Response of Mediterranean coralline algae to ocean acidification and elevated temperature

Abstract

AbstractThe effects of elevated partial pressure of CO2(pCO2) and temperature, alone and in combination, on survival, calcification and dissolution were investigated in the crustose coralline algaLithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3 °C) and at ambient [ca. 400 parts per million (ppm)] or elevatedpCO2(ca. 700 ppm). Algal necroses appeared at the end of summer under elevated temperature first at 700 ppm (60% of the thallus surface) and then at 400 ppm (30%). The death of algae was observed only under elevated temperature and was two‐ to threefold higher under elevatedpCO2. During the first month of the experiment, net calcification was significantly reduced under elevatedpCO2. At the end of the summer period, net calcification decreased by 50% when both temperature andpCO2were elevated while no effect was found under elevated temperature and elevatedpCO2alone. In autumn and winter, net calcification in healthy algae increased with increasing temperature, independently of thepCO2level, while necroses and death in the algal population caused a net dissolution at elevated temperature andpCO2. The dissolution of dead algal thalli was affected by elevatedpCO2, being two‐ to fourfold higher than under ambientpCO2. These results suggest that net dissolution is likely to exceed net calcification inL. cabiochaeby the end of this century. This could have major consequences in terms of biodiversity and biogeochemistry in coralligenous communities dominated by these algae.