Tidal variability of CO₂ and CH₄ emissions from the water column within a Rhizophora mangrove forest (New Caledonia)

We performed a preliminary study to quantify CO₂ and CH₄ emissions from the water column within a Rhizophora spp. mangrove forest. Mean CO₂ and CH₄ emissions during the studied period were 3.35±3.62mmolCm^-2h^-1 and 18.30±27.72μmolCm^-2h^-1, respectively. CO₂ and CH₄ emissions were highly variable and mainly driven by tides (flow/ebb, water column thickness, neap/spring). Indeed, an inverse relationship between the magnitude of the emissions and the thickness of the water column above the mangrove soil was observed. δ¹³CO₂ values ranged from -26.88‰ to -8.6‰, suggesting a mixing between CO₂-enriched pore waters and lagoon incoming waters. In addition, CO₂ and CH₄ emissions were significantly higher during ebb tides, mainly due to the progressive enrichment of the water column by diffusive fluxes as its residence time over the forest floor increased. Eventually, we observed higher CO₂ and CH₄ emissions during spring tides than during neap tides, combined to depleted δ¹³CO₂ values, suggesting a higher contribution of soil-produced gases to the emissions. These higher emissions may result from higher renewable of the electron acceptor and enhanced exchange surface between the soil and the water column. This study shows that CO₂ and CH₄ emissions from the water column were not negligible and must be considered in future carbon budgets in mangroves.