The physical nature and biological implications of the polar front A value and vulnerability assessment of the polar front in the Barents Sea

Photo: Rudi Jozef Maria Caeyers, Norwegian Polar Institute

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The polar front is defined as the boundary in the Barents Sea which divides relatively warm, saline water masses of Atlantic origin and colder, fresher water masses of Arctic origin. This front is particularly distinct not only around Spitsbergen Bank and the Hopen Deep, but also around Central Bank and to some extent eastwards in the Barents Sea. Due to the physical characteristics and implications for biological activity, this area is identified as a particularly valuable and vulnerable area. This report summarises the physical characteristics of the front, and its variation in time and space. A summary is also presented of the biological activity in the polar front region and its value and vulnerability.

The report presents a more detailed overview of the structure of the polar front than has been given in previous summaries, and notes that there are three fronts around Spitsbergen Bank and the Hopen Deep, all of which have different origins and structures, and therefore also different impacts on biological production and activity. High up on the flank of Spitsbergen Bank ( depth ~50-100 m) is a front which is caused by a combination of shallow depths and strong tidal currents. This front was therefore formerly referred to not only as a tidal front, but also a summer front, as the hydrographic differences that define the front itself are mostly present only during the summer months, even though the driving forces themselves which maintain the front are present at all times. This front helps to make parts of Spitsbergen Bank a "hotspot" with regard to biological production. As this front is determined by water depth and tidal currents, it is predictable and relatively easy to map. The polar front proper, which separates Arctic waters from Atlantic waters, is typically located along the ~200 m isobate around the entire the Hopen Deep, and during the summer is identifiable from a depth of approximately 50 metres and down towards the bottom. This front is dynamically "passive" and therefore does not contribute to any increase in primary production. However, the polar front acts as a habitat limit for some species, and it can also act as an aggregation area for different species. It is closely linked to topography and therefore relatively easy to map, and it is present all year round, with relatively minor variations both during the year and between years. In addition to the two fronts referred to above, there is a meltwater front in the surface layer which is present during the spring and summer. This front is typically no more than 50 m deep and may be linked to the polar front, but not necessarily. This front is more volatile and therefore harder to map on a general basis, as it depends on the melting of sea ice and thus the ice cover during the previous winter. Like the tidal front, this front is also important for the dynamics of the spring bloom, with subsequent cascade effects further up the food chain. Overall, the frontal system which is normally referred to under the collective term ‘polar front’ is an important area for various species at different trophic levels at different times of the year. During the winter, the polar front act as an overwintering and spawning ground for various fish species. The area in general, and Spitsbergen Bank in particular, is an area with high levels of biological activity as a result of high primary production during the spring, and is therefore also an important feeding ground for seabirds. Throughout the summer, the whole area also forms part of the feeding grounds used by fish. Furthermore, both seals and whales migrate here to feed during the summer, although it has not been possible to document large accumulations over time.