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Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock with coarse sediment

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Summary

UK and Ireland classification

Description

Tide-swept boulders and cobbles, often with a mobile component to the substrata (pebbles, gravel and sand), characterized by dense stands of the brown seaweed Halidrys siliquosa. It is can be mixed with the foliose brown seaweed Dictyota dichotoma and kelp such as Saccharina latissima and Laminaria hyperborea. Below the canopy is an undergrowth of red seaweeds that are tolerant of sand-scour such as Phyllophora crispa, Phyllophora pseudoceranoides, Rhodomela confervoides, Corallina officinalis and Chondrus crispus. Other red seaweeds such as Plocamium cartilagineum, Calliblepharis ciliata, Cryptopleura ramosa, Delesseria sanguinea, Heterosiphonia plumosa, Dilsea carnosa, Hypoglossum hypoglossoides and Brongniartella byssoides may be locally abundant, particularly in the summer months. There may be a rich epibiota on Halidrys siliquosa, including the hydroid Aglaophenia pluma and ascidians such as Botryllus schlosseri. There is generally a sparse faunal component colonizing the boulders and cobbles, comprising the tube-building polychaete Spirobranchus triqueter, the crab Cancer pagurus, the starfish Asterias rubens, the gastropod Gibbula cineraria and the sea anthozoan Urticina felina. The bryozoan Electra pilosa can form colonies on the kelp.

XKHal can occur below the tide-swept Laminaria digitata zone of the sublittoral fringe bedrock or boulders (LdigT). Less stable substrata of boulders, cobbles or pebbles may support kelp and Chorda filum in the shallows (LsacChoR) or dense ephemeral seaweeds (EphR). Sand-influenced rocky outcrops in deeper water may support a Flustra foliacea community (FluCoAs). This biotope is widespread and is found on the open coast in Wales, the south-west and the English Channel as well as more sheltered tidal rapids in the Scottish sealochs. It can form extensive forests or parks in certain areas (Dorset, Sarns). In Wales, the south-west and west of England the red seaweeds Spyridia filamentosa and Halarachnion ligulatum and brown seaweeds Dictyopteris membranacea and Taonia atomaria are frequent. In Scotland, kelp occur at a greater proportion of sites, solitary ascidians such as Ascidiella spp. are more common and the feather star Antedon bifida and brittlestars Ophiothrix fragilis are found. The diversity of red seaweeds is higher in summer. (Information from Connor et al., 2004; JNCC, 2015). 

Depth range

0-5 m, 5-10 m, 10-20 m

Additional information

Little information on the ecology of this biotope was found. The ecology has been inferred from general studies of subtidal macroalgal communities (see Brawley, 1992; Hawkins et al., 1992; Vadas & Elner, 1992; Williams & Seed, 1992; Schiel & Foster, 1986).

Habitat review

Ecology

Ecological and functional relationships

  • Macroalgae provide primary productivity either directly to grazing fish and invertebrates or indirectly, to detritivores and decomposers, in the form of detritus and drift algae or as dissolved organic material and other exudates.
  • Macroalgal species compete for light, space and, to a lesser extent, nutrients, depending on the growth rates, size and reproductive pattern of each species. For example, large macroalgae such as Halidrys siliquosa and laminarians shade the substratum (depending on density) so that understorey plants tend to be shade tolerant red algae. Understorey algae, by effectively restricting access to the substratum, may also inhibit or restrict recruitment of other species of macroalgae (Hawkins & Harkin, 1985; Hawkins et al., 1992).
  • Macroalgae compete for space with sessile invertebrates such as sponges, hydroids, ascidians and bryozoans.
  • Halidrys siliquosa and, when present, laminarians provide substratum for epiphytes, depending on location, including microflora (e.g. bacteria, blue green algae, diatoms and juvenile larger algae), Ulothrix and Ceramium sp., hydroids (e.g. Aglaophenia pluma, Laomeda flexuosa, and Obelia spp.), bryozoans (e.g. Scrupocellaria spp.), and ascidians (e.g. Apilidium spp., Botryllus schlosseri, and Botrylloides leachi) (Moss, 1982; Lewis, 1964, Connor et al., 1997).
  • Sessile epiphytes, including microflora, may reduce light available for photosynthesis and hence reduce growth and reproduction of the macroalgae, or increase drag and reduce the plants flexibility resulting in increased susceptibility to storm or wave damage (Williams & Seed, 1992).
  • Amphipods, isopods and other mesoherbivores graze the epiphytic flora and senescent macroalgal tissue, which may benefit the macroalgal host, and may facilitate dispersal of the propagules of some macroalgal species (Brawley, 1992; Williams & Seed, 1992). Mesoherbivores also graze the macroalgae but do not normally adversely affect the canopy (Brawley, 1992).
  • Gastropods graze epiphytes and macroalgae directly, e.g. Steromphala cineraria, Lacuna vincta and the limpet Tectura spp. Epiphyte grazing by Tectura (as Acmaea) sp. was reported to be important to the survival of an encrusting coralline algae (Hawkins et al., 1992; Williams & Seed, 1992; Birkett et al., 1998b). Where present , laminarians are probably grazed by the blue-rayed limpet Patella pellucida.
  • Sea urchins are important general grazers (grazing drift algae, macroalgae, microalgae, and sessile fauna) in subtidal algal habitats. For example, Echinus esculentus has been shown to control the depth reached by Laminaria hyperborea biotopes in Port Erin (Kain, 1979) (see EIR.LhypR) and to significantly affect the biomass of understorey macroalgae (Schiel & Foster, 1986; Hawkins et al., 1992; Vadas & Elner, 1992: Birkett et al., 1998b).
  • The impact of sea urchin grazing depends on density and hence depth (Hawkins et al., 1992). Although Echinus esculentus and Psammechinus miliaris occur at low density in this biotope (JNCC, 1999), as evidenced by the extent of algal cover, urchin grazing probably increases the diversity of the biotope by clearing small areas for colonization by other species.
  • Mobile predators include crabs (e.g. Cancer pagurus and Necora puber) feeding on small crustaceans and gastropods, starfish such as Asterias rubens, and fish such as the corkwing wrasse Crenilabrus melops, the butterfish Pholis gunnellus and the dragnet Callionymus lyra feeding on small crustaceans, polychaetes and other invertebrates.
  • Starfish (Asterias rubens and Henricia oculata), crabs and hermit crabs probably act as scavengers within the biotope.
  • Epiphytic and benthic suspension feeders include bryozoans, sponges and hydroids together with tube worms (e.g. Spirobranchus triqueter) on boulders or Lanice conchilega or Chaetopterus variopedatus in intervening sediment, the barnacle Balanus crenatus, the long clawed porcelain crab Pisidia longicornis and the starfish Henricia oculata.
  • Seasonal and longer term change

    Little is known about temporal change in subtidal algal populations (Schiel & Foster, 1986). Most of the dominant algae within the biotope are perennial, present all year round, e.g. Halidrys siliquosa, Delesseria sanguinea, Chondrus crispus, Furcellaria lumbricalis, and Dilsea carnosa. However, they show seasonal variation in reproduction, with Halidrys siliquosa, Furcellaria lumbricalis, Chondrus crispus and Delesseria sanguinea releasing spores in the winter months, potentially enabling them to colonize free space opened up by increased wave action in winter storms and the dying back of annual species (see Kain, 1975). Annual species, e.g. Chorda filum are likely to proliferate in spring, reaching maximum abundance in summer (high insolation and temperature). Winter storms have been reported to damage Furcellaria lumbricalis plants (Austin, 1960b) and presumably could potentially damage or remove other members of the community, potentially opening space for colonization.

    Habitat structure and complexity

    • Halidrys siliquosa, together with laminarians present, form an upper canopy shading the understorey algae and substratum.
    • Halidrys siliquosa, and to a lesser extent Saccharina latissima when present support a diverse assemblage of epiphytes (see above). If present, Laminaria hyperborea may also support a diverse array of epiphytes on its stipe (see species review).
    • The understorey of smaller macroalgae is dominated by a variety of sand-scour tolerant red algae, which probably varies with location. However, Phyllophora sp., Chondrus crispus, Polyides rotunda, Delesseria sanguinea, Dilsea carnosa and Furcellaria lumbricalis typically occur. The understorey includes brown seaweeds, e.g. Dictyota dichotoma, Chorda filum and Desmarestia aculeata.
    • The surface of the substratum may support sessile invertebrates that are effective space occupiers, e.g. sponges, and barnacles (e.g. Balanus crenatus) and some anemones e.g. the dahlia anemone Urticina felina.
    • The surface of boulders or cobbles support a sparse fauna of encrusting sponges (e.g. Esperiopsis fucorum), tubeworms (e.g. Spirobranchus triqueter) barnacles, crabs and ascidians (Botryllus schlosseri, Clavelina lepadiformis, and Ascidiella spp.). The underboulder surface may support encrusting sponges, the porcelain crabs and brittlestars.
    • The substratum typically includes mobile, coarse sediment (e.g. pebbles, gravel and sand), which may support burrowing polychaetes such as Lanice conchilega or Chaetopterus variopedatus.
    • The interstices between understorey macroalgae may act as shelter or refuge for larvae and juveniles of the organisms found in the community (Birkett et al., 1998). Laboratory evidence (Johns & Mann, 1987) suggested that Irish moss (Chondrus crispus) and habitat complexity attract juvenile lobster, presumably as a refuge from predation. However, Vadas & Elner (1992) suggested that field evidence for large invertebrates or fish using macroalgal habitats as refuges or nurseries was conjectural.

    Productivity

    Studies of subtidal seaweed communities in Nova Scotia suggested that seaweed annual production exceeded the consumption rates of herbivores about 10-fold. It was suggested that most of the productivity was exported in the form of suspended particulate matter (Miller et al., 1971; cited in Vadas & Elner, 1992). A large proportion of the primary productivity of seaweeds in subtidal algal stands is, therefore, probably exported in the form of drift algae (onshore or onto the strand line), particulates, exudates of dissolved organic matter, and contributes to the productivity of surrounding communities. However, no information concerning productivity within this biotope was found.

