Mangrove ecosystems are one of the most carbon dense ecosystems worldwide. Yet, the stabilization and recalcitrance of carbon (C) and organic matter (OM) are little understood in mangroves, especially across eco-geomorphological settings and depths. Here, we characterized the sediment C and OM of Indo-Pacific mangroves, located in four distinct eco-geomorphological settings (i.e., delta, estuary, non-carbonated open coast, carbonated open coast) and at two different depths (i.e., 0-20 cm and 80-100 cm). We quantified the fraction of C within (i) mineralized associated organic matter (MAOM), and (ii) within particulate organic matter (POM). We coupled these analyses with lignin quantity and composition, as well as stable C isotopes analysis in mangrove sediments.

We found significant variation in the quantity of MAOM and POM across mangrove eco-geomorphological settings, but not across mangrove sediment depths. The terrigenous deltaic mangrove exhibited up to three times more MAOM than the carbonate open coast mangrove, which was dominated by POM. Mangroves of the carbonate coast type had higher C content than other eco-geomorphic types. The  was not different across mangrove eco-geomorphologies, but was different across mangrove sediment depths. Regarding OM recalcitrance, the lignin content displayed strong variations across the different eco-geomorphologies, however, there was no clear pattern of lignin degradation stage across depths. Finally, an inverse correlation between sediment C recalcitrance (i.e., lignin content) and stabilization (MAOM) processes were determined across mangroves.

Our findings suggest that the processes leading to OM preservation differ among mangroves in various eco-geomorphological settings. Those results have important implications to guide mangrove restoration for carbon persistence and to model carbon pools across mangrove areas.

How to cite: Maceiras, M., Arnaud, M., Lovelock, C., Pearse, A., Dang, H., Robin, S., Marchand, C., Felbacq, A., Abiven, S., Lebrun Thauront, J., Bottinelli, N., Mishra, A. K., Hilal Farooq, S., Bhadra, T., and Rumpel, C.: Carbon recalcitrance and stabilization processes vary across mangrove eco-geomorphologies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8023, https://doi.org/10.5194/egusphere-egu24-8023, 2024.

Mangroves have high CO₂ sequestration capacity, storing large amounts of carbon on their biomass and sediments/soil. Mangrove carbon is also transported to the ocean, i.e. outwelling or lateral fluxes, where it can be stored for long time scales. Here, we used radium isotopes (²²⁴Ra and ²²³Ra) to resolve carbon and alkalinity outwelling to the ocean from two mangrove seascapes in Brazil. We sampled porewaters to define the source composition, mangrove creek waters to resolve tidal cycles, and performed transects away from the mangrove into continental shelf to trace mangrove carbon across the seascape. High-resolution observations of radium isotopes in the creek indicated that tidal pumping is the main driver of carbon exchange. Low pH (6.8 – 7.0) and high ²²⁴Ra activities (165 – 290 dpm 100L^-1) were found during low tides, indicating mangrove porewater exchange. Radium mass balance models revealed porewater exchange at 20.0 ± 25.4 cm d^-1 in the tropical mangrove and 3.0 ± 2.0 cm d^-1 at the sub-tropical mangrove. Radium-derived transport rates of mangrove porewater to the continental shelf were higher in the mesotidal tropical (667 ± 313 m d^-1) than the microtidal subtropical (371 ± 168 m d^-1) seascape. Radium isotopes were positively correlated (p < 0.05) with dissolved inorganic (DIC), organic (DOC) and particulate organic (POC) carbon across the entire seascape. DIC as bicarbonate (HCO₃^-) was the main form of carbon on all scales in both mangrove seascapes, representing 57 – 82% of the total carbon pool. DOC and POC accounted for 5 – 12% and 1 – 7% of total carbon, respectively. Although mangrove waters emitted CO₂ to the atmosphere (38.4 – 142.9 mmol m^-2 d^-1), both bays and continental shelves were a CO₂ sink (-1.9 – -0.6 mmol m^-2 d^-1). Porewater-derived carbon outwelling exceeded carbon fluxes at the mangrove-bay and bay-shelf interfaces, indicating carbon transformations across the seascape continuum. Total carbon outwelled from mangroves were 3 – 4 times higher than soil carbon burial at both mangrove sites. Bicarbonate outwelling (31.0 – 71.6 mmol m^-2 d^-1) reaching the continental shelves increased mangrove soil carbon sequestration capacity by 234% in these mangrove systems. Hence, overlooking outwelling as a blue carbon sink mechanism would underestimate the role of mangroves in sequestering CO₂ and mitigating climate change.

How to cite: Cabral, A., M. S. Reithmaier, G., Y. Y. Yau, Y., C. Cotovicz Jr., L., Barreira, J., Viana, B., Hayden, J., Bouillon, S., Brandini, N., E. de Rezende, C., L. Fonseca, A., and R. Santos, I.: Large porewater-derived carbon outwelling across two mangrove-seascapes revealed by radium isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19077, https://doi.org/10.5194/egusphere-egu24-19077, 2024.

