Studies on nitrogen dynamics in seepage lakes are seldom undertaken, yet our understanding of the complex pattern of the nitrogen (N) cycle is complicated by its temporal and spatial heterogeneity. This research investigated temporal variation in N concentration, considering different forms of N (NO₃^–, NO₂^–, NH₄⁺, dissolved organic N, particulate organic N) in a Spanish flowthrough seepage lake (Colgada Lake) receiving high N loads. The study was based on monthly data collected over the period 2003–2005 from lake inputs and outputs, vertical profiles at a single representative site in the middle of the lake and fluxes of NH₄⁺ and NO₃^– at the sediment–water interface. The distribution of total N (TN) inputs and outputs did not follow a clear temporal pattern. TN inputs varied from 27.70 to 125 tonnes N month^–1, 75–84% of which is NO₃^–. Temporal variation of concentration profiles for different N forms measured showed significant differences owing to stratification. Ammonium always entered the sediment, whereas sediments acted as either a sink or source of NO₃^–. Fluctuation in N dynamics in this lake was more influenced by external factors, such as the input variability, than by the turnover of nitrogenous substances in the water body. Comparing seasonal N dynamics, there seems to be temporal differences between seepage lakes and drainage lakes. In seepage lakes, dissolved inorganic nitrogen (DIN) peaks were reached in early spring and after overturn, whereas in drainage lakes, the autumn minimum gradually increased to maxima in late winter and during the stratification period.