Growing Seaweed and Shellfish Helps Balance and Purify Seawater
The world’s oceans and seas absorb about 30% of the carbon dioxide that is released into the atmosphere, according to the National Oceanic and Atmospheric Administration. As human beings maintain or increase certain activities, such as burning fossil fuels and clearing land of trees, the amount of CO2 is increased—and more is absorbed in the world’s waters.
When CO2 is absorbed by seawater, it sets in motion a series of chemical reactions that can result in acidification, or an increase in hydrogen ions. Acidification is believed to be generally harmful to marine species because it can reduce calcification in some species and affect some fish’s ability to find predators, for example, there has been an ongoing debate about clownfish being impacted. Acidification can also contribute to coral bleaching.
Today's average ocean water pH stands at 8.1. Before the Industrial Revolution in the late 1700s, when humans began using fossil fuels for manufacturing, the level was 8.2. The US Environmental Protection Agency says this drop in pH looks small, but it means that “the acidity of the ocean today, on average, is about 25% greater than it was during preindustrial times.”
The Intergovernmental Panel on Climate Change in 2016 estimated that the pH level* of the oceans could decrease, or become more acidic, by up to 0.287–0.29 pH units by 2081–2100 under a high emissions (RCP8.5) scenario relative to 2006–2015.
That’s why the work of Havhøst, a Danish membership organization, is so important. Literally translated, its name means “ocean harvest.” It promotes regenerative ocean farming, combining seaweed, which can help alleviate the effects of ocean acidification, and shellfish, which can purify the water.
What is Regenerative Ocean Farming?
Bodil Sofie Espersen, project manager at Havhøst, explains: “Regenerative ocean farming is cultivation of edible marine organisms that have an overall net positive impact on the surrounding ecosystem. So, it’s zero-inputs cultivation—no fertilizers, no pesticides, no medicine, no nothing added to the water. The whole regenerative idea is to always give more than you take, so leave a positive print on whatever ecosystem you’re working with.”
“When you cultivate mussels or oysters, they work as edible biofilters, so they filter the water. They take out excess nutrients, which is a big issue, especially in Danish coastal waters, because we have very heavy agricultural activities on the land,” Sofie Espersen adds.
“When you cultivate mussels or oysters, they work as edible biofilters, so they filter the water. They take out excess nutrients, which is a big issue, especially in Danish coastal waters, because we have very heavy agricultural activities on the land.”
Excess nutrients, she says, cause nutrification and loss of oxygen, leading to “ocean deserts.” The term refers to massive areas of ocean that don’t have enough nutrients for marine life to thrive fully. The deserts tend to lie about 30 degrees on either side of the equator—far from the biologically productive landmasses—and are also estimated to be the largest biome on Earth, occupying about 40% of its surface, according to the University of New Hampshire.
However, seaweed turns CO2 into oxygen, and, combined with oysters and mussels, creates a “positive circular effect.”
Benefits
Havhøst’s role is to help set up community sea gardens across Denmark, which has more than 5,000 miles of coastline, offering excellent conditions for cultivating blue mussels, seaweed, and oysters.
Many sea gardens are run on a voluntary basis, but more small business owners and local sustainable fishers are becoming involved.
“We want lots of small-scale ocean farms, with fishers using the business to supplement other sources of income,” comments Sofie Espersen.
Apart from economic advantages, the local community can benefit too, no matter what their reasons for getting involved. “Some [people] are together just to get food on the table. […] and some of them are fishermen trying to keep living on the ocean. […] But all have the same outcome—that they support local blue biodiversity, create healthy ecosystems—and they tell the story of the future,” says Sofie Espersen.
Environmental Factors
Before establishing new projects, environmental factors are considered.
“We’re very aware not to put a farm on top of areas that have seagrass because seagrass is a key species, just like mussels and oysters and seaweed,” says Sofie Espersen. The risk is that shade may be created, which will affect the seagrass.
