Effect of temperature and trace metal exposure on early life stages of European flat oysters and Pacific oysters.

Ocean warming and metal pollution pose a threat to coastal ecosystems worldwide. In the German Bight, efforts to restore biogenic reefs using the native European flat oyster (Ostrea edulis) face challenges due to environmental conditions and potential pollutants of the North Sea. Besides O. edulis, the non-native Pacific oyster (Crassostrea gigas) inhabits the North Sea. Larval stages of bivalves are known to be sensitive to pollution. In this study, we investigate the effect of the trace metals copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in combination with water temperatures of 18° and 24 °C on the embryo-larval development of C. gigas and acute mortality of C. gigas and O. edulis D-larvae. This multi-stressor approach revealed that Cu was the most toxic metal, regardless of temperature, species or life stage. While elevated temperatures mitigated the negative effects of metal exposure on embryo-larval development, larval mortality was species- and metal-dependent at the tested temperatures. O. edulis D-larvae demonstrated a greater absolute tolerance to metal exposure at both temperatures, but a species comparison showed that O. edulis D-larvae had lower relative tolerance to the combined stress of warming and metal exposure than C. gigas. Based on the resulting toxicity thresholds, an environmental risk assessment for Cu was conducted to identify potentially hazardous areas for O. edulis restoration to be included in future habitat suitability studies and site selection for restoration. The identified areas may also indicate problematic environmental conditions for larval stages of other invertebrate species or fish.