A 3-D coupled biophysical model ECOSMO (ECOSystem MOdel) has been developed. The biological module of ECOSMO is based on lower trophic level interactions between two phyto- and two zooplankton components. The dynamics of the different phytoplankton components are governed by the availability of the macronutrients nitrogen, phosphate and silicate as well as light. Zooplankton production is simulated based on the consumption of the different phytoplankton groups and detritus. The biological module is coupled to a nonlinear 3-D baroclinic model. The physical and biological modules are driven by surface forcing at temporal scale of 6 h using atmospheric re-analysis data. The model was integrated for 1984 and 1986. The simulated fields for 1984 were used to investigate the annual spatial distribution of phytoplankton and zooplankton biomass and their production in the North Sea. A detailed validation showed that the model, based on consideration of limiting processes, is able to reproduce the observed spatial and seasonal variability of the North Sea ecosystem e.g. the spring bloom, summer sub-surface production and the fall bloom. Distinct differences in regional characteristics of diatoms and flagellates could be modeled and their different roles in the seasonal cycle were resolved by ECOSMO. Moreover, the model was able to describe seasonal and regional characteristics of zooplankton biomass. In contrast to earlier models ECOSMO was able to identify frontal environments as zones of high productivity, and the simulations characterized the dynamics of different zooplankton feeding environments with special emphasis on the role of frontal production. For the second trophic level the regional increase of production in the frontal zone was found to be several times higher than for the first trophic level. (c) 2006 Elsevier B.V All rights reserved.