Carbon sequestration assessment and analysis in the whole life cycle of seaweed

Methods for carbon sequestration are warranted to tackle climate change caused by greenhouse gases released from anthropogenic activities. Seaweed is a type of marine plant that utilizes carbon dioxide for photosynthesis and has a substantial capacity to sequestrate carbon. Despite the huge potential, the long-lasting carbon sequestration (LLCS) of seaweed has never been calculated throughout its whole life cycle (nursery, temporary rearing, maturation, harvesting, and processing). In this paper, we use a life cycle assessment (LCA) approach to calculate the LLCS of seaweed, which can be understood as the difference between carbon fixation and released carbon throughout the life cycle of seaweed. Using kelp (Laminaria japonica) as an example of seaweed, the present study validates the procedure of calculating the LLCS of seaweed throughout its whole life cycle in Ailian Bay from nursery to processing into biochar (fertilizer) as the final product. The results showed that the carbon sequestration (full life cycle) of kelp in Ailian Bay was 97.73 g C m−2 year. Biomass carbon accounts for approximately 86% of the total value (982.53 g C m−2 year) of carbon absorption source of kelp in Ailian Bay, with the remaining 14% consisting of recalcitrant dissolved organic carbon and sedimentary carbon. Moreover, we calculated the amount of biomass carbon that was sequestrated by seaweed production in China from 2010 to 2020. Thus, the present study demonstrates that the mass production of seaweed can be utilized as an efficient method to sequestrate carbon and a feasible method for evaluating the effect of kelp farms on climate change.


Introduction
Since the United Nations released a report on blue carbon in 2009 (Nellemann et al 2009), the term 'blue carbon' has gradually been recognized and valued. Blue carbon refers to absorbing and storing organic carbon in marine ecosystems, especially coastal vegetation ecosystems (Macreadie et al 2019). The carbon sink potential of seaweed gives seaweed the characteristics needed to be classified as a blue carbon habitat and a large-scale carbon sink (Yong et al 2022). Sufficient available area will help increase the area of seaweed culture and carbon sink compared to other blue-carbon plants (Gao et al 2022a). In addition, in integrated multi-nutrient farming of seaweed, seaweed has shown a solid ability to mitigate eutrophication, acidification, and deoxygenation caused by shrimp farming (Gao et al 2022b) and provide feedstock for food and biofuels (Duarte et al 2022). Therefore, constructing a seaweed farm can become an essential and feasible method for enhancing the capacity of the ocean to sink carbon while supporting the development of the ocean economy.
Many experts in China and abroad have researched seaweed carbon sinks. In terms of seaweed carbon sink potential and intensity, a report from UN Global Compact states that the cultivation of seaweed is a nature-based climate solution (UN Global Compact 2021). Hill (2015) and Trevathan-Tackett et al (2015) suggested that large amounts of carbon are stored in the living macroalgae and contribute to climate change mitigation. Smale et al (2018), Nellemann et al (2009), Duarte et al (2017), and Tsai et al (2017) demonstrated, both theoretically and experimentally, that seaweed can store large amounts of carbon in a short period and play an important role in the carbon cycle as a biological carbon sink. Gao et al (2022c) found that the carbon sink potential of seaweed plays a vital role in achieving China's carbon neutrality goals.
Regarding the calculation of seaweed biomass carbon, the calculation methods to account for seaweed carbon sinks mainly focus on the growth stage of seaweed. The Chinese Ministry of Natural Resources (2021) has published a standard specification for seaweed carbon sinks in the Chinese marine industry. Duarte et al (2017) and Sondak et al (2017) used the same method to estimate seaweed biomass carbon by measuring the weight of cultivated kelp. Wu and Li (2022) also gave a formula for calculating the carbon sink of seaweeds and estimated the existing carbon sink in ten coastal provinces of China from 2008-2019.
