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Abstract

We measured bacterial abundance, biomass, production rates, and grazing losses to protozoans in three oligotrophic British Columbia lakes and analyzed bacteria abundance and lake productivity data from 11 other lakes, most fertilized with inorganic nutrients to increase their productivity for juvenile salmon. Bacteria contributed about 24% to the phytoplankton-bacteria carbon biomass in the most ultraoligotrophic lakes, and their relative contribution declined to < 11% with increasing lake productivity. At increasingly high nutrient loadings to the lakes, bacteria abundance increased and was closely correlated with phytoplankton biomass and productivity. Heterotrophic nanoflagellate (HNF) abundance was positively correlated with bacterial numbers. Grazing experiments revealed that HNF were the primary pathway for moving bacterial production to higher trophic levels in oligotrophic British Columbia lakes, and predation by the ciliate and rotifer microzooplankton community appeared to exert top-down control over the abundance of HNF and the transfer of carbon from bacteria. The HNF and microzooplankton, in turn, were affected by the abundance of crustacean mesozooplankton, principally copepods in our experiments.