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1. Comprehensive assessments of contemporary diatom distributions across the Arctic remain scarce. Furthermore, studies tracking species compositional differences across space and time, as well as diatom responses to climate warming, are mainly limited to paleolimnological studies due to a lack of routine monitoring in lakes and streams across vast areas of the Arctic. 2. The study aims to provide a spatial assessment of contemporary species distributions across the circum-Arctic, establish contemporary biodiversity patterns of diatom assemblages to use as reference conditions for future biomonitoring assessments, and determine pre-industrial baseline conditions to provide historical context for modern diatom distributions. 3. Diatom assemblages were assessed using information from ongoing regulatory monitoring programmes, individual research projects, and from surface sediment layers obtained from lake cores. Pre-industrial baseline conditions as well as the nature, direction and magnitude of changes in diatom assemblages over the past c.200 years were determined by comparing surface sediment samples (i.e. containing modern assemblages) with a sediment interval deposited prior to the onset of significant anthropogenic activities (i.e. containing pre-1850 assemblages), together with an examination of diatoms preserved in contiguous samples from dated sediment cores. 4. We identified several biotypes with distinct diatom assemblages using contemporary diatom data from both lakes and streams, including a biotype typical for High Arctic regions. Differences in diatom assemblage composition across circum-Arctic regions were gradual rather than abrupt. Species richness was lowest in High Arctic regions compared to Low Arctic and sub-Arctic regions, and higher in lakes than in streams. Dominant diatom taxa were not endemic to the Arctic. Species richness in both lakes and streams reached maximum values between 60°N and 75°N but was highly variable, probably reflecting differences in local and regional environmental factors and possibly sampling effort. 5. We found clear taxon-specific differences between contemporary and pre-industrial samples that were often specific to both ecozone and lake depth. Regional patterns of species turnover (β-diversity) in the past c.200 years revealed that regions of the Canadian High Arctic and the Hudson Bay Lowlands to the south showed most compositional change, whereas the easternmost regions of the Canadian Arctic changed least. As shown in previous Arctic diatom studies, global warming has already affected these remote high latitude ecosystems. 6. Our results provide reference conditions for future environmental monitoring programmes in the Arctic. Furthermore, diatom taxa identification and harmonisation require improvement, starting with circum-Arctic intercalibrations. Despite the challenges posed by the remoteness of the Arctic, our study shows the need for routine monitoring programmes that have a wide geographical coverage for both streams and lakes.

1. Arctic freshwaters are facing multiple environmental pressures, including rapid climate change and increasing land-use activities. Freshwater plankton assemblages are expected to reflect the effects of these stressors through shifts in species distributions and changes to biodiversity. These changes may occur rapidly due to the short generation times and high dispersal capabilities of both phyto- and zooplankton. 2. Spatial patterns and contemporary trends in plankton diversity throughout the circumpolar region were assessed using data from more than 300 lakes in the U.S.A. (Alaska), Canada, Greenland, Iceland, the Faroe Islands, Norway, Sweden, Finland, and Russia. The main objectives of this study were: (1) to assess spatial patterns of plankton diversity focusing on pelagic communities; (2) to assess dominant component of β diversity (turnover or nestedness); (3) to identify which environmental factors best explain diversity; and (4) to provide recommendations for future monitoring and assessment of freshwater plankton communities across the Arctic region. 3. Phytoplankton and crustacean zooplankton diversity varied substantially across the Arctic and was positively related to summer air temperature. However, for zooplankton, the positive correlation between summer temperature and species numbers decreased with increasing latitude. Taxonomic richness was lower in the high Arctic compared to the sub- and low Arctic for zooplankton but this pattern was less clear for phytoplankton. Fennoscandia and inland regions of Russia represented hotspots for, respectively, phytoplankton and zooplankton diversity, hereas isolated regions had lower taxonomic richness. Ecoregions with high α diversity generally also had high β diversity, and turnover was the most important component of β diversity in all ecoregions. 4. For both phytoplankton and zooplankton, climatic variables were the most important environmental factors influencing diversity patterns, consistent with previous studies that examined shorter temperature gradients. However, barriers to dispersal may have also played a role in limiting diversity on islands. A better understanding of how diversity patterns are determined by colonisation history, environmental variables, and biotic interactions requires more monitoring data with locations dispersed evenly across the circumpolar Arctic. Furthermore, the importance of turnover in regional diversity patterns indicates that more extensive sampling is required to fully characterise the species pool of Arctic lakes. α diversity, β diversity, ecoregions, latitude, taxonomic richness, temperature

