Ecological Applications


Mark W Williams
Department of Geography and Institute of Arctic and Alpine Research University of Colorado, Boulder

Kathy Tonnesson
Air Resources Division National Park Service Denver, CO


We suggest an empirical approach for determining critical loads for inorganic N deposition in wetfall to the central Rocky Mountains. We define "critical loads" as a deposition amount above which natural resources can be negatively affected. The arithmetic average from 1992 to 1996 of annual inorganic N deposition in wetfall at the eight NADP sites located at elevations greater than 2,500\|m in the central Rocky Mountains ranged from 2.5 to 3.5\|@kghayr@. In contrast, inorganic N deposition was less than 2.5\|@kghayr@ at all 23 NADP sites less than 2,500\|m in elevation. At the Niwot Ridge NADP site in the Colorado Front Range, a simple linear regression of inorganic N in wetfall with time shows a significant increase in deposition of inorganic N in wetfall at the rate of of 0.32\|@kghayr@ (@r2@ = 0.62; p < 0.001, n = 13). In turn, the increasing amount of inorganic N in wetfall is causing episodic acidification in headwater catchments of the Green Lakes Valley in the Colorado Front Range, with acid neutralizing capacity (ANC) values below 0\|@mueq@ in surface waters during snowmelt runoff at 9-ha and 42-ha sampling sites. At present rates of ANC decrease, we can expect the 9-ha and 42-ha sites to become chronically acidified within the next decade and the 220-ha basin of Green Lake 4 to become episodically acidified. A synoptic survey in 1995 of 91 high-elevation lakes in the central Rocky Mountains suggests that water quality is being affected by inorganic N in wetfall throughout the region. Federal land managers are required to "err on the side of protection" when assessing the amount of deposition that will alter ecosystem processes. However, given the political and economic ramifications of policy decisions, land managers are aware of the need to provide a scientific basis for these decisions and to balance conflicting needs. To achieve this balance and to allow for natural resource protection, we make a conservative recommendation that critical loads of inorganic N in wetfall to Class 1 areas in in the central Rocky Mountains be set at 4\|@kghayr@. Target loads my be set at lower levels of inorganic N deposition in wetfall to allow a margin of safety to protect extremely sensitive natural resources.