The Effects of Temperature and CO2-induced Acidification on Skeletal Morphology of the Tropical Reef-building Coral Siderastrea siderea

Coral reefs are threatened by increasing sea surface temperatures and decreasing surface seawater pH. Although numerous experimental studies have examined the effects of these global scale stressors on corals, few have quantified the effects of temperature and acidification on coral skeletal morphology. We conducted controlled laboratory experiments to investigate the effects of temperature (25, 28, 32°C) and CO₂-induced ocean acidification (pCO₂ = 324, 477, 604, 2553 µatm) on skeletal morphology of the highly resilient Caribbean reef-building coral Siderastrea siderea over a 95-day interval. Post-treatment S. siderea corallites from nearshore, backreef, and forereef colonies were imaged via stereo microscopy to evaluate impact of warming and acidification on corallite height and infilling. Both an increase and decrease in temperature relative to the control (i.e., near-present-day temperatures) resulted in increased corallite height but decreased skeletal infilling. In contrast, corals reared under the lowest (i.e., pre-industrial) and highest pCO₂ treatments (i.e., extreme pCO₂) exhibited both decreased corallite height and skeletal infilling relative to the control. We observed no difference in corallite height or infilling across reef zones, either within or across treatments. Interestingly, the warming projected for the end of the 21^st century (32°C) resulted in increased corallite height and reduced corallite infilling. Acidification projected for the same interval (pCO₂ = 604 µatm) also resulted in increased corallite height and decreased infilling. Collectively, our results suggest that these two global stressors will result in S. siderea corallites that are taller yet less infilled by the end of the 21^st century. Changes in S. siderea arising from warming and acificiation may exacerbate observed declines in coral health across Caribbean reef systems.