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Underwater light availability and exposure of ultraviolet radiation (UV) in mountain lakes is mainly controlled by dissolved organic matter and ice cover. However, both of these factors are affected by climate warming and other anthropogenic pressures. Still, little is known of the impacts of long-term fluctuations in underwater light conditions onto functional distribution of diatoms, species sensitive to changes in climate and UV penetration in mountain lakes. Two mountain lakes in the Italian Alps were analysed using paleolimnological approaches to investigate impacts of anthropogenic pressures (e.g., climate warming, acidification) on underwater light availability, exposure to UV and diatom functional group distributions, focusing on the post-industrial era. Contemporary diatom communities were collected from the main habitats in the shallow, high irradiance littoral zones of these lakes and geochemical proxies describing the development of lake-water transparency and carbon dynamics were analyzed from the sediment cores covering the time period from ca. 1400 CE to present. The geochemical data indicate that cultural acidification decreased lake-water organic carbon concentrations in both lakes, which suggests increased light availability and UV exposure during the past century. The responses of the studied lakes to anthropogenic pressures varied, as the lake situated at a lower altitude with a larger catchment showed only a few biotic changes indicating higher resilience, whereas the lake situated at higher altitude showed distinct changes in its ecological status. In this alpine lake, almost 30% of the diatom functional groups shifted from benthic to planktic during the post-industrial era. The role of increased light availability and UV exposure as a driver of diatom functional group distributions could not be unambiguously separated, and such diatom assemblage changes have been shown in many regions and most closely linked to a warming climate. However, low guild functional groups, prevalent also in the contemporary samples from the shallow littoral zone, dominated the diatom communities throughout the studied period, suggesting high tolerance to UV radiation.

Algal communities act as sensitive indicators of past and present climate effects on northern lakes, but their responses can vary considerably between ecosystems. Functional trait-based approaches may help us better understand the nature of the diverse biotic responses and their underlying ecosystem changes. We explored patterns in diatom (Bacillariophyceae) growth forms and species composition during the Neoglacial in two shallow lakes typical of subarctic regions, including a dark-colored woodland lake and a clear tundra lake. Sediment carbon and nitrogen elemental and isotope biogeochemistry and spectral indices were used to track broadscale changes in lake productivity, the inflow of organic carbon from land, and benthic substratum over the past three millennia. The biogeochemical indices tracked declines in land-lake connectivity as well as lake-water and sediment organic enrichment above and below the subarctic treeline driven by Neoglacial cooling. This broadscale environmental transition was intercepted by periods of elevated primary production associated with transient Neoglacial warm anomalies and, in particular, the twentieth century warming. Although the Neoglacial development of the lakes showed conspicuous similarities, diatom functional and taxonomic responses were not uniform between the lakes pointing to intrinsic differences in the development of benthic habitats and underwater-light regimes. Many of the observed biotic shifts aligned with expectations based on earlier research linking diatom functional traits to changing light and organic levels but the results also point to further research needs, particularly to better differentiate the individual and interactive effects of substratum and light. Despite distinct anthropogenic imprints in the biogeochemical record, the scale of human impact on the lakes’ biota has not, as yet, been profound, but the changes are nonetheless clear when compared to the previous three millennia of natural lake development.

Global environmental change alters the production, terrestrial export, and photodegradation of organic carbon in northern lakes. Sedimentary biogeochemical records can provide a unique means to understand the nature of these changes over long time scales, where observational data fall short. We deployed in situ experiments on two shallow subarctic lakes with contrasting light regimes; a clear tundra lake and a dark woodland lake, to first investigate the photochemical transformation of carbon and nitrogen elemental (C/N ratio) and isotope (δ13C, δ15N) composition in lake water particulate organic matter (POM) for downcore inferences. We then explored elemental, isotopic, and spectral (inferred lake water total organic carbon [TOC] and sediment chlorophyll a [CHLa]) fingerprints in the lake sediments to trace changes in aquatic production, terrestrial inputs and photodegradation before and after profound human impacts on the global carbon cycle prompted by industrialization. POM pool in both lakes displayed tentative evidence of UV photoreactivity, reflected as increasing δ13C and decreasing C/N values. Through time, the tundra lake sediments traced subtle shifts in primary production, while the woodland lake carried signals of changing terrestrial contributions, indicating shifts in terrestrial carbon export but possibly also photodegradation rates. Under global human impact, both lakes irrespective of their distinct carbon regimes displayed evidence of increased productivity but no conspicuous signs of increased terrestrial influence. Overall, sediment biogeochemistry can integrate a wealth of information on carbon regulation in northern lakes, while our results also point to the importance of considering the entire spectrum of photobiogeochemical fingerprints in sedimentary studies.

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 20th 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.

