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2.2.12 



PROCESSES OF THE SHELF ECOSYSTEM IN THE 

 TEXAS-LOUISIANA REGION 



Dr. Gilbert T. Rowc and Dr. Rezneat M. Darnell 

 Department of Oceanography 



Texas A&M University 

 College Station, TX 77843 



Major previous ecological investigations of the TEXLA shelf include the following: South Texas Outer 

 Continental Shelf Study, Strategic Petroleum Reserve Study, Buccaneer Gas and Oil Field Study, East and West 

 Flower Garden Banks Studies, Offshore Ecology Investigations (off Timbalicr Bay, LA), Central Gulf Platform 

 Study, and Biofouling Community Studies. Additional investigations have dealt with the distribution of plankton, 

 nekton, the demersal invertebrates, fishes, and with physical oceanography. There has been amassed a large 

 inventory of the species present and their distribution patterns, primarily off the eastern half of Louisiana and 

 on the southwestern half of the Texas shelf. Much is also known about the distribution of sediment types and 

 of the various chemical species (including hydrocarbons, pollutants, and trace metals). Although very little 

 quantitative information has been accumulated concerning ecological processes of the area, a great deal may be 

 inferred from the historical studies. 



The dominant influence on the TEXLA shelf is the Mississippi-Atchafalaya River System. Freshwater, nutrients, 

 and sediments brought to the shelf by this system effectively set up a series of biological and ecological gradients 

 as shown in Figure 10. Phytoplankton standing crops and primary productivity are highest near the Mississippi 

 River and lowest down-coast, and they are highest nearshore and lowest offshore. The same general situation 

 applies to the meiofauna, macrofauna, and demersal species. A surface-to-bottom gradient exists in relation 

 to phytoplankton and the biofouling species. 



Existing data point to the importance of the element nitrogen as the factor limiting phytoplankton growth on the 

 TEXLA shelf. Components and processes of the nitrogen cycle in the region are shown in Figure 11. Nitrogen 

 exists in both organic and inorganic forms. It enters the shelf waters through river runoff, upwelling of deep Gulf 

 water, hydrocarbon seeps, and local regeneration. These processes are most intense in the eastern half of the 

 shelf. 



A related set of processes of importance on the TEXLA shelf are those which lead to the development of hypoxic 

 nearshore bottom waters off Louisiana and eastern Texas. High phytoplankton productivity followed by bacterial 

 decay, coupled with vertical stratification and poor bottom circulation, lead to oxygen depletion during summer 

 months when the temperature is elevated. 



These and other processes which are known in general terms require detailed explanations. All involve coupling 

 of physical and biological processes and they cannot be resolved by the examination of species inventories and 

 distribution patterns. Our focus must shift, as it were, from anatomy to physiology. 



