Does the nature of the end point for a spring run (marine or riverine) affect the structure of its fish assemblage?

Springs and other groundwater-dependent ecosystems have received much less attention than rivers, lakes, and wetlands. Among freshwater springs, those that flow directly into oceans have been studied even less than those that flow into freshwater systems, the former being less common. The goal of this study was to document the fish assemblages of several springs that flow into the Gulf of Mexico and to compare these assemblages with those of springs that flow into rivers prior to flowing into the Gulf. To describe the fish assemblages, I deployed GoPro cameras along the first 650 m of each of 24 springs, five with a terminus in the Gulf and 19 that had a terminus in a river. On the 12-min footage from each camera, I counted the maximum number of individuals of each species that occurred in a frame together. From these counts, I calculated the total number of fish, species richness, and diversity. Across the geographic range of the study, springs varied in conductivity by more than an order of magnitude, but for any given spring, the conductivity was related more to the proximity of the spring head to saltwater than to a marine or riverine terminus. The total number of fish, species richness, and diversity of the springs did not differ between the two categories of springs; instead, species richness and diversity were related more to the size of the spring, the temperature (which is influenced by latitude), and the percent substrate coverage by primary producers. However, the composition of fish assemblages differed, with the addition of euryhaline species and some loss of sunfish (Lepomis sp.) in springs with more saline water. In addition, the springs in the most urban area had relatively high abundances and species richness of nonnative fishes. Therefore, the physical, chemical, and biological characteristics of springs, and the potential for human-caused disturbance, may be more important than the springs' exact terminus in predicting the composition of the fish assemblage. In particular, connection to saltwater and high salinity is important for migration of euryhaline fish species into a spring, but this connection need not be direct.