Lake superior sedimentary diatom profiles from the Medieval Climate Anomaly and twentieth century suggest recent assemblage changes reflect novel environmental conditions

We examined diatom assemblages in a sediment core from Lake Superior (Canada/USA) that spans the interval ~ 500–1350 CE and includes the Medieval Climate Anomaly (MCA), 950–1250 CE. We sought to determine whether diatom assemblages responded to climate change during that warm, dry period. From 500 to 1350 CE, diatom assemblages were dominated by Lindavia ocellata and Lindavia comensis and there were no significant changes in the diatom community during the MCA. In another core from Lake Superior, which spans the period ~ 1815–2010 CE, we documented significant changes in diatom assemblages, which began ca. 1940. We used recently established relationships between planktonic diatom taxa and climate and lake physicochemical variables from monitoring data, to assess the drivers of change in diatom assemblages during the past century. Results from the two cores suggest that multiple environmental drivers, including both climate change and human-mediated influences on nutrient cycling, led to shifts in recent diatom communities in Lake Superior. The lack of change in diatom assemblages during the MCA supports the conclusion that environmental changes in Lake Superior during the past century are novel and were driven largely by human activities.