| 08 Dec 2025
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.
General comments:
This manuscript presents a valuable investigation identifying the origin and pathways of spring water and associated nutrients (N, P, Si) in different backwater systems by measuring ion composition, 222Rn concentration, stable hydrogen and oxygen isotope ratios (δ2H–H2O and δ18O–H2O) in water, and chromophoric dissolved organic matter (CDOM) concentration at the spring. The study addresses an important research gap with a robust tracer approach, but major revisions are needed to resolve methodological ambiguities, strengthen data interpretation, and clarify limitations.
Below I provide specific comments organized by manuscript section.
specific comments：
Introduction
Your introduction is very general and elaborate.
The literature review adequately covers relevant studies but could not better highlight the novelty of this work.
The rationale for focusing on small floodplain water bodies is not enough.
Materials and methods
Specify the number, depth, and spatial distribution of shallow groundwater wells relative to the backwaters. Clarify how these wells represent the aquifer’s lateral and vertical heterogeneity, as this is critical for validating groundwater-endmember calculations.
Since CDOM was only measured in November 2022, justify why this single snapshot is sufficient to infer seasonal contributions of river water, groundwater, and sewage. Provide details on sensor calibration, analytical precision, and replicate measurements to support CDOM as a reliable mixing tracer.
Discussion：
L327：The inference of denitrification in Backwater C relies on the 2:1 δ15N:δ18O enrichment ratio, but additional evidence (e.g., dissolved N₂ gas concentrations, sediment denitrification potential assays, or microbial marker genes) is needed to confirm this process. Explain why denitrification is prominent in Backwater C but not in other backwaters, considering factors like organic matter availability, hydrological residence time, and sediment properties.
Clarify the potential influence of the factory drainage (600 m upstream of Backwater A) on water chemistry. Provide data on the drainage’s nutrient and tracer concentrations (e.g., CDOM, ions) to rule out or quantify its contribution to Backwater A’s unique ion composition (high Na⁺ and SO₄²⁻).
L389：Define "DIN" (Dissolved Inorganic Nitrogen) upon first use, as it includes NO₃⁻, NH₄⁺, and NO₂⁻—a distinction critical for readers unfamiliar with aquatic biogeochemistry.
There is no Conclusion.