Investigating the effects of temperature variability and heatwaves on pond zooplankton communities

Background/Question/Methods

While recent studies have extensively focused on the effects of climate change through rising mean temperatures, there is less known about the impact of projected changes in temperature variability and extreme temperature events like heatwaves. Freshwater ponds provide an ideal system to study responses to varying shifts in temperature regimes because they are relatively closed and thermally homogeneous. Ponds are also ecologically significant as sources of biodiversity with high productivity per their size and relatively understudied compared to lakes and streams. We performed a fully-factorial mesocosm experiment in 208 L tanks over nine weeks with three factors: 1) long-term thermal variation (four source ponds with similar average temperature but daily thermal ranges varying 4-11°C); 2) short-term thermal variation (insulated and non-insulated tanks); and 3) a four-day heatwave treatment (+3.6°C) halfway through the experiment. We sampled zooplankton community, chlorophyll a (proxy for algal biomass), and physicochemical factors throughout the experiment, and assayed the upper thermal limit (CTmax) and metabolism of individuals immediately before and after the heatwave. Our aim was to determine how short and long term thermal variability and heatwaves influence zooplankton community composition and organismal physiology.

Results/Conclusions

Before the heatwave, the short-term thermal variability treatment resulted in mesocosms that varied in their daily thermal fluctuations (insulated = 8.8°C, not insulated = 11.9°C), but this difference did not affect zooplankton diversity, abundance, or composition (diversity LME, p=0.719; abundance LME p=0.544; PERMANOVA p=0.437). However, the heatwave influenced the community composition by reducing the abundance of small zooplankton species, with a stronger effect on the more stable source pond than the more variable source pond (LME, p=0.0004). Physiologically, the heatwave increased CTmax in populations originating from thermally variable ponds, and decreased CTmax in populations originating from more thermally stable ponds (LME, p=0.006). Metabolic rates increased following the heatwave and were higher in the insulated, less variable mesocosms (LME, p=0.004). Overall, our data suggests that short-term temperature variability does not have an effect on zooplankton community composition but does have an effect on individuals’ metabolic rate; whereas long term variability (i.e., source pond) affects individuals’ CTmax. Finally, heatwaves could disproportionately affect smaller zooplankton species abundance. The incorporation of different scales of thermal variability will allow us to more accurately understand and predict how climate change will affect freshwater communities.