Some aquatic plants such as genus Nuphar which contains many endangered species, occur frequently in acidic waters where the rate of free CO₂ is relatively high. However, these species sometimes occur in water of high pH value. We tried to clarify the reason why these species can grow in water of relatively high pH value.

The results of pH drift experiment for 18 aquatic plants showed that these species can be classified into two groups: low final pH(8.65-9.04) group and high final pH(10.27-10.7) group. The former contains species such as Nuphar spp. which prefer acidic water, and are aquatic plants that can use only free CO₂ for photosynthesis. The results of pH and RpH measurements throughout the year showed that the waters of habitats of Nuphar populations were oversaturated stably by dissolved CO₂. The species are found in river courses where spring waters are seeping from river beds and pH values are lower than RpH values, indicating that the waters are oversaturated with dissolved CO₂. We concluded that the spring water oversaturated with dissolved CO₂ stimulate photosynthesis of the endangered aquatic plants and help their growth in relatively high pH value water.