Salt marshes are highly productive intertidal wetlands providing important ecological services for maintaining coastal biodiversity, buffering against oceanic storms, and acting as efficient carbon sinks. However, about half of these wetlands have been lost globally due to human activities and climate change. Inundated periodically by tidal water, salt marshes are subjected to strong surface water and groundwater interactions, which affect marsh plant growth and biogeochemical exchange with coastal water. This paper reviews the state of knowledge and current approaches to quantifying marsh surface water and groundwater interactions with a focus on porewater flow and associated soil conditions in connection with plant zonation as well as carbon, nutrients, and greenhouse gas fluxes. Porewater flow and solute transport in salt marshes are primarily driven by tides with moderate regulation by rainfall, evapotranspiration and sea level rise. Tidal fluctuations play a key role in plant zonation through alteration of soil aeration and salt transport, and drive the export of significant fluxes of carbon and nutrients to coastal water. Despite recent progress, major knowledge gaps remain. Previous studies focused on flows in creek‐perpendicular marsh sections and overlooked multi‐scale 3D behaviors. Understanding of marsh ecological‐hydrological links under combined influences of different forcing factors and boundary disturbances is lacking. Variations of surface water and groundwater temperatures affect porewater flow, soil conditions and biogeochemical exchanges, but the extent and underlying mechanisms remain unknown. We need to fill these knowledge gaps to advance understanding of salt marshes and thus enhance our ability to protect and restore them.

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m⁻² d⁻¹ in mangroves and 57 ± 104 mmol m⁻² d⁻¹ in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.

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.

We quantified whether pore‐water exchange flushes out saltmarsh sediment carbon, driving carbon outwelling into the ocean and outgassing into the atmosphere. Radon‐derived pore‐water exchange released 1.8 times more sediment carbon in the wet than in dry season. Both crab burrow flushing and delayed seepage of surface water infiltrating sediments during the spring tide released sediment carbon to surface waters. The outwelling flux of dissolved inorganic carbon exceeded dissolved organic carbon. Carbon dioxide and methane emissions were 169 and 0.25 mmol m⁻² d⁻¹, respectively. Pore‐water carbon fluxes exceeded carbon outwelling. This requires some carbon processing within the saltmarsh (e.g., degradation or outgassing to the atmosphere) before pore‐water carbon is exported to the ocean. Overall, pore‐water exchange and outwelling are key components of saltmarsh carbon budgets and should be considered when assessing their carbon sequestration potential and strategies to mitigate climate change.

Terrestrial groundwater travels through subterranean estuaries before reaching the sea. Groundwater‐derived nutrients drive coastal water quality, primary production, and eutrophication. We determined how dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved organic nitrogen (DON) are transformed within subterranean estuaries and estimated submarine groundwater discharge (SGD) nutrient loads compiling > 10,000 groundwater samples from 216 sites worldwide. Nutrients exhibited complex, nonconservative behavior in subterranean estuaries. Fresh groundwater DIN and DIP are usually produced, and DON is consumed during transport. Median total SGD (saline and fresh) fluxes globally were 5.4, 2.6, and 0.18 Tmol yr⁻¹ for DIN, DON, and DIP, respectively. Despite large natural variability, total SGD fluxes likely exceed global riverine nutrient export. Fresh SGD is a small source of new nutrients, but saline SGD is an important source of mostly recycled nutrients. Nutrients exported via SGD via subterranean estuaries are critical to coastal biogeochemistry and a significant nutrient source to the oceans.