There is a remarkable biodiversity within the kelp bed ecosystems of the UK. Over 1500 benthic faunal species have been reported to date (March 1998) from kelp biotopes in UK waters. In addition, there are possibly 400-500 macroalgal species and unknown numbers of microflora, microfauna, protozoan and fungal species.

Most biological surveys have, of necessity, been restricted to readily observable and readily identifiable species. The difficulty of obtaining statistically representative samples of the flora and fauna within a kelp bed is daunting. With so many different microhabitats within a site, establishing the range of species that might be representative is very difficult indeed.

The MNCR species list provides a good foundation from which lists of species anticipated at a site within a known kelp biotope may be constructed. However, the species present at a particular location are a reflection of the unique environmental conditions at that site and, as such, the database is only a starting point for the determination of the biodiversity of a site.

The concept of diversity indices may be useful as a preliminary approach to establishing a baseline of diversity for monitoring specific sites. However, the diversity index alone is not a sufficiently sensitive tool for monitoring changes within a biotope as the reproducibility and reliability of the method is strongly related to the time, effort, and specific personnel conducting the survey.

The wide variety of species which are known to be found within kelp biotopes could provide a base-line from which it may in future be possible to select species with known:

• distribution patterns
• chemical sensitivities
• temperature and salinity limits
• seasonal patterns of growth and reproduction
• irradiance requirements
• suspended or settling silt tolerances
• hydrographic requirements

These selected species would then serve as sensitive biological indicators of the conservation status of the habitat within a monitoring programme. Unfortunately, at present the information listed above is known for only a handful of the thousands of species found within kelp biotopes.

• The research into and the development of the use of biological indicator species within monitoring programmes should receive a high level of priority.

Within the kelp biotopes in UK waters are species which are representatives of almost every taxonomic phylum in the classification systems presently in use. The correct identification of species is needed in order to determine the biodiversity of a site, but:

• in many kelp beds, at certain times of the year, it is even difficult to distinguish between L. hyperborea and L. digitata without taking tissue samples
• the numbers of different but closely related species within kelp biotopes are large
• the consistent recognition and identification of any organisms, other than the most obvious and common, requires extensive taxonomic expertise (in each kingdom and phylum)
• much more attention to species identification will be required if the true biodiversity of species in UK kelp beds is to be monitored
• the levels of expertise required for correct and consistent results are restricted to taxonomic specialists ("an increasingly rare commodity", J. Breen, pers. comm.).

The most serious drawback of the high levels of expertise needed for species identification is that a monitoring programme needing to distinguish between closely related species can initially be time-consuming and costly. However,

• shifts in the population structure or the distribution of closely related species may be excellent markers of change in the conservation status of a site or of damage related to pollution, etc.

There are no other biotopes in UK coastal waters which display the species diversity, complexity of ecological relationships and net primary productivity (mg C m^-2 yr^-1) of kelp beds. Other seaweed dominated habitats such as rocky shores, have equally complex ecological patterns and relationships, but do not have as high diversity and productivity as kelp biotopes. Salt marsh areas dominated by Spartina spp. are also productive areas (the lower plant biomass of Spartina beds compared to kelp beds is partially compensated for by the higher rates of net primary production, mg C mg^-1 yr^-1), but Spartina beds are species-poor in comparison to kelp beds. Subtidal beds of Zostera spp. (eel-grass) are also relatively productive systems but again, lack the biomass of the kelp forests as well as the diversity of habitat niches that are found in kelp beds.

Although the species of kelps and their morphology and biology differ in kelp beds throughout the world, there are broad similarities in the ecology of kelp beds and in the range of niches and habitats found within them. Kelp beds:

• have high primary productivity per unit area (mg C m^-2 yr^-1)
• export large amounts of POM and DOM
• have a high biodiversity
• have a commercial value based on kelp species
• are the habitat and food source for other commercially valuable species (seaweeds, lobsters, crabs etc.)
• dampen the force of the waves arriving at the shore

Maggs (1986) has prepared a biogeographical analysis of marine algae (with emphasis on the Scottish flora but applicable throughout the UK). The British Isles intersect the boundaries of the arctic, lusitanian and boreal floristic provinces and as such, the diversity of algae found within the kelp biotopes of the UK can vary greatly with location. Of the kelp species, Alaria esculenta, L. hyperborea, L. digitata and L. saccharina are found throughout the British Isles. In the northern parts of Scotland, Alaria (a boreal species) will over-winter as a recognisable plant, whereas on the southern coasts of England this species usually dies back to a nearly unrecognisable holdfast (T. Holt, pers.obs.). L. ochroleuca is only found on the south western coasts of England, where winter water temperatures are just high enough for the species to complete its life cycle. As a generalisation, the further south and west a kelp bed is located in UK waters, the more algal species are likely to be found (under ideal local conditions of salinity, silt load and water movement).

