Dec. 11, 2000 Preliminary research results on the hyporheic zones of streams—the region in which stream and ground waters mix—in different types of environments show that hyporheic activity accounted for up to 70 percent of stream respiration as oxygen demand and 10 percent to 50 percent of nitrate uptake. The results suggest that the retention of nitrate in the stream increases with the size of the hyporheic zone. Steven A. Thomas, a research scientist in biology at Virginia Tech, and colleagues at Virginia Tech, the University of New Mexico, and the Environmental Science Division at Oak Ridge National Laboratory have been examining the hyporheic zone beneath streams, where surface and subsurface waters mix. This region is a "hot spot," Thomas said, because biological activity there may be a significant player in stream nitrogen dynamics.
Thomas and his fellow researchers are midway through a three-year grant from the National Science Foundation, and their results tend to show that about half the nitrogen retained in streams occurs in the hyporheic zones. They are examining the ability of such areas to take the nitrogen out of the water. "Streams that have high amounts of surface-subsurface exchange tend to retain more nitrogen," Thomas said.
While retention might be temporary, it is ultimately desirable that the nitrogen be returned to a gaseous dinitrogen, he said. Retention delays the sweeping of the materials downstream, but if denitrification does not occur, the material will be transported downstream, where it may alter environmental conditions. "We're trying to determine whether hyporheic zones are important denitrification areas and therefore important in the global nitrogen cycle," he said.
For example, agricultural areas often use fertilizer, containing nitrogen and phosphorus, which ultimately runs off into nearby streams. This process results in higher levels of nitrogen than naturally occur in the streams and may cause species shifts, or the growth of organisms that are not representative of that area.
Also, if this nitrogen makes its way into coastal areas and lakes, it can cause eutrophication, or excessive growth of weeds, causing a wide variety of water-quality problems.
Thomas and his colleages—H. Maurice Valett and Jackson R. Webster of Virginia Tech's biology department, Clifford Dahm of the University of New Mexico, and Patrick J. Mulholland of Oak Ridge National Laboratory—added a stable isotope of nitrogen, as sodium nitrate, along with chloride, to four streams. Two were in the mesic forests of North Carolina and Tennessee and two in the semi-arid region of New Mexico.
The use of Na15NO3, a heavier isotope of sodium nitrate, enabled the researchers to distinguish the introduced nitrogen nitrates from naturally occurring forms existing in the water and to avoid changing the magnitude of the nitrogen pool. They used the ratio of nitrate and chloride to quantify the nitrate uptake, or the amount of nitrates needed by the stream, and to determine whether that uptake occurred in the surface and subsurface areas of the stream.
Streams and associated habitats, i.e., hyporheic zones, may be important control points for remediating excess nitrogen produced by human activities, Thomas said. Denitrification is an anaerobic process that occurs under very low levels of oxygen and can be common in some aquatic environments, Thomas said. Bacteria can live in such environments by using nitrate, rather than oxygen, for respiration. In this activity, nitrate is converted to dinitrogen gas (N2), a process referred to as denitrification.
Thomas and his colleagues are assessing the nitrogen dynamics of the hyporheic zone. "If we find a lot of denitrification in the headwaters, it's a cue that we may need to pay particular attention to those areas when we plan development," Thomas said. The studies could affect the priorities of development in those regions, he said. "They may be key ecological areas that deserve a greater level of protection if it turns out they are major denitrification zones." Thomas and his colleagues suspect that small streams may be very important, since they are so numerous and cover such a large area.
Thomas presented the first year's research results at the Geological Society of America's meeting in Reno in November.
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