USGS Washington Water Science Center
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9722-D8P - Developing Critical Loads for Atmospheric Deposition of Inorganic Nitrogen to North Coast and Cascades Network Lakes - Completed FY2011
Problem - Excessive nitrogen from atmospheric dry and wet deposition is a key cause of eutrophication in many ecosystems. Nitrogen limitation of alpine lakes makes alpine ecosystems especially sensitive to additional inputs of atmospheric nitrogen because of their inability to utilize atmospheric nitrogen for plant growth and to retain nitrogen in soils. In Rocky Mountain National Park, atmospheric nitrogen deposition increased and induced near-synchronous changes from oligotrophic to mesotrophic ecosystems, detected through changes in diatom communities, in the 1950s when atmospheric deposition averaged about 1.5 kg/ha/yr. The long-term wet average deposition of atmospheric nitrogen to four low-elevation sites in western Washington ranged between 0.9 and 1.8 kg/ha/yr. Monitoring in other regions suggest that deposition of N at higher elevations is greater because of higher precipitation rates at higher elevations, yet little high-elevation data are available for comparison in the North Coast and Cascades Network (NCCN) parks. If the ecosystems of alpine waters in western Washington respond to atmospheric nitrogen depositions as those in the Rocky Mountain National Park (ROMO) do, it is likely that some alpine lakes in the NCCN may be showing the beginnings of the shift in water quality from oligotrophic to mesotrophic ecosystems.
Objective - The objective of this proposal is to determine if alpine lakes within the network are receiving atmospheric N at rates sufficient to alter their trophic state and, if so, to identify the critical load - the deposition rate at which the change occurred. As part of this objective, we will determine if atmospheric deposition of nitrogen to alpine lakes in the NCCN is greater than the deposition measured at the low-elevation NADP sites. If a gradient of atmospheric deposition and ecosystem alteration exists, we will determine the amount of atmospheric deposition at which altered ecosystems are likely to occur in alpine lakes given their vulnerability, as measured by the ability of the watershed to utilize atmospheric nitrogen. If any lake ecosystem has already been altered, we will determine the present day amount of atmospheric deposition to the lake and estimate when the lake was altered through diatom populations in sediment cores.
Relevance and Benefits - This project is consistent with both the USGS Strategic Plan and the goals set forth in the USGS-National Park Service Water-Quality Partnership. Specifically, this research provides the hydrologic information needed to understand changes in ecosystems as a result of atmospheric transport of nitrogen compounds.
Approach - This study will be conducted in four phases: 1) site selection, during which existing atmospheric and hydrologic data will be analyzed and interpreted to choose lakes over a gradient of expected nitrogen deposition; 2) broad-scale sampling of 15 lakes and measurements of atmospheric deposition over the summer and water quality of the lake in the fall; 3) intensive study, the relation between water quality during the summer at three lakes and atmospheric deposition will be examined over week-scale intervals; and finally 4) the relations of lake trophic state and watershed vulnerability with atmospheric deposition will be evaluated.