Abstract
We examined changes in a microbial community in reaction to shifts in predation pressure and resources in a meso-eutrophic reservoir. Manipulations consisted of size fractionation and either in situ incubation or transplantation from the dam area to the river area, which differed in P-availability. Three treatments, in which samples were incubated for 96 h in dialysis bags, were used: a bacterivore-free (<0.8 µm) treatment, an enhanced flagellate grazing (<5 µm) treatment and a control treatment (all bacterivores present). We monitored bacterial abundance, mean cell volume (MCV), bacterial production, protistan grazing, as well as bacterial community composition (BCC) using fluorescence in situ hybridisation (FISH) with oligonucleotide probes with different levels of specificity. Populations transplanted to the relatively P-rich river area showed a 3- to 6-fold increases in bacterial and heterotrophic nanoflagellates (HNF) abundance and bacterivory compared to the corresponding treatments incubated at the dam area. In the transplanted community, nearly all phylogenetic groups of bacteria distinguished showed increased growth rates, even in the top-down manipulation treatments of increased bacterivory. In contrast, at the more resource-limited dam, the top-down manipulations induced significant changes in bacterial community composition. Thus, we found that BCC changes with predation in resource-limited bacteria but that predation plays a minor role when resource limitation is relaxed.
Highlights
A topic of hot debate in aquatic ecology today is the relative importance of different mechanisms in controlling bacterioplankton stocks and community structure
The data illustrate the difference between the 2 sites with total phosphorus (TP) and dissolved reactive phosphorus (DRP) lower by a factor of 18 and 48, respectively in the dam waters compared to the river site
Samples taken directly from the dialysis bags at the end of incubations showed markedly higher concentrations of TP and DRP in the samples transplanted to the river compared to those incubated at the sampling site, while the bulk of DOC showed no clear trend at both sites (Table 1)
Summary
A topic of hot debate in aquatic ecology today is the relative importance of different mechanisms in controlling bacterioplankton stocks and community structure. Selective protistan bacterivory has been suggested as one of the key factors regulating bacterial community composition (BCC) of natural freshwater assemblages (e.g. Jürgens et al 1999, 2imek et al 1999, 2001b, Langenheder & Jürgens 2001). The interplay of selective and unselective predation is probably quite complex and changes in the grazer community may have varying effects on BCC. The metazooplankton community can alter the size structure and community composition of the protistan grazers Pace et al 1990, Jürgens 1994), which suggests that the entire framework of the top-down forces directly or indirectly shaping BCC may be quite complex The metazooplankton community can alter the size structure and community composition of the protistan grazers (e.g. Pace et al 1990, Jürgens 1994), which suggests that the entire framework of the top-down forces directly or indirectly shaping BCC may be quite complex
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