HR: 13:40h
AN: H53F-01 INVITED     [Abstracts]
TI: The Relative Effects of Hydrology, Ecology, and Climate on Temporal Trends and Spatial Patterns of Stream Nitrate Concentrations in the Catskill Mountains, New York, USA
AU: * Burns, D A
EM: daburns@usgs.gov
AF: U.S. Geological Survey, 425 Jordan Rd., Troy, NY 12180 United States
AU: Lovett, G M
EM: lovettg@ecostudies.org
AF: Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545 United States
AU: Murdoch, P S
EM: pmurdoch@usgs.gov
AF: U.S. Geological Survey, 425 Jordan Rd., Troy, NY 12180 United States
AB: The Catskill Mountains of New York receive 10 to 15 kg ha$^{-1}$ yr$^{-1}$ of nitrogen (N) in atmospheric deposition, among the highest rates in eastern North America. Consequently, streams in the Catskills have relatively high nitrate (NO$_{3}$$^{-}$) concentrations (mean of stream surveys = 20 to 30 $\mu$mol L$^{-1}$ at baseflow), which contribute to chronic and episodic stream acidification. Stream chemistry monitoring in the 1980s showed an increasing trend in NO$_{3}$$^{-}$ concentrations that was attributed to N saturation, a condition whereby continued high rates of atmospheric N deposition in combination with a maturing forest result in gradually decreasing ecosystem N retention. This increasing trend reversed itself during the early 1990s, a pattern that was partially attributed to an intense region-wide soil freezing event in December 1989. Stream NO$_{3}$$^{-}$ concentrations remained relatively low at long-term monitoring sites from 1992-03 despite relatively constant atmospheric N deposition rates from the 1980s through 2003, suggesting that a simple interpretation of the N saturation model is not valid on decadal time scales. The discovery of groundwater seeps with relatively high NO$_{3}$$^{-}$ concentrations in the 1990s led to a hypothesis that the presence of seeps in the Catskills may be controlling spatial variability in stream NO$_{3}$$^{-}$ concentrations, which range from near 0 to about 50 $\mu$mol L$^{-1}$ at baseflow. Subsequent research indicated these variations in stream NO$_{3}$$^{-}$ concentrations are likely controlled by variations in tree species dominance. Nitrate concentrations in drainage waters are highest in stands of sugar maple and yellow birch and lowest in red oak and hemlock stands. One study, however, showed that NO$_{3}$$^{-}$ concentrations in shallow groundwater were correlated with a topographic index that is a surrogate for soil moisture suggesting that covariance of tree species and soil moisture may amplify the apparent differences in NO$_{3}$$^{-}$ concentrations previously attributed solely to differences in tree species. A conceptual model of controls on spatial variation of stream NO$_{3}$$^{-}$ concentrations in the Catskills indicates that these values are established in the shallow soil at the time of recharge as a result of differences in relative nitrification rates caused by differences in tree species effects on the relative recalcitrance and C/N ratio of litter. Groundwater seeps serve mainly as conduits that drain a deeper reservoir that can provide seasonally higher NO$_{3}$$^{-}$ concentrations during the summer growing season. In spring 2004, stream NO$_{3}$$^{-}$ concentrations reached 125 $\mu$mol L$^{-1}$ in Biscuit Brook, among the highest values ever recorded at this stream during 21 years of monitoring. A preliminary analysis finds no apparent disturbance or extreme climatic event that may have caused such high concentrations. This result suggests that much remains to be learned about the controls on temporal patterns and trends in stream NO$_{3}$$^{-}$ concentrations in this region. Whereas a viable conceptual model explains most spatial variability in NO$_{3}$$^{-}$ concentrations among streams, a simple interpretation of the N saturation model that would indicate a long-term trend of increasing stream NO$_{3}$$^{-}$ concentrations does not explain the observed temporal patterns. A more complex model that includes the role of climate, hydrology, and disturbances caused by insects and tree disease may be necessary to adequately predict long-term trends.
DE: 1803 Anthropogenic effects
DE: 1806 Chemistry of fresh water
DE: 1871 Surface water quality
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting