Estimates of bacterial and phytoplankton mortality caused by viral lysis and microzooplankton grazing in a shallow eutrophic lake

1. Since viral lysis and zooplankton grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both mortality factors. In this study, an adapted version of the dilution technique was applied to simultaneously estimate the impact of both viral lysis and zooplankton grazing on the mortality of heterotrophic bacteria, eukaryotic algae, unicellular cyanobacteria, prochlorophytes and especially filamentous cyanobacteria in a shallow eutrophic lake. 2. Four dilution experiments were performed in December 2004, January 2005, and March and April 2006. Viral and heterotrophic bacterial abundances were obtained by epifluorescence microscopy and abundances of different phytoplankton groups by flow cytometry and light microscopy. 3. Viral lysis was identified as the main mortality cause during the December 2004 and January 2005 experiments, apparently removing between 84% and 97% of the potential filamentous cyanobacterial production and up to 101% of the potential heterotrophic bacterial production. Microzooplankton grazing was estimated to remove between 90% and 99% of the potential unicellular cyanobacterial production and up to 46% of the potential heterotrophic bacterial production during the spring 2006 experiments. 4. In some cases, no significant impact of viral lysis or zooplankton grazing was detected. Contrary to expectations, the apparent growth rate of filamentous cyanobacteria was even sometimes observed to decrease significantly upon dilution of microzooplankton. 5. The dilution technique can give valuable insight into the impact of zooplankton grazing and viral lysis on the mortality of different plankton groups but require some caveats require special care to be taken when comparing and interpreting results.