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Author(s): E. M. Krümmel (corresponding author) [1]; R. W. Macdonald [2]; L. E. Kimpe [1]; I. Gregory-Eaves [1]; M. J. Demers [1]; J. P. Smol [3]; B. Finney [4]; J. [...]

Threats to biodiversity are fostering new collaboration between aquatic ecologists and palaeolimnologists, who have traditionally asked ecological questions on different time scales. While the differences between surface sediment and water column or snapshot sampling are well understood, less so are the consequences of comparing the predominant drivers of aquatic assemblages resulting from these two types of sampling., Using diatom data from the 2007 USEPA National Lakes Assessment (NLA) program (468 lakes), we compared the main environmental and spatial drivers of diatom community composition between samples derived from the water column and surface sediments. We hypothesised that, in explaining community variation across the conterminous United States, the effect of environment would be stronger in diatom assemblages preserved in surface sediments because of the inclusion of benthic members and temporal integration. We used a combination of ordination overlays and variation partitioning to examine differences in community drivers between palaeolimnological (surface sediment) and water column sampling., We found that these two types of sampling were significantly correlated with respect to the drivers of community composition in addition to having congruent patterns of ordination. Congruency between sampling methods further increased when the water column data were temporally integrated and may be explained by variation in seasonally dynamic taxa., To our knowledge, this is the first study that has tested for differences in environmental structuring patterns between palaeolimnological and water column samples using such a highly replicated and landscape-level approach. On the basis of our results, we encourage ecologists to consider the joint analysis of these two types of data sets where data are available. [ABSTRACT FROM AUTHOR]

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., (© 2024 The Author(s). Journal of Phycology published by Wiley Periodicals LLC on behalf of Phycological Society of America.)

Plastics are pervasive in aquatic ecosystems, in which they circulate in the water column, accumulate in sediments, and are taken up, retained, and exchanged with their biotic environment via trophic and non-trophic activities. Identifying and comparing organismal interactions are a necessary step to improve monitoring and risk assessments of microplastics. We use a community module to test how abiotic and biotic interactions determine the fate of microplastics in a benthic food web. Using single-exposure trials on a trio of interacting freshwater animals (the quagga mussel Dreissena bugensis, a filter feeder; the gammarid amphipod Gammarus fasciatus, a deposit feeder; and the round goby Neogobius melanostomus, a benthivorous fish), we quantify the (1) uptake of microplastics from environmental routes (water, sediment) under six exposure concentrations, (2) the depuration capacities over 72 h, and (3) the transfer of microbeads via trophic (predator-prey) and behavioral interactions (commensalism, intraspecific facilitation). Under 24 h exposures, each animal of our module acquired beads from both environmental routes. The body burden of filter-feeders was higher when they were exposed to particles in suspension, whereas detritivores had similar uptake from either route. Mussels transferred microbeads to amphipods, and both invertebrates transferred beads to their mutual predator, the round goby. Round gobies generally displayed low contamination from all routes (suspension, sedimented, trophic transfer) with a higher microbead load from preying on contaminated mussels. Higher mussel abundance (10-15 mussel per aquaria, i.e., ~200-300 mussels·m 2 ) did not increase individual mussel burdens during exposure, and neither did it increase the transfer of beads from mussels to gammarids via biodeposition. Our community module approach revealed that the feeding behavior of animals allows microplastic uptake from multiple environmental routes, whereas trophic and non-trophic species interactions increased their burden within their food web community., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: We received no additional funding outside government agencies and academic institutions (see funding section for details)., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Understanding the environmental conditions and taxa that promote the occurrence of cyanobacterial toxins is imperative for effective management of lake ecosystems. Herein, we modeled total microcystin presence and concentrations with a broad suite of environmental predictors and cyanobacteria community data collected across 440 Canadian lakes using standardized methods. We also conducted a focused analysis targeting 14 microcystin congeners across 190 lakes, to examine how abiotic and biotic factors influence their relative proportions. Microcystins were detected in 30 % of lakes, with the highest total concentrations occurring in the most eutrophic lakes located in ecozones of central Canada. The two most commonly detected congeners were MC-LR (61 % of lakes) and MC-LA (37 % of lakes), while 11 others were detected more sporadically across waterbodies. Congener diversity peaked in central Canada where cyanobacteria biomass was highest. Using a zero-altered hurdle model, the probability of detecting microcystin was best explained by increasing Microcystis biomass, Daphnia and cyclopoid biomass, soluble reactive phosphorus, pH and wind. Microcystin concentrations increased with the biomass of Microcystis and other less dominant cyanobacteria taxa, as well as total phosphorus, cyclopoid copepod biomass, dissolved inorganic carbon and water temperature. Collectively, these models accounted for 34 % and 70 % of the variability, respectively. Based on a multiple factor analysis of microcystin congeners, cyanobacteria community data, environmental and zooplankton data, we found that the relative abundance of most congeners varied according to trophic state and were related to a combination of cyanobacteria genera biomasses and environmental variables., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Protists play key roles in aquatic food webs as primary producers, predators, nutrient recyclers, and symbionts. However, a comprehensive view of protist diversity in freshwaters has been challenged by the immense environmental heterogeneity among lakes worldwide. We assessed protist diversity in the surface waters of 366 freshwater lakes across a north temperate to subarctic range covering nearly 8.4 million km 2 of Canada. Sampled lakes represented broad gradients in size, trophic state, and watershed land use. Hypereutrophic lakes contained the least diverse and most distinct protist communities relative to nutrient-poor lakes. Greater taxonomic variation among eutrophic lakes was mainly a product of heterotroph and mixotroph diversity, whereas phototroph assemblages were more similar under high-nutrient conditions. Overall, local physicochemical factors, particularly ion and nutrient concentrations, elicited the strongest responses in community structure, far outweighing the effects of geographic gradients. Despite their contrasting distribution patterns, obligate phototroph and heterotroph turnover was predicted by an overlapping set of environmental factors, while the metabolic plasticity of mixotrophs may have made them less predictable. Notably, protist diversity was associated with variation in watershed soil pH and agricultural crop coverage, pointing to human impact on the land-water interface that has not been previously identified in studies on smaller scales. Our study exposes the importance of both within-lake and external watershed characteristics in explaining protist diversity and biogeography, critical information for further developing an understanding of how freshwater lakes and their watersheds are impacted by anthropogenic stressors. IMPORTANCE Freshwater lakes are experiencing rapid changes under accelerated anthropogenic stress and a warming climate. Microorganisms underpin aquatic food webs, yet little is known about how freshwater microbial communities are responding to human impact. Here, we assessed the diversity of protists and their myriad ecological roles in lakes varying in size across watersheds experiencing a range of land use pressures by leveraging data from a continental-scale survey of Canadian lakes. We found evidence of human impact on protist assemblages through an association with lake trophic state and extending to agricultural activity and soil characteristics in the surrounding watershed. Furthermore, trophic state appeared to explain the distributions of phototrophic and heterotrophic protists in contrasting ways. Our findings highlight the vulnerability of lake ecosystems to increased land use and the importance of assessing terrestrial interfaces to elucidate freshwater ecosystem dynamics.

