Sweep net, surface sediment, and down core relative Cladocera assemblages from Finnish Lapland

Aim: To understand modern and past aquatic community responses to climate-induced shifts in productivity and ultraviolet radiation (UV) exposure. Location: Treeline ecotone from north boreal forest to subarctic tundra in northeastern Finnish Lapland. Taxon: Cladocera (Crustacea: Branchiopoda). Methods: 31 small and shallow lakes were examined for summer epilimnetic communities (SEC) and surface sediment fossil integrative communities (FIC) of Cladocera for species distribution and their environmental correlations. A 700-year down-core sediment sequence from a treeline lake (Námmájávri) was analyzed for FICs and cladoceran-inferred UV-absorbance (ABSUV, indicative of melanin pigmentation) for evidence of long-term community and photoprotective responses and compared with records of paleotemperature, solar intensity, and composite sediment biogeochemistry by variance partitioning analysis. Results: The SECs were primarily correlated with specific UV absorbance (indicative of UV exposure) and total phosphorus and FICs by mean July air temperature and total nitrogen. The Námmájávri FICs showed subtle changes with a directional shift between the 19th and 21st centuries and were mostly explained by solar intensity. ABSUV exhibited increases during the 18th and 20th centuries, being related to variation in sediment biogeochemistry, which was indicative of changes in auto- vs. allochthonous production. Main conclusions: The ecotonal distribution of cladocerans is sensitive to temperature, nutrients, and allochthonous carbon, which is closely linked with UV exposure. The long-term community shifts and photoprotection have been governed by solar intensity and biogeochemical shifts through lake water optics, attributable to direct UV impact or climate-mediated intensification in photodegradation of allochthonous carbon. Estimations of the dual effects and mechanisms of increasing temperatures and UV on subarctic lakes and their biota remain challenging as their individual impacts on key species were partly contradictory.