Modelled phytoplankton dynamics in the Gulf of Finland

The three-dimensional water quality and flow model was used to simulate phytoplankton biomass and dissolved nutrient concentrations in the Gulf of Finland. In the model, transport of nutrients is based on calculated wind and river flow-induced currents. Water quality data from two points off the Finnish coast were used for the model calibration. Nitrogen is the limiting nutrient in most parts of the Gulf of Finland, but in the eastern Gulf phosphorus plays a more important role. According to the model results, load reductions are needed everywhere along the coast in order to counteract eutrophication. Nitrogen reduction is more important for the whole Gulf of Finland, while phosphorus reduction is most effective in coastal regions, particularly in the Neva estuary. A sensitivity analysis was made to the water quality model to study the effects of parameters and nutrient loads on calculated phytoplankton biomass. The influence of some parameters was found to be widely varying in different areas of the Gulf depending on the limiting nutrient. Also the dynamics of the phytoplankton growth—e.g. timing of the spring bloom peak—was strongly dependent on model parameter values.