Lake and stream ecosystems integrate conditions within their catchments and regional climatic changes. Results of prior studies in high elevation aquatic ecosystems of the Sierra Nevada have indicated changes in the importance of phosphorus to biological activity. The proposed research will test the hypothesis that the changing snow regime and atmospheric inputs of phosphorus are driving ecological changes in high elevation lakes. Continued measurements of chemical and biological conditions in lakes and streams in the southern Sierra Nevada, application of advanced techniques to determine atmospheric inputs, and analyses of phosphorus biogeochemistry in soils and sediments will be conducted to test the hypothesis. High elevation lakes are excellent indicators of regional environmental changes and are increasingly being used by regulatory agencies to establish critical loads for atmospheric pollutants. Further, the importance of montane snowpacks to freshwater supplies of the western states mandates that particular attention be paid to these systems. Continuation of more than two decades of meteorological, hydrological, chemical and biological measurements in sites at high elevation will increase their value as detectors of conditions as regional variations occur. The proposed studies will continue to be used by the National Park Service in interpretative programs to demonstrate the role of scientific research in the National Parks. Engagement of local high school students in the studies fosters an interest in natural environments and encourages these students to attend college and pursue professional careers.
Lakes have long attracted interest by many people for economic, recreational, aesthetic and scientific reasons. Because of the high degree of interconnectedness of lakes to soils and the atmosphere, lakes integrate conditions within their catchments and are sentinels of environmental change. Mountain lakes, in particular, are recognized as excellent indicators of conditions in the atmosphere and their watersheds. Emerald Lake and the Tokopah Valley in the Sierra Nevada (California) have been the focus of our ecological research since the early 1980s. Knowledge gained through this research has transformed our understanding of how the thousands of lakes and watersheds in the Sierra Nevada are responding to their environment. Studies have been motivated by concerns over the impact of acid deposition on aquatic ecosystems, lake and watershed responses to atmospheric deposition of nutrients and to climate variability. Our research activities have benefited air quality regulation in California, Federal management of wilderness areas in the Sierra Nevada and student education spanning K-12, undergraduate and graduate levels. Through funding from the NSF-LTREB program, long-term observations have continued and this research has provided new insights into how altered climate, variable snowfall and changes in atmospheric composition are influencing high-elevation ecosystems in the Sierra Nevada. Summer-time air temperatures at Emerald Lake have been increasing over the last 30 years at a rate faster than average global increases. Though the water content of the snowpack has varied by a factor of five and rainfall amounts have varied by a factor of 20, no long-term trends in precipitation are evident. Nevertheless, such hydrological variability is an important driver of both seasonal and inter-annual variations in nitrate concentrations and sources, acid neutralizing capacity of water and lake metabolism. Atmospheric particulate matter derived from local and regional wildfires add considerable amounts of phosphorus to the watershed, and metal ratios suggest a mixture of dust from regional agriculture and long-range transport of dust from Asia reaches the high-elevation Sierra Nevada. Within the soils of the watershed microbes retain phosphorus under snow which highlights a process important to ecosystem fertility sensitive to climate changes. One likely consequence of climate change for high-elevation lakes is more precipitation as rain, and we observed considerable ecological change as a consequence of an especially large rain storm that occurred during an "Atmospheric River" event. Large quantities of material washed into the lake, reducing the transparency of the lake water, increasing nitrate concentrations by over 100 times, and shifting the lake's metabolism to be consume more dissolved oxygen than it produced. Paleolimnological investigations at Emerald, Pear and other Sierra Nevada lakes suggest that acid deposition impacted the alkalinity of some lakes early on in the 20th century. Acid deposition effects appear to peak in the 1960s and 1970s and we have noted a pronounced recovery of alkalinity, most likely due to improvements in air quality brought about by the Clean Air Act and stringent air pollution controls implemented by the State of California.
