The life cycle impact assessment applied to a coastal lagoon: the case of the Slimane lagoon (Tunisia) by the study of seasonal variations of the aquatic eutrophication potential.
Résumé
The life cycle impact assessment (LCIA) is a standard tool for the study of the environmental impact on a system including all activities in connection with manufacturing a product or detailing a service, from extraction of raw materials to disposal of final waste. Here, the system is not only anthropogenic, but also includes both natural behaviours and pollution aspects from human activity: it is a coastal lagoon with varied activities and waste disposals nearby. This stagnant zone is highly subjected to solar exposure and tends to dryness during summer, thus offering ideal conditions to algal proliferation. We have taken into account the trophic state of the lagoon assessed by the LCIA methodology, on the basis of the aquatic eutrophication potential (AEP). We have considered the concentrations of the phosphorus and nitrogen compounds for the calculation of the AEP and their spatial and temporal variations in the lagoon. The results show that the AEP of the phosphorus exceeds systematically the AEP of nitrogen and that the contents of both are systematically higher in summer than in winter. Nitrogen is the limiting factor for the algae growth. Ammonia and phosphates are the most important nutrients for the AEP in summer, whereas nitrates dominate in winter. In addition, the spatial and temporal variations of the N and P nutrients of the surface waters allow to distinguish three areas in the lagoon: a transition zone between the sea- and the lagoon waters; a zone reflecting directly the influence of the O. Bey creek- and the treated-waste waters and one representing the most isolated part of the lagoon and consequently the less contaminated by nutrient inputs.