In addition, there are other studies about algal carbon sinks. Figure 1 illustrates the carbon sequestration process of seaweed, from which it can be seen that in addition to seaweed biomass carbon, recalcitrant dissolved organic carbon (RDOC) driven by microorganisms and deposited carbon are also essential components of seaweed carbon sink (Ren 2021). Zhang et al (2017) and Jiao et al (2010), Jiao and Azam (2011) studied the production and development of RDOC. Wada and Hama experimentally estimated the concentration of dissolved organic carbon (DOC) in macroalgae, demonstrating that seaweed had a measurable contribution to DOC production. Studies on the burial of carbon in sediments by seaweeds are also ongoing. A study by Queirós et al (2019) found that every square meter of seaweed cultivation area in sediments can store 58.74 g of organic carbon per year. Besides, the sequestration of seaweed from a global perspective was analyzed by Krause-Jensen and Duarte (2016). Krause-Jensen found that macroalgae can sequester around 173 Tg of carbon per year globally, and about 10% of the isolation is buried in coastal sediments.
In addition, as shown in figure 1, DOC is part of organic carbon, which is abundant in marine and freshwater systems and is one of Earth's largest organic matter cycle reservoirs. DOC can originate from within or outside any given body of water. DOC arising from within the body of water is known as autochthonous DOC and typically comes from aquatic plants or seaweed, while DOC originating outside the body of water is known as allochthonous DOC and typically comes from soils or terrestrial plants (Kritzberg et al 2004). The DOC described in this paper is from seaweed. RDOC denotes recalcitrant dissolved organic carbon. Depending on the origin and composition of DOC, the behavior and cycling of DOC are different. The unstable part of DOC decomposes rapidly through microbial or photochemically mediated processes, while refractory DOC is resistant to degradation and can persist in the ocean for thousands of years. This part of refractory DOC is known as RDOC (Hansell and Carlson 2014). Microbial carbon pump (MCP) refers to the conversion of organic carbon from the available active state to the unavailable inert dissolved organic carbon by the physiological ecological and biogeochemical processes of marine microorganisms (Jiao and Azam 2011).
Based on the above review, the current studies have focused on the potential and intensity of seaweed carbon sinks, measurement of seaweed biomass carbon, sedimentary seaweed carbon, and RDOC. However, studies have yet to be conducted to assess and analyze the carbon sinks of seaweeds throughout their life cycle. Therefore, to provide a calculation method for long-term carbon sequestration, this paper reports a life cycle carbon assessment method that provides a cradle-to-grave perspective, i.e. from the seaweed nursery process to the final process (production of seaweed biochar). This life cycle carbon assessment approach helps assess large-scale kelp farms' carbon sequestration contribution while using new case data from a kelp farm in China. This study contributes to an improved cradle-to-grave understanding of the climate benefits of large-scale kelp farms and kelp carbon accounting.

Process flow of the whole life cycle of seaweed products
Currently, seaweed farming in China is dominated by kelp (Laminaria japonica), nori, gracilaria, and wakame. It includes small-scale farming of hijiki, sage, unicorn, stone lily, reef membrane, prolifera, and long-stemmed grape fern (Zhang 2018). The cultivation processes of different seaweed are different, but they can be generally summarized as a nursery, temporary raising, growing, harvesting and processing procedures. The relevant data changes of seaweed production in China during the 10 years (2010-2020), according to the statistics of the 'China Fishery Statistical Yearbook' , are summarized and plotted in figure 2.
It can be seen from figure 2 that kelp (Laminaria japonica) accounts for the vast majority of seaweed aquaculture in China. Therefore, this paper takes kelp as the primary research object. A process flow diagram of the whole life cycle of kelp culture was drawn, as shown in figure 3. The specific method is: (1) Select suitable kelp seedlings for land cultivation.
(2) After about 80 d, move the kelp seedlings into the sea for temporary cultivation.
