Kenta Watanabe

Understanding the decomposition kinetics of organic carbon (OC) is key to accurately assessing blue carbon sequestration by seagrass and macroalgal beds. In this study, mesh-bag decomposition experiments were conducted in flow-through tanks supplied with natural coastal seawater across multiple coastal regions of Japan. Temporal changes in bulk OC remaining in the mesh bags, representing its persistence, were determined over 2 years for numerous seagrass and macroalgal species, and decomposition dynamics were subsequently modeled to estimate carbon persistence in long-term litter at annual, decadal, and centennial timescales.,Seagrass species exhibited substantially slower decomposition than most macroalgal species, retaining greater than 20% of initial bulk carbon after 1 year. In contrast, brown macroalgae of the order Laminariales, particularly edible species, showed rapid initial decomposition, losing more than 90% of bulk carbon within 3 months. Green and red algae exhibited relatively limited initial decomposition but ultimately declined to levels comparable to those of brown algae at the end of the 2-year experiment.,Statistical analyses indicated that taxonomic order influenced early-stage decomposition, whereas environmental effects, particularly water temperature, on residual proportions after 1 month and 1 year remained limited. Decomposition dynamics were successfully reproduced using a dual-exponential Multi-G model and a mixed model incorporating a reactivity continuum framework. Although estimated residual proportions during the 2-year experimental period were similar between models, projections diverged markedly at longer timescales of 10 and 100 years.

Dissolved organic carbon released from marine macrophytes is an important carbon sequestration pathway, but its recalcitrant fraction remains poorly quantified. Here, we quantified dissolved organic carbon release rates and recalcitrance using empirical data from macroalgal and seagrass species across cold-temperate to subtropical coastal Japan. Dissolved organic carbon release rates ranged from 5 to 462 µmol g-DW ⁻¹ d ⁻¹ , with similar averages between macroalgae and seagrasses. Using degradation dynamics simulated with a reactivity continuum model, we estimated the mean recalcitrant fractions over 100-year timescale as 25% (17–34%, 95% credible interval) for seagrasses and 14% (11–16%) for macroalgae, corresponding to 8% (4–12%) and 4% (3–6%) of annual net primary production, respectively. These regionally constrained estimates indicate that macrophyte-derived recalcitrant dissolved organic carbon generation represents a major carbon storage process comparable to particulate organic carbon storage beyond the source ecosystems.

Blue carbon ecosystems, classically defined as mangroves, tidal marshes and seagrasses, but increasingly expanded to include ecosystems such as tidal flats, macroalgal forests and shelf sediments, contribute to climate change mitigation and biodiversity support. Here, seven years after the last global assessment of research priorities, we conducted a priority-setting exercise to identify persistent knowledge and implementation gaps, and the strategic priorities that must be addressed to enable scalable, high-integrity and equitable management of blue carbon ecosystems in a rapidly evolving policy and finance landscape. The highest priority focuses on managing blue carbon ecosystems to support coastal communities while integrating traditional ecological knowledge, emphasizing the essential role of social legitimacy and equity in enabling scalable, long-lasting outcomes. Additional priorities focus on developing cost-effective restoration methods, improving the accuracy of greenhouse gas flux estimates, quantifying the impacts of human activities on carbon cycling and integrating co-benefits such as biodiversity and coastal protection into natural capital frameworks. Emerging technologies like remote sensing, machine learning and data-sharing platforms are also highlighted as transformative tools to fill knowledge gaps and scale solutions. Collectively, these priorities highlight the complexity of blue carbon science and the need for inclusive interdisciplinary approaches that support the resilience and livelihoods of coastal communities.

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.

Accurate measurement of the partial pressure of CO₂ (pCO₂) is essential for evaluating CO₂ fluxes in aquatic coastal environments. However, compact and low-power instruments suitable for both underway and moored deployments remain limited. We developed a battery-powered pCO₂ system incorporating a gas-permeable membrane equilibrator. The equilibrator employs a modified commercial membrane module with its outer housing removed, allowing direct contact between the membrane surface and ambient water while carrier gas flows through the interior. This configuration minimizes clogging risk and enhances gas exchange efficiency. In laboratory tests, the system achieved a response time of 7.1 ± 2.8 min, comparable to or faster than conventional devices. Underway observations in Tokyo Bay recorded pCO₂ values ranging from 200 to 1200 µatm, showing good agreement with a commercial analyzer. In brackish areas, where pCO₂ varied by approximately 1000 µatm over 4 km, slower ship speeds are recommended to achieve sufficient spatial resolution. Moored observations in a seagrass meadow revealed pCO₂ values ranging from 74 to 206 µatm, with nighttime values approximately 1.3 times higher than daytime values. The system supports both active (pumped) and passive (moored) modes and can operate continuously for approximately 60 h using six D-size batteries, enabling reliable capture of diel pCO₂ variations. These features make it suitable for high-frequency estimation of air–water CO₂ exchange in dynamic aquatic environments. Its compact design and cost-effectiveness enhance the accessibility of autonomous pCO₂ monitoring, contributing to improved assessment of coastal carbon dynamics and a better understanding of local CO₂ fluxes for environmental management.

