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Eco-metrics scientists advance understanding of nitrogen cycling in stream networks |
Ashley Helton's
research modeling nitrogen cycling in river networks has certainly made
a big "splash." As an M.S. Student sponsored by Eco-metrics, Inc
at the University of Georgia’s Odum School of Ecology, Ms.
Helton worked under the direction of Geoffrey Poole and UGA researcher Judy Meyer, as part of the "LINX-II"
project. The LINX-II project was a collaboration among 31 aquatic scientists from different U.S.
universities to study how nitrate, a form of nitrogen pollution, is
transported within and removed from stream water. Results from their work, including Helton’s model results, are reported in the March 13th issue of the journal Nature. A less technical summary of the research implications also appears in the same journal.
In the first phase of their study, the research scientists tagged
nitrate with a rare, non-radioactive isotope of nitrogen, 15N. They
then added the tagged nitrate to 72 different streams reaches across
the U.S. and Puerto Rico, carefully measuring how rapidly the tagged
nitrate was carried downstream. As the nitrogen passed through the
experimental stream reaches, the scientists found that the nitrate was
removed from stream water by tiny organisms such as algae, fungi and
bacteria living on the streambed. In addition, a considerable fraction
was permanently removed from streams by a bacterially-mediated process
known as denitrification, which converts nitrate to nitrogen gas that
then escapes harmlessly to the atmosphere. The researcher’s concluded
that streams and rivers converted a substantial amount of nitrate to
nitrogen gas, and the rate of conversion increased as nitrate
concentrations rose. However, conversion rates did not rise as
fast as increases in nitrogen concentration. Thus, the efficiency of
denitrification (the fraction of nitrate in stream water that is
converted to nitrogen gas) dropped rapidly as nitrate concentrations
increased.
Although these field experiments showed what happened to nitrate in
individual stream reaches, the scientists turned to Ms. Helton to help
them understand what the findings meant when considering entire stream
networks, which develop as small streams flow together to form larger
streams and eventually rivers. Based on the field experiments, Helton
developed a computer simulation model to study nitrate removal from
water within river networks. The model showed that streambed biota
were most effective at removing nitrate from stream networks when the
streams were not overloaded by nitrate pollution from sources such as
fertilizers and wastes from human activities. Additionally, she found
that most of the nitrate removal occurred in small headwater streams
when nitrate pollution rates were low. In contrast, under higher
nitrate pollution levels, the biota in small streams could be
overwhelmed with nitrate, allowing the nitrate to flow downstream and
be removed by organisms in larger rivers. If the nitrate pollution
levels were sufficient to overwhelm the large rivers, too, then most of
the nitrate would make its way to lakes or coastal oceans, where it can
cause noxious algal blooms and lead to oxygen depletion and death of
fish and shellfish. Such oxygen starved “dead zones” have been
reported recently in the Gulf of Mexico. Ms. Helton’s work, along with
that of the larger group of scientists, was funded by a grant from the
National Science Foundation.
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