Accurately identifying the species present in an ecosystem is vital to lake managers and successful bioassessment programs. This is particularly important when monitoring cyanobacteria, as numerous taxa produce toxins and can have major negative impacts on aquatic ecosystems. Increasingly, DNA-based techniques such as metabarcoding are being used for measuring aquatic biodiversity, as they could accelerate processing time, decrease costs and reduce some of the biases associated with traditional light microscopy. Despite the continuing use of traditional microscopy and the growing use of DNA metabarcoding to identify cyanobacteria assemblages, methodological comparisons between the two approaches have rarely been reported from a wide suite of lake types. Here, we compare planktonic cyanobacteria assemblages generated by inverted light microscopy and DNA metabarcoding from a 379-lake dataset spanning a longitudinal and trophic gradient. We found moderate levels of congruence between methods at the broadest taxonomic levels (i.e., Order, RV=0.40, p < 0.0001). This comparison revealed distinct cyanobacteria communities from lakes of different trophic states, with Microcystis, Aphanizomenon and Dolichospermum dominating with both methods in eutrophic and hypereutrophic sites. This finding supports the use of either method when monitoring eutrophication in lake surface waters. The biggest difference between the two methods was the detection of picocyanobacteria, which are typically underestimated by light microscopy. This reveals that the communities generated by each method currently are complementary as opposed to identical and promotes a combined-method strategy when monitoring a range of trophic systems. For example, microscopy can provide measures of cyanobacteria biomass, which are critical data in managing lakes. Going forward, we believe that molecular genetic methods will be increasingly adopted as reference databases are routinely updated with more representative sequences and will improve as cyanobacteria taxonomy is resolved with the increase in available genetic information., (Copyright © 2022. Published by Elsevier B.V.)

