Community Structure within Kelp Biotopes

Pelagic habitat in a kelp bed

Kelp forest habitat

Kelp parkland habitat

Kelp plants as a habitat

Benthic habitat

As with terrestrial forests, the kelp beds contain a series of stratified habitats within them, and the flora and fauna associated with a kelp forest may occupy one or more of these vertical subdivisions. The relationships within and between the strata are complex because each will modify the water flow and irradiance available to the other layers. Furthermore, the species composition of a habitat stratum will vary not only with the geographical location of the biotope but also with depth within a single kelp bed. It is clear that kelp biotopes often have an extremely diverse fauna and, possibly as a result of this (daunting) diversity, there does not appear to have been any single investigation of the entire range of species in a single location. The epibenthic flora and fauna is now relatively well known and could possibly be used to demonstrate some environmental trends and gradients; the fauna of holdfasts is also fairly well known from a few locations. However, the meiofauna and the ambient plankton populations in kelp beds around the British Isles are more or less unknown.

Pelagic habitat in a kelp bed

Planktonic

Very little specific information is available about the plankton communities in areas dominated by kelps (or indeed other algae). It can be assumed that these communities will be similar to the general plankton of inshore areas but with larger inputs of larval stages from species with bentho-pelagic life cycles. Additional differences from the plankton in non-kelp biotopes would be the contribution from species with tidal or diurnal behaviour patterns. The former will lead to large seasonal inputs of spores, gametes and larval stages from the biota of the kelp biotopes and this input will be diluted with distance from its habitat of origin. The tidal or diurnal input will add some additional diversity to the plankton of the kelp biotopes on a cyclical basis; but to date this has been documented only in sea-grass communities (Bell et al., 1983) and not in kelp biotopes. It is possible that some planktonic species such as mysids and the arrow worm Spadella may be more common in kelp areas than just outside them (P.J.S. Boaden, personal observations). The abundance of filter-feeding and impingement-feeding organisms (e.g. sponges and bryozoans) within kelp biotopes highlights the importance of planktonic input to the benthic community. Wulff & Field (1983) have demonstrated the particular importance of phytoplankton during downwelling conditions in Benguela kelp beds.

Nektonic

There is a paucity of information on the roles of fish, diving birds and mammals in the kelp biotopes of northern Europe, although many divers have observed that conspicuous fish such as wrasse and pollock are common amongst kelp, in addition to more or less benthic and cryptic fish species such as blennies and gobies. Such fish can be important in the diet of diving birds such as cormorants and of seal and some otter populations. Elsewhere, the importance of the sea-otter and of benthic wolffish in the functioning of kelp ecosystems has been documented (Mann, 1982). These animals consume large numbers of sea urchins in the kelp habitats of North America.

Kelp forest habitat

The term "kelp forest" is generally used to refer to the part of the kelp biotope which lies between the lowest tides and the depths where kelp plants become less densely distributed. In the main forest area, kelp plants are densely packed and the blades of the tallest plants effectively form a canopy, casting smaller kelp plants, stipe epiphytes and understorey algal species into deep shade. The kelp canopy may intercept as much as 90% of the incident irradiance, with the result that many algal species which are normally confined to greater depths with low irradiances, are able to migrate into shallower waters.

In the upper part of a kelp forest, several kelp species may be found, depending on the wave exposure at a particular site, however, the comments in this section refer to L. hyperborea unless otherwise stated.

Flora

There is a zonation pattern of the epiphytic algae down the stipes of kelp plants, especially on plants of L. hyperborea. and this zonation pattern shifts and the component species change, depending on the stipe length and the depth at which the kelp holdfast is attached to the substratum (Harkin, 1981; Whittick, 1983; see also section III.C.2.e.i, and appendices 3 & 4). The epilithic species that are found vary with geographic location, water depth, canopy density and benthic irradiance, and also as a result of interactions with the grazing species of the site. As a result the species that are found form a diverse population. A list of species is given in the MNCR data set (Appendix 5).

Fauna

Erwin et al. (1986; 1990) described six depth-related categories of faunal distribution which could be distinguished among the common species in this densely populated part of a kelp biotope.

Table - Common faunal species with different depth distribution patterns within kelp forests in Northern Ireland

Kelp parkland habitat

In the lower infralittoral, where a suitable substratum continues into deeper water, the density of the kelp plants is reduced, probably as a result of light limitation. The extension of kelp biotopes into deeper water may, however, be prevented by the pressure of urchin grazing or by the increased rate of sedimentation due to reduction of water movement with depth. Individual kelp plants may become very large in this part of the biotope.

Flora

In the lower parts of the kelp forest and into the park biotopes there is little change in the patterns of species distribution on the kelp stipes. However, the biomass of individual epiphytic plants increases and in the habitat as a whole (rather than on individual stipes) the diversity of epiphytic algae increases.

