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Cyanobacterial blooms in freshwater systems are a global threat to human and aquatic ecosystem health, exhibiting particularly harmful effects when toxin-producing taxa are present. While climatic change and nutrient over-enrichment control the global expansion of total cyanobacterial blooms, it remains unknown to what extent this expansion reflected cyanobacterial assemblage due to the scarcity of long-term monitoring data. Here we use high-throughput sequencing of sedimentary DNA to track ∼100 years of changes in cyanobacterial community in hyper-eutrophic Lake Taihu, China's third largest freshwater lake and the key water source for ∼30 million people. A steady increase in the abundance of Microcystis (as potential toxin producers) during the past thirty years was correlated with increasing temperatures and declining wind speeds, but not with temporal trends in lakewater nutrient concentrations, highlighting recent climate effects on potentially increasing toxin-producing taxa. The socio-environmental repercussions of these findings are worrisome as continued anthropogenic climate change may counteract nutrient amelioration efforts in this critical freshwater resource.

Semi-arid areas of northern China are under increasing pressures from anthropogenic activities and climate change. Although wetland areas in these drylands have experienced dramatic, unidirectional shifts in their ecological status in recent centuries, fundamental driving forces are poorly quantified. Here, we examine changes in sedimentary proxies (diatoms, spectrally-inferred chlorophyll-a, stable isotopes) preserved in a radiometrically-dated core from Tianchi Lake, an alpine lake within the margin of the East Asian Summer Monsoon (EASM) limit in China’s southwestern Loess Plateau. Our algal trends were compared with regional instrumental records, changes in EASM intensity, and with previously published paleolimnological data from this same lake to determine the principal drivers of regional ecological changes. We found no clear evidence that geochemical and biological proxies were strongly affected by deforestation and other human activities. Major environmental changes during the past ~ 200 years were found to be predominantly driven by climatic fluctuations, extreme precipitation events, and changes in EASM intensity. Prior to ~ 1965 CE, diatom assemblages indicate an oligotrophic, clear water state. Shifts in dominance between benthic Staurosirella pinnata and planktonic Lindavia comensis were likely controlled by ice-cover dynamics. Between ~ 1965 and 1980 CE an abrupt shift to a turbid water state during a period of extreme precipitation events was caused by excessive nutrient-laden soil erosion in the already susceptible deforested catchment. This turbid period was evidenced by a rapid increase to dominance of Achnanthidium minutissimum, a sharp decline in oligotrophic Lindavia comensis, increased primary production, and peaks in sediment grain size and SiO2 content. Post- ~ 1980 CE, we provide evidence that a shift towards planktonic diatom dominance can best be explained by changes in climate and EASM intensity, despite substantial nitrogen deposition in the region during the past few decades. Specifically, a drier and warmer climate together with weakened EASM wind strength resulted in decreased erosion and a return to a clear water state, coupled with enhanced thermal stability. Collectively, these observations expand our understanding of how changes in climate, extreme precipitation events, and fluctuations in EASM intensity influence semi-arid alpine lakes in northern China, as well as climate’s leading role in driving ecological change over the past two centuries, despite the intensification of human disturbances during recent decades.

The Athabasca Oil Sands Region (AOSR) in northeast Alberta contains Canada’s largest reserve of hydrocarbons, and the third largest in the world. Extraction and processing operations generate contaminants and nutrients that are aerially deposited across the region. However, structured environmental monitoring efforts were only initiated decades following the establishment (1967) and escalation (~ 1980) of commercial operations. We examined whether diatom assemblages preserved in 210Pb-dated sediment cores retrieved from strategically selected lakes have responded to aerial deposition of contaminants. The relative amount of contaminant inputs was tracked using sedimentary dibenzothiophene (DBT) concentrations and DBT enrichment factors, which are established proxies for AOSR activities. We observed no relationship between diatom assemblage changes and DBT enrichment. The nature of the diatom changes differed among lakes, regardless of DBT enrichment, suggesting that diatom responses were related to site-specific conditions. Moreover, diatom assemblage changes at several sites tracked trends in whole-lake primary production. Collectively, these records indicate that regional warming is likely the primary driver of recent diatom assemblage changes in these shallow, closed-basin lakes.

