Photosynthesis of most seagrass species seems to be limited by present concentrations of dissolved inorganic carbon (DIC). Therefore, the ongoing increase in atmospheric CO₂ could enhance seagrass photosynthesis and internal O₂ supply, and potentially change species competition through differential responses to increasing CO₂ availability among species. We used short-term photosynthetic responses of nine seagrass species from the south-west of Australia to test species-specific responses to enhanced CO₂ and changes in HCO₃ ^-. Net photosynthesis of all species except Zostera polychlamys were limited at pre-industrial compared to saturating CO₂ levels at light saturation, suggesting that enhanced CO₂ availability will enhance seagrass performance. Seven out of the nine species were efficient HCO₃ ^- users through acidification of diffusive boundary layers, production of extracellular carbonic anhydrase, or uptake and internal conversion of HCO₃ ^-. Species responded differently to near saturating CO₂ implying that increasing atmospheric CO₂ may change competition among seagrass species if co-occurring in mixed beds. Increasing CO₂ availability also enhanced internal aeration in the one species assessed. We expect that future increases in atmospheric CO₂ will have the strongest impact on seagrass recruits and sparsely vegetated beds, because densely vegetated seagrass beds are most often limited by light and not by inorganic carbon.