Synchronised changes of planktonic cyanobacterial and diatom assemblages in North German waters re- duce seasonality to two principal periods

The seasonal dynamics of cyanobacterial and diatom assemblages in 11 water bodies along a mesoto hypertrophic gradient were compared. The cyanobacterial and diatom communities are characterised according to representative species of each algal class, selected by data reduction following principal component analyses and discriminant analysis. The cyanobacterial and diatom assemblages of the four seasons show different similarities in their species composition. In this respect the assemblages of the four seasons can be distinguished as only two different periods: winter/spring and summer/autumn. Species show a preference of growth either in the period of winter/spring (Limnothrix redekei, Nitzschia acicularis, Stephanodiscus neoastraea) or of summer/autumn (Aphanizomenon flos-aquae, several species of Microcystis and Anabaena, Planktothrix agardhii, Aulacoseira granulata/italica/ambigua, Actinocyclus normanii). The significant compositional changes for both algal classes during the transition from spring to summer and from autumn to spring were discussed in relation to synchronised seasonal variations of TN:TP ratios.