Bathymetric and bottom effects on CZCS chlorophyll-like pigment estimation: data from the Kerkennah Shelf (Tunisia)

The Coastal Zone Color Scanner (CZCS)-retrieved chlorophyll-like pigments f or the Mediterranean Sea show zones of high concentrations (> 10 mg m(-3)) such as the shallow Kerkennah Shelf, off the southern Tunisian coast. This occurrence-pointing to probable erroneous estimates, exaggerated by the eff ect of the bottom signal-is discussed in the general framework of the appli cability of water colour techniques to coastal areas, as a source of enviro nmental data suitable for inclusion in a multi-purpose Geographic Informati on System (GIS). The state of knowledge on the Kerkennah Shelf test site, o rganized into a regional gee-referenced data base (including CZCS and TM im agery), is presented. A two-flow model has been applied to in situ reflecta nce measurements, to derive some information on the interplay between water colour and bottom typology. The results have been used to evaluate nature, intensity and extent of the sedimentary and vegetated sea bottom effect on the chlorophyll-like pigment concentrations derived from CZCS in the Kerke nnah area. Finally, the applicability of satellite-derived ocean colour dat a to coastal zone mapping is discussed in some detail.