Global production of farmed aquatic plants, overwhelmingly dominated by seaweeds, grew in output volume from 13.5×10^6 t (13,300,000 long tons; 14,900,000 short tons) in 1995 to just over 30×10^6 t (30,000,000 long tons; 33,000,000 short tons) in 2016. As of 2014, seaweed was 27% of all marine aquaculture.
An American kelp farmer, Bren Smith of GreenWave explains his farming methods, including the symbiotic relationship kelp has with other seafood he grows.
The earliest seaweed farming guides in the Philippines recommended the cultivation of Laminaria seaweed and reef flats at approximately one meter's depth at low tide. They also recommended cutting off seagrasses and removing sea urchins before farm construction. Seedlings are then tied to monofilament lines and strung between mangrove stakes pounded into the substrate. This off-bottom method is still one of the primary methods used today.
The cultivation of seaweed in Asia is a relatively low-technology business with a high labor requirement. There have been many attempts in various countries to introduce high technology to cultivate detached plant growth in tanks on land to reduce labor, but they have yet to attain commercial viability.
Seaweed is an extractive crop that has little need for fertilisers or water, meaning that seaweed farms typically have a limited environmental footprint compared to other forms of agriculture or fed aquaculture. Nonetheless, many environmental problems can result from seaweed farming. For instance, seaweed farmers sometimes cut down mangroves to use as stakes for their ropes. This, however, negatively affects farming since it reduces the water quality and mangrove biodiversity due to depletion. Farmers may also sometimes remove eelgrass from their farming areas. This is also discouraged as it adversely affects water quality.
Seaweed farming can also pose a biosecurity risk, as farming activities have the potential to introduce or facilitate invasive species to new environments. For this reason, only native varieties of seaweed may be grown in many regions, such as in the UK, Maine and British Columbia.
While seaweed farms may have some negative environmental impacts, they may also have a variety of positive environmental effects. For instance, seaweed farms may support positive ecosystem services such as nutrient cycling, carbon uptake, and habitat provision, and so provide additional value through these other benefits that complement crop production. Notably, however, many of the impacts of seaweed farms, both positive and negative, remain understudied and uncertain.
Seaweed culture can be used to capture, absorb, and eventually incorporate excessive nutrients into living tissue. "Nutrient bioextraction" is the preferred term for bioremediation involving cultured plants and animals. Nutrient bioextraction (also called bioharvesting) is the practice of farming and harvesting shellfish and seaweed to remove nitrogen and other nutrients from natural water bodies. (See main article Nutrient pollution.)
Similarly, seaweed farms may provide an additional positive service by creating habitat that enhances biodiversity. In this sense, seaweed farms have been suggested to help preserve coral reefs by increasing diversity where the algae and seaweed have been introduced, providing an added niche for local species of fish and invertebrates. Farming may be beneficial by increasing the production of herbivorous fish and shellfish in the area.Pollnac 1997b reported an increase in Siginid population after the start of extensive farming of eucheuma seaweed in villages in North Sulawesi, Indonesia. However, the biodiversity benefits of seaweed farms remain widely uncertain and seaweed farming often has the potential to negatively impact local biodiversity instead.
In Japan alone, the annual production value of nori amounts to US$2 billion and is one of the world's most valuable crops produced by aquaculture. The high demand for seaweed production provides plentiful opportunities and work for the local community.
A study conducted by the Philippines showed that plots of approximately one hectare could have a net income from Eucheuma farming that was 5 to 6 times that of the minimum average wage of an agriculture worker. In the same study, they also saw an increase in seaweed exports from 675 metric tons (MT) in 1967 to 13,191 MT in 1980, which doubled to 28,000 MT by 1988.
Farmed seaweed is used in several different industrial products: directly as food, as an ingredient in animal feed, and as source material for biofuels.
Algae fuel, algal biofuel, or algal oil is an alternative to liquid fossil fuels that uses algae as its source of energy-rich oils. Also, algae fuels are an alternative to commonly known biofuel sources, such as corn and sugarcane. When made from seaweed (macroalgae) it can be known as seaweed fuel or seaweed oil. It is also carbon negative unless the dead plant matter is burned, as the energy (stored as hydrogen gas) is produced by solar photosynthesis and comes from the sun. The emissions from burning the hydrogen make up only water and air.
Climate change mitigation
There has been considerable attention to how large-scale seaweed cultivation in the open ocean can act as a form of carbon sequestration to mitigate climate change. A number of academic studies have demonstrated that nearshore seaweed forests constitute a source of blue carbon, as seaweed detritus is carried by wave currents into the middle and deep ocean thereby sequestering carbon. Moreover, nothing on earth sequesters carbon faster than Macrocystis pyrifera (also known as giant kelp) which can grow up to 60 m in length and as rapidly as 50 cm a day in ideal conditions. It has therefore been suggested that growing seaweeds at scale can have a significant impact on climate change. According to one study, covering 9% of the world’s oceans with kelp forests “could produce sufficient biomethane to replace all of today’s needs in fossil fuel energy, while removing 53 billion tons of CO2 per year from the atmosphere, restoring pre-industrial levels”.
