Then, the seawater temperature was elevated to a higher level (31 °C), and the response of coral–algal symbiosis was compared between the two nutrient conditions. The temperature of 31 °C was sometimes observed in the summer around the Ryukyu Archipelago, Japan, and was recently analyzed to be near the upper threshold of coral bleaching for the genus Acropora ( Muko et al., 2012). This experiment was designed to simulate effects on corals living under different nutrient loading and subsequently exposed to an occasional high seawater temperature.