Alternative blooming of Aphanizomenon flos-aquae or Planktothrix agardhii induced by the timing of the critical nitrogen:phosphorus ratio in hypertrophic riverine lakes

Großer Müggelsee and Langer See are limnologically similar but during an investigation from 1990 until 1993 marked differences were observed in the species that dominated during the summer blooms. Two distinct planktonic communities could be recognised, one dominated by ”Planktothrix agardhii” and the other by ”Aphanizomenon flos-aquae/Microcystis spp.”. A sensitive moment for the differentiation of the plankton development to the one or the other cyanobacterial summer bloom was the time, when the total nitrogen to total phosphorus ratio (TN:TP ratio) dropped to a critical value of 16:1. In addition, the phytoplankton composition at the time when this ratio is reached is of decisive importance. Rapid growth of the N2-fixing A. flos-aquae was favoured at TN:TP<16:1 in both lakes, when the timing of the critical TN:TP ratio and low biomass of P. agardhii coincided (clear water phase). Throughout all the investigated years in Großer Müggelsee, the rapid growth of the heterocyst-forming cyanobacteria A. flos-aquae started at the time when TN:TP=16:1, even if this critical ratio was delayed by several weeks. However, if P. agardhii biovolume exceeded 6 mm3 l-1 at the time when the critical TN:TP ratio was reached, then the mass development of this cyanobacteria continued during summer and into autumn, whereas A. flos-aquae were only present in traces. Therefore, A. flos-aquae reached high biovolumes only when TN:TP<16:1, while the growth of P. agardhii seemed to be independent of seasonal variation of the TN:TP ratio. Differences between ”P. agardhii” and ”A. flos-aquae/Microcystis spp.” dominated years were not restricted to cyanobacterial species, but were also seen in the diatom assemblages. A comparison, based on biovolumes of 15 dominant cyanobacteria and diatoms, showed that seasonal changes in phytoplankton in ”P. agardhii”-years were more gradual than the drastic shifts observed in ”A. flos-aquae/Microcystis spp.”-years. Authors’ addresses: K. Teubner, Institut für Limnologie, Österreichische Akademie der Wissenschaften, Gaisberg 116, A-5310 Mondsee. – R. Feyerabend, M. Henning, A. Nicklisch, P. Woitke and J.-G. Kohl, Humboldt-Universität zu Berlin, Inst. für Biologie, FB Ökologie, Unter den Linden 6, D-10099 Berlin 326 K. Teubner et al.