Characterization of variability of water and nutrient cycles in small floodplain water bodies using a geochemical multi-tracer

River floodplains contribute to river ecosystems by supporting high biological productivity and biodiversity. Backwaters, locally called Wando in Japanese, are semi-enclosed water bodies formed along rivers that serve as habitats for aquatic organisms. However, research on the origin of the seepage water supplied to backwater and the associated nutrient supply remains limited. In this study, we investigated the origins and pathways of water and the internal nutrient dynamics (sources, concentrations and composition ratios) using multiple geochemical tracers – ion balance, chromophoric dissolved organic matter (CDOM), 222 Rn, stable hydrogen and oxygen isotope ratios in water ( δ 2 H and δ 18 O–H 2 O), and stable nitrogen and oxygen isotope ratios in nitrate ( δ 15 N and δ 18 O–NO 3 ) – at three distinct backwaters sites within a 5 km section of the middle reaches of an urban river (Tama River) in Tokyo. Each geochemical tracer exhibited significantly different values between the surrounding shallow groundwater and the main river along the backwaters, serving as an effective indicator for evaluating the contribution of both sources to the water supplied to the backwaters. The water sources differed not only among the three backwaters locations within a short river section (5 km) but also across seasons. River water exhibited relatively high phosphate concentrations (3.4–12.4 µmol L −1 ) and low dissolved silicate (DSi) concentrations (157–218 µmol L −1 ), whereas shallow groundwater exhibited lower phosphate (0.7–1.3 µmol L −1 ) and higher DSi concentrations (236–730 µmol L −1 ). Although no significant difference in DIN (NO3- + NH4+) concentrations was observed between river water and groundwater, the use of δ 15 N and δ 18 O–NO 3 revealed differences in the sources of NO 3 between urban rivers and groundwater, as well as the possibility of denitrification in the floodplain subsurface of some backwaters. Furthermore, it was suggested that differences in the geology and topography of the floodplains surrounding each backwater, which influence seasonal changes in river flow and microbial activity, may also lead to variations in N : P ratios among individual backwaters. This study, based on multiple geochemical tracer analyses, suggests that, in Japan's steep-gradient urban rivers, hydrological connectivity and microbial activity can vary even within short river sections, and that backwaters exhibit spatially and temporally diverse nutrient environments.