Abstract
The aims of this study were 1) to clarify the purification function of an immersed plant, indian rice (makomo in Japanese: Zizania latifolia Trucz.) by monitoring the water-quality change in open-water and porewater, and 2) to estimate the possible amount of nutrient removal from river system by reaping Z. latifolia. The study area was selected in a small semi-natural river, the Juuroh-gawa River located in suburbs of Kohfu, Yamanashi. Water quality of open-water was measured at three points in the middle of the river course from upstream to downstream through a year, in whose intervals Z. latifolia were dominated along the river side. Porewater for the analysis were sampled in summer and winter from inside of Z. latifolia growing riverbeds and none-vegetation ones as contradistinctions, where water depth and flux were as similar as the former ones. In results, water quality of open-water had no change among three points; purification function of the species was not clear. In the result of porewater, however, total N was lower in Z. latifolia growing sites significantly than none-vegetation ones especially in summer. This means habitats of the species have purification function when the species grows. Additionally the maximum amount of nutrient absorption of the species was measured by analyzing the nutrient contents in the aboveground biomass reaped in summer when the growth is reached to the maximum. In the result, per cover area contents of N and P were 82 gNm-2 and 26 gPm-2 respectively. When the absorption rate during growing period was compared to other species after previous researches, these values were similar to the results of other immersed plants measured under field conditions. However, when the possible amount of nutrient removal from river system was estimated under the hypothesis that all individuals of the species growing within study area could be reaped and brought out from riverbeds, it was considered to be too low to purify all effluent loadings run into from the catchments area. It was concluded that Z. latifolia and its growing habitat have purification function, but it would be effective in the case just degree of eutrophication is low.
