Effects of lowered pH and elevated nitrate on coral calcification

Abstract

Both CO2 chemistry and nutrient concentrations of seawater affect coral calcification. The relative effects of these factors on growth of corals were studied using coral tips or 'nubbins' of the hermatypic coral Porites compressa. Coral nubbins were grown over 5 wk in different combinations of pCO2 (760 and 3980 μatm), HCO3- (1670 and 1520 μM), CO32- (110 and 20 μM), and NO3- (0.42 to 5.66 μM). The pCO2 was increased and CO32- decreased by adding HCl to normal seawater; NO3- was increased by adding KNO3 to ambient seawater. Corals growing in seawater at a reduced pH of 7.2 calcified at half the rate of control corals at pH 8.0, indicating that coral growth is strongly dependent on the concentration of CO32- ions in seawater. Reduction of calcification from lowered pH and CO32- was greater than reduction from nitrate additions. Corals in low pH treatments recovered their initial calcification rates within 2 d of re-introduction to ambient seawater, indicating the effects of CO2 chemistry are immediate and reversible. Changes in calcification from increases in atmospheric CO2, and hence decreases in CO32-, may be larger than local effects from elevated nutrients.