Spatiotemporal and multi-isotope assessment of metal sedimentation in the Great Lakes.

This study investigates spatiotemporal dynamics in metal sedimentation in the North American Great Lakes and their underlying biogeochemical controls. Bulk geochemical and isotope analyses of n = 72 surface and core sediment samples show that metal (Cu, Zn, Pb) concentrations and their isotopic compositions vary spatially across oligotrophic to mesotrophic settings, with intra-lake heterogeneity being similar or higher than inter-lake (basin-scale) variability. Concentrations of Cu, Zn, and Pb in sediments from Lake Huron and Lake Erie vary from 5 to 73 mg/kg, 18-580 mg/kg, and 5-168 mg/kg, respectively, but metal enrichment factors were small (<2) across the surface- and core sediments. The isotopic signatures of surface sediment Cu (δ65Cu between -1.19‰ and +0.96‰), Zn (δ66Zn between -0.09‰ and +0.41‰) and Pb (206/207Pb from 1.200 to 1.263) indicate predominantly lithogenic metal sourcing. In addition, temporal trends in sediment cores from Lake Huron and Lake Erie show uniform metal concentrations, minor enrichment, and Zn and Pb isotopic signatures suggestive of negligible in-lake biogeochemical fractionation. In contrast, Cu isotopic signatures and correlation to chlorophyll and macronutrient levels suggest more differentiation from source variability and/or redox-dependent fractionation, likely related to biological scavenging. Our results are used to derive baseline metal sedimentation fluxes and will help optimize water quality management and strategies for reducing metal loads and enrichment in the Great Lakes and beyond.

Diatom species responses along gradients of dissolved inorganic carbon, total phosphorus, and lake depth from lakes across Canada.

Diatoms are key components of freshwater ecosystems and are regularly used for paleolimnological reconstructions, in which defining species optima and tolerances is fundamental for interpreting assemblage shifts in a sediment record. Here, we examined responses of diatoms across three major environmental gradients-dissolved inorganic carbon (range: 0.1-230.5 mg · L-1), total phosphorus (range: 3-326 µg · L-1), and maximum lake depth (range: 0.9-55.0 m)-taken from 158 lakes from across Canada. The lakes were sampled as part of the LakePulse Network, which conducted a standardized sampling of lakes spanning 12 Canadian ecozones. Hierarchical logistic regression was used to model the species responses of 37 common taxa, and species optima and tolerances were calculated with weighted average modeling. The most common response detected was the symmetrical unimodal model, suggesting we likely captured the full environmental ranges for many species, although skewed unimodal responses were also common. Indicator species analyses identified taxa with high predictive values and fidelities to particular ecozones, with high-nutrient-adapted taxa such as Stephanodiscus spp. and Cyclotella meneghiniana characteristic of the agriculturally productive Prairie region. The Prairies stood out in the dataset as the region with the most unique flora from the local contribution to beta diversity analysis. Overall, the autecological data provided by our study will allow for improved interpretations of paleolimnological records and other biomonitoring efforts, addressing management concerns and contributing to a better understanding of our changing environment.

Assessing long-term diatom changes in sub-Arctic ponds receiving high fluxes of seabird nutrients.

Algal bioindicators, such as diatoms, often show subdued responses to eutrophication in Arctic lakes because climate-related changes (e.g., ice cover) tend to be the overriding factors influencing assemblage composition. Here, we examined how sub-Arctic ponds historically receiving high nutrient inputs from nesting seabirds have responded to recent climate change. We present diatom data obtained from 12 sediment cores in seaduck-affected ponds located on islands through Hudson Strait, Canada. All study cores show consistently elevated values of sedimentary ẟ15N, an established proxy for tracking marine-derived nutrients, indicating seabirds have been present on these islands for at least the duration of the sediment records (~100 to 400 years). We document diverse epiphytic diatom assemblages to the base of all sediment cores, which is in marked contrast to seabird-free Arctic ponds-these oligotrophic sites typically record epilithic diatom flora prior to recent warming. Diatoms are likely responding indirectly to seabird nutrients via habitat as nutrients promote the growth of mosses supporting epiphytic diatom communities. This masks the typical diatom response to increased warming in the Arctic, which also results in habitat changes and the growth of mosses around the pond edges. Changes in sedimentary chlorophyll a were not consistently synchronous with large changes in ẟ15N values, suggesting that primary production in ponds is not responding linearly to changes in seabird-derived nitrogen. Across all ponds, we recorded shifts in diatom epiphytic assemblages (e.g., increases in % relative abundance of many Nitzschia species) that often align with increases in chlorophyll a. The changes in diatoms and chlorophyll a, although variable, are most likely driven by climate change as they are generally consistent with longer ice-free conditions and growing seasons. Together, our results show that to effectively use diatoms in animal population reconstructions across the sub-Arctic and Arctic, a strong understanding of eutrophication and climate change, based on supplementary proxies, is also required.

