¹Fisheries & Oceans Canada, 867 Lakeshore Road, Burlington, Ontario, Canada, L7R 4A6
²Plankton Canada, Burlington, Ontario, Canada

Summary

         We undertook a comprehensive analysis of the microbial food web of Lake Superior, including bacteria, autotrophic picoplankton, phytoplankton, heterotrophic nanoflagellates and ciliates, during the spring and summer of 2001. Total microbial loop biomass increased from 215 mg m^-3 (spring) to 320 mg m^-3 (summer). Bacteria did not show any change from spring to summer, while autotrophic picoplankton, heterotrophic nanoflagellates and ciliates all showed small but significant increases. Phytoplankton biomass did not change significantly from spring (˜950 mg m^-3) to summer (˜1070 mg m^-3). The composition of the phytoplankton community did change, however, with an increase in the biomass and relative composition of Diatomeae observed during summer. This was supported by a rise in net plankton (> 20 µm) biomass. Primary productivity was extremely low in both seasons (< 1 mg C m^-3 h^-1) but greatly exceeded bacterial productivity (< 0.02 mg C m^-3). The microbial food web, when expressed as organic carbon, was overwhelmingly autotrophic (˜ 90%) in spring and summer. Carbon turnover rates (or P/B) were highest for picoplankton in both seasons (˜ 0.5 d^-1) and dominated by a single species, Chroococcus dispersus var. minor. Carbon turnover rates of heterotrophic bacteria (0.06 d^-1) were higher in the summer than autotrophic net plankton (0.05 d^-1) and nanoplankton (0.04 d^-1) but were still an order of magnitude less than picoplankton. Our 2001 survey of Lake Superior demonstrated that Lake Superior remains a cold stenothermic and ultra-oligotrophic ecosystem. While we observed that energy transfer was predominantly autotrophic, the significance and importance of heterotrophs in the physiological ecology of oligotrophic lakes deserves more attention. Research should be focussed on exploring sources of energy (autochthonous vs. allocthonous) and ultimately the linkages between lower and higher trophic levels.