By Jonathan Fildes

Science and technology reporter, BBC News, San Francisco



The Oregon researchers watched crab populations crash

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That is the conclusion of researchers investigating "dead zones" off the coast of the US, where populations of marine life were suddenly wiped out.

These vast graveyards occur where there are disturbances to currents driven by coastal winds, they say.

Dead zones have been recorded off the coast of California and Oregon every year for the last five years.

The most intense event, which left the ocean floor littered with the carcasses of crabs, happened in 2006.

"It was unlike anything that we've measured along the Oregon coast in the past five decades," said Dr Francis Chan of Oregon State University (OSU).

Dead zones have also been seen in the waters off Chile, Namibia and South Africa.

Plant bloom

The common factor between all of these areas is that marine currents off the coast rise from the deep ocean.

These upwelling zones bring nutrient-rich water up from the deep ocean, triggering plankton blooms that underpin the coastal food chain. Nearly 50% of the world's fisheries are in these areas.

Changes could be long lasting

Dead zones

Using observations from the west coast of the US, researchers believe that this upwelling is being disrupted; there have been changes in its timing and intensity.

For example, in 2005 the upwelling was delayed, which meant that the plankton blooms did not occur, leading to a collapse in fish populations.

This particularly hit migrating salmon, which annually pass along the coast in April and May.

"In 2005, they found nothing to eat," said Dr Bill Peterson of the National Oceanic and Atmospheric Administration (Noaa). "By the time upwelling started, they were dead."

Huge graveyard

An even more catastophic event occurred in 2006 when the amount of upwelling doubled, leading to a huge influx of nutrients and a supercharged plankton bloom.

When these sank to the ocean floor, they stripped the water column of oxygen, creating a 3,000 sq km (1,150 sq mile) dead zone, where creatures unable to swim away suffocated en masse.

Dr Francis Chan used underwater cameras to survey the area two months after the event.

"We were shocked to see a graveyard," he said. "Frame after frame of carcass, carcass, carcass."

Crabs, worms and sea stars all perished in the anoxic water. On the surface, sea birds were also affected by the lack of food.

Scientists found the carcasses of migratory seabirds such as auklets washing up on the beaches.

Uncertain future

The researchers believe the cause of these events was changes in the intensity of the coastal winds, perhaps brought about by global warming.

"What we know from the climate change models is that the land will warm more than the sea," Jack Barth or Oregon State University told the BBC News website.

Scientists used underwater cameras

"As you intensify that gradient - that will drive the stronger winds."

To confirm this link to climate change, the researchers say they need another 10 to 15 years of data. Other work also needs to be done to see if there is a common link between the US dead zones and those seen elsewhere.

In the meantime, they add, we must change our approach to managing and using these ecosystems, particularly for fish stocks.

"The most prudent course of action is to begin to think differently about what is happening," said Dr Jane Lubchenco, also of OSU.

"Climate models predict increasing uncertainty with wild fluctuations. We should expect more surprises."

The research was presented at the American Association for the Advancement of Science (AAAS) annual meeting in San Francisco, US.