The interactive roles of temperature, nutrients, and zooplankton grazing in controlling the winter-spring phytoplankton bloom in a temperate, coastal ecosystem, Long Island Sound

We examined biological, chemical, and physical processes controlling the onset and demise of the winter-spring bloom in the third largest US estuary, Long Island Sound (LIS), during 2010 and 2011. The spring bloom in LIS initiated in the absence of thermal stratification and peaked in early February when water temperatures were at an annual minimum (≤1.0˚ C). Bloom initiation occurred when phytoplankton cellular growth exceeded zooplankton grazing and bloom demise occurred as temperatures rose and grazing exceeded algal growth. During the bloom collapse, nutrients were drawn down by >90% and the phytoplankton community was nutrient limited. A highly significant (r² = 0.81, p = 0.001), multivariate linear regression model using the percentage of primary production consumed daily by zooplankton and dissolved silicate concentrations hind-casted the timing and magnitude of the bloom within 10% of observed values each year. Mesocosm experiments performed during the winter-spring bloom demonstrated that higher seawater temperatures (+13.0˚ C) increased zooplankton densities and grazing rates and decreased phytoplankton biomass. The winter-spring bloom in LIS was controlled by the interaction of phytoplankton growth and zooplankton grazing but not thermal stratification. Phytoplankton growth and zooplankton grazing were, in turn, influenced by nutrient availability and temperature. Continued climatic warming may lessen the intensity of the winter-spring bloom in this and other ecosystems in the future.