With the increase of global carbon emissions since the industrial revolution, atmospheric CO₂ levels are expected to be twice the present level by the end of this century, reaching 750 μL/L. The water carbonate chemistry of the environment may change notably because of this increase in CO₂ levels, which would in turn have a dramatic effect on aquatic ecosystems. Studies related to the impacts of atmospheric CO₂ elevation on plankton mainly focus on oceanic, rather than fresh water ecosystems. Lake Taihu is a highly eutrophic lake, and carbon limitation due to intense cyanobacterial blooms might develop, even in the presence of high concentrations of dissolved inorganic carbon. Increased atmospheric CO₂ concentrations could cause a shift in the chemical equilibrium of aquatic ecosystems. There is increasing evidence that the concentration of dissolved inorganic carbon (DIC) increases because of the uptake of CO₂ from the atmosphere, whereas the pH and CO₃^2- concentrations in the water decrease. This change in environmental conditions is likely to affect the physiology of phytoplankton, and has the potential to influence species composition and competition. Furthermore, elevated pCO₂ levels are expected to cause an increase in carbon-rich compounds stored within phytoplankton, and a shift in their overall elemental composition toward higher carbon to nutrient ratios. These changes in the carbon/nutrient stoichiometry of primary producers can have cascading effects on zooplankton. The effect of CO₂ enrichment on zooplankton communities in eutrophic lakes is an important topic from both scientific and lake management perspectives. Laboratory studies of both the direct and indirect effects of CO₂ are particularly suitable for determining the physiological thresholds and mechanisms of the response of plankton to rising CO₂ levels, but cannot address how zooplankton respond in their natural environment. In contrast, in situ controlled experiments are an important tool for studying the effects of water acidification on pelagic community dynamics under near-to-natural conditions. In this study, in situ simulated experiments were carried out at the Lake Taihu Ecosystem Station in the spring and summer, to explore the impacts of changes in CO₂ concentration on the community structure of Cladocera. This experiment included three treatments with CO₂ concentrations of (270±40), (380±20) and (750±50) μL/L, which represent CO₂ concentrations at pre-industrial levels, the present level, and the predicted level at the end of this century. Our results showed that CO₂ enrichment could increase the biomass of Scenedesmus, and could change the community structure of Cladocera. High CO₂ levels favored the growth of Bosmina and Diaphanosoma, but were unfavorable for the growth of Ceriodaphnia. Elevated CO₂ concentrations may likewise cause an increase in C:P ratios due to elevated carbon fixation, which would be beneficial to Cladoceran populations with low phosphorus contents. Thus, the cladoceran community structure was governed by food quality, rather than food quantity. The extrapolation of our results to the context of natural ecosystems indicates that rising CO₂ concentrations will intensify phytoplankton blooms in eutrophic waters. This may further enhance the productivity of these waters. Increases in phytoplankton carbon:nutrient ratios can therefore alter the structures of food webs. Our results imply that rising atmospheric CO₂ levels will have dramatic consequences for eutrophic ecosystems. Our conclusions will help predict likely changes in the plankton community in Lake Taihu due to climate change.