Spectral reflectance as a potential tool for detecting stressed corals

Abstract

We observed the reflectance spectra of naturally bleached corals and of experimentally stressed corals exposed to high temperature and strong solar radiation, and analyzed changes in reflectance spectra, photosynthetic capacity, and zooxanthellae status. In response to stress, the corals became dominated by degraded, shrunken zooxanthellae, and their photosynthetic capacity decreased. Degraded, pale-transparent zooxanthellae dominated in the naturally bleached corals, which had low photosynthetic capacity. Differences in the type of degraded zooxanthellae in the experimentally and naturally bleached corals suggest that the degradation pattern reflects the mode of stress (acute or chronic). In both experimentally stressed and naturally bleached corals, the red edge in the reflectance spectra shifted to a shorter wavelength. In the stress experiment, the shift was observed prior to visible bleaching. We propose that spectral reflectance is a useful indicator of coral stress.