By the end of the century, climate changes are predicted to occur through increases in sea and air temperature, sea level, water acidification and severe fluctuations in salinity. The worst scenario predicted by the Intergovernmental Panel on Climate Change (IPPC) forecasts a 4°C increase in temperature and a decrease in pH to levels below to what was previously predicted (pH 7.7). Ocean warming and acidification are the direct key stressors of anthropogenic climate change, responsible for causing deleterious effects on marine biological and physiological processes which, consequently, affect bivalve production. Most of the studies have focused mainly on the variation of only one factor of climate change, even though, in the natural environment, organisms are affected by several combined factors. Therefore, this study aimed to address the combined effects of temperature and pH on the physiological and growth performance of Magallana gigas spat.
Four treatments with different pH (pH 7.6 and pH 8.2) and temperatures (22 ± 1ºC; 26 ± 1ºC) were tested in triplicate. The treatment pH 8.2; 22ºC was considered as the control group since it represents the regular conditions. Juveniles of Magallana gigas (9.29 ± 0.58 mm shell length; 86.63 ± 11.12 mg total weight), were separated into twelve groups of 200 individuals and placed into the experimental tanks for 30 days. Oysters were fed daily with a mixed microalgae diet at a ratio of 6% of oyster dry weight (g) in algal dry weight (mg). Environmental parameters and mortality were registered daily. Condition index, biometric measurements, and biochemical composition (proteins, total lipids and glycogen content) were evaluated every 15 days.
Survival and shell growth were not affected by any of the experimental conditions. However, after 30 days of exposure, the soft tissue weight of spat exposed to higher temperature treatments (26°C) was significantly lower, regardless of the pH. Likewise, oysters exposed to higher temperatures exhibited significantly lower condition index, with the control treatment (pH 8.2; 22ºC) showing the best condition. The biochemical composition did not show significant differences among treatments. The Principal Component analysis (PCA) revealed that temperature had a negative correlation with soft tissue weight (fresh and dry) as well as with condition index, while pH did not significantly correlate with any other variable.
This study highlights that the physiological performance of bivalves is more affected by ocean warming than ocean acidification. Consequently, the increase in the frequency and intensity of heat waves may have severe implications for bivalve aquaculture.
Acknowledgements: Broderick J. House was supported by the Harding Distinguished Postgraduate Scholarship.