Evaluation of coral bleaching condition in situ using an underwater pulse amplitude modulated fluorometer

Abstract

Photosynthetic activities of hermatypic corals (28 genera, c. 68 species and c. 92 specimen) were measured in November 2000 at Sekisei lagoon, Okinawa, Japan, using an underwater pulse amplitude modulated fluorometer. Maximum quantum yield (Fv/Fm) ranged from 0.61 to 0.70 and had an average value of 0.664 (standard deviation [SD], 0.0289). Following the measurement of Fv/Fm, quantum yield (Fm′-F)/Fm′ at eight-leveled actinic-light intensities were obtained from all specimens. Values of electronic transport rates (ETR) were nearly the same under low photosynthetic active radiation (PAR; μmol; quanta/m2 per s) condition and varied with increasing PAR. The value of ETR at 100 μmol quanta/m2 per s ranged from 21.3 to 30.0 and had an average of 27.8 (SD, 2.09). In August 2001, a coral bleaching event was observed in Sekisei Lagoon. During the event, photosynthetic activities of 21 specimens of eight species (seven genera) were measured. By applying two indicators, Fv/Fm and ETR at 100 μmol quanta/m2 per s obtained in August 2000, 18 of 21 specimens were suggested under the bleaching process. In case of Porites lutea, seven colonies were visually evaluated for their condition of bleaching and were found to have two 'normal', three 'slightly bleached' and two 'heavily bleached'. From the density of zooxanthellae, colonies evaluated 'normal' or 'slightly bleached' did not show an apparent difference. However, they were clearly identified by Fm/Fv and ETR at 100 μmol quanta/m2 per s.