    Recruitment processes

    The propagules of most macroalgae tend to settle near the parent plant (Schiel & Foster, 1986; Norton, 1992; Holt et al., 1997). For example, the propagules of fucales are large and sink readily and red algal spores and gametes and immotile. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition (zygotes or spores being thrown against the substratum). For example, spores of Ulva sp. have been reported to travel 35km, Phycodrys rubens 5km and Sargassum muticum up to 1km, although most Sargassum muticum spores settle within 2m. The reach of the furthest propagule and useful dispersal range are not the same thing and recruitment usually occurs on a local scale, typically within 10m of the parent plant (Norton, 1992).

    The presence of sessile invertebrates (e.g. sponges) or coralline algae, sand or sediment cover and grazing gastropods may inhibit settlement or attachment of propagules and the survival of the germlings. Fucalean algae showed greater recruitment to areas cleared of low lying algae, and coralline algae have been shown to inhibit the settlement of a number of sessile kelp forest species (Schiel & Foster, 1986). Vadas et al. (1992) noted that post-settlement mortality of algal propagules and early germlings was high, primarily due to grazing, canopy and turf effects, water movement and desiccation (in the intertidal) and concluded that algal recruitment was highly variable and sporadic. For example, Sousa et al. (1981) reported that experimental removal of sea urchins significantly increased recruitment in long-lived brown algae. In experimental plots cleared of algae and sea urchins in December, Halidrys dioica colonized the plots, in small numbers, within 3-4 months. Plots cleared in August received few , if any recruits, suggesting that recolonization was dependant on zygote availability and therefore the season. Halidrys dioica did not colonize plots grazed by urchins in their experiments (Sousa et al., 1981).

    When bare substratum becomes available for colonization, for instance following storm events, it is expected that algal recruitment and succession would follow a predictable sequence (Hawkins & Harkin, 1985). Initial colonizers on bare rock are often epiphytic species, suggesting that it is competition from canopy forming algae that usually restricts them to their epiphytic habit (Hawkins & Harkin, 1985). Gradually, the original canopy or turf forming species, in this case Furcellaria lumbricalis and Chondrus crispus, then become established. These findings suggest that interactions between macrophytes are often more important than grazing in structuring algal communities (Hawkins & Harkin, 1985).

    Halidrys siliquosa can float if detached, suggesting another potential route for dispersal. However, although some long range dispersal must occur in macroalgae (resulting in colonization of oil rigs and similar structures), van den Hoek (1987) and Norton (1992) suggested that it is probably ineffective for most species of macroalgae. Wernberg et al. (2001) suggested that the lack of long range dispersal success in Halidrys siliquosa was responsible for its regional distribution in the north east Atlantic.

    Epiphytic and sessile fauna, such as sponges, hydroids, bryozoans and ascidians, have pelagic but short lived larvae with relatively short effective dispersal ranges, depending on the local hydrography. However, most epiphytic species are widespread and ubiquitous and would probably recruit rapidly from adjacent or nearby populations.

    Time for community to reach maturity

    Kain (1975) noted that on a single block cleared every two months, most biomass belonged to Rhodophyceae in winter, Phaeophyceae in spring and Chlorophyceae in late summer. On blocks cleared and monitored for five years, the red algae colonized quickly and the community (including Laminaria hyperborea) had reached a condition similar to the pre-clearance community within 2 years and nine months (Kain, 1975). Furcellaria lumbricalis species grows very slowly compared to other red algae (Bird et al., 1979) and takes a long time to reach maturity. For example, Austin (1960b) reported that in Wales, Furcellaria lumbricalis typically takes 5 years to attain fertility. This would mean that, following perturbation, recovery to a mature reproductive community would take at least 5 years. Similarly, Halidrys siliquosa does not reproduce until the end of its second year, and the population would therefore, take at least 2 years to begin recovery if removed. However, it grows rapidly, a maximum summer growth rate of 2cm/month being reported by Moss & Lacey (1963), so that damaged but surviving individuals would probably regain prior condition is within a year, depending on season. Recovery of Chondrus crispus was monitored after a rocky shore was totally denuded by ice scour in Nova Scotia, Canada, its original biomass returning within 5 years. (Minchinton et al., 1997). Several fucoids have been shown to recolonize cleared areas readily, especially in the absence of grazers (Holt et al., 1995, 1997). For example, Fucus dominated areas may take 1-3 years to recolonize in British waters (Holt et al., 1995).

    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.

    Overall, therefore, it is likely that the understorey and large fucoids such as Halidrys siliquosa and laminarians where present may recolonize and recover their biomass within at least 5 years. However, although epiphytic species may recruit rapidly, it may take longer (up to 10 years) for them to recover their original biomass and the biotope to return to its prior species richness.

    Additional information

    None entered.

Preferences & Distribution

Habitat preferences

Depth Range 0-5 m, 5-10 m, 10-20 m
Water clarity preferences
Limiting Nutrients Nitrogen (nitrates), Phosphorus (phosphates)
Salinity preferences Full (30-40 psu)
Physiographic preferences Open coast
Biological zone preferences Infralittoral
Substratum/habitat preferences Bedrock, Large to very large boulders, Small boulders, Cobbles, Coarse sediments
Tidal strength preferences Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferences Moderately exposed
Other preferences Sediment abrasion.

Additional Information

Halidrys siliquosa dominated communities may occur below the shallow water kelp dominated belt or form extensive beds where silt accumulation prevents kelps such as Laminaria hyperborea or Laminaria digitata becoming dominant e.g. Weymouth Bay (Dixon et al., 1978). Halidrys siliquosa dominated communities are characterized by species tolerant of silt and sediment abrasion and wave sheltered conditions but die out as the sediment substratum grain size decreases (i.e. shingle or coarse gravel) or water flow increases. However, this community is often associated with the entrance (or exit) of tidal rapids in Lochs, e.g. up to 1 -2 knots (0.5 -1m/sec) in Loch Yeor, west Uist (Lewis, 1964; Thorpe et al., 1998). With increasing water flow, Himanthalia elongata abundance within the biotope increases. At higher water flow rates found in rapids, or increased wave action, the biotope is replaced by Laminaria digitata or Laminaria hyperborea biotopes (e.g. EIR.LhypR or MIR.Ldig.Ldig) (Lewis, 1964; Connor et al, 1997a).

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope

-

Additional information

The MNCR recorded ca 734 species within this biotope, although not all species occurred in all records of the biotope. Halidrys siliquosa dominated communities are also described by Lewis (1964) and additional records provided by Dixon et al. (1978).

Sensitivity review

Sensitivity characteristics of the habitat and relevant characteristic species

IR.HIR.KSed.XKHal is within the sediment-affected or disturbed kelp and seaweed communities (IR.HIR.KSed) habitat complex. This tide-swept biotope is characterized by a canopy of Halidrys siliquosa. The canopy can also be mixed with the kelps Saccharina lattissima (formerly Laminaria saccharina) and Laminaria hyperborea. Below the canopy, the brown seaweed Dictyota dichotoma can be frequent mixed with scour tolerant red seaweeds, such as Phyllophora crispaPhyllophora pseudoceranoidesRhodomela confervoidesCorallina officinalis and Chondrus crispus. There may be a rich epibiota on the Halidrys siliquosa sporophytes, including Aglaophenia pluma, ascidians including Botryllus schlosseri, and sponges. The understorey faunal community is not diverse, typically limited to Spirobranchus triqueter and other scour tolerant fauna (Connor et al., 2004). The associated red algal species and invertebrates occur across a range of rock biotopes and are not considered to be key structuring or functioning species in this biotope. The sensitivity of these species is considered only generally for the sensitivity assessments.

For this sensitivity assessment, Halidrys siliquosa is the primary focus as the key characterizing species defining the biotope, the kelps Saccharina lattissima and Laminaria hyperborea are also considered specifically within assessments as important species to the biotope. Examples of other important species groups are mentioned where appropriate. High sediment scour and mobility is also a key environmental process that defines this biotope and limits the dominance of the kelps Laminaria hyperborea.

Resilience and recovery rates of habitat

Limited information concerning recruitment in Halidrys siliquosa was found. Halidrys siliquosa is a perennial brown seaweed (Pederson et al., 2005) distributed across the North East Atlantic from Morocco to Northern Norway (Moss & Lacy, 1963; Algae Base, 2015). Sporophytes reach maturity in 1-2 years (Moss & Lacy, 1963). From early spring (March), germlings develop on rock substrata and undergo a period of rapid vegetative growth. Within the first year of growth, the sporophyte develops lateral branches but lacks air vesicles and reproductive structures. In the second year of growth from August to December, reproductive tissues (known as receptacles) develop and from December to March gametes are released (Moss & Lacy, 1963). Sporophytes can reach a maximum length of 120 cm (Bunker et al., 2012). Halidrys siliquosa has comparatively large eggs (approximately 150µm in diameter) when compared to other macro-algae (Moss & Shreader, 1973), which may cause eggs to settle out and enhance local recruitment. Recovery following disturbance could, therefore, be influenced by the proximity of mature individuals producing viable eggs.

Halidrys dioica was shown to recruit to cleared areas within 3-4 months in the absence of sea urchins on the California coast (Sousa et al., 1981). Similarly, Halidrys siliquosa became a dominant alga three years after the removal of kelps in Norway (summary only, Svendsen, 1972). Several fucoids have been shown to recolonize cleared areas readily, especially in the absence of grazers (Holt et al., 1995, 1997). For example, Fucus dominated areas may take 1-3 years to recolonize in British waters (Holt et al., 1995). Overall, Halidrys siliquosa is highly fecund and widespread in British waters. If a population is damaged or reduced in abundance it is likely that local recruitment will be good, especially in the winter months and prior abundance may return within a few years. Should the population be destroyed, then recruitment from the surrounding area and subsequent growth may take longer, possibly up to five years.

