Laminaria hyperborea forest with a faunal cushion (sponges and polyclinids) and foliose red seaweeds on very exposed upper infralittoral rock
Image Bernard Picton - Laminaria hyperborea forest with a faunal cushion (sponges and polyclinids) and foliose red seaweeds on very exposed upper infralittoral rock. Image width ca 20 m.
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Ecological and functional relationships
This is an extremely dynamic biotope with the main rock cover species that occupy it competing for space and significant seasonal changes occurring. In shallow depths, suspension feeding animal species may out-compete algae and dominate even in well-lit areas. Also, sea urchins are often absent from shallow (say, less than 10m) depths due to strong wave action thus allowing a much more lush growth of algae than would be the case if grazing was occurring.
Kelps are major primary producers and up to 90 percent of kelp production enters the detrital food web so that kelp is probably a major contributor of organic carbon to surrounding communities (Birkett et al. 1998b). Kelp fronds, stipes and holdfasts provide substrata for distinct communities of species, some of which are found only or especially on kelp plants. Hiscock & Mitchell (1980) list 15 species of algae associated with kelp stipes in the UK. The stipes also support 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. An account of holdfast fauna is given by Hayward (1988).
Where sea-urchins occur, they graze the undercanopy and understorey algae, including juvenile kelp sporophytes, together with epiphytes and epifauna on the lower reaches of the laminarian stipe. Sea urchin grazing may maintain the patchy and species rich understorey epiflora/fauna by preventing a small number of species from becoming dominant. 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. Echinus esculentus grazing probably controls the lower limit of Laminaria hyperborea distribution in some location, e.g. in the Isle of Man (Jones & Kain 1967; Kain et al. 1974; Kain 1979). Other ecological relationships are:
Epiphytes and understorey algae are grazed by a variety of amphipods, isopods and gastropods, e.g. Littorina spp., Acmaea spp., Aplysia and rissoid gastropods (Birkett et al. 1988b).
Lobsters (Homarus gammarus), crabs and some fish species (e.g. the wolffish Anarchicas lupus) and perhaps otters are known to consume gastropod and echinoderm grazers.
Kelp communities and the interaction between kelp, urchins and predators has been studied in Nova Scotia, Norway, southern California and the UK (Mann 1982; Kain 1979; Sivertsen 1997; Vadas & Elner 1992; Elner & Vadas, 1990; Schiel & Foster, 1986).
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.
The composition of the holdfast fauna has been show 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 species diversity or 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 (1973a&b). 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 Amphithoe 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 Monohystera 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).
Seasonal and longer term change
The species present in the biotope are believed to be mainly present throughout the year. However, many algae will show a seasonal change from recruitment of ephemeral species and re-growth of perennial species in the spring, through growth of epibiota on the algae in summer and degeneration of fronds in many species in the autumn and winter. 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 (Birkett et al 1998b; Elner & Vadas 1990; Mann 1982).
- 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.
Habitat structure and complexity
Kelp beds are diverse species rich habitats and over 1,800 species have been recorded in the UK kelp biotopes (Birkett et al.
1998b). Kelp forest provides a variety of habitats and refugia in a similar way to terrestrial forests. Kelps also reduce current flow producing a sheltered microclimate . 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 light intensity decreases, the kelp density decreases forming a kelp park (Norton et al.
1977). 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 (Birkett et al.
1998b; Erwin et al.
1990; Norton et al.
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).
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 plants produce 2.7 times their standing biomass per year. Kelp detritus, as broken plant tissue, particles and dissolved organic material supports soft bottom communities outside the kelp bed itself. 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. (Please 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 polychides
, Alaria esculenta
spp., Laminaria hyperborea
, Laminaria digitata
, Saccharina latissima
(studied as Laminaria saccharina
) and un-specified Rhodophyceae at 0.8m. Saccorhiza polychides
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 polychides
; 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 polychides
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 larva that swim for only 2-3 hours (Berril 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). Kain (1979) noted that grazing would slow recovery as few sporophytes survived after 1 year in the presence of Echinus esculentus
. However, the presence of other kelps and Desmarestia
spp. (the latter is distasteful to grazers due to presence of sulphuric acids in its tissue) may act as refugia from grazing for developing Laminaria hyperborea
juveniles that eventually out compete the other species. 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. Re-growth partly due to growth of viable juveniles remaining 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. Older plants have larger holdfasts. Shrimp, lobster, hermit crabs, Echinus esculentus
and Strongylocentrotus droebachiensis
were associated with holdfasts in control areas but absent from harvested areas. Control areas had a more diverse benthic macroflora and macrofauna. Dredged areas exhibited growth of opportunistic kelps e.g. Alaria esculenta
and also Desmarestia
spp. while the bottom was covered by coralline algae between young Laminaria hyperborea
after 3 years. Control areas had a more diverse bottom community. Overall his results suggest that full biological recovery, or maturation, may take at least 10 years.
This review can be cited as follows:
Laminaria hyperborea forest with a faunal cushion (sponges and polyclinids) and foliose red seaweeds on very exposed upper 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=44&code=1997>