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Covering 55% of Canada’s total surface area and stretching from coast to coast to coast, the Canadian boreal zone is crucial to the nation’s economic and ecological integrity. Although often viewed as relatively underdeveloped, it is vulnerable to numerous stressors such as mining, forestry, and anthropogenic climate change. Natural archives preserved in lake sediments can provide key insights by quantifying pre-disturbance conditions (pre-1850 CE) and the nature, magnitude, direction, and speed of environmental change induced by anthropogenic stressors over the past ~150 years. Here, we paired a review of paleolimnological literature of the Canadian boreal zone with analyses of published sediment core data to highlight the effects of climate change, catchment disturbances, and atmospheric deposition on boreal lakes. Specifically, we conducted quantitative syntheses of two lake health indicators: elemental lead (Pb) and chlorophyll a. Segmented regressions and Mann-Kendall trend analysis revealed a generally increasing trend in elemental Pb across the boreal zone until ~1970 CE, followed by a generally decreasing trend to the present. Snapshot comparisons of sedimentary chlorophyll a from recent and pre-industrial sediments (i.e., top-bottom sediment core design) revealed that a majority of sites have increased over time, suggesting a general enhancement in lake primary production across the boreal zone. Collectively, this body of work demonstrates that long-term sediment records offer a critical perspective on ecosystem change not accessible through routine monitoring programs. We advocate using modern datasets in tandem with paleolimnology to establish baseline conditions, measure ecosystem changes, and set meaningful management targets.

Despite decades of research and mitigation efforts, declines in freshwater quality resulting from anthropogenic nutrient input remain a persistent issue worldwide. Canada has the greatest number of freshwater lakes in the world, yet we have a limited understanding of the magnitude and scale at which most lakes have been affected by human activities, namely Land Use/Land Cover (LULC) alterations. In response, the NSERC Canadian Lake Pulse Network has compiled the first nationwide systematic database of lake quality metrics by surveying 664 lakes across 12 ecozones over three years. To assess the influence of catchment development on water quality and its spatial variation, we built models quantifying the association between watershed LULC and water quality. We found that agricultural and urban land use explained the greatest proportion of variation in water quality among LULC categories (R2= 0.20–0.29). Overall, our study highlights that drivers of water quality are similar across regions; however, baseline conditions vary, so freshwater ecosystem management strategies must consider their geographic context to better predict where water quality thresholds will be surpassed.

Accurately identifying the species present in an ecosystem is vital to lake managers and successful bioassessment programs. This is particularly important when monitoring cyanobacteria, as numerous taxa produce toxins and can have major negative impacts on aquatic ecosystems. Increasingly, DNA-based techniques such as metabarcoding are being used for measuring aquatic biodiversity, as they could accelerate processing time, decrease costs and reduce some of the biases associated with traditional light microscopy. Despite the continuing use of traditional microscopy and the growing use of DNA metabarcoding to identify cyanobacteria assemblages, methodological comparisons between the two approaches have rarely been reported from a wide suite of lake types. Here, we compare planktonic cyanobacteria assemblages generated by inverted light microscopy and DNA metabarcoding from a 379-lake dataset spanning a longitudinal and trophic gradient. We found moderate levels of congruence between methods at the broadest taxonomic levels (i.e., Order, RV=0.40, p0.0001). This comparison revealed distinct cyanobacteria communities from lakes of different trophic states, with Microcystis, Aphanizomenon and Dolichospermum dominating with both methods in eutrophic and hypereutrophic sites. This finding supports the use of either method when monitoring eutrophication in lake surface waters. The biggest difference between the two methods was the detection of picocyanobacteria, which are typically underestimated by light microscopy. This reveals that the communities generated by each method currently are complementary as opposed to identical and promotes a combined-method strategy when monitoring a range of trophic systems. For example, microscopy can provide measures of cyanobacteria biomass, which are critical data in managing lakes. Going forward, we believe that molecular genetic methods will be increasingly adopted as reference databases are routinely updated with more representative sequences and will improve as cyanobacteria taxonomy is resolved with the increase in available genetic information.