AMER. ZOOL., 30:3-6 (1990)
Gulf of Mexico: Environmental Overview and History of
Environmental Research1
REZNEAT M. DARNELL
Department of Oceanography, Texas A&M University, College Station, Texas 77843
AND
RICHARD E. DEFENBAUGH
Minerals Management Service, Gulf of Mexico OCS Region, Environmental Studies Section,
1201 Elmwood Park Blvd., New Orleans, Louisiana 70123-2394
INTRODUCTION
The National Environmental Policy Act
of 1969, as amended, mandated that all
Federal agency activities that might significantly affect the quality of the human environment must be preceded by the preparation of environmental impact statements
detailing, among other things, any potential adverse effects and irreversible and
irretrievable commitments of resources. In
response to this and other mandates and
in relation to massive technological intrusion into the Gulf of Mexico waters, Federal agencies have, in the past decade and
a half, sponsored over $75 million in
research studies of the northern Gulf of
Mexico. Most of these studies were sponsored by the U.S. Department of the Interior (in relation to oil and gas development
on the continental shelf and slope),
although significant studies have also been
funded by the U.S. Navy, Army Corps of
Engineers, Department of Commerce,
Department of Energy, and Environmental Protection Agency, among others. As
a result, our knowledge of the biology and
ecology of the Gulf has increased dramatically during the past several years. Unfortunately, most of this knowledge is locked
up in the various technical reports submitted to the sponsoring agencies, and only
a small fraction has appeared in the professional journal literature.
Department of the Interior provided a
grant to the American Society of Zoologists to subsidize a special session of contributed papers at the December 1987
annual meetings of the Society and to
underwrite publication costs of these papers
in the Society's journal. It is the purpose
of this special session to present to the
national scientific audience summaries and
highlights of the recent Gulf of Mexico
research and also to provide appropriate
references to the technical reports so that
interested scientists may obtain access to
the more complete information contained
within the technical documents. As background for this series of articles, we present herewith a brief environmental overview of the Gulf and a history of
environmental research.
ENVIRONMENTAL OVERVIEW
The Gulf of Mexico occupies a mediterranean-type basin bounded by the North
American continent and the island of Cuba.
It is connected with the Caribbean Sea by
the Yucatan Strait and with the Atlantic
by the Straits of Florida. Its maximum eastwest length is 1,575 km, and its northsouth width is about 900 km. The Gulf
occupies a surface area of 564,200 km2 of
which 35.2 percent is underlain by continental shelf (0-200 m depth) and 24.3 percent is underlain by very deep water
To partially alleviate this situation the (> 3,000 m). The maximum depth of 3,850
Minerals Management Service of the U.S. m, known as the Sigsbee Deep, is situated
in the west-central sector. The depth of the
sill underlying the Yucatan Strait is 1,500—
1
1,900 m, and the controlling depth of the
From the Special Session on Ecology of the Gulf of
Mexico organized by Rezneat M. Darnell and Richard
Straits of Florida is only about 800 m. The
E. Defenbaugh and presented at the Annual Meeting
Gulf of Mexico basin was formed by seaof the American Society of Zoologists, 2 7 - 3 0 Decemfloor spreading in pre-Cretaceous time,
ber 1987, at New Orleans, Louisiana.
R. M. DARNELL AND R. E. DEFENBAUGH
TABLE 1. Prominent early biological investigations of the Gulf of Mexico (1850-1939).
Ships
1850-1880
1867-1868
Corwin, Bibb
1872-1878
Blake
1884-1885
Albatross
1895-1917
Fish Hawk
1917
Grampus
1926
1929-1939
Explorations
Biological survey of Florida coral reefs by Louis Agassiz.
Dredging for bottom fauna between Florida and Cuba (to depths of 1,550
m).
Extensive dredging off north and west coasts of Cuba, west of Florida
shelf, and north of Yucatan shelf (to depths of 3,658 m).
Dredging in Yucatan Channel, off northern Cuba, on Florida shelf, and
east of Mississippi River Delta.
Extensive estuary and inshore surveying (Florida sponge grounds, oyster
bottoms, and fishery grounds; south Texas through Florida).
Extensive surveying of continental shelf shrimp and fishery grounds (Texas through Florida).
Oyster bottom surveys of Texas bays and estuaries by Paul Galtsoff.
Life history studies of estuary-related commercial shrimp and fish species
by John Pearson.
probably between 240 and 170 million
years ago (Permian and early Jurassic).
Faulting and subsidence have resulted in
its present depth. The oceanic crust is
overlain by thick sedimentary layers in
which are embedded (in much of the western and southern sectors) one or more deep
layers of Jurassic salt. Since the beginning
of the Tertiary, extensive accumulations of
clastic sediments have been deposited, primarily on the northern shelf and slope, a
process that greatly accelerated with the
melting of the Pleistocene ice sheets. These
rapidly accumulated sediments are often
unstable and subject to slumping into the
deep Gulf. Off the Texas-Louisiana coast
the sediments are over 17 km thick, making the Gulf of Mexico one of the world's
deepest basins.