Simmatis B, Nelligan C, Ruhland KM, Jeziorski A, Castro V, Paterson AM, Smol JP. 2020. Tracking ∼200 years of water quality in Muskrat Lake, a eutrophic lake trout lake in Ontario (Canada) with cya...

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

Diatom assemblages are excellent indicators for environmental monitoring. However, enumerating diatoms using fine-level taxonomy takes considerable effort, which must be undertaken by specialist taxonomists. One alternative is to enumerate assemblages using morphological traits. In this study, we compared the accuracy of models using 20 morphological traits with those using species assemblages to infer lake water pH, salinity, depth, and total phosphorus concentrations in four data sets, each comprising over 200 lakes. Assemblages aggregated by trait combinations were used to predict environmental variables via weighted averaging regressions, and richness of trait combinations was regressed against the environmental variables. Trait-based weighted averaging regressions showed slightly lower accuracy than species-level analyses and higher accuracy than analyses at the family and sometimes genus level. Richness of trait combinations showed relationships with pH, salinity, and lake depth that were marginally stronger than relationships using species richness. Although species-level analyses are the best approach when time and budgets allow, we suggest that trait combinations could provide an alternative method for water quality assessment programs, where funds do not allow the use of specialist taxonomists or where diatoms are being used as part of a multi-indicator analysis.

Temperature–oxygen profiles, collected biweekly to monthly for ∼40 years, were used to calculate end-of-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in six small lakes located in south-central Ontario, Canada. Coherent decreases in thermocline depth and increases in hypolimnetic volume, mean hypolimnetic dissolved oxygen (DO) concentration, and VWHO were observed in five of the six study lakes. All lakes underwent an abrupt increase in VWHO and mean hypolimnetic DO after 2010. In four of the six study lakes, the highest hypolimnetic DO concentrations were observed in years where chlorophyll a concentrations were low, whereas at five of the six study lakes the highest hypolimnetic volumes were observed when dissolved organic carbon concentrations were relatively high. Warmer spring or winter air temperatures were associated with higher hypolimnetic DO concentrations at two sites, and longer ice-free periods were associated with smaller hypolimnetic volumes at two sites. These results suggest that the recent VWHO increases in the studied south-central Ontario lakes may be a function of multiple drivers that include changes in primary production, lake water transparency, and regional climatic factors.

Dickson Lake, located in Algonquin Provincial Park, Ontario, is a remote, oligotrophic lake, where cyanobacterial blooms of the genus Dolichospermum (Ralfs ex Bornet & Flahault) P. Wacklin, L. Hoffmann and J. Komarek were reported for the first time in the fall of 2014, and subsequently in the late spring of 2015. To investigate the potential environmental triggers of these bloom events, we assessed long-term trends in water quality using a multi-proxy paleolimnological approach, examining sedimentary diatoms, chironomids, cladocerans, spectrally inferred chlorophyll a, and cyanobacterial akinetes preserved in a 210Pb-dated sediment core. Assemblage changes were modest in all biological proxies. A subtle increase in the abundance of warm-water chironomid taxa (Topt > 15 °C) commences in the year ~ 2000, with further increases in the most recent years of the sediment record (~ 2013–2015). End-of-summer volume-weighted hypolimnetic oxygen concentrations (CI-VWHO), inferred from chironomid remains, reveal a decline in oxygen concentrations over the last two decades coincident with the highest levels of sedimentary chlorophyll a and cyanobacterial akinetes in the sediment record. These paleolimnological findings corroborate observed reports of the onset of cyanobacterial blooms in Dickson Lake in late 2014 and are consistent with increasingly favourable bloom-forming conditions over the past few decades that are related to warmer air temperatures, sharp declines in wind speed, and a lengthening of the ice-free season by 2 weeks since 1975. It is plausible that late ice-out and a quick onset to stratification in 2014 may have resulted in incomplete spring mixing, early onset of hypolimnetic anoxia, and increased internal nutrient loading, that, occurring during a period when climate conditions were particularly ideal for cyanobacterial proliferation, may have fueled the unprecedented algal blooms in this remote lake. Collectively, the factors causing algal blooms in remote lakes such as Dickson Lake are not yet fully understood, and it is worrisome that with continued warming the triggering conditions may become a more common feature of Algonquin Park and other minimally impacted Boreal Shield lakes in the coming years.

