Widespread pH increase and geochemical trends in fifty boreal lakes: Evidence, prediction and plausible attribution to climate and permafrost thaw impacts across northeastern Alberta

This study focuses on 50 boreal lakes and catchments situated in northeastern Alberta, Canada between 55.68°N – 59.72°N and 110.02°W – 115.46°W. Evidence for trends in chemical composition of lakes, including pH increases are provided using Mann-Kendall statistics, geochemical modelling, δ 18 O, δ 2 H, and δ 13 C, which are compared to trend statistics for climate, water balance, and groundwater indicators. Groundwater contributions are generally found to be increasing with water yield and carbon inputs as sites advance along the thaw trajectory. The exception to this is shield lakes which continue to be surface water dominant. Statistical analyses suggest widespread trends, both significant and non-significant, in geochemical parameters across the lake network including pH increases in 46 of 50 lakes. In shale-dominated plateau areas, pH trends are adequately described by changes in HCO 3 - , attributed mainly to carbon input associated with permafrost thaw. For these lakes, prediction improves little if other variables are considered, whereas for post thaw areas, prediction of pH trends improves if water yield trends are also considered. In sub-regions with appreciable carbonate, pH trend prediction improves significantly if values of δ 13 C DIC and Dissolved Inorganic Carbon (DIC) are also considered. We postulate that recent pH trends across the region may only be temporary and that lake acidification may yet occur once permafrost thaw and related carbon imports diminish. • Geochemical trends described for 50 lake network in northeastern Alberta. • Trend analysis supported by Mann-Kendall, PCA and PHREEQC modelling. • Concurrent trends presented for climate, water balance, groundwater, carbon sources. • Analysis of pH drivers used to develop and compare pH prediction models. • Regional response consistent with evolution along a permafrost thaw trajectory.