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The increasing rate of glacier retreat and turbid glacial runoff can have a strong influence on freshwater ecosystems. Seton and Anderson lakes (British Columbia, Canada) are sockeye salmon (Oncorhynchus nerka) nursery systems. Since the 1940s, the Bridge River Diversion (BRD) introduced glacially turbid water into Seton Lake. To assess the impact of the BRD on the production of Seton Lake, we combined data from limnological surveys with the analysis of subfossil cladocerans and diatoms from sediment cores, using Anderson Lake as a reference. The modern data indicate that the euphotic zone is 14 m shallower and the cladoceran density and biomass are significantly lower in Seton Lake in comparison with Anderson Lake. The paleo-data indicate that following the BRD, the sedimentary fluxes of cladoceran and diatom declined 2- to 10-fold in Seton Lake and remained low thereafter. Together, our data support declines in primary and secondary producers following the BRD, likely due to changes in light penetration and (or) other indirect influence, and provides insights into the impact of turbid meltwater on the biological production of downstream lakes. [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]

The inference of past temperatures from a sedimentary pollen record depends upon the stationarity of the pollen-climate relationship. However, humans have altered vegetation independent of changes to climate, and consequently modern pollen deposition is a product of landscape disturbance and climate, which is different from the dominance of climate-derived processes in the past. This problem could cause serious signal distortion in pollen-based reconstructions. In the north-central United States, direct human impacts have strongly altered the modern vegetation and hence the pollen rain since Euro-American settlement in the mid-19th century. Using instrumental temperature data from the early 1800s from Fort Snelling (Minnesota), we assessed the signal distortion and bias introduced by using the conventional method of inferring temperature from pollen assemblages in comparison to a calibration set from pre-settlement pollen assemblages and the earliest instrumental climate data. The early post-settlement calibration set provides more accurate reconstructions of the 19th century instrumental record, with less bias, than the modern set does. When both modern and pre-industrial calibration sets are used to reconstruct past temperatures since AD 1116 from pollen counts from a varve-dated record from Lake Mina, Minnesota, the conventional inference method produces significant low-frequency (centennial-scale) signal attenuation and positive bias of 0.8-1.7°C, resulting in an overestimation of Little Ice Age temperature and likely an underestimation of the extent and rate of anthropogenic warming in this region. However, high-frequency (annual-scale) signal attenuation exists with both methods. Hence, we conclude that any past pollen spectra from before Euro-American settlement in this region should be interpreted using a pre-Euro-American settlement pollen set, paired to the earliest instrumental climate records. It remains to be explored how widespread this problem is when conventional pollen-based inference methods are used, and consequently how seriously regional manifestations of global warming have been underestimated with traditional pollen-based techniques. [ABSTRACT FROM AUTHOR]

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]

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]

Aim (1) To resolve theoretical debates regarding the role of environment versus dispersal limitation, the conservatism of niches across distances and the prevalence of environmental specialists in diatom communities. (2) To provide guidance on the use of diatom communities and other microbial analogues to analyse ecological response to environmental change. Location Eight hundred and ninety-one lakes in five regional datasets from north-western Europe and four regional datasets from north-eastern North America. Methods Lacustrine diatom communities were analysed at three scales: inter-continental, intra-continental and regional. Nested partial redundancy analyses (RDAs) were used to determine spatial versus environmental components of community variation. Weighted-averaging (WA) regression and calibration, as well as logistic and quadratic regressions, were used to detect niche conservatism and the prevalence of environmental specialists. Results Community patterns indicate that dispersal limitation acts predominantly at the inter-continental scale, while at the regional (less than c. 1,000,000 km2) scale, a single environmental variable (pH) explains more than five times the community variation as spatial (dispersal-related) variables. In addition, pH niche components appear to be conserved at the inter-continental scale, and environmental specialization does not impose relative rarity, as specialists apparently readily disperse to suitable environments. Main conclusions Analysis at multiple scales is clearly important in determining the influences of community variation. For diatom communities, dispersal limitation acts most strongly at the broadest scales, giving way to environment at the scales considered by most analyses. The availability of a wide variety of propagules with consistent niches across regions indicates that diatom communities reflect the succession of taxa according to local environmental conditions, rather than disequilibrium with the environment or adaptation of local populations. While multi-scale analyses must be undertaken for other groups to resolve debates over community drivers and determine appropriate scales for prediction, for diatoms (and probably other microbial communities), responses to environmental change can be inferred using analogue datasets from large geographic areas. [ABSTRACT FROM AUTHOR]

