olynyas are areas of open water surrounded by sea ice. In the Arctic, polynyas occur in the midst of the thick (>2 m) ice pack that covers the Arctic Ocean and adjacent seas during 9 or 10 of  the 12 months of the year. Polynyas can vary in size from less than a few km² to immense areas such as the North Water that can spread over 50 000 km²  (map of Canadian Arctic polynyas). Some polynyas, such as the North Water, occur at the same time and place each year. Because animals can adapt their life strategies to this regularity, such recurring polynyas are of special ecological significance. In winter, their ice-free waters provide a predictable overwintering ground for marine mammals such as walruses, narwhals and belugas that do not migrate south. In spring, the thin or absent ice cover allows the penetration of light in the surface layer as soon as the winter night ends, which triggers the early blooming of microalgae that are at the basis of the marine food chain. Hence, recurring polynyas are suspected to be focal points for the intense and early production of the planktonic herbivores that ensure the transfer of solar energy fixed by planktonic microalgae to Arctic cod, seals, whales, polar bear and man (simplified Arctic food web). This early spring productivity would explain the concentrations in recurring polynyas of huge populations of feeding, mating and spawning marine birds.

In seasonally frozen seas, the ice cover forming in early autumn prevents the atmospheric carbon dioxide dissolved in the surface waters and assimilated in summer by marine microalgae to return to the atmosphere in winter. Therefore, seasonally ice-covered areas (in particular around highly productive polynyas) could contribute disproportionately to the sequestration of atmospheric CO₂ and the slowing down of global warming. The study and modeling of the role of the North Water in fixing and sequestering carbon will help scientists clarify the role of the Arctic Ocean in climate regulation and forecast its response to climate change.

How do polynyas form?

Polynyas are maintained free of ice by several physical processes including currents, tides, upwellings and winds.

Sensible Heat Polynyas form when deep, relatively warm (~2^oC) water is upwelled towards the surface where it melts the ice cover or prevents ice from forming. The upwelling of warm water can occur when deep water is brought to the surface to replace surface waters that have been pushed away from land by offshore blowing winds. It can also occur when local topography deviates a current towards the surface. In both cases, if sufficient oceanic heat erodes the underside of the ice cover, a polynya will form. Once open, the polynya will rapidly lose heat to the atmosphere but the sustained upwelling of warm water may be sufficient to prevent refreezing.

Latent Heat Polynyas form in areas where ice is transported away by winds or currents as soon as it is formed, thus preventing a consolidated pack from forming locally. In such an ice factory, a tremendous amount of heat is lost from the sea to the atmosphere in the freezing process. As the ice forms, much of the salt content of the freezing water is rejected in the underlying stratum of very cold water. Thus, cold (near-freezing) and salty water of high density is formed, which tends to sink at depth, a process known as the thermohaline circulation.
 

Biological season in sensible and latent heat polynyas

The mechanism by which a polynya or part of a polynya opens will affect the duration and intensity of the biological production taking place within its boundaries.

In sensible-heat polynyas, the input of nearly fresh water resulting from the melt of the ice cover will produce a thin layer of diluted water floating on the more saline and heavier deep water. Once stratified in this way, the water column is no more mixed vertically and the upper layer quickly warms up under the sun. The microalgae growing in this layer become permanently exposed to sunlight and an  algal bloom quickly follows. The growth of plankton animals is stimulated by the warmer waters and the abundant algal food available. The time interval between this early explosion of life in spring and the return of the polar night in autumn may be up to six months.

In latent heat polynyas, the wind that carries the ice away also churns the water column, thus preventing the warming of the surface layer and the growth of algae which are entrained into deeper waters where darkness prevails. The algal bloom will be delayed until the sun warms up the surface layer (thermal stratification) and the season of biological productivity can be several weeks shorter than in a sensible-heat polynya.

In areas surrounding the polynya, the interval between the break-up of the ice cover in mid-summer and the return of the polar night may be as short as 2 months. Hence, the season during which solar radiation reaches the surface layer to trigger algal production and sustain the development of the food web could be as long as 6 months in a sensible-heat polynya, 4 months in the latent-heat polynya and as short as 2 months in non-polynya areas of the Arctic. The comparative study of the response of the Arctic food web to the different regimes of irradiance, temperature and vertical mixing in sensible heat and latent heat areas of the North Water as well as outside of the boundaries of the polynya is a major objective of NOW.