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Low Oxygen Zones in Oceans; Climate Impact on Oceans
Topic Started: Mar 8 2010, 11:34 PM (478 Views)
Xnn

Here's a news article on growing areas of low oxygen in the oceans.
What's different about this finding is that it is not related to nutrients run off from land.
Instead it appears to be related to circulation of the oceans.
This is consistent with most models of global warming.
However, the time frame is too short to infer how much is from rising CO2 levels and how much are from other factors (including natural variations).

In brief, warming of surface waters acts to impede circulation with deeper waters.
The warm water acts as a cap and keeps the oxygen rich waters confined to the surface.

This like this have happened in the past and lead to mass extinction of many types of sea life.



http://news.yahoo.com/s/mcclatchy/20100307/sc_mcclatchy/3444187

Quote:
 
By Les Blumenthal, McClatchy Newspapers – Sun Mar 7, 12:01 pm ET
WASHINGTON — Lower levels of oxygen in the Earth's oceans, particularly off the United States' Pacific Northwest coast, could be another sign of fundamental changes linked to global climate change, scientists say. They warn that the oceans' complex undersea ecosystems and fragile food chains could be disrupted.

In some spots off Washington state and Oregon , the almost complete absence of oxygen has left piles of Dungeness crab carcasses littering the ocean floor, killed off 25-year-old sea stars, crippled colonies of sea anemones and produced mats of potentially noxious bacteria that thrive in such conditions.

Areas of hypoxia, or low oxygen, have long existed in the deep ocean. These areas — in the Pacific, Atlantic and Indian oceans — appear to be spreading, however, covering more square miles, creeping toward the surface and in some places, such as the Pacific Northwest , encroaching on the continental shelf within sight of the coastline.

"The depletion of oxygen levels in all three oceans is striking," said Gregory Johnson , an oceanographer with the National Oceanic and Atmospheric Administration in Seattle .
In some spots, such as off the Southern California coast, oxygen levels have dropped roughly 20 percent over the past 25 years. Elsewhere, scientists say, oxygen levels might have declined by one-third over 50 years.

"The real surprise is how this has become the new norm," said Jack Barth , an oceanography professor at Oregon State University . "We are seeing it year after year."

Barth and others say the changes are consistent with current climate-change models. Previous studies have found that the oceans are becoming more acidic as they absorb more carbon dioxide and other greenhouse gases.
"If the Earth continues to warm, the expectation is we will have lower and lower oxygen levels," said Francis Chan , a marine researcher at Oregon State .
As ocean temperatures rise, the warmer water on the surface acts as a cap, which interferes with the natural circulation that normally allows deeper waters that are already oxygen-depleted to reach the surface. It's on the surface where ocean waters are recharged with oxygen from the air.
Commonly, ocean "dead zones" have been linked to agricultural runoff and other pollution coming down major rivers such as the Mississippi or the Columbia . One of the largest of the 400 or so ocean dead zones is in the Gulf of Mexico , near the mouth of the Mississippi .
However, scientists now say that some of these areas, including those off the Northwest, apparently are linked to broader changes in ocean oxygen levels.
The Pacific waters off Washington and Oregon face a double whammy as a result of ocean circulation.
Scientists have long known of a natural low-oxygen zone perched in the deeper water off the Northwest's continental shelf.

During the summer, northerly winds aided by the Earth's rotation drive surface water away from the shore. This action sucks oxygen-poor water to the surface in a process called upwelling.
Though the water that's pulled up from the depths is poor in oxygen, it's rich in nutrients, which fertilize phytoplankton. These microscopic organisms form the bottom of one of the richest ocean food chains in the world. As they die, however, they sink and start to decay. The decaying process uses oxygen, which depletes the oxygen levels even more.
Southerly winds reverse the process in what's known as down-welling.

Changes in the wind and ocean circulation since 2002 have disrupted what had been a delicate balance between upwelling and down-welling. Scientists now are discovering expanding low-oxygen zones near shore.

"It is consistent with models of global warming, but the time frame is too short to know whether it is a trend or a weather phenomenon," Johnson said.

Others were slightly more definitive, quicker to link the lower oxygen levels to global warming rather than to such weather phenomena as El Nino or the Pacific Decadal Oscillation, a shift in the weather that occurs every 20 to 30 years in the northern oceans.

