Initiation of cyanobacterial blooms in a frontal region at the entrance to the Gulf of Finland, Baltic Sea

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 flos-aquae 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 saltier 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 less-saline 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 mu m) and flagellate (<20 mu m) 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. spumigena.