Heterotrophic bacteria are a key component driving biogeochemical processes in aquatic ecosystems. In 1998, we examined the role of heterotrophic bacteria by quantifying plankton biomass and bacterial and planktonic respiration across atrophic gradient in several small Minnesota lakes as well as Lake Superior. The contribution of bacteria (<1-μm fraction) to total planktonic respiration ranged from ∼10 to 90%, with the highest contribution occurring in the most oligotrophic waters. The bacterial size fraction constituted a substantial reservoir of planktonic carbon, nitrogen, and phosphorus (14-58%, 10-49%, and 14-48%, respectively), being higher in oligotrophic than in eutrophic waters. However, we saw no clear evidence for the selective enrichment of either nitrogen or phosphorus in the bacteria size fraction relative to total plankton. Carbon: nitrogen and carbon: phosphorus ratios in both the total particulate matter and <1-μm fractions were similar and above Redfield values in oligotrophic waters, but approached them in eutrophic waters. Carbon-based bacterial growth efficiencies (BGE) were variable (4-40%) but were lowest in oligotrophic systems and increased in eutrophic systems. BGE varied negatively with carbon:nitrogen:phosphorus ratios, suggesting increased maintenance costs in low-nutrient waters. In oligotrophic waters most of the organic matter is dissolved, supporting a predominantly microbial food web, whereas in eutrophic waters there is an increased abundance of particulate organic matter supporting a food web consisting of larger autotrophs and phagotrophic heterotrophs.