Sediment DNA (sedDNA) analyses are rapidly emerging as powerful tools for the reconstruction of environmental and evolutionary change. While there are an increasing number of studies using molecular genetic approaches to track changes over time, few studies have compared the coherence between quantitative polymerase chain reaction (PCR) methods and metabarcoding techniques. Primer specificity, bioinformatic analyses, and PCR inhibitors in sediments could affect the quantitative data obtained from these approaches. We compared the performance of droplet digital polymerase chain reaction (ddPCR) and high-throughput sequencing (HTS) for the quantification of target genes of cyanobacteria in lake sediments and tested whether the two techniques similarly reveal expected patterns through time. Absolute concentrations of cyanobacterial 16S rRNA genes were compared between ddPCR and HTS using dated sediment cores collected from two experimental (Lake 227, fertilized since 1969 and Lake 223, acidified from 1976 to 1983) and two reference lakes (Lakes 224 and 442) in the Experimental Lakes Area (ELA), Canada. Relative abundances of Microcystis 16S rRNA (MICR) genes were also compared between the two methods. Moderate to strong positive correlations were found between the molecular approaches among all four cores but results from ddPCR were more consistent with the known history of lake manipulations. A 100-fold increase in ddPCR estimates of cyanobacterial gene abundance beginning in ~1968 occurred in Lake 227, in keeping with experimental addition of nutrients and increase in planktonic cyanobacteria. In contrast, no significant rise in cyanobacterial abundance associated with lake fertilization was observed with HTS. Relative abundances of Microcystis between the two techniques showed moderate to strong levels of coherence in top intervals of the sediment cores. Both ddPCR and HTS approaches are suitable for sedDNA analysis, but studies aiming to quantify absolute abundances from complex environments should consider using ddPCR due to its high tolerance to PCR inhibitors., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Mejbel, Dodsworth, Baud, Gregory-Eaves and Pick.)

Phytoplankton images were collected using an Imaging Flow Cytobot moored in the mesotrophic lake Lac Montjoie (Quebec, Canada). Cystodinium -an unusual dinoflagellate genus-was found during manual classification of the images into taxonomic groups while building an automated classifier. Cystodinium 's particularity is that while it can take a typical motile dinoflagellate form, it is thought to exist primarily as an immotile photosynthetically competent parasitic cyst in the shape of a crescent moon. Observations presented here are of this immotile lunate cyst. Manually classified images revealed that the majority of the Cystodinium found (86%) were attached to other microalgae or detrital material while the rest were unattached. The established auto-classifier was only able to correctly identify unattached Cystodinium images and thus was used to generate time series as cells per 100 mL for the unattached cell subset. Our observations, coupled with a literature review, lead us to question the parasitic nature of this taxonomic group., (© The Author(s) 2021. Published by Oxford University Press.)

The reconstruction of ecological time series from lake sediment archives can retrace the environmental impact of human activities. Molecular genetic approaches in paleolimnology have provided unprecedented access to DNA time series, which record evidence of the microbial ecologies that underlaid historical lake ecosystems. Such studies often rely on single-gene surveys, and consequently, the full diversity of preserved microorganisms remains unexplored. In this study, we probed the diversity archived in contemporary and preindustrial sediments by comparative shotgun metagenomic analysis of surface water and sediment samples from three eastern Canadian lakes. In a strategy that was aimed at disentangling historical DNA from the indigenous sediment background, microbial preservation signals were captured by mapping sequence similarities between sediment metagenome reads and reference surface water metagenome assemblies. We detected preserved Cyanobacteria , diverse bacterioplankton, microeukaryotes, and viruses in sediment metagenomes. Among the preserved microorganisms were important groups never before reported in paleolimnological reconstructions, including bacteriophages ( Caudovirales ) and ubiquitous freshwater Betaproteobacteria ( Polynucleobacter and Limnohabitans ). In contrast, ultramicroscopic Actinobacteria (" Candidatus Nanopelagicales") and Alphaproteobacteria ( Pelagibacterales ) were apparently not well preserved in sediment metagenomes even though they were numerically dominant in surface water metagenomes. Overall, our study explored a novel application of whole-metagenome shotgun sequencing for discovering the DNA remains of a broad diversity of microorganisms preserved in lake sediments. The recovery of diverse microbial time series supports the taxonomic expansion of microbiome reconstructions and the development of novel microbial paleoindicators. IMPORTANCE Lakes are critical freshwater resources under mounting pressure from climate change and other anthropogenic stressors. The reconstruction of ecological time series from sediment archives with paleolimnological techniques has been shown to be an effective means of understanding how humans are modifying lake ecosystems over extended timescales. In this study, we combined shotgun DNA sequencing with a novel comparative analysis of surface water and sediment metagenomes to expose the diversity of microorganisms preserved in lake sediments. The detection of DNA from a broad diversity of preserved microbes serves to more fully reconstruct historical microbiomes and describe preimpact lake conditions., (Copyright © 2020 Garner et al.)

There is growing evidence that cyanobacterial blooms are becoming more common in different parts of the world; within this context, predictive cyanobacteria models have an essential role in lake management. Several models have been successfully used in temperate systems to describe the main drivers of cyanobacterial blooms, but relatively less work has been conducted in the Tropics. We analyzed data from six Brazilian reservoirs and from five Canadian lakes using a combination of regression tree analyses and variation partitioning to evaluate the similarities and differences between regions. Our results, together with a synthesis of the literature from different latitudes, showed that trophic state (i.e. nutrients), climatic variables (e.g., temperature and/or precipitation) and hydrodynamic regimes (i.e. water residence time) are significant drivers of cyanobacteria biomass over several scales. Nutrients came out as the primary predictor in both regions, followed by climate, but when all systems were pooled together, water residence time came out as most important. The consistency in variables identified between regions suggests that these drivers are widely important and cyanobacteria responded quite similarly in different geographical settings and waterbody types (i.e. lakes or reservoirs). However, more work is needed to identify key thresholds across latitudinal gradients. Taken together, these results suggest that multi-region syntheses can help identify drivers that predict broad-scale patterns of cyanobacteria biomass., (Crown Copyright © 2020. Published by Elsevier B.V. All rights reserved.)

