COMMUNITY-DEVELOPMENT AND MODES OF PHOSPHORUS UTILIZATION IN A LATE SUMMER ECOSYSTEM IN THE CENTRAL GULF-OF-FINLAND, THE BALTIC SEA

The development of a filamentous, nitrogen-fixing cyanobacterial bloom was followed during July-August 1990 in a stratified basin in the cen tral Gulf of Finland, Baltic Sea. Hydrography, dissolved inorganic, pa rticulate and total nutrients, chlorophyll a, alkaline phosphatase act ivity, (PO4)-P-32-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 d ominated by bacteria and picocyanobacteria (< 2 mu m) and the proporti on of uptake in the size fraction 2-10 mu m remained low (2-6%). Highe r phosphate turnover times were observed in a community showing signs of enhanced heterotrophic activity. The bloom of filamentous, nitrogen -fixing cyanobacteria Aphanizomenon flos-aquae was promoted by a nutri ent pulse with an inorganic nutrient ratio (DIN:DIP) of 15. The result s show that the quality, frequency and magnitude of the physically for ced nutrient pulses have an important role in determining the relative share of the different modes of phosphorus utilization and hence in d etermining the cyanobacterial bloom intensity and species composition in the Baltic Sea.