SeaWiFS data interpretation in a coastal area in the Bay of Biscay

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument, launched onboard the Orbimage 2 satellite, is composed of an optical scanner with eight channels that are used to interpret the ocean colour, and has been operational since September 1997. SeaWiFS data were received by the Dundee Satellite Receiving Station and processed by the Plymouth Marine Laboratory with a slight time-lag. In situ measurements of reflectance, salinity, seston and chlorophyll a were analysed during the Biomet surveys to gain a better knowledge of the dynamics of the Gironde turbid plume during this period. The results showed logarithmic relationships between the SeaWiFS normalized water-leaving radiances (nL(w)) at 490 and 555 nm (nL(w) (490) and nL(w) (555)), and the suspended particle matter concentrations. The relationship between the nL(w) (555) radiances and these concentrations is used to map the coastal terrigenous turbidities. But the chlorophyll a concentrations calculated from SeaWiFS are overestimated in the turbid waters. The nL(w) (490)/nL(w) (555) ratio decreases with increasing turbidity and with increasing chlorophyll a concentration. To distinguish the chlorophyll a in turbid waters, the nL(w) (490) radiances are calculated from nL(w) (555) considering only the effect of terrigenous turbidity. Then, the nL(w) (490) SeaWiFS image is compared to the calculated nL(w) (490), to reveal the 'negative' areas caused by the chlorophyll a and yellow substance absorption.