Effects of increased pCO2 and temperature on the North Atlantic spring bloom. II. Microzooplankton abundance and grazing.

The North Atlantic spring bloom is one of the largest annually occurring phytoplankton blooms in the world ocean. The present study investigated the potential effects of climate change variables (temperature and pCO2) on trophic dynamics during the bloom using a shipboard continu- ous culture system. The treatments examined were (1) 12°C and 390 ppm CO2 (ambient), (2) 12°C and 690 ppm CO2 (high pCO2), (3) 16°C and 390 ppm CO2 (high temperature), and (4) 16 °C and 690 ppm CO2 (greenhouse). Individually, increasing temperature and pCO2 initially resulted in sig- nificantly higher total microzooplankton abundance and grazing rates over the ambient treatment mid-experiment, with significantly greater increases still in the greenhouse treatment. By the end of the experiment, microzooplankton abundance was highest in the 2 low temperature treatments, which were dominated by small taxa, while the larger ciliate Strombidium sp. numerically dominated the high-temperature treatment. Microzooplankton community composition was dominated by small taxa in the greenhouse treatment, but total abundance declined significantly by the end after peak- ing mid-experiment. This decrease occurred concurrently with the growth of a potentially unpalat- able phytoplankton assemblage dominated by coccolithophores. Our results suggest that indirect effects on microzooplankton community structure from changes in phytoplankton community com- position as a result of changing temperature or pCO2 were likely more important than direct effects on microzooplankton physiology. Similar changes in trophic dynamics and whole plankton commu- nity composition may also be important for future climate-driven changes in the North Atlantic spring bloom assemblage.