Solar radiation fuels primary production and carbon uptake in aquatic environments, but shorter ultraviolet radiation (UVR) wavelengths may cause adverse effects and trigger photoprotective mechanisms in organisms inhabiting lakes. In sensitive mountain lakes, ice cover and dissolved organic matter (DOM) mainly control underwater light conditions. Climate warming and other anthropogenic pressures impact the length of the ice cover period and the concentration and composition of DOM. A prolonged growing season with increased light availability may change habitat availability and drive changes in the functional group distribution of phototrophic communities. However, the roles of changing habitats and possible adverse effects of UVR as drivers of micro-algae functional group distributions have not been separated, and lake studies have thus far mainly focused on species specific sensitivities.This study focuses on changes in underwater light conditions and UVR exposure during the past ~200 years and the impacts on functional group distributions of diatom (Bacillariophyceae) communities in two mountain lakes with different altitude and catchment size. Dated short sediment cores from each lake were analysed for elemental composition and stable isotopes of carbon and nitrogen, as well as sediment-inferred lake water total organic carbon (TOC) and chlorophyll-a. Diatoms were analysed to the species level and further divided into eco-morphological groups to investigate parallel changes in diatom functional group distributions. Geochemical proxies were used to describe trends in lake water transparency and duration of ice cover period. In addition, changes in diatom functional group distributions were analysed from the main habitat in the shallow littoral zone to detect diatom functional groups tolerances to high UVR.Anthropogenic pressure was identified as a likely driver of sediment-inferred lakewater TOC concentration changes within the past ~200 years, suggesting changes in light conditions and UVR. Diatom functional group distributions in the lake located at higher altitude with a smaller catchment showed an increase of planktic life-forms within the past ~100 year, which may be related to shortening of the ice cover period. Earlier studies have shown trade-offs between motility and photoprotective capacity, but in this study clear differences between occurrences of motile or non-motile functional groups in the shallow littoral zone were not found. Based on these results, further research with broader habitat sampling is needed to separate the roles of habitat availability and UVR, and their roles affecting the distribution of diatom functional groups.

A key question in aquatic elemental cycling is related to the influence of bottom water oxygen conditions in regulating the burial and release of carbon under climate warming. In this study, we used head capsules of Chironomidae larvae to assess community and diversity change between the past (estimated as Pre-Industrial Period) and present and to reconstruct changes in hypolimnetic oxygen conditions from 30 subarctic ecotonal lakes (northeastern Lapland) using the top-bottom paleolimnological approach applying surface sediment (topmost 0-2 cm) and reference (4-5 cm) samples. Subsequently, we tested the findings against dissolved organic carbon (DOC) concentration of the sites. We found that the benthic communities were statistically dissimilar between the past and the present with largest changes occurring in the more transparent oligo-mesohumic lakes. However, murky polyhumic lakes displayed uniformly a decrease in diversity. The chironomid-inferred oxygen values showed a general decrease toward the present with largest shifts in low-DOC lakes, whereas no significant changes were found in the hypolimnetic oxygen conditions of high-DOC lakes, which were often located in wet-land areas. These finding suggest that lakes associated with constant organic carbon inputs are more resilient toward climate-induced reductions in hypolimnetic oxygen. (c) 2018 Elsevier B.V. All rights reserved.

The Younger Dryas (YD) is a roughly 1,100-year cold period marking the end of the last glaciation. Climate modelling for northern Europe indicates high summer temperatures and strong continentality. In eastern Europe, the scale of temperature variation and its influence on ecosystems is weakly recognised. Here, we present a multi-proxy reconstruction of YD conditions from Lake Gościąż (central Poland). The decadal-resolution analysis of its annually varved sediments indicates an initial decrease in Chironomidae-inferred mean July air temperature followed by steady warming. The pollen-inferred winter-to-summer temperature amplitude and annual precipitation is highest at the Allerød/YD transition and the early YD (ca. 12.7–12.4 ky cal BP) and YD/Holocene (11.7–11.4 ka cal BP) transition. Temperature and precipitation were the main reasons for lake level fluctuations as reflected in the planktonic/littoral Cladocera ratio. The lake’s diatom-inferred total phosphorus decreased with increasing summer temperature from about mid YD. Windy conditions in the early YD until ∼12.3 ka cal BP caused water mixing and a short-lived/temporary increase in nutrient availability for phytoplankton. The Chironomidae-inferred summer temperature and pollen inferred summer temperature, winter temperature and annual precipitation herein are one of only a few in eastern Europe conducted with such high resolution.

The body size of aquatic invertebrates is, to a great extent, dependent on ambient temperature, but size distributions are also determined by other factors like food supply and predation. The effect of temperature on organisms is formulated in the temperature&ndash, size hypothesis, which predicts a smaller body size with increasing temperature. In this study, the effect of temperature on the subfossil remains of three littoral Cladocera (Alona affnis, A. quadrangularis, and Chydorus cf. sphaericus) was investigated. Exoskeletal remains of these species can be found in large numbers in lacustrine sediments and over a wide north&ndash, south range in Europe. The total length of both headshield and postabdomen for A. affinis and A. quadrangularis and carapace length for C. cf. sphaericus were measured to observe their response to changes in latitude and temperature. A different response to ambient temperature in the growth of body parts was observed. The size of the headshields of both Alona species and of the carapace of Chydorus was significantly larger in colder regions as opposed to warm ones. It turned out that the postabdomen was not a good predictor of ambient temperature. While the sizes of all remains increased with latitude, the sizes of the Alona remains was smaller in the mountain lakes of the Southern Carpathians than in other cold lakes, in this case in Finland, a fact indicative of the importance of other factors on size distribution. This study demonstrates that a morphological response to climate is present in littoral cladocerans, and, therefore, changes in the length of headshield and carapace may be used as a proxy for climate changes in paleolimnological records.

