Thermocline deepening and mixing alter zooplankton phenology, biomass and body size in a whole‐lake experiment

SUMMARY 1. Summer thermal stratification is thought to be one of the key structuring physical factors for north temperate lake zooplankton. Shifts associated with climate change may lead to altered thermocline depths in stratified lakes through changes to (i) wind stress and associated water column mixing or (ii) air temperature and precipitation. The effects of thermocline deepening through these two scenarios were simulated in a whole-lake experiment to assess the effects on the phenology, biomass and mean body size of the zooplankton. 2. The thermocline of a three-basin dimictic lake was deepened to 7–8 m in one basin using a lake mixer. Through heat transfer, the thermocline of the adjacent basin was deepened to 6–7 m, while the third basin served as an unmanipulated control (4–5 m deep thermocline). Zooplankton community dynamics were followed weekly in a control year and in two experimental years. A before-after-control-impact (BACI) statistical protocol was used to assess the effects of two treatments: Deepening and Deepening+Mixing. 3. Thermocline depth was the main factor influencing zooplankton changes. The biomass and late-season dominance by smaller-bodied and more fish-evasive species were favoured by deepening, leading to a decline in mean community body size, but an increase in total biomass. 4. While many effects could be attributed to thermocline deepening, additional or exacerbated responses were observed when mixing was also present. This resulted mostly from a loss of the hypolimnetic refuge for zooplankton, which accentuated predation effects by warm-water fish on larger cladocerans. Overall, our treatments promoted top-down (predation) effects that increased over the 2 years of the experiment.