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Giant Kelp

kelp and fish

Does the thought of something brown, wet and slimy that can grow up to 60 cm (2 ft) per day interest you? No? Well it should because giant kelp is a provider of valuable ecosystem services. Let's find out how it benefits us.


What is giant kelp?

kelp labelled diagrm
anatomy of giant kelp plant

Kelps, which belong to the group brown algae, are marine organisms capable of photosynthesis (technically though they are not classified as plants). Macrocystis pyrifera is the scientific name of the largest of the kelps which can grow up to 50 - 60 m (150+ ft) long.

A rootlike holdfast attaches to a solid surface. The holdfast plays no role on nutrient gathering.

A  stipe resembles a stem and blades resemble leaves.  The blades carry out both photosynthesis and mineral absorption. As it is an alga, there are no veins in the stipe or blades. Floats  are gas filled compartments that help the kelp blades to reach available light near the ocean surface.

Giant kelp increase their length at an average rate of 30cm a day but can add 60cm a day in ideal conditions.

Although giant kelp plants are perennial, the individual fronds only survive for about 6-9 months. Fronds of mature kelp plants become senile and deteriorate about 6 months after they are produced. Mature fronds continually develop, then die and break away in a process known as sloughing, giving way to the new fronds shooting up from the holdfast.1

Giant kelp distribution

Giant kelp grows on rocky reefs from the sea floor 8 metres below sea level and deeper. Its fronds grow vertically toward the water surface, in cold temperate waters where the water temperature does not exceed 20 degrees C.  Kelps growing in areas with available surfaces for attachment may form a bed (small area) or a forest (large area).

Macrocystis  occurs naturally on the west coast of the USA  and in coastal waters around Australia, New Zealand, South Africa and South America. Ideal habitat for kelp is cold, nutrient rich seawater near to the coast.


A kelp forest forms the basis of an undersea ecosystem that provides food, shelter and protection for a variety of marine organisms including plankton, sea urchins, mussels, fish and sea otters. Giant kelp is a keystone species.  It provides food and habitat for a wide range of marine invertebrates and fishes. Forests of giant kelp may support millions of individual organisms.

Kelp forests collectively exert a damping force on waves.  This provides a calmer and more livable habitat for other members of the community. It also helps to ameliorate coastal erosion.

Giant kelp is harvested as a source of alginate, an emulsifying and binding agent used in the production of many foods and cosmetics, like ice cream, cereal and toothpaste. Historically we have used kelp as a source of sodium carbonate and also of iodine.

Giant kelp itself is not eaten by humans on a regular basis like other members of the kelp family as its carbohydrate storage product is not very digestible. However we do eat many of the species that live in kelp forests e.g. abalone, lobsters, some fish.

It is given to livestock as a mineral supplement and also used in some fertilizers.

Kelp forests are popular with scuba divers who describe it as an amazing experience to dive there.


Summary of services provided for us

  • Carbon sequestration
  • Raw material for a range of products
  • Nutritional supplement
  • Prevention of erosion
  • Habitat for other organisms
  • Recreational value

Threats to the services?

A short case study of the problems faced by giant kelp forest - There used to be extensive kelp forests off the eastern coast of Tasmania, Australia. Unfortunately temperatures there have risen at nearly four times the global average, and they are now about 2C hotter than a little over a century ago. Thick canopies that once covered much of the region’s coastal sea surface have wilted in intolerably warm water. Then, there was invasion by warm-water sea urchin species.

Voracious grazers, the invaders have mowed down much of the remaining vegetation and, over vast areas, have formed what scientists call urchin barrens, bleak marine environments largely devoid of life.2

Lobsters which were previously abundant in these waters would have kept the sea urchin population under control but the lobsters had been wiped out by commercial overfishing. (Remember what I said about the Jenga analogy).

Other threats include  danger from pollution e.g. sewerage discharge, fertilizer runoff, dumping of hazardous materials.

It was estimated that by 2019, 95 per cent of the giant kelp forests along Tasmania's east coast had been lost within just a few decades.3

What can we do to retain these services?

The restoration of kelp forests globally has typically followed two broad approaches: assisted recovery and active restoration. In this case, removing the sea urchins would be an assisted recovery strategy. Active restoration is, as the name suggests, the process of building new forests. This can be challenging especially in urchin barrens and attempts so far have met with mixed success.

One of the more successful approaches has been to use the "green gravel" method to repopulate seriously depleted beds.

The method, which has been tested in a number of places including Norway, Australia, California and by us in Portugal, involves seeding small rocks with kelp spores in the laboratory and then planting them in the ocean. When the young kelps on the rocks are dropped into the ocean from a boat, they attach to the underlying reef on the seafloor. This technique is cheap, simple, and does not require scuba diving, highly trained field workers, or engineered structures. The ease of scattering the gravel from a boat means the process can be scaled up to treat large areas.4

Dig deeper

did you know graphic In the Northern Hemisphere giant kelp forests, sea otter inhabitants wrap themselves in giant kelp to keep from floating away while sleeping.


  1. Giant bladder kelp []
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  4. Restoring The Forgotten Forests In Our Ocean []
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