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Widespread ecological reorganizations and increases in organic carbon (OC) in lakes across the Northern Hemisphere have raised concerns about the impact of the ongoing climate warming on aquatic ecosystems and carbon cycling. We employed diverse biogeochemical techniques on a high-resolution sediment record from a subarctic lake in northern Finland (70°N) to examine the direction, magnitude and mechanism of change in aquatic carbon pools prior to and under the anthropogenic warming. Coupled variation in the elemental and isotopic composition of the sediment and a proxy-based summer air temperature reconstruction tracked changes in aquatic production, depicting a decline during a cool climate interval between ~1700–1900 C.E. and a subsequent increase over the 20th century. OC accumulation rates displayed similar coeval variation with temperature, mirroring both changes in aquatic production and terrestrial carbon export. Increase in sediment organic content over the 20th century together with high inferred aquatic UV exposure imply that the 20th century increase in OC accumulation is primarily connected to elevated lake production rather than terrestrial inputs. The changes in the supply of autochthonous energy sources were further reflected higher up the benthic food web, as evidenced by biotic stable isotopic fingerprints.

Global warming can induce profound changes to the functioning of northern freshwater ecosystems. Diatom (Bacillariophyceae) communities often provide early warning signs of associated ecological regime shifts, responding sensitively to alterations in underwater light climate, nutrient regimes, habitat availability and lake water acid–base balance. The underlying mechanisms are manifold and may be mediated via direct climate impact on the physical and chemical properties of lakes or via changes in the terrestrial environment and catchment‐lake coupling. To address catchment‐mediated climate effects on diatom community composition, spatial diatom distribution in the surface sediments of 31 subarctic treeline lakes displaying a broad gradient in terrestrial dissolved organic matter (tDOM) was contrasted with limnological indices of light climate, nutrient availability and lake water pH. To evaluate direct and indirect climate impacts on the long‐term development of benthic phototrophic communities at the subarctic treeline, fossil diatom assemblages in the sediments of a shallow oligotrophic lake were examined against established temperature variability and inferences of terrestrial influence over the past 600 years. The regional lake set was used to test local calibration models for reconstructing dissolved organic carbon as well as lake water pH that is a fundamental environmental determinant for diatom distribution and may echo temperature variability in dilute lakes. Across the treeline, lake water pH imposed primary control over the benthic‐dominated surface sediment diatom communities. The pH influence was connected to catchment geomorphology, soils and vegetation cover and, together with habitat controls, largely superseded tDOM impact on underwater light attenuation and nutrient levels. Similarly, temporal changes in diatom distribution in the sediment core appeared to be relatively little affected by tDOM variability. The species shifts were subtle yet occurred in distinct synchrony with centennial temperature fluctuations, attributed to changing length of the ice cover period and associated effects on lake water chemistry, nutrient regimes and physical habitats. Our results suggest that diatom flora in shallow lakes at the subarctic Fennoscandian treeline may be comparatively resilient towards climate‐driven changes in terrestrial carbon and nutrient fluxes. Diatom communities in poorly buffered lakes may, however, be susceptible to catchment greening and changes in hydrology through effects on lake water acid–base balance. While diatom responses in the sediment sequence were subtle, the palaeolimnological record indicates that periphytic diatom communities in shallow oligotrophic subarctic lakes may be sensitive to the effects of global warming.

The major climate events of the Common Era (CE) have global imprints but significant variations in their timing and magnitude have been suggested. For reliable assessments of the past climate patterns and their applications for evaluations of the ongoing changes, spatially comprehensive network of high‐fidelity paleorecords are necessary. In this study, we reconstruct summer air temperatures of the past 2000 years from northern Lapland (Utsjoki, Finland). We use fossil Chironomidae (Diptera) assemblages from sediments of a remote subarctic lake (Loažžejávri) and the transfer function approach for quantitative temperature reconstruction. The results indicate that the Chironomidae fauna were responding to air temperature and the core assemblages had good modern analogues in the calibration set allowing reliable paleoclimate reconstruction. In our reconstruction, a warm period between ∼900 and 1300 CE is synchronous with the globally defined extent of the Medieval Climate Anomaly (MCA), though beginning considerably later than previously reconstructed for eastern Scandinavia. The MCA was also relatively mild, as the temperatures were only 0.5 °C higher than the record average. Similar to eastern Scandinavia, a cold period corresponding to the Little Ice Age (LIA) was longer that typically observed in hemispheric reconstructions beginning already at ∼1400 CE and lasting very close to modern times. We also found confirming evidence that the LIA was interrupted by a short‐lived warmer period dividing it into two separate cold events in the region. Based on our results, the present is warmer than during any time of the MCA displaying how rapid and severe the ongoing climate change is.

