Abstract: Marine macroalgae cultivation represents a cost-effective and highly controllable approach to enhance the ocean carbon sink. However, the lack of a well-developed carbon sink accounting system has prevented it from receiving sufficient attention. A comprehensive investigation on the chemical compositions of marine macroalgae is crucial for improving the carbon sink accounting system and promoting its potential of carbon sequestration. The content and stable isotope compositions (δ¹³C and δ¹⁵N) of organic carbon and nitrogen of Saccharina japonica, Gracilariopsis lemaneiformis, Pyropia haitanensis, Sargassum sp., Undaria pinnatifida, and Ulva pertusa from major coastal aquaculture areas of Fujian are determined in this investigation for the examination of their interspecies variations, regional differences, and potential application for carbon sink accounting. Results reveal that, while the organic carbon content and δ¹⁵N value are similar, the δ¹³C value exhibits significant variation among marine macroalgae species, indicating differences in carbon dioxide concentration and carbon sequestration mechanisms. For the same species of marine macroalgae, δ¹³C composition is somewhat stable throughout the Chinese coasts, whereas δ¹⁵N shows significant spatial variability, reflecting its assimilation of different nitrogen sources during growth. Estimation based on the organic carbon content and yield of major macroalgae in Fujian indicates that the removable carbon sink of S. japonica is higher than that of G. lemaneiformis and P. haitanensis, while P. haitanensis exhibits the highest organic carbon content and therefore a remarkable potential as a carbon sink. What’s more, due to significant variability in chemical compositions among different species of marine macroalgae, using the chemical composition data of different macroalgae species as endmember values will also lead to considerable variation in the organic carbon source apportion results. The selection of endmember values for marine macroalgae significantly influences estimates of their relative contribution to sedimentary organic carbon in Sansha Bay. Using S. japonica or G. lemaneiformis as the sole marine macroalgae endmember would lead to an underestimation (49.6%) or overestimation (22.9%) of the contribution of marine macroalgae to sedimentary organic carbon, respectively. Therefore, in future carbon sink accounting research, it is necessary to properly set endmember values based on the species composition of local macroalgae. This study has delineated the chemical compositions of common marine macroalgae in Fujian, providing data to support a comprehensive understanding of the carbon sink potential of marine macroalgae cultivation, and technical guidance for improving the accounting system for sedimentary organic carbon sinks in macroalgae cultivation.