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Microplastics, i.e., plastic particles in the size range of planktonic organisms, have been found in the water columns and sediments of lakes and rivers globally. The number and mass of plastic particles drifting through a river can exceed those of living organisms such as zooplankton and fish larvae. In freshwater sediments, concentrations of microplastics reach the same magnitude as in the world’s most contaminated marine sediments. Such particles are derived from a unique biogeochemical cycle that ultimately influences productivity, biodiversity, and ecosystem functioning. Furthermore, microplastics act as vectors of toxic substances to invertebrates, fishes, herpetofauna, and waterfowl. We contend that the concentration of this distinct particle component is an ecologically significant parameter of inland waterbodies because of its ubiquity, environmental persistence, and interactions with key ecological processes. No environmental field survey that has searched for microplastics has yet failed to detect their presence. Standardized limnological protocols are needed to compare spatio-temporal variation in the concentration of microplastics within and across watersheds. Data obtained from such protocols would facilitate environmental monitoring and inform policy for managing plastic waste; furthermore, they would enable more accurate modeling of contaminant cycling and the development of a global plastic budget that identifies sources, distribution and circulation pathways, reservoir size, and retention times.

Covering 55% of Canada’s total surface area and stretching from coast to coast to coast, the Canadian boreal zone is crucial to the nation’s economic and ecological integrity. Although often viewed as relatively underdeveloped, it is vulnerable to numerous stressors such as mining, forestry, and anthropogenic climate change. Natural archives preserved in lake sediments can provide key insights by quantifying pre-disturbance conditions (pre-1850 CE) and the nature, magnitude, direction, and speed of environmental change induced by anthropogenic stressors over the past ~150 years. Here, we paired a review of paleolimnological literature of the Canadian boreal zone with analyses of published sediment core data to highlight the effects of climate change, catchment disturbances, and atmospheric deposition on boreal lakes. Specifically, we conducted quantitative syntheses of two lake health indicators: elemental lead (Pb) and chlorophyll a. Segmented regressions and Mann-Kendall trend analysis revealed a generally increasing trend in elemental Pb across the boreal zone until ~1970 CE, followed by a generally decreasing trend to the present. Snapshot comparisons of sedimentary chlorophyll a from recent and pre-industrial sediments (i.e., top-bottom sediment core design) revealed that a majority of sites have increased over time, suggesting a general enhancement in lake primary production across the boreal zone. Collectively, this body of work demonstrates that long-term sediment records offer a critical perspective on ecosystem change not accessible through routine monitoring programs. We advocate using modern datasets in tandem with paleolimnology to establish baseline conditions, measure ecosystem changes, and set meaningful management targets.

Little is known about the current state of freshwater biodiversity in Canada, one of the countries with the greatest amount of surface waters in the world. To address this knowledge gap, we compiled a list of all available assessments of conservation status for freshwater species (over 3000 taxa) and further evaluated the overall status of six distinct taxonomic groups, focusing on organisms reliant on fresh waters (i.e., aquatic plants, invertebrates (with a focus on freshwater mussels), fishes, herpetofauna (reptiles and amphibians), birds, and mammals). Overall, 11.7% of all freshwater species of plants and animals assessed were found to be “at risk” (i.e., listed as “Threatened”, “Endangered”, or “Extirpated”) and 17.9% identified as “Special Concern”. We found that 37.9% of species lacked sufficient data to enable their status to be assessed. Data gaps in Canada’s assessment of its freshwater species were most prevalent in invertebrates (excluding freshwater mussels). Given the alarming state of freshwater biodiversity in Canada, we conclude by providing recommendations that focus on evaluating temporal trends and informing conservation actions.

Analyses of sedimentary DNA (sedDNA) have increased exponentially over the last decade and hold great potential to study the effects of anthropogenic stressors on lake biota over time. Herein, we synthesise the literature that has applied a sedDNA approach to track historical changes in lake biodiversity in response to anthropogenic impacts, with an emphasis on the past c. 200 years. We identified the following research themes that are of particular relevance: (1) eutrophication and climate change as key drivers of limnetic communities; (2) increasing homogenisation of limnetic communities across large spatial scales; and (3) the dynamics and effects of invasive species as traced in lake sediment archives. Altogether, this review highlights the potential of sedDNA to draw a more comprehensive picture of the response of lake biota to anthropogenic stressors, opening up new avenues in the field of paleoecology by unrevealing a hidden historical biodiversity, building new paleo-indicators, and reflecting either taxonomic or functional attributes. Broadly, sedDNA analyses provide new perspectives that can inform ecosystem management, conservation, and restoration by offering an approach to measure ecological integrity and vulnerability, as well as ecosystem functioning. © 2022 The Authors. Freshwater Biology published by John Wiley & Sons Ltd.

