CO₃²⁻ concentration and pCO₂ thresholds for calcification and dissolution on the Molokai reef flat, Hawaii

Abstract. The severity of the impact of elevated atmospheric pCO₂ to coral reef ecosystems depends, in part, on how seawater pCO₂ affects the balance between calcification and dissolution of carbonate sediments. Presently, there are insufficient published data that relate concentrations of pCO₂ and CO₃²⁻ to in situ rates of reef calcification in natural settings to accurately predict the impact of elevated atmospheric pCO₂ on calcification and dissolution processes. Rates of net calcification and dissolution, CO₃²⁻ concentrations, and pCO₂ were measured, in situ, on patch reefs, bare sand, and coral rubble on the Molokai reef flat in Hawaii. Rates of calcification ranged from 0.03 to 2.30 mmol CaCO₃ m⁻² h⁻¹ and dissolution ranged from –0.05 to –3.3 mmol CaCO₃ 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 pCO₂ and CO₃²⁻ at which rates of calcification and dissolution are equivalent. Results indicate that calcification and dissolution are linearly correlated with both CO₃²⁻ and pCO₂. Threshold pCO₂ and CO₃²⁻ values for individual substrate types showed considerable variation. The average pCO₂ threshold value for all substrate types was 654±195 μatm and ranged from 467 to 1003 μatm. The average CO₃²⁻ threshold value was 152±24 μmol kg⁻¹, ranging from 113 to 184 μmol kg⁻¹. Ambient seawater measurements of pCO₂ and CO₃²⁻ indicate that CO₃²⁻ and pCO₂ threshold values for all substrate types were both exceeded, simultaneously, 13% of the time at present day atmospheric pCO₂ concentrations. It is predicted that atmospheric pCO₂ will exceed the average pCO₂ threshold value for calcification and dissolution on the Molokai reef flat by the year 2100.