Effects of warming on the ecology and physiology of the Southern Ocean pteropod, Limacina helicina antarctica

Pteropods (pelagic snails) are abundant zooplankton in the Southern Ocean where they are important grazers of phytoplankton, prey for higher trophic levels, and sensitive to environmental change. However, little is known about long-term and regional environmental impacts, such as warming and food availability, on pteropod physiology in the Southern Ocean. The western Antarctic Peninsula is a highly dynamic and productive region that has undergone rapid warming in the past half century, with unprecedented increases in air and sea surface temperature, and in ocean heat content, which have additionally led to decreasing sea ice. Superimposed on the long-term trend is considerable interannual variability and a recent plateau in the rate of warming. Using data from the Palmer Antarctica Long-Term Ecological Research (PAL LTER) program, we show that warm sea surface temperature and La Niña conditions leading to ice free water causes higher Limacina helicina antarctica abundance and may additionally have important controls on pteropod physiology. The effects of warming seawater temperatures and shifting food availability on L. antarctica metabolism (e.g., respiration and excretion) were determined by conducting shipboard experiments exposing pteropods to elevated temperature and decreased phytoplankton (food) conditions. Highest respiration and usually highest excretion rates occurred under higher temperature and lower food conditions. The proportion of total N excreted as DON significantly increased with increasing temperature while the proportion of total P excreted as DOP significantly increased with increasing food; however, these trends were not observed for DOC excretion relative to total C metabolized. Metabolic O:N ratio decreased significantly with increasing temperature, indicating a shift toward increased protein catabolism. These insights into the metabolic response of pteropods to ocean variability increase our understanding of the role of zooplankton in biogeochemical cycles and help predict future responses to climate change.