Community structure of shoreline nekton in the estuarine portion of the Alafia River, Florida: Differences along a salinity gradient and inflow-related changes

Determination of baseline biological conditions in Florida's rivers is required prior to increased water withdrawals for the rapidly growing human population. We studied the small-bodied shoreline nekton community of the estuarine portion of the Alafia River (a tributary of Tampa Bay, SW Florida) from 1998 to 2003 using 21.3-m seines of 3.2-mm stretched mesh. Community structure along the estuarine salinity gradient changed from dominance by marine and estuarine transients from river km 0 to 9.3 to greater abundances of resident low-salinity and freshwater forms near the tidal–freshwater interface (river km 9.3–14). There was evidence that the magnitude of the freshwater inflow influenced nekton community patterns along the estuary, with the center of abundance of many species shifting in response to inflow. The community structure of the nekton of river km 0–9.3 showed regular seasonal changes that were of greater importance than was temporal correlation with abiotic variables (temperature, salinity, inflow); these changes can be explained by predictable spawning patterns of transient species. In contrast, changes in the structure of the nekton community in river km 9.3–14 were well correlated with changes in salinity and, to a lesser extent, inflow. The sensitivity of the nekton to relatively small changes in salinity near the tidal–freshwater interface is of importance given that comparatively minor changes in freshwater inflow could markedly affect salinities in this upstream region.