Reef QOFircuiUtieSt The hard substrate communities present on 

 petroleum platforms within the white shrimp grounds offshore of 

 Louisiana has been described by George and Thomas (1974) and by Gallaway 

 et al. (1979). The biomass of sessile epifauna is dominated from the 

 surface to about 8-m depths by small acorn barnacles ( Balanus amphitrite 

 and JLa. improvisus) , which, in turn, are covered by a mat of 

 macroalgae, hydroids, bryozoans, and encrusting sponges. The macroalgae 

 component of the mat is restricted to zones near the surface (from 1 to 

 6 m deep) where growths may be luxuriant or sparse depending largely 

 upon turbidity and season. Oysters are usually present but seldom 

 abundant. Near bottom, hydroids dominate and a few dead barnacles and 

 serpulid worm tubes are usually found. 



The sessile epifauna of the near shore platforms provides a 

 low-relief habitat which influences the composition of the associated 

 cryptic macrofauna. Although xanthid crabs and blennies are present, 

 the motile fauna consists mostly of small species, particularly the 

 amphipods Corophium sp., Stenothoe sp. , and Caprella sp. 



The sheepshead ( Archosargus probatocephalus ) is by far the dominant 

 epifaunal grazer inhabiting coastal platforms, but is rivaled or 

 exceeded in numbers (and biomass) by several pelagic species including 

 spadefish (Chaetodioterus XaJjexl ; lookdown ( Selene vomer) ; blue runner 

 ( Caranx chrvsos) and other jacks (Carangidae) ; and bluefish ( Pomatomus 

 saltatrix) . Tropical reef fishes are sometimes encountered but seldom 

 comprise a major component of the fauna. The gray snapper (Lutianus 

 svnagris) is sometimes abundant at some of the more offshore of these 

 platforms. 



Brown Shrimp Gr o u nd Zaunal Assemblage 



Pelagic .aM soft-bottom communities. This assemblage is 

 structurally similar to the white shrimp community but is generally 

 characterized by lower levels of primary productivity and zooplankton 

 consumer biomass. Diatoms dominate phytoplankton populations, and 

 highest primary productivity occurs in spring and from fall through 

 early winter (Kamykowski et al. 1976). As in the white shrimp 

 community, Skeletonema costatum is often associated with seasonal 

 blooms. In general, water column primary productivity in the brown 

 shrimp community decreases from the north to the south, and with 

 distance offshore. These differences have been attributed to freshwater 

 runoff which, along with benthic nutrient regeneration, is considered to 

 be a significant source of nutrients supporting primary production in 

 shelf waters. Pequegnat (1976) has pointed out that highest 

 phytoplankton biomass in gulf waters is, in many cases, associated with 

 the bottom of the euphotic zones (50 to 110 m) , pycnoclines or nitrate 

 nutriclines, and that surface values are typically relatively low. 



37 



