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Lakes in the semiarid Southern Interior Plateau of British Columbia, Canada are vulnerable to future climate-driven changes in water quantity and quality. However, few long-term monitoring records exist to assess the effects of varying hydroclimate conditions on lake function. We present a high-resolution multiproxy paleolimnological record containing subfossil diatoms, μXRF via ITRAX, and chlorophyll-a that spans the past 1800 years from Roche Lake in south-central British Columbia. Generalized additive models indicate that the diatom assemblages were associated with changes in lake level and the accompanying effects of atmospheric forcing on lake thermal structure, and that these forcing mechanisms had a weak relationship with primary production. We infer two pronounced arid periods between ca. AD 600-950 and ca. AD 1730-1950 that correspond to evidence of reduced lake mixing and enhanced anoxia, but variable patterns in primary production. Synthesis of regional proxy records suggests that inferences of moist but possibly warm conditions during the Medieval Climate Anomaly may have occurred because of a complex interaction between prolonged La Niña-like conditions and higher radiative forcing. Overall, lakes in this region are sensitive to changes in hydroclimate generated by interactions between external forcing and Pacific Ocean-atmosphere processes, and the resulting changes in aridity may have potentially negative consequences on salmonid habitat. [ABSTRACT FROM AUTHOR]

Twentieth-century historic industrial activity in Cornwall, ON loaded high amounts of metals and metalloids to the St. Lawrence River. Despite industrial improvements and the eventual closure of point-source polluters, sedimentary contamination in the Cornwall waterfront remains above provincial guidelines, causing concern for ecosystem management. We analysed chironomid (Order: Diptera) subfossils from modern-day surface-sediment samples and a sediment core from the Cornwall waterfront in order to better understand historic pollution impacts and potential recovery. Modern-day chironomid assemblages were diverse (48 genera, >70 species) and overall assemblage structure was weakly related to sedimentary zinc concentrations, with Procladius , Paratanytarsus , and Ablabesmyia identified as zinc-tolerant and Rheotanytarsus , Polypedilum and Paratendipes identified as zinc-sensitive. The sediment core showed peak total mercury (THg) concentrations were an order of magnitude higher than in modern-day surface sediments. In sections of the core below 10 cm in depth, THg concentrations were high (~30–50 µg/g) and chironomids were nearly absent from the record. Above 10 cm (c. 1980) THg concentrations decreased and chironomid taxa returned in low abundances. Subsequent intervals displayed an increase in diversity, abundance, and the relative abundance of metal-sensitive taxa. This study provides evidence of ecological change which co-occurs with Cornwall's historic industrial activity and indicates progress toward related ecological goals identified in the current management strategy. [ABSTRACT FROM AUTHOR]

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]

Stager JC, Wiltse B, Cumming BF, Holsen TM, Stetler J, Laxson C, Marcillo CE, Charles DF. 2019. A novel ecological state at Bear Pond (Adirondack Mountains, NY, USA) following acidification and partial recovery. Lake Reserv Manage. 35:208–223. The pH of precipitation in the northeastern United States has risen as industrial sulfur and nitrogen oxide emissions have declined following amendment of the Clean Air Act in 1990, but the effects of this change on the region's lakes are not yet fully documented. Here we use the siliceous remains of diatoms and chrysophyte algae in a sediment core from Bear Pond (Adirondack State Park, NY), which acidified during the 20th century, to reconstruct pH variability during the last 2 centuries, evaluate the extent to which the lake has recovered from acidification, and determine the timing of that reversal to help identify its causes. Inferred pH declined erratically from a high of 6.4 during the 1860s to 5.7–5.9 during the 1930s, then decreased to the 4.9–5.4 range by 1995–2010. After 2010, inferred pH rose and remained within the 5.4–5.6 range following an abrupt rise in the pH of regional precipitation. Although the acidification trend at Bear Pond has now reversed, the lake ecosystem has not returned to its pre-impact condition. Distinctive members of the pre-acidification diatom community are still absent, chrysophytes have become unusually abundant, and water clarity (Secchi depth) has decreased by about half. Furthermore, the original fish community was lost due to stocking and piscicide treatment in 1958, and high concentrations of toxaphene residues from the piscicide are present in the sediments. Full ecological restoration of formerly acidic lakes such as Bear Pond may be unlikely due to complicating factors such as climate change and fisheries management practices. [ABSTRACT FROM AUTHOR]

