LATE‐QUATERNARY SETTING FOR THE EVOLUTION OF AN ENDEMIC DIATOM IN YELLOWSTONE LAKE, USA

Theriot, E. C. 1 , Fritz, S. C. 2 , Whitlock, C. 3 , Conley, D. J. 4 , Kilham, S. S. 5 & Interlandi, S. 51 Texas Memorial Museum of Science and History, 2400 Trinity Street, Austin, TX 78705; 2 Department of Geosciences and School of Biological Sciences, University of Nebraska, Lincoln, NE 69588‐0340, USA; 3 Department of Geography, University of Oregon, Eugene, OR 97403, USA; 4 Department of Marine Biology & Microbiology, National Environmental Research Institute, PO Box 358, DK‐4000 Roskilde, Denmark; 5 Department of Bioscience and Biotechnology, Drexel University, Philadelphia, PA 19104 The diatom Stephanodiscus yellowstonensis Theriot and Stoermer is endemic to Yellowstone Lake, where it can be an important component of the summer phytoplankton assemblage. It is closely related to Stephanodiscus niagarae Ehrenberg which is abundant in nearby Heart Lake, Lewis Lake, Jackson Lake and several regional reservoirs. Here we use the stratigraphic record of Yellowstone Lake to investigate the evolution of S. niagarae to S. yellowstonensis and to describe the limnologic and climatic conditions associated with its evolution. A dramatic morphological shift takes place between about 10,000 and 9,000 years before present (ybp). Coincident with the morphological change in the S. niagarae/S. yellowstonensis complex are changes in the diatom species assemblage, biogenic silica concentrations, sediment lithology, and regional vegetation (as indicated by pollen analyses), which suggests environmental change as an influence on speciation. The increase in biogenic silica concentrations indicates elevated siliceous algal biomass in the lake driven by increased nutrient availability, particularly Si and N. Paleoclimatic recon‐structions for southern regions of Yellowstone Park at this time suggest increased winter precipitation, as a result of wastage of the Laurentide ice sheet and the northward migration of the winter jet. Thus, the expansion of A. formosa and total diatom biomass in Yellowstone Lake was likely fostered by both increased N and Si loading driven by increased winter precipitation and earlier ice out caused by warming. The finely laminated sediments suggest elevated lake level and long periods of stable water‐column stratification, also associated with the elevated precipitation and regional warming.