Pequegnat (1976) and references therein report maximum concentrations of 

 chlorophyll .a at 50 to 110 m, often coinciding with the bottom of the 

 euphotic zone, pycnoclines or nitrate nutriclines. Zooplankton 

 assemblages probably represent a transition zone between the neritic and 

 ocean zones as described by Pequegnat (1976) for the adjacent 

 continental shelf. Parker (1960) and Defenbaugh (1976) described 

 characteristic epifaunal invertebrates; the demersal fish community was 

 characterized as similar to that of the brown shrimp assemblage by 

 Chittenden and McEachran (1976) except near the hard banks where a 

 tropical fauna is characteristic. Little has been published concerning 

 the structure and dynamics of the soft bottom communities of the outer 

 shelf. 



Reef nninmiini ties. Natural banks occurring on the outer shelf 

 (Figure 15) exhibit much variability in their sessile epifauna; this 

 variability has been variously attributed to depth, turbidity, nature of 

 the basal substrate, proximity to the Mississippi River, and other 

 factors. As recently reported by Dr. T.J. Bright (Texas A&M 

 University, April 1981; pers. comm. ) , the natural banks can perhaps 

 best be categorized biologically in terms of their biotic zonation 

 defined with respect to reef-building activity. Dr. Bright recognizes 

 seven zones: 



Diploria-Montastrea-Porites Zone : A zone 

 consisting of living, high diversity coral 

 reefs. 



II. Madracis Zone: A zone dominated by the small 

 branching coral Madracis mlrabilis , which 

 is producing large amounts of carbonate 

 substratum. 



III. Stephanocoenia Zone: A zone consisting of 

 living, low-diversity coral reefs. 



IV. Algal-Sponge Zone: A zone dominated by 

 crust ose coralline algae actively producing 

 large quantities of carbonate substratum 

 (considered here to extend downward, past 

 the depth at which algal nodules diminish in 

 abundance, to the greatest depths at which 

 coralline algal crusts are known to cover a 

 substantial percentage of the hard 

 substratum) . This is the largest of the 

 reef-building zones in terms of area of sea 

 bottom. 



42 



