High-resolution monitoring of environmental state variables in the surface layer of the Gulf of Finland (during a dynamic spring bloom in March-May 2010)

The flow-through system (Ferrybox) installed onboard a ferry cruising between Tallinn and Helsinki in the Gulf of Finland (Baltic Sea) measures temperature, salinity, chlorophyll a fluorescence, turbidity and since January 2010 also pCO
₂
. In March-May 2010, the water sampling was conducted on a weekly basis at 17 locations along the ferry route to measure nutrient concentrations (NO
₂
+NO
₃
and PO
₄
), chlorophyll a content and phytoplankton species composition and biomass. Our aim was to show that the Ferrybox technology can be successfully applied to follow the rapid changes of state variables during a very dynamic season of the year - phytoplankton spring bloom. High variability of environmental parameters has been observed both in space and time in the Gulf of Finland in spring 2010. It is suggested that both the general circulation in the surface layer and mesoscale hydrodynamic processes are influencing the bloom evolution and spatio-temporal variability. The observed coincidence of Chl a peaks with the periods of relatively fast temperature increase indirectly shows the importance of positive buoyancy fluxes (vertical stratification) for phytoplankton growth in spring. Spatio-temporal distribution of pCO
₂
was in a good accordance with the Chl a dynamics confirming that the pCO
₂
measurements can be used for the estimates of phytoplankton productivity. Our data confirm that the regular late evening fluorescence measurements can be successfully applied to determine the Chl a content in the surface waters in spring. It is concluded that autonomous high-resolution in-situ monitoring in combination with adaptive water sampling and remote sensing could give a full enough data set to assess the environmental state of the Gulf of Finland during this highly dynamic season of the year.