1. Freshwater chemistry across the circumpolar region was characterised using a pan-Arctic data set from 1,032 lake and 482 river stations. Temporal trends were estimated for Early (1970–1985), Middle (1986–2000), and Late (2001–2015) periods. Spatial patterns were assessed using data collected since 2001. 2. Alkalinity, pH, conductivity, sulfate, chloride, sodium, calcium, and magnesium (major ions) were generally higher in the northern-most Arctic regions than in the Near Arctic (southern-most) region. In particular, spatial patterns in pH, alkalinity, calcium, and magnesium appeared to reflect underlying geology, with more alkaline waters in the High Arctic and Sub Arctic, where sedimentary bedrock dominated. 3. Carbon and nutrients displayed latitudinal trends, with lower levels of dissolved organic carbon (DOC), total nitrogen, and (to a lesser extent) total phosphorus (TP) in the High and Low Arctic than at lower latitudes. Significantly higher nutrient levels were observed in systems impacted by permafrost thaw slumps. 4. Bulk temporal trends indicated that TP was higher during the Late period in the High Arctic, whereas it was lower in the Near Arctic. In contrast, DOC and total nitrogen were both lower during the Late period in the High Arctic sites. Major ion concentrations were higher in the Near, Sub, and Low Arctic during the Late period, but the opposite bulk trend was found in the High Arctic. 5. Significant pan-Arctic temporal trends were detected for all variables, with the most prevalent being negative TP trends in the Near and Sub Arctic, and positive trends in the High and Low Arctic (mean trends ranged from +0.57%/year in the High/Low Arctic to −2.2%/year in the Near Arctic), indicating widespread nutrient enrichment at higher latitudes and oligotrophication at lower latitudes. 6. The divergent P trends across regions may be explained by changes in deposition and climate, causing decreased catchment transport of P in the south (e.g. increased soil binding and trapping in terrestrial vegetation) and increased P availability in the north (deepening of the active layer of the permafrost and soil/sediment sloughing). Other changes in concentrations of major ions and DOC were consistent with projected effects of ongoing climate change. Given the ongoing warming across the Arctic, these region-specific changes are likely to have even greater effects on Arctic water quality, biota, ecosystem function and services, and human well-being in the future. biogeochemistry, eutrophication, lakes, oligotrophication, rivers

Recent anthropogenic climate change and the exponential increase over the past few decades of Saharan dust deposition, containing ecologically important inputs of phosphorus (P) and calcium (Ca), are potentially affecting remote aquatic ecosystems. In this study, we examine changes in cladoceran assemblage composition and chlorophyll-a concentrations over the past ~150 years from high-resolution, welldated sediment cores retrieved from six remote high mountain lakes in the Sierra Nevada Mountains of Southern Spain, a region affected by Saharan dust deposition. In each lake, marked shifts in cladoceran assemblages and chlorophyll-a concentrations in recent decades indicate a regional-scale response to climate and Saharan dust deposition. Chlorophyll-a concentrations have increased since the 1970s, consistent with a response to rising air temperatures and the intensification of atmospheric deposition of Saharan P. Similar shifts in cladoceran taxa across lakes began over a century ago, but have intensified over the past ~50 years, concurrent with trends in regional air temperature, precipitation, and increased Saharan dust deposition. An abrupt increase in the relative abundance of the benthic cladoceran Alona quadrangularis at the expense of Chydorus sphaericus, and a significant increase in Daphnia pulex gr. was a common trend in these softwater lakes. Differences in the magnitude and timing of these changes are likely due to catchment and lake-specific differences. In contrast with other alpine lakes that are often affected by acid deposition, atmospheric Ca deposition appears to be a significant explanatory factor, among others, for the changes in the lake biota of Sierra Nevada that has not been previously considered. The effects observed in Sierra Nevada are likely occurring in other Mediterranean lake districts, especially in softwater, oligotrophic lakes. The predicted increases in global temperature and Saharan dust deposition in the future will further impact the ecological condition of these ecosystems, Ministerio de Educación y Ciencia (MEC), Grant/Award Number: AP2007-00352; Programa Nacional de Movilidad de Recursos Humanos de Investigaci on (MICINN); Ministerio de Medio Ambiente (MMA), Grant/Award Number: 87/2007; Ministerio de Econom ıa, Industria y Competitividad (MINECO), Grant/Award Number: CGL2011-23483; Natural Sciences and Engineering Research Council of Canada

Paterson AM, Ruhland KM, Anstey CV, Smol JP. 2017. Climate as a driver of increasing algal production in Lake of the Woods, Ontario, Canada. Lake Reserv Manag. 33:403–414.Lake of the Woods (LOW) is...

