Spectral signatures of coral reefs: Features from space

The special signatures of coral reefs and related scenes, as they would be measured above the Earth's atmosphere, are calculated using a coupled atmos phere-ocean discrete ordinates radiative transfer model. Actual measured re flectance spectra from field work are used as input data. Four coral specie s are considered, to survey the natural range of coral reflectance: Montast rea cavernosa, Acropora palmata, Dichocoenia stokesii, and Siderastrea side rea. Four noncoral objects associated with reefs are also considered: sand, coralline algae, green macroalgae, and algal turf. The reflectance spectra as would be measured at the top of the atmosphere are substantially differ ent from the in situ spectra, due to differential attenuation by the water column and, most importantly, by atmospheric Rayleigh scattering. The resul t is that many of the spectral features that can be used to distinguish cor al species from their surroundings or from one another, which have been use d successfully with surface or aircraft data, would be obscured in spectral measurements from a spacecraft. However, above the atmosphere, the radianc e contrasts between most coral species and most brighter noncoral objects r emain noticeable for water column depths up to 20 m. Over many spectral int ervals, the reflectance from dark coral under shallow water is smaller than that of deep water. The maximum top-of-atmosphere radiances, and maximum c ontrasts between scene types, occur between 400 nm and 600 nm. This study s upports the conclusions of recent satellite reef mapping exercises, suggest ing that coral reef identification should be feasible using satellite remot e sensing, but that detailed reef mapping (e.g., species identification) ma y be more difficult. (C) Elsevier Science Inc., 2001.