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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]

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]

Abstract: Drought is endemic to the North American Great Plains, causing severe economic consequences. However, instrumental climate data only exist from ca AD 1890, and limited tree-ring, paleolimnological, archeological and eolian records document the last two millennia. To address this lack of monitoring and paleoclimatic data, the pollen preserved in the varved sediments of Lake Mina, Minnesota, on the northeastern border of the Great Plains, were analyzed. May and February mean monthly temperatures and “annual precipitation minus potential evapotranspiration” were reconstructed at a 4-year resolution using a pre-settlement pollen–climate calibration set. The period of the so-called Little Ice Age (LIA) (AD 1500–1870) was colder than the Medieval Climate Anomaly (MCA) (AD 1100–1500) in west-central Minnesota. Winter temperatures in the LIA declined more than summer ones. The pollen record suggests that the LIA occurred in three phases: an initial cold phase from AD 1505 to AD 1575, a warmer phase, and then a very cold phase from AD 1625 to AD 1775. There were severe droughts detected in the Lake Mina record from AD 1660 to AD 1710 and AD 1300 to AD 1400, suggesting that high-resolution pollen records can detect events previously defined from the tree-ring records. This latter century-scale drought is concurrent with the widely reported “AD 1250–1400 mega-drought”, which exceeds the severity of 20th century droughts. [Copyright &y& Elsevier]

This study investigates regional changes in primary producers in boreal head-water lakes during the warmer early-to-mid-Holocene (EMH) period, across the present-day boreal forest in northwest Ontario, a region that is adjacent to the prairie-forest ecotone. We quantified changes in algal abundance and composition over the Holocene period using pigments, spectrally-inferred chlorophyll a and diatom assemblages in well-dated sediment cores from three lakes. All three indicators showed a coherent pattern of enhanced primary producers in two of the study lakes (Gall Lake and Lake 239) during the EMH, whereas only diatom assemblages suggested higher levels of nutrients in Meekin Lake. Overall, this study supports a regional pattern of enhanced primary producers during the EMH, likely as a function of lower water-levels and warmer temperatures. Elevated concentrations of cyanobacterial pigments also occurred in two of the three lakes during the EMH, whereas pigments from purple-sulphur bacteria provide evidence of enhanced deep-water anoxia in one lake. These findings suggest that future climatic warming in boreal regions could include regional eutrophication and associated increases in cyanobacteria. [ABSTRACT FROM AUTHOR]

The past several decades have witnessed a significant expansion of mining activities in the Athabasca oil sands region, raising concerns about their impact on the surrounding boreal forest ecosystem. To better understand the extent to which distal sites are impacted by oil sands-derived airborne contaminants, we examine sources of polycyclic aromatic hydrocarbons (PAHs) in surface sediments and dated sediment cores from Saskatchewan lakes situated ∼100–220 km east–northeast of the main area of bitumen mining activities. The concentrations and fluxes of both parent and alkylated PAHs are low and show considerable variability over the past 70–100 years. Small yet discernible increases in PAH concentrations and fluxes occurred over the past 30 years, a trend which coincides with the rapid growth in bitumen production. However, several lines of evidence point to wildfires as the principal source of PAHs to these lakes: (1) the significant co-variations in most cores between retene (1-methyl-7-isopropyl phenanthrene) and other groups of parent and alkylated PAHs, (2) the similarity in compound specific δ 13 C signatures of the parent PAHs phenanthrene and pyrene in recently deposited surficial sediments and those corresponding to time intervals considerably pre-dating the large scale development of the oil sands and (3) the discernible up-core increases in the proportion of refractory carbon (i.e., char) in Rock-Eval 6 data. The collective evidence points to softwood combustion from boreal forest fires as the principal source of retene in sediments and the general increase in forest fire activity in this region over the past several decades as the source of refractory carbon. Mining activities associated with the Athabasca oil sands are thus not considered a major source of PAHs to these lakes. [ABSTRACT FROM AUTHOR]

