This review makes use of the MNCR biotope classification system in an attempt to group together kelp beds which are similar in nature.

The MNCR biotope classifications use the easily measured, stable physical parameters (depth, location, substratum etc.) in combination with observations of easily found and identifiable species that are thought to act as bioindicators of dynamic physical conditions (subsurface irradiance, sedimentation, temperature & salinity fluctuations, etc.). The broad classifications (higher codes) are further refined in terms of the presence or absence of the easily observed and characteristic plant and animal species within each biotope.

It must be stressed that the marine biotope classification is not intended to give any indication of the

• conservation status of a site
• biodiversity of the site
• dynamic patterns of species interactions that may occur at that site.

As an illustration of an equivalent situation on land, a woodland area might be classified on the basis of being upland or lowland, on acidic or calcareous rock, in a high or low rainfall area, being deciduous or coniferous (etc.). The characteristic species might include the larger understorey plants and some of the birds. But a combination of these descriptions would not necessarily give an indication of the diversity and temporal shifts in the insect, bird and plant populations found in the wood, the spring flowering bulbs and autumnal fungi, the complete range of tree species or the maturity of the trees.

Biotope classifications are a very useful, standardised descriptive tool for the mapping and definition of management areas and for suggesting the potential species to be encountered within the area of each biotope (based on previous survey experience). They can provide the underlying framework for the structure of a management plan but cannot be expected to contribute to the biological monitoring or management of a site. Due to the gaps in our present knowledge of the species interactions that may be critical to the maintenance of the favourable status of kelp ecosystems, species that are normally hidden or cryptic but are characteristic of certain biotopes may not be listed in the present biotope descriptions.

At present there are 51 different biotope codes for infralittoral kelp communities. Within these classifications are included the higher code descriptions. Of the most detailed classifications (biotope and sub-biotope), 45 separate "sub-tidal reef biotopes with kelp" are listed. These classifications are summarised with brief biotope descriptions in an appendix. The 5 biotope complexes (with their higher code categories), which together encompass the individual biotopes, are:

• Exposed infralittoral rock
• EIR.KFaR: kelp with cushion fauna, foliose red seaweeds or coralline crusts (exposed rock)
• Moderately exposed infralittoral rock
• MIR.KR: kelp with red seaweeds (moderately exposed rock)
• MIR.GzK: grazed kelp with algal crusts
• MIR.SedK: sand or gravel affected or disturbed kelp and seaweed communities
• Sheltered infralittoral rock
• SIR.K: silted kelp (stable rock)

The biological complexity of the kelp beds as habitats and their significance in the marine coastal ecosystems of the UK, can be illustrated by comparing the numbers of distinct kelp biotope descriptions with the numbers of other marine biotope codes.

• Subtidal kelp biotopes are present in 3 of the 23 coastal habitat complexes defined in the MNCR classification,
• Of the 276 biotopes and sub-biotopes defined to date, there are 45 (16%) with kelp as a characteristic component.
• The enormous numbers of species so far recorded from kelp beds around the UK are summarised in the table below, (see Appendix for complete list). Species numbers (grouped taxonomically) in the MNCR database records for all kelp biotopes in the UK are summarised in the Appendix. Within the five biotope complexes in the MNCR database (as of March, 1998) the species recorded appear to vary greatly from one site to the next, reflecting the wide geographical distribution of the component biotopes within these complexes as well as the variable quality of the data (D. Connor, pers. comm.) and the intrinsic patchiness and diversity of the fauna and flora found within kelp beds.

• For all the sites included in the MNCR database, a total of 1,819 species (taxa) have been listed (see also Appendix).
• Only 10 of these species are regularly found at most sites (more than 50% of sites; see table above).
• The "faithful" species are those which are conspicuous and easily identified, and this may account for their prominence in the database.
• Smaller and less readily identifiable species, or species that are hidden within the structure of the biotope, are less frequently recorded even if they are common and abundant at a site. The less conspicuous the species, the fewer the number of records in the database.
• In terms of management of biotopes for conservation purposes, these biases within the database and produced by the data collection methods may pose a serious problem by placing emphasis on species which may not be critical to the maintenance of the favourable status of the biotope.

The "faithful" species (those recorded at more than half of the sites) in the various kelp biotopes are shown in the table below. That so few large and common species should be recorded on a regular basis from kelp beds, suggests that the faunal and floral variation recorded from different locations as well as from different biotopes with kelp is enormous. The variation in species faithfulness that occurs within a specific biotope is very much less than the variation that occurs between biotopes grouped within a biotope complex. For example, of the suite of 11 species used to characterise the biotope "MIR.Lhyp.Ft", 8 species are found at more than 61% of all recorded sites of this biotope (T. Hill, pers. comm.)

• With only 12 demonstration SACs scattered throughout the variety of coastal locations in the UK, it is probable that some of the kelp biotopes will not be represented adequately in the proposed protected areas.

It is very important that the following points are noted when referring to the numbers and diversity of species found in kelp forests:The assemblages of species found in kelp forests are probably larger and more complex than our present knowledge would suggest. Much of the information collected during biological surveys has been collected by divers. This is an excellent method of preliminary data collection but it must be noted that it is akin to collecting data on the species diversity of a rain-forest while suspended from a helicopter!

• More than 1000 of the species reported from kelp biotopes were recorded from fewer than 1% of the sites listed under the higher code classifications (see Appendix). This might suggest that, of the nearly 2000 total species reported as living in kelp biotopes, more than half are uncommon (or rare) or confined to an uncommon biotope. However, of the species listed as "rare" or only present at a few locations even if locally common, many are either difficult or impossible to see under normal surveying circumstances or hard to recognise in situ, or are small and easily overlooked in samples or require specialist skills in order to identify them.
• Based on the patterns of species reported in the MNCR database and on the raw data obtained from several subtidal biological surveys, it is suggested that in many instances one or more of the following may have occurred:
• samples were sorted and identified by non-specialists (there is a paucity of species indifficult groups, reduced biodiversity, inaccurate or incomplete identification).
• samples were sorted and identified by a specialist (possible over-emphasis in the biodiversity of the area of expertise).
• uneven sampling or recording efforts (only the largest, the most easily recognised or the most obvious of the characteristic species of the biotope are reported).
• An understanding of how the samples were collected and identified is needed in order to set the limits of interpretation on the data (D. Connor, pers. comm.).
• For pragmatic reasons, the biotope classification method does not take into account the presence of species which are rare, cryptic, hidden within the biotope or ephemeral. These may well be the species that are key to the conservation status of the biotope
• Seasonally migrating species may be key predators within a biotope.
• The grazing activity of the population of Helcion pellucidum may control the age structure of kelp forests, but this limpet species commonly lives hidden within the kelp holdfast and is only obvious when newly settled on fronds. This species has been reported (according to the MNCR database) from only about 500 of the 3425 kelp biotope sites in the database yet phycologists report that it is abundant in L. hyperborea forests throughout the UK.

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