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.

Despite decades of research and mitigation efforts, declines in freshwater quality resulting from anthropogenic nutrient input remain a persistent issue worldwide. Canada has the greatest number of freshwater lakes in the world, yet we have a limited understanding of the magnitude and scale at which most lakes have been affected by human activities, namely Land Use/Land Cover (LULC) alterations. In response, the NSERC Canadian Lake Pulse Network has compiled the first nationwide systematic database of lake quality metrics by surveying 664 lakes across 12 ecozones over three years. To assess the influence of catchment development on water quality and its spatial variation, we built models quantifying the association between watershed LULC and water quality. We found that agricultural and urban land use explained the greatest proportion of variation in water quality among LULC categories (R2= 0.20–0.29). Overall, our study highlights that drivers of water quality are similar across regions; however, baseline conditions vary, so freshwater ecosystem management strategies must consider their geographic context to better predict where water quality thresholds will be surpassed.

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.

Environmental decision-makers and practitioners need and deserve high-quality environmental evidence for effective decision-making. We collate and share a suite of best practices for applied environmental researchers to support their capacity to inform such decision-making processes. This raises a number of important questions: What does “relevant” and informative evidence look like? How do we know when evidence has been applied? We assembled an experienced team of knowledge generators and users in Canada to identify insights that have emerged from their work and that could serve as guideposts for others who seek to apply environmental research to policy challenges. By reflecting on successes and failures, we define “success” in applied environmental science as respectfully conducted, partner-relevant research that is accessible, understandable, and shared and that can create opportunities for change (e.g., in policy, behaviour, management). Next, we generated a list of best practices for delivering “successful” applied environmental research. Our guidance emphasizes the importance of engaging early and often, in a respectful manner, with partners, generating high-quality, relevant research (which requires flexibility), having a plan for communicating and sharing outputs, and being transparent about uncertainties and limitations. Other important considerations include acknowledging partners for involvement and training early career researchers in applied partnership research. Finally, we generated a list of specific, measurable indicators for evaluating success, including quality and quantity of scientific outputs, the relationship with the partner(s), relevance and connectedness of the research, accessibility and availability of outputs to users, provision of outputs that are digestible and usable by different audiences, training and capacity building, and ultimate outcomes (e.g., including social, environmental, and economic outcomes, as well as partner satisfaction). We encourage those embarking on applied environmental research to consider embracing the strategies, to continuously reflect on progress toward shared research goals, and to be flexible. Doing so will increase the likelihood of delivering research that is “successful” and in doing so contribute to overcoming and addressing environmental issues and problems.

Lakes worldwide are under threat by a myriad of environmental stressors that have been increasing in number and magnitude. These stressors can be regional such as climate change, or local such as nutrient-rich runoff, invasive species, and road salt contamination, to name but a few. To protect lake ecosystems from further deterioration, we need long-term data to define pre-disturbance baselines and to identify stressors that are causing the greatest ecological changes. Paleolimnology is an effective approach to reconstruct limnological history, providing an important window of lake changes through time. Here, we applied paleolimnological tools to explore the pre-industrial and contemporary diatom assemblage changes of 27 lakes located in the most populated ecozone in Canada, the Mixedwood Plains. We also examined a full sediment core for Lac des Chicots (Southern Québec), aiming to disentangle the impacts of natural versus anthropogenic interactions and to assess their relative effects on the lake’s biotic structure. Our ordination analysis suggests that the Mixedwood Plains lakes have experienced varying lake-specific ecological changes over the past ~ 150 years, with two major trends across most study lakes: (1) a prevalent increase in planktic species, and (2) a rise in mesotrophic/eutrophic taxa in lakes receiving high human impacts. Our case study of Lac des Chicots identifies ecological impacts from both historical natural events and recent human activities, such as cultural eutrophication and climate warming. Overall, our study demonstrates that lakes in the Mixedwood Plains ecozone have experienced marked ecological changes that are mainly associated with human impacts.

Protists play key roles in aquatic food webs as primary producers, predators, nutrient recyclers, and symbionts. Yet, 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 extent covering nearly 8.4 million km2 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 at smaller scales. Our study exposes the importance of both within-lake and external watershed characteristics in explaining protist diversity and biogeography, critical information in further developing an understanding of how freshwater lakes and their watersheds are impacted by anthropogenic stressors.ImportanceFreshwater 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.

