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

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Biogeochemical cycling and ecological thresholds in a High Arctic lake (Svalbard) © The Author(s) 2019. Published version Luoto, Tomi P.; Rantala, Marttiina V.; Kivilä, Henriikka; Nevalainen, Liisa; Ojala, Antti E. K. Luoto, T. P., Rantala, M. V., Kivilä, H., Nevalainen, L., & Ojala, A. E. K. (2019). Biogeochemical cycling and ecological thresholds in a High Arctic lake (Svalbard). Aquatic Sciences, 81(2), Article 34. https://doi.org/10.1007/s00027-019-0630-7 2019 Vol.:(0123456789) 1 3 Aquatic Sciences (2019) 81:34 https://doi.org/10.1007/s00027-019-0630-7 RESEARCH ARTICLE Biogeochemical cycling andecological thresholds inaHigh Arctic lake (Svalbard) TomiP.Luoto1 · MarttiinaV.Rantala2· E.HenriikkaKivilä3· LiisaNevalainen1· AnttiE.K.Ojala4 Received: 4 October 2018 / Accepted: 6 February 2019 © The Author(s) 2019 Abstract 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. Keywords Bird guano· Carbon· Chironomidae· Diatoms· Microplastic· Nitrogen Introduction Climate change alters biogeochemical cycling of major elements and nutrients, especially in regions with sparse vegetation, which are particularly sensitive to changes in surface energy and water balance (Zepp etal. 2007). This phenomenon is most visible in the Polar Regions, where a recent pan-arctic greening of the tundra has been observed (Wookey etal. 2009). In addition to increasingly productive freshwater systems in the Arctic (Michelutti etal. 2005; Holmgren etal. 2010), in some regions, changes in the water balance and cryogenic processes have caused disappearance of lakes (Bouchard etal. 2013; Linderholm etal. 2018) as well as formation of new freshwater ecosystems known as permafrost thaw ponds (Vonk etal. 2015). Climate-driven biological reorganizations in the Arctic include increased primary production owing to longer summer growing seasons, increased algal habitat availability and enhanced catchment nutrient fluxes (Wrona etal. 2016). For example, melting permafrost will likely cause a release of nutrients into inlet streams driving changes in the ecosystem structure of lakes (Hobbie etal. 1999; Thienpont etal. 2013). Another factor causing nutrient enrichment of Arctic lakes and their catchments is influence of climate on bird populations, as seabirds increasingly transport nutrients Aquatic Sciences * Tomi P. Luoto [email protected] 1 Faculty ofBiological andEnvironmental Sciences, Ecosystems andEnvironment Research Programme, University ofHelsinki, Niemenkatu 73, 15140Lahti, Finland 2 Institute ofEarth Surface Dynamics, University ofLausanne, CH1015Lausanne, Switzerland 3 Department ofBiological andEnvironmental Science, University ofJyväskylä, P.O. Box35, 40014Jyvaskyla, Finland 4 Geological Survey ofFinland, Betonimiehenkuja 4, 02150Espoo, Finland T.P.Luoto et al. 1 3 34 Page 2 of 16 from the marine environment to their terrestrial nesting areas (Côté etal. 2010; Hargan etal. 2017), a phenomenon known as ornithogenic drainage (Smol 2016). Climate-mediated physical disturbances, such as changes in underwater light availability and thermal stability due to increased loading of organic or minerogenic matter from the terrestrial environment, may also lead to major disruptions to aquatic community structures (Vincent and Pienitz 1996; Nevalainen etal. 2015). In addition to enhanced biogeochemical cycling, greening terrestrial landscape and increasingly productive ecosystems (Forbes etal. 2010), accumulation of microplastic, especially in the oceans (Cole etal. 2011), is an increasing threat in the Arctic areas (Lusher etal. 2015). Besides the marine environment, microplastic particles are also transported to freshwater ecosystems, particularly in coastal areas, via the atmosphere or by biovectors, such as birds feeding in the ocean (Eerkes-Medrano etal. 2015; Horton etal. 2017; Provencher etal. 2018). Microplastic particles cause threat to freshwater organisms through physiological problems (ingestion and digestion) and ecotoxicological effects (Dris etal. 2015). Although microplastics have been encountered even in remote arctic areas (Lusher etal. 2015), their distribution in High Arctic lakes is still mostly unknown. Since observational records in the Arctic are scarce and short, indirect paleolimnological methods are required to reveal long-term environmental dynamics in high latitude lakes and their surroundings (Smol 2016). The paleolimnological record in surface and downcore lake sediments is based primarily on various physical and biogeochemical proxies and biological indicators, such as diatoms (Bacillariophyta) (Rühland etal. 2003; Rantala etal. 2017), chironomids (Diptera) (Quinlan etal. 2005; Luoto etal. 2019) and cladocerans (Crustacea) (Sweetman etal. 2008; Thienpont etal. 2015; Nevalainen etal. 2016). Physical proxies provide valuable lithological information, whereas biogeochemical proxies are particularly useful in tracking elemental cycling and lake-catchment interactions, such as bird effects using the sediment δ15N signature (Stewart etal. 