Long-term temporal response of eastern canadian lakes chemistry along a large spatial gradient of atmospheric sulfate deposition

A major consequence of the Industrial Revolution was the acidification of continental water bodies by sulfates (SO42−) and nitrates deposited over long-range distance from atmospheric emissions. Regulation policies were implemented in the 1980s leading to the general decrease of SO42− concentrations in freshwaters and progressive recovery from acidification, a complex process that is still ongoing. The surface water SO42− decrease has been linked to declining calcium (Ca2+) and increasing dissolved organic carbon (DOC) concentration. Here, up to 40 years (1983–2023) of chemical data in 46 lakes of Eastern Canada are analyzed for long-term trends and spatial patterns along a large gradient of SO42− deposition. The average lake SO42− concentration decreased along the distance from sulfur dioxide sources. SO42− concentration decreased significantly in every lake and faster close to the emission sources. In 70% of the lakes, the SO42− decline drove concurrent but slower Ca2+ and magnesium declines, leading to an overall increase in the Ca2+/SO42− ratio, in pH and in alkalinity for 72% of the lakes. The Ca2+ concentration is now below the crucial threshold for large zooplankton species in most lakes. Increasing trends in lake sodium concentration suggest an intensification of mineral weathering in catchment soils, potentially linked to rising temperatures, which could be involved in the observed recovery. DOC increased in 67% of lakes which drove a concurrent increase in aluminum in some lakes. The numerous environmental changes in progress across temperate and boreal lakes will deeply modify the structure and functioning of these systems, especially in the context of climatic changes. [Display omitted] •40-year trends for 46 eastern Canadian lakes were analyzed.•Lake SO4 concentration and its rate of decline were higher close to SO2 emissions.•Strong but incomplete lake recovery from acidification was observed.•Sodium increases suggest an increase of soil mineral weathering.•Calcium concentration declined below crucial thresholds for large zooplankton.