Marine ecosystem models often have difficulties in matching observed production rates of particulate organic carbon (POC). For example, errors are likely to be associated with an excessive carbon assimilation (high C:N assimilation ratio) in the absence of other nitrate.A model is formulated which decouples the formation of POC from particulate organic nitrogen (PON). The model study provides a quantitative assessment of carbon fluxes, including inorganic carbon assimilation by phytoplankton, the exudation of polysaccharides, and their susbsequent transformation into the carbon content of larger particles such as those measured as transparent exopolymeric particles.