Alpine/subalpine Lakes in the Pacific Northwest may be subjected to environmental stressors, such as transpacific atmospheric pollutants deposition. Diatom assemblages in these lakes are sensitive to environmental changes and siliceous diatom fossils are well preserved in lake sediments. The purpose of this study is to characterize changes of sediment diatom assemblages over time. The lake sediment cores and physicochemical data were collected from 10 alpine/subalpine lakes in western Oregon. Diatom species assemblages were dominated by benthic diatoms such as Cymbella, Pinnularia, Nitzschia, Navicula, and small chain-forming Fragilaria taxa. Several planktonic taxa such as Aulacoseira alpigena (Grunow) Krammer, Fragilaria crotonensis Kitton, and Asterionella formosa Hassal were abundant in a few lakes.  Dominant taxa at the top and bottom sediment layers were overall similar in most of lakes. 

Changes in dominant taxa were evident in South Twin Lake. Our data suggest that most of sampled alpine/subalpine lakes have been relatively stable over time. Characterizing diatom assemblages in these lakes provides a bench mark on their natural conditions.