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ALKALINE PHOSPHATASE ACTIVITY AND STRUCTURE OF THE PLANKTON COMMUNITY IN THE COASTAL PREVLAKA AQUATORY (SOUTH ADRIATIC SEA) IN APRIL 2011.

Phosphorus (P) is an essential element utilized by all living organisms for growth and energy transport and critically involved in the biological productivity of aquatic systems. In limited pool of dissolved PO4, phytoplankton inductively synthesize alkaline phosphatase AP. Potential AP activity (APA) of the plankton was determined in water column of Prevlaka aquatory (South Adriatic Sea). Chl a concentration was converted into phytoplankton biomass for expression of specific APA (sAPA = APA/biomass). Alternative assessment of phytoplankton biomass (from abundance of cyanobacteria, abundance and volume of autotrophic flagellates < 10 μm) was performed to recalculate sAPA. Secchi disch depth (13 m) indicated compensation depth (39 m) and light penetration throughout whole water column. Oligotrophic conditions were documented in low TRIX (1.43 to 2.67). TIN/PO4 did not show great disturbance from Redfield's phytoplankton biomass ratio ~ 16. Phytoplankton growth was PO4/NH4 limited in surface - 20 m. APA had maximum at 10 m (103.71 nM P/h) and minimum at 20 m (35.17 nM P/h) following the trend of cyanobacteria abundance in surface – 20 m. sAPA (Chl a as biomass estimator) positively followed (p < 0.01) abundance of cyanobacteria throughout water column. sAPA minimum at 39 m (2.69 nmol P/μg C h) expressed ~ 1/4 of sAPA maximum at 10 m (10.1 nmol P/μg C h). Alternative assessment of phytoplankton biomass revealed sAPA maximum at 39 m (1.85 nmol P/μg C h), while sAPA at 10 m was 1.66 nmol P/μg C h. Chl a concentration was marked as unreliable phytoplankton biomass estimator.

alkaline phosphatase activity; cyanobacteria; phytoplankton; Chl a; nutrients; Adriatic Sea

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