Modern models of the Baltic Sea eutrophication describe only a bioavailable fraction of the nutrient input from land, thus introducing uncertainty into forcing. In order to alleviate this uncertainty, the coupled 3D hydrodynamical-biogeochemical St. Petersburg Eutrophication Model (SPBEM) has been expanded with variables representing dissolved organic nutrients. The model modification involves an explicit description of the labile and refractory fractions of dissolved organic nitrogen and phosphorus, in addition to their particulate forms, represented by the detritus variables. The modified SPBEM-2 allows for a full account of the total amounts of nutrients reported in field measurements and presented in environmental documents. Particularly, a model description of detritus, as the only bulk organic matter variable, has been replaced by more realistic parameterizations with adequate rates of settling and mineralization. The extensive validation and verification of the model performance in the Gulf of Finland from 2009 to 2014, based on over 4000 oceanographic stations, shows that SPBEM-2 plausibly reproduces all the major large-scale features and phenomena of the ecosystem dynamics in the Gulf of Finland, especially in its surface productive layer. These demonstrated capabilities of SPBEM-2 make the model a useful tool, both in studies of biogeochemical interactions and in historical and scenario simulations.
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