Simmatis B, Jeziorski A, Zemanek A, Selbie DT, Hyatt K, Fryer JK, Cumming BF, Smol JP. 2018. Long-term reconstruction of deep-water oxygen conditions in Osoyoos Lake: implications for Okanagan River sockeye salmon. Lake Reserve Manage. 34:392-400. Osoyoos Lake is the primary nursery lake supporting sockeye salmon (Onchorhynchus nerka) originating from the Okanagan River system in south-central British Columbia, Canada. By the early 1960s, Osoyoos Lake had experienced significant cultural eutrophication related to rapid human development of the watershed, increasing total phosphorus concentrations, and frequent algal blooms. Concern exists regarding recent (post-1990s) declines in hypolimnetic oxygen concentrations and the potential degradation of salmon habitat in this multi-basin lake. However, reliable observations are largely unavailable for oxygen conditions prior to the mid-1990s. We reconstructed late-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in Osoyoos Lake from the late 1800s to 2009 by examining past chironomid (Diptera: Chironomidae) subfossil assemblages from 210Pb-dated lake sediment cores. Chironomid-inferred VWHO revealed only slight variations since the late 1800s (mean values for the north and south basins are 4.4 and 2.9 mg/L, respectively), with seemingly little response to the cultural eutrophication of the mid-20th century. The chironomid-inferred VWHO trends suggest that the current management target of 5.8 mg/L dissolved oxygen for both basins of Osoyoos Lake may require revision to better reflect reference conditions and natural seasonal anoxia. [ABSTRACT FROM AUTHOR]

Recent surface-water surveys suggest that high nutrient concentrations and nuisance algae remain issues in the St. Lawrence River Area of Concern (AOC) at Cornwall, Ontario, specifically in the tributaries and nearshore zones of Lake St. Francis (LSF). In particular, it is unclear whether management actions designed to reduce nutrient inputs, first implemented in the 1990s as part of the Remedial Action Plan for the AOC, have reduced algal production or influenced assemblage composition. To address this issue, a paleolimnological approach was used to provide a historical context for the present-day nutrient concentrations and to quantify the extent of change in water quality in LSF since the early 1990s. A sediment core was collected near the north shore of LSF and was examined for changes in the concentrations and compositions of fossil diatoms and pigments, as well as stable isotope (δ 15 N and δ 13 C) values. Analyses of diatom and pigment concentrations indicated that overall algal abundance has risen in the last few decades, including trends of increasing occurrences of potentially toxic cyanobacteria, despite ongoing remediation efforts. Temporal patterns of stable isotope signatures in the core suggest a steady increase in nutrient influx since the mid-20th century, with the post-1990 increase in algal production likely attributable to recent inputs associated with land-use changes in local contributing watersheds. These patterns suggest that the AOC delisting goals for the LSF tributaries will not be reached without a drastic change in land management practices. [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]

A high-resolution diatom record from Wolf Lake, a minimally disturbed 'heritage' lake, provides insights into the hydroclimatic history of the Adirondack Mountains of northern New York during the last c. 1600 years. Three pronounced dry periods occurred during c. AD 490-610, 780-870, and 1010-1080, and low precipitation generally prevailed during the warm Medieval Climate Anomaly (c. AD 950-1350), a finding that fills an important gap in knowledge of the spatial extent of droughts across North America during that period. During the cooler 'Little Ice Age' interval (c. AD 1350-1800), inferred water balance was generally more positive. Seven peaks in charcoal abundance represent fire events during both wet and dry periods. Unusually high charcoal and inorganic sediment deposition c. AD 1700 could reflect human activity in the watershed, as might an abrupt rise in the relative abundances of planktonic and tychoplanktonic diatoms in Wolf Lake during the AD 1860s. The diatom record displays periodicities of c. 256 and 512 years in addition to high-frequency fluctuations, suggesting that significant precipitation variability is likely to continue to disrupt climatic trends in this region. [ABSTRACT FROM AUTHOR]

