The phytoplankton spring bloom in the Baltic Sea in 1985, 1986: multitude of spatio-temporal scales

Abstract

The spatio-temporal development of the phytoplankton spring bloom in the Baltic Sea for two consecutive years is analysed. Quasi-continuous, on-track measurements of particle concentration, fluorescence, temperature and salinity with a resolution of the spatial scales from ≈400 m to basin-wide were supplemented with quantitative samples of the phytoplankton abundance, pigments, and vertical CTD/fluorescence profiles. The improved spatial and temporal resolution allowed us to distinguish variability on different time and space scales. Year-to-year differences were found that include not only the timing of the bloom but also the size distribution of the plankton (composition of the phytoplankton assemblage). Contrary to conventional understanding, the bloom does not start due to the establishment of the vertical thermal stratification as the vertical density profile is controlled by the salinity stratification. The well-known massive diatom bloom is preceded by an initial growth of unidentified small-sized (1–4 μm) phytoplankton. As the bloom usually starts when the surface temperature is still below the temperature of maximum density (about 2.4°C for the salinity in the central Baltic), warming of the surface layer during that period has in fact a destabilizing effect on the stratification. The expansion of the bloom does not appear as a smooth, wave-like propulsion in the northeastern direction but rather as centripetal movements in the form of eddies and filaments from the more stratified coastal areas towards the center; hence, in the northern Baltic proper the progression is roughly in the southerly direction. The central eastern Gotland Basin with the least likelihood of vertical stratification in the photic layer is the last where the bloom commences. There the bloom starts in a mosaic of filaments and eddies that provide the vertical stability. The filaments with reduced salinity and increased temperature probably originate from the coastal areas and represent transformed coastal water.