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Sedimentary diatoms have been used to quantitatively reconstruct climate-related variables, such as temperature at different timescales. Even though temperature is often less of a key driver of diatom ecology than other environmental parameters (water chemistry), diatom inference models have been shown to be reliable in deducing past temperature trends. In addition, the oxygen isotope composition (δ18Odiatom) preserved in buried diatom frustules has demonstrated its potential to reflect climatic and hydrological conditions at the time of frustule formation. This study combines results from both diatom-based climate proxies to reconstruct summer water and mean annual air temperatures, and hydrological trends in Nettilling Lake, Baffin Island, from ca. 5000 to 500 cal a bp. Diatom-inferred temperatures revealed an overall ca. 2 °C cooling throughout the Late-Holocene. The δ18Odiatom values showed an increasing trend up to ca. 1900 cal a bp, where they reached their highest values (+24.8‰ at 15 cm) and thereafter decreased to their lowest values (+21.4‰ at 4 cm). These trends were linked to meltwater inflows associated with Penny Ice Cap thaw rate that was in turn controlled by regional climatic conditions which went from intensified cooling during the Neoglacial period to slight warming thereafter. Our results suggest that diatom- and diatom-isotope-based temperature and hydrological reconstructions can identify trends related to the natural climate system variability. The diatom oxygen isotopes are useful for paleoenvironmental studies of terrestrial aquatic ecosystems, but not for all hydrological systems are the ideal temperature proxy. Hence, the combination of proxies helps to disentangle temperature and hydrological effects for paleoclimatic reconstructions and may support future studies of postglacial environmental change in northern lakes.

High latitude freshwater ecosystems are sentinels of human activity and environmental change. The lakes and ponds that characterize Arctic landscapes have a low resilience to buffer variability in climate, especially with increasing global anthropogenic stressors in recent decades. Here, we show that a small freshwater pond in proximity of the archaeological site “Native Point” on Southampton Island (Nunavut, Arctic Canada) is a highly sensitive environmental recorder. The sediment analyses allowed for pinpointing the first arrival of Sadlermiut culture at Native Point to ~ 1250 CE, followed by a dietary shift likely in response to the onset of cooling in the region ~ 1400 CE. The influence of the Sadlermiut on the environment persisted long after the last of their population perished in 1903. Presently, the pond remains a distorted ecosystem that has experienced fundamental shifts in the benthic invertebrate assemblages and accumulated anthropogenic metals in the sediment. Our multi-proxy paleolimnological investigation using geochemical and biological indicators emphasizes that direct and indirect anthropogenic impacts have long-term environmental implications on high latitude ecosystems.

Jacques O, Pienitz R, Ibrahim G. 2020. Paleolimnological assessment of long-term changes in a boreal recreational lake of the Fermont mining region (subarctic Quebec, Canada). Lake Reserv Manage. XX:XXX–XXX. Lac Carheil is a boreal recreational lake located near Fermont, a remote town of subarctic Quebec built in the 1970s to support iron ore mining activities. Harmful cyanobacterial blooms were recently observed at its surface, suggesting that it suffered from eutrophication. However, long-term water quality data were lacking to properly understand its current condition. We therefore undertook a paleolimnological study to investigate past limnological changes in the lake based on the multiproxy analysis of sediment cores. The cores retrieved from Lac Carheil were marked by numerous distinct laminae representing past episodes of high-energy events in the lake catchment. Furthermore, diatom assemblages of the sediments revealed that the limnological conditions of the lake had been relatively stable for more than a thousand years before beginning to change around 1840 AD. This was mainly evidenced by the emergence of species indicative of nutrient enrichment (e.g., Aulacoseira subarctica, Fragilaria crotonensis), which suggested an early eutrophication potentially attributable to climate change. The period of modern settlement (1971–2015) was characterized by a decrease in the carbon to nitrogen ratio (C/N) of the sediments, and marked increases in their organic matter content, diatom total abundance, and diatom-inferred total phosphorus concentrations, which indicated an enhanced primary production. Nevertheless, our results suggested that the disturbance level of the lake was weak and that its prospect for recovery is good. Our study provides valuable reference data to assist the management of Lac Carheil, and reveals first insights into the sedimentological and limnological history of the Fermont mining region.

More than a century (1877–2011 CE) of asbestos mining activities in the Thetford Mines region have resulted in the accumulation of gigantic mineral waste piles on the banks of the Becancour River (southern Quebec, Canada). This river widens downstream from the mining sites to form a chain of lakes, successively: Stater Pond, Trout Lake, Lake William and Lake Joseph. A previous paleolimnological investigation revealed that waste erosion and transport strongly modified and polluted Trout Lake. However, questions remain about the extent of the mining contamination within the Becancour River Basin and its impacts at other spatial scales. Here, we aimed to address this lack of knowledge by analyzing the sedimentological evolution of Stater Pond, Lake William, Lake Joseph and Lake Becancour (upstream reference site). Radiometric dating (210Pb, 137Cs, 14C) and analyses of geochemical composition (ICP-AES/ICP-MS), computed tomography, magnetic susceptibility, loss-on-ignition and grain size were performed on sediment cores retrieved at these sites. In contrast to Lake Becancour, yet similar to Trout Lake, we found that Stater Pond and Lake William have received high mineral matter loads since the creation of the Lake Asbestos Mine during the 1950s. Recent lake sediments at these downstream sites were highly enriched in magnesium, chromium and nickel. Comparison of their geochemical signature with that of sedimentary source materials from within the drainage basin demonstrated that they predominantly originate from mining waste erosion. Consequently, Stater Pond and lakes Trout and William are nowadays exposed to very high sediment accumulation rates (up to 1.4 cm yr−1; 0.6 g cm−2 yr−1) and heavy metal enrichment. Evidence for contamination was also found in Lake Joseph, thereby indicating that wastes are transported and deposited over ≥25 km downstream from the former mining sites. Our study highlights the high risks and dangers associated with asbestos pollutants in aquatic ecosystems.

