Laminaria digitata and piddocks on sublittoral fringe soft rock

Researched byJacqueline Hill Refereed byDr Eunice Pinn
EUNIS CodeA3.2113 EUNIS NameLaminaria digitata and piddocks on sublittoral fringe soft rock


UK and Ireland classification

EUNIS 2008A3.2113Laminaria digitata and piddocks on sublittoral fringe soft rock
EUNIS 2006A3.2113Laminaria digitata and piddocks on sublittoral fringe soft rock
JNCC 2004IR.MIR.KR.Ldig.PidLaminaria digitata and piddocks on sublittoral fringe soft rock
1997 BiotopeIR.MIR.KR.Ldig.PidLaminaria digitata and piddocks on sublittoral fringe soft rock


Soft rock, such as chalk, in the sublittoral fringe characterized by Laminaria digitata and rock-boring animals such as piddocks (Barnea candida, Pholas dactylus and Petricola pholadiformis), the bivalve Hiatella arctica and worms Polydora spp. Beneath the kelp forest, a wide variety of red seaweeds, including Corallina officinalis, Palmaria palmata, Chondrus crispus, Membranoptera alata and Halurus flosculosus, may occur. Empty piddock burrows are often colonised by the anemones Sagartia elegans or by the sand-tube building worm Sabellaria spinulosa. The undersides of small chalk boulders are colonised by encrusting bryozoans, colonial ascidians and the keel worm Pomatoceros lamarcki. The boulders and any crevices within the chalk provide a refuge for small crustaceans such as Carcinus maenas, young Cancer pagurus, Pagurus bernhardus and Porcellana platycheles. (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Recorded distribution in Britain and Ireland

Recorded from a few sites on the south Kent coast and north Wales.

Depth range

Lower shore

Additional information


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Habitat review


Ecological and functional relationships

Kelp habitats are dynamic ecosystems where competition for space, light and food result in patchy distribution patterns of flora and fauna. Kelp biotopes are diverse species rich habitats and over 1,200 species have been recorded in UK moderately exposed kelp biotopes (MIR.KR) (Birkett et al., 1998b). Kelps are major primary producers; up to 90% of kelp production enters the detrital food web and is probably a major contributor of organic carbon to surrounding communities (Birkett et al., 1998b). Major interactions are thought to be the effects of competition for space, shading, herbivory and predation.
  • In most kelp biotopes there is evidence of strong competition for space, especially for space on a favourable substratum. Competition may between individual plants of the same species, between kelps and substratum-colonizing species of animals and other algae and between colonial animals and encrusting algae. Competition for space between individuals and species is dynamic, resulting in a constantly changing patchwork of species covering any suitable substrata within the biotope.
  • The blades of Laminaria digitata plants form a canopy layer which may cut off much of the incident irradiance. This restricts the development of species with high light demands so that the understorey of plants becomes dominated by shade tolerant red algae. It also allows species normally restricted to the lower infralittoral in kelp-free areas to compete more effectively in the reduced light levels of the kelp bed and so are found at shallower depths.
  • Within kelp beds there are relatively few species that are directly grazing either the kelp or the understorey algae as the enzymes required to directly utilise algae as food are not common. Those species able to graze directly on the kelp include the gastropods: Gibbula spp., Littorina spp., Haliotis tuberculata (in the Channel Islands only), Helcion pellucidum, Lacuna spp. and the Rissoidae, together with some amphipods and isopods. Helcion pellucidum grazes epiphytes and the kelp tissue directly, forming pits similar to the home scars of intertidal limpets. The larger, laevis form excavates large cavities in the holdfast of Laminaria spp. which creates tissue damage weakening the adult plant and possibly contributes to its loss due to wave action and storms (Kain, 1979). Infestation with Helcion pellucidum varies between sites and decreases with depth.
  • Burrowing species such as the piddocks, including the common piddock Pholas dactylus, and the tube worm Polydora ciliata are characteristic of this biotope and contribute to the creation of a relatively high silt environment through burrowing activities. The abundance of filter feeding organisms such as sponges, bryozoans and tunicates within kelp biotopes indicates the importance of planktonic input to the benthic community. Although very little information is available about planktonic communities in kelp beds it can be assumed that there will be larger inputs of larval stages from species with bentho-pelagic life cycles than in the general plankton (Birkett et al., 1998b).
  • Predation within kelp beds has not been well studied in the UK. Although some species are known to prey on others, such as starfish on mussels, very little is known of the predator-prey relationships for the many species occurring in kelp beds.
  • Kelp plants are exploited as a habitat; the holdfast, stipe and frond each support a different type of community consisting of possibly thousands of individuals from hundreds of species; holdfasts shelter a particularly rich diversity of animals from a wide range of taxa (see Habitat complexity).
  • Seasonal and longer term change

