SEASONAL PROTISTAN GRAZING IN NARRAGANSETT BAY

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The impact of heterotrophic protist grazing on phytoplankton abundance was measured in Narragansett Bay, RI, USA, a coastal estuary, from January 2010 to February 2011. Plankton samples were collected within the long-term phytoplankton monitoring project in Narragansett Bay, initiated in the 1950s. Concurrent with weekly dilution experiments, samples were assessed for phytoplankton species composition and environmental conditions at the sampling site were recorded. Over the year, grazing removed an average of 94% (range 20 - 200%) of daily primary production, with peaks in both phytoplankton growth and heterotrophic grazing rates occurring during the summer. Phytoplankton growth rates averaged 0.69 ± 0.58 day-1 for the year, while protistan grazing rates averaged 0.66 ± 0.61 day-1. Phytoplankton growth rates were negative in both winter and spring. Negative growth rates in the winter did not result from nutrient limitation, although nutrient limitation was evident during the summer. There was no relationship between protistan grazing rates and ambient chl a concentration. Grazing rates were related to temperature as well as changing phytoplankton community composition. Seasonal patterns of protistan grazing and phytoplankton community composition and abundance may be better understood when examined in relation to species composition and environmental conditions rather than bulk measures of biomass, including chl a. Overall, results suggest that grazing by heterotrophic protists accounts for a large proportion of phytoplankton mortality in Narragansett Bay.