Removing Significant Amounts Of Nitrogen Prevents Downstream "Dead Zones"
By Beth Potier, Media Relations
March 12, 2008
This stream in the Luquillo Mountains of Puerto Rico drains relatively intact
forest with little human activity. Note the relatively clear water and thick
riparian vegetation. Credit: William McDowell
Small streams play a significant role in retaining human-generated nitrogen,
serving as the kidneys of watersheds by removing nitrogen before it ends up
in estuaries and oceans, finds a paper published this week in the journal Nature.
UNH professor William McDowell and research scientist Jody Potter, both in
the department of natural resources, are among the co-authors of the study,
led by Patrick Mulholland of the Oak Ridge National Laboratory in Tenn.
“The major finding is that streams remove significant amounts of nitrogen
that’s coming off the landscape,” says McDowell, noting that human-generated
activities such as agricultural runoff, acid rain, and the human waste stream
are major sources of nitrogen. “But while this process of denitrification – a
bacterial process that converts nitrogen to a harmless nitrogen gas -- cleans
up waterways, if we overuse it by putting too much nitrogen into the water
it’s not as effective.”
Nitrogen removal in streams is important because it reduces the potential
for eutrophication – the excessive growth of algae and aquatic plants – in
downstream lakes and coastal marine waters. Eutrophication is linked to problems
such as harmful algal blooms and oxygen depletion in places such as the Gulf
of Mexico, where the Mississippi River creates a vast “dead zone” of
oxygen depletion with adverse effects on fisheries.
A key finding of the study, titled “Stream denitrification across biomes
and its response to anthropogenic nitrate loading,” is that the effectiveness
of streams to remove nitrate was greatest if the streams were not overloaded
by nitrogen sources such as fertilizers and wastes from human activities. The
largest percentage removal occurred when nitrate entered small healthy streams
and traveled throughout the network before reaching larger rivers. As terrestrial
ecosystems become increasingly saturated with nitrogen as a result of human
activities, the authors caution that large-scale land conversion may negatively
impact streams’ ability to effectively remove nitrogen from the watershed.
The study, which undertook a huge field campaign to investigate nitrogen uptake
and denitrification in 72 streams that drain watersheds of varying land-use
types in the U.S. and Puerto Rico, is the first continental-scale assessment
of total denitrification in small streams. McDowell oversaw field sites in
Puerto Rico and Potter, who at the time of the study was McDowell’s graduate
student, worked in both Puerto Rico and Massachusetts.
The study is also unique in that it captures the entire denitrification process,
measuring not only the production of nitrous oxide but also N2, a harmless
gas that is prevalent in the Earth’s atmosphere.
Moving forward, McDowell notes that this study will inform the work of the
National Ecological Observatory Network (NEON), of which he and several other
UNH researchers are a part. McDowell will undertake long-term experimental
manipulations of streams, adding nutrients to streams in a controlled way.
He also notes that the breadth of this project will carry forward. “We’ve
developed this network of stream ecologists, and we can now answer questions
at the continental scale,” he says.
This study is published in the March 13 issue of Nature. It was funded by
the National Science Foundation.