In the framework of the developments of the European system RODOS (Real-time On-line DecisiOn support System) for emergency response to nuclear accident, the computer code POSEIDON, that was developed to assess the radiological consequences of radioactive releases into marine environment, was adapted to cope with emergency conditions, in situations of radioactive discharges into the oceans from direct deposition from the atmosphere, sunken ships and containers, from discharges of rivers and estuaries and from coastal run-off. Based on the box model developed within the 'Marina' project, POSEIDON can calculate the dose effects from radionuclide releases in the coastal waters of Europe integrated over long time periods. A dynamic food chain model was implemented to deal with the short-term dynamical uptake of radioactivity by specific marine plants and organisms. POSEIDON has been installed on a UNIX platform to be fully compatible with RODOS input/output databases and on a Windows platform with an interface based on web technology. The 3D hydrodynamic model THREETOX is a part of the POSEIDON/RODOS system. It has been applied to coastal areas of the Baltic Sea, the Black Sea, and the North Sea. to derive the parameters for a flexible system of well-defined model compartments to be adapted to emergency conditions. The activity concentrations in water and in the marine food web were calculated by means of POSEIDON for radioactive fallout resulting from bomb testing, from the Chernobyl accident, and from routine discharges from nuclear facilities. POSEIDON's model results were compared with measurement data, and with calculation results from THREETOX. The model results agreed with the measurement data sufficiently.