Sustainable and scalable seaweed cultivation is critical to producing low-carbon seaweed-based products that can mitigate global greenhouse gas (GHG) emissions at the gigaton-scale (DeAngelo et al., 2023). Meeting this potential requires innovations in infrastructure materials, fuel use, and dewatering/drying to increase seaweed production with a low carbon footprint. Therefore, this section analyzes the state of the science, market, and policies involved in seaweed cultivation and dewatering/drying, associated knowledge gaps, and first-order priorities for filling said gaps.

Elements of Seaweed Cultivation

Nurseries / hatcheries

Some seaweeds pass through a hatchery and/or nursery before its grow-out phase on a farm. Hatcheries are controlled facilities where spores or cuttings are germinated, conditioned, and grown to transplant-ready seedlings under managed light, temperature, and nutrient conditions (Andersen et al., 2005).  Traditionally, these seedlings are sown onto a substrate like a rope in a nursery during its most vulnerable juvenile stage, allowing for early growth in a controlled environment.  Nurseries/hatcheries enable large-scale industrialization by supplying farms with high yields of genetically standardized seedlings, helping control genetic variability so the harvested biomass—and resulting products—remain consistent, and by providing seedlings that are free of pathogens and pests. In mature and emerging markets alike, a single nursery/hatchery often supplies multiple growers (Andersen et al., 2005).

Seaweed Cultivation approaches

Seaweed has been used by humans for over 10,000 years and cultivated for generations in certain communities (Mouritsen et al., 2024). Today, cultivation is practiced across three broad settings — nearshore, onshore, and offshore — each defined by its distance from shore, degree of environmental control, number of inputs, and suitability for different scales and end-uses. Nearshore cultivation has been the most common approach, but research and innovation in management practices and farm infrastructure has expanded cultivation on to land as well as further offshore.

Nearshore

Nearshore cultivation typically occurs within three nautical miles of the shore (Tullberg et al., 2022). They range in size from 0.1–1 hectare low-input smallholder farms to 15,000-hectare high-input commercial farms.

Onshore

Onshore seaweed cultivation grows seaweed in terrestrial facilities, operating in open or closed systems to circulate nutrients through tanks, raceways, photobioreactors, or ponds (Buschmann et al., 2017). It permits tighter operational and environmental controls than near- or offshore cultivation.

Offshore

Offshore seaweed cultivation, defined as farming beyond three nautical miles but within 200 nautical miles, can supply biomass at a scale that nearshore and onshore cultivation cannot match (ARPA-E, 2025; Golberg et al., 2018; Ross et al., 2025). Studies suggest that if offshore cultivation was conducted at scale, roughly 650 million hectares or 1.8% of ocean area, it could produce ~6.5 billion tons of dry seaweed per year; however, the area needed for cultivated could reach up to 20 billion hectares given geographic variation in nutrient limitations (Arzeno-Soltero et al., 2023; Spillias et al., 2023).

Harvesting

Harvesting practices remove seaweed from the farm for transport to processing sites that develop the raw biomass into end-products; strategy varies according to the species lifecycle, location, and seasonality (Amponsah et al., 2024; Buschmann et al., 2014; Vásquez et al., 2012). For example, seaweed destined for agricultural supplements requires harvesting systems timed to peak bioactive compound concentration and rapid onsite stabilization (Hafting et al., 2015).

Cleaning

Cleaning begins as soon as seaweed is harvested and scales in intensity with product quality. At its simplest (e.g., in making construction materials), cleaning involves a deck hose rinse to remove sand and surface debris. At its most rigorous (e.g., in blue food development), it encompasses sequential washes, manual quality selection, heat treatment to neutralize biotoxins, and chemical and microbial analysis to verify compliance with regulatory standards (Radulovich et al., 2015).

Dewatering and Drying

Seaweed holds 70–90% water by weight; it must be dried for cost-effective storage, transport and downstream processing. Because of this, drying can account for as much as 75% of  a product’s net emissions and critically impacts the carbon footprint of a seaweed end-product (Albright & Fujita, 2023; Nilsson et al., 2022). In practice, dewatering and drying are two sequential steps rather than alternatives. Mechanical dewatering — using screw presses, belt presses, or centrifuges — removes bulk water first, typically reducing moisture to ~50–60% without heat; this reduces transport weight and the load placed on subsequent drying. Thermal or passive drying then brings moisture down to 10-20%.

