The latest IPCC assessment report highlights once more the need for negative emissions via carbon dioxide removal (CDR) measures to reach ambitious mitigation goals. In particular ecosystem-based CDR measures are currently the focus of national net-zero strategies and novel carbon crediting efforts. Using ecosystem-based carbon removal measures in marine environments as an example, we here highlight key challenges concerning the monitoring and evaluation of blue carbon fluxes for carbon crediting. Challenges specific to ecosystem-based CDR measures are i) the definition of baseline natural carbon fluxes, which is necessary for ii) clear anthropogenic CDR signal attribution, as well as iii) accounting for possible natural or anthropogenic disturbances of the carbon stock and hence an assessment for the durability of the carbon storage. In addition, the marine environment poses further monitoring and evaluation challenges due to i) temporal and spatial decoupling of the carbon capturing and sequestration processes, combined with ii) signal dilution due to high ecosystem connectivity, and iii) large pre-existing carbon stocks which makes any human-made increase in carbon stocks even harder to quantify. To increase the scientific rigor behind issued carbon credits, we propose a concentration of monitoring efforts on carbon sequestration rather than capturing processes, and baseline establishment for natural carbon sequestration in diverse ecosystems. Finally, we believe that making carbon credits subject to dynamic adjustments over time, will increase their credibility.

How to cite: Mengis, N., Paul, A., and Fernández-Méndez, M.: Counting (on) Blue Carbon - Challenges and Ways forward for carbon accounting of ecosystem-based carbon removal in marine environments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1168, https://doi.org/10.5194/egusphere-egu23-1168, 2023.

The blue carbon system generally refers to the carbon sink environment that can be stored in the ocean system, and these environments are mainly mangroves, seagrass beds and salt marshes. This study investigates the second-largest seagrass bed in Kenting in Southern Taiwan. In addition to the advantages of high ecological diversity, seagrass beds are also considered to be a high carbon storage environment, which is more capable of sequestering carbon in the atmosphere than green carbon systems. In risk assessment, green carbon system may have fire risks, causing the sequestered carbon in plants to be released back into the atmosphere. Therefore, we believe that research on coastal blue carbon systems and carbon sequestration issues are better development goal and direction. To understand how much total organic carbon can be sequestered in seagrass bed sediments under natural growth, and to estimate how many tons of carbon equivalent (CO₂e) in the atmosphere the carbon sequestered in this area are our ultimate goal. In the choice of sampling sites, we collected two seagrass bed sediment cores about 40 cm long, namely core A (BH2-SG)(coring in the seagrass area), and core B (BH1-NSG)(coring in the bare area on the seagrass bed). The analysis results showed that the organic carbon content of sediment core A was 0.184-0.298 wt%, with an average content of 0.237 wt%, and that of sediment core B was 0.188-0.401 wt%, with an average content of 0.318 wt%. After plugging in the organic carbon accumulation content formula (MgC *ha^-1= (TOC(%)*depth(cm)*BD(g/cm³)), we can get the organic carbon accumulation values of sediment core A (13.539 MgC*ha^-1) and sediment core B (18.405 MgC*ha^-1). For now, we can only evaluate the carbon accumulation of the upper 40 cm seagrass bed sediments in this area. The average accumulated carbon content of the two cores is multiplied by the total area of the Kenting seagrass bed (about 4.38 ha), and then multiplied by the carbon dioxide equivalent coefficient 3.67 represents its carbon dioxide equivalent (CO₂e) (the content value is 256.49 CO₂e). At last, we consider that the area is a major factor affecting the amount of carbon storage. If we can increase seagrass area, more carbon can be stored in the sediment.

Keywords: Kenting, Taiwan, blue carbon system, seagrass bed, organic carbon content (TOC%), carbon dioxide equivalent (CO₂e)

How to cite: Tang, Z.-W. and Chen, H.-F.: Estimating sediment carbon stocks in the environment of Taiwan's coastal blue carbon system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2547, https://doi.org/10.5194/egusphere-egu23-2547, 2023.

Many continental shelves host sediment depocenters which act as natural, long-term (>100 yr) carbon sinks. Human activities can strongly affect the efficiency with which carbon is sequestered in these depocenters, either through direct disturbances of the seafloor, or indirectly through climatic, light- or nutrient-induced changes, thereby affecting habitat and ecosystem functioning. In this study, we address the short- and long-term impacts of sea-use on carbon burial in the North Sea. Specifically, we focus on the role of bottom trawling as a crucial disturbance of seafloor sediments and benthic biota. In order to quantify the large-scale impact on carbon sequestration, we employ a numerical coastal ocean model to simulate the effects of demersal fishing gear on sediment transport, bioturbation efficiency and their interactions. Based on the results, the effects of potential management scenarios are discussed.

