Laminaria hyperborea with dense foliose red seaweeds on exposed infralittoral rock.
Image Anon. - Laminaria hyperborea forest with dense foliose red seaweeds on exposed upper infralittoral rock (EIR.LhypR.Ft). Image width ca 3 m in foreground.
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Ecological and functional relationships
Kelps are major primary producers, up to 90 percent of kelp production enters the detrital food web and kelp is probably a major contributor of organic carbon to surrounding communities (Birkett et al. 1998b).
Kelp beds are diverse species rich habitats and over 1,800 species have been recorded in the UK kelp biotopes (Birkett et al., 1998b).
Sea-urchins graze the undercanopy and understorey algae, including juvenile kelp sporophytes, together with epiphytes and epifauna on the lower reaches of the laminarian stipe. Wave action and abrasion between stipes probably knocks urchins off the upper stipe. Sea urchin grazing may maintain the patchy and species rich understorey epiflora/fauna by preventing dominant species from becoming established. Vost (1983) examined the effect of removing grazing Echinus esculentus and found that after 6-10 months the patchiness of the understorey algae had decreased and the species richness and biomass of epilithic species increased. Strongylocentrotus droebachiensis and Paracentrotus lividus also graze kelp beds but are less common in the British Isles than Echinus esculentus. Echinus esculentus grazing probably controls the lower limit of Laminaria hyperborea distribution in some locations, e.g. in the Isle of Man (Jones & Kain, 1967; Kain et al., 1975; Kain, 1979).
Helcion pellucidum grazes epiphytes and the kelp tissue directly, forming pits similar to the home scars of intertidal limpets (see Kain & Svendsen, 1969 for photographs). The older, laevis form excavates large cavities in the holdfast. This tissue damage weakens the adult plant and contributes to its loss due to wave action and storms (Kain, 1979; Birkett et al. 1988b). Infestation with Helcion pellucidum varies between sites and decreases with depth, e.g. infestation may reach up to 50 % on mature plants in shallow water in the Isle of Man, whereas <20 % was found (on kelps of any age group and depth) in England and Scotland (Kain, 1979).
Laminaria hyperborea is grazed directly by Lacuna vincta in Norway, the individuals forming deep pits in the lamina (Kain, 1979).
Kelp fronds, stipes and holdfasts provide substrata for distinct communities of species, some of which are found only or especially on kelp plants. Kelp holdfasts provide both substrata and refugia (see Detailed ecology).
Epiphytes and understorey algae are grazed by a variety of amphipods, isopods and gastropods, e.g. Littorina spp., Acmaea spp., Haliotis tuberculata, Aplysia and rissoid gastropods (Birkett et al., 1988b).
Predators within kelp beds have not been well studied in the UK. Lobsters (Homarus gammarus), crabs and some fish species (e.g. the wolffish Anarhichas lupus) are known to consume gastropod and echinoderm grazers. In Scotland, the Eurasian otter Lutra lutra, is commonly found feeding on fish in kelp beds (see e.g. Kruuk et al, 1990).
Kelp communities and the interaction between kelp, urchins and predators has been studied in Nova Scotia, Norway, southern California and the UK (Kain, 1979; Mann, 1982; Schiel & Foster, 1986; Elner & Vadas, 1990; Vadas & Elner, 1992; Sivertsen, 1997).
Birkett et al. (1998b) suggest that juveniles of animals present in kelp beds as adults probably use the habitat as a nursery and unknown numbers of species are likely to use the habitat during their life cycle. Rinde et al. (1992, cited in Birkett et al., 1998b) state that Norwegian kelp beds are nurseries for gadoid species.
Seasonal and longer term change
New blades of Laminaria hyperborea
grow in winter between the meristem and the old blade, which is shed in early spring or summer together with associated species growing on its surface.
Larger and older kelp plants become liable to removal by wave action and storms due to their size and weakening by grazers such as Helcion pellucidum
. There is therefore likely to be a reduced abundance of kelps following the winter. Loss of older plants results in more light reaching the understorey, temporarily permitting growth of algae including Laminaria hyperborea
Areas of kelp may become denuded of macroalgae at intervals and the substrata dominated by encrusting corallines. These areas are often associated with an increase in urchin numbers forming 'fronts' of small and large urchins that remove large quantities of algae including the kelps themselves forming 'urchin barrens'. Sea urchin grazing is an important factor in kelp beds and, as part of the biotope, the following suggested factors affecting sea urchin populations are presented.
- Several predators have been suggested as controlling sea urchin populations e.g. sea otters, lobsters, crabs or wolffish, however the evidence is equivocal ( Mann, 1982; Elner & Vadas, 1990; Birkett et al., 1998b).
