The phytoplankton communities of nine lakes, Waikato, New Zealand: a comparative study of floristics, seasonal dynamics and the influence of herbivory
The structure and seasonal dynamics of the phytoplankton communities of nine shallow lakes located in relatively close proximity to one another within the Waikato Basin, and of similar age and origin, were studied for one year (July 1983 to July 1984); the lakes were Kainui, Mangahia, Mangakaware, Maratoto, Ngaroto, Rotokauri, Rotomanuka North and South, and Rotoroa. There was a broad range of physico-chemical regimes, a direct result of differing morphometries and quantities of allochthonous dissolved humic material from adjacent peatland and/or swamps, and varying degrees of exposure to wind. Community structure was markedly influenced by humic content, in terms of both α diversity and distribution of species within the major classes, with total numbers of species found per lake ranging from 113 to 210. Temporal patterns of community composition were directly related to specific mixing regimes and morphometry. Lakes which had minimal thermal stability throughout summer were dominated throughout the entire year by one or two algal classes, and it was shown statistically that the major phytoplankters were large, K- selected species (e.g., Botryococcus braunii and Microcystis aeruginosa). Unpredictable sequences of domination, one class replacing another after brief periods of importance, were recorded in lakes where ephemeral periods of summer stratification alternated with either equally short periods of holomixis or weakly developed stratification; the dominant species in these lakes were r- strategists such as Asterionella formosa, Acanthoceras zachariasi and Tetrastrum triangulare. Marked seasonal periodicity (including autogenic succession) occurred in Lake Rotomanuka North, the only warm monomictic lake within the series. Total α diversity (402 species) was higher than that of any other group of intensively studied New Zealand lakes, but compositional overlap at both generic and specific levels was low, with only 24 species being found in all nine lakes. Of these, the most common and ecologically versatile phytoplankters were Closterium acutum var. variabile, Cryptomonas marssonii and C. ovata, Cyclotella stelligera, Monoraphidium contortum and Trachelomonas volvocina. Mean total phytoplankton biomass and density ranged from 0.7 to 72.2 g m³ and 0.4 to 11.6 pu 1⁻¹, respectively, and comparisons with other quantitative phytoplankton studies indicate that some of the study lakes (Maratoto, Ngaroto and Rotomanuka South) are the most productive recorded in New Zealand to date. Eutrophic waters and rich organic sediments, together with a combination of both stratified and frequently mixed water columns, resulted in relatively high euglenophyte species diversity. Conversely, the number of desmid species was relatively low. Zooplankton exclusion experiments using limnocorrals were carried out in an almost clear-water lake (Rotomanuka North) and in a darkly-stained lake (Maratoto). The results suggest that a reduction in grazing pressure markedly influenced phytoplankton community structure in the former, causing shifts (in terms of biomass) from communities dominated by K- strategists (Microcystis aeruginosa or Peridinium cinctum) to smaller, fast-growing species (Cyclotella stelligera or Coelastrum microporum). Reduced herbivory also permitted increases in the number of taxa and total biomass of small edible phytoplankters (GALDs ≤ 20 μm), together with an increase in the number of euchlorophyte species. Similar responses were not apparent in dystrophic Lake Maratoto, suggesting that the stressful physico-chemical regime was the major regulator of community structure, and that trophic organisation within the two systems differed markedly, with energy transfer in the latter operating largely through a heterotrophic-based food chain.
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