Impacts of late-Holocene climate variability and watershed-lake interactions on diatom communities in Lac Brule, Quebec

A high-resolution diatom analysis of a varved sediment sequence from Lac Brule, southwestern Quebec, was used to study temporal patterns of environmental change in the late Holocene. Key periods of interest in the record included the Medieval Warm Period (similar to 800-1300 CE), the Little Ice Age (similar to 1450-1850 CE), and post-European settlement (similar to 1850-present). Subfossil diatom assemblages were compared to previously published pollen, cladocera, and sediment records from Lac Brule, revealing complex dynamics between terrestrial vegetation succession, nutrient fluxes, and trophic interactions. Generalized additive models showed a response to long-term climate variability in the diatom record, although it was not the most influential driver of community changes at Lac Brule. Catchment-mediated processes instead played the largest role in governing the structure of diatom assemblages in the lake. For example, nutrient loading following a local fire in the watershed at 1345 CE led to an abrupt and significant increase in Fragilaria spp. Human activity associated with deforestation and the Wallingford-Back Mine (1924-1972 CE) also had strong impacts on the landscape, which triggered further responses in the aquatic communities of Lac Brule.