The Distribution Characteristics and Influencing Factors of the CO₂ Fluxes Across the Sea–Air Interface in the Artificial Reefs Area of Sanheng Island

Abstract

In order to study the impact of artificial reef construction on marine carbon sinks, a one-year monitoring and analysis were carried out in the Sanheng artificial reef areas of Shengsi Island. The main parameters observed were sea surface temperature, salinity, pH, total alkalinity (TA), and dissolved inorganic carbon (DIC). The partial pressure of CO₂ (pCO_2w) in the surface water and the CO₂ flux (FCO₂) across the sea–air interface were calculated and analyzed. The results showed that the annual range of surface water pCO_2w was 34.48~501.53 μatm, and the partial pressure of CO₂ decreases in the following order: winter, spring, autumn, summer. The FCO₂ was significantly negatively correlated with temperature and significantly positively correlated with salinity and TA. The FCO₂ in the four seasons decreases in the following order: winter, spring, autumn, summer. The annual sea–air flux in the survey area ranged from −108.79 to 41.74 mmol m⁻² d⁻¹. In winter, both the reef area and the control area displayed positive FCO₂ values, indicating a source of CO₂, while in the other three seasons, CO₂ flux values were negative, indicating an overall CO₂ sink. Additionally, in autumn, the reef area exhibited a stronger CO₂ sink than the control area. The results indicated that the construction of artificial reefs has a certain carbon sink effect, with no significant difference in effectiveness between different artificial reef construction models.

1. Introduction

Indeed, oceans cover 71% of the Earth’s surface and play a crucial role as an essential sink for carbon dioxide (CO₂). It is estimated that oceans absorb approximately 20% to 40% of the CO₂ emitted by human activities during the industrial era [1]. Jiao [2] pointed out that implementing “negative emissions” in the ocean is a key pathway to achieving carbon neutrality, and there is increasing reporting on the contribution of the oceanic carbon cycle by the biologically driven pump centered around marine organisms.

By expanding biological carbon sequestration through fisheries production and enhancing the ocean‘s role in the carbon cycle, it is crucial to leverage the ocean’s capacity for climate regulation [3]. As an environmentally friendly method of marine fisheries production, marine ranching creates habitats for aquatic organisms through engineering techniques such as deploying artificial reefs, constructing algae reefs and seaweed beds, and stock enhancing and releasing fish stocks [4,5,6]. This initiative not only enhances the biodiversity and resource abundance of marine ranches, but also increases carbon sequestration by marine organisms in the sea and extends the transfer of carbon within the food chain [7,8,9]. Macroalgae are the dominant primary producers in the coastal zone with a global net primary production (NPP) of 1521 TgC yr⁻¹ [10].

With the continuous rise in global CO₂ concentration and the ongoing degradation of marine ecosystems, countries around the world are increasingly emphasizing the need to enhance biological carbon sinks to reduce CO₂ levels and protect marine ecosystems. In 2009, blue carbon was first proposed in a joint report of the United Nations Environment Program; Food and Agriculture Organization of the United Nations (FAO); and Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific and Cultural Organization (IOC–UNESCO) [11].

In around 2010, the Chinese scholar in [12] first proposed the concept of “carbon sink fisheries”, which has garnered significant attention and gradually gained acceptance worldwide in recent years [13]. In 2014, China officially launched the “Chinese Blue Carbon Initiative” [14]. China is currently the largest developing country and the largest carbon emitter globally. Accelerating The pace of China’s emission reduction is of great positive significance to the world’s response to climate change [15,16]. In response, China proposed the concept of the ‘3060 target’ in September 2020, in which regional carbon emissions will strive to peak by 2030 and peak again by 2060. This highlights the urgent need to strengthen carbon reduction in China [17,18].Current research on CO₂ exchange flux (FCO₂) at the sea–air interface is mainly focused on the open ocean and the continental shelf margins where human activities are concentrated [19]. The East China Sea has complex biogeochemical conditions [20]. Ye [21] conducted shipboard observations to explore the surface CO₂ partial pressure (pCO_2W) and distribution characteristics, as well as seasonal variations in the East China Sea region. They found that the CO₂ partial pressure in the East China Sea region exhibits differential characteristics of “coastal area—continental shelf area—offshore area”, with overall seasonal trends showing CO₂ uptake. Peng [22] studied the spatial and temporal variations in the CO₂ exchange flux at the sea–air interface in the Kaozhou oyster farming area and found that the peak farming season in the marine farming area is a CO₂ source, while the off-peak farming season overall acts as a weak CO₂ sink.

