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Spatial Distribution of Nitrogen Deposition in Rocky Mountain National Park

David Clow*
USGS, Denver Federal Center, MS 415, Denver CO 80225

Mark Fenn
USDA Forest Service, 4955 Canyon Crest Drive, Riverside, CA 92507

Don Campbell, Leora Nanus
USGS, Denver Federal Center, MS 415, Denver CO 80225


High-elevation streams and lakes in Rocky Mountain National Park are being adversely impacted by atmospheric deposition of nitrogen (N). Previous studies have documented high concentrations of nitrate in surface waters, particularly on the east side of park, and resultant changes in species composition of aquatic biota. Prevailing winds are from the west, but transport of pollutants from urban and agricultural sources to the east probably occurs during upslope meteorological conditions that are common during spring and summer. Higher N deposition rates on the east side of the park have been suggested as an explanation for the higher surface-water nitrate concentrations there, but there is little quantitative information about spatial patterns in N deposition in the park, largely due to the difficulty of access. Ninety-five percent of the park is managed as wilderness, which precludes motorized access.

In this study, a relatively new method using ion-exchange resin (IER) collectors is being tested as an inexpensive means of collecting spatially extensive deposition data in these wilderness settings. IER collectors use a 7 inch-diameter plastic funnel to capture rain, which flows downward through a column containing ion-exchange resin. Columns are swapped twice per year, and N compounds are eluted from the resins in the lab and analyzed. IER collectors were deployed in pairs from late July, 2006 to early October, 2006 along several east-west transects in the park. Valid samples were obtained from 11 sites on the eastern slope and 5 sites on the western slope. The median difference between pairs of co-located samplers was 13% for total N deposition, indicating good reproducibility. Nitrate deposition measured by IER collectors co-located with the Loch Vale NADP site matched that of the NADP collector (0.48 kg/ha); ammonium deposition was slightly higher from the IER collectors (0.60 kg/ha) than from the NADP collector (0.50 kg/ha). On average, ammonium accounted for 60 ± 7% of total N in the bulk deposition captured by the IER collectors, compared to 45% in wet deposition at the two NADP sites in the park. CASTNET data indicates that dry deposition of N occurs primarily as nitrate compounds, so the difference in ammonium deposition probably is not due to dry deposition. The IER collectors may be less prone to loss of ammonium via nitrification than NADP collectors.

Average N deposition for the 85-day exposure period at the eastern slope sites was 0.85 ± 0.21 kg/ha, and at west-slope sites was 0.69 ± 0.12 kg/ha. An ANOVA analysis indicated that differences in the means of east- and west-side sites were not statistically different at p < 0.05, however, these results should be interpreted with caution due to the small sample size. Sampling is continuing and additional sites have been added on the west side of the park to provide improved statistical power.