The effects of predation by planktivorous juvenile fish on the microbial food web.
The feeding impact of planktivorous fish on microbial organisms is still poorly understood. We followed the seasonal dynamics of the food web in two natural fishponds for two years: one was stocked with planktivorous whitefish while the other had no planktivorous fish. The aim of the study was the simultaneous assessment of the feeding behaviours of planktivorous fish and of bacterivorous meta-/protozooplankters. We hypothesized that in the presence of planktivorous fish there would be fewer metazooplankton, more protozoans and decreased numbers of bacteria. Our results showed that the amount of metazooplankton eaten by the fish was indeed negatively correlated with metazooplankton biomass. The feeding impact of planktivorous fish in shaping the microbial loop was remarkable. The main grazers of bacteria in the fishpond were ciliates, whereas in the pond without fish these were heterotrophic nanoflagellates. In the fishless pond the role of the top predator shifted to the predaceous metazooplankter Leptodora kindtii which controlled the abundance of herbivorous metazooplankters. We found a negative relationship between the number of bacteria and flagellates in the fishless pond, while the number of bacterivorous ciliates was suppressed by predaceous ciliates. Therefore the bacteria-grazing activity was higher in the absence of planktivorous fish.
Centre for Limnology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, 61117 Rannu, Tartu, Estonia. Electronic address: katrit.karus@emu.ee.
Katrit
K
Karus
Department of Aquaculture, Institute of Veterinary Medicine and Animal Sciences, Estonian University of Life Sciences, 48 Kreutzwaldi Street, Tartu, Estonia.
Tiit
T
Paaver
Centre for Limnology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, 61117 Rannu, Tartu, Estonia.
Helen
H
Agasild
Centre for Limnology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, 61117 Rannu, Tartu, Estonia; Department of Aquaculture, Institute of Veterinary Medicine and Animal Sciences, Estonian University of Life Sciences, 48 Kreutzwaldi Street, Tartu, Estonia.
Priit
P
Zingel
D000818:Animals; D058491:Bacterial Load; D018407:Bacterial Physiological Phenomena; D016798:Ciliophora / Q000502:physiology; D003445:Crustacea / Q000502:physiology; D020387:Food Chain; D010839:Phytoplankton / Q000502:physiology; D011235:Predatory Behavior / Q000502:physiology*; D012487:Salmonidae / Q000502:physiology*; D014871:Water Microbiology; D015048:Zooplankton / Q000502:physiology*
Bacteria; Ciliates; Heterotrophic nanoflagellates; Microbial loop; Planktivorous fish; Predation; Zooplankton