Photosynthesis of amphibious and obligately submerged plants in CO-rich lowland streams.

Small unshaded streams in lowland regions receive drainage water with high concentrations of free␣CO, and they support an abundant growth of amphibious and obligately submerged plants. Our first objective was to measure the CO regime during summer in a wide range of small alkaline Danish streams subject to wide variation in temperature, O and CO during the day. The second objective was to determine the effect of these variations on daily changes in light-saturated photosynthesis in water of a homophyllous and a heterophyllous amphibious species that only used CO, and an obligately submerged species capable of using both HCO and CO. We found that the median CO concentrations of the streams were 11 and 6 times above air saturation in the morning and the afternoon, respectively, but stream sites with dense plant growth had CO concentrations approaching air saturation in the afternoon. In contrast, outlets from lakes had low CO concentrations close to, or below, air saturation. The amphibious species showed a reduction of photosynthesis in water from morning to afternoon along with the decline in CO concentrations, while increasing temperature and O had little effect on photosynthesis. Photosynthesis of the obligately submerged species varied little with the change of CO because of HCO- use, and variations were mostly due to changes in O concentration. Independent measurements showed that changes in temperature, O and CO could account for the daily variability of photosynthesis of all three species in water. The results imply that CO supersaturation in small lowland streams is important for the rich representation of amphibious species and their contribution to system photosynthesis.