Nitrogen deposition induced changes in DOC : NO3-N ratios determine the efficiency of nitrate removal from freshwaters

With a unique data set comprising 1041 boreal forested and low human impacted lakes included in three Swedish lake inventories for 1995, 2000 and 2005 and with time series for 12 of the lakes from 1988 to 2008 we show that nitrate-nitrogen (NO3-N) is accumulated in freshwaters along with increasing atmospheric nitrogen deposition (Ndep). At the same time we observe decreasing DOC : NO3-N ratios in the water column. We suggest that NO3-N is accumulated in freshwaters when denitrifying bacteria are limited by their energy source rather than the availability of NO3-N, i.e. at low DOC : NO3-N ratios. We obtained further support for a close relationship between Ndep driven DOC : NO3-N ratios and the efficiency of nitrate removal by using a published global data set on measured nitrate removal rates in unproductive reference streams. Owing to the currently decreasing Ndep in large regions of, for instance, Northern Europe, this process is now reversed, resulting in increasing DOC : NO3-N ratios and more efficient nitrate removal from freshwaters. Depending on NOx emissions, nitrogen limited regions may expand with an immediate effect on nitrate concentrations in freshwaters.