Optimization of phytoplankton monitoring in the Baltic Sea

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 < 0.05) Bray-Curtis similarity for the station pairs to be mutually representative according to the species composition and biomass. Our results indicated that in areas of similar hydrological conditions, it is sufficient to have sampling stations in 50-65 km distance between one another, but in the areas with varying hydrographical conditions, i.e. coastal areas, higher spatial coverage may be needed. Whenever possible, it is recommended to combine monitoring efforts between the Baltic Sea countries in the offshore areas, including SOOP. The need for further harmonization of phytoplankton analysis between countries is addressed in order to have comparable data sets by different countries.