13. Northern Gulf of Mexico

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.