UNRAVELING PAST HYDROCLIMATE DYNAMICS IN A SIERRA NEVADA LAKE, CALIFORNIA. INSIGHTS FROM DIATOM COMMUNITY ASSEMBLAGES AND FIRE RECONSTRUCTIONS

Increasing climate variability in the Sierra Nevada, California, has resulted in warmer temperatures, leading to changes in winter precipitation and fire regimes. Yet, the Sierra Nevada spring meltwater is an important water source for both 40 million people in the San Joaquin Valley, and for sustaining agriculture and tourism. High-resolution paleoenvironmental records are a powerful tool for understanding the natural variability of aquatic ecosystems and how they respond to changes in their surroundings. With the aim of understanding the impact that human activities and changes in hydroclimate have had in lakes in the Sierra Nevada, we collected a series sediment cores from Gull Lake, located at the western edge of the Great Basin in eastern California. Through the use of diatoms as a biological indicator and charcoal as a fire proxy, we aim to identify changes in water quality in relation to human occupation of the basin, as well as anthropogenic climate change. The sediment record spans the last ~265 years and can be characterized by three distinct zones of ecological change. Zone 1 (42-35 cm) is defined by the abundance of Stephanodiscus coruscus and Asterionella formosa. Zone 2 shows a shift towards the dominance of Stephanodiscus minutulus and Fragilaria crotonensis. Zone 3 is defined by an increase in diatom diversity, with an increase in abundance of Aulacoseira granulata var. angustissima accompanied by an increase in the tychoplankton. The transition from Zone 1 to Zone 2 seems to correlate to an increase in the fire intensity in the basin, as interpreted by the charcoal data. These data suggest that species turnover in this ecosystem may be subject to nutrient cycling and changes in nutrient input from the catchment as a response to fire. The preliminary age model suggests that this transition took place in the turn of the 20^th century. We infer this is associated with hydrological disconnection to June Lake as water level regressed as a response to Industrial Era climate change.