Diatom responses to long‐term climate and sea‐level rise at a low‐elevation lake in coastal British Columbia, Canada

Diatom assemblages from Lake Stowell, a low‐elevation lake in coastal British Columbia, were used to assess limnological responses associated with changes in relative sea level (RSL) and climate over the past 14,500 yr. Marine diatom taxa dominated the early record and disappeared abruptly by 14,000 cal yr BP in response to glacio‐isostatic uplift and an ensuing decrease in RSL. A brackish‐freshwater phase from 14,200 to 13,400 cal yr BP was marked by increases in several diatom taxa with tolerance for low‐to‐moderate salinity, after which assemblages became strictly freshwater. Biostratigraphic changes in both diatom and chironomid assemblages from Lake Stowell occurred more or less simultaneously throughout the record in response to long‐term changes in climate forcings, such as temperature. Increases in cold‐adapted diatom (Staurosirella pinnata, Staurosira construens, Staurosirella dubia) and chironomid (Sergentia) taxa between 12,900 and 11,700 cal yr BP coincided with Younger Dryas cooling, suggesting a direct link with summer temperature. Shifts in aquatic biota also reflected indirect mechanisms through which climate affected the Lake Stowell ecosystem, for example, by altering thermal stratification, disturbance regimes, and nutrient concentrations. For instance, high summer insolation and warm summer temperatures from 11,700 to 8500 cal yr BP were associated with greater abundances of Stephanodiscus hantzschii and especially Aulacoseira subarctica. A concurrent opening of the forest canopy triggered by warming would have exposed the lake to strong winds and enhanced mixing, favoring these more heavily silicified taxa as an indirect consequence of climatic changes. Diatom assemblages appear to have been impacted by the deposition of Mount Mazama tephra at 7600 cal yr BP; a notable increase in Aulacoseira tenella suggests a response to increased silica availability in association with tephra deposition. In southern British Columbia, patterns of post‐glacial climate have been inferred at local and regional scales from paleo‐records of terrestrial pollen (Heusser et al. 1985, Pellatt et al. 2001, Walker and Pellatt 2003, Galloway et al. 2007) and aquatic biota (Palmer et al. 2002, Rosenberg et al. 2004, Lemmen and Lacourse 2018). Paleoclimate reconstructions show that temperature oscillated over rather broadscales; for instance, a generally cooler late‐glacial period culminated in warming during the B？lling/？ller？d interstadial (14,640–12,850 cal yr BP; Kuehn et al. 2014), followed by a short‐term cooling during the Younger Dryas (12,900–11,700