Ocean acidification conditions suppressed the metabolism of an Antarctic pteropod by approximately 20 percent in some instances. However, the effect on metabolism depended on abundance of phytoplankton in the region and the pteropods' baseline level of metabolism. Pteropod populations may be compromised by climate change, both directly by acidification-related suppression of metabolism and indirectly by changes in the phytoplankton community. Long-term observations over all four seasons and multi-faceted laboratory analyses were necessary to reveal the complex response of this polar pteropod to ocean acidification. (Laboratory study)