Fauna

All the species listed in Table 12, except those in the first category, also occur in the kelp parkland, both on and among the kelp plants. In addition, there are many species which are more or less characteristic of this lower part of the infralittoral zone and other species which extend upwards from the circalittoral zone. Hence, the kelp parkland biotopes are frequently the most species-rich areas to be found in the sublittoral zone. The precise array of animal species will depend on a number of factors, principal among which are the types and patterns of substrata and the localised hydrodynamic regimes. For example, many species are found more frequently within kelp biotopes that are on bedrock than within those that are on boulders; a few species show the reverse preference. Erwin et al. (1986) cite examples which can be seen in the following linked table:

Table - Fauna with apparent substratum preferences within kelp parkland biotopes in Northern Ireland

Kelp plants as a habitat

Kelp plants themselves can form the habitat for numerous species of understorey algae, and a large number of invertebrate species exploit the plants as a secure habitat - in much the same way as forest trees support a variety of other forms of plant and animal life.

Epiflora

A wide variety of species of algae can be found on the stipes of L. hyperborea. Epiphytic algae are not as commonly found on the stipes of other kelp species and only the most rapidly growing of the opportunistic species are able to colonise the blades of kelps. In the upper infralittoral, where the kelp plants are exposed to wave action, surf and surge, where the upper portions of the stipe may be exposed to the atmosphere during periods of low water and where irradiance levels may become high, there are fewer epiphytic algae than may be found in the deeper parts of the kelp bed. In addition to the mechanical disturbance, irradiance and grazing pressure are thought to affect the distribution of epiphytic algae.

Table – Kelp species in UK waters as habitats for epiflora and epifauna

Blades: In many habitats the blades of kelps are remarkably free from macroalgal epiphytes, however, the blades can be important hosts for microalgae as epiphytes and endophytes. Myrionema corunnae is only found on Laminaria blades while Pogotrichum filiforme and Chilionema spp. are mainly restricted to kelp blades. When extension growth of the blade has slowed, a number of opportunistic species may colonise the distal portion of the blades, particularly in areas where there is little wave action. Enteromorpha spp., Ulva spp., Ectocarpus spp. and Pilayella littoralis have been noted (D. Birkett, pers. obs. Strangford Lough).

Stipes: The stipes of the long-lived L. hyperborea are frequently heavily epiphytised by a wide range of smaller seaweeds, especially foliose red algae. The epiphytic species which are found vary with the geographic location of the kelp bed. The distribution patterns of species on kelp stipes follows a pattern related to the light available on the stipe and the number of species increases with the age and length of the stipe. As a result, the structure of the epifloral community varies with the holdfast depth and the height of the adult kelp plant (see Appendix 3). The spacing of the kelp stipes of the host plants also influences the epiphytic algae - where stipes are closely spaced, the foliose epiphytes are abraded as the stipes are agitated by the waves and currents. The algae are not evenly distributed over the stipes, but there is a biomass concentration in the top 10-20 cm of the host stipe.

Epifauna and endofauna

There are several physically distinct parts of the kelp plant which are exploited by animals as a habitat. Each supports a different type of community consisting of possibly thousands of individuals from hundreds of different species, and all physically supported by a single individual kelp plant. The complexity of the kelp plant as a habitat has long been recognised but little modern research has been published, so the available information is mostly of a descriptive nature. Relatively little information on kelp epifauna is available but it can be assumed the species concerned show niche segregation and competitive hierarchy as does the epifaunal population of Fucus serratus (Seed & O’Connor, 1981).

Two important points that should be noted with regard to epifaunal and endofaunal populations are that:

  • it is possible that different kelp species support a different selection of epifaunal and endofaunal species.
  • it is probable that some if not many of the animal species found have specific sensitivities which would be of use in an environmental monitoring and conservation management role.

Specific elements of kelp anatomy which are exploited by epifauna and endofauna

Microflora and microfauna

Nothing is known of the possible components, associations or ecological importance of the fungi, unicellular algae, protozoans or bacteria which may be associated with kelp plants.

Benthic habitat

Introduction

Kelps occur under a wide variety of hydrodynamic conditions, hence the substrata may differ widely (igneous or sedimentary bed rock; sand or mud with embedded pebbles, Connor et al., 1995). A number of different substrata often occur within small areas of the sea bed; fissured bed rock may protrude in an area of boulders lying on shell gravel, for example. The benthic fauna in such sites would include species characteristic of rock, rock crevices, boulders (upper and undersides), sediment and phytal habitats.

