Dynamic of organic pollution in two Benin coastal lagoons, Lake Nokoué and Porto-Novo lagoon (Ramsar site 1018)

The Lake Nokoué and Porto-Novo lagoon complex is the most important lagoon system in Benin. Situated in the heart of the urban agglomeration, this complex is faced with environmental stresses, notably pollution. The aim of this study was to assess the dynamics of organic pollution in this complex. A total of 270 water samples were taken at 15 stations over six campaigns from November 2022 to October 2023. Water depth, Water temperature, pH, Dissolved oxygen and Salinity were measured in situ. Ammonium, Nitrite and Phosphate concentrations were measured using a Spectrophotometer, and Biochemical Oxygen Demand using the respirometric method. The Organic Pollution Index (OPI) was calculated to assess the level of organic pollution. The results showed strong pollution (OPI=2.75) in Lake Nokoué and moderate pollution (OPI=3.00) in the PortoNovo lagoon. In Lake Nokoué, Lake center, Cotonou Channel and Tchonvi recorded moderate pollution (OPI=3.25) during high water. In Porto-Novo lagoon, strong pollution (OPI=2.75) was observed during flooding, and Djassin showed strong pollution (OPI=2.75) during slight rise water. In conclusion, there is a gradient of strong pollution from Lake Nokoué to moderate pollu tion in the Porto-Novo lagoon. These results will have great emphasis on the comprehension of the functioning of the lagoon complex.