Katarina Vrede

Viral and bacterioplankton dynamics in two lakes with different humic contents

Viral and bacterioplankton dynamics were investigated, together with the temporal variation of phage-infected bacterioplankton in two oligotrophic lakes, one humic and the other clearwater. Bacterial abundance was significantly higher in the humic lake, while the abundance of virus-like particles (VLP) was significantly higher in the clearwater lake. There were no differences in either the frequency of infected bacterial cells (FIC), or in burst size between the lakes. Because of the higher bacterial abundance in the humic lake, a larger number of bacteria were lyzed in this lake. FIC showed large seasonal changes, varying between 9 and 43%, which covers almost the entire range of previously published data from both lacustrine and marine environments. The temporal changes in VLP abundance and FIC were slow in both the humic and clearwater lakes. The burst size was low in both lakes (average value, nine in each case), probably because of the oligotrophic status of the lakes. The chlorophyll a concentrations were higher and positively correlated with VLP numbers in the clearwater lake, indicating that a significant proportion of the viruses in this lake may be phytoplankton viruses.

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

Abstract.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 µgC L−1d−1) and was isotopically distinct (−11.7 ‰) from lakewater organic C (<−27.2‰). All enclosures were also enriched with the same amounts of inorganic nitrogen (30 µgN L−1d−1 as NH4NO3) and inorganic phosphorus (2 µgP L−1d−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.δ15N 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.

Contrasting “top-down” effects of crustacean zooplankton grazing on bacteria and phytoflagellates

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.