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.
ABSTRACT
Canada is home to more lakes than any other nation, but there is a fragmented and limited understanding of the ecological status of these water bodies. Zooplankton are excellent ...bioindicators of lake health, given their central food web position. To date, many studies have investigated the effect of individual stressors on zooplankton communities, mediated through changes in water quality (e.g., macronutrients, temperature, or chemicals). Increasingly, stressors act simultaneously in lakes, often over extended periods of time. As part of the NSERC Canadian Lake Pulse Network project, pelagic zooplankton were sampled in 624 lakes across Canada, spanning six continental drainage basins. We evaluated the effect of 40+ environmental variables on zooplankton diversity and community composition, considering both taxonomic and functional approaches. We also tested specific hypotheses on the relationships between zooplankton communities and environmental conditions, including eutrophication, calcium, chloride, and fish predation. We found that lake morphometry variables were among the most important predictors of zooplankton diversity, while water quality metrics were more critical in explaining variation in community composition. Our results also reveal significant heterogeneity across Canada, with contrasting effects of environmental drivers among continental watersheds, highlighting that response models cannot be assumed to apply universally.
Crustacean zooplankton occupy a key position as primary consumers within lake food webs, acting as early indicators of wider ecosystem shifts that can cascade to higher trophic levels. Their ...communities can be assessed via water column net hauls or via sediment samples containing preserved subfossil remnants from various time periods; the latter probably integrate multiple years of data, habitats within lakes, and seasonal patterns. The degree to which such samples do or do not correspond has never been assessed across a wide variety of lake types and gradients.
As part of a larger sampling of over 600 lakes across lake‐rich Canada, 85 lakes were selected along a gradient of human impact, according to contemporary catchment land use, to analyse cladoceran zooplankton assemblage relatedness among snapshot water column, contemporary and pre‐industrial sediment samples.
We used a combination of ordinations and matrix correlations (RV coefficients), along with partial redundancy analyses, to determine the degree to which the different sample types were comparable and whether they responded similarly to >30 environmental variables. We also compared conclusions obtained using the full cladoceran assemblages versus pelagic‐only taxa and whether our results varied according to degree of human impact.
Overall, while all sample types were correlated, congruence was highest between contemporary and pre‐industrial sediment sample types. This result could arise from the under‐representation of littoral taxa in water column samples relative to those from the sediment. In high impacted catchments, however, we observed greater similarity between water column and contemporary sediment samples, as water quality stressors potentially structure assemblages in parallel directions over time. In these modern‐day sample types and among measured variables, we identified lake depth and pH as the most important structuring variables across Canada.
In this first study comparing cladoceran assemblages in water column, contemporary and pre‐industrial sediments across many lakes, we conclude that water column and sediment samples respond similarly to environmental drivers but should not be considered entirely analogous or redundant.
Aim
While it is now well accepted that human activities are having pronounced effects on natural ecosystems, regional variation in the rate and magnitude of various human impacts is unclear. ...Moreover, the effects of land use change on natural aquatic communities have only relatively recently begun to be explored. Our goal was to understand how and where assemblages of a central food web component of freshwater lakes have changed over the course of industrialization in relation to land use.
Location
Canada.
Time period
Pre‐1880 AD to present.
Major taxa studied
Cladoceran zooplankton.
Methods
As part of the Natural Sciences and Engineering Research Council (NSERC) Canadian Lake Pulse Network, we selected 101 lakes across Canada along a gradient of human impact to analyse subfossil cladoceran assemblages from sediment cores. We examined relationships between taxonomic and functional indicators of cladoceran assemblage change among lakes and through time.
Results
Contemporary assemblages were taxonomically and functionally less diverse locally relative to pre‐industrial assemblages (α‐diversity) and were structured by the degree of human impact in watersheds. Local α‐diversity losses were greatest in highly impacted lakes where agriculture and pasture are prevalent. While spatial homogenization (spatial β‐diversity) did not increase since pre‐industrial times as expected, temporal turnover (temporal β‐diversity) showed a non‐significant but increasing trend in highly impacted lakes, especially in urbanized watersheds.
Main conclusions
Cladoceran assemblages have changed significantly over the course of the Industrial period, and especially in more highly impacted watersheds, underscoring the important role of watershed land use in shaping diversity. However, indicators of cladoceran function have remained relatively conserved over time and land use change showed little impact on β‐diversity, despite important environmental variation. Overall, this research furthers our understanding of the health status of Canadian lakes and the consequences of human activities, especially agriculture and urbanization, on lake zooplankton.
