Introduction to ecosystems
Introduction to ecosystems

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Introduction to ecosystems

3.2.2 Life in the polar seas

Life in the polar sea ice forms part of a web of interactions, which Dr Mark Brandon discusses with Brett Westwood as he considers the tiny life trapped in the sea ice that is the foundation for the entire food chain at the poles.

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Life in the polar seas

INTERVIEWER
Mark, ice doesn't behave the same way at the different poles, does it? So how does it vary?
DR. MARK BRANDON
Well, one of the things that affects the ice most of all is the basic geography. In the Arctic, you've got land and then a deep ocean in the middle. Whereas in the Antarctic, you've got land in the middle and deep ocean around the edge. If you go back to the Arctic, in winter, all of that Arctic Ocean gets frozen over with what we call sea ice, which is a very thin layer of ice, only perhaps two or three metres thick. It extends out - about 15 million square kilometres of the ocean gets covered by this sea ice. And it is a bit like if you think of polystyrene floating on the sea, it gets blown about by the wind. So the sea ice is constantly moving and constantly drifting around and grinding up. So if you hear about ice being thicker that three metres, it's usually two ice flows of three metre thickness that bumped into each other, one on top of the other.
INTERVIEWER
So all this floating ice in the Arctic, it does collide, I would imagine. It forms ridges, does it?
DR. MARK BRANDON
It does. And if you actually go online, there's the International Arctic Ocean Buoy Project, IOEB. And you can actually look at the drift tracks of buoys that are actually out there right now, sending back weather data from the Arctic Ocean, that drift with the sea ice. and so you can look at these fantastic movies of how the ice drifts.
INTERVIEWER
How does ice behave in the Antarctic, not in the same way then?
DR. MARK BRANDON
Well, the seasonal change between the Arctic- in the Arctic, about 2/3 of the ice disappears between the summer and the winter. In the Antarctic, almost all of the ice disappears in Antarctic summer.
INTERVIEWER
Where does it go to?
DR. MARK BRANDON
It melts away. So there are only a couple of small areas, mainly the Ross Sea and the Weddell Sea and quite close to the coast. Because when I say most of the ice disappears, there's about 2 million square kilometres of ice still in the Antarctic. But compared with Antarctic winter, that 2 million square kilometres of ice grows another 15 million square kilometres. So it really is just the remnants of a vast amount of ocean covered by ice.
INTERVIEWER
And for the permanent ice, we're talking about some incredible thicknesses as well, aren't we, in the Antarctic particularly?
DR. MARK BRANDON
Well, if you look at Antarctica, it's a continent. It's land. And then snowfall, over millions of years, gets compressed and turned into ice. And individual layers of this snow build up thickness of ice. And this started happening, this snowfall, maybe 35 million years ago. And the thickness of ice now, on East Antarctica, the east part of Antarctica, is about three kilometres thick. And that's made entirely of snow. So it's fresh ice. It's the sort of stuff that, when it reaches the edge of the continent, this land ice, it can either form an ice shelf, which is a large, thick shelf of ice, perhaps 200 or 300 metres thick. Or the glaciers can fall straight into the sea and form icebergs. Whereas the sea ice is just frozen seawater, and so anything that's in the sea water gets trapped within the ice. And it's quite a porous thing, sea ice, compared with land ice, because it's formed from ice crystals growing in the water, rather than snowfall being compressed.
INTERVIEWER
So there's life within this sea ice.
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Seawater freezes at −1.9° C, but because of the anomalous relationship between the density and temperature of water, ice floats, insulating the water underneath from the cold air above. Except in very shallow areas, the sea-ice does not extend to the sea-bed, even at the North Pole. Storms and currents sometimes break up the ice, creating many temporary, and some permanent, areas of open water even at high latitudes in mid-winter. Such turbulence also oxygenates the water and admits more light, making the environment much more hospitable to larger organisms.

The movements of ocean currents are complex and may change erratically from year to year. This often results in an upwelling of deep water rich in nutrients and promotes high primary productivity in the sea. In most arctic regions, the sea is both warmer and more productive than the land. So at high latitudes there are many more organisms in the sea than on land, at least during the brief summer, and, as in the case of the baleen and sperm whales, some are very large.

Krill

You heard previously how Dr Mark Brandon and colleagues studied krill under the sea ice. In this video scientists from the British Antarctic Survey (BAS) are trawling for krill and sorting them for later analysis – some task!

As you watch listen out for answers to the following questions:

  • What is the role of krill in the Antarctic food chains?
  • How do the food chains in the polar seas compare with those introduced earlier by Professor David Streeter in the oak wood?
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Krill

DR. DAVID ROBINSON: This huge net is being used to trawl the polar seas for krill, as part of a research programme. In the folds at the bottom of the net are thousands of these prawn-like crustaceans. Krill are a key component of the main food web in the southern oceans.

MALE SPEAKER 1: Do you want me to--

MALE SPEAKER 2: In the net too.

MALE SPEAKER 1: Yeah.

MALE SPEAKER 2: I think there'll be more water on deck tomorrow morning.

DR. DAVID ROBINSON: They're an essential source of food for numerous animals, from fish and birds to the largest of the whales, the blue whale. In recent years, krill have even been harvested commercially.

FEMALE SPEAKER: Yeah.

MALE SPEAKER 3: Is there anything on that or is it just a surface layer of krill?

DR. DAVID ROBINSON: Sorting the sample is a painstaking task, as the scientists pick out individual krill to place them in trays for later analysis.

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