Research on Carbon Sink Effect of Marine Shellfish and Algae in China

Abstract

Global warming has increasingly become a widespread concern of the international community, and one of the key approaches to achieving carbon neutrality goals lies in the carbon sequestration capacity of oceans. Therefore, scientifically and accurately measuring the carbon sink capacity of marine fisheries and studying its spatial effects are particularly crucial for mitigating global climate change. Marine fisheries encompass categories such as fish, shellfish, algae, and crustaceans. Given that marine fisheries-based carbon sinks are non-feed fisheries, with cultivated shellfish and algae being highly representative, this paper primarily focuses on the carbon sink capacity of shellfish and algae as the main assessment criteria for marine fisheries carbon sinks, aiming to apply this research to other countries worldwide to assist in addressing global warming. Thus, based on panel data of shellfish and algae cultivation in nine coastal provinces of China from 2007 to 2021, this paper employs the “removable carbon sink” model to calculate the carbon sink capacity of Chinese marine shellfish and algae aquaculture industry and utilizes the spatial Durbin model to analyze its spatial effects. The research findings are as follows: (1) The spatial distribution of carbon sink capacity in China’s marine shellfish and algae is uneven. (2) Moran’s Index indicates that the carbon sink capacity of marine shellfish and algae exhibits positive spatial correlation, but the degree of spatial agglomeration is unstable. Fujian Province has the highest average carbon sink capacity at 446,451.21 tons, while regions such as Hainan, Hebei, and Jiangsu have relatively lower average carbon sink capacities, with Hainan Province’s being only 3627.57 tons, sufficiently demonstrating the characteristic of uneven spatial distribution. (3) Through decomposition using the spatial Durbin model, it is found that the direct effects of marine shellfish and algae aquaculture production, technological input, technological promotion, and fishery disaster situations are positive, with the result for marine shellfish and algae aquaculture production being 1.617, significantly positive at the 1% level. The result for labor input is −0.847, with a negative direct effect. From the perspective of indirect effects, the indirect effects of marine shellfish and algae aquaculture production, technological input, and technological promotion are positive, with aquaculture production at 1.185, still significantly positive at the 1% level. The result for labor input is −2.140, with a negative indirect effect. These research conclusions provide important references for the formulation of global marine carbon sink-related policies, helping countries optimize resource allocation, strengthen regional collaboration, and increase investment in science and technology. Consequently, they can promote the sustainable development of marine shellfish and algae aquaculture industries, and contribute to enhancing marine carbon sink capacity and achieving global carbon neutrality goals.

1. Introduction

Since the 21st century, there has been a sharp increase in greenhouse gas emissions such as carbon dioxide, leading to climate change phenomena like global warming, which pose non-traditional security threats to human society [1]. In the face of the increasingly severe greenhouse effect, emission reduction and carbon sequestration have become global focal points. In addition to adjusting the energy structure, promoting industrial upgrading and transformation, and developing a low-carbon economy, marine carbon sequestration is also an important means to address global warming [2]. Therefore, measuring and enhancing the carbon sink capacity of marine shellfish and algae in China is a key pathway to achieving carbon neutrality goals. Fisheries, as a fundamental way for humans to exploit marine resources, play a crucial role in coastal carbon cycling, and fisheries carbon sequestration is an indispensable part of marine carbon sequestration [3]. Enhancing marine carbon sequestration capacity is an urgent global task, of great significance for addressing climate change, protecting marine ecosystems, and promoting sustainable development. With the continuous rise in atmospheric carbon dioxide concentrations, the ocean, as the largest carbon sink on Earth, plays a vital role in carbon sequestration and storage. By enhancing marine carbon sequestration capacity, we can effectively mitigate the impacts of climate change and buy more time for humanity to address this global challenge. Additionally, improving marine carbon sequestration capacity also helps protect and restore marine ecosystems, maintain biodiversity, and provide suitable habitats for marine life. Meanwhile, strengthening scientific research and technological innovation in the field of marine carbon sequestration can also promote the healthy development of the marine economy and foster new industrial growth points. Therefore, based on the research foundations of scholars, we should strive to enhance the carbon sequestration capacity of marine shellfish and algae and promote high-quality marine development. This is one of the important measures for the Chinese government to fulfill its international commitments, address climate change, and balance the relationship between economic development and carbon emissions.