Other important algae species within IR.HIR.KSed.XKHal includes; Saccharina lattissima (formerly Laminaria saccharina), Laminaria hyperborea and scour tolerant red seaweeds such as Chondrus crispusSaccharina lattissima can reach maturity relatively quickly in 15-20 months (Sjøtun, 1993), however, Laminaria hyperborea requires 1-6 years to reach maturity (Kain, 1979; Fredriksen et al., 1995; Christie et al., 1998). The understorey red seaweed communities are often dependent on the presence of canopy-forming species, without which the understorey red seaweed community may become bleached or outcompeted by opportunistic annuals (Hawkins & Harkin, 1985; Jenkins et al., 2004). Recovery of Chondrus crispus was monitored after a rocky shore was totally denuded by ice scour in Nova Scotia, Canada. The original biomass of Chondrus crispus returned within five years, but if holdfasts remained it was able to recover cover within 18 months (Minchinton et al., 1997).

The turf forming red algae may recover through repair and regrowth of damaged fronds from bases or via recolonization of rock surfaces where all the plant material is removed. All the red algae (Rhodophyta) exhibit distinct morphological stages over the reproductive life history. This phenomenon is known as heterotrichy and describes cases where the algal thallus consists of two parts, a prostrate creeping system exhibiting apical growth and functioning as a holdfast. The thalli can regrow from these crusts where they remain supporting the recovery of the biotope (Mathieson & Burns, 1975; Dudgeon & Johnson, 1992). The basal crusts are perennial, tough, resistant stages that prevent other species from occupying the rock surface and allow rapid regeneration and are therefore a significant recovery mechanism.

The scour tolerant fauna such as Spirobranchus (syn. Pomatocerostriqueter and Electra pilosa are early successional species characteristic of disturbed environments. Hiscock (1983). for example, noted a community consisting of fast-growing species such as Spirobranchus triqueter under conditions of scour and abrasion from stones and boulders moved by storms, Off Chesil Bank, the epifaunal community dominated by Spirobranchus triqueter, Balanus crenatus and Electra pilosa, decreased in cover in October as it was scoured away in winter storms, and was recolonized in May to June (Warner 1985).  Recovery of the sparse associated fauna is considered to be ‘High’ for any level of impact.

Resilience assessment. The limited evidence suggests that Halidrys siliquosa may recover within 2-3 years (Moss & Lacy, 1963; Svendsen, 1972). Other important algal species such as Saccharina lattissima may recover within 1-2 years (Sjøtun, 1993), however, Laminaria hyperborea may require 1-6 years to recover if disturbed (Kain, 1979; Minchinton et al., 1997). For pressures that would likely cause a significant loss of the macroalgal canopy (e.g., resistance is ‘None’, ‘Low’ or ‘Medium’) resilience has been assessed as ‘Medium’. Where resistance is ‘High’, then resilience is ‘High’.

Climate Change Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Medium Very Low Medium
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

Kelp and fucoids can be particularly sensitive to temperature change because of their sessile nature (Wahl et al., 2015). Growth, reproduction and recruitment patterns of seaweeds are often seasonally controlled and regulated by changes in temperature (Harley et al., 2012; Hobday et al., 2016; Atkinson et al., 2020). Therefore, seaweeds can be extremely sensitive to ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020).

Halidrys siliquosa is a large brown alga restricted to the North East Atlantic, and recorded from northern Norway, Scandinavia, the Baltic Sea, Helgoland and the Netherlands south to the Bay of Biscay, north Portugal and the Canary Islands (John et al., 2004). Halidrys siliquosa has an upper survival limit of 25°C (Luning, 1984). Zygote germination and growth are temperature dependent. Moss & Sheader (1973) reported 50-97% germination success at 3 and 10°C, falling considerably to 8-52% at 20°C and to zero at 22°C. Growth increased with temperature up to 20°C but germlings developed abnormally at 20°C (Moss & Sheader, 1973).

Halidrys siliquosa occurs in the sublittoral and in rockpools (Lima et al., 2009). The physico-chemical conditions of rockpools vary considerably, with different parameters than the main body of the sea (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993; Metaxas et al., 1994). The temperature of rockpools fluctuates considerably with air temperature and sunlight, and tend to warm throughout the day, especially if in direct sunlight (Daniel & Boyden, 1975; Goss-Custard et al., 1979). Therefore, Halidrys siliquosa is unlikely to be affected by long-term temperature changes within the British Isles.

Lima et al. (2009) reported the geographical shift of Halidrys siliquosa to be inconsistent with the general predictions of species migrations under warming climate conditions. In addition, Mieszkowska et al. (2013) reported Halidrys siliquosa to have increased in abundance in estuaries in the Western English Channel, where other species of macroalgae had declined. 

Saccharina latissima has an optimal growth temperature between 10 and 15°C, with growth reducing by 50-70% at 20°C, and all experimental specimens disintegrating after seven days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23-25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. 

Laminaria hyperborea has an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). At 17°C gamete survival is reduced (Steinhoff et al., 2008) and gametogenesis is inhibited at 21°C (Dieck, 1992). Therefore, Laminaria hyperborea recruitment could be impaired at a sustained temperature increase above 17°C. However, sporophytes can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998b).

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima would move northwards, retreating from the coast of Portugal, France and the southwest coast of the UK. The authors projected that, under RCP 2.6, 13% suitable Laminaria hyperborea habitat would be lost from the Western English Channel, while under the RCP 8.5 emission, 87 % of suitable habitat was expected to be lost.

Sensitivity assessment. Halidrys siliquosa has an upper survival limit of 25°C and therefore should be able to tolerate UK summer seawater temperatures. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988), and maybe crucial in the persistence of this species around the UK, as seawater temperatures rise. However, Laminaria hyperborea is already growing at suboptimal temperatures in the southern UK, based on evidence of decreased productivity comparative to Scotland (Pessarrodona et al., 2018; Smale et al., 2020b), and predictions have estimated Laminaria hyperborean to be lost from the UK by 2100 as a result of warming (Brodie et al., 2014). 

Under the middle emission scenario, a rise of 3°C could lead to maximum summer high temperatures of 22°C in the south of the UK. Populations of Halidrys siliquosa, Saccharina latissima and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C. However, this is above the upper thermal limit of 21°C for Laminaria hyperborea (Bolton & Lüning, 1982), and is likely to lead to loss of this species from the south of England. Furthermore, biomass and plant sizes are expected to decrease as waters warm, with Scottish Laminaria hyperborea stipe lengths decreasing to lengths observed in southern England, leading to a decline in carbon assimilation, productivity and habitat quality (Pessarrodona et al., 2018). However, the dense stand of Halidrys siliquosa, the main characteristic of the biotope will probably survive. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario.

For the high and extreme emission scenario where sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century, Halidrys siliquosa is likely to be able to survive these temperatures, however, growth and germination is likely to be impacted. At the predicted sea temperatures Saccharina latissima and Laminaria hyperborea are likely to be lost from southern England. The northward retreat of the distribution of Laminaria hyperborea is expected to increase. Under the high emission scenario it is expected to be lost from 30% of the coastline around the UK (Assis et al., 2018), and under the extreme scenario even more is projected to be lost. Populations of Laminaria hyperborea that remain around the UK are predicted to become less productive. However, as Halidrys siliquosa, the main characteristic of the biotope will probably survive, under these scenarios, resistance is assessed as ‘Medium, and resilience is assessed as ‘Very Low’. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario. 

Medium Very Low Medium
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

Kelp and fucoids can be particularly sensitive to temperature change because of their sessile nature (Wahl et al., 2015). Growth, reproduction and recruitment patterns of seaweeds are often seasonally controlled and regulated by changes in temperature (Harley et al., 2012; Hobday et al., 2016; Atkinson et al., 2020). Therefore, seaweeds can be extremely sensitive to ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020).

Halidrys siliquosa is a large brown alga restricted to the North East Atlantic, and recorded from northern Norway, Scandinavia, the Baltic Sea, Helgoland and the Netherlands south to the Bay of Biscay, north Portugal and the Canary Islands (John et al., 2004). Halidrys siliquosa has an upper survival limit of 25°C (Luning, 1984). Zygote germination and growth are temperature dependent. Moss & Sheader (1973) reported 50-97% germination success at 3 and 10°C, falling considerably to 8-52% at 20°C and to zero at 22°C. Growth increased with temperature up to 20°C but germlings developed abnormally at 20°C (Moss & Sheader, 1973).

Halidrys siliquosa occurs in the sublittoral and in rockpools (Lima et al., 2009). The physico-chemical conditions of rockpools vary considerably, with different parameters than the main body of the sea (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993; Metaxas et al., 1994). The temperature of rockpools fluctuates considerably with air temperature and sunlight, and tend to warm throughout the day, especially if in direct sunlight (Daniel & Boyden, 1975; Goss-Custard et al., 1979). Therefore, Halidrys siliquosa is unlikely to be affected by long-term temperature changes within the British Isles.

Lima et al. (2009) reported the geographical shift of Halidrys siliquosa to be inconsistent with the general predictions of species migrations under warming climate conditions. In addition, Mieszkowska et al. (2013) reported Halidrys siliquosa to have increased in abundance in estuaries in the Western English Channel, where other species of macroalgae had declined. 

Saccharina latissima has an optimal growth temperature between 10 and 15°C, with growth reducing by 50-70% at 20°C, and all experimental specimens disintegrating after seven days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23-25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. 

Laminaria hyperborea has an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). At 17°C gamete survival is reduced (Steinhoff et al., 2008) and gametogenesis is inhibited at 21°C (Dieck, 1992). Therefore, Laminaria hyperborea recruitment could be impaired at a sustained temperature increase above 17°C. However, sporophytes can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998b).

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima would move northwards, retreating from the coast of Portugal, France and the southwest coast of the UK. The authors projected that, under RCP 2.6, 13% suitable Laminaria hyperborea habitat would be lost from the Western English Channel, while under the RCP 8.5 emission, 87 % of suitable habitat was expected to be lost.