While decadal predictability is one of the key information demands on marine management and conservation endeavors, the lack of long-term observed records and complex climate variability challenges understanding it. Seagrass beds are not only important blue carbon sinks but also crucial habitats and feeding grounds for diverse marine organisms. This study uses in-situ data from 2001 to 2021 to investigate the primary decadal environmental control, the Pacific Meridional Mode, on seagrass growth in southern Taiwan. Two primary seagrass metrics, aboveground biomass and cover, were examined against various environmental and meteorological variables. Our initial findings reveal a significant correlation between PMM and seagrass growth. Aboveground biomass exhibits a robust negative correlation with PMM, while cover displays a weaker yet positive association. Further examination of regional climate dynamics unveils notable shifts in surface solar radiation, temperature, and rainfall concerning seagrass. Specifically, increased aboveground biomass coincides with reduced solar radiation, lower temperatures, and enhanced rainfall in southern Taiwan, resembling a negative PMM-like pattern. This pattern underscores the sensitivity of aboveground biomass to large-scale climatic fluctuations across the Pacific basin. Conversely, seagrass cover demonstrates opposing patterns compared to aboveground biomass but with less statistical significance. This suggests that cover growth is influenced by a broader array of factors, resulting in a more nonlinear response. In essence, our research underscores the vital role of PMM and regional climate conditions in shaping tropical seagrass growth, offering further insights for marine conservation efforts.

How to cite: Hsu, L.-Y., Tseng, W.-L., Hsu, P.-C., Chen, K.-Y., Wang, S.-Y., and Lin, H.: On the decadal linkage of seagrass growth in southern Taiwan with the Pacific Meridional Mode, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4577, https://doi.org/10.5194/egusphere-egu24-4577, 2024.

This paper presents a model of competition between the arborescent canopy-forming algae Cystoseira s.l., and the smaller size algae forming turf on rocky bottoms. The two algal assemblages are both grazed by sea urchins. The algae and invertebrates involved in the model play an important role in the health of coastal ecosystems and particularly in the conservation of biodiversity-rich habitats of temperate nearshore rocky reefs: the model presented is intended to help describing real ecological processes of canopy degradation and recovery. The model is a 3-species space implicit food web. Its distinctive feature is the inclusion of community border effects in interactions, as only the individuals laying along the canopy border of Cystoseira s.l. forests take part in recruitment and competition with algal turfs, and undergo grazing by sea urchins. Also endogenous recruitment of turf involves only border individuals. These border effects are taken into account in the same way of herd behavior in the mathematical ecology of animal species. Some important features of the model phase space are investigated, focussing on the capacity of the system to mimic the transitions among ecologically different states of the rocky bottom (canopy, turf, and barren).

How to cite: Materassi, M.: Modelling Cystoseira s.l.-turf competition via a space-implicit model with border dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5113, https://doi.org/10.5194/egusphere-egu24-5113, 2024.

Saline wetlands play a crucial role in climate regulation through their robust cooling effect, attributed to rapid carbon sequestration and minimal methane production. Despite this, a comprehensive understanding of the mechanisms supporting such carbon exchange, and their specific contributions to greenhouse gas mitigation potential, is lacking, particularly in salt marshes facing the impacts of global climate changes. Here, we test the effects of water table levels, grazing, and plant community composition on CO₂ and CH₄ fluxes during the growing season of salt marshes by a controlled manipulative experiment and an in situ experiment. Rising water table levels resulted in higher CH₄ emissions but reduced photosynthesis and ecosystem respiration. Conversely, grazing enhanced ecosystem respiration but suppressed plant photosynthesis. Furthermore, CH₄ emissions from Phragmites-dominated communities were nearly a thousand times higher compared to Spartina-dominated communities. Our findings indicate that, throughout the growing season, lower salt marshes function as carbon sinks, whereas grazed Phragmites-dominated salt marshes are carbon sources. Our study accounted for CH₄ fluxes, CO₂ uptake and emission together, and identified the mechanisms controlling carbon exchange, an approach that is crucial for evaluating the potential of saline tidal wetlands as net atmospheric carbon sinks and developing scientifically sound climate mitigation policies.

How to cite: Yang, D., Jensen, A., Sorrell, B., Brix, H., and Eller, F.: The impact of water table level and grazing on greenhouse-gas exchange in salt marshes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5983, https://doi.org/10.5194/egusphere-egu24-5983, 2024.