Predictions of the extent of forest fires under warmer and drier conditions in boreal regions require knowledge of long-term relationships between fire and climate. However, many long-term studies that utilize the remains of total charcoal in lacustrine sediments fail to demonstrate a relationship between climate change and fire activity. A new approach to reconstruct the relative-area burned based on specific types of charcoal particles (the charcoal-morphotype (CM) fire index) has shown significant correlations to recorded forest fires. Here we assess the utility of the CM derived from an analysis of charcoal morphotypes in sediment cores from Opatcho Lake (British Columbia, Canada) using two independent paleoclimate proxies over the last 400 years, and since the mid Holocene. Over the past 400 years, significant correlations between the CM fire index and independent climate reconstructions (diatom-inferred salinity, dendroclimatic reconstructions of temperature and precipitation, and reconstructions of the Pacific Decadal Oscillation) range from 0.35 to 0.42. Similarly, since the mid Holocene the correlation between the CM fire index and independent proxies of past climate (diatom-inferred salinity and temperature inferences from chironomids from the southern interior of British Columbia) range from 0.70 to 0.76. These significant correlations strongly contrast with the very low and insignificant correlations between the CM fire index and total charcoal, suggesting that this approach provides paleofire information not available from traditional techniques. The CM fire index suggests that fires were at least two- and five-fold larger than those observed during the instrumental period, over the last 400 and 6000 years, respectively. [ABSTRACT FROM AUTHOR]

A high-resolution, independent pollen-inferred paleoclimate record and direct algal seasonality data from the actual time of sediment deposition are used to interpret the high-resolution diatom and chrysophyte record of varved Lake Mina, west-central Minnesota, USA during AD 1116-2002. This direct algal seasonality information was obtained by splitting varves into constituent winter-spring and summer lamina, and separately analyzing the siliceous algae in each layer. Analyses of integrated, continuous four-year diatom and chrysophyte samples from a sedimentary sequence show that the time period AD 1116-1478 (ie, the Atlantic-centered 'Medieval Climate Anomaly' (MCA)) was characterized by periods of vigorous and prolonged spring mixing, suggesting that ice-out occurred early. However, the warm summer temperatures in the MCA, particularly in a massive drought spanning AD 1300-1400, frequently caused the lake to stratify strongly, leading to nutrient depletion. During AD 1478-1870 (ie, the Atlantic-centered 'Little IceAge' (LIA)), Lake Mina was characterized by weak spring circulation and increasing nutrient depletion, suggesting late ice-out conditions. Strong summer stratification and/or nutrient depletion in both time periods is shown by the occurrence of the nutrient-poor oligotrophic taxon Cyclotella pseudostelligera. The diatom and chrysophyte assemblages of the period of Euro- American settlement AD 1870-2002 show higher nutrient availability and increased spring mixing intensity, as a result of forest clearance and increasingly earlier ice-out (documented in regional historical records). [ABSTRACT FROM AUTHOR]

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]

Abstract: A paleolimnological study of lake-level changes in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area in northwestern Ontario, indicates large fluctuations have occurred over the Holocene. Analyses are based on diatoms, the proportion of chrysophyte scales to diatoms and organic matter content from near-shore sediment cores. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several transects in Lake 239. Declines of ∼1–3 m occurred during the late Holocene, whereas declines of at least 8 m occurred during the more arid mid-Holocene. These results provide the first substantive evidence of large declines in lake level in northwestern Ontario during the mid-Holocene. Conditions during the mid-Holocene may provide a partial view of future conditions under increasing global temperatures. [Copyright &y& Elsevier]

Aim The bias in modern North American pollen assemblages by landscape disturbance from Euro-American settlement has long been overlooked in the construction of pollen–climate transfer functions. Our aim is to examine this problem and to develop an unbiased pre-settlement pollen–climate transfer function, and to test its performance and inference power in comparison with commonly used techniques. Location Minnesota, USA, is of palaeoclimatic interest because within the state are located two continental-scale ecotones, controlled by temperature and available moisture. Shifts of these ecotones can be tracked using palaeoecological techniques. Methods Using a data set of pre-settlement pollen assemblages from 133 lakes, which were coupled to climate data from the earliest instrumental records (i.e. 1895–1924), a pre-settlement pollen–climate data set was developed that lacked the influence of anthropogenic landscape disturbance. A corresponding modern pollen data set (from lake sediment core tops) and a modern climate (i.e. 1961–90) data set were also developed. The two pollen sets were compared to demonstrate the effects of landscape disturbance from human activities. Ordination (redundancy analysis with Monte Carlo permutation tests) and regression techniques (generalized linear modelling) were used to establish the relationships between the early instrumental climate variables and pre-settlement pollen assemblages and individual taxa, respectively. Transfer functions for the most suitable climate variables (i.e. those forming a minimal set of non-collinear climate variables that explained the greatest amount of pollen variance) were developed from the pre-settlement data set using bootstrapping. Results Comparison of pre-settlement pollen and modern pollen showed an over-representation of Ambrosia, Chenopodiaceae and Poaceae, and an under-representation of arboreal taxa (e.g. Pinus, Quercus, Ostrya) in the modern assemblages. Not surprisingly, ordination and regression techniques showed a strong relationship between the early instrumental climate variables and pre-settlement pollen assemblages and taxa. Transfer functions were developed for May and February mean temperature and available moisture. Pre-settlement transfer functions substantially improved the root mean squared error by 37–72% in comparison with modern transfer functions inferring pre-settlement conditions, suggesting that the modern transfer functions have poorer predictive abilities. Main conclusions For climatic reconstructions, there can be a serious distortion of inferences based solely on modern pollen–climate data sets in regions where anthropogenic landscape disturbance has occurred. By using historical climate data, coupled with pre-disturbance pollen assemblages, robust transfer functions for temperature and effective moisture were developed. [ABSTRACT FROM AUTHOR]

