ABSTRACT: The importance of predation as a limiting factor for net growth rates of organisms within the pelagic microbial food web was studied in a truncation experiment performed at a coastal station in the northern Baltic Sea. To remove potential predators, seawater was fractionated into 4 size groups: <0.8 µm (bacteria); <5 µm (bacteria + small flagellates + small phytoplankton); <10 µm (bacteria + flagellates + phytoplankton); and <90 µm (bacteria + flagellates + phytoplankton + ciliates). The samples were incubated in situ in dialysis bags with a cut-off of 12 to 14 kDa, allowing nutrients and macromolecules to pass in and out of the incubation bags. The development of the plankton community was followed over 8 d. Heterotrophic bacteria and flagellates were found to be predation-limited, as removal of grazing increased their initial net growth rates from 0 to 0.5 and 0.4 d^-1, respectively. Picoeukaryotic autotrophs increased their net growth rates from 0 to 0.6 d^-1 when flagellates and ciliates were removed. Other phytoplankton and ciliates did not show any initial response to predator exclusion, indicating that they were not predation-limited. The main trophic links within the microbial food web seemed to be from heterotrophic bacteria to small heterotrophic flagellates, from small heterotrophic flagellates and autotrophic picoeukaryotes to intermediate protozoa (medium-sized flagellates and small ciliates) and from intermediate protozoa to large protozoa (large flagellates and large ciliates). Removal of predators caused no quick (<1 d) indirect response in the form of trophic cascades. The data indicate omnivory among flagellates and ciliates. A model of the microbial food web is presented, showing the main trophic links and interconnection between autotrophic and heterotrophic organisms.

KEY WORDS: Bacteria · Flagellates · Ciliates · Phytoplankton · Predation limitation · Growth rates · Direct and indirect effects · Trophic links

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Published in AME Vol. 30, No. 3 (2003) on January 23
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2003