Wetlands are recognized as critical sinks of carbon among terrestrial ecosystems. The demand for atmospheric carbon mitigation, exacerbated by climate change, underscores the importance of wetland carbon sequestration. Meanwhile, due to the strong affinity of mercury (Hg) with organic matter, Hg in the atmosphere can also be scavenged and buried in wetlands. However, increased anthropogenic activities, such as urbanization, disrupt sediment dynamics in coastal wetlands, affecting their capacity to sequester carbon and Hg. To explore the urbanization impacts, One-meter sediment cores were collected from five salt marshes in Connecticut and Massachusetts, USA. Sediment samples were collected every 2 cm, and then processed through freeze-drying, sieving, acidification, and weighing. Subsequent laboratory analyses include total organic carbon (TOC), stable carbon isotope (δ¹³C), carbon/nitrogen (C/N) ratio, and mercury (Hg) concentrations throughout the core. The TOC source (terrestrial vs. marine) was determined using an isotopic mixing model with Monte Carlo simulation. The data indicate a temporal decrease in terrestrial TOC and an uptick in marine-derived TOC. Within the terrestrial fraction, local vegetation is the primary contributor. This shift in TOC source, driven by reduced terrestrial contributions, suggests wetland degradation and a potential decline in carbon sequestration due to urbanization. Furthermore, Hg analysis reveals a negative correlation with TOC in disturbed salt marshes, highlighting Hg dilution by marsh-derived organic matter and the effects of anthropogenic point source releases of Hg. Notably, two sites uniquely showed extremely high Hg concentrations and decoupling between Hg and TOC, tracing back to localized Hg releases from industrial activities in Danbury, Connecticut, during the 19^th century. This study demonstrates the temporal shifts in sources of TOC, a decline in carbon sequestration, and historical Hg contamination across Northeast US salt marshes, underscoring anthropogenic impacts on these ecosystems. Future work will incorporate Lead-210 and radiocarbon dating of sediment cores to better understand the temporal variation of carbon sequestration. 

How to cite: Zhang, H., Zhang, M., shrestha, B., Tsao, S.-E., Vaughn, D., Liu, M., and Raymond, P.: Investigation of carbon sequestration and mercury contamination in the Northeastern US salt marshes and the potential impact of anthropogenic activities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6747, https://doi.org/10.5194/egusphere-egu24-6747, 2024.

Global warming, accelerating at an alarming rate, has thrust climate change into the forefront of global concerns. The recent warning from the World Meteorological Organization (WMO) about the record-low Antarctic sea ice coverage serves as a stark reminder of the urgency surrounding environmental issues in the twenty-first century. Climate change is undeniably one of the most pressing challenges facing humanity today, with far-reaching implications for our survival and socio-economic development.In response to the escalating crisis, there has been a global call to action, urging nations to limit the rise in global temperatures to 1.5 to 2 degrees Celsius. Oceans, as the primary regulators of climate change, emerge as pivotal players, holding approximately 93 percent of the Earth's CO2. In exploring solutions, the concept of "blue carbon" has emerged, drawing attention to coastal ecosystems' carbon sequestration potential. However, the effective management of blue carbon presents a myriad of challenges, necessitating a holistic approach. There is a growing consensus that international standards for assessing marine carbon sinks are lacking. Experimental methods, including those proposed by the Intergovernmental Panel on Climate Change (IPCC) and the United Nations Environment Programme (UNEP), have been explored. However, the absence of a universally accepted framework impedes progress. It is within this context that the First Institute of Oceanography (FIO), Ministry of Natural Resources (MNR), China, has embarked on a pioneering initiative, developing China's first comprehensive marine carbon sink accounting standard. The standard, structured into five parts—Scope, Documents, Definitions, Accounting, and Appendix—provides a vital foundation for research, development, and management of blue carbon projects. Key terms and definitions, including ocean carbon sinks, mangroves, salt marshes, seagrass beds, phytoplankton, macroalgae, and shellfish, contribute to a robust scientific framework for the comprehensive understanding of marine ecosystems.Despite these advancements, challenges persist in blue carbon management, requiring focused attention. From a scientific perspective, understanding carbon sink mechanisms, potential, and capacity is essential. At the technical level, the development of observation systems, monitoring data, and international standards is crucial. On a practical level, conducting high-level dialogues, implementing international blue carbon plans, and establishing global blue carbon governance structures are imperative for improving the quality and functioning of marine ecosystems. In conclusion, the journey towards effective blue carbon management is a complex but imperative one. Standardizing practices, promoting international cooperation, and encouraging transactions related to carbon sink accounting are pivotal steps in our collective efforts to mitigate the impacts of climate change and safeguard the health of marine ecosystems worldwide.

How to cite: Liu, D. and Dong, T.: Blue Carbon Management for Climate Resilience, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7022, https://doi.org/10.5194/egusphere-egu24-7022, 2024.