The spatial distribution of marine carbon sequestration in coastal provinces is not isolated. Differences in marine environments, aquaculture areas, fisheries policies, aquaculture species, and aquaculture technologies among provinces lead to strong spatial correlations in marine carbon sequestration capacity [4]. Marine fisheries carbon sequestration is an important component of marine carbon sequestration, referring to the processes and mechanisms that directly or indirectly reduce carbon dioxide concentrations in the atmosphere through marine fisheries production activities. Shellfish and algae aquaculture, as a major category within marine fisheries, play a crucial role in enhancing marine carbon sequestration capacity [5]. As the marine carbon sequestration of one province can directly or indirectly affect the marine carbon sequestration capacity of neighboring provinces through transmission mechanisms such as factor mobility, technology spillover, and policy diffusion, there exists a spatial spillover effect. In summary, the spatial correlation and spillover effects of marine carbon sequestration are of great significance for the high-quality development of marine industries in coastal provinces and the effective protection of regional marine environments. Scientifically and accurately measuring the carbon sink capacity of marine shellfish and algae is crucial for addressing global warming and deepening carbon neutrality goals. The main objective of this paper is to calculate the carbon sequestration capacity of marine shellfish and algae in each coastal province, analyze the main influencing factors, and put forward practical countermeasures and suggestions, so as to make a significant contribution to alleviating the problem of global climate change.

2. Literature Review

2.1. Research on the Theory of Marine Carbon Sequestration

In 2009, Nellemann et al. defined the carbon captured and stored by mangroves, seagrass beds, and salt marshes in coastal zones as ocean carbon sequestration [6]. Li Jing et al. believed that ocean carbon sequestration is an important mechanism in the global carbon cycle, which utilizes the ocean to fix and store carbon dioxide [7]. Tang Qisheng et al. considered marine fishery carbon sequestration as the process and mechanism by which aquatic organisms absorb or utilize greenhouse gases such as carbon dioxide in the water, and then remove the carbon that has been converted into biological products from the water or cause it to settle to the bottom through biological precipitation [5]. It is an important component of blue carbon sequestration. If these carbon products are reused or stored, the result will enhance the ability of water bodies to absorb and store atmospheric carbon dioxide, thereby better exerting the carbon sequestration function. Hamilton et al. conducted research on the carbon sequestration role of mussels and cyanobacteria in various aquatic environments, suggesting that mussels can facilitate carbon cycling in these waters [8]. Raquel et al. approached the analysis of the pivotal role of eukaryotic microorganisms in marine carbon sequestration from the perspective of “ribosome active” eukaryotes in marine sediments, and posited that marine sediments represent one of the largest sources of carbon sequestration on Earth [9]. Scholar Liu Mengyao also pointed out that shellfish farming can provide a large amount of high-quality protein for people’s livelihoods, possessing a dual effect of carbon sequestration and protein supply. Meanwhile, filter-feeding shellfish influence the offshore carbon cycle through their own growth, metabolic activities, and biological deposition [10].

2.2. Research on Marine Carbon Sequestration Accounting

Due to the complexity of the marine ecological environment and the differences in climate across different sea areas, the Chinese marine carbon sink verification system still needs further improvement [11]. Currently, the main methods for measuring marine fisheries carbon sinks include the weighing method, indoor incubation method, carbon footprint method, and sea–air interface carbon dioxide flux estimation method, which are primarily used to estimate the carbon sink generated by shellfish and algae aquaculture [12]. Yue Dongdong et al. calculated the carbon sink generated by marine aquaculture shellfish and analyzed the relationship between the yield of marine aquaculture shellfish and the resulting carbon sink quantity in nine coastal provinces of China [13]. Based on the principles of algae aquaculture carbon sink accounting, Yue Dongdong designed three different algae aquaculture scenario models to study the relationship between algae yield and carbon sink [14]. Shao Guilan et al. studied the carbon balance status of marine fisheries in Shandong Province by accounting for the carbon emissions of marine fishing and the carbon sink of marine shellfish and algae aquaculture. According to the difference between carbon emissions from marine fishing and carbon sinks from marine aquaculture shellfish and algae, the carbon balance status was classified into three categories: carbon surplus, carbon balance, and carbon deficit [13]. Turolla et al. estimated, using the life cycle assessment method, that 1 ton of clams can generate a net carbon sink of 54.50 kilograms [15]. Overall, existing research mainly focuses on the fishing of shellfish and algae, lacking carbon sink accounting methods for marine sediments and attached organisms. Therefore, the carbon sink capacity of marine fisheries is greater than the current estimated value.

2.3. Research on Factors Influencing Marine Shellfish and Algae Carbon Sequestration

With the development of the marine shellfish and algae carbon sequestration field, most existing research is based on the concept of “removable carbon sequestration” and models to estimate marine fishery carbon sequestration. Relevant research indicates that the annual carbon sequestration capacity of macroalgae in continental shelf areas can reach 700 million tons, accounting for approximately 35% of the global ocean’s annual net carbon sequestration. Scholar Erlania employed a combined approach of eDNA metabarcoding and sampling of seabed sediments to confirm the presence of macroalgal carbon in nearshore coastal sediments off southeastern Australia [16]. After estimating the carbon sequestration capacity of mariculture, Shao Guilan and her colleagues used the Logarithmic Mean Divisia Index (LMDI) model to analyze it and concluded that aquaculture scale and structure are the main factors influencing carbon sequestration capacity [17]. Sun Kang and his team employed the LMDI decomposition method to analyze the factors influencing marine fishery carbon sequestration capacity across coastal provinces in China, finding that scale effect is the most significant factor [18]. REN, based on estimates of marine fishery carbon sequestration, utilized the LMDI model to investigate the factors influencing marine fishery carbon sequestration capacity in Chinese coastal provinces [19]. Xu Jingjun and others, based on the results of “removable carbon sequestration” calculations, further explored the significant impact of fishery output value, labor input, and fishery disaster-affected areas on the carbon sequestration of marine aquaculture [20].

