance (at least in this study) to give up what they considered stable employment, for

the roller coaster ride associated with oil and gas employment. Interestingly, the mid

1990's shortage of skilled workers in shipbuilding was so extreme that firms offered

workers from outside the area temporary housing and mimicked the practice of the

offshore oil and gas industry with a 7 days onI7 days off work schedule (Pulsipher et al.

1998).

Industrial and Business Linkages

While the industry was rapidly developing, resources associated with human and social

capital, skills, knowledge, experience, teamwork, and networks of supply and distribution

and physical infrastructure began to interact and develop quickly as well (Gramling

1996). Adaptation occurred within oil and gas company organizations, while

independent offshore supply companies quickly emerged. Oil and gas production

companies did not, nor could not, operate without a multitude of suppliers. Small

businesses within the oil and gas industry wanted to fill particular niches within the

industry for which they felt there was a need, so there was a tendency toward

specialization. Existing businesses adapted in a variety of ways, which might have

included refitting their equipment to meet the needs of the industry. For example, there

were mechanic shops that shifted attention to a new oil-related consumer base and spent

considerable amounts of money on equipment to become marine diesel repair facilities.

New specialty businesses opened to accommodate the growing industry's needs. Linked

activities, upstream (those that supply the primary activity) and downstream (those that

use the product produced by the primary activity) generated jobs and capital. Industry

growth created a need for thousands of vendors along the Gulf coast that derived at least

a portion of their income from offshore operations, and this is still true today (Applied

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Technology Research Corporation 1994). In the mid 1950's in southern Louisiana, there

were 1,187 principal businesses serving the oil and gas industry. An analysis of

telephone directories in the early 1970's identified over 3,500 businesses in coastal

Louisiana, directly serving the petroleum industry, an increase of more than 100

businesses per year (Davis and Place 1983).

During the 1970's in particular, the emphasis in the industry was on speed, not costs. The

quicker a company could supply equipment, products, or labor, the faster the industry

grew. A basic rule of economics states that when the demand for a product or service is

high, such as skilled labor, the result is higher wages. This was certainly the case in the

growth of offshore activities, where profit margins were so high companies did not need

to be concerned with expenses or becoming expert business managers. It was a

phenomenon remarked upon by one respondent in a 1992 study, "People made money in

spite of themselves" (Freudenburg and Gramling 1992).

Today oil and gas exploration and production companies operating in the Gulf are usually

classified as either major corporations or independent companies. Major corporations are

integrated or engaged in all phases of the industry: exploration, production,

transportation, manufacturing and refining, and retailing. They have the most money, the

largest assets, revenues, and reserves, and employ the most people. Independent

companies are comparatively small and vary in the degree of integration, from relatively

little downstream integration activities, to a great deal. A 1995 study found that

nonintegrated independent businesses were becoming more involved in the offshore Gulf,

while major and integrated independent companies were maintaining a steady rate of

activity or reducing their involvement in the Gulf (Seydlitz et al. 1995). Businesses'

activities tended toward becoming more involved in downstream integration operations.

The same study also found that there were more companies involved in Gulf activities in

1994 than 1986, but major and large integrated independent corporations reportedly had

smaller workforces in 1994 as compared to the mid 1980's. About 66% of the study's

respondents reported an increase in the use of outside contractors or service companies

for various activities, while more joint ventures existed between companies (Seydlitz et

al. 1995).

Industrial Restructuring

Since the mid 1980's, a major restructuring of the oil and gas industry in the Gulf of

Mexico has occuned. Cost cutting, along with financial and organizational restructuring,

has occurred among integrated oil companies, but these same measures in varying

degrees have been taken by most companies that survived the slowdown of the mid

1980's. Measures to reduce costs have included reductions in the number of management

layers, and tighter control by headquarters over budgets of home offices, and field

offices. Remote monitoring and control technology for offshore production platforms has

made possible reductions of offshore maintenance and production staff. The proliferation

ofjoint ventures between two or more companies, particularly in deepwater drilling, has

been seen as a way for each company involved to reduce financial risks by sharing those

risks and their own resources (Laska et al. 1993).
Tobin: Socioeconomics

Restructuring in the oil service industry, distinct from the integrated and independent oil

companies, has been consolidating since 1983 through bankruptcies, mergers, and

takeovers. Much of the work offshore is perfonned by subcontractors, whose tasks

include drilling, processing drilling mud, well service, and workover. Among others, oil

service companies are engaged in transportation (vessels and helicopters), supply (mud,

tubing, catering), rental, and manufacturing (metal fabrication and ship and boat building

and repair). Because service company employment is dependent on the exploration and

production budgets of offshore oil producers, any reductions in exploration and

production budgets of oil companies contribute to further consolidation (Laska et al.

1993).

Relationships to Physical Environment

The development of the oil and gas industry along the Gulf coast, by its very nature, has

significantly impacted the natural and physical environments of the region. Any

extractive activity involving a natural resource must by necessity be interconnected to its

surroundings, whether earthly or manmade. The effects of this relationship are numerous

and varied. Early offshore oil and gas activities required navigation channels and canals

to be dredged for oil and gas extraction, dramatically altering the hydrology of the coastal

area. According to the Texas Coastal Management Program in 1996, construction of oil

and gas exploration and production facilities were believed to have disturbed or destroyed

wetlands, seagrass communities, and oyster reefs in Texas (U.S. Department of

Commerce 1996), arid the Louisiana Coastal Wetlands Conservation and Restoration

Task Force noted in 1998 that although dredge and fill activities for petroleum

exploration, pipelines, and canal developments had by that time almost completely

halted, those activities directly and indirectly contributed to marsh destruction (Louisiana

Coastal Wetlands Conservation and Restoration Task Force and the Wetlands

Conservation and Restoration Authority 1998).

A growing environmental concern among coastal states of the Gulf coast region has been

the high rate of coastal land loss. Within the last 50 years in Louisiana alone, coastal

land loss rates have exceeded 40 square miles per year, and in the 1990's the rate has been

estimated to be between 25 and 35 square miles each year, or about 80% of the coastal

wetland loss in the entire continental United States (Louisiana Coastal Wetlands

Conservation and Restoration Task Force and the Wetlands Conservation and Restoration

Authority 1998). Factors that cause marshes to change to open water are complex,

combining natural processes like subsidence and hurricanes with human actions. Along

with loss of the acres of coastal marshes, swamps, and islands, goes the loss of functions

and values associated with the wetlands: commercial harvests of fisheries, furbearers and

alligators; recreational fishing and hunting, and ecotourism; habitats for threatened and

endangered species; water quality improvement; navigation corridors and port facilities;

flood control; and the intangible value of land passed from generation to generation.

These losses affect public usage, but more significantly, they are immeasurable in terms

of cultural and heritage losses.

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Regardless of reasons for coastal land loss, federal, state, and local community agencies

along the Gulf coast not only recognize the importance of preserving and protecting

coastal resource, but are taking measures to slow the erosion process. Loss of this natural

habitat threatens the resilience of commercial fisheries and seafood, endangering not only

that industry, but the economic sustainability of coastal communities.

Agriculture

Oil development activities have impacted the agricultural community as well. Navigation

channels and canals that were once dredged to expedite oil and gas activities have

allowed saltwater intrusion into the rich alluvial land along the Louisiana coast. Today,

citrus growers in Plaquemines Parish in south Louisiana are experiencing crop losses due

to this saltwater intrusion, and rice growers in the central Acadiana region of Louisiana

are concerned about the continued supply of fresh water for their crops (Louisiana

Coastal Wetlands Conservation and Restoration Task Force and the Wetlands

Conservation and Restoration Authority 1998). Agriculture and its related activities have

historically conflicted with other land uses and activities, and oil and gas development is

no exception. As noted before, some farmers sold their land to oil-related companies as

the industry expanded along the Gulf coast. But according to the Vermilion Historical

Society of Vermilion Parish in south Louisiana, the development of the oil industry in

that area was a great boon to the fanners, many of whom had heavy mortgages from

depression years (Vermilion Historical Society 1983). In a recent (forthcoming) study of

a small Louisiana coastal community with a strong oil industry presence, it was noted

that many farmers in the area supplemented their agricultural income with royalties

received from oil production. They were able to make improvements to their lands and

operations, but during the slowdown of the 1980's, the funding needed for farm

improvements dried up.

Recreational Uses of the Coast and Oil

Land usage along the Gulf coast is impacted by offshore activities in yet other ways. If

we look at state maps of Texas, Louisiana, Mississippi, Alabama, and Florida, we will

notice that among them, only Louisiana has no coastal highway. Louisiana has an

unusual coastline compared to most coastal states in the United States, in that it is lined

with a broad and sometimes nearly impenetrable band of coastal marshes. Virtually none

of the state's population lives on or near the coast, and it is often impossible to get within

10 miles of the coast by road; in fact, it has been estimated that only 12.26% of the

Louisiana coast is accessible by roads. This contrasts sharply with the other Gulf coast

states, where the opposite situation exists: most of their population lives on or near the

coast because it's readily accessible by road. In Louisiana, there is no beautiful visual

coastal imagery as is found in other areas like Florida and California, where the coasts are

viewed as things of beauty and easily accessible for recreational pursuits of visitors and

nearby residents (Freudenburg and Gramling 1994).

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The importance of tourism in the economy of Florida has already been noted in this

chapter, and increasingly, the central and western Gulf coast states are following

Florida's lead in realizing the potential for economic growth in the tourist and

recreational industries. In recent years, the states of Mississippi and Alabama have begun

to emphasize tourism as a ma] or component of their economy, and in Texas, community

leaders, government officials, and parks and recreation professionals too have begun to

appreciate the economic value of tourism. This emphasis began during the economic

downturn of the mid 198 Os, when community leaders and economic development

planners began to capitalize on the potential of recreational resources in their areas,

realizing recreation and tourism could create jobs, encourage a more diversified

economy, and help moderate recessions (U.S. Department of Commerce 1996).

However, a study by Gramling et al. (1995) found that individuals most closely

associated with coastal tourism in those states believed it would be, or could be,

negatively impacted by offshore oil and gas activity either due to an offshore accident

that would foul the beaches or that tourists would find the presence of rigs irritating.

Therefore, states along the Gulf of Mexico will have to balance the socioeconomic

benefits of the oil and gas industry with the increasing socioeconomic potential of coastal

tourism.

