Influence of seawater acidification on larvae of the European flat oyster Ostrea edulis

The emissions of carbon dioxide into the atmosphere have risen strongly since the beginning of the industrialisation. About 48% of the anthropogenic C02 emissions are taken up by the world's oceans. The excess carbon dioxide interferes with the ocean's capacity to buffer changes in pH, the carbonate system, and causes the acidification of the seawater. A decrease in the pH value ofthe ocean has negative consequences for calcifying metazoans, e.g. bivalves, because their calcification process may be negatively influenced. Larval stages are more sensitive to declining pH values than adults, but still there are only few studies on larvae. In the present study, the effects of acidified seawater on growth, survival, and shell calcium content of larvae of the European oyster (Ostrea edulis L.) were investigated. The seawater was acidified by addition of HCI, and five pH values below the recent pH were created, decreasing in steps of 0.25 units. One treatment was left at the natural condition, another treatment was alkalised to simulate a pre-industrial pH value. The experiment was performed in 2 liter glass bottles with separate aeration for each treatment. Air from the headspaces of the bottles was pumped into the water, thus a mixing with outside air was prevented. Acidified seawater had a significantly negative effect on larval shell length. The smallest shells and slowest growth were found in larvae cultivated at the lowest pH value. Survival rates were not significantly affected by the pH value, although larvae in the alkalised treatment survived seven days longer than in all the other treatments. An effect of the pH value on the calcium content as an indicator of shell calcification could not be proven, although this may be due to the processing of the samples