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.
