Factors related to heterotrophic bacterial and flagellate abundance in temperate rivers

Thirty-one rivers of varying trophic status located in Ontario and Quebec, Canada, were sampled in order to determine the factors related to heterotrophic bacterial and flagellate abundance. Bacterial abundance ranged from 1.80 X 106 to 9.92 X 106 cells ml-' and flagellate abundance ranged from 1.18 X 103 to 17.4 X 103 cells ml-' There was a highly significant, positive relationship between bacterial abundance and both total phosphorus (R2 = 0.551 and chlorophyll a (R2 = 0.55). Bacterial abundance was not related to dissolved organic carbon concentration which ranged from 4.6 to 13.0 mg 1-' A significant positive relationship was observed between heterotrophic flagellate abundance and both heterotrophic bacterial abundance (R2 = 0.46) and total phosphorus (R2 = 0.52). Neither bacterial nor flagellate abundance was significantly related to water residence time. No relationship between zooplankton biomass and bacterial or flagellate abundance was observed, possibly because of the low biomass of zooplankton in the rivers (mean = 11.3 pg 1.' dry mass). Results suggest that in rivers there is little transfer of energy from the microbial food web to the planktonic food web due to the scarcity of metazoan zooplankton.