Diversity of conditions within kelp beds

Patchiness of distribution

Kelp plants as a habitat

Species present in infralittoral biotopes with kelp

A small area of a kelp biotope (only a few meters square) may contain a variety of different small habitat areas within it, differentially exploited by various species; the substratum may be generally bedrock but with small patches (less than 1 m²) that are:

• clear bedrock,
• pockets of sediment,
• a mobile boulder,
• open rock that is flat or fissured,
• overhangs of rock,
• steep or vertical faces,

and these different substratum conditions will be advantageous to some species and disadvantageous to others. In addition, the kelp plants themselves may modify the local environment within a biotope. Kelps plants may

• act as energy dampers, ameliorating the surge effects of waves
• reduce current flow in areas of dense kelp forest
• reduce the light available to the deeper parts of the kelp bed
• reduce ambient levels of macronutrients within the kelp beds
• increase levels of DOM and POM within the kelp bed

again enabling some species to flourish at the expense of others on a small scale and leading to a patchwork pattern of species distribution within the biotope.

One of the characteristic features of kelp beds throughout the world is the patchwork of different species and groups of species that occur within the biotopes. This is particularly obvious in areas of low siltation, high water movement, and high kelp productivity (mg C m^-2 yr^-1). The bedrock or other substratum in such areas can appear to be covered by a multicoloured mosaic of encrusting algae, sponges, colonial tunicates and kelp holdfasts - with multitudes of other sessile and mobile species on and between them. The inherently dynamic nature of the patchiness of kelp biotopes adds to the difficulty of developing reliable and effective monitoring programmes.

Long term (ongoing) studies in South Africa appear to show that the patches in the kelp beds of are not stable (R. Anderson, pers. comm.), but that the intense competition for space results in slow but continuous changes in the patchwork and the species that form it. The complexity of the environmental structure suggests that small changes in the local conditions might result in a shift in the balance between the competing species and result in more rapid shifts in the patchwork or the development of a different series of dominant species. It is possible that minor environmental changes which allow urchin grazing to increase may permit encrusting algae to dominate for a period but that, over time, the balance would be restored.

The time scales involved in the natural dynamic variation in the species patchwork appear to be on the scale of decades to centuries in South African habitats. The apparently natural development and recovery of NW Atlantic kelp forests from urchin overgrazing also appears to take several decades.

• The corresponding time scales for the species of kelps and urchins and patch forming species of algae and animals in UK waters are not known.

Kelp communities are three dimensional in structure. The kelp plants themselves usually become the habitat for other marine species as the mature plants:

• provide a vertical addition to the sea bed, effectively increasing the surface area and habitat variety
• provide a surface to which other seaweeds can attach; other organisms then feed on these seaweeds
• can be grazed directly by organisms such as Helcion pellucidum
• provide attachment sites for sessile species of animals which in turn are food for other animals
• holdfasts shelter many species of animals from a wide range of taxa

The most comprehensive data sets available on the species found in UK kelp biotopes are the results of the MNCR surveys that have been undertaken during recent years. The database was consulted during the preparation of this report, in order to obtain some indication of the variety of animals and plants that are found within kelp biotopes. The complete species list as of March 1998 (separated into the 5 biotope complexes) is included as a reference source in Appendix 5. The results of other surveys of kelp beds which may have been undertaken around the UK are not published or not readily available.

The MNCR database aims to provide an indication of the species to be expected within a given kelp biotope and will of course list the characteristic species for that biotope. In addition to the limitations discussed in section I.E., the following specific weaknesses of the species list for kelp biotopes should be noted:

• errors of data entry have occurred (intertidal algae and lichens appear on the list)
• polychaete species are very sparsely reported although they make a major contribution to the biodiversity, biomass and community structure within the kelp bed (Rinde et al., 1992). Since many polychaetes are small, live in burrows or within the holdfast, or are only active at night, it is perhaps not surprising that the frequency with which they are recorded during biological surveys does not reflect their significance or diversity in the kelp biotopes.
• holdfasts were not systematically sampled during MNCR surveys so the database does not reflect the diversity of species within this intensively exploited niche (D. Connor, pers. comm.)
• Homarus gammarus (lobster) is reported at only a small percentage of the survey sites, yet lobster fishermen will frequently set pots for preference in areas of kelp forest and reported catches suggest the species is more common than the database would suggest there are some examples of incomplete identification within the database, e.g. coralline algae (as "Corallinaceae") have been recorded at more than 50% of all sites within the kelp biotopes.

However, only 19 of the more than 45 species of coralline red algae present in UK waters have been listed, whereas the taxonomic records (Irvine & Chamberlain, 1994) suggest that few species of coralline algae in the UK are found outside kelp biotopes

An additional weakness is that of the temporary or seasonal presence or absence of some species in kelp beds, either due to a migratory habit or because of a heteromorphic life history. Of the species listed it is possible that some are found in kelp beds only on a temporary basis while other species may not appear in the database having been absent or cryptic during the period of data collection.

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