Seaweed Farms May Fight Climate Change While Quietly Suffocating Coastal Waters

Seaweed farming has become one of the fastest-growing sectors of the global blue economy, and it is increasingly marketed as a natural weapon against climate change. Macroalgae cultivation now accounts for 57 percent of world mariculture production, and China alone produced roughly 3.0 million tonnes of farmed seaweed in 2024. But a new perspective article published in Discover Oceans by Yingxu Wu, Yanmei Liu and Di Qi of Jimei University argues that the climate story is far more complicated than the industry’s carbon-friendly image suggests. The authors synthesize evidence showing that while seaweed farms can genuinely boost carbon sequestration and biodiversity, they can also trigger eutrophication, oxygen depletion and acidification, especially in semi-enclosed bays where water barely circulates.

The core of the paradox lies in how seaweed handles carbon. As primary producers, macroalgae convert dissolved inorganic carbon into organic biomass through photosynthesis, drawing on both freely dissolved CO2, which diffuses across cell membranes, and bicarbonate, which is actively transported into cells via carbon-concentrating mechanisms. By lowering the partial pressure of CO2 in surface seawater, farms steepen the gradient that pulls atmospheric CO2 into the ocean. Based on carbon contents of roughly 20 to 35 percent of dry weight, China’s 2024 harvest alone removed an estimated 600,000 to 1,050,000 tonnes of carbon from seawater, while South Korea’s Porphyra industry removes about 80,000 to 140,000 tonnes annually. Photosynthesis also releases oxygen: large-scale cultivation can generate around 2.53 million tonnes of O2 per year, raising dissolved oxygen in the upper three metres of the water column by about 21 percent.

Carbon storage, however, depends on where the carbon ends up, not just how much is fixed. One pathway runs through the microbial carbon pump, in which bacteria transform the labile dissolved organic carbon released by seaweed into refractory dissolved organic carbon that resists degradation and can persist for long periods. The share of refractory material appears to grow as the cultivation season progresses, from as little as 3 to 4 percent of released dissolved organic carbon early in the cycle to estimates approaching 50 percent or more in long-term degradation experiments. Remarkably, the quantity of refractory carbon exported from farming areas to neighbouring waters appears comparable to the carbon removed in harvested biomass, underscoring how central microbial processing is to the seaweed carbon budget.

Sediments provide another sink. A recent global survey of 20 seaweed farms found a mean organic carbon burial rate of 1.87 plus or minus 0.73 tonnes of CO2-equivalent per hectare per year, roughly double the rate in adjacent non-farming areas. Aquaculture-derived sedimentary carbon accounts for less than 35 percent of total coastal stocks, but its millennial-scale residence time gives it outsized significance. Globally, about 90 percent of macroalgal-sequestered carbon is stored through deep-sea export, as detached fragments with buoyant pneumatocysts or structurally buoyant thalli drift with currents, sink through submarine canyons and reach deep sediments. Yet effective burial requires exchange with the open ocean, and many farms sit on shallow shelves in the Yellow, Bohai and East China Seas, where complex tides and circulation can resuspend carbon rather than lock it away.

The ecological benefits extend beyond carbon. Kelp forests and farms create structural habitat that supports juvenile fish, gastropods, limpets and crabs, and aquaculture infrastructure itself becomes a temporary biogenic reef for invertebrates and fish. In Gracilaria cultivation areas, plankton communities show significantly higher biodiversity at the peak of seaweed biomass than at nearby control sites, with elevated abundance, species richness and evenness. Macroalgal detritus also fertilizes low-productivity sandy habitats, feeding microbial and faunal communities through organic deposition. These habitat and food-web effects are a major part of the appeal of seaweed as a nature-based solution.

