Differences in efficiency of carbon transfer from dissolved organic carbon to two zooplankton groups: an enclosure experiment in an oligotrophic lake

We added dissolved organic carbon (C) in various amounts to 6 enclosures in an oligotrophic subarctic lake to assess how bacterioplankton growth on dissolved organic C affects the growth of calanoid copepod (Eudiaptomus graciloides) and cladoceran (Daphnia longispina) zooplankton. Organic C was added as glucose (12.5 to 400 mu gC L-1 d(-1)) and was isotopically distinct (-11.7 parts per thousand) from lakewater organic C (<-27.2 parts per thousand). All enclosures were also enriched with the same amounts of inorganic nitrogen (30 mu gN L(-1)d(-1) as NH4NO3) and inorganic phosphorus (2 mu gP L(-1)d(-1) as Na3PO4). The results showed a direct relationship between bacterial growth on dissolved organic C and incorporation of bacterial biomass into crustacean zooplankton. After 9 days, D. longispina and E. graciloides contained glucose-C in all treatments and the incorporation of glucose-C by zooplankton was strongly correlated with bacterial growth on glucose-C.delta N-15 data revealed different trophic positions of the two crustaceans, suggesting that D. longispina fed directly on bacteria while E. graciloides incorporated bacterial C by consumption of bacterivorus protozoans. Greater incorporation of glucose-C in D. longispina than in E. graciloides was explained by higher individual growth rates in D. longispina, and this difference between the two zooplankters increased as the bacterial production increased. Thus, the results show that the transfer of dissolved organic C through the food web can be more efficient via cladocerans than via calanoid copepods and that the effect becomes more pronounced as bacterial energy mobilization increases.