dependent upon hyposaline estuaries decline in abundance resulting in 

 changes in the relative abundance of species within at least the 

 nearshore community. 



Within the Northwestern Gulf Province defined above, I believe 

 (after reviewing data describing both soft- and hard-bank communities) 

 that three characteristic faunal assemblages are represented on the 

 Texas-Louisiana shelf — the white shrimp ground (inner shelf) assemblage, 

 the brown shrimp ground (intermediate shelf) assemblage, and the outer 

 shelf assemblage (Figure 19). Of these, the latter should be considered 

 as representative of a separate province of a tropical nature. The 

 estuarine-dependent white shrimp ground assemblage is considered to be 

 generally delimited by the 20-m depth contour except near the 

 Mississippi River Delta where the shelf is narrow and the community 

 extends into deep water. This division (Figure 19) corresponds to the 

 inner shelf assemblage of Parker (1960), and, generally, to the inner 

 shelf assemblage plus the pro-delta fan assemblage of Defenbaugh (1976). 

 The brown shrimp ground assemblage of the Texas- Louisiana shelf (Figure 

 19) is considered to extend from 20 to 60-m depths, corresponding to the 

 intermediate shelf assemblages of Defenbaugh (1976) and Parker (1960). 

 As indicated above, disjunct pink shrimp communities are represented 

 within the brown and white shrimp grounds where appropriate sediments 

 occur. 



The outer-shelf, tropical assemblage is considered generally 

 representative at depths greater than 60 m as shown by Figure 19. With 

 the exception of some of the hard-bank communities, the outer-shelf 

 assemblage is the least well-known of all the assemblages in terms of 

 community composition and dynamics. Synoptic accounts of soft-bottom 

 and hard-bank communities for each of the recognized assemblages are 

 provided below. As with any effort of this nature, it should be kept in 

 mind that these accounts represent generalized, broad-brush treatments 

 and variation and exceptions are the rule rather than the exception. 



White Shrimp Ground Fau n al Assemblage 



Pelagic .and. soft-bottom communities. The white shrimp grounds 

 (particularly offshore of Louisiana) are characterized by high primary 

 productivity as well as by high zooplankton consumer biomass levels — all 

 greatly influenced by Mississippi River discharge. Fucik and El-Sayed 

 (1979) reported results of phytoplankton ecological studies performed in 

 the white shrimp grounds offshore of Louisiana. They found the diatoms 

 to dominate phytoplankton populations over most or all of the year, 

 particularly Skeletonema costatum during spring and fall months. They 

 reported the surface standing crop of phytoplankton to be several times 

 higher than in the open gulf, and integrated standing crop values to be 

 double that normal for the gulf. In terms of primary productivity, the 

 Louisiana white shrimp grounds were indicated to be more than an order 

 of magnitude more productive than the oceanic gulf. 



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