- Evidence suggests that sea urchin recruitment is sporadic and may be enhanced by low temperatures (Birkett et al., 1998b).
- Sea urchin recruitment is also enhanced by the presence of 'urchin barrens' presumably due to the lack of suspension feeders that would otherwise consume their larvae (Lang & Mann, 1978).
- Sea urchin diseases, such as 'bald-urchin' disease, encouraged by high water temperatures drastically reduce the urchin population (Lobban & Harrison, 1997). However, although parasitic infections are found in Echinus esculentus, no evidence of sea urchin disease has been found in the UK.
- Sivertsen (1997) examined grazing of west and north Norwegian coast Laminaria hyperborea beds by Strongylocentrotus droebachiensis and Echinus esculentus. He concluded that seven environmental factors contributed to the distribution of kelp beds and 'barrens': depth gradient, latitude, time of sampling, nematode infection (in Strongylocentrotus droebachiensis), wave exposure, coastal gradient and substratum.
The factors controlling sea urchin populations and 'urchin barrens' in kelp beds is poorly understood, especially in the UK. However, it is likely that the local urchin population is controlled by a number of factors that vary between sites and biotopes; including predators, competition for food with other grazers, variation in sea urchin recruitment, and parasitic infection or disease.
Periodic storms are likely to remove older and weaker plants creating patches cleared of kelp and increasing the local turbidity. While cleared patches may encourage growth of sporelings or gametophyte maturation, they may also enhance sea urchin recruitment. No studies of storm effects in the UK were found, however, Birkett et al. (1998b) cite observations by Dayton et al. (1992) of the results of an intense storm (possibly the most severe for 200 years) in a giant kelp forest off Point Loma, San Diego, California. The storm changed the age-specific kelp mortality, caused the first large-scale mortality in the understory and removed drift algae resulting in intense local sea urchin grazing and, therefore, reduced kelp recruitment.
Kelp biotopes should be viewed as dynamic systems that exhibit mixed age kelp stands, together with a patchy distribution of understory flora/fauna and 'urchin barrens'. Kelp communities may exhibit a 15-20 year cycle between 'urchin barrens' and kelp dominated phases (Elner & Vadas, 1990; Lobban & Harrison, 1997) although such clear cycles have not been observed in Britain and Ireland.
Habitat structure and complexity
Kelp forest provides a variety of habitats and refugia in a similar way to terrestrial forests. Kelps also reduce current flow and their canopy shades the understorey vegetation and substrata producing a particular microclimate depending on the depth and density of the kelp plants. In kelp forest (e.g. EIR.LhypR.Ft) the kelp density produces a canopy which excludes up to 90 % of incident light allowing many deeper water, shade tolerant algae, mainly reds, to invade. In deeper water, as irradiance decreases, the kelp density decreases forming a kelp park. Kelp beds are patchy and dynamic with areas devoid of kelp (due to storms, wave surge or grazing) in the process of expansion or recolonization in different stages of succession. Species diversity changes with depth, between forest & park, with exposure, substratum and turbidity (Norton et al.
, 1977; Erwin et al.
, 1990; Birkett et al.
, 1998b). Erwin et al.
(1990) noted that species richness increased in the kelp park (as lower infralittoral and upper circalittoral species overlapped) and was higher in boulder fields in which sand-scour and substratum heterogeneity provided more niches for colonization.
Kelp beds exhibit a series of stratified habitats, and a patchwork of species depending on the substratum, light, water flow and exposure.
- Planktonic: Spores and larvae from algae and benthic organisms within the bed, as well as from the surrounding area, probably form an import food source given the number of suspension feeding organisms in kelp beds
- Nekton: wrasse and pollock have been observed associated especially with kelp forests and epibenthic predatory or herbivorous fish are also found, e.g. blennies, gobies and wolffish (Anarhichas lupus).
- Kelp blades support microalgal epiphytes or endophytes such as Pogotrichum filiforme, Chilionema sp. and Myrionema corunnae which is only found on Laminaria blades, as well as Helcion pellucidum and opportunistic hydroids (e.g. Obelia geniculata) and bryozoans (e.g. Membranipora membranacea).