Peri-urban lakes offer many valued ecosystem services, but their vulnerability to climate change and anthropogenic disturbances increases with increasing human populations the effects and interactions of multiple stressors on lakes can lead to unexpected outcomes, affecting societal and ecological values, it is necessary to evaluate ecosystem trajectories and respective drivers in peri-urban lakes. Better management practices could thus be applied to preserve ecosystem services of peri-urban lakes. We conducted a multi-proxy paleolimnological study on Cultus Lake, British Columbia, a Canadian peri-urban lake experiencing cultural eutrophication, to reconstruct a comprehensive ecological trajectory of the lake over the past ~ 200 years. We also integrated historical data as well as historical archival information to identify the potential drivers of the changes. We identified ca. 1800–1900 CE as a reference period, reflected in muted variations across most paleo-indicators. Minor increases in sedimentary δ15N ca. 1880–1940 CE coincided with the onset of anthropogenic modifications to the Cultus Lake watershed. Signs of early eutrophication were evident by ca. 1940 CE, as indicated by increases in all sedimentary pigments. By ca. 1970–1990 CE, elevated concentrations of sedimentary cyanobacterial pigments and changes in diatom species assemblages highlighted the potential interactive effects of multiple stressors, including cultural eutrophication, climate warming and declines in the endangered Cultus Lake sockeye salmon population. Recent (ca. 1990–2008 CE) declines in sedimentary pigments and increases in cladoceran fluxes suggested an increase in top-down control of the lake food web. From the collection of changes observed in the past ~ 200 years in our study, it is clear that Cultus Lake and its associated ecosystem services would benefit from abatement of nutrient loadings from terrestrial and atmospheric sources. Our study emphasizes the complexity and interactivity of drivers in peri-urban lake ecosystems and the necessity of long-term perspectives to contextualize modern ecological conditions to inform lake and watershed management.

Stoco Lake (Tweed, Ontario, Canada) has a history of industrial contamination and is heavily influenced by inflow from the Moira River. Stoco Lake is frequently affected by nuisance algal blooms (including cyanobacteria), which have largely been attributed to cultural eutrophication. To further our understanding of the environmental dynamics of Stoco Lake, we applied paleolimnological techniques to examine long-term trends in subfossil invertebrate assemblages, geochemistry, and inferred trends in whole-lake primary production from a sediment core representing ~ 70 years of environmental history. We examined past trends in Simuliidae abundance, geochemical variables, and historical river flow records to examine past hydrological conditions in Stoco Lake. Inferred whole-lake primary production and sediment core organic content increased between the late 1950s and early 1970s, likely reflecting increased eutrophication (e.g. from agricultural intensification and increased catchment activities), with some stabilization in the post-1970s sediments coinciding with the installation of a sewage treatment plant. The geochemical composition of the core highlighted the decrease in catchment-derived sediment inputs and an increase in percentage of organic content towards the top of the core. In addition, declines in the proportion of As and Ni coincided with the closure of the upstream Deloro Mine Site in the 1960s. Chironomidae and Chaoboridae (Diptera) assemblages indicate expansion of littoral habitat after the 1960s and a general stability of low-oxygen conditions in the hypolimnion over the past ~ 70 years. Several Simuliidae (Diptera: Nematocera) remains, indicators of fluvial conditions rarely discussed in paleolimnological studies, were present at their highest abundances in the pre- ~ 1975 sediments of the core, but disappeared after the early 1990s. Our findings suggest that changes in river flow (discharge, velocity, or seasonality) may have contributed to recent algal blooms. Overall, this study highlights the potential of Simuliidae in paleohydrological studies to help understand the role of river flow on lake conditions.

Horizon scanning is a systematic approach increasingly used to explore emerging trends, issues, opportunities, and threats in conservation. We present the results from one such exercise aimed at identifying emerging issues that could have important scientific, social, technological, and managerial implications for the conservation of inland waters in Canada in the proximate future. We recognized six opportunities and nine challenges, for which we provide research implications and policy options, such that scientists, policy makers, and the Canadian society as a whole can prepare for a potential growth in each of the topic areas we identified. The issues spanned a broad range of topics, from recognizing the opportunities and challenges of community-enabled science and the need to consider the legal rights of nature, to the likely increase of pharmaceuticals in wastewater due to an aging population. These issues represent a first baseline that could help decision makers identify and prioritize efforts while simultaneously stimulate new research avenues. We hope our horizon scan will pave the way for similar exercises related to the conservation of biodiversity in Canada.

