DISTRIBUTION OF CARBON IN A TROPICAL HYPERSALINE ESTUARY, THE CASAMANCE (SENEGAL, WEST-AFRICA)

The Casamance estuary, on the coast of Senegal, is an inverse hypersal ine estuary: salinity increases landward, and dry season salinity valu es are up to 172 psu due to the evaporation of seawater. Dissolved ino rganic carbon (DIC) concentrations decreased landward as a negative li near function of salinity. Thermodynamic modelling and the absence of CaCO3 in the sediments indicate that this loss of DIC was not due to c alcite precipitation in the main water body. The innermost, almost lan dlocked, waters contained high phytoplankton biomass (50-300 mu g chi l(-1)) and high concentrations of allochthonous dissolved organic carb on. Photosynthetic uptake of DIC and subsequent particulate organic ca rbon sedimentation is proposed as hypothetical explanation of the rela tionship between DIC and salinity; localized overheating in shallow wa ters might also be involved.