Impact of macroalgae farming on the carbonate system and biogenic sulfur dynamics in Sansha Bay, China

Macroalgae aquaculture plays a crucial role in carbon sequestration and mitigating coastal eutrophication, yet its impacts on carbon and sulfur cycling during the harvesting process is still poorly understood. Here, the effects of the harvesting on the carbonate system and biogenic sulfur release were examined in November, corresponding to the Laminaria japonica - Gracilariopsis lemaneiformis rotational aquaculture zones in Sansha Bay, China, in 2022. Our findings indicate that the significant release of dissolved organic carbon after harvesting occurred alongside the remineralization of organic carbon, making the aquaculture area act as a net atmospheric CO₂ source during the observation period. The average sea-to-air flux was 6.48 mmol m^-2 d^-1, which was 14.43 to 34.71 times higher than adjacent non-cultivated waters. Concurrently, the sinking macroalgal debris promoted dimethylsulphoniopropionate production in the bottom layers, elevating biogenic sulfur concentrations in the aquaculture area. In contrast, in-situ mesocosm cultivation showed that fresh G. lemaneiformis was mainly characterized by photosynthetic carbon fixation and sulfur release. The main difference is that the mesocosm system was production-dominated, controlled by photosynthetic carbon fixation and biogenic sulfur release from fresh macroalgae, whereas the field system was primarily dominated by debris input and organic carbon remineralization, which regulated carbon and sulfur cycling. These findings highlight the role of macroalgae in the carbon and sulfur cycle at harvest and highlight the need for optimized management strategies to more accurately assess the long-term impacts of macroalgae cultivation on the water carbon and sulfur cycle and its climate change.