Anthropogenic climate change and the recent increase of Saharan dust deposition has had substantial effects on Mediterranean alpine regions. We examined changes in diatom assemblage composition over the past ~180 years from high-resolution, dated sediment cores retrieved from six remote lakes in the Sierra Nevada Mountains of Southern Spain. In all lakes, changes in diatom composition began over a century ago, but were more pronounced after ~1970 CE, concurrent with trends in rising regional air temperature, declining precipitation, and increased Saharan dust deposition. Temperature was identified as the main predictor of diatom assemblage changes, whereas both Saharan dust deposition drivers, the Sahel precipitation index and the winter North Atlantic Oscillation, were secondary explanatory variables. Diatom compositional shifts are indicative of lake alkalinization (linked to heightened evapoconcentration and an increase in calcium-rich Saharan dust input) and reduced lake water turbulence (linked to lower water levels and reduced inflows to the lakes). Moreover, decreases in epiphytic diatom species were indicative of increasing aridity and the drying of catchment meadows. Our results support the conclusions of previous chlorophyll-a and cladoceran-based paleolimnological analyses of these same dated sedimentary records which show a regional-scale response to climate change and Saharan dust deposition in Sierra Nevada lakes and their catchments during the 20th century. However, diatom assemblages seem to respond to different atmospheric and climate-related effects than cladoceran assemblages and chlorophyll-a concentrations. The recent impact of climate change and atmospheric Saharan deposition on lake biota assemblages and water chemistry, as well as catchment water availability, will have important implications for the valuable ecosystem services that the Sierra Nevada provides.

Ecosystem managers have often considered the nuisance diatom species, Didymosphenia gemi¬nata (didymo) as being introduced to the natural environment through human activities; however, observations from early 20th-century surveys challenge this characterization. We use diatoms pre¬served in lake sediment cores to investigate the history of didymo blooms from Gaspesie, Quebec. Relative abundances of diatoms were examined from the dated sediments of Lac Humqui and Lac au Saumon (a lake with an inflowing river currently supporting blooms). Didymo was observed throughout the Lac au Saumon core, demonstrating that it has been present in the region well before the first reports of blooms in 2006. Lac Humqui diatom assemblages experienced a shift in composition with declines in benthic taxa (attached to substrates) and increases in planktonic (free floating) diatoms that began ~1970. Strong relationships between this diatom shift, and increases in regional air temperatures and earlier river ice-out dates were consistent with the expected effects of climate warming on aquatic systems. Our paleolimnological evidence shows that climate warming, rather than human introduction, likely plays an important role in triggering didymo blooms.

We provide the first assessment of regional water chemistry and plankton (phytoplankton and crustacean zooplankton) for a suite of lakes near the Sutton River region of the north-central Hudson Bay Lowlands (HBL). We use ordination analyses to examine the spatial variation in water chemistry and plankton across lakes, and to explore the factors that may explain this variation. Based on data collected during summer from 2009 to 2011, we found that in addition to geology, water chemistry was strongly influenced by a lake's proximity to salt water and the degree of permafrost development within its catchment. Phytoplankton composition varied across lakes based on differences in water depth and nutrient concentrations, with non-filamentous cyanobacteria and chlorophytes more common in shallow lakes, and deeper lakes dominated by planktonic diatoms or filamentous cyanophytes. Crustacean zooplankton community composition and richness in the HBL lakes was similar to communities found in Ontario lakes in...

