Seawater carbonate chemistry and biomass and pteropod metabolism

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. The Western Antarctic Peninsula (WAP) is a highly dynamic and productive region that has undergone rapid warming, but little is known about how environmental changes there will affect pteropod physiology. In this study, the effects of warming seawater temperatures and shifting food availability on Limacina helicina antarctica metabolism (respiration and excretion) were determined by conducting shipboard experiments that exposed pteropods to a range of temperatures and phytoplankton (food) concentrations. Highest respiration (up to 69 μmol O2/gDW/h) and usually highest excretion rates occurred under higher temperature with more limited metabolic response to food concentration, indicating these factors do not always have an additive effect on pteropod metabolism. The proportion of dissolved organic matter (DOM) to total organic and inorganic dissolved constituents was high and was also significantly affected by shifts in temperature and food. Dissolved organic carbon, nitrogen, and phosphorus (DOC, DON, and DOP) were on average 27, 51, and 11.5% of the total C, N, and P metabolized, respectively. The proportion of total N excreted as DON and the proportion of total P excreted as DOP were significantly affected by a combination of shifting temperature and food concentrations. There were no effects of temperature or food on DOC excretion (mean 8.79 μmol C/gDW/h; range 0.44 to 44) as a proportion of total C metabolized. Metabolic O2:N ratio ranged from 2 to 9 and decreased significantly with increasing temperature and food, indicating a shift toward increased protein catabolism. Metabolic ratios of C, N, and P were all below the canonical Redfield ratio, which has implications for phytoplankton nutrient uptake and bacterial production. Respiration rates at ambient conditions of other WAP pteropods, and excretion rates for Clio pyramidata, were also measured, with respiration rates ranging from 24.39 (Spongiobranchaea australis) to 28.86 (L. h. antarctica) μmol O2/gDW/h. Finally, a CO2 perturbation experiment measuring L. h. antarctica metabolism under pre-industrial and elevated dissolved pCO2 conditions showed no significant change in mean L. h. antarctica respiration or excretion rates with higher pCO2. 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.