Remote Detection of Coral 'Bleaching' Using Pulsed-Laser Fluorescence Spectroscopy

Despite their biological and economic importance coral reefs are increasingly threatened by human activities. Recently, 'bleaching' of reefs, i.e. loss of photosynthetic pigmentation, has occurred at numerous globally-distributed sites. A number of environmental stressors, including increased water temperature, can induce bleaching. Several investigators have suggested that the widespread occurrence of coral bleaching represents an early warning signal of global greenhouse warming. Regardless of the cause, the extent of coral bleaching, both regionally and globally, needs to be documented and monitored. We conducted laboratory studies to evaluate the potential of using remotely-sensed laser-induced fluorescence to monitor coral pigmentation. Five species of Caribbean corals were collected, transported to the laboratory, and maintained in aquaria. Coral samples were irradiated at either 532 nm or 337 nm with pulsed-laser light and spectral scans of fluorescence were collected at 1 nm intervals. Dlstinct chlorophyll fluorescence peaks at 685 and 740 nm were observed in all species examined. In corals exposed to temperature-induced stress, incipient changes In the fluorescence spectra could be detected prior to visible bleaching and coral death. Remotely-measured (aircraft or ship) laser-induced fluorescence provides a potentially useful tool for monitoring the pigmentation status of coral reefs. Field validation will be necessary, but our results suggest that it may be possible to distinguish bleaching, or even partial bleaching, over large geographical areas with sensitivity and rapidity.