Changes in midge assemblages reflect climate and trophic gradients across north temperate and boreal lakes since the pre-industrial period

1. North temperate and boreal lakes are affected by a complex suite of anthropogenic stressors, yet little is known about how these ecosystems are faring across the most lake-rich nation, Canada. The NSERC Canadian Lake Pulse Network is the first large-scale national survey of its kind, with the aim of assessing the health of Canadian lakes, determined by the deviation of key variables from baseline pre-industrial conditions (pre-1880 CE).2. We examined midge remains (chironomids and Chaoborus) from modern and pre-industrial sediment intervals in 67 lakes, spanning a range of sizes, ecozones, and degrees of catchment land-use change. A marked northwest/southeast divide was evident in the modern chironomid assemblages, which was partially associated with water quality, physiographical and climatic variables, with a small portion of the variance explained by geography alone.3. Between the pre-industrial and modern sediment intervals, taxa typically found in warmer waters increased across the country (p < 0.001); this pattern also was evident in the littoral assemblages (p = 0.01), providing additional support for the influence of warming.4. Turnover in chironomid assemblages varied across trophic classes. Mesotrophic lakes showed increases in macrophyte-associated taxa, whereas eutrophic lakes showed declines in littoral taxa, perhaps reflecting increased turbidity, and the increase of low-oxygen-tolerant profundal taxa in eutrophic lakes with high human impacts.5. Overall, the distribution and dynamics of chironomid assemblages across Canadian lakes appear to be strongly linked to changes in climate and deep-water oxygen conditions.