The Hudson Bay Lowlands (HBL) region of the far north of Ontario (Canada) is expected to undergo considerable physical, chemical and biological change as a result of ongoing climatic change. Previous research in the region has shown marked limnological changes during the past ~ 20 years in relatively deep lakes that have been attributed to increased air temperatures and changes in sea ice phenology in Hudson Bay since the mid-1990s. Here, we present diatom assemblage, primary production and geochemical data from lake sediments documenting recent limnological change in two shallow sub-arctic lakes in the Sutton River region of the HBL. Both lakes recorded increased whole-lake production and diatom diversity changes that are consistent with a longer ice-free period and growing season. Changes in diatom composition at Wolfgang Lake were characterized by a response amongst benthic/periphytic taxa whereas a modest increase in planktonic diatoms was observed at Sam Lake. Geochemical changes (δ15N, C/N and %N) were temporally coherent with diatom assemblage changes, but showed different responses in the two study lakes. Thus, although the biological and geochemical changes were consistent with recent warming, differences in the nature and timing of these shifts illustrate the heterogeneous nature of shallow lakes, and suggest that local (catchment-specific) factors are important determinants of the trajectory of limnological change in these sensitive systems.

We used high-resolution diatom-based paleolimnological techniques to assess the effects of shoreline development and recent climate warming on three large embayments of Georgian Bay, Lake Huron (two impacted and one minimally impacted by shoreline development). The sedimentary diatom assemblages recorded subtle shifts at the impacted sites (North and South Bays) as a result of the establishment of permanent settlements and recreational resorts around the turn of the 20th century. No turn-of-the-century changes were observed at the reference site (Tadenac Bay). The abrupt increase in epiphytic Cocconeis placentula and benthic fragilarioid taxa during the ~1950s at the shallower impact site (South Bay) was likely due to increased habitat provided by macrophytes. The increase in relative abundances of pennate (Asterionella formosa and Fragilaria crotonensis) and/or small centric (Cyclotella ocellata, C. comensis, and C. gordonensis) planktonic diatoms across the three sites after the 1970s suggests that recent warming-mediated changes to thermal properties (and related effects) are driving biological changes in these embayments. These recent diatom compositional changes are consistent with similar trends reported across Ontario and the Laurentian Great Lakes, where warming and its effects on aquatic ecosystem processes often favor small-celled centric diatoms. Differences in the timing and nature of diatom responses to similar environmental stressors across the three embayments highlight the importance of understanding site-specific characteristics when interpreting changes in diatom assemblages in paleolimnological records from the Laurentian Great Lakes.

Human activities over the last ∼100 years have fundamentally changed the biogeochemistry of the global nitrogen cycle. For example, increased nitrogen deposition from industrial and agricultural sources has been linked to lake acidification and nutrient fertilization, and thus it has the potential to significantly influence lake ecosystems. Records of anthropogenic nitrogen deposition from alpine lakes are sparse in China, which limits our understanding of its effects on remote alpine lake ecosystems. In this study, we analyzed multiple geochemical proxies at Tianchi Lake (2430 m a.s.l.), which is part of the Liupan Mountains National Nature Reserve in the central Chinese Loess Plateau (CLP). We measured total nitrogen (TN) concentrations, C/N ratios and nitrogen stable isotope ratios (δ15N), as well as a biological proxy (spectrally-inferred chlorophyll a), from a well-dated sediment core spanning the past ∼200 years. Our aim was to examine anthropogenic nitrogen deposition to the lake and to assess its implications for lake primary production. We found that δ15N was stable prior to ∼1980, but it decreased significantly thereafter. This is consistent with documented changes in anthropogenic nitrogen deposition over the past ∼200 years, and it indicates that the δ15N record of Tianchi Lake likely reflects anthropogenic nitrogen deposition, at least in this region. Prior to ∼1980, the trend in primary production of Tianchi Lake agrees with the trend of reconstructed regional temperature, when reactive nitrogen emissions in China were very low. This suggests that temperature was likely the main factor driving lake primary production before ∼1980. Primary production has increased significantly since ∼1980, consistent with the continued rising temperature and with enhanced nitrogen deposition, indicating that both factors are simultaneous drivers. Although the current data for Tianchi Lake are insufficient to unequivocally determine which of the two stressors is more important after the 1980s, temperature likely played the more significant role in driving primary production. This is because the lake is phosphorus-limited today, and in addition there is a strong association between temperature and chlorophyll a over the past ∼200 years. Continuing global warming and increasing nitrogen deposition in the future will likely further affect the fragile alpine ecosystems in the region.