Microplastics are pervasive pollutants in fresh waters, but their distribution, abundance, and diversity in fluvial environments remain poorly documented. Previous research indicated that large polyethylene microbeads were abundant in the freshwater sediments of the St. Lawrence River. Here we extend this work by quantifying the abundance of a broad range of sizes and types of microplastics in sediments and surface water samples, and we relate these metrics to environmental variables. We sampled 21 sites for sediments that spanned a land use gradient, and 10 surface water stations above and below wastewater effluent sites, along the fluvial corridor of the St. Lawrence River between Montreal and Quebec City from July to August 2017. Microplastics were removed from sediments using an oil extraction protocol and enumerated under fluorescent microscopy. We tested predictions that environmental filters and known point sources affect microplastic concentrations in the river. The mean concentration of microplastics across all sediment sampling sites was 832 (±150 SE) plastics per kg dry weight (range 65-7562 plastics per kg dry weight), which is among the highest recorded (in the top 25%) for the world's freshwater and marine systems. Microplastic concentrations in the sediments were significantly related to a suite of environmental variables including land use and sediment particle characteristics. Particle characteristics, proximity to point sources (urban land use), and environmental filters (sediment compositional variables, % organic carbon, % inorganic carbon and distance from shore) each explained a significant fraction of variation in the microplastic composition in the sediment, with environmental filters having the greatest influence. We present a protocol that could be used to efficiently and accurately detect a broad range of microplastics until a standardized protocol is established for large-scale monitoring., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The distribution and quality of water resources vary dramatically across Canada, and human impacts such as land-use and climate changes are exacerbating uncertainties in water supply and security. At the national level, Canada has no enforceable standards for safe drinking water and no comprehensive water-monitoring program to provide detailed, timely reporting on the state of water resources. To provide Canada's first national assessment of lake health, the NSERC Canadian Lake Pulse Network was launched in 2016 as an academic-government research partnership. LakePulse uses traditional approaches for limnological monitoring as well as state-of-the-art methods in the fields of genomics, emerging contaminants, greenhouse gases, invasive pathogens, paleolimnology, spatial modelling, statistical analysis, and remote sensing. A coordinated sampling program of about 680 lakes together with historical archives and a geomatics analysis of over 80,000 lake watersheds are used to examine the extent to which lakes are being altered now and in the future, and how this impacts aquatic ecosystem services of societal importance. Herein we review the network context, objectives and methods., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)

Accelerated soil erosion has become a pervasive feature on landscapes around the world and is recognized to have substantial implications for land productivity, downstream water quality, and biogeochemical cycles. However, the scarcity of global syntheses that consider long-term processes has limited our understanding of the timing, the amplitude, and the extent of soil erosion over millennial time scales. As such, we lack the ability to make predictions about the responses of soil erosion to long-term climate and land cover changes. Here, we reconstruct sedimentation rates for 632 lakes based on chronologies constrained by 3,980 calibrated 14 C ages to assess the relative changes in lake-watershed erosion rates over the last 12,000 y. Estimated soil erosion dynamics were then complemented with land cover reconstructions inferred from 43,669 pollen samples and with climate time series from the Max Planck Institute Earth System Model. Our results show that a significant portion of the Earth surface shifted to human-driven soil erosion rate already 4,000 y ago. In particular, inferred soil erosion rates increased in 35% of the watersheds, and most of these sites showed a decrease in the proportion of arboreal pollen, which would be expected with land clearance. Further analysis revealed that land cover change was the main driver of inferred soil erosion in 70% of all studied watersheds. This study suggests that soil erosion has been altering terrestrial and aquatic ecosystems for millennia, leading to carbon (C) losses that could have ultimately induced feedbacks on the climate system., Competing Interests: The authors declare no competing interest., (Copyright © 2019 the Author(s). Published by PNAS.)

Dams are recognised to impact aquatic biodiversity, but the effects and conclusions diverge across studies and locations. By using a meta-analytical approach, we quantified the effects of impoundment on fish communities distributed across three large biomes. The impacts of dams on richness and diversity differed across biomes, with significant declines in the tropics, lower amplitude but similar directional changes in temperate regions, and no changes in boreal regions. Our analyses showed that non-native species increased significantly in tropical and temperate regulated rivers, but not in boreal rivers. In contrast, temporal trajectories in fish assemblage metrics were common across regions, with all biomes showing an increase in mean trophic level position and in the proportion of generalist species after impoundment. Such changes in fish assemblages may affect food web stability and merit closer study. Across the literature examined, predominant mechanisms that render fish assemblages susceptible to impacts from dams were: (1) the transformation of the lotic environment into a lentic environment; (2) habitat fragmentation and (3) the introduction of non-native species. Collectively, our results highlight that an understanding of the regional context and a suite of community metrics are needed to make robust predictions about how fish will respond to river impoundments., (© 2019 John Wiley & Sons Ltd/CNRS.)

