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Despite the biologically damaging impacts of solar ultraviolet radiation (UV) in nature, little is known about its natural variability, forcing mechanisms, and long-term effects on ecosystems and organisms. Arctic zooplankton, for example the aquatic keystone genus Daphnia (Crustacea, Cladocera) responds to biologically damaging UV by utilizing photoprotective strategies, including pigmentation. We examined the preservation and content of UV-screening pigments in fossil Daphnia remains (ephippia) in two arctic lake sediment cores from Cornwallis Island (Lake R1), Canada, and Spitsbergen (Lake Fugledammen), Svalbard. The aims were to document changes in the degree of UV-protective pigmentation throughout the past centuries, elucidate the adaptive responses of zooplankton to long-term variations in UV exposure, and estimate the potential of fossil zooplankton pigments in reconstructing aquatic UV regimes. The spectroscopic absorbance measurements of fossil Daphnia ephippia under UV (280–400 nm) and visible light (400–700 nm) spectral ranges indicated that melanin (absorbance maxima at UV wavebands 280–350 nm) and carotenoids (absorbance maxima at 400–450 nm) pigments were preserved in the ephippia in both sediment cores. Downcore measurements of the most important UV-protective pigment melanin (absorbance measured at 305 and 340 nm) showed marked long-term variations in the degree of melanisation. These variations likely represented long-term trends in aquatic UV exposure and were positively related with solar radiation intensity. The corresponding trends in melanisation and solar activity were disrupted at the turn of the 20th century in R1, but remained as strong in Fugledammen. The reversed trends in the R1 core were simultaneous with a significant aquatic community reorganization taking place in the lake, suggesting that recent environmental changes, likely related to climate warming had a local effect on pigmentation strategies. This time horizon is also concurrent with previously recorded major ecological shifts in circumpolar lakes when human induced changes in ecological processes of sensitive arctic ecosystems started to occur. The current centennial record of UV-induced melanisation of sedimentary Daphnia ephippia presents unique reference material for assessing UV impacts in arctic aquatic ecosystems before human influence and during the 20th century climate change and provides potential for assessing past aquatic UV regimes.

The developmental history, from the Oldest Dryas to the Late Holocene, of a paleolake and mire located in a river valley in Central Poland was studied using high-resolution records of pollen, macrofossils, Cladocera, sedimentology, and geochemistry from a 14 C dated sediment core. We inferred temperature and water level dynamics using cladocerans and palynological and geochemical methods were employed to investigate natural fluvial activity, particularly for these elements that are difficult to find through sedimentological methods. Our research demonstrates how climate fluctuation in the Late Glacial and Holocene periods influenced the development of a paleolake and mire in a small river valley and shows the importance of long-term research to establish hydroclimatic conditions in biogenic sediments. We propose that water level fluctuations were important in modifying biotic assemblages. The results of our multiproxy analysis clearly suggest that the main driver of changes in aquatic and terrestrial ecosystems, as well as in geomorphological processes in the valley, was climate: reconstructed temperatures generally follow the known trends, showing an increase at the beginning of the Bolling, Allerod, and Holocene and a decrease at the beginning of the Oldest and Younger Dryas. Similarly, periods of water level increase are synchronous with climatic changes. However, the local environmental forces were a significant factors affecting biotic diversity in the studied time span. Fish predation, macrophyte structure, pH, and calcium content partly influence the results of the cladoceran-based water depth reconstruction in particular. Interestingly, the cladoceran-inferred summer temperature reconstruction for Pawlowa appear realistic. Although the temperature trends can be considered reliable, our results indicate the important role of local factors which control the species composition of the subfossil assemblages.

We investigated a Holocene (since ∼10 500 cal yr BP) sediment core from a high-altitude, UV-sensitive tundra lake in northwestern Finnish Lapland to disentangle long-term dynamics in underwater UV exposure, lake-catchment coupling processes, and aquatic community development under changing climate. We analyzed biogeochemical and paleobiological proxies indicative of catchment impact, lake production and water transparency, including inferences of lake-water total organic carbon (TOC), sediment chlorophyll-a and UV-induced cladoceran (Crustacea) melanin (ABSUV), cladoceran communities, and elemental and isotopic fingerprints of sediment organic matter (OM). The results depicted a general climate-mediated ontogeny of early, mid- and late Holocene limnological regimes. The rapid postglacial temperature increase and the regionally early Holocene temperature maximum initiated lacustrine changes with rapid community turnovers, increasing benthic-dominated autochthonous production and high UV exposure due to low input of terrestrial OM (TOC ∼1–2 mg L−1) from the deglaciated catchment. The mid-Holocene regime was characterized by a general stabilization of the catchment and a benthic-dominated aquatic community that became gradually more productive under the warm and dry climate conditions. Superimposed on late Holocene climate cooling and increasing effective moisture, TOC increased to the highest Holocene values (>3 mg L−1), which was associated with abundant aquatic macrophytes and a phytophilous Acroperus harpae dominated cladoceran community around 4500–3000 cal yr BP. Over the past three millennia, TOC declined to low modern values (∼2 mg L−1) and UV-resistant Alonopsis elongata became the dominant taxon indicating a regime of increased underwater UV exposure. ABSUV was consistently higher around ∼1500–1000 cal yr BP after the late Holocene TOC decline, suggesting a photoprotective response to the new UV regime, possibly through a reduction of available UV refugia among aquatic macrophytes. While ABSUV did not show a straightforward relationship with TOC or solar forcing over the Holocene, the cladoceran community shift suggested a clear limno-ecological response to altering TOC and underwater UV regimes.

