Developing Critical Loads for Atmospheric Deposition of Nitrogen to Alpine Lakes in the Pacific Northwest using Sediment Diatoms

Richard W. Sheibley*, James R. Foreman, Patrick W. Moran and Anthony J. Paulson
U.S. Geological Survey
Washington Water Science Center
934 Broadway, Suite 300
Tacoma, WA, 98402

Excessive nitrogen from atmospheric deposition can be an important component of eutrophication in some aquatic ecosystems. Alpine lake ecosystems are nitrogen limited and especially sensitive to additional inputs of atmospheric nitrogen because they have adapted to an oligotrophic environment and may be sensitive to additional inputs. In Washington State, long term National Atmospheric Deposition Program (NADP) monitoring at low elevation (<1,500 feet) has shown deposition (loads) similar to the 1.5 kg/ha/yr effects-level determined for Rocky Mountain National Park (ROMO) in Colorado. Deposition data for higher elevations in Washington is lacking. The U.S. Geological Survey initiated a study with the National Park Service to address this data gap and work to determine effects-level critical loads for Washington State. Like ROMO, our approach to determine a critical load for nitrogen deposition is based on shifts in sediment diatom community composition in 12 lakes in Mount Rainier, North Cascades, and Olympic National Parks. Sites were at elevations above 4,000 feet with minimal forest cover to reduce non-atmospheric inputs of nitrogen subsequently increasing the potential effect from atmospheric deposition of nitrogen to these lakes. During summer 2008, bulk nitrogen deposition was determined using ion exchange resin collectors. Total N deposition rates ranged from 0.6 to 2.5 kg-N/ha/yr across all sites, and typically were higher (by about 0.5 kg/ha/yr) than the low elevation NAPD sites in Washington during the same period. The lowest summer deposition was in Olympic National Park and highest summer deposition was in North Cascades National Park. In summer 2009, each lake was sampled for nutrients and a sediment core was collected for diatom analysis. All lakes were extremely oligotrophic and at most sites ammonium, nitrate, and phosphate were at or below detection limits. Initial diatom analysis of top and bottom sections of each sediment core was done to identify the most affected lakes. Of the nine top-bottom diatom analyses completed, only two lakes showed signs of degradation. These lakes were in Olympic National Park where the lowest N deposition was measured. These cores will have a more complete diatom profile analyzed, and the sediment will be dated to indicate when major diatom changes have occurred, which will allow us to relate these changes to N deposition at nearby NADP sites to identify a critical load for N. Results from this ongoing project will be available by early 2011.

*U.S. Geological Survey, Washington Water Science Center, 934 Broadway, Suite 300, Tacoma, WA, 98402. 253-552-1611;