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 itrogen in rain and airborne particles contributes
as much as 15 to 35 percent of the nitrogen in the coastal streams
that flow into US estuaries, according to a new study from the
US Geological Survey, the National Oceanographic and Atmospheric
Administration (NOAA), and the Blackland Research Center at Texas
A&M University.
 The
study, published by the American Geophysical Union (AGU) and
released last month, sheds new light on the role of the atmosphere
as a source of pollutants to downstream estuaries.
Nutrients,
especially nitrogen, are needed to sustain the productivity of
estuaries, but too much nitrogen causes the excessive growth
of algae, bacteria and microscopic water animals known as zooplankton,
depriving fish and plants of much-needed oxygen. A recent assessment
by NOAA identified such problems in many US estuaries, especially
in those along the Gulf of Mexico and Mid-Atlantic coasts. Until
now, scientists have had incomplete information on the sources
of nitrogen entering these estuaries. Beyond nitrogen contributions
from agricultural and wastewater sources, it was unclear how
much nitrogen came from other sources like cars, trucks and power
plants because no comprehensive national studies had been done
to quantify those contributions. Moreover, little was known about
the amounts of airborne nitrogen that make their way to coastal
streams and estuaries once the nitrogen is deposited in watersheds.
"This
study provides important new information about how coastal watersheds
process atmospheric nitrogen and about the amounts of atmospheric
nitrogen that enter estuaries," said Richard Alexander,
a USGS hydrologist and expert on nutrients who coauthored the
study. "There are important scientific and policy questions
about the role of atmospheric nitrogen in causing coastal water-quality
problems. This information can help local resource managers determine
sources of nitrogen entering estuaries. This improved understanding
of the links between air deposition of nitrogen and coastal water
quality also leads to a better scientific basis for steps to
minimize coastal pollution sources."
This national
study of the watersheds that drain to 40 major US estuaries reported
the highest atmospheric contributions -- a quarter to about a
third of the nitrogen in streams along the northeastern and Mid-Atlantic
coasts, including streams flowing to the Long Island Sound and
Chesapeake Bay. Atmospheric contributions were also nearly this
large in many streams along the Louisiana Gulf coast, which corresponded
to the locally high levels of atmospheric deposition of nitrogen
in this region. A USGS study published earlier this year reported
that the atmosphere accounts for a similar percentage of the
nitrogen (nearly 20 percent) in the waters of the Mississippi
River that flow into the Gulf of Mexico.
The new national
study also confirmed that estuaries receive much of their nitrogen
from non-atmospheric sources, including farms, pastureland, and
wastewater treatment plants. Agricultural runoff contributed
the largest share, more than one-third in most of the coastal
watersheds studied. The contributions from municipal and industrial
wastewater are similar to those from the atmosphere in many watersheds,
but represent the largest share of nitrogen -- more than a third
in several densely populated watersheds along the North Atlantic
and Gulf coasts.
The results
of the joint USGS/NOAA/BRC study by scientists Richard Alexander,
Richard Smith, Gregory Schwarz, Stephen Preston, John Brakebill,
Raghavan Srinivasan, and Percy Pacheco, entitled Atmospheric
Nitrogen Flux From the Watersheds of Major Estuaries of the United
States: An Application of the SPARROW Watershed Model, was
part of a larger collaborative effort convened by NOAA, including
scientists from more than 15 federal, state, and academic institutions,
to quantify atmospheric nitrogen inputs to the watersheds and
water surfaces of US estuaries. The results of the entire investigation
appear in the American Geophysical Union (AGU) Monograph 57,
entitled "Nitrogen Loading in Coastal Water Bodies: An Atmospheric
Perspective" and edited by Richard Valigura, Richard Alexander,
Mark Castro, Tilden Meyers, Hans Paerl, Paul Stacey, and R. Eugene
Turner. The Monograph was released to the public at the AGU Fall
Meeting, December 15-19 in San Francisco, California.
The eight
chapters of the Monograph include discussions of the ecological
effects of atmospheric inputs to estuaries, the chemical components
of nitrogen deposition to estuaries and their watersheds, alternative
methods for determining the watershed contributions of atmospheric
nitrogen, and an evaluation of the accuracy of the various methodologies.
The larger study underscores the importance of the atmosphere
as a pathway for nitrogen, based on its finding that, in about
a third of the estuaries studied, the amounts of airborne nitrogen
deposited directly onto the water surface are as large or nearly
as large as the amounts of nitrogen carried to estuaries by streams.
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Abstract
and SPARROW links:
water.usgs.gov/nawqa/sparrow/coast/
agu_sparrow.html;
NOAA Study "National Estuarine Eutrophication Assessment:
Effects of Nutrient Enrichment in the Nation's Estuaries":
www.publicaffairs.noaa.gov/releases2000/
jun00/noaa00055.html;
USGS Study "Effect of stream channel size on the delivery
of nitrogen to the Gulf of Mexico":
www.usgs.gov/public/press/public_affairs/
press_releases/pr1162m.html
As the
nation's largest water, earth and biological science and civilian
mapping agency, the USGS works in cooperation with more than
2,000 organizations across the country to provide reliable, impartial,
scientific information to resource managers, planners, and other
customers. This information is gathered in every state by USGS
scientists to minimize the loss of life and property from natural
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of the nation's natural resources, and enhance the quality of
life by monitoring water, biological, energy, and mineral resources.
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