Long-term trends (1975–2014) in the concentrations and export of carbon from Finnish rivers to the Baltic Sea: organic and inorganic components compared.

Finnish rivers exported annually on average 1.2 M t carbon, and total organic carbon (TOC) comprised the major share (nearly 80 %) of this export. The mean area specific carbon export was 4.5 g C m−2 year−1. The highest organic carbon export originated from peat dominated catchments, whereas rivers draining agricultural catchments had the highest area-specific inorganic carbon fluxes. Between 1975 and 2014 total inorganic carbon (TIC) concentrations rose more steadily than the respective TOC concentrations. There was an overall decrease in TOC concentrations between 1975 and the mid-1990s. Decreased point source loading contributed to this pattern, although decreases were also detected in rivers without any major pollution sources. From the mid-1990s TOC concentrations started to rise and the increase was even more pronounced than the earlier decrease. The upward trend was ubiquitous, both in time and space, and it was not possible to link the changes to any specific catchment characteristics or another single driver. Warming climate, changes in hydrology and decreases in acidic deposition were the major driving factors although their contribution varied geographically. At the same time both TOC and TIC export increased slightly, but the strong upward trends in TOC concentrations were not reflected as clearly in TOC export trends. This was because changes in water flow had a dominant influence on TOC export to the sea and any changes in concentrations were masked behind the variation in flow.