Spotlight: Kelp – Underwater Forests
The intention of this column is to throw a SPOTLIGHT on individual plants and animals – not
to blind you with science but to reveal important and fascinating aspects of specific organisms.
Brown underwater forests surround the coasts of Britain. These forests are made up of densely packed,
large, tree-like seaweeds called kelp but, unless you are a diver or have a glass-bottomed boat, you won’t
be able to see them. The only time you might catch a glimpse of the edge of the forest is at low tide
when you may see Laminaria digitata growing in a narrow fringe around the low water mark. Below it and just showing on the very lowest tides - is Laminaria hyperborea, which forms extensive forests
wherever there is underwater bedrock or large boulders.
In spite of their tree-like appearance, kelps are algae, a relatively primitive group of organisms ranging in
size from microscopic single cells to the giant kelp Macrocystis which grows to over 50 m long. Larger
seaweeds are classified according to their colour (pigments) into ‘green’ (Chlorophyta), ‘red’
(Rhodophyta) and ‘brown’ (Phaeophyta). They all contain the green pigment chlorophyll a, but red algae
also contain phycobilins and brown algae contain xanthophylls (reddish- and brownish-coloured
pigments, respectively). Kelps are extremely successful, colonising shallow seawater habitats throughout
temperate and cold water areas wherever they find a hard substrate to which they can attach. This
SPOTLIGHT column looks at some of the particular features which help kelps to survive, and thrive, in
apparently harsh and often storm-battered environments.
THE KELP ‘PLANT’
Although not strictly comparable in structure to flowering plants which are divided into root, stem and
leaf, kelps such as Laminaria hyperborea have evolved a remarkably tree-like shape (see Figure 1, (p.11).
The highly branched kelp holdfasts are root-like, not in order to keep the plant anchored in soil, nor to
aid the absorption of nutrients, but simply to attach the kelp firmly to the rock surface and enable it to
withstand the pull of the waves. The specialised tissues of flowering plants evolved largely in response to
the need to conserve moisture and the need to absorb water and nutrients from the soil and transport
them to other parts of the plant. Kelps have no such needs because seaweeds, surrounded by the
nutritive medium of seawater, can absorb and excrete directly through their tissues.
The long, stiff but fairly flexible stipe carries a large, fan-shaped frond, divided into many sections. The
frond is flattened, as are leaves, to expose a large area to the light - first one side then the other as it
waves in the swell. The stipe supports the frond above the seabed, which prevents it from being
Reference: Hiscock, S. (March 1991), Biological Sciences Review, pgs 7-11
smashed against the rocks by the action of the waves and also takes it nearer the light for efficient
photosynthesis.
IMPORTANT FACTORS
In common with other organisms which photosynthesise, kelps need light as an energy source, together
with dissolved gases and nutrients from the surrounding seawater. The amount of sunlight reaching the
kelp depends on the amount of light reaching the sea surface and on the clarity of the water. Light is
absorbed directly by the water, and is also scattered by particles suspended in the water so, even in the
clearest water, light penetration is at least 2,000 times less than it is through air. The maximum depth at
which kelps can grow around Britain varies from only a metre or two below low water in muddy
estuaries, to over 30 metres in the clear, more oceanic type of water around St Kilda and other remote
Scottish islands. This lower limit of kelp roughly corresponds with the depth where light is 1% of that at
the surface.
Kelps need water movement so that new supplies of nutrients can be brought to them and waste
products can be taken away. Laminaria hyperborea, for example, cannot grow in very sheltered locations
unless there is some tidal flow to compensate for the constant wave action on exposed coasts.
As on land, there are seasons in the sea, with temperature, light levels, day length and nutrient
concentrations all changing according to a fairly predictable pattern. Important nutrients for growth,
such as nitrates, become depleted by plankton blooms and seaweed growth during spring, and may then
be reduced to levels which would be limiting for most plant growth. Kelps, however, can continue to
grow into the summer. Although external nitrates have then become limiting, kelps contain large
reserves of nitrates in their tissues which they can mobilise at this critical time. They can also store
excess products of summer photosynthesis as laminarin, a polysaccharide which is used by the plant in
winter when light and temperature conditions are unfavourable for growth, but when nitrate levels are
again high in the surrounding seawater. This ability to store photosynthetic products gives kelp an
advantage over other seaweeds, whose growth is limited by lack of light and nutrients at certain times of
the year.
THE FOREST
In addition to being admirably suited to their environment, and successful enough to form dense forests
over great stretches of coastline, kelps also provide a home for a host of other animals and plants. The
layered structure of a kelp forest is analogous to that of land forests, with different organisms associated
with, and characterising, the different layers. Between the kelp hold-fasts, rock surfaces are covered with
Reference: Hiscock, S. (March 1991), Biological Sciences Review, pgs 7-11
smaller red and brown seaweeds, attached animals like hydroids, sponges and anemones, and a host of
small, mobile animals living in and among the undergrowth. This ‘forest-floor’ is a relatively sheltered
environment, with wave action dampened by the presence of the kelp canopy above.
Kelp holdfasts are highly branched, with sheltered nooks and crannies for many small animals. The
number of amphipod species (i.e. small crustaceans such as sand hoppers) present in the holdfast, and
their quantity, can be useful indicators of water turbidity, and of the degree of exposure to waves and
currents. The blue-rayed limpet grazes a hollow in the centre of the holdfast, sometimes weakening it so
much that the kelp plant is carried away by the waves during a storm, taking the limpet’s home with it.
