Less oxygen dissolved in the water is often referred to as a “dead
zone” (in red above) because most marine life either dies, or, if they
are mobile such as fish, leave the area. Habitats that would normally be
teeming with life become, essentially, biological deserts. (Credit:
NOAA)
A dry spring in portions of the Midwest is expected to result in the
second-smallest Gulf of Mexico "dead zone" on record in 2012, according
to a University of Michigan forecast just released.
The U-M prediction calls for a 2012 Gulf of Mexico dead zone of about
1,200 square miles, an area the size of Rhode Island. If the forecast
is correct, 2012 would replace 2000 (1,696 square miles) as the year
with the second-smallest Gulf dead zone. The smallest Gulf
oxygen-starved, or hypoxic, zone was recorded in 1988 (15 square miles).
"While it's encouraging to see that this year's Gulf forecast calls
for a significant drop in the extent of the dead zone, we must keep in
mind that the anticipated reduction is due mainly to decreased
precipitation in the upper Midwest and a subsequent reduced water flow
into the Gulf," said aquatic ecologist Donald Scavia, professor at the
U-M School of Natural Resources and Environment. "The predicted 2012
dead-zone decline does not result from cutbacks in nitrogen use, which
remains one of the key drivers of hypoxia in the Gulf."
The U-M prediction is one of two Gulf dead zone forecasts released
June 21 by the National Oceanic and Atmospheric Administration, which
funds the research. The other NOAA-supported team, from the Louisiana
Universities Marine Consortium and Louisiana State University, predicts a
2012 Gulf dead zone of 6,213 square miles.
The Michigan forecast model is based solely on 2012 spring nutrient
inputs from the Mississippi River, which are significantly lower than
average due to drought conditions throughout much of the watershed. The
Louisiana State University forecast model takes into account last year's
above-normal nutrient load, which the Louisiana researchers say can
remain in bottom sediments and can result in a "carryover effect" that
increases the size of the 2012 dead zone.
Last year, the Gulf dead zone measured 6,765 square miles. The
largest Gulf hypoxic zone measured to date occurred in 2002 and
encompassed more than 8,400 square miles. The Gulf dead zone has
averaged about 6,000 square miles over the past five years.
This year, for the first time, Scavia's U-M team was able to forecast
the volume of hypoxic water contained in the dead zone. The most likely
2012 scenario corresponds to a volume of 2.6 cubic miles, according to
the U-M team.
Scavia, director of the Graham Sustainability Institute and special
counsel to the U-M president for sustainability, also released his
annual Chesapeake Bay dead zone forecast. It calls for an average-size
dead zone in the Bay this year, down significantly from last summer's
record-setter.
"These dead zones are ecological time bombs," he said. "Without
determined local, regional and national efforts to control nutrient
loads, we are putting major fisheries at risk."
In 2009, the dockside value of commercial fisheries in the Gulf of
Mexico was $629 million. Nearly 3 million recreational anglers further
contributed more than $1 billion to the Gulf economy, taking 22 million
fishing trips.
Farmland runoff containing fertilizers and livestock waste -- some of
it from as far away as the Corn Belt -- is the main source of the
nitrogen and phosphorus that cause the annual Gulf of Mexico hypoxic
zone. Each year in late spring and summer, these nutrients flow down the
Mississippi River and into the Gulf, fueling explosive algae blooms
there.
When the algae die and sink, bottom-dwelling bacteria decompose the
organic matter, consuming oxygen in the process. The result is an
oxygen-starved region in bottom and near-bottom waters: the dead zone.
"This forecast is a good example of NOAA, USGS and university
partnerships delivering ecological forecasts that quantify the linkages
between the watershed and the coast," said NOAA Administrator Jane
Lubchenco. "While the occurrence of a low-flow year following a year
with major flooding will help us to evaluate any carryover effect from
prior years, we should not lose sight of the ongoing need to reduce the
flow of nutrients to the Mississippi River and thus the Gulf."
According to U.S. Geological Survey estimates, the Mississippi and
Atchafalaya rivers transported 58,100 metric tons of nitrogen (in the
form of nitrite plus nitrate) to the northern Gulf in May 2012, an
amount that is 56 percent lower than average May nitrogen loads
estimated in the last 33 years.
"These forecasts are the product of decades of research, monitoring
and modeling on how decisions we make in the vast drainage basin of the
Mississippi and its tributaries translates into the health of the
coastal zone of the Gulf of Mexico," said U.S. Geological Survey
Director Marcia McNutt. "Comparing the actual hypoxic zone against the
predictions will help scientists better understand the multiyear memory
of this complex land-sea system, and ultimately better inform options
for improving ecosystem productivity."
About a thousand miles northeast of the Gulf of Mexico in Chesapeake
Bay, this year's hypoxic zone is expected to measure about 1.5 cubic
miles, Scavia said. That's about average compared to measured volumes
since 2000 but much smaller than last year's record-setter of 2.75 cubic
miles, which was due to spring storms that washed large amounts of
nutrients into rivers that feed the Bay.
So far in 2012, rainfall in the Chesapeake Bay watershed has been 50-to-75 percent of normal, Scavia said.
The actual size of the 2012 Gulf hypoxic zone will be announced
following a NOAA-supported monitoring survey led by the Louisiana
Universities Marine Consortium between July 27 and Aug. 3.
The amount of nitrogen entering the Gulf of Mexico each spring has
increased about 300 percent since the 1960s, mainly due to increased
agricultural runoff. The Gulf of Mexico/Mississippi River Watershed
Nutrient Task Force has targeted 1,900 square miles as a long-term goal
for the size of the Gulf dead zone.
No comments:
Post a Comment