Aerially transported contaminants from the industrial development of the bituminous sands in the Athabasca Oil Sands Region (AOSR) of western Canada may threaten the water quality and community structure of the region’s lakes. This environmental threat is further compounded by the region’s changing climate (i.e. increased temperatures and reduced moisture). Because environmental monitoring began ~ 30 years after the initiation of the region’s bitumen-based industry (~ 1967), a paleolimnological approach is required to document pre-disturbance conditions and to determine how lakes have changed, if at all, in response to environmental stressors. Our bottom–top (before and after) study of dated sediment records compared pre-disturbance (~ 1850) and modern subfossil diatom assemblages from 18 shallow, isolated lakes (which are typical of the region), located along a spatial gradient (up to ~ 110 km) relative to the main area of local industry. Despite the region’s substantial environmental stressors, the changes in mostly benthic-dominated diatom communities were minor at 15 of the 18 lakes. We conclude that the muted biological responses may be a consequence of naturally high nutrient concentrations and/or the typically subtle nature of changes in assemblages dominated by benthic generalist taxa. At all sites, including three lakes with marked changes, we found no evidence of a diatom assemblage response to airborne inputs of contaminants (i.e. dibenzothiophenes) and nutrients (i.e. bioavailable nitrogen) from the AOSR industry. Instead, it is likely that regional warming played a role in the modest diatom assemblage changes observed in these shallow lakes, with responses mediated by lake-specific characteristics.

Lake of the Woods (LOW) is a large, morphologically and hydrologically complex lake of international importance, located in the provinces of Ontario and Manitoba and the state of Minnesota. A high-resolution sedimentary sequence retrieved near Kenora, Ontario, and spanning at least the past ~11,000 cal yr BP (calibrated years before present), was analysed for multiple environmental proxies with an emphasis on diatom assemblage composition and spectrally-inferred chlorophyll a. These biological proxies indicate that northern LOW was relatively nutrient-rich soon after its isolation from glacial Lake Agassiz ~10,000 cal yr BP. The post-glacial hydrological and environmental history of LOW was found to be controlled by both climate and isostatic rebound. During the low water phase of the mid-Holocene dry and warm period, abrupt and synchronous shifts across all proxies suggest that the northern basin had a relatively deep and well-mixed water column that experienced increases in nutrients and whole-lake algal production. This differs from recent limnological changes associated with warming since the late-1970s, where primary production increased concurrently with large shifts in diatoms indicative of increased thermal stability, but with little change in nutrients. The millennial-scale context of this study provides evidence that climate has long played an important role in algal dynamics in LOW, with implications for lake management strategies concerning recent increases in nuisance algal blooms on LOW.

The copper-zinc smelter at Flin Flon (Manitoba) operated between 1930 and 2010 and emitted large amounts of metal(loid)s and sulphur dioxide into the atmosphere, damaging the surrounding terrestrial landscapes and depositing airborne industrial pollutants into aquatic ecosystems. However, the extent of biological impairment in regional lakes is largely unknown. Here, we analysed biological and geochemical proxies preserved in a dated sediment core from Phantom Lake, collected seven years after the smelter closed in 2010. Our objectives were to determine how smelting history affected long-term trends in (1) sedimentary elements, (2) biota across multiple trophic levels, and (3) spectrally-inferred chlorophyll a and lake-water total organic carbon. The effects of smelting activities were clearest in the diatom record, in concordance with modest responses in chironomid and cladoceran assemblages. Several metal(loid)s were naturally high and exceeded sediment quality guidelines during the pre-smelting era. With the opening of the smelter, metal(loid) concentrations in sediments increased through the 1930s, peaked in the 1960s, and declined thereafter with technological improvements but remained above background to this day. Although modest declines in inferred lake-water total organic carbon indicate reduced terrestrial carbon supply following sulphate deposition in the catchment, the diatom record showed no evidence of acidification as the lake was and remained well-buffered. Pre-smelting diatom and invertebrate assemblages were diverse and indicated oligo-mesotrophic conditions. Smelting was associated with the loss of metal-sensitive biological indicators and the emergence of assemblages dominated by metal-tolerant, generalist taxa. Diatoms tracked substantial reductions in aerial emissions since the 1990s, particularly after the smelter closed, but also indicated that the biological effects of metal pollution persist in Phantom Lake. Examining the effects of a base metal smelter on a well-buffered lake offered insights into multi-trophic level responses to severe metal contamination and potential recovery without the confounding effects of concurrent changes in lake acidity.

