Sensitivity of Pacific Northwest Mountain Lakes to Eutrophication Effects of Atmospheric Nitrogen Deposition
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Mountain lakes are among the most sensitive and policy-relevant ecological indicators of atmospheric nitrogen deposition, but limited information is available about the effects of nitrogen deposition on Pacific Northwest mountain lakes. This dissertation assesses the sensitivity of Pacific Northwest mountain lakes to nitrogen deposition. The research herein is designed to generate information needed by air quality managers in federal land management agencies. Three aspects of N deposition effects in the Pacific Northwest are addressed. Chapter 2 evaluates the operational performance of several air quality models frequently used by researchers and air quality managers to estimate N deposition rates and test for critical load exceedances (model deposition > critical load). Results demonstrate that model uncertainty is frequently large relative to critical load values for diatoms and lichens in the Pacific Northwest, and could potentially cause false positive or false negative exceedance test results. Chapter 2 includes several recommendations regional air quality managers could use to minimize uncertainty in exceedance tests. Chapter 3 describes results of in situ nutrient enrichment bioassays conducted in mountain lakes within Mount Rainier, North Cascades, and Olympic National Parks. Experiments characterized phytoplankton species and biomass responses to nitrogen enrichment, and associated nitrogen concentration thresholds. This information is necessary to calculate critical loads and assess lake sensitivity to N deposition. Approximately 75% of sampled lakes across the three parks have concentrations below experimentally-defined thresholds, suggesting many park mountain lakes are sensitive to future N deposition increases. Chapter 4 tests for differences in nitrate chemistry, phytoplankton biomass, and phytoplankton community structure between lakes with and without a glacier in their watershed at North Cascades National Park (NOCA). Glacier melting has increased nitrate concentrations in downstream streams and lakes in many mountainous regions, but its effects in the Pacific Northwest are not known. If glacier melting enriches mountain lakes with nitrate, nitrate may reduce sensitivity of glacier-fed lakes to future N deposition increases. Glacier-fed lakes at NOCA had slightly higher nitrate concentrations than snow-fed lakes, but there were no differences in phytoplankton biomass or community structure between lake types.