Because Louisiana's coast is inaccessible in all but a few areas, it has few beach-oriented

tourist attractions, so the offshore presence of drilling platforms is a non-issue in that

regard. There is some eco-tourism in the swamps and marshes, where one can take an air

boat tour of the area, looking for alligators, spotting numerous types of birds, and

admiring huge cypress groves, but those activities, along with duck and goose hunting

and fresh and saltwater fishing, cany on it spite of the network of oil and gas pipelines

and presence of other offshore infrastructures.

Industrial Development and Infrastructure

An examination of oil industry facilities sites by Davis and Place (1983) noted two forms

were evident: clustered facilities and interspersed facilities, both forms of which involved

land use conflicts. In the first type, it was found that residential areas were mixed with

heavy industry or commercial activities and were actually competing for the available

higher ground along watercourses. In some cases, research showed this was because

there was only one road along the bayou, and the community had been there first, but

offshore oil facilities needed deep draft access, so they became neighbors. In the second

type, family businesses were located on land already owned, which happened to also be

the place of residence. Both of these two types of facilities related to oil activities can be

found almost anywhere along the Gulf of Mexico. In Corpus Christi, Texas, there are

some neighborhoods where oil refinery holding tanks are as close a neighbor as the house

next door (Corpus Christi Caller-Times 1998). In 1993, more than 2,000 refinery-area

residents united in a class-action suit with property damage claims against 11 refinery

row industries. As a result of the litigation, still in the courts, little business development

has occurred in those areas. Since 1985, refineries in the area have bought about

500 homes along their boundaries in order to clear a buffer zone they need, which has

caused a sharp decline in the appraised home property values. A struggle over land use

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has resulted in this particular city, but it appears unlikely these residential areas will ever

rebound from oil and gas industrialization.

The 1990's has seen a resurgence in the oil and gas industry that has resulted from the

discovery of oil and gas in the deepwater fields of the central Gulf of Mexico.

Subsequent deepwater royalty tax relief and continued technological improvement has

brought about an increase in offshore and onshore activities in the coastal regions of the

Gulf states. 1-lowever, the volume of activity that is, and will be, associated with

deepwater has raised concerns about infrastructural capabilities of communities involved

in offshore oil industry activities. A 1998 study by the Economic Development

Administration found that coastal leaders are concerned that recent economic growth due

to deepwater development has exacerbated the problems of the deteriorating

infrastructural network found in the region (Louisiana Coastal Wetlands Conservation

and Restoration Task Force and the Wetlands Conservation and Restoration Authority

1998).

Infrastructural networks may be both physical and cultural. Physical infrastructure

usually refers to capital facilities and land assets that are necessary to support

development while protecting public health, safety, and well-being. Among others, this

includes water supply and wastewater disposal, transportation (ports, roads, bridges,

airports, rail, navigation, highways), solid waste disposal, drainage, flood protection,

industrial parks, electricity, and oil and gas structures. Social infrastructure generally

refers to social services, educational facilities, hospitals, and parks.

In order to understand the concerns of today's leaders, we have to look back at the

infrastructural needs of coastal zone activities as the industry developed, and

consequences thereof for the Gulf coast. Communities caught up in rapid development

found they were unable to keep up with the demands for adequate infrastructure.

Significant population increases strained utility and sewage systems, medical facilities,

social service services, and created housing shortages. The shortage of available land in

the 1950's for residential sites around Morgan City, Louisiana was alleviated by creating

a new community, Bayou Vista, in former sugar cane fields. By 1970 it was an

incorporated community of 5,000. The education system was severely strained during

periods of rapid oil and gas development; between 1950 and 1970, St. Mary Parish

experienced a 115% increase in public school registration. The educational system,

perpetually underfunded, experienced crowded classrooms, scarcity of supplies, and

teacher shortages (Gramling and Freudenburg 1992).

At the same time, oil industry development also led to infrastructural improvements in

local medical facilities, and water and sewer systems, among others. Public bonds

ftinded new construction projects that communities hoped would draw new enterprises,

but with the decline in production value and activity associated with exploration in the

mid-1980's, these facilities constructed to attract new businesses became liabilities for the

communities instead, as they had to continue making payments on those facilities,

regardless of economic downturns. Property values declined, in some instances there was

high outmigration, and businesses of all types failed (Gramling and Freudenburg 1992).

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The concerns that coastal leaders have today of the physical infrastructure involve roads,

highways, and bridges that are not only deteriorating with age, but are concomitantly

becoming more congested as regular offshore oil and deepwater activities increase.

Many of these are vital hurricane evacuation routes for coastal residents (Louisiana

Coastal Wetlands Conservation and Restoration Task Force and the Wetlands

Conservation and Restoration Authority 1998).

Port Fourchon

Located 21 miles south of Port Fourchon, Louisiana, in Lafourche Parish, is home of the

Louisiana Offshore Oil Port (LOOP), the nation's only deepwater superport, providing

tanker offloading and temporary storage services for crude oil transported on some of the

largest tankers in the world. This port is the geographic and economic center of offshore

drilling efforts along Louisiana's Gulf coast. In 1995, LOOP handled over 250 million

barrels (bbl) of crude oil, or an estimated 685,000 bbls of oil per day. Louisiana

Highway 1 is Port Fourchon's only land-based access, and it was estimated in 1998 that

each month 30,000 trucks and over 200,000 passenger vehicles traveled that road.

Traffic flow figures predict an increase of between 3% and 6% annually in the next

decade with continued anticipated expansion of the port (Louisiana Coastal Wetlands

Conservation and Restoration Task Force and the Wetlands Conservation and Restoration

Authority 1998). A report in The Advocate, a Baton Rouge, Louisiana newspaper

(Guarisco 1998), estimated that 21% of Highway 1 was in poor condition and 98% was in

need of some kind of improvement. Furthermore, most people agreed in the Port

Fourchon area that the Leeville Bridge, on which every vehicle must cross heading south

to the port, was the most critical point on the route. This 30 year old deteriorating bridge

must lift to allow boats to pass below on Bayou Lafourche; when it is open, or broken,

traffic backs up. There is no alternate route.

The primary worries of leaders and residents of Lafourche Parish at the time of the

newspaper article dealt with both present and future concerns: there were only five

deepwater rigs in operation, but another 78 were on the drawing boards (Guarisco 1998).

Because deepwater activity takes place so far offshore, beyond state waters, the federal

government keeps the taxes collected on production. Not much is returned to the states,

so without this tax base, local and state economies find needed infrastructural

improvements problematic, unless local and state economies are further burdened.

Coastal states are therefore hoping to have legislation passed that would give more of the

tax proceeds to the states. Besides road and bridge maintenance and repairs,

infrastructural concerns included nonexistent available housing, with waiting lists for

those wanting rental property to lease, and availability of fresh water; the Lafourche

water company was then selling millions of gallons of fresh water to Port Fourchon and

the surrounding area daily, and demand was anticipated to increase as deepwater projects

continued.

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Oil Spills and Flazardous Waste

Growth of offshore oil activities in the Gulf coast has been accompanied by other

environmental concerns, those of potential oil spills and hazardous waste disposal. The

Louisiana Department of Transportation (1999) Internet website cites the LOOP facility

in Louisiana as having had a satisfactory safety and performance record, according to the

Executive Director of the facility, noting cumulative volume of spills in 1996 totaled

1.5 gallons. Furthermore, since 1980, OCS operators have produced 4.7 billion bbl of oil

and spilled only 0.00 1% of this oil, or 1 bbl for every 81,000 bbl produced. In the last

15 years, according to MMS (1999), there have been no spills greater than 1,000 bbl from

an OCS platform or drilling rig, and the spill risk related to a diesel spill from drilling

operations is even less. From 1976-1985, there were 80 reported diesel spills greater than

1 bbl associated with drilling activities, compared with 11,944 wells drilled, or a 0.7%

probability of occurrence. Interestingly, natural seepage of oil in the Gulf of Mexico is

far more extensive. Researchers have estimated a natural seepage rate of about

120,000 bbl per year from one area of 23,000 square kilometers offshore of Louisiana.

The lead state agency in Texas for the prevention of and response to oil spills in the

marine environment is the Texas General Land Office. In 1995, the program responded

to approximately 1,250 reported oil spills, noting that despite the increasing amount of oil

handled in Texas waters, the number of oil spill responses decreased due to the program's

aggressive enforcement program, aerial surveillance activities, and increased harbor and

vehicle patrols (U.S. Department of Commerce 1996).

However, as offshore activities have developed, there have been reports of oil spills in the

Gulf of Mexico that have either involved ship/barge collisions, or pipeline ruptures. In

1993, three vessels collided at the entrance to Tampa Bay, Florida, resulting in the release

of an estimated 328,000 gallons of oil which formed a 17-mile oil slick. High winds

drove the oil to shore, impacting 2 miles of beach park and wildlife found in the area

along the Gulf of Mexico. Within a month all loose oil had been recovered. This

particular incident was significant as it was the first major demonstration of the U.S.

Coast Guard vessel response plan requirements. The cleanup was financed by owners of

the barges involved in the collision, who divided tasks and geographic sectors among

several contractors. Approximately 1,800 professionals and volunteers assisted in

cleaning up the oil that came ashore (Environmental Protection Agency [EPAJ 1999).

In 1997, there was a 5,000 bbl oil spill in the saltwater Lake Bane, off the Louisiana

coast. The spill was the result from the rupture of an offshore pipeline bringing oil

onshore. A study conducted Pulsipher et al. (1998) at the time of the oil clean-up showed

that the short-term social and economic consequences of the oil spill were modest. This

has been aifributed to three factors. The first is that there is a flexible adaptive

cooperative coalition in the oil spill cleanup industry that springs into action when large

spills occur along the Gulf coast. The presence of this coalition was noted during

clean-up activities. Second, cleanup activities were fortunately of relatively short

duration, which further limited the short-term economic arid social impact. Third,

because the spill site was geographically isolated, most coastal users were able to avoid

the spill site.

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Hazardous waste disposal has been somewhat more problematic for the Gulf coast

region. Drilling and production activities generate liquil and solid wastes, including

debris, drill cuttings, drilling fluids, and wastewater, particularly produced water or brine.

Additionally, activities of companies directly linked to the oil industry frequently produce

contaminants that are potentially environmentally harmful. Although most agree that

tighter government control along with close monitoring of waste disposal has helped

reduce potential environmental problems, costly federal cleanup operations of numerous

sites along the Gulf coast point to problems arising from the past. Improper disposal of

oil and gas activity contaminants in some areas have threatened groundwater of some

communities. In Abbeville, Louisiana, for example, two adjacent sites placed oil drilling

muds, salt water, and other drilling fluids into either on-site tanks, or earthen pits. EPA

tests in late 1985 revealed, among other compounds, lead, arsenic, benzene, and toluene

in an area where about 2,600 people obtained their drinking water, and 1,000 acres of

cropland were being irrigated by private wells within 3 miles of the site. Another

9,000 acres were being irrigated with surface water, also potentially threatened by the

sites (EPA 1999).

