Convection and the Timing of Phytoplankton Spring Blooms in the Western Baltic Sea

The influence of water column stability on the occurrence of phytoplankton spring blooms in an estuarine system (western Baltic Sea) was studied by means of a coupled 3-D model. The model comprised a mesoscale circulation model and a simple biochemical model. The onset of the spring bloom in the western Baltic Sea is known to show a time lag from west to east. Simulations of the onset and course of spring blooms were done to investigate the reasons for this time lag. Spatio-temporal variations were simulated in accordance with observations for the years 1994 to 1996. Simulations showed that the bloom begins after the convective mixing period in spring has ceased. The spring convection is a common feature of brackish waters. It occurs due to the non-linear behaviour of density with respect to temperature, but only if water temperatures had fallen below the temperature of maximum density in winter. The density maximum temperature is higher in the less saline eastern Baltic, as this critical temperature increases with decreasing salinity. Thus the period of convective mixing increases from west to east in the Baltic Sea. The increased mixing period in the eastern regions explains the observed west-east time lag in the spring bloom initiation. The example of an exceptional mild winter in 1994-95 further confirms this reasoning. Because the water temperatures did not drop below the temperature of maximum density during this mild winter, no spring convection occurred and the regional variations of the spring bloom disappeared.