The Missing Greenhouse Gas - The Ripple Effect: The Fate of Freshwater in the Twenty-First Century - page 78

America's biggest dead zone lurks in the Gulf of Mexico , a result of the outflow of the
Mississippi River. “It's bad enough that little Muddy Creek impacts Chesapeake Bay,”
Hirsch said. “But think of what happens when the Mississippi and Atchafalaya flow into
the Gulf of Mexico. Those are much bigger systems, by several orders of magnitude; the
effects of nitrogen and phosphorus runoff on them is that much bigger, too.”
At 2,320 miles, the Mississippi River—“the Big Muddy”—is the second-longest river
in the United States; when it combines with the Missouri, it creates the biggest river sys-
tem in North America. The Mississippi's watershed, its size exceeded only by those of
the Amazon and the Congo, drains 41 percent of the continental United States, or 1.245
million square miles, drawing water from parts of thirty-one states and two Canadian
provinces before the big river empties into the Gulf of Mexico, about a hundred miles
downstream of New Orleans.
Every year farmers apply approximately 7 million metric tons of nitrogen , in fertil-
izers, to the Mississippi Basin. (This number does not include nitrogen from human
sewage, animal manure, the atmosphere, or natural deposits from woodlands.) Accord-
ing to a USGS study, nine states—Illinois, Iowa, Indiana, Missouri, Arkansas, Kentucky,
Tennessee, Ohio, and Mississippi—account for 75 percent of the nitrogen and phos-
phorus flowing into the Gulf from the Atchafalaya and Mississippi River Basin. Agri-
culture accounts for an estimated 70 percent of nitrogen delivered to the Gulf; about 9
percent originates in urban areas.
Nitrogen is “slippery,” meaning it is mobile in the environment; rather than sticking
to the soil, where the farmers want it, as much as 20 percent of the fertilizer is washed of
the land and flows downstream to the Gulf, where instead of producing corn it stimu-
lates algae growth. Wetlands help to filter and absorb some of this runoff, but they have
largely been filled in and destroyed by man. In the Midwest, about 85 percent of wet-
lands have been drained and filled in to create farmland or room for development.
Fossil fuels burned by cars, factories, and power plants are another major problem,
creating low-lying smog and high-altitude air pollution. When it rains, nitrogen oxide
is scrubbed from the air, drops to the ground in acid rain, and is eventually washed into
freshwater ways and the sea.
Carried into the Gulf of Mexico on warm, fresh Mississippi and Atchafalaya currents,
nutrients collect near the Gulf's surface, where sunshine causes the massive algae mon-
ster to bloom. When those algae die, they settle into the deeper, colder salt water, where
they decay and use up all of the oxygen near the bottom. Down there, the algae create
the Gulf dead zone, which has averaged 6,046 square miles since it was first mapped in
1985. The lack of oxygen in the Gulf's deep waters makes it impossible for fish and other
aquatic life to survive.