A comparison of community structure in regulated and unregulated reaches in the Upper Eel River, California

Flow regulation of rivers by impoundments and diversions can reduce hydrological variability. As a result, densities of algae and benthic macroinvertebrates often increase, but many sensitive taxa are lost, causing shifts across the food web. In Northern California, dams that reduce winter peak floods can result in the primary consumer community becoming dominated by grazers that are relatively invulnerable to predation, which can reduce the amount of energy transferred up the trophic levels. The steeper slope of the spring hydrograph recession limb downstream of a dam can also greatly impact instream diversity of periphyton, invertebrates and fish. The dam and diversion system on the upper Mainstem Eel River in Northern California has direct impacts on endangered salmon populations, but the effects of the dams on the greater ecosystem are not well understood. This study compared the seasonal algal and benthic macroinvertebrate communities from the Mainstem Eel River below Cape Horn Dam to that of the unregulated Middle Fork Eel River. The 2017 water year had above average rainfall, with several bankfull flows observed in the winter as well as elevated base flows in the summer in both rivers. Despite the wet water year in 2017, the regulation of flows by the dams still likely produced a shorter spring recession limb and, more importantly, a delay in peak summer temperatures in the Mainstem relative to the Middle Fork. Although the abundance and diversity of invertebrates were not notably different between the regulated and unregulated rivers, there did appear to be a variation in the food webs. By mid-summer, the unregulated Middle Fork developed into an ecosystem predominated by Cladophora and its epiphytes with numerous invertebrate grazers. Yet the Mainstem, especially immediately below the dam with the encroachment of vegetation, had less growth of the filamentous green algae (t(20)=4.61, p=0.0002) with lower mid-summer algal richness ((20)=2.53, p= 0.020), resulting in an invertebrate community more reliant on filtering fine organic matter with far fewer grazers (pCladophoraas well as the abundance and development of some key macroinvertebrate taxa, such as midges and small minnow mayflies relative to an unregulated reach. As the Middle Fork is an inherently warmer system, future studies are needed across a longitudinal gradient of the Mainstem over multiple water years to capture interannual variation and to ultimately determine the influence of the dams on riverine ecosystems