The Irish Sea presents an ideal location for developing coupled physical-biological models. It is semi-enclosed, contains stratified and well-mixed regions controlled by tidal currents, has nutrient enrichment from river inputs, and a biological system not dominated by a single plankton or zooplankton species. A computationally efficient 3-dimensional modelling system (POLCOMS) has been developed which acts as `host' to ecosystem dynamics. This system has been applied at eddy-resolving lengthscales (1.5 km) to the Irish Sea and the annual cycle of nutrients, primary and secondary production investigated. The structure of the modelling system allows different ecosystem formulations to be explored and 2 different approaches — a model with two compartments each for phytoplankton and zooplankton based on the ecosystem model of Anderson & Williams 1998 and the more complex multi-compartment European Regional Seas Ecosystem Model based on Baretta et al, 1995. From direct comparisons of the two approaches with data we can investigate the complexity required of ecosystem models to reproduce the observed biological functioning and explore the (sometimes subtle) physical-biological interactions occurring in the Irish Sea.