The Northern Gulf of Mexico’s hypoxic (oxygen-deficient)
waters represent one of the Western Hemisphere’s largest “dead zones”—areas 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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