Freshwater ecosystems are threatened by multiple anthropogenic stressors acting over different spatial and temporal scales, resulting in toxic algal blooms, reduced water quality and hypoxia. However, while catchment characteristics act as a 'filter' modifying lake response to disturbance, little is known of the relative importance of different drivers and possible differentiation in the response of upland remote lakes in comparison to lowland, impacted lakes. Moreover, many studies have focussed on single lakes rather than looking at responses across a set of individual, yet connected lake basins. Here we used sedimentary algal pigments as an index of changes in primary producer assemblages over the last ~200 years in a northern temperate watershed consisting of 11 upland and lowland lakes within the Lake District, United Kingdom, to test our hypotheses about landscape drivers. Specifically, we expected that the magnitude of change in phototrophic assemblages would be greatest in lowland rather than upland lakes due to more intensive human activities in the watersheds of the former (agriculture, urbanization). Regional parameters, such as climate dynamics, would be the predominant factors regulating lake primary producers in remote upland lakes and thus, synchronize the dynamic of primary producer assemblages in these basins. We found broad support for the hypotheses pertaining to lowland sites as wastewater treatment was the main predictor of changes to primary producer assemblages in lowland lakes. In contrast, upland headwaters responded weakly to variation in atmospheric temperature, and dynamics in primary producers across upland lakes were asynchronous. Collectively, these findings show that nutrient inputs from point sources overwhelm climatic controls of algae and nuisance cyanobacteria, but highlights that large-scale stressors do not always initiate coherent regional lake response. Furthermore, a lake's position in its landscape, its connectivity and proximity to point nutrients are important determinants of changes in production and composition of phototrophic assemblages., (© 2018 The Authors. Global Change Biology Published by John Wiley & Sons Ltd.)

Enhanced phosphorus (P) export from land into streams and lakes is a primary factor driving the expansion of deep-water hypoxia in lakes during the Anthropocene. However, the interplay of regional scale environmental stressors and the lack of long-term instrumental data often impede analyses attempting to associate changes in land cover with downstream aquatic responses. Herein, we performed a synthesis of data that link paleolimnological reconstructions of lake bottom-water oxygenation to changes in land cover/use and climate over the past 300 years to evaluate whether the spread of hypoxia in European lakes was primarily associated with enhanced P exports from growing urbanization, intensified agriculture, or climatic change. We showed that hypoxia started spreading in European lakes around CE 1850 and was greatly accelerated after CE 1900. Socioeconomic changes in Europe beginning in CE 1850 resulted in widespread urbanization, as well as a larger and more intensively cultivated surface area. However, our analysis of temporal trends demonstrated that the onset and intensification of lacustrine hypoxia were more strongly related to the growth of urban areas than to changes in agricultural areas and the application of fertilizers. These results suggest that anthropogenically triggered hypoxia in European lakes was primarily caused by enhanced P discharges from urban point sources. To date, there have been no signs of sustained recovery of bottom-water oxygenation in lakes following the enactment of European water legislation in the 1970s to 1980s, and the subsequent decrease in domestic P consumption., Competing Interests: The authors declare no conflict of interest.

Increases in atmospheric temperature and nutrients from land are thought to be promoting the expansion of harmful cyanobacteria in lakes worldwide, yet to date there has been no quantitative synthesis of long-term trends. To test whether cyanobacteria have increased in abundance over the past ~ 200 years and evaluate the relative influence of potential causal mechanisms, we synthesised 108 highly resolved sedimentary time series and 18 decadal-scale monitoring records from north temperate-subarctic lakes. We demonstrate that: (1) cyanobacteria have increased significantly since c. 1800 ce, (2) they have increased disproportionately relative to other phytoplankton, and (3) cyanobacteria increased more rapidly post c. 1945 ce. Variation among lakes in the rates of increase was explained best by nutrient concentration (phosphorus and nitrogen), and temperature was of secondary importance. Although cyanobacterial biomass has declined in some managed lakes with reduced nutrient influx, the larger spatio-temporal scale of sedimentary records show continued increases in cyanobacteria throughout the north temperate-subarctic regions., (© 2015 John Wiley & Sons Ltd/CNRS.)