(3) When the kelp grows to about 30 cm, transport it back to land for artificial isolation. (4) After completing this series of processes, the kelp culture rope is transferred to the kelp farming area, where they grow. (5) After the mature kelp is harvested, it is processed into seaweed products.

Calculation method for carbon sequestration in seaweed
In this article applying the LCA approach, the CO 2 removal process from the cradle to the grave must be included when assessing the carbon sequestration of seaweed (Hasselström and Thomas 2022). The longterm carbon sequestration in the whole life cycle of seaweed is the difference between the total amount of absorption and emission sources in the production, transportation, and processing of seaweed throughout the life cycle. Long-lasting sequestration refers to storing CO 2 by seaweed stably and safely for at least a century. The formula for calculating the carbon sequestration (∆C) in the whole life cycle of seaweed (XPRIZE 2021) is: where, ∆C is the CO 2 fixed amount in the whole life cycle of seaweed (ton yr −1 ); C i is the ith CO 2 uptake during the whole life cycle of seaweed (ton yr −1 ); ω i is the proportion of CO 2 released back to the atmosphere during the ith carbon absorption of the seaweed throughout its life cycle; C j is the jth CO 2 emission in the whole life cycle of seaweed (ton yr −1 ).

Calculation of carbon uptake
As a marine plant, seaweed is highly efficient at photosynthesis, absorbing CO 2 from the ocean and converting inorganic carbon into organic carbon for storage in the seaweed. During the growth phase, seaweed will produce detritus as a result of wind and waves, grazing, decay, and other factors. Some of this detritus is transported to neighboring ecosystems in the form of dissolved organic carbon (DOC) and particulate organic carbon (POC) (Paine et al 2021), which is converted to RDOC through the 'MCP' theory (Ahmad et al 2021). The RDOC produced by the 'MCP' can be stored in the ocean for  up to 4000-6000 years and can remain suspended in the sea for a long time (Watanabe et al 2020). A further part is deposited in the deep sea, where it is fixed in the form of sedimentary carbon (Paine et al 2021). Sedimentary carbon is sequestered in the deep ocean, unlike emissions in the atmosphere to achieve sequestration. Thus, RDOC and sedimentary carbon can be considered long-lasting sequestration by seaweed. In addition to these two forms of carbon, mature seaweed contains large amounts of carbon, known as biomass carbon. Whether this carbon can be sequestered in the long term will be explained in section 3.2, but it is still part of the carbon absorbed by the seaweed. Carbon absorbed by seaweed is the sum of biomass carbon, RDOC, and sedimentary carbon (Wenhan 2021) where, C Absorption is the amount of CO 2 absorbed across the whole life cycle of seaweed (ton yr −1 ); C biomass is the amount of biomass carbon contained in mature seaweed throughout its life cycle (ton yr −1 ); C sedimentary is the amount of sedimentary carbon produced by the seaweed throughout its life cycle (ton yr −1 ); C recalcitrant is the amount of RDOC produced by the seaweed throughout its life cycle (ton yr −1 ). Different types of seaweed have different carbon content rates. The carbon content of some significant seaweed is listed in table 1 (China Fisheries Statistical Yearbook 2010-2020). Zhang et al (2005) used the carbon content in the dry weight of kelp to be 31.2% for calculation. Sondak et al (2017) selected the average 30% of the carbon content of seaweeds as the calculation data. Duarte et al (2017) where W is the yield (wet weight) of seaweed; R is the dry-wet ratio of mature seaweed; C aa is the carbon content of seaweed; F cef is the relative molecular mass ratio of carbon dioxide to carbon (44/12). The final carbon fixed by seaweed in the form of RDOC is mainly related to the rate of DOC released by seaweed and the rate of transformation of DOC to RODC. Thus the equation of seaweed refractory carbon can be summarized as follows (in CO 2 equivalent): where Q is the release amount of DOC (g C m -2 d -1 ); S is the planting area of the seaweed zone (m 2 ); D is the seaweed culture cycle (d); α is the conversion of seaweed DOC to RDOC ratio; and F cef is the relative molecular mass ratio of carbon dioxide to carbon (44/12). Some studies have shown that the detritus produced by seaweed, which is buried in the deep sea during depositional processes, contributes to carbon storage (Wada andHama 2013, Abullah et al 2017). Seaweed sedimentary carbon is not only related to the deposition rate of debris but also to the content of organic carbon in the debris (Zhang et al 2017, Queirós et al 2019. C sedimentary can be summarized by the formula (in CO 2 equivalent): where V is the particle deposition rate (g C m −2 d -1 ); β is the organic carbon content of the sediment; S is the planting area of seaweed (m 2 ); D is the seaweed culture cycle (d); F cef is the relative molecular mass ratio of carbon dioxide to carbon (44/12).

