MISSING DIATOMS – HOW LAKE BATHYMETRY AFFECTS THE DISTRIBUTION OF DIATOM ASSEMBLAGES AND THE INTERPRETATION OF LAKE LEVEL VARIABILITY IN SMALL SUBALPINE LAKES

Small lakes at middle- to high-elevations (1,500-3,500 m asl) can be sensitive recorders of both regional climate change and local conditions. For example, basin morphology of these small lakes is known to have a significant effect on the thermal structure of the water column, as well as the duration and spatial extent of winter ice cover. In turn, these conditions may affect the species composition of the diatom flora found in these lakes. In an effort to better characterize diatom microfossil variability in small lakes, this study explores the relationship between lake bathymetry and diatom assemblages. The lakes in this study have been divided into three categories: Type I - shallow (< 2 m deep), flat-bottomed; Type II - deep central basin (>5 m deep) surrounded by a shallow shelf (“inverted sombrero”); and Type III - deep (>6 m deep) with steep sides. Shallow, flat-bottomed lakes are characterized by a diverse benthic assemblage including motile, non-motile, and prostrate epiphytic species. Lakes with a shallow shelf and deep basin are often dominated by tychoplanktonic taxa which form large mats in shallow water. As these mats break up, the constituent diatoms are then deposited in deeper water. Deep lakes with steep sides are often dominated by planktic diatoms. As lake levels change over time, the basin morphology of individual lakes can also change. For example, a Type I lake may become a Type III lake as it fills, eventually culminating in a Type II lake. This leads to variations in the timing and magnitude of thermal stratification and ice cover, which in turn can lead to changes in the dominant members of the diatom assemblage. Variations in bathymetry may limit or eliminate entire groups of diatoms. For example, as water levels vary, a type III lake (deep water; planktic diatoms) can change to a type I lake (shallow; benthic diatoms) without ever containing the tychoplanktonic assemblages characteristic of a type II lake. With an understanding of the relationship between basin bathymetry and modern diatom flora, changes in the diatom assemblage preserved in sediment cores can then be used to infer past variations in lake levels. Data from lakes in California and eastern Nevada will be presented as examples of modern variability and as potential archives of lake level change.