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Abstract:Based on the Intergovernmental Panel on Climate Change interpretation of carbon sink and carbon source, and the characteristics of carbon sequestration by aquatic plants, this review revises the definition of fisheries carbon sink and carbon sink fisheries proposed in 2010. We emphasize the basic expression of the function of fisheries carbon sink and the important role of aquatic plants in them. We further explain the process and mechanism of promoting aquatic organisms to "remove and store" greenhouse gases (such as carbon dioxide) through algae culture, filter-feeding shellfish and fish aquaculture, fishing and enhancement of fishery stock, and other means. We analyzed the process and mechanism of carbon dioxide use by non-fed shellfish aquaculture by filtering phytoplankton and particulate organic matter, such as organic debris. Further, the characteristics of carbon usage, removal, storage, and release, and their quantitative relationships with energy budget are discussed. The analyses confirmed that shellfish aquaculture enhances the carbon sink capacity of aquatic ecosystem, and is a carbon sink rather than a carbon source. Our results revealed that with the continuous development of mariculture production, the carbon sink of shellfish mariculture in the coastal ocean of China has substantially increased in the past 20 years. The total carbon sink increased from 3.94 million tons in 2001 to 6.59 million tons in 2020. This included an average of 6.48 million tons/year in the past three years (2018—2020), equivalent to 870 000 hectares of compulsory afforestation per year. The net carbon sink increased from 2.55 million tons to 4.3 million tons in 2020, with an average of 4.22 million tons/year in the last three years (2018—2020), equivalent to 560 000 hectares of compulsory afforestation per year. Finally, relevant suggestions for sustainable and further development of carbon sink fishery are proposed.

Abstract:Global warming caused by human CO2 emissions has endangered the sustainable development of human society. To deal with global warming, the United Nations General Assembly formulated the “United Nations Framework Convention on Climate Change” in 1992, integrating the control of CO2 and other greenhouse gas emissions and the adverse effects of global warming on human economy and society into the framework of international law. As a responsible country, China committed to achieving a carbon peak by 2030 and carbon neutrality by 2060 at the 75th UN General Assembly in September 2020. However, it is limited to achieving carbon neutralization only by controlling CO2 emissions; the most effective way to achieve this is to reduce emissions while increasing foreign exchange. As the largest carbon pool in the world, the ocean plays an essential role in regulating global climate change and has significant potential to increase sinks. To increase the ocean sink, scientists put forward the “blue carbon” strategy: the restoration and protection of mangroves, salt marsh wetland, seagrass bed, and other three types of coastal ecosystems, to strengthen the organic carbon burial, and slow down climate warming. As most macroalgae grow in a batholith environment, the carbon burial process is hindered, and they are excluded from the “blue carbon” system. However, macroalgae also have a strong carbon sink. The algae-released dissolved organic carbon (DOC) can be transformed into refractory DOC (RDOC) by the action of a microbial carbon pump (MCP). Owing to the stable chemical properties of RDOC, carbon sequestration can be achieved on a millennium scale, thus effectively alleviating global warming. Aquaculture activity is highly controllable. If we can increase the carbon sink through algae cultivation, we can achieve a win-win situation of economic and ecological benefits. Preliminary estimates show that the annual DOC release of cultured algae in China has reached 82.2×104~91.5×104 t C/a. The DOC released by cultured algae can generate more than 600,000 tons of RDOC per year through MCPs, approximately 1.7 times the carbon storage capacity of “blue carbon” in China´s coastal zone, with significant carbon sink potential. The DOC released by macroalgae is an important part of the offshore DOC pool, accounting for approximately 20% of the offshore DOC pool. However, the environmental regulation mechanism of DOC released by macroalgae remains controversial. To investigate the effects of light and nutrients on the release of DOC from young S. japonica seedlings, the release rate of DOC in young S. japonica seedlings was measured under different light intensities [0, 83, 165, and 250 μmol photons/(m2·s)] and different nutrient conditions (nitrogen enrichment, phosphorus enrichment, co-enrichment, and natural seawater). Two hypotheses of DOC release from algae, the “overflow” and “diffusion” hypotheses, were tested. According to the “overflow” hypothesis, DOC released by algae is positively correlated with light intensity, and its components are mainly high molecular weight substances; according to the “diffusion” hypothesis, the release of DOC from algae is positively correlated with nutrients and mainly low molecular weight substances. The experimental method is as follows: the first part is the illumination experiment. Four light intensities of 0, 83, 165, and 250 μmol photons/(m2·s) were set. Six parallel light intensities were used for each light intensity. Each parall楥獬†晳潡牭⁰歬敥氠灷⁡捳甠汰瑬楡癣慥瑤椠潩湮⸠੡ 2-L glass bottle, and the S. japonica young seedlings without loss were cultured in a light incubator at different light intensities for 6 to 8 h. At the same time, three parallel samples without S. japonica young seedlings were set in each light treatment as a blank control. Water samples (100 mL) were collected before and after culture; one for the determination of DOC content and absorption spectrum and one for the determination of dissolved oxygen (DO). The second was a nutrient experiment, in which four experimental groups were set up: nitrogen enrichment, phosphorus enrichment, co-enrichment, and natural seawater. Standard solutions of 100 and 1000 μg/L were prepared with potassium monohydrogen phosphate and potassium nitrate, respectively, for enrichment. The light intensity was 250 μmol photons/(m2·s). Other treatments were the same as those described above. The results showed that the DOC release rate of young S. japonica seedlings was positively correlated with light under natural seawater conditions. The highest value is (24.31±5.84) μmol/(g·h) in the 250 μmol photons/(m2·s) light condition, about four times that in the dark. The single nitrogen and phosphorus enrichment condition had no significant effect on the release of DOC in the young S. japonica seedling, with releasing rate of (23.04±4.24) μmol/(g·h) and (18.18±4.59) μmol/(g·h), respectively. The co-enrichment of nitrogen and phosphorus significantly increased the DOC releasing rate of young S. japonica seedlings to (37.15±6.77) μmol/(g·h), about three times that in natural seawater. In conclusion, there are likely two regulatory mechanisms of “overflow” and “diffusion” in the release of DOC from young S. japonica seedlings. Under the oligotrophic condition, the “overflow” mechanism is dominant; under the eutrophication condition, the “diffusion” mechanism is dominant. The significance of this study is to clarify the environmental regulation mechanism of DOC release from young S. japonica seedlings and provide a scientific bas

Abstract:China has the largest population and contributes the most to greenhouse gas emissions in the world. Given the background of low-carbon emissions elsewhere, how to carry out emission reduction activities scientifically and rationally is a question that individuals, enterprises, governments, and countries must seriously consider. The carbon footprint refers to the total amount of greenhouse gases emitted by a commodity or service during the entire life cycle of the product, including production, transportation, use, and disposal. The carbon sink effect of cultured macroalgae in coastal waters is receiving considerable attention. However, international research on macroalgal carbon sinks is still poor, especially the carbon footprint of cultured macroalgae, which makes it impossible to include the carbon sinks of macroalgae within the scope of emission reductions such as “blue carbon.” Therefore, by calculating the carbon footprint of macroalgae, the carbon emissions of each stage in the entire life cycle can be determined, and subsequently scientific emission reduction measures can be formulated based on the calculated carbon footprint results of each stage to reduce emissions. Kelp (Saccharina japonica Areschoug) is the main macroalgae cultured in China. It has obvious advantages in aquaculture resources and has a very large potential for the development of carbon sinks. As a primary producer in the sea, organic matter is generated through photosynthesis, and carbon sequestration occurs during the kelp growth phase. However, CO2 is released during seedling growth, electricity utilizing of equipments, fuel consumption on boats, and facilities for culture. To explore the sources and sinks of CO2 emissions from kelp throughout the entire culture cycle and to establish a standard system for evaluating the carbon footprint of macroalgae production, based on the life cycle assessment theory, a carbon footprint calculation method for raft-cultured kelp was established in this study. The cradle-to-gate carbon footprint of cultured kelp in Sanggou Bay was calculated, and the main influencing factors of the carbon footprint and possible sources of error were analyzed. The life cycle assessment method included four parts: Goal and scope definition, inventory analysis, impact assessment, and interpretation of results. One ton of produced kelp was recorded as the functional unit of the carbon footprint of cultured kelp, and the entire life cycle of cultured kelp to form a kelp product was divided into three phases: Breeding, transport, and culture. The carbon footprints of the three stages were analyzed. The results showed that the carbon footprint of 1 t of kelp farming is –95.93 kgCO2e, which indicates that the entire process from breeding to growth and harvest is a carbon sink process. Among them, the carbon emission is 74.30 kgCO2e, and the carbon absorption is 170.23 kgCO2e. A carbon sink of 79.9% is in the form of kelp biomass carbon, 14.1% exists in the form of deposited buried carbon, and 6.0% exists in the form of refractory dissolved organic carbon (RDOC). Deposited buried carbon and RDOC can accumulate in the deep sea or on the seafloor for a long time. Previous studies on the carbon sink capacity of primary producers have primarily focused on biomass carbon formed by them. Further research confirmed that DOC released during the growth stage of kelp and RDOC formed by detritus under the action of microorganisms and deposited carbon are all important parts of fishery carbon sinks and are also important forms of long-term stable carbon pools in the ocean. If RDOC and deposited carbon are not considered, the carbon sink of cultured kelp will be underestimated by approximately 20%. Of course, differences in culture conditions, species, and modes in different seas make the formation rate of deposited carbon different. In addition, the formation process and mechanism of RDOC require further study. Aquaculture facilities were the main carbon source, and their carbon emissions accounted for 93.81%. Our research found that emission reduction can be achieved by extending the service life of aquaculture facilities. Each year of service life extension can reduce the emissions by 8%. The carbon emissions from diesel and electricity accounted for 5.05% and 1.14%, respectively. Sanggou Bay is a typical coastal water; therefore, the demand for energy during the breeding process is low. When the aquaculture area expands to the open sea, the proportion of the energy carbon footprint will greatly increase, and even become the main carbon source. Fertilizer and transportation account for only one ten-thousandth of carbon emissions. The kelp seedlings in the breeding area of Sanggou Bay come from Rongcheng; therefore, the amount of CO2 released during transportation was not high. Insufficient numbers of nurseries for kelp breeding will result in the seeds coming from other places, and the amount of CO2 released during transportation will also increase greatly. Therefore, strengthening the overall layout of the industrial chain is of great significance in reducing carbon emissions during transportation. With further understanding of the carbon sink function of cultured seaweeds, macroalgal cultures will play a more important role in ocean emission reduction. This study provides technical support for the establishment of carbon footprint evaluation procedures and standard systems for macroalgal farming.

