Seasonal dynamics of runoff pathways and in-stream retention of nitrogen in a northeastern European lowland catchment

Although nitrogen and phosphorus emissions have decreased dramatically after the changes in northeastern Europe in the beginning of the 1990s, the reaction of stream water quality is limited or absent. To explain this, more knowledge is needed about the spatial and temporal distribution of transfer and retention processes. Using a novel combination of existing models we simulated 1991-2004 total nitrogen (Ntot) inputs and in-stream retention in an Estonian catchment. In-stream retention removed 9.5% of the yearly Ntot inputs, and was limited to the summer months. In contrast to other studies, our results indicate that most retention does not occur in lower, but in higher order streams, where low gradients and high sinuosity promote long residence times. We conclude that large soil N pools, long groundwater transit times and possibly increasing autumn and winter runoff due to climate change explain the weak reaction in Ntot loads to the emission decreases.