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Warming triggers 'dead zones'
07/08/2006 07:19 - (SA)
Oregon - Bottom fish and crabs washing up dead on Oregon beaches are being killed by a recurring "dead zone" of low-oxygen water that is larger than in previous years and may be triggered by global warming, scientists said.
There are signs it is spreading north to Washington's Olympic Peninsula.
Scientists studying the 113-kilometre-long zone of oxygen-depleted water, along the Continental Shelf between Florence and Lincoln City, conclude that it is being caused by explosive blooms of tiny plants known as phytoplankton, which die and sink to the bottom, then are eaten by bacteria which use up the oxygen in the water.
The recurring phytoplankton blooms are triggered by northerly wind, which generates a process known as upwelling in which nutrient-rich water is brought to the surface from lower depths.
"We are seeing wild swings from year to year in the timing and duration of the winds that are favourable for upwelling," Jane Lubchenco, professor of marine ecology at Oregon State and a member of the Pew Oceans Commission, said from Corvallis.
"This increased variability in the winds is consistent with what we would expect under climate change."
Scientists first noticed a dead zone off Newport in 2002. That one was traced back to a rare influx of cold water rich in nutrients and low in oxygen that had migrated from the Arctic, said Jack Barth, professor of oceanography.
Dead zones have returned each summer since then, but these have been marked by intense bursts of upwelling that were followed by calm periods, when the water contains lower nutrient levels, Barth added.
'Also occur off Namibia and South Africa'
This year, the upwelling started strongly in April, stalled in May and picked up again in late June. Following the upwellings, scientists found the oxygen levels lower.
"We know it's not pollution. It's not a toxic algal bloom. The simple fact is there's not enough oxygen," said Francis Chan, a research professor of zoology at Oregon State who has been measuring ocean oxygen levels.
Oxygen levels are generally lower in deeper water, said Lubchenco, but what is unusual about this condition is that it is moving into relatively shallow water, about 15 metres deep, and moving toward shore, where the richest marine ecosystems are.
Deep water fish, such as ling cod, wolf eels and rockfish, are showing up in Oregon tide pools, apparently driven toward shore by the advancing dead zone, said Lubchenco.
Although the dead zone has been documented along 113 kilometres of coast, dead crabs and fish also have been showing up along Washington's Olympic Peninsula, Barth said.
"If we continue like we are now, we could see some ecological shifts," Barth said. "It all depends on what happens with the warming and the greenhouse gases."
Dead zones in other places around the country, such as Hood Canal in Washington and the Mississippi River Delta off Louisiana are caused by agricultural runoff fuelling blooms of algae that rot and deplete oxygen levels, said Lubchenco. But dead zones like the one off Oregon also occur off Namibia and South Africa in the Atlantic and off Peru in the Pacific.
"We're not really sure what is down the road. If it's just for a short period of time, it will not be as devastating as if it starts lasting a significant fraction of summer," she said.
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