Size-fractionated alkaline phosphatase activity in the hypereutrophic Villerest reservoir (Roanne, France)

Seasonal changes in size-fractionated alkaline phosphatase activity (APA) w ere studied in the hypereutrophic Villerest reservoir (Roanne, France) to d etermine whether the APA showed seasonal variations coupled to abiotic and biotic variables. Samples were collected at different depths from November 1992 to October 1993. The phosphatase activities were measured spectrophoto metrically with p-nitrophenyl phosphate as the substrate and related to pro tein concentrations. No correlations were found between APA and soluble rea ctive phosphorus concentrations during an annual cycle. Free phosphatases w ere particularly high in the epilimnion (54.5% of total activity was at 5 m ) and decreased with depth (4% of total APA at 40 m). The contribution of s mall microorganisms to APA increased with depth. It was concluded that APA is not a valid quantitative test of the phosphorus status of aquatic microo rganisms.