It is clear that oil and gas development in the Gulf of Mexico has significantly affected

and altered land use patterns; residential, industrial, recreational, and occupational. In

some coastal communities, there is ready acceptance and acquiescence of oil industry

development needs and demands, in others, there is uneasy give-and-take, and in yet still

other coastal communities, there is absolute rejection. The industry in effect asks of these

communities to make a considerable investment in their own future, a future over which

they actually have very little control.

Impacts on Families and Individuals

Researchers have known that rapid technological development in the oil industry has

been accompanied by significant impacts on individuals and their families Some of

these have taken place rather quickly, while other impacts have been spread along a

continuum over the many decades of Gulf coast oil involvement. Most research has

shown that communities experiencing rapid industrial development have an improvement

in their economic position (Laska et al. 1993); therefore, it follows that individuals

residing within those communities should benefit from increases in employment

opportunities and higher wages. Offshore oil industry activities in the coastal

communities of the central Gulf of Mexico are no different; they are widely believed to

be connected to positive economic benefits which includes generating new jobs within

the industry itself, and general economic spinoffs. Conviction that these activities are

associated with positive economic benefits stemmed from either past experiences, or a

general understanding that it was logical for industrial activity to stimulate job growth

(Gramling et al. 1995). From 1959 to 1969 the new jobs created by the oil and gas

industry increased the median income of families in urban areas of 2,500 or more

inhabitants by several thousand dollars per year (Bobo and Charlton 1974; Davis and

Place 1983). Another example of economic improvement of individuals within a

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particular community can be shown by the growth of per capita personal income in

Lafayette, Louisiana from 1973-1980 when it increased at an annual rate of 8.1% during

the period of rapid development in the offshore industry. This exceeded the total

Louisiana rate by a factor of two and by a factor of four over the United States rate, and

was the result of the widespread real growth of Lafayette (Manuel 1983). This

unparalleled economic prosperity, and significant economic gains, were felt not only in

Louisiana coastal communities and their residents, but as was pointed out previously, in

Texas as well.

Although offshore oil extraction shares similarities with Western U.S. energy

development projects, among them the cyclical nature of extraction, creation ofnewjobs,

and the influx of immigrants into an area seeking job opportunities, there are differeilces:

new employment opportunities associated with offshore oil development may not be

located near the source of the oil, and jobs on oil rigs may be filled by non-local

individuals who are able to commute long distances due to concentrated work scheduling

(Gramling and Brabant 1986; Gramling 1989; Forsyth and (iauthier 1991). But

Gramling and Brabant (1986) also found that in the case of East St. Mary Parish in

Louisiana, there was a strong secondary and tertiary service sector that developed,

employing local people working under traditional conditions and hours.

In some cases, traditional economic activities of long-term residents were replaced by the

development and expansion of economic activities associated with offshore oil that

appeared to be more highly paid. This was exemplified in the case of the Morgan City

shrimp industry, which ceased to exist with the rapid expansion of offshore activities and

subsequent growth of oil and gas related services sector.

Oil and Louisiana Cajuns

An ethnic group in Louisiana whose traditional economic activities were greatly affected

by oil industry development was the "Cajuns." These exiled Acadians from Nova Scotia,

Canada, began moving into south Louisiana in the 1750's. Acadian pioneers were

characterized by individualism, adaptability, pragmatism, industriousness, egalitarian

principles, and an ability to pull together when threatened. They possessed extended

families, distinctive language and speech patterns, and lived an agrarian way of life

(Brasseaux 1992). All immigrants to Louisiana's wetland landscapes, according to Davis

(1994), developed cultural practices tied to the annual-use cycle that is still linked to the

region's natural resource base.

Getschow and Petzinger of the Wall Street Journal (1984) printed a series of articles on

the oil industry's impact on Louisiana's Cajun culture, concluding that extraction had

depleted that culture. Specifically, the articles focused on how the oil boom brought oil

men in the 1930's to an area where life was "frozen in time," and fishing was "in the

blood." These outsiders, or "Texians" as they were called, at first disparaged the Cajuns

because of their quaint customs and clumsy English. The Cajun "swampers" and

subsistence farmers gave up their traps and leased or sold their land to take part in the

economic boom of rapid oil development. Many became the backbone of the oil industry
Tobin: Socioeconomics

in south Louisiana, toiling under the harsh sun to help extract a resource that was needed

by an entire nation. But the price that was paid by this ethnic group was high, according

to the Wall Street Journal. The concentrated work schedules pulled apart family ties,

fishing grounds were despoiled by years of furious oil aiid gas production, and the Cajun

society was rio longer self-sufficient, but rather one dependent on oil.

Not everyone agrees with this depiction. Gramling (1994) did not concur with the Wall

Street Journal's articles, instead remarking that people he interviewed gave the offshore

industry credit for helping to hold the Cajun culture together. In times of prosperity, oil

industry development may actually have helped sustain locally valued ways of life. Long

extracting the region's natural resources, Cajuns viewed oil as another opportunity for

employment and business development. The concentrated work scheduling allowed the

offshore workers the chance to earn good wages, while continuing to enjoy a home life

on days off in an area they felt comfortable with. It is well known to anyone living in

south Louisiana that many of the traditional ways of life are alive and well. In our

experience, when interviewing respondents who reveal they are of Cajun heritage (and

they usually do), they frequently pride themselves as having a heritage that breeds

stubbornness and toughness in times of turmoil and trouble. Today they continue to have

a strong cultural presence in the south Louisiana region.

Economic Impacts on Individuals and Families

Because the oil industry is cyclical, there are times of slowdown, with decreased

exploration, drilling, arid production activities. This impacts cormnunities, families, and

individuals negatively, and we would expect to see this reflected in certain economic

measures. Tolbert (1995) examined family income inequality trends in coastal Louisiana

parishes adjacent to the developed OCS, and coastal counties of the Florida panhandle

where there has been no significant onshore or offshore development. A comparative

inequality analysis of data from the 1970, 1980, and 1990 U.S. Censuses revealed that

inequality in Louisiana exhibited a great deal of volatility from the 1970's period of rapid

oil and gas expansion through the 1980's period of decline. Although Florida inequality

trended downward over time, by 1990 in selected Louisiana parishes, inequality was

higher in several cases than it had been in 1970. These patterns suggest a substantial

impact of onshore and offshore industry activity on coastal families in the middle to

upper-middle portions of the income distribution.

Increased opportunities for the types of highly paid employment frequently associated

with offshore oil activities usually suggests widespread prosperity within those

communities. However, there are other reasons than those previously mentioned why not

all local individuals will benefit from the development; those with few skills or who are

not willing to train for work in energy-related jobs will not benefit from the improved

economic conditions (Manuel 1983). Those that are economically marginal at the time of

rapid growth and development in the offshore oil industry are impacted in numerous

ways. Brabant (1993) found that in four Louisiana parishes, those who were poor prior to

development were similar in that they were local people, uneducated, and they lacked

skills. With the tremendous growth in the offshore oil industry, there was a significant

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decline in the mean number of households receiving food stamps by the mid 197OTs. Not

only were those poor who wanted to work able to extricate themselves from welfare rolls,

they were able to earn substantial incomes even if skills and education were deficient.

Yet, not all locals benefited from the growth. Brabant found that the situation for the

elderly poor worsened because the cost of living went up, which impacted them

financially. Fixed incomes and low incomes meant housing could be problematic for

families because of housing shortages or higher housing costs, unless they were home

owners prior to the increased activities.

There are other negative economic impacts on individuals and their families during this

period of rapid oil industry development. These include inflationary prices on goods

purchased in the community which contributes to increased costs of living, and economic

issues with regards to "amenity" uses of the coast, such as tourism, versus "consumptive"

uses (Gramling 1995). This has been at the heart of the argument against offshore

development by residents and communities of the Florida Gulf coast. There, tourism is

not simply a dominant element in the economy; it is the economy. Offshore oil

development seen as unwanted competition over limited coastal resources, and is

aesthetically incompatible with the amenity utilization of the coastal areas (Gramling and

Freudenburg 1995).

During times of decline in oil industry activities, impacts on individuals and their families

include loss of employment or reduction in wages/number of hours worked, and

increased poverty. In the 1950's and 1960's, migrants came to the Louisiana coast

seeking jobs in the oil industry, and many of them had no familial or other ties to the

area. The local social service agencies that provided Food Stamps and Welfare

Assistance reported that many of these were hopeful and desperate to find employment,

and were unaware that social service provisions in this particular part of the country were

not as available as they had been in the North Central "Rustbelt" economies. The state

was simply not prepared to deal with this influx of new residents needing social services,

and when the slowdown came, the state retracted its expenditures at the local level (Laska

et al. 1993). Brabant (1993) found that the mean number of households in Lafayette

Parish receiving food stamps in 1983 increased 20.7%, and in St. Mary, 29.9%. These

households consisted of previous welfare recipients, and those referred to as "new poor";

individuals who had once enjoyed high-paying jobs in the oil industry, but had

subsequently lost them, and consequently found themselves unable to meet their financial

obligations. These were people who had never been poor before. The number of people

in Texas living below the poverty level grew by more than 500,000 in the decade of the

1980's, to 2.7 million in 1990 from 2.2 million 10 years earlier. During the same time,

demands for social services increased (Hodel and Deitz 1993). In a recent study

(forthcoming) of a small coastal Louisiana community, executives with oil-field related

businesses frequently reported that in the late 1980's, a period of severe oil industry

slowdown, in an effort to avoid letting employees go, would greatly reduce employees

hours, eliminate bonuses and paid vacations, and asked employees if they were willing to

take a cut in pay. Although these appear to be drastic measures, it was their attempt to

delay as long as they could, laying off their employees.

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Social Problems

There has been some research investigating the relationship between rapid oil and gas

development and various social problems. It is widely accepted by researchers that rapid

social change is often accompanied by social disorder. Factors such as increased

economic inequality and inmigration are believed to be partly responsible for breakdowns

in social control that lead to increases in social problems. Seydlitz et al. (1993), using

data from 1956-1990, found in her analyses of Louisiana parishes that rapid increases in

oil and gas activities do disrupt social controls that serve to inhibit social problems. For

instance, increases in suicide and homicide rates between periods of rapid growth of

activities and severe industry retractions were found to be greater in these parishes

between 1956-1990 than those in the U.S. overall. However, there appeared to be a cycle

of disruptions and reequilibriums, with increases in social problems followed by

decreases, and not just one large fluctuation related to rapid growth and abrupt decline.

