A one-dimensional, vertically resolved, physical-biochemical upper ocean model is utilized to study plankton productivity and nitrogen cycling in the central Black Sea region characterized by cyclonic gyral circulation. The model is an extension of the one given by Oguz et al. (1996, J, Geophys. Res. 101, 16585-16599) with identical physical characteristics but incorporating a multi-component plankton structure in its biological module, Phytoplankton are represented by two groups, typifying diatoms and flagellates, Zooplankton are also separated into two groups: microzooplankton (nominally <200 mu m) and mesozooplankton (0.2-2 mm), The other components of the biochemical model are detritus and nitrogen in the forms of nitrate and ammonium. The model incorporates, in addition to plankton productivity and organic matter generation, nitrogen remineralization (ammonification) and ammonium oxidation (nitrification) in the water column. Numerical simulations ape described and compared with the available data from the central Black Sea. The main seasonal and vertical characteristics of phytoplankton and nutrient dynamics inferred from observations appear to be reasonably well represented by the model. Fractionation of the biotic community structure is shown to lead to increased plankton productivity during the summer period following the diatom-based early spring (March) bloom. The annual nitrogen budget for the euphotic zone reveals the substantial role of recycled nitrogen in the surface waters of the Black Sea. (C) 1999 Elsevier Science Ltd. All rights reserved.