L'oxygène dissous en Lagune Ebrié: Influences de l'hydroclimat et des pollutions

The hydrology of the Ebrie coastal lagoon in Abidjan area is summarized. The authors describe the oxygenation in that area during the two extreme seasons of the hydrological cycle: the low-water season (March-April) and the high-water season (Sept-Oct). The influences of the continental and oceanic waters, photosynthesis, exchanges with the atmosphere and pollution are considered. The oxigen consumption of primary organic pollution represents from 9 to 12% of the content of the waters that circulates in the area. It is geographically very heterogeneous. The central basin, swept by strong marine and fresh water currents, shows a rather high level of water oxygenation. In the peripheric bays, water circulation and mixing are less important and pollution accelerates the natural eutrophic processes. During the low-water season, a vertical stratification is responsible for a bottom anoxic layer and the deposit of reduced organic silts. On the contrary, supersaturations, up to 200%, are recorded on the surface layer. During the high-water season the break of the vertical stratification sets the loose reduced silts into suspension and partly reoxygenates the bottom waters. A classification of the different areas, based on the oxygen vertical profiles is proposed.