2013; Hargan etal. 2017). Diatoms are known to respond to pH and nutrient conditions (Tammelin etal. 2017; Pla-Rabés and Catalan 2018), chironomids to hypolimnetic oxygen and temperature (Quinlan and Smol 2002; Engels etal. 2014) and cladocerans to water quality and habitat changes (Jeppesen etal. 2011; Nevalainen 2012). The use of these paleolimnological proxies has enhanced understanding of the trajectories of climate-induced changes in northern aquatic ecosystems. Applying the multiproxy paleolimnological approach it is possible to assess long-term interactions between aquatic systems and their watersheds, i.e. lake-catchment coupling, and to find connections between different environmental realms, including the terrestrial, marine and atmospheric environments. In this study, we analyzed fossil algal (diatoms) and invertebrate (chironomids, cladocerans and oribatid mites) communities together with physical (magnetic susceptibility, organic content), biogeochemical (carbon, nitrogen and their stable isotopes) and ecotoxicological (microplastics) proxies from a sediment profile collected from a High Arctic Lake Revvatnet in Svalbard (77°N). The aim was to build holistic understanding on long-term Arctic biogeochemical cycling and lake ecosystem shifts under the climate warming since the Little Ice Age. We hypothesize that long-term changes are climate-driven, but expect a complex interplay between varied environmental controls and multiple responses of the Arctic ecosystem. The study provides insights into linkages between the atmospheric, terrestrial and freshwater environments but also on the marine-derived influence, since extensive seabird colonies occupy the lake catchment area. Study site Lake Revvatnet (77.022°N, 15.368°E) is located in Hornsund, High Arctic Svalbard (Fig.1a). Revvatnet is a glacial lake situated close to the Polish Polar Station Hornsund in an area characterized by pristine Arctic tundra and polar desert. The lake has infertile rocky shores, and a maximum depth of 26m in the southern main basin (Fig.1b). Based on epilimnetic limnological measurements at the end of July 2013, the water color was 0 PCU, pH 7.6, dissolved oxygen (DO) content 9.1mgl−1, specific conductivity 30µgl−1 and total dissolved solids (TDS) 10µgl−1. Arctic char (Salvelinus alpinus) were observed living in Revvatnet. Continuous water mixing occurs in Revvatnet during the summer, while it ceases during the winter when the lake freezes over (Nowiński and Wiśniewska-Wojtasik 2006). Revvatnet, which lies at an elevation of 30m a.s.l. and has a surface area of about 0.9km2, is an overflow lake (Karczewski etal. 1981) with a network of streams and creeks entering from north and an outlet (Revelva) draining to Hornsund fjord (ocean bay) in the south (Fig.1c). The average present-day summer air temperature (June–August) in the area is 4.4°C and the average annual precipitation is < 400mm (Marsz and Styszyńska 2013). An increase in summer air temperature (~ 2°C) since 1979 has been meteorologically observed (Marsz and Styszyńska 2013), but the biologically active vegetation period still lasts only ~ 2months. The periglacial tundra catchment of Revvatnet lies on the Revbotnen and Revdalen post-glacial marine terraces between the Hornsund fjord and the mountain summits (Fig.1c). The terrain consists of outwash plains and undulating ground moraine with sporadic marginal and lateral ridges, whereas hillsides feature solifluction lobes and talus cones. Barnacle geese (Branta leucopsis) are abundant in the adjacent Fuglebergsletta and extensive Little Biogeochemical cycling andecological thresholds inaHigh Arctic lake (Svalbard) 1 3 Page 3 of 16 34 Auk (Alle alle) colonies are present on the mountain slopes of Revdalstoppen and Rotjesfjellet, which drain into the lake. A recent expansion of bird colonies along the Hornsund coast has been observed (Wojczulanis-Jakubas etal. 2008; Zmudczyńska etal. 2009). The expansion of Little Auk colonies in the area appears to have begun during the early twentieth century (Gąsiorowski and Sienkiewicz 2019). Materials andmethods Sediments, chronology andsedimentological analyses Of the collection of several sediment cores from Lake Revvatnet by Ojala etal. (2016), a 30-cm sediment profile RE2, taken from the southern part of the main basin, was used in the present study. We selected this specific core for the present study because of its distant location from the network of streams in the north (Revbotnen, Fig.1) to avoid the dominant effect of stream sediments and to capture a variety of environmental changes and lake-catch- ment dynamics. The sampling was performed in June 2013 from a boat with a Kajak corer (Renberg 1991) and the sediments were subsampled at 0.5–1cm intervals at the lake shore. Water depth at the coring site was 23.5m. For chronological control, 137Cs analysis was performed at the Geological Survey of Finland using an EGandG Ortec ACE TM—2K gamma spectrometer equipped with a four-inch NaI/TI detector. The core was logged for magnetic susceptibility (Dearing 1999) with a Bartington MS2E1 surface-scanning sensor and subsampled for loss on ignition (LOI, + 550°C for 2h) at 1cm resolution (Dean 1974). Biogeochemical andmicroplastic analyses Prior to the carbon analyses from sediment bulk organic matter, the fresh sediment was subjected to acid fumigation to remove carbonates, whereas nitrogen analyses were performed from natural sediment. Subsamples of 2–4mg of freeze-dried and homogenized lake sediments were weighed and packed into tin capsules for elemental and stable isotope (δ13C, δ15N) composition of organic matter. 