Effects of climate and atmospheric deposition on a boreal lake chemistry: A synthesis of 36 years of monitoring data

Reduction in SO42- and NO3- atmospheric deposition in the past decades has improved surface water quality in several catchments but recent studies suggest an increasing influence of climate and dissolved organic carbon (DOC). Here, we report on long-term trends in climate variables, strong acid anions and base cations concentrations in precipitation and at the lake outlet (stream) of a boreal catchment in Quebec, Canada, and assess the combined effects of these trends on streamchemistry. Annual SO42- and NO3- depositions respectively decreased by similar to 85% (from 23 to similar to 3 kg ha(-1)) and similar to 70% (from 18 to similar to 5 kg ha(-1) yr(-1)) from 1981 to 2016. As a response, stream SO42- and Ca2+ concentrations decreased by 50% (from 3.9 to 1.9 mg L-1) and similar to 35% (from 2.4 to 1.5mg L-1), respectively. Stream NO3- concentration decreased by similar to 89% (from 0.6 to 0.07 mg L-1) mainly due to the decline in NO3- deposition and possibly to increased vegetation N uptake. Unexpectedly, stream alkalinity decreased, likely due to the decline in Ca2+ concentration and to an increase in DOC concentration. Variations in stream pH and Na+ concentrations were best explained by climatic changes than by changes in acid deposition, likely reflecting the effect of climate change on chemical weathering in the region. In addition, the average daily temperature between May and September had a strong influence on stream Ca2+ concentration in the last two decades (negative relationship), suggesting an increasing vegetation nutrient uptake caused by improved growth conditions. Overall, decreased acidic deposition resulted in a general recovery of surface water although the parallel increase in DOC concentration prevented from an increase in water alkalinity. Our data also indicate an increasing influence of climate on water chemistry at the study site, probably mediated by increasing weathering rate and vegetation nutrient uptake. (C) 2020 Published by Elsevier B.V.

Automated summary

Reduction in SO42- and NO3- atmospheric deposition in the past decades has improved surface water quality in several catchments, but an increasing influence of climate and dissolved organic carbon (DOC) has been observed. Decreased acidic deposition resulted in a general recovery of surface water, although the parallel increase in DOC concentration prevented from an increase in water alkalinity. The study also indicates an increasing influence of climate on water chemistry at the study site, probably mediated by increasing weathering rate and vegetation nutrient uptake.