Species Selection

There are over 10,000 species of seaweed that vary substantially in their life cycles, biochemical compositions, and how they interact with marine biodiversity. That variation impacts the full life cycle of a product, determining how well it performs and ultimately whether and how it can contribute to gigaton-level decarbonization of an industry. However, only about eight genera are responsible for more than 96% of current global seaweed production, and more than 90% of that production is used for the food and pharmaceutical industries (Chopin & Tacon, 2021; Albright and Fujita, 2023; The State of World Fisheries and Aquaculture, 2024; Table 1).

Table 1. Top cultivated seaweed genera in the world, as of 2020. Adapted from Chopin and Tacon (2021).

Species selection in practice is shaped by farm type and context. Nearshore farms generally cultivate endemic species suited to local growing conditions – Kappaphycus and Eucheuma in the Coral Triangle are the clearest example — though non-endemic cultivation does occur (e.g., Saccharina japonica in China; Duarte et al., 2022).  Onshore operations select for species that perform well under tightly controlled environmental conditions (CO₂, pH, nutrients, water motion), making species endemism a secondary consideration relative to product performance. For offshore cultivation, the high cost of farm deployment and maintenance drives selection toward large-bodied, buoyant, and fast-growing species — including Sargassum, Saccharina, Laminaria, Macrocystis, and Ulva — that can generate commercially viable biomass yields. This is occurring in China and other parts of the world. For example, Ocean Rainforest grows Saccharina latissima in the Faroe Islands nearshore and are researching scalable cultivation techniques in the open ocean off of the United States (Camarena Gómez and Lähteenmäki-Uutela, 2024). Sea6 Energy grows Kappaphycus alvarezii in the open ocean off of Indonesia (Albright & Fujita, 2023).

Climate mitigation goals are increasingly shaping species selection, but some major seaweed species are already near the limits of their temperature optima—and in some cases their physiological tolerance (Corrigan et al., 2025). The most direct example is Asparagopsis taxiformis, whose high bromoform content makes it effective as a livestock feed supplement to reduce enteric methane emissions; this has led to selective breeding programs focused on maximizing bromoform yield and growth rate in controlled cultivation environments (e.g., Symbrosia; Albright & Fujita, 2023).

Hatcheries/Nurseries

For species that are cultivated during their sexually reproductive life cycles, modern hatcheries clone individuals as “broodstock” for their desirable traits (e.g., fast growth, disease resistance). This reduces the time needed to produce genetically stable, high-performing seedstock, shortens the nursery stage, and cuts production costs (e.g., Li et al., 1999). The broodstock are maintained in controlled tanks, where they release reproductive cells onto seeding substrates; the resulting seedlings are grown to transplant size (typically 2–5 cm) in nurseries before transfer to farms, or frozen for future use(Buschmann et al., 2017).

For species with vegetative reproduction lifecycle phases (e.g., Kappaphycus), cuttings are placed directly onto cultivation lines or in tanks to grow. This is simpler and more cost-effective, but decades of repeated cloning have narrowed genetic diversity, contributing to yield declines of up to 15% in Indonesian Kappaphycus and Eucheuma farms in recent decades Rimmer et al., 2021).

Recent advances are accelerating hatchery productivity on several fronts. This includes testing ways to optimize process steps from fertilization to seedling retention on substrates and creating protocols to standardize these steps (Aldridge, 2025). Space and energy savings are being pursued through drawer-based spool incubation systems. In countries in Asia, nurseries often employ seed collector frames (thin twine wound around frames and packed at high density, leaving only narrow gaps for light) that increase packing efficiency by orders of magnitude (see Figure 1). Hatchery processes are also being optimized with eventual farm yield in mind. Chinese summer seedling techniques, like growing seedlings to 5–20 cm over summer before autumn sea deployment to increase resilience, are being trialed in Europe with early promising yield results. Selective breeding programs, a standard practice in China, South Korea, the Philippines, and Japan for over 50 years, are now being developed in the Western countries, with genomic tools such as gene sequencing and statistical breeding pair selection accelerating strain improvement for yield, nutritional composition, and climate tolerance (Aldridge, 2025).

