by Les Blumenthal
March 8, 2010
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
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 Niņo 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
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."