Low Arctic Lakes Research Articles

Effects of long-term climate trends on the methane and CO2 exchange processes of Toolik Lake, Alaska

Methane and carbon dioxide effluxes from aquatic systems in the Arctic will affect and likely amplify global change. As permafrost thaws in a warming world, more dissolved organic carbon (DOC) and greenhouse gases are produced and move from soils to surface waters where the DOC can be oxidized to CO2 and also released to the atmosphere. Our main study objective is to measure the release of carbon to the atmosphere via effluxes of methane (CH4) and carbon dioxide (CO2) from Toolik Lake, a deep, dimictic, low-arctic lake in northern Alaska. By combining direct eddy covariance flux measurements with continuous gas pressure measurements in the lake surface waters, we quantified the k600 piston velocity that controls gas flux across the air–water interface. Our measured k values for CH4 and CO2 were substantially above predictions from several models at low to moderate wind speeds, and only converged on model predictions at the highest wind speeds. We attribute this higher flux at low wind speeds to effects on water-side turbulence resulting from how the surrounding tundra vegetation and topography increase atmospheric turbulence considerably in this lake, above the level observed over large ocean surfaces. We combine this process-level understanding of gas exchange with the trends of a climate-relevant long-term (30 + years) meteorological data set at Toolik Lake to examine short-term variations (2015 ice-free season) and interannual variability (2010–2015 ice-free seasons) of CH4 and CO2 fluxes. We argue that the biological processing of DOC substrate that becomes available for decomposition as the tundra soil warms is important for understanding future trends in aquatic gas fluxes, whereas the variability and long-term trends of the physical and meteorological variables primarily affect the timing of when higher or lower than average fluxes are observed. We see no evidence suggesting that a tipping point will be reached soon to change the status of the aquatic system from gas source to sink. We estimate that changes in CH4 and CO2 fluxes will be constrained with a range of +30% and −10% of their current values over the next 30 years.

Macrophyte performance in a low arctic lake: effects of temperature, light and nutrients on growth and depth distribution

Submerged macrophytes are important contributors to primary production in clear water arctic and low arctic lakes. However, their production and potential coverage are hampered by the harsh climate conditions. In this study, we investigated the potential for increased macrophyte production and coverage in arctic lakes in a future warmer climate. In situ growth experiments with Callitriche hamulata were performed at 2, 4, 8 and 12 m depth in combination with nutrient assay experiments at 2 m depth. In addition, growth experiments were performed in the laboratory at four temperatures (5, 10, 15 and 20 °C) under saturated and light-limited conditions (150 and 25 µmol m−2 s−1). The results show that macrophyte growth in the low arctic lake, Badeso, is phosphorus limited, but they also indicate that nutrients are not the limiting factor for the macrophyte depth distribution. Rather the short growing season combined with low summer temperatures may limit the expansion of C. hamulata. Our study also shows that C. hamulata is a very temperature-sensitive plant, particularly around 10 °C. In a future warmer climate in the arctic, the thermocline in clear lakes is expected to expand deeper into the water column. Thus, at the present light conditions we can expect an expansion of both colonisation depth and coverage, which may affect the overall primary production in arctic lakes and the carbon flux and carbon cycling in the lake systems. Our findings strongly support recent predictions of increased growth and a more northerly distribution range of cold-temperature submerged macrophytes.

Characterization of perfluoroalkyl substances in sediment cores from High and Low Arctic lakes in Canada