Fodrie [23] conducted research on the carbon source–sink functions of intertidal and subtidal oyster reefs and found that oyster reefs exhibit different source–sink characteristics in different distribution areas. Currently, there is limited research on the source–sink characteristics of artificial reefs, which are an important component of marine ranches.

Artificial reef areas, as nearshore artificially restored and regulated ecosystems, not only enhance the quality of aquatic products and protect the marine environment but also increase the carbon sequestration capacity of the sea area [5]. The ecological effects of reef construction and related technologies are continuously receiving attention. Coastal regions throughout China are actively engaging in the construction of artificial reefs. Although researchers both domestically and internationally have conducted extensive studies on marine biological carbon sequestration, there is relatively little research on the expansion of marine carbon sinks through artificial reefs [24,25].

Sanheng Island is located in Shengsi, Zhoushan, Zhejiang Province. As early as 2004, Shengsihas carried out the construction of artificial reefs, and the first artificial reef area is located in the sea area west of Ma’an Archipelago and southeast of Lvhua Island, known as Dongkushan. At the end of 2020, Shengsi Ma’an Archipelago Marine Ranch had cumulatively deployed 164,750 cubic meters of artificial reefs. The confirmed sea area of the ranching zone covers approximately 1060 hectares. This study takes the artificial reef zone and the control area of Sanheng Island in the Shengsi Ma’an Archipelago Marine Ranching as the research objects. It estimates and analyzes the seawater CO₂ partial pressure (pCO₂) and CO₂ exchange flux (FCO₂) from indicators such as sea surface temperature (SST), salinity (S), dissolved inorganic carbon (DIC), pH, and total alkalinity (TA), aiming to provide a reference for comprehensively evaluating the carbon sink effect of island and reef waters and the construction of artificial reefs.

2. Materials and Methods

2.1. Study Area and Sampling Sites

The main study area is the Sanheng-Dongku National Marine Ranch (Figure 1), where pCO_2W analysis was conducted in the artificial reef and control zones to explore the carbon sequestration effect of artificial reef areas.

2.2. Sample Collection and Processing

This study conducted sampling and monitoring of the artificial reef area and the control area in the Sanheng Marine Ranching area in summer and autumn of 2022, as well as winter and spring of 2023, respectively. On-site measurements were taken using a multiparameter water quality probe (SBE19plus, Sea-bird, Bellevue, Washington, America) to directly determine dissolved oxygen (DO), sea surface temperature (SST), salinity (S); pH was measured on-site using a Mettler pH meter, and Total Alkalinity (TA) was measured using the pH method. Water samples are collected on-site and filtered through glass fiber filters (Whatman GF/F). The filtered water samples are then transferred into brown glass bottles (pre-burned at 450 °C for 6 h), where a 0.02% saturated HgCl₂ solution is added to fix and store the samples in the dark [26]. Subsequently, the Dissolved Inorganic Carbon (DIC) is determined using the Total Organic Carbon Analyzer (TOC-L, Himadzu, Kyoto, Japan) [27].

2.3. Estimation of Carbon Flux

The estimation of carbon flux in this study is based on the relationship equation provided by Dickson and Miller [28]

This formula is applicable for estimating carbon flux in seawater with salinity ranging from 20 to 40 and SST ranging from 2 to 35 °C [28].

