Historic forest management practices led to widespread aerial application of insecticides, such as dichlorodiphenyltrichloroethane (DDT), to North American conifer forests during ∼1950–1970. Lake basins thus may provide an important archive of inputs and aquatic responses to these organochlorines. We use dated sediment cores from five study lakes in multiple watersheds in New Brunswick (NB), Canada, to provide a regional paleo-ecotoxicological perspective on this potential legacy stressor in remote lake ecosystems. Peak sedimentary levels of p,p′- and o,p′-DDT (ΣDDT) and breakdown products ΣDDE (dichlorodiphenyldichloroethylene) and ΣDDD (dichlorodiphenyldichloroethane) generally occurred during the 1970s to 1980s. Sediments exceeded probable effect levels (PELs) by ∼450 times at the most impacted lake. Modern sediments in all study lakes still contained levels of DDT-related compounds that exceed PELs. For the first time, we show that aerial applications of DDT to eastern Canadian forests likely resulted in large shifts to primary consumers within several lake food webs, principally through lake-specific impacts on zooplankton community composition. Modern pelagic zooplankton communities are now much different compared to communities present before DDT use, suggesting that a regional organochlorine legacy may exist in the modern food webs of many remote NB lakes.

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The development of cyanobacteria blooms is of increasing concern in many lakes worldwide, and as a result, modeling their predictors is vital for understanding where and why they occur. In this study, we developed and analyzed a 640‐lake data set that spans Canada and 12 ecozones to identify the drivers of cyanobacteria biomass and of several key toxin‐ and bloom‐forming genera (Microcystis, Aphanizomenon, and Dolichospermum). The database consisted of an exhaustive list of potential predictors (n = 55), including water chemistry, land‐use, and zooplankton variables. We applied a series of empirical modeling approaches to identify significant predictors and thresholds (generalized linear and additive models, mixed effect regression trees), all while accounting for ecozone variability. Across all modeling approaches, and ecozones total phosphorus was identified as the most important predictor of total cyanobacterial and focal genera biomass. In addition, cyanobacteria across Canada showed significant associations with increasing dissolved organic and inorganic carbon, and several ions. Despite the widely held notion that cyanobacteria are often toxic and/or a poor food source for zooplankton, we found a positive relationship between cyanobacteria and zooplankton, particularly with daphnid and copepod biomass. Localized top‐down forces and evolutionary adaptations resulting from long‐term exposure in eutrophic lakes are among the possible explanations for this observed positive association. By considering a suite of complementary modeling approaches, we found that nonlinear models provided greater predictive power and the random ecozone effect was minor due to the overarching importance of local abiotic and biotic factors.

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 <i>a</i>. 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 <i>a</i> 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.