Sensitivity assessment. Halidrys siliquosa has an upper survival limit of 25°C and therefore should be able to tolerate UK summer seawater temperatures. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988), and maybe crucial in the persistence of this species around the UK, as seawater temperatures rise. However, Laminaria hyperborea is already growing at suboptimal temperatures in the southern UK, based on evidence of decreased productivity comparative to Scotland (Pessarrodona et al., 2018; Smale et al., 2020b), and predictions have estimated Laminaria hyperborean to be lost from the UK by 2100 as a result of warming (Brodie et al., 2014). 

Under the middle emission scenario, a rise of 3°C could lead to maximum summer high temperatures of 22°C in the south of the UK. Populations of Halidrys siliquosa, Saccharina latissima and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C. However, this is above the upper thermal limit of 21°C for Laminaria hyperborea (Bolton & Lüning, 1982), and is likely to lead to loss of this species from the south of England. Furthermore, biomass and plant sizes are expected to decrease as waters warm, with Scottish Laminaria hyperborea stipe lengths decreasing to lengths observed in southern England, leading to a decline in carbon assimilation, productivity and habitat quality (Pessarrodona et al., 2018). However, the dense stand of Halidrys siliquosa, the main characteristic of the biotope will probably survive. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario.

For the high and extreme emission scenario where sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century, Halidrys siliquosa is likely to be able to survive these temperatures, however, growth and germination is likely to be impacted. At the predicted sea temperatures Saccharina latissima and Laminaria hyperborea are likely to be lost from southern England. The northward retreat of the distribution of Laminaria hyperborea is expected to increase. Under the high emission scenario it is expected to be lost from 30% of the coastline around the UK (Assis et al., 2018), and under the extreme scenario even more is projected to be lost. Populations of Laminaria hyperborea that remain around the UK are predicted to become less productive. However, as Halidrys siliquosa, the main characteristic of the biotope will probably survive, under these scenarios, resistance is assessed as ‘Medium, and resilience is assessed as ‘Very Low’. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario. 

Medium Very Low Medium
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

Kelp and fucoids can be particularly sensitive to temperature change because of their sessile nature (Wahl et al., 2015). Growth, reproduction and recruitment patterns of seaweeds are often seasonally controlled and regulated by changes in temperature (Harley et al., 2012; Hobday et al., 2016; Atkinson et al., 2020). Therefore, seaweeds can be extremely sensitive to ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020).

Halidrys siliquosa is a large brown alga restricted to the North East Atlantic, and recorded from northern Norway, Scandinavia, the Baltic Sea, Helgoland and the Netherlands south to the Bay of Biscay, north Portugal and the Canary Islands (John et al., 2004). Halidrys siliquosa has an upper survival limit of 25°C (Luning, 1984). Zygote germination and growth are temperature dependent. Moss & Sheader (1973) reported 50-97% germination success at 3 and 10°C, falling considerably to 8-52% at 20°C and to zero at 22°C. Growth increased with temperature up to 20°C but germlings developed abnormally at 20°C (Moss & Sheader, 1973).

Halidrys siliquosa occurs in the sublittoral and in rockpools (Lima et al., 2009). The physico-chemical conditions of rockpools vary considerably, with different parameters than the main body of the sea (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993; Metaxas et al., 1994). The temperature of rockpools fluctuates considerably with air temperature and sunlight, and tend to warm throughout the day, especially if in direct sunlight (Daniel & Boyden, 1975; Goss-Custard et al., 1979). Therefore, Halidrys siliquosa is unlikely to be affected by long-term temperature changes within the British Isles.

Lima et al. (2009) reported the geographical shift of Halidrys siliquosa to be inconsistent with the general predictions of species migrations under warming climate conditions. In addition, Mieszkowska et al. (2013) reported Halidrys siliquosa to have increased in abundance in estuaries in the Western English Channel, where other species of macroalgae had declined. 

Saccharina latissima has an optimal growth temperature between 10 and 15°C, with growth reducing by 50-70% at 20°C, and all experimental specimens disintegrating after seven days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23-25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. 

Laminaria hyperborea has an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). At 17°C gamete survival is reduced (Steinhoff et al., 2008) and gametogenesis is inhibited at 21°C (Dieck, 1992). Therefore, Laminaria hyperborea recruitment could be impaired at a sustained temperature increase above 17°C. However, sporophytes can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998b).

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima would move northwards, retreating from the coast of Portugal, France and the southwest coast of the UK. The authors projected that, under RCP 2.6, 13% suitable Laminaria hyperborea habitat would be lost from the Western English Channel, while under the RCP 8.5 emission, 87 % of suitable habitat was expected to be lost.

Sensitivity assessment. Halidrys siliquosa has an upper survival limit of 25°C and therefore should be able to tolerate UK summer seawater temperatures. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988), and maybe crucial in the persistence of this species around the UK, as seawater temperatures rise. However, Laminaria hyperborea is already growing at suboptimal temperatures in the southern UK, based on evidence of decreased productivity comparative to Scotland (Pessarrodona et al., 2018; Smale et al., 2020b), and predictions have estimated Laminaria hyperborean to be lost from the UK by 2100 as a result of warming (Brodie et al., 2014). 

Under the middle emission scenario, a rise of 3°C could lead to maximum summer high temperatures of 22°C in the south of the UK. Populations of Halidrys siliquosa, Saccharina latissima and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C. However, this is above the upper thermal limit of 21°C for Laminaria hyperborea (Bolton & Lüning, 1982), and is likely to lead to loss of this species from the south of England. Furthermore, biomass and plant sizes are expected to decrease as waters warm, with Scottish Laminaria hyperborea stipe lengths decreasing to lengths observed in southern England, leading to a decline in carbon assimilation, productivity and habitat quality (Pessarrodona et al., 2018). However, the dense stand of Halidrys siliquosa, the main characteristic of the biotope will probably survive. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario.

For the high and extreme emission scenario where sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century, Halidrys siliquosa is likely to be able to survive these temperatures, however, growth and germination is likely to be impacted. At the predicted sea temperatures Saccharina latissima and Laminaria hyperborea are likely to be lost from southern England. The northward retreat of the distribution of Laminaria hyperborea is expected to increase. Under the high emission scenario it is expected to be lost from 30% of the coastline around the UK (Assis et al., 2018), and under the extreme scenario even more is projected to be lost. Populations of Laminaria hyperborea that remain around the UK are predicted to become less productive. However, as Halidrys siliquosa, the main characteristic of the biotope will probably survive, under these scenarios, resistance is assessed as ‘Medium, and resilience is assessed as ‘Very Low’. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario. 

Low Low High
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: Medium

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persists for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Marine heatwaves are known to impact both kelp and fucoid communities (Smale et al., 2019; Mieszkowska et al., 2020). A heatwave in the summer of 2018 caused physiological damage to several fucoid species including Pelvetia canaliculata, Fucus spiralis and Fucus vesiculosus (Mieszkowska et al., 2020). 

No evidence for the impacts of marine heatwaves on Halidrys siliquosa was found. However, Halidrys siliquosa has an upper survival limit of 25°C (Luning, 1984), abnormal development occurs at 20°C and zygote germination is inhibited at 22°C (Moss & Sheader, 1973). Therefore, marine heatwaves could impact the survival and reproduction of Halidrys siliquosa, especially if the marine heatwave coincides with the release of gametes or with germination.

In Baja California, Mexico, an extreme heat even between 2014– 2016, led to both a decrease in density of Macrocystis pyrifera and a decrease in the number of fronds per individual in Baja California, Mexico (Arafeh-Dalmau et al., 2019). Additionally, there was a significant change to the understory algal composition, and half of the fish and invertebrates associated with this habitat disappeared. The same heatwave, coupled with a loss of starfish through disease and an increase in urchin grazing, led to the loss of > 90% of Macrocystis pyrifera from 350 km of coastline in northern California (Rogers-Bennett & Catton, 2019).

Under experimental conditions, Nepper-Davidson et al. (2019) exposed a northern (Denmark) population of Saccharina latissima to a simulated three-week heatwave of three different intensities; 18, 21 and 24°C. When exposed to heatwaves of 18 and 21°C there was a decrease in photosynthesis and growth. When a 24°C was simulated, 91% of sporophytes were dead within a week, and the fronds of the few survivors were disintegrating, so the experiment was terminated (Nepper-Davidsen et al., 2019). These results suggest that this species is unlikely to survive heatwaves of the length and magnitude predicted by the end of this century for both the middle and high emission scenarios in the southern UK.

Laminaria hyperborea is a cold-temperate species of kelp with an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). Germination success can decrease by almost two thirds at temperatures as low as 17°C. Therefore, it is expected that similar to other kelp species, Laminaria hyperborea will be highly sensitive to marine heatwaves.

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Halidrys siliquosa has an upper survival limit of 25°C, therefore Halidrys siliquosa might be able to tolerate a heatwave of this magnitude, but reproduction could be impacted. However, Laminaria hyperborea and Saccharina latissima are unlikely to survive a heatwave of this magnitude and likely to suffer severe mortality in the south. Although, In Scotland, where these biotopes occur, temperatures are not predicted to rise above 20°C, and therefore, Laminaria hyperborea and Saccharina latissima are likely to survive a heatwave of this magnitude. Nevertheless, the southern assemblages may be impacted. However, as Halidrys siliquosa, the main characteristic of the biotope will probably survive, under this scenario, resistance is assessed as ‘Medium’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Medium.’ This biotope IR.MIR.KT.XKT is assessed as having ‘Medium’ sensitivity to marine heatwaves under the middle emission scenario

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England. Halidrys siliquosa, Laminaria hyperborea and Saccharina latissima are unlikely to survive a heatwave of this magnitude, as temperatures are likely to reach > 21°C in Scotland under this scenario, there is likely to be mortality throughout these species’ UK biogeographic distribution. Therefore, resistance has been assessed as ‘Low’. As a further heatwave is likely to affect this habitat before full recovery (under the pressure benchmark definition), resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Medium Medium Medium
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: Medium

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persists for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Marine heatwaves are known to impact both kelp and fucoid communities (Smale et al., 2019; Mieszkowska et al., 2020). A heatwave in the summer of 2018 caused physiological damage to several fucoid species including Pelvetia canaliculata, Fucus spiralis and Fucus vesiculosus (Mieszkowska et al., 2020). 