In summary, current research primarily focuses on theoretical studies, capacity estimates, and factor analysis of marine carbon sequestration. The spatial effects of marine shellfish and algae carbon sequestration, which mainly consists of shellfish and algae carbon sequestration (excluding fish and crustaceans as they are not within the scope of non-fed fisheries), remain unclear. Therefore, this paper focuses on the representative shellfish and algae carbon sequestration. Based on data from nine coastal provinces in China between 2007 and 2021, this paper analyzes the regional differences and temporal variations in carbon sequestration capacity for marine shellfish and algae. Spatial models are employed to assess the presence of spatial correlation among samples, and an in-depth study is conducted on their spatial effects and influencing factors. The aim is to promote a virtuous cycle in regional marine ecological environments, advance marine ecological civilization construction, achieve coordinated development of marine shellfish and algae aquaculture across regions, and provide a reference for formulating differentiated policies for marine carbon sequestration development.

3. Material and Methods

Conducting an assessment of marine fishery carbon sequestration capacity is an important precursor to enhancing carbon sequestration in marine fisheries. To gain a deeper understanding of the distribution pattern and evolution of marine fishery carbon sequestration capacity, this paper employs a highly accurate and operational material quantity assessment method to estimate marine fishery carbon sequestration capacity.

3.1. Measurement of Carbon Sequestration Capacity of Marine Shellfish and Algae

With reference to the research by Shao Guilan and others, as well as Sun Kang and others, this paper utilizes the material quantity assessment method, based on the relationship between marine organism’s “carbon sequestration coefficient–production–carbon sequestration amount,” to calculate the carbon sequestration capacity of marine shellfish and algae in coastal provinces. The calculation steps for the carbon sequestration capacity of marine shellfish and algae are shown in

Table 1. Measurement method for carbon sequestration capacity of marine shellfish and algae.

Category	Calculation Method for Carbon Sequestration Capacity
Shellfish	Shellfish carbon sink capacity = Shell carbon sink capacity + Soft tissue carbon sink capacity
	Shell carbon sink capacity = Shellfish production × Dry-wet ratio × Shell specific gravity × Shell carbon sink coefficient
	Soft tissue carbon sink capacity = Shellfish production × Dry-wet ratio × Soft tissue specific gravity × Soft tissue carbon sink coefficient
Algae	Algae carbon sink capacity = Algae production × Dry-wet ratio × Algae carbon sink coefficient
Total amount	Marine fisheries carbon sink capacity = Shellfish carbon sink capacity + Algae carbon sink capacity

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3.2. Data Sources

This paper selects the marine shellfish and algae carbon sequestration capacity of nine coastal provinces as the research object (Tianjin and Shanghai are not included in the research scope due to their limited marine fishery aquaculture activities and zero data recorded in the “China Fishery Statistical Yearbook”). The data used in this paper are all sourced from the “China Fishery Statistical Yearbook” and “China Fishery Yearbook” spanning from 2007 to 2021.

3.3. Descriptive Statistics

Descriptive statistics are used to describe the degree of difference or fluctuation among data. A larger standard deviation indicates greater variations in the quantities of shellfish and algae across different years and regions. Such variations may be attributed to factors such as climate, environment, and local policies implemented, which hold research and exploration value. If the variations are smaller, the research significance is also relatively low. According to

Table 2. Descriptive statistical analysis of shellfish algae in different regions.

Province	Hebei	Liaoning	Jiangsu	Zhejiang	Fujian	Shandong	Guangdong	Guangxi
Mean standard error	10,743.46	91,831.13	13,893.51	23,185.79	267,590.72	185,296.42	452,272.99	18,370.18
Median	600.00	115,725.00	300.00	27,932.00	658.00	690.00	282,385.00	727.00
Standard deviation	18,608.23	159,056.18	24,064.26	40,158.97	463,480.73	320,942.82	783,359.80	31,818.08
Skewness	1.73	1.73	0.76	1.73	1.73	1.73	1.73	1.73
Deviation standard error	1.23	1.23	1.23	1.23	1.23	1.23	1.23	1.23

,through analysis, it is found that the standard deviation values for different regions are relatively large, with the standard deviation for shellfish and algae in Guangdong Province reaching 783,359.80. The deviation standard error can effectively represent the overall deviation, with a larger value indicating lower reliability and vice versa. Through descriptive analysis, it is concluded that the deviation standard error for different regions is 1.23, suggesting a high overall reliability of the data, which in turn indicates the significant research value of this paper.

