Development of Critical N Loads Using Diatom Proxies: Distribution of Indicator Species in Lakes of the Sierra Nevada of California

Dr. Danuta Bennett1, Andrea Heard2, Delores Lucero2 and Dr. James O. Sickman2

Mountain lakes are recognized as excellent indicators of regional environmental change and are increasingly being used by regulatory agencies to establish critical loads for atmospheric pollutants. Long-term monitoring of lakes throughout the montane western United States indicates that aquatic ecosystems are responding to increasing nutrient inputs most likely from atmospheric deposition. In the Rocky Mountains, several species of lake diatoms have been identified as indicators of recent eutrophication by N and include: Cyclotella bodanica var. lemanica, Asterionella formosa and Fragilaria crotonensis. However, relatively little is known of the current distribution of these indicator species in the Sierra Nevada of California which contains over 6,000 alpine and subalpine lakes. In order to explore whether a similar trend in changes of diatom species composition is occurring in Sierra Nevada alpine lakes we present preliminary results from an ongoing paleolimnology investigation designed to establish critical N loads for the region. This study is using data from a calibration set of 50 high-elevations lakes with large variation in nitrate concentrations, along with long sediment cores from four lakes in the Central Sierra Nevada. The diatom flora and nutrient chemistry in a subset of the calibration lakes will be described and compared, with special attention paid to the occurrence and distribution of nitrogen enrichment indicator species to evaluate if lakes in the Sierra Nevada have been affected by atmospheric deposition of N during the 20th Century.

1 Marine Science Institute, University of California, Santa Barbara CA 93106,Email: , Phone: (805) 804-7230
2 Department of Environmental Sciences, University of California, Riverside, CA 92521