|The distribution and dynamics of planktonic rotifers, at various spatial and temporal scales, were investigated in lakes of the North Island, New Zealand. The overall aim was to provide a better understanding of the relative influence of various environmental gradients in determining the composition of planktonic rotifer communities in New Zealand.
Potential methods for collection and concentration of samples were assessed. For investigating seasonal and vertical distributions in lakes, sedimentation provided significantly greater (P>0.01) estimates of total and abundant rotifer densities than filtration with 40 μm mesh. For collection of depth integrated samples, various net designs and a tube sampler gave differing estimates of the proportions of species. Overall nets were preferred because they required less time and effort than tubes, and nets with reducing cones provided results more similar to tubes than those without.
The seasonal and vertical dynamics of rotifer communities were investigated in four central North Island lakes of contrasting trophic state and mixing regime: Lakes Tikitapu, Rotoiti, Okaro and Ngaroto. Total rotifer abundances were consistently lowest in winter. The temporal occurrence of species assemblages within lakes related to mixing pattern, with different communities in stratified (or relatively stable) and holomictic periods. Multivariate techniques (Cluster analysis, MDS, CCA) indicated that species distributions were most strongly associated with temperature. The predominant warm monomictic mixing pattern in New Zealand is likely to have prevented the occurrence of truly cold-water assemblages. No distinct hypolimnetic community was recognised in any lake, indicating that New Zealand’s frequently strong winds are likely to prevent persistence of winter species in the hypolimnion. Rotifer densities were lower in the hypolimnion. Total rotifer abundance was generally greater in lakes of higher trophic state.
The geographical distribution of rotifer species in 42 North Island lakes was investigated using multivariate techniques. Distribution patterns were most strongly associated with trophic state gradients. Based on this relationship, bioindicator indices were developed using Weighted Averaging techniques using rotifer communities for inferring trophic state. Some species, for example Keratella australis and Conochilus exiguus, appear to be limited in distribution by poor dispersal abilities. Rotifer diversity within lakes, previously thought to be low in New Zealand, was found to be comparable with that of northern temperate lakes.
The methods for estimating rotifer resting egg densities from lake sediment, and the use of emergence for determining community composition from lakes, were investigated in Lakes Tikitapu and Okaro. Significantly lower (P<0.01) estimates of resting egg densities were obtained by incubation of sediments than egg counts. Significantly higher (P<0.01) densities of resting eggs were found from the eutrophic Lake Okaro sediments. Incubation of sediments did not result in the emergence of all the planktonic species found in the seasonal study.
The taxonomy and biogeography of North Island and New Zealand species were investigated based on the species recorded from the current study and from New Zealand to date. The level of endemism in New Zealand appears to be low. Fewer species have been recorded from the North Island than the South Island, and fewer in New Zealand than Australia. This variation can be largely explained by differences in sampling effort or biogeographical processes. No typically cold-water species, e.g., Notholca species, are known from the North Island. New Zealand species records have affinities with the Australian fauna.