High control of bacterial production by viruses in a eutrophic oxbow lake

The aim of the study was to test the hypothesis that the magnitude of viral control on bacterial production in a eutrophic oxbow lake of the River Danube would be higher than all average values reported so far in the literature, This assumption was based on the findings of low grazing of heterotrophic nanoflagellates (HNF) in this system, accounting on average for 5 % of the bacterial mortality. Several approaches (viral decay method, estimation of the frequency of infected bacterial cells) to determine viral control of bacterial production were applied on a comparative basis. All system-specific parameters necessary to describe virus-bacteria interactions (burst size, bacterial production, contact rates) were monitored simultaneously. The average viral control of bacterial production determined by the different approaches was similar, ranging from 55.7 to 62.7 %, and prevailing over HNF grazing by a factor of more than 11, For individual events, however, we observed large variations between the methods, indicating that the use of one single method is not reliable to decide whether a detected trend is representative of a specific system. We discuss error sources of the applied methods and mathematical models, and accounted for them when calculating the contribution of viruses to bacterial mortality, We demonstrated that viruses could control more than 100 % of the bacterial production in the Alte Donau, which implies that occasionally up to 1.6% h(-1) of the bacterial standing stock was removed from the water column, High bacterial mortality due to viruses indicated that a large amount of dissolved organic carbon might be recycled from bacteria by phage-induced cell lysis. On average 15.2 mug C l(-1) d(-1), corresponding to some 46% of the bacterial secondary production (BSP), was released into the water column due to viral lysis of bacterial cells and again became available for microheterotrophic consumption.