Influence of the marsh edge on the structure and trophic ecology of the fish and macroinvertebrate community in a Louisiana estuarine ecosystem

Habitat degradation of saltmarsh ecosystems reduces the suitability of the marsh as permanent environments for resident species and seasonal nursery grounds for transient fauna. This study was conducted in Terrebonne-Timbalier bays near Cocodrie, Louisiana. Fishes and macroinvertebrates were collected using a drop sampler in marsh-edge and open-water habitat types, in three locations (i.e., pond, channel, and bay), and four seasons (Fall of 2000, Winter of 2000-2001, and Spring and Summer of 2001). The specific objectives of this study were: (1) to determine the difference in composition and structure of fish and macroinvertebrate communities, (2) to establish the food habits of the fish community, and (3) to determine the dependence on Spartina alterniflora of fishes and macroinvertebrates through the use of stable isotope techniques. Densities of resident and transient species including darter goby, naked goby, and brown shrimp were detectably different among habitat types, seasons, and locations. The naked goby consumed higher abundances of harpacticoid copepods along the marsh edge than in the open water. Bay anchovy and naked goby had detectable differences in prey utilization among seasons and ingestion was associated with changes of environmental variables. Despite the presence of detritus in the stomachs of several fish species, it was relatively rare. Naked goby was the only species that had detectable differences in carbon stable isotopes between mash-edge and open-water habitat types. Significant seasonal variations were identified in the carbon isotopic values of naked goby, daggerblade grass shrimp, and blue crab, and in the nitrogen isotopic values of brown shrimp. In the field experiments with naked goby, differences in prey utilization and in carbon or nitrogen isotope values were not detectable among habitat types. The marsh edge is essential for food and refuge for estuarine nekton, but macroinvertebrates densities may be more sensitive to marsh loss than fish densities. Nevertheless, marsh loss may have an effect on the feeding ecology of juvenile fishes that rely primarily on benthic prey along the marsh edge. The relative contribution of Spartina alterniflora to support the nekton community was less than 35% in the study area