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]

The relationships between diatom assemblages and important limnological variables were investigated in 494 lakes from northeastern North America (Pennsylvania, USA, to Nova Scotia, Canada). The limnological variable most significantly related to diatom assemblages was lake water pH, although dissolved organic carbon and nutrients were also important. Based on these strong relationships, highly significant diatom-based inference models were developed to reconstruct key limnological variables based on diatom assemblages using weighted averaging (WA), maximum likelihood (ML), and modern analogs technique (MAT). The performances of the pH-inference models were high, similar, and significant (WA: r2boot = 0.89, root mean squared error of prediction (RMSEP) = 0.43; ML: r2boot = 0.89, RMSEP = 0.45; MAT: r2boot = 0.89, RMSEP = 0.46). In addition, distribution of sites along a pH gradient did not have the anticipated bias, especially with respect to the WA model, although an evenly distributed study set did result in slightly less noise. While some regionally specific information may be lost by utilizing a large number of lakes from a wide geographic area, the broad limnological gradients allow a more realistic, accurate, and complete description of the ecological characteristics of diatom species. In addition to providing new autecological data on diatoms for northeastern North America, these inference models can now be used to infer accurately and precisely lake water pH and associated variables for limnologists and lake managers. [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]

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]

Faunal records of 20 common midge species (Diptera: Chironomidae) in 32 African surface waters with salinities ranging from 20 to 41 000 µS·cm–1 were used to develop inference models for quantitative reconstruction of past salinity variations from larval chironomid fossils preserved in lake sediments. Weighted-averaging regression and calibration models using presence–absence data (P/A) and presence–absence data with tolerance down-weighting (P/Atol) produced bootstrapped coefficients of determination (r2) of 0.78 and 0.81, respectively, and root mean squared errors (RMSE) of prediction of 0.42 and 0.39 log conductivity units. Historical conductivity data from African lakes are scarce. Therefore, model performance was tested in time by comparing chironomid-inferred conductivity estimates with the corresponding diatom-inferred estimates in sediment records of two fluctuating lakes in the Rift Valley of Kenya. A hybrid procedure in which presence–absence calibration models were applied to abundance-weighted fossil data yielded significantly higher correlation between chironomid- and diatom-inferred time series (Lake Oloidien AD 1880–1991, r2 = 0.76–0.78; Crescent Island Crater AD 900–1993, r2 = 0.56–0.61) than by applying the same models to presence–absence fossil data (r2 = 0.47–0.56 and 0.26–0.42, respectively). Overall, model performance confirms that Chironomidae are valuable bioindicators for natural and man-made changes in the water balance of African lakes. [ABSTRACT FROM AUTHOR]

1. Lake managers suspect that taste and odour-causing algal blooms are increasing in frequency and intensity, although long-term monitoring records are scarce, and a number of critical scientific and management questions remain unanswered. 2. In nutrient-poor lakes and reservoirs, these events are caused primarily by sporadic outbreaks of some chrysophyte algae, which leave identifiable markers in lake sediments. We examine the siliceous remains of these organisms in more than fifty boreal lakes at broad temporal and spatial scales. 3. Colonial scaled chrysophytes, including the taste and odour-causing Synura petersenii, have increased markedly in more that 90% of the lakes examined since pre-industrial times. 4. Detailed stratigraphic analyses of two lakes show a rise in the abundance of colonial taxa in the 1930s to 1950s, with a sharp increase over the past two decades. 5. An examination of biogenic silica and biological ratios in Crosson Lake, Ontario, Canada, indicate that these changes represent true increases in the absolute abundance of colonial chrysophytes. 6. Rapid increases over the past two decades indicate that these trends are the result of one or more anthropogenic stressors that are operating at a broad, regional scale. [ABSTRACT FROM AUTHOR]

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]

