Prokaryotic community structure and heterotrophic production in a river-influenced coastal arctic ecosystem

Spatial patterns in prokaryotic biodiversity and production were assessed in the Mackenzie shelf region of the Beaufort Sea during open-water conditions. The sampling transect extended 350 km northwards, from upstream freshwater sites in the Mackenzie River to coastal and offshore sites, towards the edge of the perennial arctic ice pack. The analyses revealed strong gradients in community structure and prokaryotic cell concentrations, both of which correlated with salinity. Picocyanobacterial abundance was low (10(2) to 10(3) cells ml(-1)), particularly at the offshore stations that were least influenced by the river plume. Analysis by catalyzed reporter deposition for fluorescence in situ hybridization (CARD-FISH) showed that the dominant heterotrophic cell types were beta-Proteobacteria at river sites, shifting to dominance by alpha-Proteobacteria offshore. Cells in the Cytophaga-Flavobacter-Bacteroides and gamma-Proteobacteria groups each contributed <5% of total counts in the river, but >10% of counts in the marine samples. Archaea were detected among the surface-water microbiota, contributing on average 1.3% of the total DAPI counts in marine samples, but 6.0% in turbid coastal and riverine waters. H-3-leucine uptake rates were significantly higher at 2 stations influenced by the river (1.5 pmol l(-1) h(-1)) than at other marine stations or in the river itself (<= 0.5 pmol(-1) h(-1)). Size-fractionation experiments at 2 coastal sites showed that >65% of heterotrophic production was associated with particles >3 mu m. These results indicate the importance of particle-attached prokaryotes, and imply a broad functional diversity of heterotrophic microbes that likely facilitates breakdown of the heterogeneous dissolved and particulate terrestrial materials discharged into arctic seas.