Released carbon sources
Seaweed is harvested and processed after maturity, and some of the biomass carbon from seaweed will be re-released into the atmosphere due to different processing methods. Therefore, seaweed biomass carbon only indicates seaweed's great carbon sequestration potential and the re-released carbon source must be taken into consideration in predicting the long-lasting carbon sequestration (LLCS) of seaweed. In contrast, RDOC and sedimentary carbon from seaweed are buried to the deep ocean and can be sequestered for a long time, so the proportion of carbon re-released from these two components is zero. Hence, in terms of the carbon re-released from seaweed, only seaweed biomass carbon was analyzed. According to statistics, the vast majority of global seaweed comes from Asia, and the processing of seaweed is mainly concentrated in food consumption, of which 31%-38% are directly used for human food, animal feed or plant fertilizer (Naylor et al 2021). After the seaweed is harvested and used for food processing, its biomass carbon is separated into the food chain and broken down by other organisms. For example, when seaweed is used as feed for other animals, the biomass carbon contained in the seaweed reenters the food chain. It is eventually partially released back into the atmosphere. However, by making seaweed into seaweed biochar as fertilizer, the proportion of carbon rereleased from seaweed will be greatly reduced. Figure 4 shows a processing facility for seaweed that converts seaweed into biochar. Bird et al (2011) pointed out that seaweed biomass carbon can be converted into biochar, a carbon storage tool with substantial advantages in improving soil potential. By slow pyrolysis under anaerobic conditions to form LLCS biomass, biochar can stably store carbon for hundreds of years, in which carbon is difficult to decompose after mineralization (Roberts et al 2015). Sun et al (2021) also pointed out that seaweed biochar can be stored in the soil for thousands of years; Firmly locking solid carbon will lead to the removal and storage of atmospheric carbon. As a form of fertilizer, the processing of seaweed biochar can avoid the release of carbon. The seaweed biochar can make carbon permanently isolated.
In contrast to other biochar, although some experts have noted the carbon sequestration potential of algal biochar, few have conducted detailed studies. Pariyar et al (2020) believed that biochar yield depends on biomass's physical and chemical composition and the pyrolysis conditions, such as temperature, heating rate, duration, and type of pyrolysis. According to relevant experimental studies, the average biochar yield of seaweed fertilizer is about 62% de Nys 2016, Dang et al 2023). Thus for seaweed biomass carbon, its CO 2 re-release rate (ω i ) is 38%.

Calculation of carbon emissions
The whole life cycle of seaweed has corresponding carbon emissions. According to the greenhouse gas carbon emission factor method, the carbon emission of seaweed's whole life cycle is calculated.
The equation (Eggleston et al 2006) for fuel carbon emissions during seaweed transport is: where E is the CO 2 emission from fossil fuel combustion; AD i denotes activity data for the ith fossil fuel of the year; EF i denotes the CO 2 emission factor of the ith fossil fuel.

