Proteolytic and glycolytic activities in size-fractionated surface water samples from an oligotrophic reservoir in relation to plankton communities

From February to August 1996, the following parameters were determined in surface water samples from an oligotrophic reservoir: concentrations of dissolved combined amino acids (DCAA), monosaccharides (DFCHO) and polysaccharides (DCCHO); activities of glycolytic and proteolytic enzymes associated with three size fractions (0-0.2 μm, 0.2-2 μm, and 2-100 μm); densities and biomass of free-living and attached bacteria, nanoflagellates, and metazoan zooplankton. The mean concentrations of DCAA, DFCHO and DCCHO were 2.7, 0.5 and 1.4 mg C l-1 respectively. Both βGlcA and αGlcA showed a high substrate affinity. The enzyme activities associated with all size fractions fluctuated throughout the year. Highest activities were generally associated with the 2-100 μm fraction; the mean percentages of leucine-aminopeptidase (LeuA), β-glucosidase (βGlcA), α-glucosidase (αGlcA) and β-N-acetylglucosaminidase (βGlcNAcA) activities were 58%, 62%, 76%, and 59%, respectively. Activities of αGlcA and βGlcNAcA in the 0.2-2 μm fraction were correlated with the abundance of free-living picobacteria (bacteria <2 μm). The biomass-specific activity of attached bacteria was 18 to 64 fold higher for attached compared to free-living bacteria. The abundances of autotrophic and mixotrophic nanoflagellates (ANF) were correlated with LeuA (r = 0.59, P < 0.05) and the βGlcA (r = 0.64, P < 0.05) activities in the 2-100 μm fraction.