To determine pre-disturbance limnological conditions, evaluate the impact of environmental stressors (surface water acidification, nutrient inputs, climate change, and winter deicing salt), and set realistic recovery targets for lake management strategies, a rapid assessment paleolimnological approach was used to determine the amount (and likely causes) of environmental changes over the past ∼100–150 years in 51 urban lakes from Halifax, Nova Scotia (Canada). Diatom assemblages from lake sediment cores were used with “top” (recently deposited, surface) samples being matched to measured limnological conditions, and “bottom” (generally from >15 cm deep) samples used to infer pre-disturbance limnological conditions such as pH, total phosphorus (TP), specific conductance, and shifts due to changing climatic conditions. Environmental change was assessed by calculating the metric of change in species composition between present-day and pre-disturbance diatom assemblages, and inferences from quantitative estimates of diatom-inferred pH, specific conductance, and TP. All 51 study lakes have experienced floristic changes in diatom species composition since pre-disturbance times, but different environmental stressors were implicated: 8 of the 51 lakes underwent significant (i.e., >2 times the root mean-squared error [2X RMSE] of the inference model) decreases in diatom-inferred pH; 8 lakes had significant increases in diatom-inferred TP; and 19 otherwise relatively pristine lakes had increases in planktonic taxa consistent with observations linked to changes in lake seasonality and limnological changes most closely linked to climate warming in Nova Scotia and other regions. The remaining 16 lakes did not have large and consistent changes in diatom flora or changes in diatom-inferred TP or pH >2X RMSE of the prediction of the models. Lake-specific factors were related to these inferences, and the lakes that acidified were mainly the currently most acidic sites, whereas those that experienced issues related to eutrophication were generally among the most alkaline sites. Of our 51 lakes, 22 (including some experiencing pH, TP, or showing floristic changes linked to climate changes) had increases in measured conductivity (1980–2002) and, correspondingly, increased relative abundances of halophilic diatom taxa. These lakes, often with catchments containing high surface areas of impervious surfaces, are examples of a trend of increasing salinity in northeastern North American lakes likely related to winter application of deicing (road) salt. The application of this paleolimnological approach enabled us to identify which lakes have undergone significant changes in diatom assemblages, as well as which environmental stressor(s) were most probable. This information can help lake managers develop more targeted and effective management strategies. [ABSTRACT FROM AUTHOR]

Many lakes within the Boreal Plain and Grassland regions of Canada are currently eutrophic to hypereutrophic. Limited paleolimnological work has been conducted to define water quality trajectories of lakes within this region. A 25-year intermittent monitoring and a ∼150-year paleolimnological time series from Baptiste Lake, Alberta, were analyzed using a combination of trend, correlation, and multivariate analyses. The temporal overlap between the monitoring and paleolimnological time series provides an opportunity to assess coherence between the data sources. Diatom assemblages in the sediment core show that eutrophic conditions were present for at least 150 years. Monitored water chemistry data since the early 1980s show that total Kjeldahl nitrogen (TKN) has increased by approximately 50% since the early 1990s, whereas concentrations of total phosphorus (TP) have remained stable. Further, measured TKN is significantly correlated to measurements of chlorophyllaover the monitoring period and to diatom-inferred TKN values, suggesting nitrogen limitation in Baptiste Lake. In contrast, measured TP was not correlated to chlorophyllaor diatom-inferred TP. Changes in land use over the past 100 years is the most parsimonious explanation for the nutrient changes. No statistical support for climatic change as a linear predictor of nutrient dynamics was found. Our contemporary and paleolimnological analysis provides an important perspective on the timing and magnitude of nutrient dynamics over ∼150 years. Future government and community decisions on Baptiste Lake management would benefit from testing nutrient limitation and detailed modeling of nutrient runoff from the watershed. [ABSTRACT FROM AUTHOR]

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Abstract: Changes in vegetation were tracked from a well-dated sediment core from a boreal lake, Lake 239, at ∼200-year resolution over the Holocene. This presently oligotrophic lake is located ∼100-km east from the present-day parkland-forest ecotone in northwestern Ontario. Near-shore sediment core transects from Lake 239 have previously shown this lake was at least 8-m lower than present in the mid-Holocene, or ∼58% less lake volume in comparison to today. Large shifts were expected in the terrestrial vegetation if the low lake levels were related to climate. The core from Lake 239 shows increases in the relative abundance and concentration of pollen such as Cupressaceae and Ambrosia, indicating a more open boreal forest between ∼4500–8000 cal yr BP. Pollen-based inferences of average, summer and winter temperatures suggest that temperatures were on average up to 1–2 °C warmer than today, with winter temperatures up to 4 °C warmer. The pollen inferences also suggest enhanced precipitation, likely in the summer, but with an overall increase in evaporation and evapotranspiration resulting in reduced effective moisture. To assess regional climate changes, pollen-based reconstructions of temperature and precipitation were developed and synthesized from sediment cores from eight previously published lakes, from which pollen sites were available to both the west and east of Lake 239, spanning present-day prairie lakes to forested lakes up to 300 km east of the prairie-boreal ecotone. All sites show shifts in pollen assemblages that indicate a warm mid-Holocene period; prairie sites west of the Experimental Lakes Area (ELA) show mid-Holocene decreases in precipitation relative to today, whereas sites near or east of ELA show consistent increases in precipitation, but with increased temperatures and enhanced evaporation during the mid-Holocene. [Copyright &y& Elsevier]

