Phosphorus pool variations and their relation to cyanobacteria development in the Baltic Sea: A three-year study

Diazotrophic cyanobacteria are the dominant organisms in the phytoplankton community in summer in the Baltic Sea, and can form huge surface blooms. The excess of dissolved inorganic phosphorus (DIP) after the spring phytoplankton bloom in May and its decline until summer has been used in calculations and modelling of nitrogen fixation rates. However, this method was based mainly on data obtained in May and July or August. The period of cyanobacteria development was not covered. In 2001, 2004 and 2005, from May until August, phosphorus and other pool concentrations were investigated at a higher resolution, and related to cyanobacteria development to restore this temporal inconsistency. This study addresses the importance for cyanobacteria development of (a) winter convection, (b) the DIP excess in May, and (c) organic phosphorus pools. Our results did not show a relationship between DIP concentrations in May, as a result of convection processes in winter and assimilation by phytoplankton in the spring bloom, and the occurrence or intensity of cyanobacteria blooms in summer. The DIP excess in May was not additively stored as particulate organic phosphorus (POP) during its disappearance. It did, however, equate the current POP in the same order of magnitude as POP was exported. Thus, the POP pool remained constant until June. The DIP excess supported only between 46.8% and 84.6% of the new phytoplankton production in June, requiring an additional phosphorus source for summer autotrophy. Cyanobacteria seem to develop in July on the basis of POP measured in June. In 2001, this source was enough to explain cyanobacteria biomass increase, which was accompanied by an increase in particulate C:P ratio. In 2005, this process was unable to explain the biomass production in July supporting the requirement for additional phosphorus. In summary, the use of DIP excess in May is insufficient for modelling cyanobacteria blooms and the calculation of nitrogen fixation by cyanobacteria in summer: using May excess DIP results in an underestimation of the possible summer fixation rate. Additional phosphorus sources, which are not included in the calculations, seem to be necessary.