Sunlight fuels the drawdown and evasion of carbon in shallow northern lakes. Amplified polar warming is altering the sunlit transport and transformation of aquatic carbon at an alarming rate entailing potential for climate feedbacks. We combined experimental and retrospective approaches to explore the synoptic interlinks between underwater light, aquatic carbon biochemistry, landscape carbon cycling and climate change in two shallow subarctic lakes with divergent light and carbon regime (a clear lake low in organic carbon and a dark organic rich lake). In situ enclosures (treatments under full sunlight, sunlight without the ultraviolet [UV] spectrum, no light) were first deployed on the lakes to decipher the effect of photochemical alteration on the spectral, elemental and isotopic properties of lake water organic carbon pools under short term (four weeks) exposure. We then focused on elemental, isotopic and spectral fingerprints archived in the sediments of the lakes to trace coeval variability in aquatic primary production, terrestrial carbon transport, and underwater light under centennial climate fluctuations. We observed distinct differences in carbon biochemistry between the experimental treatments illustrating the importance of sunlight, and particularly the UV spectrum, in shaping the carbon pools of the lakes already over short time scales. Over the past centennia, sediment biogeochemical composition carried signatures of change in carbon origins (algal vs terrestrial) and shifting underwater light regime. The results shed light on how climate change and sunlight shape carbon flows in shallow northern lakes over short and long time scales.

High latitude freshwater systems are facing changes in catchment-mediated allochthonous input, as well as physical and chemical controls triggered by on-going climate change, which may alter their carbon processing and ecological characteristics. To explore changes in chironomid functional responses and carbon utilization in relation to long-term environmental change, we studied a sediment core covering ca. 2000 years from a tundra lake in northern Finland, which was analysed for sediment geochemistry, isotopic composition of chironomid remains and their functional assemblages. We aimed to relate changes in chironomid functional feeding assemblages and resource utilization, based on Bayesian stable isotope modelling, and determined that the long-term resource utilization was more controlled by sediment geochemistry (resource availability) and climatic variables, reflecting changes in habitat and lake ontogeny, rather than the functional feeding assemblage composition. Change horizons were observed for both sediment geochemistry and functional assemblage composition. However, different timing of these changes suggests different drivers affecting the dynamics of primary production and chironomid community functionality. We also compared the recent warming period to Medieval Climate Anomaly (MCA), observing divergent patterns, which suggests that MCA may not be a good analogue for changes induced by on-going climate warming.

Due to the lack of knowledge on dynamics of the North Atlantic Oscillation (NAO) prior to the last millennium, synchronized records of air temperature and precipitation variability are needed to understand large-scale drivers of the hydroclimate. Here, we use completely synchronized paleolimnological proxy-based records of air temperature and effective precipitation from two Scandinavian lakes with ∼2000-year sediment profiles. We show that the relationship between air temperature and precipitation (T/P ratio) is synchronous in both study sites throughout the records suggesting warm and dry conditions at ∼300–1100 CE and cold and wet conditions at ∼1200–1900 CE. Owing to the significantly increased air temperatures, the most recent T/P ratio has again turned positive. During the first millennium of the Common Era, the T/P mimics patterns in Southern Oscillation index, whereas the second millennium shows response to the NAO index but is also concurrent with solar irradiance shifts. Since our T/P reconstruction is mostly linked with the NAO, we propose the T/P ratio as an indicator of the NAO. Our results from the coherent records provide first-time knowledge on the long-term temperature-precipitation relationship in Northern Europe that increase understanding of the comprehensive hydroclimate system in the region and the NAO dynamics also further back in time.

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 (Loažžejavri) 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.

This paper provides a high-resolution temperature reconstruction for the last Millennium from Lake Atnsjøen, SE Norway (61°52′31″N, 10°10′37″E). The sedimentary record reveals strong influence of the large-scale global climate patterns on the local climate in southern part of Eastern Norway. We reconstructed mean July air temperature using Chironomidae-based transfer function and fossil Chironomidae assemblages. The reconstruction was supported by a selection of climate-sensitive geochemical and paleoecological sedimentary proxies of terrestrial and aquatic origin, including Cladocera, pollen and macrofossils. Presented results revealed that summer temperatures were 1–2 °C warmer than the mean Millennial temperatures during the 11th, 13th, 15th and 20th centuries and 1–2 °C lower during the 12th, 14th, 17th–18th centuries. A persistent cold period, the Little Ice Age (LIA), occurred between 1550 and 1800 CE, was interrupted by a short warming at 1650 CE. The recognized regional climate fluctuations during the last Millennium affected the lake and its catchment, of which the strongest impact was caused by the LIA cooling. During the LIA the catchment vegetation was impacted by climate deterioration and the lake productivity reached its lowest level during the last Millennium. The current temperature reconstruction is in agreement with a previous continental scale temperature reconstruction for Europe. From obtained results it emerges that during the LIA the climate of western Scandinavia has been dictated by the atmospheric patterns originating from the North Atlantic. peerReviewed