A multi-proxy paleolimnological survey was performed on 13 lakes in the Hudson Bay Lowlands (HBL) of northern Ontario in order to provide a regional analysis of recent environmental changes in this poorly studied sub-Arctic region. In contrast to the amplified warming experienced by most of the circumpolar Arctic since the mid-19th century, the climate of the Hudson Bay (HB) region has remained relatively cool and stable for hundreds of years. However, since approximately the 1990s, the HBL has experienced rapid and large increases in air temperature and declines in sea ice. Diatom, cladoceran, and chironomid remains preserved in the recent (surface) and pre-1850 sediments of 13 lakes were used to examine whether this new climate regime has resulted in species assemblage changes across multiple trophic levels. Our results indicate clear limnological responses to warming among the freshwater biota of HBL lakes; however, the magnitude of this change varied among both biological indicators and sites...

There are two distinct views of the north: one as frontier, the other as homeland. —Thomas Berger (Berger, 1977) The general study region examined in this special issue is the drainage to the south...

This study presents the results of a multi-proxy analysis of a Holocene lacustrine sediment sequence recovered from a small tundra lake in the central Canadian Arctic and links climatic variations there to the North Atlantic. Chironomid and pollen-based reconstructions of mean July air temperature (MJAT) document notable deviations at 9.3 ka and 8.2 ka. Chironomid-inferred July air temperature is depressed by 1.4 °C at 9.3 ka and 1.7 °C at 8.2 ka relative to the long-term chironomid-inferred Holocene average of 9.4 °C. These cooling events also correspond to an increase in Gramineae (Poaceae) and a decrease in Betula pollen, reflecting a decrease in the density of dwarf birch cover. Concurrent, abrupt increases in several diatom taxa, including aerophilic epiphytes, and Sphagnum indicate an expansion of moss habitats as a result of wetter conditions. The expression and timing of the cooling evidenced at TK2 during the early Holocene is consistent with abrupt climate events identified in the North Atlantic and coincides with intervals of North Atlantic cooling and weakened Atlantic Meridional Overturning Circulation (AMOC). This research, which improves our understanding of the spatial extent and magnitude of the 9.3 ka and 8.2 ka events in the central Canadian Arctic, serves as a benchmark for coupled-ocean atmosphere climate models simulating the response of the climate system to abrupt climate events and provides insight in marine-atmosphere teleconnections in the circum-North Atlantic region.

We examine the role of local environmental and spatial factors in explaining variation in the composition of cladoceran assemblages from surface sediments within a set of 50 lakes spanning a broad southwest to northeast transect across the central Canadian Arctic treeline region from Yellowknife (Northwest Territories) to the northern boundary of the Thelon Game Sanctuary (Nunavut Territory). Within each lake, the cladoceran fauna was identified based on the subfossil exoskeletal remains preserved in recently deposited lake sediments. Physical and chemical limnological data were measured in August of 1996 and 1998. Spatial data were generated based on latitude and longitude using Principal Coordinates of Neighbors Matrices analysis (PCNM). The relationships between cladocerans and the measured environmental and spatial variables were examined using both unconstrained (Principal Components Analysis, PCA) and constrained (Redundancy Analysis, RDA) ordination techniques. Variance partitioning, based on partial RDAs, was used to identify the relative importance of significant environmental and spatial explanatory variables. Three environmental variables were identified as significantly influencing cladoceran community structure: surface water temperature, dissolved organic carbon (DOC), and total phosphorus (TP). Five PCNM-generated spatial variables were also significant in explaining cladoceran distributions. Variance partitioning attributed 14% of the variance in the distribution of Cladocera to spatial factors, an additional 10% to spatially-structured environmental variables, and 8% to environmental factors that were not spatially-structured. Within the central Canadian Arctic treeline region, spatial and other environmental processes had an important influence on the distribution of cladoceran communities. The strong influence of spatial factors was related to the large ecoclimatic gradient across treeline. The distribution patterns of cladocerans suggest that they have potential for use in paleoenvironmental assessments of northern ecosystems, a region of considerable interest for environmental change research.

The premise of this article is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework. To this end, the energy (E)-mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere; and indirect influx of water, suspended particles, and dissolved substances from the catchment. Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers. In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.

Arctic and subarctic ecosystems have undergone considerable environmental changes in recent years as the result of climate warming. Fossil records of freshwater diatoms in lakes throughout the circumpolar Arctic, including the central Canadian Arctic treeline region, have revealed marked directional shifts in diatom assemblages in recent lake sediments. These algal changes have been linked to longer growing seasons, decreased duration of ice cover, and/or increased thermal stability. The effects of these recent environmental changes on higher trophic levels, such as the Cladocera, are still unclear. Using cladoceran remains preserved in the sediments of 50 lakes, which were previously examined for changes in diatoms, we show that significant changes in cladoceran species composition have occurred from pre-industrial times to the present. However, these changes are considerably muted compared to the more substantial changes observed in the diatom record. We found no consistent patterns of change i...