Six high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the northern prairies of North America, show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia. The predominant broad-scale pattern seen at all sites is a major shift in moisture regimes from wet to dry, or vice versa (depending on location), that occurred after a period of relative stability. These large-scale shifts at the different sites exhibit spatial coherence at regional scales. The three Canadian sites record this abrupt shift between anno Domini 500 and 800, and subsequently conditions become increasingly variable. All three U.S. sites underwent a pronounced change, but the timing of this change is between anno Domini 1000 and 1300, thus later than in all of the Canadian sites. The mechanisms behind these patterns are poorly understood, but they are likely related to changes in the shape and location of the jet stream and associated storm tracks. If the patterns seen at these sites are representative of the region, this observed pattern can have huge implications for future water availability in this region. [ABSTRACT FROM AUTHOR]

A new diatom record from Lake Victoria’s Pilkington Bay, subsampled at 21- to 25-year intervals and supported by 20 AMS dates, reveals a ∼10,000 calendar year environmental history that is supported by published diatom and pollen data from two nearby sites. With their chronologies adjusted here to account for newly documented ancient carbon effects in the lake, these three records provide a coherent, finely resolved reconstruction of Holocene climate change in equatorial East Africa. After an insolation-induced rainfall maximum ca. 8800–8300 cal yr B.P., precipitation became more seasonal and decreased abruptly ca. 8200 and 5700 yr B.P. in apparent association with northern deglaciation events. Century-scale rainfall increases occurred ca. 8500, 7000, 5800, and 4000 yr B.P. Conditions after 2700 yr B.P. were generally similar to those of today, but major droughts occurred ca. 1200–600 yr B.P. during Europe’s Medieval Warm Period. [Copyright &y& Elsevier]

Inferences of past climatic conditions from a sedimentary record from Big Lake, British Columbia, Canada, over the past 5,500 years show strong millennial-scale patterns, which oscillate between periods of wet and drier climatic conditions. Higher frequency decadal- to centennial-scale fluctuations also occur within the dominant millennial-scale patterns. These changes in climatic conditions are based on estimates of changes in lake depth and salinity inferred from diatom assemblages in a well dated sediment core. After periods of relative stability, abrupt shifts in diatom assemblages and inferred climatic conditions occur approximately every 1,220 years. The correspondence of these shifts to millennial-scale variations in records of glacial expansion/recession and ice-rafting events in the Atlantic suggest that abrupt millennial-scale shifts are important to understanding climatic variability in North America during the mid- to late Holocene. Unfortunately, the spatial patterns and mechanisms behind these large and abrupt swings are poorly understood. Similar abrupt and prolonged changes in climatic conditions today could pose major societal challenges for many regions. [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.

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]

Throughout the twentieth century, waterfront industries in Cornwall (Ontario, Canada) discharged significant quantities of mercury (Hg) and other industrial effluents to the St. Lawrence River (SLR), which accumulated in downstream sediments and currently persist in some nearshore areas. While a great deal of research has investigated the bioavailability and movement of this legacy Hg up the food web, considerably less is understood about its impacts on the algae that live on these contaminated sediments. This study examined diatom (Bacillariophyceae) responses to present-day and historical sedimentary contamination on the SLR at Cornwall. Surface sediments collected from contaminated zones and upstream reference sites were analyzed alongside a dated sediment core from a contaminated area to evaluate the diatom community assemblage shifts and cell deformations (teratologies) in response to elevated concentrations of sedimentary Hg and other contaminants. Results suggest that elevated sedimentary metal concentrations have a small but significant effect on diatom community assemblage structure and incidence of teratologies. However, it appears that excess nutrient loading from waterfront industry was historically a more important driver of diatom assemblage structure than other industrial effluents. [ABSTRACT FROM AUTHOR]

Descriptions of regional climate expression require data from multiple lakes, yet little is known of how variation in records within morphometrically complex lakes may affect interpretations. In northeast Ontario (Canada), this issue was addressed using records of pollen, pigments, and diatoms in three sediment cores from two small boreal lakes spanning the last ~6000 years. Pollen analysis suggested warm conditions between ~6000 and ~4000 cal yr BP, coherent with previous assessments from boreal eastern Ontario and western Quebec. Analysis of phototrophic communities from fossil pigments and diatom valves suggested relatively eutrophic conditions with lower lake-levels during this interval. Generalized additive model trends identified significant regional changes in pollen assemblages and declines in pigment concentrations after ~4000 cal yr BP consistent with cooler and wetter climate conditions that resulted in regional lake oligotrophication and increased lake levels during the late-Holocene. Despite contemporaneous changes in pollen and pigment biomarkers across lakes, cores collected from adjacent basins of the same lake (Green Lake) did not show similar trends in fossil pigments likely reflecting preferential deposition of clay-rich allochthonous material in the deeper central basin and suggesting that regional signals in climate may be complicated by lake- or basin-specific catchment processes. [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]

