Regional Overview & Fact Sheet
The Northern Gulf of Mexico Region contains over 60 percent of the tidal marshes of the United States, freshwater inputs from thirty-seven major rivers, numerous nursery habitats for fish, the Flower Gardens Banks, and also the so-called ‘dead zone’ resulting from increased loads of organic material from the extensive Mississippi River watershed. The region extends from Gullivan Bay on the west coast of Florida to north of the Rio Pánuco in the state of Tamaulipas, and includes the coastal areas of the US states of Alabama, Mississippi, Louisiana, and Texas. The region comprises the northern portion of the Gulf of Mexico (hereafter referred to as the Greater Gulf)—a semi-enclosed sea with tropical currents and a high nutrient load. Most of the oceanic input to the Greater Gulf is from the Caribbean Sea through the Yucatan Channel, forming the Loop Current, which winds north then east through the Greater Gulf, outflowing through the Straits of Florida. A broad continental shelf covers about a third of the Greater Gulf.
The region is defined physiographically by the enclosed nature of the Gulf of Mexico, as well as by a sea surface temperature differential with the Southern Gulf of Mexico in the winter.
14–24°C (winter), 28–30°C (summer)
Loop Current (cyclonic), Florida Current, Tamaulipan Gyre (anticyclonic)
Broad continental shelf along Florida narrowing to a steep thin shelf fronting the Mississippi River outlet, widening again along the Texas coast, narrowing in the Mexican southern portion; much of the near shore waters are divided into bays and estuaries behind barrier islands.
shelf (roughly 0–200 m): 56%
slope (roughly 200–2,500/3,000 m): 40%
abyssal plain (roughly 3,000+ m): 4%
Sand barriers; silt and mud, with clays in central Gulf coast; sandy muds in Texas; and sand and carbonate muds in Florida.
Mangrove ecotone, patchy seagrass communities, oyster reefs, isolated salt domes with coral reefs and deep Lophelia coral mounds, deltaic systems, coastal lagoons, estuaries, salt marshes, river inlets, dwarf mangroves, serpulid worm reefs.
Moderately high productivity ecosystem (150-300 gCm-2yr-1). Conditions range from eutrophic in the coastal waters to oligotrophic in the deeper ocean.
Kemp’s ridley turtle, green turtle, loggerhead turtle, Gulf sturgeon, speckled hind, dusky shark, sand tiger shark, night shark, largetooth sawfish, smalltooth sawfish, Texas pipefish, opossum pipefish, dwarf seahorse, diamondback terrapin, Alabama shad, and saltmarsh topminnow, groupers, Johnson's seagrass.
Areas of the broad continental shelf, coastal lagoons and estuaries, river inlets, cypress swamps, mangroves swamps, seagrass beds, oyster reefs, tidal freshwater grasses, salt marsh, tidal freshwater marsh flats, intertidal scrub forest, muddy bottom habitats, Coquina beach, rock marshes and bars, intertidal/subtidal beaches and bars, serpulid worm reefs.
Tourism expansion; urban expansion; ports; oil and gas exploration and recovery; liquefied natural gas (LNG) terminals; shipping; coastal industrialization and infrastructure; natural and anthropogenic loss of wetlands; coastal erosion; fishing activities cause additional impacts, including benthic habitat alteration by shrimp trawling; fishing along slope kills sea turtles and other threatened species as bycatch; the goliath grouper was subjected to severe fishing pressure on spawning aggregations; destruction of marsh habitat by conversion or land erosion is eliminating the habitat of the saltmarsh topminnow; ecological changes have also been wrought by the deliberate introduction of whiteleg shrimp.
Physical and Oceanographic Setting
The Northern Gulf of Mexico is characterized as semi-tropical due to the seasonal pattern of its temperature regime, which is influenced mainly by tropical currents in the summer and temperature continental influences during the winter. It has a distinct sea surface temperature gradient from north to south (up to 7oC) in winter. The Gulf of Mexico presents a seasonally changeable wind regime with predominance of the northeast trades.
The Northern Gulf is characterized physiographically by a broad continental shelf extending out to 250 km from the coastline, a steep continental slope and a small section of the large central abyssal plain of the Greater Gulf. Distinctive bathymetric and morphological processes and features of the region have had a great influence on the makeup and functioning of the region. For example, the Flower Gardens Banks—surface expressions of salt domes—off the Louisiana/Texas coast are home to the northernmost coral reefs in the United States, natural gas seeps, and numerous tropical fishes, manta rays, turtles and sharks. The entire southern portion of the state of Louisiana and part of eastern Texas was formed by delta building and river-switching processes of the Mississippi River. The Chenier Plain and Mississippi River Birdsfoot Delta were deposited by deltaic and plume transport processes of the Mississippi River. And finally, the extensive barrier island system that runs from the Florida Panhandle, through Alabama, Texas, and northern Mexico—created by longshore transport and deposition of sands—forms many lagoons and sheltered areas that serve as refugia and spawning grounds.
