Spatio-temporal perspectives on aquatic carbon dynamics and limnoecological change in climate-sensitive lakes

Northern lakes are displaying widespread ecological reorganizations in response to global change. These sensitive freshwater ecosystems are an integral component of subarctic and arctic landscapes and play a critical role in regional and global biogeochemical cycles. To understand lake ecosystem responses to direct and indirect climate forcing, and to estimate future trajectories of environmental change, we must look for past analogies of climate-lake interaction recorded in lake sedimentary archives. The present, in turn, holds the key to understand the past. This work examines patters and drivers of limnoecological change in shallow subarctic lakes across the treeline ecotone in northern Finland. A diverse set of neolimnological and paleolimnological tools and approaches were employed to explore aquatic ecosystem responses to landscape variability and direct climate forcing through space and time. Particular emphasis was placed on aquatic organic carbon that regulates vital biogeochemical processes in lakes and couples them to the global carbon cycle. To address landscape-mediated climate impact on shallow subarctic lakes, catchment controls on pelagic (lake water) and benthic (sediment) carbon pools, nutrients and productivity, periphytic algal (diatom) communities, and carbon utilization of benthic macrofauna (Cladocera) were assessed in 31 lakes spanning the treeline. To investigate long-term ecosystem development and carbon dynamics under natural and anthropogenic climate variability, two downcore sediment sequences from shallow and oligotrophic treeline lakes, covering the postglacial period (~11 500 years) and the late neoglacial (~600 years), were examined for diverse biogeochemical and paleobiological indices. Across the treeline, lake water carbon pools were fundamentally shaped by wetland influence. The adverse effect of terrestrial colored organic carbon on underwater light availability was diminished in the shallow waters and superseded by nutrient stimulation of primary production. Catchment and soil geomorphology and related hydrological processes were further identified as important controls over aquatic carbon pools, nutrient regimes, and lake water acid-base balance, carrying effects on aquatic primary production and diatom community structure. The late neoglacial paleolimnological record demonstrated a strong coupling between centennial temperature fluctuations, the structure and productivity of aquatic communities, and carbon sequestration. The changes were attributed primarily to alterations in the length of the ice cover period, and were most distinct over the Anthropocene. The postglacial sediment sequence demonstrated repeated diatom turnovers over the Holocene, attributed primarily to millennial moisture fluctuations driving changes in the depth and turbulence of the water column. Overall, the results strengthen recent notions related to the strong terrestrial-aquatic coupling as well as to the susceptibility of northern lakes to climate change, providing new aspects into the underlying processes, drivers of heterogeneity, and biotic interactions. Collectively, the findings build towards an improved understanding of the responses and resilience of northern lake ecosystems to a hierarchy of environmental drivers under the ongoing global change.Ilmastonmuutos vaikuttaa voimakkaasti pohjoisten järvien ekologiseen tilaan. Nämä herkät vesiekosysteemit ovat erottamaton osa arktista ja subarktista luontoa ja merkittävä osatekijä globaalissa biogeokemiallisessa kierrossa. Nykyinen ymmärryksemme ilmaston moninaisista vaikutusmekanismeista sekä järvien vasteista niihin on riittämätön. Uhkina ovat ainutlaatuisten pohjoisten järviekosysteemien katoaminen ilmaston lämpenemisen seurauksena sekä hiilen kiertoon liittyvät ilmaston takaisinkytkennät. Tutkimalla ilmaston ja järvien välisiä kytköksiä järvien pohjasedimentteihin tallentuneiden aikasarjojen sekä alueellisten vertailujen pohjalta, voidaan paremmin ymmärtää nykyisten muutosten luonnetta, suuntaa ja nopeutta. Tässä työssä pyrittiin selvittämään ilmastovaikutuksia matalien subarktisten järvien tilaan hyödyntäen sekä alueellisia että pitkän ajan vertailuaineistoja. Erityishuomion kohteena oli valuma-alueelta peräisin oleva eloperäinen hiili, joka kontrolloi tärkeitä biogeokemiallisia prosesseja järvissä sekä sitoo ne vahvasti globaaliin hiilen kiertoon. Ilmaston epäsuoria, valuma-alueen muutokseen liittyviä vaikutuksia havainnoitiin tutkimalla vaihtelua Lapin puurajan järvien limnologiassa sekä pohjasedimenttien biologisissa ja geokemiallisissa ominaisuuksissa suhteessa valuma-alueen piirteisiin. Muuttuvan ilmaston viimeaikaisia (∽600 vuotta) sekä pitkän aikavälin (∽11 500 vuotta) vaikutuksia järvien ekologiaan ja hiilen kiertoon tutkittiin kahdesta sedimenttiaikasarjasta hyödyntäen sedimentin geokemiallisia sekä paleobiologisia arkistoja. Valuma-alueen suopeite nousi tutkimuksen alueellisessa tarkastelussa tärkeimmäksi järviveden hiilen määrää ja laatua määrääväksi muuttujaksi havumetsävyöhykkeeltä tundralle. Soilta peräisin olevan, valon läpäisyä vedessä vaimentavan, eloperäisen hiilen vaikutus järvien perustuotantoon oli vähäistä tutkituissa matalissa järvialtaissa. Samanaikainen valuma-alueen ravinnekuorma näkyi sen sijaan perustuotannon kasvuna. Valuma-alueen geomorfologiset piirteet sekä niihin liittyvät hydrologiset prosessit osoittautuivat vastaavasti tärkeiksi vesipatsaan kemiaa sekä hiili- ja ravinnepitoisuuksia määrittäviksi tekijöiksi. Tulokset lyhyemmästä, myöhäisen neoglasiaalin kattavasta sedimenttisarjasta indikoivat voimakasta kytköstä lämpötilavaihteluiden, perustuotannon, piilevien yhteisökoostumuksen, sekä hiilen varastoitumisen välillä. Todennäköisimpänä muutoksia ajavana tekijänä pidettiin lämpötilan vaikutusta jääpeiteajan kestoon. Muutokset pidemmässä, koko jääkauden jälkeisen ajan kattavassa sedimenttisarjassa sen sijaan heijastivat erityisesti sadannan vaihteluita, jotka näkyivät ennen kaikkea piileväyhteisöjen rakenteessa. Tärkeimpänä taustalla olevana prosessina pidettiin kosteusolojen vaikutusta vesipatsaan turbulenssiin sekä vedenpinnan korkeuteen. Kaikkiaan, tutkimus nosti esiin kirjon joko valuma-alueen vaihteluun tai suoremmin lämpötilan ja sadannan muutoksiin liittyviä tärkeitä mekanismeja. Samalla tulokset lisäävät tietoa näihin prosesseihin liittyvistä biologisista vasteista ja vuorovaikutuksista sekä merkityksestä järvien hiilen kierron kannalta. Tutkimuksen tulokset rakentavat parempaa ymmärrystä globaalin muutoksen vaikutuksista matalien subarktisten järviekosysteemien tilaan

