Some cold-water coral (CWC) species are important ecosystem engineer, forming complex three-dimensional reefs in the deep sea. These reefs consist of both live corals and dead skeletons and are associated with high biodiversity. However, CWCs are threatened by climate change. Previous laboratory studies mainly focused on the short-term effects of single environmental factors on adult CWCs, especially elevated temperatures and reduced pH. So far, little is known about the effects of reduced oxygen concentration and food availability on CWCs, the long-term and combined effect of all these environmental drivers as well as their effect on different life stages.

Therefore, I have conducted two long-term (6 and 12 months) aquarium experiments to investigate the combined effect of reduced pH, elevated temperature, reduced oxygen concentration and reduced food supply on three life stages of the solitary CWC Caryophyllia huinayensis and the colony-forming CWC Lophelia pertusa (syn. Desmophyllum pertusum). During the experiments, I have determined coral mortality, calcification, respiration, and energy reserves of live corals. I have also examined dissolution rates of dead L. pertusa skeletons under different ocean acidification scenarios using micro-computed tomography (µCT) to better predict how ocean acidification will affect the structural integrity of CWC reefs in the future. In both experiments, I have observed a delay in response, presumably because the effects only become visible once energy reserves are depleted, suggesting that short-term experiments overestimate coral resilience.

In the long-term, acidification alone had no effect on C. huinayensis, but warming and reduced food availability lowered their survival and calcification rates. The magnitude of change differed between life stages as calcification rates declined more in juvenile than in adult corals. Calcification rates of L. pertusa were lowest in the multiple driver treatments, reaching negative values after more than three months, presumably because the dissolution of skeletal parts not covered with tissue exceeded the growth rate of live polyps at aragonite undersaturation. In addition, the dissolution rate of dead coral skeletons increased with reduced seawater pH. Overall, the findings highlight the importance of considering interactive effects of multiple drivers, appropriate duration of experiments and potential ontogenetic differences when investigating CWC susceptibility to climate change. I also conclude that live CWCs may be able to cope with future environmental changes to a certain extent, whereas increased skeletal dissolution due to ocean acidification will lead to structural weakening of the dead skeletal framework and potential crumbling of CWC reefs in the long term.