PHYSICAL-CHEMICAL FORCES 



Marine biological systems are controlled, or regulated, by a number 

 of physical and chemical variables including light, nutrients, 

 temperature, salinity, and dissolved oxygen, as well as the physical 

 nature of the habitat. Each of the above variables is, in turn, greatly 

 influenced by riverine discharge and marine circulation patterns. 

 Generalized surface current patterns, hydrography and prominent 

 topographic features of the Texas-Louisiana shelf are described below. 



Currents 



The principal inflow of water into the gulf is from the Caribbean 

 Sea through the Yucatan Channel. These waters are a mixture of South 

 Atlantic water (transported northwestward by the Guiana and Equatorial 

 current systems) with North Atlantic water (Pequegnat 1976). The ratio 

 of South to North Atlantic water has been estimated to be between 1:4 

 and 1:2 (Harding and Nowlin 1966). The generalized current pattern for 

 the gulf is shown by Figure 3. The Yucatan Current branches into three 

 major flows (1) the Loop Current which dominates eastern gulf 

 circulation and entrains some western gulf waters, (2) the Central Gulf 

 Current (Moore 1973) which flows towards the Mississippi River Delta and 

 to the west, and (3) the West Gulf Current which flows over and around 

 the Yucatan Shelf to the west until it is forced to the north by the 

 curvature of Mexico (Brucks 1971). 



While the general flow of water along the Texas-Louisiana shelf is 

 to the west, seasonal variation occurs and nearshore currents are 

 greatly influenced by the shoreline and regional winds. Currents on the 

 nearshore shelf are primarily driven by local and regional wind systems, 

 passage by diurnal tide, and impingement of regional gulf currents onto 

 the shelf. Results of drift bottle studies by Kimsey and Temple (1962) 

 and Kimsey (1963) showed that, just west of the Mississippi River Delta, 

 nearshore currents moved south, west, and east (Figure 4). Under 

 conditions of an easterly flow, currents were deflected by the delta 

 which resulted in a large-scale eddy development in the embayment west 

 of the delta. 



Additional detail describing the nearshore currents of the 

 Louisiana shelf was provided by Oetking et al. (1979). These 

 investigators summarized their findings as follows: 



• The net movement of the water column, which 

 was driven primarily by regional winds, was 

 easterly in the summer and westerly from 

 winter through early spring. 



