MICROZOOPLANKTON GRAZING DYNAMICS IN RUN-OF-RIVER VERSUS STORAGE RESERVOIRS

Microzooplankton grazers influence phytoplankton growth in freshwater ecosystems. In rivers, microzooplankton grazing dynamics may be influenced by variations in flow rates, temperature, etc. 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 Lake, North Santiam River) to assess how microzooplankton grazing and phytoplankton growth rates varied between these sites over a growing season. There were significant effects of dilution based on changes in chl-a from May to August in the storage reservoir; however, in the ROR reservoir significant effects based on chl-a 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. Results show a clear pattern of microzooplankton grazer preference for diatoms, with likely higher grazing impacts in ROR reservoirs with shorter residence times, and have implications for reservoir management, especially where cyanobacteria blooms are a concern.

Presentation Preference: Oral

Primary Presenter: McKenzie Frazier, Washington State University (mckenziefrazier97@gmail.com)

Authors:

Gretchen Rollwagen-Bollens, Washington State University  (rollboll@wsu.edu)

Stephen Bollens, Washington State University  (sbollens@wsu.edu)