Traditionally, nurseries in Asia have relied on incubation—growing seedlings on thin twine in controlled tanks before transferring them to cultivation rope. Direct seeding — applying seedlings directly onto cultivation rope before sea deployment and bypassing the nursery stage entirely — has attracted interest as a lower-cost alternative, but has shown inconsistent results in Western contexts, with yields in some locations up to four times lower than nursery-incubated seedlings due to the greater vulnerability of seedlings to biofouling and dislodgement before holdfast attachment (Aldridge, 2025).

Cultivation

Cultivation approaches vary in the infrastructure and operational management needed to maintain consistent high-quality yields and a productive working environment.

Nearshore (Technological Readiness Level 9)

Farming methods depend on species and geography and use three broad classes of cultivation structure – lines, rafts or nets (see Figure 2). Fixed and floating systems — including off-bottom long-lines, hanging long-lines, and floating rafts — dominate in tropical smallholder operations across Southeast Asia, where low capital is a primary constraint. All three structures are sensitive to stronger wave energy and weather events; fixed off-bottom systems also experience higher seedling loss and difficulty managing epiphytic growth, while floating rafts carry ecological risk from drifting over large distances.

Long-line and grid systems — horizontal and vertical long-lines and horizontal grid/raft configurations — are used in temperate East Asian commercial operations for Saccharina and Undaria cultivation. They offer better light distribution and space efficiency, and horizontal grids are suited to large-scale operations, but all require more labor than fixed and floating systems. Horizontal grids also carry a risk of shading benthic habitats below the farms.

Net systems — floating turnover nets, semi-floating nets, and fixed nets — are used almost exclusively for Pyropia cultivation in East Asia. Their advantage is flexibility across tidal depths; their main operational cost is the need to raise nets daily to mitigate biofouling. For detailed configuration specifications, species coverage, and geographic distribution, see Hatch Blue Seaweed Insights (2026).

There is active research on how seaweed cultivation can grow in integrated multi-trophic aquaculture (IMTA) systems, where seaweed can enhance productivity and play a buffering role against eutrophication, ocean acidification, and deoxygenation (Li et al., 2016; Tullberg et al., 2022).

Onshore (Technological Readiness Level 7–8)

Onshore cultivation systems are currently being developed by businesses looking to either bring more control to the cultivation process or because they are unable to grow seaweed in regional waters (e.g., CH₄ Global, Figure 3). Operators and researchers are pursuing several approaches to reduce operational costs and improve overall efficiency; Table 2 below summarizes current innovations, examples, and readiness/status levels.

Table 2. Onshore cultivation innovations, current examples, proposed benefits of the innovations, and technological readiness. Sources: Albright and Fujita (2023), Camarena Gómez and Lähteenmäki-Uutela (2024); Buschmann et al. (2017), and Hafting et al. (2015)

Offshore (Technological Readiness Level 2–6)

Offshore seaweed cultivation remains limited to a small number of pilot and demonstration sites, primarily off the coasts of Norway, the United States, and the Faroe Islands (Fisheries, 2025; van den Burg et al., 2013; Marine Biological Laboratory; NOAA National Centers for Coastal Ocean Science).

Offshore farms must endure difficult water conditions alongside nutrient and light limitations. Current farm designs use floating long-lines, rafts, and net systems with controlled buoyancy, like the tension leg platforms developed by the Korean Institute of Ocean Science and Technology for Saccharina japonica designed to withstand 50-year typhoon conditions (Chung et al., 2015). An active R&D focus is depth cycling and artificial upwelling to improve nutrient access and reduce storm exposure, yielding tradeoffs between growth and quality. For example, Navarette et al., 2021 found that depth-cycled Macrocystis pyrifera grew faster than fixed-method and naturally growing kelp, but with greater morphological variation. Table 3 summarizes the current state of offshore structural and operational innovations (Buck & Buchholz, 2004; Chung et al., 2015).

Table 3. Offshore cultivation innovations, current examples, proposed benefits of the innovations, and readiness/status.

