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

Abstract. River floodplains contribute to river ecosystems by supporting high biological productivity and biodiversity. Within floodplains, semi-enclosed water bodies develop, among which those partially connected to the river are known as backwater (i.e., locally called Wando in Japanese). Although backwater serves as habitats for aquatic organisms, studies on the origin of spring water within backwater and the associated nutrient supplies remain 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), ²²²Rn, stable hydrogen and oxygen isotope ratios in water (δ²H & δ¹⁸O–H₂O), and stable nitrogen and oxygen isotope ratios in nitrate (δ¹⁵N & δ¹⁸O–NO₃)—at three distinct backwaters sites within a 5-kilometer 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). While no significant difference in DIN concentration was observed between rivers and groundwater, the increase in δ¹⁵N & δ¹⁸O–NO₃ observed in one backwater site, coinciding with the decrease in nitrate concentration, suggested denitrification occurring in subsurface flow paths. As a result, in the backwaters, strongly influenced by the urban river, nutrient conditions reflected inputs from treated wastewater, leading to relatively stable N : P ratios in space and time. In contrast, the backwaters, which was primarily replenished by groundwater, showed pronounced seasonal fluctuations in N : P ratios due to variations in microbial activity, fertilizer inputs, and river inflow rates. Given the influence of nutrient environments on microbial communities and primary producers, the ecological functions of backwater can be better understood through intensive research focused on water-quality processes.