Table 4.17. Primary productivity (g dry wt/m /yr) in open water areas of Calcasieu Lake. 



Table 4.18 Average annual values (mg/1) of total 

 organic carbon for four areas in Cam- 

 inada and Barataria bays. 



4.8.5 ENERGY BUDGET OF AQUATIC HABI- 

 TATS 



Figure 4-29 shows the relative importance of dif- 

 ferent organic energy sources to the aquatic habitats 

 and indicates the ways in which this organic matter is 

 used. In the inland open water habitat, the three 

 major sources of organic energy (phytoplankton, ben- 

 thic algae, and detritalinput from wetlands) are about 

 equal in importance. The largest fraction of this or- 

 ganic production supports the benthic community. 

 This evidence, as well as the predominance of ben- 

 thic feeding organisms among the larger consumers 

 (fish, birds), emphasizes the importance of the benthic 

 community in shallow inland waters. 



In the nearshore Gulf habitat the major organic 

 producer is phytoplankton, but some supplemental 

 input of detritus is contributed from the estuaries. 

 Fishes, which feed primarily on small benthic animals, 

 account for less than 5% of the total energy flow in 

 this habitat. 



4.8.6 THE DETRITUS SYSTEM AND THE BEN- 

 THIC COMMUNITY 



Detritus has been defined in a biological sense as 

 dead organic material, usually finely divided, which is 

 used as a food source by many aquatic organisms. 

 The process by which raw detritus becomes enriched 

 by microorganisms and consumed by small crustacean 

 scavengers was discussed in part 4.2, wetland habitats. 

 Detritus is equally important in the shallow aquatic 

 habitats of the Chenier Plain. It is ingested as 

 suspended particulate material by filter-feeding ani- 

 mals such as the Gulf menhaden, or it can be eaten 

 by deposit feeders such as shrimp, after it settles to 

 the bottom. 



Except for insects such as dragonflies, water sur- 

 face bugs, and mosquitoes, many aquatic invertebrates 

 spend their entire life cycle as benthos. These popu- 

 lations are usually most abundant near marsh inter- 

 faces; thus, the edges of inland open water bodies are 

 particularly productive (fig. 4-30). Benthic pro- 

 ductivity reaches a peak in early spring, providing a 

 food supply for spring concentrations of shrimp and 

 Gulf metihaden (fig. 4-31). The benthos tend to be 

 concentrated near the surface sediment layer, with 

 only a small fraction of the community found deeper 

 in the sediment (fig. 4-32). Thus, the productivity of 

 benthic invertebrates is primarily dependent on the 

 surface area of bottom sediment and the physical 

 conditions during the early spring season. 



The importance of detritus feeders in the overall 

 food web of estuarine waters was demonstrated for 

 Lake Pontchartrain, Louisiana (Darnell 1961). The 

 most common fishes and shellfishes (menhaden, 

 mullet, and shrimp) in this system fed predominantly 

 on detritus or on small benthic scavengers which 

 themselves were detritus feeders. The bay anchovy 

 was the only abundant estuarine fish that was not a 

 detritivore, its diet was composed largely of zooplank- 

 ton. The largest group of fishes occurring in the 

 Chenier Plain consists of bottom-feeders and most 

 fish are dependent upon the detrital system (appen- 

 dix 6.3). 



199 