Biostratigraphical and geochemical evidence for late Holocene water quality and limnoecological changes in a small humic lake in southern Finland

Multiple anthropogenic stressors on lake ecosystems demand effective measures towards improved protection and management of water bodies. The Water Framework Directive defines a common goal for sound water management and obliges EU member countries to monitor and protect the ecological status and water quality of all relevant surface waters. However, major problems hindering the attempts for effective water management are the lack of long-term observational data on reference status and an inadequate understanding of the responses of lake ecosystems to environmental pressures. With this regard, paleolimnological techniques are invaluable as they provide means to seek for past analogies of lake-environment interactions. Long-term development in the water quality and ecological status of Lake Storträsk, located in southern Finland, was assessed using a variety of paleolimnological proxies. The aim was to determine the reference status of this dystrophic lake, and to attain holistic understanding of late Holocene environmental changes and their influences on the lake's status. The principal hypothesis of the study was that late Holocene climate changes, catchment development and contemporary human activities have affected the status of the lake leaving records in the abiotic and biotic features of the sediment deposits. A 43-cm sediment core was obtained from the lake basin and studied for its physicochemical and biological attributes. The core was dated with radiometric methods, and a time frame of ca. 4500 years was established for the sediment sequence. Loss-on-ignition and magnetic susceptibility were measured and the elemental content of the sediment was assessed by ICP-MS- and CNS-analyses. Fossil diatom assemblages were studied to reconstruct long-term development in lake-water pH. Ordination techniques and diversity indices were applied to identify temporal patterns and relationships in the bioassemblages, and Spearman's correlation coefficients were calculated to assess statistical relationships between the studied abiotic and biotic parameters. The results reveal that, whereas long-term climate changes have had the most profound impact on the water quality and ecology of Lake Storträsk, the status of the lake has also been altered by early catchment disturbances and historical human activities as well as more intensive anthropogenic disturbances after the establishment of intensive agriculture in the area. The base of the sediment core reflects the transition from the warm and dry Holocene Thermal Maximum to the cooling and increasingly humid late Holocene. After the initial phase, a more stable development took place, disrupted by possible signs of forest fires and early clearance periods in the area. From the late Middle Ages onwards, human activities within the vicinity of the lake became more intensive leaving highly distinct marks in the sediment, but were receded towards the present. Regardless of the current location of Lake Storträsk in a conservation area, and the apparent inability of the barren catchment to support any intensive agricultural practices, it is clear that human influences on the lake have been significant. The results of this study highlight the importance of long-term perspective in the assessment of lake reference conditions since lake ontogeny is often far from linear. Furthermore, this study emphasizes the importance of combining information from multiple paleolimnological proxies as they provide a more robust and holistic basis for understanding lake-environment dynamics particularly in humic