By Les Blumenthal, McClatchy Newspapers
Les Blumenthal, Mcclatchy Newspapers –
Sun Mar 7,
12:01 pm ET
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."
ON THE WEB
The
Partnership for Interdisciplinary Studies of Coastal
Oceans
The
National Oceanic and Atmospheric
Administration
Oregon
State University's College of Oceanic and Atmospheric
Sciences
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