    Most species in the biotope are perennial and seasonal changes are likely to be in condition of individuals rather than presence or absence.
    • Growth rate of Laminaria digitata is seasonally controlled with a period of rapid growth from February to July and one of slower growth from August to January. Increased wave exposure and storms in winter months are likely to erode Laminaria digitata blades so that they appear tattered in winter months and overall standing biomass is reduced. Periodic storms are also likely to remove older and weaker plants creating patches cleared of kelp and increasing the local turbidity. Cleared patches may encourage growth of sporelings or gametophyte maturation. Growth of understorey algae is also reduced in the winter months.
    • 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 recognizable in the foliose phase of their life cycles. The occurrence of such algae is often seen as the occurrence of 'ephemeral' algae. 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 medusae.
    • With a life span of less than a year and a reproductive period of 3-4 months in the spring or summer numbers of Polydora ciliata are likely to be fairly seasonal with highest abundance of individuals after recruitment in the summer and autumn.
    • Pholas dactylus live to approximately 14 years of age with a maximum shell length of 75 mm (Pinn et al., 2005), although earlier work has recorded maximum shell lengths of 125-150 mm (Jeffries, 1865; Turner, 1954). Spawning usually occurs between May and September with settlement and recruitment of juvenile piddocks occuring between November and February. It is likely that populations of Pholas dactylus will not be subject to significant seasonal changes in abundance.
    It should be emphasized that present understanding of the natural fluctuations in the species assemblages, populations, distribution and diversity of species in kelp habitats is very limited.

    Habitat structure and complexity

    The structure of the biotope is complex with many different microhabitats. They include bedrock, crevices, sediment pockets, the holdfast, stipe and blade of Laminaria digitata plants themselves, undersides of boulders and empty piddock burrows.
    • Holdfasts provide refuge to a wide variety of animals supporting 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.
    • Kelp fronds are grazed by molluscs such as the blue-rayed limpet Helcion pellucidum.
    • Older Laminaria digitata stipes provide a substratum for a large number of epiphytic flora and fauna and it has been estimated that rugose stipes provide one and a half times that surface area provided by the bedrock (Jones et al., 2000).
    • Empty burrows of piddocks, such as the common piddock Pholas dactylus, create additional refugia which are recorded as being colonized by vagile species such as Littorina littorea, Gibbula cineraria, Porcellana platycheles and Eulalia viridis (Pinn et al., in press). Sabellidae and Lithothamnia spp. Are examples of sessile species utilising the burrows.
    • The understorey of red algae and crevices in the bedrock provide space for many cryptic fauna.
    • In areas of mud tubes built by Polydora ciliata can agglomerate and form layers of mud up to an average of 20 cm thick, occasionally to 50 cm. These layers can eliminate the original fauna and flora, or at least can be considered as a threat to the ecological balance achieved by some biotopes (Daro & Polk, 1973).


    Kelp plants are the major primary producers in the marine coastal habitat. Within the euphotic zone kelps produce nearly 75% of the net carbon fixed and large kelps often produce annually well in excess of a kilogram of carbon per square metre of shore. However, only about 10% of this productivity is directly grazed. Kelps contribute 2-3 times their standing biomass each year as particulate detritus and dissolved organic matter that provides the energy supply for filter feeders and detritivores, such as piddocks and polychaetes like Polydora ciliata, in and around the kelp bed. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local subtidal ecosystems, or be exported further offshore. Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.