The scarcity of research in NE China on lake ecosystem responses to large-scale climate oscillations since the last deglaciation limits our abilities for informing conservation practices and policies in the context of recent global warming. Here, a high-resolution, sedimentary diatom record covering the past ∼17,500 years was retrieved from Lake Moon, a small, hydrologically closed crater lake in the remote central part of the Great Khingan Mountain Range in NE China. We compare diatom changes with geochemical data from the same core, and with regional palynological and dust deposition records, to better understand the influences of long-term environmental and climatic variability on aquatic ecosystems in this climatically sensitive region. Several abrupt and pronounced shifts in dominance among diatom taxa corresponded to marked fluctuations in the climate regime since the last deglaciation. During the close of the Last Glacial Period, a series of short-lived diatom shifts that were indicative of an increase in nutrients, signaled the transition from the cold period of the Heinrich event 1 (∼17,500–∼14,700 cal yr BP) to the warmer Bolling - Allerod interstadial period (∼14,700–∼12,900 cal yr BP). The onset of the cold Younger Dryas period was marked by a brief rise to dominance of benthic taxa (∼12,900–∼11,800 cal yr BP), followed by a pronounced lake ecosystem shift to a new trophic state at the Pleistocene-Holocene transition (∼11,800–∼9900 cal yr BP), evidenced by an abrupt rise to dominance of several eutrophic diatom indicators (e.g. Cyclostephanos dubius, Aulacoseira ambigua and Stephanodiscus parvus). This nearly complete compositional turnover was indicative of an increase in phosphorus supply to the lake with the onset of a warmer and moister climate and an increased frequency of strong dust storms. During the mid-Holocene (∼6000 cal yr BP), a striking increase in the relative abundance of Discostella pseudostelligera and Asterionella formosa, at the expense of previously dominant eutrophic indicators, signified another lake ecosystem change from well-mixed, turbid and phosphorus-rich conditions to a lower nutrient state with longer open-water periods and increased thermal stability. The turnover to oligotrophic diatom taxa was likely in response to variations in seasonal temperature, precipitation and dust deposition. The diatom shifts of Lake Moon during the past ∼17,500 years were directly or indirectly mediated by climate change that affected thermal stratification, productivity, lake level and trophic state. Our results indicate that climate change had an overarching control on aquatic ecosystem changes in the mountain regions of NE China since the last deglaciation.

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

Anderson JP, Paterson AM, Reavie ED, Edlund MB, Ruhland, KM. 2017. An introduction to Lake of the Woods – from science to governance in an international water body. Lake Reserv Manage. 33:325–334.Lake of the Woods (LOW), a vast lake covering over 3850 km2, is a waterbody of current and historical significance that spans the borders between Minnesota and the Canadian provinces of Ontario and Manitoba. Over the past 2 decades, there has been concern for the water quality of LOW, sparked by anecdotal evidence of increased intensity and frequency of algal blooms by the public and resource managers. Minnesota's waters of LOW were declared impaired in 2008 due to exceedances of eutrophication criteria, initiating a Total Maximum Daily Load study. LOW's impairment declaration spurred several contrasting challenges and opportunities for water quality management across this diverse lake and basin. Governance challenges are emphasized by the large number of government agencies (>25) with land or water autho...

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...

