Exchange potentials of phosphorus between sediments and water coupled to alkaline phosphatase activity and environmental factors in an oligo-mesotrophic reservoir.

We investigated the exchange potentials of phosphates at the water-sediment interface together with in situ benthic-chamber fractionated alkaline phosphatase activity and bacteria estimates during September and October 1998 at two stations: station 1, which received immediately the urban inputs from the Taounate city, and station 2, located in the centre of the Sahela reservoir (Morocco). The results showed that low oxygenation enhanced both the bacterial abundance and the alkaline phosphatase activity. Size-fractionated (0.65-100 microm) bacteria attached to dead organic matter together with algae and zooplankton contributed strongly (78%) to the total alkaline phosphatase synthesis in the two sampled stations, suggesting that attachment to organic particles stimulated phosphatase activities. The appearance of anoxic conditions and the decrease of pH supported the dissolution of particulate phosphorus and the release of soluble reactive phosphorus. This latter, together with persisting discharges of organic matter, sewage, and olive mill waste will exacerbate the eutrophication of the reservoir.