The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0–180°W; north of ∼60°N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6±0.8°C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12–10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet.

Changes in diatom assemblage composition since preindustrial times were analyzed in a landscape paleolimnological study of 50 lakes and also in a more detailed analysis of a dated sediment core from Slipper Lake in the Canadian central arctic treeline region. The most apparent taxonomic shift was toward a higher relative abundance of the planktonic Cyclotella stelligera complex and a lower relative abundance of benthic Fragilaria taxa (F. pinnata, F. construens var. venter, F. construens, and F. brevistriata) in the modern versus the older sediments. Diatom assemblage composition in Slipper Lake recorded a marked change in the top 5.0 cm (ca. mid-1800s) of the core with a clear shift to a more planktonic diatom assemblage characterized by higher percentages of the Cyclotella stelligera complex. Possible causative factors, such as recent anthropogenic acidification, nutrient enrichment, or atmospheric deposition of contaminants, do not appear sufficient to explain these species changes. Instead, t...

Measured environmental variables from 56 lakes across the Central Canadian Treeline Region exhibited clear limnological differences among subpolar ecozones, reflecting strong latitudinal changes in biome characteristics (e.g. vegetation, permafrost, climate). Principal Components Analysis (PCA) clearly separated forested sites from tundra sites based on distinct differences in limnological characteristics. Increases in major ions and related variables (e.g. dissolved inorganic carbon, DIC) were higher in boreal forest sites in comparison to arctic tundra sites. The higher values recorded in the boreal forest lakes may be indirectly related to differences in climatic factors in these zones, such as the degree of permafrost development, higher precipitation and runoff, duration of ice-cover on the lakes, and thicker and better soil development. Similar to trends observed in DIC, substantially higher values for dissolved organic carbon (DOC) were measured in boreal forest lakes than in arctic tundra lakes. This was likely due to higher amounts of catchment-derived DOC entering the lakes from coniferous leaf litter sources. Relative to arctic tundra lakes, boreal forest lakes had higher nutrient concentrations, particularly total nitrogen (TN), likely due to warmer conditions, a longer growing season, and higher precipitation, which would enhance nutrient cycling and primary productivity. Results suggest that modern aquatic environments at opposite sides of the central Canadian arctic treeline (i.e. boreal forest and arctic tundra) exhibit distinct differences in water chemistry and physical conditions. These limnological trends may provide important information on possible future changes with additional warming.

An extensive compilation of previously published Holocene proxy climate records from the Arctic. This database is the product of the PAGES endorsed Arctic Holocene Transitions (AHT) project. ABSTRACT: We present a systematic compilation of previously published Holocene proxy climate records from the Arctic. We identified 170 sites from north of 58° N latitude where proxy time series extend back at least to 6 cal ka (all ages in this article are in calendar years before present BP) are resolved at submillennial scale (at least one value every 400 ± 200 years) and have age models constrained by at least one age every 3000 years. In addition to conventional metadata for each proxy record (location proxy type reference) we include two novel parameters that add functionality to the database. First "climate interpretation" is a series of fields that logically describe the specific climate variable(s) represented by the proxy record. It encodes the proxy climate relation reported by authors of the original studies into a structured format to facilitate comparison with climate model outputs. Second "geochronology accuracy score" (chron score) is a numerical rating that reflects the overall accuracy of 14C based age models from lake and marine sediments. Chron scores were calculated using the original author reported 14C ages which are included in this database. The database contains 320 records (some sites include multiple records) from six regions covering the circumpolar Arctic: Fennoscandia is the most densely sampled region (31 of the records) whereas only five records from the Russian Arctic met the criteria for inclusion. The database contains proxy records from lake sediment (60) marine sediment (32) glacier ice (5) and other sources. Most (61) reflect temperature (mainly summer warmth) and are primarily based on pollen chironomid or diatom assemblages. Many (15) reflect some aspect of hydroclimate as inferred from changes in stable isotopes pollen and diatom assemblages humification index in peat and changes in equilibrium line altitude of glaciers. This comprehensive database can be used in future studies to investigate the spatio temporal pattern of Arctic Holocene climate changes and their causes. The Arctic Holocene data set is available from NOAA Paleoclimatology. © Author(s) 2014.

