Carbon dynamics in seawater and sediment: A case study of shellfish and seaweed mariculture systems

Shellfish and seaweed, the primary mariculture species in China, generate significant amounts of dissolved organic matter (DOM) during growth. This production significantly influences the carbon cycle in the marine environment. In the present study, we evaluated the DOM changes during growth in both seawater and sediments in Nan'ao, Guangdong Province, southern China. The results showed that both shellfish and seaweed growth increased organic carbon content in seawater and sediments. DOM and water-extractable organic matter in the seaweed cultivation area exhibited greater aromaticity and hydrophobicity, indicating that seaweed-produced organic matter is more difficult to decompose and resistant to consumption. This implies a potential to expand the refractory dissolved organic carbon (RDOC) pool in the marine environment. We also estimated carbon removal and carbon sequestration by shellfish and seaweed culture in Guangdong Province from 2012 to 2021. Average carbon removal by shellfish cultivation is at 227.81 Gg C yr^-1, and the release of carbon is at 205.71 Gg C yr^-1. Carbon removal by seaweed cultivation is at 22.95 Gg C yr^-1 with carbon sequestration of 11.89 Gg C yr^-1. Compared with shellfish, seaweed has a large carbon sequestration potential. The integrated aquaculture of shellfish and seaweed in adjacent areas, given the environmental and socioeconomic benefits of absorbing nitrogen and phosphorus nutrients, mitigating eutrophication, and ocean acidification, is advisable for coastal developing countries to promote shellfish-seaweed farming.