    Recruitment processes

    Most species in this biotope produce planktonic propagules annually and so recruitment is often from distant sources and is frequent.
    • Benthic species, plant and animal, that possess a planktonic stage: gamete, spore or larvae, are likely to be influenced by kelp mediated alteration of fluid and particulate, and consequently larval fluxes. Kelp canopies also affect the physical environment, such as the substratum, experienced by actively settling planktonic larvae. The substrata beneath kelp plants for example, are often dark and sediment laden, conditions likely to affect larval settlement and post settlement survival. Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates driven by such factors.
    • Laminaria digitata plants are fertile all year round with maximum production of spores in July - August and November - December. Young sporophytes (germlings) appear all year with maxima in spring and autumn. Chapman (1981) demonstrated that substantial recruitment of Laminaria digitata plants to areas barren of kelp plants was possible up to 600m away from reproductive plants.
    • Pholas dactylus spawns between May and September with settlement and recruitment of juvenile piddocks occuring between November and February (Knight, 1984; Pinn et al., in press).
    • The spawning period for Polydora ciliata varies, from February until June in northern England for example, and from April - September in the Black Sea. Larvae are substrate specific selecting rocks or sediment according to their physical properties settling preferentially on substrates covered with mud
    • Among sessile organisms, patterns fixed at settlement, though potentially altered by post settlement mortality, obviously cannot be influenced by dispersal of juveniles or adults.
    • Some of the species living in kelp beds do not have pelagic larvae, but instead have direct development producing their offspring as 'miniature adults'.

    Time for community to reach maturity

    Kain (1975) examined the recolonization of cleared concrete blocks by kelp plants and other algae and found that Laminaria digitata plants were re-established within 2 years and that red algae returned with a year. Although there is no information available on colonization times or growth rates for the common piddock the other main rock borer, Polydora ciliata is able to rapidly (within months of reproductive period) colonize a suitable area. Recruitment of other species to the kelp bed may take longer. However, maturity is likely to be reached within five years.

    Additional information


Preferences & Distribution

Recorded distribution in Britain and IrelandRecorded from a few sites on the south Kent coast and north Wales.

Habitat preferences

Depth Range Lower shore
Water clarity preferences
Limiting Nutrients Nitrogen (nitrates)
Salinity Full (30-40 psu)
Physiographic Open coast
Biological Zone Sublittoral fringe
Substratum Bedrock
Tidal Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave Moderately exposed
Other preferences

Additional Information

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope


Additional information

Sensitivity reviewHow is sensitivity assessed?


Laminaria digitata and Pholas dactylus are the key structuring species and also give the name to the biotope. The fronds, stipe and holdfast of the kelp plants create structural complexity within the biotope. The piddock Pholas dactylus is a key structuring species because of the refugia provided by the holes it makes. These have been known to increase species diversity (Pinn et al., in press). The burrowing polychaete Polydora ciliata is functionally important because tube building can modify the substratum and in areas of mud tubes can agglomerate, forming layers of mud that can sometimes be thick enough to eliminate the original fauna and flora, or at least can be considered as a threat to the ecological balance achieved by some biotopes (Daro & Polk, 1973). The understorey of red algae in the biotope is represented by Palmaria palmata which is frequently found both on rocks and sometimes on older Laminaria digitata stipes.

Species indicative of sensitivity

Community ImportanceSpecies nameCommon Name
Key structuralLaminaria digitataOarweed
Important characterizingPalmaria palmataDulse
Key structuralPholas dactylusCommon piddock
Key functionalPolydora ciliataA bristleworm

