The influence of solar radiation on bleaching of shallow water reef corals in the Andaman Sea, 1993-1998

Historically, elevated sea-temperature excursions have been used almost exclusively to explain incidences of natural coral bleaching. Little attention, however, has been paid to instances where environmental conditions appear conducive to bleaching but none has been observed. In this paper we examine contemporaneous records of sea temperature and solar radiation (photosynthetically active radiation; PAR, 400-700 nm) over a 6-year period at a site in the Andaman Sea where regular monitoring of shallow water reef coral communities has been conducted since 1979. Four years (1991, 1995, 1997, and 1998) when anomalous sea temperatures were recorded in May are compared. We suggest a complex interaction of PAR and sea temperature, whereby elevated solar radiation prior to, sea-temperature maxima may actually protect corals against subsequent bleaching. In addition, we demonstrate the important role that sea-level anomalies play in modifying the underwater light regime to bring about such conditions.