Solar ultraviolet radiation (UV) induces photodegradation of optically and functionally important organic compounds in lakes and may negatively impact aquatic biota. We disentangled UV impacts on dissolved organic matter (DOM) transformation, and algal and zoobenthic micro-organisms in two shallow subarctic lakes in NW Finnish Lapland; in a high-UV + low-DOM (tundra, Iso-Jehkas) and a low-UV + high-DOM (mountain birch woodland, Mukkavaara) system. In addition to site and seasonal comparisons, in situ experiments with three treatments (DARK, photosynthetically active radiation [PAR], UV + PAR) were set up floating on the lakes for four weeks during midsummer. Lake water and experimental lake water were analyzed for basic limnology, optical properties (dissolved organic carbon [DOC], specific UV absorbance [SUVA], colored DOM [CDOM], and DOM compounds) as well as for photosynthetic and photoprotective pigments in algae and microzoobenthos. DOC concentrations remained largely unchanged after the exposure period in seasonal and experimental samples in both lakes yet the biochemical composition of the carbon pools was distinctly altered. CDOM and SUVA decreased seasonally and under UV exposure in the experiments, and terrestrial DOM compounds decreased in the experiments, suggesting UV induced photodegradation of large molecular size DOM of terrestrial origin. Higher seasonal and experimental (UV + PAR vs. PAR) proportional CDOM degradation occurred in Iso-Jehkas (32%, 29%) than in Mukkavaara (19%, 9%). Accordingly, the high-UV + low-DOM lake was more sensitive to photodegradation despite originally low CDOM relative to the low-UV + high-DOM system where DOM biodegradation likely prevailed. Experimental results showed elevated algal carotenoid/chlorophyll ratios and microzoobenthic melanin under UV exposure indicating photoinhibition and photoprotective pigmentation. UV has a significant impact on aquatic food webs of subarctic lakes altering the biogeochemical composition of organic matter and organisms through mechanisms of photodegradation, photoinhibition and photoprotection.

The morphology of one of the smallest chydorid cladocerans in the world, Unapertura latens Sarmaja-Korjonen, Hakojärvi & Korhola, 2000 (Branchiopoda: Cladocera: Anomopoda) is studied herein. External features and postabdomen of the adult parthenogenetic female are discussed in detail for the first time, based on an intact specimen from northern Finland. Originally described on Quaternary fossil remains, we investigate the validity of the monotypic genus Unapertura and its position within the Aloninae subfamily, in particular in relation to Rhynchotalona Norman, 1903. As a result, we rename the taxon as Rhynchotalona latens comb. nov., based on morphology. The species, a glacial relict, seems one of the rarest cladocerans in contemporary waters in Finland, yet intact specimens are clearly overlooked and recent remains in surface sediments suggest an Arctic-Alpine distribution in oligotrophic habitats in the Holarctic.

Lakes are a dominant feature of the Arctic landscape and a focal point of regional and global biogeochemical cycling. We collected a sediment core from a High Arctic Lake in southwestern Svalbard for multiproxy paleolimnological analysis. The aim was to find linkages between the terrestrial and aquatic environments in the context of climate change to understand centennial-long Arctic biogeochemical cycling and environmental dynamics. Two significant thresholds in elemental cycling were found based on sediment physical and biogeochemical proxies that were associated with the end of the cold Little Ice Age and the recent warming. We found major shifts in diatom, chironomid and cladoceran communities and their functionality that coincided with increased summer temperatures since the 1950s. We also discovered paleoecological evidence that point toward expanded bird (Little Auk) colonies in the catchment alongside climate warming. Apparently, climate-driven increase in glacier melt water delivery as well as a prolonged snow- and ice-free period have increased the transport of mineral matter from the catchment, causing significant water turbidity and disappearance of several planktonic diatoms and clear-water chironomids. We also found sedimentary accumulation of microplastic particles following the increase in Little Auk populations suggesting that seabirds potentially act as biovectors for plastic contamination. Our study demonstrates the diverse nature of climate-driven changes in the Arctic lacustrine environment with increased inorganic input from the more exposed catchment, larger nutrient delivery from the increased bird colonies at the surrounding mountain summits and subsequent alterations in aquatic communities.

We conducted a paleolimnological study to examine how changes in human activity interacted with climate to influence productivity and biodiversity over the past millennium in oligotrophic Lake Atnsjøen, SE Norway. The study included analyses of sediment geochemistry, subfossil diatoms and cladocerans, and macrofossils. Results were compared with the historical record of human activities in the catchment, pollen analysis and paleoclimate inferences from the lake. During the first 750 years of the record (1000–1750 CE), a time of relatively low human activity, lake productivity and biodiversity were strongly related to climate. During the Little Ice Age (1550–1800 CE), lake productivity and diatom diversity were constrained by cold climate. A century of climate warming (1780–1880 CE) initiated an increase in productivity. Accelerated human settlement after 1850 CE, however, had an even stronger impact on productivity, mediated by increased agriculture and/or forestry, which led to greater nutrient loading of the lake. Similarly, diatoms in the lake responded to the rise in temperature, but increasing human activity also had a moderate impact on the diatom community, which displayed weak signs of nutrient enrichment. From 1980 to 1990 CE onwards, lake productivity declined as a consequence of a recent decrease in human activity and changing land use. The human-induced increase in lake productivity starting ca. 1850 CE propagated through the food web and increased consumer productivity, as reflected by greater accumulation rates of cladocerans, trichopterans and turbellarians. The cladoceran community was likely under top-down control of fish, as indicated by changes in size structure and diversity. Our study showed that increasing human activity during the settlement period had a stronger impact on lake productivity than did climate. Furthermore, the slight human-mediated increase in nutrient loading had different impacts on productivity and biodiversity in the study lake. This study demonstrates that even relatively small changes in human activities in watersheds can have measurable impacts on nutrient-poor lakes.

