The Arctic Ocean represents a complex system of currents with influx and efflux of water (see Figure 5-1).  Cold and relatively less saline water enters the Arctic Ocean through the narrow Bering Strait between Alaska and Siberia, while warmer, more saline surface waters from the Atlantic penetrate the Arctic Ocean and are cooled as they move through the Greenland Sea and the Norwegian Sea. Water reaching the Arctic Ocean basin is swept into a huge circular current — driven by strong winds — the Beaufort Gyre. Siberian and Canadian rivers drain into the circular marine current to create a captured internal reservoir of relatively fresh water. Periodically, the circular current weakens, allowing large volumes of fresh water to leak out and cross the Arctic in the Transpolar Current. The water exits the Arctic Ocean via several “gateways.” It can flow through the Fram Strait, between northeast Greenland and Svalbard Island, and then branch around either side of Iceland. It can flow around the west side of Greenland through Baffin Bay and out Davis Strait. It can also flow through a maze of Canadian islands and out Hudson Strait. These mixes of salty and fresh water also generate Arctic haloclines, a vertical effect in which the cold fresh water lies atop warmer saltier water.

Figure 5-1. Prevailing currents in the Arctic Ocean (Source: Woods Hole Oceanographic Institution; http://www.divediscover.whoi.edu

The transport routes carry microorganisms into the Arctic from a complex mixture of seawater and river sources, with the North-Atlantic current (number 6 in Figure 5-1) as the single most important influx transport route. This results in cosmopolitanism of microorganisms in the Arctic Ocean and the presence of sometimes unexpected microorganisms in this cold marine environment (Hubert et al. 2009).

Significant to the discussion of currents, water masses, and effects on microbial population structures, a considerable part of the Arctic consists of one- or multi-year ice coverage. Since marine ice is frozen seawater the microbes frozen into the ice are transported with ice movement. This movement essentially follows two routes; the Beaufort Gyre circulating clockwise around the North Pole and the Transpolar Drift Stream, where ice moves from the Siberian coast of Russia across the Arctic basin, exiting into the North Atlantic off the east coast of Greenland (number 5 in Figure 1).  During ice growth, brine pockets are generated in the ice, since ice crystal generation is almost devoid of impurities (Petrich and Eicken 2010).  Brine accumulations are first generated as pockets, and later as chimney-like tubes termed brine channels. Microbes require a fluid environment for active metabolism, and these brine channels represent a fluid microenvironment within the ice mass. These channels therefore become important ecological niches for microbes which are able to survive in the high salinity and subzero temperatures in these systems. Eventually, contact with seawater will result in nutrient transport and reduced salinity in the channels. If channels are wide enough (approximately 2 mm in diameter), they are able to support counter-flow, i.e. simultaneous upward and downward movement inside the same channel (Lake and Lewis 1970).  Despite the potential for flow and mixing with bulk marine water, brine salinity tends to increase with distance to the seawater.   These salinity gradients in brine channels may result in gradients of microbial population density.  In winter, the apparent effect is higher bacterial and algal concentrations in the lower than the upper parts of the ice (Gradinger and Zhang 1997). However, in late spring and summer with rising air temperatures, the opposite situation is observed. The ice is warmed up from the top, becoming more porous, and melt water penetrates the porous ice and generates a linear bulk salinity profile that is commonly observed in the upper 20 to 50 cm of the ice during melt. The organisms in the brine channels are therefore exposed to fluctuating and physiologically challenging salinity conditions during the periods of ice freezing and melting (Faksness et al. 2011). Since much of the brines are drained off the ice during the first year after freezing, multi-year ice tends to have lower salinity and density than first-year ice (Tang et al. 2007), resulting in fewer brine pockets and also fewer possibilities for active microbial metabolism in the ice.

The population structures of bacteria in Arctic seawater are comparable to those in seawater from temperate regions, with the predominance of Alphaproteobacteria, Flavobacteria/Bacteroidetes,Gammaproteobacteria and Verrucomicrobia constituting more than 90% of the communities (Comeau et al. 2011; Teske et al. 2011; Ghiglione et al. 2012). It is also relevant to compare the microbial communities in the Arctic and Southern Oceans, since the two geographical areas have similar climatic conditions. Although some comparative studies of the microbial communities in Arctic and Antarctic marine environments indicated a high degree of resemblance (e.g. Bano et al. 2004; Brinkmeyer et al. 2003), a recent pole-to-pole study of surface and deep marine bacterial communities revealed significant differences, with 78% of operational taxonomic units (OTUs) unique to the Southern Ocean and 70% unique to the Arctic Ocean. Despite these dissimilarities, polar ocean bacterial communities were more similar to each other than to lower latitude pelagic communities (Ghiglione et al. 2012).

At a more localized scale, sea ice microbial communities may differ from those in the surrounding seawater. A study of first-year Arctic ice showed differences between ice and source seawater populations that were characterized by an increased abundance of Gammaproteobacteria and a lower abundance of Bacteroidetes in sea ice relative to seawater.  Winter sea ice communities were more similar to autumn water communities; species-specific die off processes were not observed in sea ice.  The implication was that winter sea ice communities are stable, whereas seawater communities undergo seasonal succession. The Alphaproteobacteria dominated in both environments (Collins et al. 2010). A study of multi-year ice revealed overall dominance of Gammaproteobacteria, as well as an increase ofBacteroidetes in the sea ice when compared to source seawater; the abundance of Alphaproteobacteria was greatly reduced in sea ice relative to the abundance in surrounding seawater (Bowman et al. 2012). A number of studies have also investigated the microbial communities in Arctic sediments, which may be regarded as pristine environments compared to most other sediments investigated. Studies from an Arctic fjord (Svalbard) showed that the Alphaproteobacteria predominated in the overlying seawater, while Gammaproteobacteria were more abundant in surface sediments. Deeper anoxic sediment layers were dominated by Deltaproteobacteria, which include common genera of strictly anaerobic bacteria associated with marine sediments (Ravenschlag et al. 2001; Teske et al. 2011).