Loss of zooxanthellae in a coral under high seawater temperature and nutrient enrichment

To investigate the effect of nutrient concentrations on coral-algal symbiosis under thermal stress, the abundance and release of zooxanthellae in the coral Acropora tenuis were quantified under laboratory conditions. The coral fragments were first cultured in either low-nutrient (LN) or high-nutrient (HN) seawater condition at 27°C for 25days and the seawater temperature was then elevated and kept at 31°C for 1week for both nutrient conditions. Nutrient enrichment at 27°C increased the densities of symbiotic algae and chlorophyll a and the algal release rates (AR) from the host coral to the ambient seawater. The percentage of algal release rate to the standing stock (AR%) was approximately 0.015%h^-1 for both nutrient conditions at 27°C. After the seawater temperature was elevated to 31°C, the densities of zooxanthellae in the corals decreased by 21-61%, and AR and AR% increased. The corals in HN lost more zooxanthellae per unit surface area than those in LN, but the lost percentages were not significantly different between LN and HN. AR% was also not significantly different between LN and HN at 31°C. These results indicated that the percentage rate of symbiotic algal loss was not affected by the nutrient condition, implying that nutrient incorporation itself would not accelerate coral bleaching susceptibility.