1-cm sample resolution was used in less compacted surface samples at 0–8cm depth, whereas the lower part of the core was analyzed with 0.5cm resolution. The analyses were performed with a FlashEA 1112 elemental analyser coupled with a Thermo Finnigan DELTA plus Advantage mass spectrometer. The results are expressed as delta values δ13C and δ15N (‰), described as δ = (Rsample/ Rstandard − 1) × 1000, where R equals 13C/12C and 15N/14N, respectively. The reference standards are Vienna Pee Dee Belemnite for C and atmospheric N2 for N. Proportions of organic C and total N (%) in organic matter were also used in the calculation of the Corg/Ntot mass ratio that can be used as a source (allochthonous/autochthonous) indicator of organic matter (Meyers and Teranes 2001). Topmost 10cm were analyzed for microplastic particles using microscopic separation for identification and 736 628 692 416 434 Revvatnet Revbotnen Revdalen Revelva Tuvbreen Fuglebreen Fuglebergsletta Rotjespynten Polish Polar Station 300 300 300 400 400 400 500 600 500 600 200 200 200 100 100 77.00°N 15.27°E 01.0 km Barents Sea Arctic Ocean Svalbard 10°E 20°E 80°N 78°N Greenland Sea Revvatnet x <8 8-16 16-24 >24 Depth (m) Coring site Revdalstoppen Rotjesfjellet Glacier Hornsund fjord (a) (b) (c) Fig. 1 The study site Revvatnet in Hornsund, Svalbard (a), bathymetric map and coring site (white arrow) (b) and catchment characteristics (c). Locations of Little Auk colonies are marked with bird symbols T.P.Luoto et al. 1 3 34 Page 4 of 16 classification (Karlsson etal. 2017). Plastic contamination from sampling and storage was avoided and controlled visually at identification. The separated size fractions included 100–500µm, 500–1000µm and > 1000µm (Löder and Gerdts 2015). Therefore, since particles smaller than 100µm were not analyzed, sample contamination from nanoplastics originating from e.g. clothing, sample storage and preparation was minimized. The results are expressed as number of plastic particles found per 1cm3 of dry sediment. Diatom analysis Samples for diatom analysis were prepared following standard procedures as described in Battarbee etal. (2001). The samples were analyzed using a 2-cm resolution. Organic matter was removed by oxidizing sediment samples with hydrogen peroxide (30% H2O2) followed by removal of carbonates with hydrochloric acid (37% HCl). Coarse minerogenic matter was removed physically by swirling the sample solution in a beaker and decanting the diatom suspension. The sample residue was checked for absence of diatom valves prior to disposal. Samples were dried on coverslips and mounted with Naphrax, and a minimum of 300 diatom valves per sample were identified with a light microscope at 1000× magnification. Taxonomic determination was mainly based on the flora of Krammer and Lange-Bertalot (1986, 1988, 1991a, b), with nomenclature updated where relevant due to taxonomic refinements. Chironomid andcladoceran analyses Standard methods were applied to fossil chironomid analysis (Brooks etal. 2007). The samples were analyzed using a 1-cm resolution. The wet sediment was sieved through a mesh (100-µm) and the residue was examined under a stereomicroscope (25× magnification). Larval head capsules were extracted and mounted permanently with Euparal on microscope slides. Taxonomic identification following Brooks etal. (2007) was performed under a light microscope (400× magnification). The minimum chironomid head capsule number per sample was set to 50 (Heiri and Lotter 2001; Larocque 2001; Quinlan and Smol 2001). Alongside chironomid analysis, remains of cladocerans were picked and identified according to Szeroczyñska and Sarmaja-Korjonen (2007) and also oribatid mites were calculated following the procedure for environmentally extreme downcore sites (Luoto etal. 2013). Statistical methods anddata utilization Hierarchical clustering was applied to separate stratigraphical diatom and chironomid zones. In the constrained cluster analysis, we used the unweighted paired group method with arithmetic mean (UPGMA) as the algorithm and Bray-Curtis as the similarity index (dissimilarity threshold of 0.5 for a zone to be included). The clustering was carried out using the program Past3 (Hammer etal. 2001). Due to linear nature of the assemblage data, principal component analysis (PCA) was used to examine variation in diatom and chironomid communities. The species data were log10 transformed prior to these analyses. The PCAs were carried out using the program Canoco 5 (Šmilauer and Lepš 2014). Diversity was assessed using the N2 effective number of occurrences (Hill 