Spectral Discrimination of Healthy and Non-Healthy Corals Based on Cluster Analysis, Principal Components Analysis, and Derivative Spectroscopy

Reports of global mass coral bleaching are of major concern, but the scientific basis of these reports is questionable. There exists no objective measure of coral health, so that individual perceptions of the paleness of an individual coral head, or an entire coral reef, are the foundation of coral bleaching reports. It is understood that coral bleaching results from an expulsion or reduction of the algae housed in the individual polyp, which causes the coral to lose its color. Satellite or airborne remote sensing may be a feasible means of mapping and monitoring coral reefs over large geographic areas if a quantitative means of remotely determining coral health can be developed. In an effort to remotely detect coral stress, in situ spectral reflectance of healthy and bleached Fijian scleractinian corals was measured with a hand-held spectroradiometer. Principal components and cluster analysis revealed that there is a spectral distinction between healthy and bleached coral based largely on magnitude of reflectance. Spectral derivative analysis was used to determine the specific wavelength regions ideal for remote identification of substrate type. These results are encouraging with respect to using an airborne spectroradiometer to identify areas of bleached corals thus enabling accurate monitoring over time.