But the same biology that sequesters carbon can destabilize coastal chemistry. During senescence and decay, macroalgae release large quantities of labile organic carbon that microbes rapidly remineralize, consuming dissolved oxygen and releasing CO2. In open coastal systems, tides flush these byproducts away within days. In semi-enclosed bays, however, prolonged water residence times turn the system into something like a bioreactor, allowing organic matter to decompose completely in place. Observations from China’s North Yellow Sea show that summer macroalgae cultivation induces coupled oxygen depletion and acidification in benthic zones, and the world’s largest seaweed farm has documented emergent seasonal hypoxia and acidification linked to polyculture operations.

The numbers are striking. Kelp can export up to 98 percent of its production as detritus, and in natural kelp ecosystems detrital production averages about 82 percent of annual primary productivity. Timing matters enormously: Norwegian Saccharina latissima farms that harvest in June lose only 8 to 13 percent of annual production to erosion, but if biomass is left in the water until August, cumulative losses can reach 49.4 percent as senescence intensifies. Decomposing detritus beneath farms creates microenvironments prone to hypoxia, particularly where low-energy hydrodynamics restrict oxygen renewal. Acidification follows a similar pattern, with decomposition-driven CO2 accumulation lowering pH in subsurface and bottom waters, and seasonal warming further shifting carbonate equilibria toward higher CO2 and lower pH late in the growing season.

Hydrodynamics emerge as the decisive variable separating success from harm. Sanggou Bay, a semi-enclosed farming region with short water-exchange times, maintains year-round carbon sink functionality and shows none of the acidification seen elsewhere, while Sansha Bay, where flushing is far weaker, records dissolved oxygen as low as 44 percent saturation and functions predominantly as a net carbon source on an annual scale despite brief seasonal sink periods. The authors also flag subtler risks: shading from farm infrastructure can cut light beneath farms by 75 to 90 percent, contributing to seagrass decline in Indonesia; non-native species such as Kappaphycus alvarezii raise invasion concerns in Brazil; and translocation of farmed algae can introduce pathogens, parasites and genetic introgression with wild populations, a phenomenon documented in coastal China and Japan.

The perspective outlines management tools that could tip the balance. Integrated multi-trophic aquaculture pairs seaweed with finfish and shellfish so that one species’ waste becomes another’s resource: macroalgal detritus feeds filter-feeding bivalves, while algae absorb the CO2 from shellfish respiration, and calcified shells create net carbon storage. Fish-seaweed polycultures produce fast-sinking fecal pellets that bypass water-column remineralization, though fish feed itself can worsen oxygen and pH conditions if poorly managed. Artificial upwelling, which pipes nutrient-rich deep water into the sunlit zone, has shown promise in field trials in Qiandao Lake and the East China Sea, but sudden nutrient injections risk triggering phytoplankton blooms and must be calibrated to local conditions.

The authors’ central demand is a comprehensive evaluation framework that treats water-exchange efficiency and biogeochemical vulnerability as first-order criteria, alongside carbon yields. Photosynthesis and remineralization may balance stoichiometrically over a full cycle, following the Redfield ratio, but their spatial and temporal decoupling means surface gains can coexist with subsurface oxygen deficits and pH declines. Current sustainability assessments that focus narrowly on harvested carbon, the paper warns, overlook the benthic-pelagic coupling that ultimately determines ecosystem resilience. The proposed answer is hydrodynamically guided site zoning combined with adaptive management of cultivation densities and harvest timing. Seaweed farming, the authors conclude, is a double-edged strategy: capable of meaningful carbon capture and coastal benefits, but only if placed and managed with the physics of each bay in mind.

Subject of Research: Climate mitigation potential and ecological risks of coastal seaweed farming

Article Title: Reassessing the climate mitigation benefits and environmental risks of coastal seaweed farming

Article References: Wu, Y., Liu, Y., & Qi, D. (2026). Reassessing the climate mitigation benefits and environmental risks of coastal seaweed farming. Discover Oceans, 3(1), Article 37. https://doi.org/10.1007/s44289-026-00144-2

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00144-2

Keywords: seaweed farming, macroalgae, marine carbon dioxide removal, blue carbon, hypoxia, acidification, eutrophication, microbial carbon pump, hydrodynamics, aquaculture, coastal ecosystems, benthic-pelagic coupling