Abstract:Seagrass meadows are one of the most abundant systems in the coastal area. They absorb and sequester a large amount of atmospheric carbon dioxide (CO2) forming an important blue carbon system. The strong ability of seagrass beds to absorb CO2 and store organic carbon (Corg) could be attributed to their ability to convert CO2 into plant biomass and to reduce water flow and sediment re-suspension, capturing organic particles from outside the ecosystem. Thus, the source and the stock of carbon have attracted much attention. In certain areas, this value exhibits a large range. In some meadows, endogenous organic matter (OM) such as seagrass litter (leaves and roots) and epiphytes account for a larger contribution to carbon burial, while in most other grass beds, exogenous OM such as phytoplankton, suspended particulate matter, and mangroves in adjacent systems dominate. This is primarily because all marine ecosystems exchange energy and matter with the surrounding systems through water flow, and carbon is no exception. Sanggou Bay is one of the main seagrass distribution areas in northern China. It is also a typical mariculture bay in China. In this bay, the long-term large-scale bivalve culture activities have given a special ecological scene to the adjacent culture ecosystem and seagrass beds. Based on stable isotopes, this study analyzed the sources of organic carbon in the sediments of the two eelgrass meadows in Sanggou Bay and evaluated the organic carbon stock. The results showed that the isotope δ13C of the eelgrass bed in Sanggou Bay was in the range of (20.31~ –21.99)‰, compared to 12.30‰ of the eelgrass itself. The difference (8.2‰) shows the typical characteristics of allochthonous organic carbon. The estimation from the Isosource 1.3 isotope mixing model software showed that the surface organic carbon in the two eelgrass beds mainly originated from the phytoplankton (34.0%~41.4%). Bio-deposit from cultured bivalve also contributed 23.9%~25.3%, while eelgrass itself only contributed about 8.3%~17.1%. The contribution of shellfish bio-deposit was about 23.9%~25.3%, while that of macroalgae was about 25.0%. Around the Chudao eelgrass bed, the carbon output from the eelgrass contributed about 5.2% to 10.7% to the organic carbon deposited on the surface site within 2 km. The carbon stock estimation showed that the organic carbon storage at the depth of 0~30 cm in the two grass beds was 2.01 and 3.75 Mg C/hm2, with an average of 2.88 Mg C/hm2, and about 0.71 Mg C/hm2 was from bivalve deposition. In addition, eelgrass also contributed (average 5.2%~10.7%) to sediment carbon in the surrounding system. The contribution of eelgrass to surface sediment organic carbon in the study site was lower than that of the average contribution of eelgrass in the north temperate eelgrass beds (20%~40%). This result could be linked to the fact that both seagrass beds are in an environment with strong nearshore hydrodynamics. Under the strong hydrodynamic action, the exfoliated materials such as leaves are carried out of the grass bed and get accumulated in the surrounding environment, while the bio-deposit of the surrounding cultured shellfish are carried into the meadows by the resuspension. Strong water flow presents a weaker rate of carbon accumulation due to the high microbial decomposition. Compared to the average carbon stock in the temperate eelgrass beds (27.2 Mg C/hm2) in the world, the carbon stock in Sanggou Bay eelgrass meadows is low. The primary reason is the substrate characteristics of the eelgrass bed. The average grain size is relatively large, with relatively low mud content, which results in strong microbial decomposition, making it difficult for organic carbon from various sources to accumulate. The decline of seagrass will lead to a large loss of sedimentary organic carbon and eelgrass bed restoration could be an effective means to curb this trend. Strengthening the protection of seagrass in Sanggou Bay and carrying out effective transplantation and restoration could be an important measure to increase the carbon storage and would help to provide more ecological services such as increasing biodiversity and maintaining environmental health. This study shows that the carbon storage of the seagrass system in Sanggou Bay is relatively low. Bio-deposit from farmed shellfish is an important source of organic carbon in eelgrass beds. The results provide an in-depth analysis of the source of sedimentary carbon sink in the eelgrass bed in Sanggou Bay and the contribution of large-scale mariculture activities to the seagrass blue carbon.

Abstract:Ulva prolifera is the main species in green tides and also an important marine carbon sink organism. It is characterized by fast growth, diverse reproductive modes, and strong resistance to stress. It can form a large-scale biomass in a short time. We review the carbon fixation features in the growth and drift of U. prolifera. It possesses a unique high pH induced HCO3– assimilation mechanism, which can promote CO2 absorption from the air by floating U. prolifera. Moreover, it utilizes the Hatch-Slack Cycle (C4) to enhance carbon fixation rate under high light irradiation conditions. The diverse carbon assimilation and sequestration mechanisms enhance photosynthetic carbon sequestration ability and help the quick accumulation of floating algal biomass. These abilities make the carbon sequestrating efficiency of U. prolifera significantly higher than that of Saccharina japonica, Undaria pinnatifida, and Porphyra haitanensis. Since 2007, the largest green tide in the world occurred in the Yellow Sea and persisted over 15 years. The average annual distribution area of U. prolifera was greater than 30 000 square kilometers, with an average annual coverage area in excess of 500 square kilometers. The average annual outbreak of U. prolifera had a biomass greater than 1.5 million tons. The net carbon sequestration from 2008 to 2020 was estimated to be 25 000~275 000 tons, averaging over 78 000 tons. This figure is higher than that of Gracilaria, P. haitanensis, and U. pinnatifida, and only second to that of S. japonica. The great biomass and strong carbon sequestration ability of U. prolifera has made it a potential new and important marine carbon sink and carbon storage pathway. We suggest strengthening the salvage and resource utilization of U. prolifera. This approach is a win-win situation for carbon utilization and eutrophication removal. These measures will promote the green tide U. prolifera carbon sink in joining the carbon market as soon as possible, to become a new low-carbon economic resource.

