Phytoplankton and bacterial alkaline phosphatase activity in the northern Adriatic Sea

The importance of bacterial, phytoplankton and dissolved alkaline phosphatase activity (APA) in the northern Adriatic was investigated during 2006. In surface waters total APA increased from early spring (0.07-0.08 μmol l−1 h−1) to late spring (up to 4.64 μmol l−1 h−1) and remained relatively high during the summer (0.46-0.71 μmol l−1 h−1), due to an increase in specific phytoplankton (up to 30 nmol μg C−1 h−1) and bacterial APA (up to 17.11 nmol μg C−1 h−1). Activity of free enzymes was not important. During late spring and summer both communities exploited dissolved organic phosphorus although, taking into account biomass, phytoplankton activity usually dominated over bacterial activity. In autumn an extra P supply from deeper waters drastically reduced phytoplankton APA, though not bacterial APA, in upper waters. Probably in these months bacteria that were degrading phytoplankton produced organic matter were P limited. In deeper waters APA was low and mainly due to the activity of free enzymes.