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Greater Yellowstone Ecosystem Report

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Sediment records from several high alpine lakes in the Grand Teton National Park (GRTE), Wyoming were examined for stable isotopic signatures δ15N and diatom community composition because atmospheric deposition of reactive nitrogen (Nr) is known to be altering ecosystem functioning in other lakes of the Rocky Mountain Range. Alpine lakes exposed to greater Nr impacts in Colorado have higher N:P ratios in the water column, indicating an excess of N, thus GRTE sites were selected across a range of N:P values, spanning measures indicative of nitrogen to phosphorus limitation. Sediment cores were analyzed for diatom relative abundances, concentrations of carbon, nitrogen and phosphorus, and stable isotopic signatures of δ13C and δ15N. Every sediment record showed progressive δ15N depletion, evidence of increasing Nr deposition during the past forty years. In the GRTE, benthic flora dominated the community composition without changes to the fossil diatom taxonomy, a response atypical of other Nr impacted sites in the Rocky Mountain Range that have exhibited a marked shift towards nitrophilous planktonic diatoms Asterionella formosa and Fragilaria crotonensis. The suite of GRTE lacustrine sediment records exhibited a continuum of increasing nutrient enrichment from low to high N:P ratios, suggesting that lakes sensitive to Nr enrichment exhibit elevated ratios of N:P. The long-term impact of Nr deposition has not reached a critical threshold, but monitoring of the GRTE lakes needs to incorporate assessment of the N:P ratios in advance of greater ecological impacts.