- The stipes support a diverse fauna and flora, especially foliose red algae (see e.g. Harkin, 1981), depending on age of the stipe, kelp density (stipes in close proximity may abrade each other) and depth. Epiphytes show greater biomass on the top 10-20 cm of stipe and exhibit a zonation pattern down the stipe which changes with depth (Birkett et al., 1998b). Norton et al. (1977) found the greatest biomass at 3m depth near Lough Ine. Whittick (1983) showed that epiphyte biomass was significantly greater in plants over 5yrs old, with Palmaria palmata(dulse) dominating the top of the stipe from 1-2m, being replaced by Ptilota plumosa between 6-10m, while Membranoptera alata and Phycodrys rubens dominate below 12m or present at lower parts of the stipe. Hiscock & Mitchell (1980) list 15 species of algae associated with kelp stipes in the UK. The stipes also supports epifaunal bryozoa and hydroids (Norton et al., 1977).
- Holdfasts support a diverse fauna that represents a sample of the surrounding mobile fauna and crevice dwelling organisms, e.g., polychaetes, small crabs, gastropods, bivalves, and amphipods. Jones (1971) lists 53 macrofaunal invertebrates in holdfasts and Moore (1973) reports 389 species from holdfasts collected in the north east coast of Britain. A useful account of holdfast fauna is given by Hayward (1988).
- The composition of the holdfast fauna has been shown to vary with turbidity (natural and anthropogenic in origin), between kelp species (due to holdfast architecture and volume), and with location around the coast of the British Isles (Moore, 1973a&b; Moore, 1978; Edwards, 1980; Sheppard et al., 1980). Moore (1973a&b) identified groups of species that were found in most cases, or restricted to either turbid or clear waters. Moore (1978) noted that the species diversity of amphipods decreased with increasing turbidity, partly due to the increased dominance of a few species. Edwards (1980) noted that holdfast fauna in south-west Ireland were numerically dominated by suspension feeders with decreasing numbers of omnivores and carnivores respectively. Edwards (1980) noted that holdfasts were dominated by Pomatoceros triqueter in the most turbid sites, although these were not as turbid as sites examined by Moore (1973 a&b). Sheppard et al. (1980) examined 35 sites around the Britain Isles and demonstrated a correlation between heavy metal pollution, turbidity and location. Along the North Sea coast species number and diversity increased with increased clarity, however where heavy metals were a factor species number and diversity decreased with increasing heavy metal pollution. They were able to distinguish groups of species characteristic of all sites, or clear or turbid sties. Along the west coast both heavy metals and turbidity were important. Where turbidity and heavy metals increased suspension feeders increased in abundance while other trophic groups decreased. However, along the south coast longitude was the most important factor, and they suggested that natural variation in temperature, salinity and water flow were responsible for variation between holdfast communities (Sheppard et al., 1980). Moore (1985) also demonstrated that the amphipod fauna varied with water flow rate (resulting from wave action and currents); for example sites of increased exposure were dominated by Ampithoe rubricata, Lembos websteri and Jassa falcata whereas Gitana sarsi, Dexamine thea and Corophium bonnellii flourish in wave sheltered environments.
- A few meiofaunal species may burrow into kelp tissue, e.g. the nematode Monhystera disjuncta (Birkett et al., 1998b).
- The understorey flora varies with location, depth, exposure, hydrographic regime, turbidity and siltation and may be sparse or species rich. Birkett et al. (1998b; Appendix 5) list 52 common kelp biotope understorey algae in the UK including characterizing species such us Delesseria sanguinea, Dictyota dichotoma, Phycodrys rubens, Cryptopleura ramosa, Plocamium cartilagineum, and Callophyllis laciniata.
- The benthic fauna varies with depth, exposure, location and substratum, however, no species are specific to kelp forest. Norton et al. (1977) demonstrate the zonation of 22 epibenthic species. However, many species, both fixed and mobile, are present and probably under recorded (Birkett et al., 1998b).
Kelps are the major primary producers in UK marine coastal waters producing nearly 75 percent of the net carbon fixed annually on the shoreline of the coastal euphotic zone (Birkett et al., 1998b). Kelp detritus, as broken plant tissue, particles and dissolved organic material supports soft bottom communities outside the kelp bed itself. As a result, kelp plants can contribute 2-3 times their own biomass to the biomass of the coastal ecosystem over one year (Birkett et al., 1998b). The kelps reduce ambient levels of nutrients, although this may not be significant in exposed sites, but increase levels of particulate and dissolved organic matter within the bed.
