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To: Hypoxia <hypoxia@lists.iatp.org>
From: mmuller@iatp.org
Date: 2004-12-20 10:01:08
Subject: Direct link between ag runoff and algal blooms in Sea of Cortez
EARTH OBSERVATION
Direct Link Discoverd Between Agricultural Runoff And Algal Blooms In
Sea Stanford CA (SPX) Dec 09, 2004 @
http://www.spacedaily.com/news/eo-04zzzzzzi.html
Scientists have found the first direct evidence linking large-scale
coastal farming to massive blooms of marine algae that are potentially
harmful to ocean life and fisheries.
Researchers from Stanford University's School of Earth Sciences made
the discovery by analyzing satellite images of Mexico's Sea of Cortez,
also known as the Gulf of California-a narrow, 700-mile-long stretch of
the Pacific Ocean that separates the Mexican mainland from the Baja
California Peninsula. Immortalized in the 1941 book Sea of Cortez, by writer John Steinbeck
and marine biologist Edward Ricketts, the region remains a hotspot of
marine biodiversity and one of Mexico's most important commercial
fishing centers. The results of the Stanford study will be presented at the annual
meeting of the American Geophysical Union (AGU) in San Francisco on
Dec. 13. Algal blooms Algal blooms occur naturally when cold-water upwellings bring from the
seafloor to the surface nutrients that stimulate the rapid reproduction
and growth of microscopic algae, also known as phytoplankton. These
events often benefit marine ecosystems by generating tons of algae that
are consumed by larger organisms. But several phytoplankton species produce harmful blooms, known as red
or brown tides, which release toxins in the water that can poison
mollusks and fish. Excessively large blooms can also overwhelm a marine
ecosystem by creating oxygen-depleted "dead zones" in the ocean. Scientists have long suspected that many harmful blooms are fueled by
fertilizer runoff from farming operations, which in many regions pour
tons of excess nitrogen and other nutrients into rivers that eventually
flow into coastal waters. However, some agricultural industry groups
contend that there is not enough evidence to link farm runoff to red
tides or dead zones. Satellite imagery To assess the impact of agriculture on marine algae, Stanford
scientists turned their attention to one of Mexico's most productive
coastal farming regions-the Yaqui River Valley, which drains into the
Sea of Cortez. "The Yaqui Valley agricultural area is 556,000 acres [225,000 hectares]
of irrigated wheat," said Pamela A. Matson, the dean of Stanford's
School of Earth Sciences and co-author of the AGU study. "The entire
valley is irrigated and fertilized in very short windows of time during
a six-month cycle. The excess water from irrigation runs off through
streams and channels into the estuaries, and then out to sea." Matson and her colleagues wondered if each fertilization and irrigation
event would trigger a noticeable phytoplankton bloom near the mouth of
the Yaqui River, which is located on the mainland side of the Sea of
Cortez. To find out, the researchers analyzed a series of images from an
orbiting NASA satellite called SeaWiFS, which is equipped with special
light-sensitive instruments that can detect phytoplankton floating near
the surface of the sea. "These instruments measure the level of greenness in the water,"
explained Kevin R. Arrigo, an associate professor of geophysics at
Stanford and co-author of the AGU paper. "The greener the water, the
more phytoplankton there are." Dramatic results Stanford doctoral candidate Mike Beman carefully analyzed dozens of
SeaWiFS images taken over the Sea of Cortez from 1998 through 2002. The
results were dramatic. "I looked at five years of satellite data," said Beman, lead author of
the study. "There were roughly four irrigation events per year, and
right after each one, you'd see a bloom appear within a matter of days." Each bloom was enormous, he said, covering from 19 to 223 square miles
(50 to 577 square kilometers) of the Sea of Cortez and lasting several
days.
"Sometimes eddies actually pulled the plumes across the gulf, from the
mainland side all the way to the Baja Peninsula," Beman added. "Mike found that immediately following each one-week window in which
much of the valley was irrigated, there was a response in the ocean off
the coast of the Yaqui Valley," Matson explained. "We were quite surprised," Arrigo added, noting that the AGU paper
marks the first time that scientists have documented a "one-to-one
correspondence between an irrigation event and a massive algal bloom." Red tides and dead zones According to the researchers, artificially induced algal blooms could
have major impacts on recreational and commercial fishing, major
industries in the Sea of Cortez. Red tides, for example, can cause
outbreaks of life-threatening diseases, such as paralytic shellfish
poisoning, which can shut down mussel and clam harvesting for long
periods of time. Another concern is hypoxia, or oxygen depletion, which is caused by
excessive algae growth. As the algal mass sinks, it is consumed by
bacteria, which use up most of the oxygen in the water as they multiply.
The result is an oxygen-depleted dead zone at the bottom of the sea
where few creatures can survive. A massive dead zone appears every summer in the Gulf of Mexico off the
coast of Louisiana and Texas. Scientists believe that agricultural
runoff from the Mississippi River plays a pivotal role in creating this
annual dead zone, which measured 8,500 square miles (22,000 square
kilometers) in 2002-an area bigger than the state of Massachusetts. "In the Sea of Cortez, there's the possibility that hypoxia could occur
at a local scale, which could be devastating to the shrimp and
shellfish industries," Matson said. "Shrimp fisheries are very important economically, and they're already
under a lot of stress from overfishing and aquaculture. It is possible
that agricultural runoff could cause additional stress if it does lead
to toxic blooms or hypoxia." She and her colleagues plan to conduct
follow-up studies to assess the ecological impact of Yaqui Valley
runoff events. "The availability of high-resolution satellite data has opened up a
whole new opportunity to look at the importance of what's going on on
land in the sea," Matson added. "This study shows that you have to pay
attention to the land-sea connections."
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