system. The seasonal movements of this gyre were described by Pequegnat 

 (1976) and more recently by Smith (1980). During the winter, the 

 counterclockwise gyre is typically located just northeast of Brownsville 

 (Figure 5). It begins moving north during spring and often penetrates 

 as far as Galveston by late s umm er-early fall. This phenomenon allows 

 water from as far south as the Gulf of Campeche to move as far north as 

 Galveston and perhaps beyond. The convergence of the Brownsville Gyre 

 and coastal waters coming southwestward from Louisiana serve to 

 transport nearshore water carrying living and non-living organic 

 materials to the offshore and out of the shelf system. The Brownsville 

 Gyre is also characterized by a transient summer upwelling phenomenon 

 bringing cooler, saltier water onto the inner shelf (Smith 1980). 



Salinity 



Salinity of gulf waters over the Texas-Louisiana shelf is greatly 

 influenced by the generalized current patterns and by river 

 discharge — namely the Mississippi and the Atchafalaya Rivers. Although 

 variable, highest discharge of freshwater runoff into the northwestern 

 gulf occurs during March through May (peak in April) and is lowest in 

 September (Figure 6). Although the annual runoff is only about 0.1? of 

 the annual volume of the Florida Current (Figure 3) exiting the gulf, it 

 represents "\0% of the volume of the water on the continental shelf in 

 the western gulf (El-Sayed et al. 1972). In general, surface water 

 salinity increases on the shelf from the east to the west and southwest, 

 as well as with distance offshore (Temple et al. 1977, Figure 7). 

 During periods of peak river discharge in spring, the effects of runoff 

 (low salinity) sometimes are evident over much of the shelf, extending 

 to the south and west to beyond Galveston (Figure 8a) . During periods 

 of low flow (late summer-fall), the effects of river discharge on 

 salinity are less pronounced. The most prominent salinity phenomenon 

 during this period is the northward extension of high salinity water 

 from the extreme southwestern gulf north to Galveston and beyond (Figure 

 8b, Temple et al. 1977). 



The salinity regime of the water column over the shelf often shows 

 marked stratification. In shallow, nearshore waters, the effect of 

 lower salinity from freshwater discharge may reach throughout the water 

 column, but farther offshore, the less saline water forms a band of 

 shallow, surface water overlying higher salinity water below (e.g. 

 Gallaway et al. 1980). In addition high-salinity, deep water from the 

 open gulf is sometimes upwelled and advected inshore along the bottom of 

 the shelf, also contributing to stratification by producing a bottom 

 layer of high salinity water. 



The pycnoclines or areas of interface between layers of water 

 having different densities serve as accumulation points for suspended 

 materials producing turbid layers in the water column (Figure 9). In 

 the example shown by Figure 9» which depicts conditions offshore of 

 Galveston during spring 1979, relatively low-salinity, turbid water 



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