DYNAMICS OF LITTORAL FISHES AND DECAPODS ALONG A COASTAL RIVER-ESTUARINE GRADIENT

We examined community dynamics of nekton along a coastal river-estuarine gradient in Old Fort Bayou Mississippi, U.S.A. Distribution, abundance and similarity of nekton varied spatially and seasonally along the gradient in concert with varying physical-chemical conditions. The tidal freshwater (TFW) and oligohaline (OH) habitats had greater diversity and evenness than the mesohaline (MH) habitat, due to the interfacing of the dominant freshwater and the estuarine/marine faunas. Stepwise multiple regression indicated that water temperature was the most important predictor of species richness within TFW and MH when the entire species complement was considered. However, separation of freshwater and estuarine/marine species suggested that water temperature (positive relationship) and salinity (negative relationship) were the best predictors of freshwater species at TFW while the estuarine/marine species complement was best predicted by salinity and water temperature at TFW and MH. Models based on abundance data indicated that there was a significant negative relationship between salinity and turbidity and freshwater species abundance while a positive relationship was documented between estuarine/marine species abundance and salinity and turbidity. The dynamic nature of saline marsh nekton depends, in part, on the species complement, the salinity, and the rate of salinity fluctuation within various sections along the estuarine gradient. Although it is dogmatic that many estuarine habitats are physiologically 'harsh' due to fluctuating physical-chemical conditions, low-salinity nekton were found to tolerate greater fluctuations in salinity and pH than higher-salinity nekton. These habitats act as conduits between freshwater and estuarine habitats and serve a nursery function for estuarine-dependent fishes and decapods; they are thus functional components of the estuarine system. © 1991.