A palaeolimnological investigation of changes in water quantity and quality in Lake 239, Experimental Lakes Area, in northwestern Ontario indicates marked changes in limnological conditions during the Holocene. Water quantity changes are based on the analysis of diatoms and inferences of lake-level changes from a nearshore sediment core. Changes in water quality are based on the analysis of diatom assemblages and associated quantitative inferences of total phosphorus (TP) from a deep central core, as well as changes in diatom accumulation rates, the proportion of chrysophyte scales to diatoms and changes to organic matter content. Lake levels at least 8 m lower than today were inferred from the nearshore core and were concurrent with an increase in nutrient-rich diatom assemblages, an increase in diatom accumulation and a decrease in the chrysophytes relative to diatoms in the central core. Fragilaria crotonensis and Aulacoseira subarctica were two of the dominant nutrient-rich taxa that contributed to an increase in diatom-inferred TP during the mid Holocene. Results from this study provide strong empirical evidence that water levels much lower than today can dramatically change trophic status, as occurred during the dry mid Holocene, and may provide a good analogue for understanding future impacts of climatic changes due to anthropogenic activities. [ABSTRACT FROM AUTHOR]

The article focuses on the study which tracks anthropogenic- and climatic-related environmental changes in the habitat lakes of the endangered Atlantic whitefish through palaeolimnological techniques. The study examines the three lakes near Bridgewater, Nova Scotia using palaelimnological techniques to infer their historic limnological conditions and compare the results with pre-industrial conditions. The results of the study reveal that acidic deposition has no important impact on the lakes, and their nutrient statuses have never changed. It notes that the lakes are not yet been affected by acidic precipitation. However, it considers climatic warning and associated limnological changes as potential stressors to the habitats.

Paleolimnological approaches using sedimentary diatom assemblages were used to assess water quality changes over the last approximately 200 years in three lakes from King's County, Nova Scotia. In particular, the role of recent shoreline development in accelerating eutrophication in these systems was assessed. Sediment cores collected from each lake were analyzed for their diatom assemblages at approximately 5-year intervals, as determined by 210Pb dating. Analyses showed that each system has changed, but tracked different ecosystem changes. Tupper and George lakes recorded shifts, which are likely primarily related to climatic warming, with diatom assemblages changing from a preindustrial dominance by Aulacoseira spp. to present-day dominance by Cyclotella stelligera. In addition to the recent climatic-related changes, further diatom changes in the Tupper Lake core between approximately 1820 and 1970 were coincident with watershed disturbances (farming, forestry, and construction of hydroelectric power infrastructure). Black River Lake has recorded an increase in diatom-inferred total phosphorus since about 1950, likely due to impoundment of the Black River system for hydroelectric generation and subsequent changes in land runoff. Before-and-after (i.e., top-bottom) sediment analyses of six other lakes from King's County provided further evidence that the region is being influenced by climatic change (decreases in Aulacoseira spp., increases in planktonic diatom taxa), as well as showing other environmental stressors (e.g., acidification). However, we recorded no marked increase in diatom-inferred nutrient levels coincident with shoreline cottage development in any of the nine study lakes. Paleolimnological studies such as these allow lake managers to place the current limnological conditions into a long-term context, and thereby provide important background data for effective lake management. [ABSTRACT FROM AUTHOR]

