We investigated the effects of temperature and CO 2 variation on the growth and elemental composition of cultures of the diatom Pseudo-nitzschia subcurvata and the prymnesiophyte Phaeocystis antarctica , two ecologically dominant phytoplankton species isolated from the Ross Sea, Antarctica. To obtain thermal functional response curves, cultures were grown across a range of temperatures from 0 °C to 14 °C. In addition, a competition experiment examined the relative abundance of both species at 0 °C and 6 °C. CO 2 functional response curves were conducted from 100 to 1730 ppm at 2 °C and 8 °C to test for interactive effects between the two variables. The growth of both phytoplankton was significantly affected by temperature increase, but with different trends. Growth rates of P. subcurvata increased with temperature from 0 °C to maximum levels at 8 °C, while the growth rates of P. antarctica only increased from 0 °C to 2 °C. The maximum thermal limits of P. subcurvata and P. antarctica where growth stopped completely were 14 °C and 10 °C, respectively. Although P. subcurvata outcompeted P. antarctica at both temperatures in the competition experiment, this happened much faster at 6 °C than at 0 °C. For P. subcurvata , there was a significant interactive effect in which the warmer temperature decreased the CO 2 half saturation constant for growth, but this was not the case for P. antarctica . The growth rates of both species increased with CO 2 increases up 425 ppm, and in contrast to significant effects of temperature, the effects of CO 2 increase on their elemental composition were minimal. Our results suggest that future warming may be more favorable to the diatom than to the prymnesiophyte, while CO 2 increases may not be a major factor in future competitive interactions between Pseudo-nitzschia subcurvata and Phaeocystis antarctica in the Ross Sea.