Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 429

9.4.2.4 Advection-Dispersion Modeling (MIKE21 AD)

The calibration of the transport module of MIKE21 was performed on the basis of
18 months of salinity measurements at 22 points in the lagoon. Good correspondence
between measured and simulated salinity variations ensured accurate calculations
of water exchange between the Baltic and the Vistula Lagoon. The diffusion coef-
ficient obtained during calibration was used later in the eutrophication step. After
several numerical experiments the specific values for calibration parameters were
found, which provided acceptably accurate simulations for both the sea-lagoon water
exchange and saltwater transport toward the remote eastern lagoon corner, where
the Kaliningrad Channel meets the Pregel River . The best simulation salinity results
were obtained after using a dispersion coefficient of 45 m 2 /s.

9.4.2.5 Eutrophication Model (MIKE21 EU)

The input data for the eutrophication model covered the whole of 1994 and included:

• Initial concentrations of simulated chemicals in the Vistula Lagoon
• Concentrations of nitrogen and phosphorus in organic and inorganic forms
and dissolved oxygen for all rivers and point sources
• Concentrations of nitrogen and phosphorus in organic and inorganic forms
and dissolved oxygen at the open boundary (lagoon entrance)
• Water temperature
• Solar radiation

The eutrophication module describes the relation between available nutrients
(nitrogen and phosphorus) and the algae growth in coastal and open waters. It
simulates carbon, nitrogen, and phosphorus content in phytoplankton, zooplankton,
detritus, and, with regard to nitrogen and phosphorus, the concentration of inorganic
nitrogen and phosphorus in the water. It is assumed that dissolved carbon is present
in excessive amounts and it is not included explicitly in the water.
The eutrophication module considers the chlorophyll a concentration calculated
from phytoplankton carbon (biomass). Chlorophyll a concentration is included in
the model because it is the most easily measured parameter representing phytoplank-
ton biomass.
Nutrient loading was used for eutrophication module calibration along with
results of direct measurements of nutrient and chlorophyll a at monitoring stations
in the Vistula Lagoon. Nearly 50 simulations with durations of 6 to 36 h were
conducted during eutrophication module calibration. This resulted in identifying the
set of internal parameters of the model that best fits the field data. 10
Simulations were carried out with a 3-hour time step and grid spacing of
1 km 1 km. Simultaneously, hydrodynamic and transport calculations were carried

out with a 5-min time step and with the same grid spacing. A longer time step for
eutrophication was used because of different time scales of the processes and also
to minimize CPU time.

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