Effects of elevated water temperature, reduced salinity and nutrient enrichment on the metabolism of the coral Turbinaria mesenterina

Water quality is declining in many coastal areas, which has caused coral degradation worldwide. In addition, reduced water quality may aggravate the impacts of seawater temperature. In this study the effects of increased temperature (31 °C), nitrate enrichment (+5 μM NO 3 −), low salinity (20) and combinations of these stressors were investigated compared to ambient water (25 °C, 30, 0.3 μM NO 3 −) on the metabolism and survival of the coral Turbinaria mesenterina from the Tonkin Gulf, Vietnam. The results showed that all specimens exposed to a combination of all three stressors (i.e. high temperature + high nitrate + low salinity) died after 24 h exposure, while those that had been exposed to high nitrate + low salinity at ambient temperature did not show any effects on the metabolism or survival. Furthermore, corals exposed to low salinity + high temperature displayed a decrease in gross primary production/respiration (GP/R) ratio and the mortality rate was 50%. In addition, all corals exposed to increased temperature, alone or in combination with another stressor, displayed a GP/R 24h ratio below 1.0, suggesting that they depend on stored energy to cover their metabolic requirements. The results showed that corals may tolerate short-term exposure to stressors such as low salinity + high nitrate concentration in ambient temperature, while additional increased temperature lead to rapid mortality, hence suggesting a synergistic effect. Thus, the effect of climate change might be more severe in nearshore coastal areas where corals already are exposed to several disturbances.