Diatoms and other siliceous microfossils were examined from a 386-cm-long peat core, covering the last ca. 7200 yr, from north-central Siberia to gain insights into peatland developmental history, ...

23 páginas, 7 figuras, 1 tabla., Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship., The authors acknowledge project support from GRACCIE (CSD2007-00067), NITROPIR (CGL2010- 19373), OCUPA (088/2009), the European Research Council (Starting Grant Project, 239858), the Natural Sciences and Engineering Research Council of Canada, the US Department of the Interior, the Commission for Scientific Research in Greenland, the Austrian Science Foundation (FWF R 29N10, FWF J 1963-Geo), the Alpine Research Programme of the Austrian Academy of Sciences (project DETECTIVE), and the Czech Science Foundation (project GACR 526/09/0567).

Chironomid and Chaoborus (midge) remains preserved in a dated sediment core from Poplar Bay, Lake of the Woods (LOW), Ontario, Canada, were used to assess the effects of multiple stressors (e.g., recent warming and shoreline development) on water quality over the past ~200 years. As monitoring data for LOW do not extend beyond recent decades, paleolimnological methods are used to reconstruct long-term limnological trends and to establish pre-disturbance conditions. The effects of recent warming and shoreline development on Poplar Bay water quality are examined using an index of hypolimnetic oxygen (O2) status based on the ratio of Chaoborus to chironomid remains (chaob:chir) and a midge-inferred volume-weighted hypolimnetic oxygen (VWHO) model. Our paleolimnological data indicate that hypolimnetic [O2] in Poplar Bay have been historically hypoxic (1-4 mg O2 L-1) but have declined further (generally

[1] Evidence is mounting that abrupt climatic shifts occurred during the last glacial-interglacial transition (LGIT) in the North Atlantic and other regions. However, few high-resolution climatic records of the LGIT exist from the high latitudes of the North Pacific rim. We analyzed lake sediments from southwestern Alaska for biogenic silica, organic carbon, organic nitrogen, diatom assemblages, and compound-specific hydrogen isotopes. Results reveal climatic changes coincident with the Younger Dryas, Intra-Allerod Cold Period, and Pre-Boreal Oscillation. However, major discrepancies exist in the paleoclimate patterns of the Bolling-Allerod interstadial between our data and the GISP2 δ18O record from Greenland, and causes are uncertain. These data suggest that the North Pacific and North Atlantic experienced similar reversals during climatic warming of the LGIT but that the Bolling-Allerod cooling trend in the GISP2 δ18O record is probably not a hemispheric or global pattern.

[1] The Himalayan region of northern India depends on monsoon rains, together with snow and glacial melt, to supply life-sustaining water to one of the world's most densely populated areas. Here we provide high-resolution pollen and diatom evidence from a peat deposit in the Pinder Valley that shows a synchronous and abrupt ecosystem turnover toward a wetter state in the last two centuries that exceeded changes recorded over the last three millennia. Contrary to expectations, there was no relationship between recent proxy changes and summer monsoon precipitation. Strong relationships, however, were found with winter climate data. We link this recent unprecedented wetness to marked warming at higher elevations resulting in increased seasonal runoff and associated climatic feedbacks in this snow and ice-melt dominated region. In contrast to the expected desiccation and decomposition of most peat systems with warming, this site has instead become the wettest in its ca. 3500-year history.

Fifty-five paleolimnological records from lakes in the circumpolar Arctic reveal widespread species changes and ecological reorganizations in algae and invertebrate communities since approximately anno Domini 1850. The remoteness of these sites, coupled with the ecological characteristics of taxa involved, indicate that changes are primarily driven by climate warming through lengthening of the summer growing season and related limnological changes. The widespread distribution and similar character of these changes indicate that the opportunity to study arctic ecosystems unaffected by human influences may have disappeared.

