The distribution and abundance of viruses in the Southern Ocean during spring

The concentrations of viruses, bacteria, chroococcoid cyanobacteria and chlorophyll a were determined in surface waters of the Southern Ocean during spring. Viral concentrations declined southward from around 4 x 10(6) ml(-1) near Tasmania to a minimum of around 1 x 10(6) ml(-1) at the Polar Front. South of the Front, virus concentrations increased again, reaching around 4 x 10(6) ml(-1) in the sea-ice zone south of 60 degreesS. Bacterial concentration decreased southwards across the Southern Ocean from around 6.5 x 10(5) ml(-1) near Tasmania to < 1.0 x 10(5) ml(-1) in the sea-ice zone. Cyanobacteria accounted for < 8% of the prokaryotes. There was no significant relationship between viral abundance and either cyanobacterial or chi a concentration. Viral and bacterial concentrations were not significantly correlated north (P {0.10 < r < 0.20}) or south (P {0.20 < r < 0.5}) of the Polar Front. The virus to bacteria ratio (VBR) was between 3 and 15 in the open ocean but varied between 15 and 40 in the sea-ice region. These virus concentrations and VBRs indicate that viruses are no less important in Southern Ocean ecosystems than elsewhere in the world's oceans.