Functional diatom responses to Neoglacial environmental change in a dark and a clear shallow subarctic lake

Algal communities in northern lakes respond sensitively to climate changes but their responses vary considerably between ecosystems. Functional approaches may help us better understand the nature of the biotic responses to environmental change, though presently this approach has rarely been used in northern lake environments. We explored patterns in diatom (Bacillariophyceae) species and functional composition during the Neoglacial in two shallow oligotrophic lakes typical of the Fennoscandian subarctic region. Sediment carbon and nitrogen isotope (δ¹³C, δ¹⁵N) and elemental biogeochemistry and spectral (visible-near infrared [VNIR] inferred lake-water total organic carbon [TOC] and sediment chlorophyll a) indices were used to track broad-scale environmental transitions over the past three millennia. A number of congruent change patterns were observed indicative of centennial to millennial scale changes in lake productivity, the inflow of organic carbon from land, and sediment organic carbon content. Both the dark colored woodland lake and the clear tundra lake displayed a gradual decline in lake water TOC concentrations attributed to Neoglacial cooling and transient increases in primary production associated with warmer periods and, in particular, the 20^th century warming. Although the Neoglacial evolution of the lakes showed similarities, diatom functional responses were not uniform between the lakes. In the dark woodland lake, functional shifts appeared most strongly connected to declining lake-water TOC and sediment organic carbon content, and were reflected, most notably, as a decline in motile species affiliated with high organic levels and low-light conditions. In the clear tundra lake, changes in lake productivity and sediment organic carbon were reflected most distinctly in the abundance of attached and colonial life forms but the relationships were more ambiguous. While many of the observed shifts aligned with expectations based on earlier research linking diatom functional traits to changing light and organic carbon levels, discrepancies among the lakes and functional groups call for further refinement to detect ecologically meaningful traits in divergent aquatic environments. Both species and functional composition in the two lakes indicated that, despite distinct anthropogenic imprints in the biogeochemical record, human impact on the lakes’ diatoms has not, as yet, been profound.