Climate change is causing rapid permafrost degradation across Arctic and subarctic regions, resulting in changes in the size, abundance, and structure of thermokarst (thaw) ponds and lakes. The main objectives of this study were to analyze periphytic diatom communities and their affinity to vegetation substrates in thermokarst ecosystems located in the eastern Hudson Bay region and to establish a first inventory of diatom assemblages and the associated littoral vegetation in these systems. Some generalist diatom species, including Tabellaria flocculosa, occupied all ecological niches in the water bodies. In contrast, genera such as Eunotia and Pinnularia were more specialized and generally concentrated on moss substrates. Shoreline vegetation and thermokarst pond/lake littoral morphology (slope) resulted in limnological conditions that differed between sites and ultimately affected diatom community structure. Our results show that both shoreline vegetation and diatom communities are diverse in thermokarst ecosystems, and their species composition depends mostly on site-specific properties (available microhabitats, local pond/lake morphology) rather than limnological conditions that are closely aligned with regional ecoclimatic conditions.

Ecotones mark zones of rapid change in ecological structure at various spatial scales. They are believed to be particularly susceptible to shifts caused by environmental transformation, making them key regions for studying the effects of global change. Here, we explored the variation in assemblage structure of aquatic primary producer and consumer communities across latitudinal transects in northeastern North America (Quebec-Labrador) to identify spatial patterns in biodiversity that indicated the location of transition zones across the landscape. We analyzed species richness and the cumulative rate of compositional change (expressed as beta-diversity) of diatoms and chironomids to detect any abrupt shifts in the rate of spatial taxonomic turnover. We used principal coordinates analysis to estimate community turnover with latitude, then applied piecewise linear regression to assess the position of ecotones. Statistically significant changes in assemblage composition occurred at 52 and 55°N, corresponding to the transition between closed- and open-crown forest, and to the southern onset of the forest tundra (i.e., the forest limit), respectively. The spatial distribution of ecotones was most strongly related to air temperature for chironomids and to vegetation- and soil-related chemical attributes of lake water for diatoms, including dissolved organic carbon content and water color. Lakes at mid- to high-latitudes currently face pressures from rapidly rising temperatures, accompanied by large increases in organic carbon inputs from their catchments, often leading to browning and its associated effects. The biota at the base of food webs in lakes located in transition zones are disproportionately affected by the cascading effects of these multi-factorial changes, concurrent with pronounced terrestrial greening observed in these regions. Similar patterns of biotic shifts have been observed along alpine aquatic transects, indicating the potential for widespread restructuring of cold, high-altitude and high-latitude freshwater communities due to global change.

International audience; The levels and speciation of dissolved, particulate and gaseous Se have been measured in five thermokarst ponds in a sporadic and discontinuous permafrost region of northern Québec (Canada) during summer oxygen stratification. Evolution of Se concentrations with depth was investigated in sediment cores collected in three different ponds. The potential for inorganic Se transformation in natural pond waters was investigated by experimental incubations of isotopic species-specific tracers of selenite and selenate. Experimental and monitoring observation revealed that high dissolved carbon concentration, suspended particle matter concentration, heterotrophic activity and periphytic biofilms have a significant role in the formation of gaseous selenium species. In sediment, Se is mainly associated with organic matter in the concentration range 0.14–1.6 mg kg−1. Despite low outgassing rates of volatile Se toward the atmosphere (1–97 ng Se m−2 day−1), the large surface occupied by ponds in northern Canada may lead to important exchange rates (ca. 1 ton year−1) at the global scale. Detailed measurements of Se speciation, including volatile compounds, and its reactivity toward heterotrophic activity in selected thaw ponds from the Canadian Subarctic provides new insight to better constrain the biogeochemical pathways leading to Se removal from the water column via atmospheric gas exchange and sediment accumulation.

The hydrology of shallow lakes (and ponds) located in the western Hudson Bay Lowlands (HBL) is sensitive to climate warming and associated permafrost thaw. However, their biological characteristics are poorly known, which hampers effective aquatic ecosystem monitoring. Located in northern Manitoba along the southwestern coast of Hudson Bay, Wapusk National Park (WNP) encompasses numerous shallow lakes representative of the subarctic zone. We analyzed the distribution and diversity of diatom (microscopic algae; class Bacillariophyceae) assemblages in surficial sediments of 33 lakes located in three different ecozones spanning a vegetation gradient, from NE to SW: the Coastal Fen (CF), the Interior Peat Plateau (IPP), and the Boreal Spruce Forest (BSF). We found significant differences (P

