Effects of Reservoir Type (Run-of-River vs. Storage) on Zooplankton Assemblages

Dams hydrologically alter riverine systems by lengthening water residence time and affecting water quality. Two main dam types occur within the Columbia River Basin (CRB) of North America: run-of-river reservoirs, which have relatively short residence times (days), and storage reservoirs, which have longer residence times (months). Zooplankton are vital primary and secondary consumers in these systems, yet it is currently unknown how reservoir type may affect zooplankton assemblages, including their abundance, diversity, and composition. To address this knowledge gap, we undertook a field study of four CRB reservoirs, including two run-of-river reservoirs (Bonneville and The Dalles) and two storage reservoirs (Cougar and Detroit), to address this knowledge gap. Triplicate zooplankton tows and associated environmental samples were collected at each site bimonthly over two years (November 2022-September 2025). We examined spatial and temporal variability in zooplankton and environmental conditions, and we applied PERMANOVA, non-metric multidimensional scaling (NMDS) ordination, hierarchical clustering analysis, indicator species analysis, and ENVFIT to assess zooplankton assemblage patterns and their environmental associations. We found zooplankton abundances to be lower in run-of-river reservoirs than in storage reservoirs (with marked seasonal variability), and zooplankton diversity to be lower in storage systems (dominated by copepods) compared to run-of-river reservoirs. Water temperature, dissolved oxygen, phosphate concentration, combined nitrate and nitrite concentration, conductivity, Secchi depth, and residence time were significantly associated with zooplankton assemblage structure in the study reservoirs. These associations may be attributed to direct effects (e.g., via zooplankton physiological rates), as well as indirect effects via trophic dynamics (e.g. nutrients increasing the phytoplankton prey base for zooplankton). Lastly, two invasive zooplankton species (the copepod Pseudodiaptomus forbesi and the cladoceran Eubosmina coregoni) were only found in the run-of-river reservoirs, likely due to shipping introducing invasive species in the mainstem Columbia River. This study is the first to directly compare differences in zooplankton abundance, diversity, and composition between run-of-river versus storage reservoirs and provides novel insights into how the distinct hydrological alterations produced by dams may influence freshwater food webs and ecosystem dynamics.