Blooms of Baltic Sea Aphanizomenon sp (cyanobacteria) collapse after internal phosphorus depletion

We studied C:N:P stoichiometry, heterocyst frequency, and biomass of the N-2-fixing cyanobacteria Aphanizomenon sp. and Nodularia spumigena from May to September 1999 and 2000 at an offshore station in the NW Baltic Proper. In 2000, we included iron (Fe) and molybdenum (Mo) contents and biomass-specific C-14 uptake. We also complemented published C:N:P stoichiometry data from 1998 with heterocyst frequency. We found a drastic increase in C:P and N:P ratios in Aphanizomenon sp., indicating severe P deficiency at the biomass maximum. N. spumigena also had high C:P and N:P ratios at high abundances. In 2000, and 1998, the total amount of P stored in Aphanizomenon sp. biomass in early summer equalled that at the bloom peak. Only a small part of the DIP surplus remaining after the spring bloom of diatoms and dinoflagellates ended up in the peak biomass of the subsequent cyanobacterial bloom. Aphanizomenon sp. heterocyst frequency peaked in early summer when the C:P ratio was near the Redfield value, and then decreased with increasing C:P ratio, initially perhaps due to increased temperature, and later to P limitation. This decrease occurred parallel to a decrease in Aphanizomenon sp. Mo content. We found no indication of Fe limitation since there was no clear decrease in Aphanizomenon sp. Fe content as the bloom progressed. N. spumigena had considerably higher Fe content, but a high Fe:Mo ratio suggests that Fe adsorbed to cell surfaces biased the measurements. An expected reduction in growth rate due to the high C:P ratio was not mirrored in a decreased specific C uptake of Aphanizomenon sp. as measured in short-term C-14 incubations. This may indicate short-term C storage with subsequent respiration or excretion of excess carbon when growth is P limited.