4:15 PM - 4:30 PM
[HTT16-04] Understanding anthropogenic N loading in the Tama River using δ15N of multiple N compounds and examining their characteristics as indicators of N pollution
Keywords:River ecosystems, Nitrogen pollutions, Nitrogen stable isotopes, Multiple tracers
Nitrogen (N), which takes various forms in water, is one of the elements that causes eutrophication in rivers and adjacent coastal areas. Therefore, N management in the river ecosystem (i.e., identifying N sources and understanding N forms used by algae) is essential for conserving riverine and coastal environments. In this study, we focused on the nitrogen stable isotope ratio (δ15N) and estimated the source and actual bioavailability of N in the Tama River, based on: 1) δ15N in nitrate (NO3-) and ammonium (NH4+), which are used by macro- and micro-algae, 2) Particulate organic matter (POM), which reflects nitrogen used by phytoplankton, and 3) epiphytic algae and in situ incubated macroalgae, which record time-averaged N source information. Based on the results of these analyses, we will discuss the N dynamics in the Tama River, and properties and effectiveness of each δ15N in various N compounds as a pollution indicator.
NO3- concentrations, and δ15N-NO3- increased downstream and correlated with CDOM (colored dissolved organic matter) concentrations, suggesting that N from treated sewage water affects river water quality. It was also shown that CDOM, which is easily measured by loggers with high spatio-temporal resolution, can be used as a simple indicator of NO3- concentration in the Tama River. The δ15N-NO3- at each site did not change from the observations in 1994 (Kumazawa et al., 2000), suggesting no significant change in the main nitrogen source in the Tama River. Variations in δ15N-epiphytic algae and δ15N- NH4+ were consistent along the river, indicating that, despite low NH4+ concentrations, riparian epiphytic algae took up more NH4+ than NO3- on a time-averaged basis. On the other hand, δ15N-POM showed a similar shift to δ15N-NO3-, suggesting the utilization of high concentrations of NO3- by phytoplankton. However, the fact that δ15N-POM was 5~10‰ lower than δ15N-NO3-probably reflects the carryover effect of POM with low δ15N and other factors and was not considered an effective environmental indicator at the sampling site. Different δ15N values were observed in each N compound, even at the same site, Therefore, it is important to understand and use the characteristics of δ15N of each N compound for proper N management in the rivers.
NO3- concentrations, and δ15N-NO3- increased downstream and correlated with CDOM (colored dissolved organic matter) concentrations, suggesting that N from treated sewage water affects river water quality. It was also shown that CDOM, which is easily measured by loggers with high spatio-temporal resolution, can be used as a simple indicator of NO3- concentration in the Tama River. The δ15N-NO3- at each site did not change from the observations in 1994 (Kumazawa et al., 2000), suggesting no significant change in the main nitrogen source in the Tama River. Variations in δ15N-epiphytic algae and δ15N- NH4+ were consistent along the river, indicating that, despite low NH4+ concentrations, riparian epiphytic algae took up more NH4+ than NO3- on a time-averaged basis. On the other hand, δ15N-POM showed a similar shift to δ15N-NO3-, suggesting the utilization of high concentrations of NO3- by phytoplankton. However, the fact that δ15N-POM was 5~10‰ lower than δ15N-NO3-probably reflects the carryover effect of POM with low δ15N and other factors and was not considered an effective environmental indicator at the sampling site. Different δ15N values were observed in each N compound, even at the same site, Therefore, it is important to understand and use the characteristics of δ15N of each N compound for proper N management in the rivers.