89 



2.2.15 



ECOSYSTEMS OF THE TEXAS-LOUISIANA OCS REGION 



Dr. Nancy N. Rabalais 



Louisiana Universities Marine Consortium 



Chauvin, LA 70344 



The oil and gas development scenario for the Gulf of Mexico is expected to continue as in previous decades with 

 over 90% of all offshore drilling and production occurring there (Ray 1987). In the area of the TEXLA shelf, 

 exploration of deeper waters on the continental slope will predominate, but a large number of wells will continue 

 to be drilled on the inner and mid-shelf. Most of the drilling and production will take place in the Minerals 

 Management Service (MMS) central planning region, but activity will also increase in the western Gulf lease area, 

 with new exploration occurring both inshore and offshore. An understanding of the ecosystems in the above 

 mentioned areas is necessary for the proper development and management of oil and gas resources within the 

 context of the biological resources of the area. 



The summary that follows provides a brief description of the dominant features and processes of the continental 

 shelf environments of Texas and Louisiana and a focus on the benthic communities. Without diminishing the 

 importance of water column communities or populations of larger demersal sea bed fauna, this review 

 emphasizes the macroinfaunal communities for several reasons. The benthos has received emphasis in studies 

 of the effects of oil spills and other discharges because of their susceptibility, longevity, and relative immobility. 

 Future studies will likely concentrate on the benthos. The benthos of parts of the TEXLA shelf have been 

 extensively surveyed; still other areas have received no attention. An outline of the studies completed or ongoing 

 in the area of interest are given in Table 9 and located in Figure 12. A large area of the southwestern Louisiana 

 and upper Texas continental shelves has not been investigated with regard to the benthic communities. Off the 

 southeastern Louisiana coast, the outer shelf ecosystem is unknown as is the shelf break and upper slope for the 

 entire area. Features of the benthic nepheloid layer, as illustrated from the east Texas shelf (Sahl et al. 1987), 

 and the potential for transport of sediment-adsorbed contaminants further illustrate the need to understand these 

 ecosystems. 



A matrix comparing the dominant features and processes of U.S. continental shelf areas, including those from 

 the Mississippi River delta to the Texas-Mexico border, has been compiled by Rabalais and Boesch (1987). The 

 region is not directly influenced by major ocean currents, except for the passage of anticyclonic gyres which spin 

 off the LOOP Current and travel westward along the outer shelf. Circulation on the shelf proper is more 

 affected by wind forcing, tides, and river discharges (Murray 1972, 1976). A net westward (Louisiana) and 

 southwesterly (Texas) flow along the shelf characterizes the predominant conditions from fall to early spring 

 (Smith 1980). In summer, the flow is to the west and southwest form Louisiana to about 95° W where it 

 converges with an opposing flow to the north and northeast. A clockwise eddy is frequently found just west of 

 the Mississippi River delta. This eddy advects part of the river's effluent back toward shore where it may be 

 entrained in a coastal boundary layer. An easterly flowing counter current and energetic cross-shelf currents 

 were also observed near the shelf break by McGrail and Carnes (1983). The large freshwater discharges of the 

 Mississippi and Atchafalaya Rivers influence the hydrography of the northwestern Gulf shelf. The influence is 

 especially prominent in the reduced salinity of inner shelf waters as far west as Galveston (Nowlin 1971) and 

 occasionally down the Texas coast (Smith 1978). Related to the density stratification influenced by late spring 

 river discharges and the nutrient inputs is the development of depressed levels of dissolved oxygen in bottom 

 waters of the inner shelf during summer (Turner and Allen 1982; Rabalais et al. in press). Hypoxia in bottom 

 waters is recurrent but ephemeral off the southwestern Louisiana shelf (Gaston 1985; Pokryfki and Randall 1987) 

 and is known to occasionally extend at least to Freeport, Texas (Harper et al. 1981a). 



The continental shelf from the Mississippi River delta to the Rio Grande is gently sloping and wide, over 200 

 km off the TEXLA border (Emery and Uchupi 1972; Shepard 1973). There are many more physiographic 

 irregularities in the central part of the shelf than to the east and southwest. Topographic features include many 

 channels, most of which are associated with longitudinal ridges, and largely filled extensions of large rivers across 

 the shelf. The topography of the northwestern Gulf north of Matagorda Bay is marked by numerous 

 protuberances which have been shown in most cases to be caused by salt or shale diapirs (Emery and Uchupi 

 1972; Rezak et al. 1983). The stage of sedimentary evolution (Curray 1965) grades from allochthonous at the 