No evidence for the impacts of marine heatwaves on Halidrys siliquosa was found. However, Halidrys siliquosa has an upper survival limit of 25°C (Luning, 1984), abnormal development occurs at 20°C and zygote germination is inhibited at 22°C (Moss & Sheader, 1973). Therefore, marine heatwaves could impact the survival and reproduction of Halidrys siliquosa, especially if the marine heatwave coincides with the release of gametes or with germination.

In Baja California, Mexico, an extreme heat even between 2014– 2016, led to both a decrease in density of Macrocystis pyrifera and a decrease in the number of fronds per individual in Baja California, Mexico (Arafeh-Dalmau et al., 2019). Additionally, there was a significant change to the understory algal composition, and half of the fish and invertebrates associated with this habitat disappeared. The same heatwave, coupled with a loss of starfish through disease and an increase in urchin grazing, led to the loss of > 90% of Macrocystis pyrifera from 350 km of coastline in northern California (Rogers-Bennett & Catton, 2019).

Under experimental conditions, Nepper-Davidson et al. (2019) exposed a northern (Denmark) population of Saccharina latissima to a simulated three-week heatwave of three different intensities; 18, 21 and 24°C. When exposed to heatwaves of 18 and 21°C there was a decrease in photosynthesis and growth. When a 24°C was simulated, 91% of sporophytes were dead within a week, and the fronds of the few survivors were disintegrating, so the experiment was terminated (Nepper-Davidsen et al., 2019). These results suggest that this species is unlikely to survive heatwaves of the length and magnitude predicted by the end of this century for both the middle and high emission scenarios in the southern UK.

Laminaria hyperborea is a cold-temperate species of kelp with an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). Germination success can decrease by almost two thirds at temperatures as low as 17°C. Therefore, it is expected that similar to other kelp species, Laminaria hyperborea will be highly sensitive to marine heatwaves.

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Halidrys siliquosa has an upper survival limit of 25°C, therefore Halidrys siliquosa might be able to tolerate a heatwave of this magnitude, but reproduction could be impacted. However, Laminaria hyperborea and Saccharina latissima are unlikely to survive a heatwave of this magnitude and likely to suffer severe mortality in the south. Although, In Scotland, where these biotopes occur, temperatures are not predicted to rise above 20°C, and therefore, Laminaria hyperborea and Saccharina latissima are likely to survive a heatwave of this magnitude. Nevertheless, the southern assemblages may be impacted. However, as Halidrys siliquosa, the main characteristic of the biotope will probably survive, under this scenario, resistance is assessed as ‘Medium’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Medium.’ This biotope IR.MIR.KT.XKT is assessed as having ‘Medium’ sensitivity to marine heatwaves under the middle emission scenario

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England. Halidrys siliquosa, Laminaria hyperborea and Saccharina latissima are unlikely to survive a heatwave of this magnitude, as temperatures are likely to reach > 21°C in Scotland under this scenario, there is likely to be mortality throughout these species’ UK biogeographic distribution. Therefore, resistance has been assessed as ‘Low’. As a further heatwave is likely to affect this habitat before full recovery (under the pressure benchmark definition), resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

Increasing levels of CO2 in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on emission scenario. Marine autotrophs will generally benefit from ocean acidification, though an increase in the availability of aqueous COfor photosynthesis (Koch et al., 2013). 

Halidrys siliquosa occurs in the sublittoral and in rockpools (Lima et al., 2009). The physicochemical conditions of rockpools vary considerably, with different parameters than the main body of the sea (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993; Metaxas et al., 1994). In rockpools, there are daily and seasonal fluctuations in pH levels (Daniel & Boyden, 1975), which suggests Halidrys siliquosa has some tolerance to fluctuating pH levels. 

Research on most kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012, Fernández et al., 2015, Nunes et al., 2015, Iñiguez et al., 2016b, a), except for one study, that found ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Laminaria hyperborea appears to be undersaturated in respect to carbon dioxide, although they can generally utilise HCO3 and have external carbonic anhydrase for extracellular dehydration of HCO3 to CO2 (Koch et al., 2013). This was confirmed for Laminaria hyperborea by Olischläger et al. (2012) who found that ocean acidification at levels expected for the end of this century (700 µatm CO2; a value between the middle and high emission scenario) led to an increase in female gametogenesis and increasing net photosynthesis and growth of sporophytes.

Under experimental COenrichment at levels expected by the end of this century, germination rates of Saccharina latissima were the same as control samples but gametophyte size increased, suggesting a benefit for juvenile stages of this species (Roleda et al., 2012). Nunes et al. (2015) found that experimental exposure of adult Saccharina latissima to enhanced CO2 led to an increase in net primary production, whilst Gordillo et al. (2015) found that enhanced CO2 led to increased photosynthesis and growth. In contrast, Iñiguez et al. (2016) found no increase in carbon fixation under elevated CO2 conditions. Whilst contrasting in findings, these studies suggest that ocean acidification will not negatively impact Saccharina latissima

Studies on the impacts of ocean acidification on the calcareous tube structure of serpulid polychaete worms observed reductions of tube elongation, elasticity and strength when exposed to reduced pH conditions (Chan et al.,2012; Li et al., 2014; Dıáz-Castanẽda et al., 2019). Dıáz-Castanẽda et al (2019) observed pH to affect trochophore size and post-settlement tube growth, with post-settlement tubes half the size of those at current ocean pH levels. Li et al (2014) observed changes to the structure, volume and density of the tubes, with reductions in hardiness and elasticity, in addition to tube crushing force reduced by 64%. The reduction in tube size and hardiness could impact the survival of the species with increased predation and reduction in the ability to withstand wave force (Chan et al., 2012; Li et al., 2014). 

Sensitivity assessment. Kelp forests live in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Laminaria hyperborea and Saccharina latissima are not expected to be impacted by ocean acidification at levels predicted for the end of this century. As Halidrys siliquosa can occupy rockpools where pH fluctuation occurs regularly, Halidrys siliquosa is not expected to be impacted by ocean acidification. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’

High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

Increasing levels of CO2 in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on emission scenario. Marine autotrophs will generally benefit from ocean acidification, though an increase in the availability of aqueous COfor photosynthesis (Koch et al., 2013). 

Halidrys siliquosa occurs in the sublittoral and in rockpools (Lima et al., 2009). The physicochemical conditions of rockpools vary considerably, with different parameters than the main body of the sea (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993; Metaxas et al., 1994). In rockpools, there are daily and seasonal fluctuations in pH levels (Daniel & Boyden, 1975), which suggests Halidrys siliquosa has some tolerance to fluctuating pH levels. 

Research on most kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012, Fernández et al., 2015, Nunes et al., 2015, Iñiguez et al., 2016b, a), except for one study, that found ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Laminaria hyperborea appears to be undersaturated in respect to carbon dioxide, although they can generally utilise HCO3 and have external carbonic anhydrase for extracellular dehydration of HCO3 to CO2 (Koch et al., 2013). This was confirmed for Laminaria hyperborea by Olischläger et al. (2012) who found that ocean acidification at levels expected for the end of this century (700 µatm CO2; a value between the middle and high emission scenario) led to an increase in female gametogenesis and increasing net photosynthesis and growth of sporophytes.

Under experimental COenrichment at levels expected by the end of this century, germination rates of Saccharina latissima were the same as control samples but gametophyte size increased, suggesting a benefit for juvenile stages of this species (Roleda et al., 2012). Nunes et al. (2015) found that experimental exposure of adult Saccharina latissima to enhanced CO2 led to an increase in net primary production, whilst Gordillo et al. (2015) found that enhanced CO2 led to increased photosynthesis and growth. In contrast, Iñiguez et al. (2016) found no increase in carbon fixation under elevated CO2 conditions. Whilst contrasting in findings, these studies suggest that ocean acidification will not negatively impact Saccharina latissima

Studies on the impacts of ocean acidification on the calcareous tube structure of serpulid polychaete worms observed reductions of tube elongation, elasticity and strength when exposed to reduced pH conditions (Chan et al.,2012; Li et al., 2014; Dıáz-Castanẽda et al., 2019). Dıáz-Castanẽda et al (2019) observed pH to affect trochophore size and post-settlement tube growth, with post-settlement tubes half the size of those at current ocean pH levels. Li et al (2014) observed changes to the structure, volume and density of the tubes, with reductions in hardiness and elasticity, in addition to tube crushing force reduced by 64%. The reduction in tube size and hardiness could impact the survival of the species with increased predation and reduction in the ability to withstand wave force (Chan et al., 2012; Li et al., 2014). 

Sensitivity assessment. Kelp forests live in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Laminaria hyperborea and Saccharina latissima are not expected to be impacted by ocean acidification at levels predicted for the end of this century. As Halidrys siliquosa can occupy rockpools where pH fluctuation occurs regularly, Halidrys siliquosa is not expected to be impacted by ocean acidification. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’

Medium Very Low Medium
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al. 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and have adapted to low-light conditions (Gerard, 1990).

This biotope (IR.HIR.KSed.XKHal) occurs on moderately wave exposed infralittoral bedrock, boulders or cobbles with coarse sediment from 0-20m (JNCC, 2015). Understanding how sea-level rise will affect tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Although the distribution of Laminaria hyperborea is positively correlated with wave exposure (Pedersen et al., 2012), and Saccharina latissima is abundant at both turbid and deep sites (Gerard, 1990), the biotopes characterizing species Halidrys siliquosa could be impacted. As an increase in water flow may remove the fine sand sediment and favour larger kelps typical of stable bedrock biotopes, a reduced abundance of Halidrys siliquosa would result in the loss of this biotope. An increase in wave exposure, from moderately exposed to exposed is likely to increase the level of scour, resulting in a change in the biotope and its potential replacement by IR.HIR.KSwed.XKScR. Similarly, a decrease in wave exposure from exposed to sheltered conditions would reduce scour and favour more sheltered kelp dominated biotope, e.g. IR.MIR.KT.XKT (Connor et al., 2004).

