Preprints
https://doi.org/10.5194/egusphere-2025-1478
https://doi.org/10.5194/egusphere-2025-1478
08 Apr 2025
 | 08 Apr 2025

Multidecadal trends in CO2 evasion and aquatic metabolism in a large temperate river

An Truong Nguyen, Gwenaël Abril, Jacob S. Diamond, Raphaël Lamouroux, Cécile Martinet, and Florentina Moatar

Abstract. Rivers play a critical role in the global carbon cycle. However, the environmental and hydro-climatic factors that control the sign and magnitude of river CO2 fluxes across seasons and multi-decadal periods are less constrained. The origin of excess river CO2—delivered by soils, wetlands and groundwater or produced by aquatic respiration of organic matter—remains an important unknown in linking terrestrial and aquatic carbon budgets. To address these knowledge gaps, we report on a 32-year high-frequency dataset (1990–2021) from the Loire River, a large, temperate river that underwent a shift from a eutrophic, phytoplankton-dominated regime to an oligotrophic, macrophyte-dominated regime in ca. 2005. We estimated daily river-atmosphere CO2 flux (FCO2) and river net ecosystem productivity (NEP) from hourly pH, alkalinity, dissolved oxygen, water temperature and solar radiation. We demonstrate that: i) annual FCO2 varied an order of magnitude among years (range = 200–2600 g C m2 yr-1); ii) the mean annual contribution of aquatic metabolism to total FCO2 was 40 %, but this also varied according to year and trophic regime, ranging from negative to 100 % contribution; iii) the river occasionally acted as a CO2 sink (FCO2 < 0) during summer, especially during the eutrophic period of 1990–2000, but this flux was negligible (-0.6 % of the FCO2 budget); and iv) FCO2 exhibited hysteresis with discharge, with FCO2 levels ranging from 1.5 to 2 times higher in autumn compared to spring at equivalent discharge rates, and the degree of which was depended on trophic regime. This study makes clear that river FCO2—and the source of this CO2—is dynamic within and across years and that global changes affecting the river trophic regime control the balance between internal and external CO2 production.

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Journal article(s) based on this preprint

25 Sep 2025
| Highlight paper
Multidecadal trends in CO2 evasion and aquatic metabolism in a large temperate river
An Truong Nguyen, Gwenaël Abril, Jacob S. Diamond, Raphaël Lamouroux, Cécile Martinet, and Florentina Moatar
Biogeosciences, 22, 4923–4951, https://doi.org/10.5194/bg-22-4923-2025,https://doi.org/10.5194/bg-22-4923-2025, 2025
Short summary Co-editor-in-chief
An Truong Nguyen, Gwenaël Abril, Jacob S. Diamond, Raphaël Lamouroux, Cécile Martinet, and Florentina Moatar

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-1478', Anonymous Referee #1, 16 May 2025
    • AC1: 'Reply on RC1', Truong An Nguyen, 11 Jun 2025
  • RC2: 'Comment on egusphere-2025-1478', Anonymous Referee #2, 18 May 2025
    • AC2: 'Reply on RC2', Truong An Nguyen, 11 Jun 2025

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-1478', Anonymous Referee #1, 16 May 2025
    • AC1: 'Reply on RC1', Truong An Nguyen, 11 Jun 2025
  • RC2: 'Comment on egusphere-2025-1478', Anonymous Referee #2, 18 May 2025
    • AC2: 'Reply on RC2', Truong An Nguyen, 11 Jun 2025

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
ED: Reconsider after major revisions (17 Jun 2025) by Ji-Hyung Park
AR by Truong An Nguyen on behalf of the Authors (07 Jul 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (10 Jul 2025) by Ji-Hyung Park
ED: Publish subject to minor revisions (review by editor) (27 Jul 2025) by Ji-Hyung Park
AR by Truong An Nguyen on behalf of the Authors (28 Jul 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (06 Aug 2025) by Ji-Hyung Park
AR by Truong An Nguyen on behalf of the Authors (13 Aug 2025)

Journal article(s) based on this preprint

25 Sep 2025
| Highlight paper
Multidecadal trends in CO2 evasion and aquatic metabolism in a large temperate river
An Truong Nguyen, Gwenaël Abril, Jacob S. Diamond, Raphaël Lamouroux, Cécile Martinet, and Florentina Moatar
Biogeosciences, 22, 4923–4951, https://doi.org/10.5194/bg-22-4923-2025,https://doi.org/10.5194/bg-22-4923-2025, 2025
Short summary Co-editor-in-chief
An Truong Nguyen, Gwenaël Abril, Jacob S. Diamond, Raphaël Lamouroux, Cécile Martinet, and Florentina Moatar
An Truong Nguyen, Gwenaël Abril, Jacob S. Diamond, Raphaël Lamouroux, Cécile Martinet, and Florentina Moatar

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Short summary
This 32-year study of France’s Loire River shows cleaner water reduced carbon dioxide emissions by 62 %, despite increased contributions from aquatic plant activity. Seasonal emissions were higher in autumn than spring, while long-term declines were driven by reduced external carbon inputs from groundwater and soils. Results highlight how ecosystem changes influence rivers' role in global carbon cycles and climate management.
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