On the occurrence, causes and potential consequences of low zooplankton to phytoplankton ratios in New Zealand lakes

Abstract

SUMMARY. 1. New Zealand lakes are shown to have lower average zooplankton biomasses than north‐temperate lakes of similar average phytoplankton biomass, expressed as cell volume or chlorophyll a, or similar average total phosphorus concentration, typically by a factor of 5 or more. 2. Evidence suggests that the relatively low zooplankton biomasses of New Zealand lakes may be related to a tendency for them to be dominated by large algae that are not directly available as food for zooplankton, with oligotrophy lakes in particular differing from north‐temperate lakes in this respect. 3. This difference in turn may be related largely to their mixing regimes. All of the New Zealand lakes are polymietic or monomietic, whereas the northern lakes used for comparison are mostly dimietic. Also, hetero‐cystous cyanobacteria are favoured by the low inorganic nitrogen concentrations that are typical of New Zealand lakes. 4. Poor nutritional quality of the phytoplankton, relating to nitrogen limitation in many New Zealand lakes, might supplement the effects of cell size. 5. Low exploitation of phytoplankton by zooplankton can be expected to produce a shift in the metabolism of New Zealand lakes towards the sediments. Among the potential consequences of this effect are increased hypolimnetic oxygen demand with increased susceptibility to development of large internal loads of nutrients, and consequently, increased sensitivity to accelerated eutrophication from any increase in external nutrient loads.

New Zealand lakes research, 1967–91

“…Paerl & MacKenzie (1977) showed also that the proportional contribution of "net" phytoplankton (20-90 |im) to total phytoplankton fixation changed during the day and pointed out that this lack of uniformity could affect assessments of long-term trends in eutrophication that were based on short-term daylight measurements of 14 C fixation. Studies of phytoplankton associations of New Zealand lakes over the past 25 years suggest that they are similar to those elsewhere although there are some differences in distribution patterns of diatom genera (Viner & White 1987), and large-sized taxa may be relatively more abundant in New Zealand (Malthus & Mitchell 1989). Dense populations of Chlorella spp.…”

Phytoplankton species predictability increases towards warmer regions

“…This suggests that environments with reduced grazing losses (by zooplankton) and diminished competition (for nutrients and light) might lead to higher predictability and is furthermore consistent with the idea that trophic interaction-driven chaos is a source of unpredictability. Grazing by zooplankton that might drive chaos is considered to be less in warmer lakes (Malthus and Mitchell 2006). Indeed, in this same set of lakes we have found low grazing pressures on phytoplankton and absence of large herbivorous zooplankton in the warmer lakes (Kosten et al 2009b;G.…”

“…Data analysis-To explore the explained variance in species occurrence, we used binomial multiple logistic regression. In this case we expressed species occurrence as presence-absence (Mc Cullagh and Nelder 1989) and calculated the Nagelkerke R 2 coefficient of determination to evaluate their explicability by the environmental variables (Nagelkerke 1991). To analyze the predictability of the biomass of species across our lakes, we used ordinary multiple linear regressions.…”

Calanoid copepod grazing on phytoplankton: seasonal experiments on natural communities

“…Implications for the structure of New Zealand plankton communities Malthus & Mitchell (1990) have drawn attention to the way in which New Zealand plankton communities have lower average zoop1ankton:phytoplankton ratios than northern temperate lakes, and the tendency for New Zealand lakes of all trophic states to be dominated by large phytoplankton species. They explained these characteristics mainly on the basis of the distinctive characteristics of mixing in New Zealand lakes , which produce conditions conducive to continual resuspension of large algae, and frequent nitrogen limitation in many New Zealand lakes, which would tend to favour the larger cyanobacteria.…”