Aquatic community organization in a diverse floodplain river fish fauna of the southeastern United States

I investigated the ecology of native fishes resident to the unregulated Sipsey River in west-central Alabama for 20 consecutive months. Using a synoptic sampling approach, I collected > 19,000 fishes representing 77 of 88 species on record within 3 % of the river's 187 km length. Channel and floodplain contributions to abundance and biomass showed floodplain habitat harbored a majority of small recruits whereas channel habitat harbored the majority of fish biomass, supported greatest numbers of species and supplied smaller individuals for several species. Species exchange between habitats facilitated by flooding events occurred during winter and late spring periods. I provide evidence that several regression models of relative species abundances and body size, a proxy for metabolic constraints, did not conform to prevailing theory. These results support my prediction that theoretical expectations of energetic constraints are best examined in local assemblages as opposed to regional models, and indicate resource use increases with body size rather than supporting the assertion that energy is utilized equally across different body sizes. The habitat-based food web identified broad similarities in channel and floodplain prey consumption, with the exception of microcrustaceans in the floodplain. The vast majority of links were weak associations constituting < 24 % of mean prey biomass in fish diets. Aquatic insects were the most heavily utilized prey category, with chironomids consumed in largest proportion in the channel, second only to consumption of cladocerans in the floodplain. The Sipsey provided a rare opportunity to investigate species-specific associations of unionid mussel larvae which parasitize fish hosts during development to juveniles. Infections were highest in early spring and summer seasons, and Lampsilini species were carried on twice as many fishes as other tribes, but at low abundance. Unionid species packaging glochidia into conglutinates expressed highest host specialization when standardized by fish species relative abundances compared to unionid species that broadcast larvae or use mantle lures to transmit glochidia. These results will contribute to our understanding of some major ecological components of a naturally free-flowing floodplain river, which will provide important perspective into the management of southeastern drainages and a useful comparative example for watershed conservation.