Recruitment processes of key characteristic or dominant species are described here.
produces vast numbers of spores, however they need to settle and form gametophytes within about 1 mm of each to ensure fertilisation and therefore may suffer from dilution effects over distance. Gametophytes can survive darkness and develop in the low light levels under the canopy. However, young sporelings develop slowly in low light. Loss of older plants provides the opportunity to develop into adult plants. Recruitment in Echinus esculentus
is sporadic or annual depending on location and may benefit from the presence of 'urchin barrens'. Helcion pellucidum
is an annual species, larvae settling in the lower eulittoral and juveniles migrating to kelp, via several algal species, as they grow. (View individual key information reviews for details.) Epifaunal larvae probably contribute to the plankton of the kelp bed and many are lost to the suspension feeding epifauna. Kelp beds also provide nurseries for larvae and fish species (see above). Recruitment of epiphytes and epiflora are dependant on dispersal and settlement of algal spores and survival of early post-settlement stages. Norton (1992) suggests that spore dispersal in primarily dependant on currents and eddies. Settlement of algal spores is partly dependant on their motility (if any) and adhesive properties together with preferences for topography (surface roughness), the chemical nature of the substratum and water movement (Norton, 1992; Fletcher & Callow, 1992). Vadas et al.
(1992) suggested that survival of early post settlement stages is dependant on grazing, the algal canopy and turf effects together with desiccation and water motion, and they further suggest that recruitment is likely to be episodic, variable and to suffer from high mortality of early stages.
Kain (1975) examined recolonization of artificially cleared areas in a Laminaria hyperborea
forest in Port Erin, Isle of Man. Cleared concrete blocks were colonized by Saccorhiza polyschides
, Alaria esculenta
spp., Laminaria hyperborea
, Laminaria digitata
, Saccharina latissima
(studied as Laminaria saccharina
) and un-specified Rhodophyceae at 0.8m. Saccorhiza polyschides
dominated within 8 months but had virtually disappeared with 77 weeks to be replaced by laminarians, including Alaria esculenta
. After about 2.5 years, Laminaria hyperborea
standing crop, together with an understorey of red algae (Rhodophyceae), was similar to that of virgin forest. Rhodophyceae were present throughout the succession increasing from 0.04 to 1.5 percent of the biomass within the first 4 years. Colonizing species varied with time of year, for example blocks cleared in August 1969 were colonized by primarily Saccharina latissima
and subsequent colonization by Laminaria hyperborea
and other laminarians was faster than blocks colonized by Saccorhiza polyschides
; within 1 year the block was occupied by laminarians and Rhodophyceae only. Succession was similar at 4.4m, and Laminaria hyperborea
dominated within about 3 years. Blocks cleared in August 1969 at 4.4m were not colonized by Saccorhiza polyschides
but were dominated by Rhodophyceae after 41 weeks, e.g. Delesseria sanguinea
and Cryptopleura ramosa
. Kain (1975) cleared one group of blocks at two monthly intervals and noted that Phaeophyceae were dominant colonists in spring, Chlorophyceae (solely Ulva lactuca
) in summer and Rhodophyceae were most important in autumn and winter.
Animal species are likely to recruit mainly from the plankton although some species such as polyclinid tunicates may have only a short lived (2-3 hours) larva (Berrill, 1950). Or no larval stage (amphipods). Little is known about the reproductive biology and dispersal of some species but information from clearance experiments (see 'Time for community to reach maturity) suggests that sponges may be slow to settle.
Time for community to reach maturity
Experimental clearance experiments in the Isle of Man (Kain, 1975; Kain, 1979) showed that Laminaria hyperborea
out-competed other opportunistic species (e.g. Alaria esculenta
, Saccorhiza polyschides
spp.) and returned to near control levels of biomass within 3 years at 0.8 m but that recovery was slower at 4.4m (see above). Studies of the effects of harvesting in Norway (Svendsen, 1972, cited in Birkett et al.
, 1998b) showed that kelp biomass returned 3-4 years after harvesting, although the plants were small (about 1m) and the age class was shifted towards younger plants. Sivertsen (1991, cited in Birkett et al.
, 1998b), showed that kelp populations stabilise about 4-5 years after harvesting. Current advice suggests that kelp forest should be left 7-10 years for kelp and non-kelp species to recover (Birkett et al.
, 1998b). Detailed studies in Norway by Rinde et al.
(1992, cited in Birkett et al.
, 1998b) examined recovery of non-kelp species. The epiphyte community in control areas about 10 years old was richer and more extensive than on replacement plants in harvested areas. Of the epifauna, Halichondria
sp. were only found on 10 year old plants and tunicates on plants 6 years post harvesting. Holdfast fauna was more abundant richer in 10 year old plants in control areas than younger plants in previously harvested area. Overall his results suggest that full biological recovery, or maturation, may take at least 10 years.
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This review can be cited as follows:
Laminaria hyperborea with dense foliose red seaweeds on exposed infralittoral rock..
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=171&code=1997>