Bringing seaweed back into the food chain

Nutrients and minerals from the earth’s crust flow into the sea and are absorbed by seaweed.  Duncan Smith outlines the benefits of closing the loop and bringing seaweed back into your food chain.
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Seaweeds are Earth’s most ancient and essential organisms, originating about three and half billion years ago, and their benefits were realised thousands of years ago. There are several thousand seaweed species, generally grouped as green, brown and red. New Zealand waters host over 900 species. 

Seaweeds are simple structures but are not true plants. They’re non-flowering aquatic organisms called macroalgae; some microscopic, like free-floating phytoplankton; some huge, like giant kelps. Most attach themselves to surfaces using a holdfast and have a blade which can be divided into fronds. Larger seaweeds have a flexible stalk (stipe) between the blade and holdfast. Some brown algae, like Sargassum, are free-floating, drifting in large, island-like masses over oceans. Found in oceans, rivers and some lakes, seaweeds require clear sea or brackish water with good sunlight penetration for photosynthesis. Most prefer rocky shores so that they can anchor themselves. 

Seaweeds contain chlorophyll for photosynthesis. Brown and red seaweeds have additional pigments for photosynthesis as they live deeper under the sea, where sunlight doesn’t penetrate as easily. During photosynthesis, they absorb carbon dioxide from the water and liberate oxygen released into the atmosphere, producing 50 to 80% of Earth’s oxygen.  

About 2 billion years ago, a microscopic blue-green alga, Prochlorococcus, produced Earth’s first oxygen molecules. It’s our smallest photosynthetic organism and produces up to 20% of our oxygen, more than all our tropical rainforests combined.

Land-based plants obtain nutrients from soil via their roots; seaweeds gather theirs across all their surfaces directly from the water. Many gases, nutrients and other substances in seawater are derived from the Earth’s crust, sea floor or from decaying dead plants and animals.  

Until recently, seaweeds were the world’s most sustainable food source, needing clean water and sunlight to thrive. However, seaweed farming has increased in recent years, and sustainability problems are beginning to emerge. 

Uses of seaweeds 

 Until now, the most extensive use of seaweeds has been fodder for animals, cropping and human consumption (no known poisonous or toxic seaweeds are found in New Zealand waters).  

With modern technology, their uses have gone further but must be carried out sustainably and responsibly. These include:

  • Uses in medicine, cosmetics and food production 
  • As bio-plastics and plastic alternative that replace fossil fuels used to create packaging.  
  • As biofuels.  
  • Seaweeds are the fastest-growing plants on the planet and can capture vast amounts of carbon in underwater seaweed forests, locking it in to macroalgal carbon in deep-sea sediments.  
  • Underwater plantings of seaweed can help replace fish stocks, providing safe havens for eggs and hatchlings, and supporting commercial fishing. 
Common types of edible seaweeds  
  • Wakame – Brown seaweed is cooked in stews and soups or eaten fresh as a salad. 
  • Nori – Red seaweed, regularly sold in sheets, used to roll sushi or eaten as a snack. 
  • Dulse – Dried red seaweed, used to flavour dishes and eaten as a slightly chewy snack.  
  • Kelp – Brown seaweed, typically dried into sheets and added to many dishes; an excellent gluten-free alternative to noodles. 
  • Kombu – A strong-flavoured kelp, pickled or used in soups.  
  • Arame – A firm textured kelp with a sweet, mild flavour, used in many dishes and baked goods. 
  • Sea Lettuce – A green type of nori used raw in salads or soups. 
  • Chlorella  and  Spirulina - These are freshwater algae, a good source of proteins, fats, carbohydrates, fibre, chlorophyll, minerals, and vitamins. According to the literature, only Chlorella contains “active” B12 that we can absorb.  

Left: Judy Smith, foraging for fertiliser on Gisborne Beach.

Human consumption 

The highest consumers of seaweeds are the Chinese, Japanese and Koreans, and it is becoming popular in Western cultures, both dried and fresh.  

Seaweeds have all the essential nutrients necessary for healthy diets. They are rich in vitamins, minerals and trace elements and are nature’s best source of iodine. They are more nutrient dense than land plants, being 10 to 20 times more concentrated. Their ratio of carbohydrates to proteins varies between the species and where they are grown. Generally, one cup of seaweed (15 grams) will provide approximately 5 grams of protein and 8 grams of carbohydrates.