Circulation in the Gulf of Mexico is dominated by the Loop Current, which enters
through the Yucatan Strait, bends eastward and southward, and exits through the
Straits of Florida. The northward extension of this current varies over time, but it
has been known to intrude upon the continental shelf of the northern Gulf east of
the Mississippi River Delta. This current
has a measured speed of 127-177 cm sec"1.
Periodically, this current spins off large,
clockwise-rotating eddies, which proceed
west or southwest toward Tampico, Mexico, and which may persist for many
months. The southward-flowing arm of the
Loop Current may impinge upon the outer
Florida shelf. Most of the currents of the
continental shelves are strongly coupled
with the wind. Throughout most of the
Gulf the continental shelves and slopes are
floored by terrigenous, riverborne deposits of clay, silt, and silicious sand, but the
broad platforms off west Florida and Yucatan are largely covered with biogenic carbonate sand and rubble.
The Gulf annually receives via river and
stream flow about 10.6 x 1011 m3 yr"1 of
freshwater, of which 85 percent comes
from the United States and 64 percent from
the Mississippi River drainage alone. This
flow varies from one year to another and
is highly seasonal, with most of the flow
coming in the spring months. The Mississippi River also brings annually to the Gulf
about 4.1 x 108 metric tons of sediment
including about 3 x 109 kg of dissolved
and paniculate organic carbon. Relative to
waters of the Gulf, the Mississippi River is
quite rich in nitrates, phosphates, and silicates, which stimulate heavy phytoplankton growth in waters affected by the river
outflow. Considerable loads of heavy metals and other pollutants are also brought
to the Gulf by the Mississippi. Most of the
sediment is deposited on the slope and in
the deep Gulf, but a significant amount is
distributed westward on the continental
shelves off Louisiana and east Texas. Estimated subsurface mineral reserves of the
northern Gulf (Rio Grande to DeSoto Canyon) include 1.4-7.2 x 108 bbl of petroleum and 4.4-22.3 x 1010 m3 of natural
gas.
G U L F OF MEXICO: ENVIRONMENTAL OVERVIEW
I
TABLE 2. Major broad-scale and interdisciplinary investigations of the U.S. Gulf of Mexico conducted since I960.
Dales
1961-1965
1965-1967
1970-1976
1972-1974
1974-1979
1975-1980
1975-1985
1977-1980
1978-1981
1980-1987
1981-1985
1981-present
1983-present
Sponsor, location, and nature of study
National Marine Fisheries Service (NMFS). Rio Grande to Mobile Bay. Monthly transects across shelf. Hydrography, plankton, and trawl fishery survey.
State of Florida. Hourglass Cruises. Monthly transects across Florida shelf off Tampa
and Charlotte Harbor. Hydrography, plankton, trawl, dredge, trap, and handline
survey.
Texas A&M Univ. Defenbaugh dissertation. Gulfwide except west Florida and Yucatan.
Trawl survey of benthic invertebrate fauna of U.S. and Mexican continental shelves.
Gulf Universities Research Consortium (GURC). Offshore Ecology Investigation. Louisiana
bays and inner shelf. Sedimentology, hydrography, microbiology, plankton, benthic
flora and fauna, bio-fouling communities, trace metals, and hydrocarbons.
BLM. Mississippi-Alabama-Florida (MAFLA) Baseline Study. Continental shelf off Mississippi, Alabama, and Florida. Sedimentary, geological mapping, hydrography and physical oceanography, plankton, neuston, benthos, demersal fishes, and trace metals and
hydrocarbons (in water, sediments, and biota).
BLM. South Texas Outer Continental Shelf Baseline Study. Transects off south Texas (Rio
Grande to Matagorda Bay). Sedimentology, geologic mapping, hydrography, plankton, neuston, benthos, demersal fishes, histopathology, and trace metals and hydrocarbons (in water, sediments, and biota).
BLM/MMS.* Topographic Features Program. Study of submarine banks off Texas, Louisiana, and Florida (Florida Middle Grounds). Geologic mapping and hydrographic
and biological surveys.
NMFS, EPA. Buccaneer Gas and Oil Field Program. Inner shelf off Galveston, Texas. Sedimentology, hydrography and circulation, benthos, bio-fouling communities, artificial reef study, and toxicology.
BLM. Central Gulf Outer Continental Shelf Platform Study. Continental shelf off Louisiana.
Sedimentology, microbiology, benthos, trace metals and hydrocarbons (in water, sediments, and biota), histopathology, and platforms as artificial reefs.
BLM/MMS. Southwest Florida Shelf Marine Ecosystems Study. Continental Shelf off southwest Florida from Fort Myers to 25th parallel (below the Everglades). Sedimentology,
geologic mapping, hydrography, biological survey, benthos, biological processes, and
productivity.
U.S. Department of Energy (DOE). Strategic Petroleum Reserve Studies. Nearshore shelf
off Cameron, Louisiana, and Freeport, Texas. Hydrography, plankton, benthos, demersal fishes, sediment chemistry.