New Brunswick lakes are subjected to multiple environmental stressors, such as atmospheric acid deposition and climate change. In the absence of long-term environmental data, the impacts of these stressors are not well understood. Long-term effects of environmental change on diatom species assemblages were assessed in the sediments of 16 New Brunswick lakes using paleolimnological approaches. A regional trend of increasing Cyclotella stelligera Cleve & Gunrow and decreasing Aulacoseira species complex was recorded in most lakes. Detailed paleolimnological analyses of Wolfe, Cundy, and West Long lakes revealed varying degrees of species change, with assemblage shifts beginning ca. 1900 CE (common era). These species trends are not consistent with acidification. However, linear regression of mean July temperature with time for two New Brunswick historical instrumental temperature records revealed statistically significant warming over the past century. The shift from heavily silicified tychoplanktonic Aulacoseira species to small planktonic diatom species, such as C. stelligera, is consistent with paleolimnological inferences of warming trends recorded in several other lake regions of the Northern Hemisphere. These assemblage shifts are likely due to recent climate change and may be mediated by reduced ice cover and (or) increased thermal stability (decreased lake mixing) during the open water period. [ABSTRACT FROM AUTHOR]

Fire is an important mechanism of disturbance in boreal ecosystems; however, the effects of fire on lake ecosystems are still not well understood. This study provides a detailed assessment of the impacts of fire on the limnology of a small oligotrophic lake (Lake 42), located approximately 200 km northwest of Thunder Bay, Ont. The study lake is characterized by a small drainage ratio (watershed area : surface area) and a relatively long water residence time. Age establishment and fire scar analyses determined that at least one, and perhaps two, major fires had burned to the lake's shoreline in the past ca. 150 years. Using a paleoecological approach, diatoms were examined in a [sup 210] Pb-dated sediment core. Following watershed fires, minimal changes were noted in the diatom species assemblage. These findings may be explained by the low sedimentation rates and small drainage ratio of the study lake, although other studies suggest that the biological response may be minimal compared with physical–chemical responses in some ecosystems. Beginning in the early 1980s, however, distinct changes were noted in the species assemblage and in diatom-inferred total phosphorus. Our findings suggest that the study lake may be more sensitive to precipitation inputs of nutrients than to inputs resulting from watershed disturbances.Key words: paleolimnology, diatoms, forest fire, water quality.Le feu est un important mécanisme de perturbation dans les écosystèmes boréaux; cependant, les effets du feu sur les écosystèmes lacustres demeurent encore mal connus. Les auteurs présentent un évaluation détaillée des impacts du feu sur la limnologie d'un petit lac (Lake 42), localisé à environ 200 km au nord-ouest de Thunder Bay, en Ontario. Le lac étudié se caractérise par un faible ratio de drainage (aire du bassin de drainage : aire de la surface) et un temps de résidence de l'eau relativement prolongé. L'analyse des données sur l'âge et sur les traces d'incendies montre qu'au moins un, et peut être deux, incendies majeurs ont brûlé jusqu'au bord du lac au cours des ca. 150 dernières années. Dans une approche paléo-écologique, les auteurs ont conduit une analyse des diatomées dans une carotte datée au [sup 210] Pb. À la suite des incendies dans le bassin versant, on note des changements minimes dans l'arrangement des espèces de diatomées. Ces constatations peuvent s'expliquer par les faibles taux de sédimentation et le faible ratio de drainage du lac étudié, bien que d'autres études suggèrent que la réaction biologique pourrait être minimale comparativement aux réactions physico-chimiques, dans certains écosystèmes. Cependant, avec le début des années 1980, on note de nets changements dans l'arrangement des espèces et du phosphore total indiqué par les diatomées. Ces constatations suggèrent que le lac sous étude pourrait être plus sensible aux apports par précipitation de nutriments qu'aux apports provenant de perturbations dans le bassin de drainage.Mots clés : paléolimnologie, diatomée, feu de forêt, qualité de l'eau.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Interest in climate change research has taken on new relevance with the realization that human activities, such as the accelerated release of the so-called greenhouse gases, may be altering the thermal properties of our atmosphere. Important social, economic, and scientific questions include the following. Is climate changing? If so, can these changes be related to human activities? Are episodes of extreme weather, such as droughts or hurricanes, increasing in frequency? Long-term meteorological data, on broad spatial and temporal scales, are needed to answer these questions. Unfortunately, such data were never gathered; therefore, indirect proxy methods must be used to infer past climatic trends. A relatively untapped source of paleoclimate data is based on hindcasting past climatic trends using the environmental optima and tolerances of algae (especially diatoms) preserved in lake sediment profiles. Paleophycologists have used two approaches. Although still controversial, attempts have been made to directly infer climatic variables, such as temperature, from past algal assemblages. The main assumption with these types of analyses is that species composition is either directly related to temperature or that algal assemblages are related to some variable linearly related to temperature. The second more commonly used approach is to infer a limnological variable (e.g. water chemistry, lake ice cover, etc.) that is related to climate. Although paleolimnological approaches are broadly similar across climatic regions, the environmental gradients that paleophycologists track can be very different. For example, climatic inferences in polar regions have focused on past lake ice conditions, whereas in lakes near arctic treeline ecotones, paleophycologists have developed methods to infer past lakewater-dissolved organic carbon, because this variable has been linked to the density of coniferous trees in a drainage basin. In closed-basin lakes in arid and semiarid regions, past lakewater salinity, which can be robustly reconstructed from fossil algal assemblages, is closely tied to the balance of evaporation and precipitation (i.e. drought frequency). Some recent examples of paleophycolgical work include the documentation of striking environmental changes in high arctic environments in the 19th century believed to be related to climate warming. Meanwhile, diatom-based reconstructions of salinity (e.g. the Great Plains of North America and Africa) have revealed prolonged periods of droughts over the last few millennia that have greatly exceeded those recorded during recent times. Marked climatic variability that is outside the range captured by the instrumental record has a strong bearing on sustainability of human societies. Only with a long-term perspective can we understand natural climatic variability and the potential influences of human activities on climate and thereby increase our ability to understand future climate. [ABSTRACT FROM AUTHOR]

