Variable dissolved organic carbon trends in the tributaries of the St. Lawrence River driven by natural geographic features and human activities

Over the past decades, dissolved organic carbon (DOC) concentrations in boreal rivers have increased broadly. At the regional scale, however, the magnitude of these trends varies, likely reflecting differences in natural and anthropogenic catchment features that remain poorly understood. Here we examined long-term DOC trends data from 89 stations located on 54 tributaries of the St. Lawrence River (Qc, Canada) over 35 years, assessing DOC changes among catchments with contrasting natural characteristics and land-use patterns. We observed a significant increase in DOC in 46 stations (0.08–1.54% per year), and a significant decrease in 19 stations (−0.25 to −1.59% per year). When considering climatic variables and land cover features, the proportion of forested area within the watershed emerged as the most significant factor; DOC tended to increase at forest cover >60% and to be stable or decrease at forest cover <60%. Watershed slope, water area, and annual mean precipitation were also significantly associated with DOC changes. This study emphasizes the importance of accounting for land-use differences when interpreting regional patterns of river DOC trends.