A review of existing and potential blue carbon contributions to climate change mitigation in the Anthropocene

Carbon removal, sequestration and release by mariculture in an important aquaculture area, China

To combat with climate change, most countries have set carbon neutrality target. However, our understanding on carbon removal, release and sequestration by mariculture remains unclear. Here, carbon removal, release and sequestration by maricultured seaweeds, shellfish and fish in Shandong Province during 2003-2022 were assessed using a comprehensive method that considers the processes of biological metabolism, seawater chemistry and carbon footprint. Saccharina japonica productivity has been largely enhanced since 2014, resulting in increased production and CO2 removal and sequestration. Seaweeds removed 172 Gg C and sequestered 62 Gg C in 2022. CO2 removal and release by shellfish demonstrated a slow increase trend, ranging from 231 to 374 Gg C yr-1 and 897 to 1438 Gg C yr-1 during 2003-2022, respectively. Contrary to seaweed and shellfish, maricultured fish added CO2 to seawater due to the use of feeds. The added CO2 by fish culture achieved the peak of 60 Gg C in 2011 and decreased to 25 Gg C in 2022. Most of this added CO2 was released to atmosphere by microbial mineralization and it was in the range of 21-52 Gg C yr-1 during 2003-2022. After summing up the contribution of seaweeds, shellfish and fish, both total CO2 removal (from 110 to 259 Gg C yr-1) and total CO2 release (from 929 to 1429 Gg C yr-1) increased remarkably during the past 20 years. To neutralize CO2 release by shellfish and fish, Pyropia yezoensis needs the largest culture area (1.65 ± 0.15 × 106 ha) while Gracilariopsis lemaneiformis requires the smallest area (0.11 ± 0.03 × 106 ha). In addition, there are enough available areas for culturing G. lemaneiformis, Ulva prolifera and Sargassum fusifarme to neutralize total CO2 emission in Shandong Province. This study elucidates carbon removal, release and sequestration capacities of mariculture and indicates that seaweed culture has a tremendous potential to achieve carbon neutrality target in Shandong.

Ocean acidification significantly alters the trace element content of the kelp, Saccharina latissima

Seaweeds are ecosystem engineers that can serve as habitat, sequester carbon, buffer ecosystems against acidification, and, in an aquaculture setting, represent an important food source. One health issue regarding the consumption of seaweeds and specifically, kelp, is the accumulation of some trace elements of concern within tissues. As atmospheric CO2 concentrations rise, and global oceans acidify, the concentrations of elements in seawater and kelp may change. Here, we cultivated the sugar kelp, Saccharina latissima under ambient (~400 μatm) and elevated pCO2 (600-2400 μatm) conditions and examined the accumulation of trace elements using x-ray powder diffraction, sub-micron resolution x-ray imaging, and inductively coupled plasma mass spectrometry. Exposure of S. latissima to higher concentrations of pCO2 and lower pH caused a significant increase (p < 0.05) in the iodine and arsenic content of kelp along with increased subcellular heterogeneity of these two elements as well as bromine. The iodine-to‑calcium and bromine-to‑calcium ratios of kelp also increased significantly under high CO2/low pH (p < 0.05). In contrast, high CO2/low pH significantly reduced levels of copper and cadmium in kelp tissue (p < 0.05) and there were significant inverse correlations between concentrations of pCO2 and concentrations of cadmium and copper in kelp (p < 0.05). Changes in copper and cadmium levels in kelp were counter to expected changes in their free ionic concentrations in seawater, suggesting that the influence of low pH on algal physiology was an important control on the elemental content of kelp. Collectively, these findings reveal the complex effects of ocean acidification on the elemental composition of seaweeds and indicate that the elemental content of seaweeds used as food must be carefully monitored as climate change accelerates this century.