Criminal court cases, homicide rates, suicide rates, and rates ofjuvenile commitments

were not consistently statistically significant among parishes differing in level or type of

involvement in oil and gas activities. Although Seydlitz et al. (1993) did find a

relationship between oil and gas development and social problems, they point out that it

is not clear, consistent, nor strong.

Concentrated Work Scheduling

Among the biggest socioeconomic impacts of offshore oil development and activities on

individuals and families has been the concentrated temporal scheduling of work. There

are a number of reasons. First, concentrated work scheduling (7 days on, 7 days off, etc.)

attracts employees from a larger geographical area, and diminishes the potential

concentration of settlement around this economic activity, oil and gas extraction

(Gramling 1989). This allows families of offshore workers the flexibility of living in a

traditional place of residence while the offshore worker goes where needed. Decisions

concerning the site selection of oil activities can be made with less regard to existing

human settlement patterns because of the large number of these long-distance commuters.

Gramling and Brabant (1986) found that employment patterns in East St. Maiy Parish in

Louisiana prior to the offshore industry development did not emphasize this type of

nontraditional work scheduling. Therefore, local residents appeared reluctant to work

offshore in a concentrated work scheduling environment. Because the rapidly growing

support sector offered available jobs, offshore work was often filled by commuters

outside the area. For some, this pattern of concentrated work scheduling has become a

way of life. By working offshore, they may be able to escape fluctuations in local or

regional economies; for example, if they or their family reside in an economically

depressed area, concentrated work scheduling elsewhere allows them the possibility of

escaping the local economic impacts arid participating in a wider labor market. In

addition, there is the possibility of upward mobility without familial dislocation.

A major aspect of concentrated work scheduling is that participation in the nuclear family

and its social networks is interrupted. While other family members operate on more

traditional time/space schedules, the offshore worker experiences a totally different

non-standardized work schedule. A 1991 study by Forsyth and Gauthier reported that

312
313

Tobin: Socioeconomics

families of offshore workers frequently experienced problems with integration; only

through changing their basic structure and organization could they remain viable social

systems. Adaptation to that type of work scheduling remains a source of stress for

individuals and families.

Those individuals working offshore find transportation between work sites offshore and

support bases onshore requires boats or helicopters and takes time. With deepwater

drilling developing in areas hundreds of miles offshore, transportation time will increase.

Concentrated work schedules may change to accommodate those longer distances and

increased time in that workers' periods of "on days" and "off days" may be extended.

Although offshore housing has a number of amenities, there are still problems associated

with living offshore, including physical risk and strenuous labor in sometimes hostile

environments, limited recreational facilities, and loneliness (Gramling 1989).

Impacts of Oil on Education, Migration, and Population

As oil industry activities developed along the Gulf of Mexico, and residents of

communities began to enjoy the positive economic benefits associated with the rapid

economic growth of the industry, individuals began to make career decisions based on the

expectation that the oil industry would stay at that high level of development. Becker

(1971) notes that people make conscious decisions to invest in education: factors such as

costs of obtaining an education, rate of return, market influences, and supply and demand

conditions determine an individual's decision. Costs vs. benefits are weighed.

Furthermore, people are more likely to migrate/relocate than invest in education when

higher paying jobs can be obtained simply by moving and higher education is not

necessary for those jobs. There was such a growth in activities along the Gulf coast that

more highly paid jobs become available without greater education, which reduced the

perceived benefits of higher education. Seydlitz and Laska (1994) found that greater

petroleum industry activity in Louisiana parishes from 1956-1990 was associated with

higher percentages of students completing high school, but lower percentages of high

school graduates enrolling in college. Trade schools, in response to industry demand,

taught skills that were marketable for the offshore activity (Gramling and Reilly 1980).

Because educational attainment of the residents of an area has a strong impact on future

development opportunities, localities would benefit from a well-educated populace. One

of the concerns of Louisiana economic development specialists is that the success and

embeddedness of the oil and gas industry has actually served as a deterrent to education.

The development of the oil industry over an extended period of time has attracted people

to the area because of the available work in the industry that promised financial rewards.

A 49 county and parish study of the Texas, Louisiana, Mississippi, and Alabama Gulf

coast showed that there was a 20.78% increase in population from 1960-1970; from

1970-1980 there was a 27.88% increase. The average annual population growth rate of

2.79% across those states was nearly twice the national average of 1.15% during the same

period. Oil and gas activities are used to explain why the population in this central and

western Gulf of Mexico region increased by 54.46% from 1960-1980. During this same
Tobin: Socioeconomics

20 year period, the central Louisiana coast alone gained 58% (McKenzie et al. 1993).

Unprecedented net inrnigration of 126,007 persons to Louisiana occurred in the 1970's,

which was the first and only decade since 1870 that Louisiana had experienced any

substantial net inmigration, and it was clearly related to the oil boom and accelerated job

growth that began in the early 1970's and extended through the early 1980's (Maruggi and

Wartenberg 1996).

One impact attributed to the decline in oil and gas activities along the Gulf of Mexico

coast has been population decline. In 1981, 5 of the 49 counties and parishes within the

McKenzie et al. (1993) study area experienced negative net migration. In other words,

there were more people moving out of the area than moving in. By 1984, the number of

counties and parishes experiencing negative net migration had increased to 35. In

southwest Louisiana, the 1981-1982 percent change of 3.33 in population shifted from a

rate three times the national average to a net population loss within 2 years. The loss of

the high-paying oilpatch jobs in the 1980's resulted in an outmigration from Louisiana of

411,099 persons, and according to Maruggi and Wartenberg (1996), the underlying cause

of the net migration reversal was not so much from an increase of people moving out of

Louisiana, but from the virtual cessation of people moving into the state.

With the 2000 Census on the horizon, researchers will have access to much data,

including population data from the 1990's. It will be interesting to see what changes have

taken place in coastal communities along the Gulf of Mexico in the past decade. At least

until the slowdown of the late 1990's, the authors of the Louisiana Economic Outlook

were forecasting that the Houma Metropolitan Statistical Area would add almost

6,000 jobs during the 1998-1999 fiscal year. Behind their belief in rapid growth was the

oil and gas exploration and production resurgence in the Gulf, especially in the "deep

water Gulf' (Pulsipher et al. 1998). Because of the recent slowdown in the oil and gas

industry, forecasts such as this one may have been overly optimistic.

Conclusions

No other industry in recent history has had more of an impact on individuals arid families

living along the central and western Gulf of Mexico coastal region than oil and gas.

Particularly along the Louisiana Gulf coast, nearly everyone knows someone who earns a

living as an employee of an oil- and gas-related company, and many people can relate a

story involving some aspect of offshore development. The effects are so pervasive it

would be hard for most people residing in the area to imagine what it would be like

without the presence of oil industry activities.

The brief review presented here of the socioeconomic impacts of oil industry activities on

the Gulf of Mexico coastal region has primarily focused on those resulting from shallow

water activities, not deepwater. Because deepwater activities are fairly new along the

historical continuum of oil and gas development, and therefore little socioeconomic

research exists for those activities, there is a recognition among researchers that before

gaps in data become too large, research efforts need to step up their pace. Ascertaining

314
Tobin: Socioeconomics

the differences and similarities of socioeconomic impacts upon individuals, families,

communities, and states between shallow and deepwater oil industry activities will not be

easy because of the integration of respective activities by oil- and gas- related companies.

With few exceptions, those areas most likely to be affected are presumed to have already

become immersed in such activities.

Some of the differences we have noted between deepwater and shallow or coastal zone

operations, in terms of social and economic impacts, are derived from the need for

cost-effective measures, and technological innovations. These may include lengthened

concentrated work schedules for those employed offshore in deepwater; joint ventures

between companies affecting labor demand; and stricter industry safety standards

necessitating a greater need for skilled labor. Additionally, activities supporting

deepwater development strain community infrastructures, and communities must have an

adequate tax base with which to meet both public and private needs. Federal revenue

from taxes levied on deepwater production must be fairly distributed to compensate for

demands placed on Gulf coast communities due to OCS activities.

Information needs continue to be similar to those noted in Carney (1998). The goal of

this socioeconomic research is to better understand events of the past, placing them in

some theoretical perspective, and then through further use of empiricism, explain societal

stmctures in such a way as to benefit the future. For instance, industrial labor surveys

would provide information on industry behavior, and community capacity evaluations

and subsequent action could help mitigate potential problems.

Deepwater development is now a technological advancement that will serve to strengthen

the commitment of individuals and families within this area to the oil and gas industry.

Regardless of problems brought forth by oil industry activity, and the rhetoric of needed

economic diversification, it is a fact that oil and gas development along the OCS in the

Gulf of Mexico has been a significant force that in many respects has benefited the entire

region.

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Chapter 12: Synthesis

Neal W. Phillips

(with contributions by authors of Chapters 2-11)

Introduction 321

Operational Contrasts and Implications 325

High Production Rates 325

Shift in Types of Structures 325

Changes in Drilling and Production 326

Transportation Challenges 327

Socioeconomic Implications 327

Environmental Contrasts and Implications 328

Physical/Chemical Environment 328

Water Column Ecosystems 330

Benthic Ecosystems 333

Data Gaps and Information Needs 334

Deepwater Technology 334

Geology 334

Physical Oceanography 335

Chemical Oceanography 336

Water Column Biology 336

Benthic Communities 337

Protected Species 337

Fishes and Fisheries 338

Socioeconomics 338

Literature Cited 339

Introduction

The rapidly increasing level of deepwater development in the Gulf of Mexico poses a

number of environmental, socioeconomic, and technological issues. Both new and old

(existing) technologies are being introduced into a relatively unfamiliar environment.

The effects of new technologies and deepwater development are not limited to that

environment, but also have implications for onshore, coastal, arid shelf environments

where the existing infrastructure lies.

As noted by Carney (1998), the Minerals Management Service (MMS) is highly

experienced in evaluating potential impacts of existing technologies in coastal and shelf

environments. He identified two main problems posed by deepwater development:

1. "Identify those aspects of deepwater development for which the possibility of

environmental or socioeconomic impact is no different than that now experienced in

shallow water. Such activities may be evaluated and regulated largely within the

current scope of knowledge, and MMS' needs for new information should be modest

and easily met."

