Evaluating regional patterns in nitrate sources to watersheds in National Parks of the Rocky Mountains using nitrate isotopes.

In the Rocky Mountains, there is uncertainty about the source areas and emission types that contribute to nitrate (NO3) deposition, which can adversely affect sensitive aquatic habitats of high-elevation watersheds. Regional patterns in NO3 deposition sources were evaluated using NO3 isotopes in five National Parks, including 37 lakes and 7 precipitation sites. Results indicate that lake NO3 ranged from detection limit to 38 microeq/L, delta18O (NO3) ranged from -5.7 to +21.3% per thousand, and delta15N (NO3) ranged from -6.6 to +4.6 per thousand. delta18O (NO3) in precipitation ranged from +71 to +78% per thousand. delta15N (NO3) in precipitation and lakes overlap; however, delta15N (NO3) in precipitation is more depleted than delta15N (NO3) in lakes, ranging from -5.5 to -2.0 per thousand. delta15N (NO3) values are significantly related (p < 0.05) to wet deposition of inorganic N, sulfate, and acidity, suggesting that spatial variability of delta15N (NO3) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO3 and delta15N (NO3) are more enriched in lakes and precipitation from the southern Rockies and at higher elevations compared to the northern Rockies. The correspondence of high NO3 and enriched delta15N (NO3) in precipitation with high NO3 and enriched delta15N (NO3) in lakes, suggests that deposition of inorganic N in wetfall may affect the amount of NO3 in lakes through a combination of direct and indirect processes such as enhanced nitrification.