All seaweeds contain proteins, and some have more than other foods on a dry weight basis. Red and green seaweeds usually have more protein than browns, and up to five times more protein, per acre, than soybeans. Brown seaweeds have about 8-12% protein; reds 13-40%; and sea lettuce about 15% protein. 

Seaweed has varying nutrient profiles, with components depending on species, location, season, condition at time of harvest etc.  The biggest determining factor is the purity of the water it grows in.  Seaweeds can absorb pollutants just as easily as nutrients.   

As an example, a study in France found seaweed contained 70% organic compounds (of which 57% were carbohydrates, 4% lipids and 9% proteins), 20% mineral compounds, and 10% water. 

An analysis by New Zealand Southern Pacific Seaweed Company Ltd of the Brown NZ Kelp Macrocystis Pyrifera which is found in the waters around the South Island, showed it contained over 54 minerals, vitamins and trace elements. See table below.

There is an ongoing debate about whether our bodies can absorb and use B12 from seaweeds, as they are primarily in an analogue form rather than an active form, so our bodies can’t utilise them. However, the B12 found in Nori (red laver seaweed) and Chlorella, a freshwater alga, are active and considered the best sources of B12 for vegans.

New developments with seaweed 

 Agrisea has been at the forefront of research and development of innovative uses for seaweed in New Zealand for over 25 years. After a three-year collaboration with Scion Research in Rotorua, they’ve developed a novel hydrogel from seaweed. Hydrogels are materials that can absorb and retain large quantities of water. They are used in burn dressings; biomedical engineering applications; drug delivery; cosmetics; baby’s nappies; breast implants; contact lenses; agriculture, and many more applications. 

In Germany, non-polluting coatings for fast food packaging have recently been developed from seaweed. It could replace plastic coatings and other environmentally harmful chemicals currently used in grease-resistant fast-food packaging. 

Typical analysis of Macrocystis Pyrifera (Brown NZ Kelp)

Iodine(mg/kg) 845 
cis-Oleic acid(g/100g)0.29
Nitrogen (total)(g/100g)1.95
Omega 3(g/100g)0.15
Palmitic acid(g/100g)0.38
Vitamin B12(mcg/100g)7.93
Vitamin E(IU/100g)4.15
Plus trace elements of Niacin (B3), Riboflabin (B2), Cyanocobalamin (B12), Thiamine (B1), Pyridoxine (B6) and Fucoidan.

Effects on soils

Plants depend on healthy soil microorganisms, like bacteria, fungi and others, to break down organic forms of minerals like nitrogen, phosphorus and sulphur into a form that they can readily use. Seaweeds, fresh, dried or liquid, have been demonstrated to feed these beneficial microorganisms, leading to abundant soil biology, enhanced soil health, improved plant health, open soil structures and increased productivity. Plants have been proven to function better when nutrients are obtained from natural microbial pathways rather than artificial fertilisers. 

Seaweeds also have unique plant biostimulants like cytokinins, auxins, abscisic acid, and ethylene that act upon plant growth, ageing, cell division, germination, and stress management. 

Seaweeds enhance seed germination, increase root and plant growth, crop yields, protein and quality, and improve soil tilth and other properties. Studies in Australia have shown that seaweed extracts can increase plants’ resistance to insects and diseases, improve drought and frost resistance, and increase the shelf life of produce.  

New Zealand farmers could improve their environmental footprint by using seaweed products instead of relying solely on artificial nitrogen (N), phosphorous (P) and potassium (K) fertilisers. The N, P and K in seaweed are found in their proteins, and soil microorganisms break these down into a chelated form that plants can use straight away.  

Animal health  

 There are significant benefits of feeding seaweed to livestock if they have a nutritional imbalance. They’ve been shown to enhance nutrient absorption; endocrine systems; hormone production; immune systems and reproductive systems; growth and stress response; nervous systems energy, and stress-damaged organs are repaired.  

Scientists at Davis University in California found that feeding a species of red seaweed to cattle reduced the methane they burp up by over 80%. Worldwide investigations have confirmed this in other ruminants. 

Benefits of seaweed 

Seaweed biostimulants have manifold benefits.

Plants have improved root systems, flowering, leaf development, fruit set, and higher yields. They are healthier and can tolerate insects, diseases, cold, frost and drought, and harden off better. 
Soils have improved soil structure, water-holding capacity and soil biology.
Livestock has improved health and productivity.  
Bees and hives come through winter healthier, move into spring faster, and are more productive. 
Humans have enriched health, including enhanced thyroid function, gut and heart health, stabilised blood sugar levels, boosted immune systems and nutrition, thanks to seaweed’s high profile of vitamins and minerals. 