Long-term data are often lacking to effectively assess patterns of lake acidification and recovery. Fortunately, paleolimnological techniques can be used to infer past changes in lakewater acidity and related variables by means of biological indicators, such as diatom valves and chrysophyte scales, preserved in 210Pb-dated sediment cores. We summarize paleolimnological data that we have gathered from 36 Sudbury (Ontario) and 20 Adirondack Park (New York) lakes to estimate the magnitude of lake acidification and any subsequent recovery in these lake systems. In both regions, many lakes were shown to have acidified considerably, some over two pH units, since the 1850s. Although some recovery was noted in both lake regions, Sudbury lakes generally showed larger increases in inferred lakewater pH with recent declines in sulfur emissions. Possible explanations of these differences include the greater decrease in sulfate deposition in the Sudbury area, as well as generally longer residence times of lakes in Sudbury, perhaps allowing for more in-lake alkalinity generation. In addition, Sudbury lakes generally had higher pre-industrial pH levels, suggesting that lakes with higher natural buffering capacities are more likely to recover more quickly with declines in deposition, even if they had been acidified to a great extent. [ABSTRACT FROM AUTHOR]

The relationship between 23 scaled chrysophyte taxa (Chrysophyceae and Synurophyceae) and measured limnolgoical variables in 62 Adirondack, New York, drainage lakes were examined by canonical correspondence analysis (CCA). The major proportion of variation in chrysophyte species distributions was strongly related to total monomeric Al (Al[SUBm]) and Mg concentrations, and their close correlates pH, Na, Ca, and acid-neutralizing capacity (ANC). Total monomeric Al concentrations explain a greater proportion of species variation than pH, suggesting that al[SUBm] concentrations may be more important in governing the distribution of chrysophyte laxa in these lakes. Gaussian logit (GL) and linear logit (LL) regressions of the relative percentages of individual chrysophyte taxa to lakewater pH and Al[SUBm] concentrations and the examination of pH-Al[SUBm] response surfaces show that many chrysophyte taxa exhibit unique responses to these environmental gradients; taxa can be characterized as alkaline, circumneutral, acidic, and pH indifferent. Within each of these groups, taxa can be characterized further based upon their optima and tolerances to Al[SUBm] concentrations.Chrysophyte indicator species (i.e. a taxon with a strong statistical relationship to the environmental variables of interest, a well-defined optimum, and a narrow tolerance to the variable of interest) for pH include Mallomonas hindonii, M. crassisquama, M. pseudocoronata, and Synura uvella; M. hindonii, M. crassiquama, M. pseudocoronata, S. petersenii, and S. Spinosa are good indicators of Al[SUBm] concentrations. Highly significant predictive models were developed to infer lakewater pH and Al[SUBm] concentrations from the relative percentages of chrysophyte scales in the study lakes. Model evaluation was based on th eir correlation coefficients and the root-mean-squared error of predictions (RMSE) derived from bootstrapping. Weighted averaging regression and calibration with tolerance downweighting (i.e. weighting taxa inversely to their variance) produced superior results when compared to the computationally and data-demanding maximum likelihood methods and to simple weighted average regression and calibration. [ABSTRACT FROM AUTHOR]