Muddy clay-silts and muddy sands dominate bottom substrates of the region throughout the entire shelf, slope and plain off the Louisiana, Texas and Mexico coasts. From Alabama east to Florida, sand, gravel and shell dominate the region, and on the shelf off of Florida, the carbonate limestone substrate is interspersed with gravel-rock and coral reefs. The carbonate limestone substrate of the Western Florida Estuarine Area (level III region), however, is overlain by sand and silt and supports extensive seagrass beds dominated by turtlegrass that in turn are nursery, feeding and spawning areas for several important fishes.
The entire region is subjected to a mixed semi-diurnal tidal regime of low amplitude, generally between 5 and 30 cm. Several persistent current features—including the Loop Current, the Coastal Countercurrent, and the Florida Current—mark the greater Gulf waters. The Gulf Stream, which has such an immense effect on the waters of the Atlantic, also originates in this region. In the Eastern Gulf (level III region), the eastern edge of the Loop Current interacts with the shallow shelf to create zones of upwelling and onshore currents. These nutrient rich events promote high phytoplankton growth and support high biological activity.
The region in the central north is strongly influenced and dominated by the Mississippi River and its tributary, the Atchafalaya River, which carries 30 percent of the flow of the Mississippi. Together these rivers’ discharge exceeds all other Gulf rivers by an order of magnitude. In addition to these major rivers, several other large rivers and estuaries are found in the region. Fresh water inputs—including both the quantity and quality—from these systems significantly impact the coastal physico-chemical characteristics and biological communities of the region.
Along with several smaller fresh water sources these rivers deliver freshwater to the northern Gulf during the wet/flood season from May through November. During this period, the region is marked by large plumes of turbid water which become entrained in a westward flowing longshore current and carried westward to the Texas border, and can become entrained in the Loop Current and carried eastward to southern Florida.
Recently, the delivery and deposition of increased loads of terrestrial organic material from the extensive Mississippi River watershed has been a concern within the Greater Gulf. These increased loads affect even relatively deep offshore waters, and have resulted in organic accumulations, bacterial degradation, and a region of oxygen demand, often resulting in severe oxygen depletions in the bottom waters, and the frequent appearance of a so-called ‘dead zone’—an area where large populations of benthic fauna are killed by low oxygen concentrations.
In the western Gulf of Mexico, freshwater input is low and bracketed by the low-flowing Rio Grande (USA)/Rio Bravo (Mexico) to the south, and the Brazos (USA) to the north, and subsidized by Laguna Madre (Mexico/USA). Low fresh water inputs from this system result in clear, high salinity water in this region of low tidal amplitude. The Laguna Madre is noted in particular for the long extent of coastline with almost no fresh water input. The region is characterized by a number of estuaries, mostly behind barrier island formations. The micro-tidal western gulf results in a low-energy, broad sandy beach substrate. Seasonally low fresh water input in this highly evaporative area can result in hypersaline conditions and salt pan formation.
A major climatic feature of the Greater Gulf and adjacent areas is the occurrence of hurricanes, which greatly affect the physical, biological and human systems of the region. Several severe hurricanes have caused widespread disaster and loss of life along the Gulf coast; however, most biological systems recover relatively quickly from hurricane impacts. The passage of strong wind and storm events are thought to be important to the ecology of this otherwise low-energy region because these episodic inputs of energy rework sediments, redistribute biological seed material and remove accumulated toxics, promoting healthier communities.
In terms of climate, the region is considered semi-tropical to tropical, and consequently the coastal communities range from salt marshes to seagrasses, and mangrove systems to salt pans, with scarce and isolated coral reef formations. Key habitats in this diverse areas include areas of the broad continental shelf, coastal lagoons and estuaries, river inlets, bald cypress swamps, mangrove swamps, seagrass beds, oyster reefs, tidal freshwater grasses, salt marsh, tidal freshwater marsh flats, intertidal scrub forest, muddy bottom habitats, Coquina beach, rock marshes and bars, intertidal/subtidal beaches and bars, and serpulid worm reefs. Productivity in the Northern Gulf of Mexico ranges from eutrophic conditions in coastal waters to oligotrophic in the deeper ocean.
