Feasibility study for using diatom assemblages from a small dilute subalpine lake as an indicator of past megadroughts in the Sierra Nevada

The Medieval Climate Anomaly (MCA) describes a period of extreme drought across the Great Basin and Sierra Nevada during the 12 th and 13th centuries CE. While the MCA has been well documented, the precise timing, severity, and spatial variability of individual droughts remain poorly constrained. Using a previously documented Medieval lowstand in Fallen Leaf Lake (FLL) as an independent calibration point, this study attempts to refine the regional record of the MCA by testing the sensitivity of the diatom record in FLL to the paleohydrographic variability in the Lake Tahoe watershed. Half cm-thick samples were taken at 4-cm intervals from piston core BOLLY-FLL10-2D-1K-1, which was recovered from the lake depocenter. These samples were analyzed for diatoms, providing an approximately 10 year composite snapshot every 70 years. A total of 180 diatom species were identified and are documented herein. Down-core diatom assemblages are dominated by phytoplankton, tychoplanktonic fragilariods and small monoraphids. Heavy, tychoplanktonic centrics (e.g. Aulacoseira subarctica) show the greatest variability in the species assemblage. Statistical analysis, including correspondence analysis, non-metric multidimensional scaling, agglomerative cluster analysis, and bivariate regression, suggests that the diatom assemblage responds to major disturbances in the lake followed by reestablishment of relatively static conditions, as opposed to known climate variables. The diatom assemblages appear to show little sensitivity to the MCA lowstand. Ordination results indicate that changes in lake stratification are the primary control on species assemblage variation, followed by temperature, yet regression against Palmer Drought Severity Index and Northern Hemisphere temperature anomalies did not show a strong correspondence. Additional work, including finer sampling resolution and detailed thermal modeling of the lake, would provide greater insight into the environmental factors controlling diatoms in Fallen Leaf Lake, and potentially allow a more direct link to climate variation.