Sensitivity assessment.  The biotope is recorded from 0 to 20 m in depth (JNCC, 2015). This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is probable that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-20 m in depth, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance may be ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al. 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and have adapted to low-light conditions (Gerard, 1990).

This biotope (IR.HIR.KSed.XKHal) occurs on moderately wave exposed infralittoral bedrock, boulders or cobbles with coarse sediment from 0-20m (JNCC, 2015). Understanding how sea-level rise will affect tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Although the distribution of Laminaria hyperborea is positively correlated with wave exposure (Pedersen et al., 2012), and Saccharina latissima is abundant at both turbid and deep sites (Gerard, 1990), the biotopes characterizing species Halidrys siliquosa could be impacted. As an increase in water flow may remove the fine sand sediment and favour larger kelps typical of stable bedrock biotopes, a reduced abundance of Halidrys siliquosa would result in the loss of this biotope. An increase in wave exposure, from moderately exposed to exposed is likely to increase the level of scour, resulting in a change in the biotope and its potential replacement by IR.HIR.KSwed.XKScR. Similarly, a decrease in wave exposure from exposed to sheltered conditions would reduce scour and favour more sheltered kelp dominated biotope, e.g. IR.MIR.KT.XKT (Connor et al., 2004).

Sensitivity assessment.  The biotope is recorded from 0 to 20 m in depth (JNCC, 2015). This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is probable that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-20 m in depth, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance may be ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al. 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and have adapted to low-light conditions (Gerard, 1990).

This biotope (IR.HIR.KSed.XKHal) occurs on moderately wave exposed infralittoral bedrock, boulders or cobbles with coarse sediment from 0-20m (JNCC, 2015). Understanding how sea-level rise will affect tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Although the distribution of Laminaria hyperborea is positively correlated with wave exposure (Pedersen et al., 2012), and Saccharina latissima is abundant at both turbid and deep sites (Gerard, 1990), the biotopes characterizing species Halidrys siliquosa could be impacted. As an increase in water flow may remove the fine sand sediment and favour larger kelps typical of stable bedrock biotopes, a reduced abundance of Halidrys siliquosa would result in the loss of this biotope. An increase in wave exposure, from moderately exposed to exposed is likely to increase the level of scour, resulting in a change in the biotope and its potential replacement by IR.HIR.KSwed.XKScR. Similarly, a decrease in wave exposure from exposed to sheltered conditions would reduce scour and favour more sheltered kelp dominated biotope, e.g. IR.MIR.KT.XKT (Connor et al., 2004).

Sensitivity assessment.  The biotope is recorded from 0 to 20 m in depth (JNCC, 2015). This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is probable that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-20 m in depth, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance may be ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

Hydrological Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Medium Medium Medium
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Lüning (1984, 1990) reported an upper survival temperature of 25°C after one week exposure in Halidrys siliquosa. It did not survive at the higher temperatures studied. Zygote germination and growth are temperature dependent. Moss & Sheader (1973) reported 50-97% germination success at 3 and 10°C, falling considerably to 8-52% at 20°C and to zero at 22°C. Growth increased with temperature up to 20°C but germlings developed abnormally at 20°C (Moss & Sheader, 1973; Tyler-Walters, 2002).

Halidrys siliquosa is distributed from northern Norway to northern Portugal and also occurs in rock pools, which may experience a relatively large temperature range. Therefore, it is unlikely to be affected by long-term temperature changes within the British Isles. Short-term acute change may have adverse effects if the change increased the temperature over 20-25°C, especially if the change coincided with the release of gametes or the germination of zygotes. However, Halidrys siliquosa releases gametes and zygotes in the winter months (December to March).

Kain (1964) observed Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0 - 20°C. Upper and lower lethal temperatures have been estimated at between 1-2°C above or below the extremes of this range (Birkett et al., 1988). Above 17°C gamete survival is reduced (Kain, 1964 & 1971) and gametogenesis is inhibited at 21°C (Dieck, 1992). It is, therefore, likely that Laminaria hyperborea recruitment will be impaired at a sustained temperature increase of above 17°C. Sporophytes, however, can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998).

The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima geographic distribution (Müller et al., 2009). Gametophytes can develop in ≤23°C (Lüning, 1990) however the optimal temperature range for sporophyte growth is 10-15°C (Bolton & Lüning, 1982). Bolton & Lüning (1982) experimentally observed that sporophyte growth was inhibited by 50-70% at 20°C and following 7 days at 23°C all specimens completely disintegrated. In the field Saccharina latissima has shown significant regional variation in its acclimation to temperature changes, for example Gerard & Dubois (1988) observed sporophytes of Saccharina latissima which were regularly exposed to ≥20°C could tolerate these temperatures whereas sporophytes from other populations which rarely experience ≥17°C showed 100% mortality after 3 weeks of exposure to 20°C. Therefore, the response of Saccharina latissima to a change in temperatures is likely to be locally variable.

Sensitivity assessment. IR.HIR.Ksed.XKHal is distributed throughout the UK (Connor et al., 2004). Northern to southern Sea Surface Temperature (SST) ranges from 8-16 oC in summer and 6-13°C in winter (Beszczynska-Möller & Dye, 2013). The effect of this pressure is likely to be regionally variable. An increase in 5°C may increase winter temperatures to above 10°C, which may negatively affect, but would not inhibit, Halidrys siliquosa germination. Halidrys siliquosa is probably tolerant to changes of temperature in British waters. A chronic change (2°C for a year) outside the normal UK temperature range for a year may reduce Laminaria hyperborea recruitment and growth, and cause mass mortality of Saccharina lattissma ecotypes which are not acclimated to similar temperatures.  Therefore, resistance has been assessed as ‘Medium’, Resilience as ‘Medium’. Sensitivity has been assessed as ‘Medium’.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Little information concerning the effects of low temperatures on Halidrys siliquosa was found. Lüning (1984, 1990) reported that it survived at 0°C for one week, and Moss & Sheader (1973) reported that the lower limit of germination was not reached at 3°C but that no gametes were released from fertile receptacles at -4°C. Overall, Halidrys siliquosa is recorded from northern Norway and is probably tolerant to decreases of temperature likely to occur in British waters (Tyler-Walters, 2002).

Moss & Shreader (1973) observed experimentally that Halidrys siliquosa germination was not affected at 3-10⁰C. The lower temperature limit for Halidrys siliquosa germination was not tested however receptacles kept at -4⁰C bore no viable gametes, and, therefore, the lower temperature threshold is likely to be between -4⁰C and 3⁰C.

Laminaria hyperborea and Saccharina lattissima have boreal distributions throughout the arctic. Saccharina lattissima and Laminaria hyperborea have lower temperature thresholds for sporophyte growth at 0°C (Kain 1964; Lüning, 1990). Subtidal red algae can survive at -2°C (Lüning, 1990; Kain & Norton, 1990). These temperatures are well below that considered within this pressure benchmark.

Sensitivity assessment. Northern to southern Sea Surface Temperature (SST) ranges from 8-16°C in summer and 6-13 °C in winter (Beszczynska-Möller & Dye, 2013). Both a long-term and acute temperature decrease of 2-5°C combined with low winter temperatures are considered unlikely to have a significant effect on Halidrys siliquosa or Laminariales and is, therefore, unlikely to have a significant effect on IR.HIR.Ksed.XKHal. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

Not relevant (NR) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

There was limited information on the upper and lower salinity tolerances of Halidrys siliquosa. However, Halidrys siliquosa can be found growing in rock pools (Moss, 1982). High air temperatures cause surface evaporation of water from rock pools so that salinity steadily increases. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). It is, therefore, likely that Halidrys siliquosa can tolerate hypersaline conditions, at least in the short-term however the long-term effects of hypersalinity are unknown.

Lüning (1990) suggest that “kelps” are stenohaline, their general tolerance to salinity as a phenotypic group covering 16-50 PSU over a 24 hr period. Optimal growth probably occurs between 30-35 PSU (MNCR category-Full Salinity) and growth rates are likely to be affected by periodic salinity stress. Birkett et al. (1998) suggested that long-term increases in salinity may affect Laminaria hyperborea growth and may result in loss of affected kelp.

Karsten (2007) tested the photosynthetic ability of Saccharina latissima under acute 2 and 5 day exposure to salinity treatments ranging from 5-60 psu. A control experiment was also carried at 34 psu. Saccharina latissima showed high photosynthetic ability at >80% of the control levels between 25-55 PSU. The affect of long-term salinity changes (>5 days) or salinity >60 PSU on Saccharina latissima’ photosynthetic ability was not tested.

Sensitivity assessment. The evidence suggests that Halidrys siliquosa and Saccharina latissima can tolerate short exposure to hypersaline conditions of ≥40‰ (MNCR full salinity range=30-40‰), and Laminaria hyperborea may be more affected at long-term salinity increases. However, there is insufficient information on the hypersaline tolerance of Halidrys siliquosa to assess this pressure for IR.HIR.KSed.XKHal.

Medium Medium Medium
Q: Medium
A: High
C: High
Q: Medium
A: Low
C: High
Q: Medium
A: High
C: High

There was limited information on the upper and lower salinity tolerances of Halidrys siliquosa. However, Halidrys siliquosa has been recorded growing at 28‰ in Limfjorden, Denmark (Pederson et al., 2005) but is absent further into the Baltic sea where average salinity is 7.4‰ (Meier & Kauker, 2003). Halidrys siliquosa can also be found growing in rock pools (Moss, 1982) Freshwater runoff and rain can cause a dilution effect within rock pools so that salinity steadily decreases. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air (Pyefinch, 1943). It is, therefore, likely that Halidrys siliquosa can tolerate hyposaline conditions, at least in the short-term, however, the long-term effects of hyposalinity are unknown.

