In a joint project (1997-2000), the University of Hamburg (Germany) and the Ocean University of Qingdao (China) have investigated the circulation, water mass structure, nutrient fluxes and phytoplankton dynamics in the Bohai Sea. A hierarchy of coupled circulation, transport and production models for the seasonal scale has been developed and tested against field data from two ship cruises in autumn and spring. The models are available now for scenario calculations under the impact of environmental change.