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 (delta C-13) and nitrogen (delta N-15) 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 (delta C-13, delta N-15, 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.

A long hidden chydorid (Chydoridae, Cladocera) taxon, first found as fossil specimens and recently redefined as Rhynchotalona latens (Sarmaja-Korjonen et al., Hydrobiologia 436: 165-169, 2000) is investigated for its biogeography and ecology. Late Holocene sediment sequence from Lake Sylvilampi, NE Finnish Lapland, and R. latens spatial distribution in relation to limno-climatic attributes in Finland were examined. Principal component analyses of fossil cladoceran communities showed that R. latens is mostly affiliated with Alonella excisa-Alonopsis elongata-Alonella nana species pool. Generalized linear modeling of R. latens responses to limno-climatic variation indicated that it prefers acidic, mesotrophic, humic and shallow lakes with organic sediments in NE Lapland and has a north boreal-subarctic climatic affiliation. At the northern end of its geographical distribution (NE Lapland), it reproduces with abundant gamogenesis under environmental stress. The specialized taxon is a benthic detritivore and scraper and has a Holarctic northern-alpine distribution. It is a glacial relict associated with modern analogs of periglacial aquatic environments, and it occurs in semi-aquatic wetlands, lush lake littorals and clear and cold waters. Examination of chydorids as bioindicators, especially those with restricted niches, allow us to understand biodiversity responses of lake littorals under changing limno-climatic regimes.

In this study, we aim to quantify summer air temperatures from sediment records from Southern, Central and Northern Finland over the past 2000 years. We use lake sediment archives to estimate paleotemperatures applying fossil Chironomidae assemblages and the transfer function approach. The used enhanced Chironomidae-based temperature calibration set was validated in a 70-year high-resolution sediment record against instrumentally measured temperatures. Since the inferred and observed temperatures showed close correlation, we deduced that the new calibration model is reliable for reconstructions beyond the monitoring records. The 700-year long temperature reconstructions from three sites at multi-decadal temporal resolution showed similar trends, although they had differences in timing of the cold Little Ice Age (LIA) and the initiation of recent warming. The 2000-year multi-centennial reconstructions from three different sites showed resemblance with each other having clear signals of the Medieval Climate Anomaly (MCA) and LIA, but with differences in their timing. The influence of external forcing on climate of the southern and central sites appeared to be complex at the decadal scale, but the North Atlantic Oscillation (NAO) was closely linked to the temperature development of the northern site. Solar activity appears to be synchronous with the temperature fluctuations at the multi-centennial scale in all the sites. The present study provides new insights into centennial and decadal variability in air temperature dynamics in Northern Europe and on the external forcing behind these trends. These results are particularly useful in comparing regional responses and lags of temperature trends between different parts of Scandinavia.

Summary Functional diversity (FD) as a biodiversity measure has an explicit role in ecosystem functioning because the effects of environmental changes in ecosystems are determined by biological functions, such as feeding type and trophic position, of particular species. We evaluated the usability of functional characterization and FD of an aquatic keystone group (Crustacea: Cladocera) for enhancing the understanding of long-term lake functional responses to environmental changes. The aims were to separate ecologically significant functional groups, investigate succession of such functional groups during nutrient enrichment process and determine the relationship between FD and lake productivity using a palaeolimnological approach. We selected two eutrophicated study lakes from southern Finland for down-core investigations, one with a centennial (past c. 350 years) and the other with a decadal (past c. 70 years) nutrient enrichment record. Cladoceran microfossils in the sediment cores were used to determine the taxonomic structure of the past communities. Ecologically relevant functional characteristics were determined to separate functional groups by utilizing a functional dendrogram and a weighted community-based FD index together with a set of multidimensional FD indices. The indices were applied to the down-core assemblages. The functional dendrogram separated cladocerans into functional groups where habitat type principally separated open-water filterers and predators from epibenthic scrapers and detritivores. Further separation in the pelagic branch was based on body size and feeding and among the benthic branch body shape. Functional assemblages changed markedly during the nutrient enrichment process. In the early stage of eutrophication, the largest functional changes were caused by small planktonic filterers and predators. Small filterers and epibenthos responded strongest during the eutrophic–hypereutrophic succession. FD had a positive long-term relationship with lake trophic status until eutrophic conditions that was likely caused by diversifying resources and versatile food webs. Under hypereutrophic conditions, and especially at a decadal temporal resolution, alternating predation regimes caused variance to FD. In the current records, cladoceran FD was positively related to lake productivity and bottom-up controls during the early stages of eutrophication in the long-term record but top-down controls apparently were more important at a decadal scale and under hypereutrophic conditions.