3.4. Calculation Results and Analysis of Carbon Sink Capacity in Marine Shellfish and Algae

3.4.1. Analysis of the Temporal Changes in Chinese Marine Shellfish and Algae Carbon Sink Capacity

In Tianjin, there are traditional economic shellfish and algae species in marine shellfish and algae aquaculture, such as Rapana venosa and Crassostrea gigas. However, due to coastal urbanization development and excessive fishing, the resources have declined. In Shanghai, a large-scale sterile seaweed ecological restoration demonstration area has been established for marine aquaculture. Seaweeds that are tolerant to low salinity and low light are selected for ecological restoration, and the marine ecological environment is improved through multiple combined approaches. In view of the fact that the data by year in Tianjin and Shanghai are severely lacking, they have not been included in the actual research scope.

The temporal variation trend of Chinese marine shellfish and algae carbon sink capacity from 2007 to 2021 is shown in Figure 1. Although there were certain fluctuations in the carbon sink capacity of the nine coastal provinces in China, it still showed an overall upward trend. From 2007 to 2009, it first rose and then fell, reaching the lowest value in 2009. From 2010 to 2021, it showed a continuous upward trend. The reason for this is that in 2009, China experienced over a hundred occurrences of storm surges, waves, sea ice, red tides and other marine disasters, which had a significant impact on marine shellfish and algae, thus affecting the marine shellfish and algae carbon sink capacity of coastal provinces. In 2010, China was transitioning from the “11th Five-Year Plan” to the “12th Five-Year Plan”, and the release of the “National Marine Economic Development ‘12th Five-Year Plan’” played a positive role in promoting the development of Chinese marine shellfish and algae, resulting in a rapid increase in carbon sink capacity. During the “12th Five-Year Plan” period from 2011 to 2015, the state paid high attention to the “three rural issues” and implemented high-investment policies for marine shellfish and algae. Through increasing fixed asset investment in marine shellfish and algae, diesel subsidies for fishery, and subsidies for fishery resource protection, the scale of marine fishery shellfish and algae aquaculture expanded rapidly, promoting a rapid increase in marine shellfish and algae carbon sink capacity. From 2016 to 2020, that is, after entering the “13th Five-Year Plan” period, the growth rate of Chinese marine shellfish and algae carbon sink capacity slowed down significantly. In some areas, the density of aquaculture was too high, approaching the limit that the marine environment could bear. The coastal water environment deteriorated day by day, and various disasters occurred frequently, posing a huge threat to the sustainable development of regional marine shellfish and algae. At the same time, the government began to adjust its policy orientation. According to the “National Marine Economic Development ‘13th Five-Year Plan’”, during the “13th Five-Year Plan” period, China will focus on promoting the optimization and upgrading of marine industries, especially traditional marine industries such as marine shellfish and algae, and implement a policy of negative growth in nearshore fishing production. Therefore, the growth rate of marine shellfish and algae carbon sink capacity was relatively slow.

3.4.2. Analysis of Regional Distribution of Carbon Sink Capacity in Chinese Marine Shellfish and Algae

As seen in Figure 2, the carbon sink capacity of Chinese marine shellfish and algae displays uneven regional distribution, with Fujian and Shandong having higher average capacities, while Hainan and Hebei have lower capacities. Fujian is a province with rapid development in marine fisheries. Adjacent to Southeast Asia, it possesses unique fisheries resources and a high level of openness to the outside world and enjoys numerous national preferential policies. These favorable social development foundations provide advantageous conditions for the development of marine shellfish and algae. Shandong is a traditional major province in marine shellfish and algae, with vast mariculture areas and strong scientific and technological expertise. In recent years, Shandong has achieved remarkable results in pollution control in its maritime areas, significantly improving the quality of the marine environment. Aquaculture technology and models have been continuously innovated, and aquaculture production has continued to increase, promoting the green development of marine shellfish and algae and achieving sustainable development. Hainan, despite belonging to the region with optimized development of marine shellfish and algae, has obvious disadvantages in fisheries resource endowment compared to other coastal provinces, resulting in a relatively low level of marine shellfish and algae development. Hebei is located in the industrially developed Bohai Rim region. Although it possesses marine resources, the scale of its marine shellfish and algae is relatively small, and the structure of the fisheries industry needs to be improved. It has a strong dependence on traditional fisheries models, and prominent marine environmental pollution problems. These factors collectively contribute to the low level of marine shellfish and algae development in Hebei.

4. Analysis of Spatial Effects on Marine Shellfish and Algae Carbon Sink Capacity

Based on the measurement of marine shellfish and algae carbon sink capacity, this paper further analyzes the spatial effects of Chinese marine shellfish and algae carbon sink capacity and deeply reveals the key influencing factors of marine shellfish and algae carbon sink capacity.