Perfluoroalkyl substances (PFASs) are synthetic environmentally-persistent pollutants that are amenable to long-range transport and accumulation in remote Arctic ecosystems. In this study, historical inventories of twenty-three PFASs (i.e. C4-C14, C16 perfluoroalkane carboxylic acids (PFCAs); C4, C6-C8, C10 perfluoroalkane sulfonic acids (PFSAs); perfluoro-4-ethyl-cyclohexane sulfonic acid (PFECHS); dodecafluoro-3H-4,8-dioxanonanoic acid (ADONA); 8-chloro-perfluoro-1-octane sulfonic acid (8-Cl-PFOS); chlorinated polyfluorinated ether sulfonic acids (Cl-PFESAs) including 9-chlorohexadecafluoro-3-oxanonane-1-sulfonic acid (6:2 Cl-PFESA) and 11-chloroeicosafluoro-3-oxaundecane-1-sulfonic acid (8:2 Cl-PFESA); as well as perfluorooctane sulfonamide (FOSA)) are determined in two intact sediment cores collected from Lake Hazen, located in northern Ellesmere Island at 82° N in 2012 and Lake B35, located in central Nunavut at 64° N in 2009. In Lake Hazen, fluxes of perfluorooctanoic acid (PFOA), perfluorodecanoic acid (PFDA), perfluorobutane sulfonic acid (PFBS), and perfluorooctane sulfonic acid (PFOS) increased during 1963–2011. In Lake B35, fluxes of perfluoroheptanoic acid (PFHpA), PFOA, perfluorononanoic acid (PFNA), and perfluoroundecanoic acid (PFUnDA) increased during 1952–2009. The temporal trends for PFASs in Lake Hazen and Lake B35 sediments are consistent with the continuous annual delivery of PFASs to the Arctic of Canada. Temporal trends in sediment cores appear to follow historical market changes in PFAS manufacturing inventory. The doubling time of PFAS fluxes are faster in Lake Hazen sediments than Lake B35 sediments. In Lake Hazen, this may be attributed to the enhanced delivery of sediment and historically-archived PFASs promoted by climate-induced glacier melting in the Lake Hazen watershed post-2005. Exponentially increasing PFAS temporal trends in High and Low Arctic lakes in Canada stress the importance of developing effective global regulatory policies for PFAS manufacturing and highlights the potential for climate change-induced contaminant release from melting glaciers in the Arctic.

The initiation and development of small peat‐forming ecosystems adjacent to lakes in the north central Canadian low arctic during the Holocene

Small peat-forming ecosystems in arctic landscapes may play a significant role in the regional biogeochemistry of high-latitude systems, yet they are understudied compared to arctic uplands and other major peat-forming regions of the North. We present a new data set of 25 radiocarbon-dated permafrost peat cores sampled around eight low arctic lake sites in northern Manitoba (Canada) to examine the timing of peat initiation and controls on peat accumulation throughout the Holocene. We used macrofossils and charcoal to characterize changes in the plant community and fire, and we explored potential impacts of these local factors, as well as regional climatic change, on rates of C accumulation and C stocks. Peat initiation was variable across and within sites, suggesting the influence of local topography, but 56% of the cores initiated after 3000 B.P. Most cores initiated and remained as drier bog hummock communities, with few vegetation transitions in this landscape. C accumulation was relatively slow and did not appear to be correlated with Holocene-scale climatic variability, but C stocks in this landscape were substantial (mean = 45.4 kg C m−2), potentially accounting for 13.2 Pg C in the Taiga Shield ecozone. To the extent that small peat-forming systems are underrepresented in peatland mapping, soil organic carbon (SOC) stocks may be underestimated in arctic regions. Mean fire severity appeared to be negatively correlated with C accumulation rates. Initiation and accumulation of soil C may respond to both regional and local factors, and substantial lowland soil C stocks have the potential for biogeochemical impacts on adjacent aquatic ecosystems.