Harvesting

Harvesting falls into two strategic approaches — total and partial — and two operational modes — manual and automated. These choices interact: partial harvests favor manual or semi-automated methods for precision; total harvests favor automated approaches. Operationally, harvesting is carried out manually at smallholder and quality-sensitive farms, and by mechanized or automated systems at large-scale and offshore operations (see Table 4 for a summarized comparison of approaches).

Table 4. Comparison of total versus partial harvesting approaches

Offshore

The harsh conditions of offshore cultivation and high operational costs are driving research into automated harvesting. For example, Sea6 Energy has developed an automated harvester that trims younger growth, removes and reseeds the tube nets with a portion of the trimmed younger growth for future harvest (Albright & Fujita, 2023).

Dewatering/drying

The appropriate dewatering and drying processes vary with final product, farm location, and operational scale. Smallholder nearshore farms typically rely on passive drying without mechanical pre-treatment (e.g., Dineshkumar et al., 2024); large commercial operations use screw press dewatering (e.g., in agriculture supplement production) followed by thermal drying. Dewatering/drying processes typically occur at or near to the cultivation site given the short shelf life of fresh seaweed (Paine et al., 2021).

Emerging technologies are focused on integrating multiple methods, reducing drying time, final moisture content, and energy consumption for small- and large-scale operations (Santhoshkumar et al., 2023). For example, MAVUNOLAB developed a low-cost solar-powered dryer to dry seaweed and fish on-site, helping small-scale farmers avoid post-harvest losses from spoilage (Royal Academy of Engineering, accessed 2026). In 2025, the United States’ Department of Energy sent out a funding call for at-sea dewatering and preservation technologies to reduce water content as low as 10% and prevent decomposition for ≥14 days, including non-evaporative thermal, ultrasound-based, and freeze-drying approaches (ARPA-E, 2025).

Table 5. Seaweed dewatering and drying technologies used, the status of their commercial readiness, seaweed final moisture content, and R&D innovation focus (Source: Santhoshkumar et al., 2023).

Post-drying preservation and storage

Low-carbon methods of preservation such as ensiling (the preservation of wet seaweed biomass through lactic acid fermentation under anaerobic conditions, or by direct acid addition) and process optimization such as cascading biorefineries, and/or co-locating harvest and processing operations, are being explored as ways to bypass the energy costs of mechanical dewatering and drying while still preserving seaweed biomass for downstream processing (Nilsson et al., 2022).

A seaweed farm’s net GHG emissions are determined by the emissions generated by farm operations and the net sequestration of GHGs that occur on and around seaweed farms. Achieving the mitigation potential of low-carbon seaweed products is thus dependent on keeping a seaweed farm’s operational emissions low and understanding and robustly reporting its net sequestration.

This section explores the net emissions of a seaweed farm. Most emissions stem from infrastructure materials, vessel fuel use and post-harvest dewatering/drying, the latter of which is discussed separately. Furthermore, seaweed farms’ carbon sequestration potential through carbon burial would only offset a fraction of emitted emissions. Figure 5 and Table 6 summarizes the operational emissions and net sequestration potential of seaweed farms covered in this section.

Operational emissions

There are currently few cradle-to-harvest lifecycle analyses (LCAs) of seaweed cultivation, and the majority are concentrated in the Global North, covering smaller pilot-scale farms that represent approximately 1% of global production (FAO, 2024). This limits confident generalization about the emissions profile of production at current or projected scale (for example, see the table below). If we assume that the average GHG emissions (~108 kg CO₂e/ton fresh weight, FW) for commercial seaweed farms in China, for which we have high-quality LCAs, applied to 2023 global seaweed production (~36 million tons FW; FAO, 2024), then global seaweed farms in 2023 would have released roughly 4.9 million tons CO₂e, excluding emissions from dewatering/drying.

Dewatering and drying

Dewatering and drying seaweed is the largest single contributor to the climate impact of most seaweed-based products. For example, Nilsson et al. (2022) estimated that the climate impact of producing Saccharina latissima as biorefinery feedstock increased 40-fold from 0.16 kg CO₂e per kg FW to 6.12 kg CO₂e per kg dry weight after drying (Table 6). Using this value at scale, and assuming that 10 pounds of seaweed make only 1 pound of dry seaweed (following calculations from Li et al., 2017), drying all the seaweed produced in 2023 would have released more than 22 million tons of CO₂e.

