Catchment properties and the photosynthetic trait composition of freshwater plant communities

Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO₂) to compensate for the low diffusivity and potential depletion of CO₂ in water. Concentrations of bicarbonate and CO₂ vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO₂ concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO₂ concentrations may alter the species compositions of freshwater plant communities.