“We’re very aware not to put a farm on top of areas that have seagrass because seagrass is a key species, just like mussels and oysters and seaweed.”
Havhøst also concentrates on growing species in areas that can sustain production, taking into account the ocean’s current, the depth of the water, and potential pollution.
Drawbacks
But there are hurdles to overcome. For example, it can be hard to obtain the right permission to set up a community garden, because regulations often favor large-scale industries rather than small businesses. It’s also often difficult to obtain start-up funding. However, Havhøst managers have built up a wealth of expertise and can help groups get over these potential barriers.
Community Gardens
Some twenty-six community gardens, involving 15 to 200 members, have been created since Havhøst started out ten years ago. Small groups of people who decide they want to start a project can approach Havhøst for help with obtaining permission, applying for financial aid, and getting started.
Bælthaven in Middelfart is the twelfth and largest association-based maritime garden in Denmark. It was created through a collaboration involving stakeholders such as the local municipality and the Nature Centre Hindsgavl.
Its members farm mussels using long “socks” that hang from a platform over the edge of the sea, and seaweed is grown on ropes. Apart from cultivating sustainable marine food, the association organizes events where the public can find out more about regenerative ocean farming. For example, lunches and food tastings are arranged, and lectures held. As part of Nature Day in September 2023, local people were also invited to collect and cook mussels caught by local sea farmers.
Education
Part of Havhøst’s agenda involves imparting knowledge to students. It currently works with forty-six schools in Copenhagen alone, and there are nine “satellites” around the country. Pupils aged between ten and seventeen participate in activities such as ocean farm work and dissecting mussels. More than 6,000 students have attended Havhøst’s educational programs over the years.
The seaweed field school course is just one educational initiative. It offers students the chance to visit Havhøst’s floating platform Bølgemarken, which showcases the sea’s edible produce. During the visit, students learn about the lifecycle of seaweed and the work of sea farmers. They also harvest seaweed from tang lines and learn about its role in counteracting the climate crisis. In the kitchen, pupils prepare and taste food, such as pesto and waffles made from seaweed.
“We use the ocean farms as a platform to discuss a whole range of topics, from ocean ecosystems and organisms to global food production and sustainable development goals.”
Sofie Espersen says: “We use the ocean farms as a platform to discuss a whole range of topics, from ocean ecosystems and organisms to global food production and sustainable development goals.”
Nordic Initiative
The message is also being disseminated across the Nordic countries through the Cool Blue project, a new collaboration between four partners, Havhøst, the University of Gothenburg in Sweden, Aktion Österbotten (Action Ostrobothnia) in Finland, and s.Pro, a specialist consultancy from Germany.
The project aims to establish a network of small-scale regenerative ocean farming initiatives in Sweden, Denmark, and Finland. As part of the project, Havhøst will contribute its expertise in building community-based activities.
Maria Bodin, Cool Blue project coordinator from the University of Gothenburg, outlines how the network aims to foster knowledge exchange: “We wanted to create the network so if people want to start something they can contact us and one person in each country can help them […]. We also want to increase ocean literacy to work more on how we can use the sea in a sustainable way.”
New Technology
The Cool Blue team is interested in how the scheme might use new technology, such as artificial intelligence monitoring systems, to gauge the positive environmental impact of regenerative ocean farming activities. A camera could, for example, be positioned on one of the farms to see if more fish and other species are attracted.
Emphasizing the importance of promoting regenerative ocean farming more widely, Sofie Espersen says: “We’ve been talking about sustainability for thirty years, but it’s no longer enough to just not make things worse. Now we need to actively regenerate the ecosystem.”
*Yasmin Prabhudas is a freelance journalist working mainly for non-profit organizations, labor unions, the education sector, and government agencies.
Editorial Note:
The pH scale measures the relative amount of free hydrogen and hydroxyl ions in water. It runs from 0 to 14, with 7 being neutral, below 7 being acidic, and above 7 being alkaline. A change in pH by 1 represents a concentration change by a factor of 10.