Resolution in space: The horizontal grid size of the numerical model is 20x20 square kilometer, the vertical resolution is 5 m for the upper 50 m and increasing layer thickness below 50 m up to maximal 19 layers. Fig. 1 illustrates the model boundaries and bathymetry of the North Sea.

Fig. 1: Model boundaries (thick line) and bathymetry (depth in m) of the North Sea.

State variables: Fig. 2 describes the conceptual model with state variables for the nutrients phosphate (dissolved inorganic phosphorus (DIP)) and dissolved inorganic nitrogen (DIN), phytoplankton (chlorophyll), and detritus at the bottom. Copepods (zooplankton) were prescribed by observations. Pelagic detritus and dissolved organic phosphorus in the water column were indirectly used. Primary production is limited in the model by solar radiation, the triggering nutrient and zooplankton grazing due to prescribed monthly mean copepod biomass. DIP and DIN regeneration occured in the pelagic and through a simple parameterization at the bottom via a benthic detritus pool.

Equations: The equations are presented in (Moll, 1998)

Fig. 2: Conceptual diagram of ECOHAM1 state variables and processes.

Forcing: The forcing for the model is as close as possible for an actual year (1985-1994), to include the physical short term variability, that determines the phytoplankton system. The probably most prominent feature of the North Sea dynamics for annual studies is the residual circulation (integration over the tidal cycle). A three-dimensional baroclinic circulation model (Pohlmann 1996) provided the daily mean velocities and eddy diffusivities that were used to advect and diffuse the compontens of the biological model. Solar radiation was calculated every 30 minutes for each grid point by an octa-model using daily cloud data (Pätsch 1994). The under water light is modified by self-shading. Monthly mean river loads stem from RWS (1992) for 11 rivers around the North Sea. Copepod biomass for 1° by 2° boxes of the North Sea were derived after Moll (1995). Fig. 3 describes all necessary forcing data.

Fig. 3: Scetch of the forcing data sets.

• MOLL, A. (1995):
  Regionale Differenzierung der Primärproduktion in der Nordsee: Untersuchung mit einem drei-dimensionalen Modell. Berichte aus dem Zentrum für Meeres- und Klimaforschung; Reihe B: Ozeanographie, 19, 151pp.
• MOLL, A. (1998):
  Regional distribution of primary production in the North Sea simulated by a three-dimensional model. Journal of Marine Systems, 16(1-2):151-170.
• PÄTSCH, J. (1994):
  MOCADOB a Model Generating Synthetical Time Series of Solar Radiation for the North Sea. Berichte aus dem Zentrum für Meeres- und Klimaforschung; Reihe B: Ozeanographie, 16, 67pp.
• POHLMANN, T. (1996):
  Calculating the annual cycle of the vertical eddy viscosity in the North Sea with a three-dimensional baroclinic shelf sea circulation model. Continent. Shelf Res., 16, 147-161.
• RWS - Rijkswaterstaat (1992):
  Guidance Document for the NSTF modelling workshop 6-8 May 1992. Report: Directoraat-Generaal Rijkswaterstaat, The Hague. 41pp.