The physical and biological mechanisms controlling the winter-spring phytoplankton bloom in Long Island Sound

Abstract

The spring phytoplankton bloom is an annual event that occurs at middle and high latitudes, in the world's oceans, is an important source of organic matter for marine food webs, and can influence global carbon cycles. The spring bloom is controlled by many physical and biological factors. This study examined the biological and physical mechanisms controlling the onset and demise of the spring phytoplankton bloom in Long Island Sound (LIS) during 2010 and 2011 with a focus on zooplankton grazing, phytoplankton growth, and the effects of increased seawater temperature on these factors. During 2010 and 2011, the spring bloom initiated when there was no stratification of the water column (ΔT from surface to bottom = -0.02 ??C and -0.28 ??C, respectively), and peaked in early February when temperatures were at the annual minimum (1.0 ??C and 0.8 ??C). The bloom magnitude and duration were a function of phytoplankton growth and zooplankton grazing, with bloom initiation occurring when cellular growth exceeded grazing (net growth rates were 0.34 d-1 and 0.35 d-1), and the bloom demise occurring when grazing exceeded growth (>100% of primary productivity grazed per day). During the bloom collapse, nutrients were drawn down and the phytoplankton community was nitrogen-limited, suggesting the bloom demise was due to both top-down and bottom-up effects. Over the entire study, measured percentages of primary production consumed daily by microzooplankton were capable of accurately forecasting the occurrence of the spring bloom during both study years. Mesocosm experiments demonstrated that experimentally increased seawater temperature (+3C??) increased zooplankton grazing and decreased phytoplankton biomass. This study demonstrates that the winter-spring bloom in LIS is controlled by the interaction of phytoplankton growth and zooplankton grazing but not water column stratification, and that phytoplankton growth and zooplankton grazing are, in turn, controlled by temperature and nutrient availability.