Embedded emissions

Across Global North LCAs, the embedded emissions of cultivation infrastructure and their materials can make up the second-largest proportion of emissions after dewatering/drying; this is lower in the Global South (Langlois et al., 2012; Taelman et al., 2015; Thomas et al., 2021; Nilsson et al., 2022; Zhang et al., 2022). Using our 2023 assumptions, they would have been responsible for roughly 2.5 million tons CO₂e. In commercial farms in the Global South, successful attempts have been made to reduce emissions. Li et al. (2023) found that adopting heat exchangers to recycle water through flow‑through systems could reduce cooling‑related energy use by 20–30%. Furthermore, recycling existing cultivation lines and buoys, and manufacturing new ones from recycled materials, could reduce the farm’s climate impact by 60%.

Vessel fuel use

Vessel fuel use produced between 7 and 25 kg CO₂e / ton of fresh seaweed in Chinese commercial farms (Wu et al., 2025; Sun et al., 2026). If applied to 2023 global seaweed production and the LCAs analyzed, this energy hotspot would have contributed nearly 675,000 tons CO₂e. Offshore dewatering, onsite sensors and mechanized harvest systems can substantially reduce a farm’s carbon footprint. Sea6 Energy’s integrated automated harvester, which simultaneously trims, removes, and reseeds cultivation lines in a single pass, is an example of this approach in practice (Albright & Fujita, 2023).

Table 6. Comparison of selected cultivation data reported in LCAs of seaweed production. Infrastructure refers to cultivation infrastructure including ropes, buoys, and mooring equipment; values vary by material type (e.g., wood, metal, concrete, polyurethane). GWP after drying data was only available for Nilsson et al. (2022); drying data was not reported in the remaining four studies.

Carbon cycling on seaweed farms

We summarize current understanding of cycling pathways in this section.

Burial of seaweed detritus

Seaweed farms may function as carbon sinks through the sinking and burial of seaweed detritus in sediments below and adjacent to farm sites. Duarte et al. (2022) analyzed sediment cores from 20 seaweed farms and estimated that farms buried an additional 1.06 ± 0.74 tons CO₂e/hectare per year; globally, this would equal almost 300,000 tons in incidental CO₂e sequestered/year (see Figure 5 above). However, carbon cycles beyond farm limits, and so this is an imperfect measure of sequestration.

Remineralization of biomass after grazing and/or product use

Seaweeds fix dissolved carbon dioxide or bicarbonate into biomass during the cultivation. However, when the biomass is consumed by grazing, or used by consumers as a product, that carbon is re-released to the atmosphere as carbon dioxide (Pessarrodona et al., 2024).

Methane production

Seaweed detritus may be consumed by deep-sea microorganisms that convert carbon to methane; while it should be factored into the life cycle analysis, relatively little is known about this pathway of seaweed decomposition (Ross et al., 2023).

Changes in total alkalinity of surrounding waters

Beyond direct carbon burial, seaweed farms may contribute to mitigation by increasing total alkalinity in surrounding waters. Denitrification and reduction of metals and sulphates during the anaerobic mineralization of seaweed biomass releases alkalinity; if transferred to the water column, this can shift the dissolved inorganic carbon equilibrium and enhance ocean carbon drawdown (Ross et al., 2023). Fakhraee and Planavsky (2026) estimated that this process could remove on average 0.85 tons of CO₂e per hectare each year; if applied to 2023 total ocean area used for farming, this would equal almost 230,000 tons CO₂e in incidental carbon sequestration (see Figure 5 above).

Indirect impacts from nutrient competitions with marine primary producers

A seaweed farm competes with primary producers (e.g., phytoplankton) for nutrients, possibly suppressing their net primary productivity. Therefore, understanding the net carbon footprint of a seaweed farm requires knowing the counterfactual of what the unfarmed ocean would have sequestered in its absence. The magnitude of both effects under realistic large-scale farm scenarios is not yet quantified, but studies have shown 30-100% of a seaweed system’s carbon fixation is not “new” removal but instead reallocation of nutrients away from existing natural sinks (Bach et al., 2021; Hurd et al., 2022; Ross et al., 2023).