Aim To understand modern and past aquatic community responses to climate‐induced shifts in productivity and ultraviolet radiation (UV) exposure. Location Tree line ecotone from north boreal forest to subarctic tundra in northeastern Finnish Lapland. Taxon Cladocera (Crustacea: Branchiopoda). Methods Thirty‐one 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 tree line lake (Námmájávri) was analysed 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 palaeotemperature, 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‐ versus 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.

1. Aquatic organisms are adversely influenced by ultraviolet radiation (UV) and utilise photoprotective strategies, including pigmentation. We examined UV-protective melanin pigmentation of aquatic meiofauna (Cladocera) in relation to the UV exposure across 25 tree line lakes in Finland to address the potential effects of increased UV and altered input of UV-screening terrestrial dissolved organic carbon (DOC) on aquatic organisms. 2. Bio-optical parameters, including concentration of DOC, the coloured dissolved organic matter (CDOM) fraction, a suite of carbon quality indices and chlorophyll a, were analysed from lake water, and their role in controlling underwater UV environment (measured as diffuse UV attenuation coefficient K d at 305 and 340 nm) was examined. Cladoceran (Alona affinis) carapaces were extracted from the surface sediments, and their melanisation was assessed with spectroscopic UV-visible light absorbance measurements. 3. DOC, CDOM and specific UV absorbance (SUVA) had strong positive relationships with the attenuation of UV in the lakes, suggesting that terrestrial organic carbon controls underwater UV exposure in the examined lakes. The absorbance measurements indicated the presence of melanin in the cladoceran carapaces, the degree of melanisation varying strongly among the lakes. Melanisation had significant relationships with SUVA and fluorescence index (FI). It was higher in lakes with low SUVA and high FI, indicating that cladocerans exhibit strong melanisation in lakes with low contribution of UV-attenuating allochthonous DOC (i.e. high UV exposure). 4. The results suggest that cladoceran meiofauna respond to UV by utilising photoprotective pigmentation and that the degree of pigmentation is affected by site-specific underwater UV exposure, which is ultimately controlled by UV-attenuating DOC of terrestrial origin. 5. Although cladoceran meiobenthos are able to adapt to varying underwater UV doses, climate change with its multiple consequences on hydrology, limnology and catchment vegetation in the tree line zone may cause major changes in underwater UV environment for the organisms to adapt.

Climate warming and consequent greening of subarctic landscapes increase the availability of organic carbon to the detrital food webs in aquatic ecosystems. This may cause important shifts in ecosystem functioning through the functional feeding patterns of benthic organisms that rely differently on climatically altered carbon resources. Twenty‐five subarctic lakes in Finnish Lapland across a tree line ecotone were analysed for limnological and optical variables, carbon (δ13C) and nitrogen (δ15N) stable isotope (SI) composition of surface sediment organic matter (OM) and fossil Chironomidae (Diptera) remains to examine environmental controls behind chironomid functional feeding group (FFG) structure and their isotopic associations for assessing ecosystem functioning and carbon utilisation. We hypothesise that the chironomid SI signatures reflect increased allochthony with increasing allochthonous input, but the resource use may be altered by the functional characteristics of the assemblage. Multivariate analyses indicated that carbon geochemistry in the sediments (δ13C, δ15N, C/N), nutrients, indices of productivity (chlorophyll‐a) and lake water optical properties, related to increasing presence of OM, played a key role in defining the chironomid FFG composition and isotopic signatures. Response modelling was used to examine how individual FFGs respond to environmental gradients. They showed divergent responses for OM quantity, dissolved organic carbon and nutrients between feeding strategies, suggesting that detritivores and filter feeders prefer contrasting carbon and nutrient conditions, and may thus hold paleoecological indicator potential to identify changes between different carbon fluxes. Benthic production was the primary carbon source for the chironomid assemblages according to a three‐source SI mixing model, whereas pelagic and terrestrial components contributed less. Between‐lake variability in source utilisation was high and controlled primarily by allochthonous OM inputs. Combination of biogeochemical modelling and functional classification is useful to widen our understanding of subarctic lake ecosystem functions and responses to climate‐driven changes in limnology and catchment characteristics for long‐term environmental change assessments and functional paleoecology.

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