Abstract
The large, oligotrophic lakes in the South Island of New Zealand are an iconic part of the sub-alpine landscape, and are vital for a large number of ecosystem services, including water supply, power generation, and tourism. These lakes face numerous pressures, including a changing climate, that are likely to affect phytoplankton, the main pelagic primary producers. The aim of this study was to predict likely impacts of climate change on phytoplankton biomass and composition in two of these lakes. First, the physical dynamics of Lakes Wanaka and Wakatipu were modelled under several climate change scenarios using the lake model DYRESM. Second, results from modelling were combined with information on the current dynamics and limitations of phytoplankton from field surveys and experimental data.
Lake modelling suggests that for Lakes Wanaka and Wakatipu a predicted air temperature increase of 1°C or 2°C by 2040 or 2090 (Ministry for the Environment 2008) is likely to result in warmer lake temperatures, and a shallower, longer thermal stratification, negating the cooling influences of predicted increased rainfall and river flows. However, wind speeds may increase by up to 10% (Ministry for the Environment 2008), which could counteract the effects of air warming on thermocline depth. In Lake Wanaka, warmer winters could increase phytoplankton biomass in winter and spring, whereas in Lake Wakatipu warmer winters and the predicted earlier onset of stratification are likely to result in an earlier phytoplankton spring ‘bloom’. Due to apparent nutrient limitation and the absence of light limitation in the mixed layer in summer, a shallower and warmer epilimnion is unlikely to increase summer biomass. However, nutrient inputs to the lakes are likely to increase with climate change, as a result of which phytoplankton biomass can increase, highlighting the need to carefully control nutrient inputs to the lakes.
Small, photosynthetic picocyanobacteria (PCB), the dominant phytoplankter in Lakes Wanaka and Wakatipu in the 1990s, have diminished in their contribution to total phytoplankton biomass and productivity. PCB contribution to primary production was negatively affected by nutrient additions, especially phosphorus. Thus, any nutrient enrichment associated with climate change is likely to disadvantage PCB compared to eukaryotic phytoplankton. Changes in PCB contribution, and other changes in phytoplankton composition, can have far reaching consequences for food web dynamics and trophic efficiency. In Lake Wanaka, a notable increase in the centric diatom Cyclotella (or Discotella) was observed, potentially linked to warming air temperatures, as an increase in centric diatoms recorded in several Northern Hemisphere lakes has been attributed to decreased turbulence due to lake warming (Rühland et al 2008; Winder et al 2009). However, changes in phytoplankton composition may also be linked to the recent introduction of Daphnia ‘pulex’ or changes in nutrient inputs.
Even morphologically similar lakes, situated in the same geographical area, such as Lakes Wanaka and Wakatipu, can differ in their plankton ecology, and thus in their likely response to climate change. My findings highlight that a thorough understanding of pelagic plankton dynamics is fundamental to predict impacts of climate change on lakes, and a lake-specific management approach is required.