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Originally published Vol.
1, Issue 5 (November 2003)
Hypoxia in the Gulf: Addressing Agriculture's
Contribution
Marc
Ribaudo
The Northern Gulf of Mexico’s
hypoxic zone represents one of the Western Hemisphere’s
largest areas of oxygen deficient waters, where
lack of oxygen kills fish, crabs, and other marine
life. The size of the zone varies but at its peak,
it stretches along the inner continental shelf from
the mouth of the Mississippi River westward to the
upper Texas coast, covering about 7,000 square miles,
an area as large as New Jersey. Long-term consequences
to biodiversity, species abundance, and biomass
in the Gulf are not yet known, but experience with
other coastal dead zones has shown significant ecological
deterioration and depleted fisheries.
Scientists believe that Gulf hypoxia
is caused by nitrogen loads from the Mississippi
River. Nitrogen fuels the rapid growth of large
populations of algae and plankton. When they die
and sink to the bottom, their decay robs the water
of oxygen.
Because two-thirds of the nitrogen
in the Mississippi River comes from use of fertilizer
and manure on agricultural lands, reducing agricultural
nitrogen is a major component of the strategy for
controlling the hypoxic zone. Two basic approaches
can be taken: (1) induce changes in the application
and management of nitrogen fertilizer on farm fields,
or (2) restore wetlands along rivers and streams
to intercept and filter out the nitrogen before
it reaches surface waters. Because the geographic
scale of the problem is so large, any policy to
reduce nitrogen from agriculture will affect commodity
prices, and consequently farmers and consumers both
inside and outside the basin.
An ERS analysis of the two approaches
found farm-based controls on nitrogen fertilizer
use to be more cost-effective than restoring wetlands
when up to 1.2 million metric tons (26 percent)
of basinwide nitrogen losses (nitrogen leaving the
land and entering the water system) must be eliminated.
Until that point, crop yields are little affected
by the controls on nitrogen use. But when nitrogen
losses must be cut by more than 1.2 million metric
tons, a turnaround occurs and wetland restoration
becomes the more cost-effective strategy. The reason
for the turnaround is that when reduction in nitrogen
use reaches a certain point, crop yields decline
significantly, causing subsequent increases in prices
of some agricultural products. The price increases
also result in more intense production of the commodities
outside the Mississippi Basin, increasing erosion
and nutrient runoff in those regions. However, these
calculations don’t include (because of insufficient
data) other environmental benefits of wetlands not
related to nitrogen reduction, such as increased
habitat for wildlife. Inclusion of these benefits
would cause the wetland option to become the more
cost-effective approach at a lower level of nitrogen
reduction.
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