1973). In addition to taxonomic assemblages, the diatom and chironomid data were examined for functional classification using ecological guilds and applying methodologies for algal (Rimet and Bouchez 2012) and macroinvertebrate (Schmera etal. 2017) functional ecology. The functional classification of diatoms was based on ecological guilds as delineated by Passy (2007) and Rimet and Bouchez (2012), including low profile taxa positioned at the bottom of the biofilm firmly attached to their substrate, high profile taxa extending to the upper layers of the biofilm (including colony forming diatoms), motile taxa capable of fast movement, and planktonic taxa. Each guild comprises taxa having developed diverse strategies to exploit resources and adapt to abiotic factors in a given environment, particularly with reference to nutrients, light, and physical disturbance. The chironomid feeding groups, including collector-gatherers and collector-filterers, were based on Merritt and Cummins (1996) and Mandaville (2002). As indicators for bird-impact, we used relative percentages of nitzschioid diatoms (Jones and Birks 2004; Keatley etal. 2009) and chironomids typical for lakes with significant bird influence in Svalbard, such as Orthocladius trigonolabis-type, O. consobrinus-type and Metriocnemus eurynotus-type (Brooks and Birks 2004; Luoto etal. 2016; Luoto and Ojala 2018). To represent climate variability in Svalbard over the recent centuries, we used the data published by D’Andrea etal. (2012). Summer (June–August) temperature reconstructions are based on alkenone unsaturation in Lake Kongressvatnet, western Svalbard. To depict general trends, we used LOESS smooth with a span 0.2. The temperature data were obtained from the World Data Center for Paleoclimatology and NOAA’s National Climatic Data Center, Paleoclimatology Branch website (http://www.ncdc.noaa.gov/ paleo /paleo .html). The chronologies were matched using the 137Cs peak in the Revvatnet RE2 sediment profile and further extrapolated deeper into the past. Results The 137Cs stratigraphy of the present sediment core RE2 was found to be very similar with other 137Cs stratigraphies in Svalbard (Appleby 2004; Chu etal. 2006; Luoto etal. 2015) Biogeochemical cycling andecological thresholds inaHigh Arctic lake (Svalbard) 1 3 Page 5 of 16 34 as well as other cores taken from Revvatnet and nearby Svartvatnet (Ojala etal. 2016). The 137Cs activity in the RE2 core is well resolved with a single peak at the depth of 5.5–4.5cm (Fig.2). The peak can be linked with the atmospheric testing of nuclear weapons, with the onset of cesium fallout in the early 1950s and maximum fallout in 1963 CE. Therefore, the age horizon of ~ 1950 CE was assigned to the sample at 5cm. Chronological extrapolation provides an age estimate of ~ 1720 CE for the bottom core, but since the lower part of the sediment profile lacks chronological control, this estimate is uncertain. Considering the potential increase in recent sedimentation rates (Ojala etal. 2016), the extrapolated ages are more likely older than younger. The physical and biogeochemical proxies showed rather consistent changes in the Revvatnet sediment profile (Fig.3) and were partly linkable with changes in diatom and chironomid assemblages and summer temperature increase. In the initial part of the core (29–19cm), magnetic susceptibility was low (10–14 SI × 10−5) but began to increase towards the present (> 20 SI × 10−5). A similar pattern was observed with δ13C (from − 29 to − 25‰). In contrast, organic matter content (measured as LOI) was high in the initial phase (6–11%) but low between 18 and 0cm (4–6%). Similar to organic matter, also total organic C (− 0.7 to 1.4%), total N (0.1–0.2%) and Corg/Ntot (− 8.2 to 8.5) showed lower values in the upper sediment profile, with thresholds at 18 and 6cm. δ15N values had a deviating pattern showing a decreasing trend from the bottom core (from ~ 3 to 1‰) until a general increase in values from 10cm onwards (mostly > 3‰). From the sediment samples of Revvatnet, 89 diatom taxa were identified. The most abundant taxa included Cyclotella rossii-comensis-tripartita complex (mean abundance 18.7%, maximum abundance 48.0%), Pseudostaurosira brevistriata (10.6%, 21.9%) and Achnanthidium minutissimum (7.0%, 20.3%). According to the cluster analysis, four diatom zones (I–IV) were separated (Fig.4). In zone I between 28 and 16cm, C. rossii-comensis-tripartita complex dominated and also P. brevistriata and Stauroneis anceps were common. In zone II between 14 and 10cm, P. brevistriata became the most abundant taxon, while C. rossii-comensis-tripartita Atmospheric testing of nuclear weapons, maximum fallout 1963 CE 0 5 10 15 20 500100 150200 ( ,Sediment depth cm) 13 7- 1 Cs activity (Bqkg) Fig. 2 137Cs activity in the sediment profile from Revvatnet, Svalbard 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 14 18 22 Magnetic susceptibility 6810 Organic matter δ13CδN 15 Organic C Total NC/N orgtot -1 0123 Chironomid PC axis 1/2 -1 012 Diatom PC axis 1/2 (SIx10 ) -5 (Sedimetn depth, cm) ~1950 CE (LOI, %) (‰) (‰) (%) (%) 