Kelp - cool brown forests

Kelps are big brown palm-shaped seaweeds which grow on underwater rocks from low tide downwards all around Scotland's coasts. Kelps form dense underwater forests, which are just as valuable for wildlife as woodlands on land. Kelp forests provide food and shelter for many marine animals, and play a large part in recycling coastal nutrients, supporting food chains which include species of great commercial significance to Scotland's coastal communities.

What is kelp?

The name 'kelp' originally referred to the ash from burnt seaweeds, used in former times to make glass and soap. Now 'kelp' is widely used for a group of similar big brown seaweeds in the genera Laminaria, Saccharina and Saccorhiza. Four kelps are common in Scottish waters. The familiar tangle of the seashore (Laminaria digitata) grows in a narrow band around low water, while just below it, visible only on the lowest tides, is the main forest-forming kelp in Scotland, cuvie (Laminaria hyperborea), usually known simply as kelp. Sugar kelp Laminaria saccharina only forms forests in very sheltered places, including sealochs, and furbelows kelp (Saccorhiza polyschides) is a quick-growing, annual kelp which can occasionally form forests after storms have removed other kelps.

Kelps have no roots; instead they attach to the rocks with tough, branched holdfasts. In place of true stems, they have stiff stipes, which hold them up towards the sunlight. These are topped by palm-like fronds, which can be long and strap-shaped or split into several strap-like lobes, depending on the species.

Underwater forests

Dense forests of cuvie kelp develop wherever there is bedrock or large, stable boulders for attachment. They grow best in strong water movement, from either waves or currents. They need good levels of sunlight, so they only grow to depths of around 5m in dark, sheltered sealochs, but can grow to depths of more than 30m in the clear waters around St Kilda

If not grazed heavily by urchins, the sheltered 'forest floor' beneath the kelp is full of life: sea anemones, sea firs, sea squirts, sponges and a variety of red seaweeds all survive where the kelp stipes give them some protection from the waves. Small fish, like the two-spotted goby, dart amongst the fronds, and wrasse hunt small prey. Shoals of pollack or saithe often hover over the forest. Kelp forests may well provide some coasts with a degree of protection from storms, by absorbing some of the energy of the waves.

Slippery fronds

Kelp fronds produce slippery mucus which makes it difficult for animals to attach. Even so, by the end of the summer, the fronds can become covered in a fuzz of sea firs and sea mats, which block sunlight and hinder photosynthesis. So, in late winter and spring, the kelp sheds its frond, together with all its attached animals, and grows a new one. However little blue-rayed limpets, that graze the fronds all summer, climb down the stipe and into the holdfast before the old frond is shed, so avoid being discarded by their host. The old frond is eaten by urchins, or degraded by bacteria, releasing nutrients back into the water for the next season's growth.

Stiff stipes

The rough stipe of cuvie kelp provides attachment for a range of smaller seaweeds, and for animals including sea mats, sea squirts, and soft corals. Small amphipods and prawns often live amongst the seaweeds, and common urchins climb the stipes during calm periods to graze the seaweeds

Handy holdfasts

Many more creatures live in the sheltered nooks and crannies in the kelp holdfast. As many as 50 different species (and many hundreds of individual creatures) can make a home here. Worms, brittlestars, amphipods (small crustaceans) molluscs, anemones and seasquirts are amongst the commonest. Small fish including clingfish and butterfish also hide there, and larger fish including scorpionfish and lumpsuckers lay their eggs amongst the holdfasts.