Arctic warming drives striking twenty-first century ecosystem shifts in Great Slave Lake (Subarctic Canada), North America's deepest lake.

Great Slave Lake (GSL), one of the world's largest and deepest lakes, has undergone an aquatic ecosystem transformation in response to twenty-first-century accelerated Arctic warming that is unparalleled in at least the past two centuries. Algal remains from four high-resolution palaeolimnological records retrieved from the West Basin provide baseline limnological data that we compared with historical phycological surveys undertaken on GSL between the 1940s and 1990s. We document the rapid restructuring of algal community composition ca 2000 CE that is consistent with recent increases in regional air temperature and declines in ice cover and wind speed, that collectively altered habitats for aquatic biota. This new limnological regime initiated the first observation of scaled chrysophytes and favoured the rapid proliferation of small planktonic cyclotelloid diatoms which replaced the long-established dominance of large filamentous Aulacoseira islandica in West Basin sedimentary records. Such abrupt transformations in the primary producers of this socioecologically valuable 'northern Great Lake' may have widespread implications for the entire food web with unknown consequences for aquatic ecosystem functioning and fisheries, which First Nations, Métis and other northern communities depend upon, pointing to the need for new studies.

Tracking pollution from fur farms using forensic paleolimnology.

Eutrophication, which remains one of the greatest threats to water quality worldwide, is particularly acute in agricultural areas. Here we assessed long-term drivers of potential pollution inputs to lakes in southwest Nova Scotia (Canada), a region marked by fur farming (mainly mink) and other agricultural activities. We used a BACI (before-after-control-impact) study design with sediment cores collected from 14 lakes selected based on their proximity to mink farms. We combined economic data, mink faecal samples, and a series of geochemical markers in dated sediment cores, including sterols, δ15N, visible reflectance spectroscopy (VRS)-inferred chlorophyll-a, and heavy metals, to relate changes in sediment geochemistry to the growth of mink farms in the region. Sterol biomarkers (cholesterol and ß-sitosterol) measured in a range of samples (i.e. mink faeces and feed, aquaculture feed), were elevated where mink farms were located close to each study lake. Mink-related sterols (cholesterol, ß-sitoserol), δ15N measurements, VRS chlorophyll-a, and heavy metals As, Cu, Sr increased in the 1980s coeval with a ∼400% increase of mink farms in the region, especially near Nowlans Lake. Agricultural impacts were subtler in other lakes. Our study expands on prior applications of geochemical fingerprinting in forensic paleolimnology when direct monitoring data are incomplete. This multi-proxy approach has promising applications for environmental pollution assessments in other lake ecosystems experiencing water quality issues.

Climate change amplifies the risk of potentially toxigenic cyanobacteria.

Cyanobacterial blooms pose a significant threat to water security, with anthropogenic forcing being implicated as a key driver behind the recent upsurge and global expansion of cyanobacteria in modern times. The potential effects of land-use alterations and climate change can lead to complicated, less-predictable scenarios in cyanobacterial management, especially when forecasting cyanobacterial toxin risks. There is a growing need for further investigations into the specific stressors that stimulate cyanobacterial toxins, as well as resolving the uncertainty surrounding the historical or contemporary nature of cyanobacterial-associated risks. To address this gap, we employed a paleolimnological approach to reconstruct cyanobacterial abundance and microcystin-producing potential in temperate lakes situated along a human impact gradient. We identified breakpoints (i.e., points of abrupt change) in these time series and examined the impact of landscape and climatic properties on their occurrence. Our findings indicate that lakes subject to greater human influence exhibited an earlier onset of cyanobacterial biomass by 40 years compared to less-impacted lakes, with land-use change emerging as the dominant predictor. Moreover, microcystin-producing potential increased in both high- and low-impact lakes around the 1980s, with climate warming being the primary driver. Our findings chronicle the importance of climate change in increasing the risk of toxigenic cyanobacteria in freshwater resources.

