Anthropogenic nitrogen deposition induces rapid ecological changes in alpine lakes of the Colorado Front Range (USA)

Recent sediments from two alpine lakes (> 3300 m asl) in the Colorado Front Range (USA) register marked and near-synchronous changes that are believed to represent ecological responses to enhanced atmospheric deposition of fi xed nitrogen from anthropogenic sources. Directional shifts in sediment pro xies include greater representations of mesotrophic diatoms and increasingl y depleted delta N-15 values. These trends are particularly pronounced sinc e similar to 1950, and appear to chronicle lake responses to excess N deriv ed from agricultural and industrial sources to the east. The rate and magni tude of recent ecological changes far exceed the context of natural variabi lity, as inferred from comparative analyses of a long core capturing the en tire 14,000-year postglacial history of one of the lakes. Nitrogen depositi on to these seemingly pristine natural areas has resulted in subtle but det ectable limnological changes that likely represent the beginning of a stron ger response to nitrogen enrichment.