[1] Snowmelt is a crucial source of water for many shallow subarctic lakes, but climate models predict that snowfall will decrease in some regions, with profound ecological consequences. Here we use lake water isotope data across gradients of terrestrial vegetation cover (open tundra to closed forest) and topographic relief to identify lakes that are vulnerable to desiccation under conditions of low snowmelt runoff in two subarctic landscapes—Old Crow Flats, Yukon, and Hudson Bay Lowlands, Manitoba (Canada). Lakes located in low-relief, open tundra catchments in both landscapes displayed a systematic, positive offset between directly measured lake water δ18O over multiple sampling campaigns and lake water δ18O inferred from cellulose in recently deposited surface sediments. We attribute this offset to a strong evaporative 18O-enrichment response to lower-than-average snowmelt runoff in recent years. Notably, some lakes underwent near-complete desiccation during midsummer 2010 following a winter of very low snowfall. Based on the paleolimnological record of one such lake, the extremely dry conditions in 2010 may be unprecedented in the past ~200 years. Findings fuel concerns that a decrease in snowmelt runoff will lead to widespread desiccation of shallow lakes in these landscapes.

Warwick F. Vincent1,2,3, Isabelle Laurion1,4, Reinhard Pienitz1,2,5, and Katey M. Walter Anthony6 1Center for Northern Studies (CEN), Quebec City, Canada 2Takuvik Joint International Laboratory, Universite Laval Canada—Centre national de la recherche scientifique (CNRS) (France) 3Departement de biologie, Universite Laval, Quebec City, Canada 4Institut national de la recherche scientifique—Centre Eau Terre Environnement (INRS-ETE), Quebec City, Canada 5Departement de geographie, Universite Laval, Quebec City, Canada 6Water and Environmental Research Center, University of Alaska, Fairbanks, USA

Paleolimnological evidence from some Arctic lakes suggests that longer ice-free seasons have been experienced since the beginning of the nineteenth century. It has been inferred from some additional records that many Arctic lakes may have crossed an important ecological threshold as a result of recent warming. In the instrumental record, long-term trends exhibit increasingly later freeze-ups and earlier break-ups, closely corresponding to increasing air temperature trends, but with greater sensitivity at the more temperate latitudes. Broad spatial patterns in these trends are also related to major atmospheric circulation patterns. Future projections of lake ice indicate increasingly later freeze-ups and earlier break-ups, decreasing ice thickness, and changes in cover composition, particularly white-ice. For rivers, projected future decreases in south to north air-temperature gradients suggest that the severity of ice-jam flooding may be reduced but this could be mitigated by changes in the magnitude of spring snowmelt.

Arctic climate change is leading to accelerated melting of permafrost and the mobilization of soil organic carbon pools that have accumulated over thousands of years. Photochemical and microbial transformation will liberate a fraction of this carbon to the atmosphere in the form of CO2 and CH4. We quantified these fluxes in a series of permafrost thaw ponds in the Canadian Subarctic and Arctic and further investigated how optical properties of the carbon pool, the type of microbial assemblages, and light and mixing regimes influenced the rate of gas release. Most ponds were supersaturated in CO2 and all of them in CH4. Gas fluxes as estimated from dissolved gas concentrations using a wind-based model varied from 220.5 to 114.4 mmol CO2 m22 d21, with negative fluxes recorded in arctic ponds colonized by benthic microbial mats, and from 0.03 to 5.62 mmol CH4 m22 d21. From a time series set of measurements in a subarctic pond over 8 d, calculated gas fluxes were on average 40% higher when using a newly derived equation for the gas transfer coefficient developed from eddy covariance measurements. The daily variation in gas fluxes was highly dependent on mixed layer dynamics. At the seasonal timescale, persistent thermal stratification and gas buildup at depth indicated that autumnal overturn is a critically important period for greenhouse gas emissions from subarctic ponds. These results underscore the increasingly important contribution of permafrost thaw ponds to greenhouse gas emissions and the need to account for local and regional variability in their limnological properties for global estimates.

The physical and chemical variabilities as well as the distribution of diatoms of six boreal lakes in the Laurentian Mountains (southern Quebec, Canada) were studied. The lakes are located along an altitudinal gradient and were sampled at a biweekly resolution from May through October, 2002. In general, we found later onset and weaker lake stratification under colder climates. Lake circulation and SiO2 are strongly correlated and together significantly explain the distribution of diatoms of the individual lakes. Diatoms that accumulated in the sediment traps were mostly composed of benthic species, suggesting resuspension. However, diatom flux and lake circulation were not significantly correlated, the diatom assemblages in the sediment traps were similar in two consecutive years, and species–environment relationships were comparable among lakes, which indicates that the effects of resuspension were minimal. In addition, we found that one lake was more productive due to forest logging. The forest in the catchment of Lake Maxi was entirely clear-cut shortly prior to our sampling. Mean total phosphorus, dissolved organic carbon, and chlorophyll a concentrations were significantly higher when compared to the other five study lakes. This study seeks to improve our understanding of how diatoms in boreal lakes respond to climate change and forest clear-cut.

