Extracellular APA in the water samples was determined
using a modified procedure by Gage and Gorham (1985)
and Boon (1989). Triplicates of 5 mL water samples were
supplied with Tris–HCl buffer (pH=8.5, final concentration
13 mmol L−1), Na3N (final concentration 5 mmol L−1), and
p-nitrophenyl phosphate (pNPP, final concentration
0.3 mmol L−1), and the samples were incubated at 37°C
for 4 h. Absorbancy of p-nitrophenol was measured
spectrophotometrically at 410 nm and corrected for blanks.
Autoclaved lake water was used as the blank. APA was
determined in unfiltered sample (APAT) and in the filtrates
through 3.0 μm and 0.45 μm membrane filters (APA3.0 μm=APAT−APA