Coastal macroalgae aquaculture reduces carbon dioxide emission in a subtropical enclosed bay: Insights from eddy covariance measurements

Macroalgae aquaculture has been increasingly recognized as a promising nature-based solution to enhance carbon sinks towards climate change mitigation. However, a limited understanding of the temporal patterns of air-sea carbon dioxide (CO₂) fluxes and their environmental controls across time scales poses an enormous obstacle to the carbon sink potential assessment of macroalgae aquaculture. Here, we utilized the eddy covariance (EC) approach to acquire continuous and high-frequency measurements of net ecosystem exchange (NEE) of CO₂ over the macroalgae aquaculture in a subtropical enclosed bay in southeast China, throughout one full year from April 2023 to March 2024. The results showed (a) this ecosystem acted as a CO₂ source in most months with the strongest source and sink occurring at the beginning of autumn and winter, respectively; (b) annually this ecosystem emitted 58.9 g C m⁻² of CO₂ into the atmosphere with nighttime source contributing 84.7 %; (c) macroalgae aquaculture of Saccharina japonica and Gracilariopsis Lemaneiformis tended to reduce CO₂ emission from this ecosystem, while the extent of the reduction varied with aquaculture types and growth stages; (d) temporal variability of NEE was most correlated with air temperature, while faster tidal currents tended to stimulate CO₂ emission during both flood and ebb tides. The strong temporal variability of NEE highlights the importance of high-frequency EC measurements in improving the understanding of temporal patterns of air-sea CO₂ fluxes over the macroalgae aquaculture ecosystems. This study suggests that macroalgae aquaculture has the potential to mitigate CO₂ emission, although the ecosystem itself overall functions as a net CO₂ source on an annual time scale.