Viral lysis and microzooplankton grazing of phytoplankton throughout the Southern Ocean

We investigated microzooplankton grazing and viral lysis of the pico- and nanophytoplankton community in the Southern Ocean during the “Sensitivity of Sub-Antarctic Zone Waters to Global Change” cruise to the Australian sector (January–February 2007) and the Antarctic–XXIV/3 (February–March 2008) expedition to the Greenwich Meridian and the Drake Passage. A dilution assay was employed in concert with flow cytometry at stations that spanned the sub-Antarctic, Polar Frontal, and Antarctic Zones and in the Weddell Gyre. Both viral lysis and microzooplankton grazing played a significant role in controlling cyanobacterial populations, except in the sub-Antarctic west of Tasmania, where their growth was regulated by grazing alone. Three main groups of eukaryotic algae were detected, and these were calculated to have average cell diameters of 0.8, 2.1, and 3.7 µm. Grazing was significant in the control of their growth at all locations, whereas viral lysis of eukaryotic algae was comparatively minor, as it was detected in only 40% of the experiments. Throughout the surveyed sites no pattern between geographical location and growth rates and/or mortality rates could be established for any of the phytoplankton groups. Estimates of iron regeneration indicated that as a result of its widespread occurrence, microzooplankton grazing is a more consistent source of regenerated iron to the Southern Ocean but that viral lysis can be responsible for significant pulses of iron into the dissolved pool