A ONE-DIMENSIONAL PHYSICAL-BIOLOGICAL MODEL STUDY OF THE PELAGIC NITROGEN CYCLING DURING THE SPRING BLOOM IN THE NORTHERN NORTH-SEA (FLEX-76)

A one-dimensional model of the pelagic ecosystem was developed and app lied to the spring bloom in the northern North Sea making use of the d ata set obtained during the Fladenground experiment FLEX '76. The phys ical submodel is the second-order turbulence closure model of level 2 type developed by Mellor and Yamada(1974, 1982). The biological submod el 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 p arameter set employed by Fasham et al. did not yield satisfying result s. However, using a parameter set adapted to the North Sea ecosystem w e obtained a realistic overall description of the development of the N orth Sea ecosystem during the spring bloom. We were able to hindcast s uccessfully the onset, duration, magnitude and daily variability of th e net primary production, the magnitude of the PON export flux to the sea bottom, of the bacterial production and of the nitrogen regenerati on within the water column. From the results of the simulation a mass budget of nitrogen fluxes within the euphotic zone and the deeper wate r layers as well as between them was derived. The results of the simul ation 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 microzoo plankton was mainly responsible for the breakdown. The depth dependenc e of the vertical particulate flux obtained from the simulation exhibi ts the hyperbolic character recently found in different oceanic region s. The vertical particulate nitrogen flux shows a stronger decrease th an typically observed for the particulate carbon Aux. This is in corre spondence with the observation that there was a remarkable increase of the C/N ratio of POM with depth during FLEX '76.