In situ optical characterization of a coastal lagoon in Tunisia for ecological assessment

Ghar El Melh is a shallow lagoon (average depth is 0.8 m), undergoing an eutrophication process, due to growing human pressures. To assess the relations between its optical properties and its trophic state, downwelling and upwelling visible spectral irradiances were measured in 22 sampling stations across the basin; then Apparent Optical Properties (AOPs), namely reflectance R(λ) and diffuse attenuation coefficient Kd(λ) were calculated. From water samples the concentrations of OASs (Optically Active Substances), chlorophyll a, particulate suspended matter and colored dissolved organic matter, was assessed, showing elevated values both for particulate and dissolved matter (13.2 ≤ [SPM] ≤ 46.8 mg/l; 0.2 ≤ aCDOM(440) ≤ 0.6 m-1). The use of an optical classification for basins rich in SPM and CDOM, integrating AOPs and OASs, highlighted a great spatial heterogeneity, with waters from clear to very turbid, the latter dominating where water exchanges were slower. The application of the Ecological Evaluation Index (EEI), based on macrophyte-cover distribution in shallow environments, where phytoplankton is not the main primary producer, indicated an intermediate ecological condition. So the integrate use of both methods permitted a complete, repeatable characterization, detecting more impacted sectors and suitable to understand links between optical and trophic classification in coastal basins.