Owing to high nutrient load in the Bibi River,
Phalaris arundinacea shoots overgrew and clogged the stream by forming floating masses, resulting in the decline of submerged macrophytes. To restore submerged macrophytes such as
Ranunculus nipponicus and
Sparganium emersum, we partially removed the floating masses of
P. arundinacea by adjusting the stream width to where the submerged macrophytes remained. Immediately after the partial removal of
P. arundinacea, velocity of the stream increased whereas mud depth decreased and the bottom sediment became coarser in the riverbed. However, water depth decreased 4 years after the partial removal of
P. arundinacea. Furthermore, coverage of
R. nipponicus increased every year after the partial removal of
P. arundinacea, whereas coverage of
S. emersum increased the year after the partial removal of
P. arundinacea but remained nearly stable for the next 6 years. By contrast, after the partial removal of
P. arundinacea, the coverage of emerged macrophytes
P. arundinacea and
Cicuta virosa continued to decrease, and they disappeared by the fifth year. Canonical correspondence analysis revealed that the coverage of
R. nipponicus positively correlated with the increase in surface velocity and coarsening of the bottom sediment, but negatively correlated with mud depth. The coverage of
P. arundinacea and
Cicuta virosa negatively correlated with surface velocity and coarsening of the bottom sediment, whereas the cover of
S. emersum positively correlated with water depth. Accordingly, changes in aquatic macrophytes would be the result of changes in physical environments following partial removal. The partial removal of
P. arundinacea is considered to be an effective countermeasure because it suppressed the growth of
P. arundinacea in the stream for at least 7 years.
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