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The St. Lawrence River at Cornwall, Ontario, Canada, received substantial inputs of mercury from local, shoreline‐based industries through much of the 20th century. Although emission controls were implemented in the late 20th century to reduce the influx of mercury and other metals entering the river, legacy contamination of riverine sediments continues to be a concern. Monitored natural recovery was prescribed in 2005 to remediate contaminated sediments; however, few surveys have been undertaken to examine its effectiveness on shallow, nearshore sediments in contaminated areas. Surface sediments were collected at shallow, nearshore sites in contaminated zones and upstream reference areas to evaluate the current state of sedimentary contamination of mercury and other metals. A Getis–Ord Gi* "hot spot" analysis was employed to assess the spatial distribution of contaminants. In addition, 3 sediment cores were collected from contaminated zones and dated using radioisotopes (210Pb) to assess sedimentation patterns over time. Results indicated that surface sediments from contaminated zones remained elevated in mercury relative to reference sites but spatial distribution of contaminants was highly heterogeneous. Dated sediment cores suggested that sedimentation was not occurring consistently across all areas; variable sedimentation and resuspension patterns over small spatial scales were likely factors driving heterogeneous sedimentary contamination. Such patterns complicate remediation strategies because unburied sediments may serve as continuing sources of contaminants to the ecosystem. Environ Toxicol Chem 2021;40:1788–1799. © 2021 SETAC [ABSTRACT FROM AUTHOR]

Cladocera serve as important bio‐ and paleo‐indicators of lake food webs and environmental conditions. The ecological optima of cladocerans are often established by regional‐scale calibration sets, with subsequent comparisons to limnological variables. However, due to logistical constraints when sampling large numbers of lakes, this approach often limits the length of the environmental gradients that can be examined. To extend spatial and limnological gradients, we combined 20 datasets (388 lakes) containing both cladoceran and environmental data, spanning multiple ecoregions across Canada and Alaska. These data were collected over c. 20 years using similar techniques in a single laboratory. Our continental‐scale analysis examined the main chemical and physical variables that influenced cladoceran assemblages, and identified critical environmental thresholds structuring assemblages.Multivariate analyses were used to examine the influences of six environmental variables (depth [either maximum depth or coring depth, which are very similar], surface area, pH, calcium [Ca], total phosphorus [TP], and dissolved organic carbon) and ecoregion classifications on cladoceran assemblages. Gradients of pH and Ca were strongly related. Daphnia longispina spp. and Chydorus brevilabris/biovatus were associated with higher pH and Ca concentrations, while Bosmina spp. and Holopedium spp. were more common in lakes with lower pH and Ca. Dissolved organic carbon was highly correlated with TP. Chydorus brevilabris/biovatus was associated with higher‐nutrient systems. Littoral chydorids, D. longispina spp., Holopedium spp., and Daphnia pulex spp. were associated with intermediate TP and dissolved organic carbon concentrations, and Bosmina spp. was more closely associated with oligotrophic systems.Physical limnological variables influenced taxonomic composition on a continental scale. For example, D. longispina spp., D. pulex spp., Bosmina spp., and Holopedium spp. were associated with larger and/or deeper systems, and littoral chydorids were associated with smaller and/or shallower lakes. A multivariate regression tree identified two thresholds important for structuring assemblages based on pH (<7 and ≥7, probably closely tied to lakewater Ca concentrations) and depth (<6.45 m and ≥6.45 m at pH < 7, and <4.75 m and ≥4.75 m at pH ≥ 7).Our results highlight the sensitivity of several cladoceran taxa to multiple chemical and physical gradients. We demonstrate that the ecological responses of cladocerans to environmental variables—often established through regional‐scale surveys—are applicable across ecoregions and broad limnological gradients, reinforcing the value of cladocerans as bioindicators of key chemical and physical variables in freshwaters and as paleoindicators for assessing human impacts on aquatic systems through time. [ABSTRACT FROM AUTHOR]

