Spatial heterogeneity and dynamics of vernal phytoplankton species in the Baltic Sea in April–May 1986

Abstract

The mesoscale phytoplankton distribution was studied as part of an international joint Baltic Sea Patchiness Experiment (PEX'86) during April–May 1986 in the open Baltic Proper. The study period covered the peak phase of the phytoplankton spring bloom. The spatio-temporal dynamics of four dominating phytoplankton species, Achnanthes taeniata, Chaetoceros spp., Skeletonema costatum and Thalassiosira levanderi were studied within an area of 20×40 nmi with grids of 2 and 4 nmi spatial resolution. The results showed highly varying spatial distributions for all species, and the variability was accentuated on the synoptic space scale before the development of the seasonal thermocline. The maxima of Chaetoceros spp. and Thalassiosira levanderi coincided with the anticyclonic and cyclonic eddies prevailing in the area. Skeletonema costatum was found in high abundances only within a warmer water mass of higher salinity advecting into the area. The results pointed out that different successional stages can simultaneously be found even in adjacent water masses and both the phytoplankton growth and composition during the bloom peak phase in the Central Baltic depend on complex factors, mainly those connected with mesoscale hydrodynamic features (eddies, frontal zones, jet currents).