Impacts of marsh loss and fragmentation on microhabitat use by estuarine nekton in southwest Louisiana

In Louisiana, the extensive loss and fragmentation of coastal marshes has prompted inquires into the impacts these processes may have on estuarine-dependant nekton. To date, research on nekton response to marsh loss and fragmentation has been limited to landscape-level studies which focus on the relationship between nekton productivity and the availability of marsh edge. These studies have relied on the assumption that marsh edges provide the same level of support to nekton regardless of the degree of surrounding marsh loss or fragmentation. This study tested this assumption by investigating the impacts of marsh loss and fragmentation on marsh-edge characteristics and their associated nekton assemblages. The effects of marsh loss at the 1 km2 scale were examined by stratifying three brackish marsh management units located in the Chenier Plain of western Louisiana into three 1 km2 treatment squares, each representing one of three levels of marsh loss: Low (10%-35% water), Medium (40%-65% water), and High (70-95% water). Within each treatment square, nekton assemblage (density, diversity, body condition) and marsh-edge characteristics (water quality, submerged aquatic vegetation biomass, sediment organic matter, and emergent stem density) were sampled concurrently at six randomly established sampling points during the spring and fall of 2008. Variables representing marsh loss and fragmentation were also quantified within 1 ha squares centered on each sampling point. Relationships between nekton assemblage characteristics and environmental variables were explored with a canonical correspondence analysis. Data analysis revealed differences in sediment organic matter, ha scale percent marsh, and ha scale fragmentation in the spring sampling season. The passage of Hurricane Ike between sampling seasons may explain why only ha scale percent marsh differed between treatments in the fall. Despite differences in these environmental variables between treatments, nekton assemblage characteristics were not found to differ between treatments. This may be partially explained by the lack of strong relationships between nekton assemblage characteristics and environmental variables as indicated by the canonical correspondence analysis. The results of this study do not indicate that nekton support provided by marsh edges is influenced by the degree of marsh loss at the 1 km2 scale