Hydrological changes and air temperature variability are reconstructed from the sediments of Lake Pieni-Kauro, eastern Finland during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) using transfer functions derived from midge (Insecta: Nematocera)-based calibration models. The reconstructions are compared with a regional tree-ring chronology and sediment physical properties are determined to track depositional changes. An objective of the study is to examine the long-term relationship between the North Atlantic Oscillation (NAO) and climate (temperature, precipitation). Our results show that the MCA was characterized by warm and dry summer conditions, which were accompanied by changes in the sediment magnetic susceptibility values most likely representing major forest fires during the tree-ring indicated MCA megadrought. However, the midge-based stream flow reconstruction shows increased values during the MCA, thus implying enhanced spring floods after snowy winters. During the LIA, the tree-ring data indicate that a generally wetter climate prevailed during summers, but the stream flow reconstruction indicates less snowy winters. In the terms of long-term climatology, the present results show support to the concept that the NAO has a positive correlation between winter precipitation and annual temperature and a negative correlation between summer precipitation in eastern Finland. Thus, the results may serve as important background data for global change assessments.

In the present study I utilize subfossil chironomid and chaoborid distributions in surface sediments of 68 shallow lakes. The aim is to develop a calibration model for past water-level reconstructions by applying weighted averaging-partial least squares (WA-PLS) techniques and to evaluate its potential applications and limitations. This study considers water depth at sampling sites, rather than maximum lake depth. The best of the water depth models developed uses three components and has a cross-validated coefficient of determination ( r 2 jack ) of 0.68 and root-mean-squared error of prediction (RMSEP) of 0.78 m. The model performance is tested on the sediment sequence of a previously studied lake from southern Finland that is known to have experienced past fluctuations in its water level. The water levels inferred are compared with results of chironomid-inferred air temperature reconstruction to ease separation of the effects of the variables. The reconstruction shows consistent results similar to those of previously published cladoceran planktonic:littoral ratios (P:Ls) from the same lake. However, the results indicate that factors other than depth and temperature, such as pollution, may possibly distort the inference results. The results suggest that in applying the midge-based water depth calibration model, it would be advantageous to use it together with an inference model for temperature and preferably in a multiproxy content where changes in water chemistry may be detected. The model can be useful in studies on past effective moisture variability that is closely related to climatic changes.

International audience; Climate exerts strong control over the functioning of northern freshwater ecosystems, yet their resilience and responses to climate forcing may vary. We examined postglacial development patterns in subarctic Lake Varddoaijavri to discern the impact of direct climate controls, catchment influence, and ontogenic processes on the ecological functioning of the lake over the Holocene. Subfossil diatom assemblages together with the elemental and stable isotopic (delta C-13, delta N-15) composition of sediment organic matter were used to examine climate-induced changes in the structure of the phototrophic community and transport of terrestrial organic matter from the catchment. Stable isotopic composition (delta C-13, delta N-15) of subfossil Cladocera (Crustacea) was further used to assess how the changes were reflected higher up the food web. The diatom assemblages and sediment geochemistry closely mirrored the established climate patterns of the Holocene, confirming the strong climate coupling evidenced by earlier studies from lakes across the circumpolar Arctic. Our record indicates overarching influence of moisture fluctuations, superimposing the impact of light limitation by terrestrial organic carbon and temperature-driven alterations to lake physical regimes, which have been emphasized by recent research. The millennial changes in humidity were reflected as shifting dominance between planktonic and benthic diatom life forms, related to changes in the depth of the water column, vertical mixing patterns, and underwater light conditions. Despite the marked regime shifts at the base of the food web, zooplankton carbon utilization was little changed over the Holocene, likely attributable to selective feeding strategies. Overall, our results propose that the projected increases in precipitation in high-latitude regions may have marked impact on the structure and functioning of aquatic communities in shallow subarctic lakes.