4.1. Specification of Spatial Econometric Model

4.1.1. Spatial Correlation Test

The prerequisite for conducting spatial econometric analysis is to test for the existence of spatial correlation between variables. Spatial autocorrelation can reflect the agglomeration characteristics among geographical units. Most spatial econometric models choose Moran’s I as a common indicator for testing the correlation of spatial elements. Through this test, we can determine whether there is a spatial relationship in the marine shellfish and algae carbon sink capacity among the nine coastal provinces.

$${\mathrm{Moran}}^{\prime }\mathrm{s} I={\displaystyle \frac{n{{\displaystyle \sum }}_{i=1}^n{{\displaystyle \sum }}_{j=1}^n{W}_{ij}\left(X_i-\overline{X}\right)\left(X_j-\overline{X}\right)}{{{\displaystyle \sum }}_{i=1}^n{{\displaystyle \sum }}_{j=1}^n{W}_{ij}{\left(X_i-\overline{X}\right)}^2}}$$

The value of Moran’s I is usually in the range of [−1, 1]. When the value is less than 0, it indicates that there is a negative spatial correlation in marine shellfish and algae carbon sink capacity; when it equals 0, it indicates that there is no spatial correlation in marine shellfish and algae carbon sink capacity; and when it is significantly greater than 0, it suggests that there is a positive spatial correlation in marine shellfish and algae carbon sink capacity.

4.1.2. Construction of Spatial Econometric Mode

Spatial econometric models primarily include the spatial autoregressive model (SAR), Spatial Error Model (SER), and spatial Durbin model (SDM). The spatial autoregressive model (SAR) directly reflects the spatial autocorrelation of the dependent variable, meaning that the dependent variable in one region is influenced by that of its neighboring regions. The Spatial Error Model (SEM) indirectly captures spatial effects through the spatial correlation of error terms, implying a correlation among the unexplained error terms in neighboring regions. In addition to these functions, the spatial Durbin model (SDM) places greater emphasis on spatial spillover effects, which can reflect the interdependent relationships of the dependent variable among neighboring regions. In the context of this paper, the SDM not only considers the impact of explanatory variables within the same province on the carbon sequestration capacity of marine shellfish and algae but also takes into account the influence of lagged explanatory variables and the lagged carbon sequestration capacity of marine shellfish and algae in neighboring provinces. To explore the influencing factors of the carbon sequestration capacity of marine shellfish and algae as well as the spatial spillover effects of these factors, we have established an SDM tailored to the carbon sequestration capacity of marine shellfish and algae.

$$\begin{gathered} CS{C}_{it}=\rho {\displaystyle \sum }_{j=1}^n{\omega }_{ij}CS{C}_{jt}+{\beta }_{1}D{I}_{it}+{\beta }_{2}L{I}_{it}+{\beta }_{3}C{A}_{it}+{\beta }_{4}D{T}_{it}+{\beta }_{5}ST{P}_{it} \\ +{\beta }_{6}ST{I}_{it}+{\theta }_1{\displaystyle \sum }_{j=1}^n{\omega }_{ij}D{I}_{jt}+{\theta }_2{\displaystyle \sum }_{j=1}^n{\omega }_{ij}L{I}_{jt}+{\theta }_3{\displaystyle \sum }_{j=1}^n{\omega }_{ij}C{A}_{jt} \\ +{\theta }_4{\displaystyle \sum }_{j=1}^n{\omega }_{ij}D{T}_{jt}+{\theta }_5{\displaystyle \sum }_{j=1}^n{\omega }_{ij}ST{P}_{jt}+{\theta }_6{\displaystyle \sum }_{j=1}^n{\omega }_{ij}ST{I}_{jt} \end{gathered}$$

In the equation, $i$ and $j$ represent provinces, $t$ represents time, CSC is the explained variable representing carbon sink capacity, DI stands for the development level of marine shellfish and algae, $LI$ represents labor input in marine shellfish and algae, CA denotes the culturing area of marine shellfish and algae, DT signifies disaster situations in marine shellfish and algae, STP indicates the promotion of marine shellfish and algae science and technology, and STI represents the investment in marine shellfish and algae science and technology.

4.2. Selection of Influencing Factors

In addition to spatial spillover effects, there are other influencing factors that contribute to marine shellfish and algae carbon sink capacity. Drawing on the research by Xu Jingjun et al. [21], Ji Jianyue et al. [22], and Zhang Lixi et al. [23], the following variables have been selected: the development level of marine shellfish and algae, labor input in marine shellfish and algae, the culturing area of marine shellfish and algae (CA), disaster situations in marine shellfish and algae, the promotion of marine shellfish and algae science and technology (STP), and investment in marine shellfish and algae science and technology (STI). Specific explanations of these variables are provided in

Table 3. Selection and explanation of factors influencing the carbon sink capacity of marine fisheries.

