Impact of the pollution by heavy metals and eutrophication on the southern lagoon of Tunis before its ecological rehabilitation

The Southern lagoon of Tunis, (northern Tunisia, central Mediterranean), has been strongly affected by nutrients and heavy metals. Since the Middle Ages, at least, the lagoon collected natural and anthropic pollutants and both categories of pollution considerably increased concomitant to urban industrialization and, were the cause of important dystrophic crises. With regard to this ecosystem, a previous article was provided by who given thorough data allowing to suggest that local benthic communities were submitted to both natural and anthropic pollutants pressure. These unfavorable patterns reached a high level eutrophication and ended in a collapse stage according to. Before ecological rehabilitation of southern lagoon, investigations were conducted in order to assess and to delineate the environment state of pollution and its effects in the area. We conducted a comparative study on the pollution by heavy metals and nutrients in the water column, surface sediment and in various organisms such as the green algae Ulva rigida, the two mussels Ruditapes decussatus and Mytilus galloprovincialis, the three species of fish Anguilla anguilla, Chelon labrosus and Mugil cephalus .The following metals were studied (Cd, Cu, Zn, Pb, Cr, Ni, Fe, Mn, and HgTot). Samples are collected seasonally during five years 1995-1999 and treated according to international norms and analysed by Atomic Absorption Spectrophotometer (A.A.S.). The results showed a high level of contamination by heavy metals of surface sediment. The values for mercury, cadmium and lead oscillate respectively between (0.17 and 5μg/g dry weight; 0.13 and 30μg/g dry weight; 3.97 and 698 μg/g dry weight). The sediment of the lagoon constitute before the restoration program a great reserve of toxic metallic pollutants strongly correlated with the nature and the importance of the industrial activity. In the same way, ecotoxicological survey revealed that relatively high concentrations of lead, mercury and cadmium occur in Ulva rigida, although concentrations in water were generally, except during the decomposition of algae in summer, less than the detection limit. The variation of metals concentrations in the cosmopolitan green algae Ulva rigida, characteristic of many eutrophic coastal environments, seems to reflect the fluctuations of the above metals in the sediment. In fact, contents of this 'bio-sensor species cover large intervals of values according to stations considered reaching in certain cases very high concentrations. Concerning the animal organisms, the seasonal study has allowed us to observe the bioaccumulation complex, dependent on a multitude of factors such as abiotic parameters, the species and its metabolism, the organ analysed as well as the time of exposure to the pollutants. Facing the stress caused by this pollution, these animals, although belonging to the similar environment, develop and present different behaviours. This state of pollution is to the origin of a deplorable ecological situation in the lagoon, characterised by the relatively low specific diversity and the colonisation of the ecosystem by the indicators of pollution and extreme conditions