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Les sources de phosphore de la lagune de Porto-Novo (50 km 2 de superficie, situee au sud-est du Benin) sont de deux types : les apports diffus (principalement le ruissellement sur les terres cultivees) et les apports ponctuels (rejets industriels ou domestiques). Cette lagune est victime d’une pollution qui se justifie par le developpement anarchique de la flore aquatique (eutrophisation). Objectifs: Etudier les differentes fractions biodisponibles du phosphore au niveau des sediments de la lagune et les conditions de leur relargage dans la colonne d’eau. Methodologies et resultats: Les eaux et sediments de la lagune ont ete preleves en cinq (5) points differents et diverses analyses ont ete effectuees. Le fractionnement du phosphore a ete effectue par la methode d’extraction sequentielle. Les resultats issus de nos analyses ont ete traites avec le logiciel Microsoft Excel et interpretes. Les resultats du fractionnement du phosphore ont permis de constater que la fraction liee a la matiere organique est la plus representative (10612,73 Bg-P/g soit 68,76% de la valeur moyenne du phosphore total), les autres fractions telles que, la fraction particulaire (P-Fe, P-Al, P-Ca) et celle facilement echangeable (P-labile) sont egalement presentes dans les sediments preleves mais a de faibles pourcentage. En moyenne, 58,85Bg-P/g soit 0,38% pour le p-labile ; 20,97Bg-P/g pour P-Fe, 35,42Bg-P/g pour le P-Al et 2,67Bg-P/g pour le P-Ca, soit respectivement 0,13%, 0,22%, 0,017% de la valeur moyenne du phosphore total. Ce pourcentage eleve de P-Org serait du a l’utilisation des engrais organiques dans l’agriculture sur le bassin versant qui comporte la lagune, et a la presence de matieres organiques animales et vegetales, vivantes ou en voie de degradation. En outre, l’effet de la variation du pH sur les echantillons ayant subi le fractionnement apres 96 heures de remises en suspension revele en general une augmentation des differentes fractions du phosphore donc un relargage de ces fractions dans la colonne d’eau (56,84% de P-Al est remobilisable, 96,11% pour le p- Ca, 80,72% pour le p-labile, 33,78% pour le P-Org). Conclusions et applications: Ces resultats pourraient servir de base aux politiques environnementales nationales et autres qui visent a proteger les lagunes cotieres et les organismes qui y vivent. Mots cles: Biodisponibilite, mobilite, relargage, fractionnement, extraction sequentielle English Title:  Bioavailability and mobility of phosphorus in the sediments of the Porto-Novo Lagoon English Abstract Phosphorus sources in the Porto-Novo lagoon (50 km 2 of area, located in southeast Benin) are of two types: diffuse inputs (mainly runoff on cultivated land) and point sources (industrial or domestic). This lagoon is therefore the victim of pollution, which is justified by the radical development of the aquatic flora (eutrophication). Objective: To study the different bioavailable fractions of phosphorus in the sediments of the lagoon and the conditions of their release in the water column. Methodology and results: To achieve this, the waters and sediments of the lagoon were collected in five (5) different points and various analyses were carried out. Phosphorus fractionation was performed by the sequential extraction method. The results of phosphorus fractionation revealed that the organic matter fraction is the most representative (10612.73 μg-P/g, either 68.76% of the average value of the total phosphorus), the other fractions (P-Fe, P-Al, P-Ca) and the easily exchangeable (P-labile) are also present in the collected sediments but at a low percentage (On average, 58.85μg-P / g either 0 , 38% for P-labile; 20.97μg-P / g for PFe, 35.42μg-P / g for P-Al and 2.67μg-P / g for P-Ca, respectively 0 , 13%, 0.22%, 0.017% of the mean value of total phosphorus). This high percentage of P-Org could be explained by the use of organic fertilizers in agriculture on the watershed that includes the lagoon, and also the presence of living and degrading organic and animal matter Furthermore, the effect of the pH variation on the samples which have been fractionated after 96 hours of resuspension generally reveals an increase in the various fractions of the phosphorus, thus releasing these fractions into the water column (56, 84% P-Al is remobilizable, 96.11% for p-Ca, 80.72% for plabile, 33.78% for P-Org). Conclusions and applications: These results could serve as a basis for national and other environmental policies aimed at protecting coastal lagoons and the organisms that live there. Keywords: Bioavailability, mobility, release, fractionation, sequential extraction

Biodisponibilité et mobilité du phosphore des sédiments de la lagune de Porto-Novo