Today, as the “dual carbon” goals and marine carbon dioxide removal (mCDR) attract widespread attention, the carbon‑sequestration capacity of nearshore bivalves has become a central topic of academic debate. However, existing research has largely focused on the carbon‑sequestration contributions of filter‑feeding bivalves—such as oysters—under conditions of external nutrient input, while critical knowledge gaps remain regarding natural or aquaculture settings without sustained external nutrient supply, as well as the roles of benthic organisms with different feeding strategies, such as deposit‑feeding mud snails.
Recently, the research team led by Professor Dong Shuanglin at Ocean University of China published a study in the Journal of Environmental Management. Through a 120-day mesocosm enclosure experiment, the study revealed significant differences between the Pacific oyster (Crassostrea gigas) and the mud snail (Bullacta exarata) in their roles in regulating marine carbon sequestration.
Why can’t we assess the carbon sequestration of shellfish by looking only at “shell carbon”?
Regarding whether shellfish can serve as a carbon sink, current assessments primarily adopt two perspectives: the individual‑level and the ecosystem‑level.
At the individual level, shellfish respiration and calcification may release CO₂, making it insufficient to assess their carbon sequestration contribution solely based on shell carbon.
At the ecosystem level, processes such as filter feeding, nutrient excretion, bioturbation, and harvesting may further enhance atmospheric CO₂ uptake and organic carbon export.
Therefore, the net carbon effect of bivalves must be assessed by comprehensively accounting for CO₂ exchange at the air–water interface, community production and respiration, net community calcification, as well as changes in the water column, sediments, and the bivalve biomass carbon pool.
Figure 1：The location of mesocosms
How to conduct the research
The experiment was conducted in a coastal pond in the southern Bohai Sea from June 8 to October 6, 2023, lasting a total of 120 days. The experiment included four treatment groups, each with three replicates:
Oyster group: 57.67 ± 0.57 g·m⁻²;
Oyster control group: no oysters were stocked;
Mud snail group: 52.70 ± 0.45 g·m⁻²;
Mud snail control group: no mud snails were stocked;
Oysters are suspended in the water column, while mud snails are placed on the bottom of the enclosures. Sampling and monitoring are conducted every 20 days between 8:00 a.m. and 10:00 a.m. (It should be noted that, to meet the dietary requirements of the mud snails, their sediment is pre-mixed with dry chicken manure (to increase organic matter content); however, no fertilizers are applied to the culture water throughout the entire period.)
Indicator measurements: CO₂ flux at the air–water interface, chl-a, PP, R, PP/R, SD, TN, TP, seawater PCO₂, TA, DIC, NCP, NCC, as well as DOC, POC, SOC, RSOC, and biological carbon, among others.
Core monitoring method: The study employed the PS-9000 Portable Soil CO₂ Flux System (Beijing Lijia United Technology Limited), which uses the dynamic chamber method to quantify CO₂ exchange fluxes at the air–water interface. Researchers mounted an automated respiration chamber on a floating platform and placed it on the water surface, connecting it to a gas analyzer to continuously measure changes in CO₂ concentration within the chamber. CO₂ uptake flux was then calculated by integrating these measurements with environmental parameters.
Figure 2. Seawater physicochemical parameters
Figure 3. Seawater CO₂ intrusion flux and carbonate system
Figure 4. Organic carbon pools and carbon sequestration in the system
Figure 5. Biogeochemical processes regulating carbon sequestration and their influencing factors
Key Findings
Compared with the oyster control group, the oyster group exhibited a significant decrease in chl-a concentration, while PP, PP/R, and SD all increased significantly. During the experiment, the oyster group’s PP/R ratio rose from 1.48 to 1.98, indicating an enhanced degree of autotrophy in the system. This was attributed to the oysters’ top-down grazing on phytoplankton, which alleviated the self‑shading effect among phytoplankton populations.
The nutrient excretion of mud snails promotes the proliferation of phytoplankton; however, due to the absence of filter-feeding regulation, excessive algal accumulation intensifies the self‑shading effect in the water column and community respiration, causing the PP/R ratio to decrease from 1.48 to 1.18. This indicates a significant reduction in their autotrophic capacity and that they function as a source of atmospheric CO₂.
In oyster systems, primary production and calcification are the dominant processes; both consume DIC, leading to a decrease in seawater pCO₂ and enhancing atmospheric CO₂ uptake.
In the mud snail system, due to its extremely weak calcification capacity, respiration and organic matter mineralization dominate, continuously releasing CO₂ into the water column, thereby increasing seawater pCO₂ and exerting a negative regulatory effect on the ocean’s capacity to absorb atmospheric CO₂.
Oysters promote the accumulation of RSOC through biogenic sedimentation, with a combined treatment effect of 109.14 mg C·m⁻²·d⁻¹ for shell carbon and RSOC; in contrast, mud snails reduce carbon sequestration due to sediment disturbance and potential remineralization, resulting in −2.69 mg C·m⁻²·d⁻¹. The overall carbon budget indicates that the CIMδ values for oysters and mud snails are +52.47 and −24.68 mg C·m⁻²·d⁻¹, respectively, suggesting that under the experimental conditions, they exert positive and negative carbon sequestration effects, respectively.
RF model analysis indicates that SOC is the key environmental factor regulating both CIM and RSOC, accounting for 8.12% and 5.33% of the variance, respectively, thereby underscoring the central role of sediment carbon pool management in assessing shellfish carbon sequestration.
Peroration
Whether shellfish can serve as a carbon sink hinges not on the mere formation of shells, but on their integrated effects on community metabolism, gas–water CO₂ exchange, and the sedimentary carbon pool. Using monitoring tools such as the PS‑9000, this study elucidated, at the ecosystem scale, the markedly distinct carbon cycling pathways of oysters and mud snails, thereby providing a scientific basis for assessing shellfish‑driven carbon sequestration and for coastal ecological management. Future research will need to incorporate long-term in situ flux measurements, analyses of sedimentary carbon transformation processes, and investigations under varying aquaculture densities and environmental conditions, in order to further clarify the mechanisms underlying shellfish carbon sequestration and its applicable limits, thus supporting evidence‑based strategies for enhancing carbon uptake and achieving precision management in coastal ecosystems.
Figure 6. The impact of different dietary habits among shellfish on marine carbon sequestration pathways
Journal Published: Journal of Environmental Management [Impact Factor: 9.2]
Research institutions: Ocean University of China, Yanzhou Bay Innovation Research Institute of Hainan Tropical Ocean College, and others.
Study site: A seawater pond on the southern coast of the Bohai Sea, China
Equipment used: PS-9000 Portable Soil CO₂ Flux System
DOI: https://doi.org/10.1016/j.jenvman.2026.130782