Multiple anthropogenic drivers behind upward trends in organic carbon concentrations in boreal rivers

Increases of riverine organic carbon concentrations have been observed across the northern hemisphere over the past few decades. These increases are the result ofmultiple environmental drivers, but the relative importance of the drivers is still unclear.We analyzed a dataset of>10 000 observations of riverine total organic carbon (TOC) concentrations and associatedwater chemistry and hydrological observations from1993 to 2017. The observations span a∼600 kmnorth–south gradient from30 individual river systems in Finland.Our data show significantly increasing TOC concentrations in 25 out of 30 systems, with an average increase from12.0 to 15.1 mg l. The observed increase in riverine TOC concentrations led to an increase of 0.28Mt in annual TOC load to the Baltic Sea from1993 level to 2017 level.We analyzed the role of three putative environmental drivers of the observed TOC trends.Multiple regression analysis revealed that themost common driver was discharge, which alone explainedTOC increases in 13 rivers, whereas pH and temperature were less important drivers (sole predictor in one and zero rivers, respectively). Different permutations of these three drivers were also found to be significant; the combination of discharge and pHbeing the most common (4 rivers). Land usewas not in general linkedwith trends in TOC, except for the proportion of ditched land in the catchment, whichwas significantly correlatedwith increases in TOC concentration. Land use showed significant relationships with trends in discharge and pH.We also found that catchment characteristics are regulating the extent of these regional or global environmental changes causing the upward trends of riverine organic carbon.