Nutrient limitation of cyanobacterial blooms: an enclosure experiment from the coastal zone of the NW Baltic proper

In an enclosure experiment with nutrient additions performed in a western archipelago of the brackish Baltic Sea, cyanobacterial development was stimulated by phosphate additions. Large increases in particulate nitrogen, corresponding to 2.3 g nitrogen m(-2) over the 25 d experiment, occurred in 2 enclosures receiving phosphorus but not nitrogen, which indicates a high capacity of nitrogen-fixing cyanobacteria to use molecular (N-2) rather than inorganic nitrogen as a source for growth in brackish waters. The amount of nitrogen fixed during the experimental period exceeded values reported from freshwater systems. At the end of the experiment, the phytoplankton community in enclosures receiving phosphate but not nitrate was dominated by filamentous nitrogen-fixing cyanobacteria. Enclosures receiving both phosphate and nitrate reached about the same concentration of phytoplankton biomass but had a lower abundance of nitrogen-fixing cyanobacteria than in bags to which phosphate but not nitrate was added, We hypothesize that the strong response of nitrogen-fixing cyanobacteria to additions of phosphate might be explained by the presence of a high concentration of bioavailable iron (20 mug l(-1)), as well as favorable weather conditions.