Mangroves, as well as other coastal ecosystems, perform a fundamental role as sinks of carbon in their soils for long periods of time (hundreds to thousands of years). This capacity for sequestering carbon is controlled by several factors, such as mesoscale meteorology, structural complexity of the forest, hydrological regimes, and microtopography. Therefore, quantifying the carbon on these forests is fundamental to understand their potential for climate change mitigation and local adaptation. The Ria Celestun Biosphere Reserve is a natural protected area located in a karstic region in Mexico with a shallow slope and a strong environmental gradient that allows the presence of different ecological types of mangroves. Based on the analysis of sedimentary cores collected in mangrove areas and dated with the ²¹⁰Pb method, we assessed the soil and carbon accumulation rates in the upper 50 centimeters in four ecological types of mangroves (fringe, basin, dwarf and peten). According to preliminary results, basin mangrove dominated by Rhizophora mangle and Avicennia germinans showed higher values of carbon stocks (360 MgCha^-1), than fringe and peten mangroves (240 MgCha^-1 and 270 MgCha^-1, respectively). Taking into account gaps in mangrove knowledge in karst regions and among mangrove associations, the results could be used as a tool for decision-making and priority-setting of conservation actions.

How to cite: Canul Cabrera, J. A., Herrera Silveira, J. A., Díaz Asencio, M., Pech Poot, E., Gasser, B., and Masque, P.: Soil and carbon accumulation rates in different ecological types of mangroves in a karstic region (Celestún, Yucatán), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13829, https://doi.org/10.5194/egusphere-egu24-13829, 2024.

Globally, salt marshes play a pivotal role as efficient carbon (C) sinks, absorbing and storing substantial C in the soil. Despite their high C sequestration potential, understanding C exchange mechanisms in salt marshes along the Pacific coast in western Canada remains limited, resulting in uncertainties in estimating regional C budgets. This study addresses this gap by analyzing two years of eddy covariance (EC) measurements in a temperate salt marsh, revealing a shift from being a C sink (-185.3 gC m^-2 yr^-1) in the first year to a weak C source (10.4 gC m^-2 yr^-1) in the second year.

Exploring the annual patterns of Gross Primary Productivity (GPP) and ecosystem respiration (R_eco) sheds light on the dynamics of C exchange within the ecosystem. GPP of the first year is significantly higher than that of the second year during the growing season, from April to October. The light response curve indicated that the second year had lower light use efficiency and light-saturated net photosynthesis rate than the first-year data. Moreover, we found similar values in temperature sensitivity of soil respiration (Q₁₀) for both years using R_eco and soil temperature, with the first year slightly higher. Notably, the annual estimates for 2021 reveal a GPP of 1488.9 gC m^-2 yr^-1 and R_eco of 1303.6 gC m^-2 yr^-1. Conversely, for 2022, GPP was 1147.8 gC m^-2 yr^-1, and R_eco was 1158.2 gC m^-2 yr^-1. The contrasting GPP values between the two years suggest a significant influence of GPP over R_eco on the overall C balance of the ecosystem, which predominantly controls the variations in NEE.

In delving into the relationship between environmental factors and C exchange patterns, it becomes evident that water availability emerges as a key determinant at this site. The rainfall during the growing season of the first year closely matched 30-year averages from nearby meteorological stations, approximately 15% above the Climate Normals. In contrast, the second growing season precipitation was below average, only 52% of the long-term average. Additionally, it is noteworthy that in 2022, the growing season had a significantly higher Vapor Pressure Deficit (VPD), which led to lower GPP. This salt marsh demonstrates enhanced C uptake capabilities in a specific range of VPD, with peak efficiency observed at VPD levels ranging from 3 to 10 kPa. Conversely, CO₂ absorption capacity diminishes when VPD falls outside this range.

Water scarcity negatively impacts plant life, potentially leading to ecosystem instability and reduced C uptake capacity under climate change. A focused exploration of influencing factors is warranted to enhance our understanding of the observed transition from a C sink to a weak C source in the second year. Considering the broader implications of water scarcity on plant and ecosystem health could inform strategies to mitigate climate-induced stress. Addressing these areas will advance our knowledge of C dynamics in salt marsh ecosystems, guiding conservation and management efforts.

How to cite: Lu, T.-Y., Russell, S., and Knox, S.: Controls on Interannual Variability of Carbon Dioxide (CO2) in a Coastal Wetland Ecosystem: Insights from Two-year Measurements in a Temperate Salt Marsh , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14700, https://doi.org/10.5194/egusphere-egu24-14700, 2024.

