Effects on river water environment by communities of aquatic plants -: Role of emergent plants in the Tama River

To estimate effects of emergent plants in urban rivers, we investigated the physical and chemical environments of the vegetation of emergent plants and the standing crop of plankton and epiphyton in the Tama River, a typical urban river in Japan. Muddy sediment was accumulated on the stones of the riverbed in the backwater of the dam which was investigated, and communities of emergent plants were in the sediment. Within the areas of vegetation it was dark, and the concentration of Chi-a and the total number of phytoplankton both were lower than outside. Diatoms with a diameter of 50 mum or more showed particularly marked reduction. The density of epiphyton increased in places where the vegetation density was low. The water quality showed a marked decline in the concentration of particulate matter within the area of emergent plants, compared with that outside. For example, the decline in concentration of particulate phosphorous within the area of vegetation averaged 25%. A decline in concentration of NH4+-N within the vegetation averaged 13%, and it was understood that nitrification had occurred within the area of vegetation. However, we found that the communities of emergent plants in the backwater area nitrified very small amounts of NH4+-N in the river water compared with the riverbed. We thought it important that proliferation of phytoplankton was prevented by shade provided by plant bodies in the backwater area. The existence of emergent plants gives physical and chemical diversity to the water environment of rivers and influences the density of organisms.