Aim
We examined variation in crustacean zooplankton composition and diversity across Canada, the most lake‐rich country in the world. In addition to α‐diversity patterns, we explore mechanisms behind ...β‐diversity spatial variation, using taxonomic and functional metrics. Our goal was to explore geographical gradients and related mechanisms shaping zooplankton distribution across different spatial scales.
Location
Canada.
Taxon
Crustacean zooplankton.
Methods
Pelagic zooplankton was sampled in and characterized for 624 lakes across Canada, spanning 12 ecozones (defined by climatic, vegetation and geological differences) or 6 continental drainage basins as part of the NSERC Canadian Lake Pulse Network project. We compared taxon and trait distributions, as well as spatial (longitudinal and latitudinal) patterns of community composition and diversity. We computed taxonomic and functional spatial β‐diversity indices, decomposing these into taxon replacement and richness differences. Finally, species (or traits) and lake uniqueness (contributions) to β‐diversity (SCBD and LCBD) were estimated by ecozone and continental basin.
Results
In all, 90 crustacean zooplankton species were identified across the country. Differences in zooplankton taxonomic and functional composition were more distinct when considered by continental basin than by ecozone. α‐diversity varied greatly across space, with greatest diversity in eastern Canada. β‐diversity was greatest when based on taxonomy and was driven by richness differences across all spatial and biodiversity dimensions.
Main conclusions
Structuring factors influencing zooplankton taxon and trait assemblages vary across spatial scales, being more important at the broadest scale considering continental hydrodynamics. Our results point to the combined effects of physical barriers to longitudinal dispersal and climate change in shaping zooplankton taxonomic and functional biogeography across Canadian lakes.
Mercury (Hg) contamination in the environment is a persistent issue as emissions from industry are ongoing and legacy cycling is prolonged. To quantify total mercury (total Hg) and methylmercury ...(MeHg) concentrations in lake sediments across a wide suite of temperate to subarctic lakes, measurements and modeling were conducted for 320 sites across Canada. Total Hg varied from 7 to 567 ng g−1 dry sediment with a mean (median) value of 148 (128) ng g−1 dry sediment, with lowest concentrations observed in eutrophic lakes of the Prairies and Boreal Plains. Average total Hg concentrations have increased twofold in contemporary sediments relative to their preindustrial pair. MeHg concentrations were generally low, with a mean of 0.8 ng g−1 dry sediment and this form representing on average 0.6% of the total contemporary mercury concentrations. Water column variables related to eutrophication were among the strongest predictors of contemporary total Hg, MeHg, and total Hg enrichment in the sediments. In lakes where sediment radiometric data were collected, contemporary total Hg accumulation rates were strongly related to longitude(+) and Hg atmospheric deposition(+). Latitude(−) and Hg atmospheric deposition(+) were the best predictors of MeHg accumulation rates. Chromophoric dissolved organic matter optical properties were key predictors of both total Hg concentrations and accumulation rates. Our work highlights the paramount importance of lake trophic state and the nature of organic matter as key predictors of lake sediment mercury metrics.
Abstract
North temperate and boreal lakes are affected by a complex suite of anthropogenic stressors, yet little is known about how these ecosystems are faring across the most lake‐rich nation, ...Canada. The NSERC Canadian Lake Pulse Network is the first large‐scale national survey of its kind, with the aim of assessing the health of Canadian lakes, determined by the deviation of key variables from baseline pre‐industrial conditions (pre‐1880 CE).
We examined midge remains (chironomids and
Chaoborus
) from modern and pre‐industrial sediment intervals in 67 lakes, spanning a range of sizes, ecozones, and degrees of catchment land‐use change. A marked northwest/southeast divide was evident in the modern chironomid assemblages, which was partially associated with water quality, physiographical and climatic variables, with a small portion of the variance explained by geography alone.
Between the pre‐industrial and modern sediment intervals, taxa typically found in warmer waters increased across the country (
p
< 0.001); this pattern also was evident in the littoral assemblages (
p
= 0.01), providing additional support for the influence of warming.
Turnover in chironomid assemblages varied across trophic classes. Mesotrophic lakes showed increases in macrophyte‐associated taxa, whereas eutrophic lakes showed declines in littoral taxa, perhaps reflecting increased turbidity, and the increase of low‐oxygen‐tolerant profundal taxa in eutrophic lakes with high human impacts.
