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 biolog ical mechanisms controlling the initiation of the late summer blooms o f the diazotrophic cyanobacteria, Aphanizomenon flos-aquae Ralfs and N odularia spumigena Mertens, in the Baltic Sea was carried out in a fro ntal region at the entrance to the Gulf of Finland in July 1993. The f ront is formed by inflowing saltier waters of the northern Baltic prop er and outflowing fresher waters from the gulf, and its position and s hape 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 mas ses differed in terms of phytoplankton community structure, both at sp ecies level and at the level of functional groups. Wind-induced vertic al mixing was instrumental in bringing nutrient pulses to the upper mi xed layer in the less-stratified, high-saline water mass. Nutrient pul ses were followed by enhancement of primary productivity and assimilat ion number (primary productivity/Chl a) in cyanobacterial (> 20 mu m) and flagellate (<20 mu m) size fractions. It is proposed that mesoscal e blooms of A. flos-aquae benefit from the nutrient-pulsing events. Ca lm weather and solar heating, as reflected by rising temperatures in t he upper mixed layer and overriding of water masses in the frontal reg ion, resulted in substantial shallowing of the upper mixed layer, whic h initiated the bloom of N. spumigena.