Anthropogenic and climatic influences on the diatom flora within the Fallen Leaf Lake watershed, Lake Tahoe Basin, California over the last millennium

This study addresses the effects of climate, land-use, and atmospheric nitrogen (N) deposition on the Fallen Leaf Lake watershed, Lake Tahoe Basin, through diatom and geochemical analyses of sediment cores. Four diatom zones are recognized from a core taken at Fallen Leaf Lake (FLL), a site moderately impacted by human activities: (1) Pre-Little Ice Age Zone (840–1385), (2) Little Ice Age Zone (1385–1810) characterized by Stephanodiscus alpinus and Aulacoseira subarctica, (3) Transitional Zone (1810–1950) of warming and anthropogenic influence with increased Lindavia rossii-ocellata group and Discostella stelligera and decreased Pseudostaurosira brevistriata, and (4) Anthropogenic Zone (1950–2010) characterized by a rapid increase of mesotrophic diatoms of the Fragilaria tenera-nanana group, Tabellaria flocculosa strain IIIP, and Nitzschia gracilis. The Transitional Zone increases in elemental Co, Zn, and Sn that may be attributed to an increase in coal burning and smelting activities in California and Nevada. Beginning around 1910 and accelerating in the 1940s, increased building, land-use, and recreation around FLL caused an increase in terrestrial sedimentary input. Down core proxies for atmospheric N deposition in the FLL watershed are, at best, weakly expressed and appear to be overshadowed by stronger signals. Lack of support for N deposition includes the asynchrony in the appearance of the N-sensitive diatom Asterionella formosa in FLL and a lower impact site at Gilmore Lake, and an uninformative δ¹⁵N record. Asterionella formosa is a dominant component in the FLL water column today, but has been present in similar abundances for at least the last 1200 years. Asterionella formosa is present in the water column at Gilmore Lake and absent from the sediment, indicating a very recent appearance. The data collected show that the FLL record is sensitive to climatic cooling during the Little Ice Age and to anthropogenic activities commencing in the 1800s that increased throughout the latter half of the twentieth century; however the effects of anthropogenic N deposition in these lakes could not be substantiated.