Photosynthesis of amphibious and obligately submerged plants in CO2-rich lowland streams.

Small unshaded streams in lowland regions receive drainage water with high concentrations of free␣CO2, and they support an abundant growth of amphibious and obligately submerged plants. Our first objective was to measure the CO2 regime during summer in a wide range of small alkaline Danish streams subject to wide variation in temperature, O2 and CO2 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 CO2, and an obligately submerged species capable of using both HCO-3 and CO2. We found that the median CO2 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 CO2 concentrations approaching air saturation in the afternoon. In contrast, outlets from lakes had low CO2 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 CO2 concentrations, while increasing temperature and O2 had little effect on photosynthesis. Photosynthesis of the obligately submerged species varied little with the change of CO2 because of HCO3-- use, and variations were mostly due to changes in O2 concentration. Independent measurements showed that changes in temperature, O2 and CO2 could account for the daily variability of photosynthesis of all three species in water. The results imply that CO2 supersaturation in small lowland streams is important for the rich representation of amphibious species and their contribution to system photosynthesis.