boreal lakes which often respond to environmental change in distinct ways.Ihmistoiminnasta aiheutuva järvien kuormituksen lisääntyminen edellyttää määrätietoista vesienhoidon suunnittelua sekä tehokkaita vesiensuojelun välineitä. Euroopan unionin vesipuitedirektiivi pyrkii osaltaan yhtenäistämään ja tehostamaan vesiensuojelun tavoitteita ja toimia edellyttäen vesistöjen tilan säännöllistä seurantaa sekä asettaen tavoitteita vesien hyvän ekologisen ja kemiallisen tilan saavuttamiseksi. Kestävän vesienhoidon esteenä on kuitenkin usein puutteellinen tieto taustatilasta sekä riittämätön ymmärrys järviekosysteemien ja ympäristön välisestä vuorovaikutuksesta. Paleolimnologiset menetelmät tarjoavat tärkeän välineen menneiden ympäristömuutosten tarkasteluun, sillä nämä muutokset heijastuvat usein järvisedimenttien fysikaalisissa, kemiallisissa ja biologisissa ominaisuuksissa. Tutkimuksessa selvitettiin Etelä-Suomessa, Östersundomissa sijaitsevan Storträskin vedenlaadun sekä ekologisen tilan kehitystä myöhäis-holoseenin aikana. Pyrkimyksenä oli määrittää järven taustatila paleolimnologisin menetelmin, sekä tutkia myöhäis-holoseenin aikaisia ympäristömuutoksia ja niiden vaikutusta järven tilaan. Tutkimushypoteesina oli, että myöhäis-holoseenin ilmastonmuutokset, järven valuma-alueen kehitys sekä ihmistoiminta alueella ovat vaikuttaneet Storträskin tilaan heijastuen sedimentin piirteissä. Järven syvännealueelta otettiin 43 cm pitkä sedimenttisarja, joka ajoitettiin radiometrisin menetelmin. Sedimentistä määritettiin hehkutushäviö sekä magneettinen suskeptibiliteetti, ja sedimentin alkuainekoostumusta tutkittiin ICP-MS -menetelmällä sekä hiilen, typen ja rikin pitoisuuksia CNS -analysaattorilla. Sedimenttisarjasta tehtiin lisäksi piileväanalyysi, jonka avulla rekonstruoitiin järviveden pH-muutoksia myöhäis-holoseenin aikana. Ordinaatioanalyysien, lajiston monimuotoisuutta kuvaavien diversiteetti-indeksien sekä korrelaatioanalyysin avulla pyrittiin hahmottamaan ajallisen muutoksen luonnetta sekä yhteyksiä tutkittujen abioottisten ja bioottisten muuttujien välillä. Tutkimus osoittaa, että myöhäis-holoseenin ilmastokehitys on ollut pääasiallinen muutosta ajava tekijä Storträskin limnologisessa ja ekologisessa kehityksessä, mutta myös valuma-alueen luonnolliset sekä ihmistoimintaan liittyvät häiriöt ovat vaikuttaneet järven tilaan merkittävällä tavalla. Sedimenttisarjan alaosa edustaa siirtymävaihetta holoseenin lämpömaksimin ja myöhäis-holoseenin välillä ja poikkeaa ominaisuuksiltaan selkeästi järven myöhemmistä kehitysvaiheista. Myöhäis-holoseeniin siirtymisen jälkeen järven kehitys on ollut kohtalaisen tasaista, joskin myös merkkejä erilaisista valuma-alueen häiriöistä on tallentunut sedimenttiin. Erityisesti intensiivisemmän maatalouden leviäminen alueelle keskiajalla sekä teollistumiseen liittyvä raskasmetallilaskeuma ovat heijastuneet huomattavina muutoksina järven ekologisessa tilassa ja vedenlaadussa huolimatta järven soistuneesta ja kallioisesta valuma-alueesta, joka rajoittaa ihmistoimintaa järven välittömässä läheisyydessä. Tulokset osoittavat Storträskin reagoivan herkästi erilaisiin ympäristön paineisiin, minkä vuoksi järven tilan seuranta on ensiarvoisen tärkeää ihmistoiminnan lisääntyessä alueella. Tutkimuksen tulokset korostavat pitkän ajan näkökulman merkitystä paleolimnologisessa tutkimuksessa ja vesistöjen taustatilan selvityksessä, sillä järviekosysteemien kehitys on usein kaukana lineaarisesta. Tulokset painottavat myös useiden paleoindikaattorien hyödyntämisen tärkeyttä, sillä ympäristömuutoksia ajaa tyypillisesti kirjo toisiinsa sidoksissa olevia muuttujia ja muutoksen ilmenemiseen vaikuttaa lisäksi eri järvityyppien sisäinen dynamiikka ja kyky puskuroida kuormitusta