This study represents a step towards developing seasonal climate inferences by using high-resolution modern data sets. The importance of seasonal climate changes is highlighted by the instrumental record of a meteorological station close to our study site (lac du Sommet in the Laurentian Mountains, Quebec, Canada): Between 1966 and 2001, May temperatures increased significantly by 3.1°C (r = 0.41, n = 35, p 0.05). Comparison of this instrumental record with fossil diatom assemblages in a sediment core from lac du Sommet showed that axis one of a principal component analysis (PCA) of the fossil diatoms was best correlated with wind velocity in June (r = 0.62, n = 19, p < 0.005) and that past diatom production was significantly enhanced in periods with colder July temperatures (r = −0.77, n = 19, p < 0.0005) and higher wind velocity in June (r = 77, n = 19, p < 0.0005). The strong impact of the spring and summer conditions on overall diatom composition and productivity suggests that seasonal lake responses to climate are more important than annual mean temperatures. However, the seasonal dynamics of diatom communities are not well understood, and seasonality is rarely inferred effectively from lake sediment studies. Our research presents a pilot study to answer a twofold question: Is it possible to identify diatom communities which are typical for warmer or colder seasonal climate using sediment traps, and if it is, can this knowledge be used to infer seasonal climate conditions from fossil diatom assemblages? To address these questions, the seasonal dynamics of diatom communities and water chemistry were studied using sediment traps and water samples at biweekly intervals in four lakes distributed along an altitudinal gradient in the Laurentian Mountains from May through October 2002. Date of ice break-up was significantly related to the diatom assemblages taken in spring and uncorrelated to other significant environmental variables. Summer water temperature, circulation of the water column and pH explained a significant part of the biological variance in summer, and total nitrogen (TN) explained most of the biological variance in autumn. To infer these variables, weighted averaging partial least squares models were applied to the seasonal data sets. Inferred ice break-up dates were significantly correlated with number of days below 0°C in April (r = 0.52, n = 19, p < 0.025), inferred circulation of the water column was significantly related to measured wind velocity in June (r = 0.64, n = 19, p < 0.005), inferred summer water temperature and inferred pH was significantly related to measured July air temperature (r = 0.50, r = −53, n = 19, p < 0.025) and inferred TN autumn concentrations had an inverse relationship to August temperatures (r = −0.53, n = 19, p < 0.01). This comparison of the historical record with diatom-inferred seasonal climate signals, based on the comparison of fossil diatom assemblages with modern sediment trap data of high temporal resolution, provides a promising new approach for the reconstruction of seasonal climate aspects in paleolimnological studies.

The effect of 33 environmental variables on the distribution of chironomids was studied in 60 lakes of northwestern Quebec (Canada). A detrended canonical correspondence analysis detected linearity in the chironomid assemblages, thus redundancy analysis was used to identify the variables affecting the chironomid communities. Lake depth, dissolved organic carbon (DOC), mean August air temperature, and surface water temperature were the four variables that best explained the distribution of chironomids. Partial least squares analysis was used to develop new inference models. Among models, the one for mean August air temperature had the highest coefficient of determination (r[Formula: see text] = 0.67) and the lowest root mean square error of prediction (= 1.17 °C). The results indicated that for downcore temperature reconstructions, it might be hard to dissociate the combined effects of temperature, DOC, and depth. Changes in taxa such as Heterotrissocladius brundini-type, Heterotrissocladius subpilosus-type, Heterotrissocladius grimshawi-type, Micropsectra radialis-type, Tanytarsus lugens-type, and Microtendipes can be attributed to changes in lake depth and (or) temperature. Changes in Heterotanytarsus, Dicrotendipes, Cryptotendipes, and Cryptochironomus might be attributed to shifts in temperature and DOC. Relationships among temperature, DOC, and lake depth should be studied in a "neo-ecology" design to better understand their impact on chironomid assemblage composition.

Analyses of sedimentary diatoms in two New England ponds reveal limnological changes during the past 2500 years that are related to climatic change, anthropogenic activities and natural disturbance. Deforestation in the lake catchments during the eighteenth and nineteenth centuries temporarily affected diatom assemblages, with subsequent recovery. However, algal communities did not return completely to presettlement conditions as a result of long-term trends in climatic change, small-scale natural disturbances and delayed watershed recovery from acidification. A short-term rise in diatominferred dissolved organic carbon (DOC) in Levi Pond was related to partial removal of vegetation by logging in the catchment, and a similar DOC increase at North Round Pond is correlated with a hurricane and increased aquatic productivity. At Levi Pond, increasing diatom-inferred DOC concentrations during the past c. 2000 years likely reflect a long-term increase in allochthonous organic matter and peat development in the watershed related to moister conditions, supported by corresponding patterns in the stable-isotope, chironomid and pollen records. These results correspond with moisture patterns in adjacent areas inferred from pollen and sediment analyses, suggesting that diatoms in Levi Pond recorded a larger regional trend in increasing moisture. This is the first study in a temperate region that links diatom-inferred DOC concentration to past changes in moisture balance, suggesting that fossil diatoms may be a promising proxy for future palaeohydrological studies in temperate regions. However, more studies are necessary to separate the effects of peatland growth and allochthonous organic-matter input on lake-DOC concentrations.