Saltmarshes protect the coast against storm surges and erosion, are important ecosystems for breeding and sheltering birds and fishes, and sequester large amounts of carbon in their soils. Natural plant colonisation of mudflats as well as future sea-level rise will contribute to new saltmarsh formation and additional managed realignment projects can improve national capacities to meet climate targets. In this study, we’re investigating the current and future extent of saltmarshes in Ireland and estimate their carbon storage potential. The areas of current saltmarshes are identified based on literature and openly available GIS data. The potential natural development and expansion of existing saltmarshes is analysed using mean and extreme water level data from marine tidal gauges and topographic elevation data of the adjacent terrestrial areas. Further, carbon storage in up to 1 m deep soils was determined in various types of saltmarshes in a nationwide field campaign and upscaled to estimate future blue carbon potential. Results show that e.g. saltmarshes in County Dublin could increase from 181 ha to 227 ha due to natural saltmarsh expansion, with a potential increase of stored carbon by 22,688 Mg C_org and avoiding the emission of 83,264 t CO₂. However, saltmarsh depth plays a significant role in carbon sequestration. Thus, when considering carbon storage in only 10 cm deep soils the estimated carbon storage increase is reduced to 1,588 Mg C_org and 5,828 t CO₂ emission avoided. Further, our results indicate that Irish saltmarsh plants sequester less carbon than species in China or the United States, thus lowering global blue carbon estimates. Results of this study will serve as a basis for managers and policymakers estimating carbon credit potentials of saltmarshes and planning Managed Realignment projects.

How to cite: Fuchs, A., Burke, S., Delamer, I., and Cott, G.: Estimating current and future saltmarsh areas and carbon storage in Irish Blue Carbon habitats, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15255, https://doi.org/10.5194/egusphere-egu24-15255, 2024.

Saltmarshes are important ecosystems for carbon capture and storage and play a vital role in carbon cycling processes. Despite their significance, the carbon fluxes within Irish saltmarshes remain poorly understood, with a notable absence of greenhouse gas flux data from coastal wetlands in the region. This study addresses this gap by employing the eddy covariance method to calculate carbon dioxide (CO₂) fluxes from Derrymore saltmarsh, County Kerry, a natural estuarine saltmarsh on the west coast of Ireland, from May 2023 to the present. The tower is equipped with an open path CO₂ and water vapour (H₂O) infrared gas analyser (LI-7500, LI-COR biosciences) and a sonic anemometer (CSAT3, Campbell Scientific) set 3.2m above the marsh surface. This method allows us to get continuous high frequency CO₂ and H₂O data with measurements being taken ten times per second.

Our findings reveal patterns in net ecosystem exchange (NEE), with higher values observed during autumn compared to summer months, attributed to reduced photosynthetic CO₂ uptake. These findings are comparable to saltmarshes in other regions. With a project duration of 12 months, our hypothesis suggests Derrymore saltmarsh will act as a modest CO₂ sink.

The use of the eddy covariance method allows us to get an overall picture as to the extent to which this saltmarsh acts as a carbon sink, giving us a better understanding of the carbon dynamics from this Kerry saltmarsh. This ongoing project contributes vital data to a broader initiative in Irish saltmarsh research, aiming to establish a scientific foundation for a comprehensive management framework. The framework will guide efforts in saltmarsh protection, restoration, and optimisation of carbon sequestration. Our research underscores the importance of understanding local variations in carbon dynamics, paving the way for informed environmental strategies in the context of climate change.

How to cite: Jessen, L., Eichelmann, E., Fuchs, A., and Cott, G.: Carbon dioxide fluxes from an Irish saltmarsh, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15567, https://doi.org/10.5194/egusphere-egu24-15567, 2024.

Recent scientific investigations have classified blue carbon ecosystems as coastal vegetated areas characterized by rooted vegetation and marine sediments, spanning coastal, continental shelf, and offshore regions. While vegetated coastal salt marshes are recognized as highly effective carbon storage ecosystems, there is a limited understanding of the blue carbon potential when these ecosystems are transformed into embanked freshwater agricultural marshes. In regions like Charente-Maritime (France), where marshes cover 15 to 20%, comprising 95,000 hectares of freshwater marshes and 15,000 hectares of saltwater marshes, human activities through farming and water control, play a pivotal role in shaping and managing these areas. This results in a blend of water and land habitats, namely ditches and meadows, crucial for biodiversity maintenance and carbon storage. Despite the well-documented green carbon storage potential of meadows, able to store up to 1200 Kg C/ha/yr, the blue carbon footprint (uptake/sequestration) of associated freshwater ditches or wet meadows under various water management practices remains inadequately understood.

The present PhD work is closely aligned with two prominent regional blue carbon initiatives in France, namely TETRAE MAVI (Living Marshes) and LRTZC (La Rochelle Zero Carbon Territory). It focuses on the impact of water management practices in Charente's marshes on biodiversity and carbon dynamics in aquatic and terrestrial ecosystems. The study targets three different habitats - ditches, ditch edges, and wet meadows- and their corresponding compartments. Efforts will be made to understand the functional diversity of aquatic organisms (pelagic and benthic) and carbon dynamics in the water column, along with associated fluxes at exchange interfaces (water-air, sediment-air) using various methods such as sensors, chambers, samplings, and laboratory analyses.

