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

Abstract

Groser Muggelsee and Langer See are limnologically similar but during an investi- gation 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 and the other by Aphanizomenon flos-aquae/Micro- cystis 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 phos- phorus 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 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 agardhii coincided (clear water phase). Through- out all the investigated years in Groser Muggelsee, the rapid growth of the heterocyst-forming cyanobacteria flos-aquae started at the time when TN:TP=16:1, even if this critical ratio was delayed by several weeks. However, if agardhii biovolume exceeded 6 mm 3 l -1 at the time when the critical TN:TP ratio was reached, then the mass development of this cyano- bacteria continued during summer and into autumn, whereas flos-aquae were only present in traces. Therefore, flos-aquae reached high biovolumes only when TN:TP<16:1, while the growth of agardhii seemed to be independent of seasonal variation of the TN:TP ratio. Differences between P. and A. flos-aquae/Microcystis spp. dominated years were not restricted to cyanobacterial species, but were also seen in the diatom assemblages. A com- parison, based on biovolumes of 15 dominant cyanobacteria and diatoms, showed that seaso- nal changes in phytoplankton in P. agardhii-years were more gradual than the drastic shifts observed in A. flos-aquae/Microcystis spp.-years.