Detecting climate-driven ecological changes in high-altitude lakes in the Sierra Nevada, California

Over the past several decades, increasing climate instability in the Sierra Nevada, California, expressed primarily as reduced winter precipitation and higher temperatures, has led to more frequent drought. High-altitude lakes in this region have been characterized as pristine ecosystems, but growing evidence suggests that they are responding acutely to climate change. To address this, we analyzed the diatom assemblages of two ²¹⁰Pb dated sediment cores (Gull and June Lakes) from the eastern Sierra Nevada with the aim of assessing their sensitivity to and timing of responses to climate change at the end of the neoglacial (~1450 CE to ~1850 CE) and identifying how climate drivers can impact diatom communities. The nutrient cycles of both lakes have been disrupted by changes in thermal stratification, driven by increasing temperatures, as interpreted from the shift from a Stephanodiscus corruscus dominated ecosystem to a Stephanodiscus minutulus dominance. In this case, the June Lake (the deeper lake) diatom assemblage shifted from an assemblage representative of well mixed conditions to one representative of a stratified system before Gull Lake as a response to increasing temperatures and a strengthened thermocline. We relate the asynchronous change in the thermocline stability to basin morphology, where the deeper lake with a deeper thermocline is more sensitive to increasing temperatures. Further, generalized additive models (GAMs) allowed us to identify the onset of ecological change in both lakes, pointing to the termination of the Little Ice Age (LIA; ~1850 CE), revealing an acute response to changing climate.