A coupled atmosphere and ocean radiative transfer model, the Gulf of Maine (GOM) model, was developed to simulate water-leaving radiance from a vertically stratified ocean containing a bloom of the coccolithophore Emiliania huxleyi. The model is based largely on atmospheric and ocean data representing the Gulf of Maine. The atmospheric submodel simulates direct sunlight and diffuse skylight illuminating the sea surface and is adjusted to account for seasonal changes in atmospheric aerosols. The optical properties of E. huxleyi, required by the ocean submodel, are derived from measurements collected in Gulf of Maine coccolithophore blooms occurring in 1989 and 1990. The modeled response of volume reflectance to the combined effects of chlorophyll and particulate calcite compares favorably with field measurements of E. huxleyi cell abundance and coastal zone color scanner (CZCS)-derived volume reflectance representing a coccolithophore bloom in the northeast Atlantic Ocean. The GOM model was used to investigate the response of normalized water-leaving radiance, modeled for visible CZCS and sea viewing wide field of view sensor (SeaWiFS) bands, to particulate calcite and chlorophyll a. Ranges in the concentrations of particulate calcite, chlorophyll a, and colored dissolved organic material (CDOM) are selected to represent conditions reported for coccolithophore blooms. Water-leaving radiance increases with increasing particulate calcite concentration, primarily because of a disproportionately large amount of backscatter from detached coccoliths (about an order of magnitude larger than is predicted using Mie theory).

As a result, CZCS plant pigment algorithms based upon radiance ratios may be corrupted more severely than previously estimated. As an alternative to radiance ratio-based algorithms, an iterative procedure (also referred to as optimization) is used to invert the GOM model in order to simultaneously retrieve particulate calcite and chlorophyll a concentrations. The approach uses normalized water-leaving radiance computed for all visible CZCS or SeaWiFS bands. Tests of the approach suggest that independent of errors associated with instrument calibration and atmospheric correction, errors in the retrieved concentrations are small, even when high concentrations of CDOM and vertical structure within the water column are neglected, i.e., with the assumptions that CDOM concentration is small and the water is vertically homogeneous. However, since there are no data sets of contemporaneous chlorophyll a concentration, particulate calcite concentration, and CZCS imagery, a rigorous test of the model and inversion technique must wait for the launch of new ocean color scanners such as the NASA SeaWiFS. ¿ American Geophysical Union 1994