This thesis aims to give an insight into the ecology of the viral community in a shallow eutrophic lake. To achieve this, the population dynamics, diversity and control of the viral community in Lake Loosdrecht were studied, as well as the impact of the viral community on plankton mortality and community composition. A seasonal study of Lake Loosdrecht revealed the virioplankton community as a dynamic component of the aquatic community, with abundances ranging between 5.5 x 107 and 1.3 x 108 virus like particles ml-1 and viral genome sizes between 30 and 200 kb. Viral community dynamics followed a distinct seasonal cycle, and were related to Chl-a concentrations, prochlorophyte numbers and cyanobacterial community composition, suggesting that a significant proportion of the viruses in Lake Loosdrecht may be phytoplankton and more specific cyanobacterial viruses. Temporal changes in bacterial abundances were significantly related to viral community assemblage, and vice versa, also suggesting an interaction between viral and bacterial communities in Lake Loosdrecht. During a series of enclosure experiments viruses were indeed identified as the major cyanobacterial mortality cause, when the drastic collapse of the dominant filamentous cyanobacterial population was accompanied by high percentages of virally infected cyanobacterial cells and increasing viral numbers. The viral lysis of the cyanobacterial bloom was accompanied by changes in both cyanobacterial and heterotrophic bacterial community composition, indicating that viral infection in Lake Loosdrecht not only has the potential to control cyanobacterial blooms, but also has a marked impact on the structure of the plankton community. To simultaneously estimate the impact of both viral lysis and zooplankton grazing on the mortality of bacteria, algae and cyanobacteria, an adapted version of the dilution technique was applied on Lake Loosdrecht water. Viral lysis was identified as the main mortality source during winter experiments, removing between 84 and 97% of the potential filamentous cyanobacterial production and up to 101% of the potential bacterial production. Viral impact on plankton mortality varied strongly between different experiments however, and care should be taken when interpreting results and to optimize the dilution technique for application on a variety of plankton groups. Loss mechanisms and rates of viruses in eutrophic lake water were estimated by following the loss in infectivity of cyanophage LPP-1 in Lake Loosdrecht water. Viral decay rates in the lake ranged between 0.85 and 0.98 d-1, corresponding to turnover times of 1.0 and 1.2 days. The addition of HMW-DOM resulted in a significant decrease in viral decay rate, suggesting that the abundant HMW-DOM in the lake prolongs viral infectivity. Eukaryotic organisms smaller than 10 m were found to be responsible for 37 to 55 % of the daily viral decay, thus implying that grazing by heterotrophic nanoflagellates was the major viral decay source in Lake Loosdrecht. This study thus identified the viral community as a dynamic and active component of the foodweb in shallow eutrophic lakes, which should therefore not be ignored in future ecosystem research. More specific methodology for virus detection is needed however to further unravel the virus-host interactions in these lakes.
|Award date||19 Sep 2007|
|Place of Publication||Utrecht|
|Publication status||Published - 19 Sep 2007|