A coupling model based on spatial characteristics and evolution of terrestrial ecosystem carbon storage: a case study of Hanzhong

Ecosystem carbon storage (ECS) is a critical consideration in reducing the impact of global warming and tackling environmental challenges, positioning it at the forefront of contemporary research. Due to the significant differences in the influence of land usage patterns on ECS in various policy contexts and China's commitment to attaining a carbon-neutral status, a model simulating different scenarios is needed to analyze the spatiotemporal characteristics and evolutionary process of carbon storage in terrestrial ecosystems accurately. To address this challenge, this study established a coupling model of "Geographical analysis -Evolution analysis -Predicting (GEP)" for assessing ecosystem ECS and analyzing its spatial characteristics and evolutionary patterns and projecting the spatial distribution of ECS under various developmental scenarios, which analyzed variations in ECS across different levels of magnitude and delineated the changing areas across a range of varying scenarios in the future additionally. The outcomes suggested that the ECS decreased by 1.17 × 106 t from 1990 to 2020, which pertaining to the utilization transfer of land in the area, whose change in ECS levels with a positive trend. It is predicted that the ECS will grow by 1.15 × 106 t and 1.44 × 106 t, in 2030 and 2060 compared with 2020 within the framework of natural development scenario (NDS), while within the framework of ecological protection scene (EPS), ECS will increase significantly, increasing by 3.06 × 106 t and 4.44 × 106 t. There will be more areas where ECS increases within the framework of EPS, by comparing with the NDS. This study offers a comprehensive analysis of Hanzhong City's carbon storage trends, demonstrating its significant impact on climate change mitigation and serving as a predictive model for similar regions, which underscores the importance of localized carbon management strategies, offering valuable insights for local governments in formulating effective climate adaptation and mitigation policies.

Diatom-mediated food web functioning under ocean artificial upwelling

Enhancing ocean productivity by artificial upwelling is evaluated as a nature-based solution for food security and climate change mitigation. Fish production is intended through diatom-based plankton food webs as these are assumed to be short and efficient. However, our findings from mesocosm experiments on artificial upwelling in the oligotrophic ocean disagree with this classical food web model. Here, diatoms did not reduce trophic length and instead impaired the transfer of primary production to crustacean grazers and small pelagic fish. The diatom-driven decrease in trophic efficiency was likely mediated by changes in nutritional value for the copepod grazers. Whilst diatoms benefitted the availability of essential fatty acids, they also caused unfavorable elemental compositions via high carbon-to-nitrogen ratios (i.e. low protein content) to which the grazers were unable to adapt. This nutritional imbalance for grazers was most pronounced in systems optimized for CO2 uptake through carbon-to-nitrogen ratios well beyond Redfield. A simultaneous enhancement of fisheries production and carbon sequestration via artificial upwelling may thus be difficult to achieve given their opposing stoichiometric constraints. Our study suggest that food quality can be more critical than quantity to maximize food web productivity during shorter-term fertilization of the oligotrophic ocean.

Microplastics - A major contaminant in marine macro algal population: Review

Microplastics (MPs) are a significant concern in this modern environment, and the marine environment is a sink for them now. Researchers have taken an interest in marine microplastic studies recently, which has opened the door to research in macroalgae and microalgae. Macroalgae are the primary producers in maritime ecosystems and are economically significant. This review aimed to identify the microplastic interactions with marine macroalgae and the impacts of microplastics on macroalgae based on existing literature while also recognizing knowledge gaps. MPs were mostly fibers and polymers with notable production and application levels; their abundance differed among species. More MPs were found in filamentous species than in other types. The results of this study indicated that, in maritime environments, macroalgae contribute to MP biomagnification and bioaccumulation. Adequate studies are needed to fill the research gaps in this area of MPs in macroalgae and their effects.

