Post: Recent Report on the Dead Zone
Posted by Mike B. on 1/31/08
Most of Gulf Dead Zone Nutrients Come from Just Nine States
RESTON, Virginia, January 30, 2008 (ENS) - Nine states in
the Mississippi River Basin contribute the majority of
nutrients to the northern Gulf of Mexico, threatening the
economic and ecological health of one of the nation's
largest and most productive fisheries, according to a
federal government report released Wednesday.
The states of Illinois, Iowa, Indiana, Missouri, Arkansas,
Kentucky, Tennessee, Ohio and Mississippi make up only one-
third of the 31-state Mississippi River drainage area, but
contribute more than 75 percent of nitrogen and phosphorus
to the Gulf, U.S. Geological Survey, USGS, researchers
Excess amounts of these nutrients have resulted in a zone
of low dissolved oxygen or hypoxia, caused by the growth of
large amounts of algae. This can stress and cause death in
bottom-dwelling organisms in the Gulf, so it is commonly
called the dead zone.
The study found that agricultural nonpoint sources
contribute more than 70 percent of the nitrogen and
phosphorus delivered to the Gulf. By comparison, only about
nine to 12 percent originates from urban sources.
Corn and soybean cultivation is the largest contributor of
nitrogen to the Gulf. Animal manure on pasture and
rangelands and crop cultivation are the largest
contributors of phosphorus.
These are among the new findings in the
report, "Differences in Phosphorus and Nitrogen Delivery to
the Gulf of Mexico from the Mississippi River Basin," by
the U.S. Geological Survey, USGS, published in the
journal "Environmental Science and Technology. "
"This study is important because it reveals new details
about sources of phosphorus," said Richard Alexander, USGS
scientist and lead investigator. "The report shows that
animal manure on pasture and range lands contribute nearly
as much phosphorus as cultivated crops, 37 versus 43
The study suggests that phosphorus associated with the
wastes of unconfined animals is a much larger source of
phosphorus in the Mississippi River Basin than previously
recognized. Current animal manure management emphasizes
controlling nutrients primarily from confined animal
Delivery of nutrients to the Gulf was found to be highest
from watersheds in the central and eastern portions of the
Mississippi River Basin that are drained by large, fast
flowing rivers with very little natural removal.
Runoff entering Fall Creek in Indianapolis, Indiana (Photo
by Charles Crawford courtesy USGS)
Alexander concludes that nutrient reductions in the Gulf
may be most efficiently achieved by managing nutrients in
watersheds drained by large rivers.
Reservoirs, particularly common in the Tennessee Valley and
along the Missouri River, are effective at removing
phosphorus from watersheds in the Mississippi Basin, but
Alexander and his team found that this creates water
quality issues in the reservoirs themselves.
The joint federal-state Gulf of Mexico Hypoxia Task Force
is evaluating recommendations by the U.S. Environmental
Protection Agency's Science Advisory Board to set reduction
targets of at least 45 percent for both nitrogen and
The targets would be set in an effort to shrink the size of
the dead zone in half to 5,000 square kilometers by 2015.
USGS Associate Director for Water Dr. Robert Hirsch
says "on-the-ground" water monitoring is essential to
provide credible, comparable data to verify computer
modeling predictions across large regions, such as the
Mississippi River Basin.
Yet surface water quality monitoring stations are
disappearing. Only 35 stations exist today, down from about
425 stations in the early 1990s, Hirsch said.
Hirsch says this study shows that nutrient issues are
complex and management of animal and crop production,
control of nutrient sources in close proximity to large
rivers, and consideration of reservoir effects on
phosphorus all are necessary to reduce the nutrient burden
flowing in the Mississippi River Basin.
Copyright Environment News Service (ENS) 2008. All rights
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