Overall, the distribution and dynamics of chironomid assemblages across Canadian lakes appear to be strongly linked to changes in climate and deep‐water oxygen conditions.
The development of cyanobacteria blooms is of increasing concern in many lakes worldwide, and as a result, modeling their predictors is vital for understanding where and why they occur. In this ...study, we developed and analyzed a 640‐lake data set that spans Canada and 12 ecozones to identify the drivers of cyanobacteria biomass and of several key toxin‐ and bloom‐forming genera (Microcystis, Aphanizomenon, and Dolichospermum). The database consisted of an exhaustive list of potential predictors (n = 55), including water chemistry, land‐use, and zooplankton variables. We applied a series of empirical modeling approaches to identify significant predictors and thresholds (generalized linear and additive models, mixed effect regression trees), all while accounting for ecozone variability. Across all modeling approaches, and ecozones total phosphorus was identified as the most important predictor of total cyanobacterial and focal genera biomass. In addition, cyanobacteria across Canada showed significant associations with increasing dissolved organic and inorganic carbon, and several ions. Despite the widely held notion that cyanobacteria are often toxic and/or a poor food source for zooplankton, we found a positive relationship between cyanobacteria and zooplankton, particularly with daphnid and copepod biomass. Localized top‐down forces and evolutionary adaptations resulting from long‐term exposure in eutrophic lakes are among the possible explanations for this observed positive association. By considering a suite of complementary modeling approaches, we found that nonlinear models provided greater predictive power and the random ecozone effect was minor due to the overarching importance of local abiotic and biotic factors.
Freshwater ecosystems are increasingly exposed to co‐occurring anthropogenic stressors that can alter food web interactions and organismal life histories. We examined the individual and combined ...effects of climate warming and microplastic pollution on the growth rate and predatory performance of an invasive freshwater fish, the round goby (Neogobius melanostomus). In temperature‐controlled chambers, we exposed 160 juvenile gobies to one of six scenarios over 37 d, combining three environmentally relevant concentrations of microplastics (63–75 μm polyethylene microbeads) with two temperature regimes representing contemporary (18°C) and projected mean summer maxima (26°C) in their current range in the Great Lakes–St. Lawrence River basin. Exposure to elevated temperature reduced the growth and predatory performance of round gobies. Their decline in predatory performance was greatest at the highest microplastic concentration, regardless of temperature. The effects of environmentally relevant microplastic concentrations on the growth and performance of gobies were weaker than the effects of thermal stress. Given that the round goby is an abundant and widely distributed bottom‐dwelling fish in nearshore areas of the Great Lakes–St. Lawrence River basin, its responses to these co‐occurring stressors could have cascading effects on food webs.
Lake sedimentation rate represents a synthetic metric of ecosystem functioning. Many localized studies have reported a significant association between land use/land cover changes and lake sediment ...mass accumulation rates, with a few global syntheses echoing these findings at larger scales. In the literature, studies evaluating lead‐210 (210Pb) for establishing sediment chronologies will report at least one of three dating models, but the constant rate of supply (CRS) model is the most widely used. However, it is often unclear how or why this model is selected, despite its influence on the interpretation of many subsequent analyses about ecosystem dynamics and functioning. It would thus be advantageous to design an objective and semi‐automated way of choosing among dating models. We measured radioisotopic activities in 37 sediment cores across four ecozones of Eastern Canada and developed an approach to assess model fit for the three commonly applied dating models. The derived chronologies were then used to evaluate the spatial and temporal variation in sedimentation rates across four ecozones in Canada (covering a surface area of 2.2 × 106 km2). We observed a recent increase in lake sedimentation rates across most lakes, as has been observed globally, albeit with significant differences in the magnitude of sedimentation rates across ecozones. Across all lakes, we found that regional human population counts and mean annual air temperatures were significant temporal predictors of variation in mass accumulation rates. Overall, this analytical framework offers an objective approach for assessing fit and selecting among sediment age models, which contributes to a more robust quantification of sedimentation rates. With this first application, we provide a quantitative assessment of how lake sedimentation rates have varied across a northern lake‐rich region and have responded to environmental change.
We developed a methodological framework for assessing the fit of three of the main sediment dating models to measured 210Pb radioisotopic activities from cores from Eastern Canada. Using this method, we were able to track recent accelerations in lake sediment mass accumulation rates recorded in all four ecozones that we investigated. This acceleration was correlated with the intensity of anthropogenic activities within lake watersheds, such as human population growth and agricultural land use.
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