Effects of phytoplankton on the distribution of submerged macrophytes in a small canal

Submerged macrophytes have been disappearing from the Kanto Plain, Japan since the 1960s. This disappearance is usually attributable to the interaction between macrophytes and phytoplankton. Phytoplankton contributes to shading of the available light and changes the availability of inorganic carbon from free CO₂ to HCO ₃ ⁻ for use in photosynthesis. However, limited information is available about the interaction between carbon fraction and submerged macrophytes through phytoplankton abundance. In this short note, we observe the distribution of submerged macrophytes and phytoplankton in a small canal. We found that, despite high photosynthetically active radiation (PAR) in the downstream region, low free CO₂ concentration through phytoplankton abundance can deplete free CO₂ for submerged macrophytes. In contrast, the upstream region exhibited macrophytes in an environment with high free CO₂ concentration. The stable carbon isotope ratio of submerged macrophytes follows this pattern, with more positive values occurring in the downstream region and more negative values in the upstream region. It has been reported that phytoplankton limits light availability for submerged macrophytes, but carbon availability could also be a factor in the distribution of submerged macrophytes. Because the source of water for submerged macrophytes is groundwater, its preservation possibly plays a key role for the restoration of submerged macrophytes.