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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]

Predictive models relating ecological assemblages to environmental conditions are widely used in environmental impact assessment and biomonitoring. Such models are often parameterized using comprehensive ecological sampling and taxonomic identification efforts., Limited resources mean that expensive sampling and analytical procedures should be planned to maximize information gain and minimize unnecessary expense. However, there has been little consideration of cost-effectiveness in parameterizing predictive models using ecological assemblages and no explicit consideration of cost-effectiveness in balancing investment in the crucial aspects of sample size and taxonomic resolution., Using lacustrine diatom (Bacillariophyceae) assemblages from four large-scale ( c. 77 000-1·3 million km2) data sets containing between 207 and 493 lakes, we address the following questions: (1) how does taxonomic resolution affect information content; (2) how does sample size affect information content for different taxonomic resolutions; and (3) what are the most cost-effective strategies for constructing environmental assessment models using diatom assemblages across a range of budgets? We use weighted averaging regression models for p H, phosphorus, salinity and lake depth and realistic data collection costs to examine the relationship between cost and model information content ( R2 and root mean squared error of prediction)., For diatom-based models, finer taxonomic resolutions almost always provide more cost-effective information content than collecting more samples, with (morpho)species being the most appropriate taxonomic resolution for nearly all budget scenarios. Information content exhibits an asymptotic relationship with sample size and budget, with greatest information gain during initial sample size increases, and little gain beyond c. 100 samples. Smaller sample sizes can also achieve surprising predictive power in some cases, suggesting low-cost regional models may be achievable. However, caution is necessary in such an approach, because spatial dependencies in predictions may be missed and analogues with predicted assemblages may be poor., Synthesis and applications. We demonstrate the utility of explicitly considering cost estimates to determine optimal sampling effort and taxonomic resolution for ecological assemblage models. For large, regional biomonitoring programmes, cost-effective sampling could save millions of dollars. Our framework for determining optimal trade-offs in ecological assemblage models is easily adaptable to other taxa and analytical techniques used in biomonitoring and environmental assessment. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the modern distribution of fossil midges within a dimictic lake and explored downcore patterns of inferred lake depths over the last 2000years from previously published proxies. Modern midge distribution within Gall Lake showed a consistent and predictable pattern related to the lake-depth gradient with recognizable assemblages characteristic of shallow-water, mid-depth and profundal environments. Interpretations of downcore changes in midge assemblages, in conjunction with quantitative lake-depth inferences across a priori defined (based on diatom data) ~500-yr wet and dry periods, demonstrated that both invertebrate and algal assemblages exhibited similar timing and nature of ecological responses. Midges were quantified by their relative abundance, concentrations and an index of Chaoborus to chironomids, and all showed the greatest differences between the wet and dry periods. During the low lake-level period of the Medieval Climate Anomaly (MCA: AD 900 to 1400), profundal chironomids declined, shallow-water and mid-depth chironomids increased, chironomid-inferred lake level declined and the Chaoborus-to-chironomid index decreased. The coherence between multiple trophic levels provides strong evidence of lower lake levels in Gall Lake during the MCA. [Copyright &y& Elsevier]

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

Calcium (Ca) concentrations of many softwater lakes on the Canadian Shield have been in decline for decades as a response primarily to regional acid deposition and repeated cycles of forest harvesting. These Ca declines are of ecological interest, as many lakes have fallen to Ca levels detrimental to the fitness of ecologically important Ca-rich cladoceran taxa such as Daphniapulex. However, distinguishing the impacts of reduced Ca from acidification (i.e., decreasing pH) on Ca-demanding fauna has proven difficult due to strong correlations between pH and Ca in softwater lakes. Here, we examine cladoceran sedimentary assemblages from low Ca lakes (mean present-day Ca < 2.0 mg l −1) in the Experimental Lakes Area (ELA) of Ontario, Canada, where Ca concentrations have declined since the 1980s, despite being closed to forestry and remote from (i.e., not downwind of) major sources of acid deposition. Differences between present-day and pre-industrial cladoceran assemblages were greatest among planktonic taxa, including Bosmina spp. and Holopedium glacialis. However, daphniid remains were present in only three of the ten study lakes with minimal directional trends. Overall, in these ELA lakes, daphniid abundance was low historically, and impacts possibly attributable to Ca declines have been obscured by the effects of regional climate warming. [ABSTRACT FROM AUTHOR]

The Winnipeg River Drainage Basin (WRDB), within the boreal forest region of northwest Ontario, is a region that is expected to be negatively affected by climate warming. Inferences of droughts over the past two millennia from Little Raleigh Lake were based on two nearshore sediment cores. The core locations were from depths of ~12 and 15 m and were based on sufficient nearshore sediment accumulation and distance from the modern benthic-to-planktonic diatom boundary, where a distinct shift from dominance of benthic taxa changed to dominance of planktonic taxa in surficial sediments at ~11.8 m. Diatom-inferred depth was based on a model developed from 60 surficial sediments within the study lake. Depth inferences indicate that prolonged periods of aridity occurred from ~ad 950 to 1300 (corresponds to ‘Medieval Climate Anomaly’) and from ~ad 1625 to 1750 (aridity during ‘Little Ice Age’). We found that the core collected from a depth closer to the benthic-to-planktonic diatom boundary was more sensitive to changes in lake level than the deeper core where planktonic diatoms dominated the assemblage. The inferred low-water stands of the past two millennia are well outside of the range of the past ~100 years, suggesting that recent drought history may not be a good estimate of future extremes. [ABSTRACT FROM PUBLISHER]

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