MEPS 546:75-84 (2016)  -  DOI: https://doi.org/10.3354/meps11655

ABSTRACT: Climate change stress on coral reefs occurs as a result of increased temperature and ocean acidification. However, these stressors do not act uniformly: acidification is a ‘press’ disturbance characterized by chronic increases in CO₂, whereas thermal stress is a ‘pulse’ disturbance characterized by acute episodes of anomalously warm temperatures. Therefore future episodes of thermal stress will develop within the context of pre-existing acidification. Many studies have investigated the effect of combined temperature and CO₂ on corals, but no study has yet investigated whether pre-exposing corals to elevated CO₂ affects their response to high temperature. We investigated this for the first time using replicate fragments of the Caribbean coral Porites porites preconditioned to either 390 ppm or 900 ppm CO₂ at 26°C for 3 mo. After this period, half of the corals from each CO₂ level were exposed to 31°C (i.e. 31°C/390 ppm or 31°C/900 ppm) for 2 mo, while the other half were maintained in their original treatments (26°C/390 ppm or 26°C/900 ppm). Calcification, feeding rate, and photochemical efficiency were measured. Corals preconditioned to high CO₂ before thermal stress (i.e. 31°C/900 ppm) showed 44% lower calcification rates than the control group, but single stress treatment groups did not experience significant growth reductions. Feeding rates increased for corals exposed to either high CO₂ or high temperature singularly, but not when thermal stress was applied following CO₂ preconditioning. Photochemical efficiency decreased by 25% for all treatment groups compared to the control. Together, these data suggest that preconditioning to elevated CO₂ worsens holobiont response to thermal stress, potentially exacerbating the effects of climate change stressors on coral reefs.

KEY WORDS: Climate change · Ocean acidification · Coral bleaching · Calcification · Heterotrophy · Photochemical efficiency