EVALUATING THE VALIDITY OF SeaWiFS CHLOROPHYLL ALGORITHM

The SeaWiFS ocean colour sensor on board the SeaStar (Orbview 2) satellite is able to derive the chlorophyll concentration in the surface sea water. The chlorophyll algorithm employed is based on the ratio of radiance or reflectance measured in the blue and green spectral bands. Regression between the band ratio and the in-situ measurements of chlorophyll values gives the coefficients for the retrieval equation. It is commonly known that the standard SeaWiFS chlorophyll algorithm (OC4) is generally applicable only in open ocean (case 1 waters). This algorithm is not expected to be valid in coastal waters (case 2 waters) containing other constituents such as sediments and dissolved organic matter. In this study, we simulate SeaWiFS reflectance spectra of coastal waters using an analytical model based on the quasi-single scattering approximation. The OC4 algorithm is then applied to the simulated spectra to derive chlorophyll concentration. The derived chlorophyll value are compared with the actual chlorophyll values used to generate the spectra. The results show that the OC4 algorithm is valid only for sea waters with low dissolved organic matter, but the accuracy deteriorates for typical coastal waters with high dissolved organic matter.