A five channel chlorophyll concentration algorithm applied to SeaWiFS data processed by SeaDAS in coastal waters

Chlorophyll-a concentration derived from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) after applying the current SeaWiFS Data Analysis System (SeaDAS) processing tools appears to be higher than reality in coastal areas, particularly from late summer to early spring when optical properties of water are dominated by yellow substances and suspended matter. As a complement to the SeaWiFS standard procedure addressing clear water, empirical algorithms can bring immediate progress for observing the coastal domain. This paper proposes to modify the SeaWiFS Ocean Colour 4 band algorithm (OC4) by including the 412 and 555 channels. The effect of the suspended matter on the ratios used as inputs in OC4 is revealed by the 555 channel whereas the atmospheric over-correction and the absorption by yellow substances are related to the 412 channel. Based on a dataset located in the English Channel and on the continental shelf of the Bay of Biscay, a parametrization of the relationship between the OC4 ratio and the 412 and 555 bands has been empirically proposed for different chlorophyll concentrations. Application of a lookup table, relating triplets (OC4 band ratio, 412 and 555 bands) to chlorophyll-a concentration, provides realistic concentration maps likely to satisfy the needs of researchers involved in environmental surveying or fishery management.