ECOHAM is a three-dimensional model for estimating the annual primary production in the North Sea (Moll (1998), Skogen and Moll (2000)).

Phytoplankton distribution: The simulation of the annual cycle for 1986 shows the development of the spring phytoplankton bloom starting in the continental coastal zone, extending first into the area of the Baltic outflow along the Norwegian Trench and covering the whole of the North Sea already in April. Due to shortage of nutrients caused by stratification the phytoplankton concentration decreased at first in the southern central and then in the northern central North Sea. Heavy depletion was reached in July in the central North Sea, which covered the total area except for the coastal margins around the North Sea, from Scotland to southern Norway. As an example of the simulation results Fig. 4 showed the simulated temporal development of the spatial distribution of monthly mean surface phytoplankton concentration for February to October 1986. The effects of advection and diffusion on the amounts of annual primary production were investigated by Moll and Pohlmann (1997). It could be shown that only a combination of the physical processes of horizontal advection and vertical turbulent diffusion achieved a nutrient distribution which led to an annual primary production as if the full three-dimensional water movements (including horizontal diffusion) were acting. Horizontal and vertical turbulent diffusion increased the strongly reduced locally (i.e. one-dimensionally) caused primary reduction only a little. This demonstrates that the structure of the currents is very important for the amounts of annual primary production in the North Sea.

Fig. 4: Simulated monthly mean surface phytoplankton concentration with contour intervalls of 50 mgC per cubic metre.

Regional primary production: The amount of annual primary production was largest in the continental coastal zone (> 300 gC m^-2 y^-1), decreasing towards north-west, and in the northern central North Sea primary production showed its minimum (< 100 gC m^-2 y^-1). Using realistic atmospheric forcing for driving the circulation the variability of annual primary production was estimated for a simulation range of ten years (1985 - 1994) (Skogen and Moll (2000)). The comparison between the primary production estimates from ECOHAM and from NORWECOM gave the important result that the regional differences in primary production were larger than the differences between the two models in each of the areas inspected. The model ECOHAM suggested that the interannual variability of primary production in the North Sea was around 15 % of the mean.

The Figure below illustrates the regional distribution of the annual net primary production of the model region, simulated for the year 1986.

Fig. 5: Simulated annual depth-integrated net primary production for the North Sea with contour lines of 100, 125, 150, 200, 250 and 300 gC per square metre and year.

Phosphorus budget: A phosphate budget derived from an annual simulation for 1986 was given in Moll (1997).

Validation: Several attemps were made to validate the ECOHAM1 using observed chlorophyll (Moll, 1998), phosphate concentrations (Moll, 2000) and primary production values (Skogen and Moll, 2000). ECOHAM1 took part in a model-data comparison (OSPAR et al., 1998); the model was validated by using two larger data sets, one from ICES and one from the ERSEM project.

The three-dimensional model ECOHAM1 is the simplest model for the analysis of the phytoplankton dynamics, the nutrient transports, and the primary productivity in the North Sea. The model based on a simple phosphorus/nitrogen cycle, took only four state variables into account. Three pelagic variables: phytoplankton, phosphate (DIP), nitrogen (DIN) and one for benthic detritus. The model used the simplest parameterization for pelagic and benthic regeneration, and prescribed the copepod biomass for the trophic edge of zooplankton. It impresses by the results despite its simplicity.

• LUFF, R., MOLL, A. & POHLMANN, T. (1997):
  Animation of three-dimensional model results of the North Sea. EOS, 77, 529.MOLL, A. (1997) Phosphorus budget of the North Sea from the three-dimensional Model ECOHAM1. LOICZ Meeting Report, No. 29, 126pp.
• 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.
• MOLL, A. (2000):
  Assessment of three-dimensional physical-biological ECOHAM1 simulations by quantified validation for the North Sea with ICES and ERSEM data. ICES Journal of Marine Science, 57(4):1060-1068.
• MOLL, A. & POHLMANN, T. (1997):
  Influence of advection and diffusion on pirmary production in the North Sea derived from ECOHAM1. Berichte aus dem Zentrum für Meeres- und Klimaforschung; Reihe Z: Interdisziplinäre Zentrumsberichte, No. 2:182-185.
• OSPAR; Villars, M.; de Vries, I.; Bokhorst, M.; Ferreira, J.; Gellers-Barkman, S.; Kelly-Gerreyn, B.; Lancelot, C.; Mensguen, A.;Moll, A.; Pätsch, J.; Radach, G.; Skogen, M.; Soiland, H.; Svendsen, E. and Vested, H. J. (1998):
  Report of the ASMO modelling workshop on eutrophication Issues, 5-8 November 1996, The Hague, The Netherlands. OSPAR Commission Report, 102pp.
• 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.
• SKOGEN, M. and MOLL, A. (2000):
  Interannual variability of the North Sea primary production: comparison from two model studies. Continental Shelf Research, 20(2):129-151.