The Athabasca Oil Sands Region (AOSR) has been intensely developed for industrial bitumen extraction and upgrading since the 1980s. A paucity of environmental monitoring prior to development raises questions about baseline conditions in freshwater systems in the region and ecological responses to industrial activities. Further, climatic changes prompt questions about the relative roles of climate and industry in shaping aquatic ecosystems through time. We use aquatic bioindicators from multiple trophic levels, concentrations of petrogenic contaminants (dibenzothiophenes), and spectrally-inferred chlorophyll-a preserved in well-dated sediments of a closed-basin, shallow lake ~50km away from the main area of industry, in conjunction with climate observations, to assess how the biotic assemblages of a typical AOSR lake have changed during the past ~75years. We examine the contributions of the area's stressors in structuring aquatic communities. Increases in sedimentary measures of petrogenic contaminants provide clear evidence of aerial contaminant deposition from local industry since its establishment, while climate records demonstrate consistent warming and a recent period of reduced precipitation. Quantitative comparisons of biological assemblages from before and after the establishment of regional industry find significant (p0.05) differences; however, the magnitude and overall timing of the changes are not consistent with a threshold-type shift in response to the onset of regional industry. Rather, biotic assemblages from multiple trophic levels suggest transitions to an increasingly complex benthic environment and relatively warmer waters, which, like the increasing trends in inferred primary production, are consistent with a changing climate. These findings highlight the important role of climate conditions in regulating primary production and structuring aquatic communities in these shallow systems.

Historically, warm periods enhanced the Asian summer monsoon—increased rainfall brought additional nutrients to freshwater ecosystems and increased production. However, anthropogenic aerosols have weakened the monsoon and altered lake ecosystems. Anthropogenic aerosol increases over the past few decades have weakened the Asian summer monsoon1,2,3 with potentially far-reaching socio-economic and ecological repercussions. However, it is unknown how these changes will affect freshwater ecosystems that are important to densely populated regions of Asia. High-resolution diatom records and other proxy data archived in lake sediment cores from the Chinese Loess Plateau allow the comparison of summer monsoon intensity, lake trophic status and aquatic ecosystem responses during warming periods over the past two millennia. Here we show that an abrupt shift towards eutrophic limnological conditions coincided with historical warming episodes4,5, marked by increased wind intensity and summer monsoon rainfall leading to phosphorus-laden soil erosion and natural lake fertilization. In contrast, aerosol-affected Anthropocene warming catalysed a marked weakening in summer monsoon intensity leading to decreases in soil erosion and lake mixing. The recent warm period triggered a strikingly different aquatic ecosystem response with a limnological regime shift marked by turnover in diatom species composition now dominated by oligotrophic taxa, consistent with reductions in nutrient fertilization, reduced ice cover and increased thermal stratification6. Anthropogenic aerosols have altered climate–monsoon dynamics that are unparalleled in the past ∼2,000 years, ushering in a new ecological state.

Aquatic microorganisms are commonly used as indicators in environmental assessments. Two key aspects of sampling design for these studies are the number of sites sampled and the enumeration effort of organisms within samples. However, there has been no rigorous determination of a cost-effective balance between them. Here, we use regional diatom data sets from 207 to 493 lakes to determine the influence of sample size (lake number) and enumeration effort (valve count) on the accuracy and cost-effectiveness of commonly used environmental assessment analyses. We find that both lake number and valve count can be considerably reduced from the original data sets. However, there is considerable variation among analyses, such that a single most cost-effective sampling configuration cannot be recommended. For weighted-averaging assemblage versus environment models, we recommend 70–100 lakes and 200 valves for retaining high accuracy. For other analyses, we recommend iteratively building up data sets, using initially high (300 or more) and consistent valve counts and using subsampling to determine when the influence of either lake number or valve count plateaus. This technique is applicable to other types of microorganisms used in environmental assessments.

Nelligan C, Jeziorski A, Ruhland KM, Paterson AM, Smol JP. 2016. Managing lake trout lakes in a warming world: A paleolimnological assessment of nutrients and lake production at three Ontario sites. Lake Reserve Manage. 32:315−328.Recent declines in hypolimnetic dissolved oxygen (DO) concentrations in many lakes throughout Ontario have prompted concern regarding the health of lake trout (Salvelinus namaycush) populations. We used diatom assemblages and spectrally inferred chlorophyll a from the dated sediments of 3 lake trout lakes to track trends in total phosphorus (TP) and whole-lake production over the past ∼150 years. Eagle Lake is considered a sensitive lake trout lake because DO concentrations have been lower than the provincial standard (>7 mg/L) since the early-2000s; however, a lack of monitoring data impedes the evaluation of current conditions within a historical context. For a regional perspective, we compared trends in Eagle Lake with those of 2 additional lake trout lakes ∼200 km aw...

