Agriculture Reference
In-Depth Information
THE GULF OF MEXICO'S HYPOXIC “DEAD ZONE”
Every summer, a large area of the Gulf of Mexico near the mouth of the Mississippi River loses most of its dissolved
oxygen and thus its ability to support nearly all species of marine life. It has been appropriately named the “dead
zone.” The size of the dead zone varies, but in recent years it has been alarmingly large; in 2002 it encompassed
about 8500 square miles, nearly the size of New Jersey. The dead zone has many direct negative effects on human
society, most notably threatening the important commercial fisheries of the Gulf coast region by killing fish and
shrimp directly, compromising the ability of many species to reproduce, and altering migration patterns.
The dead zone is a direct result of massive amounts of nitrogen and phosphorus leaching out of the agricultural
lands of the Mississippi River basin and causing excessive growth (“blooms”) of phytoplankton in the Gulf. When
the phytoplankton die, their decomposition by bacteria uses up much of the oxygen dissolved in the water. The
relatively calm summer weather prevents mixing of the water column, resulting in the sustained hypoxic (low
oxygen) conditions that kill fish and bottom-dwelling organisms.
The dead zone phenomenon shows the multifaceted and interrelated ways in which conventional agriculture impacts
the environment. Irrigation, intensive tillage, monoculture, over-application of inorganic fertilizer, and factory farming
of animals all play a role in causing unnaturally large amounts of nitrogen and phosphorus to flow into the Gulf of Mexico.
A little more than half of the excess nitrogen (an estimated 56%) comes from the inorganic fertilizer applied
to fields in Kansas, Missouri, the Dakotas, Arkansas, and the other agricultural states in the Mississippi's vast
watershed. Much of this nitrogen leaches into the region's rivers because much more nitrogen is applied than can
be taken up by plants or chemically bound in the soil; excess fertilizer is applied because monocropped high-yield
varieties require it for maximum production. And even more nitrogen ends up in the rivers because of irrigation
and the erosion caused by intensive tillage.
About 25% of the excess nitrogen, and an even greater proportion of the excess phosphorus, comes from the
animal waste produced by hog, poultry, and cattle CAFOs. These nutrients find their way into the rivers from
manure spills, leaching of manure-treatment lagoons, and leaching from the excess treated manure applied to fields.
Ironically, if the Mississippi River and its tributaries were not so thoroughly engineered for human purposes —
dammed for flood control and irrigation, channelized and locked for shipping — its healthy aquatic and wetland
ecosystems and functioning floodplains would be able to remove much of the excess nitrogen and phosphorus from
the rivers before these nutrients reached the Gulf of Mexico. Since much of the altering of the rivers in the Mississippi's
watershed was done for the sake of agriculture — irrigation and transport of agricultural commodities — this is just
one more way in which conventional agriculture is implicated in a continuing environmental disaster with huge
impacts on human society.
FIGURE 1.7 Satellite image of the “dead zone” in the Gulf of Mexico. The darker areas indicate highly turbid waters with high
concentrations of phytoplankton fed largely by agricultural runoff from the huge Mississippi River basin. The phytoplankton in the
blooms will die and sink to the bottom, causing bacterial decay that removes oxygen from the surrounding water. Source : NASA.
 
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