| Summary: | Background: Although arctic lakes have responded sensitively to 20 th-century climate change, it remains uncertain how these ecological transformations compare with alpine and montane-boreal counterparts over the same interval. Furthermore, it is unclear to what degree other forcings, including atmospheric deposition of anthropogenic reactive nitrogen (Nr), have participated in recent regime shifts. Diatom-based paleolimnological syntheses offer an effective tool for retrospective assessments of past and ongoing changes in remote lake ecosystems. Methodology/Principal Findings: We synthesized 52 dated sediment diatom records from lakes in western North America and west Greenland, spanning broad latitudinal and altitudinal gradients, and representing alpine (n = 15), arctic (n = 20), and forested boreal-montane (n = 17) ecosystems. Diatom compositional turnover (b-diversity) during the 20 th century was estimated using Detrended Canonical Correspondence Analysis (DCCA) for each site and compared, for cores with sufficiently robust chronologies, to both the 19 th century and the prior,250 years (Little Ice Age). For both arctic and alpine lakes, b-diversity during the 20 th century is significantly greater than the previous 350 years, and increases with both latitude and altitude. Because no correlation is apparent between 20 th-century diatom b-diversity and any single physical or limnological parameter (including lake and catchment area, maximum depth, pH, conductivity, [NO3 2], modeled Nr
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