Developing Critical Loads for Atmospheric Deposition of Nitrogen to High Alpine Lakes in the Pacific Northwest: Preliminary Results

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. Nitrogen limitation of alpine lakes makes these ecosystems especially sensitive to additional inputs of atmospheric nitrogen because they have adapted to an oligotrophic environment and may have less capacity to use additional inputs. Previous work at Rocky Mountain (ROMO) National Park in Colorado indicated that nitrogen deposition at levels greater than 1.5 kg/ha-yr was sufficient to alter trophic state in high alpine lakes. In Washington State, long term National Atmospheric Deposition Program (NADP) monitoring at low elevation (< ~1,500 feet) has shown deposition amounts around the 1.5 kg/ha-yr effects level determined for ROMO. However, deposition data for higher elevations in Washington is lacking. It is hypothesized that at higher elevations, greater amounts of precipitation leads to greater nitrogen deposition when compared to low elevation NADP sites. The U.S. Geological Survey (USGS) initiated a study with the National Park Service to address this question and work to develop critical loads for the Pacific Northwest. Similar to ROMO, our approach to develop a critical load for nitrogen deposition is based on shifts in sediment diatoms at 12 lakes in Mount Rainier, North Cascades, and Olympic National Parks. Sites were located at elevations higher than 4,000 feet with minimal forest cover to increase the potential nitrogen deposition to these lakes. Historical data from the sites showed that surface water was low in nitrogen and phosphorus indicating that the lakes are oligotrophic and potentially susceptible to increases in these nutrients. The DIN:TP ratio of surface water, an indicator of nutrient limitation for phytoplankton, ranged from 0.2 to 12.8 across all sites. Olympic National Park was the only park that showed consistently low DIN:TP ratios and evidence of nitrogen limitation. During summer 2008, bulk nitrogen deposition was determined using ion exchange resin collectors and are currently being analyzed. Five collectors were installed at each lake, and rainwater was collected in a collapsible bag attached to the outlet of the resin column at three of the five collectors. Precipitation volumes were variable within and across the three parks indicating the heterogeneity in precipitation in this region. During summer 2009, a sediment core from each lake will be collected for diatom analysis and dated to identify if and when shifts to nitrophilic species occur. This presentation will outline preliminary data collected to date and discuss work planned for the final field season in 2010.

* , phone: 253-552-1611