Abundant northern lakes have an intrinsic role in the transport, sequestration, and mineralization of terrestrial organic carbon. The quantity and quality of this carbon control vital aquatic biogeochemical processes, and influence the metabolic balance of lakes with subsequent impact on the global carbon cycle. We measured concentrations and type of dissolved organic matter and elemental and stable isotopic composition of carbon and nitrogen in 31 subarctic lakes with varying catchment types across the treeline in northern Finland, integrating both the pelagic (lake water) and the benthic (surface sediments) carbon pools for a comprehensive understanding of landscape influence on aquatic carbon dynamics. Wetland cover was identified as the primary catchment control over the aquatic carbon pools, reflected particularly in the bio-optical properties of lake water. Landscape influence on sediment carbon content and composition, mirroring largely the structure and productivity of the aquatic communities, was primarily connected to allochthonous nutrient inputs fueling autotrophic production. Basin depth and benthic production were identified as important internal controls on the surface sediment geochemistry. Overall, our results suggest that shallow subarctic lakes will be particularly susceptible to climate-mediated changes in the export of terrestrial organic matter from wetlands. Whether the landscape influence will promote the channeling of terrestrial carbon into the atmosphere via aquatic ecosystems will strongly depend on the interplay between the biogeochemical characteristic of the allochthonous carbon inputs, terrestrial nutrient fluxes, and the depth of the recipient ecosystems.

Paleolimnology is a promising approach to reconstruct past carbon cycle in lakes and its response to global changes. Here, we test the potential of the combined use of sedimentary geochemical proxies and δ13C analysis of subfossil chironomid (δ13CHC) in a sediment core retrieved from a boreal lake. Characteristics of sedimentary organic matter appeared to be strongly variable over time, corresponding to periodic decreases in aquatic organic matter contribution to lake sediments, and this dynamic was attributable to climatic changes occurring during the late Holocene. Results revealed also that δ13CHC values were lower than those of organic matter, and these differences were greater when lake sediments were depleted in aquatic organic matter. Thus, chironomid feeding behavior seems to be dependent on the organic matter quality, showing a strong affinity for aquatic organic matter even if this resource is not the most available in sediments. Based on this methodological strategy, our results indicate (i) the relatively poor nutritive quality of allochthonous materials for benthic chironomid larvae, (ii) the strong influence of climate variability on the whole lake functioning, and (iii) the high potential of the combined use of this methodology to reconstruct the past carbon cycle in boreal lakes.

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.

We examined functional climate responses of two small taiga lakes in eastern Scandinavia with the aim to compare biogeochemical shifts in the hydrologically different basins over the past 3500 years. Sediment cores from one seepage and one drainage lake were studied for C and N elemental and stable isotopic composition to track organic matter origin, elemental cycles, and trophic structure. Fossil Cladocera assemblages and functional grouping of Chironomidae were determined to understand food web structure, biotic interactions, and habitat quality. In the seepage lake, the warm Medieval Climate Anomaly (MCA, ~1400 to 1000 cal BP) induced thermal stratification, hypolimnetic anoxia, and increased in-lake production, similar to the 20th century development. The Neoglacial cooling, with a high precipitation period at ~2200 to 2000 cal BP, and the cold Little Ice Age (LIA, ~700 to 300 cal BP) increased terrestrial carbon inputs and reduced productivity. The drainage lake was shallow, productive, and humic during the warm and dry period at ~4000 to 3000 cal BP but cooler and moister climate induced higher lake level with stratification and hypolimnetic anoxia around 2500 cal BP. The MCA was characterized by increased in-lake production and altered food web structure, as the keystone zooplankton taxon Daphnia was extirpated, and LIA by intensified anoxia and reduced planktonic production due to longer ice-cover period. Coherent responses to climate warming included increased planktonic production, stronger stratification, and hypolimnetic anoxia with anaerobic processes in both lakes, but responses to moisture were site-specific and dependent on basin hydromorphology.

We investigated the influence of long-term external forcing on aquatic communities in Alpine lakes. Fossil microcrustacean (Cladocera) and macrobenthos (Chironomidae) community variability in four Austrian high-altitude lakes, determined as ultra-sensitive to climate change, were compared against records of air temperature, North Atlantic Oscillation (NAO) and solar forcing over the past ~400years. Summer temperature variability affected both aquatic invertebrate groups in all study sites. The influence of NAO and solar forcing on aquatic invertebrates was also significant in the lakes except in the less transparent lake known to have remained uniformly cold during the past centuries due to summertime snowmelt input. The results suggest that external forcing plays an important role in these pristine ecosystems through their impacts on limnology of the lakes. Not only does the air temperature variability influence the communities but also larger-scale external factors related to atmospheric circulation patterns and solar activity cause long-term changes in high-altitude aquatic ecosystems, through their connections to hydroclimatic conditions and light environment. These findings are important in the assessment of climate change impacts on aquatic ecosystems and in greater understanding of the consequences of external forcing on lake ontogeny.

