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 for the Mediterranean Sea show zones of high concentrations (10mgm-3) such as the shallow Kerkennah Shelf, off the southern Tunisian coast. This occurrence pointing to probable erroneous estimates, exaggerated by the effect of the bottom signal is discussed in the general framework of the applicability of water colour techniques to coastal areas, as a source of environmental data suitable for inclusion in a multi-purpose Geographic Information System (GIS). The state of knowledge on the Kerkennah Shelf test site, organized into a regional geo-referenced data base (including CZCS and TM imagery), is presented. A two-flow model has been applied to in situ reflectance 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 Kerkennah area. Finally, the applicability of satellite-derived ocean colour data to coastal zone mapping is discussed in some detail.