1:45 PM - 3:15 PM
[HTT15-P03] Study on the variation of water and biogeochemical cycles at backwaters in the floodplain of the Tama River
Keywords:Tama River, Nutrient, Backwater, Oxgen and Hydrogen Stable Isotope Ratios
Wando is a stagnant water area formed by river that bends into the floodplain, and is known to function as a habitat for aquatic organisms. For this reason, it is included in the “Neo-Natural River Reconstruction” program promoted by the government and is a place where contributions to the river ecosystem through creation are desired. However, the chemical water quality of Wando and the mechanism that created Wando has not been clarified, and it is necessary to clarify the nutrient dynamics (origin, supply, concentration, and composition ratio) that affect primary production in order to assess its function in the river. In this study, we focused on the water supplied to the Wando and analyze the water cycle using 222Rn concentration, stable isotope ratios of hydrogen and oxygen (δD and δ18O) and fluorescence dissolved organic matter (FDOM) at the Wando spring area, in addition to measuring various dissolved ion concentrations and nutrient concentrations. The objective of this study is to clarify the characteristics of water and material cycle in Wando areas.
Three Wando areas (Wando A, Wando B, Wando C) located in the middle reaches of the Tama River (Fuchu City, Tokyo) were selected for the study (Figure 1). Sampling areas in each Wando were categorized into two areas; “spring area” where water from the spring were often observed, and “downstream area” where the water flows out to the river. In addition, water sampling was conducted in the river near the Wando areas. Shallow groundwater was collected from several locations in the city as endmembers of the water flowing into the Wando, and rainfall water was also collected in Fuchu city.
The 222Rn concentration at the spring area of each Wando was higher than that of the lateral river, but lower than that of shallow groundwater. There was a negative correlation between 222Rn concentration and precipitation in Wando A and C. Therefore, it is considered that a mixture of subterranean river water and shallow groundwater flows into the Wando. The plots of δD and δ18O at the spring area approached those of shallow groundwater from spring to fall, suggesting that the mixing rate of river and shallow groundwater may show seasonal changes. Mixing rates were calculated for each Wando using δD, δ18O, and Cl-, indicating that subterranean river water predominates in the Wando A and C, while shallow groundwater predominates in the Wando B. The FDOM concentrations in each Wando were consistent with this trend, indicating that FDOM may be also useful in estimating mixing rates (i.e., the detailed origin of the water supplied to Wando). Nutrient loadings varied in each Wando, particularly PO43- and NO3-, and were influenced by the surrounding environment and the origin of water. As a result, DIN:DIP showed the possibility of PO43- limitation for primary production due to excess NO3- loading in the Wando A and B, and DIP:DSi ratio showed the possibility of NO3- limitation in the Wando C due to the constant supply of high concentration of DSi from shallow groundwater (Figures 2 and 3).
Wando areas in the middle reaches of the Tama River are subject to different nutrient dynamics due to the different supply rates of subterranean river water and shallow groundwater in each Wando, as well as nutrient loading caused by the surrounding environments. Therefore, different nutrient dynamics in each Wando (Figure 4) may affect internal phytoplankton composition, aquatic plant community, and their primary productions.
Three Wando areas (Wando A, Wando B, Wando C) located in the middle reaches of the Tama River (Fuchu City, Tokyo) were selected for the study (Figure 1). Sampling areas in each Wando were categorized into two areas; “spring area” where water from the spring were often observed, and “downstream area” where the water flows out to the river. In addition, water sampling was conducted in the river near the Wando areas. Shallow groundwater was collected from several locations in the city as endmembers of the water flowing into the Wando, and rainfall water was also collected in Fuchu city.
The 222Rn concentration at the spring area of each Wando was higher than that of the lateral river, but lower than that of shallow groundwater. There was a negative correlation between 222Rn concentration and precipitation in Wando A and C. Therefore, it is considered that a mixture of subterranean river water and shallow groundwater flows into the Wando. The plots of δD and δ18O at the spring area approached those of shallow groundwater from spring to fall, suggesting that the mixing rate of river and shallow groundwater may show seasonal changes. Mixing rates were calculated for each Wando using δD, δ18O, and Cl-, indicating that subterranean river water predominates in the Wando A and C, while shallow groundwater predominates in the Wando B. The FDOM concentrations in each Wando were consistent with this trend, indicating that FDOM may be also useful in estimating mixing rates (i.e., the detailed origin of the water supplied to Wando). Nutrient loadings varied in each Wando, particularly PO43- and NO3-, and were influenced by the surrounding environment and the origin of water. As a result, DIN:DIP showed the possibility of PO43- limitation for primary production due to excess NO3- loading in the Wando A and B, and DIP:DSi ratio showed the possibility of NO3- limitation in the Wando C due to the constant supply of high concentration of DSi from shallow groundwater (Figures 2 and 3).
Wando areas in the middle reaches of the Tama River are subject to different nutrient dynamics due to the different supply rates of subterranean river water and shallow groundwater in each Wando, as well as nutrient loading caused by the surrounding environments. Therefore, different nutrient dynamics in each Wando (Figure 4) may affect internal phytoplankton composition, aquatic plant community, and their primary productions.