Seasonal in situ microcosm experiments were carried out in Lake Taihu to assess the seasonal effects of CO₂ on the carbon:nutrient stoichiometry of phytoplankton in freshwater ecosystems with different trophic status. The experimental treatments were three concentrations of CO₂, 270, 380 and 750 ppm, which represent the level for preindustrial, present and the prediction for the end of this century, respectively. Although nutrient concentrations were higher in the Meiliang Bay than those in the East Taihu, the later one has generally higher major nutrient contents of phytoplankton. Meanwhile, the nutrient contents of phytoplankton showed seasonal variation in the Meiliang Bay, while relatively stable in the East Taihu. The rise of CO₂ could largely increase the C:P ratio of phytoplankton and slightly increase the N:P ratio of phytoplankton of microcosms in the Meiliang Bay, but did not have significant impact on those two stoichiometric values in the microcosms of the East Taihu. The rise of elemental ratios were due to the increase of C and N content in phytoplankton under high CO₂ treatments. Rising CO₂ concentration is likely to have a more significant impact on particulate elemental ratios if phytoplankton is in nutrients-limited status, regardless of the nutrient concentrations of surrounding waters.