We document the rapid transformation of one of the Earth's last remaining Arctic refugia, a change that is being driven by global warming. In stark contrast to the amplified warming observed throughout much of the Arctic, the Hudson Bay Lowlands (HBL) of subarctic Canada has maintained cool temperatures, largely due to the counteracting effects of persistent sea ice. However, since the mid-1990s, climate of the HBL has passed a tipping point, the pace and magnitude of which is exceptional even by Arctic standards, exceeding the range of regional long-term variability. Using high-resolution, palaeolimnological records of algal remains in dated lake sediment cores, we report that, within this short period of intense warming, striking biological changes have occurred in the region's freshwater ecosystems. The delayed and intense warming in this remote region provides a natural observatory for testing ecosystem resilience under a rapidly changing climate, in the absence of direct anthropogenic influences. The environmental repercussions of this climate change are of global significance, influencing the huge store of carbon in the region's extensive peatlands, the world's southern-most polar bear population that depends upon Hudson Bay sea ice and permafrost for survival, and native communities who rely on this landscape for sustenance.

High-resolution paleoclimate data and numerous instrumental records report that, at higher elevations, Himalayan temperatures over the last ca. 100 years have been rising (Liu and Chen 2000, Thompson et al. 2000). However, many instrumental records from various lower elevation localities are reporting cooler temperatures in recent decades (Balling 2001). This recent cooling trend is not only opposite to the global warming trend but also with the accelerated retreat of the Himalayan glaciers. The effect that warming conditions and melting glaciers have on downslope ecosystems has not been fully explored. Here we present compelling evidence that, over the last 200 years, accelerated glacier retreat and increased melt waters have played a central role in changing the ecology (and perhaps local climate) of downslope sites. The Pinder Valley (~30° 50' 30° 52' N, 78° 47' 78° 51’E) of the Kumaon Higher Himalaya, receives precipitation from the Asian monsoon predominantly during the summer but also during winter seasons. The valley climate is also influenced by hanging glaciers (viz. Sunder Dunga, Pindari, and Kaphni), situated along the valley head. An ~ 125-cm thick peat deposit situated in oak dominated mixed evergreen forest 2650-m altitude, was trenched and continuously sampled at 2-cm intervals. Well-dated multi-proxy paleoclimate record (pollen, diatoms, phytoliths, % organic matter, and magnetic susceptibility) from this peat sequence revealed past ca. 3500year high-resolution climate history of the area. At the beginning of the peat sequence (ca. 3500 to ca. 3300 cal yr BP), the pollen assemblage was almost exclusively represented by conifers suggesting that the regional climate was cool and wet. A sudden increase in the dominance of grass, alder (Alnus nepalensis) and brown oak (Quercus semecarpifolia) pollen at ca. 3300 cal yr BP indicates an abrupt climatic shift towards drier conditions that continued until ca 2300 cal yr BP. The increase in silver fir (Abies pindrow) and other conifers [Himalayan spruce (Picea smithiana) and blue pine (Pinus wallichiana)] at the expanse of grasses, and the first appearance of diatoms ca. 2300 cal yr BP mark the end of the dry period. The progressive decrease in grass pollen for the following ~200 years reveals the improved wetness of climate until ca. 2100 cal yr BP. Variations in our proxy indicators throughout the remainder of the peat sequence suggest a stepwise increase in wetness until the present. A major shift in all of the proxy indicators at ca. 680 cal yr BP (~ 1320 AD) indicates an increase in SW monsoon at 1300 AD, evidence of which is widespread throughout the Asian monsoon region (Morrill et al. 2003). Around this time the once dominant Fragilaria diatoms abruptly diminished to trace abundances, Navicula minima appeared for the first time in the record in relatively high abundances, and epiphytic diatom taxa became more prominent. Circa 1580 to 1730 AD, a substantial cooling was inferred with a reduction to trace levels of the epiphytic proportion of the diatom assemblage and the strong re-appearance of Fragilaria species to abundances of 80 – 90%, not seen before or afterwards in the peat profile. A comparison of the last ~400 years of the peat record with reconstructed spring temperature trend derived from a series of 12 Himalayan tree-ring chronologies (Yadav and Singh 2002) is consistent with our proxy-based inferences from ca. 1600 AD onwards. The high rate of glacier melting over the last few centuries in the Himalayan mountains is a clear demonstration of a warming climate. Our data show that increased glacial meltwaters under this warmer climatic scenario has had a substantial impact on down-slope ecosystems. It appears that the glacial retreats are consistent with the global climate scenario but the resulting melt waters are having localized to regional cooling effects at lower elevation sites over the last few decades.