Heterogeneity in Permeability and Particulate Organic Carbon Content Controls the Redox Condition of Riverbed Sediments at Different Timescales

• Published in: Geophysical research letters Vol. 51; no. 11
• Main Authors: Chen, Kewei, Guo, Zhilin, Zhan, Yang, Roden, Eric E., Zheng, Chunmiao
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.06.2024; American Geophysical Union (AGU); Wiley
• Subjects: Aquatic ecosystems; Biogeochemistry; Carbon; Carbon content; Ecosystems; Heterogeneity; hydrologic exchange flow; Hyporheic zone; Hyporheic zones; Metabolism; Organic carbon; Particulate organic carbon; Permeability; reactive transport; River beds; River sediments; Riverbeds; Sediment; sediment redox condition; Sediments
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL107761

The hydrological and biogeochemical properties of the hyporheic zone in stream and riverine ecosystems have been extensively studied over the past two decades. Although it is widely acknowledged that sediment heterogeneity can influence biogeochemical reactions, little effort has been made to understand the role of heterogeneity on the spatiotemporal variability of riverbed redox conditions under changing flow dynamics at different timescales. Here we integrate a mechanistic model and field data to demonstrate that heterogeneity in permeability plays a vital role in modulating sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescales, whereas heterogeneity in particulate organic carbon (POC) content only has a comparable influence on redox conditions at the seasonal timescale. These findings underscore the importance of accurately characterizing sediment heterogeneity, in terms of permeability and POC content, in quantifying biogeochemical dynamics in the riverbed and hyporheic zones of riverine ecosystems. Plain Language Summary The redox condition of riverbed sediments is subject to the combined influence of hydrologic exchange flow and biogeochemical processes and is important for regulating the functioning of riverine ecosystems. Current understanding of the spatiotemporal pattern of sediment redox conditions especially with heterogeneity in consideration is limited, partially due to the lack of measurements and quantitative models. In this study, we integrate a mechanistic model and field data to reveal the role of sediment heterogeneity in controlling the redox condition under dynamic flow conditions. We demonstrate that heterogeneity in permeability modulates sediment redox condition at both seasonal and event timescales, and heterogeneity in particulate organic carbon is most prominent over multi‐month time intervals that reflect the balance between particulate organic carbon (POC) metabolism and time‐integrated oxygen influx. These findings highlight the importance of accurate characterization of sediment heterogeneity in both permeability and POC for predicting the dynamic redox shifts in riverbed sediments. Key Points A reactive transport model was developed to quantify the impact of heterogeneity in permeability and particulate organic carbon (POC) concentration on sediment redox conditions Heterogeneity in permeability controls sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescales The effects of heterogeneity in POC occur over the monthly timescale, reflecting a balance between POC metabolism and the influx of oxygen