Cascading river damming amplifies photosynthetic organic matter production and DOC transport

Dengming He; Wanfa Wang; Yuanbi Yi; Jun Zhong; Khan M G Mostofa; Xuan Hu; Wenhong Shi; Ding He; Si-Liang Li

doi:10.1016/j.watres.2025.124036

Cascading river damming amplifies photosynthetic organic matter production and DOC transport

Water Res. 2025 Jun 16:284:124036. doi: 10.1016/j.watres.2025.124036. Online ahead of print.

Authors

Dengming He¹, Wanfa Wang², Yuanbi Yi³, Jun Zhong⁴, Khan M G Mostofa⁴, Xuan Hu¹, Wenhong Shi¹, Ding He⁵, Si-Liang Li⁴

Affiliations

¹ Key Laboratory of Karst Georesources and Environment, Ministry of Education, Guizhou University, Guiyang 550025, China; College of Resources and Environmental Engineering, Guizhou Karst Environmental Ecosystems Observation and Research Station, Ministry of Education, Guiyang 550025, China.
² Key Laboratory of Karst Georesources and Environment, Ministry of Education, Guizhou University, Guiyang 550025, China; College of Resources and Environmental Engineering, Guizhou Karst Environmental Ecosystems Observation and Research Station, Ministry of Education, Guiyang 550025, China. Electronic address: wfwang@gzu.edu.cn.
³ Department of Ocean Science and Center for Ocean Research in Hong Kong and Macau, The Hong Kong University of Science and Technology, Clear Water Bay, New Territories, Hong Kong 999077, China; State Key Laboratory of Marine Pollution, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China. Electronic address: yuanbiyi@ust.hk.
⁴ Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China.
⁵ Department of Ocean Science and Center for Ocean Research in Hong Kong and Macau, The Hong Kong University of Science and Technology, Clear Water Bay, New Territories, Hong Kong 999077, China; State Key Laboratory of Marine Pollution, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China.

PMID: 40544568
DOI: 10.1016/j.watres.2025.124036

Abstract

Large-scale damming of global rivers has fundamentally altered hydrological connectivity and carbon cycling dynamics. However, cascade damming effects on the transport and transformation processes of dissolved organic matter (DOM) remain unclear. To address this, we integrated water chemistry, nutrient concentrations, optical properties, dual carbon isotopes (δ¹³C and Δ¹⁴C), and structural equation modeling (SEM) to elucidate organic carbon dynamics and underlying controlling mechanisms across the Wujiang cascade reservoirs. Results demonstrated intensified photosynthetic OM production during the thermally stratified period (April-October), evidenced by higher epilimnetic chlorophyll-a (8.0 ± 8.5 vs 1.0 ± 0.7 µg L^-1) and increased DOC concentrations (1.3 ± 0.5 vs. 0.9 ± 0.1 mg L^-1) compared to the vertical mixing period (January). Concurrently, isotopic evidence from enriched δ¹³C_DIC values and modern Δ¹⁴C_POC signatures in the epilimnion confirmed intensification of the biological carbon pump (BCP) effect, coinciding with dissolved inorganic carbon (DIC) uptake (29.0 ± 2.5 vs. 26.5 ± 5.2 mg L^-1) and nutrient assimilation (nitrate, 3.2 ± 0.6 vs. 2.7 ± 1.1 mg L^-1; phosphate, 92.1 ± 61.2 vs. 65.8 ± 76.3 µg L^-1). The SEM analysis identified DIC as the primary regulator of BCP strength across the cascade reservoirs (r = -0.679, p < 0.001). It is estimated that the annual DOC flux increased 17-fold from upstream (1076 ± 142 t yr⁻¹) to downstream reservoirs outlet (18,476 ± 667 t yr⁻¹), primarily governed by riverine DOM dynamics and hydrological regulation. Our findings reveal that cascade reservoirs profoundly alter biogeochemical processes related to DOM. With the increasing number of global reservoirs, elucidating the organic carbon dynamics of cascade reservoirs is crucial for accurate carbon budgeting and science-based reservoir management.

Keywords: Biological carbon pump; Carbon flux; Carbon isotope; Cascade reservoirs; Dissolved organic carbon.