Left: Judy Smith fertilisers and feeds her potatoes with a thick layer of seaweed mulch.

Gathering for the garden 

Whether you are a keen gardener or a large farmer/grower, the benefits of using seaweed biostimulants are precisely the same.  

Anyone can collect seaweed off our beaches except from Marine Reserves, MātaitaiReserves (areas where the Tangata whenua manage all non-commercial fishing), or Taiāpure (estuarine or coastal areas that are significant for food, spiritual, or cultural reasons). Following the 2016 earthquakes, there is still a ban on collecting seaweed along the Kaikōura Coast, from Cape Campbell to the Conway River.  

Beach-cast seaweed anywhere around New Zealand can be a wonderful mix of different species, all great for your garden. Gather it from below the high tide mark, as seaweed helps prevent beach erosion when mixed with vegetation, driftwood and other materials found there. Seaweeds are not toxic per se, but some, like sea lettuce, can become toxic if they’ve absorbed toxins from polluted waters. Always collect it from clean, uncontaminated seas/beaches. Process it as soon as possible as it breaks down quickly, and you want to capture all their nutrients for your garden or compost. 

Brown kelps are generally considered the best selection –  twenty-five years ago, Agrisea NZ Seaweed Ltd, seaweed processors in Paeroa, researched many New Zealand seaweeds before settling on the brown kelp Ecklonia radiata as their primary input – but green and red seaweeds work well too.

Adding seaweed to the garden 


Unless you have high sodium in your soil, there is no need to wash seaweed first. Apply a 10 – 15 cm layer around your plants, keeping it clear of their stems; reapply a similar amount a week later. Seaweed doesn’t contain weed seeds; keeps soils moist; saves watering; helps repel slugs and snails; feeds soil biology, and enriches and improves soils. Pull out and highlight or box if you want or leave in text. 

Seaweed tea

Collect a range of green, brown and red seaweeds if you can, as each type has a different nutrient profile. Fill a bucket 3/4 full, cover them with water, put a lid on, let it brew for two to three weeks and occasionally stir to help release their nutrients. It smells as it ferments, so store it away from your house!  

When required, dilute to the colour of weak tea and apply it around your plants. Immersing transplants or soaking seeds in seaweed tea before planting/sowing will boost them. 

Foliar spray

Seaweed tea can also be used as a foliar spray, even on house plants, but filter it to prevent your sprayer nozzle from blocking.   

The spray enters plants within minutes through their stomata and the microscopic pores between leaf cells on the upper and lower surfaces of leaves. In comparison, on average, soil-applied fertilisers can take up to two weeks for plants to respond fully.  


Seaweed is an excellent addition to compost, adding many extra nutrients and helping speed up the composting process. Layer it in equal amounts with your usual organic materials. It is unnecessary to wash it first, as it adds other elements to the compost that are not present in seaweed. Shredding it with a lawnmower helps it break down faster, speeding up the composting process.  


Seaweed can be air-dried and stored in a cool, dry place to use later. 

Underwater kikuyu 

Two invasive seaweeds have been found in New Zealand waters – and MPI are asking for public vigilance to prevent them spreading. Caulerpa brachypus and Caulerpa parvifolia are exotic seaweeds which spread rapidly forming vast, dense, beds on the seafloor up to 30 metres deep.  

It has recently been found in bays around Great Barrier Island and Great Mercury Island. Check gear (anchors especially) and ensure you don’t transport it to a new zone  – you can take it home and compost it though.  Report it if you see it growing elsewhere (0800 80 99 66 or online at report.mpi.govt.nz)  

Duncan Smith studied agriculture and has an MSc in Plant Pathology. He was a research agronomist for Cedenco Foods and carried out independent research for Crop and Food Research and international seed companies. Duncan’s wife, Judy, studied horticulture and permaculture and lectured in organics at the Tairawhiti Polytechnic, Gisborne. Judy is a winemaker and Master Cider-maker. For 26 years, they created a 23-acre certified organic lifestyle block in the Waimata Valley, Gisborne, growing a wide range of vegetables and fruit, producing a gold medal-winning chardonnay and olive oil, and hosting a number of organics and permaculture courses.   

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