African tropical lakes provide vital ecosystem services including food and water to some of the fastest growing human populations, yet they are among the most understudied ecosystems in the world. The consequences of climate change and other stressors on the tropical lakes of Africa have been informed by long-term analyses, but these studies have largely focused on the massive Great Rift Valley lakes. Our objective was to evaluate how recent climate change has altered the functioning and services of smaller tropical lakes, which are far more abundant on the landscape. Based on a paired analysis of 20 years of high-resolution water column data and a paleolimnological record from a small crater lake in western Uganda, we present evidence that even a modest warming of the air (∼0.9°C increase over 20 years) and changes in the timing and intensity of rainfall can have significant consequences on the dynamics of this common tropical lake type. For example, we observed a significant nonlinear increase (R(2) adj  = 0.23, e.d.f. = 7, p<0.0001) in thermal stability over the past 20 years. This resulted in the expansion of anoxic waters and consequent deterioration of fish habitat and appears to have abated primary production; processes that may impair ecosystem services for a vulnerable human population. This study on a system representative of small tropical crater lakes highlights the far-reaching effects of global climatic change on tropical waters. Increased research efforts into tropical aquatic ecosystem health and the development of sound management practices are necessary in order to strengthen adaptive capabilities in tropical regions.

Observational data from the past century have highlighted the importance of interdecadal modes of variability in fish population dynamics, but how these patterns of variation fit into a broader temporal and spatial context remains largely unknown. We analyzed time series of stable nitrogen isotopes from the sediments of 20 sockeye salmon nursery lakes across western Alaska to characterize temporal and spatial patterns in salmon abundance over the past ∼500 y. Although some stocks varied on interdecadal time scales (30- to 80-y cycles), centennial-scale variation, undetectable in modern-day catch records and survey data, has dominated salmon population dynamics over the past 500 y. Before 1900, variation in abundance was clearly not synchronous among stocks, and the only temporal signal common to lake sediment records from this region was the onset of commercial fishing in the late 1800s. Thus, historical changes in climate did not synchronize stock dynamics over centennial time scales, emphasizing that ecosystem complexity can produce a diversity of ecological responses to regional climate forcing. Our results show that marine fish populations may alternate between naturally driven periods of high and low abundance over time scales of decades to centuries and suggest that management models that assume time-invariant productivity or carrying capacity parameters may be poor representations of the biological reality in these systems.

Biodiversity losses over the next century are predicted to result in alterations of ecosystem functions that are on par with other major drivers of global change. Given the seriousness of this issue, there is a need to effectively monitor global biodiversity. Because performing biodiversity censuses of all taxonomic groups is prohibitively costly, indicator groups have been studied to estimate the biodiversity of different taxonomic groups. Quantifying cross-taxon congruence is a method of evaluating the assumption that the diversity of one taxonomic group can be used to predict the diversity of another. To improve the predictive ability of cross-taxon congruence in aquatic ecosystems, we evaluated whether body size, measured as the ratio of average body length between organismal groups, is a significant predictor of their cross-taxon biodiversity congruence. To test this hypothesis, we searched the published literature and screened for studies that used species richness correlations as their metric of cross-taxon congruence. We extracted 96 correlation coefficients from 16 studies, which encompassed 784 inland water bodies. With these correlation coefficients, we conducted a categorical meta-analysis, grouping data based on the body size ratio of organisms. Our results showed that cross-taxon congruence is variable among sites and between different groups (r values ranging between -0.53 to 0.88). In addition, our quantitative meta-analysis demonstrated that organisms most similar in body size showed stronger species richness correlations than organisms which differed increasingly in size (radj(2) = 0.94, p = 0.02). We propose that future studies applying biodiversity indicators in aquatic ecosystems consider functional traits such as body size, so as to increase their success at predicting the biodiversity of taxonomic groups where cost-effective conservation tools are needed.

Humans have more than doubled the amount of reactive nitrogen (Nr) added to the biosphere, yet most of what is known about its accumulation and ecological effects is derived from studies of heavily populated regions. Nitrogen (N) stable isotope ratios ((15)N:(14)N) in dated sediments from 25 remote Northern Hemisphere lakes show a coherent signal of an isotopically distinct source of N to ecosystems beginning in 1895 ± 10 years (±1 standard deviation). Initial shifts in N isotope composition recorded in lake sediments coincide with anthropogenic CO(2) emissions but accelerate with widespread industrial Nr production during the past half century. Although current atmospheric Nr deposition rates in remote regions are relatively low, anthropogenic N has probably influenced watershed N budgets across the Northern Hemisphere for over a century.

