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Top-down control by micro- and mesozooplankton on winter dinoflagellate blooms of Heterocapsa rotundata.
Winter dinoflagellate blooms in Chesapeake Bay tributaries can account for over 50% of a system’s annual primary production, potentially more than the spring diatom bloom. Research on winter blooms has focused on environmental conditions that result in blooms, but little focus has been given to the potential importance of zooplankton grazers. We investigated the impact of microzooplankton and mesozooplankton (copepods) grazing on the population of winter phytoflagellates in the Choptank River, MD, in 2012 to 2013 and 2013 to 2014. We estimated community microzooplankton and copepod grazing rates on the dominant phytoflagellate species, and measured daily gross primary production (GPP) rates. The chlorophyll a concentration and the abundance of the dinoflagellate Heterocapsa rotundata were significantly higher in 2013 to 2014 compared with 2012 to 2013, but average daily GPP was similar between the 2 yr. However, the average percentage of daily GPP removed by grazers in 2013 to 2014 was lower than in 2012 to 2013, despite average environmental conditions and nutrient concentrations not differing between years. We hypothesize that the observed release from grazing pressure is one of the main factors controlling winter dinoflagellate bloom formation in these and other coastal temperate systems.
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