Monitoring suspended particulate matter fluxes and patterns with the AVHRR/NOAA-11 satellite: application to the Bay of Biscay

AVHRR/NOAA-11 satellite images of the C1 (580-680 nm) and C2 (725-1100 nm) channels were used to quantify the suspended particulate matter discharged into the ocean by the Gironde estuary (France). The calibration method is b ased on a comparison between satellite values (AVHRR reflectances) and susp ended particulate matter (SPM) concentrations at the sea surface, measured in situ. To correct the aerosol variations a C1-C2 image subtraction is app lied. However, when the SPM concentration is greater than about 15 mg l(-1) , the C2 channel image is affected by water turbidity and the C1-C2 correct ion cannot be used. The C1 channel is more sensitive, and we were able to d istinguish SPM concentration variations greater than 0.5 mg l(-1). A subtra ction of two images recorded with a 24 h interval was carried out to evalua te vertical and horizontal fluxes. The residual image shows the quantity of particles leaving the surface layer. To assess horizontal displacements, t urbid fronts were mapped by means of an image gradient analysis. Major and minor fronts recorded each day, superimposed to outline displacements (grad ients and orientations), were displaced from the Gironde estuary to the Wes t and the Northwest. During the ECOFER cruises (1989-1991), several AVHRR scenes were processed using an atmospheric correction based on the low SPM concentrations ("ocean pixel subtraction"). Generally, surface particulate matter is confined to the inner continental shelf. Higher values were observed during the ECOFER 4 cruise. Suspended particulate matter concentrations with the maximum exte nsion appeared during the ECOFER 1 cruise. Concentrations were very low dur ing the ECOFER 5 cruise, even in the lower estuary. This was due to low sum mer discharges that occurred during the dry period of 1988-1991 in France. During these surveys, estimated SPM concentrations were less than 1.5 mg l( -1) in the ECOFER area. (C) 1999 Elsevier Science Ltd. All rights reserved.