Analysis of the diffuse attenuation coefficients for radiance and the implications for retrieval of the spectral signature of submerged tropical corals

This is the International Year of the Reef (1997), yet the basis for reef mapping is existing Admiralty charts and air photos. The spectral information of airborne or satellite digital imagery is needed to retrieve information regarding stress on corals that may be caused by climate variability or change, and/or pollutant stress. The challenge is to adjust the digital imagery for the optical attenuation through water to depth with corrections that will vary with the density of intervening sediment or plankton. In this paper the author reports on a study in Fiji in which the vertical profiles of the diffuse attenuation coefficient were measured for the downwelling irradiance and upwelling radiance in the SeaWiFS channels. Over forty profiles have been obtained over coral reefs, debris surfaces, and sand surfaces, as well as `blue water´ with depths far beyond the range of the 100 metre cable for the dropsonde radiometer. The author derives an algorithm for determination of the bottom reflected radiance independent of image characteristics