Modified dilution technique to estimate viral versus grazing mortality of phytoplankton: limitations associated with method sensitivity in natural waters

This study tested the validity of the modified dilution method approach to partition natural phytoplankton mortality into grazer versus virus-induced fractions. A series of experiments was conducted weekly between August and October 2005 from an offshore site in the western English Channel. The phytoplankton community during this period was dominated numerically by Synechococcus spp. and picoeukaryotes. Despite highly significant regression results, no discernible viral lysis of Synechococcus spp. or picoeukaryote populations was apparent during the 3 mo period. Microzooplankton grazing was the major source of mortality for Synechococcus spp. and picoeukaryotes during this time. Statistical analysis indicates that, using our experimental design, the modified dilution approach was not sensitive enough to be able to measure significant viral lysis during the experimental period. Sensitivity analysis indicated that viral mortality occurred at rates of <0.1 d(-1). Methodological impacts on phytoplankton growth rates may also have affected the ability to measure virus-induced mortality of picoeukaryotes, as there was increasing suppression of picoeukaryote growth rate in relation to dilution with the 30 kDa filtrate and a corresponding depression of quantum photosynthetic efficiency (F-v/F-m) in the 30 kDa dilutions compared to the 0.2 mu m series. The implications and limitations of the dilution approach for estimating viral versus grazing mortality of natural phytoplankton populations are discussed.