A significant innovation of our work lies in the simultaneous consideration and measurement of blue carbon (aquatic) and green/brown carbon (terrestrial), leading to an integrative and comprehensive assessment. For example, one management approach, i.e. the dredging of a marsh (ditch), will enable a coupled analysis of blue carbon and green/brown carbon capture and sequestration, addressing the dual challenge of biodiversity and carbon balance on the terrestrial part too, considering all ecosystem components, and associated human activities.

The aim of this presentation is to present my PhD work in the framework of MAVI and LRTZC projects and to discuss the measurements and methodologies employed in our approach. Anticipated results will revolve around evaluating functional diversity and carbon capture/sequestration in the three studied habitat systems, considering human activities. To achieve this, a two-year on-site monitoring, coupled with laboratory measurements, will be conducted within an experimental unit in Charente-Maritime, encompassing diverse freshwater marsh habitats and ensuring logistical instrumentation and reliable data acquisition. Standardizing carbon footprint analysis methodologies is crucial for a consistent and reliable assessment of the carbon balance as intended here in this work presentation that would benefit from discussions to optimize the associated efficiency and overall benefit to the blue carbon community.

How to cite: Perdrau, M. A., Polsenaere, P., Mzali, L., and Dupuy, C.: Carbon footprint (uptake/sequestration) of freshwater marshes according to agricultural water management practices, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17518, https://doi.org/10.5194/egusphere-egu24-17518, 2024.

As an ecosystem of high socioeconomic and ecological importance, global loss and degradation of salt-marshes has multifaceted detrimental impacts. Salt-marshes are recognised as blue carbon ecosystems and play a notable role in the global carbon cycle, due to their enhanced ability to efficiently uptake and store organic carbon over long time scales. The characterisation of salt-marsh carbon sequestration capacity across spatial and temporal scales, however, is challenging due to the inherent complexities of the factors which dictate these processes. To capture the spatial variability, it is necessary to formulate relationships between halophyte specie above and below ground biomass and subsoil organic carbon content and then examine how this interacts with hydraulic regimes, geomorphological context and external pressures. In this study, seven typically occurring associations of halophyte species in the Venice Lagoon (Italy) were selected and the relative magnitude of above and below ground biomass as well as their relationship to soil organic carbon content was examined for each association. Specifically, the analysis examined measurements of AGB, BGB, LOI, SOC and Bulk Density at 54 sample sites over two different years for species associations dominated by Inula crithmoides, Sarcocornia fruticose, Juncus maritimus, Limonium narbonense, Spartina maritima, Spartina anglica, and Salicornia veneta. The results confirmed that the association type is influential and must be considered when mapping carbon sequestration capacity. The halophyte evolutionary differences paired with geomorphology and external forcings play a key role in determining the spatial variability of carbon sequestration capacity. Above ground biomass has a stepped increase on the transition between low and high marsh with the greatest densities being found in the high marsh, often adjacent to channels. Whereas, below ground biomass and soil organic carbon content peaked in the middle marsh zone. Furthermore, the below ground to above ground biomass ratio depends strongly on the association, the higher ratios being found in the low to middle marsh. Overall, the derived patterns in halophyte biomass and sequestration capacity across diverse associations of species is vital for predicting below ground biomass and organic carbon content based on above ground biomass. These results support salt-marsh monitoring and modelling endeavours, particularly in a sustainable coastal management and blue carbon assessment context.

How to cite: Blount, T., Silvestri, S., Marani, M., and D'Alpaos, A.: The diverse influence of halophyte species on carbon sequestration capacity in salt-marshes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18253, https://doi.org/10.5194/egusphere-egu24-18253, 2024.

A human-in-the-loop approach to monitor blue carbon ecosystems at scale with Copernicus Sentinel-2 imagery

Blue carbon ecosystems, such as seagrass meadows, mangroves or coral reefs play an essential role for food provision, erosion control, disaster resilience, biodiversity and as habitats, and in addition they serve as important natural sinks for carbon. With increasing climate pressures and human impacts related to eutrophication, overfishing and habitat fragmentation, the coverage and health of these coastal habitats have declined globally. To address this, it requires first and foremost an accurate quantification of the distribution and quality of these ecosystems. This poses challenges from multiple angles (i.e., lack of manpower, fiscal limitations, etc.). By exploiting the full capacity of Copernicus Sentinel-2 imagery and AI technology, we have developed a cloud-based interactive platform, MCSAV – short for Mangrove, Coral and Submerged Aquatic Vegetation (Figure 1) – to map and improve the planning, management and monitoring of blue carbon ecosystems worldwide. The platform is designed with a focus on making coastal mapping as easy as possible for users with greater local knowledge of blue carbon ecosystems but without expert knowledge in satellite image processing or machine learning. The entire mapping process, from the selection of suitable satellite imagery to the final classification, can be executed in just a few clicks with the platform. The backbone of the classification model that is integrated into the backend of the platform, is a pretrained convolutional neural network (DeepResUNet). A Human-in-the-Loop component allows fine-tuning of the pre-trained classification model with additional training data.

