High nutrient concentrations are generally known to adversely affect coral calcification. This reduction in calcification rate is often associated with increased zooxanthellae densities, but little is known about the mechanism underlying calcification inhibition. In this study, we assessed the limiting effects of dissolved inorganic carbon (DIC) on growth rates of Stylophora pistillata before and after five weeks of nitrogen and phosphorus enrichment. Nutrient enrichment resulted in a significant increase in zooxanthellae density and inhibition of calcification, measured using the alkalinity anomaly technique. DIC limitation was the main causative factor for this inhibition; a doubling of the bicarbonate concentration not only restored but greatly enhanced calcification rates of colonies with elevated zooxanthellae densities. At high bicarbonate concentration, no significant negative effect of nutrient enrichment on coral growth was found. The causal mechanism behind calcification inhibition due to nutrient enrichment is most likely increased competition for dissolved inorganic carbon, either among the zooxanthellae or between the coral host and its symbiotic dinoflagellates. This highly limiting effect of DIC on coral growth at elevated nutrient concentrations has important implications for coral aquaculture and husbandry.

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