The seasonality of physical, chemical, and biological water variables is a major characteristic of temperate, dimictic lakes. Yet, few investigations have considered the potential information that is encoded in seasonal dynamics with respect to the paleolimnological record. We used a one-year sequence of diatoms obtained from sediment traps and water samples, as well as the sedimentary diatom record covering the past ca. 1000 years in Bates Pond, Connecticut (USA), to investigate which variables influence the seasonal distribution of diatoms and how this can be used for the interpretation of the fossil record. The seasonal patterns in diatom assemblages were related to stratification and, to a lesser extent, to nitrate, silica, and phosphorus. During mixing periods in spring and autumn, both planktonic and benthic species were collected in the traps, while few lightly silicified, spindle-shaped planktonic diatoms dominated during thermal stratification in summer. Changes in fossil diatom assemblages reflected human activity in the watershed after European settlement and subsequent recovery in the 20th century. A long-term trend in diatom assemblage change initiated before European settlement was probably related to increased length of mixing periods during the Little Ice Age, indicated by the increase of taxa that presently grow during mixing periods and by application of a preliminary seasonal temperature model. We argue that the analysis of seasonal diatom dynamics in temperate lakes may provide important information for the refinement of paleolimnological interpretations. However, investigations of several lakes and years would be desirable in order to establish a more robust seasonal data set for the enhancement of paleolimnological interpretations.

With depleted ozone levels and the possibility that climate change might lower dissolved organic matter (DOM) concentrations, the risk of exposure of fresh waters in Canada to ultraviolet B (UV-B) was evaluated. First, the distribution of a UV-B-sensitive zooplankton genus, Daphnia, was examined as a function of DOM concentration and maximum depth (Zmax) in 258 systems. Distribution was not restricted by UV-B, although very clear, shallow systems are underrepresented. Secondly, the depth at which 1% of surface radiance at 320 nm occurs (Z320,1%) was compared with Zmax in over 1000 aquatic systems in 15 ecozones to determine the proportions of optically clear systems (Zmax ≤ Z320,1%) and systems that may become clear (i.e., are "at risk") should DOM decrease by 50%. South of the treeline

The objective of this study was to evaluate the effect of agricultural pollution on periphyton in streams and rivers of southern Québec. We sampled benthic algae incubated from mid-July to mid-August on artificial substrates at 29 sites and analysed the variations in community structure and total community biomass. Diatom community structure as well as total benthic algae community were analysed. Water samples were taken to provide background chemical information, and land use data were also obtained. Preliminary tests showed that colonisation of the artificial substrates (unglazed ceramic tiles) resulted in biomass levels (Chlorophyll a and ash-free dry weight) and species composition that were not statistically different from those on natural rock substrates. The canonical correspondence analyses showed that pH, conductivity and suspended solids were the most significant environmental variables accounting for variations among sites and diatom community structure. No additional resolving power was obtained by including cyanobacteria, green algae and flagellates. This total community analysis substantially increased variance and sample processing time while reducing the relationship with environmental variables. These results indicate that an analysis based exclusively on diatoms provided the optimal approach. Traditional nutrient measurements (phosphorus and nitrogen) did not explain a significant part of the variance in the species composition among sites. The ordination analyses clearly separated agriculturally-impacted streams from reference sites, but no significant grouping was observed related to the intensity and type of agriculture, indicating the greater importance of local farming practices. The use of periphyton as a bioindicator provides an integrated measurement of water quality as experienced by the aquatic biota, and therefore offers a useful addition to physico-chemical water quality monitoring strategies.

Changes in the concentration of chromophoric dissolved organic matter (CDOM) were determined over short (hours to days) and medium (weeks) time scales during summer in a shallow, polymictic lake in subarctic Quebec, Canada. CDOM fluorescence (F CDOM ) decreased linearly from mid-June until early August, consistent with the zero-order kinetics expected for photodegradation of organic matter in a regularly mixed lake in which most of the photochemically-active radiation is absorbed by CDOM. A major rainfall event then resulted in a sharp increase in F CDOM back to levels recorded at the start of the study. These changes showed that CDOM concentrations were controlled by the balance between catchment inputs and subsequent photodegradation processes within the lake. Shorter term changes in CDOM concentration were monitored by measuring the penetration of ultraviolet radiation (UVR) using a Satlantic TACCS system moored near the center of the lake and set to monitor down-welling 380 nm irradiance at 10 min intervals. The diffuse attenuation coefficient K d (380) decreased by 12 % during the deployment indicating increased penetration of UVR and the change paralleled the drop in F CDOM over the same period. Many of the daily records showed a significant decline in K d (380) in the near-surface waters over the course of the morning and early afternoon indicating diurnal photobleaching of CDOM, and an increase later in the day consistent with the breakdown of the diurnal thermocline and entrainment of deeper waters. The results illustrate the short and medium term dynamics of CDOM, the importance of diurnal stratification and mixing for photochemical processes, and the variability that aquatic organisms must experience in UV exposure in their natural environment.

