A comparison of benthic algal and macroinvertebrate communities in a dammed and undammed Mediterranean river (Eel River watershed, California, USA)

Abstract Flow regulation by dams can reduce hydrologic variability, and subsequently, alter river food webs. This study compared the summer algal and macroinvertebrate assemblages from the mainstem Eel River (Mainstem) below Cape Horn Dam to that of the unregulated Middle Fork Eel River (Middle Fork). Despite above average rainfall in 2017, flow regulation produced a steeper spring flow recession limb with only 15.72 cubic meter per second in the dammed Mainstem in late May, which was about half the undammed Middle Fork's discharge of 32.28 cubic meter per second at the same time of year. Peak summer water temperatures in the Mainstem (mid‐August) were later relative to the Middle Fork (late July) based on past years of monitoring. The regulated Mainstem supported less Cladophora with lower mid‐summer algal richness, resulting in a macroinvertebrate community less reliant on Cladophora , with fewer key invertebrates such as Chironominae compared to the unregulated Middle Fork. Dam systems that reduce flow variation in Mediterranean rivers may shift the downstream community away from seasonal proliferation of dense green filamentous algae and invertebrate grazers typical of rivers in this climate, and towards adnate epilithic algae and cyanobacteria and generalist invertebrates, due to the encroachment of riparian vegetation and reduced thermal variation.