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), CO₃^2- (110 and 20 µM), and NO₃^- (0.42 to 5.66 µM). The pCO₂ was increased and CO₃^2- 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 CO₃^2- ions in seawater. Reduction of calcification from lowered pH and CO₃^2- 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 CO₃^2-, may be larger than local effects from elevated nutrients.

KEY WORDS: Calcification · Scleractinian corals · Dissolved inorganic carbon · Nutrients · Eutrophication

Published in MEPS Vol. 188 (1999) on November 3
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999