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

Abstract

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 ( 222 Rn) mass balance model. The SGD fluxes in Maowei Sea were estimated to be 4.9 × 10 7 (0.36 ± 0.33 m/day) and 2.6 × 10 7 m 3 /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 À2 ·day À1 ) 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.Plain Language Summary Mangrove soils sequester large amounts of carbon, but little is known on the loss of carbon via groundwater pathways. Here we quantified submarine groundwater discharge using radon and related dissolved inorganic and organic carbon fluxes in a mangrove-dominated tropical bay in China (Maowei Sea). Combining our results with literature data revealed that groundwater fluxes in mangroves are significant and should be considered in blue carbon assessments.

Submarine Groundwater Discharge: Updates on Its Measurement Techniques, Geophysical Drivers, Magnitudes, and Effects

“…If extrapolated to the global mangrove area, these exchange rates would be enough to filter the entire continental shelf volume in ∼150 years and are equivalent to ∼1/3 of the annual volume of river water entering the oceans (Tait et al, 2016). Because mangrove and saltmarsh porewaters are often highly enriched in carbon and greenhouse gases (Santos et al, 2019), the input of dissolved carbon to the oceans via mangroves may be comparable to the input from global rivers (Chen et al, 2018b). Therefore, muddy mangrove and saltmarsh systems that are widespread on global shorelines deserve additional attention and may disproportionally contribute to SGD and related biogeochemical inputs to the ocean.…”

“…By considering SGD in the DIC budget in the ocean, the sink or source regions of CO 2 have been re-evaluated (Cai et al, 2003;Dorsett et al, 2011;Liu et al, 2012). The importance of SGD-derived DIC fluxes to the oceans have been particularly emphasized in carbon-rich mangrove forest areas (Chen et al, 2018b) and have been suggested to exceed regional river inputs in Florida (Liu et al, 2012) and in an Australian embayment (Stewart et al, 2015). In addition, DIC concentrations in STEs and their associated contributions to the ocean showed large seasonal variations (Wang et al, 2015).…”

Carbon outwelling and outgassing vs. burial in an estuarine tidal creek surrounded by mangrove and saltmarsh wetlands

“…Because the transport of biogeochemical compounds via advective flushing can be orders of magnitude faster than molecular diffusion (Santos et al ), mass balances overlooking advective pore water transport may underestimate the role played by intertidal wetlands in coastal carbon budgets. A recent global literature summary suggests that pore‐water–derived carbon exports from mangroves can be equivalent to ~ 30% of global river inputs to the oceans (Chen et al ).…”

Net Submarine Groundwater‐Derived Dissolved Inorganic Nutrients and Carbon Input to the Oligotrophic Stratified Karstic Estuary of the Krka River (Adriatic Sea, Croatia)

“…Assuming uniformly distributed karst aquifers around the globe, which have similar DIC concentration in their groundwater, the global net SGD‐derived DIC flux from karst aquifers would be (0.18–1.06) × 10 13 mol/year. This number equals 8–48% of the DIC flux via SGD in the global mangrove ecosystem that has high DIC export rates and account for 2.7–9.6% of the global riverine DIC inputs (Chen et al, ; Maher et al, ). Just as reported in Chen et al (), if the total SGD‐derived DIC flux is used, then the estimated SGD‐derived DIC flux in global karst aquifers along the coasts equals 17–97% of the flux in the global mangrove ecosystem.…”

“…This number equals 8–48% of the DIC flux via SGD in the global mangrove ecosystem that has high DIC export rates and account for 2.7–9.6% of the global riverine DIC inputs (Chen et al, ; Maher et al, ). Just as reported in Chen et al (), if the total SGD‐derived DIC flux is used, then the estimated SGD‐derived DIC flux in global karst aquifers along the coasts equals 17–97% of the flux in the global mangrove ecosystem. Therefore, SGD from the DIC‐enriched coastal karst aquifers is likely to be an important but easily ignored source for the carbon budget in the coastal seas worldwide.…”