The Polar Regions

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The differences in the physical ocean and the ocean circulation between the Arctic and the Antarctic.

Let us take the differences in the physical oceans first. The Arctic Ocean is surrounded by land with few openings to the wider ocean. The Antarctic is open to the major oceans of the world. The Arctic is one of the oldest ocean basins in the world. The Antarctic was formed with the breakup of Gondwanaland. The Arctic is one of the smallest oceans whilst the Antarctic is over 35 million km square. The Arctic Ocean has an average depth of 1000m with some shallow areas such as the Chakchi Sea and broad continental shelves.

The Antarctic is deeper with narrow continental shelves.The Arctic produces smaller ice bergs on average than the Antarctic. Seasonal Arctic ice cover is usually thicker than the thinner ice found in Antarctic due to more ocean influences in the south. The Arctic has more freshwater run off entering it from surrounding rivers and hence a siltier ocean floor than Antarctica. There is heavier snowfall in the Antarctic As far as circulation is concerned we see that ocean circulation is much more restricted in the Arctic than the Antarctic.. In the Arctic 80% of water flows through the Greenland Sea and 20% through the Bearing Straits. There are two major ocean circulation patterns in
the Arctic with the Beaufort gyre over the Canadian Basin and the current which flows through the Fram Strait.

In contrast in the Antarctic there is a much less restricted circulation systems. In fact the Antarctic has the world's largest ocean current - the Antarctic Circumpolar Current flowing in east. This current marks the boundary with the warmer oceans to the north such as the Indian and Pacific in an area known as the Antarctic Convergence Zone. However, south of latitude 60 degrees South the current turns west as it flows around the continent of Antarctic itself with notable eddies in the Ross and Weddell Seas. To add complexity to the ocean circulation systems the Antarctic has a deep water circulation system with the cold Antarctic Bottom Water moving north. We shall see later that these circulation systems are vital in areas of upwelling for marine life.

Arctic and Antarctic ice cover

There are several key differences in Arctic and Antarctic Ice Cover. Firstly there is the difference in extent of this cover. In the Arctic we see an approximate range of cover from 15 million square kilometers at maximum to 7 million at minimum. In Antarctica we see a range between 18 million square kilometers at maximum and 3 million square kilometers at minimum.

In the Arctic there is an year round permanent covering of ice in the central area surrounded by pack ice. This pack ice is persistent rather than permanent. Because the Arctic Ocean is relatively landlocked in comparison to the Antarctic there is less movement and the ice becomes thicker than at the Antarctic where the ice is freer to float northwards and melt. Also there is less seasonal variation in extent of ice cover in the Arctic as we noted above because of this relative lack of movement. In addition in the Arctic there is less snow fall but in the Antarctic the ice can be covered by deep snow. Finally there are notable differences in the size and form of icebergs between the Arctic and Antarctic. The Arctic has smaller icebergs which can possess a variety of colors depending on sediments and soils. Icebergs in the Antarctic tend to be much bigger and tabular in shape. The Arctic ice cover also seems to be more influenced than the Antarctic by global warming so far.

The characteristics of polar organisms and the effects of ice on life.

There are several main characteristics of polar organisms. Firstly many polar species have variable growth rates with very slow rates in winter and faster rates in summer, usually due to increases in primary production (blooming phytoplankton) and subsequent availability of food. Some species have generally lower metabolic rates. There are also relatively many long lived species perhaps correlated to this slow growth rate. e.g. Bowhead Whales in the Arctic and Antarctic sponges. Size is perhaps another characteristic of polar organisms with either a tendency to gigantism or dwarfism. For example polar bears, seals and walruses are large animals and some bivalve species and sponges grow to large size in the polar regions.

In contrast certain gastropod molluscs in the Antarctic are tiny with thin shells which take less energy to develop. Other methods of dealing with the immense cold in both regions are varied. Some animals have developed blubber e.g whales, walruses and others thick fur e.g. polar bear. Other species use biological antifreeze (glycopeptides) or have other blood and organ adaptation e.g. Ice Fish ( Channichthyidae sp.) The effects of ice on life are usually negative in that ice is a great destroyer as it scours and freezes. In both polar regions most life has to be mobile as the ice does not allow permanent shelter. However there are exceptions to this. Many organisms live in association with ice e.g. ice algae. There are over 200 species of bacteria, algae and unicellular organisms living in the ice in Antarctica. Krill and other planktonic life such as larval fish may graze on this algae beneath the ice.

In this sense the ice actually protects some species. Also Antarctic ice is generally thinner than Arctic ice so more light will penetrate for photosynthesis. In addition a covering of ice may protect the waters beneath and also the substrate. There are more benthic species in the Antarctic than Arctic. The Arctic specializes in burrowing or infauna species whereas the Arctic in general has more species dwelling on the surface of the substrate (epifauna). Ice can also act as a shelter in certain circumstances for larger forms of life such as the Ringed seals who excavate small caves to protect their pups from polar bear predation. Therefore we can conclude that although ice is usually negative toward life there are many exceptions to this in both polar regions.

Reproductive strategies of polar organisms.

Polar organisms adopt several different reproductive strategies. These can be loosely grouped into 1) variations in method of reproduction 2) timing 3) speed of reproduction and 4) location of reproduction.. Let us look at variation is methods of reproduction first. Most benthic species produce large numbers of planktonic larvae but in the polar regions they follow Thorson's rule with non pelagic development and brooding of young. This seems mainly a response to food availability. As an example, of the 66 species of bivalve only two show planktonic reproduction in the polar regions. However, when food becomes more available they can switch to production of planktonic larvae. This lead us to the second point about timing. In addition some species produce eggs with large yolks (lecithotrophic) 2) Many animals time egg production to occur with planktonic blooms in spring and summer.

Higher up the food chain Polar bears give birth to cubs in the winter so when spring comes they are more ready to exploit feeding opportunities. Most penguins breed in summer months but Emperor penguins breed in winter and chicks become independent just as the Antarctic summer begins again to exploit feeding options. 3) Speed of production is another reproductive strategy. Some species have very slow reproduction e.g. Aegia antarctica takes 10 years to develop sexual maturity and embryonic development takes 20 months. 4) Location of reproduction is also a variable. e.g. Humpback whales breeding in the warmer ocean waters and Euphausia superba releasing eggs in deep ocean areas so as they rise they will find food at a later stage. These are just some of the reproductive strategies adopted in polar areas.

Dr Simon Harding