The Mississippi and Atchafalaya 

 Rivers combined discharge about 2.2 x 

 1013 cubic meters of water annually. 

 The region's next largest rivers, the 

 Pascagoula and Pearl, each carry about 

 three orders of magnitude less water 

 (1.2 x 10 10 and 1.0 x 10 10 m 3 /yr, res- 

 pectively). Smaller streams and bayous 

 are also important transporters of water 

 and nutrients. Most of the water flow 

 to Lac des Allemands in the swamp forest 

 of the upper Barataria basin is carried 

 through swamp bayous (Day et al . 1977). 

 The transport of organic matter from 

 wetland and terrestrial producers to 

 aquatic consumers is a primary func- 

 tional role of natural bayous and 

 streams. A Mississippi coastal plain 

 stream studied by de la Cruz and Post 

 (1977) carried between 36,000 and 

 538,000 kg of organic carbon per year. 

 Greater channelization in the upper 

 basin probably reduces the flow through 

 natural channels and results in more 

 rapid water movement out of the basin 

 (Hopkinson and Day 1979). 



The input of organic matter from 

 terrestrial sources is important for 

 river and stream habitats (Hynes 1970). 

 De la Cruz and Post (1977) calculated an 

 input of particulate organic matter of 

 about 400 g/m 2 /yr into a Mississippi 

 stream. Day et al. (1981) estimated 

 annual organic matter loading into swamp 

 bayous in the des Allemands region of 

 Barataria basin at 2.5% of net primary 

 production (about 1700 g dry wt/m 2 /yr). 



Respiration typically exceeds pri- 

 mary production in rivers, streams, and 

 bayous. Day et al. (1977) estimated 

 bayou gross primary production at 229 g 

 dry wt/m 2 /yr and community respiration 

 at 298 g dry wt/m 2 /yr. Bayou production 

 is generally less than lake production 

 because of shading. 



We assumed for lack of quantitative 

 information that major rivers and bayous 

 are similar in community composition and 

 structure to fresh open water areas. 

 Smaller streams take up an insignificant 

 area and are considered a minor compo- 

 nent of the rivers, streams, and bayou 

 habitat. 



As indicated in the description of 

 canal habitat, natural tidal creeks are 

 different from canals of similar width. 

 There is some evidence that tidal creeks 

 are themselves different from the very 

 shallow pools of water that occur on the 

 surface of wetlands. The latter tempo- 

 rary habitat is used by nursery ground, 

 or estuarine-dependent juvenile nekton 

 at higher densities than are tidal 

 creeks (Day et al. 1982). This implies 

 that the ecological or functional role 

 of the river, stream, or bayou habitat 

 is primarily as a conduit, or transpor- 

 tation route by which many nektonic 

 animals migrate to their primary feeding 

 areas, rather than their primary habitat. 



ESTUARINE AQUATIC BED (15) 



Estuarine aquatic beds (EAB) , also 

 commonly called seagrass beds, or marine 

 meadows, are subtidal beds of rooted 

 aquatic vegetation (Figure 32). This 

 habitat occupied 14,319 ha (35,368 

 acres) in the MDPR in 1978 (Table 5). 

 Although this area is relatively small, 

 grassbeds may play an important role 

 because of their value as a refuge and 

 feeding ground for some commercially 

 important nekton species (e.g., blue 

 crabs, menhaden) and as a food source 

 for waterfowl and nekton, either in its 

 living form, or after it decomposes to 

 detritus. 



Studies of EAB in the MDPR have 

 mostly been descriptive, i.e., mapping 

 and identification (Eleuterius 1973; 

 Montz 1978; Turner et al. 1980). The 

 principal submergent macrophytes encoun- 

 tered in Louisiana are Ruppia maritima , 

 Vallisneria americana , Najas guadalu- 

 pensis , and Potamogeton perfoliatus . 

 Halodule beaudettii , Halophila engel- 

 manii , and Cymodocea f iliformis also 

 occur. 



The main factors that limit the 

 distribution and production of submerged 

 grassbeds are salinity, nutrient con- 

 centrations, and light. The two species 

 of submergent macrophytes most commonly 

 reported in estuarine waters are eel- 

 grass ( Zostera marina ) from temperate 



66 



