At this point, it is necessary to provide some definitions of the terms that will be used in this review.

Kelp is the term at one time used for the ash obtained from burning seaweeds (400,000 t yr^-1 fresh weight in Northern Europe around the year 1800, Jensen 1979), the ash was used as a source of soda and potash for the glass and soap industries and later as a source of iodine. Folk memory (Northern Ireland) suggests that the stipes were the main component burned, the Laminaria blades together with the other species collected (Ascophyllum nodosum and Fucus vesiculosus for the main part) being valuable as a soil conditioner and fertiliser. In time the name "kelp" was transferred to the main constituents that were burned - Laminaria spp. - and since then, kelp has become a common colloquial term applied to the large brown seaweeds of the taxonomic order Laminariales, throughout the world.

Kelp plants are composed of distinct parts; the blade or lamina is the very obvious, largely thin, flat, flag-like portion of the plant. This is supported by the stipe, the stalk of the plant which may vary in length and thickness between different plants. The plant is attached to the substratum by the holdfast, which resembles a collection of roots but serves only as an anchor, having no nutrient gathering role. The root-like structures are known as haptera. Some kelp species produce sporophylls which are additional structures above the holdfast and below the blade which may resemble small, thicker blades or may be flattened outgrowths from the stipe. The large kelp plants in coastal waters are the sporophytes, they are all diploid ("2N", they have two sets of chromosomes and produce male or female spores by meiosis). The gametophyte plants are microscopic and are haploid ("N", have one set of chromosomes and produce eggs and sperm by mitosis).

Kelp forest is the term generally used to describe the nature of the kelp biotope in the upper infralittoral, where the plants are densely distributed and the large laminae overlap each other to form a canopy that excludes light. The upper limit of the forest is determined by the effects of storm waves, the tidal cycle (desiccation stress) and competition with other macroalgal species for space and light.

Kelp parkland is the term used to describe the kelp biotope in the lower infralittoral, where the kelp plants are more widely spaced and the laminae do not form a canopy cover. The lower limit of the parkland may be determined by the available light, the available substratum or by grazing pressure from sea urchins.

A biotope is defined as the habitat (i.e. the environment’s physical and chemical characteristics) together with its recurring associated community of species, operating together at a particular scale.

The habitat is taken to encompass the substratum and the particular conditions of wave exposure, salinity, tidal streams and other factors which contribute to the overall nature of the location. The term community is here used to signify a similar degree of association of species which regularly recurs in widely-separated geographical locations; the degree of similarity will vary, depending on the scale considered" (Connor et al, 1997).

• It should be noted that in the EC Habitats Directive, the term habitat is used in a much wider sense, to include the species or community living in the habitat; the common usage of this term is therefore synonymous with the term "biotope".

Bioindicators are plant or animal species which can be used to deduce environmental characteristics without the need to measure those characteristics continually. For example, rhododendrons are terrestrial plants that are indicators of acidic soils.

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