Integrated water pollution assessment of the Ebrie Lagoon, Ivory Coast, West Africa

Abstract An environmental pollution assessment of the Ebrie lagoon, the largest coastal ecosystem in Western Africa, was executed by applying the Driving force-Pressure-State-Impacts-Response (DPSIR) framework. The domestic and industrial activities in Abidjan and agricultural activities in the wider catchment area were identified as the main driving forces. Two-thirds of Biological Oxygen Demand (BOD) loads and 95% of total nitrogen (N) and phosphorous (P) loads of Abidjan are from domestic effluents, with industry making up the rest. Outside of the direct influence of Abidjan, nutrient levels in the lagoon are governed by the influx of nutrients from the rivers Comoe, Me and Agneby, with nutrient land runoff as the key factors. Total annual N loads to the lagoon for 2000 are estimated at 33 kt, of which 45% from urban sources, 42% from land runoff and 13% from atmospheric deposition. Estimates for P are 2.5 kt, 39%, 48% and 13%, respectively. Scenario analysis has shown that autonomous growth, without pollution reduction measures, would result in an estimated five-fold increase in nutrient inputs to the lagoon over the period 1980–2050. Nutrient concentrations in the lagoon would consequently increase by a factor of 3 1/2, which could escalate to a dramatic level of eutrophication for the complete system. Pollution reduction policies aimed at non-point sources would be most effective in reducing nutrient concentrations. Point-source pollution reduction would improve conditions around Abidjan, but not substantially in the other sections of the lagoon. The approach taken in this study has proven efficient under conditions of relative data scarceness, and sufficiently reliable to allow for policy level conclusions to be drawn.

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