Cylindrospermopsis raciborskii is a toxin-producing species of cyanobacteria that in autumn 2003 was recorded for the first time in three shallow (max. depth 5 m) Waikato lakes and a hydro-electric dam on the Waikato River, New Zealand. It formed water blooms at densities >100 000 cells/ml in Lakes Waahi and Whangape. Net rates of population growth >0.2 day -1 were recorded for C. raciborskii in Lakes Ngaroto, Waahi, and Karapiro, based on comparisons of low numbers (detection of <10 cells/ml) from initial samples and its presence at bloom densities (>15 000 cells/ml) in the subsequent sample "x"-"y" days later. C. raciborskii may be well adapted to rapid proliferation in the Waikato lakes, which are eutrophic to hypertrophic, with high light attenuation, and where nitrogen (N) fixation may provide it with a competitive advantage over non-nitrogen fixing algae under N-limited conditions.

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Relationships between phytoplankton assemblages and lake trophic state, mixing regime and light climate were investigated in 40 North Island, New Zealand, lakes. We tested the hypothesis that mixing regime is more important than trophic state or light climate in determining the community composition of phytoplankton assemblages which were represented as presence/ absence of cyanobacteria and diatom genera. Mixing regime, characterised as a function of lake size, more accurately predicted summer phytoplankton composition than indices related to trophic state or light regime. Analysis of similarities indicated distinct differences in phytoplankton assemblages between mixed lakes and those that intermittently or periodically stratify. In mixed lakes, a greater variety of cyanobacteria genera than diatom genera were recorded compared with lakes that stratify stably in summer. Conversely, relatively high numbers of diatom to cyanobacteria genera were found in lakes that stratify compared with mixed lakes. Lakes with different trophic states but similar mixing regimes shared very similar summer phytoplankton assemblages. None of the three variables tested was a good predictor of winter phytoplankton composition. Although phytoplankton biomass may be regulated to a large extent by nutrient status, this study shows that mixing regime plays a more important role in regulating phytoplankton composition than either nutrient status or light climate.

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River floodplains are focal points for human development worldwide due to the ecosystem services they provide, especially for urbanization and agriculture (Tockner et al., 2008). To facilitate this development, floodplains of many of the world's large rivers have been subjected to extensive physical modifications