BLM/MMS. Northern Gulf Continental Slope Study. Outer Continental Shelf and upper
slope, Gulfwide. Biological surveys, and benthos and chemosynthetic communities
studies.
MMS. Mississippi-Alabama Shelf Marine Ecosystems Study. Continental shelf from Mississippi River Delta to DeSoto Canyon. Geologic mapping, and hydrographic and biological surveys.
* The Minerals Management Service (MMS) was formed by merger of units from the Bureau of Land
Management (BLM) and the U.S. Geological Survey (USGS) in 1981-1982.
The shelf biota of the southern Gulf of
Mexico is essentially tropical and is derived
from that of Central America and the West
Indies. The biota of the northern Gulf shelf
is Carolinean. These two groups intergrade along the east coast of Mexico and
along the west coast of Florida. Active coral
reefs are present along the Mexican coast
and the southern half of the Florida shelf,
but isolated active coral reef formations
are also situated on topographic features
of the Texas outer continental shelf. Tropical biota is associated with outer shelf
topographic features and offshore oil and
gas structures. Along the northern Gulf
the commercial fishing industry annually
lands about 7.6 x 105 metric tons of fishes
and shellfishes valued at $4.3 x ^ " . M e n haden and penaeid shrimp dominate the
catch.
HISTORY OF ENVIRONMENTAL RESEARCH
The history of environmental studies of
the Gulf of Mexico can be divided into
three periods: early exploration, local
coastal studies, and finally large-scale, mul-
R. M. DARNELL AND R. E. DEFENBAUGH
tidisciplinary investigations and synthesis.
Each of these will be addressed briefly
below with emphasis on the biological studies.
Exploratory period
(1850-1939).
During the latter half of the nineteenth
and the first part of the twentieth centuries, major biological explorations were
conducted in the Gulf, primarily under the
auspices of the U.S. Coast Survey, Fish
Commission, and Bureau of Fisheries
(Table 1). These investigations were carried out in both shallow and deep water.
Toward the end of this period, in his studies of the life histories of penaeid shrimp
and fishes, Pearson demonstrated the
important biological relationships of estuaries and continental shelves. Major hydrographic studies during this period were
undertaken by the Coast and Geodetic Survey and the U.S. Navy Hydrographic
Office.
Local coastal study period
(1940-1959)
by Galtsoff (1954). It summarized existing
knowledge of the origin, waters, and
marine life of the Gulf. Extensive studies
of the geology, sedimentology, and ecology of the Texas-Louisiana shelf were supported by the American Petroleum Institute (Shepard et al, 1960).
Multidisciplinary investigation and
synthesis period (1960-present)
Since 1960 additional marine laboratories have been established and our knowledge of the local estuarine and neighboring shelf areas has continued to increase.
Most significantly, however, a series of
large, multidisciplinary investigations,
especially on the continental shelves and
slopes of the Gulf, has been undertaken or
completed, primarily through State and
Federal agency sponsorship, and this information is being synthesized into major ecological reports. Some of the more important of these investigations are listed in
Table 2, and the resulting reports are referenced in the accompanying articles. The
fact that much of this information has been
obtained to satisfy management needs renders it no less interesting to the scientist.
It is hoped that the present series of articles
will make this recently obtained information more broadly available to the scientific
community.2
During and following the Second World
War, over a dozen marine laboratories,
affiliated with Federal and State agencies
and with academic institutions, were established in the States bordering the Gulf of
Mexico. Prominent among these were laboratories located at St. Petersburg and
Alligator Harbor, Florida; Ocean Springs
REFERENCES
and Pascagoula, Mississippi; Grand Isle,
Louisiana; and Galveston and Port Aran- Galtsoff, P. S. (ed.) 1954. Gulf of Mexico, its origin,
sas, Texas. These facilities resulted in a
waters and marine life. U.S. Fish. & Wild. Serv.,
Fish. Bull., 55 (89). 604 pp.
great expansion of our biological knowlF. P., F. B. Phleger, and T. H. van Andel.
edge of the local estuaries and adjacent Shepard,
(eds.) 1960. Recent sediments, northwest Gulf of
continental shelves. A clear exception to
Mexico. Am. Assoc. Petrol. Geol. Tulsa, Oklathe local focus has been that of the National
homa. 394 pp.
Marine Fisheries Service (NMFS) laboratory at Pascagoula, Mississippi. Established
1
Inquiries concerning the availability of Minerals
in 1951, the Pascagoula laboratory with its
ships, the FRS Alaska, Oregon I, and Oregon
II, conducted exploratory fishing operations throughout the Gulf of Mexico and
related waters, resulting in a Gulfwide
expansion of our knowledge of faunal distribution. During this period there
appeared the important publication edited
Management Service documents dealing with the Gulf
of Mexico (Contract Studies Reports and Programmatic Documents of the Environmental Studies Program) should be addressed to the following office:
U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, Public
Information Unit (OPS-3-4), 1201 Elmwood Park
Blvd., New Orleans, Louisiana 70123-2394. (Telephone: 504/736-2519).