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

Reduction in SO and NO 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 Québec, Canada, and assess the combined effects of these trends on stream chemistry. Annual SO and NO depositions respectively decreased by ~85% (from 23 to ~3 kg ha) and ~70% (from 18 to ~5 kg ha yr) from 1981 to 2016. As a response, stream SO and Ca concentrations decreased by 50% (from 3.9 to 1.9 mg L) and ~35% (from 2.4 to 1.5 mg L), respectively. Stream NO concentration decreased by ~89% (from 0.6 to 0.07 mg L) mainly due to the decline in NO deposition and possibly to increased vegetation N uptake. Unexpectedly, stream alkalinity decreased, likely due to the decline in Ca 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 Ca 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.