Hopkin & Kain (1978) tested Laminaria hyperborea sporophyte growth at various low salinity treatments. Laminaria hyperborea sporophytes could grow “normally” at 19 psu, growth was reduced at 16 psu and did not grow at 7 psu. A decrease in one MNCR salinity scale from 'Full' salinity (30-40psu) to 'Reduced' salinity (18-30 PSU) would result in a decrease of Laminaria hyperborea sporophyte growth.

Karsten (2007) tested the photosynthetic ability of Saccharina latissima under acute 2 and 5 day exposure to salinity treatments ranging from 5-60 psu. A control experiment was also carried at 34 PSU. Saccharina latissima showed high photosynthetic ability at >80% of the control levels between 25-55 PSU. Hyposaline treatment of 10-20 PSU led to a gradual decline of photosynthetic ability. After 2 days at 5 PSU Saccharina latissima showed a significant decline in photosynthetic ability at approx. 30% of control. After 5 days at 5 PSU Saccharina latissima specimens became bleached and showed signs of severe damage. The effect of long-term salinity changes (>5 days) or salinity >60 PSU on Saccharina latissima’ photosynthetic ability was not tested. The experiment was conducted on Saccharina latissima from the Arctic, and at extremely low water temperatures (1-5°C) macroalgae acclimation to rapid salinity changes could be slower than at temperate latitudes. It is, therefore, possible that resident Saccharina latissima of the UK maybe be able to acclimate to salinity changes more effectively.

Sensitivity assessment. Halidrys siliquosa may tolerate minor decreases in salinity below 30-40‰ however Halidrys siliquosa absence from the Baltic sea combined with a lack of IR.HIR.KSed.XKHal records in “reduced salinity” (Connor et al., 2004) suggests a decrease in one MNCR salinity scale from “Full Salinity” (30-40 PSU) to “Reduced Salinity” (18-30 psu) may diminish the abundance of Halidrys siliquosa in IR.HIR.KSed.XKHal. Similarly, Laminariales would likely tolerate minor reductions in salinity however at the lower end of the “reduced salinity” category (e.g. 18-25psu) photosynthesis and growth may be inhibited. Resistance has been assessed as ‘None’, as the biotope would be lost if the abundance of Halidry siliquosa was reduced significantly.  Resilience is probably ‘Medium’. The sensitivity of this biotope to a decrease in salinity has been assessed as ‘Medium’.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

IR.HIR.KSed.XKHal is found from 1.5m/sec-weak tidal streams. Halidrys siliquosa decreases in abundance with increasing water flow, so that in tidal rapids with current speeds of 2-3 m/sec, it is replaced by Laminaria digitata, Laminaria hyperborea and Saccorhiza polyschides communities (Lewis, 1964; Schwenke, 1971). However, an increase of 0.1-0.2 m/s would not likely have a significant effect on IR.HIR.KSed.XKHal.

Peteiro & Freire (2013) measured Saccharina latissima growth from 2 sites, the first had maximal water velocities of 0.3m/sec and the second 0.1m/sec. At site 1 Saccharina latissima had significantly larger biomass than at site 2 (16  kg/m to 12 kg/m respectively). Peteiro & Freire (2013) suggested that faster water velocities were beneficial to Saccharina latissima growth. However, Gerard & Mann (1979) measured Saccharina latissima productivity at greater water velocities and found Saccharina latissima productivity is reduced in moderately strong tidal streams (≤1m/sec) when compared to weak tidal streams (<0.5m/sec).

However, changes in the water flow are likely to alter the sedimentation regime and increase or decrease scour. An increase in water flow may remove the fine sand sediment and favour larger kelps typical of stable bedrock biotopes, a reduced abundance of Halidrys siliquosa , and result in loss of this biotope. A decrease in water flow may allow greater deposition of fine sediments, and hence increases scour, so that the biotope would be potentially replaced by IR.HIR.Ksed.XKScR. Sediment transport processes are influenced by a range of site-specific factors including local sediment supply and topography. A generic assessment is not possible and this indirect effect is not assessed for this pressure, although the siltation and changes in sediment type pressures indicate sensitivity to habitat changes. It should be noted also that wave action may also be a contributory factor with local tidal currents for sediment transport in this biotope.

Sensitivity assessment. An increase and/or decrease of 0.2 m/s is not likely to be significant in examples of IR.HIR.KSed.XKHal from moderate flow (0.5-1.5 m/s). A reduction in flow by 0.2 m/s may however significantly reduce flow in examples of IR.HIR.KSed.XKHal from weak flow. A decrease in tidal flow may decrease sediment scour and increase sediment stability, which may, therefore, facilitate kelps to dominate and change the biotope. A change of 0.1m/s to 0.2m/s is not likely to dramatically affect IR.HIR.KSed.XKHal structure. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

Medium Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Low
C: High
Q: Low
A: Low
C: Low

IR.HIR.KSed.XKHal is recorded from 0-30m Below Chart Datum (BCD) (Connor et al., 2004). Shallow (0-5m) examples of this biotope may be affected by changes in emergence. An increase in emergence will increase exposure of the biotope to air and hence may increase desiccation. Therefore, the upper extent of several species within the biotope, most notably Halidrys siliquosa, Saccharina latissima and Laminaria hyperborea and hence the upper extent of the biotope is likely to be reduced. IR.MIR.KT.LdigT is typically found in the sublittoral fringe, at a higher tidal elevation to IR.HIR.KSed.XKHal. If IR.HIR.KSed.XKHal was elevated IR.MIR.KT.LdigT may replace IR.HIR.KSed.XKHal. Providing suitable substrata are present, IR.HIR.KSed.XKHal is likely to re-establish further down the shore within a similar emergence regime to that which existed previously.

Sensitivity assessment. Resistance has been assessed as ‘Medium’. Resilience as ‘Medium’. The sensitivity of this biotope to a change in emergence is considered as ‘Medium’.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

IR.HIR.KSed.XKHal occurs at extremely-moderately wave exposed sites (Connor et al., 2004). An increase in wave exposure, from e.g. moderately exposed to exposed is likely to increase the level of scour, resulting in a change in the biotope and its potential replacement by IR.HIR.KSwed.XKScR. Similarly, a decrease in wave exposure from exposed to sheltered conditions would reduce scour and favour more sheltered kelp dominated biotope, e.g. IR.MIR.KT.XKT (Connor et al., 2004). However, an increase in nearshore significant wave height of 3-5% is not likely to have a significant effect on biotope structure.

Sensitivity assessment. Resistance has been assessed as ‘High’, Resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’ at the benchmark level.

Chemical Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This pressure is Not assessed but evidence is presented where available

Holt et al. (1995, 1997) reported that fucoids and other algae were capable of retaining and concentrating heavy metals, so much so that Fucus spp. are used as indicators of heavy metal pollution. Alginates found in fucoids (and in Halidrys siliquosa) strip heavy metals and some radionuclides from seawater and store them in inert forms. Hence, adult plants are considered to be relatively tolerant of heavy metal contamination. However, younger stages may be more intolerant. For example iron ore dust interfered with the interaction between eggs and sperm in Fucus serratus (Boney, 1980; cited in Bryan, 1984). Bryan (1984) also reported that heavy metals retarded growth in brown algae and suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. Heavy metals have been shown to effects on sporophyte development, growth and respiration in Laminaria hyperborea (Hopkin & Kain, 1978) and in Laminaria digitata (Axelsson & Axelsson, 1987).

Cole et al. (1999) suggested that Cd was very toxic to Crustacea (amphipods, isopods, shrimp, mysids and crabs), and Hg, Cd, Pb, Cr, Zn, Cu, Ni, and As were very toxic to fish. Bryan (1984) reported sub-lethal effects of heavy metals in crustaceans at low (ppb) levels. Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. short-term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se were the least toxic. However, he suggested that gastropods were relatively tolerant of heavy metal pollution.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This pressure is Not assessed but evidence is presented where available.

IR.HIR.KSed.XKHal is protected from the direct effects of oil spills due to its subtidal habit, although it may be exposed to water soluble components of the oil or oil adsorbed on to particulates. No information concerning the effects of oil on Halidrys siliquosa was found. However, Holt et al. (1997) suggested that other fucoids, e.g. Fucus sp. had limited intolerance to oil but noted that studies on long-term exposure were limited. Saccharina latissima (studied as Laminaria saccharina) was observed to show no discernible effects from oil spills, largely due to poor dispersion into the water column and high levels of dilution (Holt et al., 1995).

O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination, possibly due to the susceptibility of phycoerythrins to destruction. Laboratory studies of the effects of oil and dispersants on several red algal species, including Delesseria sanguinea and Plocamium cartilagineum, concluded that they were all sensitive to oil/ dispersant mixtures, with little difference between adults, sporelings, diploid or haploid life stages (Grandy, 1984; cited in Holt et al., 1995). long-term effects of continuous doses of the water accommodated fraction (WAF) of diesel oil were determined in experimental mesocosms (Bokn et al., 1993). Mean hydrocarbon concentrations tested were 30.1 µg/l and 129.4 µg/l. After 2 years, there were no demonstrable differences in the abundance patterns of Chondrus crispus. Kaas (1980; cited in Holt et al., 1995) reported that the reproduction of adult Chondrus crispus plants on the French coast was normal following the Amoco Cadiz oil spill. However, it was suggested that the development of young stages to adult plants was slow, with biomass still reduced 2 years after the event. O'Brien & Dixon (1976) also noted that hydrocarbon exposure reduced photosynthesis in algae.

Oil spills and hydrocarbon exposure in the intertidal results in loss of gastropod or crustacean grazers (Southward, 1982; Suchanek, 1993). Loss of grazers may allow development of more ephemeral green algae and a change in the algal community. However, although Bokn et al. (1993) could not demonstrate direct effects of chronic hydrocarbon contamination in their mesocosms, they concluded that chronic effects of oil on Littorina littorea and perhaps other herbivores may require more than 2 years to develop.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This pressure is Not assessed but evidence is presented where available.

