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Les échanges de phosphore à l'interface eau-sédiment ont été étudiés dans la retenue Al Massira. La campagne de prélèvements a été menée pendant la faible oxygénation des eaux profondes au mois d'octobre 1991. Les mesures des échanges de phosphore ont été réalisées in situ en chambre benthique.Les valeurs de pH sont élevées et reflètent clairement l'influence de la nature géologique du bassin versant sur la composition chimique des eaux. Ce bassin versant, formé essentiellement de roches sédimentaires calcaires, est vraisemblablement à l'origine des concentrations élevées en calcium mesurées dans les chambres benthiques.En période de faible oxygénation, on note une augmentation des teneurs en phosphore particulaire dans les eaux surnageantes en raison de la présence de complexes calcium-phosphore, fer-phosphore et manganèse-phosphore. En période d'anoxie, la réduction de ces complexes est accompagnée d'une aug- mentation des teneurs en phosphore.La densité bactérienne et la concentration en phosphore total et notamment en orthophosphates suivent une évolution temporelle comparable. Une fraction non négligeable du phosphore particulaire serait liée aux bactéries.Des mesures drastiques de déphosphatation du tributaire Oum Errbia doivent être prises pour éviter la dissociation des complexes Ca, Fe, Mn-Phosphore et le relargage de phosphore biodisponible.Phosphorus exchange at the sediment-water interface was studied in the Al Massira reservoir. This reservoir, located on the Oum Errbia river, is the most important freshwater resource in Morocco. It provides drinking water to the whole population of Casablanca and contributes to farming and to the functioning of several industrial plants. Our investigations were made in October 1991 and coincided with low oxygen concentrations in the deep layers. The measurements of phosphorus exchange have been made in sitn with a benthic chamber device.The results showed that these exchanges were regulated by a combination ofthe physical chemical variability of the environment and the geological composition of the catchment basin.The pH, which permanently exceeded 7 in Massira, suggests that calcium-bound phosphorus was primarily CaHFO4. Moreover, given the high phosphonrs inputs into the reservoir (570 tonnes. year-1), and despite the fact that, as soon as it reached the reservoir, a large fraction of this element was associated with to calcium, it's likely that the Massira reservoir is subject to eutrophication.In low oxygenated deep layers, we observed an increase in particulate phosphorus concentrations of the overlying water, due to the simultaneous presence of calcium-phosphorus, iron-phosphorus and manganese-phosphorus forms. Under anoxic conditions, the reduction ofthese complexes resulted in an increase of phosphorus concentrations.Bacterial cell numbers and total phosphorus concentratioru showed a comparable temporal behaviour. This would suggest that attached bacteria contributed at least partially to the total particulate phosphoms concentrations.We suggest that a drastic control of phosphorus inputs into the waters must be done through a program of dephosphorylation of tributaries to avoid the dissocation of calcium-, iron-, manganese-bound phosphorus with release upwards of bioavailable phosphorus

Étude préliminaire sur les échanges de phosphore à l'interface eau- sédiment au sein de la retenue d'Al Massira (Maroc)