"It's a large disturbance in the ecosystem that could have huge biological changes," said Steve Bograd , an oceanographer at NOAA's Southwest Fisheries Science Center in Southern California .
Bograd has been studying oxygen levels in the California Current, which runs along the West Coast from the Canadian border to Baja California and, some scientists think, eventually could be affected by climate change.

So far, the worst hypoxic zone off the Northwest coast was found in 2006. It covered nearly 1,200 square miles off Newport, Ore. , and according to Barth it was so close to shore you could hit it with a baseball. The zone covered 80 percent of the water column and lasted for an abnormally long four months.

Because of upwelling, some of the most fertile ocean areas in the world are found off Washington and Oregon . Similar upwelling occurs in only three other places, off the coast of Peru and Chile , in an area stretching from northern Africa to Portugal and along the Atlantic coast of South Africa and Namibia .

Scientists are unsure how low oxygen levels will affect the ocean ecosystem. Bottom-dwelling species could be at the greatest risk because they move slowly and might not be able to escape the lower oxygen levels. Most fish can swim out of danger. Some species, however, such as chinook salmon, may have to start swimming at shallower depths than they're used to. Whether the low oxygen zones will change salmon migration routes is unclear.
Some species, such as jellyfish, will like the lower-oxygen water. Jumbo squid, usually found off Mexico and Central America , can survive as oxygen levels decrease and now are found as far north as Alaska .

"It's like an experiment," Chan said. "We are pulling some things out of the food web and we will have to see what happens. But if you pull enough things out, it could have a real impact."
















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sylas old account

Xnn
Mar 8 2010, 11:34 PM
Here's a news article on growing areas of low oxygen in the oceans. [...]
Wow. That news story came out on Sun March 7; and it already has 3131 comments! This board should be so active!

I've long felt that the effects on ocean chemistry of shifting climates are likely to be some of the most pragmatically significant, but I don't know much about hypoxia. The article lists a number of different scientists working on this, and links to three relevant research groups, but not to any specific papers. So I had a quick look around myself. The story mentions Gregory Johnson, of NOAA. He wrote about this in 2008, in


Unfortunately only the abstract is open access. The full article seems to be available open access, as Stramma et al 2008, full text, at science magazine
Stramma et al 2008 abstract
 

Oxygen-poor waters occupy large volumes of the intermediate-depth eastern tropical oceans. Oxygen-poor conditions have far-reaching impacts on ecosystems because important mobile macroorganisms avoid or cannot survive in hypoxic zones. Climate models predict declines in oceanic dissolved oxygen produced by global warming. We constructed 50-year time series of dissolved-oxygen concentration for select tropical oceanic regions by augmenting a historical database with recent measurements. These time series reveal vertical expansion of the intermediate-depth low-oxygen zones in the eastern tropical Atlantic and the equatorial Pacific during the past 50 years. The oxygen decrease in the 300- to 700-m layer is 0.09 to 0.34 micromoles per kilogram per year. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies.


I'd like to know more about the actual physical/chemical mechanisms involved in hypoxia. It seems to be linked with circulation. The key phrase in your news story in relation to the mechanisms seems to be this:
Growing low-oxygen zones in oceans worry scientists newstory
 

"If the Earth continues to warm, the expectation is we will have lower and lower oxygen levels," said Francis Chan , a marine researcher at Oregon State .

As ocean temperatures rise, the warmer water on the surface acts as a cap, which interferes with the natural circulation that normally allows deeper waters that are already oxygen-depleted to reach the surface. It's on the surface where ocean waters are recharged with oxygen from the air.


Edited by sylas old account, Mar 9 2010, 01:20 AM.
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SkyHunter

I wonder how the "warm cap" effecting the pH of the surface layers?

Warmer water means CO2 will be dissolved at a slower rate, yet the surface waters will have more exposure to the atmosphere, hence more time for CO2 to dissolve.

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sylas
Board owner
[moderate=sylas]
I have spilt off two posts here specifically on the oil spill in the Gulf of Mexico, and made them a new thread in "General discussion". The oil spill impact is potentially a major topic in its own right, and well worth its own thread.

You can find the new thread at Impacts of oil spill in Gulf of Mexico .[/moderate]
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SkyHunter

Here is a new paper in Science. Changing Oceans

I am just now reading it, thought it might be helpful in this thread.
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