Submarine Groundwater Discharge-Derived Carbon Fluxes in Mangroves: An Important Component of Blue Carbon Budgets?

Mangroves are blue carbon systems characterized by high soil carbon storage and sequestration. Soil carbon losses via groundwater or pore water pathways are potentially important yet poorly understood components of mangrove carbon budgets. Here we quantified submarine groundwater discharge (SGD) and associated dissolved inorganic carbon (DIC) and organic carbon (DOC) fluxes into a mangrove-dominated tropical bay (Maowei Sea) using a radon (²²²Rn) mass balance model. The SGD fluxes in Maowei Sea were estimated to be 4.9 × 10⁷ (0.36 ± 0.33 m/day) and 2.6 × 10⁷ m³/day (0.20 ± 0.18 m/day) for the wet and dry seasons, respectively, implying that SGD may respond to precipitation. The SGD-derived DIC and DOC fluxes (mol·m⁻²·day⁻¹) in the wet season (DIC: 0.70 ± 0.82; DOC: 0.31 ± 0.30) were higher than those in the dry season (DIC: 0.25 ± 0.24; DOC: 0.25 ± 0.23). These SGD-derived carbon fluxes exceed local river inputs and constituted >70% of the total DIC and DOC input into the bay. If scaled up to the global weighted mangrove area in combination with data from other 32 study sites, carbon fluxes via SGD into mangroves may be equivalent to 29–48% of the global riverine input into the ocean. Therefore, we suggest that SGD is a major component of coastal carbon budgets and that accounting for SGD helps to reduce uncertainties in mangrove blue carbon budgets.