The extensive peatlands and lakes of the Far North of Ontario warrant committed scientific attention given their status as a significant carbon sink. Economic interest in this region has recently increased due to the discovery of vast mineral deposits (mainly chromite and nickel) known as the “Ring of Fire”. Mineral exploration and infrastructure planning are underway, but environmental monitoring is only beginning. Detailed baseline ecological information is required to assess the impacts of future resource extraction within the context of multiple environmental stressors (including recent regional climate warming). Here we use sediment cores from two relatively deep lakes (Zmax ~ 10 m) and two shallow lakes (Zmax ~ 2 m), all located in the vicinity of the Ring of Fire, to examine biotic responses to warming prior to the commencement of mining activities. Our data show that, over the past ~150 years, diatom and cladoceran sedimentary assemblages have transitioned from dominance by littoral/benthic forms to greater abundances of planktonic cladoceran (an increase of ~3 to 34 %) and diatom taxa (an increase of ~3 to 22 %). Increased relative abundances of planktonic taxa are consistent with warming-induced changes in lake properties including longer ice-free periods and increased production by planktonic algae. The response of diatom assemblages in shallow lakes to warming preceded the deeper lakes by ~45 to 60 years, and substantial increases in aquatic production (~4 to 15 times higher than in sediments deposited prior to 1900) were observed in the shallow lakes, in agreement with previous analyses demonstrating the heightened sensitivity of shallow systems to climate warming. These data provide important information necessary to distinguish potential ecological impacts related to resource extraction from natural variation and the ongoing responses to regional climate warming.

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.

The freshwater diatom Asterionella formosa Hassall is a widely distributed species whose populations have been increasing in the recent sediments of numerous lakes worldwide. This taxon’s proliferation has commonly been associated with nutrient enrichment and, more recently, with twentieth century increases in atmospheric nitrogen deposition. However, a growing number of studies have reported increases in A. formosa in the absence of increased nutrients and have thus raised questions about whether alternative causative factors (such as recent climate warming) may be important. Here we assess the relative importance of nutrients and climate warming on A. formosa by comparing regional air temperature trends and 20 years of annually measured lake water nutrients (total inorganic nitrogen, total nitrogen, and total phosphorus) with past changes in relative abundance of this taxon from a sediment core obtained from a soft-water, Boreal Shield lake (George Lake, Killarney, Ontario). Measured lake water total phosphorus has always been very low and remains so to the present (

Nuisance Didymosphenia geminata (Lyngbye) M. Schmidt (didymo) blooms were first reported in 2006 from the Restigouche River Watershed (RRW), eastern Canada. Although government agencies and recreational users are concerned about these blooms, little is known about the impact on biota and (or) the structure and function of the relatively pristine riverine systems of the RRW. Here, we assess whether didymo blooms affect overall benthic diatom assemblage composition by examining epilithic samples from middle stretches of the Patapedia and Upsalquitch rivers. Significant (albeit minor) differences (P < 0.05) in diatom assemblage composition between sites, with and without didymo blooms, were only observed from the Patapedia River. Rarefied diatom species diversity (Hill’s N2) and rarefied richness did not differ significantly among sites, regardless of the presence or absence of blooms. Our data show that didymo blooms have minimal effect on benthic diatom assemblage composition in the RRW.

