Water Constituents Assessment at the Sassandra River Mouth (Côte d’Ivoire): An Outline Based on Remote Sensing Reflectances

Abstract

Optical remote sensing of surface waters has been used to monitor the impact of the Sassandra River-one of the four main rivers of Cote d'Ivoire-along the coast of the Gulf of Guinea. A 5-year (2003-2007) series of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) images was chosen to identify (spectrally) the major constituents of coastal waters under the influence of river discharges. The percent contributions of yellow substance, i.e. coloured dissolved organic matter (CDOM), chlorophyll-like pigments of phytoplankton (CHL) and suspended detritus (DET) to the total reflectance (approximated by the sum of reflectances at 443, 555 and 670 nm) were calculated. Ternary plots of these percentages provided some general indications about the relative proportions of CDOM, CHL and DET, according to the impact attributed to each water constituent. The analysis showed that water constituents follow trends linked to the ``marine{''} seasons of the area. The CHL and DET contributions seem to correlate with the timing of cooler (upwelling) periods, while CDOM seems to correlate better with that of warmer periods. The relationship with the Sassandra river runoff is not as evident, even though CDOM increased contributions appear to mirror the weakening of upwelling and the discharge of local rivers. This broad ecosystem outline should be extended to the entire Cote d'Ivoire coastline, to better classify marine waters impacted by upwelling processes or anthropogenic pollution due to river runoff.