Blue carbon assessments of seagrass and mangrove ecosystems in South and Southeast Asia: Current progress and knowledge gaps

Coastal blue carbon ecosystems can be an important nature-based solution for mitigating climate change, when emphasis is given to their protection, management, and restoration. Globally, there has been a rapid increase in blue carbon research in the last few decades, with substantial investments on national scales by the European Union, the USA, Australia, Seychelles, and Belize. Blue carbon ecosystems in South and Southeast Asia are globally diverse, highly productive and could represent a global hotspot for carbon sequestration and storage. To guide future efforts, we conducted a systematic review of the available literature on two primary blue carbon ecosystems-seagrasses and mangroves-across 13 countries in South and Southeast Asia to assess existing national inventories, review current research trends and methodologies, and identify existing knowledge gaps. Information related to various aspects of seagrass and mangrove ecosystems was extracted from 432 research articles from 1967 to 2022. We find that: (1) blue carbon estimates in several countries have limited data, especially for seagrass meadows compared to mangrove ecosystems, although the highest reported carbon stocks were in Indonesia and the Philippines with 4,515 and 707 Tg within mangrove forest and 60.9 and 63.3 Tg within seagrass meadows, respectively; (2) there is a high difference in the quantity and quality of data between mangrove and seagrass ecosystems, and the methodologies used for blue carbon estimates are highly variable across countries; and (3) most studies on blue carbon stocks are spatially biased towards more familiar study areas of individual countries, than several lesser-known suspected blue carbon hotspots. In sum, our review demonstrates the paucity and variability in current research in the region, and highlights research frontiers that should be addressed by future research before the robust implementation of these ecosystems into national climate strategies.

Mariculture structure adjustment to achieve China's carbon neutrality and mitigate climate change

China is responsible for the biggest shellfish and macroalgae production in the world. In this study, comprehensive methods were used to assess the CO2 release and sequestration by maricultured shellfish and macroalgae in China. Through considering CaCO3 production and CO2 release coefficient (Φ, moles of CO2 released per mole of CaCO3 formed) in different waters, we find that cultured shellfish released 0.741 ± 0.008 Tg C yr-1 through calcification based on the data of 2016-2020. In addition to calcification, maricultured shellfish released 0.580 ± 0.004 Tg C yr-1 by respiration. Meanwhile, shellfish sequestered 0.145 ± 0.001 and 0.0387 ± 0.0004 Tg C yr-1 organic carbon in sediments and shells, respectively. Therefore, the net released CO2 by maricultured shellfish was 1.136 ± 0.011 Tg C yr-1, which is about four times higher than that maricultured macroalgae could sequester (0.280 ± 0.010 Tg C yr-1). To achieve carbon neutrality within the mariculture system, shellfish culture may need to be restricted and meanwhile the expansion of macroalgae cultivation should be carried out. The mean carbon sequestration rate of seven kinds of macroalgae was 174 ± 6 g m-2 yr-1 while some cultivated macroalgae had higher CO2 sequestration rates, e.g. 356 ± 24 g C m-2 yr-1 for Gracilariopsis lemaneiformis and 331 ± 17 g C m-2 yr-1 for Undaria pinnatifida. In scenario 0.5 (CCUS (Carbon Capture, Utilization and Storage) sequesters 0.5 Gt CO2 per year), using macroalgae culture cannot achieve China's carbon neutrality by 2060 but in scenarios 1.0 and 1.5 (CCUS sequesters 1.0 and 1.5 Gt CO2 per year, respectively) it is feasible to achieve carbon neutrality using some macroalgae species with high carbon sequestration rates. This study provides important insights into how to develop mariculture in the context of carbon-neutrality and climate change mitigation.

Removable carbon and storage carbon of golden tides

Due to ever-increasing global warming, ocean acidification, and inshore eutrophication, the outbreak of golden tides with Sargassum horneri has increased in the Yellow sea, where the biomass carbon enters three main carbon pathways: a. Removal of carbon from seawater by salvage, known as removable carbon; b. Biomass carbon is deposited to the seafloor through POC and RDOC through Biological Carbon Pump and Microbial Carbon Pump; c. Re-entering the carbon cycle through the food chain or re-entering the atmosphere through the action of microbes. Estimating carbon fixation (removable carbon) and storage (particulate organic carbon (POC) and refractory dissolved organic carbon (RDOC)) is vital in studying the global carbon cycle. In this research, it was observed that the C content of S. horneri was high, and the utilization rate of dissolved organic carbon (DOC), RDOC, and POC was also high in the eutrophication environment, where only 2.71 % of algal biomass carbon was converted to RDOC, and only 0.20 % converted to POC. The C + N + P combination has a restart effect on the seasonal accumulation of RDOC in relevant sea areas. It is suggested that the salvage and resource utilization should be strengthened to effectively control the golden tide and reduce the substantial economic losses to realize the win-win situation of carbon sink and environmental restoration.