321
Phillips et al.: Synthesis

2. "Identifi those aspects of deepwater development for which the modes of

development and/or the environments subject to impact possibly pose novel threats of

impact. For such activities the adequacies of current knowledge for evaluation and

regulation must be seriously questioned. In many cases, the absence of management

information will be the result of major gaps in scientific understanding of the deep

ocean...."

Previous chapters of this report have discussed deepwater technology (Chapter 2), the

physical and chemical environment (Chapters 3-5), biological communities and fisheries

(Chapters 61O), and socioeconomics (Chapter 11). This Synthesis Chapter draws

together information from the preceding chapters to contrast deepwater operations and

deepwater environments with their shallow water counterparts. Important differences are

noted and their implications discussed. Such contrasts provide a basis for addressing the

two problems cited by Carney (1998) and for identifiing data gaps and future study

needs. Table 12.1 summarizes selected operational and environmental contrasts

discussed in this chapter.

The chapter is organized into three main sections:

Operational contrasts and implications. Based on Chapter 2 (Deepwater

Tecirnology) and other sources, this section highlights the main differences in oil and

gas operations between deepwater and shelf environments, and discusses implications

for assessing potential enviromnental and socioeconomic impacts.

Environmental contrasts and implications. This section focuses on significant

differences between deep and shallow ecosystems that could affect the nature of

environmental and socioeconomic impacts.

Data gaps and information needs. This section identifies what types of information

are needed that are currently not available to support decisions regarding oil and gas

operations in deepwater environments.

322
Operational

Transportation challenges

Onshore activities

Environmental

Complex, rugged topography, steep

slopes, etc.

Energetic currents (various classes)

Complex mesoscale circulation

features

Phillips et al.: Synthesis

Table 12.1. Operational and environmental contrasts between deepwater

and shelf settings, and their implications

Technological challenges for transportation and

processing

Increased volumes of produced water discharge

Potential increase in spill risk (and size)

Epipelagic fishes should be attracted to deepwater

structures (similar to shelf), but fewer structures

needed

Possible future reduction in explosive platform

removals

Benthic impacts will still occur due to

mooring/anchoring systems, flowlines, etc.

Increased use of synthetic drilling fluids

(environmental impacts are being studied)

Drilling impacts lessened if wells are fewer, more

dispersed spatially (e.g., subsea wells tied to

floating production structure), and in deeper water

(enhanced dispersion)

Flowline chemicals pose new spill risks

Longer and heavier pipelines needed, with impacts

to deepwater benthic environment

Increasing cost of pipelines may favor FPSOs

(environmental risks beillg studied)

Distance from shore affects work schedules,

MMS inspections

Sophisticated technology, stringent safety

requirements increase demand for skilled labor

Increasing strain on coastal infrastructure

Concerns over future economic downturns

Novel geohazards, engineering challenges

Challenges for designing facilities, predicting fate

of leaks and spills

Effects on productivity of biological communities

in water column (and benthos?)

323

High oil and gas production rates

Shift from bottom-founded to

floating production structures

(tension leg platforms, spars, etc.)

and subsea systems

Changes in drilling and production

Contrast Implications
Phillips et &.: Synthesis

Table 12.1. Operational and environmental contrasts between deepwater

and shelf settings, and their implications (Continued)

Environmental (continued)

Hydrocarbon seepage

Biological "hot spots" of increased

primary and secondary productivity

in the water column

Migratory pathways for epipelagic

fishes present in deepwater

Fisheries include extensive surface

and bottom longlining operations

Concentrations of fish eggs and

larvae in deepwater

Much larger suite of up to

28 cetacean species (vs. 2-3 species

on shelf) including deep-diving and

cryptic species

Some cetaceans associated with

mesoscale oceanographic features

Lower adult turtle abundance, with

leatherbacks outnumbering

loggerheads

Hatchlings and juveniles of all five

turtle species may be present in

Sargassuin and flotsam

High diversity of deepwater benthos

Lack of understanding of benthic

processes including benthic-pelagic

linkages

Topographic complexity of benthic

environment, much of which has

not been explored

Widespread presence of

chemosynthetic communities

Background of hydrocarbon presence in relatively

uncontaminated environment

Presence of chemosynthetic communities

(requiring protection/avoidance)

Localized concentrations of plankton, fishes,

cetaceans (potential for impacts)

Attraction of epipelagic fishes to structures may

have regional impacts on migration and spawning

Potential space-use conflicts with fishing industry

Potential for impacts due to spills

Many more cetacean species may come in contact

with oil and gas operations (e.g., potential for

colliding with vessels)

Cetacean preferred feeding areas are not

geographically fixed. Complicates protection, but

also offers possibility of avoiding some impacts by

coupling project schedule/location with remote

sensing

Potential for some impacts reduced (especially

those associated with structures and benthic

activities)

Potential for impacts due to spills

Potential impacts on benthic biodiversity

Complicates predicting impacts and recovery

(e.g., do benthic hot spots occur, and if so, are they

spatially predictable?)

Potential for unusual benthic communities

(e.g., associated with canyons)

Dense assemblages require protection and

avoidance

324

Contrast Implications
Phillips et aL: Synthesis

Operational Contrasts and Implications

Deepwater operations differ from their shelf counterparts in several key aspects, as

discussed in Chapter 2. Differences are also highlighted in proceedings of the MMS

Deepwater Workshop (Carney 1998) and in an MMS report by Cranswick and Regg

(1997).

high Production Rates

Large, productive reservoirs exist in the deepwater Gulf of Mexico. As discussed in

Chapter 2, flow rates for wells completed in reservoirs typical of shallow water locations

have generally been in the range of up to 2,500 barrels of oil per day (BOPD). In

contrast, daily production rates as high as 50,000 BOPD have been reported from the

deepwater Gulf of Mexico. Such rates are due to the characteristics of deepwater

reservoirs and advances in well technology.

High production rates are among the characteristics that make deepwater development

economically attractive. But they pose technological challenges for transporting and

processing oil and gas (Chapter 2). In addition, rates of produced water discharge are

expected to increase (Hays and Ray 1998). Risks of oil spills may increase, since such

risks are calculated relative to the volume of oil handled (Anderson and LaBelle 1994).

Spill size may increase due to the higher flow rates handled by larger pipelines (Rainey

1998). The environmental implications are well known to MMS and are a matter of

degree rather than a novel problem.

Shift in Types of Structures

With increasing water depth has come a shift from primarily bottom-founded structures

(fixed platforms and compliant towers) to floating production systems (tension leg

platforms [TLPs], spars, semisubmersibles), and subsea production systems (Chapter 2).

It has also been suggested that the overall numbers of production structures per unit area

will be lower in deepwater due to more productive reservoirs and increased use of subsea

wells (Hays and Ray 1998).

Oil and gas platforms on the shelf typically develop a fouling community and attract

epipelagic fishes (Gallaway and Lewbel 1982; LGL Ecological Research Associates, Inc.

and Science Applications International Corporation 1998). While fouling community

development is expected to decline with increasing water depth, deepwater oil and gas

structures in the Gulf of Mexico will certainly attract epipelagic fishes of all kinds. The

attraction of fishes to floating structures in the upper water colunrn should be similar to

what is observed on the shelf (Chapter 10).

A shift away from bottom-founded structures should tend to reduce future impacts of

platform removal. On the shelf, platforms are usually removed using explosives to sever

platform legs just below the seabed (MMS 1997). Explosive platform removals pose

risks to marine mammals and turtles (Klima et al. 1988; Gitschlag and Herczeg 1994)

that have been effectively mitigated by NMFS monitoring requirements (National

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Research Council 1996). Deepwater structures that are not bottom-founded may not

require explosive removal. Cranswick and Regg (1997) also mention the possibility of

mid-water abandonments, in which structures might be cut off in mid-water, leaving the

bottom half in place. Ecological implications have not been studied.

The shift away from bottom-founded structures will not avoid all benthic impacts of

structure installation. TLPs, spars, and semisubmersibles are held in place by mooring

systems that connect the floating production system to piled foundation systems or

anchors on the seafloor. Subsea wells are connected to the floating production system via

flowlines, which rest for some length on the seafloor. Impacts on benthos in the

deepwater Gulf of Mexico have not been studied. The MMS is initiating a monitoring

study (Effects of Oil and Gas Exploration and Development at Selected Continental

Slope Sites in the Gulf of Mexico) that should provide relevant information.

Changes in Drilling and Production

Deepwater drilling is expected to make increasing use of synthetic based drilling fluids

(Hays and Ray 1998). In the last decade, a variety of synthetic drilling fluids have been

developed in an effort to provide the oil and gas industry with "environmentally safe"

alternatives to oil-based muds. While potential impacts of water-based drilling fluids

have been studied extensively (National Research Council 1983; Neff 1987), impacts of

synthetic drilling fluids are less well documented. The MMS has just initiated a Gulf of

Mexico Comprehensive Synthetic Based Fluids Monitoring Program to provide

information on the environmental distribution, fates, toxicity and ecological effects of

cuttings from synthetic based drilling fluid systems near platforms in the western and

central Gulf of Mexico. This information will be used by the U.S. Environmental

Protection Agency and the oil industry to develop effluent limitation guidelines and other

regulations for these discharges.

As noted in Chapter 2, with increasing use of subsea wells tied to a floating production

system, the wells can be spread out and distributed to penetrate the reservoir where

needed. This may result in simpler, shorter, more direct wells, as compared with having

to drill wells from a comnion surface location with significant horizontal segments to

reach different parts of the reservoir. Where surface locations are spread out, drilling

fluid and cuttings discharges from initial spudding of a well (i.e., before the riser is set,

allowing drilling fluids to be recirculated to the surface) should also tend to be more

diffuse. On the other hand, Hays and Ray (1998) noted that greater total volumes of

cuttings may be expected in deepwater due to multiple wells and horizontal well

segments. Also, the dispersion of discharged cuttings will differ due to the increasing use

of synthetic drilling fluids, which adhere strongly to cuttings particles; cuttings from

synthetic based systems tend to clump and sink rapidly to the seafloor. The

aforementioned MMS monitoring study of selected exploration and development sites is

expected to provide relevant information.

A major technological challenge is ensuring continuous flow of unprocessed oil and gas

from subsea wells through the flowlines on the seafloor (Chapter 2). Technology is not

presently available for subsea processing of oil and gas produced from subsea wells.

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Thus, the oil and gas must flow unprocessed through the flowlines to the manifold and

the host. Due to cold temperatures at the deep ocean bottom, waxes in oils and gas

hydrates can cause flow restrictions or complete plugging of flowlines and pipelines.

