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The effect of single nutrient additions (ammonia or phosphate) on the biomass and phos- phorus dynamics of natural Baltic Sea phytoplankton bloom communities dominated by nitrogen-fix- ing Cyanobacteria was studied. The aim was to elucidate bloom-time phosphorus deficiency charac- teristics of the community. In addition, the intention was to study the effect of an increased phosphate supply on phosphate uptake in different size fractions and to test the hypothesis of increased phos- phate supply favoring large nitrogen-fixing Cyanobacteria during general phosphate deficiency. Based on specific phosphate uptake affinity and specific alkaline phosphatase activity, the bloom communities showed varying degrees of phosphorus deficiency. The bulk of phosphate uptake was dominated by the 8 to 20 µm size fraction. However, if the community initially showed signs of phos- phorus deficiency, phosphate addition increased the relative amount of phosphate taken up by the size fraction dominated by nitrogen-fixing Cyanobacteria. The 2 main bloom-forming filamentous diazotrophic species, Aphanizomenon sp. (considered non-toxic in the Baltic Sea) and Nodularia spumigena (toxic), showed differing levels of phosphorus deficiency. N. spumigena constantly had a higher percentage of the population expressing alkaline phosphatase activity, as indicated by enzyme-labeled fluorescence assays. Approximately one-fifth of added phosphate was estimated to be taken up by the diazotrophic Cyanobacteria during phosphorus deficiency.

Phosphate addition to phosphorus-deficient Baltic Sea plankton communities benefits nitrogen-fixing Cyanobacteria