Concentrations of dissolved organic carbon (DOC) are increasing in many surface waters across Europe. Two of the main mechanisms proposed to explain this increase are declines in sulfate (SO42−) deposition and changes in climate. Many of the reductions in SO42− have already occurred; climate change related effects are occurring now and will continue in the future. This paper presents the first application of a new version of INCA-C, the Integrated Catchments model for Carbon, which simulates the effects of both climate and SO42− deposition on surface water DOC concentration ([DOC]). The model was applied to Valkea-Kotinen, a small headwater catchment in Finland, where it was able to simulate present-day (1990–2007) trends in [DOC] in the lake and catchment outflow as functions of observed climate and European Monitoring and Evaluation Programme (EMEP)-modelled SO42− deposition. Using a parameter set derived from a present-day calibration, the model was run with two climate scenarios from the Special Report on Emissions Scenarios (SRES) and three EMEP deposition scenarios to simulate surface water [DOC] between 1960 and 2100. The results show that much of the historical increase in [DOC] can be explained as a result of historical declines in SO42− deposition and that surface water [DOC] will continue to increase as climate changes.

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