A 2200-year record of Andean Condor diet and nest site usage reflects natural and anthropogenic stressors.

Understanding how animals respond to large-scale environmental changes is difficult to achieve because monitoring data are rarely available for more than the past few decades, if at all. Here, we demonstrate how a variety of palaeoecological proxies (e.g. isotopes, geochemistry and DNA) from an Andean Condor (Vultur gryphus) guano deposit from Argentina can be used to explore breeding site fidelity and the impacts of environmental changes on avian behaviour. We found that condors used the nesting site since at least approximately 2200 years ago, with an approximately 1000-year nesting frequency slowdown from ca 1650 to 650 years before the present (yr BP). We provide evidence that the nesting slowdown coincided with a period of increased volcanic activity in the nearby Southern Volcanic Zone, which resulted in decreased availability of carrion and deterred scavenging birds. After returning to the nest site ca 650 yr BP, condor diet shifted from the carrion of native species and beached marine animals to the carrion of livestock (e.g. sheep and cattle) and exotic herbivores (e.g. red deer and European hare) introduced by European settlers. Currently, Andean Condors have elevated lead concentrations in their guano compared to the past, which is associated with human persecution linked to the shift in diet.

Reconstructing historical time-series of cyanobacteria in lake sediments: Integrating technological innovation to enhance cyanobacterial management.

The global rise of cyanobacterial blooms emphasizes the need to develop tools to manage water bodies prone to cyanobacterial dominance. Reconstructing cyanobacterial baselines and identifying environmental drivers that favour cyanobacterial dominance are important to guide management decisions. Conventional techniques for estimating cyanobacteria in lake sediments require considerable resources, creating a barrier to routine reconstructions of cyanobacterial time-series. Here, we compare a relatively simple technique based on spectral inferences of cyanobacteria using visible near-infrared reflectance spectroscopy (VNIRS) with a molecular technique based on real-time PCR quantification (qPCR) of the 16S rRNA gene conserved in cyanobacteria in 30 lakes across a broad geographic gradient. We examined the sedimentary record from two perspectives: 1) relationships throughout the entire core (without radiometric dating); 2) relationships post-1900s with the aid of radiometric dating (i.e., 210Pb). Our findings suggest that the VNIRS-based cyanobacteria technique is best suited for reconstructing cyanobacterial abundance in recent decades (i.e., circa 1990 onwards). The VNIRS-based cyanobacteria technique showed agreement with those generated using qPCR, with 23 (76%) lakes showing a strong or very strong positive relationship between the results of the two techniques. However, five (17%) lakes showed negligible relationships, suggesting cyanobacteria VNIRS requires further refinement to understand where VNIRS is unsuitable. This knowledge will help scientists and lake managers select alternative cyanobacterial diagnostics where appropriate. These findings demonstrate the utility of VNIRS, in most instances, as a valuable tool for reconstructing past cyanobacterial prevalence.

The impacts of whole-lake acidification and eutrophication on the accumulation of lead in sediments from manipulated lakes in the Experimental Lakes Area (IISD-ELA).

Acidification and eutrophication are common limnological stressors impacting many water bodies across the globe. While the negative impacts of these stressors on limnetic communities are generally known, their influence on the accumulation of specific sediment constituents, such as metals, remains unclear. Benefitting from past research and long-term monitoring, lakes at the International Institute for Sustainable Development - Experimental Lakes Area (IISD-ELA) in northwestern Ontario, Canada are invaluable to understand the extent to which these two common lake stressors can influence the accumulation of metals in lacustrine sediment. To address these issues, sediment cores were retrieved from six lakes: four were subjected to past experimental acidification or eutrophication and two were reference lakes. Focusing on elemental lead (Pb), a metal known to have accumulated in lake sediments worldwide and generally exhibiting a relatively small fraction of terrigenous input, we assessed the hypothesis that greater accumulation of Pb would be observed in lakes subjected to eutrophication, while the reverse was expected for lakes subjected to acidification experiments. Our analyses support this hypothesis, whereby relatively low enrichment was recorded in sediments deposited in the acidified lake during the manipulation era. On the other hand, eutrophied lakes demonstrated a strong enrichment in Pb during experimental manipulation. When investigating the mechanisms behind these divergent responses, we found epilimnetic dissolved organic carbon (DOC) and conductivity were associated with a relative increase in Pb accumulation in sediments. Acidic pH is also expected to mediate these responses by decreasing epilimnetic DOC concentrations leading to reduced Pb accumulation in the sediment.

