A 2-week multidisciplinary study of the physical, chemical, and biological mechanisms controlling the initiation of the late summer blooms of the diazotrophic cyanobacteria, Aphanizomenon jlos-aqua2 Ralfs and Nodularia spumigena Mertens, in the Baltic Sea was carried out in a frontal region at the entrance to the Gulf of Finland in July 1993. The front is formed by inflowing sa'itier waters of the northern Baltic proper and outflowing fresher waters from the gulf, and its position and shape are largely controlled by wind conditions. In general, the waters of the northern Baltic proper are less stratified than the outflowing lesssaline waters. At the time of the study, the two major water masses differed in terms of phytoplankton community structure, both at species level and at the level of functional groups. Wind-induced vertical mixing was instrumental in bringing nutrient pulses to the upper mixed layer in the less-stratified, high-saline water mass. Nutrient pulses were followed by enhancement of primary productivity and assimilation number (primary productivity/Chl a) in cyanobacterial (> 20 pm) and flagellate (< 20 vrn) size fractions. It is proposed that mesoscale blooms of A. flos-aquae benefit from the nutrient-pulsing events. Calm weather and solar heating, as reflected by rising temperatures in the upper mixed layer and overriding of water masses in the frontal region, resulted in substantial shallowing of the upper mixed layer, which initiated the bloom of N.spumifena.

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The development of a deep chlorophyll maximum (DCM) at a depth of 30-35 m was followed during a 15-d case study in July 1998 at the entrance to the Gulf of Finland. The study consisted of three 18-24-h periods of biological (chlorophyll a, phytoplankton, primary production), chemical (nitrate, phosphate) and physical (CTD, turbulence, vertical particle size distribution) measurements at an anchor station and six mesoscale towed CTD/ fluorometer mappings over the surrounding area. Exceptionally cold and windy weather led to a red tide of the dinoflagellate Heterocapsa triquetra instead of the cyanobacterial bloom that frequently occurs in late summer.Comparison of the estimated amount of nitrogen required for an H. triquetra bloom biomass with external loading affirmed that the bloom had been formed on the basis of the nitrate pool below the thermocline. The development of the bloom, therefore, led to the extremely deep nitracline. The DCM formed by H. triquetra developed at the top of the nitracline at an illumination of Ͻ0.1% of the sea surface illumination. A temperature-salinity analysis showed that the DCM was not caused by intrusions from inshore regions. It was concluded that the DCM was formed as a result of changing migratory behavior of H. triquetra after an upwelling event that fertilized the upper layer with phosphorus.

Horizontal, vertical, and temporal variability of chlorophyll a (Chl a), stratification, currents, and pelagial biology (phytoplankton, size-fractionated primary productivity and Chl a, ciliates, picocyanobacteria, bacterial production) were followed at the entrance to the Gulf of Finland during a 12-day cast 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 flos-aquae (Linne) Ralfs, Anabaena lemmermannii Richter, and Noduluria 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 Chl a changes in the layer above the thermocline. Subtraction of the Chl a change due to physical processes, mainly advection, gave a biologically induced mean decrease of 0.18 mg Chl a m-3 d-l. 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 Chl a was due to a size-specific grazing of ciliates on the dominating flagellates Chrysochromulina spp. 2-5 µm and Ochromonas spp. 7-8 µm 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 Chl a maximum in the thermocline.Horizontal and vertical inhomogeneities in waterbodies and their role in sustaining favorable growth conditions for opportunistic plankton species are of increasing interest in marine and freshwater science. Subsurface Chl a maxima (SCM) are well-documented phenomena in tropical, subtropical, and temperate seas. Chl a is a summarizing parameter of the pigment from several phytoplankton species and functional groups, and typically, the vertical distributions of single species show more distinct subsurface maxima than the SCM, even in decimeter scale (Bjørnsen and Nielsen 1991; Carpenter et al. 1995). The role of SCMs in the population dynamics of harmful algal species has recently been addressed by Nielsen et al. (1990) and Maestrini and Graneli (1991).Usually, SCMs develop in waters where, or when, the Acknowledgments We thank the officers and crew of RV Aranda for their cooper-

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