Model analyses of the future water quality of the eutrophicated Ghar El Melh lagoon (Northern Tunisia)

Abstract

Ghar El Melh (Northern Tunisia) is a shallow eutrophicated lagoon (total surface area 35.6 km2) which was one of the primary sites within the MELMARINA project. It is connected to the Mediterranean through a dredged opening and receives untreated sewage from two towns and a catchment of 131 km2. During the past decade the lagoon’s water quality has deteriorated and vegetation has changed. Ruppia sp. has diminished and green macroalgae (mainly Cladophora sp.) has increased. Frequent resuspension of fine alluvial sediment has reduced both water transparency and angiosperm (Ruppia sp.) vegetation cover. Flash floods during winter deliver most of the load of sediment and nutrients from the catchment to the lagoon. Options for restoring the angiosperm cover in the lagoon have been analysed using a model system developed for the main part of the lagoon consisting of a 2-D hydrodynamic model coupled to an ecological model. The ecological model includes a combined description of eutrophication and sediment transport with rooted vegetation (Ruppia sp.), benthic macroalgae, benthic microalgae, phytoplankton and suspended matter and nutrients. A separate sediment module includes pools of nutrients, organic carbon and fine sediment. Deposition and resuspension of the sediment pools are dependent on current and wind-generated shear stress. The load of nutrients and fine sediment was estimated through calibrating the model against measured water quality data and sediment deposition rates. The anthropogenic load from July 2003 to July 2004 was 33,000 tonnes fine sediment, 182 tonnes total N and 26 tonnes total P. A net import of sediment and nutrients from the sea was simulated, driven by the relatively high concentrations of suspended matter and particulate-bound N and P measured in coastal Mediterranean water outside the lagoon. Scenario simulations indicate that the lagoon acts as a deposition area for sediment N and P for boundary concentrations down to 50% of the measured concentrations at the coast outside the lagoon. A range of simulations with decreasing load of sediment and nutrients indicate that angiosperm vegetation will slowly recover with loads of 25% of current loads, combined with increased filtration by bivalves which would decrease suspended matter concentrations and increase transparency. Elevated concentrations of suspended matter and nutrients in the sea outside the lagoon are discussed in relation to a 30-year flash flood of the Mejerda River in February 2003. Coastal erosion as well as nutrients and sediment delivered by this river probably account for a general elevation of these concentrations close to the coast.