Abstract:Carbon sequestration and carbon transfer through the food chain are important aspects of the carbon cycle in marine fisheries, and an essential part of the blue carbon sink of marine organisms. It includes not only the carbon used in shellfish and macroalgae farming at lower trophic levels in the food web, but also by certain organisms through feeding and growth activities. In marine ecosystems, macroalgae are one of the most important primary productive forces and one of the most efficient carbon-fixing organisms. They directly absorb carbon dioxide from seawater through photosynthesis, increasing the ocean carbon sink. Moreover, they promote and accelerate the diffusion of atmospheric carbon dioxide into seawater, helping to reduce it in the atmosphere. Macroalgae support many marine biota, including amphipods. Amphipods not only use the macroalgae habitat as shelter and nursery, but also as a source of nutrition. Moreover, amphipods provide a critical food source for other marine animals, such as fish, crustaceans, cephalopods, and even gray whales. Therefore, amphipods play an essential role in the material circulation and energy transfer in the food chain of the marine ecosystem. As primary consumers, the amphipods may also play an important role in the carbon sink process of marine fisheries by transferring the macroalgae fixed carbon to senior consumers. Additionally, amphipods prioritize ‘delicious’ macroalgae rather than treat them equally like many other invertebrates. They also reduce the biomass accumulation of this macroalgae and even affect its community structure. Consequently, studying the amphipods feeding selectivity to macroalgae is essential to understanding the relationship between macroalgae and algae-dwelling animals. Based on the above research background, this study investigated the feeding selectivity characteristics of Eogammarus possjeticus, an amphipod from the Shandong Peninsula, in relation to five different macroalgae, including Ulva prolifera, U. intestinalis, U. compressa, Chaetomorpha linum, and Cloniophora sp. The potential amphipods’ carbon sink characteristic was preliminarily discussed. The results showed that the feeding rates of E. possjeticus on U. intestinalis and U. prolifera were the highest, with daily feeding rates of 0.81 g of fresh weight/(g·d) and 0.80 g of fresh weight/(g·d), respectively, while the feeding rate of E. possjeticus on C. linum was the lowest of 0.19 g of fresh weight/(g·d). The proportion of E. possjeticus individuals living in macroalgae was the highest in Cloniophora sp., followed by U. intestinalis and U. prolifera. We analyzed the correlation between total organic carbon (TOC), total nitrogen (TN), carbon/nitrogen ratio (C/N), and dry weight/fresh weight ratio (DW/FW) of the macroalgae, as well as with the E. possjeticus feeding rate. A significant positive correlation was observed between the feeding rate and the macroalgae TOC concentration and C/N ratio (P<0.05). Nonetheless, the feeding rate negatively correlated with the TN concentrations and DW/FW ratio (P<0.05). These results suggested that the feeding selectivity of E. possjeticus to macroalgae was significantly correlated with TOC, TN, C/N, and DW/FW. It seemed that amphipods prefer to inhabit filamentous algae with complex structures and dense branches. In fact, amphipods give priority to Enteromorpha genus species with rapid growth rate and high carbon sequestration, which can accelerate the carbon transfer process of macroalgae to a higher trophic level species. The carbon transfer process enables marine animals at the top of the food chain to store carbon in the form of biological pumps. With the harvest of fisheries, some marine animals are removed from the seawater to promote carbon removal, while other animals not captured by humans continue to conduct carbon uptake and food chain transmission. In conclusion, amphipods have feeding selectivity to macroalgae, which may play the important role of carbon transfer channel in accelerating carbon sinks in marine fisheries.

Abstract:An important driver of coastal marine ecosystem processes involves seaweed photosynthesis converting atmospheric CO2 into organic carbon and internally storing it to achieve carbon storage, which is of great significance for the realization of the goal of carbon neutrality and carbon peak in China. Seaweed fields transport a large amount of organic detritus to the ocean during the peak period of seaweed withering, providing primary productivity for the surrounding area and realizing the function of a carbon sink after settlement. The transport and settlement processes of seaweed detritus is one of the key dynamic processes connecting the circulation of important biogenic elements in different coastal habitats. It not only affects the primary productivity of the sea area around the seaweed field, but also affects the temporal and spatial distribution pattern of its carbon sink function. In this study, a three-dimensional Ocean numerical model (Estuarine Coastal Ocean Model, semi-implicit, ECOM-si) coupled to a Lagrange particle tracking module were combined with tidal elevation, current velocity, and drift buoy trajectory to investigate the dynamic mechanism and carbon sink function of organic detritus transport and settlement processes in a natural seaweed field on the northwest of Gouqi Island. The model revealed the spatial distribution pattern of the organic detritus contributing to the carbon sink, which presents a high value area centered on the seaweed field and decreases in the northwest-southeast direction. The difference in the hydrodynamic conditions and particle size of the detritus leads to significant variation in the spatial distribution of the settlement. During spring tides, the highest carbon sink was 699 g C/(hm2·d) and the carbon sink contribution of seaweed detritus was not limited to the adjacent sea area. The smaller the particle size of the detritus, the larger its contribution range, contributing approximately 7.5~33.7 kg C of organic carbon daily to the sea area 5 km away from the center of the seaweed area. During a neap tide (with lower flow velocity) the contribution of the seaweed area to the carbon sink was concentrated in the coastal area. The radiation range was relatively small, but the carbon sink intensity was high, reaching approximately 817 g C/(hm2·d). The tidal current direction when seaweed detritus drops and the geographical location of the seaweed site are also important factors affecting the contribution of the seaweed field to the carbon sink and ecological radiation function. When the seaweed was located in relatively open terrain with less shielding by the tidal current, the settlement and distribution range of the organic detritus was wider and provided a greater contribution to the carbon sink in the surrounding sea area. The Lagrange method is feasible for investigating the offshore transportation and settlement mechanism of organic detritus from a seaweed field. This method can effectively evaluate the contribution of a typical nearshore seaweed field to the carbon sink in the surrounding sea area and the role of the main dynamic factors. The research results reveal that the temporal and spatial characteristics of seaweed detritus transport and settlement and the carbon sink function are affected by many factors, such as tidal current magnitude, seaweed particle size (with different settlement velocities), detritus shedding time, and the geography of the location of the seaweed field. This research provides an important scientific reference for investigating seaweed fields and evaluating ecological radiation range in China.

Abstract:Bivalve filter feeders, such as oysters, clams, and scallops, are an economically important species in China, with a total production of up to 14.8×106 tons in 2020, accounting for more than 69.3% of mariculture production. In recent years, the density and scale of shellfish mariculture have continued to increase in some areas owing to economic benefits. The crude farming strategies have a series of negative effects on the cultured organisms and marine ecosystems, such as inhibiting the individual growth rate and increasing individual mortality. The negative effects could then lead to the attenuation of phytoplankton, changing the structure of the phytoplankton community and benthic environment. To manage shellfish farms and the industry properly, it is important to understand the interaction between shellfish aquaculture and the marine environment and evaluate the ability of the marine ecosystem to support shellfish production. At the ecosystem level, the carrying capacity of shellfish is defined as the maximum mariculture density that does not significantly affect the structure and function of marine ecosystems. To evaluate the ecological carrying capacity of shellfish, a method that assesses the overall impact of cultured shellfish on the structure, function, and other functional groups of the ecosystem is required. Based on the principle of nutrition dynamics, the Ecopath model (Ecopath with Ecosim software, Version 6.5) is a scientific and effective tool for evaluating the ecological carrying capacity of shellfish from the perspective of material and energy balance, and fully considers predation, competition, and ecological transformation efficiencies between functional groups. The Ecopath model has been widely used to evaluate the ecological carrying capacity of shellfish in mariculture ecosystems. In addition to their important economic value, bivalve filter feeders play an important ecological role in carbon sequestration, storage, and water purification by ingesting particles and changing the nutrient cycle. In offshore ecosystems, the primary productivity is very high. Filter-feeding bivalves ingest a large amount of phytoplankton to convert particulate organic carbon into feces or pseudo-feces and accelerate the transportation of organic carbon to the seabed. Moreover, calcium carbonate shells are formed through absorption of inorganic carbon from seawater, which plays an important role in long-term carbon storage. Jiaozhou Bay is an important aquaculture base for the Manila clam, Ruditapes philippinarum, in North China. Studies have found that the cultivation of Manila clams caused disturbance to the benthic ecosystem of Jiaozhou Bay. To explore the ecological carrying capacity and ecological service functions of R. philippinarum in the bay, Ecopath with Ecosim models were applied, and the effects of further expansion of clam biomass on the community structure and functional characteristics of the Jiaozhou Bay ecosystem were evaluated. Furthermore, the effects of Manila clam aquaculture on carbon cycling were quantified. Results showed that: The ecological carrying capacity of R. philippinarum in Jiaozhou Bay was 239.9 t/km2; the average biomass of cultured clam has not yet reached the ecological carrying capacity, but the clam biomass in some areas has far exceeded this limit. When the clam biomass increased up to the level of the ecological carrying capacity, the total system throughput, capacity, ascendency, and Finn cycling index would also increase by 16.0%, 3.9%, 47.1%, and 103.0%, respectively, whereas the entropy would decrease by 10.4%, suggesting that the increase in clam biomass would lead to an increase in system maturity and stability; however, continued expansion to 10 times the ecological carrying capacity would adversely affect the ecosystem. At the individual level, a clam ingested approximately 3310.1 mg of carbon during a farming cycle, of which approximately 46.2% was deposited and 13.2% was removed through harvesting; at the population level (scaled to ecological carrying capacity), 15 000 tons of carbon could be deposited, and 6000 tons be harvested annually. These results provide theoretical guidance for the sustainable development of clam aquaculture and its ecological service functions.

