Low control of bacterial production by heterotrophic nanoflagellates in a eutrophic backwater environment

In an investigation in the backwater system of the Alte Donau (Vienna, Austria) on the impact of heterotrophic nanoflagellates (HNF) as bacterivores and on their role in the pelagic carbon cycle, it was shown that from April 1995 to dune 1996, HNF abundance ranged from 0.2 x 10(2) to 39.2 x 10(2) cells ml(-1), with a yearly mean of 5.8 x 10(2) cells ml(-1). Mean HNF biomass was 7.4 mu g C l(-1), ranging from 0.1 to 41.1 mu g C l(-1). Ingestion rates on heterotrophic bacteria, estimated on the basis of uptake of fluorescently labeled bacteria, were 2.2 to 26.5 cells HNF-1 h(-1) (mean: 7.5 cells HNF-1 h(-1)). Resulting grazing rates only made up between 0.3 and 20 % (mean: 5 %) of bacterial secondary production, and it was evident that heterotrophic nanoflagellates were not able to control bacterial production, indicating that factors other than HNF grazing must play an important role for the mortality of bacteria in the investigated system. A consequence of the fact that only a mean of 0.09 mu g bacterial carbon l(-1) h(-1) was consumed by HNF was that transfer of picoplanktonic carbon to higher trophic levels via HNF was negligible. HNF could not function as a link between the microbial compartment and the classical pelagic food chain, even if nanoflagellates themselves were under strong predatory control, which was indicated by experiments conducted to compare flagellate gross and net growth rates. Another finding was that HNF covered only a mean of 21 % of their carbon demand by the consumption of bacteria, despite high abundance of bacterial carbon as a food source. The error potential of the applied methods was estimated in view of the carbon budget of the bacteria-HNF interaction. This still implied that HNF had to use other carbon sources to a remarkable degree in order to maintain production. The possibility that dissolved organic matter (DOM) may represent a major energy source covering HNF carbon demand is discussed.