The portion and total quantity of 

 wetland primary production exported to 

 adjacent water bodies is lowest in the 

 headwater areas and greatest in the salt 

 marsh. Approximately 2% of produced 

 carbon is exported from the swamp 

 forest; 30% of salt marsh net production 

 is exported to adjacent water bodies. 

 Total and areal organic carbon loading 

 also increase in a downstream direction. 

 Total loading increases from about 1 x 

 10 10 to 20 x 10 10 gC/yr (swamp to salt 

 marsh), and areal loading from 110 to 

 246 g C/m z /yr (swamp to salt marsh). 

 Production and allochthonous carbon 

 inputs exceed respiration and sedimen- 

 tation in all aquatic habitats of Bara- 

 taria basin. Precipitation exceeds 

 evapotranspiration by about 33% on an 

 average annual basis, which results in a 

 runoff surplus (Sklar 1980). Rainwater 

 surplus is a driving mechanism that 

 links upstream and downstream habitats. 



The Role of Marshes and Estuaries 

 in Fisheries Production 



Louisiana has the greatest area of 

 coastal wetlands and the largest com- 

 mercial fishery in the United States 

 (Bahr et al. 1932). Coastal wetlands 

 are thought to play an important role in 

 supporting fisheries (Turner 1977; Bahr 

 et al. 1982). Gunter (1967) called the 

 area around the Mississippi delta the 

 "fertile fisheries crescent." Many of 

 the references that suggested a positive 

 relation between wetlands and fishery 

 production failed to describe the spe- 

 cific manner in which wetlands enhance 

 fisheries production. Studies in the 

 Barataria basin illustrate the way fish 

 species use wetlands. 



Eight species make up 80% to 95% of 

 the nekton of the lower Barataria basin 

 (Gunter 1936, 1938a, 1938b; Perrett et 

 al. 1971; Wagner 1973; Sabins and 

 Truesdale 1974; Chambers 1980): 



bay anchovy ( Anchoa mitchilli ) 

 croaker ( Micropogonias undulatus ) 

 sea catfish ( Arius felis ) 

 striped mullet ( Mugil cephalus ) 

 spot ( Leiostomus xanthurus ) 

 menhaden ( Brevoortia patronus ) 



silverside (Menidia beryllina ) 

 penaeid shrimp ( Penaeus spp.) 



The bay anchovy is thought to com- 

 plete its entire life cycle within the 

 estuary. The other species spawn off- 

 shore and use the estuary as a nursery/ 

 feeding ground. 



Chambers (1980) constructed a dia- 

 gram showing a systematic pattern of use 

 of the Barataria basin by different 

 nekton groups (Figure 52). In the fall 

 and early winter, juveniles and adults 

 of freshwater species move southward 

 into brackish areas as marine species 

 emigrate. As salinities and water tem- 

 peratures increase in the late winter 

 and spring, the marine species begin 

 their upbasin migration while the 

 freshwater species retreat to the 

 fresher water of the northernmost lakes. 

 During the warmer months, mesohaline 

 juveniles of some truly estuarine spe- 

 cies move up to the mid-basin during 

 periods of high salinity. They return to 

 the lower bays and gulf in the late fall 

 and winter as salinities decrease. 



The data of Wagner (1973) and 

 Chambers (1980) suggest that euryhaline 

 marine-spawned juveniles migrate pref- 

 erentially into low salinity waters and 

 slowly move into higher salinity waters 

 as they grow. Nekton biomass in lower 

 salinity areas consisted primarily of 

 large numbers of juveniles (Wagner 

 1973). By contrast, nekton biomass in 

 the more saline areas consisted pri- 

 marily of large juveniles or adults. 



The results of studies from the 

 Barataria basin show that a large pro- 

 portion of nekton found in estuaries 

 spend only a part of their life cycles 

 in the basin. These species, especially 

 larval and juvenile forms, seek out 

 shallow water such as marsh ponds, tidal 

 creeks, and the marsh edge. Data from 

 the Barataria basin and Lake Pontchar- 

 train showed that nekton biomass was 6.8 

 to 11.5 times higher in shallow water 

 marsh areas than in open water (Table 

 20). Several workers on the east coast 

 have also found that shallow tidal 

 creeks and marsh shoals harbor dense 

 populations of juvenile marine species 



126 