Abstract: A sediment core was obtained from Moira Lake to study the legacy of contamination and remediation at the Deloro industrial site which includes 95-years of operations involving gold mining, mineral processing, and arsenic-based pesticide production resulting in high levels of arsenic, cobalt, and nickel. A timeline for the sediment core was established by 210Pb dating and used to evaluate the geochemical record and the impact on primary production and subfossil cladocerans. In the early 1800s, there was an initial increase in the arsenic, cobalt and nickel concentrations due to industrial development. By the 1850s, the rate of enrichment increased due to the conglomeration of small-scale operations. In the 1960s, the concentrations of those metal(loid)s decreased following the cessation of the industrial activity at Deloro and the initiation of a clean-up effort. Primary production, inferred by chlorophyll-a concentrations, initially decreased as the metal(loid)s concentrations increased. This was followed by a recovery of the chlorophyll-a concentrations and further increases in production to higher levels than recorded prior to the Deloro years. Secondary production, inferred by cladoceran assemblage structure, was initially dominated by bosminids. The assemblage then changed to one dominated by chydorids and daphnids with the change occurring contemporaneous with the change in chlorophyll-a. However, the changes in primary and secondary production occurred during the period of accelerated metal(loid) enrichment, suggesting limited impact of contamination on primary and secondary producers. Loss on ignition results revealed that during the period of accelerated arsenic enrichment, the carbonate content of the sediments increased while the percent organic content decreased. This work contributes to ongoing research to establish the environmental legacy of historical industrial activities within complex ecosystems. Furthermore, the combination of geochemical (i.e. 210Pb, ICP-OES, XANES) and ecological analysis provides a more complete picture of the complex interactions that have occurred in Moira Lake. Graphical abstract Image 1 Highlights • The effects of the Deloro site were evaluated using Moira Lake sediment records. • Elevated trace metals concentrations corresponded to Deloro years. • Primary and secondary production proxies exhibited shifts. [ABSTRACT FROM AUTHOR]

As the industrial footprint of the Athabasca Oil Sands Region (AOSR) continues to expand, concern about the potential impacts of pollutants on the surrounding terrestrial and aquatic ecosystems need to be assessed. An emerging issue is whether recent increases in lake production downwind of the development can be linked to AOSR activities, and/or whether changing climatic conditions are influencing lake nutrient status. To decipher the importance of pollutants, particularly atmospheric deposition of reactive nitrogen (Nr), and the effects of climate change as potential sources of increasing lake production, lakes from both within and outside of the nitrogen deposition zone were analyzed for historical changes in diatom assemblages. Lake sediment cores were collected from a priori defined nitrogen (N) - and phosphorus (P) - limited lakes within and outside the N plume associated with the AOSR. Diatom assemblages were quantified at sub-decadal resolution since ca. 1890 to compare conditions prior to oil sands expansion and regional climate warming, to the more recent conditions in each group of lakes (Reference and Impacted, N- and P-limited lakes). Analyses of changes in assemblage similarity and species turnover indicates that changes in diatom assemblages were minimal both within and across all lake groups. Small changes in percent composition of planktonic taxa, particularly small centric taxa (Discostella and Cyclotella species) and pennate taxa, such as Asterionella formosa and Fragilaria crotonensis, occurred in some of the lakes. While these changes were consistent with potential climate effects on algal growth, water column stability and other factors; the timing and direction of biotic changes were variable among sites suggesting that any apparent response to climate was lake dependent. The absence of a consistent pattern of diatom changes associated with receipt of reactive nitrogen or intrinsic nutrient-limitation status of the lake suggest that downwind AOSR emissions had no demonstrable effect on diatom composition. [ABSTRACT FROM AUTHOR]

Charcoal accumulation rates and fire-return intervals were calculated from total charcoal and charcoal morphotypes over the Holocene, from a well-dated sediment core from Lake 239 located in north-western Ontario, and compared with previously published independent climate reconstructions. Both total and morphotype analysis show a two-to-three fold increase in accumulation rates in the early-to-mid Holocene (range: 1 to 6 pieces cm[sup -2] year[sup -1]) compared with the early and late Holocene (range: 0 to 2 pieces cm[sup -2] year[sup -1]). Fire-return intervals and fire frequencies calculated during these periods, based on peak analysis, showed very different trends. The fire-return interval based on Type M charcoal, a morphotype associated with primary charcoal deposition, was high during the early and late Holocene, and low from ~7500 to 4000 cal year BP, with high inferred fire frequency during the warm mid-Holocene (~12.5 fires per 1000 years), compared with <5 fires per 1000 years over the rest of the Holocene, whereas fire-return interval and fire frequency based on total charcoal did not show patterns consistent with climate. These results suggest that a two- to three-fold increase in fire frequency would not be unexpected in the future under a predicted warmer climate. Total charcoal and charcoal morphotypes were used to assess changes to fire regimes in NW Ontario - increases during the mid-Holocene warm period suggest increased fire activity. Charcoal morphotypes are consistent with independent climate reconstructions, unlike total charcoal. Findings suggest a two- to three-fold increase in reconstructed fire frequency. [ABSTRACT FROM AUTHOR]

