Predation limitation in the pelagic microbial food web in an oligotrophic aquatic system

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.