Long-term, large-scale perspectives are necessary for understanding climate variability and its effects on ecosystems and cultures. Tree ring records of the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) have documented major hydroclimatic variability during the last millennium in the American West, but fewer continuous, high-resolution hydroclimate records of the MCA-LIA period are available for eastern North America, particularly during the transition from the MCA to the LIA (ca. A.D. 1250–1400). Diatoms (micro-algae with silica cell walls) in sediment cores from three Adirondack (NY, USA) lakes and a hiatus in a wetland peat deposit in the Adirondack uplands provide novel insights into the late Holocene hydroclimate history of the Northeast. These records demonstrate that two of the region's most extreme decadal-scale droughts of the last millennium occurred ca. A.D. 1260–1330 and ca. A.D. 1360–1390 during a dry-wet-dry (DWD) oscillation in the Adirondacks that contributed to forest fires and desiccation of wetlands in New York and Maine. The bimodal drying was probably related to more extreme droughts farther west and coincided with major events in Iroquoian and Abenaki cultural history. Although the causes of the DWD oscillation in the Adirondacks remain uncertain, changing sea-surface temperatures and solar variability are likely to have played a role. [ABSTRACT FROM AUTHOR]

Inputs of glacial meltwater and changes in climate can profoundly influence lake ecosystems. Anderson and Seton lakes, two morphologically and chemically similar fjord lakes within the Fraser River Basin, British Columbia, experience a common biogeoclimatic setting, yet contrasting turbid-water influences from a hydroelectric development which diverts glacially-turbid water into Seton Lake, but not Anderson Lake. We conducted a comparative paleolimnological study of these two lakes to infer climatic and hydro-system influences affecting the freshwater algal community over the past ~ 200 years. Paleolimnological analysis of multiple cores for sedimentary diatom assemblages from Seton Lake revealed substantial diversion-related reductions in diatom concentrations and fluxes following the completion of the Bridge River Diversion (ca. 1950). Diatom compositional changes in Seton Lake were consistent with decreased light penetration due to increased turbidity. These changes did not occur in Anderson Lake, indicating the changes in the Seton Lake cores were likely driven by inflow of the glacially-turbid waters. Both lakes exhibited diatom compositional changes ca. 1980, with a rise in Lindavia comensis coincident with significant increases in local mean annual air temperatures and presumably associated limnological changes. Modern phytoplankton data, collected as part of this study, provides support for the occurrence of different L. comensis morphs throughout the sampling period (May–October) in Anderson Lake and in the fall in Seton Lake. The rise of L. comensis in both Anderson and Seton lakes is conceivably linked to the recent ice-free conditions enabling this taxon to persist throughout the year. [ABSTRACT FROM AUTHOR]

Evaluation of large lake response to centennial changes in land use and climate can be complicated by high spatial and hydrological complexity within their catchments, particularly in regions of low relief. Furthermore, large lakes can exhibit abrupt changes in structure and function that obscure causes of eutrophication.We provide the first quantification of historical trends in lake production, cyanobacterial abundance, sediment geochemistry and diatom composition since c. 1800 in Lake Manitoba, the 29th largest lake in the world, and compared them to Lake Winnipeg, a morphologically similar, adjacent basin with a 10‐fold larger catchment and an abrupt increase in production around 1990.Before 1900, Lake Manitoba was mesotrophic, with low sedimentary concentrations of carbon, phosphorus, nitrogen, cyanobacteria and algal pigments, as well as assemblages of low‐light‐adapted benthic diatoms. Analysis of pigment time‐series with hierarchical generalised additive models revealed that Lake Manitoba eutrophied during 1900–1930 as a consequence of the development of intensive agriculture within its local catchment, but thereafter exhibited stable cyanobacterial densities with limited expansion of N2‐fixing cyanobacteria despite persistent eutrophication.Lake Manitoba did not undergo an abrupt change as seen in Lake Winnipeg.These findings suggest that catchment size had little influence on water quality degradation and that nutrient influx from proximal agricultural sources was sufficient to initially degrade these large prairie lakes. The abrupt change in Lake Winnipeg around 1990 required additional intensification of local land use that did not occur in the Lake Manitoba catchment. [ABSTRACT FROM AUTHOR]

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]

