COCOA will investigate physical, biogeochemical and biological processes in a combined and coordinated fashion to improve the understanding of the interaction of these processes on the removal of nutrients along the land-sea interface. The results from the project will be used to estimate nutrient retention capacity in the coastal zone of the entire Baltic Sea coast. An ensemble of biogeochemical models will be used in combination with field studies at seven different coastal study sites around the Baltic Sea. The present report is a deliverable of COCOA work package 5 (WP5). Within the objective of WP5 process understanding and process descriptions will be improved in state-of-the-art biogeochemical models of the Baltic Sea coastal zone. This report presents brief information about the models available for the COCOA project and defines the needed input to the models that will be set-up at several learning sites. The aim is to perform ensemble modelling at several sites, using at least two different models at each site. A pilot study to estimate nutrient retention capacity in the Stockholm Archipelago with the existing Swedish model system is ongoing and first results are presented and the concept of nutrient retention is briefly discussed. The existing models for different learning sites presented in the report are; 1) The Swedish model system SCM (Öre river estuary and the Stockholm archipelago) - A multi-box-model approach 2) The Danish model system FLEXSEM (Roskilde fjord) - A combined box-model and 3-D model approach 3) The Finnish model system ESIM-BFMSI (Tvärminne Archipelago) - A 1D box-model approach 4) The Polish model system M3D UG/ProDeMo (Puck Bay) - A 3-D model approach. Operational model. 5) The Lithuanian model system SYFEM/AQUABC (Curonian Lagoon) - A combined box-model and 3-D model approach 6) The Swedish open Baltic model system RCO-SCOBI (for the open Baltic Sea and the Gulf of Gdansk/Vistula). - A 3-D model approach In addition a biogeochemical model (Boudreau, 1996) for the Gulf of Finland (Russian State Hydrometeorological University model) is used to study the quantitative effect of Marenzelleria on the Gulf of Finland ecosystem. Process studies at selected sites will be performed with a reactive transport model developed at Utrecht University. Focus will be on the role of iron and phosphorus cycling. Process studies with the Danish model system will support the development of new parameterizations of nutrient fluxes taking benthic habitat into account. The new parameterizations of the nutrient fluxes will in addition also be implemented into SCM and the models will be used to estimate nutrient fluxes, retention times and the filter capacity of the coastal zones. The In Kind contributions from previously (in the literature) well described open Baltic Sea models RCO-SCOBI, BALTSEM, ERGOM and SPBEM that will be used for the description of open sea conditions are also briefly mentioned in the report with references to the relevant literature. 6) The Swedish open Baltic model system RCO-SCOBI (for the open Baltic Sea and the Gulf of Gdansk/Vistula). - A 3-D model approach In addition a biogeochemical model (Boudreau, 1996) for the Gulf of Finland (Russian State Hydrometeorological University model) is used to study the quantitative effect of Marenzelleria on the Gulf of Finland ecosystem. Process studies at selected sites will be performed with a reactive transport model developed at Utrecht University. Focus will be on the role of iron and phosphorus cycling. Process studies with the Danish model system will support the development of new parameterizations of nutrient fluxes taking benthic habitat into account. The new parameterizations of the nutrient fluxes will in addition also be implemented into SCM and the models will be used to estimate nutrient fluxes, retention times and the filter capacity of the coastal zones. The In Kind contributions from previously (in the literature) well described open Baltic Sea models RCO-SCOBI, BALTSEM, ERGOM and SPBEM that will be used for the description of open sea conditions are also briefly mentioned in the report with references to the relevant literature.