Both chemical and mechanical tecimiques are used to assure flow. With increasing use

of flowline chemicals, potential impacts of spills or leaks in the deepwater environment

will need to be evaluated.

Transportation Challenges

In deepwater, pipelines tend to be larger to handle the high production rates, and thicker

walled pipe is necessary to resist the higher water pressures and operating pressures

(Chapter 2). Thus, the pipelines have become very heavy, and different installation

methods have been developed so that the laying process can be carefully controlled.

Environmental impacts of pipeline installation in the deepwater Gulf of Mexico have not

been studied.

Deepwater pipelines are expensive (e.g., $1 million per mile), and pipeline costs can be a

significant factor in the overall cost of a deepwater development (Chapter 2). As new

discoveries are made that are farther and farther from the existing pipeline infrastructure,

floating production storage and offloading facilities (FPSOs) can be an attractive option

for the Gulf. Oil can be produced to an FPSO, stored, and then offloaded onto a shuttle

and shipped to shore. This transportation option can save the cost of expensive

deepwater oil pipelines. The MMS is conducting a study to prepare an Enviromnental

Impact Statement for the use of tanker-based FPSOs in the Gulf (George et al. 1999).

The MMS is also sponsoring a comparative risk analysis that will compare the risks of an

FPSO operating in the Gulf against the risks of other deepwater production systems

currently operating in the Gulf (Gilbert and Ward 2000). Both studies should be

completed in late 2000, and will be used by the MMS in their permitting decisions.

Socioeconomic Implications

As development moves farther offshore, the increasing distance results in increased travel

time for everything from crew changes to MMS inspections (Oynes 1998). As noted in

Chapter 11, concentrated work scheduling (e.g., seven days on, and seven days off) is

among the biggest socioeconomic impacts of OCS development and activities on

individuals and families. With increasing deepwater development, concentrated work

schedules may change to accommodate longer travel distances. Although offshore

housing has a number of amenities, there are still problems associated with living

offshore, including physical risk, strenuous labor, and limited recreational facilities.

The increasingly sophisticated technology used in deepwater development and

increasingly stringent industry safety requirements are expected to increase the already

strong demand for skilled labor. It has also been noted that the number ofjoint ventures

between companies is expected to affect labor demand (Chapter 11).

Activities supporting deepwater development strain the infrastructure of coastal

communities, which are concerned about receiving their fair share of proceeds from
Phillips et aL: Synthesis

deepwater development. Coastal communities are particularly concerned about roads,

highways, and bridges that are not only deteriorating with age, but are becoming more

congested as deepwater activities increase. These concerns are exemplified by the

discussion of Port Fourchon, Louisiana in Chapter 11. Besides road and bridge

maintenance and repairs, infrastructural concerns include housing shortages and

availability of fresh water.

Historically, the social and economic effects of offshore oil and gas development, as

reviewed in Chapter 11, have included dramatic economic and social changes. No other

industry in recent history has had a more pervasive impact on individuals and families

living along the central and western Gulf of Mexico. While deepwater development is

currently booming, history shows that there are times when the oil industry is in a

slowdown, with significant negative impacts on communities, families, and individuals.

Environmental Contrasts and Implications

Physical/Chemical Environment

Geology

Roberts (1998) noted that, "twenty-five years ago our knowledge of processes on the

continental slope in the northern Gulf of Mexico was so limited that it was viewed simply

as an accreting sedimentary structure." In contrast, the synthesis in Chapter 3 reinforces

the contemporary view that this is an extremely complex and dynamic environment.

The Gulf of Mexico is unique in the construction and evolution of its northwestern

continental margin and in particular the continental slope off Texas and Louisiana

(Chapter 3). The processes that determined the physiography of the continental slope are

almost completely dominated by the halokinesis of allochthonous salt. Bathymetric maps

reveal the presence of over 90 intraslope basins with relief in excess of 150 m.

Intraslope-interlobal and intraslope-supralobal basins occupy the upper/middle and lower

continental slope, respectively. In addition to the many structural effects discussed in

Chapter 3, salt movement creates subsurface conduits that allow liquids and gases to seep

into near-surface sediments and the overlying water column. The extent of seepage on

the slope as compared with the shelf is important from the standpoint of both chemistry

(Chapter 5) and the chemosynthetic communities associated with seeps (Chapter 8).

The engineering and geological constraints on the continental slope off Texas and

Louisiana related to hydrocarbon recovery will require both novel geological and

geophysical surveys and engineering methods. Significant seafloor engineering problems

in deep water include slope instabilities, both short-term (slump) and long-term (creep);

pipeline spanning problems; mass transport from unknown causes; and unusual stiffness

and strength conditions. The geohazards (engineering and geologic constraints) present

in and on the central and western continental slope are many in number and are mainly

due to the activity of salt and rapid sedimentation. Specific examples cited in Chapter 3

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include: faults (sediment tectonics, halokinesis); slope stability (slope steepening, slumps,

creep, debris flow); gassy sediments (sediment strength reduction, hydrates, sediment

liquefaction); fluid and gas expulsion features; diapiric structures (salt, mud, hydrates);

seafloor depressions (blowouts, pockrnarks, seeps); seafloor features (sediment waves,

differential channel fill, brine-low chanmels, seabed furrows); shallow waterfiow; and

deep water high-velocity currents (mega-furrows, seabed erosion).

Physical Oceanography

Physical oceanographic characteristics of the deepwater Gulf may be of concern to the

MMS from three perspectives: (1) facilities design; (2) pollutant transport; and

(3) biological consequences, such as intensified primary and secondary production

associated with cyclonic eddies and confluence zones between Loop Current eddies

(LCEs).

Chapter 4 discusses five classes of energetic currents in the deepwater portion of the Gulf

of Mexico of potential concern from a design perspective to those involved with offshore

oil and gas production and transportation. The first class is a familiar one that has long

been of interest to the MMS and oil industry operators on the continental shelf. The

others are specific to, or more associated with, the deepwater environment.

currents resulting from energetic, episodic atmospheric events, including cold air

outbreaks, extratropical cyclones, and tropical cyclones such as hurricanes;

surface-intensified currents arising from the ma] or surface circulation features (the

Loop Current, the anticyclonic LCEs derived therefrom, and both cyclonic and

anticyclonic eddies spun up in the Gulf);

currents extending from about 1,000 m through the deeper water column with little

depth variation (e.g., those believed to be associated with topographic Rossby waves),

sometimes with bottom intensification;

high-speed, subsurfaceintensified currents or jets; and

currents responsible for large, linear mega-furrows discovered along the base of the

continental slope in some locations of the northwestern Gulf.

From the standpoint of pollutant transport, numerical modeling discussed in Chapter 4

offers the possibility of increasingly accurate predictions. Predicting the fate of spills or

leaks from subsea pipelines, flowlines, and/or wellheads will require an understanding of

currents throughout the water column.

Oceanographic studies such as the Louisiana-Texas Shelf Physical Oceanography

Program (LATEX), the Northeastern Gulf of Mexico Physical Oceanography Program

(NEGOM), and GulfCet I and II have produced an understanding of the circulation

dynamics of the deepwater Gulf and its relationship to biological productivity. This

research has shown that the continental slope is characterized by a complex array of

cyclonic and anticyclonic eddies of varying time and length scales. These features have

substantial impacts on nutrient concentrations, plankton and micronekton communities,

and cetacean distribution (Davis et al. 2000).

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Chemical Oceanography

In general, the chemistry of the deepwater Gulf of Mexico is not well studied (Chapter 5).

En many ways, the deep Gulf is buffered from the direct influence of coastal waters and

processes. This is reflected in the low levels of anthropogenie contaminants present in

the dissolved and particulate phases of the water column. However, oceanographic

mechanisms can transport low-salinity, nutrient-rich waters across the shelf and into the

deepwater environment. This has been shown to occur off the Mississippi River mouth,

for example, when a seaward-moving surface flow confluence is created by deepwater

cyclone-anticyclone circulation pairs (Chapter 6). Riverine input of dissolved and

particulate materials is important to the chemistry of the deepwater basin. Secondary

movement of materials downslope is an important process as well. The sparse

information on sediments in the area suggests that the benthic environment is mostly

unaffected by anthropogenic impacts. One important feature of the study area is the

widespread occurrence of natural seepage of oil, gas, and brines in near-surface

sediments and up through the water column. This unusual "natural background" of

hydrocarbons and other disturbances is important to consider when attempting to detect

impacts related to oil and gas exploration and production activities in the area.

Water Column Icosystems

Plankton

It has been known for years that both standing stocks arid productivity of plankton are

low seaward of the shelf-slope break in the Gulf of Mexico. Chapter 6 supports this

description of the mean condition but also shows that "hot spots" in primary production

occur whenlwhere nutrient availability is locally enhanced, even in deepwater. The

available evidence from the Gulf of Mexico indicates that deepwater "hot spots" that are

temporally persistent (even if spatially variable) have higher stocks of zooplankton and

micronekton.

The occunence of "hot spots" in the deepwater Gulf of Mexico is relevant not only to

plankton, but to fisheries and apex predators like marine mammals. The presence of

sizeable populations of apex predators in the deepwater Gulf of Mexico implies a reliable

supply of lower trophic level prey resources and suggests that underlying physical

processes allow "oases" of biological productivity to develop in the mostly oligotrophic

deepwater environment.

Chapter 6 shows that whenlwhere they occur over deepwater in the Gulf, anticyclonic

and cyclonic hydrographic features play an important role in determining biogeographic

patterns of and controlling secondary productivity. The potential enhancement of

fisheries productivity associated with the cyclones and the frontal zones of both types of

eddies is becoming better understood, now that these have been identified as deepwater

concentrating mechanisms for higher trophic levels and apex predators. Continued study

and assessment of zooplankton and nekton abundance within these mesoscale circulation

features is warranted, since these organisms ultimately serve as food stocks for higher

trophic levels.

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Fishes and Fisheries

Taxonomically, the epipelagic and demersal fish assemblages are similar at the familial

and higher levels between shelf and deepwater environments (Chapter 10). Deepwater

mesopelagic and bathypelagic assemblages consist of phylogenetically older taxa that

have proliferated in the deep pelagic waters that are cold and devoid of light. Also the

species composition of the shelf fauna tends to be more geographically restricted than

deepwater faunas, which include many cosmopolitan species. One particular difference

in feeding mode between shelf and deepwater environments is the prevalence of filter

feeding in shelf species. In shelf waters, plankton filtering is an important feeding mode

that supports extremely productive stocks such as gulf menhaden. This mode of feeding

is not common in the deepwater environment.

