DEVELOPMENT OF A SUBSURFACE CHLOROPHYLL MAXIMUM AT THE ENTRANCE TO THE GULF OF FINLAND, BALTIC SEA

Horizontal, vertical, and temporal variability of chlorophyll a (Chl a ), stratification, currents, and pelagial biology (phytoplankton, size -fractionated primary productivity and Chi a, ciliates, picocyanobacte ria, bacterial production) were followed at the entrance to the Gulf o f Finland during a 12-day case study in July-August 1994. The study wa s carried out during weak wind conditions when hydrodynamic activity w as minimal and when an intense blooming of diazotrophic cyanobacteria Aphanizomenon pos-aquae (Linne) Ralfs, Anabaena lemmermannii Richter, and Nodularia spumigena Mertens was in its decaying phase. The propert y conservation equation was used to differentiate the contributions of physical and biological processes to the local Chi a changes in the l ayer above the thermocline. Subtraction of the Chi a change due to phy sical processes, mainly advection, gave a biologically induced mean de crease of 0.18 mg Chl a m(-3) d(-1). Sampling data confirmed the calcu lated decrease and revealed the basin-wide character of the phenomenon . The observed plankton dynamics showed that the pelagial system was ' 'top-down'' controlled and that about half the decrease of Chi a was d ue to a size-specific grazing of ciliates on the dominating flagellate s Chrysochromulina spp. 2-5 mu m and Ochromonas spp. 7-8 mu m in the u pper layer. The grazers were confined to the layer above the thermocli ne, while the flagellates inhabited both the thermocline and the layer above. This led to the formation of a subsurface Chi a maximum in the thermocline.