Emergent seasonal hypoxia and acidification risks induced by seaweed and fish polyculture in the world's largest seaweed farm

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 CO 2 sources. δ 13 C DIC 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.