Preprints
https://doi.org/10.5194/egusphere-2025-335
https://doi.org/10.5194/egusphere-2025-335
30 Jan 2025
 | 30 Jan 2025
Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Aquatic metabolism influences temporal variations of water carbon and atmospheric carbon dioxide fluxes in a temperate salt marsh

Jérémy Mayen, Pierre Polsenaere, Aurore Regaudie de Gioux, Jonathan Deborde, Karine Collin, Yoann Le Merrer, Élodie Foucault, Vincent Ouisse, Laurent André, Marie Arnaud, Pierre Kostyrka, Éric Lamaud, Gwenaël Abril, and Philippe Souchu

Abstract. Salt marshes are blue carbon (C) ecosystems characterized by intense atmospheric CO2 uptake and C sequestration but also organic and inorganic C exports through the tide. However, uncertainties on main biotic factors controlling vertical and horizontal C fluxes imply studying simultaneously terrestrial and aquatic metabolisms at small timescales (diurnal and tidal) and distinguish their contributions to net ecosystem CO2 exchanges (NEE). Within a temperature salt marsh, four sampling 24-h cycles were performed to measure water biogeochemical parameters (carbon and nutrients) and planktonic metabolism simultaneously to NEE successively at high tide (imported coastal waters influenced by the continental shelf) and low tide (exported channel waters influenced by the marsh). At high tide, water CO2 oversaturation due to aquatic heterotrophy was able to significantly reduce marsh atmospheric CO2 uptake at the ecosystem scale (NEE) during the highest immersion levels. At low tide, water pCO2 were also mainly controlled by marsh biological activity inducing large water CO2 oversaturation in winter due to heterotrophy and large water CO2 undersaturation in spring and summer due to autotrophy. In winter, the highest increases of dissolved inorganic carbon (DIC; from 2354 to 3963 µmol kg-1), total alkanity (TA; from 2508 to 4016 µmol kg-1) and dissolved inorganic nitrogen (DIN; from 27.7 to 68.4 µM) were measured at low tide night probably due to intense anaerobic respiration processes in channel waters and/or sediments resulting in the highest water pCO2 (up to 1461 ppmv). On the contrary, in spring and summer, large water pCO2 decreases and dissolved organic carbon (DOC) increases from high to low tide could be related to intense autochthonous and allochthonous aquatic primary production. Over the 24-h cycles, planktonic metabolism strongly influenced water pCO2 variations, especially at low tide, though planktonic communities did not play a major role in the atmospheric C balances at the ecosystem scale (NEE), accounting for only 10 % in spring.

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Jérémy Mayen, Pierre Polsenaere, Aurore Regaudie de Gioux, Jonathan Deborde, Karine Collin, Yoann Le Merrer, Élodie Foucault, Vincent Ouisse, Laurent André, Marie Arnaud, Pierre Kostyrka, Éric Lamaud, Gwenaël Abril, and Philippe Souchu

Status: open (until 15 Mar 2025)

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Jérémy Mayen, Pierre Polsenaere, Aurore Regaudie de Gioux, Jonathan Deborde, Karine Collin, Yoann Le Merrer, Élodie Foucault, Vincent Ouisse, Laurent André, Marie Arnaud, Pierre Kostyrka, Éric Lamaud, Gwenaël Abril, and Philippe Souchu
Jérémy Mayen, Pierre Polsenaere, Aurore Regaudie de Gioux, Jonathan Deborde, Karine Collin, Yoann Le Merrer, Élodie Foucault, Vincent Ouisse, Laurent André, Marie Arnaud, Pierre Kostyrka, Éric Lamaud, Gwenaël Abril, and Philippe Souchu

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Short summary
In a salt marsh, we performed seasonal 24-h cycles to look for aquatic metabolism influence on water carbon dynamics and net ecosystem CO2 exchanges (NEE). From high to low tide in winter, marsh anaerobic respiration induced the highest levels of dissolved inorganic carbon and alkalinity. On the contrary, in spring and summer, marsh primary production led to CO2-depleted water exportations downstream. Aquatic heterotrophy at high tide can influence NEE during the highest immersion levels only.
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