AbstractResults from thirteen years of weekly observations are presented on the nitrogen cycle in Lough Neagh. The data comprised catchment and atmospheric inputs, output via the outflow and calculated losses by sedimentation and denitrification. Nitrate‐nitrogen in the rivers is the dominant input fraction and the nitrate loading has increased over the period observed. 52 % of the input N sediments to the lake bottom, but 65 % of this is lost by denitrification. In spite of increasing nitrogen inputs, the summer soluble nitrate concentrations have decreased due to uptake by a perpetual crop of the cyanobacteriumOscillatoria agardhiiGOMON