Ancient DNA reveals potentially toxic cyanobacteria increasing with climate change.

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.

Lakes in Hot Water: The Impacts of a Changing Climate on Aquatic Ecosystems.

Our planet is being subjected to unprecedented climate change, with far-reaching social and ecological repercussions. Below the waterline, aquatic ecosystems are being affected by multiple climate-related and anthropogenic stressors, the combined effects of which are poorly understood and rarely appreciated at the global stage. A striking consequence of climate change on aquatic ecosystems is that many are experiencing shorter periods of ice cover, as well as earlier and longer summer stratified seasons, which often result in a cascade of ecological and environmental consequences, such as warmer summer water temperatures, alterations in lake mixing and water levels, declines in dissolved oxygen, increased likelihood of cyanobacterial algal blooms, and the loss of habitat for native cold-water fisheries. The repercussions of a changing climate include impacts on freshwater supplies, water quality, biodiversity, and the ecosystem benefits that they provide to society.

Scavenging gulls are biovectors of mercury from industrial wastes in Nova Scotia, Canada.

Seabirds are important biovectors of contaminants, like mercury, moving them from marine to terrestrial environments around breeding colonies. This transfer of materials can have marked impacts on receiving environments and biota. Less is known about biotransport of contaminants by generalist seabirds that exploit anthropogenic wastes compared to other seabird species. In this study, we measured total mercury (THg) in O-horizon soils at four herring gull (Larus smithsoniansus) breeding colonies in southern Nova Scotia, Canada. At colonies with dry substrate, THg was significantly higher in soils collected from gull colonies compared to nearby reference soils with no nesting gulls. Further, THg was distinct in soils among study colonies and was likely influenced by biotransport from other nesting seabird species, most notably Leach's storm-petrels (Hydrobates leucorhous). Our research suggests gulls that scavenge on anthropogenic wastes at local industrial sites are biovectors moving THg acquired at these sites to their colonies and may increase the spatial footprint of contaminants from these industries.

Pervasive changes in algal indicators since pre-industrial times: A paleolimnological study of changes in primary production and diatom assemblages from ~200 Canadian lakes.

Anthropogenic stressors affect lakes around the world, ranging in scale from catchment-specific pollutants to the global impacts of climate change. Canada has a large number and diversity of lakes, yet it is not well understood how, where, and when human impacts have affected these lakes at a national scale. The NSERC Canadian Lake Pulse Network sought to create the first nationwide database of Canadian lake health, undertaking a multi-year survey of 664 lakes spanning 12 ecozones across Canada. A key objective of the network is to determine where, by how much, and why have Canadian lakes changed during the Anthropocene. To address this objective, we compared sedimentary chlorophyll a and diatoms from modern and pre-industrial sediment intervals of ~200 lakes. The lakes spanned a range of sizes, ecozones, and degrees of within-catchment land use change. We inferred the quantity of chlorophyll a, its isomers and main diagenetic products using visible reflectance spectroscopy. We found widespread increases in primary production since pre-industrial times. Primary production increased, on average, across all ecozones, human impact classes, and stratification classes. Likewise, an increase in planktonic diatom taxa over time was detected in the majority of sampled lakes, likely due to recent climate warming. However, regional factors (ecozones) explained the most variation in modern diatom species assemblages as well as their temporal turnover. Furthermore, lakes with high human impact (i.e., higher weighted proportions of human land use in the catchment) exhibited greater taxonomic turnover than lakes with a low human impact class. The greatest diatom turnover was found in the agriculture-rich Prairies and the lowest in the sparsely populated Boreal Shield and Taiga Cordillera ecozones. Overall, our study highlights that drivers operating at different geographic scales (i.e., climatic and land-use changes) have led to significant alterations in algal indicators since pre-industrial times across the country.

Developing diatom-based inference models to assess lake ecosystem change along a gradient of metal smelting impacts: Sudbury lakes revisited.

