Carbon sink function and carbon footprint assessment of kelp cultivation

With increasing global seaweed production in recent decades, it has attracted wide attentions for its environmental benefits, especial for its contribution to blue carbon.  China's seaweed production ranks first in the world. Kelp (Saccharina japonica Areschoug) is the main macroalgae cultured in China. It has a very large potential for carbon removal. However, few studies focused on the quantification of carbon sink function and carbon emissions during the kelp cultivation processes. In this study, a carbon footprint (CFP) assessment framework for kelp cultivation was established using cradle-to-gate life cycle assessment and applied in Sanggou bay, the most famous and typical seaweed mariculture site in China. The functional unit of the CFP of kelp cultivation was defined as producing one ton of kelp, and its life cycle was divided into three phases: Breeding, transport, and culture. The results showed that the CFP of producing 1 t kelp was –95.93 kgCO₂e with the carbon emission of 74.30 kgCO₂e and the carbon absorption of 170.23 kgCO₂e, which indicated that the entire process from breeding to growth and harvest is a carbon sink process. A carbon sink of 79.9% was in the form of kelp biomass carbon, 14.1% existed in the form of deposited buried carbon, and 6.0% existed in the form of refractory dissolved organic carbon (RDOC). Previous studies on the carbon sink capacity of primary producers have primarily focused on biomass carbon formed by them. If RDOC and deposited carbon were not considered, the carbon sink of cultured kelp would be underestimated by approximately 20%. Aquaculture facilities were the main carbon source, and their carbon emissions accounted for 93.81%. The carbon emissions from diesel and electricity accounted for 5.05% and 1.14%, respectively. Fertilizer and transportation accounted for only one ten-thousandth of carbon emissions. The kelp seedlings were from local city, so the amount of CO₂ released during transportation was very low. Our suggestions for carbon emission reduction are that extending the service life of aquaculture facilities to reduce materials consumption, and strengthening the overall layout of the industrial chain to reduce energy consumption during transportation. This study provides data basis and scientific basis for carbon sink function and carbon sink trading of seaweed cultivation.