The conversion formula between pH_NBS (America National Bureau of Standards: ANSI) and pH_SWS (Potential of hydrogen) used in this study with the NBS scale is as follows [29]:

$${pH}_{SWS}={pH}_{NBS}+{p{K}^{\prime }}_{1SWS\left(M\right)}-{p{K}^{\prime }}_{1NBS\left(M\right)}$$

$${p{K}^{\prime }}_{1SWS\left(M\right)}={\displaystyle \raisebox{1ex}{$3670.7$}\!\left/ \!\raisebox{-1ex}{$T$}\right.}-62008+9.7944lnT-0.0118\times S+0.000116\times S^2$$

$${p{K}^{\prime }}_{1NBS\left(M\right)}=13.7201+0.031334\times T+{\displaystyle \raisebox{1ex}{$3235.76$}\!\left/ \!\raisebox{-1ex}{$T$}\right.}+0.000013\times T\times S-0.1032\times S^{0.5}$$

$$T\left(k\right)=t\left(°\mathrm{C}\right)+273.15$$

In the equation, T represents absolute temperature (K); S represents salinity; t represents temperature in degrees Celsius.

In this study, we assumed the CO₂ system to be in equilibrium in the artificial reef area. The pCO_2W is determined based on parameters such as temperature, salinity, pH, and total alkalinity (TA) of the seawater.

$${pCO}_2={\displaystyle \frac{CA\times {\alpha }_{H^{+}}^2}{{\alpha }_s\times k_1^{\prime }\times \left({\alpha }_{H^{+}}+2K_2^{\prime }\right)}}$$

$$CA=TA-{\displaystyle \frac{B_T\times K_B^{\prime }}{{\alpha }_{H^{+}}+K_B^{\prime }}}$$

$$\begin{array}{ll}\hfill {lnK}_B^{\prime }=& \left(-8966.90-2890.51S^{0.5}-77.942S+1.726S^{1.5}-0.0993S^2\right)/T\\ & +\left(148.0248+137.194S^{0.5}+1.62247S\right)\\ & -\left(24.4344+25.085S^{0.5}+0.247S\right)\times lnT+0.053105S^{0.5}\times T\end{array}$$

In the equation, CA represents carbonate alkalinity; ${\alpha }_{H^{+}}$ represents hydrogen ion activity; $k_1^{\prime }$ and $k_2^{\prime }$ are the apparent dissociation constants of carbonic acid; $K_B^{\prime }$ is the first apparent dissociation constant of boric acid; $B_T$ is the total boron concentration in seawater; and $B_T$ = 0.000416 (S/35) mol kg⁻¹. TA is in units of mmol dm³. The value of $K_B^{\prime }$ is calculated using the value provided by Dickson [30], and ${\alpha }_s$ is the solubility coefficient [mol kg⁻¹·atm⁻¹], calculated using the value provided by Weiss [31].

The FCO₂ at the air–sea interface refers to the net exchange of CO₂ between the atmosphere and the ocean per unit area per unit time, expressed in units of mmol m⁻²·d⁻¹. The calculation formula is as follows:

$${FCO}_2=k\times a_s\times ∆{pCO}_2=k\times a_s\times \left({pCO}_{2W}-{pCO}_{2air}\right);$$

$$k=0.31\times U_{10}^2\times {\left({\displaystyle \raisebox{1ex}{$S_c$}\!\left/ \!\raisebox{-1ex}{$660$}\right.}\right)}^{-0.5};$$

In the equations above, k (m s⁻¹)represents the gas transfer velocity, calculated using the formula provided by Wanninkhof [32] with a coefficient of 0.31; ${\alpha }_s$ (mol kg⁻¹ atm⁻¹) is the solubility coefficient of CO₂ in seawater; and ΔpCO₂ is the partial pressure difference of CO₂ between the surface seawater and the atmosphere. The atmospheric CO₂ partial pressure (pCO_2w) is based on the monitoring results from the Global Monitoring Laboratory-Earth System Research Laboratories of NOAA, and in this study, a value of 413.42 μatm is used for atmospheric CO₂ concentration. $U_{10}^2$(m s⁻¹) is the wind speed at 10 m above sea level, based on daily and monthly average wind speeds provided by Shengsi Meteorological Station (58472). Sc is the Schmidt coefficient, which is a function of temperature, Sc = 2073.1 − 125.62 T + 3.6276 T² − 0.043219 T³. When F > 0, it indicates seawater releasing CO₂ to the atmosphere; when F < 0, it indicates seawater absorbing CO₂ from the atmosphere.