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 use a range of environmental variables to explore the possible drivers influencing the biota, especially the composition of aquatic invertebrates, during the Younger Dryas (YD) and Early Holocene (EH) in different river valley sites: a well-developed meandering river and the confluence zone of headwater streams. Using pollen, macrofossil, cladoceran, and chironomid as well as geochemical and lithological data as proxies for environmental factors (i.e., water depth and temperature), we attempt to show that these different sites display similar hydroclimatic signals (especially floods). The geochemical records in the studied valley are correlated with environmental factors, such as the evolution of the vegetation, the intensification phase of slope processes, and fluvial activities. Cladocerans and chironomids potentially play important roles in the recognition of flood episodes in the study sites, and any shift in their diversity and, especially, the co-occurrence of planktonic, sediment-associated cladocerans with rheophilic chironomids could be valuable indicators of floods and flow episodes. The variable changes in water levels at the sites reconstructed by the cladoceran-based water-depth model were positively associated with changes in lithophilic elements (K, Na, Al, Mg, and Ti), SiO2(biog + ter) and Fe/Mn, and our estimates of the paleohydraulic parameters for the study sites are generally consistent with changes in the composition of aquatic invertebrates. The recorded YD floods appear to be synchronous, regardless of the size and the different geological and geomorphological settings of each part of the river catchment. Our investigation demonstrates that flood magnitude increases in the downstream direction along the valley. The common increases in the water level at all study sites from the Grabia River valley are significant from 12,400 to 12,200 and from 11,900 to 11,800 cal BP and are comparable with those from elsewhere in Poland and Europe.

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.

To fully utilize palaeolimnological archives for tracking past environmental changes, methodological advances are necessary. We present a novel method to quantitatively reconstruct past summer air temperatures based on fossil chydorid (Cladocera: Chydoridae) ephippia assemblages. We examined a 76-lake training set from Finland for chydorid ephippia distribution to develop a transfer function for summer air temperatures. Subsequently, we tested its usability in downcore reconstructions by inferring Holocene temperatures from Lake Arapisto in southern Finland. Significant climate-related variation was found in the distribution and abundance of chydorid ephippia in the training set. A transfer function for palaeotemperature reconstructions was developed using the partial least squares technique, which showed favourable performance statistics, enabling the test use of the model in a downcore reconstruction. The ephippia assemblages were responding to the Holocene climate development in the Lake Arapisto core. The ephippium-inferred temperatures correlated closely with the previously available independent palaeoclimate data from Lake Arapisto and from an adjacent lake, although the early Holocene samples had poor modern analogues in the training set. The use of fossil chydorid ephippia in palaeoclimatological reconstructions is a promising tool and it can contribute as a valuable addition for multiproxy assessments of past climate changes.

We examined long-term relationships between Cladocera and limnogeological and climate variables in a late Holocene lake sediment sequence to assess cladoceran responses to climate change and reconstruct past climate variations in southern Finland, near the Baltic Sea coast. Elemental composition, organic matter, and inferred water quality variables were used to constrain paleoenvironmental (until 4500 cal BP) conditions and human impact. Fossil Cladocera assemblages, ephippia, and body size trends were utilized to estimate mean July air temperature (T Jul) variability, open-water season length, and water-temperature regimes. Results revealed stable cladoceran communities, dominated by Eubosmina, until 500 cal BP, followed by major assemblage changes including increases in Chydorus cf. sphaericus, Alonella spp., and Bosmina longirostris. Pb and Cu concentrations were associated temporally with increasing human impact and warming climate during the past few centuries, and were the most significant factors in explaining cladoceran community changes, based on redundancy analysis. A Cladocera-based T Jul reconstruction estimated elevated temperatures for the end of the Holocene Thermal Maximum, a cooler period during ~3000–2000 cal BP, slightly increased temperatures during 1200–800 cal BP corresponding to the Medieval Climate Anomaly (MCA), and lower temperatures during the Little Ice Age (LIA), 800–200 cal BP. The reconstruction also suggests that significant climate warming took place during the twentieth century. Cladocerans mostly associated with warm periods included B. longirostris and Pleuroxus uncinatus, whereas Alona affinis, A. quadrangularis, and C. cf. sphaericus were associated with colder climate. Compared to a reference Chironomidae-inferred T Jul reconstruction from the same region, discrepancies were apparent in amplitude of temperature change, as the temperature variability in the cladoceran-based record was muted until ~200 cal BP. During the LIA, increased ephippia and body size agreed with the temperature reconstruction, but suggested a severely shorter open-water season and reduced water temperatures compared to the preceding late Holocene episodes, even though the cladoceran T Jul showed quite similar temperatures for these periods. Our results suggest that fossil cladoceran assemblages, ephippia, and morphological attributes respond sensitively to long-term climate fluctuations and this record reflects well the major climate events of the late Holocene and provides realistic paleoclimatic estimates for maritime southern Finland.

Summary 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. 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 Kd 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. 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). 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. 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.