Abstract:In recent years, aquaculture has rapidly developed in many countries, playing a positive role in ensuring food security and promoting economic development. However, it has also produced negative effects, such as water pollution and eutrophication. As the key species in integrated aquaculture systems, bivalves not only improve space utilization and provide economic benefits, but also regulate nutrient cycling, reduce water body eutrophication, increase the ability of blue carbon sinks to capture and hold carbon, improve system stability, and perform various ecosystem services, including nutrient removal and carbon sequestration. However, as a resource-dependent aquaculture industry, high-density and unreasonable bivalve aquaculture produces a strong downlink control effect on the phytoplankton community structure, which in turn restricts the carbon sink function of shellfish aquaculture ecosystems and negatively affects the ecosystem. China is the dominant country in terms of shellfish farming. In 2020, the total output of marine shellfish was 14.80 million tons, ranking first in the world. In China, the main target of marine shellfish farming is bivalves, which account for 95% of the total marine shellfish output. The total output of Ruditapes philippinarum is over 3 million tons, accounting for 90% of the global output. Jiaozhou Bay is an important large-scale aquaculture base for R. philippinarum in northern China, with a clam output of 325 000 tons, accounting for 91.5% of the total output from this base. In April 2017, the core purpose of the proposal for the Chinese Academy of Engineering entitled "Proposal on Promoting Green Development of Aquaculture Industry" was to call for the establishment of an aquaculture capacity management system. In this context, research on the carrying capacity of shellfish is of theoretical and applied significance. The ecosystem dynamics approach assesses carrying capacity based on different evaluation criteria by constructing ecosystem models to simulate key biogeochemical processes and their interactions with important biogenic elements. As our understanding of the concept of carrying capacity and ecosystem structure and function continues to improve, ecosystem dynamics methods that can describe in more detail the physical, biological, and chemical processes and their interactions involving culture organisms in aquaculture ecosystems have become the mainstream direction for global carrying capacity researchers. Although these methods are now widely used in several aquaculture bays around the world, they remain rare in China. We estimated the carrying capacity of R. philippinarum in Jiaozhou Bay based on the Dame indices and Herman model, although the assessment method used portrayed coarse lines of ecological processes, which mainly considered shellfish feeding on phytoplankton, lacking the depiction of other biological roles, and the core parameters were not sufficiently comprehensive. In the present study, the individual growth model for R. philippinarum and a biogeochemical model were coupled to build the nutrient–phytoplankton–zooplankton–detritus–clams (NPZD-C) dynamic ecosystem model of Jiaozhou Bay, and the carrying capacity of R. philippinarum was estimated dynamically. The individual growth model for R. philippinarum in Jiaozhou Bay was constructed based on the dynamic energy budget (DEB) theory following model parameterization and validation. The simulated results from the dynamic ecosystem model well fit the observed results. Regression analysis showed a significant (P < 0.01) linear correlation between the simulated and observed values of clam wet weight and phytoplankton concentration (R2 = 0.934 8 and 0.926 4, respectively). The results of carrying capacity estimation showed that the final clam yield was 10.5, 15.6, 18.9, 21.6, and 23.2 t/hm2 and the maximum phytoplankton (carbon) concentration was 231.3, 176.9, 147.6, 125.1, and 109.8 mg/m3 when the initial seeding density was 300, 500, 700, 1000, and 1500 clams/m2, respectively. Of note, carrying capacity assessed based on the ecosystem dynamics model varies depending on concerns regarding environmental quality, yield, and economic benefits, and there is no uniform standard. In the present study, the criteria used for the assessment of carrying capacity included the maximum stocking density to achieve the minimum size of commercial shellfish and the aquaculture density to maximize economic benefits within a limited period. Our assessment results can help farmers develop aquaculture management strategies. The seeding density should be less than 1000 clams/m2 if individuals with a wet weight of 5 g or more are harvested within the expected 10-month aquaculture period. According to the maximum economic and ecological benefits, the most suitable seeding density is 550~750 clams/m2. This study attempted to construct the NPZD-C ecosystem dynamics model for Jiaozhou Bay by coupling the individual growth model for R. philippinarum and a biogeochemical model and assess the carrying capacity of R. philippinarum in Jiaozhou Bay by considering economic and ecological benefits as the assessment criteria, proposing farming management suggestions for clam seeding density. The results are expected to provide data support and reference in decision-making for planning the development of the local R. philippinarum aquaculture industry and provide a theoretical basis and scientific guidance for managing shellfish aquaculture at the ecosystem level and exploiting the carbon sink function of shellfish.

Abstract:In this study, the following spatial and temporal distribution characteristics of surface sediments in Sanggou Bay were analyzed: grain size, total organic carbon (TOC), total nitrogen (TN), and carbon-nitrogen stable isotopes (δ13C and δ15N). Additionally, the contribution of shellfish-seaweed biodeposition to the organic matter in coastal sediments was estimated. The results showed that the sediment composition of Sanggou Bay is primarily silt, and that particle size distribution is closely related to mariculture and hydrodynamic conditions. The seasonal variation and horizontal distribution of TOC and TN showed similar patterns: highest in spring and lowest in autumn, with little seasonal difference, and higher values in shellfish seaweed, fish shellfish, and shellfish culture areas. However, there were larger differences in the seasonal variation and horizontal distribution of δ13C and δ15N. According to the correlation analysis, there was a significant positive correlation between TOC and TN (r=0.955, P<0.001), indicating that TOC and TN in the surface sediments of Sanggou Bay were homologous. In addition, according to the comprehensive analysis of δ13C and C/N, the main sources of sedimentary organic matter are shellfish biodeposition, kelp, and soil organic matter. Using a three-end-member mixed model, it is estimated that the contribution rates of shellfish biodeposition, soil organic matter, and kelp are 67.52%, 26.47%, and 5.97%, respectively. Therefore, marine shellfish-seaweed biodeposition has a significant impact on coastal carbon burial.

Abstract:In this study, columnar sediments taken from the shellfish aquaculture area of Sanggou Bay in 2014 were analyzed to determine the mass fraction of total carbon (TC), total organic carbon (TOC), and total nitrogen (TN) in each layer of sediments, and to calculate the mass fraction of total inorganic carbon (TIC), marine organic carbon (Cm), shell inorganic carbon (Shell-IC), and their contribution ratios to TC. The accumulation rate (or burial flux, BF) of each component was estimated. High-resolution records of various carbon accumulation rates in sedimentary carbon pools in the last 80 years were obtained using the 210Pb dating method. The average contents of TC, TIC, TOC, Cm, and Shell-IC were 1.09%, 0.75%, 0.34%, 0.15%, and 0.06%, respectively. Results showed that TIC was the main form of TC, with a contribution ratio greater than 60% between 1960 and 2010. The mass fraction of Cm did not fluctuate significantly before 2010, but increased significantly after 2010, significantly increasing Cm/TC and TOC/TC. Shell-IC remained at a low level from the start of aquaculture activities until 2000. The carbon accumulation rate of each component responded to human marine aquaculture activities from 1960 to 2000. Due to the modification of aquaculture scale and pattern after 2000, BFCm, BFTOC, and BFTC increased significantly, BFTIC decreased, and BFshell-IC first increased and then decreased. Marine aquaculture activities in Sanggou Bay influenced the composition and accumulation rate of the carbon pool in the shellfish aquaculture area, and the carbon components responded to changes in aquaculture activities. The research results describe the effects of human aquaculture activities on the accumulation rate of sedimentary carbon pools in detail over 50 years after the start of aquaculture activities. This study also provides a reference for rational planning of aquaculture activities in continental shelf areas in the future.