2.4. Data Analysis

All data were expressed as mean ± standard error, and SPSS is used for correlation analysis. When p < 0.01, the difference is extremely significant; when p < 0.05, the difference is significant. Data visualization was carried out using ArcGIS 10.8.

3. Results and Analysis

3.1. Surface Seawater Environment Characteristics

The basic environment parameters of surface seawater in different seasons in the study area in

Table 1. Basic parameters of surface seawater in different seasons.

Season	Region	SST (°C)	Salinity (pSU)	pH	TA (mg/L)	DIC (mg/L)
Autumn	A	20.78	32.20	8.33	134.59	19.05
	B	20.98	32.75	8.24	132.97	19.13
	C	20.37	33.3	8.07	146.98	19.21
Winter	A	11.31	32.13	7.82	153.77	20.33
	B	11.15	32.15	7.81	152.95	20.01
	C	11.26	31.50	7.81	146.36	19.37
Spring	A	19.93	28.80	8.06	133.87	26.10
	B	19.35	29.10	8.04	136.52	22.00
	C	18.97	29.5	8.07	132.29	26.49
Summer	A	27.18	27.54	8.27	97.65	25.88
	B	27.48	28.84	8.25	87.94	22.55
	C	27.41	27.93	8.36	80.03	25.75

A—united artificial reefs (site: S1–S3); B—artificial seamount (site: S4, S5); C—control area (site: S6–S8).

. The annual SST in the study area ranges from 11.10 °C to 27.48 °C, with the highest temperatures in summer and the lowest in winter. The differences between the highest and lowest temperatures in the four seasons of summer, autumn, winter, and spring are 0.62 °C, 1.90 °C, 2.14 °C, and 1.90 °C, respectively.

The pH of surface seawater ranges from 7.80 to 8.41, with the highest pH in summer and the lowest pH in winter. From the perspective of distribution area, the overall performance is that the pH in the artificial reef is higher than that in the control area in autumn, while in other seasons, the pH in the control area is higher than that in the artificial reef area.

There are significant differences in total alkalinity (TA) of surface seawater among different seasons (p < 0.05). The highest values were observed in winter and the lowest in summer. Except for autumn, the performance in the artificial reef was consistently greater than in the control area. Additionally, the TA differences between united artificial reefs and artificial seamount were not significant.

The entire study area showed a relatively small range of salinity variations. However, there were significant seasonal differences, with the highest salinity in winter and the lowest in summer. Except for autumn, the salinity in the artificial reef area was generally higher than the control area. Additionally, the difference in TA (total alkalinity) between the united artificial reefs and the artificial seamount was not significant.

The annual average DIC concentration for the entire area was 22.16 ± 3.08 mg/L. Seasonally, the DIC concentration was higher in spring and summer than in autumn and winter (Figure 2). In Figure 2, S1–S3 are united artificial reefs area while S5, S6 are artificial seamount area and S6–S8 are control area. From a spatial perspective, the DIC concentration in the artificial reef area and the control area was basically the same in autumn and winter, while in spring and summer, the DIC concentration observed in the unite artificial reefs area and the control area is similar, with lower DIC concentrations observed in the artificial seamount area.

