Distinct Anthropocene biosphere recorded by the rise of green algae and chrysophytes in varved sediments of Crawford Lake (Ontario, Canada).

The Crawford Lake biosphere evolved in response to both natural and anthropogenic stressors since water filled this karstic basin on the Niagara Escarpment. It was substantially impacted by hydrologic/limnological changes attributed to environmental conditions that define the Northgrippian and Meghalayan Ages, but the greatest impact was human activity in its small catchment. Cultural eutrophication from Indigenous agriculture in the late 13th century altered the lake chemistry, promoting seasonal precipitation of calcite that allows sub-annual resolution. Global signatures of the Great Acceleration are recorded in varved sediments deposited since the mid-20th century, despite little direct human impact on the Crawford Lake catchment since 1900 CE. A sharp increase in green algae and chrysophytes that rely on passive diffusion of CO2 for photosynthesis closely tracks global CO2 emissions concentrations since the mid-20th century. Elevated pCO2 affects lake biospheres in numerous and complex ways, with synergistic responses with limiting nutrients. The reported widespread increase in both groups of non-RuBisCO algae suggests that algal phyla limited by Holocene CO2 concentrations are favoured by atmospheric conditions not experienced since the middle Miocene. Novel lake biospheres are an additional argument for adding an Anthropocene epoch and Crawfordian age to the geologic time scale. This article is part of the theme issue 'The biosphere in the Anthropocene'.