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Abstract
Temporal variations of phosphorus concentrations, bacterial abundance and activity of fractionated (i.e: size fractions: 0.22 to 0.65 μm; 0.65 to 100 μm and 0.22 to 100 μm) alkaline phosphatase (APA) were studied in situ in a benthic chamber placed in a station near urban inputs in the Sahela reservoir (Morocco) on September and October 98. Results show that low oxygen concentrations induced enhanced concentrations of orthophosphates, and bacterial numbers and APA activity reached their highest values. Anoxic conditions and the decrease of pH favour the dissolution of particulate phosphorus and the release of soluble reactive phosphorus. This release is accompanied simultaneously by an increase in the abundance of anaerobic bacteria and a progressive reduction in APA. Lastly, the release of phosphorus together with urban inputs from the city of Taounate should be considered as potential inducers of the eutrophication of the reservoir.