Mining and smelting activities have strongly influenced the Sudbury region (Ontario, Canada) since the late 19th century, leading to acidification and metal contamination in many local ecosystems. Regulations on restricting acidic emissions were enacted in the 1970s, after which a considerable volume of paleolimnological work was completed to study the impacts of acidification on Sudbury-region lakes and their subsequent biological recovery. Twenty years after the last regional diatom-based assessment, many lakes have undergone large changes in limnological variables, including increases in pH and dissolved organic carbon concentrations, as well as decreases in metal concentrations. Additionally, these lakes are under the potential impacts of newly emerging environmental stressors such as climate warming and road salt contamination. Here, we revisited a suite of Sudbury-region lakes (n = 80) to examine the relationships between their current water chemistry and diatom assemblages preserved in surface sediments using a canonical correspondence analysis. Although the pH gradient in our study lakes is shorter (pH ~1.4) than in earlier calibration studies conducted in this region, lake water pH was still identified as the strongest environmental variable shaping diatom distributions and was used to construct a robust inference model (R2boot = 0.73; RMSEP = 0.32). By assessing ecological changes experienced by a subset of these Sudbury-region lakes (n = 33) over the past few decades, we identified two major trends: an overall increase in diatom-inferred pH and a rise in the relative abundance of planktonic taxa. Our study provides useful insights into the autecology of major diatom taxa in acidified waters and highlights the importance of considering other anthropogenic stressors when assessing the recovery response of acid-impacted systems.

Climate oscillations drive millennial-scale changes in seabird colony size.

Seabird population size is intimately linked to the physical, chemical, and biological processes of the oceans. Yet, the overall effects of long-term changes in ocean dynamics on seabird colonies are difficult to quantify. Here, we used dated lake sediments to reconstruct ~10,000-years of seabird dynamics in the Northwest Atlantic to determine the influences of Holocene-scale climatic oscillations on colony size. On Baccalieu Island (Newfoundland and Labrador, Canada)-where the world's largest colony of Leach's storm-petrel (Hydrobates leucorhous Vieillot 1818) currently breeds-our data track seabird colony growth in response to warming during the Holocene Thermal Maximum (ca. 9000 to 6000 BP). From ca. 5200 BP to the onset of the Little Ice Age (ca. 550 BP), changes in colony size were correlated to variations in the North Atlantic Oscillation (NAO). By contrasting the seabird trends from Baccalieu Island to millennial-scale changes of storm-petrel populations from Grand Colombier Island (an island in the Northwest Atlantic that is subjected a to different ocean climate), we infer that changes in NAO influenced the ocean circulation, which translated into, among many things, changes in pycnocline depth across the Northwest Atlantic basin where the storm-petrels feed. We hypothesize that the depth of the pycnocline is likely a strong bottom-up control on surface-feeding storm-petrels through its influence on prey accessibility. Since the Little Ice Age (LIA), the effects of ocean dynamics on seabird colony size have been altered by anthropogenic impacts. Subsequently, the colony on Baccalieu Island grew at an unprecedented rate to become the world's largest resulting from favorable conditions linked to climate warming, increased vegetation (thereby nesting habitat), and attraction of recruits from other colonies that are now in decline. We show that although ocean dynamics were an important driver of seabird colony dynamics, its recent influence has been modified by human interference.

Metal contamination in alkaline Phantom Lake (Flin Flon, Manitoba, Canada) generates strong responses in multiple paleolimnological proxies.

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.

Early presence of Bythotrephes cederströmii (Cladocera: Cercopagidae) in lake sediments in North America: evidence or artifact?

The spiny water flea (Bythotrephes cederströmii), a freshwater crustacean considered to be the world's best-studied invasive zooplankter, was first recorded in North America in the Laurentian Great Lakes during the 1980s. Its arrival is widely considered to be the result of ocean-going cargo ships that translocated contaminated ballast water from Eurasia to the Great Lakes during the 1970-1980s. The subsequent first discovery of the species in inland lakes is consistent with the hypothesis that propagules dispersed initially from established Great Lakes populations. Here we present evidence of exoskeletal remains, including mandibles, tail spines, and resting eggs, in 210Pb-dated lake sediment cores, which suggests that B. cederströmii was already resident in four inland North American lakes (two in Minnesota, USA; two in Ontario, Canada) by at least the early 1900s. Densities of exoskeletal remains were low and relatively steady from first appearance until about 1990, after which time they increased in all cores. The earliest evidence that we found was a mandible at 33-cm depth (pre-1650) in the sediments of Three Mile Lake, Ontario, Canada. These unexpected findings challenge the current paradigm of B. cederströmii invasion, renew uncertainty about the timing and sequence of its colonization of North American lakes, and potentially question our ability to detect invasive species with traditional sampling methods. We attempted to eliminate errors in the dated stratigraphies of the exoskeletal remains that might have been introduced either methodologically (e.g., core-wall smearing) or naturally (e.g., bioturbation). Nonetheless, given the very low numbers of subfossils encountered, questions remain about the possible artifactual nature of our observations and therefore we regard our results as 'preliminary findings' at this time.

