Zollner E, Santer B, Boersma M, Hoppe HG, Juergens K
Bacteria, mesozooplankton, protists, trophic cascade, mesocosm, freshwater, Lake Shosee, Germany, 2.5m3, Daphnia, Copepod
1. We performed a mesocosm experiment to investigate the structuring and cascadingeffects of two predominant crustacean mesozooplankton groups on microbial food webcomponents. The natural summer plankton community of a mesotrophic lake was exposedto density gradients of Daphnia and copepods. Regression analysis was used to reveal top–down impacts of mesozooplankton on protists and bacteria after days 9 and 15.2. Selective grazing by copepods caused a clear trophic cascade via ciliates tonanoplankton. Medium-sized (20–40 lm) ciliates (mainly Oligotrichida) were particularlynegatively affected by copepods whereas nanociliates (mainly Prostomatida) became moreabundant. Phototrophic and heterotrophic nanoflagellates increased significantly withincreasing copepod biomass, which we interpret as an indirect response to reducedgrazing pressure from the medium-sized ciliates.3. In Daphnia-treatments, ciliates of all size classes as well as nanoflagellates were reduceddirectly but the overall predation effect became most strongly visible after 15 days athigher Daphnia biomass.4. The response of bacterioplankton involved only modest changes in bacterial biomassand cell-size distribution along the zooplankton gradients. Increasing zooplanktonbiomass resulted either in a reduction (with Daphnia) or in an increase (with copepods) ofbacterial biovolume, activity and production. Patterns of bacterial diversity, as measuredby polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE),showed no distinct grouping after 9 days, whereas a clear treatment-coupled similarityclustering occurred after 15 days.5. The experiment demonstrated that zooplankton-mediated predatory interactionscascade down to the bacterial level, but also revealed that changes occurred rather slowlyin this summer plankton community and were most pronounced with respect to bacterialactivity and composition.