Photosynthetic response to globally increasing CO 2 of co‐occurring temperate seagrass species

Photosynthesis of most seagrass species seems to be limited by present concentrations of dissolved inorganic carbon (DIC). Therefore, the ongoing increase in atmospheric CO 2 could enhance seagrass photosynthesis and internal O 2 supply, and potentially change species competition through differential responses to increasing CO 2 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 CO 2 and changes in HCO 3 − . Net photosynthesis of all species except Zostera polychlamys were limited at pre‐industrial compared to saturating CO 2 levels at light saturation, suggesting that enhanced CO 2 availability will enhance seagrass performance. Seven out of the nine species were efficient HCO 3 − users through acidification of diffusive boundary layers, production of extracellular carbonic anhydrase, or uptake and internal conversion of HCO 3 − . Species responded differently to near saturating CO 2 implying that increasing atmospheric CO 2 may change competition among seagrass species if co‐occurring in mixed beds. Increasing CO 2 availability also enhanced internal aeration in the one species assessed. We expect that future increases in atmospheric CO 2 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.