A comparative study of ancient environmental DNA to pollen and macrofossils from lake sediments reveals taxonomic overlap and additional plant taxa

We use 2nd generation sequencing technology on sedimentary ancient DNA (sedaDNA) from a lake in South Greenland to reconstruct the local floristic history around a low-arctic lake and compare the results with those previously obtained from pollen and macrofossils in the same lake. Thirty-eight of thirty-nine samples from the core yielded putative DNA sequences. Using a multiple assignment strategy on the trnL g–h DNA barcode, consisting of two different phylogenetic and one sequence similarity assignment approaches, thirteen families of plants were identified, of which two (Scrophulariaceae and Asparagaceae) are absent from the pollen and macrofossil records. An age model for the sediment based on twelve radiocarbon dates establishes a chronology and shows that the lake record dates back to 10,650 cal yr BP. Our results suggest that sedaDNA analysis from lake sediments, although taxonomically less detailed than pollen and macrofossil analyses can be a complementary tool for establishing the composition of both terrestrial and aquatic local plant communities and a method for identifying additional taxa.

Stable isotopes reveal that chironomids occupy several trophic levels within West Greenland lakes: Implications for food web studies

Stable isotope analyses of nitrogen (δ15N) and carbon (δ13C) were conducted on individual chironomid taxa from low arctic lakes to investigate the range in trophic levels covered by this diverse insect group. Five lakes were sampled, including two freshwater, two oligosaline, and one glacier influenced lake, representing the major lake types in southwest Greenland. There was a large difference in the offset of isotopic values among the different lake types. In the oligosaline lakes, we ascribe low δ13C values to the use of recycled CO2, while the high δ15N signal is possibly controlled by microbial processes. In order to compare among lakes, the δ15N signal of the chironomid taxa was normalized to common primary consumers (Psectrocladius sordidellus group and Psectrocladius limbatellus groups) that show consistently low δ15N values and belong to the subfamily Orthocladiinae. Normalized δ15N values spanned more than 5‰, indicating that the chironomid community covered at least two trophic levels in these low arctic lakes. The δ15N values of different chironomid taxa within a single habitat differed by 1‰ to 5‰. The results suggest that chironomids should not be considered as one group in food web studies. Even interpretation of trophic position on subfamily level can be problematic as a result of the large differences observed in their δ15N values. The level of detailed trophic information on individual chironomid taxa presented here has not previously been reported. Chironomids are an abundant and important group of organisms in arctic lakes, and the observed variation in their trophic level indicates a high complexity of the food web structure of arctic lakes.

Limnological controls on stable isotope records of late-Holocene palaeoenvironment change in SW Greenland: a paired lake study

Stable isotope records are increasingly being used in palaeoenvironmental studies of Arctic lakes. Here we compare stable isotope and elemental records (δ13C, δ15N, C/N) with high resolution XRF-derived geochemical and colour data from low Arctic lakes (SS1220 and SS85) in southwest Greenland. Lake SS1220 sediments are laminated gyttja whereas SS85 consist of homogeneous gyttja, both records cover the last c. 5000 years. δ13C and carbon content suggest that organic matter in both lakes is predominantly autochthonous. The C/N variability, ranging between 11 and 15, is interpreted to be a function of nitrogen deficiency which is further supported by δ15N values (at SS1220) falling close to atmospheric δ15Natmosphere. Although some changes in the geochemical stratigraphies presented here can be related to regional environmental change (i.e. hydrology, effective precipitation, aeolian activity), these effects are filtered through the lake catchments. These changes alter the nutrient supply and hence lake productivity which appear to be the primary controls on the stable isotope data. We suggest that information of past environmental change can only be gained from the stable isotope composition of lake sediments when there is a reasonable understanding of limnological processes, and records may be lake specific.

Diatom-inferred climatic and environmental changes over the last ∼ 9000 years from a low Arctic (Nunavut, Canada) tundra lake

