The Heat Is Online

Warming-Driven Wind Shifts Create Marine "Dead Zones"

Wind shifts devastate ocean life, Feb. 16, 2007


The delicate interplay between the oceans and atmosphere is changing with catastrophic consequences. Entire marine ecosystems have been wiped out, devastating populations of sea birds and larger marine mammals.

These "dead zones" occur where there are disturbances to the nutrient-rich ocean currents, which are driven by coastal winds.

Extreme marine suffocations have occurred off the west coast of the US 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).

Other coastal countries including Chile, Namibia and South Africa have also been affected.

Plant bloom

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

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

The currents are driven by winds that move surface water away from the coast, drawing more up from the deep.

But now, observations along the west coast of the US suggest that the upwelling is being disrupted, changing 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 pass along the coast in April and May every year.

"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 catastrophic 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 miles) 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.

The event was so severe that the researchers fear that marine life cannot return to the area.

"In previous years, fish that have escaped the low-oxygen area appear to have returned once the oxygen was renewed," said Dr Jane Lubchenco, also of OSU.

"This year may be different, however, because unlike earlier years, the living habitat was also suffocated."

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," colleague Jack Barth told the BBC News website.

It is this difference in temperature and pressure that drives the winds.

"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.

In the meantime, they say, 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 Lubchenco.

"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.