The mangrove CO₂ pump: Tidally driven pore‐water exchange

Abstract

Atmospheric carbon dioxide (CO2) is fixed by mangrove vegetation and stored in its biomass and sediments. Part of the sediment carbon can be exported to coastal waters via tidally driven pore‐water exchange. Here, we quantify pore water‐derived dissolved CO2 export using in situ, high‐resolution observations of 222Rn and CO2 over a spring‐neap tidal cycle in a mangrove‐fringed estuary (Coffs Creek, Australia). 222Rn‐derived pore‐water exchange rates were 11.5–34.9 cm d−1 (23.0 ± 6.7) over 30 tidal cycles. Pore‐water exchange released CO2 from intertidal sediment at rates of 61–213 (136 ± 43) mmol m−2 d−1. This is equivalent to ~ 94% of the total CO2 input into the estuary and approximately two times of the water‐atmosphere CO2 emission. These observations reveal that tidal pumping is a major regulator of both mangrove pore‐water exchange and associated dissolved CO2 export to the ocean. Combining our estimates with literature data, a first‐order global pore water‐derived dissolved CO2 export from mangroves was estimated to be 83 ± 50 Tg C yr−1. This is higher than an earlier estimates of global mangrove CO2 emissions to the atmosphere (34.1 ± 5.4 Tg C yr−1) and carbon burial in sediments (18.4–34.4 Tg C yr−1), implying that pore water‐derived CO2 escapes to the atmosphere within and beyond mangrove waters. Overall, CO2‐rich pore water seems to be a widespread, important pathway of CO2 into mangrove‐dominated estuaries and should be considered in mangrove carbon assessments in the context of global climate change and blue carbon.

Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes

“…Porewater acidification following CO 2 production from organic matter degradation can drive metabolic carbonate dissolution, which also produces TA 15 . Drainage of intertidal wetland sediments by tidal pumping (porewater export driven by tidal variations in hydraulic gradients) and bio-irrigation (porewater export driven by benthic organisms) transports DIC- and TA-enriched porewater to surface waters and eventually to the coastal ocean 16 – 18 .…”

Changes in DIC/TA ratio by tidal asymmetry control <i>p</i>CO₂ over a spring-neap tidal cycle in a subtropical mangrove forest in Japan

“…Spatially, extremely high pCO2 values were observed upstream of the mangrove creek at Sites M11 and M12 (Figure 4 and Table S1). This indicates that the influence of mangrove soils became more pronounced upstream of the mangrove creek (Chen et al, 2021;Maher et al, 2013).…”

“…The former is the microbial decomposition of the soil organic matter and porewater export through tidal pumping, whereas the latter includes a primary production by phytoplankton and degradation of DOC. Previous studies have reported that 90-100% of the exported DIC and TA from mangrove forests originate from porewater in a mangrove swamp via tidal pumping (Chen et al, 2021;Maher et al, 2013;Taillardat et al, 2018). The mean value of Chl.…”

Tidal Pumping Controls Dissolved Organic Matter Properties and Outwelling From Mangrove Groundwater to Coastal Water

“…Groundwater in mangroves is a mixture of seawater infiltration and inland freshwater (Chen et al., 2021). Both terrestrial plant litter and soil are important sources of DOM to groundwater (McDonough et al., 2022).…”

“…Groundwater flow in mangroves plays an important role in enhancing above‐ground growth (Hayes et al., 2019) and is often driven by tidal flows (Tait et al., 2016). The concept of “tidal pumping” highlights the significance of water exchange on soluble outwelling from mangroves to nearby coastal waters (Chen et al., 2021; Santos et al., 2021; Zheng et al., 2016). Tidal pumping requires that surface water infiltrates into the sediments during flood tides, and that groundwater mixed with surface water eventually discharges when sea level is lower.…”