Food Web Shifts Down River Networks: Where Are Predators Supported by Algae, and Why

Predators in river food webs derive increasing proportions of their carbon and energy from algae as one moves downstream through river networks.  We are studying how the length of the algal-based food chain responds to the interplay of disturbance (bed mobilization) and ecosystem productivity, which both increase downstream with substrate fining, discharge, and channel and valley width.  The major control exerted by these biophysical factors on food webs may be through their impact on predator resistant grazers. Disturbance removes armored, defended grazers, releasing resilient grazers from competition for limiting algae for a window of time.   During this time window, predators derive energy indirectly from algae, a superior food resource.  This window of opportunity may open earlier, but close sooner in more productive sunlit habitats downstream, if primary productivity and warmth accelerate recoveries of first vulnerable and then invulnerable primary consumer taxa, following disturbance.