Name of Influencing Factors	Explanation
Level of Marine Shellfish and Algae Development	Total Economic Output of Mariculture (in CNY 10,000)
Aquaculture Output of Marine Shellfish and Algae	Aquaculture Production (in Tonnes)
Aquaculture Area of Marine Shellfish and Algae	Aquaculture Area (in Hectares)
Labor Input in Marine Shellfish and Algae	Number of Employees in Marine Shellfish and Algae (in Persons)
Investment in Marine Shellfish and Algae Science and Technology	Marine Shellfish and Algae Aquaculture Technology Promotion Institutions
Promotion Status of Marine Shellfish and Algae Science and Technology	Promotion Funding for Marine Shellfish and Algae Aquaculture Technology (in CNY 10,000)
Marine Shellfish and Algae Disasters	Affected Aquaculture Area in Marine Shellfish and Algae (in Hectares)

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4.3. Results and Analysis of Spatial Effects on Marine Shellfish and Algae Carbon Sink Capacity

4.3.1. The Results of Spatial Correlation Test

In this paper, Stata Statistical Software, Release 15 was used to calculate the global Moran’s I for marine shellfish and algae carbon sink capacity in nine coastal provinces of China from 2007 to 2021, with the results shown in Figure 3. The results in Figure 3 indicate that the global Moran’s I for carbon sink capacity from 2007 to 2021 has always been positive, with some local fluctuations, but an overall upward trend. This suggests that marine shellfish and algae carbon sink capacity exhibits positive spatial correlation and spatial agglomeration, but the degree of spatial agglomeration is unstable.

To further observe the degree of spatial agglomeration of marine shellfish and algae carbon sink capacity, a scatter plot of local Moran’s I for 2021 was created, as shown in Figure 4. The four quadrants of the Moran’s I scatter plot represent four spatial correlation patterns: “High–High (H–H)”, “Low–High (L–H)”, “Low–Low (L–L)”, and “High–Low (H–L)”. From Figure 4, it can be seen that Zhejiang and Fujian are in the first quadrant. Both provinces have suitable mariculture environments and experience relatively minor impacts from climate change on marine shellfish and algae. Additionally, their mariculture models and technologies are relatively advanced, resulting in a spatial agglomeration trend of mutual promotion and development, belonging to the High–High (H–H) cluster. Liaoning and Jiangsu are in the second quadrant, belonging to the Low–High (L–H) cluster. Shandong, Guangdong, Guangxi, and Hainan are in the third quadrant. Although these provinces have good mariculture foundations, their marine shellfish and algae carbon sink potential is limited, leading to a negative spatial agglomeration trend and belonging to the Low–Low (L–L) cluster. Hebei is the only province in the fourth quadrant, belonging to the High–Low (H–L) cluster. Coastal provinces are mainly concentrated in the first and third quadrants, indicating that there is a spatial correlation of “High–High and Low–Low agglomeration” in marine shellfish and algae carbon sink capacity among coastal provinces.

4.3.2. Analysis of Regression Results from the Spatial Durbin Model

The specific form of the spatial econometric model was identified through Hausman, LR, and Wald tests, with the results shown in

Table 4. Hausman, LR, and Wald test results.

Type of Test	Statistical Value	p-Value
Hausman	30.83	0.00
LR-lag	35.02	0.00
LR-error	48.30	0.00
Wald-lag	212.63	0.00
Wald-error	127.43	0.00

. The Hausman test result was 30.83, which was significant at the 1% level, indicating that a fixed effect should be used. Both the LR and Wald tests were significant at the 1% level, suggesting that the spatial Durbin model cannot be reduced to a Spatial Lag Model or a Spatial Error Model. Therefore, this paper selects the spatial Durbin model with fixed effects.

The regression results of the spatial Durbin model are shown in

Table 5. Regression results of SDM mode.

Variable	SDM	Spatial Effect	Variance
lnDI	0.137 (0.50)
lnAP	0.930 *** (0.00)
lnCA	−0.143 (0.49)
lnLI	0.100 (0.70)
lnSTI	0.360 *** (0.00)
lnSTP	−0.057 (0.52)
lnDT	0.040 (0.15)
rho		0.254 *** (0.00)
sigma2_e			0.053 *** (0.00)

Note: The superscript *** indicates significance at the 1% confidence level.

. The spatial autoregressive coefficient (rho) for marine shellfish and algae carbon sink capacity is 0.254 and is significant at the 1% level, indicating that there is a significant positive spatial spillover effect in marine shellfish and algae carbon sink capacity among coastal provinces. In other words, an increase in carbon sequestration in neighboring provinces will have a positive impact on the carbon sequestration capacity of the province in question, and an improvement in the carbon sequestration capacity of the province in question will also positively affect the carbon sequestration capacity of neighboring provinces. Coastal provinces are not isolated entities; with the exchange of material and non-material resources such as policy impacts, resource transfers, and knowledge diffusion, coupled with increasing transportation convenience, the relationships between provinces have become increasingly close. These intricate connections determine that a province’s marine shellfish and algae carbon sink capacity cannot exist independently and is often influenced by surrounding provinces. Furthermore, marine resource elements have their uniqueness and are more mobile in economic activities, which provides sufficient conditions for the spatial spillover of marine shellfish and algae carbon sink capacity. Therefore, to enhance the overall marine shellfish and algae carbon sink capacity, joint governance of coastal provinces is required.

