The Invisible Tax on Reef Building: Carbonate Loss Through Dissolution

Coral reefs are vital to the long-term viability of coastal society, providing economic, recreational, and aesthetic value from which coastal communities thrive.  Coral reef communities develop over thousands of years as calcifying organisms form skeletons that result in the 3-dimensional structure of reefs, and carbonate sediments as their skeletons degrade post-mortem.  Carbonate sediments form the reef foundation as they cement reef structure, provide substrate for coral growth, and enable continued upward growth of the reef to keep up with rising sea level.  Ocean acidification, resulting from increased atmospheric pCO₂, causes a decrease in seawater pH, carbonate ion concentration, and carbonate mineral saturation state.  Recent studies indicate that changes in seawater chemistry resulting from ocean acidification reduce calcification rates of marine organisms.   However, fewer studies have quantified the impact of ocean acidification on dissolution of carbonate sediments.  Whether or not coral reefs will be able to continue to grow at a rate that is fast enough to keep up with rising sea level depends, in part, on how seawater pCO₂ affects the balance between calcification and dissolution of carbonate sediments, and sediment accumulation rates.

Rates of coral reef community calcification and carbonate sediment dissolution were measured in Biscayne National Park, the U.S. Virgin Islands, Puerto Rico, Florida Bay, and Hawaii using a large benthic incubation chamber.  Total alkalinity, pH, and total carbon dioxide were measured every four hours during 24-hour incubation periods.  Calcification and dissolution rates were calculated using the alkalinity anomaly method.  Diurnal trends in calcification were observed at all locations with calcification occurring primarily during light hours and dissolution of carbonate sediments observed at night.  Dissolution rates ranged up to 68% of calcification rates.  The average pCO₂ threshold for calcification/dissolution for all substrate types was 585 μatm, and the average CO₃^2- threshold was 203 μmol kg^-1.  Atmospheric pCO₂ is predicted to reach 780 μatm by the year 2100, surpassing the average pCO₂ threshold for these substrate types, and indicating that a significant amount of sediment in coral reef ecosystems may be lost due to carbonate sediment dissolution.  Implications for carbonate sediment accumulation rates on reefs will be discussed.