Altered Coral‐Turf Algae Competition Leads to the Passive Dissolution of Coral Skeletons Under Ocean Acidification

ABSTRACT

Ocean acidification (OA) is expected to lower seawater pH and aragonite saturation (Ω _arag ), altering reef carbonate budgets through combined effects on calcification, dissolution, and species interactions. However, OA may benefit low‐profile algal turfs, comprised of several epilithic species, that tend to settle onto exposed coral skeletons following disturbance events. Despite the future predicted prevalence of turf algae across ecosystems, their contribution to future reef ecology and carbonate budget remains poorly understood. Coral‐turf algae competition and carbonate dynamics under future high p CO ₂ conditions were investigated by using standardized coral‐turf algae microcolonies representative of future community‐level interactions. Experiments were conducted using two coral species, Acropora solitaryensis and Porites heronensis , in both controlled aquaria and a natural CO ₂ seep environment in a marginal coral community. Turf algae were competitively favored under high p CO ₂ conditions, highlighting the importance of biotic interactions in shaping future reef communities. The local pH microenvironment under the turf, resulting from associated biological activity, combined with OA, promoted the chemical passive dissolution of the coral carbonate skeleton. Turf proliferation under climate change and OA will influence carbonate budgets through a combination of biotic and abiotic mechanisms, contributing to the shift of reefs towards a state of net dissolution.