Diatom assemblage changes over the Holocene were examined from a 14C-dated sediment core retrieved from Lake TK-2, a small low Arctic lake located ca. 200 km north of the forest–tundra ecotone in mainland Nunavut, Canada. Notable changes in the diatom assemblages were recorded throughout the core, suggesting that the Holocene epoch in this region has been environmentally and climatically dynamic. The earliest diatom assemblages (ca. 9000 cal yr BP) were dominated by taxa that are atypical of post-glacial assemblages commonly recorded throughout the Arctic, and may suggest that early Holocene conditions at Lake TK-2 were relatively warmer. A shift to dominance by small, benthic, opportunistic Fragilaria taxa followed (after ca. 8550 cal. yr BP), more typical of Arctic assemblages during initial lake ontogeny, suggesting the onset of cooler, more alkaline conditions. An abrupt and short-lived marked decrease in diatoms between ca. 8550 and 8500 cal. yr BP, with corresponding changes in physical and chemical indicators (e.g., sedimentation rate, siliciclastic content, % organic matter content), provides potential evidence for the 8.2 k cooling anomaly, an event rarely recognized from other paleolimnological studies in the Canadian north. Following ca. 7000 cal. yr BP, a substantial shift occurred to a more complex and diverse diatom assemblage that now included more acidophilic taxa. This compositional change is likely indicative of a natural, long-term loss of alkalinity in the lake, and marks the onset of the Holocene Thermal Maximum, consistent with the timing of this warm period for this region. The relatively stable diatom assemblage composition during the Neoglacial period was punctuated by fluctuations in key species potentially correlative with the so-called Medieval Warm Period and Little Ice Age. In the most recent sediments, the post-industrial expansion of the small, planktonic Cyclotella stelligera complex, and a concurrent decline in the heavily-silicified Aulacoseira lirata complex, are similar to shifts that are increasingly being recognized as geographically widespread diatom responses to recent climate warming. The Lake TK-2 diatom record provides important insights into the Holocene environmental history of this understudied region of the Canadian Arctic. Furthermore, it is one of the few Arctic lakes in which the 8.2 k cold event is possibly expressed.

Climate Versus In-Lake Processes as Controls on the Development of Community Structure in a Low-Arctic Lake (South-West Greenland)

The dominant processes determining biological structure in lakes at millennial timescales are complex. In this study, we used a multi-proxy approach to determine the relative importance of in-lake versus indirect processes on the Holocene development of an oligotrophic lake in SW Greenland (66.99°N, 50.97°W). A 14C and 210Pb-dated sediment core covering approximately 8500 years BP was analyzed for organic–inorganic carbon content, pigments, diatoms, chironomids, cladocerans, and stable isotopes (δ13C, δ18O). Relationships among the different proxies and a number of independent controlling variables (Holocene temperature, an isotope-inferred cooling period, and immigration of Betula nana into the catchment) were explored using redundancy analysis (RDA) independent of time. The main ecological trajectories in the lake biota were captured by ordination first axis sample scores (18–32% variance explained). The importance of the arrival of Betula (ca. 6500 years BP) into the catchment was indicated by a series of partial-constrained ordinations, uniquely explaining 12–17% of the variance in chironomids and up to 9% in pigments. Climate influences on lake biota were strongest during a short-lived cooling period (identified by altered stable isotopes) early in the development of the lake when all proxies changed rapidly, although only chironomids had a unique component (8% in a partial-RDA) explained by the cooling event. Holocene climate explained less variance than either catchment changes or biotic relationships. The sediment record at this site indicates the importance of catchment factors for lake development, the complexity of community trends even in relatively simple systems (invertebrates are the top predators in the lake) and the challenges of deriving palaeoclimate inferences from sediment records in low-Arctic freshwater lakes.

An experimental investigation of phytoplankton nutrient limitation in two contrasting low arctic lakes

We investigated whether phytoplankton communities in two lakes in SW Greenland were phosphorus or nitrogen limited. The study lakes have contrasting water chemistry (mean conductivities differ ten fold) and are located near Kangerlussuaq, SW Greenland (~67°N, 51°W). A microcosm nutrient enrichment experiment was performed in June 2003 to determine whether nitrate or phosphate addition stimulated phytoplankton growth. Samples were analysed for species composition, biomass, and alkaline phosphatase activity (APA). Initially, both lakes had extremely low total phosphorus but high total nitrogen concentrations and high APA, suggesting that the phytoplankton were phosphorus limited prior to the start of the experiment. The phytoplankton composition and biomass (mainly Ochromonas spp.) responded to phosphate but not to nitrate addition. In both lakes, chlorophyll a increased significantly when phosphate was added. Furthermore, APA was significantly lower in the two lakes when phosphate was added compared to the control and the nitrogen addition treatment. The dominance of mixotrophic phytoplankton and high DOC values suggest that these lakes may be regulated by microbial loop processes.

