The Blue Planet - Episode 04: Frozen Seas

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Broadcast 4 October 2001, the next episode compares oceanic life in the Arctic and Antarctica. The winter in these regions brings temperatures of minus 50°C and frozen seas that create the biggest challenge. However, there are polynyas in the Arctic, which are free of ice owing to the pressure of currents on either side, and such places do provide refuge for some species, like the walrus and the bowhead whale. A pod of belugas is shown: their movements are limited to a single hole in the ice — therefore putting them at risk of attack from polar bears. Everything changes with the arrival of summer, when melting ice brings a variety of migratory visitors. At the other end of the planet, in the Antarctic, winter is even more harsh, but emperor penguins and Weddell seals stay throughout. Under the sea ice, krill shrink in size and revert to their juvenile form in order to save energy. Chinstrap penguins overwinter to the north, beyond the ice, but return during the spring to breed. Having managed to get ashore, they have to walk a great distance to find a nest site, and the most favoured is Zavodovski Island, an active volcano whose warmth keeps ice from forming. Further south, as the icebergs break up, humpback and minke whales appear, their target the abundant krill. The leopard seal is the Antarctic's top predator. It is most effective underwater, and emperor penguins propel themselves at speed through its territory. Nonetheless, it almost invariably makes a kill.

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