Eutrophication, sediment Phosphorus fractionation and short-term mobility study in the surface and under profile sediment of a water dam. (Okpara dam, Benin, West Africa)

Okpara dam supplies the drinking water treatment station of Parakou town. Aquatic plants proliferation and chemical stratification of the water column in this reserve let see clear manifestation of eutrophication process. Phosphorus concentration in sediments was estimated in the range of 13.479 mg/g to 35.46 mg/g dry wet of sediment. These values are high and not common in the literature. The measured Phosphorus and a-chlorophyll values in the water column show hyper-eutrophication. From N/P ratio, Nitrogen is the limiting factor of eutrophication in Okpara dam, but this does not stop eutrophication. Phosphorus fractionation according to Rydin and Welch (1998) under oxic conditions for surface results sediment and under profile sediment was studied. It was demonstrated that: Fe-bound-P, Al-bound-P and Organic-bound-P comprise the largest phosphorus pool (30% to 51% of Total Phosphorus). High soluble reactive phosphorus content was measured in sediment column compared to other study cases. Organic phosphorus fraction represents 90% of the Total Extracted Phosphorus content. Water and sediment column acidity could allowed iron and humic acids production from the high organic matter amount (21.42% to 28.30%) in the dam to modulate P releasing from sediment. Phosphorus short term remobilization study lets know that deeper sediment organic matters are more mineralized and released more phosphorus (95%) than surface sediment (20%). This fact is not suitable for sediment dredging to 1metter deeper because of eutrophication resumption. In oxic medium aluminum and iron orthophosphate are precipitated more in acidic solution. Phosphorus constitutes iron mobility factor in the dam.