This study presents a novel approach to investigating the spatial relationship between land use and river water quality by applying Geographically Weighted Regression (GWR), which explicitly accounts for the nested structure of sub-watersheds—a factor that has been frequently overlooked in previous studies. The Yura River watershed in Japan was selected as the study site, and electrical conductivity (EC) was used as a comprehensive indicator of water quality. To reflect local land-use impacts, we introduced the difference in EC between upstream and downstream sampling points (ΔEC) and allocated it to individual sub-watershed polygons. By analyzing both irrigation and non-irrigation seasons, the study found that key land-use types, such as paddy fields, water bodies, and evergreen broadleaved forests, exert varying influences on water quality depending on the season and location. The GWR model outperformed global regression models in capturing spatial heterogeneity and reduced residual spatial autocorrelation, thereby validating its effectiveness in watershed-scale environmental analysis. Importantly, this study is the first to integrate GWR with ΔEC while considering the hierarchical structure of sub-watersheds. This framework enables more accurate identification of localized land-use effects on water quality, which are often masked in global models. The findings underscore the need for region-specific land-use management and offer methodological insights for improving watershed conservation strategies in heterogeneous landscapes. By highlighting both seasonal variation and spatial dependency, this study provides a useful toolset for environmental monitoring and supports the development of targeted, evidence-based watershed policies.

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.

The outwelling of remineralized dissolved inorganic carbon (DIC) from the subsurface soil layer through tidal pumping has recently been noted for its importance in the blue carbon budget of mangroves. However, the age of soil organic carbon (SOC) that has been decomposed into DIC remains uncertain. In this study, two distinct models (a two endmembers model and three endmembers model) were utilized to verify the age of the decomposed SOC, employing radiocarbon (Δ¹⁴C) as a tracer. We conducted a 24‐h measurement of DIC and DIC isotopes (δ¹³C_DIC and Δ¹⁴C_DIC), in addition to vertical profiles of Δ¹⁴C of SOC, in a subtropical mangrove ecosystem in Japan. The Δ¹⁴C expected for the source SOC that decomposed into DIC exhibited a comparable trend for the two distinct models, with mean values ranging from −86.6‰ to −16.8‰. The Δ¹⁴C values were converted to calendar age using the OxCal program, with the calibration curve from IntCal20 being employed. This conversion revealed that DIC in the mangrove creek originated from SOC buried over a century ago. Given that mangroves are confined to the intertidal zone, it is likely that there are spatial constraints on the burial of SOC in mangrove ecosystems. Consequently, the transition from SOC to the DIC carbon pool in the ocean could be a key process in supporting long‐term carbon sequestration over millennia.

Gene flow is a crucial concept in the delineation of conservation units for natural populations of a species. Seagrasses are marine species targeted for conservation because their abundance has declined worldwide during the last century. However, knowledge of how to delineate seagrass conservation units based on their genetic structure is inadequate. In this study, the genomes of Zostera marina L. (eelgrass) populations in three semi-enclosed areas with different spatial scales in Japan were analyzed using single nucleotide polymorphisms within genome-wide loci, and their genetic structures were explored in the context of their isolation caused by geographical distance and barriers. Isolation by distance was apparent at spatial scales of 200–2000 km. A barrier apparently due to a narrow strait in an area with linear dimensions less than 200 km led to greater genetic isolation than did distance. On a more local scale, we found that heterogeneity among sites and intra-site variability rather than isolation by distance or barriers governed the genetic structure of eelgrass on spatial scales less than ~ 20 km. Our results imply that future seagrass conservation efforts should make more use than before of genomic tools so that greater attention can be focused on the complex structures formed on local scales. Identifying the ecological importance of genetic heterogeneity and the functional meaning of genetic differences on local scales will be among the next challenges to the delineation of seagrass conservation units.