Determining the date of ice-melt for low Arctic lakes along Søndre Strømfjord, southern West Greenland

The length of ice cover has considerable influence on the functioning of lake ecosystems, particularly so in continental and high-latitude regions where lakes freeze annually. Long-term trends in the length of ice cover and the date of ice break-up can be related to regional weather patterns, such as the North Atlantic Oscillation. It is this relationship to weather patterns that has generated considerable interest in the use of long-term icerecords as climate proxies. Although it is reasonable to assume a relationship between the length of the icefree period and lake productivity, it is unclear if this relationship influences the sedimentary record. Whether these ice-climate interactions can be identified in the sediment record is important for distinguishing long-term palaeoclimatic trends from variations in the sediment record (Anderson et al. 2000).

The Holocene vegetational development of the Godthåbsfjord area, West Greenland

Holocene pollen and macrofossil diagrams from four low arctic lakes at Godthåbsfjord are presented. Each core has been divided into radiocarbon-dated palaeovegetation zones, based on the remnants of terrestric plants. The PY zones are physiognomically similar, but differences as to the composition and frequency of species can be seen between the two lakes in the interior and the two lakes from the outer coast area. The vegetation which invaded the deglaciated soil was open but rich in species, and 64 species or genera have been determined from the pioneer stage (c. 9400 - 8000 B.P.). Open soil plants were dominating, but dwarf-shrubs entered the vegetation, with species from snow-patches and snow-covered heaths dominating in the beginning. By 8000 B.P. Salix glauca and S. herbacea immigrated, and gradually the pioneer plants and chionophilous dwarf-shrubs were decimated. This Salix-Cyperaceae stage lasted until c. 6300 B.P., when Betula nana spread all over the area within a few centuries. A Betula nana-Juniperus stage lasted until c. 3500 B.P. In the subcontinental interior this was followed by an Alnus crispa-Berula nana stage, which in turn was replaced by a Beutla nana-Ericales stage around 1800 B.P. Alnus has never been able to grow at the maritime outer coast, where Betula, Cyperaceae, Empetrum and other Ericales dominated after c. 3500 B.P. Later on, Empetrum, Cypcraceae and snowbed plants gradually spread at the expence of Betula nana. After the deglaciation the temperature increased, reaching today's values between 8000 and 7500 B.P. At which time during the coming millennia the temperature curve peaked is not known, but it may have been fairly late, presumably during the Benita nana-Juniperus stage. Major climatic changes are registered in the interior at 3900-3600 and 1800 B.P., and at the outer coast at c. 3600 and 2500-2000 B.P. From around 8000 B.P. the development of the lakes is fairly independent of the physical conditions of the surroundings, being dependent mainly on the trophic stages of the lakes. These pass through a succession: highly productive, eutrophic – less productive, mesotrophic - very poor, oligotrophic. As well as in the flora and fauna, these stages are reflected in the sediment, which at the beginning was a clay gyttja followed by a jelly-like gyttja and, finally, by a loose, watery gyttja consisting mainly of precipitated humus. Chemical analyses of one of the cores confirm the oligotrophication. The pollen influx in the pioneer stage is less than 100 grains per cm2 per year, increasing during the Hypsithermal to c. 300 in three of the lakes and c. 1000 in the richest one, but since then the influx decreases somewhat upwards. A survey of the immigration or first appearance of some species palynologically important to South and West Greenland shows big time lags in the spreading of some species, e.g. Thalictrum and Angelica, whereas others, like Empetrum and Juniperus, have a more effective dispersal capacity.