An obvious difference in fish habitat types is the extensive occurrence of hard

bottonilreef habitat on the shelf. In Contrast, structured hard bottom habitats are rare in

the deepwater demersal environment. However, most sampling of the deepwater

demersal environment has been on featureless sedimentary bottoms amenable to trawl

gear.

The fishing and offshore energy industries have coexisted and developed amicably for

many years in shelf waters of the northern Gulf of Mexico. However, now that interest

has extended beyond the shelf and into deep waters (>300 m), the potential for conflicts

between differing technologies and fishing practices exists. All phases of the deepwater

offshore energy industrygeophysical surveys, exploratory drilling,

development/production, and abandonmentcould conflict with current deepwater

fishing practices. Current deepwater fishing practices in the northern Gulf of Mexico

include trapping for golden crab, trawling for royal red shrimp, bottom longlining for

groupers and tilefishes, and surface longlining for sharks and tunas. Of these gear types,

the pelagic longline presents the greatest possibility for interactions or space-use

conflicts.

Another concern with oil and gas activities in deepwater is the potential for interference

with migratory routes. Such interference could occur because surface structures and

attendant mooring lines will act as a fish attracting devices (FADs). The FAD effect

would be most pronounced for epipelagic fishes such as tunas, dolphin, billfishes, and

jacks (Chapter 10). The concern is that these highly migratory species would be diverted

from their normal migratory routes and, consequently, from traditional spawning or

feeding areas. Because of the highly migratory nature of many epipelagic species, these

effects could extend to the regional scale (hundreds of kilometers). At present, spawning

areas have been identified for the eastern Gulf of Mexico, but likely exist in the central

and western Gulf as well.

Eggs and larvae of many epipelagic and mesopelagic fishes are commonly found in the

surface waters of the deepwater Gulf. These ichthyoplankton could be exposed to

contaminants discharged from surface facilities, such as produced water and domestic

discharges. The potential for impacts from discharges or spills could be higher where

eggs and larvae are unusually concentrated, as may occur in association with certain

oceanographic features.
Phillips et al.: Synthesis

Cetaceans and Turtles

Cetacean species inhabiting continental shelf waters of the Gulf of Mexico are relatively

few in number (primarily bottlenose dolphins, Atlantic spotted dolphins, and perhaps

Bryde's whales within certain areas of the outer shelf and shelf edge). In comparison, the

deepwater environment is inhabited by a relatively rich assemblage of up to 28 cetacean

species (Chapter 9). The assemblage includes resident population(s) of endangered

sperm whales and several cryptic" species (beaked whales, dwarf and pygmy sperm

whales) whose habits are poorly known due to their avoidance of aircraft and ships.

Therefore, a much greater number of cetacean species may come in contact with routine

oil and gas operations in deepwater.

Conversely, the majority of historical sea turtle sightings, in terms of both numbers of

species and numbers of individuals, have been recorded on the continental shelf.

Historically, deepwater sea turtle sightings have consisted primarily of adult leatherbacks,

with few loggerheads and Kemp's ridleys (to the shelf edge only) (Chapter 9). Offshore

waters of the Gulf (which may include outer continental shelf, slope, and abyssal waters)

may also serve as developmental habitat for hatchling and juvenile sea turtles of the five

Atlantic species, and may be used by all species as a transitory habitat.

The oceanographic processes of the continental shelf of the Gulf of Mexico are

dominated by shallow water, wind-driven circulation and riverine outflow influences. In

comparison, the waters of the continental slope and abyss are enriched by the presence or

passage of dynamic mesoscale oceanographic features such as cold-core and warm-core

rings and eddies which originate fTom the Loop Current (Chapter 4). In addition, the

slope waters south of the Mississippi River mouth appear to be primarily influenced by

the volume of river outflow. The presence and persistence of these deepwater mesoscale

features may strongly influence the distributions of protected species (Davis et al. 2000).

In some respects, the spatial variability of these features makes a geographical

designation of cetacean habitat (e.g., preferred feeding areas) difficult because it may

move. On the other hand, knowledge of the position and persistence of these mesoscale

features, as well as the ability to detect and track them through remote sensing, may

provide a tool for mitigating some impacts of oil and gas operations in the deepwater

Gulf.

Potential impacts to protected species may change due to differences in operational or

logistical requirements for oil and gas operations in deepwater environments (Chapter 2).

For example, in cases where oil will be transported to shore by vessel, the increased

volume of vessel traffic may increase the chances for vessel collision with cetaceans

(especially deep-diving species which may spend greater periods of time resting on the

surface) or sea turtles. It is assumed that explosive structure removals, a concern for both

turtles and mammals, will not be needed in most deepwater operations. And in the event

of a deepwater oil spill, there is a much lower probability of the spilled oil reaching land

or sensitive coastal, or nearshore waters when compared to such operations on the

continental shelf

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Benthic Ecosystems

Compared with continental shelf benthos, the continental slope has lower abundance and

biomass and higher diversity of soft bottom macrofauna and megafauna (Chapter 7). As

is the case on the shelf, the Eastern and Western Gulf differ in species composition due to

a difference in substrate (carbonate vs. terrigenous). Faunal diversity of the macrofauna

is high but it is not characterized by a middepth maximum (unlike some other ocean

basins). The low biomass of the macrofauna and megafauna suggest that the biota is food

limited. Budgets of carbon cycling on the slope suggest that most of the organic matter is

cycled through the smaller organisms of the communities (i.e., meiofauna and bacteria).

The state of knowledge concerning the deepwater benthos (other than seep communities)

has not kept up with understanding of processes in the water column ecosystem. In part,

this reflects the enormous expense of collecting and processing benthic samples from the

deep sea. Most benthic studies have been descriptive, focusing on abundance, biomass,

species composition, and zonation. While the GulfCet program has shown that

oceanographic mesoscale features strongly affect water colunm primary and secondary

productivity ("hot spots"), this understanding has not been extended to the benthos. It is

reasonable to suspect that where such "hot spots" persist spatially, benthic enrichment

may occur. Studies during GulfCet II showed these features can be persistent but

spatially variable. The implications for the deepwater benthos have not yet been

investigated. The MMS recently initiated the Northern Gulf of Mexico Continental Slope

Habitats and Benthic Ecology Study. One of the hypotheses being tested by the study is

whether benthic communities underlying persistent "hot spots" are different from those in

other areas.

On the continental shelf, topographic features have been of much interest to the MMS

from the standpoint of environmental protection due to the unusual biological

communities associated with some of these features (e.g., East and West Flower Garden

Banks). Compared with the continental shelf', the northern Gulf of Mexico continental

slope offers complex and rugged morphology including numerous submarine canyons

and salt structures. Due to the water depth, true reef communities such as those seen at

the Flower Garden Banks will not be present. However, there are some physiographic

features whose benthic communities are practically unknown, including the Sigsbee and

Campeche Escarpments. As additional information is obtained through MMS studies or

other research on the continental slope, new habitats and resources may be encountered,

arid the MMS will need to evaluate their need for protection from oil and gas operations.

Chemosynthetic communities are a distinctive and widespread feature of the deepwater

environment (Chapter 8). They are not found on the continental shelf. Dependence upon

seeping hydrocarbons places Gulf of Mexico chemosynthetic fauna in a locality that may

be affected by human activities. The amount of seafloor influenced by seepage is quite

small compared to the extent of the subbottom hydrocarbon system, and industry

engineers generally strive to avoid the unstable substrate at seeps. In addition, through a

Notice-to-Lessees (NTL 88-1 1), the MMS requires site-specific surveys for proposed

bottom disturbing activities in water depths greater than 400 m to evaluate the potential

for chemosynthetic communities. When such areas are identified, they are subsequently

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protected from physical disturbance from anchors, pipelines, chains, and templates

(MMS 1997). Current interest lies in improving the capacity to predict where seep

communities will occur and in understanding processes that contribute to either stability

or change in this environment so that anthropogenic changes could be distinguished from

natural processes.

Data Gaps and Information Needs

Deepwater Technology

Many technical challenges have been discussed in Chapter 2. These include extending

the depth range of floating production systems; improving subsea production systems

including subsea pressure boosting and flow assurance; ensuring continuous flow of

unprocessed oil and gas from subsea wells through flowlines on the seafloor; and

constructing and maintaining increasingly long and heavy deepwater pipelines across the

rugged terrain of the continental slope.

Among the other technical challenges and needs, some of the more significant topics

include drilling wells in areas prone to shallow water flows, deepwater risers and

moorings, and geotechnical properties of the deep ocean bottom. Research and

development activities are underway, and future needs will continue to be addressed

through projects conducted and/or sponsored by individual companies, through joint

industry funded projects, and through broad industry sponsored programs such as

DeepStar and the Offshore Technology Research Center. Technology assessment and

research and development in support of deepwater development and regulations are also

being sponsored by the MMS through their Technology Assessment and Research

Program.

Geology

Previous geological studies and surveys of the continental slope and rise off Texas and

Louisiana were limited to regional scales. Detailed bathymetric maps cover diversified

geological areas and offer a blueprint for further studies. However, the spatial resolution

of present bathymetric maps is still limited to 25 m, 50 m, and 300 m, depending on the

data source. We need to fill the resolution gap as well as using other instruments, like

deep-towed seismic systems or Autonomous Underwater Vehicle (ALlY) platforms and

3-D seismic systems, to detail geological structures. There is also a need to incorporate

the old and new data into a database for intradisciplinary and inter-disciplinary studies.

The recent discovery of mega-furrows at the base of the Sigsbee Escarpment (described

in Chapter 3) is the first such observation for the Gulf of Mexico. It shows that, working

in very complex geological environments such as the continental slope and rise off Texas

and Louisiana, any increase in resolution of geophysical, geological and geotechnical

tools will increase the possibility of finding something new and totally unexpected. The

existence of mega-furrows in deepsea settings suggests that bottom currents exist in the

range of 1 to 100 cms' and higher, and they are inferred to be formed due tohelical

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secondary circulation. The significance of these features is the high water velocities

necessary for their formation and the large amount of erosion occurring at the base of the

Sigsbee Escarpment. There are indications that a single massive abyssal storm may have

created the vast furrow field south of Green Knoll. The possible impact of such an event

on seafloor structures is beyond imagination. More detailed high resolution geophysical

deep-tow or AUV surveys and water current studies are needed to determine the nature,

timing, and magnitude of the forces that created the mega-furrows.