How to cite: Porz, L., Yilmaz, R., Zhang, W., and Schrum, C.: Carbon Burial in Shelf Sea Sediments – Anthropogenic Effects and Implications for Management, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5295, https://doi.org/10.5194/egusphere-egu23-5295, 2023.

Salt marshes sequester carbon at rates significantly exceeding those found in terrestrial environments. This ability arises from the in-situ production of plant biomass and the effective trapping and storage of both autochthonous and allochthonous organic carbon. The importance of this blue carbon store for mitigating increasing atmospheric carbon dioxide depends on both the rate at which carbon is buried within sediments and the rapidity with which that carbon is remineralised. It has been hypothesized that carbon burial rates, in turn, depend on the local rate of sea-level rise, with faster sea-level rise providing more accommodation space for carbon storage. This study addresses these three key aspects in a salt-marsh sediment study from Lindisfarne, northern England. We quantify rates of carbon accumulation by combining a Bayesian age-depth model based on ²¹⁰Pb and ¹³⁷Cs activities with centimetre-resolution organic carbon density measurements. A Bayesian isotope mixing model pinpoints terrestrial sources as providing the majority of stored carbon. We compare two approaches for assessing the relative proportions of labile and recalcitrant carbon based on a two-pool modelling approach and thermogravimetric analysis. Preliminary results indicate that during the 20^th century more carbon was stored at Lindisfarne salt marsh during decades with relatively high rates of sea-level rise.

How to cite: Gore, C., Gehrels, R., Smeaton, C., Andrews, L., McMahon, L., Hibbert, F., and Garrett, E.: Accumulation rates of salt-marsh blue carbon at Lindisfarne, northern England, and their relationship with sea-level change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6103, https://doi.org/10.5194/egusphere-egu23-6103, 2023.

Effective and large-scale atmospheric carbon capture is essential in limiting global warming to within 1.5 degrees Celsius as outlined by the Paris Agreement. The oceans make up two thirds of the Earth’s surface and already absorb approximately a quarter of anthropogenic emissions annually, therefore it is imperative to maximise their carbon sequestration ability through large-scale Carbon Dioxide Removal (CDR). One technique that aims to improve the efficiency of oceanic carbon uptake is Marine Biomass Regeneration (MBR), otherwise known as Ocean Iron Fertilisation (OIF). MBR is grounded on evidence that the introduction of certain key nutrients to nutrient depleted areas of the ocean can enhance primary productivity and regenerate ocean biomass, which then acts as a carbon sink. The ocean’s ability to circulate nutrients has been hindered by the over-exploitation of whales, which naturally regulate oceanic nutrient levels by feeding at a depth of 150-200m and defecating at the ocean surface through the whale cycle. Their faeces are rich in nutrients such as nitrates, phosphates and iron, and act as a natural fertiliser. It will take decades to restore the whale population to pre-whaling numbers, therefore, to catalyse the biomass regeneration of oceans, it is proposed that artificial whale faeces are deployed to mimic the whale cycle.

A two-dimensional carbon and heat cycling box model with meridional overturning circulation is extended, to include biological processes and nutrient cycling. This model has previously been used to carry out climate projections, by investigating the ocean’s carbon and thermal response to annual anthropogenic emissions, but there has been no investigation on how the changing meridional overturning circulation impacts the biological carbon pump. A simple nutrient-phytoplankton-zooplankton (NPZ) biological model is introduced to model the impact of macronutrient concentrations on phytoplankton and zooplankton growth. Further to this, some basic parameterisations for iron cycling will be added, based off the iron box models of Parekh et al. (2004) and Lefèvre and Watson (1999).  Using the extended model, it will be possible to undertake MBR experiments with different nutrient ratios and concentrations, mimicking the whale cycle, and investigate the impact these parameters have on the oceanic carbon and heat uptake and distribution from anthropogenic carbon emissions. The model also accounts for slower meridional overturning with increased ocean warming, which allows for the investigation of the effect of slower circulation on the biological carbon pump, primary productivity and nutrient distribution.

How to cite: Baltas, E., Katavouta, A., and Hunt, H.: Marine Biomass Regeneration: Simple Modelling of Large-Scale Ocean Carbon Dioxide Removal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9877, https://doi.org/10.5194/egusphere-egu23-9877, 2023.

