Microbial production, utilization, and enzymatic degradation of organic matter in the upper trophogenic layer in the pelagial zone of lakes along a eutrophication gradient

The major aim of this study was to evaluate the relationships between the rates of microbial activities (phytoplankton primary production, bacterial secondary production, bacterial utilization of organic matter, enzymatic activities, protozoan grazing on bacteria), bacterial numbers, and dissolved organic carbon concentrations and the trophic state index (TSI) of lakes in the upper trophogenic water layer in the pelagial zone along the trophic gradient (from oligo/mesotrophy to hypereutrophy) in 19 lakes of the Mazurian Lake District (northeastern Poland). Multiple regression analysis (analysis of variance-ANOVA) on all collected data and the TSI along eutrophication gradient showed that all studied microbial processes and parameters were very tightly coupled to the trophic conditions of the studied lakes. All studied microbial processes involved in utilization and enzymatic degradation of organic matter were strongly positively dependent on the intensity and rates of photosynthetic organic matter production and exudation that markedly increased along the eutrophication gradient of lakes. V-max of alkaline phosphatase, aminopeptidase, and nonspecific esterase showed significant correlation with the TSI of the studied lakes. Protozoans removed a significant portion of bacterial production, i.e., from similar to 20% to 75-85% of newly produced bacterial biomass was simultaneously consumed by protozoans along the eutrophication gradient. These observations suggest that the importance of protozoan grazing on bacteria on regulation of bacterial production depends on lake productivity. The general working hypothesis that the intensity of microbial processes of organic matter can be tightly coupled to increasing eutrophication was proven in these studies.