Variation in water residence time is the primary determinant of phytoplankton and zooplankton composition in a Pacific Northwest reservoir ecosystem (Lower Snake River, USA)

Abstract

We assessed the degree to which advection affected the relative importance of abiotic and biotic factors on plankton composition in reservoirs of the Lower Snake River, a system that is characterized by extremely low water residence time (range 7.9 to 18.8 days). Phytoplankton and zooplankton communities in reservoirs of this run-of-the-river reservoir system were sampled approximately monthly between 2008 and 2011. Phytoplankton communities in the reservoirs were largely dominated by meroplanktonic diatoms whose populations increased as water residence time decreased. Peak phytoplankton biovolume usually occurred during late summer and corresponded with decreased hydrologic infl ows and outflows. Zooplankton biomass of the reservoir system usually was very modest but rapidly increased at stations adjacent to dams as inflows and outflows slowed. Rotifer abundances were higher under turbulent conditions associated with high inflows and outflows. Canonical correlation analyses suggested that as water residence time and temperature increased during late summer, both phytoplankton and zooplankton populations increased dramatically. Advection was likely the major factor suppressing the phytoplankton biovolume and zooplankton biomass except when flow rates decreased enough to allow biotic factors to become important.