Microzooplankton Grazing Dynamics in Two Contrasting Reservoirs (Run-of-River versus Storage) in the Columbia River Basin, USA

Microzooplankton grazers influence phytoplankton growth in freshwater ecosystems. In rivers, microzooplankton grazing dynamics may be influenced by variations in abiotic factors such as flow rates and temperature, as well as biotic factors such as phytoplankton abundance and composition. Columbia River Basin (CRB) rivers are heavily impounded by run-of-river (ROR) dams, managed for hydropower/navigation, with low reservoir residence times and high flows, and by storage dams, managed for irrigation and drinking water, with higher reservoir residence times and lower flows. We conducted monthly dilution experiments from May-October 2023 in a ROR reservoir (Bonneville, Columbia River) and a storage reservoir (Detroit, North Santiam River) to assess how microzooplankton grazing and phytoplankton growth rates varied within and between these sites over a growing season. There were significant microzooplankton grazing effects based on changes in biomass (chl-a concentration) during the experiments from May to August in the storage reservoir; however, in the ROR reservoir significant chl-a-based grazing effects were only observed in June. Taxonomic analysis indicated that microzooplankton (mostly ciliates and rotifers) exerted positive grazing pressure on diatoms in both reservoirs, but with higher grazing rates in the ROR reservoir. Microzooplankton also exhibited negative taxon-specific grazing rates on all other phytoplankton groups (including cyanobacteria) in both reservoirs, suggesting the potential for a trophic cascade effect within our dilution experiments that at times indirectly favored non-diatom taxa. Overall, our results show a pattern of microzooplankton grazer preference for diatoms, with likely higher grazing impacts in ROR reservoirs with shorter residence times, and diminished grazing and growth dynamics in storage reservoirs, that are more highly altered riverine systems.