International audience; Sediment Accumulation Rate (SAR; measured as mm yr(-1)) and Mass Accumulation Rate (MAR; measured as g cm(-2) yr(-1)) data were collected from published lake core records that spanned the past similar to 150 years, from approximately 500 sites worldwide. For each lake, key watershed characteristics including watershed size, slope, land use and climate were extracted, with the goal of quantifying the relative importance of these variables as drivers of SAR and MAR. General additive models provided evidence of accelerated global lake sediment infilling rates after AD 1950. Whereas the onset of sedimentation acceleration varied across ecoregions, global lake MAR values were found to have increased similar to threefold on average, since baseline conditions pre-1900 (i.e. mu(baseline) = 0.040 +/- 0.044 g cm(-2) yr(-1) and mu(modern) = 0.13 +/- 0.22 g cm(-2) yr(-1)). The significant drivers, identified through Linear Mixed Effect modeling of MAR time series, were watershed population density (log-transformed) and watershed cropland density (log-transformed). Our results highlight important spatial heterogeneity in SAR and MAR among lakes, precluding the use of simple modeling approaches. SAR and MAR were found to be moderately correlated to one another, despite the potential for post-depositional disparities between segments of the sediment cores. We identified organic matter content (loss-on-ignition, LOI) as a significant co-variate that could be used to correct inflated very recent SAR rates. Our empirical analyses suggest that, despite a wide range of natural variability among lakes, both SAR and MAR increased globally and the increases appear to be mainly the result of enhanced watershed activities associated with agriculture and urbanization.

Molecular genetic approaches applied to environmental DNA have great potential for biodiversity research and ecosystem monitoring. A metagenome contains genetic information from all organisms captured in an environmental sample. It has been primarily used to study bacteria and archaea, but promising reports focusing on metazoan diversity are emerging. However, methodological uncertainties remain, and studies are required to validate the power and the limitations of such an approach when applied to macro-eukaryotes. Here, we analyzed water sample metagenomes to estimate zooplankton diversity in 22 freshwater lakes across Eastern Canada. We tested the coherence of data based on morphologically identified zooplankton taxa and molecular genetic data derived from shotgun sequencing of environmental DNA collected at the same time. RV coefficients showed a significant correlation between the relative abundance of zooplankton families derived from small subunit rRNA genes extracted from the metagenomes and morphologically identified zooplankton. However, differences in congruence with morphological counts were detected when varied bioinformatic approaches were applied to presence-absence data. This study presents one of the first diversity assessments of a group of aquatic metazoans using metagenomes and validates the coherence of the community composition derived from genetic and classical species surveys. Overall, our results suggest that metagenomics has the potential to be further developed to describe metazoan biodiversity in aquatic ecosystems, and to advance this area we provide key recommendations for workflow improvement.

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.

Freshwater biodiversity loss is one of the greatest environmental threats in our changing world. Although declines have been reported extensively in the literature, much less attention has been devoted to solving the freshwater biodiversity crisis relative to other ecosystems. The recently proposed Emergency Recovery Plan for Freshwater Biodiversity (Tickner et al., 2020, BioScience, 70(4), 330–342) outlines an ambitious but necessary set of overarching actions that can help “bend the curve” for freshwater biodiversity declines. This plan is timely given the present opportunity to adjust freshwater biodiversity targets in international biodiversity agreements and to encourage meeting targets of relevant Sustainable Development Goals. Yet, relying solely on a trickle down from such agreements to national and local scales will likely take too long, given the immediate urgency of the situation. Here, we advocate for a broader, concerted effort from all actors to ensure the Emergency Recovery Plan meaningfully influences the actions of practitioners at a local scale. We outline the roles and responsibilities of actors involved with policy, research, professional bodies and societies, advocacy, and industry, as well as practitioners themselves, in achieving this goal. It is our hope that this overview facilitates the real‐world actions needed to execute the Emergency Recovery Plan so that we can indeed “bend the curve” for freshwater biodiversity.