Several recent studies have successfully applied diatom-based paleolimnological techniques to infer past hydrological changes in arctic and subarctic regions. For example, we summarize arctic studies that attempt to determine changes in peat water content, flood frequency, river discharge, effective moisture and ice cover in northern regions. Some of the investigations are still in preliminary stages, but represent innovative approaches to study arctic and subarctic paleohydrology. New data demonstrate that lake depth, which may be related to changing hydrological conditions, is a significant variable influencing the distributions of diatom taxa in lake surface sediment calibration sets from Wood Buffalo National Park (WBNP), on the border of Alberta and the Northwest Territories, Canada, and from Fennoscandia (mainly northwest Finland). Weighted averaging regression and calibration methods were used to develop quantitative inference models for lake depth using diatom assemblages preserved in surface sediments. The predictive abilities of the transfer functions were relatively high (for WBNP r 2 = 0.70 and RMSE = 2.6 m, and for Fennoscandia r 2 = 0.88 and RMSE = 1.8 m). However, evaluating the transfer functions using jack-knifing procedures indicated lower predictive abilities, possibly reflecting the relatively small sample size and/or short gradients used in these calibration sets. Such transfer functions can be used to track overall trends in lake levels, and provide an objective assessment as to directions of changing lake levels. Any interpretations of inferred lake levels, especially those related to climate change, must be made cautiously and must include some understanding of the local, present-day hydrological system.

Knowledge of the composition and ecological preferences of diatom assemblages in northern regions is important for paleoenvironmental reconstructions of variables related to climate and/or anthropogenic disturbances. Relatively little is presently known about diatoms from circumpolar treeline areas, especially for lakes located in northern Russia. Our study set included lakes in the Siberian tundra, forest-tundra, and boreal forest regions close to Norilsk, of which nine sites were subject to anthropogenic disturbances, such as mining and housing developments. We enumerated surficial sediment diatom assemblages and used multivariate statistical techniques to investigate which environmental variables were important in explaining the variation in diatom assemblages within our lake set. Deeper lakes were associated with higher abundances of planktonic centric taxa, such as Cyclotella Kutzing species. Warmer, higher conductivity forested lakes were associated with higher abundances of planktonic pennate taxa,...

Paleolimnological analyses of two lakes located near the northern treeline in the central part of the Canadian Northwest Territories document a history of abrupt postglacial climatic and limnological changes. A diatom-based transfer function, based on weighted-averaging partial least squares regression (WA-PLS) techniques, was used to give quantitative estimates of past trends in lakewater dissolved organic carbon (DOC), a limnological variable strongly aligned with catchment vegetation and soils. The diatom record from the sediment cores provides evidence for profound limnologic change during the mid-Holocene, corresponding to maximum forest-tundra development between 5000 and 3000 '4C yr BP in both Queen's and Toronto lakes, with a diatom-inferred relative change in lakewater DOC of up to 5.8 mg L-1 between the mid-Holocene and the present-day. Comparison of the diatom-inferred environmental changes with other proxy data (pollen, stable isotopes) from the same lakes provides strong evidence for an episode of climatic amelioration and lake responses to the associated vegetational changes (from tundra to forest-tundra) at sites near the central Canadian treeline. This study illustrates the usefulness of diatoms as quantitative indicators of past climate-related environmental change in northern treeline regions, and implies that aquatic ecosystems at high latitudes might respond with extreme sensitivity to climate warming.

Data on the water chemistry and limnology of 24 lakes located between Yellowknife (62 8279N, 1148219W) and Contwoyto Lake (658309N, 1108009W) in the central Northwest Territories were examined using principal components analysis and other statistical techniques. The study sites were mostly shallow ( Zmax = 2.5-25 m; mean = 8.2 m), nutrient-poor (3.4- 12.7 mg total phosphorus/L; mean = 6.6 mg · L -1 ), low-alkalinity lakes. Water was typically low in solutes (specific conductance near 0-100 mS · cm -1 ), with slightly acidic to alkaline pH (6.2-8.9). Levels of all nutrients and major ions showed identical trends of decreasing concentrations with increasing latitude, with the highest concentrations generally in lakes with conifer-forested catchments in the south. Resume : Les donnees sur les caracteristiques chimiques et limnologiques de 24 lacs situes entre Yellowknife (62 8279N, 1148219W) et le lac Contwoyto (658309N, 1108009W) dans le centre des Territoires du Nord-Ouest (T.N.-O.) ont ete examinees a l'aide de l'analyse des composantes principales. Les sites d'etude etaient pour la plupart des lacs peu profonds ( Zmax = 2,5 a 25 m; moyenne = 8,2 m), pauvres en elements nutritifs (3,4 a 12,7 mg · L -1 en phosphore totale, moyenne = 6,6 mg · L -1 ) et de faible alcalinite. De maniere typique, l'eau etait pauvre en solutes (conductivite electrique pres de 0 a 100 mS · cm -1 ), et son pH variait de legerement acide a alcalin (6,2 a 8,9). Les concentrations de tous les elements nutritifs et des ions majeurs ont montre des tendances identiques, c.-a-d. qu'ils diminuaient avec l'augmentation de la latitude, les concentrations les plus elevees se trouvant generalement dans les lacs meridionaux avec forets de coniferes dans le bassin versant. (Traduit par la Redaction)

We identified, enumerated, and interpreted the diatom assemblages preserved in the surface sediments of 59 lakes located between Whitehorse in the Yukon and Tuktoyaktuk in the Northwest Territories (Canada). The lakes are distributed along a latitudinal gradient that includes several ecoclimatic zones. It also spans large gradients in limnological variables. Thus, the study lakes are ideal for environmental calibration of modern diatom assemblages. Canonical correspondence analysis, with forward selection and Monte Carlo permutation tests, showed that maximum lake depth and summer surface-water temperature were the two environmental variables that accounted for most of the variance in the diatom data. The concentrations of sodium and calcium were also important explanatory variables. Using weighted-averaging regression and calibration techniques, we developed a predictive statistical model to infer lake surface-water temperature, and we evaluated the feasibility of using diatoms as paleoclimate proxies. This model may be used to derive paleotemperature inferences from fossil diatom assemblages at appropriate sites in the western Canadian Arctic.