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 3456 Summer temperature (°C) CE -28 -27 -26 -25 0 246 0120.10.2 0.3 -5 0510 Fig. 3 Physical and biogeochemical sediment proxies compared with chironomid and diatom principal component (PC) axes 1 (black) and 2 (grey) scores. The temperature series using LOESS smoothing (span 0.2) is the sedimentary alkenone-based June–August air temperature reconstruction from Svalbard (D’Andrea et al. 2012). The secondary axis is aligned with the 137Cs horizon of the Revvatnet record (grey dashed horizontal line). Older extrapolated sediments are not reliably dated T.P.Luoto et al. 1 3 34 Page 6 of 16 complex markedly decreased. P. brevistriata continued to thrive in zone III between 8 and 6cm, where taxa such as A. minutissimum and Hippodonta costulata also increased. Zone IV between 4 and 0cm (from ~ 1950 CE until present) was dominated by A. minutissimum and Nitzschia spp. with simultaneous disappearances of the generally most abundant taxa C. rossii-comensis-tripartita complex and P. brevistriata. The Revvatnet invertebrate stratigraphy consisted of 14 chironomid taxa, 1 cladoceran taxon and sporadic findings of oribatid mites. The most abundant chironomids included Oliveridia tricornis (mean abundance 50.7%, maximum abundance 90.4%), Micropsectra radialis-type (25.1%, 81.8%) and Hydrobaenus lugubris-type (11.6%, 38%). Similarly and almost concurrently with diatoms, four chironomid zones (I–IV) were separated (Fig.5). Zone I between 29 and 19cm was dominated by O. tricornis and M. radialistype. In zone II between 18 and 11cm, M. radialis-type disappeared and O. tricornis continued to dominate with H. lugubris-type. Zone III between 10 and 4cm resembled zone I, as M. radialis-type returned to the stratigraphy with high abundances. In the topmost zone I between 3 and 0cm, previously predominant O. tricornis disappeared and M. radialis-type decreased. Orthocladius trigonolabis-type distinctly increased together with another member from the same genus, O. consobrinus-type. Similar to O. tricornis, the only cladoceran taxon, Chydorus sphaericus-type, disappeared permanently from the stratigraphy at 3cm. Due to relatively short gradient lengths in the diatom (2.0 SD) and chironomid (2.4 SD) data, linear ordination method (PCA) was used (Šmilauer and Lepš 2014). The first diatom PC axis (λ1 = 0.37) explained 37.0% and the second axis (λ2 = 0.16) 16.3% of all variance. The first chironomid PC axis (λ1 = 0.40) explained 39.9% and the second axis (λ2 = 0.30) 30.2% of the total variance. According to the primary axis scores, both diatoms and chironomids had negatives score in the initial part of the stratigraphy and high scores at the topmost samples (Fig.6). In case of axis 2 scores, diatoms showed a decreasing trend from the bottom samples until the scores began to increase towards the 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 20 40 Cyclotella rossii-comensis- tripartita complex 20 Achnanthidium minutissimum Karayevia suchlandtii Gliwiczia calcar 20 Diploneis ovalis Amphora libyca Reimeria sinuata Pseudostaurosira microstriata 20 Pseudostaurosira brevistriata Staurosira pseudoconstruens Encyonema reichardtii Navicula cincta Navicula vitabunda Hippodonta costulata 20 Sellaphora laevissima 20 Stauroneis anceps 20 40 Pinnularia nodosa Pinnularia interrupta Pinnularia mesolepta 20 Nitzschia spp. ~1950 CE (Relative abundance, %) I II III IV (Sediment depth, cm) Planktonic Low profile High profile Motile Fig. 4 Diatom stratigraphy of the most common taxa (N ≥ 10, min ≥ 5) in Revvatnet grouped according to their ecological guilds. The floral zones (I–IV) were established using cluster analysis and the age horizon (grey dashed line) using 137Cs analysis Biogeochemical cycling andecological thresholds inaHigh Arctic lake (Svalbard) 1 3 Page 7 of 16 34 present at 4cm owing to distinct increases in A. minutissimum and Nitzschia spp. The chironomid axis 2 scores were low in the early phase of the stratigraphy (29–24cm), after which they increased until a new decrease at 10cm. The functional classification of diatoms (Fig.7) showed that planktonic taxa were abundant in the initial part of the sediment profile, between 28 and 16cm, followed by a marked decrease that lasted until the present day. In contrast, low profile diatoms had their maximum abundances at the topmost samples between 8 and 0cm, where high profile diatoms decreased. Motile diatoms were common throughout the stratigraphy but their highest abundances occurred between 14 and 0cm. Nitzschioids showed a progressively increasing trend from the bottom of the core towards the present, with a significant shift at ~ 1950 CE when diatom taxonomic diversity (measured as N2) also peaked. Only two feeding guilds of chironomids were encountered from the stratigraphy (Fig.8). Collector-gatherers were present throughout the core, but collector-filterers were absent from the stratigraphy between 15 and 11cm. Chironomids indicative of bird presence were absent in the initial phase of the sediment profile but showed moderate abundances (~ 10–20%) between 23 and 10cm. After a short absence period, bird indicators reappeared at 7cm and became highly abundant (> 80%) in the topmost samples (2–0cm), where also the chironomid taxonomic diversity was highest. Microplastic particles were found in the topmost sediment layers beginning from 2cm (~ 1990s) upwards (Fig.9). The highest microplastic accumulation (7.4particles/cm3) was enumerated in the surface sample. Size fraction 100–500µm was the most common in all samples where microplastics were encountered. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 ~1950CE 20 40 60 80 100 Oliveridia tricornis 20 40 60 80 Micropsectra radialis-type 20 40 Orthocladius trigonolabis-type 20 Paratanytarsus austriacus-type 20 40 Hydrobaenus lugubris-type 20 40 Orthocladius consobrinus-type 20 Diamesa zernyi/cinerella-type Limnophyes 20 Metriocnemus eurynotus-type 20 Chydorus sphaericus-type Oribatida (Relative abundance, %) I II III IV (Sediment depth, cm) Collector-gatherers Collector-filterers Fig. 5 Chironomid stratigraphy of the most common taxa (N ≥ 2, min ≥ 2) in Revvatnet grouped according to their feeding guilds. The faunal zones (I–IV) were established using cluster analysis and the age horizon (grey dashed line) using 137Cs analysis. The relative abundances of cladoceran Chydrorus sphaericus-type and oribatid mites are calculated from the total sum of invertebrates T.P.Luoto et al. 1 3 34 Page 8 of 16 Discussion Elemental cycling Magnetic susceptibility increased in Revvatnet at two stages (Fig.3), at 18cm and at 6cm, representing roughly the end of the Little Ice Age and the 1950s, respectively. Increases in magnetic mineral content of lake sediments are typically derived from more intense catchment erosion (Thompson etal. 1975; Dearing 1999; Ojala etal. 2017). In High Arctic environment with valley glaciers, magnetic susceptibility can also closely track changes in glacier oscillations through erosional effects (Nesje etal. 2001; Carlson etal. 2017). The recently increased values in Revvatnet correspond with the observed thinning rates in western Svalbard glaciers (Kohler etal. 2007) suggesting increasing melt water discharges (Fig.1) and causing more intense erosion and transportation of mineral material delivery of melt waters into the Revvatnet basin from the northern inlets. The influence of glacier retreat, which began following the Little Ice Age, is also reflected in the marine sediment records from Hornsund fjord, but as the major glaciers fronts apparently retreated rapidly to the inner bays, the iceberg discharge to the fjord center became quickly limited (Pawłowska etal. 2016). In Lake Revvatnet, previous studies suggest a development towards a more turbid environment (Sienkiewicz etal. 2017). According to a contemporary survey (Ojala etal. 2016), the northern basin of Revvatnet (Fig.1b) is significantly more turbid (11 formazin turbidity units, FTU) during summer open water season than the southern basin (4 FTU), from where the current core RE2 was collected. There is also a clear difference in the temperatures of the two basins, as the shallower northern basin is ~ 2°C warmer. Despite the distinct difference in turbidity and temperature, no vertical stratification was observed suggesting continuous mixing during the summer. Since the Revvatnet basins are separated by a limnological and bathymetric sill, it inevitably has ecological significance. The clear decrease in organic matter content at 18cm (Fig.3) is likely more related to increased transport of mineral matter from the catchment, as suggested by the magnetic 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 -2 3 -2 2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Present Present Diatoms Chironomids Fig. 6 Principal component analysis axis 1 and 2 scores for diatom (black) and chironomid (grey) samples of the Revvatnet sediment record 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 20 40 Planktonic 20 40 Low profile 20 40 High profile 20 40 60 Motile 20 40 Nitzschioids -1 012 PC axis 1 -1 012 PC axis 2 46810 N2 (Relative abundance, %) (Sedimetn depth, cm) ~1950CE Fig. 7 Ecological guilds (grey) of diatoms (all diatoms included), relative share of nitzchioids (white pattern fill), principal component (PC) axis scores and effective diatom diversity (N2) in Revvatnet. The age horizon established using 137Cs analysis is marked with a dashed line Biogeochemical cycling andecological thresholds inaHigh Arctic lake (Svalbard) 1 3 Page 15 of 16 34 Luoto TP, Ojala AEK, Arppe L, Brooks SJ, Kurki E, Oksman M, Wooller MJ, Zajączkowski M (2018) Synchronized proxy-based temperature reconstructions reveal mid- to late Holocene climate oscillations in High Arctic Svalbard. J Quat Sci 33:93–99 Luoto TP, Rantala MV, Kivilä EH, Nevalainen L (2019) Recent changes in chironomid communities and hypolimnetic oxygen conditions relate to organic carbon in subarctic ecotonal lakes. Sci Total Environ 646:238–244 Lusher AL, Tirelli V, O’Connor I, Officer R (2015) Microplastics in Arctic polar waters: the first reported values of particles in surface and sub-surface samples. Sci Rep 5:14947 Mandaville SM (2002) Benthic macroinvertebrates in freshwaters— taxa tolerance values, metrics, and protocols. In: Soil and Water Conservation Society of Metro Halifax, Nova Scotia, p 47 Mariash HL, Smith PA, Mallory M (2018) Decadal response of Arctic freshwaters to burgeoning goose populations. Ecosystems 21:1230–1243 Marsz AA, Styszyńska A (eds) (2013) Climate and climate change at Hornsund, Svalbard. Gdynia Maritime University, Gdynia, p402 Merritt RW, Cummins KW (eds) (1996) An introduction to the aquatic insects of North America. Kendall-Hunt, Dubuque Meyers PA, Lallier-Vergès E (1999) Lacustrine organic matter records of Late Quaternary paleoclimates. J Paleolimnol 21:345–372 Meyers PA, Teranes JL (2001) Sediment organic matter. In: Last WM, Smol JP (eds) Tracking environmental change using lake sediments, physical and geochemical methods, vol2. Kluwer Academic Publishers, Dordrecht, pp239–269 Michelutti N, Wolfe AP, Vinebrooke RD, Rivard B, Briner JP (2005) Recent primary production increases in arctic lakes. Geophys Res Lett 32:19 Michelutti N, Hermanson MH, Smol JP, Dillon PJ, Douglas MS (2007) Delayed response of diatom assemblages to sewage inputs in an Arctic lake. Aquat Sci 69:523–533 Moe B, Stempniewicz L, Jakubas D, Angelier F, Chastel O, Dinessen F, Gabrielsen GW, Hanssen F, Karnovsky NJ, Rønning B, Welcker J, Wojczulanis-Jakubas K, Bech C (2009) Climate change and phenological responses of two seabird species breeding in the high-Arctic. Mar Ecol Prog Ser 393:235–246 Nesje A, Matthews JA, Dahl SO, Berrisford MS, Andersson C (2001) Holocene glacier fluctuations of Flatebreen and winter-precipita- tion changes in the Jostedalsbreen region, western NorWay, based on glaciolacustrine sediment records. Holocene 11:267–280 Nevalainen L (2012) Distribution of benthic microcrustaceans along a water depth gradient in an Austrian Alpine lake–Sedimentary evidence for niche separation. Limnologica 42:65–71 Nevalainen L, Luoto TP (2017) Relationship between cladoceran (Crustacea) functional diversity and lake trophic gradients. Funct Ecol 31:488–498 Nevalainen L, Rantala MV, Luoto TP, Rautio M, Ojala AEK (2015) Ultraviolet radiation exposure of a high arctic lake in Svalbard during the Holocene. Boreas 44:401–412 Nevalainen L, Rantala MV, Luoto TP, Ojala AE, Rautio M (2016) Long-term changes in pigmentation of arctic Daphnia provide potential for reconstructing aquatic UV exposure. Quat Sci Rev 144:44–50 Nowiński K, Wiśniewska-Wojtasik B (2006) Diversity of abiotic properties of water in shallow lakes in Hornsund area (SW Spitsbergen). Limnol Rev 6:215–222 O’Hanlon NJ, James NA, Masden EA, Bond AL (2017) Seabirds and marine plastic debris in the northeastern Atlantic: a synthesis and recommendations for monitoring and research. Environ Pollut 321:1291–1301 Ojala AEK, Arppe L, Luoto TP, Wacker L, Kurki E, Zajączkowski M, Pawłowska J, Damrat M, Oksman M (2016) Sedimentary environment, lithostratigraphy and dating of sediment sequences from Arctic lakes Revvatnet and Svartvatnet in Hornsund, Svalbard. Pol Polar Res 37:23–48 Ojala AEK, Luoto TP, Virtasalo JJ (2017) Establishing a high-reso- lution surface sediment chronology with multiple dating methods—testing 137Cs determination with Nurmijärvi clastic-bio- genic varves. Quat Geochronol 37:32–41 Passy SI (2007) Diatom ecological guilds display distinct and predictable behavior along nutrient and disturbance gradients in running waters. Aquat Bot 86:171–178 Paul CA, Douglas MSV, Smol JP (2010) Diatom-inferred Holocene climatic and environmental changes in an unusually subsaline high Arctic nunatak pond on Ellesmere Island (Nunavut, Canada). J Paleolimnol 44:913–929 Pawłowska J, Zajaczkowski M, Lacka M, Lejzerowicz F, Esling P, Pawlowski J (2016) Palaeoceanographic changes in Hornsund Fjord (Spitsbergen, Svalbard) over the last millennium: new insights from ancient DNA. Clim Past 12:1459–1472 Perren BB, Anderson NJ, Douglas MSV, Fritz SC (2012) The influence of temperature, moisture, and eolian activity on Holocene lake development in West Greenland. J Paleolimnol 48:223–239 Peterson CG, Stevenson RJ (1992) Resistance and resilience of lotic algal communities: importance of disturbance timing and current. Ecology 73:1445–1461 Pla-Rabés S, Catalan J (2018) Diatom species variation between lake habitats: implications for interpretation of paleolimnological records. J Paleolimnol 60:169–187 Provencher JF, Vermaire JC, Avery-Gomm S, Braune BM, Mallory ML (2018) Garbage in guano? Microplastic debris found in faecal precursors of seabirds known to ingest plastics. Sci Total Environ 644:1477–1484 Quinlan R, Smol JP (2001) Setting minimum head capsule abundance and taxa deletion criteria in chironomid-based inference models. J Paleolimnol 26:327–342 Quinlan R, Smol JP (2002) Regional assessment of long-term hypolimnetic oxygen changes in Ontario (Canada) shield lakes using subfossil chironomids. J Paleolimnol 27:249–260 Quinlan R, Douglas MSV, Smol JP (2005) Food web changes in arctic ecosystems related to climate warming. Glob Change Biol 11:1381–1386 Rantala MV, Luoto TP, Weckström J, Rautio M, Nevalainen L (2017) Climate drivers of diatom distribution in shallow subarctic lakes. Freshw Biol 62:1971–1985 Renberg I (1991) The HOK—Kajak sediment corer. J Limnol 6:167–170 Rimet F, Bouchez A (2012) Life-forms, cell-sizes and ecological guilds of diatoms in European rivers. Knowl Manag Aquat Ecosyst. https ://doi.org/10.1051/kmae/20120 18 Rühland K, Priesnitz A, Smol JP (2003) Paleolimnological evidence from diatoms for recent environmental changes in 50 lakes across Canadian Arctic treeline. Arct Antarct Alp Res 35:110–123 Rühland K, Paterson AM, Smol JP (2015) Lake diatom responses to warming: reviewing the evidence. J Paleolimnol 54:1–35 Saros JE, Anderson NJ (2015) The ecology of the planktonic diatom Cyclotella and its implications for global environmental change studies. Biol Rev 90:522–541 Scherer C, Brennholt N, Reifferscheid G, Wagner M (2017) Feeding type and development drive the ingestion of microplastics by freshwater invertebrates. Sci rep 7:17006 Schmera D, Heino J, Podani J, Erős T, Dolédec S (2017) Functional diversity: a review of methodology and current knowledge in freshwater macroinvertebrate research. Hydrobiologia 787:27–44 Sienkiewicz E, Gąsiorowski M, Migała K (2017) Unusual reaction of diatom assemblage on climate changes during the last millennium: a record from Spitsbergen lake. J Paleolimnol 58:73–87 T.P.Luoto et al. 1 3 34 Page 16 of 16 Šmilauer P, Lepš J (2014) Multivariate analysis of ecological data using CANOCO 5. Cambridge University Press, Cambridge Smol JP (2010) The power of the past: using sediments to track the effects of multiple stressors on lake ecosystems. Freshw Biol 55:43–59 Smol JP (2016) Arctic and Sub-Arctic shallow lakes in a multiplestressor world: a paleoecological perspective. Hydrobiologia 778:253–272 Smol JP, Douglas MSV (2007a) Crossing the final ecological threshold in high Arctic ponds. Proc Natl Acad Sci 104:12395–12397 Smol JP, Douglas MSV (2007b) From controversy to consensus: making the case for recent climate change in the Arctic using lake sediments. Front Ecol Environ 5:466–474 Stewart EM, Michelutti N, Blais JM, Mallory ML, Douglas MSV, Smol JP (2013) Contrasting the effects of climatic, nutrient, and oxygen dynamics on subfossil chironomid assemblages: a paleolimnological experiment from eutrophic High Arctic ponds. J Paleolimnol 49:205–219 Sweetman JN, LaFace E, Rühland KM, Smol JP (2008) Evaluating the response of Cladocera to recent environmental changes in lakes from the central Canadian Arctic treeline region. Arct Antarct Alp Res 40:584–591 Szeroczyñska K, Sarmaja-Korjonen K (2007) Atlas of subfossil cladocera from central and northern Europe. Friends of lower Vistula society, Poland Tammelin M, Kauppila T, Viitasalo M (2017) Factors controlling recent diatom assemblages across a steep local nutrient gradient in central-eastern Finland. Hydrobiologia 799:309–325 Tapolczai K, Bouchez A, Stenger-Kovács C, Padisák J, Rimet F (2016) Trait-based ecological classifications for benthic algae: review and perspectives. Hydrobiologia 776:1–17 Thienpont JR, Rühland KM, Pisaric MF, Kokelj SV, Kimpe LE, Blais JM, Smol JP (2013) Biological responses to permafrost thaw slumping in Canadian Arctic lakes. Freshw Biol 58:337–353 Thienpont JR, Korosi JB, Cheng ES, Deasley K, Pisaric MF, Smol JP (2015) Recent climate warming favours more specialized cladoceran taxa in western Canadian Arctic lakes. J Biogeogr 42:1553–1565 Thompson R, Battarbee RW, O’sullivan PE, Oldfield F (1975) Magnetic susceptibility of lake sediments. Limnol Oceanogr 20:687–698 Vincent WF, Pienitz R (1996) Sensitivity of high-latitude freshwater ecosystems to global change: temperature and solar ultraviolet radiation. Geosci Can 23:4 Vonk JE, Tank SE, Bowden WB etal (2015) Reviews and syntheses: effects of permafrost thaw on Arctic aquatic ecosystems. Biogeosci 12:7129–7167 Wagner M, Scherer C, Alvarez-Muñoz D etal (2014) Microplastics in freshwater ecosystems: what we know and what we need to know. Environ Sci Eur 26:12 Wojczulanis-Jakubas K, Jakubas D, Stempniewicz L (2008) Avifauna of Hornsund area, SW Spitsbergen: present state and recent changes. Pol Polar Res 29:187–197 Wookey PA, Aerts R, Bardgett RD, Baptist F, Bråthen KA, Cornelissen JH, Gough L, Hartley IP, Hopkins DW, Lavorel S, Shaver GR (2009) Ecosystem feedbacks and cascade processes: understanding their role in the responses of Arctic and alpine ecosystems to environmental change. Glob Change Biol 15:1153–1172 Wrona FJ, Johansson M, Culp JM, Jenkins A, Mård J, Myers-Smith IH, Prowse TD, Vincent WF, Wookey PA (2016) Transitions in Arctic ecosystems: ecological implications of a changing hydrological regime. J Geophys Res 121:650–674 Zepp RG, Erickson Iii DJ, Paul ND, Sulzberger B (2007) Interactive effects of solar UV radiation and climate change on biogeochemical cycling. Photochem Photobiol Sci 6:286–300 Zmudczyńska K, Zwolicki A, Barcikowski M, Barcikowski A, Stempniewicz L (2009) Spectral characteristics of the Arctic ornithogenic tundra vegetation in Hornsund area, SW Spitsbergen. Pol Polar Res 30:249–262 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.