EFFECT OF CALCIUM-CARBONATE SATURATION OF SEAWATER ON CORAL CALCIFICATION

The carbonate chemistry of seawater is usually not considered to be an important factor influencing calcium-carbonate-precipitation by coral s because surface seawater is supersaturated with respect to aragonite . Recent reports, however, suggest that it could play a major role in the evolution and biogeography of recent corals. We investigated the c alcification rates of five colonies of the zooxanthellate coral Stylop hora pistillata in synthetic seawater using the alkalinity anomaly tec hnique. Changes in aragonite saturation from 98% to 585% were obtained by manipulating the calcium concentration. The results show a nonline ar increase in calcification rate as a function of aragonite saturatio n level. Calcification increases nearly 3-fold when aragonite saturati on increases from 98% to 390%, i.e., close to the typical present satu ration state of tropical seawater. There is no further increase of cal cification at saturation values above this threshold. Preliminary data suggest that another coral species, Acropora sp., displays a similar behaviour. These experimental results suggest: (1) that the rate of ca lcification does not change significantly within the range of saturati on levels corresponding to the last gracial-interglacial cycle, and (2 ) that it may decrease significantly in the future as a result of the decrease in the saturation level due to anthropogenic release of CO2 i nto the atmosphere. Experimental studies that control environmental co nditions and seawater composition provide unique opportunities to unra vel the response of corals to global environmental changes. (C) 1998 E lsevier Science B.V. All rights reserved.