Egeria densa is an often-found invasive species in Japan, which has spread widely in the past two decades in rivers where no macrophytes had previously been found. As a result, these ecosystems have now become dominated by E. densa. The habitat preference for E. densa colony formation was investigated using the tissue concentrations of hydrogen peroxide (H 2 O 2 : a reactive oxygen species) under varying conditions in rivers and laboratory conditions. The empirical equations that can describe the macrophyte tissue H 2 O 2 formation under various velocity and light conditions were produced. The H 2 O 2 concentrations of dark-adapted plants are proportional to the flow velocity, and the surplus H 2 O 2 concentration in the light-exposed condition corresponded to the photosystems produced H 2 O 2. When the H 2 O 2 concentration exceeds 16 µmol/gFW, plant tissue starts to deteriorate, and biomass declines, indicating the critical values required for long-term survival of the plant. The empirically obtained relationships between flow velocity or light intensity and the analysis of H 2 O 2 concentration for different slopes and depths of channels found that the H 2 O 2 value exceeds the critical H 2 O 2 concentration in channels with above 1/100 at around 0.6 m depth. This agrees with the observed results where colonies were not found in channels with slopes exceeding 1/100, and biomass concentration was the largest at depths of 0.6 to 0.8 m. H 2 O 2 concentration is quite applicable to understanding the macrophyte condition in various kinds of macrophyte management.

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