Photosynthetic use of inorganic carbon among primary and secondary water plants in streams

1. We analysed photosynthetic rates and inorganic carbon use of thirty-five vascular macrophyte species collected submerged in eight nutrient- and CO₂-rich Danish lowland streams. The species were classified in four groups as mainly terrestrial, homophyllous and heterophyllous amphibious and truly submerged. These groups represent plant species differently adapted to life in water. 2. Photosynthetic rates measured in water increased in the gradual transition from mainly terrestrial, through amphibious to truly submerged species. Species normally in contact with air adapted to submergence by increasing the photosynthetic rate at limiting CO₂. Photosynthetic rates of submerged parts of heterophyllous amphibious species were close to those of submerged species. Submerged species with thin or finely dissected leaves had the highest photosynthetic rates, probably because of low diffusional resistance to uptake of nutrients and gases. 3. In contrast to submerged species, terrestrial and amphibious species were unable to use HCO₃^?. Extensive oversaturation with CO₂ in the streams allows, however, many amphibious species to photosynthesize well under water, based on CO₂-use alone. Amphibious CO₂-users, with very few structural adaptations to life under water, can therefore be as dominant in the submerged vegetation of lowland streams as HCO₃^?-using water plants. Moreover, the streams provide open space for colonization from the dense vegetation ashore. 4. Among the 1265 Danish herbaceous species no less than seventy-five terrestrial species occasionally grow submerged, forty-five species are amphibious, and fifty-one are true water plants. These numbers suggest that adaptation to permanent or temporary submergence is an ongoing process involving many species and that the land-water interface does not represent as difficult a barrier as often believed.