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Since water quality monitoring can be rather costly, it is important to properly design the monitoring network so that maximum information can be received with moderate effort. One component of monitoring is measuring the phytoplankton community composition and detecting whether that has changed. From April to October 2012, by using ships-of-opportunity (SOOP), we collected simultaneous samples from 15 stations in the Gulf of Finland and the Baltic Proper to study the spatial variability in the phytoplankton species composition and biomass throughout the growing season. The analysis was performed with 10 dominant taxa constituting 83–97% of the total phytoplankton biomass. We set the criterion of the statistically significant (p

The response of nutrient and chlorophyll fields to climate change by the end of the twenty-first century was evaluated in the Gulf of Finland (Baltic Sea) using comparison of a hindcast simulation for 1997–2006 and future climate forcing, assuming an A1B greenhouse gas emission scenario and business-as-usual riverine nutrient load for 2090–2099. The comparison of simulated oxygen, phosphate and nitrate levels from the hindcast model with the measurements indicated a good performance of the 3D ecosystem model, except for overestimated near-bottom layer nitrates. The mean chlorophyll level was slightly overestimated by the model, whereas the variability in the surface layer chlorophyll level was well reproduced. Future projection simulations indicate no considerable changes in the upper layer oxygen concentrations compared with the hindcast simulation and observations, but deeper near-bottom layers were projected to become anoxic, causing an increase in phosphate and a decrease in nitrate concentrations in these layers. The increase in surface layer phosphate and the decrease in nitrate concentrations lead to an increase in summer cyanobacteria blooms and an increase in nitrogen fixation, which therefore led to an increase in the annual mean chlorophyll content in the upper layer.