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Understanding the relative effects of phytoplankton assemblage and temperature on heterotrophic protist grazing rates remains underdeveloped due to seasonal constraints that result in concurrent changes in both variables. In order to separate effects of temperature and community composition on microherbivory, we used the dilution method to measure grazing rates at in-situ and cooled incubation temperatures conducted in parallel during summer/autumn 2012, in Narragansett Bay, Rhode Island, USA. Chain-forming diatoms dominated the microphytoplankton, whereas aloricate ciliates dominated the microzooplankton. Weekly environmental variability –not primarily characterized by temperature– had a significant effect on phytoplankton-species composition. Initial autotrophic biomass averaged 127 g C L ± 149 and heterotrophic biomass averaged 459 g C L ± 281. Temporal change was the principal factor associated with assemblage structure differences, having a greater effect than temperature and incubation. Total autotrophic biomass increased significantly, 600% at ambient and >200% at cooled temperatures, resulting in a significant change in the phytoplankton assemblage structure over the incubation period. Ambient phytoplankton growth and grazing rates averaged 1.77 d ± 0.53 and 0.63d ± 0.41, respectively. Temporal changes in phytoplankton species composition did not have a significant effect on grazing rates. An average 6.4°C decrease in temperature significantly lowered rates by an average of 1.9-fold for growth and 3.3fold for grazing. The percent primary production consumed was on average 1/3 lower in the cooled treatment. These results suggest that temperature plays a larger role in regulating grazing magnitude than phytoplankton prey species composition.

SEPARATING THE EFFECTS OF TEMPERATURE AND COMMUNITY COMPOSITION ON THE MAGNITUDE OF HETEROTROPHIC PROTIST GRAZING RATES IN NARRAGANSETT BAY