Ocean warming and acidification synergistically increase coral mortality

Ocean warming (OW) and ocean acidification (OA) are major threats to marine organisms that build calcium carbonate structures, such as corals. While both stressors are driven by rising CO2, their combined effects on coral health and growth are not fully understood. This study investigated how the interaction between water temperature and pH affects the mortality and calcification rates of three Mediterranean coral species—a solitary symbiotic coral, a solitary asymbiotic coral, and a colonial asymbiotic coral—using a natural volcanic CO2 vent as a laboratory for environmental gradients.

Researchers transplanted coral specimens to four sites along a natural pH gradient (range 7.4–8.1) near a CO2 vent off Panarea Island. This setup allowed the team to observe corals under varying pH levels while maintaining natural conditions like currents and irradiance. Over several seasonal periods, the team monitored polyp mortality and net calcification (the balance between CaCO3 production and environmental dissolution) across a temperature range of 16–26 °C. Statistical models were then used to disentangle the separate and combined (synergistic) effects of temperature and acidity.

Results show that both temperature and pH significantly impact coral survival. Notably, the researchers identified a synergistic adverse effect where higher seawater temperatures exacerbated the negative impact of lower pH on mortality rates. In warmer months, mortality reached up to 60%.

Regarding calcification, the response was species-dependent. The symbiotic coral Balanophyllia europaea maintained its net calcification rate regardless of pH and temperature, suggesting a higher tolerance to acidified, warming conditions potentially linked to its symbiotic relationship with zooxanthellae. In contrast, the two asymbiotic species (Leptopsammia pruvoti and Astroides calycularis) showed decreased calcification rates as acidity increased and temperatures rose. These findings highlight that as climate change progresses, combined stress from warming and acidification will likely have severe, compounding consequences for shallow-water coral ecosystems.