Climate change and the phytoplankton spring bloom: warming and overwintering zooplankton have similar effects on phytoplankton

Indoor mesocosms were used to study the combined effect of warming and of different densities of overwintering mesozooplankton (mainly copepods) on the spring development of phytoplankton in shallow, coastal waters. Similar to previous studies, warming accelerated the spring phytoplankton peak by ca. 1 day degrees C-1 whereas zooplankton did not significantly influence timing. Phytoplankton biomass during the experimental period decreased with warming and with higher densities of overwintering zooplankton. Similarly, average cell size and average effective particle size (here: colony size) decreased both with zooplankton density and warming. A decrease in phytoplankton particle size is generally considered at typical footprint of copepod grazing. We conclude that warming induced changes in the magnitude and structure of the phytoplankton spring bloom cannot be understood without considering grazing by overwintering zooplankton.