Polar Regions are often thought of as cold, desolate expanses; blasted by wind and devoid of life. In some areas this may be true but both the Arctic and Antarctic support great diversity of physical and biological importance. They contain mountain ranges, glaciers, desert-like areas and sheltered coves. An important feature at both poles is the existence of polynyas which can be seen as ‘oases of life’ in these extreme environments.
The aim of this blog is to show that the existence of polynyas is vital to the polar trophic web, focusing on their importance to marine mammals (Figure 1) and using specific case studies from both poles to address this. It will conclude by summarising polynya importance and threats that they face, which could be detrimental to the survival of many marine mammal species.
What is a polynya?
Polynyas are areas of persistent open water, surrounded by thick pack ice, which given the surrounding climatology should be frozen (Martin, 2001). These open water areas can be present annually or seasonally, usually in spring. However, in exceptionally cold years, they may become frozen (Gilchrist and Robertson, 2000), affecting species population levels which rely on them. They vary in size from a few kilometres to 85,000km2, and are biological hotspots with early or enhanced primary production (Stirling, 1997; Barber et al., 2010). Due to this productivity, aggregations of marine mammals (Figure 2) are often found in and around polynyas.
How are they formed?
Polynyas (Figure 3) form due to local conditions, including tidal currents (eddies, gyres and upwellings), fast moving surface currents and localised winds (cyclones) (Jacobs, 1998). The water remains unfrozen due to processes which prevent sea-ice formation or remove ice from the area as it forms. Where polynyas occur and sea-ice is formed, salt is expelled increasing the salinity and density of the water (Martin, 2001). Increasing density causes the water to sink which influences thermohaline circulation (Martin, 2001). There are two types of polynyas, coastal and open-ocean, which differ in formation (Williams et al., 2007).
These form adjacent to a land mass or pack ice usually on the leeward shore, where persistent winds move the pack ice away from the coast (Martin, 2001). New ice is removed as fast as it forms, such as North Water polynya where currents reach 10-30 cm/s (Fissel et al., 2012). These coastal polynyas usually reach lengths of 500km (Martin, 2001) and are called latent heat polynyas, as the heat lost from the ocean goes into ice formation. In the Antarctic, these polynyas contribute to globally important water masses (Martin, 2001), such as Antarctic Deep Water which plays a major role in thermohaline circulation.
Open ocean polynyas
These are called sensible heat polynyas and are less common. They form as a result of warm water upwellings, transferring their heat to surface waters causing sea-ice to melt (Melling et al., 2001). These polynyas are self-maintaining as the heat lost from the surface waters into the atmosphere, cools the water, causing it to become dense and sink, continuing convection (Martin, 2001).