Carbon sequestration of Ailian Bay kelp throughout its life cycle
Ailian Bay is located in Weihai, Shandong Province, northern China. The bay is semi-enclosed and leads eastward to the north of the Yellow Sea. The mouth of Ailian Bay borders Sanggou Bay with similar water quality conditions. Like Sanggou Bay, Ailian Bay also carries out a long-term integrated mariculture model with two main species of shellfish and seaweed culture . Ailian Bay produces varieties of kelp, and one of mature Ailian Bay kelp was shown in figure  5 . In this paper, we choose the kelp of Ailian Bay to start the analysis, and the data related to Ailian Bay is shown in table 2. The whole carbon sequestration of seaweed can be described in three parts, including the biomass carbon sequestration of seaweed, the carbon sequestration of RDOC, and the sedimentary carbon sequestration in Ailian Bay.
The average dry to wet ratio of seaweeds (Laminaria japonica, Undaria pinnatifida, Pyropia spp etc) is about 9.6%, while the dry to wet ratio of kelp (Laminaria japonica) is about 14% (Mao et al 2018). Ailian Bay can harvest 300 000 tons (in column  (1) and (3), the total CO 2 carbon sequestration of biomass carbon is 48 048 tons. However, two-thirds of the seaweed in Ailian Bay is used in food processing and is not counted as part of the seaweed carbon sequestration. Note that the 100 000 t seaweed is (1/3) of the total harvested seaweed (300 000 tons, in column 3 of  (1) and (3)), the biochar that can be fixed for a long time by the kelp in Ailain Bay is 9931.5 tons of CO 2 . The amount of biochar carbon sequestration accounting for 20.7% (9931.5/48 048) of the total biomass carbon of seaweed. Some literature (Gao et al 2021, Paine et al 2021, Weigel and Pfister 2021 indicated that the release of DOC from kelp ranged from 470.1-1030 mg C m -2 d -1 , and the conversion of seaweed DOC to RDOC ranged from 37.8% to 78%. Since the uncertainty of DOC release and conversion coefficient of kelp leads to uncertainty of C sequestration, the upper and lower limits of DOC release and conversion coefficient of kelp respectively can be calculated, respectively. According to equation (4), the maximum value (1030 mg C m -2 d -1 ; 78%) of CO 2 fixed by RDOC of kelp in Ailian Bay is 2828 tons CO 2 and the minimum value (470.1 mg C m -2 d -1 ; 37.8%) is 625.5tons CO 2 . This means that the carbon sequestration of RDOC of kelp changing range is 625.5 ∼ 2828 tons CO 2 . To conservatively evaluate the carbon sequestration of RDOC of kelp in Ailian Bay, 625.5 t CO 2 (minimum value) was selected. RDOC can be sequestrated for a long time, hence CO 2 rerelease rate (ω i ) values of RDOC is zero. According to equation (1), the RDOC that can be sequestrated for a long time by the kelp in Ailain Bay is 625.5 t CO 2 (minimum value).
In this paper, the average particle sedimentation rate of 162.34 g C m −2 d −1 is used based on the (Zhang et al 2017). According to equation (5), we combined the total area of seaweed culture in Ailian Bay (3.2 km 2 ) and the percentage of the organic carbon content of sediment in the culture area (0.17%). It can be estimated that the annual organic carbon deposition flux in the Ailian Bay kelp culture area is 265 tons C yr −1 , corresponding to the amount of CO 2 deposited in Ailian Bay, which is 972 (265 × 44/12) tons CO 2 yr −1 . Sedimentary carbon is buried in the seafloor, hence CO 2 re-release rate (ω i ) values of sedimentary carbon is zero. According to equation (1), the sedimentary carbon that can be sequestrated for a long time by the kelp in Ailain Bay is 972 tons CO 2 . The sedimentary carbon sequestration of seaweed in Ailian Bay (plating area 3.2 km 2 ) is 972 tons CO 2 yr −1 , which is corresponding to 82.8 g C m −2 yr −1 . Note that this value is different from 58.74 g C m 2 yr −1 in the area of English Channel (Queirós et al 2019), due to sedimentary carbon sequestration values change with the hydrology conditions of plating sea areas.
