MEPS 388:27-40 (2009)  -  DOI: https://doi.org/10.3354/meps08134

ABSTRACT: 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 continuous 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 significantly 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 peaking mid-experiment. This decrease occurred concurrently with the growth of a potentially unpalatable phytoplankton assemblage dominated by coccolithophores. Our results suggest that indirect effects on microzooplankton community structure from changes in phytoplankton community composition 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 community composition may also be important for future climate-driven changes in the North Atlantic spring bloom assemblage.

KEY WORDS: Microzooplankton · Herbivory · Temperature · pCO₂ · North Atlantic spring bloom · Climate change