Submerged macrophytes are a central component of lake ecosystems; however, little is known regarding their long-term response to environmental change. We have examined the potential of diatoms as indicators of past macrophyte biomass. We first sampled periphyton to determine whether habitat was a predictor of diatom assemblage. We then sampled 41 lakes in Quebec, Canada, to evaluate whether whole-lake submerged macrophyte biomass (BiomEpiV) influenced surface sediment diatom assemblages. A multivariate regression tree (MRT) was used to construct a semiquantitative model to reconstruct past macrophyte biomass. We determined that periphytic diatom assemblages on macrophytes were significantly different from those on wood and rocks (ANOSIM R = 0.63, P < 0.01). A redundancy analysis (RDA) of the 41-lake data set identified BiomEpiV as a significant (P < 0.05) variable in structuring sedimentary diatom assemblages. The MRT analysis classified the lakes into three groups. These groups were (A) high-macrophyte, nutrient-limited lakes (BiomEpiV ≥525 μg · L(-1) ; total phosphorus [TP] <35 μg · L(-1) ; 23 lakes); (B) low-macrophyte, nutrient-limited lakes (BiomEpiV <525 μg · L(-1) ; TP <35 μg · L(-1) ; 12 lakes); and (C) eutrophic lakes (TP ≥35 μg · L(-1) ; six lakes). A semiquantitative model correctly predicted the MRT group of the lake 71% of the time (P < 0.001). These results suggest that submerged macrophytes have a significant influence on diatom community structure and that sedimentary diatom assemblages can be used to infer past macrophyte abundance., (© 2011 Phycological Society of America.)

Background: A hallmark of the latter half of the 20(th) century is the widespread, rapid intensification of a variety of anthropogenically-driven environmental changes--a "Great Acceleration." While there is evidence of a Great Acceleration in a variety of factors known to be linked to water quality degradation, such as conversion of land to agriculture and intensification of fertilizer use, it is not known whether there has been a similar acceleration of freshwater eutrophication., Methodology/principal Findings: Using quantitative reconstructions of diatom-inferred total phosphorus (DI-TP) as a proxy for lake trophic state, we synthesized results from 67 paleolimnological studies from across Europe and North America to evaluate whether most lakes showed a pattern of eutrophication with time and whether this trend was accelerated after 1945 CE, indicative of a Great Acceleration. We found that European lakes have experienced widespread increases in DI-TP over the 20(th) century and that 33% of these lakes show patterns consistent with a post-1945 CE Great Acceleration. In North America, the proportion of lakes that increased in DI-TP over time is much lower and only 9% exhibited a Great Acceleration of eutrophication., Conclusions/significance: The longer and more widespread history of anthropogenic influence in Europe, the leading cause for the relatively pervasive freshwater eutrophication, provides an important cautionary tale; our current path of intensive agriculture around the world may lead to an acceleration of eutrophication in downstream lakes that could take centuries from which to recover.

Several recent studies have highlighted the importance of salmon as a means to deliver biomagnifying contaminants to nursery lakes. There is a lack of studies, however, which demonstrate empirically how this source has varied through time. This is of great significance because past salmon-derived contaminant loading was potentially greater than it is today. By analyzing radiometrically dated sediment cores collected from ten lakes in Alaska and British Columbia (B.C.), we relate historical numbers of sockeye salmon spawners to SigmaPCB concentrations and delta(15)N values (a paleolimnological proxy for past salmon-derived nitrogen) in the sediments. The results confirm that sockeye salmon have provided an important route for PCBs to enter the lakes in the past, a finding that is especially evident when the data of all lakes are pooled. Significant relationships between sockeye salmon numbers and delta(15)N, as well as SigmaPCB concentrations and delta(15)N in sediments, were also found. However, it is difficult to establish relationships between salmon numbers, SigmaPCBs and delta(15)N in individual lakes. This may be due to a number of factors which may influence contaminant loadings to the lakes. The factors include: a) changing salmon contaminant loads over time resulting from a lag in the upper ocean reservoir and/or changing salmon feeding locations; b) greater importance of atmospheric transport in lakes with relatively low salmon returns; and c) increased PCB scavenging due to higher algae productivity in the lakes in recent years.

Recent observations and model simulations have highlighted the sensitivity of the forest-tundra ecotone to climatic forcing. In contrast, paleoecological studies have not provided evidence of tree-line fluctuations in response to Holocene climatic changes in Alaska, suggesting that the forest-tundra boundary in certain areas may be relatively stable at multicentennial to millennial time scales. We conducted a multiproxy study of sediment cores from an Alaskan lake near the altitudinal limits of key boreal-forest species. Paleoecological data were compared with independent climatic reconstructions to assess ecosystem responses of the forest tundra boundary to Little Ice Age (LIA) climatic fluctuations. Pollen, diatom, charcoal, macrofossil, and magnetic analyses provide the first continuous record of vegetation fire-climate interactions at decadal to centennial time scales during the past 700 years from southern Alaska. Boreal-forest diebacks characterized by declines of Picea mariana, P. glauca, and tree Betula occurred during the LIA (AD 1500-1800), whereas shrubs (Alnus viridis, Betula glandulosa/nana) and herbaceous taxa (Epilobium, Aconitum) expanded. Marked increases in charcoal abundance and changes in magnetic properties suggest increases in fire importance and soil erosion during the same period. In addition, the conspicuous reduction or disappearance of certain aquatic (e.g., Isoetes, Nuphar, Pediastrum) and wetland (Sphagnum) plants and major shifts in diatom assemblages suggest pronounced lake-level fluctuations and rapid ecosystem reorganization in response to LIA climatic deterioration. Our results imply that temperature shifts of 1-2 degrees C, when accompanied by major changes in moisture balance, can greatly alter high-altitudinal terrestrial, wetland, and aquatic ecosystems, including conversion between boreal-forest tree line and tundra. The climatic and ecosystem variations in our study area appear to be coherent with changes in solar irradiance, suggesting that changes in solar activity contributed to the environmental instability of the past 700 years.