The landscape of northeast Ontario, Canada, is dominated by glacial and postglacial landforms including numerous boreal lakes that formed following the retreat of the Laurentide Ice Sheet (LIS) and Glacial Lake Ojibway. This study uses micro-X-Ray Fluorescence elemental analysis to examine changes in sediment cores from two boreal lakes from northeast Ontario with the goal of inferring changes in terrestrial inputs and redox status of these lakes over the middle to late Holocene. These headwater lakes were selected as they have small watersheds that minimizes the complexity of hydrological responses, and as they are relatively proximal (∼130 km distance between lakes) to allow the detection of regional signals related to changes in climate over the Holocene. Strong correlations between Si, K, Ca, and Ti in Hogback Lake and Green Lake cores suggest that these elements represent proxies of terrestrial input. Standardized composite trends of these elements were very similar between the two lakes and were combined to represent a regional signal, with the highest terrestrial inputs between ∼6000 and ∼4000 cal yr BP, that subsequently declined through the Holocene. These results suggest that regional patterns in terrestrial input are linked to changes in hydroclimate and likely increased terrestrial stability over the middle to late Holocene. These composite trends are compared to similar geochemical composite trends from Iceland suggesting climate teleconnections between northeast North America and the North Atlantic. Cool conditions in the North Atlantic result in increased soil erosion in Iceland and correspond to cool and dry conditions in boreal northeast Ontario with corresponding low inputs of terrestrial material, and vice versa, over multidecadal time scales over the middle to late Holocene. • Run-off was controlled by hydroclimate over the Holocene of northeast Ontario, Canada. • There were similar patterns in erosion in Ontario and Iceland over the Holocene. • Climate teleconnections occur between northeast North America and the North Atlantic. • North Atlantic climate alterations control these teleconnections. [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]

For the Western North America Mercury Synthesis, we compiled mercury records from 165 dated sediment cores from 138 natural lakes across western North America. Lake sediments are accepted as faithful recorders of historical mercury accumulation rates, and regional and sub-regional temporal and spatial trends were analyzed with descriptive and inferential statistics. Mercury accumulation rates in sediments have increased, on average, four times (4 ×) from 1850 to 2000 and continue to increase by approximately 0.2 μg/m 2 per year. Lakes with the greatest increases were influenced by the Flin Flon smelter, followed by lakes directly affected by mining and wastewater discharges. Of lakes not directly affected by point sources, there is a clear separation in mercury accumulation rates between lakes with no/little watershed development and lakes with extensive watershed development for agricultural and/or residential purposes. Lakes in the latter group exhibited a sharp increase in mercury accumulation rates with human settlement, stabilizing after 1950 at five times (5 ×) 1850 rates. Mercury accumulation rates in lakes with no/little watershed development were controlled primarily by relative watershed size prior to 1850, and since have exhibited modest increases (in absolute terms and compared to that described above) associated with (regional and global) industrialization. A sub-regional analysis highlighted that in the ecoregion Northwestern Forest Mountains, < 1% of mercury deposited to watersheds is delivered to lakes. Research is warranted to understand whether mountainous watersheds act as permanent sinks for mercury or if export of “legacy” mercury (deposited in years past) will delay recovery when/if emissions reductions are achieved. [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]

Abstract: The carbon cycle of a lake is a balance between supply from the atmosphere and catchment, and the net demand exerted by primary producers, minus losses back to the atmosphere and to sediment storage. Evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi-proxy approach. One promising technique is to explore the carbon-isotope composition (δ13Cdiatom) of organic matter incorporated within the silica frustules of diatom algae. Here we present a 25,000-year record of δ13Cdiatom from the sediments of crater Lake Challa on the eastern flank of Mt. Kilimanjaro, and along with other proxy data we make inferences about the three major phases in the history of the lake''s carbon cycle. From 25 ka to 15.8 ka years BP, δ13Cdiatom is positively correlated with the δ13C of bulk sediment organic matter (δ13Cbulk), indicating that high diatom productivity, as recorded by high % biogenic silica at this time, was preferentially removing 12C and enriching the δ13C of lake-water dissolved inorganic carbon. From 15.8 to 5.5 ka the correlation between δ13Cdiatom and δ13Cbulk breaks down, suggesting carbon supply to the lake satisfied or exceeded the demand from productivity. From 5.5 ka BP the positive correlation resumes, indicating an increase in the internal demand for carbon relative to external supply. Diatom frustule-bound carbon isotopes offer an original tool in examining long-term fluctuations in a lake''s carbon budget and how the balance between supply and demand has changed through time. [Copyright &y& Elsevier]