3.2. Surface Seawater pCO_2w and FCO₂

The amplitude of annual values of pCO_2w (Figure 3a) in the surveyed was 467.05 μatm, with the lowest average value in summer (73.33 ± 30.23 μatm) and the highest in winter (477.36 ± 21.74 μatm). Overall, except for autumn when the control area’s partial pressure was higher than the artificial reef area, in the other seasons, the pCO_2w in the artificial reef area was generally higher than in the control area. However, the difference between the two types of artificial reef areas was not significant.

The pCO_2w in surface seawater shows a pattern of winter > spring > autumn > summer. In winter, both the artificial reef area and the control area exhibit positive FCO₂ values, indicating a source of CO₂. In autumn, spring, and summer, the FCO₂ values are negative, indicating an overall CO₂ sink, with the artificial reef area showing a stronger CO₂ sink than the control area, particularly in autumn. See

Table 2. The pCO_2w (μatm) and FCO₂ in the artificial reef area in different seasons.

Item	Area	Autumn	Winter	Spring	Summer
pCO2w	Total area	159.32 ± 67.13	477.36 ± 21.74	228.55 ± 17.03	73.33 ± 30.23
	united artificial reefs	130.50 ± 73.31	487.12 ± 25.51	230.57 ± 8.54	91.65 ± 32.22
	artificial seamount	109.22 ± 11.79	486.31 ± 21.53	243.18 ± 18.83	79.19 ± 8.79
	control area	221.54 ± 64.34	461.63 ± 13.31	216.79 ± 17.03	51.10 ± 18.49
FCO2	Total area	−86.3 ± 21.99	25.89 ± 9.62	−66.86 ± 6.59	−98.61 ± 8.22
	united artificial reefs	−95.94 ± 24.39	30.16 ± 11.24	−65.45 ± 6.07	−93.95 ± 10.69
	artificial seamount	−102.22 ± 4.38	29.93 ± 9.68	−61.79 ± 4.00	−95.26 ± 1.43
	control area	−66.06 ± 20.96	18.94 ± 9.62	−71.67 ± 6.61	−104.83 ± 5.00

.

The FCO₂ showed the same distribution pattern as pCO_2w (Figure 3b). In summer, the FCO₂ value was the lowest (−104.83 ± 5.00) mmol m⁻² d⁻¹, with an average of −98.61 ± 8.22 mmol m⁻² d⁻¹, indicating a CO₂ sink from the atmosphere. In autumn, it was (−86.3 ± 21.99) mmol m⁻² d⁻¹, in spring, it is −66.86 ± 6.59 mmol m⁻² d⁻¹, and in winter, it was the highest at 25.89 ± 9.62 mmol m⁻² d⁻¹. The annual range of the sea–air interface FCO₂ was −104.83~30.16 mmol m⁻² d⁻¹.

The results of two-way ANOVA showed that there is a significant difference in pCO_2w, and FCO₂ values in the study area of Sanheng Island across the four seasons (p < 0.01).

3.3. The Relationship Between Surface Seawater pCO_2w and Environmental Factors

Correlation analysis of SST, salinity, DIC, TA, and pH in surface seawater across four seasons (

Table 3. The correlation between the partial pressure of carbon dioxide (pCO₂) in surface seawater, air–sea CO₂ flux, and environmental factors.

	SST	Salinity	pH	DIC	TA	pCO2w	FCO2
SST	1
Salinity	−0.60 **	1
pH	0.89 **	−0.39 *	1
DIC	0.44 *	−0.73 **	0.29	1
TA	−0.83 **	0.68 **	−0.70 **	−0.36 *	1
pCO2w	−0.95 **	0.47 **	−0.97 **	−0.34	0.75 **	1
FCO2	−0.91 **	0.42 *	−0.95 **	−0.32	0.67 **	0.99 **	1

** Significant at the 0.01 level (two-tailed). * Significant at the 0.05 level (two-tailed).

), it was found that pCO_2w and FCO₂ were both significantly negatively correlated with SST and pH, pCO_2w was significantly positively correlated with salinity and TA (p < 0.01), whereas FCO₂ was significantly positively correlated with TA and positively correlated with salinity.

