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 filamentous, nitrogen-fixing cyanobacterial bloom was followed during July-ugust 1990 in a stratified basin in the central Gulf of Finland, altic Sea . Hydrography, dissolved inorganic, particulate and total nutrients, chlorophyll a, alkaline phosphatase activity, 32 P04-uptake and phytoplankton species were measured . The study period was characterized by wind-induced mixing events, followed by marked nutrient pulses and plankton community responses . Phosphate uptake was highest throughout the study period in the size fraction dominated by bacteria and picocyanobacteria ( 2 am) and the proportion of uptake in the size fraction 2-10 pm remained low (2-6%) . Higher phosphate turnover times were observed in a community showing signs of enhanced heterotrophic activity. The bloom of filamentous, nitrogen-fixing cyanobacteria phanizomenon flos-aquae was promoted by a nutrient pulse with an inorganic nutrient ratio (DIN :DIP) of 15 . The results show that the quality, frequency and magnitude of the physically forced nutrient pulses have an important role in determining the relative share of the different modes of phosphorus utilization and hence in determining the cyanobacterial bloom intensity and species composition in the altic Sea .

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