Aquatic palynomorphs have the potential to provide valuable insights into past environments but little has been done to assess the utility of freshwater dinoflagellate cysts as palaeoecological proxies of lacustrine environments. This study is the first to compare the distribution of freshwater dinoflagellate cysts found in surface sediments from 32 boreal lakes with varying physical and chemical characteristics. Cysts of Fusiperidinium wisconsinense , Parvodinium umbonatum , and Peridinium willei were nearly ubiquitous, but of the remaining cyst taxa, only Parvodinium inconspicuum , Peridinium limbatum , and Peridinium volzii were abundant but present in a smaller number of lakes. Assemblage composition broadly clustered into three groups: 1) an assemblage (primarily Peridinium spp.) associated with relatively shallow, mesotrophic lakes; 2) an assemblage dominated by F. wisconsinense in mesotrophic lakes with intermediate depth and relatively high pH and alkalinity; and 3) an assemblage dominated by P. umbonatum in deeper oligotrophic lakes. Moreover, changes in dinoflagellate cyst assemblage were assessed in a sediment core spanning the Holocene from nearby Gall Lake and interpreted alongside other previously published paleoecological proxies from this core. High abundances of many dinoflagellate cysts including P. limbatum , P. willei , F. wisconsinense , and P. inconspicuum were observed when lake production was high at a time when lake-levels were low and climate was more arid than today. This study suggests that dinoflagellate cysts in lacustrine sediments are, when abundant, promising both in modern and core samples, and the relationships between assemblages today and in the past can contribution additional information in understanding past ecological conditions. • Calibration set quantifying cyst distribution in relation to environmental variables. • Depth, trophic state and pH were variables related to modern cyst distribution. • Cyst assemblage was also compared with established proxies from a Holocene core. • Change in cyst diversity covary with lake level (diatoms) and production (pigments). • Cysts provide complementary interpretations suggesting their potential as proxies. [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]

Boreal regions and their freshwater ecosystems may be susceptible to future climate change under projected warmer conditions. Northwest Ontario is a boreal region adjacent to the climatically sensitive prairie-forest ecotone (PFE). Pollen records spanning the Holocene from near the Manitoba/Ontario border to lakes up to ~300 km east of the PFE indicate a warmer and possibly wetter mid-Holocene period across northwest Ontario from ~8000 to 4500 cal. yr BP. To date, only one Holocene-scale record of changes in effective moisture, as indicated through diatom-inferred changes in lake level (Experimental Lakes Area (ELA) Lake 239), is available from this region. Our study expands the regional context of Holocene climate changes, with the analysis of diatom assemblages in sediment cores from two additional lakes, which span a distance of over 200 km across the present-day boreal forest, from 80 km west of ELA Lake 239 to ~150 km to the northeast. In cores from both lakes, benthic taxa predominate in the early-to-mid-Holocene period, with a low abundance of planktonic taxa, suggesting lower lake levels by ~2–5 m. Increases in the abundance of planktonic taxa to >50% occurred in both lakes beginning ~4500–4000 cal. yr BP suggesting positive water balance over the last 4000 years in comparison with the mid-Holocene period. This new evidence supports a regional mid-Holocene period of aridity, with reduced water levels across the boreal region of northwest Ontario. If future climate change results in lower effective moisture, then conditions could become similar to the mid-Holocene period aridity, leading to real challenges for the management of water resources across the region. [ABSTRACT FROM AUTHOR]

Pollen and diatom assemblages from well-dated sediment cores from three lakes forming a west-to-east transect across the boreal forest in northwest Ontario (Canada) were used to evaluate the timing and nature of the movement of the prairie-forest ecotone (PFE) across the Holocene. Changes in vegetation, temperature, and effective moisture were inferred from pollen and pollen-based transfer functions. Analyses indicated site-specific vegetational and climate changes across short spatial distances, with prolonged prairie-like conditions during the middle Holocene at the westernmost site. Increased reconstructed temperatures at this westernmost site occurred from ∼9000 to 3000 cal yr BP, alongside increases in diatom-inferred lake levels beginning at ∼6000 cal yr BP. The abundance of Quercus peaked concurrently with rising lake levels before declining to trace levels by ∼3000 cal yr BP. Increases in the abundance of non-arboreal pollen between ∼8500 and ∼4500 cal yr BP at the more eastern lakes suggest relatively delayed and truncated PFE influence, before the reestablishment of primarily boreal taxa by ∼4500 cal yr BP, coincident with diatom-inferred increases in lake levels. This study shows that the PFE moved both farther east and north than previously determined, but generally agrees with established patterns in vegetation from other studied regions along the PFE. [ABSTRACT FROM AUTHOR]

