Drivers of protistan grazing pressure: seasonal signals of plankton community composition and environmental conditions

Abstract

Rates of heterotrophic protist grazing and phytoplankton growth were measured weekly to bi-weekly in Narragansett Bay, Rhode Island, USA, from January 2010 to February 2011. In situ sensor data and species composition were collected concomitantly to link patterns in plankton dynamics with ancillary environmental and biological processes. Annual average phyto- plankton growth rates were 0.69 ± 0.58 d �1 , and heterotrophic protist grazing rates were 0.79 ± 0.61 d �1 . Phytoplankton growth rates were at times negative in both winter and spring. Nutrient limitation was only detected during summer, and negative growth rates in winter did not result from nutrient limitation. On an annual average, grazing removed 96% (20 to 200%) of primary production, with peaks in both phytoplankton growth and heterotrophic protist grazing rates dur- ing summer. There was no relationship between protistan herbivory rates and initial chlorophyll a concentration. Dominant grazer taxa changed seasonally. Heterotrophic dinoflagellates domi - nated in summer and were associated with significantly higher than average grazing rates (>1 d �1 ). Seasonal changes in grazing rates were most significantly characterized by seasonal changes in both temperature and plankton community composition. The relative effects of temperature and species composition could not be distinguished statistically. The magnitude of protistan grazing and subsequent effects on trophic transfer and primary production rates as well as phytoplankton community composition may be better understood and parameterized when grazing pressure is evaluated in relation to species composition and environmental conditions rather than bulk measures of biomass.