Scientific Significance Statement
Seaweed farming is increasingly recognized as a promising strategy for marine carbon dioxide removal (mCDR). However, its ecological sustainability, particularly in semi-enclosed bays, remains uncertain. Using data collected from Sansha Bay, Fujian, China, the world’s largest seaweed farming site, our study reveals an inherent trade-off: in highly sheltered coastal environments, especially when integrated with algae-fish polyculture, seaweed farming can induce significant hypoxia and acidification risks through organic carbon degradation. Carbon isotopic tracing further demonstrates that seasonal shifts in organic carbon sources—from fish feed in autumn to macroalgal detritus in spring—diminish the potential of macroalgal-based carbon sequestration. These findings emphasize the complexity of coastal carbon management and highlight the critical importance of considering ecosystem health—including the system’s capacity to maintain oxygen and pH stability and sustain biogochemical functioning—when implementing seaweed-based carbon sequestration strategies.
Abstract
Seaweed farming is increasingly promoted as a carbon sequestration strategy, but its effectiveness relies on carbon burial and export to deep waters. Seaweed farms commonly occupy semi-enclosed bays, causing continuous accumulation of organic carbon (OC) and its degradation products, potentially undermining carbon sequestration and driving hypoxia and acidification. These ecological impacts may be amplified in fish–algae polyculture systems, yet they remain unclear. We investigated carbon cycling in Sansha Bay, China, the world’s largest seaweed farm and intensive algae–fish polyculture site. During aquaculture seasons, bottom waters experienced rapid OC decomposition, causing severe oxygen depletion and acidification. Vertical mixing spread these effects throughout the water column, turning surface waters into net CO2 sources. δ13CDIC carbon isotopic analyses indicated seasonal shifts in dominant OC sources, from fish feed in autumn to macroalgal detritus in spring. These findings underscore the importance of evaluating the sustainability of coastal systems when pursuing seaweed-based carbon sequestration.
Liu Y., Yang W., Wu Y., Cai W. J., Li C., Lin H., Zhuang P., Zhang J., Xu Y., Qiu H., Huang Y. & Qi D., in press. Emergent seasonal hypoxia and acidification risks induced by seaweed and fish polyculture in the world’s largest seaweed farm. Limnology and Oceanography Letters. Article.
