Benthic food supplies are used extensively. Any 

 decrease in area and productivity of the benthic com- 

 ponent will be accompanied by a decrease in depen- 

 dent fisheries. For instance, the productivity and 

 normal function of the benthic community are modi- 

 fied by hydrologic changes and dredging. Dredging 

 resuspends sediments, nutrients, and toxins in quanti- 

 ties that benthic communities cannot tolerate. A 

 common example is the smothering of oyster beds 

 with sedimentary materials. On the other hand, some 

 benefit may result from resuspending the shallow 

 buried organic material which can then enter the food 

 web. 



4.8.7 PRIMARY PRODUCTIVITY 



The capacity of a body of water to produce living 

 organisms is usually detemiined by its primary pro- 

 ductivity. Primary productivity is often measured by 

 photosynthetic rates of phytoplankton, but photo- 

 synthetic rates ofbenthic algae and submerged aquatic 

 plants also may be included. Primary productivity 

 may be expressed as gC/m^/yr or as g-cal/m^/yr. 



Data for primary productivity of the Chenier 

 Plain aquatic system or other, similar areas in the 

 northern Gulf generally are scarce and inconclusive. 

 However, seasonal differences in productivity in inland 

 brackish water and saltwater in Louisiana have been 

 documented. In a one-year study, peak productivity, 

 based upon photosynthetic rates of phytoplankton, 

 benthic algae, and submerged aquatics, occurred in 

 February to March and again in July to August (fig. 

 4-33). Cause for these peaks was not explained. In a 

 study by Sklar (1976), phytoplankton productivity 

 of the nearshore waters west of the Mississippi River 

 failed to show a second peak (in the summer). 



On the basis of Sklar's work (1976), turbid, river- 

 influenced waters in the inland open water habitat 

 tended to show higher productivity than the nearshore 

 Gulf habitat. 



150 



c" 

 CO 



E 



E 



2 100 



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u 



c 



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 O 



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 c 



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u 



w 

 Q. 



50 - 



o. 0-1- 



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3 

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-50 



-100 



February 



April 



June August 



Months -1976 



October December 



Figure 4-33. Monthly Huctuations of plant productivity in brackish and salt waters based upon deviations from 

 the 1976 mean (Day et al. 1973, Allen 1975). Adapted by R. Beck, ERCO. 



202 



