Elevated carbon dioxide can boost freshwater zooplankton abundance, while warming may shift community composition, weakening phytoplankton control

Zooplankton communities in ponds and lakes control phytoplankton growth and serve as a food source for higher trophic levels such as fish. Recent experiments have shown that several freshwater zooplankton species as well as the phytoplankton communities on which they feed can be variably affected by rising temperatures, reduced pH and higher dissolved pCO 2 associated with global environmental change. However, to understand how freshwater ecosystems may respond to the combination of these stressors, more realistic multi‐stressor tests are needed that document community‐level responses. In this study, we investigated the responses of a natural zooplankton community to combinations of elevated pCO 2 (~ 300 ppm vs. 12,500 ppm) and warming (20°C vs. 24°C) in indoor microcosms over a period of 6 weeks. Overall, both treatments resulted in strong shifts in zooplankton communities. Elevated pCO 2 promoted zooplankton abundance and biomass, while warming mainly resulted in changes in community composition and body size. Elevated pCO 2 favored the small water flea Chydorus sphaericus which dramatically increased in abundance. One species ( Eurycercus lamellatus ) became smaller under warming. The combination of warming and elevated pCO 2 reduced body size in Simocephalus vetulus , which replaced Daphnia as the dominant zooplankter. Changes in zooplankton abundance appear to be driven by changes in phytoplankton and periphyton biomass rather than changes in stoichiometry. This experiment suggests that the combination of warming and elevated pCO 2 can result in less effective phytoplankton control by zooplankton and might negatively affect higher trophic levels that rely on large zooplankton as a food source.