In Ailian Bay, the total amount of diesel consumed throughout the life cycle of seaweed is about 128 tons yr −1 , while the cultured rope used for seaweed farming is about 3500 tons yr −1 . To produce one ton of seaweed biochar, it required about 80 kW·h of electricity. Therefore, to process 100 000 tons of seaweed into seaweed biochar in Ailian Bay requires 8000 000 kW·h of electricity. Using equation (6), combined with the carbon emission factors in table 3, we can calculate that the CO 2 emission over the whole life cycle of the Ailian Bay kelp is about 10 382.2 tons (296 + 7976 + 2110.2 tons, as listed in Column 6 of table 4), of which 7976 tons of carbon dioxide are released through electricity, accounting for about 76.8% of the total carbon emissions.  (Eggleston et al 2006, Li 2018 The whole carbon sink process has been established. The calculation framework of the carbon sink of kelp in Ailian Bay can be summarized, as listed in table 4. Based on the calculation framework, the total carbon sequestration of seaweed in Ailian Bay (plating area 3.2 km 2 ) is 1146.8 tons CO 2 yr −1 , which is corresponding to 97.73 g C m −2 yr −1 . The value 97.73 g C m −2 yr −1 , includes the total value (982.53 g C m −2 yr −1 ) of absorption source of kelp and the total value (−884.8 g C m −2 yr −1 ,) of emission sources of kelp. Observing table 4, we can find that in Ailian Bay, the biomass carbon, sedimentary carbon and RDOC account for 86% (846.43/982.53), 5.5% (53.3/973.9) and 8.5% (82.8/973.9) of the total absorption value of source of kelp, respectively. In addition, table 4 also shows that 86% (846.43/982.53) of the stored C is due to the intended biochar sink, while 14% ((53.3 + 82.8)/973.9) is due to kelp-cycle internal loss processes.

Analysis and discussion
Although kelp is used as an example, the calculations presented in this paper are equally applicable to the whole-life carbon sequestration calculations of other seaweed. It is worth noting that the data for selecting RDOC and sedimentary carbon in carbon sequestration are extensive. The selected values of particle deposition rate and DOC release, etc depend on the culture area and culture species. Therefore, when calculating the carbon sequestration of seaweed, the sea area should be selected first. Since the study of DOC release from seaweed is more seriously affected by natural conditions and experimental uncertainties, there are still some uncertainty phenomena in the calculation of seaweed RDOC, which makes it challenging to quantify carbon sequestration from seaweed. For example, the range of release of DOC from kelp (Laminaria japonica) is 470-1030 mgC m −2 d. In addition, the conversion coefficient range from DOC to RDOC is 37.8%-78%. This leads to the carbon sequestration of RDOC of kelp changing range is 625.5 ∼ 2828 tons CO 2 yr −1 in the Ailian Bay plating area. Hence, the total sequestration also gets a range of 1045.9-3873.9 tons CO 2 yr −1 , which is corresponding to 89.1-330.08 g C m −2 yr −1 . This shows that the upper/lower number of the coefficient range will affect the total carbon sequestration values, making the overall C sequestration uncertain. Although there are still some uncertainties in data and coefficient selection, this paper provides methods for calculating seaweed's total carbon sequestration values.
In the case of this paper, comparing the three forms of carbon sinks in kelp, biomass carbon accounts for the largest share, about 86% of the total CO 2 uptake in kelp. Froehlich et al (2019) proposed that the deposition of mature seaweed to the deep sea could result in the permanent sequestration of biomass carbon from seaweed, considering only the aspect of increasing carbon sinks. This approach allows permanent sequestration of kelp, but it is neither practical nor economical. Based on equation (3) established above, the carbon sequestration of seaweed biomass carbon in China in this decade (2010-2020) was estimated using the wet weight data of seaweed from the China Fisheries Yearbook (2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). If all seaweed biomass carbon is permanently sequestered the results suggest that about 250 000 tons of CO 2 could be fixed by Chinese seaweed during this decade, which could reach 0.2% of China's peak carbon (He et al 2022).