Multiple stressors affect water quality and biodiversity in lakes worldwide. However, our understanding of which combinations of stressors are of greatest impact and how lakes have shifted from their pre-industrial baselines is fragmented. Questions remain regarding how multiple trophic groups are affected by global change stressors and whether region-specific differences need to be considered. Here, we apply multiple factor analyses together with latent variable modeling to quantify potential interactions within and among taxa that span multiple trophic levels (i.e. diatoms, cladocerans and chironomids) to improve our understanding of paleo-environmental dynamics from a suite of Canadian lakes spanning four ecozones: the Boreal Shield, the Mixedwood Plains, the Atlantic Highlands, and the Atlantic Maritimes. Across all ecozones, multi-trophic assemblages were distinguished in a multiple factor analysis along a land-use gradient, with mesotrophic/eutrophic diatoms and profundal chironomids tolerant of low bottom-water oxygen concentrations recorded in more disturbed sites. Functional units across three indicator groups formed distinct networks of co-responses to key environmental and land-use gradients, although cladocerans seemed to be driven by additional (i.e., residual) gradients. Applying a temporal beta-diversity approach between modern and pre-industrial assemblages, we detected significantly greater turnover of diatom functional groups in high versus low human impact sites (p = 0.03, n = 57), with similar but non-significant trends apparent with the chironomid and cladoceran groups. Ecozonal differences in temporal turnover were also evident in diatom (p = 0.003, n = 57) and chironomid functional groups (p = 0.04, n = 41). The ecozonal differences may be partially due to differential sensitivities of the pre-industrial assemblages and may also be driven by historical disturbances. Our work highlights how the magnitude and direction of ecological change vary across ecozones and can modify responses to human impacts, with a general trend of higher species turnover in sites with greater human disturbance.

Dams are recognized to impact aquatic biodiversity and ecosystem functions, but the magnitude of effects vary across studies. By using a meta-analytical approach, we examined the effects of impoundment on fish community 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 reservoirs, and no changes in boreal reservoirs. Our analysis also showed that non-native species increased significantly in tropical and temperate reservoirs, but not in boreal reservoirs. In contrast, temporal trajectories in fish assemblage metrics were common across regions, with all biomes showing an increase in mean trophic 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 factors or 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) invasive or non-native species. Collectively our results highlight that an understanding of the regional context and a suite of metrics are needed to make robust predictions about how fish will respond to river impoundments.

International audience; Ponds and shallow lakes are sensitive to eutrophication, which often increases turbidity and produces toxic algal blooms. Previous research has found that the presence of macrophytes in shallow water bodies can mitigate the effects of nutrient loading by promoting water clarity and inhibiting phytoplankton growth. However, there is still little known about how macrophyte abundance modifies the response of phytoplankton taxonomic assemblages, particularly their functional composition, to nutrient loading. We investigated how macrophyte abundance altered phytoplankton assemblages using experimental ponds that were treated with a nutrient press. Natural communities of phytoplankton, zooplankton, and fish were seeded in ponds with one of three levels of macrophyte abundance that received weekly nutrient additions. A total of nine ponds (each treatment in triplicate) were monitored for 76 days. Analyses of Shannon's diversity and functional dispersion revealed significant variation in phytoplankton assemblage composition among macrophyte treatments and through time. Phytoplankton biovolume, community metrics, and the dominant traits were generally more stable in the high macrophyte treatment ponds. Although nutrient loading did not induce a regime change to a turbid, phytoplankton-dominated state, phytoplankton assemblages did shift to a dominance of larger, inedible taxa and metaphyton became more abundant in ponds with macrophytes and zooplankton assemblages shifted to smaller species. Our study demonstrates how functional traits and functional diversity metrics can provide richer insights into the dynamics and causes of shifts in shallow freshwater ecosystems. Macrophyte densities clearly played a significant role in shaping phytoplankton assemblages. Based on the functional traits of phytoplankton distinguished among treatments, we inferred that competition for light, grazing, and, probably, allelochemicals were mechanisms through which macrophytes mediated changes in phytoplankton assemblages.

Hydrological regime alteration in reservoirs is a major anthropogenic disturbance in aquatic ecosystems. We examined winter drawdown effects on macroinvertebrate abundance and community composition in reservoirs while accounting for localized physical variables that might explain additional variability. We hypothesized that drawdown would affect the abundance and the community composition of macroinvertebrates because of the exposure of organisms to freezing and desiccation. However, we expected the detection and magnitude of these responses to be conditional on localized physical habitat variables (i.e., fetch, thermal condition and slope), which also influence abundance and community composition of macroinvertebrates. To test these hypotheses, we applied generalized linear mixed effects models and multivariate analyses to data collected from 118 samples in 15 temperate reservoirs. Sampling stations from reservoirs that experienced high winter drawdown and/or were below the thermocline showed significantly lower macroinvertebrate abundances. Changes in community composition across sites were muted as chironomids, oligochaetes and sphaeriids dominated all assemblages. Decreases in macroinvertebrate abundances with drawdown amplitude and lower temperatures (i.e., hypolimnion) could have implications for food web structure, as they substantially contribute to fish diet, and ecosystem functions (e.g., nutrient cycling).