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Diatom-based paleolimnological techniques were used to reconstruct lake acidification trends in 51 low-alkalinity Nova Scotia lakes that spanned gradients of dissolved organic carbon (DOC) concentrations and sulphate deposition. Pre-industrial, diatom-inferred pH values of these lakes were <6.8, with 31 lakes having pre-industrial pH < 6.0 and two lakes having pH < 5.5. Lakes in Kejimkujik National Park documented the greatest pH decline (–0.4 pH unit (±0.2)) since the 19th century, whereas those in northern parts of the province (e.g., Cape Breton Highlands National Park) experienced little or no acidification, with a net mean pH decline = –0.1 pH unit (±0.2). While the sulphate deposition and diatom-inferred pH changes have not been as great as those observed in other acidified areas of northeastern North America (e.g., Adirondack region of New York or New England), Nova Scotia lakes have experienced biological changes toward more acidophilous diatom assemblages, especially in lakes with low pre-industrial pH values (currently with high DOC concentrations) located in Kejimkujik National Park, which receives the highest loading of sulphate deposition in Nova Scotia. However, the generally low pre-industrial pH values inferred for most of the study lakes suggest that many of these lakes were somewhat naturally acidic, but acidified further as a result of atmospheric deposition. [ABSTRACT FROM AUTHOR]

Abstract: Quantitative analyses of variations in morphological features of charcoal were undertaken in a 210Pb-dated sediment core from Prosser Lake (British Columbia, Canada). Seven morphological types of charcoal were defined by particle shape, major–minor axis ratio, apparent porosity and progradation to unburned material. The distribution of morphotypes and total charcoal abundances were assessed as a proxy for fire events recorded between 1919 and 2000 and to subsequent mechanisms of transportation–sedimentation to lake sediments. Charcoal morphotypes showed distinct relationships to recorded area burned by fires. Fragile charcoal fragments with highly irregular porosity (termed Type M) displayed the strongest correlation to burned area (r 2 = 0.51; P = 0.0001) and did not produce any false-positive signal for fires recorded within a radius of 20 km around the lake. We infer that high porosity and low density Type M fragments are aerially transported and directly deposited on the lake, and that the fragility of Type M charcoal prevents significant quantities from being secondarily transported and incorporated into the sedimentary record. We propose that charcoal morphology is an important but underutilized technique that can yield important insights into fire type, proximity and transportation–sedimentation processes. [Copyright &y& Elsevier]

To develop models to predict past lake-water nutrient levels, the sedimentary remains of Cladocera were sampled from 53 lakes in central British Columbia, Canada. At the same time, the lakes were sampled for a suite of chemical variables. In addition, a host of physical and spatial explanatory variables were collected from each site. Canonical correspondence analysis showed that total phosphorus (TP), which ranged from 5 to 146 µg⋅L[sup-1], was the measured environmental variable that best described the differences in species composition among the lakes. Additionally, lake depth and surface water temperature were also important in explaining the distribution of cladoceran taxa. Chydorus brevilabris, Daphnia ambigua, Daphnia cf. pulex, and Graptoleberis testudinaria had a preference for eutrophic lakes, whereas Acroperus harpae, Alonella nana, Alonella excisa, Chydorus piger, Daphnia cf. dentifera, and Eubosmina spp. were found in the less productive lakes. Predictive models to estimate TP from species abundance data were developed using weighted averaging techniques. This research has produced strong and significant inference models, which can now be used to reconstruct past changes in lake trophic status from remains of Cladocera in sediment cores. [ABSTRACT FROM AUTHOR]