The region is one of the largest estuarine areas in the US, second only to Alaska, and contains over 60 percent of US tidal marshes. The delivery of high loads of terrestrial and agricultural nutrients and organic matter fuels plankton blooms and generally high biological productivity throughout the region. The nutrient input of estuaries support huge commercial fisheries—such as those of spot croaker; Atlantic menhaden; mullet; pink, brown and whiteleg shrimp—and recreational fisheries. Almost all commercially and recreationally important fish and shellfish in the region are estuarine-dependent at some time during their life cycle and use these important areas as spawning, nursery or feeding grounds. The estuarine areas also provide habitat for several threatened and endangered marine mammals, reptiles, fishes and invertebrates.
Along the coastal margin, communities composed of red, black, white and button mangroves thrive in south Florida from Florida Bay to Cape Romano (mainly within South Florida/Bahamian Atlantic region) and in smaller extensions in Texas, and northern Mexico. Scrub mangrove communities occur in Louisiana—the northern extent of mangroves in North America—limited there by temperature.
In many areas of the coastal northern Gulf, particularly in Louisiana, Texas and northern Mexico, brackish and salt marsh vegetation occurs, dominated by needlegrass rush, saltmeadow cordgrass, and smooth/saltmarsh cordgrass. Extensive seagrass beds inhabit much of the shallow coastal margin. Here communities are often dominated by turtlegrass (which harbor many important nekton species), but also include shoalweed, manateegrass, and Johnson’s seagrass. In the areas of generally lower salinity, widgeongrass is prevalent. Benthic algae, while less useful as a food source, occurs throughout the entire region from the land margin to the edge of the continental shelf. Blooms also occur in upwelling areas along the Florida shelf break and the Texas shelf. Phytoplankton is prevalent in the areas around all estuarine and freshwater inputs around the Mississippi River and the west coast of Florida.
Coral reefs occur along the mid- to outer-edge of the continental shelf off the Big Bend area of Florida, and especially at the shelf break off of Texas and Louisiana—an area that includes the well-known Flower Gardens. Extensive areas of scattered banks and coral heads occur in very shallow shelf regions around south and central Florida. Shell reefs of the eastern oyster abound in the region.
Human Activities and Impacts
The northern Gulf is highly impacted by human activity—both directly and indirectly—and conservation issues are a major concern. A quarter of the commercial shipping of the US passes through the Straits of Florida, often causing damaging anchor and grounding scars in coral communities, and potentially introducing exotic species in ballast water. The human activity associated with the major petroleum hydrocarbon formations offshore of the northern Gulf coast is also a cause for concern: a vigorous complex of offshore petroleum exploration, extraction, shipping, service, construction, and refining industries has developed over the past half century, particularly in Louisiana and Texas, resulting in severe impacts on coastal wetlands, brine discharges, heavy metal deposition in drilling muds and tailings, and large and small scale petroleum discharges and major spills. Recently, coastal Florida has been the focus of potentially large expansion of these oil and gas-related activities. Within the past few years there has been significant interest in the development of liquefied natural gas (LNG) import facilities along US coastlines, particularly in the Gulf of Mexico. The proposed use of “open-loop,” or once-through, systems for LNG regasification have raised concerns regarding the volume of water intake, generation of thermal plumes, discharge of treated water, increased turbidity, and generation of noise in the marine environment. In US Gulf of Mexico waters one open-loop terminal has recently been constructed, two others have been approved, and four more have pending license applications. Urban growth, shoreline development, freshwater reductions, and tourism also have affected the natural communities. Eutrophication in areas of high river discharges (and the expanding “dead zone” of oxygen-deprived and lifeless water caused in part by river-borne pollutants), reductions in fresh water inflow to estuaries due to upstream development and wetland loss due to subsidence and impoundment also place the region under duress. Harmful algal blooms (HABs) and fishkills occasionally affect the area—in certain areas within the region the eastern oyster populations and seagrass communities have been affected by algal blooms and turbidity events. Overfishing and bycatch and habitat impacts from shrimp trawl fisheries are a concern. Two out of 17 major US federally-managed fish stocks are overfished, with 10 of unknown or undefined status (NMFS 2007). In areas influenced by freshwater, such as in Louisiana and Texas, crayfish harvesting forms an important economic base. Other conservation issues of major concern include protection of wading bird, shorebird, seabird, and endangered sea turtle populations.