Low activity of alkaline phosphatase in two eutrophic reservoirs

Abstract

We studied recycling of phosphate by enzymatic hydrolysis in two temperate very eutrophic reservoirs. To assess the potential importance of phosphate regeneration by alkaline phosphatase, we determined the activity of this enzyme in lake water concomitantly with the determinations of the concentrations of phosphomonoesters, soluble reactive phosphate, total soluble phosphate and total phosphate. Contrary to our expectations for such productive waters where algal blooms are frequent, during the study period this process of phosphate regeneration was not significant, probably because the product of hydrolysis (contained in the soluble reactive phosphate fraction) was always abundant. We conclude that, in spite of what has been observed repeatedly in natural lakes with similar trophic characteristics, the readily available fraction of phosphate in these reservoirs is large and for that reason alkaline phosphatase production is low. Therefore hydrolysis by this enzyme is not significant for growth. What seems intriguing is the small amount of phosphomonoesters found in the water; with no phosphatase activity this phosphate fraction should always be high, unless hydrolysis takes place either during phosphomonoester release or later due to their instability.