Seaweeds are gaining recognition as a significant CO2 sink with a role in active mitigation and

climate change adaptation, and specifically so in the application of an innovative coastal CO2 removal belt, effectively utilizing seaweed habitats to mitigate the adverse effects of ocean acidification (OA). However, assessing OA modification strength requires an understanding of the multiple parameters’ potential buffering effects, especially in highly dynamic systems. Exactly how kelp might generate more favorable conditions for marine calcifiers, has not been taken into account in previous studies to date. We studied the effects of sugar kelp (Saccharina latissima) on an experimental farm at the north end of Hood Canal, Washington—a low retentive coastal system. This study can serve as a natural analogue for many coastal bay habitats where prevailing physical forcing drives chemical changes. In this field mesocosm study, pelagic and benthic calcifiers were exposed with or without the kelp’s putatively protective proximity at locations in the middle, on the edge, and outside the kelp array. Model outputs were used to identify dominating factors in spatial and temporal kelp dynamics, while wavelet spectrum analyses helped in understanding predictability patterns. We linked these results to biological assessments, including biomineralization, growth and subcellular energetics responses of the examined species. We found our studied kelp array system did not modify carbonate chemistry parameters, but changed pH autocorrelation patterns towards higher predictability that was more favorable for marine calcifiers. Kelp also improved habitat provisioning through kelp-derived particulate organic resource utilization. Because of this, the co-culture of bivalves and seaweed can protect the calcifiers from negative effects of projected near-future OA. However, our study shows that a complex combination of physical, chemical and biological processes determines the efficiency of the kelp farms for creating more favorable habitats with respect to OA. Future macrophyte studies should focus significantly on the importance of predictability patterns, which can additionally improve the conditions for marine calcifiers as well as ecosystem services, with important implications for the aquaculture industry.

How to cite: Bednarsek, N., Pelletier, G., Beck, M., Feely, R., Siegrist, Z., Kiefer, D., Davis, J., and Peabody, B.: Predictable patterns within the kelp forest can indirectly create temporary spatial refugia for ocean acidification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12012, https://doi.org/10.5194/egusphere-egu23-12012, 2023.

Mangrove associates, generally distributed in the landward fringe of mangrove forests, are one of the major carbon sinks. Mangrove associates are expected to increase in South Korea as their spatial distribution is shifting to poleward with global warming. However, understanding of carbon stocks and fluxes of mangrove associates is still limited. In this study, we estimated carbon stocks in soils and forest floors and measured carbon fluxes of soil CO₂ efflux and net photosynthesis of Hibiscus hamabo and Paliurus ramosissimus, mangrove associates which inhabit naturally in Jeju Island, South Korea from April to October, 2022. Four sites of H. hamabo (Gimnyeong – coast, Hado, Seongsan and Wimi) and P. ramosissimus (Gimnyeong – wetland and Daejeong 1 ~ 3) were selected. Soil carbon stocks at 0 – 10 cm depth from Gimnyeong – wetland, Seongsan, and Hado where soil horizons developed, and forest floor carbon stocks were quantified. In addition, soil CO₂ efflux and net photosynthesis were measured once a month. Mean soil carbon stocks (t C ha^-1) ranged from 29.0 to 30.1 while mean forest floor carbon stocks (t C ha^-1) ranged from 2.8 to 5.8. Soil CO₂ efflux rate (µmol CO₂ m⁻² s⁻¹) in August was significantly higher than that in April and October. There was a positive correlation between soil CO₂ efflux and soil (p < 0.001, r = 0.41) and air (p < 0.001, r = 0.52) temperatures compared to other factors such as soil water content (p > 0.05), and electrical conductivity (p > 0.05). Net photosynthesis (µmol m⁻² s⁻¹) was significantly high in July, and there were no significant differences among sites. Soil carbon stocks of the two species were higher than those of Quercus mongolica forests (27.8) in South Korea. Moreover, forest floor carbon stocks were higher compared to those of Q. glauca forests (1.32) in Jeju Island. Mean net photosynthesis (mean ± standard error, µmol m⁻² s⁻¹) of H. hamabo (8.9 ± 0.9) and P. ramosissimus (8.8 ± 1.3) in July were higher than that of Eleutherococcus gracilistylus (6.74 ± 0.26), a deciduous shrub inhabiting in Jeju Island. This study provides the first data base to estimate carbon stocks and fluxes of mangrove associates in South Korea and the results showed that H. hamabo and P. ramosissimus seem to be promising species for carbon sinks.

Acknowledgement

This study was carried out with the support of the National Research Foundation, Republic of Korea (Project No. 2022R1A2C1011309), and the Warm-temperate and Subtropical Forest Research Center (Project No. FE100-2022-04-2022).

How to cite: Choi, Y., Kim, G.-J., Lee, J., Kim, H.-S., Park, M., and Son, Y.: Carbon stocks and fluxes of Mangrove Associates(Hibiscus hamabo and Paliurus ramosissimus) in Jeju Island, South Korea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4623, https://doi.org/10.5194/egusphere-egu23-4623, 2023.