AME 60:43-57 (2010)  -  DOI: https://doi.org/10.3354/ame01408

ABSTRACT: The effect of single nutrient additions (ammonia or phosphate) on the biomass and phosphorus dynamics of natural Baltic Sea phytoplankton bloom communities dominated by nitrogen-fixing Cyanobacteria was studied. The aim was to elucidate bloom-time phosphorus deficiency characteristics 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 phosphate 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 phosphorus 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.

KEY WORDS: Nitrogen-fixing Cyanobacteria · Phosphate uptake · Baltic Sea · Aphanizomenon sp. · Nodularia spumigena