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.