Coral reef sediment dissolution: Insights from chamber incubations around the globe

Ocean acidification (OA) is expected to negatively affect the calcium carbonate (CaCO₃) budget of coral reefs by decreasing calcification and increasing CaCO₃ dissolution rates. Sediments represent the largest reservoir of CaCO₃ in coral reefs and form important habitats above and below the hide tide mark. Results from in situ benthic incubations at different coral reef locations around the world (Australia, Tahiti, Bermuda, Cook Islands, and Hawaii) reveal that there is a general trend between bulk seawater aragonite saturation state (Ω_ar) and net CaCO₃ sediment dissolution rates. Experimental incubations also indicate that the ratio of production to respiration (P/R) in the sediments plays a significant role in CaCO₃ dissolution, with high P/R ratios potentially offsetting the effects of human induced OA. This is most likely due to benthic microalgae photosynthesizing and consuming CO_2, which produces conditions more favourable for CaCO₃ precipitation in sediment pore waters. Despite any interactions with benthic organic metabolism, sediment dissolution could be an order of magnitude more sensitive to OA compared to the process of biogenic calcification. Increases in CaCO₃ sediment dissolution under predicted CO₂ emissions could shift the net ecosystem calcification (NEC) of coral reefs from net CaCO₃ precipitating to net dissolving by the end of this century.