Synergistic impacts of ocean acidification and temperature rise on the physiology of marine invertebrates in a latitudinal cline

Ocean warming and acidification are two of the main factors driving current ecosystem changes. Yet, the combined impacts on marine organisms are insufficiently understood and the stress response may differ at the population level across climate gradients. This thesis investigates the combined effects of rising temperature and CO2 level on energy metabolism, acid-base regulation capacity and thermal tolerance of Mytilus edulis populations along a latitudinal cline and, in combination with exercise capacity, in the Arctic population of Hyas araneus. All organisms exposed to acute temperature rise suffered from reduced animal performance likely mediated by an involvement of acid-base disturbances. The population comparison revealed a reduced heat tolerance of subarctic M. edulis in contrast to temperate ones. Apart from a lowered blood pH, moderate hypercapnia itself had only minor impacts. However, hypercapnia reduced heat tolerance and associated performance in H. araneus and M. edulis from high latitudes but not in temperate M. edulis. The findings indicate that populations living at high latitudes exhibit features of cold adaptation resulting in reduced thermal tolerance and resilience to environmental challenges, which needs to be considered in future projections.