MODELING PRIMARY PRODUCTION IN THE NORTH-SEA USING THE EUROPEAN-REGIONAL-SEAS-ECOSYSTEM-MODEL

The primary production module incorporated in the European Regional Se as Ecosystem Model (ERSEM) is described in detail. It considers two ph ytoplankton groups, diatoms and autotrophic flagellates, and four diff erent nutrients: nitrate, ammonia, phosphate and silicate (only for di atoms). All the related state variables and fluxes are represented in terms of carbon, nitrogen, phosphorus and silicon. The potential carbo n growth rate process is estimated by considering that nutrient availa bility acts as a limitation factor on the maximum growth rate which is itself a function of light and water temperature. Respiration, excret ion of organic matter, lysis and sinking are the main carbon and nutri ent loss processes. Results indicate that the model simulates well the annual phytoplankton dynamics in the central regions of the North Sea , underestimating primary production and chlorophyll in the southern N orth Sea. The model gave good correlations with the main dissolved nut rients, such as silicate, phosphate or nitrate. The primary production module proved to be especially sensitive to the flagellate/diatom int eraction and competitive behaviour for inorganic nutrients as well as with regard to grazing losses. It is suggested that a major improvemen t could be made by including a third phytoplankton group (e.g., Phaeoc ystis) in the model structure, and that comparison with other phytopla nkton growth schemes based on the Droop formulation is advisable.