Many have demonstrated that anadromous Pacific salmon are significant vectors of nutrients from the ocean to freshwaters. Recently. however, it has been recognized that salmon spawners also input significant quantities of contaminants. The objectives of this paper are to delineate the extent to which salmon-derived nutrients are integrated into the freshwater food web using delta(15)N and delta(13)C and to assess the influence of the salmon pathway in the accumulation of contaminants in rainbow trout (Oncorhynchus mykiss). We found that the delta(15)N and delta(13)C of food web components were related positively and significantly to sockeye salmon (Oncorhynchus nerka) spawner density. Contaminant concentrations in rainbow trout also positively and significantly were related to sockeye salmon spawner density. These data suggest that the anadromous salmon nutrient and contaminant pathways are related and significantly impact the contaminant burden of resident fish.

We investigated polychlorinated biphenyl (PCB) emissions to the environment from a waste treatment and transfer facility over the course of three years. We show that the facility, which undertakes PCB waste consolidation and maintains a low-yield incinerator for products such as light ballasts, acted as a point source for the spatial distribution of PCBs in vegetation. Concurrent air and vegetation sampling was performed to study the relationship between air-vegetation partitioning and the octanol-air partition coefficient (KOA). We show evidence of equilibrium partitioning for lower-chlorinated congeners (log KOA between 7 and 8.5), kinetically limited deposition on plants for intermediate congeners (log KOA between 8.5 and 11), and particle-bound deposition for congeners with high log KOA values (> 11), consistent with the McLachlan partitioning model. From spring to autumn, heavier congeners become much more concentrated in samples farther away from the facility, possibly because of higher temperatures, which enhance dispersal of these congeners. Multivariate principal components analysis showed that PCB composition in vegetation near the emission source most closely resembled the Aroclor mixtures processed by the treatment facility.

Fourteen sediment cores were collected from 10 lakes spanning a large gradient of sockeye salmon returns (0-40 000 spawners km(-2)) in Alaska and British Columbia in 1995-98 and 2002/03. The cores were analyzed for 210Pb to determine sedimentation rates and focusing factors. Polychlorinated biphenyl (PCBs) concentrations in the surface sediments (0-2 cm) were highly correlated with the number of sockeye salmon returns to each nursery lake. For 2002/03, the correlation between PCB concentration and number of salmon spawners was best with no correction factors applied, but decreased considerably when corrected for sedimentation rates, and was improved again by correcting for sediment focusing. Although sigmaPCB concentrations were similar in 1995-98 and 2002/03, the congener patterns varied. Because salmon are the dominant source of PCBs for most of these lakes, variation in sediment congener pattern likely derives from variation in congener patterns carried by the salmon. Overall, total PCB input by salmon has remained relatively constant since 1995. Unlike temperate Great Lakes contaminant studies, the North American west-coast lakes dominated by salmon bio-transport showed no sign of recent decrease in PCBs.

Historical catch records suggest that climatic variability has had basin-wide effects on the northern Pacific and its fish populations, such as salmon, sardines and anchovies. However, these records are too short to define the nature and frequency of patterns. We reconstructed approximately 2,200-year records of sockeye salmon abundance from sediment cores obtained from salmon nursery lakes on Kodiak island, Alaska. Large shifts in abundance, which far exceed the decadal-scale variability recorded during the past 300 years, occurred over the past two millennia. A marked, multi-centennial decline in Alaskan sockeye salmon was apparent from approximately 100 BC to AD 800, but salmon were consistently more abundant from AD 1200 to 1900. Over the past two millennia, the abundances of Pacific sardine and Northern anchovy off the California coast, and of Alaskan salmon, show several synchronous patterns of variability. But sardines and anchovies vary out of phase with Alaskan salmon over low frequency, which differs from the pattern detected in historical records. The coherent patterns observed across large regions demonstrate the strong role of climatic forcing in regulating northeastern Pacific fish stocks.

The effects of climate variability on Pacific salmon abundance are uncertain because historical records are short and are complicated by commercial harvesting and habitat alteration. We use lake sediment records of delta15N and biological indicators to reconstruct sockeye salmon abundance in the Bristol Bay and Kodiak Island regions of Alaska over the past 300 years. Marked shifts in populations occurred over decades during this period, and some pronounced changes appear to be related to climatic change. Variations in salmon returns due to climate or harvesting can have strong impacts on sockeye nursery lake productivity in systems where adult salmon carcasses are important nutrient sources.