Our approach has already been applied to high latitude regions with success [1] and is currently applied to Semporna in Sabah, Malaysia, as part of the United Nations Development Programme’s (UNDP) third cohort of Ocean Innovations on marine protected areas, area-based management, and blue economy.

We will give an introduction into the project, present the methods implemented in our interactive mapping approach, and demonstrate the coastal mapping tool. We will conclude with some lessons learnt and an outlook.

Figure 1 MCSAV interface showing Copernicus Sentinel-2 image of 13 March 2023 after pre-processing module has been run (top) and an example habitat classification (preliminary) and deepwater areas for Mapul region (bottom).

References

[1]          S. Huber et al., “Novel approach to large-scale monitoring of submerged aquatic vegetation: A nationwide example from Sweden,” Integr Environ Assess Manag, vol. 18, no. 4, pp. 909–920, 2022, doi: https://doi.org/10.1002/ieam.4493.

How to cite: Krüger, G., Huber, S., Nielsen, L. T., Daniel, P., Malathounis, C., and Hansen, L. B.: A human-in-the-loop approach to monitor blue carbon ecosystems at scale with Copernicus Sentinel-2 imagery, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18776, https://doi.org/10.5194/egusphere-egu24-18776, 2024.

Salt marshes are crucial eco-geomorphic features of tidal environments, providing numerous important ecological functions and delivering a wide range of ecosystem services that contribute to human well-being. Salt-marsh evolution is controlled by the interplay between hydrodynamics, geomorphology, and vegetation, as marshes accrete vertically through the deposition of both organic matter (OM) and inorganic sediments. This allows marshes to keep pace with relative sea-level rise, and likewise capture and store carbon (C), making them valuable allies in climate mitigation strategies. Thus, Soil Organic Matter (SOM) plays a key role within salt-marsh environments, directly contributing to soil formation and supporting C storage. Distribution patterns of SOM in salt marshes may vary in space and time across and within tidal wetland types depending on different factors including vegetation, sediment, and morphodynamics.

To better understand variations in SOM distribution and further comprehend physical and biological factors driving OM and C dynamics in salt-marsh soils, we analyzed soil organic content in 10 salt marshes of the microtidal sediment-starved Venice Lagoon (Italy), from 60 sediment cores to the depth of 1 m. These analyses allowed us to relate SOM patterns to soil, vegetation, and morphological variables, as well as depositional patterns testified by recent sedimentary successions.

Our results reveal two scales of variations in sedimentary OM content in salt-marsh soils. At the marsh scale, OM variability is influenced by the interplay between surface elevation and changes in sediment supply linked with the distance from tidal channels. At the system scale, OM content distribution is dominated by the gradient generated by marine and fluvial influence. Variations in inorganic and organic inputs, both autochthonous and allochthonous, sediment grain size, and preservation conditions may explain the observed variations in SOM. Our results highlight marsh importance as carbon sink environments, furthermore emphasizing that environmental conditions within a tidal system may generate strongly variable and site-specific carbon accumulation patterns, enhancing blue carbon assessment complexity.

How to cite: D'Alpaos, A., Puppin, A., Tognin, D., Ghinassi, M., Franceschinis, E., Realdon, N., and Marani, M.: Spatial variability of Soil Organic Matter and Carbon content in the salt marshes of the Venice Lagoon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19624, https://doi.org/10.5194/egusphere-egu24-19624, 2024.

Aims Coastal salt marshes are productive ecosystems that are highly efficient carbon sinks, but there is uncertainty regarding the interactions among climate warming, plant species, and tidal restriction on C cycling.

Methods Open-top chambers (OTCs) were deployed at two coastal wetlands in Yancheng, China, where native Phragmites australis (Phragmites) and invasive Spartina alterniflora (Spartina) were dominant, respectively. Two study locations were set up in each area based on difference in tidal action. The OTCs achieved an increase of average daytime air temperature of ~1.11–1.55 °C. Net ecosystem CO₂ exchange (NEE), ecosystem respiration (R_eco), CH₄ fluxes, aboveground biomass and other abiotic factors were monitored over three years.

Results Warming reduced the magnitude of the radiative balance of native Phragmites, which was determined to still be a consistent C sink. In contrast, warming or tidal flooding presumably transform the Spartina into a weak C source, because either warming-induced high salinity reduced the magnitude of NEE by 19% or flooding increased CH₄ emissions by 789%. Remarkably, native Phragmites affected by tidal restrictions appeared to be a consistent C source with the radiative balance of 7.11–9.64 kg CO₂-eq m^–2 yr^–1 because of a reduction in the magnitude of NEE and increase of CH₄ fluxes.