Physical Pressures

 IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
High High Moderate Major decline Moderate
Most of the species characteristic of this biotope are permanently attached to the substratum so would be removed upon substratum loss. These species are unable to re-attach and will be swept away so intolerance is assessed as high. The total population of Polydora ciliata is unlikely to be lost because it can reburrow into any remaining suitable substratum. Species diversity will be significantly reduced because many of the microhabitats provided by the characterizing species will be lost. Recovery of the main characterizing species Laminaria digitata is rapid with cleared rocks fully recolonized within two years (Kain, 1979). Most other characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area. Colonization of most species of fauna inhabiting kelp holdfast fauna in Norway were found as early as one year after kelp trawling (Christie et al., 1998) and on rocks the more diverse community of coralline algae joined by species of cnidarians, bryozoans and sponges seen on undredged plots was absent three years after kelp trawling (Birkett et al., 1998b). However, although full species richness and abundance may be reduced the appearance of the biotope will be much as before substratum loss and so recovery is high.
Intermediate High Low Major decline Very low
Some species, especially adult Laminaria digitata plants, are likely to protrude above any smothering material. The burrowing species such as Pholas dactylus and Polydora ciliata are able to tolerate high levels of smothering and sedimentation. However, others species such as the active suspension feeders and foliose algae are likely to be killed by smothering. Smothering can also be highly detrimental to kelp plants, in particular spores, gametophytes and young plants (Dayton, 1985) which will reduce the kelp population within the biotope and so intolerance has been assessed as intermediate. Species diversity within the biotope is likely to experience a major decline. Recovery is high because most characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area.
Intermediate High Low Decline Very low
Laminaria digitata can be found in areas of siltation although in very high silt environments the species may be out-competed by Saccharina latissima. Since many of the species, Pholas dactylus and Polydora ciliata in particular, in this biotope live in areas of high silt content (turbid water) it is expected that they would survive increased levels of silt in the water. However, very high levels of silt may clog respiratory and feeding organs of some suspension feeders such as sea squirts and may result in a decline in faunal species diversity. Increased siltation is unlikely to have a significant effect in terms of smothering by settlement in the regime of strong water flow typical of this biotope. A significant decrease in siltation levels may reduce food input to the biotope resulting in reduced growth and fecundity.
Intermediate High Low Minor decline Low
The biotope is predominantly sublittoral but does extend onto the shore and therefore has some ability to resist desiccation. Laminaria digitata in particular can return to original photosynthetic rate on reimmersion after 40-50% water loss. Species living below the kelp, such as foliose red algae, sponges and ascidians are likely to be protected from the worst effects of desiccation by the kelp canopy. However, increased desiccation equivalent to moving one biological zone up the shore is likely to result in the death of some intolerant sessile species so intolerance of the biotope is assessed as intermediate.
Intermediate High Low Decline Low
The biotope is predominantly sublittoral and so a change in the emergence regime at the benchmark level (one hour in the time covered or not covered by the sea for a period of 1 year) is likely to result in a depression of the upper limit of the biotope. Some sessile species, such as sea squirts, are unlikely to survive a long term increase in emergence. Many of the subordinate species, especially solitary sea squirts, are unlikely to survive an increased emergence regime and mobile species are likely to move away so that species diversity will decline. However, in the presence of a suitable substratum the biotope is likely to re-establish further down the shore. Kain (1975) recorded that Laminaria digitata had recolonized cleared rocks within 2 years so recovery should be high. Most other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. Growth rates of sessile species in the biotope are generally rapid. For instance, Halichondria panicea increases by about 5% per week (Barthel, 1988).
Low High Low Minor decline Low