It appears that no list is available of species which are characteristic of, or present in, areas of sediment within kelp beds, although many of such species can probably be identified from records for epilithic and epiphytic fauna. Further to this, it would seem that the ease of identifying any characteristic species, relates (negatively) to their spatial separation from the kelp. Kelp plants are usually common only where attachment to boulders or bedrock is possible. Hence the majority of records for kelp associated benthos, apply only to areas of hard substrata. The species concerned may be limited to the infralittoral or extend from or into the sublittoral fringe, or may extend from or into the circalittoral.

The principal factor governing infralittoral faunal distributions is thought to be the hydrodynamic regime; this factor has been implicitly used in attempts to classify marine benthic habitats for at least a century (Forbes 1859; Hiscock & Mitchell, 1980). Extensive lists of epilithic macrofauna are to be found in the numerous reports of various authors to the Marine Nature Conservation Review (see JNCC reviews) from 1987 onwards and many species are listed in Appendix 5.

Understorey flora

The understorey flora of kelp beds varies with depth and geographical location and may be depauperate (as in silted habitats) or very rich. The zonation patterns of the understorey algae are related to the available light rather than to the physical depth. Some indication of the complexity of the community is illustrated by Fig. 3, where the kelp forest peters out at a depth of about 7 m. and the lowest depth of algal growth is 15 m. due to the turbidity of the local water conditions. With few exceptions, algal surveys have been confined to the summer months and, although this is generally a good season for recording algae, there is the drawback that species which are recognisable only during a winter reproductive phase or which have a phase which is most conspicuous in winter (e.g. Halymenia latifolia) may not be recorded.

The kelp species forming the canopy will of course vary around the UK as well as with the local conditions of wave exposure and water movement. In the upper fringes of the kelp bed the low shore Fucus serratus may be found and in areas sheltered from wave action, the upper regions of the kelp forest may also include large plants of Halidrys siliquosa. Throughout the kelp beds of the UK, smaller plants of several species of kelp form a significant part of the understorey flora. These are not necessarily young plants (Kain, 1971) but may be small in size due to light limitation below the canopy. A similar phenomenon is well recognised in terrestrial forests, where small trees in the understorey are able to take advantage of additional irradiance available when the canopy is lost due to the removal of mature trees.

In the zonation diagram of the understorey algae of kelp forests in Helgoland (Fig. 3), Lüning (1970; 1990) illustrates more than 20 species of algae that are commonly found, excluding the kelp species themselves and the several species of encrusting algae. The species present in the understorey of UK kelp beds vary tremendously with depth within a site (if the algae are on the kelp stipes) and within a particular kelp bed (Hiscock, 1984). The species which may form part of the understorey flora show considerable variation with geographical location in the UK (Maggs, 1986) as is indicated in Appendix 4.

Benthic fauna, attached

There appear to be no species of sedentary fauna which are specific to the kelp biotopes. It is probable that kelp plants and other algae are in direct competition with the colonial tunicates, sponges etc. for space on the substratum. Any coastal marine species which can tolerate the conditions of water movement, siltation and the temperature range within the particular kelp bed is likely to be found there if space is available for attachment. As with the flora associated with kelp beds, there are some species with distinct geographical ranges, which are found in either the warmer or the cooler areas of UK coastal waters. For many of the benthic fauna however, the temperature tolerances have been assumed from recorded distributions. Inadequate reporting of the species range and miss-identification of difficult species may result in incorrect geographical ranges having been suggested. Little experimental work on temperature tolerances of the faunal species has been attempted either for the sessile adults or for the larval forms.

Benthic fauna, mobile

The wide range of habitats, algal species and potential prey species within a kelp habitat enables a wide range of mobile species to be found in kelp beds. It is likely that, for many of these species the records are not representative, mobile species are able to avoid sampling devices and respond to the presence of recording divers by hiding. A true picture of the mobile species present in terrestrial forests has been obtained by patient and intensive recording over long periods of time and seasons. As yet this has not been a practical proposition within kelp beds and effective alternative methods for data collection on mobile species have yet to be established. The MNCR database includes 81 listings for amphipods but only 3 species have been identified at more than 1% of the kelp sites recorded. Given the enormous numbers of amphipods which can be obtained by intensive sampling, this would seem to be a classic instance of the under-representation of motile benthic fauna in the records.

Figure - Typical understorey algae at different depths in the sublittoral zone off Helgoland

Fauna associated with other fauna

Most, if not all, animal species provide habitat for further species either via creation of biogenic structures or by carrying epifauna, commensals or parasites. Some of these relationships are very specific (e.g. the hydroid Hydractinia on hermit crabs or parasitic copepods in tunicates). Others may be more general, such as the occurrence of meiofaunal species on the surface of epifaunal colonies (as has been shown for F. serratus, Boaden 1995). This level of community structure is poorly understood due to the wide range of species involved and the unpopularity of such research with funding agencies and scientific journals.

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