Increases of dissolved organic carbon in temperate and boreal lakes in Quebec, Canada.

PURPOSE: Increases in dissolved organic carbon (DOC) concentrations have been reported in surface waters worldwide in the last 10 to 20 years. The causes behind these increases have been attributed to many factors, including climate change and decreasing depositions of atmospheric sulphate (SO₄²⁻). Trends in DOC concentrations and their potential causal factors were examined in a network of 30 lakes lying in undisturbed temperate and boreal catchments in the province of Quebec, Canada.
METHODS: Temporal trends in lake DOC concentrations were analysed with the seasonal Kendall test. For each lake, the variation in DOC concentration over time was assessed in light of the variation in SO₄²⁻ concentration in precipitation, air temperature, precipitation and solar radiation using the forward stepwise multiple regression.
RESULTS: Between 1989 and 2006, significant increases in DOC were observed in most of the lakes, the mean rate of change being 0.05 mg L(-1) year(-1). Lake DOC concentrations were significantly explained by different models that yielded a variance explanation ranging from 13% to 77%. The models included long-term temperature variables (i.e. averaged over intervals of 10 years), short-term precipitation variables (i.e. summed over intervals 6 months), radiation (i.e. ice-free period prior to the DOC observation) and precipitation SO₄²⁻ concentration as explanatory variables.
CONCLUSION: Temporal changes in DOC concentrations seem more consistent with the evolution of climate parameters rather than SO₄²⁻ concentrations despite the fact that most lakes were in the process of recovery, showing increases in pH.