172
K. Fennel· T. Neumann
values Po and Zo are the winter concentrations which serve also as initial concentrations. The numeric values for the biological parameters are given in
Table l.
The solid walls bordering the model area affect the circulation patterns and
influence the chemical-biological processes nearby. Winds parallel to the walls
cause upwelling of nutrients and increase the phytoplankton concentrations.
However, the spatial extent of these patterns is small. It is described by the first
baroclinic Rossby-radius, which does not exceed 10 km in this area. We assume
no significant influence on the Arkona Sea and the Pomeranian Bight, especially
during the short model period.
3
Results
3.1
Interannual and Regional Variability of the Chemical-Biological Variables
Simulations for 1994 and 1995 were carried out to investigate the interannual
and regional variations of chemical-biological variables according to the external forces. The simulations start after a spinup period of 5 days for the circulation model on 1 March. The initial distributions of temperature and salinity
were computed from climatological data and are identical for both model runs.
The initial concentrations of chemical-biological variables were set at 0.5 mmol
N/m 3 for plankton and detritus. For nutrients a winter concentration of 5 mmol
N/m 3 was assumed, following Nehring (1991). Both model runs differing only in
external forcing, i.e., wind stress, atmospheric temperature and solar radiation.
In response to the wind, a rich spectrum of mesoscale current patterns is generated
which transport and redistribute material.
Time series of the phytoplankton variable, converted to mg chlorophyll/m 3 ,
are shown in Figs. 2 and 3 for the locations of the monitoring stations 113 (Arkona Sea) and 213 (Bornholm Sea). The spring bloom can be seen in both years
with high concentrations of chlorophyll, followed by a period with nearly stationary concentrations. Only short and sporadic fluctuations in phytoplankton
values occur after the decline of the bloom.
The spring bloom event can be regarded as adjustment of the pelagic system
to the changing physical state of water column. According to Sverdrup (1953) the
bloom starts when the mixed layer is shallower than the critical depth. Generally, this criterion is not fulfilled before the formation of thermocline, which is
mainly controlled by meteorological forcing and density stratification. The high
phytoplankton concentrations during the bloom start to decrease when the limiting nutrient is exhausted. Afterwards, a nearly stationary biochemical cycle exists in the stratified water column as shown during May in Figs. 2 and 3. There is
a balance between phytoplankton sources (growing after nutrient uptake) and
sinks (losses by metabolism, grazing by zooplankton and sinking). Nutrient injections from deeper parts of the water column can disturb this balance and in-
K. Fennel· T. Neumann
values Po and Zo are the winter concentrations which serve also as initial concentrations. The numeric values for the biological parameters are given in
Table l.
The solid walls bordering the model area affect the circulation patterns and
influence the chemical-biological processes nearby. Winds parallel to the walls
cause upwelling of nutrients and increase the phytoplankton concentrations.
However, the spatial extent of these patterns is small. It is described by the first
baroclinic Rossby-radius, which does not exceed 10 km in this area. We assume
no significant influence on the Arkona Sea and the Pomeranian Bight, especially
during the short model period.
3
Results
3.1
Interannual and Regional Variability of the Chemical-Biological Variables
Simulations for 1994 and 1995 were carried out to investigate the interannual
and regional variations of chemical-biological variables according to the external forces. The simulations start after a spinup period of 5 days for the circulation model on 1 March. The initial distributions of temperature and salinity
were computed from climatological data and are identical for both model runs.
The initial concentrations of chemical-biological variables were set at 0.5 mmol
N/m 3 for plankton and detritus. For nutrients a winter concentration of 5 mmol
N/m 3 was assumed, following Nehring (1991). Both model runs differing only in
external forcing, i.e., wind stress, atmospheric temperature and solar radiation.
In response to the wind, a rich spectrum of mesoscale current patterns is generated
which transport and redistribute material.
Time series of the phytoplankton variable, converted to mg chlorophyll/m 3 ,
are shown in Figs. 2 and 3 for the locations of the monitoring stations 113 (Arkona Sea) and 213 (Bornholm Sea). The spring bloom can be seen in both years
with high concentrations of chlorophyll, followed by a period with nearly stationary concentrations. Only short and sporadic fluctuations in phytoplankton
values occur after the decline of the bloom.
The spring bloom event can be regarded as adjustment of the pelagic system
to the changing physical state of water column. According to Sverdrup (1953) the
bloom starts when the mixed layer is shallower than the critical depth. Generally, this criterion is not fulfilled before the formation of thermocline, which is
mainly controlled by meteorological forcing and density stratification. The high
phytoplankton concentrations during the bloom start to decrease when the limiting nutrient is exhausted. Afterwards, a nearly stationary biochemical cycle exists in the stratified water column as shown during May in Figs. 2 and 3. There is
a balance between phytoplankton sources (growing after nutrient uptake) and
sinks (losses by metabolism, grazing by zooplankton and sinking). Nutrient injections from deeper parts of the water column can disturb this balance and in-