Abstract:The famous “keeling curve” revealed the concentration of carbon dioxide in the earth´s atmosphere has risen from approximately 0.027% before 1700 to approximately 0.041% today. The associated global climate change has become one of the most serious issues of the 21st century. As a responsible government, China is an active participant and important contributor in the response to global climate change. In September 2020, China announced that it would aim to achieve peak CO2 emissions before 2030 and carbon neutrality before 2060 at the general debate of the 75th session of the United Nations General Assembly. Subsequently, local governments, industries, and enterprises are actively responding and working hard to formulate "timetables", "road maps" and "construction drawings" to promote the realization of the "dual carbon" goal. There are two main types of carbon sequestration: Biological and geological. Compared with the geological approach of carbon sequestration, biological carbon sequestration is technically mature, low cost, and has economic, ecological, social, and other benefits. A "fishery carbon sink" is a kind of biological carbon sink, which refers to the process and mechanism by which aquatic organisms absorb CO2 from water through fishery production activities and remove carbon from water or store it through harvesting and biodeposition. Mariculture is an important component of the "fishery carbon sink" and has recently developed exceedingly quickly. In 2020, mariculture production in China was 21.35 million tons. The structure of Chinese mariculture is distinctively characterized by species-rich diversity, dominant species concentration, multi-trophic levels, lower trophic levels, high eco-efficiency, and high yields. Shellfish, dominated by filter-feeding species, is the main mariculture species and accounts for almost 70% of the total mariculture yields in 2020. The filter-feeding shellfish and their associated coastal ecosystems are closely related to the carbon biogeochemical process. Due to the national "dual carbon" strategy and the industrial demand of "accelerating the green and high-quality development of aquaculture", it is of great theoretical and practical significance to analyze the ecological services of aquaculture bivalves from the carbon perspective. Shellfish use carbon in two ways — by using dissolved inorganic carbon to build their calcium carbonate shell, and by consuming particulate organic carbon as phytoplankton. Simultaneously, shellfish produce carbon dioxide in two ways — the chemistry of calcium carbonate production releases CO2, and CO2 is released as a waste product of metabolic processes, like other animals. Therefore, shellfish farming is proposed as both a source and sink of carbon dioxide. This article summarizes the progress of carbon sequestration research for filter-feeding shellfish aquaculture, including a series of relevant studies on the carbon removal capacity during harvesting, individual-level carbon budget, life cycle evaluation, biodeposition of particulate organic carbon, the impact of sea-air interface on CO2 exchange and so on. In summary, the current understanding of "the relationship between filter-feeding shellfish and carbon" can be divided into three categories: One is supported by the underlying logic of "reductionism", which considers filter-feeding shellfish as a carbon source based on the carbon dioxide release process of respiration and biological calcification. Another category is "holism" thinking as bottom logic support. This view mainly emphasizes that the carbon contained in the shell can be locked in the shell for a long time. With a deepening understanding of ecosystem structure and function, an increased number of scholars have realized that the interactions between shellfish, carbon flow, and nutrient cycling are complex, and understanding the magnitude of direct and feedback interactions between cultured populations and phytoplankton, particulate organic carbon, dissolved organic carbon, and nutrient dynamics is crucial for understanding the carbon cycle. Therefore, assessing the role of shellfish aquaculture in the carbon budget should be based on an ecosystem approach that accounts for the complex trophic interactions involved in dissolved and particulate organic and inorganic carbon cycling. This review analyzes the shortcomings of the existing research. The topics investigate the insufficient scientific and systematic supporting data and our insufficient understanding of key processes and mechanisms. Specifically, calcifying physiology, which is the key process in shell formation by shellfish. Previous studies generally assume that shellfish mainly use carbon from the DIC pool of the ambient water to precipitate CaCO3, without accounting for the incorporation of respired carbon into shell carbonate. The proportion of seawater DIC and respired carbon used for CaCO3 precipitation need to be distinguished. Regarding future directions, this article suggests paying more attention to the basic research of carbon sink process and mechanism, the development of methodology and a trading system, the construction, and industrial applications of the carbon sequestration amplification model. The limited space and ecological capacity of the inshore area within a depth of 20 m beneath the sea level suggests mariculture will need to expand offshore (to 20~40 m beneath the sea level). The results provide a scientific basis for further understanding and evaluating the carbon sink effect of the filter-feeding shellfish culture ecosystem and its potential to serve the national "dual carbon" strategic goals.

Abstract:In view of the current global warming trends, China has a "double carbon" goal, which reflects China′s initiative in assuming the responsibility of dealing with global climate change. Oceans play an important role in achieving carbon neutrality. Oyster reefs are typical coastal ecosystems that contain huge carbon reserves and strong carbon sequestration ability. Due to overfishing, coastal engineering construction, environmental pollution, and climate change，the global oyster reef is in a seriously degraded state and urgently needs to be restored and protected. Oyster reefs released CO2 to the atmosphere in the processes of calcification and respiration and also bury large volumes of carbon during biological and physical deposition, which makes it uncertain whether oyster reefs are a source or sink of CO2. To explore the carbon source and sink function of oyster reefs, we summarized the research status of the carbon source-sink functions by oyster reefs, analyzed the key ecological processes affecting it, and discussed the characteristics of oyster reef carbon source-sink functions in different conditions. We aim to provide opinions and suggestions for research, restoration, and protection of oyster reefs. Until recently, few studies have reported the carbon source-sink functions of oyster reefs. A study of an oyster reef in Rachel Carson Reserve of North Carolina found oyster reefs have different carbon source-sink characteristics under different environmental conditions. The reefs on intertidal sandflats were net sources of CO2 [(710±120) g C/(m2·a)], whereas shallow subtidal reefs [(–100±40) g C/(m2·a)] and saltmarsh-fringing reefs [(–130±40) g C/(m2·a)] were net carbon sinks. The concentration of seston, water temperature, depth, hydrodynamic regime, oyster density, individual size, age, reef size and structure, and sediment are important factors affecting the carbon source-sink function of an oyster reef. The oyster calcification, biological deposition, biosynthesis, and respiration processes, sediment resuspension and decomposition processes, and the physical sedimentation of oyster reefs are the key ecological processes affecting the carbon source-sink function of an oyster reef. In the process of calcification, oysters absorb bicarbonate to form calcium carbonate shells and release CO2 to the atmosphere. Whether this process is the sink or source of atmospheric CO2 is controversial. Biological deposition by oysters can transport large volumes of organic carbon to the oyster reef sediment, the organic carbon accumulation rate can reach 30~270 g C/(m2·a), which is equivalent to the carbon sink rate of blue carbon ecosystems. Meanwhile, juvenile oysters have higher biological deposition rates than older oysters. The physical sedimentation in oyster reefs is also an important process of carbon deposition, the complex physical structure of an oyster reef can slow water flow, attenuate wave energy, and facilitate the deposition of particulate organic carbon. The influence of physical sedimentation by oyster reefs reaches far beyond the boundary of the oyster reef, the area with a carbon accumulation rate higher than 100 g C/(m2·a) surrounding the reef can be over twice the size of the oyster reef. Water velocity is a key factor affecting the resuspension of oyster reef sediments. A study of an oyster reef in an estuarine intertidal zone found that most uptake of particulate material by the oyster reef took place at velocities below 15 cm/s, and the release of particulates mainly occurred at velocities above 15 cm/s. It is more conducive to achieving long-term burial of sedimentary carbon in oyster reefs with low water velocities. The highest CO2 emissions from oyster reefs may come from the oysters themselves. Therefore, oyster respiration should be one of the main sources of CO2 from an oyster reef ecosystem. An evaluation of oyster reef carbon source-sink function needs to comprehensively consider multiple and complex biological processes. The oyster reef carbon sink functions do not only include the ability of the oyster reef habitat to bury carbon, but also their ability to improve the primary and secondary productivity of other organisms. Oyster reefs can promote the growth of macro-algae or salt marsh plants in the reef area by improving water transparency, stabilizing water flow, weakening wave erosion, and accelerating the biogeochemical cycle. Oyster reefs can also improve the productivity of fish, crustaceans, cephalopods, shellfish, and other organisms in the oyster reef ecosystem. In general, oyster reefs and macroalgae, salt marsh plants, and marine animals can jointly improve the carbon sink capacity of a coastal ecosystem. If the oyster reef is not damaged, the organic carbon buried by the reef can be preserved for a long time. The serious destruction of oyster reefs by human activities causes large volumes of organic carbon buried in oyster reefs to be released, which easily decomposes and returns to the atmosphere. The estimated global carbon emission caused by the destruction of shellfish reefs is approximately 400 million Mg, which destroys the carbon sink and storage functions of oyster reefs. Therefore, protecting the existing oyster reefs and preventing them from being damaged is important for reducing global atmospheric carbon emissions. At present, the formation of oyster reef carbon sinks have not been completely clarified, and a unified evaluation method of oyster reef carbon sink functions have not been established. There is no clear conclusion whether the global oyster reef is a sink or source of CO2. To clarify the carbon sink function of oyster reefs, we suggest research of oyster reef carbon sinks should be given priority in the future, including: 1. the effects of oyster calcification on carbon exchange between seawater and the atmosphere at different time scales; 2. the dynamic carbon budget of oysters in oyster reefs; 3. the carbon metabolism beneath the taphonomically active zone of oyster reefs; 4. the synergistic carbon sequestration effects between the oyster reef and macroalgae, salt marsh, phytoplankton, and marine animals; 5. the effects of global climate change on the carbon cycle of oyster reefs; and 6. the construction of carbon sink evaluation technology for assessing oyster reefs. These researches will determine the formation mechanisms of oyster reef carbon sinks, build oyster reef carbon sink evaluation technology, and establish oyster reef protection and restoration technology to improve the carbon sink capacity of oyster reefs.

Abstract:According to the latest definition of “Carbon Sink Fisheries”, fishing is one of the three basic modes of fisheries carbon sink and carbon sink amplification. In this study, the total carbon sink and net carbon sink for capture stock in China´s coastal ocean from 1979 to 2020 were assessed by the carbon content assessment method. For determining the total carbon sink for capture stock, carbon removal was first assessed by the carbon content of capture stock and fishing yield, and subsequently, the carbon content of phytoplankton was assessed at the end according to the food web mechanism and ecological conversion efficiency of each trophic level. The net carbon sink is the sum of carbon removal and carbon storage and can be calculated as the proportion of the total carbon sink. With the development and management of the marine fishery, carbon sink for capture stock in China´s coastal ocean has changed significantly over the past 40 years. The total carbon sink increased rapidly from 1458×104 t in 1979 to 6330×104 t in 1999 and then decreased to 4983×104 t in 2020. The average total carbon sink for the last three years (2018–2020) was 5246×104 t, equivalent to 701×104 ha of compulsory afforestation per year and about 8 times the total carbon sink for shellfish and macroalgae mariculture. The net carbon sink increased rapidly from 511×104 t in 1979 to 2215×104 t in 1999 and then decreased to 1744×104 t in 2020. The average net carbon sink for the last three years (2018–2020) was 1836×104 t, equivalent to 246×104 ha of compulsory afforestation per year and about 4 times of net carbon sink for shellfish and macroalgae mariculture. The study also proposed relevant suggestions for improving the accuracy of carbon sink assessment for capture stock and for enhancing carbon sink amplification.