Long-term fluctuations in lake-water optical properties were examined using a Holocene sediment sequence and multi-proxy palaeolimnological approach in Lake Einstaken, Nordaustlandet, Svalbard. UV-absorbance of sedimentary cladoceran remains provided information on underwater UV exposure and changes in lake-catchment coupling processes were inferred from sediment geochemistry. In addition, aquatic community succession was used as an indicator for lake-water bio-optical properties and a Holocene record of sun activity (sunspots) was utilized to evaluate long-term solar forcing. The results indicated that the UV-absorbance of cladoceran remains was highest (i.e. maximum UV-induced pigmentation) for a short period during the early Holocene and for several millennia during the mid-Holocene. Sun activity was high during these time intervals, probably impacting the UV intensities, but it is probable that the amount of UV-attenuating compounds (e.g. dissolved organic carbon (DOC)) also significantly affected the underwater UV environment and were low during high UV exposure. Benthic autotrophic communities also responded to the millennial changes in lake-water optical properties. UV-resistant Nostoc cyanobacterial colonies were established during the mid-Holocene, indicative of high underwater UV intensities, and Fontinalis mosses thrived during the early Holocene, indicating a highly transparent water column. The results further suggested that underwater UV exposure decreased during the late Holocene, which is probably attributable to increased DOC and decreased solar forcing. Owing to the location of Lake Einstaken and its catchment in the periglacial barren landscape of the polar desert, the fluctuations of bio-optical lake-water properties were apparently forced by postglacial environmental processes and Holocene climate development. These factors controlled sea shoreline proximity, water discharge, ice-cover duration and littoral-benthic primary production and further affected the underwater UV environment. Although the role of solar forcing cannot be underestimated, the current record emphasizes the role of climate-mediated lake-catchment interactions in impacting bio-optical properties and UV exposure of high arctic aquatic systems.

An important factor in the ontogeny of boreal lakes is the development of their humic state through terrigenous input of organic matter (carbon) that affects strongly the functioning and structure of these ecosystems. The long-term dynamics and role of humic substances for these systems in relation to climate are not clear. In this study, a boreal lake from southern Finland was investigated using paleolimnological methods, including diatom, chironomid and geochemical analyses, for Late Holocene changes in the humic state. The aim was to examine the relationship between sediment biogeochemistry and climate variation. Consistent trends were found in diatom-inferred total lake-water organic carbon (TOC) and in the ratio of humic/oligohumic chironomids. Sediment geochemistry provided further evidence for the limnological development of the lake and related long-term climate trends in the region. The results indicated three distinct phases with differing humic state; the beginning of the record at ca. 4,500 cal year BP was characterized by extremely humic conditions coinciding with warm and dry climatic conditions, a meso-oligohumic period between ca. 3,000–500 cal year BP with increasing allochthonous organic matter transport and cooler and wetter climate, and recent period with polyhumic (TOC >10 mg L−1) lake status and warming climate. This study shows that instead of straightforward linear development, boreal lakes evolve through dynamic humic stages related to climate and lake-catchment coupling processes. As the changes in the humic state are ultimately climate-driven, the ongoing climate change probably has a major influence on boreal lakes through direct and indirect effects on organic carbon transport, utilization and accumulation.

We assessed long-term aquatic biodiversity patterns, with a focus on functional diversity (FD), by using sedimentary Chironomidae and Cladocera assemblages in four mountain lakes in the Austrian Alps. The objectives were to detect whether the invertebrate communities exhibited similar recent diversity trends in climate-sensitive alpine lakes, to observe how lake-specific limnological shifts affect biodiversity, and to test the relationship between taxonomic diversity (TD) and FD for paleolimnological biodiversity evaluations. We applied traditional TD indices to the sedimentary assemblages and used an FD index that was based on functional traits of the encountered taxa of both invertebrate groups. There were some similar long-term trends in invertebrate FD and TD among the lakes; e.g. chironomid diversity increased in two lakes and cladoceran diversity in three lakes toward the twentieth and twenty-first century, but in general the diversity patterns were variable between lakes. Overall, FD had a positive relationship with TD suggesting that functionality can be used in paleolimnological studies as a measure of biodiversity. However, cladoceran taxon richness did not correlate with FD in two lakes suggesting that TD is not always a surrogate for FD. Furthermore, TD of chironomids tended to increase more pronouncedly than FD in the top core sections indicating that increasing TD does not necessarily enhance FD and that different taxonomic entities may play similar functional roles in ecosystems. Accordingly, when evaluating biodiversity and ecological redundancy, functionality of the species pool should be carefully considered and highlighted also in paleolimnological research.

Feeding behavior of macroinvertebrates is significant in determining aquatic ecosystem functioning. However, there is a lack of knowledge on the distribution of feeding guilds, and in particular on the influence of climate on the feeding functions at large spatial and long temporal scales. In this study, lakes at different climatic zones are examined for modern abundances of midge (Diptera) feeding guilds to establish the relationships between the feeding functions and climate. In addition, 11,000-year and 700-year sediment core records are investigated for their midge feeding guilds to establish their long-term relationships with climate changes. The results indicated that feeding guilds had a significant relationship with air temperature in the modern dataset and similar climate-related patterns in trait abundances had also occurred during the important climatic intervals of the past. In addition, the long-term records agreed with the modern spatial dataset in that feeding guild diversity was decreased under warm and cold climate compared to intermediate climate conditions. The results of this study are significant in demonstrating the influence of climate on the functioning of aquatic ecosystems and in providing valuable information on long-term variability in feeding functions that is needed to better understand future changes under the ongoing climate change.