New Zealand Journal of Marine and Freshwater Research abstracts

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

E. F. Ryan
I. C. Duggan
D. P. Hamilton
D. F. Burger

Centre for Biodiversity and Ecology Research
University of Waikato
Private Bag 3105
Hamilton, New Zealand
email: dburger@waikato.ac.nz

Abstract 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.

Keywords cyanobacteria; diatoms; summer; winter; phytoplankton assemblages; community; composition

New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40: 389–398
0028–8330/06/4003–0389     © The Royal Society of New Zealand 2006
M05050; Online publication date 7 July 2006. Received 10 August 2005; accepted 23 January 2006

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