Modelling primary production in the North Sea using the European Regional Seas Ecosystem Model

The primary production module incorporated in the European Regional Seas Ecosystem Model (ERSEM) is described in detail. It considers two phytoplankton groups, diatoms and autotrophic flagellates, and four different nutrients: nitrate, ammonia, phosphate and silicate (only for diatoms). All the related state variables and fluxes are represented in terms of carbon, nitrogen, phosphorus and silicon.

The potential carbon growth rate process is estimated by considering that nutrient availability acts as a limitation factor on the maximum growth rate which is itself a function of light and water temperature. Respiration, excretion of organic matter, lysis and sinking are the main carbon and nutrient 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 North Sea. The model gave good correlations with the main dissolved nutrients, such as silicate, phosphate or nitrate. The primary production module proved to be especially sensitive to the flagellate/diatom interaction and competitive behaviour for inorganic nutrients as well as with regard to grazing losses. It is suggested that a major improvement could be made by including a third phytoplankton group (e.g., Phaeocystis) in the model structure, and that comparison with other phytoplankton growth schemes based on the Droop formulation is advisable.