The regression coefficients for marine shellfish and algae aquaculture production and technological investment are significantly positive, while other factors are not significant. This indicates that aquaculture production and technological investment have a significant positive effect on improving marine shellfish and algae carbon sink capacity. There is a direct relationship between marine shellfish and algae aquaculture production and carbon sequestration capacity, and an increase in aquaculture production will significantly enhance carbon sequestration capacity. Increased technological investment enables marine shellfish and algae to achieve greater output with fewer production factor inputs, thereby improving carbon sequestration capacity.

4.3.3. Analysis of Spatial Spillover Effects

The spatial Durbin model employs maximum likelihood estimation and introduces spatial lag terms for explanatory variables, thus effectively addressing endogeneity issues. However, due to the non-linearity of the spatial Durbin model, its regression coefficients do not directly indicate the impact of variables on marine shellfish and algae carbon sink capacity. Therefore, on this basis, referring to the partial derivative matrix method proposed by Lesage et al. [24] the impact is decomposed into direct effects, indirect effects, and total effects. The direct effect represents the impact of each influencing factor on the marine shellfish and algae carbon sink capacity of the province itself; the indirect effect, or spatial spillover effect, represents the impact on neighboring provinces’ carbon sequestration capacity; and the total effect is the sum of the two. The regression results for the direct, indirect, and total effects of various influencing factors on marine shellfish and algae carbon sink capacity are shown in

Table 6. Decomposition results of spatial spillover effects of marine carbon sinks.

Variable	Direct Effect	Indirect Effect	Total Effect
lnDI	0.113	0.580 *	0.693
lnAP	1.617 ***	1.185 ***	2.802 ***
lnCA	0.150	−0.227	−0.077
lnLI	−0.847 ***	−2.140 ***	−2.987 ***
lnSTI	0.277 *	0.880 ***	1.157 ***
lnSTP	0.245 ***	0.349 **	0.595 ***
lnDT	0.071 *	0.017	0.088

Note: The superscripts ***, **, and * indicate significance at the 1%, 5%, and 10% confidence levels, respectively.

.

4.3.4. Discussion

Based on the direct impact assessments of various factors, it is evident that marine shellfish and algae aquaculture production (AP), marine shellfish and algae scientific and technological investment (STI), marine shellfish and algae scientific and technological promotion (STP), and marine shellfish and algae disasters (DT) exert notable positive influences. Conversely, marine shellfish and algae labor input (LI) demonstrates a significant negative effect, while other factors yield insignificant impacts. An augmentation in marine shellfish and algae aquaculture production directly enhances carbon sequestration capacity. Furthermore, bolstered marine shellfish and algae scientific and technological investment and promotion propel advancements in aquaculture technology, which in turn augment marine shellfish and algae carbon sequestration capacity. Since marine shellfish and algae disasters exert minimal impact on shellfish and algae aquaculture, their production remains unhampered, ultimately not impeding the enhancement of carbon sequestration capacity. An uptick in marine shellfish and algae labor input does not necessarily signify an increase in personnel engaged in shellfish and algae aquaculture; hence, this rise in input does not contribute positively to improving carbon sequestration capacity. The indirect impact assessments reveal that the development level of marine shellfish and algae (DI), marine shellfish and algae aquaculture production (AP), marine shellfish and algae scientific and technological investment (STI), and marine shellfish and algae scientific and technological promotion (STP) exert significant positive influences. In contrast, marine shellfish and algae labor input (LI) demonstrates a notable negative effect, with other factors yielding insignificant impacts. The impact of labor input on the carbon sink capacity of marine shellfish and algae is not linear or direct but is constrained by multiple factors. On the one hand, if the increase in labor does not come with technological progress or management optimization, it may lead to stagnation or even a decline in production efficiency, as well as diminishing marginal returns in resource allocation, thereby undermining carbon sink efficiency. On the other hand, high labor input may inhibit the adoption of advanced technologies, increase management complexity, and result in delayed decision-making or resource waste. These factors indirectly reduce the technological content of the aquaculture system and its potential for enhancing carbon sink capacity, thus having a complex and potentially negative indirect impact on overall carbon sink capacity. An escalation in the development level of marine shellfish and algae and aquaculture production creates a demonstrative impact on neighboring provinces, leading to an improvement in marine shellfish and algae carbon sequestration capacity in adjacent areas and resulting in a positive spatial indirect effect. Marine shellfish and algae scientific and technological investment and promotion exhibit positive spatial spillover effects, indicating that the dissemination of information, technology, and other factors benefits adjacent provinces through technological advancements and experience sharing. This fosters a “free-rider” effect, enabling “technology sharing” among coastal provinces. Conversely, an increase in marine shellfish and algae labor input does not enhance carbon sequestration capacity but may induce imitation behavior in neighboring provinces, leading to a negative spatial indirect effect.

