Dissertation Research: The Role of Anthropogenic Acidification in Stream Nitrogen Cycling: Understanding Stress Effects on Ecosystem Function

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This research will investigate how stream acidification caused by acid rain influences the biological removal of nitrogen from surface waters. Understanding environmental controls on nitrogen uptake is essential as human activities continue to increase stream concentrations of nitrogen pollutants. While stream acidification is known to have negative effects on fish and invertebrate communities, there is little understanding of acid effects on nitrogen cycling. The investigators will study nine streams in Shenandoah National Park, VA, that differ in chronic pH to infer how nitrogen uptake processes change with acidification over time.The researchers use a basic concept of stress response - a shift in resource allocation away from growth and towards maintenance - to guide investigations at both organismal and ecosystem scales. Laboratory experiments are designed to measure biomass-specific rates of nitrogen uptake, respiration, and enzyme activity by leaf biofilms from all streams. Isotopic techniques will be used to measure the nitrogen demand of streams in situ, and these rates will be combined with whole-stream rates of respiration and microbial biomass in an effort to interpret ecosystem behavior in the physiological context of stress response. Finally, the investigators will use isotopic techniques in four streams along the pH gradient to measure nitrogen demand under experimentally-increased levels of nitrogen concentration to predict future stream uptake potential.