Algae, the dominant primary producers in many aquatic ecosystems, are critical to global biogeochemical cycling, and changes in their abundance and composition can cascade throughout aquatic food webs. Diatoms often dominate the algal communities in many freshwater systems. Their population dynamics are affected by a variety of environmental variables, many of which are linked to changes in water column properties and habitat availability, which themselves can be linked to shifts in ice cover, length of the growing season, thermal stability and stratification, vertical mixing patterns, habitat alterations, and the availability of resources such as light and nutrients. Climate has strong moderating controls on all of these fundamental aquatic processes, which can directly and indirectly alter species composition, abundance and seasonal dynamics of both periphytic and planktonic diatoms. In this review, we examine the role that climate-mediated alterations in inter-related lake processes have played on diatom community composition, dynamics and size structure, with particular attention to the recent success of planktonic diatom species relative to heavier tychoplanktonic and small benthic diatoms. We focus primarily on paleolimnological records, but also reference a wide spectrum of limnological and physiological studies to review and discuss how climate-driven shifts in lake properties may affect diatom assemblage reorganization. Understanding the limnological and historical context of these often complex diatom changes is key to making scientifically defensible interpretations of paleolimnological records. We further evaluate the plausibility of alternative explanations (e.g. atmospheric nitrogen deposition) for the recent success of small cyclotelloid species by examining trends in these planktonic diatoms from a large number of sites. Using a weight-of-evidence approach, we conclude that recent climate change is the main driver that has led to ecological tipping points resulting in the recent success of small planktonic diatoms that have been reported in many aquatic systems.

Peatlands in northern Ontario, Canada, archive multiple biological indicators, including macrofossils, algae, testate amoebae, and pollen. These proxies can provide insights concerning the timing and nature of long-term climatic and environmental changes. The Hudson Bay Lowlands (HBL) of central Canada contain one of Earth’s largest continuous peatland complexes, and thus comprehensive spatial and temporal studies are needed to understand the implications of climate change on carbon cycling. Diatom assemblages were examined in three cores retrieved from ombrotrophic bogs across two Canadian terrestrial ecozones. Comparisons were made with testate amoebae and macrofossil data previously analyzed from these cores, as well as with regional pollen records from surrounding peatlands. From ~2000 to ~600 cal. BP, changes in diatom composition likely reflect hydrosere succession within the peatland, including fluctuations in connectivity to the water table and pH changes. From ~600 cal. BP to present, the synchronous timing of changes in diatoms and testate amoebae are tracking drying conditions and subsequent microhabitat variations that occur within bogs. It is possible that diatoms are tracking subtle changes in the stability of peat microforms including bog hollows and hummocks, highlighting their sensitivity to small chemical change, whereas testate amoebae are tracking the lowering of a peatland water table and subsequent drying of the peatland. The use of multiple proxies provides a more holistic approach to interpreting past ecological succession and responses to climate within peatlands. When present and well preserved, diatoms can be applied to track drying conditions in bogs, in terms of both hydrosere succession and present climatic change.

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.

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.

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...

The Tibetan Plateau is a region that is highly sensitive to recent global warming, but the complexity and heterogeneity of its mountainous landscape can result in variable responses. In addition, the scarcity and brevity of regional instrumental and palaeoecological records still hamper our understanding of past and present patterns of environmental change. To investigate how the remote, high-alpine environments of the Nianbaoyeze Mountains, eastern Tibetan Plateau, are affected by climate change and human activity over the last ~600 years, we compared regional tree-ring studies with pollen and diatom remains archived in the dated sediments of Dongerwuka Lake (33.22°N, 101.12°E, 4,307 m a.s.l.). In agreement with previous studies from the eastern Tibetan Plateau, a strong coherence between our two juniper-based tree-ring chronologies from the Nianbaoyeze and the Anemaqin Mountains was observed, with pronounced cyclical variations in summer temperature reconstructions. A positive directional trend to warmer summer temperatures in the most recent decades, was, however, not observed in the tree-ring record. Likewise, our pollen and diatom spectra showed minimal change over the investigated time period. Although modest, the most notable change in the diatom relative abundances was a subtle decrease in the dominant planktonic Cyclotella ocellata and a concurrent increase in small, benthic fragilarioid taxa in the ~1820s, suggesting higher ecosystem variability. The pollen record subtly indicates three periods of increased cattle grazing activity (~1400–1480 AD, ~1630–1760 AD, after 1850 AD), but shows generally no significant vegetation changes during past ~600 years. The minimal changes observed in the tree-ring, diatom and pollen records are consistent with the presence of localised cooling centres that are evident in instrumental and tree-ring data within the southeastern and eastern Tibetan Plateau. Given the minor changes in regional temperature records, our complacent palaeoecological profiles suggest that climatically induced ecological thresholds have not yet been crossed in the Nianbaoyeze Mountains region.

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.