Phytoplankton assemblages in North Island lakes of New Zealand: is trophic state, mixing, or light climate more important?

that mixing regime plays a more important role in regulating phytoplankton composition than either nutrient status or light climate. 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.