Hydrologic regime and turbidity influence entrance of terrestrial material into river food webs

We used stable isotope signatures of deuterium (δD) and a Bayesian stable isotope mixing model to estimate contributions of algae versus terrestrial plants to consumers during different hydrologic phases in three Texas rivers spanning a gradient of turbidity and light penetration. In the two rivers where high-flow pulses increased turbidity, assimilation of source material by consumers varied according to discharge stage. In these rivers, algae made greater contributions to macroinvertebrates and fish biomass following low-flow periods, and terrestrial plants made greater contributions following high-flow pulses. In the river with greatest loads of suspended sediments, contributions of material from terrestrial plants also increased slightly following an extended low-flow period, possibly because of increased abundance of inedible cyanobacteria. During flow pulses, lower algal biomass and production, combined with increased inputs of terrestrial organic matter from watersheds and riparian habitats, can result in greater inputs of terrestrial material into aquatic food chains. These patterns most closely match predictions of the River Wave Concept, which posits that flow is the key process determining the source of organic matter assimilated by higher consumers in rivers. Incorporation of interactions between hydrology and turbidity into river ecosystem models should facilitate more accurate predictions of food web dynamics.