A one-dimensional model of the pelagic ecosystem was developed and applied to the spring bloom in the northern North Sea making use of the data set obtained during the Fladenground experiment FLEX '76. The physical submodel is the second-order turbulence closure model of level 2 type developed by Mellor and Yamada (1974, 1982). The biological submodel is a depth-resolved version of the nitrogen flux model of the lower trophic levels in the pelagic proposed by Fasham et al. (1990). The parameter set employed by Fasham et al. did not yield satisfying results. However, using a parameter set adapted to the North Sea ecosystem we obtained a realistic overall description of the development of the North Sea ecosystem during the spring bloom. We were able to hindcast successfully the onset, duration, magnitude and daily variability of the net primary production, the magnitude of the PON export flux to the sea bottom, of the bacterial production and of the nitrogen regeneration within the water column. From the results of the simulation a mass budget of nitrogen fluxes within the euphotic zone and the deeper water layers as well as between them was derived. The results of the simulation suggest that strong herbivorous grazing caused the decay of the bloom. The comparison with the grazing by mesozooplankton as estimated from the observations favors the hypothesis that herbivorous microzooplankton was mainly responsible for the breakdown. The depth dependence of the vertical particulate flux obtained from the simulation exhibits the hyperbolic character recently found in different oceanic regions. The vertical particulate nitrogen flux shows a stronger decrease than typically observed for the particulate carbon flux. This is in correspondence with the observation that there was a remarkable increase of the C/N ratio of POM with depth during FLEX '76.