Estimates of virus- vs. grazing induced mortality of picophytoplankton in the North Sea during summer

Abstract

Microzooplankton grazing and virally mediated mortality of the eukaryotic (3 size classes) and prokaryotic (Synechococcus) picophytoplankton was investigated across the North Sea during summer 2003. The fate of picophytoplankton differed among algal groups and their area of occurrence. Highest viral lysis rates (0.16 to 0.23 d -1 ) were recorded for specific picoeukaryotic groups in the coastal region and at a station at the deep chlorophyll a maximum (DCM). Viral lysis was insignificant at the other stations, despite a considerable percentage of dead algal cells (up to 38%), as determined by staining with the live/dead dye SYTOX Green. Microzooplankton actively grazed on picoeukaryotes (average per group 0.2 to 0.4 d -1 ), independent of the region studied. Grazing on Synechococcus was restricted to the coastal waters (0.7 d -1 ). Grazing by microzooplankton consumed 40 27% (mean SD) of the carbon produced by picophytoplankton, constituting in general the main loss factor. Viral lysis released 8 13% of the total picophytoplankton carbon production, and locally up to 32%.