Effects of lowered pH and elevated nitrate on coral calcification

Both CO2 chemistry and nutrient concentrations of seawater affect coral cal cification. The relative effects of these factors on growth of corals were studied using coral tips or 'nubbins' of the hermatypic coral Porites compr essa. Coral nubbins were grown over 5 wk in different combinations of pCO(2 ) (760 and 3980 mu atm), HCO3- (1670 and 1520 mu M), CO32- (110 and 20 mu M ), and NO3- (0.42 to 5.66 mu M). The pCO(2) was increased and CO32- decreas ed by adding HCl to normal seawater; NO3- was increased by adding KNO3 to a mbient seawater. Corals growing in seawater at a reduced pH of 7.2 calcifie d at half the rate of control corals at pH 8.0, indicating that coral growt h is strongly dependent on the concentration of CO32- ions in seawater. Red uction of calcification from lowered pH and CO32- was greater than reductio n from nitrate additions. Corals in low pH treatments recovered their initi al calcification rates within 2 d of re-introduction to ambient seawater, i ndicating the effects of CO2 chemistry are immediate and reversible. Change s in calcification from increases in atmospheric CO2, and hence decreases i n CO32-, may be larger than local effects from elevated nutrients.