4. Discussion

4.1. The Main Environmental Factors of Island Reef Areas Affecting the Surface Seawater pCO_2w

The pCO_2w is influenced by multiple factors such as water temper, salinity (mixed with saltwater and freshwater), and the activities of mariculture organisms [33]. Sea surface temperature is usually one of the main controlling factors for pCO_2w.

Temperature usually has a dual effect on the pCO_2w of seawater: on one hand, changes in temperature affect the solubility of CO₂ in seawater and the dissociation equilibrium of carbonates, meaning that an increase in temperature decreases the solubility of CO₂, leading to an increase in pCO_2w; on the other hand, temperature can influence the metabolic activities of marine organisms (such as microbial respiration, photosynthesis, coral metabolism, etc.), thereby regulating changes in pCO_2w showing higher levels in the reef area compared to the control area, except in autumn. Song [34] pointed out that in open ocean areas, the pCO_2w is positively correlated with salinity, while in water mass mixing areas, the increase in salinity due to the mixing of fresh and saltwater leads to a decrease in the pCO_2w of seawater, resulting in a negative correlation. From Table 3, we can see that pCO_2w had the significant negative correlation with SST.

In summer, the SST reaches the highest level throughout the year, leading to a decrease in CO₂ solubility, which facilitates the transfer of CO₂ from water to the atmosphere. Additionally, with the higher water temperature, nutrient species brought in by the freshwater plume from the Yangtze River stimulate the growth of phytoplankton, resulting in a significantly higher standing stock of phytoplankton compared to other seasons [35,36]. The photosynthesis of phytoplankton can reduce pCO₂ in seawater by 25–27 ppm, which may also be a reason for the lower pCO₂ in the surface seawater during summer compared to other seasons.

Additionally, Zhai [37] found a significant negative correlation between surface pCO₂ and salinity outside the Yangtze River estuary, which is consistent with the results of this study. This is likely due to the strong water exchange and mixing in the study area located at the mouth of the Yangtze River. This study found that there was a significant negative correlation between air–sea flux and temperature. Hu [38] also observed similar conclusions on the sea–air flux in the East China Sea and the Southern Yellow Sea.

Furthermore, the presence of a thermocline in summer effectively hinders vertical mixing of the water column, preventing dissolved inorganic carbon (DIC) and other substances generated by the decomposition of particulate organic carbon (POC) at the bottom from rising to the surface [39,40,41]. Overall, the complexity of the relationships with environmental factors indicates that the CO₂ exchange flux is the result of the coupled effects of physiological and ecological processes of aquaculture organisms and plankton, as well as physical processes in the marine environment. The sources and sinks of CO₂ at the air–sea interface are mainly caused by the distribution changes in surface water CO₂ partial pressure, and are influenced by physical, chemical, and biological factors such as hydrodynamic processes, sea surface temperature, acidity, and biological activity [42,43].

Although the study area acts as a carbon sink in spring, summer, and autumn, the high intensity of carbon sources in winter causes the entire research area to function as a carbon source throughout the year.

4.2. The Carbon Flux and Carbon Sequestration Capacity in Marine Ranching

Marine ranching infrastructure can alter the surrounding abiotic environment, leading to changes in the biotic environment. The flow field effects of engineered facilities such as artificial reefs can modify the fluid dynamics in the vicinity, generating upwelling that transports bottom-layer nutrients to the mid-upper water column, promoting the growth of phytoplankton and enriching the marine ranching biological. This, in turn, facilitates bait attraction, drawing more marine organisms to aggregate in the ranching area, thereby enhancing marine biodiversity and biomass in the area [4,44,45]. The improvement of the physicochemical environment of marine ranching and adjacent waters can increase primary productivity, thereby enhancing the conversion of CO₂ by photosynthesis. The placement of artificial reefs has significantly increased the biomass and density of fishery resources in marine ranching and the surrounding waters [46,47].

