Temperature Environments during Coral Bleaching Events in Sekisei Lagoon

Abstract

Coral reef ecosystems in Sekisei Lagoon, the largest coral reef complex in Japan, suffered from coral bleaching phenomena caused by elevated seawater temperature in 2001 and 2003. To investigate temperature environments during the bleaching events, seasonal changes in 3-m seawater temperature recorded at two fixed survey stations between 1998 and 2003 were analyzed with those in atmospheric temperature recorded at Ishigaki Island. The total number of days of which both daily average seawater temperature and daily average atmospheric temperature exceeded 30°C in 2001 and 2003 were larger than those in the other years. Accordingly the atmospheric temperature data in the past three decades were analyzed using an assumed bleaching subtracted temperature of 30°C and an index that was defined as a sum of the surplus value exceeding 30°C (the accumulated coral bleaching subtracted atmospheric temperature). The results indicated that coral bleaching can occur when a total number of days of which daily atmospheric temperature exceeded 30°C is larger than 30, and the accumulated coral bleaching subtracted temperature is larger than 10.