DIET CHANGES IN THE ALKALINE-PHOSPHATASE ACTIVITY OF BACTERIA AND PHYTOPLANKTON IN THE HYPEREUTROPHIC VILLEREST RESERVOIR (ROANNE, FRANCE)

The diel changes of the size fractionated alkaline phosphatase activit y (APA) were studied in relation to several abiotic and biotic factors in Villerest reservoir (located on the Loire river, near the city of Roanne, France), bihourly during two days in July 1999. The APA measur ed in this work exceeded considerably those reported in the literature , suggesting that dissolved mineral phosphorus was not available to mi croorganisms. At 1 m, the APA was primarily due to bacteria which acti vely assimilated organic P compounds released by photosynthetic algal metabolism. At 5, 10 and 20 m, the APA was predominantly algal. The hi gh concentrations in SRP (soluble reactive phosphorus) would indicate that orthophosphates were not bioavailable. The reverse (i. e. availab ility to phytoplankton) would have resulted in undetectable levels of P-PO43- due to the massive proliferation of algae in Villerest reservo ir.