Through investigations into the artificial reefs area and control area near Xiao shidao in Xi gang, Yangaodao in Mu ping, Xun shan in Weihai, and Qian shan in Rizhao, it was found that the number of fish species in the artificial reefs area was 1.8 times higher than that in the control areas. The average quantity and resource volume increased by 3.5 times and 1.9 times, respectively [48]. Feng [49] conducted trawl surveys of fishery resources in the artificial reef area of Wailingding Island, Zhuhai, and found that after the reef was deployed, the fishery resources in the reef area were 4.35 times that of the control area. Zhang [50] conducted a biological resource survey of the artificial reefs in Haizhou Bay and found that after the artificial reef was deployed, both biodiversity and evenness increased, and the CPUE in the reef area increased by one compared to before reef deployment. According to the definition of fishery carbon sink, the carbon fixed by the growing fish in the marine ranch comes from the natural sea area, playing an important role in biological carbon sequestration. Wang [51] conducted a survey on fish and large invertebrates in different habitats of the Sanheng sea area, and found that the total number of species in the artificial reef habitat was 2.06 times that of the muddy habitat, and the abundance was also significantly higher than that of the natural habitat. In addition, other scholars have studied the communities of attached mollusks in fish reef and rock reef habitats, finding that the species richness and diversity of rock reefs are significantly higher than those of artificial reef habitats, but the distribution density of certain species on artificial reefs is much higher than that on natural reefs [52,53].

The improvement of the physical and chemical environment in marine ranching areas and adjacent waters can increase primary productivity, thereby enhancing the conversion of CO₂ through photosynthesis. Filter-feeding bivalves attached to the reef structure can also sequester carbon over time. The deployment of artificial reefs significantly increases the biomass and density of fishery resources in marine ranching and adjacent radiated areas.

Research has found that, as an important component of artificial reefs, it can enhance the diversity and abundance of zooplankton, benthic organisms, and fish, thereby increasing carbon sink fisheries [54,55]. Additionally, oysters and other bivalves attached to the reef, during their growth process, produce feces and excretions that serve as nutrients for benthic organisms, acting as organic fertilizer for other blue carbon ecosystems. Studies have shown that the biomass, primary productivity, and total productivity of reefs communities and seagrass meadow co-ecosystems are 44.04 times, 5.03 times, and 5.34 times those of single systems, respectively [54,55].

Additionally, the research has found that seaweed, as an important component of marine ranching, exhibits a strong carbon sequestration effect in seaweed farming areas compared to non-farming areas, with an average decrease in CO₂ partial pressure of 58.7 ± 15.9 μatm, indicating a certain carbon sink capacity [56].

Overall, the construction of artificial reefs and marine ranching significantly improves the resource status within the marine environment, increases the biomass in the area, and effectively enhances the carbon sequestration capacity within the marine environment.

5. Conclusions

The main findings of this research were (1) the annual variation range of the partial pressure of CO₂ (pCO_2w) in the surface seawater of the Sanheng artificial reef area was found to be 34.48 to 501.53 μatm, and the partial pressure of CO₂ (pCO_2w) decreases in the following order: winter, spring, autumn, summer; (2) and the CO₂ flux (FCO₂) exhibited a highly significant negative correlation with sea surface temperature and a highly significant positive correlation with salinity and total alkalinity. Similarly, the CO₂ flux (FCO₂) in the surface seawater decreases in the following order: winter, spring, autumn, summer. (3) During winter, both the reef area and the control area served as sources of CO₂. However, in the other three seasons, the FCO₂ values were negative, indicating an overall sink for CO₂, with a more pronounced effect reef area compared to the control area in the autumn.

While this study concludes that the construction of artificial reefs contributes to carbon sequestration, the precise quantification of this effect remains unclear. Additionally, the primary factors and mechanisms driving variations in carbon sequestration across different marine regions are not yet well understood. Further research is needed to elucidate these aspects.

A comprehensive assessment of their ecological benefits and contribution to the low-carbon economy, in order to better serve the blue carbon strategy, still requires long-term and extensive monitoring.