Tracking the Limnoecological History of Lake Hiidenvesi (Southern Finland) Using the Paleolimnological Approach

Correction: Water, Air and Soil Pollution, 230 :9 (2019), art. 228 DOI: 10.1007/s11270-019-4262-2We examined a sediment record from Lake Hiidenvesi in southern Finland using paleolimnological methods to trace its limnoecological history. In our record, beginning from the 1940s, chironomid (Diptera) assemblages shifted from typical boreal taxa towards mesotrophic community assemblages at similar to 1960-1980 CE being finally replaced by eutrophic taxa from the 1990s onward. The diatom (Bacillariophyceae) assemblages reflected relatively nutrient rich conditions throughout the record showing a further increase in eutrophic taxa from the 1970s onward. A chironomid-based reconstruction of late-winter hypolimnetic dissolved oxygen (DO) conditions suggested anoxic conditions already in the 1950s, probably reflecting increased inlake production due to allochthonous nutrient inputs and related increase in biological oxygen consumption. However, the reconstruction also indicated large variability in long-term oxygen conditions that appear typical for the basin. With regard to nutrient status, chironomid- and diatom-based reconstructions of total phosphorus (TP) showed a similar trend throughout the record, although, chironomids indicated a more straightforward eutrophication process in the benthic habitat and seemed to reflect the intensified human activities in the catchment more strongly than diatoms. The DO and TP reconstructions were mostly similar in trends compared to the measured data available since the 1970s/1980s. However, the increase in TP during the most recent years in both reconstructions was not visible in the monitored data. The results of our multiproxy study emphasize the significance of including both epilimnetic and hypolimnetic systems in water quality assessments and provide important long-term limnoecological information that will be useful in the future when setting targets for restoration.Peer reviewe

