Dominance of bacterial metabolism in oligotrophic relative to eutrophic waters

Heterotrophic bacteria are a key component driving biogeochemical processes in aquatic ecosystems. In 1998, we examined the role of heterotrophic bact eria by quantifying plankton biomass and bacterial and planktonic respirati on across a trophic gradient in several small Minnesota lakes as well as La ke Superior. The contribution of bacteria (<1-<mu>m fraction) to total plan ktonic respiration ranged from similar to 10 to 90%, with the highest contr ibution occurring in the most oligotrophic waters. The bacterial size fract ion constituted a substantial reservoir of planktonic carbon. nitrogen, and phosphorus (14-58%, 10-49%, and 14-48%, respectively), being higher in oli gotrophic than in eutrophic waters. However, we saw no clear evidence for t he selective enrichment of either nitrogen or phosphorus in the bacteria si ze fraction relative to total plankton. Carbon:nitrogen and carbon:phosphor us ratios in both the total particulate matter and <1-<mu>m fractions were similar and above Redfield values in oligotrophic waters, but approached th em in eutropbic waters. Carbon-based bacterial growth efficiencies (BGE) we re variable (4-40%) but were lowest in oligotrophic systems and increased i n 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 p redominantly microbial food web, Whereas in eutrophic waters there is an in creased abundance of particulate organic matter supporting a food web consi sting of larger autotrophs and phagotrophic heterotrophs.