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Kelps (Laminaria spp)

Kelps (Laminaria spp)

The lower shore, near chart datum, on a low spring tide showing Laminarians and red algae exposed. The main kelp is Laminaria digitata

General features of the Kelp

Kelp beds are like a marine forest, with layers of algae and animals. Kelps growing in deeper water develop more pigment than those higher up to enable them to absorb what little light there is. Many organisms colonise the kelp both for space and food, e.g. sea mat, grows on the lamina to gain access to circulating water containing their food. The Blue-rayed Limpet feeds on the kelp. The holdfast is a micro-habitat containing a micro-community with plankton as the main producer, whilst the primary consumers are thousands of sedentary polychaete worms which filter the water. Most of these worms are minute. Small brittlestars also filter feed. Large carnivorous polychaetes feed on the sedentary worms. At the top of this food web is the Hairy Crab (Pilumnis hirtellus); one or two may live under each holdfast.
The Kelps can only tolerate 20 percent water loss and so are limited to the extreme lower shore where they all adapt well to the low light intensities. Their growth is prolific: most of the productivity is from the continuously growing lamina (blade) which develops more cells at the base and erodes at the tip - like a conveyor belt. Structurally, they are the most advanced of the algae, with trumpet shaped cells in the stipe which are believed to have a conducting function. They exhibit alternation of generations; the dominant phase (the kelp plant) is the sporophyte. This bears sporangia which release zoospores into the water that germinate to produce a gameteophyte stage, of which little information is known. Sporangia develop on both sides of the lamina surface. A cross section through the stipe reveals annual growth rings, the average age being about five years. Kelps are harvested commercially for alginates (used in cosmetics, non-drip paint, toothpaste and ice cream), potash, soda, iodine, fertiliser and animal fodder. In very sheltered shores where sand/mud are deposited kelp may give way to Zostera (Eel Grass). If small stones are present the Boot-Lace Weed, ( Chorda filum), grows.
Sugar Kelp, Laminaria saccharina is called the sugar kelp because when it dries a white sugary substance develops on the surface. The stipe of the Cuvie, Laminaria hyperborea surface is rough, enabling red algae to colonise it. Palmaria palmata is a common epiphyte attached to the stipe. These growths do not occur on Laminaria digitata

The three main species:

The Sugar Kelp or Sea Belt, Laminaria saccharina. Grows up to up to 2 metres in length. Unlike the other kelps the lamina is a continuous ribbon with a frilled edge. The central part is thickened. The holdfast is branched but very small in comparison with the other kelps. The stipe is thin and flexible so at low tide the plant goes limp and lies flat, remaining in the shallow water

Closeup of the frond of the Sugar Kelp
Closeup of the frond of the Sugar Kelp. Note that the uneven surface induces turbulence around the frond which ensures a good exchange of gases and nutrients. Useful in sheltered conditions.

The Tangle or Oarweed, Laminaria digitata
This grows to about a metre in length. The blade or lamina is split in to many "digits" and this splitting increases with the wave action; very sheltered areas may have no splits. The holdfast is very branched and broad giving a stronger attachment to rock than L. saccharina. The stipe is flexible so at low tide the plant goes limp and lies flat, thus remaining in the shallow water and not drying out. The stipe is smooth and oval in section and the length varies with depth. These features enable it to survive a high degree of wave action.

The Cuvie, Laminaria hyperborea
This grows between 1 - 3 metres. The lamina is split into "digits" helping to reduce the effects of wave action. The holdfast is huge and dome shaped giving the best attachment in extremes of wave action. The stipe is inflexible and supports the kelp in an upright position at low tide. This would cause the alga to dry out and so it survives, therefore, only in the sublittoral zone where it will be unaffected by the tides. The spray from extreme wave action may push a small number onto the lower shore. The stipe surface is rough, enabling red algae to colonise it. Palmaria palmata is a common epiphyte attached to the stipe. This stipe is round in section.

The kelp with the red alga, Dulse, is Laminaria hyperborea.

The kelp with the red alga, Dulse, is Laminaria hyperborea.

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