Increasing organic C and N fluxes from a northern boreal river basin to the sea

Increasing trends in dissolved organic carbon (DOC) concentrations in lakes and streams across Europe and North America have been reported. The widespread occurrence of these phenomena indicates large-scale causes, e.g. enhanced decomposition of organic soils, changes in hydrology or decreased acid deposition. The Simojoki River basin (3160 km) is located in the Northern Boreal Zone where human impacts are minor. Long-term changes (30–40 years) of organic C and N concentrations and fluxes in the Simojoki River system were studied: both TOC and TON concentrations were increasing, fluctuating between droughts and wet periods. Highest concentrations were detected in 1998–2000 during a period of very high flows, after the drought period of 1994–1997. The average TOC flux increased by 38% during the 1990s compared with the 1980s, while the average TON flux during the same period increased even more, by 42%. Runoff accounts for part but not all of the increase in the TOC and TON outputs. Since 2000, the fluxes have decreased to slightly lower levels but year-to-year variability has remained considerable. Multiple effects are probable, i.e. changes in both hydrology and in catchment soils have impacts on these increasing fluxes.