18 July 2008

Warming makes icebergs scour ocean floor

Cosmos Online
Global warming will cause more icebergs to grind against the sea floor, affecting the rich biodiversity on the Antarctic seabed. But it's not yet clear how these ice scours will affect marine life.

Sea floor scourer: Sculptured iceberg in North Bay, Rothera Point, Adelaide Island, Antarctica. Credit: Pete Bucktrout/British Antarctic Survey

SYDNEY: Global warming will cause more icebergs to grind against the sea floor, affecting the rich diversity of life found on the Antarctic seabed. But it’s unclear whether this will be a good thing or not, as little is known about how these ice scours affect marine life.

The research, conducted by the British Antarctic Survey (BAS), is detailed today in the U.S. journal Science.

Ice scour is the grinding of the seabed by the bottom of icebergs broken off from sea ice. Thought they float on water, the sheer weight of the icebergs causes them to gouge out troughs, similar to the way glaciers carve out valleys on land. Ice scour affects one-third of the world’s coastlines and is a major influence on the diversity of life on the sea bed.

First link detected

The researchers, led by Dan Smale at the BAS in Cambridge, England, tested the effect of scours by cementing a grid of concrete markers into the sea bed at different depths off the West Antarctic Peninsula.

They measured the damage to the markers over a five-year period from 2004 to 2008. They found more scours were made during years when sea ice is frozen in place for short periods of time. Once locked into the more permanent winter sea ice, icebergs are no longer able to move and scour. It might seem obvious, but this is the first empirical test of the relationship between the scours and sea ice.

“It has been suggested previously that iceberg disturbance rates may be controlled by the formation of winter sea ice, but nobody’s been able to go out and measure it before,” said Smale.

“We were surprised to see how strong the relationship between the two factors is. During years with a long sea ice season of eight months or so, the disturbance rates were really low, whereas in poor sea ice years the sea bed was pounded by ice for most of the year,” he added.

Increased ice scours are likely due to recent warming, which has reduced the amount of sea ice during Antarctic winters. Air temperatures in the Peninsula have risen by 3°C in the last 50 years, several times the global average and greater than anywhere else in the Southern Hemisphere.

Effect on sea life

On a local scale, ice scour eliminates populations of sea spiders, sea urchins and other benthic (sea floor) marine life. But on a regional scale it clears a space for marine life to grow, much like a large tree falling in the rainforest leaves room for new tree growth. Too much disturbance, however, could tip the balance and reduce diversity, the researchers say.

Previous research led by Smale, published in the journal Marine Ecology Progress Series in November last year, showed fewer species, fewer individuals and less of the seabed colonised where increased ice scouring had taken place.

Robin Beaman, marine geologist with the Collaborative East Antarctica Marine Census (CEAMARC), which surveyed the Antarctic seabed in December 2007, said the research was interesting and could be a good test of the ecological theory known as the ‘intermediate disturbance hypothesis’. The hypothesis states that low diversity is a characteristic of both dynamic environments, where animals are eliminated by disturbance, and stable environments, which allow animals to out-compete each other.

“More ice scouring could suppress the life on the Antarctic sea bed. You literally bulldoze the environment into nothingness,” said Beaman, who is based at James Cook University in Cairns, Australia.

More information:
British Antarctic Survey

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