The biotope occurs in a wide range of water flow environments, from very weak to moderately strong and so will be relatively tolerant of changes. In areas of very strong water flow it is often out-competed by Alaria esculenta and in much slower water by Saccharina latissima. Laminaria digitata is not found in areas subject to sand scouring. Water motion affects light by moving canopies and influences the impact of sedimentation and scour and importantly water motion determines the availability of nutrients. It is unlikely that species in the biotope will be killed by an increase or decrease in flow rate. Existing organisms are likely to persist although conditions will not be ideal. Decreased water flow will lead to a reduced competitive advantage for suspension feeding animals especially sponges which will decline in growth rate.
Intermediate High Low Decline Moderate
Laminaria digitata is a eurythermal species with sporophytes growing over a wide temperature range. Lüning (1984) detected a seasonal shift in heat tolerance of Laminaria digitata plants in Helgoland of 2 °C between spring and summer so the species is likely to be relatively tolerant of a long term, chronic change in temperature. However, the biotope may be intolerant of rapid changes in temperature outside its tolerance range. During an exceptionally warm summer in Norway, Sundene (1964) reported the destruction of Laminaria digitata plants exposed to temperatures of 22-23 °C. In Scotland, when spring low tides coincided with night time extreme air frosts on several consecutive days, mortality of all but the lowest shore adult Laminaria digitata plants occurred (Todd & Lewis, 1984). Therefore, the biotope is likely to be of intermediate intolerance to short term acute temperature change. Loss of plants will result in reduces species diversity.
The cold winter of 1962/63 had a dramatic effect on intertidal fauna in southern Britain including piddocks which were killed at the sites investigated (Crisp, 1964). Subsequently piddocks are now abundant throughout much of the eastern English Channel (Pinn et al., in press).
Intermediate High Low Major decline Moderate
Changes in turbidity may affect the distribution or growth rates of Laminaria digitata and other algae. Reduced turbidity may increase productivity of kelps and other algae but is not expected to increase the depth range to which the biotope extends because limiting conditions for the depth to which Laminaria digitata can grow are not usually to do with light, but due to competition with the truly sublittoral kelp Laminaria hyperborea. Increases in turbidity around a sewage treatment plant was thought to be responsible for the absence of Laminaria digitata plants in the Firth of Forth (Read et al., 1983) and has been reported to result in a reduced depth range and the fewer new plants under the kelp canopy. An increase in turbidity will reduce photosynthesis and growth of plants. On return to normal turbidity levels the growth rate would be quickly return to normal. In almost all cases not involving canopy competition, irradiance is most severely reduced by suspended particles in the water column (Dayton, 1985). There may be some clogging of suspension feeding apparatus in sea squirts, brittle stars and feather stars although those groups survive in occasionally very turbid conditions. Species richness may decline in the long-term.
Intermediate High Low Major decline Low
The biotope occurs in areas of moderate wave exposure. Although the kelp Laminaria digitata can tolerate a wide range of wave exposures a significant increase in wave exposure will have a detrimental effect on the biotope because of the friable nature of the substratum resulting in a loss of sessile species. Changes in wave exposure may also interfere with feeding for the piddocks, Polydora ciliata and other suspension feeding organisms.
Tolerant Not relevant Not relevant Not relevant Not relevant
The macroalgae characterizing the biotope have no known sound or vibration sensors. The response of macroinvertebrates is not known.
Tolerant Not relevant Not relevant Not relevant Not relevant
Macrophytes have no known visual sensors. Most macroinvertebrates have poor or short range perception and although some are likely to respond to shading caused by predators the biotope as a whole is unlikely to be sensitive to visual disturbance.
Intermediate High Low Minor decline Low
The fronds of Laminaria digitata are leathery and the whole plant is very flexible so physical disturbance by a scallop dredge or an anchor landing on or being dragged across the seabed, is unlikely to cause significant damage to the kelp bed as a whole. However, some plants may be fatally damaged or ripped off the substratum. Other algae and sessile species such as sponges and large solitary tunicates are likely to be sensitive to abrasion and so the biotope as a whole has been assessed as having intermediate intolerance.
High High Moderate Major decline Low
Displacement of Laminaria digitata, the key species, will result in loss of the biotope because plants are unable to re-attach. Species abundance and diversity will be significantly reduced because the additional habitats and refugia provided by kelp fronds, stipes and particularly the holdfast will be lost. However, recovery is good because recolonization of kelp plants to previous densities takes place within about 2 years (Kain, 1975). Similarly, displaced piddocks will be lost but empty burrows will provide additional refugia for small invertebrates (Pinn et al., in press).

