Warming oceans contain less oxygen for fish, conference told

February 24, 2007

Some areas seeing 'dead zones' of excessive nitrogen, limited oxygen

Nicholas Read

Vancouver Sun

Friday, February 23, 2007

CREDIT: John McConnico, Associated Press Files

A boat makes its way through the icebergs in Disko Bay, Greenland, where the vast icy landscape is thinning.

VICTORIA -- As the temperature of the world's oceans increases due to global warming, there may be more and more areas where oxygen in the water is either limited or absent, and that could have a deadly effect on huge numbers of marine species, a U.S. biological oceanographer warned a conference on the future of the world's oceans Thursday.

Lisa Levin, who works out of the Integrative Oceanography Division at the Scripps Institution of Oceanography in La Jolla, Calif., told an audience at the University of Victoria that most so-called "dead zones" are caused by excessive levels of nitrogen being dumped into the oceans. Nitrogen is a component of most commercial fertilizers, and rivers carry the residue of these fertilizers from farms to the ocean, she said.

When this happens, she explained, the number of nitrogen-consuming phytoplankton rises. This, in turn, prompts a concomitant rise in the number of microbes that feed on the phytoplankton -- microbes that collectively consume great quantities of oxygen.

As a consequence, the number of man-made dead zones -- areas of the ocean where oxygen is either depleted or gone -- has grown to more than 150 in the last 50 years, some of them several thousand square kilometres in size.

But now, because of global warming, Levin said, it's possible that the number of such zones, where fish that need oxygen can't thrive, could rise even higher.

"People are only beginning to study these second-order effects of global warming," she said. "But it's certainly an area that's worth watching."

One such zone that has scientists like her puzzled is a 1,000-square-kilometre site off the coast of Oregon -- known as the Oregon Dead Zone -- that appeared mysteriously five years ago. The zone lasts only five months over the summer when the ambient air is at its warmest, but during those five months fish that need oxygen -- all commercially caught species -- virtually disappear from it. Since then, smaller but similar dead zones have been identified off the coast of Washington state as well.

And it's possible, Levin said, that that may have something to do with climate change. "We don't know if it's related to large-term climate change, but we need to find out."

Warmer water causes oxygen depletion for two reasons, she explained: Oxygen is less soluble in warm water, and when water gets warmer, a fish's metabolic rate increases.

"A higher temperature means a higher metabolic rate, which means a greater oxygen requirement," Levin said. The more oxygen one fish needs, the less oxygen there is for others. Warmer air temperatures also could affect ocean circulation, she said, but that needs to be studied further as well.

So far no such dead zones have been identified off the coast of B.C., but if global warming does play a role in creating them, it's possible they could arise here too.

"We need to watch for signs of change like what we're seeing off Oregon," Levin said.

She made her comments on the second day of a two-day conference of international scientists at the university entitled "Are we killing the world's oceans?"

Other subjects addressed included ocean acidification, the effects of aquaculture on the environment, and oil and gas drilling in the North Sea.

Earlier, Andrew Weaver, a climatologist at the University of Victoria, and one of the authors of a paper on climate prediction for the Intergovernmental Panel on Climate Change, told the same audience that ocean temperatures are expected to rise between 2.5 and three degrees Celsius between now and the end of the century. And that, he said, will mean a rise in sea levels of 19 to 58 centimetres, enough to cause "huge disruptions" in the world's low-lying areas, such as Bangladesh, Florida and New Orleans.

That won't be enough to make Richmond disappear, he said, as some climate models have suggested, but it will mean a total collapse of the salmon fishery and all the other species that depend on it.

Also of concern, he said, is what a rising world temperature will do to the ice sheets of Greenland. It is generally agreed, Weaver said, that if the world's ambient temperature increases more than three degrees, Greenland will melt, and that will cause a rise of seven metres in the world's sea levels -- enough, he said, to wipe out large parts of western Europe, Manhattan and low-lying parts of the Lower Mainland.

"Beyond that [three-degree rise], its demise is inevitable," Weaver said. "No matter what we do, we've got a seven-metre rise in sea level in the cards."

However, both he and other scientists who spoke at the conference said there is still time to turn things around. That, however, depends on a political and public will to do things differently.

"Scaring people backfires," Weaver cautioned. "Because when you scare people they throw up their hands and say 'We can't do anything.' But we can. We have to."