As well as climate change mitigation, seaweed farming may be an initial step towards adapting to inevitable environmental constraints that may arise as a result of climate change in the near future. These include essential shoreline protection through the dissipation of wave energy, especially important to mangrove coasts. Carbon dioxide intake would raise pH locally, which will be highly beneficial to calcifiers (e.g. crustaceans) or in preventing the irreversibility of coral bleaching. Finally, seaweed farming and regenerative ocean farming would provide a strong oxygen input to coastal waters, thus countering the effects of ocean deoxygenation through the rising ocean temperature.
There has been considerable discussion as to how seaweeds can be cultivated in the open ocean as a means to regenerate decimated fish populations and contribute to carbon sequestration. Notably, Tim Flannery has highlighted how growing seaweeds in the open ocean, facilitated by artificial upwelling and substrate, can enable carbon sequestration if seaweeds are sunk below a depth of one kilometer. Similarly, the NGO Climate Foundation and a number of permaculture experts have posited that the offshore mariculture of seaweed ecosystems can be conducted in ways that embody the core principles of permaculture, thereby constituting marine permaculture. The concept envisions using artificial upwelling and floating, submerged platforms as substrate to replicate natural seaweed ecosystems that provide habitat and the basis of a trophic pyramid for marine life. Following the principles of permaculture, seaweeds and fish can be sustainably harvested while sequestering atmospheric carbon. As of 2020, a number of successful trials have taken place in Hawaii, the Philippines, Puerto Rico and Tasmania. The idea has received substantial public attention, notably featuring as a key solution covered by Damon Gameau’s documentary 2040 and in the book Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming edited by Paul Hawken.
In order to mitigate global warming, seaweed farming is both possible and plausible. Seaweeds remove carbon through the process of photosynthesis, taking in excess CO2 and producing O 2. About 0.7 million tonnes of carbon are removed from the sea each year by commercially harvested seaweeds. Even though seaweed biomass is small, compared to the coastal region, they remain essential due to their biotic components, the ability to provide valuable ecosystem services and high primary productivity. Seaweeds are different from mangroves and seagrasses, as they are photosynthetic algal organisms and are non-flowering. Even so, they are primary producers that grows in the same way as their terrestrial counterparts, both of which assimilate carbon by the process of photosynthesis and generates new biomass by taking up phosphorus, nitrogen, and other minerals.
The attractiveness of large-scale seaweed cultivation has been proven over the years by low-cost technologies and the multiple uses that can be made of its products. As of 2018, seaweed farming made up approximately 25% of the world's aquaculture production and its maximum potential has not been utilised.
Currently in the world, seaweeds contributes approximately 16–18.7% of the total marine-vegetation sink. In 2010 there are 19.2 × tons of aquatic plants worldwide, 6.8 × tons for brown seaweeds; 9.0 × tons for red seaweeds; 0.2 × tons of green seaweeds; and 3.2 × tons of miscellaneous aquatic plants. Seaweed is largely transported from coastal areas to the open and deep ocean, acting as a permanent storage of carbon biomass within marine sediments.
"Ocean afforestation” is a proposal for farming seaweed for carbon removal. After harvesting the seaweed decomposes into biogas, (60% methane and 40% carbon dioxide) in an anaerobic digester. The methane can be used as a biofuel, while the carbon dioxide can be stored to keep it from the atmosphere. Seaweed grows quickly and takes no space on land. Afforesting 9% of the ocean could sequester 53 billion tons of carbon dioxide annually (annual emissions are about 40 billion tons). The approach requires efficient techniques for growing and harvesting, efficient gas separation, and carbon capture and storage.
Human use of seaweed harvested from the wild date back to at least the Neolithic period. Cultivation of gim (laver) in Korea is reported in books from the 15th century. Seaweed farming began in Japan as early as 1670 in Tokyo Bay. In autumn of each year, farmers would throw bamboo branches into shallow, muddy water, where the spores of the seaweed would collect. A few weeks later these branches would be moved to a river estuary. The nutrients from the river would help the seaweed to grow.
In the 1940s, the Japanese improved this method by placing nets of synthetic material tied to bamboo poles. This effectively doubled production. A cheaper variant of this method is called the hibi method — simple ropes stretched between bamboo poles. In the early 1970s, there was a recognized demand for seaweed and seaweed products, outstripping supply, and cultivation was viewed as the best means to increase production.
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