Freshwater lakes in the Canadian Maritime provinces have been detrimentally influenced by multiple, often synergistic, anthropogenically-sourced environmental stressors. These include surface-water acidification (and a subsequent decrease in calcium loading to lakes); increased nutrient inputs; watershed development; invasive species; and climate change. While detailed studies of these stressors are often hindered by a lack of predisturbance monitoring information; in many cases, these missing data can be determined using paleolimnological techniques, along with inferences on the full extent of environmental change (and natural variability), the timing of changes, and linkages to probable causes for change. As freshwater resources are important for fisheries, agriculture, municipal drinking water, and recreational activities, among others, understanding long-term ecological changes in response to anthropogenic stressors is critical. To assess the impacts of the major water-quality issues facing freshwater resources in this ecologically significant region, a large number of paleolimnological studies have recently been conducted in Nova Scotia and southern New Brunswick. These studies showed that several lakes in southwestern Nova Scotia, especially those in Kejimkujik National Park, have undergone surface-water acidification (mean decline of 0.5 pH units) in response to local-source SO2 emissions and the long-range transport of airborne pollutants. There has been no measureable chemical or biological recovery since emission restrictions were enacted. Lakewater calcium (Ca) decline, a recently recognized environmental stressor that is inextricably linked to acidification, has negatively affected the keystone zooplankter Daphnia in at least two lakes in Nova Scotia (and likely more), with critical implications for aquatic food webs. A consistent pattern of increasing planktonic diatoms and scaled chrysophytes was observed in lakes across Nova Scotia and New Brunswick, suggesting that the strength and duration of lake thermal stratification has increased since pre-industrial times in response to warming temperatures (∼1.5 °C since 1870). These include three lakes near Bridgewater, Nova Scotia, that are among the last known habitat for critically endangered Atlantic whitefish ( Coregonus huntsmani). Overall, these studies suggest that aquatic ecosystems in the Maritime Provinces are being affected by multiple anthropogenic stressors and paleolimnology can be effective for inferring the ecological implications of these stressors. [ABSTRACT FROM AUTHOR]

Osmotic stress can accompany increases in solute concentrations because of freezing or high-salt environments. Consequently, microorganisms from environments with a high-osmotic potential may exhibit cross-tolerance to freeze stress. To test this hypothesis, enrichments derived from the sediment and water of temperate lakes with a range of salt concentrations were subjected to multiple freeze-thaw cycles. Surviving isolates were identified and metagenomes were sampled prior to and following selection. Enrichments from alkali lakes were typically the most freeze-thaw resistant with only 100-fold losses in cell viability, and those from freshwater lakes were most susceptible, with cell numbers reduced at least 100 000-fold. Metagenomic analysis suggested that selection reduced assemblage diversity more in freshwater samples than in those from saline lakes. Survivors included known psychro-, halo- and alkali-tolerant bacteria. Characterization of freeze-thaw-resistant isolates from brine and alkali lakes showed that few isolates had ice-associating activities such as antifreeze or ice nucleation properties. However, all brine- and alkali-derived isolates had high intracellular levels of osmolytes and/or appeared more likely to form biofilms. Conversely, these phenotypes were infrequent amongst the freshwater-derived isolates. These observations are consistent with microbial cross-tolerance between osmotic and freeze-thaw stresses. [ABSTRACT FROM AUTHOR]

1. Until recently, the distribution of diatom species assemblages and their attributes (e.g. species richness and evenness) in relation to water depth have been identified but not quantified, especially across several lakes in a region. Here, we examined diatom assemblages in the surface sediment across a water-depth gradient in eight small, boreal lakes in north-western Ontario, minimally disturbed by human activities. 2. Surface-sediment diatom assemblages were collected within each lake along a gentle slope from near-shore to the centre deep basin of the lake, at a resolution of ∼1 m water depth. Analysis of sedimentary samples provided an integrated view of assemblages that were living in the lake over several years and enabled a high-resolution analysis of many lakes. The study lakes ranged in water chemistry, morphology and size and are located along an east-west transect approximately 250 km long in north-western Ontario (Canada). 3. The majority of diatom species were distributed along a continuum of depth, with those taxa having similar habitat requirements forming distinct, though overlapping, assemblages. Three major zones of diatom assemblages in each lake were consistently identified: (i) a near-shore assemblage of Achnanthes ( sensu lato), Nitzschia, Cymbella ( sensu lato) and other benthic species; (ii) a mid-depth assemblage of small Fragilaria ( sensu lato) /small Aulacoseira and various Navicula taxa; and (iii) a deep-water assemblage of planktonic origin (mainly Discotella spp.). 4. The depth of the transition between assemblage zones varied between the eight lakes. The boundary between the deep-water planktonic zone and the mid-depth benthic zone varied according to water chemistry and was probably related to light attenuation. The boundary was deeper in lakes with the lower dissolved organic carbon and total phosphorus (TP) (i.e. less light attenuation) and vice versa. 5. Generally, species richness, species evenness and turnover rate of species as a function of depth were significantly lower in the planktonic assemblage zone in comparison with the two zones nearer the shore. Reproducibility of species and assemblage distributions across the depth gradient of the lakes illustrated that, despite potential for sediment transport, detailed ecological characterisation of diatom species can be gleaned from sedimentary data. Such data are often lacking, particularly for near-shore benthic species. [ABSTRACT FROM AUTHOR]

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]

