Biogeosciences CO 2 − 3 concentration and pCO 2 thresholds for calcification and dissolution on the Molokai reef flat , Hawaii

The severity of the impact of elevated atmospheric pCO 2 to coral reef ecosystems depends, in part, on how seawater pCO 2 affects the balance between calcification and dissolution of carbonate sediments. Presently, there are insufficient published data that relate concentrations of pCO 2 and CO 2− 3 to in situ rates of reef calcification in natural settings to accurately predict the impact of elevated atmospheric pCO 2 on calcification and dissolution processes. Rates of net cal-cification and dissolution, CO 2− 3 concentrations, and pCO 2 were measured, in situ, on patch reefs, bare sand, and coral rubble on the Molokai reef flat in Hawaii. Rates of calcifica-tion ranged from 0.03 to 2.30 mmol CaCO 3 m −2 h −1 and dissolution ranged from –0.05 to –3.3 mmol CaCO 3 m −2 h −1. Calcification and dissolution varied diurnally with net calcification primarily occurring during the day and net dissolution occurring at night. These data were used to calculate threshold values for pCO 2 and CO 2− 3 at which rates of calcification and dissolution are equivalent. Results indicate that calcification and dissolution are linearly correlated with both CO 2− 3 and pCO 2. Threshold pCO 2 and CO 2− 3 values for individual substrate types showed considerable variation. The average pCO 2 threshold value for all substrate types was 654±195 µatm and ranged from 467 to 1003 µatm. The average CO 2− 3 threshold value was 152±24 µmol kg −1 , ranging from 113 to 184 µmol kg −1. Ambient seawater measurements of pCO 2 and CO 2− 3 indicate that CO 2− 3 and pCO 2 threshold values for all substrate types were both exceeded, simultaneously, 13% of the time at present day atmospheric pCO 2 concentrations. It is predicted that atmospheric pCO 2 will exceed the average pCO 2 threshold value for calcifi-cation and dissolution on the Molokai reef flat by the year 2100.