Differential synergistic adverse effect of increased sea temperature and acidity on corals

Global climate change is predicted to affect marine organisms and ecosystems reliant on the accumulation of calcium carbonate structures, as coral reefs, potentially reducing the socioeconomic benefits these habitats provide. In the marine realm, two of the main stressors causing significant changes are ocean warming and ocean acidification. Projections of future climatic change estimate a 0.6-2.0°C average increase in surface ocean temperature by the end of 2100, posing a major threat for marine organisms. In temperate areas, the effect of warming is expected to be even greater. The Mediterranean Sea is already showing warming rates three times higher than the global ocean. Increased seawater temperature in the Mediterranean Sea has determined longer stratification periods associated with mass mortality events. The first well-documented Mediterranean multispecies mass mortality events were during the summers 1999 and 2003. In both years, a positive correlation was observed between mortality rates and exposure to heat stress, indicating that shallow water corals are living, at least in the North Mediterranean, near their upper thermal limits during summer. Since the frequency of abnormally warm summers is expected to increase in the next century, as a result of climate change, such mortality events in summer may also become more frequent as a direct response to elevated temperatures. Also ocean acidification is a global phenomenon which impact varies locally. The Mediterranean Sea has experienced a pH decrease of up to 0.14 units since the pre-industrial era, larger than the global average surface ocean pH decrease. Hence, understanding how enhanced acidity has already affected and how it will likely affect Mediterranean Sea ecosystems and their key taxa is urgent and crucial. Given the projected decrease of seawater pH, the mass mortality events could be exacerbated by the combination of high temperatures and low pH. Studies like this one, assessing the synergistic interaction between low pH and elevated temperatures, are essential to detect possible interactions between multiple stressors and establish to which extent corals inhabiting shallower ranges will be threatened by climate change. Here we assessed the combined effects of in situ exposure to different acidity and seasonal temperatures on the mortality and growth rates of three Mediterranean scleractinian corals; the solitary zooxanthellate Balanophyllia europaea, the solitary azooxanthellate Leptopsammia pruvoti and the colonial azooxanthellate Astroides calycularis. The corals were transplanted and observed in different seasonal conditions in proximity to a volcanic vent where water is naturally acidified to levels matching different future Intergovernmental Panel on Climate Change scenarios. The results suggest differential synergistic adverse effects of increased sea temperature and acidity and different levels of resilience/resistance to climate change among temperate coral species, probably related to different modes of nutrition and/or biomineralization processes, making symbiotic species relatively less sensitive due to the increased photosynthesis at high CO2. Acknowledgements: The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (F P7/2007-2013)/ERC grant agreement n° 249930 – CoralWarm: Corals and global warming: the Mediterranean versus the Red Sea.