Although seaweed biochar can permanently sequester some carbon, based on a comparison of biomass carbon (9931.5 tons CO 2 yr −1 ) and carbon emissions (7976 tons CO 2 yr −1 ) from seaweed in Ailian Bay, the processing of seaweed biochar also generates significant carbon emissions from thermal power generation. It can be seen that the current seaweed processing technology is still relatively backward, and if wind power is used, the carbon emission of seaweed processing will be significantly reduced. While paying attention to the seaweed carbon sequestration method, it is also essential to pay attention to the excess carbon emissions generated by this method. Only with a two-pronged approach can seaweed play a strong potential in carbon sequestration and serve China's dual carbon strategy.
The consumption of diesel fuel runs through all aspects of seaweed nursery, seedling transportation, and culture, the consumption of culture rope is mainly during the raising period, and the consumption of electrical energy is mainly during the seaweed processing. The results of the case show that the CO 2 released due to energy consumption (diesel and polyethylene) during the whole period of seaweed culture (nursery to processing) is 2406 tons, which is 23% of the total energy consumption (10 382.2 tons of CO 2 ). The CO 2 released from the aquaculture facilities during the culture period was 2110 t, accounting for 87.7% of the CO 2 released during the culture period. Therefore, controlling the use of seaweed farming facilities during the cultural period is the key to reducing emissions. Due to its high strength and good abrasion resistance, polyethylene rope is commonly used for seaweed farming rope. Among them, 116.67 kg of polyethylene rope is needed to culture 1 ton of seaweed, and the service life of polyethylene rope is ten years. Therefore, when calculating the annual carbon sink of kelp, farming one ton of kelp requires an average of 11.667 kg of polyethylene rope per year. Finding new low-carbon farming materials or increasing the service life of polyethylene rope can achieve a good emission reduction effect. In the process of seaweed farming, besides paying attention to the growth condition of seaweed, we should also strengthen the supervision and repair of seaweed farming facilities to reduce seawater erosion on the farming rope to extend the service life of seaweed farming rope.

Conclusion
Relying on the life cycle of seaweed, the calculation method of LLCS of seaweed throughout its life cycle is summarized. This article considers the sources of carbon emission and re-release during the growth of seaweed, and the calculation formula of carbon sequestration of seaweed based on the process and mechanism of the carbon cycle. The research presented in this study has important implications for studying the carbon sequestration calculation of seaweed throughout its life cycle.
The results show that the seaweed processing method will affect the final carbon sequestration. To improve the carbon sink benefits of seaweed and ensure the long-term carbon sink, research on seaweed processing methods (seaweed biochar and seaweed biofuel, etc) should be increased to mitigate global climate change.
Our calculation shows that the kelp in Ailian Bay fixes 97.73 g C m −2 yr −1 , which includes the total absorption source of kelp (982.53 g C m −2 yr −1 ) and emission sources of kelp (−884.8 g C m −2 yr −1 ). Considering that the total absorption source of kelp is 982.53 g C m −2 yr −1 in Ailian Bay, the biomass carbon, sedimentary carbon and RDOC account for 86%, 5.5% and 8.5% of the total absorption source of kelp, respectively. The carbon footprint (certain amount of gaseous emissions that are relevant to climate change and associated with human production or consumption activities) of the kelp in Ailain Bay is −1146.8 tons of CO 2 per year, and the negative value indicates that the kelp in Ailian Bay can contribute 1146.8 tons of carbon sink per year. Therefore, seaweed plays an active role in increasing carbon sinks and can contribute an essential part in serving the national dual carbon strategy. Attention must be paid to the principles and methods of carbon sequestration throughout the life cycle of seaweed, the role of seaweed farming in the carbon cycle, and the healthy and sustainable development of coastal ecosystems.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).