Combined effects of ocean acidification and elevated temperature on feeding, growth, and physiological processes of Antarctic krill Euphausia superba

Abstract

Antarctic krill Euphausia superba is a key species in the Southern Ocean, where its habitat is projected to undergo continued warming and increases in pCO2. Experiments during 2 summer field seasons at Palmer Station, Antarctica, investigated the independent and interactive effects of elevated temperature and pCO2 (decreased pH) on feeding, growth, acid-base physiology, metabolic rate, and survival of adult Antarctic krill. Ingestion and clearance rates of chlorophyll were depressed under low pH (7.7) compared to ambient pH (8.1) after a 48 h acclimation period, but this difference disappeared after a 21 d acclimation. Growth rates were negligible and frequently negative, but were significantly more negative at high (3°C, -0.03 mm d-1) compared to ambient temperature (0°C, -0.01 mm d-1) with no effect of pH. Modest elevations in tissue total CO2 and tissue pH were apparent at low pH but were short-lived. Metabolic rate increased with temperature but was suppressed at low pH in smaller but not larger krill. Although effects of elevated temperature and/or decreased pH were mostly sublethal, mortality was higher at high temperature/low pH (58%) compared to ambient temperature/pH or ambient temperature/low pH (>90%). This study identified 3 dominant patterns: (1) shorter-term effects were primarily pH-dependent; (2) krill compensated for lower pH relatively quickly; and (3) longer-term effects on krill growth and survival were strongly driven by temperature with little to no pH effect.