Physical and biological correlates of virus dynamics in the southern Beaufort Sea and Amundsen Gulf

As part of the Canadian Arctic Shelf Exchange Study (CASES), we investigated the spatial and seasonal distributions of viruses in relation to biotic (bacteria, chlorophyll-a (chl a)) and abiotic variables (temperature, salinity and depth). Sampling occurred in the southern Beaufort Sea Shelf in the region of the Amundsen Gulf and Mackenzie Shelf, between November 2003 and August 2004. Bacterial and viral abundances estimated by epifluorescence microscopy (EFM) and flow cytometry (FC) were highly correlated (r(2)=0.89 and r(2)=0.87, respectively), although estimates by EFM were slightly higher (FC=1.08 x EFM+0.12 and FC=1.07 x EFM+0.43, respectively). Viral abundances ranged from 0.13 x 10(6) to 23 x 10(6) ml(-1), and in surface waters were similar to 2-fold higher during the spring bloom in May and June and similar to 1.5-fold higher during July and August, relative to winter abundances. These increases were coincident with a similar to 6-fold increase in chl a during spring and a similar to 4-fold increase in bacteria during summer. Surface viral abundances near the Mackenzie River were similar to 2-fold higher than in the Mackenzie Shelf and Amundsen Gulf regions during the peak summer discharge, concomitant with a similar to 5.5-fold increase in chl a (tip to 2.4 mu g l(-1)) and a similar to 2-fold increase in bacterial abundance (up to 22 x 10(5) ml(-1)). Using FC, two subgroups of viruses and heterotrophic bacteria were defined. A low SYBR-green fluorescence virus subgroup (V2) representing similar to 71% of the total viral abundance, was linked to the abundance of high nucleic acid fluorescence (HNA) bacteria (a proxy for bacterial activity), which represented 42 to 72% of the bacteria in surface layers. A high SYBR-green fluorescence viral subgroup (V1) was more related to high chl a concentrations that occurred in surface waters during spring and at stations near the Mackenzie River plume during the summer discharge. These results suggest that V1 infect phytoplankton, while most V2 are bacteriophages. On the Beaufort Sea shelf, viral abundance displayed seasonal and spatial variations in conjunction with chl a concentration, bacterial abundance and composition, temperature, salinity and depth. The highly dynamic nature of viral abundance and its correlation with increases in chl a concentration and bacterial abundance implies that viruses are important agents of microbial mortality in Arctic shelf waters. (C) 2007 Elsevier B.V. All rights reserved.