Recent changes in chironomid communities and hypolimnetic oxygen conditions relate to organic carbon in subarctic ecotonal lakes

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.Peer reviewe

Cladoceran (Crustacea) Niches, Sex, and Sun Bathing—A Long-Term Record of Tundra Lake (Lapland) Functioning and Paleo-Optics

Under fundamental ecosystem changes in high latitude lakes, a functional paleolimnological approach may increase holistic understanding of lake responses and resilience to climate warming. A ~2000-year sediment record from Lake Loažžejávri in the tundra of northern Finnish Lapland was examined for fossil Cladocera assemblages to examine long-term environmental controls on aquatic communities. In addition, cladoceran functional attributes, including functional diversity (FD), UV absorbance (ABSUV) of Alona carapaces, and sexual reproduction (ephippia) in Bosmina and Chydoridae were analyzed. Cladoceran communities responded to a major change in benthic habitat quality, reflected as elevated (increasingly benthic) sediment organic matter δ13C signal since the 17th century. FD fluctuations showed association with climate oscillation, FD being generally higher during warm climate periods. These ecological changes were likely attributable to diversification of littoral-benthic consumer habitat space. ABSUV, irrespective of increases during the Little Ice Age (LIA) due to higher UV transparency of lake water, was lower under increasing autochthony (benthic production) suggesting establishment of physical UV refugia by the benthic vegetative substrata. Bosmina ephippia exhibited a decreasing trend associated with increasing benthic production, indicating favorable environmental regime, and, together with chydorid ephippia, transient increases during the climate cooling of the LIA driven by shorter open-water season

Late-Holocene variability in chironomid functional assemblages and carbon utilization in a tundra lake food web

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 longterm 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.Peer reviewe

Temperature controls organic carbon sequestration in a subarctic lake

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 degrees 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 similar to 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.Peer reviewe

Air temperature and water level inferences from northeastern Lapland (69 degrees N) since the Little Ice Age

Independent Arctic records of temperature and precipitation from the same proxy archives are rare. Nevertheless, they are important for providing detailed information on long-term climate changes and temperature-precipitation relationships in the context of large-scale atmospheric dynamics. Here, we used chironomid and cladoceran fossil assemblages to reconstruct summer air-temperature and water-level changes, during the past 400 years, in a small lake located in Finnish Lapland. Temperatures remained persistently cold over the Little Ice Age (LIA), but increased in the 20th century. After a cooler phase in the 1970s, the climate rapidly warmed to the record-high temperatures of the most recent decades. The lake-level reconstruction suggested persistently wet conditions for the LIA, followed by a dry period between similar to 1910 and 1970 CE, when the lake apparently became almost dry. Since the 1980s, the lake level has returned to a similar position as during the IAA. The temperature development was consistent with earlier records, but a significant local feature was found in the lake-level reconstruction the LIA appears to have been continuously wet, without the generally depicted dry phase during the 18th and 19th centuries. Therefore, the results suggest local precipitation patterns and enforce the concept of spatially divergent LIA conditions.Peer reviewe

Biogeochemical cycling and ecological thresholds in a High Arctic lake (Svalbard)

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.Peer reviewe

Neoglacial lake-ecosystem changes above and below the subarctic Fennoscandian treeline inferred from changes in diatom functional groups

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.Peer reviewe