The Baltic Sea is characterized by low salinity and pronounced fluctuations in pCO2, pO2 and temperature. On-line monitoring of pCO2 in 2014 in Kiel Fjord demonstrated occurrence of peak values of >2,000 µatm in summer and autumn and average values >700 µatm due to extensive wind driven upwelling of hypoxic - hypercapnic water masses. We assessed the impacts of elevated temperature (ambient temperature, ambient +3°C) and pCO2 (800, 1,500, 2,400 µatm) on a keystone species, the sea star Asterias rubens, in a fully crossed long - term experiment (N=5 replicate tanks each, 1 year duration 2013-2014). During spring and early summer (February – June), high temperature animals ingested significantly more food and spawned significantly earlier than ambient acclimated animals. Elevated pCO2 led to comparatively minor reductions in food intake and scope for growth during that period. During summer (June – August), elevated temperature >25°C caused negative energy budgets and >95% mortality in the warm acclimated groups, while mortality was low in the ambient temperature groups. Short-term heat tolerance experiments indicate that heat stress leads to reductions in coelomic fluid pO2 and, eventually, accumulation of anaerobic endproducts. Sublethal heat stress - induced reductions in feeding rates can also be related to oxygen limitation. Time series analysis indicates that 3 out of the last 15 years were characterized by heat waves of similar magnitude, indicating that future warming will constitute a strong selective pressure for sea stars in the Baltic Sea. Our results indicate that A. rubens may benefit from increased temperature during colder months, yet dramatically suffer during summer heat waves in warm years. Surprisingly, upwelling of hypoxic water masses from below the pycnocline is beneficial during summer heatwaves. Meaningful experimental approaches to assess species vulnerability to climate change need to encompass all seasons and realistic abiotic stressor levels.
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