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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.

High-resolution on-line measurements of chlorophyll a, particles, salinity and temperature supplemented with stationary vertical casts of CTD, nutrients and phytoplankton, from August 1990 were used to investigate the spatio-temporal patterns of summer cyanobacterial blooms along the axis of the Gulf of Finland, the Baltic Sea. These measurements were supplemented with information about phytoplankton species composition and the vertical distributions of chlorophyll and nutrients while the ship was stationary. The results showed that the Gulf of Finland is divided into several sub-basins which are separated from each other by quasi-stationary fronts related to the bottom topography and circulation patterns of the Gulf. The basins and fronts had their own characteristic bloom dynamics — wind-induced external energy inputs controlled bloom formation within the sub-basins, whereas the vertical, front-related transport processes had an additional impact on the bloom in frontal areas. Nutrient reserves and inorganic N:P ratios below the seasonal thermocline differed between sub-areas.

The horizontal distributions of particle size spectrum and chlorophyll a concentration across the northern edge of the North Atlantic subtropical gyre along 21°W were studied as part of the Joint Global Ocean Flux Study North Atlantic Bloom Experiment in March–April, 1989. An abrupt change in the particle size spectrum at 32°40′N was located within the broad thermohaline front between the central gyre water in the south and the Azores Current in the north. Whereas the small particulate fraction (1–10 μm) and chlorophyll a concentration were similar (within a factor of 2), the abundance of the “diatom” size fraction (28–72 μm ) showed a drastic increase by more than an order of magnitude in the Azores Current. The diatom fraction seemed to contribute little to the chlorophyll pool, indicating a postbloom condition. Different trophic pathways, i.e., grazing of the picoplankton in the gyre versus the diatom bloom with the following detrital loop in the Azores Current and northward, can be conjectured. The increased scattering versus absorption due to the different particle size spectrum and composition could bias the algorithms of pigment retrieval from remote sensing data.