High potential activity of alkaline phosphatase in the benthic nepheloid layer of a large mesotrophic lake: implications for phosphorus regeneration in oxygenated hypolimnion

Although the benthic nepheloid layer (BNL) in freshwater and marine systems is known to be an active site of material transformations, information is limited regarding the extent of and mechanisms underlying phosphorus regeneration in the BNL. We found that potential activity of particle-bound alkaline phosphatase (APase) was remarkably high in the BNL of a large (surface area: 674 km(2); maximum depth: 104 m), mesotrophic lake (Lake Biwa, Japan) during periods of thermal stratification. The enhancement in the BNL of other ectoenzyme activities was not as pronounced as that of the APase; the ratios of the particle-bound potential activities in the BNL relative to those in the upper hypolimnion were 2.6 +/- 1. 0 (mean +/- SE, n = 22) and 2.0 +/- 0.6 for P-glucosidase and leucine aminopeptidase, respectively, whereas the corresponding ratio for APase was 4.5 +/- 2.3. The APase activity increased in the BNL with increasing concentrations of soluble reactive phosphorus, inconsistent with a general notion that phosphate represses microbial synthesis of APase. Incubation experiments revealed that the net regeneration of P proceeded in waters collected in the BNL at rates comparable to the in situ accumulation rate of soluble reactive phosphorus in the hypolimnion of the basin. Our data suggest that the enzymatic cleavage of the P moiety of organic phosphorus contributes to the active regeneration of P in the oxic BNL of Lake Biwa.