Fate of excess phosphorus in the Baltic Sea: A real driving force for cyanobacterial blooms?

Abstract

The wintertime ratio of inorganic dissolved nitrogen to phosphorus (DIN/DIP) in the surface waters of the Baltic Sea is typically below the molar Redfield ratio of 16, which expresses the presence of an excess inorganic dissolved phosphorus (eDIP) reserve compared to DIN. We assessed the role of the vernal phytoplankton bloom period (VPBP) in the consumption of the potential wintertime eDIP reserve, and the role of eDIP after the VPBP as a nutritional agent for the summertime growth of the cyanobacteria in the Baltic Sea. We employed a high-frequency dataset collected by the unattended monitoring systems on board merchant ships. The dataset encompasses the Baltic basins from the Arkona Basin to the Western Gulf of Finland and the time period from 1993 to 2009. All the observed values of the wintertime DIN/DIP ratio were below the molar Redfield ratio; the ratio showed a declining trend during the study, suggesting that there is a pronounced wintertime stock of the potential eDIP in the waters of the Baltic Sea, and that this stock has lately increased in magnitude. The VPBP took up excessively DIP to DIN than calculated according to the uptake in the molar Redfield ratio, thus reducing the potential eDIP reserve. On average, 59% of the potential eDIP reserve was left in the water after the VPBP as eDIP. eDIP was typically exhausted in the time frame early June–early July, matching well the timing of the appearance of cyanobacteria in substantial numbers in the water-column. eDIP clearly fueled the cyanobacterial growth in every instance in which it was possible to clarify their relationship. The cyanobacteria must still have another DIP source than eDIP to form extensive late-summer blooms, except in the western Gulf of Finland, where eDIP remained detectable up to early August. The annual role of eDIP for cyanobacterial growth depends greatly on the weather of late spring and early summer: this may sometimes launch the cyanobacterial growth much earlier than is usually expected.