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VARIABILITY IN PHYTOPLANKTON BIOMASS, PHYTOPLANKTON GROWTH RATE, AND MICROZOOPLANKTON GRAZING RATE IN A LARGE TEMERATE RIVER (COLUMBIA RIVER, USA)
Thesis

VARIABILITY IN PHYTOPLANKTON BIOMASS, PHYTOPLANKTON GROWTH RATE, AND MICROZOOPLANKTON GRAZING RATE IN A LARGE TEMERATE RIVER (COLUMBIA RIVER, USA)

Jacie Nicole Fabela
Master of Science (MS), Washington State University
2026
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Fabela Proquest Edits
Embargoed Access, Embargo ends: 07/09/2027

Abstract

Chlorophyll Microzooplankton Phytoplankton Rivers Temporal variability
Variability in phytoplankton biomass, phytoplankton growth rates and microzooplankton grazing rates are well studied in marine systems and lakes; however, less is known about these dynamics within fully freshwater reaches of large rivers. We undertook a 7-year (2019-2025) field study of phytoplankton biomass in the lower Columbia River (CR), USA (thereby extending a previously published 14-year dataset to 20-years), and at the same location conducted a monthly set of dilution experiments to measure phytoplankton intrinsic growth and microzooplankton grazing rates over two water years (2023-2025) – the first experimental assessment of these trophic dynamics in the CR. We found striking seasonal variation in phytoplankton biomass, with peaks in spring and lower biomass in the winter months. Phytoplankton intrinsic growth rates and microzooplankton grazing rates also varied temporally, ranging from –0.26 to 0.83 d-1 and from –0.69 to 0.73 d-1, respectively. Levels of phytoplankton biomass were not significantly correlated with temperature but were negatively correlated with Secchi depth and positively correlated with river depth. Moreover, variability in phytoplankton biomass was significantly positively correlated with phytoplankton intrinsic growth rate, but not significantly correlated with microzooplankton grazing rate, suggesting a limited role for the latter in controlling phytoplankton biomass in the lower CR. Overall, our findings indicate that a variety of abiotic and biotic factors can contribute to the seasonal and interannual variability of phytoplankton biomass in the lower CR, which helps advance our understanding of planktonic food webs in rivers more broadly, as well as illustrates the power of long-term sampling programs in aquatic ecology.

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