Human impacts and Anthropocene environmental change at Lake Kutubu, a Ramsar wetland in Papua New Guinea.

The impacts of human-induced environmental change that characterize the Anthropocene are not felt equally across the globe. In the tropics, the potential for the sudden collapse of ecosystems in response to multiple interacting pressures has been of increasing concern in ecological and conservation research. The tropical ecosystems of Papua New Guinea are areas of diverse rainforest flora and fauna, inhabited by human populations that are equally diverse, both culturally and linguistically. These people and the ecosystems they rely on are being put under increasing pressure from mineral resource extraction, population growth, land clearing, invasive species, and novel pollutants. This study details the last ∼90 y of impacts on ecosystem dynamics in one of the most biologically diverse, yet poorly understood, tropical wetland ecosystems of the region. The lake is listed as a Ramsar wetland of international importance, yet, since initial European contact in the 1930s and the opening of mineral resource extraction facilities in the 1990s, there has been a dramatic increase in deforestation and an influx of people to the area. Using multiproxy paleoenvironmental records from lake sediments, we show how these anthropogenic impacts have transformed Lake Kutubu. The recent collapse of algal communities represents an ecological tipping point that is likely to have ongoing repercussions for this important wetland's ecosystems. We argue that the incorporation of an adequate historical perspective into models for wetland management and conservation is critical in understanding how to mitigate the impacts of ecological catastrophes such as biodiversity loss.

Reply to formal comment on Griffiths et al. (2017) submitted by Gajewski (2020).

Gajewski offers a formal comment on Griffiths et al. (2017), a paper that explored how microclimates and their varying ice cover regimes on lakes and ponds in Arctic regions modified the diatom assemblage responses to recent warming. One of Gajewski's main criticisms is that the microclimate classification scheme used in Griffiths et al. (2017) is merely anecdotal; a claim which ignores the value of observational evidence and misunderstands the frequency that each site was visited or surveyed. We clarify that the study sites were visited multiple times via recurrent aerial surveys and ground observations dating back to the 1970s, which supports our microclimate classification scheme. Many of Gajewski's claims regarding climate, catchment characteristics, and ice melting properties from field locations he has not visited were refuted by veteran Arctic scientists with long-term field experience in these regions. In addition, Gajewski makes several criticisms concerning radioisotopic dating, core chronology, sediment mixing, diagenesis, and preservation of bioindicators that relate more to general paleolimnological assumptions than to conclusions reached by Griffiths et al. (2017). Research from the 1980s and 1990s, when scientific consensus on these issues was first reached, readily show that the methodologies and data interpretation of Griffiths et al. (2017) are sound. We appreciate the opportunity to expound on the finer details of the Griffiths et al. (2017) paper, work based on field research by the study's co-authors spanning almost three decades, with additional observations from colleagues dating back to the 1970s. We address Gajewski's criticisms with relevant literature, expert statements, and a few clarifying comments.

Eutrophication and climatic changes lead to unprecedented cyanobacterial blooms in a Canadian sub-Arctic landscape.

Cyanobacterial blooms have been increasing in frequency and intensity but are often considered an issue restricted to temperate and tropical lakes. Here we report on one of the first occurrences of recurring cyanobacterial (Planktothrix spp.) blooms in a sub-Arctic lake from Yellowknife (Northwest Territories, Canada) and provide a long-term environmental context for the recent blooms using local meteorological data and multi-proxy paleolimnological analyses. Multiple co-occurring regional (gold mining emissions and climatic change) and local (land clearance and urbanization) stressors have impacted Jackfish Lake during the 20th and early-21st centuries, which have led to biological responses across multiple trophic levels. The unprecedented post-2013 cyanobacterial blooms were likely a cumulative response to nutrient enrichment and complex climate-mediated changes to lake thermal properties. A regional analysis of eight lakes around Yellowknife revealed that reduced ice cover duration and longer growing seasons have led to an increase in whole-lake primary production, whilst urban lakes were also fertilized by nutrients from local land-use changes in their catchments. Our findings suggest that anthropogenically nutrient-enriched sub-Arctic lakes, akin to their lower-latitude counterparts, may be vulnerable to cyanobacterial blooms in a warming world.