SS2.01 Effects of Biotic Interactions on the Structure and Function of Microbial Food Webs
Date: Wednesday, June 12, 2002
Time: 10:15:00 AM
Location: Saanich
 
JuergensK, Max Planck Institute for Limnology, D-24302 Ploen, Germany, juergens@mpil-ploen.mpg.de
Zoellner, E, , Max Planck Institute for Limnology, D-24302 Ploen, Germany, zoellner@mpil-ploen.mpg.de
Hoppe, H, G, Marine Science Institute, D-24105 Kiel, Germany, hhoppe@ifm.uni-kiel.de
 
CASCADING PREDATION EFFECTS FROM DIFFERENT MESOZOOPLANKTON COMMUNITIES TO BACTERIA - A FRESHWATER EXAMPLE
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In order to compare the impact of the two major freshwater mesozooplankton groups, copepods and cladocerans, on the structure and function of the microbial community, we performed two mesocosm experiments in a mesotrophic lake. During two time periods (spring, summer) the lake plankton community was exposed to a gradient of cladoceran (Daphnia spp.) and copepod (Eudiaptomus spp.) densities. Increasing Daphnia densities resulted in predation effects on the whole microbial food web but with strongest impact on heterotrophic nanoflagellates (HNF) whereas copepod grazing suppressed mainly oligotrichous ciliates. Mesozooplankton predation effects cascaded to the bacterioplankton but with a stronger effect on activity and taxonomic composition (assessed by DGGE and FISH) than on bacterial biomass. The increase in HNF bacterivory parallel to copepod abundance seemed to be the reason for enhanced bacterial activity and an increased proportion of grazing-resistant bacterial morphotypes (e.g., filamentous forms). The experimental results demonstrate that the composition and abundance of mesozooplankton is a key factor for the structuring of the protozoan and bacterial communities at least in more productive freshwater systems.