Abstract:Oceans are the largest carbon pool on earth, and marine food webs play an important role in the carbon cycle. Research on marine food webs has revealed many key processes in ecosystems, such as nutrient cycling and energy flow. However, the complexity of basic carbon sources complicates the study of trophic structure and carbon cycling in the food web. Many studies have focused on the contribution of food web carbon sources in rivers, lakes, estuaries, and other aquatic ecosystems, but few have considered the contribution of food web carbon sources in the sea adjacent to coastal islands. The adjacent sea area of nearshore islands is a transition area between ocean and land and plays an important role in the carbon cycle and ocean carbon sink. The Miaodao Archipelago, located in the Bohai Strait in northern China, is at the intersection of the Bohai Sea and the Yellow Sea. Its ecosystem includes sea, intertidal zones, and island land and is extremely rich in biological resources. It is an ideal location to study the carbon source and sink dynamics of the food web in seas adjacent to islands. Isotope ecology provides a unique perspective for the study of carbon sources and sinks. The isotopic composition of an animal´s tissues depends on its food and reflects the overarching environmental conditions assimilated over time. An animal´s carbon isotopic composition can reveal its food sources. Because stable nitrogen isotopes are typically enriched in the consumer´s body, the amount of 15N increases with higher trophic levels. To investigate the contribution of different carbon sources to the food web and the mechanism of carbon sequestration, we studied the basic food resources and food web in the area adjacent to the Miaodao Archipelago in autumn 2020. The stable isotope analysis in the R (SIAR) package in R 4.0.5 was used to estimate the contribution of multiple food sources to each consumer species; this model incorporates stable isotope values (δ13C and δ15N) in the siarmcmcdirichletv4 command to estimate the relative energy contributions of different base energy sources (base food sources or primary producers). Based on the stable isotope method of carbon and nitrogen, the Bayesian mixing model SIAR analyzes the relative contributions of different carbon sources (phytoplankton, macroalgae, and attached algae, suspended particulate organic matter (POM), and substrate organic matter (SOM)) to the main consumer groups (aquatic benthic invertebrates, omnivores, benthivores, and piscivores). Based on this, we discuss marine carbon sinks and carbon sink fisheries. The δ13C isotope ranges from –26.54‰ to –16.92‰, δ15N ranges from 0.38‰ to 8.58‰, and δ13C ranges from –25.43‰ to –16.74‰, δ15N ranges from 2.84‰ to 13.06 ‰ for benthic invertebrates. Fish δ13C ranges from –22.26‰ to –15.10‰, and δ15N ranges from 7.35‰ to 14.54‰. The results showed that algae (phytoplankton, macroalgae, and ancillary algae) and SOM were the main carbon sources in the autumn food web of the waters adjacent to the Miaodao Archipelago, and the contribution of algae was the highest, while the contribution of POM was relatively low. The carbon source was mainly endogenous. Macroalgae and attached algae contribute more to aquatic benthic invertebrates. With regards to carbon sink fisheries, increasing algal cultivation and mixed cultivation of shellfish or shellfish feeding fish can increase carbon sink capacity and promote the development of carbon sink fisheries. The contributions of large algae, adherent algae, and benthic algae to the marine food web are controversial. In the present isotope study, large algae and adherent algae were the main carbon source of benthic invertebrates and fish, and their contribution to the carbon source was relatively high. Although food webs play a very important role in the ocean carbon sink and carbon cycle, the specific carbon sink process is still unclear. This was a preliminary study on the ocean carbon sink from the perspective of stable isotopes, and the results contribute to our understanding of the ocean carbon sink process. Through research on the contribution of carbon sources to the food web, the carbon intake of fish can be calculated more accurately, which helps us further understand the pathways and processes of carbon sequestration in marine organisms, the carbon cycle at the sea-land interface and promotes the development of a more reasonable carbon sequestration fishery management strategy. However, the food web is a dynamic and complex process, and many uncertainties underlie the carbon sink mechanisms of marine organisms. The carbon source contribution is only a relatively simplified model, which cannot fully interpret complex carbon source mechanisms. In future studies, the selection of carbon sources should be optimized under the guidance of the carbon sink fishery concept. The biological role of food webs in marine carbon sinks should be further studied.

Abstract:Marine carbon sinking is performed by "solubility pumps" and "biological pumps" and has significant advantages related to its large carbon sequestration capacity and long-time storage characteristics. Therefore, blue carbon can effectively mitigate the greenhouse effect caused by CO2 emissions and play an irreplaceable role in combating global climate change. Fishery is the primary human activity in the ocean economy, which has an important impact on the carbon cycle in inshore waters. Fishery carbon sinking is an indispensable part of blue carbon sequestration. Marine ranching is a healthy and low-carbon fishery model that restores the habitat and conserves aquatic biological resources. Construction of marine ranching could promote the ocean's primary productivity and expand its CO2 sequestration capacity. Moreover, increasing the resources in marine ranching can improve carbon transfer efficiency among different nutrient levels in the food web, accelerating the deposition of particulate organic carbon. In marine ranching, there is no feeding behavior in fishery production, wherein photosynthesis of phytoplankton and large algae provides food for other animals. Shellfish, crustaceans, fish, and other organisms in marine ranching areas rely on plankton, benthos, and fouling organisms for nutrition. Fishing of economically important fishery resources completes the "carbon removal" in seawater, which is one of the typical models of carbon sink fisheries. Therefore, marine ranching construction is an effective new way to expand fishery carbon sink. In this study, the research status on carbon sequestration mechanisms, processes, and potential key factors of the national and international marine ranching carbon sinking was summarized based on the definition of marine ranching in China. The critical roles of seagrass bed, oyster reef, and other typical marine ranching ecosystems were analyzed concerning the inshore carbon sequestration. Fundamental research directions were suggested to further understand the carbon sequestration mechanisms and carbon cycle, and explore amplifying marine ranching technologies such as the carbon accounting method, providing support for the study of marine carbon sinking amplifications and strategies concerning carbon peaking and carbon neutrality in China.

Abstract:This methodology regulates the carbon sink project preparation of design documents and measurement monitoring and ensures that the emission reductions generated by the project meet the requirements of measurable, reportable, and verifiable outcomes, which are necessary conditions for carbon trading. In this paper, we presented an overview of the current status of carbon sink project methodology, the key content of forest carbon sink methodology, and the research progress of marine fishery carbon sink theories and standards. According to the problems faced by marine fishery carbon sink in China, specific suggestions are proposed to provide scientific reference for the establishment of marine fishery carbon sink methodology and implement it into the carbon sink trading market early. Currently, the Clean Development Mechanism (CDM) and the Verified Carbon Standard (VCS) have developed 26 forestry carbon sink methodologies. There are 6 forestry carbon-sink project methodologies being developed by China Certified Emission Reduction (CCER) in China. In 2021, 9 blue carbon-related methodologies such as mangroves, wetlands, and seagrass were launched successively. In this paper, we have briefly introduced the main content of the forestry carbon sink project methodology in order to inspire the development of marine fishery methodology. The carbon sink project methodology usually consists of 3 parts. The first part includes an introduction, applicable conditions, normative references, and definitions. The second part includes baseline and project carbon methods of calculation. The third part includes the monitoring procedures. The carbon sink calculation method is an important part of the methodology. The key to the formation of carbon sink calculated methods is to clarify the carbon sequestration function and mechanism of project activities, including the identification of carbon storage changes in carbon pools going to be caused by the project activities. The determination and selection of carbon pools and greenhouse gas emission sources is the premise of carbon sink measurement. Also, except the carbon dioxide, we should pay attention to whether there are other sources of greenhouse gas emissions and energy consumption during fishing activities. In particular, attention should be paid to the timeliness of carbon sinks. If harvesting occurs under the project scenario, the long-term change in the carbon stock of the project's wood products is equal to the carbon in the wood products still in use and going to landfill at the end of the project period or 30 years after the product is produced, while the rest is assumed to be immediately discharged when wood products are produced. There are currently no international and national standards for monitoring and calculation of carbon sinks in fisheries. Without a series of methodological systems, it is impossible to evaluate fishery carbon sink capacity and tradable volume of China comprehensively and systematically. The coupling relationship between fishery carbon sinks and economic development has not yet been established. Several industry standards are under development. However, there are still uncertainties and many challenges regarding the timeliness and measurement methods of fishery carbon sinks. The primary controversy over the carbon sink of seaweed farming is the timing of carbon sequestration. For mariculture bivalve, it is complete and needs to be considered from the perspective of an entire ecosystem. In terms of yield and scale, China is the largest mariculture country in the world. Non-feeding species such as bivalve and seaweed are mainly mariculture species. The mariculture of bivalve and seaweed is advantageous owing to low cost, high yield, and strong controllability. At the same time, bivalve and seaweed can improve the regional marine environment, such as alleviating ocean acidification and hypoxia, reducing marine eutrophication and harmful algal blooms. Recently, the marine capture stock amounted to 1000 million tons. The fishery carbon sink is a relatively new concept, and its incorporation into the global climate governance system has just started. In this regard, it is recommended to strengthen the theoretical research on fishery carbon sinks, establish a data system for fishery carbon sink measurement, establish a demonstration area for the research of fishery carbon sinks methodology, and pay attention to the rational use of harvested shellfish, seaweed, and the captured stock, to solve the current problems regarding fishery carbon sinks and promote the development of carbon sinks fisheries. The development of the carbon sink project methodology will help promote the development fishery carbon sink trading market and play the role of fisheries in coping with climate change.