The British Isles also intersect the boundaries of the arctic, lusitanian and boreal faunistic provinces and the species distribution of the marine fauna around the UK reflects these overlaps. Many species in UK coastal waters show a northern or a southern/western distributional bias (see table below; Briggs, 1974).

Kelp bed faunas are no exception to this. Indeed, the recently recorded kelp species L. ochroleuca is itself limited to south western sites and its associated biota therefore has elements of southern biased fauna (e.g. at Wembury). Sublittoral geographic distributions have been discussed by Hiscock & Mitchell (1980), Hiscock (1980) and Earll & Farnham (1983). The latter present tables listing northern and southern faunal elements and their respective southern and northern boundaries. They also noted how a number of additional factors such as substratum unsuitability may add to the impoverishment of the sublittoral fauna of the English east coast.

The seasonal changes in biodiversity which occur within a single kelp bed have not been studied. With the enormous range of species which are present, such a project would be very labour intensive and time consuming. As far as individual species are concerned, any seasonal changes in distribution are known only if that species has been studied in detail. Some species of algae have seasonally heteromorphic life histories spending a part of the year as a cryptic or encrusting growth form and only becoming recognisable in the foliose phase of their life cycles. Some hydrozoans may be present in the kelp bed in their attached, colonial form only for a part of the year, spending the rest of the year as jellyfish.

Long term fluctuations or permanent shifts in the biodiversity of kelp beds may occur in the UK but the long term monitoring of the species diversity of kelp biotopes has not been undertaken. Long term studies have taken place and are continuing at several locations around the world. A kelp bed on the Atlantic coast of Canada was investigated during the late 1960s (Mann, 1972a) and intensive research continues in the same location. The kelp beds of the south western Cape province (South Africa) and California have been under investigation for more than 20 years now, and some long term data series have been and are being collected. Temporal changes of the biota within kelp beds seem to be on a decadal scale, making long term monitoring projects a necessity.

The effect of currents on species distributions is fairly well known (Gubbay, 1988) and is well demonstrated in areas such as the Menai Straits and Strangford Narrows as well as in some more open areas. It has been shown that the array of cnidarian species in an area is dependent on current conditions (all other parameters being equal). Tubularia indivisa and Sertularia argentia are characteristic of very strong currents, S. argentia, Actinothoe sphyrodeta and Sagartia elegans of strong current, whilst species such as Halesium and Kichenpauria occur in moderate to sheltered conditions.

L. saccharina can cope with fairly strong tidal currents but given adequate substrata its distribution is generally limited to areas with modest or little wave exposure. The conditions under which L. saccharina is dominant are often such that large amounts of silt are able to settle on the kelp canopy and on the substrata, which are correspondingly faunistically poor (Hiscock, 1983). This is in complete contrast to the faunistically rich habitat of the L. hyperborea kelp forests in non-silty areas (see linked table detailed below). These disparate conditions may be found within a few hundred meters of each other where there is a kelp bed that extends from a sheltered bay to an exposed headland, for example.

Table - Tolerances of kelp bed fauna to siltation

There are several species which are deemed to be highly typical of kelp beds in the UK, including Helcion pellucidum and Obelia geniculata (D. Connor, pers. comm.). However, other than the kelp species themselves - which define the biotope - it seems possible that there are no species which are strictly confined to kelp biotopes as their habitat. The list of species that utilise kelp beds at some time as a safe habitat, or for food or as a link in the food web that supports them, may include the majority of all species found in UK waters.

The importance of rarity in the marine environment has recently been considered by Sanderson (1997). However, little or no work appears to have been compiled specifically on the rarity of species in kelp biotopes. Some species may be rare at specific sites but not in the UK or Europe as a whole, while others may be rare throughout their biogeographic range as they are an endangered species. Some species may be deemed rare because they are sparsely distributed but may be distributed extensively; others may be deemed to be rare simply because they remain unrecognised by all but the most skilled observers. In an area towards the edges of its habitat range (if these are known) a species becomes less common. In theory, therefore, if a site is known to be on the margins for a specific species, then monitoring the population of that species might be a suitable method of indicating that changes are occurring in the area. The identity and causes of the changes would, however, not be apparent from population monitoring.

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