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, picocyanobacteria, bacterial production) were followed at the entrance to the Gulf of Finland during a 12-day case study in July-August 1994. The study was carried out during weak wind conditions when hydrodynamic activity was 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 property conservation equation was used to differentiate the contributions of physical and biological processes to the local Chi a changes in the layer above the thermocline. Subtraction of the Chi a change due to physical processes, mainly advection, gave a biologically induced mean decrease of 0.18 mg Chl a m(-3) d(-1). Sampling data confirmed the calculated 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 due to a size-specific grazing of ciliates on the dominating flagellates Chrysochromulina spp. 2-5 mu m and Ochromonas spp. 7-8 mu m in the upper layer. The grazers were confined to the layer above the thermocline, 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.