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

Abstract

Mangrove-and saltmarsh-dominated estuaries have high rates of organic carbon burial. Here, we estimate soil, pore water, and surface-water carbon fluxes in an Australian estuarine tidal creek to assess whether (1) advective pore water exchange releases some of the soil carbon, (2) outwelling (lateral exports) represents a major carbon sequestration mechanism, and (3) methane emissions offset soil carbon sequestration. A radon ( 222 Rn) mass balance implied tidally driven pore-water exchange rates ranging from 5.5 AE 3.6 to 15.6 AE 8.1 cm d −1 . Pore water exchange explained most of the dissolved organic carbon (DOC) and methane surface-water fluxes but not dissolved inorganic carbon (DIC) and alkalinity. Organic carbon burial in soils derived from 239 + 240 Pu dating was 11-63 g C m −2 yr −1 . Methane and carbon dioxide emissions at the water-air interface were 0.27 AE 0.03 and 63 AE 166 mmol m −2 d −1 , respectively. When calculated as CO 2 -equivalents, aquatic CH 4 emissions converted to 19-94 g C-CO 2 m −2 yr −1 . Upscaling methane and soil carbon fluxes to representative areas revealed that CH 4 emissions could offset < 5% of soil carbon burial. DIC outwelling (12 AE 6 mmol m −2 catchment d −1 ) was less than five-fold greater than DOC and particulate organic carbon (POC) outwelling and four-fold greater than catchment-wide carbon burial. Because much of this DIC remains in the ocean after airwater equilibration, lateral DIC exports may represent an important long-term carbon sink. Recent research has focused on quantifying carbon burial rates in blue carbon habitats such as saltmarshes and mangroves. We suggest that DIC outwelling and methane outgassing should also be considered when assessing the carbon sequestration capacity of these coastal vegetated habitats.

Non‐native mangroves support carbon storage, sediment carbon burial, and accretion of coastal ecosystems

“…One challenge to this and other studies of C storage in mangrove forests is a limited ability to assess C laterally exported from mangrove stands in the form of dissolved inorganic, dissolved organic, or particulate organic C (Alongi, ). Although comprehensive estimates of the magnitude of these fluxes are still relatively rare, export in several systems has been estimated to be equivalent to or greater than the rate of sediment C burial (Maher et al, ; Santos et al, ). Total carbon storage may actually be larger given potential lateral fluxes.…”

“…Mangrove ecosystems sequester and store large amounts of carbon dioxide (CO 2 ) from the atmosphere in the form of live tree and detrital biomass (Alongi, ). Large amounts of detrital C are also stored in sediments through the daily deposition of suspended organic C from oceanic and riverine water inundation (Alongi, ; Santos, Maher, Larkin, Webb, & Sanders, ). Thus, the conservation or restoration of mangrove forests has received attention as a potential sink for atmospheric CO 2 and an important tool for climate change mitigation (Alongi, ; Crooks, Herr, Tamelander, Laffoley, & Vandever, ; Davidson, Cott, Devaney, & Simkanin, ).…”

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

“…Tidally-driven SGD has been extensively investigated following the discovery of the beach water table over-height or super-elevation (Nielsen, 1990) followed by the discovery of fresh groundwater tubes underlying upper saline plumes in beach aquifers (Robinson et al, 2006). Seawater infiltrating beaches, fractured aquifers and/or marshes at high tide creates a circulation cell that drives the return of seawater to the ocean at low tide on time scales of days to months (Robinson et al, 2009;Wilson et al, 2015;Geng and Boufadel, 2017;Santos et al, 2019). This process can account for a large fraction of SGD on a local scale, releasing solutes from the beach into the ocean.…”

“…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.…”

Impacts of land management practices on blue carbon stocks and greenhouse gas fluxes in coastal ecosystems—A meta‐analysis

“…Overall, GHG fluxes in BCEs need more attention given that they are not often assessed and are difficult to quantify, as lateral exchange in these open systems can remove large amounts of GHGs in dissolved form during high tide and via porewater exchange (Fuentes & Barr, 2015;Maher, Santos, Golsby-Smith, Gleeson, & Eyre, 2013;Santos, Maher, Larkin, Webb, & Sanders, 2019;Sippo et al, 2017).…”