Conclusions Tidal restrictions that disconnect the tidal hydrologic connection between the ocean and land may transform coastal wetlands from C sinks to C sources. This transformation may potentially be an even greater threat to coastal carbon sequestration than climate warming or invasive plant species in isolation.

How to cite: Zhou, P., Ye, S., Xie, L., W. Krauss, K., Pei, L., K. Chapman, S., Brix, H., A. Laws, E., Yuan, H., Yang, S., Ding, X., and Xie, S.: Tidal restriction likely has greater impact on the carbon sink of coastal wetland than climate warming and  invasive plant, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1293, https://doi.org/10.5194/egusphere-egu24-1293, 2024.

Mangrove forests are important in Indian Sundarban Biosphere Research (SBR) for coastal hazards and vulnerability reduction. Recent extreme natural disasters like flood inundation, cyclonic effects, shoreline change, and river bank erosion are the main threatening phenomena for coastal livelihood and forest cover change. Mangrove forest is not only a shelter for human life but also important for animals and gradually forest degradation triggers their life in serious issues. Technologies can assist in reducing those serious issues through space-based analysis, and adaptation policies and give them a sustainable life. Current space-based technologies can be applied for forest cover change analysis in the SBR area. This analysis investigates the forest cover in different years (2018 and 2022) through Sentinel-2 data. Various biophysical and ecological variables are measured because of recent cyclonic effects that have gradually affected this region. Some recent cyclones like Titli (2018), Fani (2019), Bulbul (2019), Amphan (2020), and Yass (2021) gradually triggered coastal geomorphology change, shoreline shifting, river bank erosion, and mangrove forest losses. Sentinel 2 data is applied in ArcGIS v10.8 and SNAP v9.0 for calculating those outcomes. The highest NDVI values are observed at 0.72 (2018) and 0.53 (2022), while the highest TNDVI values are also remarkable observations like 1.11 (2018) and 1.02 (2022) respectively. During cyclonic effects, those regions are affected by flood inundation, increased soil salinity, bank erosion, and huge economic losses observed. Similarly, high SAVI values are 1.08 (2018) and 0.81 (2022). The forest areas mainly decrease in G-plot, L block, some parts of Kultali block, and Jambu Dweep areas, while Blacky Island, HaLF-FiSH Island, and near Kakdwip block have increased mangrove forest areas. The high NDSI values observed were 0.42 (2018) and 0.49 (2022) because of saltwater intrusion which is triggering the crop dynamics and production losses in those regions. The S2REP and IRECI, both chlorophyll estimation indices indicate that the forest cover areas are lost during the study periods. The forest degradation index (FDI) values and threshold-based forest health index are also warned for adopting those regions, otherwise, the mangrove environment is gradually destroyed by natural extreme events and some man-made activities. Mangrove forest protection is essential for the planners, policy-makers, and stakeholders for safe forest life as well as coastal environment and coastal livelihood. Some adaptation strategies like cyclone shelters, mangrove plantations, early warning systems, river bank erosion reduction, and awareness can help to reduce the risk of extreme natural hazard events.

How to cite: Al-Ramadan, B., Halder, B., Yaseen, Z. M., and Juneng, L.: High resolution Sentinel-2 data-based ecological and biophysical variables analysis in Indian part of Sundarban Mangrove Forest, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4233, https://doi.org/10.5194/egusphere-egu24-4233, 2024.

Seagrass meadows are globally important blue carbon sinks, accumulating organic carbon within their beds from both seagrass and the burial of non-seagrass organic matter.  To date, substantial attention has been attracted on this nature-based carbon offsetting solution, especially for countries with large blue carbon resources (e.g., Australia, Indonesia, and USA). However, the carbon sink potential of small coastal regions characterized by high carbon densities of blue carbon ecosystems and its contribution to regional climate change mitigation efforts remain largely unclear. This study focusses on the quantification of seagrass carbon stock in an urbanized coastal area, Hong Kong. We collect 1-meter sediment cores from four sites (i.e., San Tau, Yam O, Sheung Pak Nai and Ha Pak Nai) covering two seagrass species (i.e., Halophila beccarii and Halophila ovalis). Our investigation encompasses the analysis of total organic carbon (OC%), organic carbon sources (δ¹³C and δ¹⁵N), labile and recalcitrant organic carbon pools, and organic carbon accumulation rates (Pb²¹⁰) at various depths within the sediment cores. The results of this study will shed light on the extent to which seagrass ecosystems can contribute to Hong Kong's Nationally Determined Contributions, as assessed under the IPCC Tier-II framework.

How to cite: Zhao, M. and Gaitán-Espitia, J. D.: Assessing Blue Carbon Storage in an Urbanized Coastal Region: Insights from Hong Kong's Seagrass Ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14230, https://doi.org/10.5194/egusphere-egu24-14230, 2024.