Abstract unavailable [ABSTRACT FROM AUTHOR]

The Holocene Thermal Maximum (HTM) is relatively well-understood both in terms of its external drivers and timing of its expression in different regions across the globe, and has previously been used to investigate lake ecosystem response to a warm and drier climate. However, connections between mid-to-late Holocene climate, associated drivers, and lake response are less understood, particularly in western North America (WNA) where complex topography and differences in proximity to the Pacific coast result in spatially variable responses to ocean-atmosphere processes. Here, we present a multi-proxy paleolimnological record spanning 11,000 years from a small lake in the semi-arid southern Interior Plateau (SIP) of British Columbia. Sub-fossil diatom, trace element (μXRF via ITRAX), and pollen data indicate that our study lake was shallow, nutrient-poor, and poorly oxygenated during the HTM. High effective moisture (EM) is inferred throughout the mid-Holocene (∼6500–2000 calibrated years before present (cal yr BP)), followed by a return to low EM after 2000 cal yr BP. Synthesis of additional SIP diatom records show similarities in EM throughout the Holocene, but most notably, the decline in EM at ∼2000 cal yr BP. Comparison of patterns in the SIP with additional climate proxy data across WNA, including lake sediment δ18O records that are interpreted to represent changes in Pacific ocean-atmosphere processes, suggest that shifts in the mean state of the Pacific Decadal Oscillation (PDO) and the El Niño Southern Oscillation (ENSO), and associated strength and position of the Aleutian Low, are linked with late Holocene declines in EM throughout the SIP. • A multi-proxy paleolimnological record of Holocene environmental change from interior B.C. is presented. • Diatom and micro-X-ray fluorescence data indicate the lake was shallow, nutrient-poor, and anoxic during the HTM. • Effective moisture was higher during the mid-Holocene (∼6500–2000 cal yr BP) compared to the late Holocene. • Diatom records from proximal interior B.C. lakes show coherent declines in effective moisture at ∼2000 cal yr BP. • WNA proxy data demonstrates the importance of PDO/ENSO dynamics on mid-to-late Holocene effective moisture. [ABSTRACT FROM AUTHOR]

Since the implementation of large-scale lake monitoring in the ~1980s, water color and dissolved organic carbon (DOC) concentrations have increased in many northern lakes (i.e., lake browning), impacting the functioning of aquatic ecosystems. In regions that formerly experienced high levels of acid deposition, this browning trend has been largely attributed to the recovery from the impacts of past acid deposition. However, the extent to which DOC levels have now returned to naturally higher, pre-industrial conditions is still poorly understood. In this study, we assessed whether DOC levels are still influenced by acid deposition in lakes near Sudbury, Ontario, a region that has been heavily affected by sulfur dioxide emissions from local metal smelting during the 20th century. We analyzed water chemistry monitoring data (1981–2018), together with comparisons between modern and pre-industrial DOC levels inferred from sediment spectroscopy, for 51 acid-sensitive and 24 buffered reference lakes across the Sudbury landscape. Since 1981, DOC concentrations doubled in acid-sensitive lakes, with a mean increase of +1.6 mg/L, whereas in more buffered reference lakes, mean DOC levels increased by only 0.8 mg/L. Similarly, sediment-inferred DOC trends indicate that current DOC levels are, on average, ~22% below pre-industrial levels in acid sensitive systems compared to only ~10% in buffered lakes. Weakening correlations between DOC and acidification-related water chemistry variables (e.g., pH, alkalinity, metals) further indicate a diminishing influence of acid deposition on DOC in Sudbury lakes. These results highlight the strong impact that acid deposition has historically had on lake-water DOC dynamics in this region, but also suggest that DOC levels are approaching natural baseline levels in less acid-sensitive lakes, and that other drivers, such as changes in climate or vegetation cover, are now becoming the dominant controls on changes in DOC concentrations. Unlabelled Image • Assessment of observed and inferred DOC trends in 75 Sudbury lakes • DOC increased by 102% in acid-sensitive compared to only 25% in buffered lakes • Weakening correlations between DOC and acidification-related chemical variables • Inferred DOC trends suggest that current DOC is still below pre-industrial levels • DOC in buffered lakes is approaching complete recovery from past acid deposition [ABSTRACT FROM AUTHOR]