Pingualuk Lake fills a deep crater in the Parc National des Pingualuit on the Ungava Peninsula (Nunavik, Canada) and is isolated from nearby surface waters. The main objectives of this study were to determine and compare the concentrations of two atmospherically derived contaminants, mercury and perfluorinated chemicals (PFCs), in the lake water column and fish of Pingualuk Lake and to assess the physical and biological factors influencing contaminant concentrations. Mercury concentrations in arctic char muscle tissue were comparable to those of char in other Arctic lakes, while the total amount of PFCs was below reported levels for remote lakes in the Arctic and elsewhere. Stable isotope and stomach content analyses were made to investigate the feeding ecology of the Pingualuk Lake arctic char population and indicated the possibility of multiple feeding groups. Genetics characteristics (MH and mtDNA) of fish from Pingualuk Lake revealed that this population is likely distinct from that of nearby Laflamme Lake. However, both arctic char populations exhibit differential variation of their allele families. Physical characteristics determined for Lake Pingualuk revealed that the water column was inversely stratified beneath the ice and extremely transparent to visible and ultraviolet radiation. The highest mercury concentrations (3 – 6 pg mL-1 TH g) occurred just beneath the ice surface in each lake. Pingualuk Lake, given its near pristine state and exceptional limnological features, may serve as a most valuable reference ecosystem for monitoring environmental stressors, such as contaminants, in the Arctic.

[1] Permafrost thaw ponds result from the irregular melting and erosion of frozen soils, and they are active sites of greenhouse gas emissions to the atmosphere throughout the circumpolar North. In the discontinuous permafrost region of Nunavik, Canada, thaw ponds show pronounced differences in color even among nearby ponds, ranging from white to green, brown and black. To quantify this optical variation and to determine its underlying controlling mechanisms, we studied the apparent and inherent optical properties and limnological characteristics of the ponds. The pond colors were well separated on a color coordinate diagram, with axis values determined from above‐water spectral reflectance measurements. Our analyses of optical properties and their empirical relationships with optically active substances showed that the differences in color could entirely be attributed to variations in the concentration of two optically active substances: dissolved organic carbon, which was a major contributor to spectral absorption, and nonalgal suspended particulate matter, which contributed to spectral scattering as well as absorption. The latter component was dominated by small sized particles that had unusually high mass‐specific absorption and scattering properties. Analysis of high spatial resolution, multispectral satellite imagery of these ponds showed that these two optically important constituents could be estimated by multivariate modeling. The results indicate that remote sensing surveys will provide valuable synoptic observations of permafrost thaw ponds across the vast subarctic region, and may allow scaling up of local greenhouse gas flux measurements to regional and circumpolar scales.

FUTURE greenhouse warming is expected to be particularly pronounced in boreal regions1, and consequent changes in vegetation in these regions may in turn affect global climate2–4. It is therefore important to establish how boreal ecosystems might respond to rapid changes in climate. Here we present palaeoecological evidence for changes in terrestrial vegetation and lake characteristics during an episode of climate warming that occurred between 5,000 and 4,000 years ago at the boreal treeline in central Canada. The initial transformation — from tundra to forest-tundra on land, which coincided with increases in lake productivity, pH and ratio of inflow to evaporation — took only 150 years, which is roughly equivalent to the time period often used in modelling the response of boreal forests to climate warming5,6. The timing of the treeline advance did not coincide with the maximum in high-latitude summer insolation predicted by Milankovitch theory7, suggesting that northern Canada experienced regionally asynchronous middle-to-late Holocene shifts in the summer position of the Arctic front. Such Holocene climate events may provide a better analogue for the impact of future global change on northern ecosystems than the transition from glacial to nonglacial conditions.

Our understanding of long-term environmental change in arctic and antarctic lakes has advanced markedly over the last two decades. This research has been driven primarily by the growing realization that circumpolar regions are critical research areas for the study of global climatic and environmental change. Logistical improvements (e.g., increased availability of aircraft, research bases, enhanced communication technology, etc.) have also made polar research safer and more feasible. Nonetheless, high costs often preclude many research initiatives, and declines in government funding programs have resulted in the closure of some key monitoring stations that are essential for understanding many of the basic environmental, limnological, and hydrological

The effect of stratospheric ozone depletion on increases in ambient levels of solar ultraviolet (UV) radiation in high-latitude regions' has raised concerns about the response of northern ecosystems to environmental change. The concentration of coloured dissolved organic material, which is derived from terrestrial vegetation and acts as a screen for ultraviolet radiation, is low in high-latitude lakes. The underwater light environment in these lakes is therefore likely to be sensitive to small variations in the supply of this material, in addition to the effects of ozone depletion. Here we use fossil diatom assemblages in combination with bio-optical models to estimate the magnitude of past variations in the underwater light regime of a lake at the boreal tree line. We find large shifts in underwater UV-B, UV-A and photosynthetically available radiation associated with changes in the input of coloured dissolved organic material into subarctic lakes during the Holocene. The inferred changes in biological exposure to UV radiation were at least two orders of magnitude greater than those associated with moderate (30%) ozone depletion. Our findings indicate that freshwater ecosystems at present located across vegetation gradients will experience significant shifts in underwater spectral irradiance through the effects of climate change on catchment vegetation and the export of coloured dissolved organic material.