Abstract:Chinese tongue sole (Cynoglossus semilaevis) growth exhibits clear sex dimorphism, and the weight of mature female fish is significantly greater than that of male fish. In fish, muscle accounts for 40%-60% of body weight, and muscle growth is the core growth trait. Myostatin (mstn), also known as GDF-8, is a member of the transcriptional growth factor (TGF-β) superfamily. mstn is a negative regulator of muscle growth and development and has an inhibitory effect on myoblast proliferation. mstn gene deletion can lead to an increase in skeletal muscle mass. In recent years, research on the mstn gene has primarily focused on the aspects of gene cloning, expression pattern analysis, and RNA interference. Minimal studies have reported the differential expression of the mstn gene between male and female fish with sex dimorphism. The aim of this study was to explore the spatiotemporal expression pattern of the mstn gene in different sexes of Chinese tongue sole during muscle growth and development. The results showed that mstn expression was detected in the liver, spleen, intestine, kidney, gonad, gill, heart, brain, and muscle of 1-year-old fish, and mstn expression was the highest in muscle. The quantitative results of muscle tissue at different developmental stages showed that the expression levels in one- and two-year-old males were significantly higher than those in females (P<0.05), however, there was no significant difference in expression levels between males and females in other selected stages. Tissue section staining showed that the muscle fiber cross sections presented an irregular polygonal shape. The results of in situ hybridization showed that mstn gene expression was found at the edge of the muscle fiber, where myoblasts proliferate and differentiate. In this study, the spatial and temporal expression patterns of mstn were systematically studied in different sexes of Chinese tongue sole, which provides a basis for further studies on the growth differences between male and female Chinese tongue sole.

Abstract:The reasonable selection of salinity acclimation is one of the most important problems in Oncorhynchus mykiss culture. Calcium metabolism in fish varies with changing environmental salinity, and is a crucial component of salt metabolism. The scale compartment constitutes a significant, readily accessible calcium source in fish, as it can contain up to 20% of the body´s total calcium. In terms of complexity, scales resemble bone better than cultured osteoblast or osteoclast cell lines. Therefore, scales are ideal models for calcium metabolism and bone research. To provide more reliable data to decipher the bone metabolism at the molecular level, an analysis of the transcriptomic response to salinity acclimation was performed on rainbow trout scales. Fish were subjected to seawater (salinity 28) acclimation or freshwater maintenance for seven days. RNA sequencing (RNA-Seq) was performed using the Illumina HiSeq 4000 sequencing platform. By setting the screening conditions for the significant differentially expressed genes (DEGs) as log2|fold change|≥1 and P<0.05, 1714 DEGs were identified, whereof 484 and 1230 were significantly upregulated and downregulated, respectively. Gene Ontology function annotation analysis showed that the DEGs were primarily annotated in biological functions such as cell membrane, cytoplasm, nucleus, transportation, signal transduction, metal ion binding, and ATP binding. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis showed that the DEGs were significantly enriched in pathways such as oxidative phosphorylation, drug metabolism-cytochrome P450, proteasome, p53 signaling pathway, and myocardial contraction. Furthermore, quantitative real-time PCR (RT-qPCR) was used to examine the expression of eight randomly-selected DEGs, and the results of the RT-qPCR and RNA-Seq were consistent, indicating the reliability of the RNA-Seq data. Unigene in rainbow trout scales are predominantly enriched in bone metabolism-related pathways, such as the MAPK, Wnt, and calcium signaling pathways, indicating that scales can be used as a model for bone metabolism research. The results of this study showed that the salinity acclimation of O. mykiss was carried out at a rate of salinity 4 per day; bone metabolism-related genes, such as Mmp-2, Mmp-9, Acp5b, Alpl, Osteocalcin, OPG and Col12a1; and bone metabolism-related signaling pathways, such as NF-kB, MAPK-(JNK, p38, ERK1/2, STAT3), Wnt/β-catenin, BMP/Smads, and OPG-RANK-RANKL, did not have a significant influence, indicating that the acclimation model adopted in this study was reasonable. The results could help clarify the regulatory mechanism of O. mykiss bone metabolism in response to altered salinity, and lay a theoretical foundation for aquaculture industry development.

Abstract:Fish cells play an important role in virus isolation, identification, functional gene analysis, and biological product preparation. Mandarin fish Siniperca chuatsi is one of the most popular aquaculture species. With a rapid increase in production, the occurrence of diseases has also increased. Moreover, there are very few cell lines of mandarin fish that can be used for virus isolation and gene function identification. In this study, the body surface of mandarin fish was disinfected with povidone-iodine and 75% alcohol. The brain tissue was removed and washed in PBS buffer containing 2´antibiotic-antimycotic 4~5 times in a biosafety cabinet. The tissue was cut into blocks of approximately 3 mm3 with sterilized ophthalmic scissors in a sterile petri dish. After digestion at 37℃ for 0.5~1 h, the tissue blocks were cleaned with L-15 medium containing 10% fetal bovine serum (FBS). The suspension was centrifuged at 1000 r/min for 5 min, and the tissue blocks were transferred into a cell culture flask. A medium containing 30% FBS, penicillin, streptomycin, bFGF, and IGF was added to the cell culture flask, which was placed in an incubator at 28℃ for primary culture to establish the brain tissue cell line. To determine the growth properties of the new mandarin fish brain (MFB) cells, the cells were seeded onto 24-well plates with L-15 medium containing 10% FBS at an initial density of 5×104 cells per well. Cell growth rates were compared under different conditions, including five different media, four different serum concentrations, and four different temperatures. The optimal culture conditions for MFB cells were L-15 containing 10% FBS at 28℃. Under the optimal culture conditions, the doubling time of MFB cell number was ~46.6 h. Chromosome numbers for the 25th generation MFB ranged from 20 to 60, with a mode of 56 and frequency of 20%. Using the total genomic DNA of MFB cells as a template, specific primers were designed for the 28S rRNA gene. A partial gene fragment of 528 bp was obtained by PCR amplification, consistent with the expected size. The PCR-amplified fragments were sequenced and compared with the GenBank database by BLAST analysis. The sequence was consistent with that published in GenBank for the mandarin fish 28S rRNA gene (EF120974). Confirming that the cell line was derived from mandarin fish. NeuN (neuronal nucleus) is a neuro-specific nuclear regulatory molecule and a unique neuronal binding protein that is widely used in the study and diagnosis of neuronal antigens after mitosis. β-tubulin Ⅲ is a signature skeletal protein of mature neurons and is mainly distributed in the synapses and cytoplasm. The purity of MFB cells was determined by immunofluorescence cytochemistry (using β-tubulin and Neu-N). The results showed that all cultured MFB cells were neuron-like. Virus research requires the establishment of a sensitive cell line that allows for the proliferation of viruses in living cells. In this study, MFB cells were used to culture common aquatic viruses. The results of the virus sensitivity test showed that MFRaIV, LMBRaIV, and GSIV could infect and produce typical cytopathic effects in MFB cells, with titers of 108.68±0.12, 108.36±0.15, and 1010.15±1.85 TCID50/mL, respectively; thus, MFB cells are sensitive to MFRaIV, LMBRaIV, and GSIV. The replication level of GSIV in MFB cells was higher than that in MFRaIV and LMBRaIV. MFB cells were insensitive to ISKNV, GCrV-I, KHV, CEV, and CyHV-2. Transfection of exogenous genes into cells is a crucial step in gene function research. After 50 passages, Lipofectamine®2000 was used to transfer pEGFP-N1 into MFB cells. Green cells were observed under a fluorescence inverted microscope 48 h after transfection. The transfection efficiency was determined as the proportion of green positive cells in five random fields; the proportion of green cells was (22.20±1.72)%. In conclusion, a cell line derived from the brain tissue of mandarin fish was successfully established in this study. It is sensitive to a variety of aquatic animal viruses and can be used for gene transfection. It not only enriches the available resources of mandarin fish cells but also provides important experimental materials for further research into infection mechanisms and the development of virus and disease prevention technologies.