Summary 1. We explored patterns of limnological variables (physical, chemical and biological) with relation to landscape position (expressed as lake order) in 86 study lakes located on shield bedrock in south-central Ontario, Canada. 2. Using anovas with lake order as the categorical variable, landscape position explained significant amounts of variation in major ion chemistry, physical and catchment characteristics, hypolimnetic oxygen, and community composition in algal (diatom, chrysophyte) and invertebrate (chironomid) assemblages preserved in surficial sediments. Several nutrient variables (TP, total phosphorus and TN, total nitrogen) and dissolved organic carbon did not have significant relationships with lake order. 3. The strongest relationships with lake order (as a fraction of variation explained in anovas) included silica concentrations (r [sup 2] = 0.40) and SO[sub 4] (r [sup 2] = 0.29) concentrations, surface area (r [sup 2] = 0.50) and hypolimnetic oxygen (r [sup 2] = 0.29). 4. Bedrock geology (carbonate metasedimentary versus non-carbonate bedrock) had strong influences on spatial gradients of pH and major ion chemistry. It was difficult to separate geological influences from spatial influences on limnological variables in this study, as drainage patterns in the region are highly influenced by surface features of underlying geological formations because of the very thin glacial till or exposed bedrock that exists in most catchments. 5. Patterns of limnological variables indicated that low-order, headwater lakes had the lowest concentrations of major ions, and, from algal inferences of pH change, had been most susceptible to acidic deposition. High-order, downstream lakes were larger and deeper, and had higher concentrations of hypolimnetic oxygen, indicating that optimal lake trout habitat was primarily located in high-order lakes. 6. Variance partitioning analyses indicated that lake order as a metric of landscape position explained comparable portions of community variation in algal and invertebrate assemblages compared with geographic position (latitude, longitude) and Cartesian coordinate position (e.g. x , y , x [sup 2] , y [sup 2] , etc.) metrics. Lake order explained more community variation in chironomid assemblages compared with other landscape metrics, possibly because of the strong relationships between lake order and lake morphometry variables. [ABSTRACT FROM AUTHOR]

Examines the scaled chrysophytes preserved in modern and fossil sediment samples from lakes in Ontario. Evaluation of the changes in water quality; Determination of chrysophyte distributions using redundancy analysis; Introduction of a multi-indicator reconstruction model.

Abstract unavailable [ABSTRACT FROM AUTHOR]

Highlights • Mean δ13C and δ15N of invertebrate fossils usually differed from bulk sediment (SOM). • Historic isotopic trends of invertebrates differed from SOM only in clearwater lakes. • Among-taxon differences in historical isotopic trends were lake-specific. Abstract Carbon and nitrogen stable isotope ratios of sub-fossil invertebrate remains are potentially powerful indicators of nutrient flux, habitat-specific resource utilization, and trophic interactions in lentic food webs, but are rarely estimated for multiple species within lakes. Here we examined historical time series of δ13C and δ15N in remains of individual invertebrate taxa representing pelagic, littoral and benthic habitats during the 20th century in five boreal lakes of central Canada. We applied a novel statistical approach based on Generalized Additive Models (GAMs) to quantify the differences in centennial means and trends (i) between invertebrate remains and sedimentary organic matter (SOM), and (ii) among different taxa within each lake to evaluate the coherence of isotope signals during the 20th century. Differences in mean δ13C and δ15N were usually significant (p < 0.05) between SOM and invertebrate taxa, and among individual taxa, reflecting selective feeding by invertebrates and differences in trophic position within food webs. In contrast, patterns of historical variance in isotope values varied among lakes with few consistent differences between long-term isotopic trends of SOM and invertebrate remains. In particular, SOM and invertebrate isotopic trends were similar in relatively dystrophic lakes, likely due to the importance of terrestrial carbon in both SOM and invertebrate diets. However, significant SOM-invertebrate trend differences were observed for both δ13C and δ15N in relatively clear-water lakes, possibly reflecting temporal variation in diets or tissue fractionation. Comparisons of historical trends in isotope values among taxa revealed few consistent patterns, likely indicating uncoupled carbon and nitrogen fluxes through invertebrate consumers with different habitat specializations or feeding modes. Together, our findings suggest that evaluation of taxon-specific δ13C and δ15N can provide valuable insights into historical tropho-dynamics in lake food webs. [ABSTRACT FROM AUTHOR]