NUTRIENT DYNAMICS IN THE NORTH-SEA - FLUXES AND BUDGETS IN THE WATER COLUMN DERIVED FROM ERSEM

Nutrient dynamics for phosphate, nitrate, ammonium and silicate have b een simulated with ERSEM, the European Regional Seas Ecosystem Model. From the model results budgets for the dissolved inorganic nutrients a nd the corresponding particulate fractions have been calculated. The a nnual cycles of the nutrients phosphate and silicate compare quite wel l with the observed ranges of variability. This does not hold for ammo nium and nitrate. Biologically mediated transformations such as nutrie nt uptake and pelagic and benthic mineralization are the dominant proc esses in changing the nutrient concentrations with the horizontal adve ctive contributions playing a minor role during the productive season. Vertical advection and vertical diffusion have a clear seasonal signa l, with a maximum in February. The decay of the advective nutrient tra nsport in summer is caused by the depletion of the upper layer of diss olved inorganic nutrients by algal uptake. The inflow of nutrients in the northwest is almost balanced by the outflow in the northeast, with out causing large nutrient transports into the shallower areas from th e north. However, from the coastal areas there is a nutrient flow towa rds the central North Sea, enhancing primary production in the central area.