Stream Size Major Factor in Nitrogen Reaching Gulf of Mexico
A U.S. Geological Survey study supports previous findings that most of the
nitrogen pollution delivered to the Gulf of Mexico by the Mississippi River
originates far upstream in the upper Midwest and Ohio Valley states. This new
USGS study also finds that within these regions there are large differences in
the percentage of nitrogen reaching the Gulf, depending on the relation of the
location of nitrogen sources to streams of different sizes in the watershed.
The study finds that the rates of nitrogen reaching the Gulf from upstream
areas near large rivers in such states as Ohio and Minnesota are much higher
than those in neighboring areas near small streams. Moreover, areas near
large rivers in these same states, located more than 1,500 miles from the Gulf,
also deliver more nitrogen to the Gulf than areas near small streams in the
states of Mississippi and Arkansas, located only a few hundred miles from the
Previously it was not clear whether a unit of nitrogen released in different
areas of the Mississippi River drainage basin has an equal chance of reaching
the Gulf. It had generally been assumed that the percentage of nitrogen
traveling downstream to the Gulf decreased as the distance increased.
However, this study, published in the February 17 issue of Nature, finds that
nitrogen pollution is naturally removed from water much more rapidly in small
streams than in large rivers. As a result, nitrogen delivery from point
and nonpoint sources in a stream drainage basin is not simply a function of the
distance between the Gulf and the nitrogen source, but a function of the amount
of time the nitrogen travels through small streams.
"We found that nitrogen is naturally removed from small streams much
more quickly than in large rivers, such as the Mississippi River and its major
tributaries," said USGS scientist Richard Alexander, one of the authors of
the study. "One of the most important ways that nitrogen is removed
from water in the bottom sediments of streams is through a natural process
called denitrification, where nitrogen is converted by bacteria to harmless
nitrogen gas and vented to the atmosphere."
The amount of nitrogen removed from water depends on the amount of water in
contact with bottom sediments. Because water in small, shallow streams has more
contact with the bottom sediments than water in deep, large rivers, more
nitrogen is expected to be removed in smaller streams than larger
rivers. The results of this study strongly support this theory.
Nitrogen increases in the Mississippi River have been cited as the leading
cause of eutrophication (excessive algal growth) and chronic hypoxia (low
dissolved oxygen) in Louisiana coastal waters and the Gulf of Mexico during the
latter half of the 20th century. This area of oxygen-depleted waters is
the largest in the western Atlantic Ocean.
"The spring inflow of nitrogen-enriched waters from the
Mississippi River into the poorly mixed, shallow Gulf waters causes excessive
algal production," Alexander explained. "Then, the decay of the algae
depletes the oxygen in the water, causing bottom-dwelling organisms to die and
stressing the fishing resources in the Gulf."
The percentage of stream nitrogen reaching the Gulf of Mexico from all areas
in the Mississippi River drainage basin is illustrated in a color map at http://www.usgs.gov/themes/nature.html.
In this map, colors representing the percentage of nitrogen reaching the Gulf of
Mexico form a dendritic pattern, similar to the veins of a leaf, with the
largest percentage near the Mississippi, Ohio, and lower Missouri River valleys.
The USGS study used data from 374 monitoring stations located on rivers and
streams in the United States, including 123 stations in the Mississippi River
basin, to quantify the rates at which nitrogen is removed from channels by
The report by USGS scientists Richard Alexander, Richard Smith, and Gregory
Schwarz and entitled "Effect of stream channel size on the delivery of
nitrogen to the Gulf of Mexico", is available in a PDF file at http://water.usgs.gov/nawqa/sparrow/nature/nature_alexetal.pdf.
SOURCE: U.S. Geological Survey