Among blue carbon ecosystems, seagrass and macroalgal beds have been managed for many years as important coastal ecosystems in the fisheries sector. Management has been carried out to ensure the sustainable use of macroalgal beds, from daily maintenance by fishermen, to maintaining and expanding their distribution in organized efforts at the fisheries cooperative and local government levels. A method for calculating CO₂ storage volume that makes it possible to visualize the value of such regional management in the fisheries sector within the framework of climate change countermeasures has been released.

Ecosystems like macroalgal beds, which provide significant benefits to humanity, are referred to as natural capital, and the benefits derived from this natural capital are called ecosystem services or "Nature's Contribution to People (NCP)." The most significant feature of macroalgal beds is the diverse benefits or co-benefits they offer. In simple terms, if natural capital such as macroalgal beds exists, it can simultaneously allow for sustainable food production, biodiversity conservation, and climate change measures to be implemented. In land areas, functions of farmland and pastureland as places for food production and functions of forests as places for environmental and biodiversity conservation are simultaneously provided. Recent international society has placed emphasis on utilizing ecosystems that can provide co-benefits as necessary for building a sustainable society, and the promotion of nature-based solutions is being proposed. For this reason, blue carbon ecosystems such as mangrove forests, salt marshes, and macroalgal beds are seen not only as carbon sink but also as ecosystems capable of achieving international goals to enhance biodiversity—such as Nature Positive¹ and 30 by 30²—and as ecosystems capable of sustainable food production. In addition, seagrass and macroalgal beds provide three simultaneous roles at once as ecosystems capable of sustainably producing food. This has led to the use of seagrass and macroalgal in sustainable blue economies worldwide.
In Japan, blue carbon ecosystems have begun to be registered in the national greenhouse gas (GHG) inventory³, with mangrove forests being registered for the first time in 2023. In April 2024, seagrass and macroalgal beds were added to the GHG inventory and reported to the UN as a carbon sink, totaling approximately 350,000 tons. If this report is accepted, macroalgal in particular would be the first in the world to be officially approved as an effective carbon sink. The inventory calculation reported to the United Nations must follow the Intergovernmental Panel on Climate Change (IPCC) guidelines. For Japan's calculations, parameters were used from an estimate of macroalgal bed areas by the Ministry of Land, Infrastructure, Transport and Tourism's Ports and Harbours Bureau and a project commissioned by the Ministry of Agriculture, Forestry and Fisheries to develop blue carbon evaluation methods and efficient macroalgal bed formation and expansion technologies (JPJ008722) which provided the estimated CO₂ sequestration per unit area of macroalgal beds. For the latter parameters, a project deliverable in the form of “a guidebook for calculating the CO₂ sequestration of seagrass and macroalgal beds" (2023, Japan Fisheries Research and Education Agency) has been published. An overview of this deliverable is provided below.

In the IPCC guidelines, the concept of CO₂ storage per unit area is called an annual emission factor. The commissioned project guidebook mentioned above also adopts an estimation method conforming to IPCC guidelines. However, since the IPCC guidelines do not specify the details of the annual emission factor, the amount of CO₂ stored for at least 100 years is calculated based on four scientifically justified carbon marine storage processes associated with macroalgal beds (Figure 1). Please refer to the guidebook for details of the estimation formula.
This guidebook aggregates Japan's seagrass and macroalgal beds, comprising at least 1,500 diverse species, into 17 types (21 types in total, including four types of cultivated seaweed) based on CO₂ storage characteristics and species composition. Furthermore, it divides the national marine areas into nine regions based on ocean environment and species composition. If each marine area had 17 macroalgal bed types, a simple calculation would yield 153 annual emission factors. However, since not all types are distributed in every region, CO₂ storage across the country is estimated using 92 annual emission factors. Additionally, the guidebook divides the annual emission factors into two parts, the maximum annual standing amount (dry weight) per unit area measured on-site in macroalgal beds and other constants compiled as parameters. This latter parameter is uniquely named the emission potential, indicating the CO₂ storage capacity, or storage amount per gram of dry weight for the targeted macroalgal bed type (Figure 2). Therefore, by observing on site the maximum standing amount per unit area of the place desired to calculate and multiplying this by the emission potential, the precise amount of CO₂ storage per unit area of the place desired to calculate, that is, the annual emission factor and its spatiotemporal variation, can be estimated.
Thus, CO₂ storage for the target macroalgal bed is calculated by observing on site the distribution area and maximum standing amount per unit area and using the emission potential values listed in the guidebook. In terrestrial forests, where CO₂ storage is estimated over time once trees are planted and growth begins, macroalgal beds, even when composed of perennial species, undergo seasonal growth and decay, causing biomass to change annually. Therefore, calculating the annual emission factor in yearly units is crucial for accurately estimating the amount of CO₂ storage. Thus, if there is an increase in the maximum standing amount per unit area and an expansion of the distribution area, improvements in CO₂ storage due to these changes can be reflected annually.

The decline of macroalgal beds, exemplified by rocky-shore denudation, is expanding extensively along with climate change. However, activities for restoring, maintaining, and expanding natural macroalgal beds continue to focus on local activities due to labor and financial issues. To overcome this issue, support for activities targeting macroalgal beds is provided within the environmental and ecosystem conservation activities of a Fisheries Agency's project that provides measures for promoting multifunctional roles in fisheries. Additionally, with the growing momentum for climate change measures, the use of the J-Blue Credit system for carbon credits is also increasing. However, as the number of fishermen is decreasing and the population aging, it is difficult to create new initiatives one after another. To encourage more initiatives and ensure they lead to solid projects, elements that can be expected to lead to further economic independence and revitalization are also needed. First, similar to the blue economy overseas, the creation of a new marine plant biomass industry that can also be used as a measure for CO₂ emission sources is desirable. For this purpose, integrating the fisheries sector, which produces biomass, with other sectors that utilize biomass is essential, and using the carbon credit system should function as a catalyst for this. With the registration of macroalgal beds in the GHG inventory, it is hoped that further disseminating the method and concept of estimating CO₂ storage in seagrass and macroalgal beds will progress, leading to the activation of both conservation and utilization of seagrass and macroalgae.

1. To halt biodiversity loss by 2030 and correct its trajectory towards increase and recovery.
2. A goal to effectively conserve more than 30% of the land and sea as healthy ecosystems by 2030.
Ministry of the Environment's Website: https://policies.env.go.jp/nature/biodiversity/30by30alliance/
3. Data summarizing the amount of greenhouse gases emitted and absorbed by Japan in one year.
Ministry of the Environment's website: https://www.env.go.jp/earth/ondanka/ghg-mrv/overview.html