Abstract: Assessment of past drought in boreal regions, a region predicted to be strongly affected by climate warming, can provide insights into future availability of water. However, limited instrumental data and paleoclimatic data are available for this assessment. To address this lack of data in the boreal region of northwest Ontario, a regional study of lakes in the Winnipeg River Drainage Basin was initiated. Diatom-inferred (D-I) depth models were developed based on surface samples collected along a depth gradient within 8 small boreal lakes. Weighted-averaging and modern analog approaches provided robust within-lake depth models for each of the study lakes, with bootstrapped r 2 values ranging from 0.90 to 0.98, and root-mean-squared-errors of prediction (RMSEP) between 1.1 and 2.5 m. Large differences in the estimated depth optima for three representative, but common diatom species across our 8 study lakes suggested that within-lake calibration datasets are more appropriate for inferring past drought based in depth models than a regional multi-lake calibration dataset, and that light and related variables are controlling factors governing the maximum depth of benthic taxa. A down-core application of the D-I depth models on a near-shore core from Meekin Lake, retrieved near the present-day ecotone between the benthic and planktonic diatom assemblages indicated highly similar trends in inferred depth (r = 0.96). The models have significant correlations with other metrics of changes in depth including diatom species richness (r = 0.74–0.78) and evenness (r = 0.76–0.8), thereby allowing a check on the strength and direction of the depth inferences down-core. Near-shore cores located near the benthic:planktonic transition is a sensitive region that can provide estimates of past droughts in lakes where such inferences have been difficult to estimate. [Copyright &y& Elsevier]

Abstract: Paleoclimatic records from the climatically sensitive Canadian prairies are relatively rare due to the scarcity of study sites with continuous Holocene stratigraphic sequences. Oro Lake, a meromictic lake in the dry grasslands of Saskatchewan (Canada), contains a continuous Holocene diatom record spanning the last ∼10,000 years. Here we present analyses at three different time scales and resolution: (1) 1–3yr resolution of the past ∼80 years, (2) century-scale analysis of the Holocene, and (3) decadal-scale analysis of the past ∼7000 years. Recent changes in the diatom assemblages and their respective salinity inferences were significantly related to measured effective moisture (precipitation minus evaporation, P−ET). The droughts of the 1930s, and a wet period during the 1950s are clearly evident in the diatom record, suggesting the Oro Lake record contains a sensitive archive of past climatic conditions. Century-scale analysis of the diatom record during the Holocene is consistent with a cool and moist climate in the early Holocene (prior to ca 9700calyrBP, 8600 14CyrBP). An abrupt increase in diatom-inferred salinity at 9600calyrBP (8500 14CyrBP) indicates the onset of an arid climate, with continuing arid conditions throughout the mid-Holocene. Decadal-scale analysis of the past ∼7000 years suggests that the mid-Holocene was more complex, with extended periods of increased variability in precipitation, particularly between ca 5800–3600calyrBP (5000–3200 14CyrBP) which is characterized by intervals of increased effective moisture. The past ∼2000 years is characterized by reduced salinities and generally wetter conditions in comparison to the mid-Holocene. The combination of the different scales of analyses in this study provides a detailed account of the dynamic nature of climate from sub-decadal to millennial scale in the Oro Lake region within the Palliser Triangle. Climate model predictions suggest that the Canadian prairie region may see a higher frequency of extreme droughts under projected global warming, potentially similar to the most arid periods seen during the mid-Holocene when many lake basins completely dried out. [Copyright &y& Elsevier]

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]

Lake Naivasha (central Rift Valley, Kenya) is regionally unique in having accumulated a continuous sediment record of climate-driven palaeohydrological change over the past 1650 years, attesting to dramatic shifts between freshwater high-stands and either fresh or saline low-stands. This study employs fossil ostracod assemblages and stable-isotope (δ18O and δ13C) signatures to disclose the lake's ecohydrological history and explain the evolving non-linearity between lake level and salinity. The original age model and lithology-based lake-level reconstruction are revised, and the diatom-inferred salinity reconstruction is expanded to the lower (pre-770 CE) part of the record. The stable-isotope time series showed limited coherence with the other proxies, probably due to complexities related to hydrological connections between different surface- and groundwater components shifting through time. Ostracod palaeoecological data indicate that between ca 350 and 1200 CE the system often veered towards hydrological closure, with frequent and high-amplitude shifts in lake level and salinity preventing long-term establishment of wetland vegetation. Low lake-level and saline conditions dominated between ca 900 and 1200 CE. After that time and particularly since the onset of more permanent Malewa River inflow ca 1430 CE, hydrologically open conditions prevailed. Salinity shifts exhibited lower amplitude and frequency, promoting the expansion of wetland vegetation and associated biota including benthivorous fish. With lake water balance strongly influenced by variation in river inflow, high-amplitude lake-level fluctuations continued throughout the freshwater phase of the past 600 years, notably creating a pronounced high-stand dated to ca 1600–1780 CE and the well-documented extreme low-stand of the early 1800s. Image 1 • Lake Naivasha's sediment record documents ca 1650 years of hydroclimatic change. • Age models for fluctuating lakes benefit from flexibility in Bayesian age-modelling. • Ostracods responded to changes in salinity, water depth and aquatic vegetation. • Pronounced aquatic-ecosystem shifts are linked to hydrological transitions. [ABSTRACT FROM AUTHOR]