5. Conclusions and Recommendations

Based on marine shellfish and algae data from nine coastal provinces in China from 2007 to 2021, this paper calculates their carbon sequestration capacity using the material quantity assessment method, identifies spatial correlations in marine shellfish and algae carbon sequestration capacity through Moran’s I test, and further explores its influencing factors and spatial spillover effects using the spatial Durbin model. The main research conclusions are as follows:

(1)

In terms of the temporal and spatial dynamics of the carbon sequestration capacity of marine shellfish and algae, from 2007 to 2009, the carbon sequestration capacity of the nine coastal provinces in China showed a trend of first increasing and then decreasing, and from 2010 to 2021, it continued to grow. This fluctuation is in line with the global goals of marine ecosystem restoration and sustainable management. At the spatial distribution level, there are significant differences in the carbon sequestration capacity among provinces. Regions such as Fujian and Shandong have relatively high average carbon sequestration capacities, while Hainan and Hebei are relatively low. This characteristic reflects the common problem of uneven distribution of marine carbon sink resources on a global scale. It also indicates that strategies for enhancing marine carbon sinks need to be formulated according to local conditions to conform to the overall plan of international marine carbon sink capacity building.

(2)

The study of spatial effects shows that there is a positive spatial correlation in the carbon sequestration capacity of marine shellfish and algae in China’s coastal provinces, but the degree of agglomeration fluctuates, presenting the characteristics of “high–high agglomeration and low–low agglomeration”. This is consistent with the international research conclusions on the spatial correlation of marine ecosystems, suggesting that when formulating marine carbon sink policies, full consideration should be given to regional synergistic effects, inter-regional cooperation should be strengthened, and an efficient marine carbon sink network should be promoted to enhance the overall marine carbon sequestration capacity and achieve the marine carbon sink goals under the United Nations Framework Convention on Climate Change.

(3)

The analysis of influencing factors shows that factors such as the aquaculture output of marine shellfish and algae, scientific and technological investment, promotion, and disaster response have a positive direct impact on the carbon sequestration capacity, while labor input has a negative direct effect. At the same time, the development level, aquaculture output, scientific and technological investment, and promotion have a positive indirect impact, while labor input has a negative indirect impact. These findings are highly consistent with the international policy orientation of promoting the development of marine carbon sinks through scientific and technological innovation and industrial upgrading. They emphasize the importance of optimizing resource allocation, increasing investment in scientific and technological research and development and promotion, and improving the industrial development level in enhancing the marine carbon sequestration capacity, providing practical evidence for realizing the release of global marine carbon sink potential and the goal of carbon neutrality.

Based on the above research conclusions, the following suggestions are proposed to promote coordinated development among coastal provinces and enhance marine shellfish and algae carbon sequestration capacity:

(1)

Optimize the structure of marine fisheries aquaculture species and increase shellfish and algae aquaculture production. Explore the value of marine fisheries aquaculture species by extending product chains and developing derivative products, increase the demand for shellfish and algae products, optimize the structure of marine aquaculture species, promote the expansion of marine fisheries carbon sequestration, and enhance the carbon sequestration function of marine fisheries. Vigorously carry out technical training and promotional activities. Organize aquaculture farmers to attend professional training courses, and invite industry experts to systematically explain knowledge such as aquaculture techniques for shellfish and algae, key points of variety breeding, and strategies for disease and pest control. Meanwhile, establish demonstration bases for aquaculture technology to showcase intuitive aquaculture models and management experiences, providing learning examples for aquaculture farmers, and widely disseminate aquaculture technologies through multiple online and offline channels.

(2)

Increase funding and improve aquaculture technology. The government should increase funding for research and projects to promote the improvement of aquaculture technology. Meanwhile, coastal provinces should strengthen technical exchanges and focus on scientific and technological innovation in marine fisheries aquaculture to increase marine fisheries production and output value. Strengthen the cooperation among domestic scientific research institutions, universities, and enterprises, establish a collaborative innovation mechanism integrating production, education, research, and application, and jointly carry out the research, development, and application promotion of aquaculture technologies. Actively engage in technical exchanges and cooperation with advanced international aquaculture institutions and enterprises, introduce foreign advanced technologies, equipment, and management experiences, and conduct digestion, absorption, and re-innovation. Establish an aquaculture technology promotion service system, improve the three-level (county, township, and village) technology promotion network, equip it with professional technical personnel to provide in-depth technical guidance and training at aquaculture bases, construct demonstration parks for aquaculture technologies to showcase advanced technological achievements, and enhance the enthusiasm of aquaculture farmers in applying technologies.

(3)

Strengthen cooperation among provinces and fully leverage marine advantages. In response to the current situation of marine fisheries carbon sequestration capacity development in China, promote the flow of factors among coastal provinces, achieve coordinated development and common progress, fully leverage Chinese marine advantages, and contribute marine power to achieving the “carbon neutrality” goal.