Ryo Sugimoto

A significant quantity of soil organic carbon (SOC) is buried in mangrove ecosystems. However, recent research has revealed that substantial amounts of carbon are exported to the atmosphere as CO ₂ or to the ocean as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). This carbon outflow highlights the need for a comprehensive understanding of carbon burial, outgassing, and outwelling to clarify the role of mangroves in net atmospheric CO ₂ removal. To elucidate the carbon cycle within a mangrove ecosystem, we quantified CO ₂ outgassing through high‐resolution measurements CO ₂ and porewater‐derived fluxes of total alkalinity, DIC, DOC, and POC using radium isotopes ( ²²⁴ Ra and ²²³ Ra) in an island mangrove ecosystem in Japan. Our findings showed that the dominant carbon flux was DIC outwelling, at 189 ± 18 mmol m ⁻² day ⁻¹ , approximately 2.3 times higher than the global median. This pronounced DIC outwelling likely reflects the presence of a vast reservoir of poorly stabilized SOC. A comparison with ecosystem‐scale CO ₂ emissions revealed that approximately 89% of the DIC was transported into the estuary without being emitted as CO ₂ . This high DIC transport appears to result from the efficient water exchange characteristic of island mangroves, with a creek residence time of ∼1 day. Surprisingly, the adjacent estuary acted as a net CO ₂ sink, surpassing CO ₂ outgassing from the mangrove creeks. These results suggest that efficient water exchange in island mangroves, coupled with high biological productivity at the adjacent estuary, promotes long‐term sequestration of mangrove‐derived DIC in the ocean.

A significant quantity of soil organic carbon (SOC) is buried in mangrove ecosystems. However, recent research has revealed that substantial amounts of carbon are exported to the atmosphere as CO2 or to the ocean as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). This carbon outflow highlights the need for a comprehensive understanding of carbon burial, outgassing, and outwelling to clarify the role of mangroves in net atmospheric CO2 removal. To elucidate the carbon cycle within a mangrove ecosystem, we quantified CO2 outgassing through high-resolution measurements CO2 and porewater-derived fluxes of total alkalinity, DIC, DOC, and POC using radium isotopes (224Ra and 223Ra) in an island mangrove ecosystem in Japan. Our findings showed that the dominant carbon flux was DIC outwelling, at 189 ± 18 mmol m-2 day -1, approximately 2.3 times higher than the global median. This pronounced DIC outwelling likely reflects the presence of a vast reservoir of poorly stabilized SOC. A comparison with ecosystem-scale CO2 emissions revealed that approximately 89% of the DIC was transported into the estuary without being emitted as CO2. This high DIC transport appears to result from the efficient water exchange characteristic of island mangroves, with a creek residence time of ~1 day. Surprisingly, the adjacent estuary acted as a net CO2 sink, surpassing CO2 outgassing from the mangrove creeks. These results suggest that efficient water exchange in island mangroves, coupled with high biological productivity at the adjacent estuary, promotes long-term sequestration of mangrove-derived DIC in the ocean.

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.

Mangroves are valuable ecosystems that facilitate primary production, carbon sequestration, and regulation of greenhouse gas (GHG) cycles in coastal sediments, with microorganisms playing key roles. Specialized bacteria and archaea compete for energy and resources in mangrove sediments to inhabit optimal ecological niches and can produce or consume methane (CH₄)—a potent GHG—in the process. CH₄ cycling in mangroves has gained growing attention, yet uncertainties regarding functional and spatial distributions of microorganisms remain. Here, we demonstrate that in a pristine mangrove forest, CH₄ concentrations and methanogen communities are concentrated within lower or below rhizosphere depths. We also reveal atypical niches for methanogens in the upper tidal salt marsh zone where vegetation is sparse and highest methanogens abundances were detected at deepest depths (4715 reads g⁻¹) despite relatively high redox potentials (> 250 mV). Pore water CH₄ concentrations were highest at the deepest depth within the mangrove forest (max. 3.40 ± 0.21 μM) and coincided with the highest sediment CH₄ fluxes (276.4 ± 54.2 μmol m⁻²d⁻¹) and methanotroph abundances at the surface (1309 reads g⁻¹). Sediment CH₄ oxidation fractions between the deepest (60 cm) and shallowest (5 cm) depths were estimated between 18.8% and 64.9%. Positive correlation between crab burrows and CH₄ fluxes suggests that CH₄ from deeper sediment and salt marsh niches can be transported via conduits to the atmosphere. The spatial data from this study highlights the importance of investigating CH₄ dynamics across estuarine ecosystem gradients to better understand the complex roles of vital coastal vegetation zones in the face of a changing climate.

External nutrient supply from the land and ocean is crucial for sustaining high primary productivity in coastal seas. Submarine groundwater discharge (SGD) is recognized as one of the most important sources of terrestrial nutrients. However, the relative importance of SGD‐derived nutrients from different sources in coastal ecosystems controlled by offshore exchange has not been well quantified. Here, we assessed water and nutrient budgets in the semi‐enclosed bay along the Sanriku ria coast, where the intrusion of nutrient‐enriched oceanic water is substantial. We conducted seasonal sampling campaigns and monitored the groundwater level throughout the year. Water and nutrient fluxes from fresh groundwater, saline groundwater, river water, and oceanic water were estimated using a hydrological method and radium (Ra) mass balance model. The results indicated that oceanic water was a dominant source, accounting for 99.5%, 86%, 97%, and 84% of the total influx of water, dissolved inorganic nitrogen, dissolved inorganic phosphorus, and dissolved silica, respectively. Although the mean fluxes of land‐derived nutrients were small, the contribution increased to 28–59% in October, when nutrient fluxes of oceanic water weakened. Of the terrestrial sources, SGD dominated (41–94%), particularly saline SGD (>99% of total SGD). We concluded that an efficient supply of the primary limiting nutrient from land to the coastal ecosystem can accelerate coastal primary production during certain seasons, even if oceanic nutrients are typically the dominant source.

The number of studies concerning Submarine Groundwater Discharge (SGD) grew quickly as we entered the twenty-first century. Many hydrological and oceanographic processes that drive and influence SGD were identified and characterized during this period. These processes included tidal effects on SGD, water and solute fluxes, biogeochemical transformations through the subterranean estuary, and material transport via SGD from land to sea. Here we compile and summarize the significant progress in SGD assessment methodologies, considering both the terrestrial and marine driving forces, and local as well as global evaluations of groundwater discharge with an emphasis on investigations published over the past decade. Our treatment presents the state-of-the-art progress of SGD studies from geophysical, geochemical, bio-ecological, economic, and cultural perspectives. We identify and summarize remaining research questions, make recommendations for future research directions, and discuss potential future challenges, including impacts of climate change on SGD and improved estimates of the global magnitude of SGD.