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 amphibio us and obligately submerged plants. Our first objective was to measure the CO2 regime during summer in a wide range of small alkaline Danish streams s ubject to wide variation in temperature, O-2 and CO2 during the day. The se cond objective was to determine the effect of these variations on daily cha nges in light-saturated photosynthesis in water of a homophyllous and a het erophyllous amphibious species that only used CO2, and an obligately submer ged species capable of using both HCO2 and CO2. We found that the median CO 2 concentrations of the streams were 11 and 6 times above air saturation in the morning and the afternoon, respectively, but stream sites with dense p lant growth had CO2 concentrations approaching air saturation in the aftern oon. In contrast, outlets from lakes had low CO2 concentrations close to, o r below, air saturation. The amphibious species showed a reduction of photo synthesis in water from morning to afternoon along with the decline in CO2 concentrations, while increasing temperature and O-2 had little effect on p hotosynthesis. Photosynthesis of the obligately submerged species varied li ttle with the change of CO2 because of HCO3- use, and variations were mostl y due to changes in O-2 concentration. Independent measurements showed that changes in temperature, O-2 and CO2 could account for the daily variabilit y of photosynthesis of all three species in water. The results imply that C O2 supersaturation in small lowland streams is important for the rich repre sentation of amphibious species and their contribution to system photosynth esis.