Physical Oceanography

Critical information is lacking regarding several topical areas of the physical

oceanography of the deepwater regions of the Gulf of Mexico. From the view of basic

science, we lack hard estimates for all terms in the salt (or freshwater) and heat budgets

for the Gulf of Mexico with error estimates. A closely related unknown is the renewal

rate (or residence time) of waters below the depths of sills connecting the Gulf with the

Caribbean Sea and open Atlantic. Recent model studies have addressed the latter issue,

but results must be confinned with observations. Closure of these information gaps is not

being addressed by known ongoing or planned studies.

Statistical information regarding energetic currents in the deep water Gulf is needed to

improve the safety, economy, and environmental integrity of deepwater petroleum

exploration and production. This will require a long-term program coupling sustained

observations with modeling. The information gaps remain large. During the past decade,

three new classes of deepwater Gulf currents have been described from direct

observations or from their effects: (1) topographic barotropic Rossby waves, (2) upper

ocean subsurface transient jets, and (3) bottom currents along the base of the Sigsbee

Escarpment. Some observational work is ongoing to address knowledge of the first of

these, and there is ongoing analysis of the second class based on existing observations.

The physics responsible for currents of classes (2) and (3) are not yet known. In fact, it is

not known if deep currents responsible for the mega-furrows along the Sigsbee

Escarpment occur at present or if the furrows result from an energetic current event in the

geologic past. It seems likely that oil and gas companies will explore the causes of these

mega-furrows. The MMS has plans to support a measurement program to better describe

and understand currents of the first two classes as well as surface-intensified currents

(e.g., those due to Loop Current Eddies or cyclonic eddies). However, it is unlikely that

these programs will be of sufficient duration to characterize statistically the energetic

currents of the deep water Gulf of Mexico.

Although the currents and water properties have been studied in the vicinity of the broad

De Soto Canyon, there have been no detailed studies of the effect of the narrower

Mississippi Canyon on the circulation over the upper slope and outer shelf; modeling

studies, however, suggest that Mississippi Canyon may generate eddies and upwelling in

the region and may cause a branching of flows.

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Chemical Oceanography

The input of non-photosynthetic carbon (i.e., from seeps) is an unknown in any

ecological model of the deepsea Gulf of Mexico. The exact areal extent and amount of

seepage are not well known. A better understanding of the distribution of this potential

nutrient and toxin in the deep sea is needed.

The vertical transport of contaminants through long water colunms is not well understood

and should be studied to determine if the contaminants would ever make it to the

seafloor. Obviously, there is also concern about subsea oil spills and their fate and

effects. The presence of gas hydrates may affect how hydrocarbons from a blowout at

the seafloor would be distributed in the deepsea environment, including enhancing their

persistence by sequestration. (At many sites on the slope where thermogenic gas hydrate

exists, it releases globules of oil on decomposition.) The innate ability of benthos to cope

with hydrocarbon exposure is not known, and the rate of microbial oxidation would be

expected to be retarded in the high-pressure and cold environment. In pristine areas,

populations of indigenous hydrocarbon-oxidizing bacteria would be expected to be low.

Although most studies of contaminants in the Gulf of Mexico have been restricted to

shelf and coastal areas, no appreciable levels of hydrocarbons or metals have so far been

detected in the water colunm or sediments of the deepwater environment (with the

obvious exception of seep areas). While there is always a need for broad surveys of

contaminant loadings in relatively unknown environments, such a survey in the

deepwater Gulf would probably produce mostly zeros and not be very rewarding.

Water Column Biology

More is known about the biology of the water column of the continental shelf than of the

biology of the water overlying the deeper regions of the northern Gulf of Mexico.

Because oil and gas exploration and production activity has been in place over the shelf

for some time, there have been many environmental impact studies and ecological

assessments of biological systems on the shelf. Now that oil and gas exploration and

production is moving into increasingly deeper water, it seems timely to consider how

deepwater and shelf regimes interact.

As emphasized in Chapter 6, the interaction of freshwater on the shelf with circulation

regimes over the outer shelf and slope is a key mechanism that can produce "hot spots" of

biological production in the deepwater Gulf of Mexico. Another key mechanism, also

mentioned in Chapter 6, is the doming of nutrients within the cyclonic slope eddies and

within the high current velocity shear zones that surround these cyclones and the

anticyclonic eddies. An improved understanding is required of the influence of these

inesoscale oceanographic features on deepwater zooplankton, micronekton, and

ichthyoplankton. Finally, Chapter 6 provides an extensive list of taxonomic studies,

which we believe are important to the interpretation of biological standing stock and

productivity data (see Cady and Wormuth 2000). Further taxonomic analysis of recent

GulfCet II Multiple Opening/Closing Net Enviromiiental Sensing System (MOCNESS)

and Isaacs-Kidd midwater trawl data, for example, would lead to an improved

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Phillips et al.: Synthesis

understanding of the biomass patterns revealed by net and acoustic sampling of

mesoscale cyclones and anticyclones in the northeastern Gulf of Mexico.

Future studies should use a combination of acoustical, optical, and net sampling for

biological surveys (for example, see Ressler 2000 and Remson and Hopkins 2000), in

order to understand the scales of variability in zooplanktonlmicronektonl ichthyoplankton

biomass. Post-cruise work should include the analysis of archival data (SEAMAP) as

well as of data gathered using newer time series monitoring techniques (i.e., volume

backscattering data from Acoustic Doppler Current Profiler moorings; see Scott et al.

2000).

We still have more "biomass" than "productivity" data. It would be useful to explore

whether the "hot spots" we describe are always areas of higher than average primary

production as well as elevated standing stock biomass (which would improve our

functional understanding of these systems). We certainly need additional primary

production measurements within these features, especially in the central and eastern

deepwater Gulf of Mexico (see Chapter 6, Figure 6.1).

Benthic Communities

Oceanographic programs such as LATEX, NEGOM, and GulfCet I and II have produced

an abundance of information about the deepwater circulation in Gulf and its relationship

to primary and secondary productivity in the water column. This level of understanding

has not been extended to the benthic environment. Information on deepwater benthic

communities consists primarily of descriptive studies focusing on abundance, biomass,

and zonation. Process-oriented studies and linkages to water column are have not been

emphasized.

This situation is currently being addressed by the MMS through its recently initiated

Northern Gulf of Mexico Continental Slope Habitats and Benthic Ecology Study. In

addition to standard sampling for community structure, the study includes process

measurements of community metabolism, meiofaunal feeding rate, sediment community

oxygen demand, and foodweb studies using stable isotopes. One of the hypotheses being

tested by the study is whether or not benthic communities underlying persistent water

column "hot spots" are different from those in otherwise similar areas.

Protected Species

The GulfCet I and II programs represent the most current research for marine mammals

and sea turtles in the oceanic Gulf of Mexico. Future research needs are discussed in

some detail in Davis et al. (2000). These include collecting additional information on

environmental variables that influence the distributions of continental shelf and shelf

edge cetacean species, such as bottlenose dolphins, Atlantic spotted dolphins, and

possibly Bryde' s whales; collecting additional data on daily movement patterns and

feeding behavior of cetaceans in association with mesoscale hydrographic features to

determine relationships between physical and biological processes and their influences on

the distributions of cetaceans. Also recommended were additional studies on the

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Phillips et aL: Synthesis

population structure, seasonal movements, and behavior ofsperm whales in the Gulf of

Mexico to ascertain the importance of lower slope water habitat south of the Mississippi

River mouth. A survey of the entire Gulf was recommended to improve the level of

understanding of the seasonal and annual distribution, abundance, and habitat

associations of cetaceans utilizing simultaneous satellite and conventional radio tracking

methods, and photo-identification of predominant species (e.g., pantropical spotted

dolphins and sperm whales).

Effects of noise from oil and gas operations are a prime subject for future research. To

date, concern has centered on geophysical surveys, which are widespread in the Gulf and

employ a variety of technologies. Davis et al. (2000) recommended conducting a

systematic study of human acoustic perturbations on the large-scale distribution of local

cetacean populations, including small-scale behavioral changes in addition to

distribution. This would include behavioral monitoring and in situ behavioral

experiments on the short-term and long-term effects of seismic and other loud industrial

sounds on the behavior and distribution of Gulf cetaceans.

Fishes and Fisheries

One of the important issues raised during the Deepwater Workshop (Camey 1998) was

the potential for space-use conflicts between deepwater fisheries and oil and gas

operations. The pelagic longline presents the greatest possibility for interactions or

space-use conflicts. There have been interactions reported that involved pelagic longlines

and drilling rigs in the Gulf. Recognizing this problem, MMS recently (October 1999)

initiated a contract study to investigate the potential problems between the deepwater

fishing and energy industries. Mappable and descriptive information on the fishing and

energy industries from the Gulf of Mexico is being gathered and synthesized in order to

develop proactive management approach that will allow MMS to avoid or lessen the

severity of space-use problems (Snyder et al. 1999). To further support the findings for

the Gulf of Mexico records of conflicts from other U.S. and international waters where

the two industries overlap are being analyzed.

Socioeconomics

Increasing deepwater development poses social and economic challenges for individuals

and communities along the Gulf of Mexico. Socioeconomic research needs continue to

be similar to those noted in the Deepwater Workshop (Carney 1998). The top priority

identified in that workshop was a large scale industry and labor force analysis, which

would include collection of data on infrastructure and community capacity. Industrial

labor surveys would provide information on industry strategies and behavior. Research

on vertical and horizontal integration among oil and gas operations would be instrumental

in assessing labor demand, an important issue associated with socioeconomic

implications of deepwater development. Community capacity evaluations and resource

assessments are necessary to mitigate potential problems that will come with expanded

deepwater oil and gas activities. Determining infrastructural capabilities of communities

and their subsequent potential for growth and sustainability would benefit not only

communities, but industry as well.

338
339

Phillips &t al.: Synthesis

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340
The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for

most of our nationally owned public lands and natural resources. This includes fostering sound use of

our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the

environmental and cultural values of our national parks and historical places; and providing for the

enjoyment of life through outdoor recreation. The Department assesses our energy and mineral

resources and works to ensure that their development is in the best interests of all our people by

encouraging stewardship and citizen participation in their care. The Department also has a major

responsibility for American Indian reservation communities and for people who live in island territories

under U.S. administration.

The Minerals Management Service Mission

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary

responsibilities are to manage the mineral resources located on the Nations Outer Continental Shelf

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute

those revenues.

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and

accurate collection and disbursement of revenue from mineral leasing and production due to Indian

tribes and allottees, States and the U.S. Treasury.

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for

all Americans by lending MMS assistance and expertise to economic development and environmental

protection.