Pii: S0038-0717(00)00173-5

In the Baltic Sea, peptidase and alkaline phosphatase activities always show the same pattern in nutrient gradients: increasing enzyme activities with decreasing dissolved inorganic nitrogen (DIN) and phosphate. The field experiments reported here suggest that peptidase activity may be regulated by DIN in a manner similar to that of alkaline phosphatase by phosphate. In nutrient gradients established in mesocosms, peptidase activity increased 5to 7-fold, whereas the alkaline phosphatase activity increased between 29and 65-fold. Parallel to the enzyme activities, phytoplankton biomass (measured as chlorophyll a), organic phosphorus and nitrogen increased, but the closest correlation was found between peptidase and alkaline phosphatase activity …r ˆ 0:90; n ˆ 18; p , 0:01†: The addition of nitrate, and nitrate plus phosphate, at the time where nutrients were depleted and both enzyme activities were high, did not result in a decrease of peptidase activity. Rather, the peptidase activity was highest after simultaneous treatment with phosphate and nitrate. The addition of 1 mg l alkaline phosphatase caused a 6-fold increase of the peptidase activity after 96 h. Several mechanisms of the stimulation of peptidase activity by alkaline phosphatase are discussed. q 2000 Elsevier Science Ltd. All rights reserved.