Abstract:To investigate the effects of different photoperiods on the growth, gonadal development, and hemolymph biochemical composition in Exopalaemon carinicauda, samples with a mass of (0.71±0.05) g and body length of (3.62±0.28) cm were studied under a water temperature of 23~25℃, salinity of 22~24, pH of 7.8~8.1, and illumination intensity of 1000 lx. The growth, gonadal development, and hemolymph biochemical components were measured under five photoperiods (where L, illumination duration; D, dark duration; whole-day illumination L, 16L:8D, 12L:12D, 8L:16D, and whole-day dark D). Results showed that the D group had the highest numerical survival rate (86.67±5.77)%, followed by 12L:12D group, the survival rate was (83.33±8.82)%, the survival rate of group 8L:16D was (80.00±3.33)%, with no significant difference between these three groups (P>0.05). The specific growth rate of group 8L:16D was (2.49±0.20)%/d, significantly higher than that of group D (P<0.05). The molting rate of group 8L:16D was the highest [(6.96±0.50) sub/(tail·d)]. The gonadal index increased gradually with a decrease in illumination time, and reached the highest value at 8L:16D [(2.37±0.04)%], significantly higher than that in other groups (P<0.05). The ovarian maturation rate of group 8L:16D was (42.93±4.57)%, significantly higher than that in group D (P<0.05), and the relative length of the ovary in group 8L:16D was significantly higher than that in other groups on the 30th day (P<0.05). The levels of protein and cholesterol in the hemolymph of group 8L:16D were significantly higher than those of group L, group 16L:8D, or group D (P<0.05), but there was no significant difference with group 12L:12D (P<0.05). The glucose concentration of group 8L:16D was significantly higher than that of group L and group D (P<0.05), but there was no significant difference with group 16L:8D and group 12L:12D (P>0.05). The triglyceride concentration in group 8L:16D was significantly higher than that in the other groups (P<0.05). The results showed that the growth, gonadal development, and hemolymph biochemical components of E. carinicauda were affected by the photoperiods. 8L:16D was the most suitable photoperiod for E. carinicauda ripening and factory farming.

Abstract:The seminal receptacle of cephalopods plays a key role in its reproduction. The number and location of seminal receptacles of different cephalopod species are different, so the seminal vesicles of cephalopods are of interest to researchers, similar to the sperm that female cephalopods store. Sepioteuthis lessoniana had an oval milky white seminal receptacle located on the buccal membrane. In this study, histological and transmission electron microscopy techniques were used to analyze the ultrastructural characteristics of the seminal receptacle of S. lessoniana for the first time. The results showed that the seminal receptacle was composed of glandular parietal tissue, a sperm storage bulb, and a central lumen. The cyst wall tissue was composed of epithelial cells, muscle tissue, and connective tissue. Epithelial cells could be identified by the visible round nucleus in the center of the cell. The demarcation between the monolayer epithelial cells and the connective tissue was obvious but the boundary between the muscle tissue and the connective tissue was not obvious. The muscle tissue was mainly smooth muscle, with a thickness of approximately 140 μm to 240 μm. There were many sperm storage bulbs, most of which were irregular in shape and a few are round or oval with diameters, ranging from 180 μm to 300 μm, which was the main part of the seminal receptacle. Two or more sperm storage bulbs joined to form a central lumen, and connective tissue and muscle tissue were scattered around the sperm storage bulb and the central lumen. A large number of sperm were present in the sperm storage bulb and central lumen, and the acrosome and flagella of the sperm were stained dark blue and red, respectively. The sperm storage bulb and central lumen were composed of monolayer epithelial cells with round or elliptical nuclei. The cytoplasm contained a variety of organelles, including endoplasmic reticulum, mitochondria, and Golgi complexes. The endoplasmic reticulum could be divided into rough endoplasmic reticulum and smooth endoplasmic reticulum. The number of mitochondria was also higher. The elliptical Golgi complex was composed of multilayered, arched flat sacs and secretory vesicles, which release a large number of secretory granules after rupture. In addition, a large number of vesicles were found, which were filled with secretory granules. There were cilia outside the epithelial cells, and many sperms could be seen in the sperm storage bulb and central lumen. The sperm were closely arranged and orderly, and the head of the sperm is directed toward the cilia. The sperm of S. lessoniana had a slender head, which was composed of a dome–shaped acrosome and a slender cylindrical sperm nucleus. The mitochondrial spur was located at the posterior end of the sperm nucleus, and the tail was a slender flagellum. The results showed that the seminal receptacle can store sperm, the epithelial cells of the sperm storage bulb can secrete sperm, and the secreted substances can attract sperm, which plays a role in the process of directing sperm into the seminal receptacle.

Abstract:Golden pompano Trachinotus ovatus is one of the most commercially important carnivorous marine species cultured in China. In recent years, the large-scale development of the aquaculture industry of T. ovatus has been rapid because of its fast growth rate and high flesh quality, and annual output has exceeded 100 000 tons. Currently, commercial formula feed for this fish still contains over 20%~30% of fishmeal (FM), the use of which precludes a sustainable aquaculture industry. As an alternative protein source, terrestrial compound proteins, which are a combination of several easily available and relatively low-priced terrestrial animal proteins and enzymatic hydrolysis or fermentation of terrestrial plant protein in a certain proportion, can alleviate the amino acid imbalance caused by a single alternative source. Terrestrial compound proteins are known to reduce the use of FM in marine carnivorous fish diets. In addition, fish protein hydrolysate is a new protein source that is processed from fish catches. In contrast to fish meal, it has an essential enzymatic hydrolysis step in the production process, so it contains more oligopeptides and free amino acids, which has been widely concerned in the industry. However, there are no reports on the application of fish protein hydrolysate in the feed of golden pompano. In order to decrease the use of FM in T. ovatus diets and evaluate the feasibility of substitution by terrestrial compound proteins and fish protein hydrolysate, this study was conducted to investigate the effects of replacing FM with compound protein and fish protein hydrolysate on growth performance, serum biochemical indexes, digestive enzyme activity, and tissue antioxidant capacity of T. ovatus. Four diets containing 42% crude protein and 12% crude fat were formulated, including D1 (control group), containing 30% fish meal; D2~D4 containing 14% terrestrial compound protein, 16%, 11%, and 6% fish meal, and 0%, 5%, and 10% fish protein hydrolysate, respectively. Methionine and lysine were added to the D2~D4 groups. T. ovatus juveniles (approximately 4 g mean initial weight) were bought from a local fish farm and then maintained for 2 months in two sea cages (2.0 m × 2.0 m × 2.0 m) at the coast near Nanao Marine Biology Station of Shantou University. A total of 360 fish (average body weight: [(7.28±0.10)g] were randomly distributed into 12 cages (1.0 m × 1.0 m × 1.5 m) and fed one of the four diets for 62 d. During the 62-d feeding trial, fish were fed to apparent satiation twice daily at 6:00 and 17:00, and the seawater temperature was 26.9℃~33.3℃, salinity was 27~33. The dead fish were collected over time and the day of death recorded. There were no significant differences in weight gain rate, specific growth rate, protein efficiency rate, feed conversion, feed rate, hepatosomatic index, viscerosomatic index, and condition factor, as well as pepsin, lipase, and amylase activity among all treatment groups (P>0.05). The whole-body protein content of groups D3 and D4 was significantly higher than those of groups D1 and D2 (P<0.05), and muscle fat content of groups D1~D3 was significantly higher than that of group D4 (P<0.05). The activity rate of aspartate aminotransferase in groups D2~D4 and alanine aminotransferase in groups D2 and D3 were significantly lower than those in group D1 (P<0.05). Total protein and globulin levels of groups D1 and D4 were significantly higher than those of groups D2 and D3 (P<0.05). Liver catalase activity, total antioxidant capacity, and muscle superoxide dismutase activity of groups D2~D4 were significantly higher than those of group D1 (P<0.05), but liver malondialdehyde content in groups D2~D4 was lower than that of group D1 (P<0.05). Overall, the whole-body protein content and tissue (liver and muscle) antioxidant capacity of group D4 were significantly higher than those of other groups, and no significant differences in growth performance were found between groups D4 and D1. The use of fish protein hydrolysate increased the level of protein metabolism and immune function of the fish and reduced the degree of liver cell damage. The results indicate that 14% terrestrial compound protein combined with 10% fish protein hydrolysate can effectively replace 80% fish meal in the diet of golden pompano when the amount of fish meal is reduced to 6%. This study is the first to investigate the feasibility of the application of fish protein hydrolysate in the formula feed of golden pompano, and the results will be relevant to the research for high-quality and inexpensive protein sources in the artificial formula feed of juvenile golden pompano.