Effects of acidification of meroplanktonic oyster settlement in a tropical estuary

Abstract

Monthly variations in salinity, temperature and pH were studied in relation to settlement of meroplanktonic oyster larvae on cultches in Vembanad Lake, a tropical estuary along the southwest coast of India. A strong correlation between monthly pH values and oyster spat density (P=0.002) was evident. When the pH values in the estuary were low, the spat settlement density was poor (4.5/cultch), which was nearly half of the value (7.8/cultch) when the pH was alkaline. Experiments were conducted to evaluate the effects of extremes of temperature (20 to 35oC) and pH (6.5 to 8.5) on metamorphosis and survival of oyster larvae. In pH 6.5 there was 100% mortality and complete dissolution of dead shells in 24 h. Survival was highest (81%) in the control temperature of 27oC and less than 50% in 25 and 20oC. Treatment-wise differences in survival were significant (P<0.01). Study indicated that, if the pH drops below 7.0 during the meroplanktonic phase of oyster, recruitment will be affected and therefore oyster spat densities can be used as an indicator for acidification in tropical estuaries.