Alkalinity export to the ocean is a major carbon sequestration mechanism in a macrotidal saltmarsh

Abstract

Saltmarshes are a blue carbon ecosystem accumulating large quantities of organic carbon in sediments. Some of this carbon can be transformed into dissolved inorganic carbon (DIC) and methane (CH 4 ) that may eventually be exported to the ocean or atmosphere. Although extensive studies have quantified specific components of the carbon budget such as carbon burial, limited attention has been given to pore-water-derived carbon and total alkalinity (TA) exports to the ocean. Here, we quantified lateral exports to the ocean (outwelling) of 202 AE 160 and 78 AE 75 mmol m À2 d À1 of DIC and TA, respectively. The TA : DIC concentration ratio in the creek waters was $ 1, implying TA production from anaerobic mineralization in sediments. The lateral TA exports were comparable to the local (94 AE 48 mmol m À2 d À1 ) and national ($ 50 mmol m À2 d À1 ) organic carbon burial. High TA exports could locally increase the ocean buffering capacity and contribute bicarbonate to the coastal ocean, acting as a long-term carbon storage. Pore water traced by radon contributed 28-37% and 58-69% of DIC and TA exports. Separating the two major DIC components (i.e., CO 2 emissions and alkalinity exports) is essential to resolve the carbon sequestration potential from saltmarshes. Here, dissolved CO 2 emissions to the atmosphere accounted for 3-5% of total DIC outwelling. CH 4 emissions played a minor role offsetting around 0.3 to 6% of the carbon sequestration. Overall, we demonstrate that alkalinity export into the ocean can be an overlooked carbon sequestration pathway in saltmarshes at rates comparable to carbon burial.

Methane emissions in seagrass meadows as a small offset to carbon sequestration

“…This has been observed in other tidally-influenced ecosystems such as mangroves and saltmarshes where higher CH 4 concentration in porewater drives the high surface water CH 4 (Call et al, 2018;Santos et al, 2019;Yau et al, 2022). Flanking saltmarshes adjacent to seagrass export CH 4 , elevating CH 4 flux in the seagrass meadows (Al-Haj et al, 2022).…”

“…Seagrass seems to emit less CH 4 than other coastal vegetated ecosystems such as mangroves and saltmarshes. For example, previous studies showed that methane emissions can offset <6% of carbon burial in a saltmarsh in China (Yau et al, 2022) and 18% in Australian mangroves receiving freshwater inputs (Rosentreter et al, 2018). Since seagrass are fully submerged and freshwater inputs are often limited, higher CH 4 oxidation in the water column could reduce CH 4 emissions relative to periodically inundated mangrove and saltmarsh systems.…”

“…Since CH 4 is a potent greenhouse gas with 45-96 times greater sustained-flux warming potential (SGWP) than CO 2 (Neubauer & Megonigal, 2015), the efficiency of seagrasses as a carbon sink can be partially offset by CH 4 emissions. Although measurements of CH 4 fluxes have been widely performed in mangroves (Call et al, 2019), saltmarshes (Yau et al, 2022), and other coastal ecosystems (Borges & Abril, 2011), CH 4 fluxes in seagrass meadows remain poorly constrained across multiple spatial and temporal scales. The air-sea and sediment-water CH 4 fluxes from seagrass ranged from 0 to 400 µmol m -2 d -1 , resulting in global upscaled fluxes of 0.18 Tg CH 4 per year (Al-Haj et al, 2022).…”

Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean

“…Late-stage green tides can also cause serious negative environmental impacts, such as the occurrence of hypoxia, acidification, and even the production of toxic hydrogen sulfide (H 2 S) in the nearshore areas covered by dense macroalgal fronds. ,− In the organic-rich hypoxic environment, total alkalinity (TAlk) can be generated through anaerobic organic matter mineralization processes (e.g., sulfate reduction and denitrification). − The generation of TAlk through sulfate reduction has been found in mangrove forests, and denitrification was considered as a dominant reaction for TAlk generation in a subterranean estuary . The increase in TAlk, a key index for the capacity of seawater to store DIC, allows more DIC to remain in the seawater as bicarbonate (HCO 3 – ) rather than degas back to the atmosphere as CO 2 . − Given that seawater TAlk usually increases evidently during green tides, − we speculate that a substantial portion of those increased DICs may persist in seawater after green tides, potentially playing a role in inorganic carbon sequestration in the ocean.…”

Carbon sequestration in aquatic ecosystems: Recent advances and challenges

“…Zhang et al (2022) performed large-scale modeling to quantify CO 2 uptake from rivers and groundwater following weathering of carbonate and silicate rocks, thus revealing the potential contribution of enhanced rock weathering as an atmospheric CO 2 sink. Yau et al (2022) built a comprehensive saltmarsh carbon budget that showed large dissolved carbon sequestration via lateral exports. Saltmarsh alkalinity production followed by porewater-derived exports to the ocean was comparable to carbon burial in local sediments.…”