Phytobenthos are often underrepresented in both limnological and paleolimnological studies but may play key roles in whole‐lake production and ecosystem processes including eutrophication, food‐web dynamics, and ecosystem state changes.Photosynthetic pigments, stables isotopes, and diatoms were quantified from surface sediments (0–1 cm) collected across a depth transect of a small, DOC‐rich, mesotrophic lake in boreal northwestern Ontario to assess spatial variation in phytobenthos abundance and production.Maximal concentrations of siliceous algae and cyanobacteria pigments occurred at ~2–6 m depth, with abundant tychoplanktonic diatoms, depleted sedimentary δ13C values, and elevated ratios of precursor chlorophyll a to product pheophytin a, all aligning well with the depths of the thermocline, epilimnetic mixing, and maximum light penetration.These patterns demonstrated the presence of three discrete community assemblages, with greatest mass accumulation occurring at intermediate depths where warm illuminated sediments provide habitat for tychoplanktonic diatoms and cyanobacteria between turbulent shallows and cold and dark depths.If widespread among boreal lakes, this tychoplanktonic zone may exert important effects on whole‐lake production, carbon sequestration, benthic‐pelagic food‐web coupling, eutrophication, and ecosystem state change. [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]

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]

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]

The introduction of salmonids into lakes of western North America for sport fishing is a widespread phenomenon. While numerous investigations have documented cascading trophic interactions upon the introduction of fish into naturally fishless systems, little research has been done to investigate the importance of natural fish status (fishless vs. fish bearing) in modulating historical food web response to dual forcing by bottom-up (resource regulation from nutrients) and top-down (planktivory from stocked fish) processes. We used the paleolimnological record to reconstruct food web changes in four lakes in interior British Columbia that have been stocked with rainbow trout since the early to mid-1900s. Analysis of pigments, diatoms, and Cladocera was undertaken in cores from all lakes. We predicted that if fish were important in structuring cladoceran abundance and composition, we would document a decline in the abundance of large daphnids post-stocking in our two naturally fishless lakes, and little change in the two fish-bearing lakes. Instead, we documented increased abundance of large daphnids after stocking in all lakes in the early to mid-1900s, a finding inconsistent with size-selective predation from planktivorous fish. Further, our data suggest that deep, low-oxygen refugia may be important in sustaining populations of large Daphnia, a process which was enhanced by increased nutrients and lake production according to sub-fossil diatom and pigment analyses. This study shows that fish stocking does not invariably result in a decrease in large-bodied Cladocera and that nutrients and lake type can modulate the response of invertebrate planktivores. [ABSTRACT FROM AUTHOR]

Historical changes in Holocene climate in northeastern Ontario were quantified using analyses of sedimentary pollen, diatoms, and pigments in a small boreal lake. Modern analog reconstructions of average temperature from Holocene pollen assemblages of Charland Lake showed temperature was ~2°C warmer than present conditions ~7800–4500 cal. yr BP, a time period consistent with the Holocene thermal maximum (HTM). Pollen data suggest a two-phase HTM: warm and dry conditions based on the presence of primarily Pinus spp., followed by warm and wet conditions based on increases in cedar. Overall, algal production was low during the HTM, as reflected by low concentrations of pigments and diatoms. In the late HTM, increases in cedar pollen and planktonic diatoms suggest sustained increases in water levels for the remainder of the Holocene. During the Post-HTM Period (~4500–2000 cal. yr BP), a period that was warmer than today but cooler than the HTM, overall pigment production was significantly higher than all other periods. However, changes in diatom species composition suggest this period was not uniform, with variation occurring between diatoms indicative of higher and lower nutrient levels. The last ~2000 cal. yr BP was less productive than the Post-HTM Period but more productive than the HTM with higher production from diatoms and cyanobacteria. This study suggests that the relationship between climate and lake water production can be quite complex, and that changes in temperature, precipitation, light, lake levels, and mixing patterns are among factors that are related to changes in subfossil phototroph assemblages. [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]

A top–bottom paleolimnological analysis of 30 undisturbed lakes in the Experimental Lakes Area (ELA) in northwest Ontario showed marked increases in the relative abundance of colonial scaled chrysophyte taxa in most lakes since pre-industrial times. The increase in abundance of colonial taxa was primarily driven by Synura sphagnicola in small, high-nutrient, and warm lakes. The colonial taxa Synura petersenii and Synura echinulata also increased in the deeper study lakes. Detailed analysis of sediment cores from six lakes revealed that these changes occurred in the early to mid-1900s. Based on the modern distribution of scaled chrysophytes relative to lake physicochemical data, we propose multiple processes that may be contributing to these changes, such as changes in lake thermal properties and historical changes in the flux of important particles and solutes to lakes. Additional mechanisms that may contribute to the observed changes are discussed, including trophic interactions and changes in limnetic pCO2. Because the ELA is relatively undisturbed, our findings provide important information on the drivers responsible for increases in colonial chrysophyte abundance, which have been documented in studies of lakes in central Ontario and northeast United States. [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]