Vernal phytoplankton bloom in the Baltic Sea: Intensity and relation to nutrient regime

The intensity of the vernal phytoplankton bloom (VPB) was quantified in the Baltic Sea in 1993-2012, and its relation to the wintertime growth-limiting nutrient pool (the dissolved inorganic nitrogen, DIN, in our case) was assessed. The intensity of the VPB (IVPB, μg/l of Chl a) was based on the integration of the measured chlorophyll a (Chl a) concentration over time and was estimated for the Arkona Basin, the Bornholm Basin, the Eastern Gotland Basin, the Northern Gotland Basin and the western Gulf of Finland. The conventional research vessel based monitoring supplemented with the ship-of-opportunity data provided a close insight into the VPB dynamics. The highly variable climatic forcing in the Baltic Sea area produces large inter-annual variation in how the VPB progresses during the spring within the frames set by available DIN. As a result, the IVPB exhibited an 8-fold variation when all the sub-regions were taken into account (3 to 24 μg/l of Chl a), compared to a 5-fold corresponding variation in DIN (2.1 to 9.7 μmol/l). Consequently, the inter-annual variation in the wintertime DIN pool explained the inter-annual variation in the IVPB well only in the Bornholm Basin (regression: p < .001), fairly in the Arkona Basin (regression: p < .05), and for the other basins no relation was detected. The quantitative relation between the IVPB and the wintertime DIN pool varies largely within the frames provided by the progress of the physical environment over the winter and spring. The performance and applicability of a trophic index which has its foundation in the intensity of seasonal phytoplankton blooms is discussed.