Nitrogen load predictions under land management scenarios for a boreal river basin in northern Finland

In Finland municipal and industrial waste water purification has effectively decreased nutrient emissions from point sources leading to improved water quality. No clear effects of decreasing non-point loading (atmospheric deposition, agriculture, forestry) are found, however, and nitrate concentrations are increasing in some rivers. The aim of this study was to determine the origin and timing of inorganic nitrogen loading to the Simojoki using the dynamic, semi-distributed INCA-N model. The simulation results showed that, at the river outlet, only about half of the inorganic nitrogen load originated from anthropogenic sources. The inorganic nitrogen load largely depended on runoff and half of the annual load was centred around the snowmelt period in April–May. There was a risk of increasing nitrogen load due to changes in agricultural land use. Water protection measures at all diffuse sources could decrease the anthropogenic part of the inorganic N load to the sea, but individual measures would only result in small reductions.