W. Kuhn et G. Radach, A ONE-DIMENSIONAL PHYSICAL-BIOLOGICAL MODEL STUDY OF THE PELAGIC NITROGEN CYCLING DURING THE SPRING BLOOM IN THE NORTHERN NORTH-SEA (FLEX-76), Journal of marine research, 55(4), 1997, pp. 687-734
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.