What's The Difference Between Kelp and Seaweed?

What's The Difference Between Kelp and Seaweed?

What’s the difference between seaweed and kelp?

In recent years, seaweed has taken the health industry and the food industry by storm. Whether as a food to address vegan diet deficiencies, or as a salt substitutes, the benefits of seaweed as a superfood are becoming clearer and clearer. This one of the reasons why it is popping up in so many vegan recipes and health products.

But not all seaweed is created equal and understanding the kind of oceanic plant life you’re enjoying is key to getting the most benefits out of your seaweed products.

Today, we’re going to be exploring the essential differences between two key terms: seaweed and kelp. What makes them different, and what should you look for from your seaweed supplier?

It can be slightly confusing when you get bombarded with different terminology when searching for seaweed. If you don’t know the difference (and who could blame you!) between seaweed and kelp, and want clarity on spirulina, algae, and sea vegetables then you’ve come to the right place!


What does seaweed include?

Firstly, seaweed is quite a general term, encompassing over 10,000 different species! Seaweed can also be called marine macroalgae – in other words large algae that grows in the sea. Seaweeds use light from the sun, carbon dioxide and nutrients from seawater to grow.  It can form many shapes, colours, and flavours! For example, the seaweed we use has the Latin name Ascophyllum nodosum and is commonly referred to as egg-wrack or knotted-wrack. It is in the wide group of brown seaweeds, with the other two main colour groups being red and green. It is quite easy to spot these varying colours on the beach around rock pools.

Ascophyllum nodosum grows in what is called the inter-tidal zone.  This is the area between low and high tide.  So, when the tide is low, the seaweed is uncovered, and when the tide is high, it is covered.  These extremes of conditions (from wet to dry, different UV levels, different predators etc), twice a day every day, are thought to contribute to the specific antioxidants and other nutrients found in our chosen species of seaweed.

Sea vegetables is just another phrase often used when talking about seaweed for food!

Sea kelp and seaweed are both terms to describe sea plants, but how are they different?

Sea kelp has a number of health benefits and a high nutritional value, making it a popular sea plant but it is not the same as seaweed.

Diver in Kelp Forest

Seaweed is a term which can be used to describe many different marine-based species of plants and algae. But sea kelp is more specific. It describes the largest subgroup of seaweed. Seaweed ranges dramatically in size, whilst sea kelp is always quite large.

You can break seaweed down into three groups — red, brown, and green. This colour has a direct effect on how much light seaweed absorbs via photosynthesis, which decides how close to the ocean’s surface it grows. Sea kelp is officially labelled as a brown seaweed, even though it can vary in colour.

The two also have different growing habitats. Seaweed can grow in any marine environment, including oceans, rivers, lochs, and lakes. The Ascophyllum seaweed found in Doctor Seaweed’s Weed & Wonderful® organic seaweed supplements is actually harvested in the pristine lochs of the Scottish Outer Hebrides. Whereas kelp is most often found along rocky coastlines, and only in saltwater. Sea kelp needs nutrient-rich waters because of its bigger size, meaning there needs to be at least a small amount of movement in the water to ensure a continuous supply of nutrients.

We mentioned the different colour groups of seaweed previously, and kelp is within the brown seaweeds (the same big group as our Ascophyllum nodosum).   However, unlike our Ascophyllum seaweed, the kelps are often larger and grow on the lowest part of the shore, being mainly below the water even at low tide. You may have heard of kelp forests, growing several meters tall – they’re beautiful!  An example of a kelp would be a species such as Laminaria digitata. A common name for kelps in food is Kombu.

The definition of kelp, and which seaweeds are described as kelp, can change depending on which country you are in so watch out for the actual species name. For example, the term kelp is sometimes (mainly in North America) used to describe all the brown seaweeds (which it actually doesn’t!)

Nutritionally all species of seaweed and kelp differ.  However, brown seaweeds generally have higher levels of iodine than red or green seaweeds, and the kelps can have extremely high levels.  The variation in levels in kelps, often sourced in China and other parts of Asia, can be huge and should be well understood before using any as a food or nutrition source.

