DIATOM-INFERRED STRATIFICATION AND FIRE HISTORY OF ISLAND LAKE, WYOMING

Island Lake, an alpine lake of glacial origin, is located in the Beartooth Mountains near Cody, Wyoming at an elevation of 3,048 meters. Like many alpine lakes, Island Lake is highly transparent with a deep chlorophyll maximum and has a maximum water depth of 33 m. Thus, it is ideally suited for researching the relationships between ecological bioindicators for mixing depth and fire. Fire is an integral ecosystem function within the Rocky Mountains. Two common fire reconstruction methods include sedimentary charcoal and fire-scarred trees.

In this study, we explore preliminary paleolimnological data from a sediment core measuring 1.54 meters, which was collected in the summer of 2013. Radiocarbon analysis revealed the core dates back to 10,000 BP. Diatom assemblages were counted for the length of the core. Two planktonic diatom genera, Discostella and Aulacoseira, were abundant throughout. Both of these genera have known ecological associations with stratification and mixing, respectively, in low-nutrient lakes from this region. Based on the relative abundances of fossil diatom species in each group, we created a diatom-inferred stratification index to reconstruct changes in lake stratification patterns over the Holocene. This stratification index was compared to dendrochronology fire history records of the area over the last 500 BP to assess whether mixing depth changes can be correlated with terrestrial fire events. Together, these paleo-environmental indicators show substantial changes throughout the Holocene at Island Lake and provide context for other regional records of fire history.