No abstract

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 .