Chemical Pressures

High High Moderate Major decline Moderate
Several of the species characteristic of the biotope are reported as having high intolerance to synthetic chemicals. For instance, Cole et al. (1999) suggests that herbicides such as Simazina and Atrazine are very toxic to macrophytic algae. Hiscock and Hoare (1974) noted that almost all red algal species were absent from areas adjacent to an acidified halogenated effluent in Amlwch Bay, North Wales. Red algae have also been found to be sensitive to oil spill dispersants (O'Brien & Dixon, 1976). Bivalve molluscs, such as piddocks are reported to be very intolerant of TBT contamination (see Pholas dactylus review) with reduced abundance and growth reported in the field and laboratory. Other species in the biotope, in particular polychaete worms, are much more tolerant of chemical pollutants. The tube dwelling polychaetes Polydora ciliata and Pomatoceros triqueter, for example, flourished close to the Amlwch Bay bromide extraction plant effluent (Hoare & Hiscock, 1974). Therefore, the result of an increase in synthetic chemicals within the biotope is likely to be the death of several of the more intolerant species, including key species such as Pholas dactylus. Abundance of other more pollution tolerant species, especially polychaete worms, is likely to increase. The overall impact is one of the probable loss of key species and a major decline in species diversity and the intolerance of the biotope is therefore reported as high. Recovery is good because recolonization of algae takes place within 2 years and most fauna have pelagic larvae and so can recolonize rapidly.
Heavy metal contamination
Intermediate High Low Minor decline Low
No information is available on the effect of heavy metals on the biotope. Intolerance of the key species is reported as intermediate, with likely effects on growth and fecundity, so biotope intolerance is assessed as intermediate. There may be a decline in overall species diversity with long term heavy metal pollution.
Hydrocarbon contamination
Intermediate High Low Major decline Moderate
No significant effects of the Amoco Cadiz spill were observed for Laminaria populations and the World Prodigy spill of 922 tons of oil in Narragansett Bay had no discernible effects on Laminaria digitata (Peckol et al., 1990). However, analysis of kelp holdfast fauna after the Sea Empress oil spill in Milford Haven illustrated decreases in number of species, diversity and abundance at sites nearest the spill (SEEEC, 1998). It is also expected that other species in the biotope will be intolerant of hydrocarbons. A proliferation of polychaete worms often follows oil spills. A major decline in species diversity within the biotope is likely and so intolerance is reported as intermediate.
Radionuclide contamination
No information No information No information Not relevant Not relevant
Changes in nutrient levels
Low High Low Decline Very low
The growth of macroalgae in temperate coastal waters is generally expected to be limited by nitrogen in the summer period. In Helgoland, where ambient nutrient concentrations are double those of the Scotland site Laminaria digitata grows in the summer months. Higher growth rates have also been associated with plants situated close to sewage outfalls. However, after removal of sewage pollution in the Firth of Forth, Laminaria digitata became abundant on rocky shores from which they had previously been absent. Therefore, although nutrient enrichment may benefit Laminaria digitata, the indirect effects of eutrophication, such as increased light attenuation from suspended solids in the water column and interference with the settlement and growth of germlings, may be detrimental. Increased nutrients may increase the abundance of ephemeral algae and result in smothering or changing the character of the biotope.
Intermediate High Low Decline Very low
Kelps are stenohaline seaweeds, in that they do not tolerate wide fluctuations in salinity (Birkett et al., 1998b) although Laminaria digitata has been reported to grow in salinities of 25psu. The biotope occurs in situations that are naturally subject to fluctuating salinity because of precipitation but kelp growth rates may be adversely affected if subjected to periodic salinity stress. Localized, long term reductions in salinity, to below 20 psu, may result in the loss of kelp beds in affected areas and thus loss of the biotope (Birkett et al., 1998). Other species within the biotope may be intolerant of large salinity changes resulting in reduced species diversity.
Low High Low Decline Low
The biotope occurs in areas where still water conditions do not occur and so some species may be intolerant of hypoxia. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l. For instance, death of a bloom of the phytoplankton Gyrodinium aureolum in Mounts Bay, Penzance in 1978 produced a layer of brown slime on the sea bottom. This resulted in the death of fish and invertebrates presumably due to anoxia caused by the decay of the dead dinoflagellates (Griffiths et al., 1979). Kinne (1972) reports that reduced oxygen concentrations inhibit both algal photosynthesis and respiration. However, on return to oxygenated conditions, rapid recovery is likely. The main characterizing species, Laminaria digitata, colonizes cleared areas of the substratum within two years (Kain, 1975) and most other characterizing species have a planktonic larva and/or are mobile and so can migrate into the affected area.