Reports of severe stratospheric ozone depletion over the Arctic have heightened concern about the potential impact of rising ultraviolet-B (UV-B) radiation on north polar aquatic ecosystems. Our optical measurements and modelling results indicate that the ozone-related UV-B influence on food web processes in the Arctic Ocean is likely to be small relative to the effects caused by variation in the concentrations of natural UV-absorbing compounds, known as chromophoric dissolved organic matter (CDOM), that enter the Arctic basin via its large river inflows. The aim of our present study was to develop and apply a simple bio-optical index that takes into account the combined effects of attenuation by atmospheric ozone and water column CDOM, and photobiological weighting for high-latitude environments such as the Arctic Ocean. To this end, we computed values for a biologically effective UV dose rate parameter ("weighted transparency" or T*) based on underwater UV measurements in high-latitude lakes and rivers that discharge into the Arctic Ocean; measured incident UV radiation at Barrow, Alaska; and published biological weighting curves for UV-induced DNA damage and UV photoinhibition of photosynthesis. The results underscore how strongly the Arctic Ocean is influenced by riverine inputs: shifts in CDOM loading (e.g., through climate change, land-use practices, or changes in ocean circulation) can cause variations in biological UV exposure of much greater magnitude than ozone-related effects.

Seasonal changes in water chemistry and periphytic diatom assemblages were monitored for a saline, a subsaline, and a freshwater lake in the central Yukon Territory. Athalassic saline lakes, such as these, are believed to be extremely rare in arctic regions. All three study lakes exhibited a gradual shoreline retreat over the season (28 May to 22 August 1992) due to evaporative water loss. As the season progressed, the saline lake exhibited a marked increase in conductivity and salinity, sim ilar to changes observed for inland salt lakes in more southern regions. The seasonal changes in water chemistry were less pronounce d in the subsaline and freshwater lakes. The periphytic diatom populations of the saline lake closely tracked changes in the lake 's salinity, exhibiting a successional shift from taxa with low salt tolerances (e.g., Nitzschia cf. commutata and N. cf. palea) to those with high salt tolerances (e.g., Amphora acutiuscula) over the study period. Periphytic diatoms in the subsaline and freshwater lakes also exhibited marked successional changes, shifting to almost complete dominance by a single species ( Cocconeis placentula and Achnanthes minutissima respectively), but these shifts were not related to lake water salinity alone.

Mercury (Hg) profiles were compared to profiles of climate indicators including microfossil remains and algal-derived or S2 carbon (C) in dated sediment cores from 14 lakes spanning latitudinal and longitudinal gradients across the Canadian high and subarctic. Hg fluxes increased postindustrialization (post-∼1850) in 11 of these lakes (postindustrialization Hg fluxes (ΔHgF(F)) = 2-24 μg m(-2) y(-1)). Correction of HgF(F) for catchment contributions demonstrated that Hg deposition originating from catchment-independent factors, such as atmospheric deposition, increased since industrialization in all 14 lakes. Several of these lakes also showed postindustrial shifts in algal assemblages consistent with climate-induced changes. Eleven lakes showed post-1850s increases in S2F(F), suggesting that lake primary productivity has recently increased in the majority of our sites (ΔS2F(F) = 0.1-4 g m(-2) y(-1)). Other studies have interpreted significant relationships between Hg:S2 concentrations in Arctic sediment as support for the algal scavenging hypothesis, which postulates that Hg fluxes to Arctic sediments are largely driven by S2. However, in six of our lakes we observed no Hg:S2 relationship, and in one lake a significant negative Hg:S2 relationship was observed due to increased Hg and decreased S2 C deposition during the postindustrialization period. In six of the seven lakes where a significant positive Hg:S2 relationship was observed, algal assemblages either did not change through time or the timing of the shifts did not correspond to changes in Hg deposition. Our results demonstrate that, although Arctic lakes are experiencing a myriad of changes, including increased Hg and S2 deposition or changing algal assemblages, increased lake primary productivity does not appear to be driving changes in Hg fluxes to sediments.

The phototrophic communities in meromictic, perennially ice-covered Lake A, on Ellesmere Island in the Canadian High Arctic, were characterized by pigment analysis using high performance liquid chromatography. Samples were taken to determine the vertical changes down the water column as well as a variation between years. These analyses showed that Lake A had distinct phototrophic communities in its oxic and anoxic layers. The pigment analyses indicated that phototrophic biomass in the upper, oxic waters was dominated by picocyanobacteria, while in the lower, anoxic layer photosynthetic green sulphur bacteria were dominant. Interannual variation in pigment concentrations was related to the penetration of photosynthetically active radiation in the water column, suggesting that light availability may be limiting the net accumulation of photosynthetic bacterial biomass in Lake A. Pigment analysis of the surface sediments indicated that deposition was dominated by the photosynthetic sulphur bacterial contribution. The sedimentary record of bacterial pigments in polar meromictic lakes offers a promising tool for the reconstruction of past changes in ice cover and therefore in climate. 2009 Elsevier B.V. and NIPR. All rights reserved.