Iodine is an essential nutrient that is especially important for women’s health and helps to maintain a healthy thyroid (regulating hormones). It is important to know how much iodine you are taking in, and that the source is well understood.  This is why we measure the iodine levels of every single batch of our harvested seaweed.

Just one of Doctor Seaweed’s Weed & Wonderful® Organic Scottish Seaweed Supplements contain enough seaweed to provide you with a good source of iodine. Also, our capsules contain ONLY 100% PureSea Organic Hebridean Ascophyllum seaweed, and nothing else.  Watch out for cheap kelp tablets that contain a very small amount of kelp, and then loads of fillers and other products to make the tablet.


Whatever colour and size they are, sea kelp and seaweed both have particular characteristics. Seaweed has a unique internal structure, differentiating it from plants that grow on land. Seaweeds have what are called holdfasts instead of roots, and these holdfasts merely hold the plant in place rather than absorbing any nutrients like roots. Seaweeds without holdfasts can easily survive by floating along with the water, carried by the hulls of ships or simply on the tides.

Sea kelp is among the largest species of seaweed. Unlike other kinds of seaweed, sea kelp fixes itself to a single spot and group together to form thick forests. These forests can become fully functioning ecosystems which often act as both food and home for different marine animals. A sea kelp forest also acts as home for a lot of other species of seaweed and algae.

Sea kelp itself can take many different forms. One of them is bull kelp, which grows along coastal fronts, can live for ten years, and can reach lengths of 45 meters, or 150 feet. Bladder kelp can grow to be a huge 50 meters in length and can weigh up to hundreds of pounds. Of course, there are smaller kinds of sea kelp like common kelp and Asian kelp which average a length of between one and two metres.

Both sea kelp and seaweed are common food ingredients in Eastern cultures, and their popularity is constantly rising in the West. But why? What health benefits do sea kelp and seaweed have?

Sea kelp and seaweed can both benefit your health with their nutritional value…

Sea kelp is a fantastic source of vitamins, as is seaweed. However, sea kelp can often have large and ranging levels of iodine which can be harmful. Whereas the seaweed harvested by Seaweed & Co. is fully traceable and sustainable and is the world’s only DNA Authenticated Seaweed. Meanwhile each batch is tested to ensure safe and consistent levels of iodine. If you don’t consume enough fish or diary then you could be at risk of iodine deficiency. However, just two of Doctor Seaweed’s Weed & Wonderful® Organic Seaweed Supplements contain as much iodine as 3 whole mackerel – a great alternative for those on a meat-free/vegan diet!

Seaweed Ingredients PureSea®

Seaweed & Co. harvest their seaweed purely from the pristine waters of the Scottish Outer Hebrides. It contains important nutrients and minerals like calcium, magnesium, iron, potassium, and iodine. The UK is one of the top ten iodine deficient countries in the world, which is worrying as iodine is a key nutrient in ensuring normal thyroid function. Seaweed can change this. Seaweed & Co.’s produce is rich enough to allow EU Approved Health Claims of normal thyroid function and normal cognitive function among many others.

And is spirulina a seaweed?

No! Spirulina is actually a biomass of cyanobacteria. And what the heck are cyanobacteria I hear you ask? Cyanobacteria are bacteria that use the sun’s light to produce food = photosynthesis! For this reason, they have, coined the name ‘blue-green algae’ even though they are not a true algae (Your brain hurting yet?). This is how seaweed and spirulina can sometimes be confused! Spirulina only grows in fresh water and not the marine environment, and is a micro, and not macro-algae.

If you’re still confused on the difference between our Organic Hebridean Ascophyllum seaweed and kelp then perhaps try Doctor Seaweed’s Weed & Wonderful® Organic Seaweed supplements, as a good source of iodine they can support normal cognitive function!

by Nick Cairns and Sophie Hoyle

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