The contribution of fish and seaweed mariculture to the coastal fluxes of biogenic elements in two important aquaculture areas, China

Carbon, nitrogen, phosphorus and oxygen (CNPO) are essential biogenic elements, driving life activities in marine environments. However, the integrated research of fish and seaweed culture on the fluxes of CNPO is scarce. To bridge the research gap, the contribution of mariculture of fish and seaweeds to the fluxes of CNPO in two important mariculture provinces, Fujian and Guangdong, in China, was investigated for the first time. Data from published literature and this study were integrated to calculate the CNPO fluxes using relative formulas. CNP release and O₂ loss caused by fish mariculture increased with year (2003-2020) and reached 185.55 ± 3.18 Gg C, 35.92 ± 0.51 Gg N, 7.27 ± 0.24 Gg P and 644.18 ± 11.05 Gg O₂ for Fujian and 215.81 ± 2.51 Gg C, 41.77 ± 0.40 Gg N, 8.47 ± 0.19 Gg P and 749.23 ± 8.71 Gg O₂ for Guangdong in 2020. The averaged P fluxes due to fish mariculture in Fujian and Guangdong during 2016-2020 are 2.2 folds of the Min River and 69 % of the Pearl River, respectively. CNP removal and O₂ generation by seaweed culture in Fujian also increased with year (2003-2020) and reached 555.74 ± 16.45 Gg C, 58.44 ± 4.83 Gg N, 7.80 ± 1.41 Gg P and 1481.97 ± 43.86 Gg O₂ in 2020. In contrast, seaweed culture in Guangdong resulted in maximal C (39.81 ± 1.43 Gg), N (4.33 ± 0.26 Gg) removal and O₂ (106.15 ± 3.82 Gg) release in 2013 and maximal P (0.41 ± 0.03 Gg) removal in 2019. The averaged N and P fluxes due to seaweed culture in Fujian during 2016-2020 are 69 % and 2.4 folds of the Min River, respectively. The different mariculture structure leads to a net CNP sink in Fujian but a net CNP source in Guangdong. The net CNP source may lead to seawater acidification, eutrophication and deoxygenation in coastal areas. These findings supply solid data for adjusting mariculture structure to achieve CNPO neutrality within mariculture.

Economic and biophysical limits to seaweed farming for climate change mitigation

Net-zero greenhouse gas (GHG) emissions targets are driving interest in opportunities for biomass-based negative emissions and bioenergy, including from marine sources such as seaweed. Yet the biophysical and economic limits to farming seaweed at scales relevant to the global carbon budget have not been assessed in detail. We use coupled seaweed growth and technoeconomic models to estimate the costs of global seaweed production and related climate benefits, systematically testing the relative importance of model parameters. Under our most optimistic assumptions, sinking farmed seaweed to the deep sea to sequester a gigaton of CO₂ per year costs as little as US$480 per tCO₂ on average, while using farmed seaweed for products that avoid a gigaton of CO₂-equivalent GHG emissions annually could return a profit of $50 per tCO₂-eq. However, these costs depend on low farming costs, high seaweed yields, and assumptions that almost all carbon in seaweed is removed from the atmosphere (that is, competition between phytoplankton and seaweed is negligible) and that seaweed products can displace products with substantial embodied non-CO₂ GHG emissions. Moreover, the gigaton-scale climate benefits we model would require farming very large areas (>90,000 km²)-a >30-fold increase in the area currently farmed. Our results therefore suggest that seaweed-based climate benefits may be feasible, but targeted research and demonstrations are needed to further reduce economic and biophysical uncertainties.