Other nektonic larvae immigrate 

 into nursery ground areas during summer 

 and fall, and emigrate offshore again in 

 winter. These animals are less tolerant 

 of low salinity water and enter the 

 basin up to brackish water zones. The 

 most important example from an economic 

 standpoint is the white shrimp ( Peneaus 

 setiferus ) . 



Freshwater nektonic adults utilize 

 the basin down into intermediate salin- 

 ity zones during late fall and early 

 winter and replace the emigrating marine 

 forms. Finally, there are some charac- 

 teristic nekton that are year-round 

 residents of the estuarine open water 

 habitat (as well as rivers streams and 

 Dayous) . A classic example is the an- 

 chovy (Anchoa mitchelli ) . These pat- 

 terns are shown in Figure 52. 



The estuarine open water habitat 

 characteristically has lower nekton 

 biomass than small creeks and ponds (see 

 Table 20). Nekton biomass ranges from 

 0.32 to 1.19 g wet wt/m 2 for estuarine 

 open water, while shallow marsh water 

 ranges from 2.57 to 46.1 (for the same 

 species) , which implies that the nek- 

 tonic organisms that use the estuarine 

 open water are strongly dependent on 

 surrounding wetlands. 



Benthos 



The benthic community in the estu- 

 arine habitat in the MDPR is not well 

 known, in that it has been examined in 

 detail only in Lake Pontchartrain and in 

 less detail in Mississippi Sound. Lake 

 Pontchartrain is probably not represen- 

 tative of the estuarine open water hab- 

 itat throughout the MDPR, for reasons 

 that are explained in the Pontchartrain 

 hydrologic unit description. 



The major functions of the benthic 

 community in estuaries may be divided 

 into four parts: (1) benthic organisms 

 affect sediment stability and water 

 turbidity by filtering the water and 

 biodepositing particulate material, by 

 burrowing through sediments, and by 

 building reefs and depositing shells and 

 mucus; (2) they consume oxygen within 

 sediments and at the surface, thus 



regulating a number of chemical pro- 

 cesses, including nutrient regeneration 

 to the water column; (3) they consume 

 carbon from sediments and the water 

 column, and upgrade it into higher 

 quality protein that is then available 

 for bottom-feeding nekton; and (4) they 

 are important socioeconomically, as a 

 resource (meat and shell) , and as bio- 

 logical indicators because of their 

 tendency to concentrate various pollu- 

 tants. 



Five areas that have been studied 

 in some detail are Lake Pontchartrain, 

 Lake Salvador, Lake Cataouatche, Little 

 Lake, and Airplane Lake. These areas 

 span the salinity and size range of the 

 habitat. They also show differing de- 

 grees of cultural stress; Lakes Pont- 

 chartrain and Cataouatche are severely 

 perturbed by dredging and/or pollution 

 of various kinds; Lake Salvador is 

 threatened by eutrophication, while 

 Little Lake and Airplane Lake are rela- 

 tively unperturbed. 



The productivity and degree of 

 eutrophication of the water bodies in 

 the Barataria basin are discussed below 

 in the Barataria (IV) section of the 

 HYDROLOGIC UNIT DESCRIPTIONS. The five 

 areas listed differ significantly in 

 their trophic status and productivity. 

 (Figures 34 and 47, and Tables 14 and 

 18.) Of these five examples of the 

 estuarine open water habitat, the high- 

 est productivity occurred in the most 

 nutrient-rich (eutrophic) water body, 

 Lake Cataouatche. Eutrophic waters also 

 are heterotrophic, i.e., more organic 

 matter is consumed annually than is 

 produced (see Table 18). Gross produc- 

 tivity ranged from 100 g C/m 2 /yr for 

 central Lake Pontchartrain to greater 

 than 600 for Lake Cataouatche. Chloro- 

 phyll levels also varied considerably, 

 from less than 5 to greater than 200 

 mg/m 3 . 



SALT MARSH (17) 



Salt marshes are among the most 

 intensively studied, and best known 

 ecological habitats in the MDPR. They 

 comprise a significant portion of the 



72 