The stipes of Laminaria hyperborea are rough and become heavily colonised by other seaweeds and
animals. Red seaweeds take advantage of an elevated position near the light, the extra water movement
and the relative safety from grazing urchins. Kelp fronds - constantly moving and with a smooth,
slippery surface - are a more difficult place for larvae and spores to settle. Young blue-rayed limpets with
their beautiful iridescent dotted-line rays, settle on the fronds before moving down the stipe to the
holdfast. By the end of summer, large areas of frond may be covered in bryozoans and hydroids.
Laminaria hyperborea has a novel way of ridding itself of these epiphytes, which interfere with
photosynthesis; it sheds its frond each spring and grows a new one from the top of the stipe.
MAN’S USE OF KELP
As well as being important in the ecology of in-shore marine areas, kelp is a valuable resource for
humans, and has been used for thousands of years as a source of food and chemicals. Direct use as food
is best developed in Japan and other parts of the Orient, where a wide variety of seaweeds, including
kelp (as kombu and wakame) are eaten - fresh, dried or cooked in soups with other ingredients. Seaweed
extracts have also become important in recent years. For example, extracts of phycocolloid (a
polysaccharide polymer) include agar, carrageenan and alginic acid. The latter is obtained from brown
algae, mainly the Laminariales (kelps) and Fucales (wracks). Alginic acid is used in a wide range of food,
cosmetic and pharmaceutical products (from ice cream to toothpaste), as an emulsifier, thickener or
stabiliser. Its water-holding properties are useful in paper and textile printing, and it has also been used
in lubricants and adhesives and as a fining for beers and wines. It has even been tried as a substitute
(‘cancer-free’) tobacco, suitably coloured and flavoured!
Kelp has fed humans indirectly through its use as an animal food or as a fertiliser (in the form of ash or
compost). Burning of kelp produces ash which is rich in sodium, potassium and iodine. When ash
production was first introduced to Orkney in 1722, the islanders reacted unfavourably, blaming the kelp
Reference: Hiscock, S. (March 1991), Biological Sciences Review, pgs 7-11
burning for bad crops and sick cattle. The matter came to a head with the 91Kelp Riot92 of 1762. Some
time later, however, ash production became a mainstay of the economy of Orkney, when seaweed ash
was a major source of soda the manufacture of glass and soaps. a; The Orkney island of North Ronald
has its own type of sheep, which live almost entirely on seaweed, including a large proportion of drift
kelp. A six-foot-high wall runs for 12 miles right around the island, keeping the sheep on the seaward
side and allowing the land inside the wall to be cultivated. In adapting to their strange diet the sheep
have become highly efficient at absorbing copper, which is present in very small quantities in seaweed.
In fact they now absorb copper so efficiently that they can actually die from copper poisoning if fed on
good pasture.
Traditionally kelp has been harvested by being cut at low tide or collected from the large amounts
thrown up on the shore after storms. For the better quality needed for commercial use, kelp must be
freshly harvested from the sea and several mechanical methods of harvesting have been developed. In
Norway, for example, Laminaria is harvested by dragging dredges through the sea, which cut the stipes
5-20 cm above the holdfast. Regeneration, from the remaining small plants, takes 3-4 years. In contrast,
the giant kelp Macrocystis pyrifera, which has fronds that float at the surface, can be harvested more easily
by barges with mowing bars 6-7 m wide which cut the fronds about 1.5 m below the surface.
Regeneration in this case is much faster, and from both the cut fronds and young plants.
Kelp has been cultivated for thousands of years in
Japan by the simple method of placing boulders in
areas where there would otherwise be no hard
substrate for them to attach to, so that kelp spores
can settle from the seawater. In recent years, the
study of the lifecycle of kelp - which involves a
microscopic, filamentous sexual phase (gametophyte)
alternating with the large, asexual kelp (sporophyte),
as shown in Figure 1 - has resulted in new methods
cultivation. The reproduction of gametophytes is
being controlled by varying the amount and quality of
light, producing young sporophytes which can be
grown in culture until they are large enough to
transfer to the sea. They are then grown on ropes
hanging from bamboo rafts, taking only one year to
reach harvestable size instead of two under natural conditions.
Figure 1: Kelp (Laminaria hyperborean) lifecycle
Reference: Hiscock, S. (March 1991), Biological Sciences Review, pgs 7-11
For the future, giant ‘ocean farms’ have been suggested for producing a large biomass of kelp. The kelp
would be used as a renewable energy source in the production of methane gas by fermentation with
anaerobic bacteria. It has already proved possible to enhance the growth of the giant kelp off California
by pumping nutrient-rich, deep water to the surface where the growing fronds float, but at present such
farms are still in the experimental stages.
So, next time you enjoy a packet of ‘kelp crunchies’ or an ice cream, why not reflect on the beautiful
underwater forests where the kelp grows, and the interesting adaptations which help to make kelp highly
successful seaweeds!
FURTHER READING
Dring, M. J. (1982) The Biology of Marine Plants, Contemporary Biology series, Edward Arnold.
Hiscock, S. (1979) A Field Key to the British Brown Seaweeds, Field Studies 5, (Reprinted 1984). Available
from Field Studies Council, Nettlecombe Court, Williton, Somerset TA4 4HT).
Hiscock, 5. (1986) A Field Key to the British Red Seaweeds, (Available as above).
Sue Hiscock
Reference: Hiscock, S. (March 1991), Biological Sciences Review, pgs 7-11
Dr Sue Hiscock is a freelance Marine Biologist and underwater photographer. She previously
worked for the Nature Conservancy Council’s ‘Marine Nature Conservation Review’ with
special responsibility for the west coast of Scotland.
Reference: Hiscock, S. (March 1991), Biological Sciences Review, pgs 7-11