Biological Pressures

Low High Low No change Moderate
There is very little information available on microbial pathogens infecting the characterizing species of the biotope. However the occurrence of hyperplasia or gall growths, seen as dark spots, on Laminaria digitata is well known and may be associated with the presence of endophytic brown filamentous algae. Fronds of Palmaria palmata frequently bear algal epiphytes and endophytes, a number of marine fungi and galls produced by nematodes, copepods and bacteria. Growth rates of algae may be impaired by such infections. However, no evidence of losses of this biotope due to disease was found and it is likely that microbial pathogens will have only a minor possible impact on this biotope.
Low High Low No change Moderate
The non-native species currently (October 2000) most likely to colonize this biotope are the Northwest Pacific kelp Undaria pinnatifida and the Japanese brown algae Sargassum muticum. Undaria pinnatifida, which has been introduced into south-west Britain in recent years, may cause displacement of native kelps including Laminaria digitata although in Brittany only areas inhabited by Saccorhiza polyschides have been affected. Sargassum muticum which is generally considered to be a 'gap-filler' has not been documented to directly displace Laminaria digitata but in France it has replaced Saccharina latissima (studied as Laminaria saccharina) through over-growing and shading of underlying species (Eno et al., 1997). The American piddock Petricola pholadiformis has become established along south and east coasts of England from Lyme Regis in Dorset to the Humber although there is no documentary evidence that the species has displaced any native piddocks (Eno et al., 1997).
Intermediate High Low Major decline Low
Extraction of Laminaria digitata does occur although there is no evidence available on the effects of Laminaria digitata harvesting on the biodiversity of kelp bed species. However, since the whole plant, including the holdfast is removed it is likely that faunal diversity will show a major decline. Given that MIR.Ldig.Pid occurs in the sublittoral fringe it is unlikely that vast amounts of Laminaria digitata will be collected although an intermediate intolerance has been suggested to reflect some loss. Palmaria palmata is used as a vegetable substitute or animal fodder although harvesting on a commercial scale only takes place in France. Recovery should be high because recolonization by Laminaria digitata on cleared rocks takes place within 2 years (Kain, 1979) and most other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. However, partially due to human collection for food, piddocks are no longer prevalent across the entire Meditterranean and the Atlantic coast of Europe, where they were once found (Michelson, 1978). They now have a reduced distribution.
Low Low Moderate No change Low

Additional information


Importance review


Habitats of Principal ImportanceSubtidal chalk [N. Ireland, England]
Habitats of Conservation ImportanceSubtidal chalk
Habitats Directive Annex 1Reefs
UK Biodiversity Action Plan PrioritySubtidal chalk


Kelp species around the world have been exploited over the years as a source of chemicals for industry. Kelp cast up on the shore has long been collected for use as an agricultural fertilizer. More recently Laminaria digitata is commercially harvested in Brittany for alginate production and in Ireland and France for sea-vegetable production.

Additional information



  1. Barthel, D., 1988. On the ecophysiology of the sponge Halichondria panicea in Kiel Bight. II. Biomass, production, energy budget and integration in environmental processes. Marine Ecology Progress Series, 43, 87-93.
  2. Birkett, D.A., Maggs, C.A., Dring, M.J. & Boaden, P.J.S., 1998b. Infralittoral reef biotopes with kelp species: an overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Natura 2000 report prepared by Scottish Association of Marine Science (SAMS) for the UK Marine SACs Project., Scottish Association for Marine Science. (UK Marine SACs Project, vol V.).,
  3. Christie, H., Fredriksen, S. & Rinde, E., 1998. Regrowth of kelp and colonization of epiphyte and fauna community after kelp trawling at the coast of Norway. Hydrobiologia, 375/376, 49-58.
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This review can be cited as:

Hill, J.M. 2008. Laminaria digitata and piddocks on sublittoral fringe soft rock. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from:

Last Updated: 03/04/2008