Fucoids, are generally quite robust in terms of chemical pollution (Holt et al., 1995, 1997), e.g. Fucus sp. seem to thrive in TBT-polluted waters (Bryan & Gibbs, 1991). However, Rosemarin et al. (1994) stated that brown algae (Phaeophycota) were extraordinarily intolerant of chlorate, such as from pulp mill or brine electrolysis effluents (Holt et al., 1997). O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination, possibly due to the susceptibility of phycoerythrins to destruction. They also reported that red algae are effective indicators of detergent damage since they undergo colour changes when exposed to relatively low concentration of detergent. Smith (1968) reported that 10 ppm of the detergent BP 1002 killed the majority of specimens in 24hrs in toxicity tests, although Ahnfeltia plicata and Chondrus crispus were amongst the algal species least affected by the detergent used to clean up the Torrey Canyon oil spill. Laboratory studies of the effects of oil and dispersants on several red algal species, including Plocamium cartilagineum, concluded that they were all sensitive to oil/ dispersant mixtures, with little difference between adults, sporelings, diploid or haploid life stages (Grandy, 1984; cited in Holt et al., 1995). Cole et al. (1999) suggested that herbicides in urban or agricultural runoff, such as simazine and atrazine, were very toxic to macrophytes. Hoare & Hiscock (1974) noted that all red algae except Phyllophora sp. were excluded from Amlwch Bay, Anglesey, by acidified halogenated effluent discharge. The evidence suggests that in general red algae are very intolerant of synthetic chemicals. Crustacean members of the fauna (mesoherbivores) are likely to be intolerant of pesticides, such as ivermecten, dichlorvos and synthetic pythrethroids (Cole et al., 1999), the exact toxicity varying with location (concentration) and species. Ascidian larval stages were reported to be intolerant of TBT (Mansueto et al., 1993 cited in Rees et al., 2001). Rees et al. (1999; 2001) reported that the epifauna of the inner Crouch estuary had largely recovered within 5 years (1987-1992) after the ban on the use of TBT on small boats in 1987. Increases in the abundance of Ascidiella sp. and Ciona intestinalis were especially noted.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No evidence was found.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This pressure is Not assessed.

High High Not sensitive
Q: Medium
A: High
C: High
Q: High
A: High
C: High
Q: Medium
A: High
C: High

Reduced oxygen concentrations can inhibit both photosynthesis and respiration in macroalgae (Kinne, 1977). Despite this, macroalgae are thought to buffer the environmental conditions of low oxygen, thereby acting as a refuge for organisms in oxygen depleted regions especially if the oxygen depletion is short-term (Frieder et al., 2012). A rapid recovery from a state of low oxygen is expected if the environmental conditions are transient. If levels do drop below 4 mg/l negative effects on these organisms can be expected with adverse effects occurring below 2mg/l (Cole et al., 1999).

Sensitivity Assessment. Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly. Resistance has been assessed as ‘High’, Resilience as ‘High’. Sensitivity has been assessed as ‘Not sensitive’ at the benchmark level.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This biotope is considered ‘Not sensitive’ at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Wernberg et al. (2001) reported that the N:P (nitrogen to phosphorus) ratio in Limfjorden Halidrys siliquosa was low in summer and high in spring, which suggested that growth was nutrient limited by P in spring and N in summer. Kindig & Littler (1980) exposed Halidrys dioica and other algae to 10% untreated sewage effluent in the field, which resulted in increased gross productivity. However, Halidrys dioica was found to be absent in the vicinity of a sewage outfall, and Kindig & Littler (1980) concluded that another component of the effluent, other than nutrient, was responsible. Overall, therefore, it would appear that moderate nutrient enrichment at the benchmark level may stimulate growth of Halidrys spp. However, excessive enrichment may lead to eutrophication, decreased oxygen levels (see relevant pressure) and the potential smothering of Halidrys sp. by microfloral epiphytes) (Tyler-Walters, 2002).

Bokn et al. (2003) conducted a nutrient loading experiment on intertidal fucoids. Within 3 years of the experiment no significant effect was observed in the communities, however, 4-5 years into the experiment a shift occurred from perennials to ephemeral algae. The results within Bokn et al. (2003) could indicate that long-term (>4 years) nutrient loading can result in community shift to ephemeral algae species.

Conolly & Drew (1985) found Saccharina latissimasporophytes had relatively higher growth rates when in close proximity to a sewage outlet in St Andrews, UK when compared to other sites along the east coast of Scotland. At St Andrews nitrate levels were 20.22 µM, which represents an approx 25% increase when compared to other comparable sites (approx 15.87 µM). Handå et al. (2013) also reported Saccharina latissima sporophytes grew approx 1% faster per day when in close proximity to Norwegian Salmon farms, where elevated ammonium could be readily absorbed by sporophytes.  Read et al. (1983) reported after the installation of a new sewage treatment  works which reduced the suspended solid content of liquid effluent by 60% in the Firth of Forth, Saccharina latissima became abundant where previously it had been absent.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) however also highlighted that macroalgal communities are relatively tolerant to contamination, but that contaminated communities can have low diversity assemblages which are dominated by opportunistic and fast growing species (Johnston & Roberts, 2009 and references therein). Organic enrichment may also result in phytoplankton blooms that increase turbidity and, therefore, may negatively impact photosynthesis.

Medium Medium Medium
Q: Medium
A: Medium
C: High
Q: Medium
A: Medium
C: High
Q: Medium
A: Medium
C: High

Bokn et al. (2003) conducted a nutrient loading experiment on intertidal fucoids. Within 3 years of the experiment, no significant effect was observed in the communities, however, 4-5 years into the experiment a shift occurred from perennials to ephemeral algae.

Conolly & Drew (1985) found Saccharina latissima sporophytes had relatively higher growth rates when in close proximity to a sewage outlet in St Andrews, UK when compared to other sites along the east coast of Scotland. Read et al. (1983) reported after the installation of a new sewage treatment works which reduced the suspended solid content of liquid effluent by 60% in the Firth of Forth, Saccharina latissima became abundant where previously it had been absent.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) however also highlighted that macroalgal communities are relatively tolerant to contamination, but that contaminated communities can have low diversity assemblages which are dominated by opportunistic and fast growing species (Johnston & Roberts, 2009 and references therein). Organic enrichment may also result in phytoplankton blooms that increase turbidity and, therefore, may negatively impact photosynthesis.

Sensitivity assessment. Conflicting evidence suggests that organic enrichment does not directly negatively affect macro-algae, however, organic enrichment could increase water turbidity and long-term exposure could cause a shift from perennial to ephemeral algae. Resistance has been assessed as ‘Medium’ due to potential changes in community composition, resilience as ‘Medium’. Sensitivity has been assessed as ’Medium’.

Physical Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

If rock substrata were replaced with sedimentary substrata this would represent a fundamental change in habitat type, which macro-algae would not be able to tolerate. The biotope would be lost.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very low’ or ‘None’. The sensitivity of this biotope to change to a sedimentary or soft rock substrata or artificial substrata is assessed as ‘High’.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant to rock substrata.

None Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: High
C: High
Q: Low
A: Low
C: Low

The biotope is characterized by abrasion in the form of scour from mobile coarse sediment, so that members of the community are probably resistant to or recover quickly from low level abrasion (e.g. potting) But abrasion of the substratum e.g. from bottom fishing gear, cable laying etc. may cause localised mobility of the substrata (e.g. pebbles, cobbles and boulders) and mortality of the resident community. The effect would be situation dependent however if bottom fishing gear were towed over a site it may mobilise a high proportion of the rock substrata and cause high mortality in the resident community.

No specific examples of anthropogenic abrasion could be found for this biotope. However, bottom fishing gear (e.g. scallop dredging) are known to cause high mortality in bycatch species by overturning sediment with resultant reductions in biodiversity (Bradshaw et al., 2001).

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘Medium’. Sensitivity has been assessed as ‘Medium’.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not Relevant, please refer to pressure “Abrasion/disturbance of the substratum on the surface of the seabed”.

Medium Medium Medium
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

Suspended Particle Matter (SPM) concentration has a negative linear relationship with sub surface light attenuation (Kd) (Devlin et al., 2008). Moss & Shreader (1973) noted that Halidrys siliquosa has comparatively large eggs (approximately 150µm in diameter) and may, therefore, have comparatively large energy stores, enabling germination and early growth to occur in total darkness. Moss & Shreader (1973) noted germling development could continue in darkness for a period of 40 days, beyond which development ceased. Germlings exposed to darkness for a period of 120 days but were returned to full light resumed normal growth, beyond 120 days of darkness the number of surviving germlings fell rapidly. However, light availability and water turbidity are principal factors in determining depth range at which macro-algae can be found (Birkett et al., 1998).

Light penetration influences the maximum depth at which Laminarians can grow and it has been reported that Laminarians grow at depths at which the light levels are reduced to 1 percent of incident light at the surface. Maximal depth distribution of Laminarians, therefore, varies from 100m in the Mediterranean to only 6-7m in the silt laden German Bight. In Atlantic European waters, the depth limit is typically 35m. In very turbid waters the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al. 1998). Laminarians show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient =0.1-0.2/m; Staehr & Wernberg, 2009).

Sensitivity assessment. An increase in water clarity from clear to intermediate (10-100 mg/l) represent a change in light attenuation of ca 0.67-6.7 Kd/m, and is likely to result in a greater than 50% reduction in photosynthesis of Laminaria spp. Therefore, the dominant kelp species will probably suffer a severe decline and resistance to this pressure is assessed as Medium at the benchmark. Resilience is probably Medium, hence, this biotope is regarded as having a sensitivity of Medium to this pressure.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Smothering by sediment e.g. 5 cm material during a discrete event, is unlikely to damage mature Halidrys siliquosa or Laminaria hyperborea and Saccharina latissima sporophytes but may provide a physical barrier to settlement and could therefore negatively impact recruitment processes (Moy & Christie, 2012). Due to their small size newly settled zoospores could be inundated with sediment, however, laboratory studies have shown that newly settled Halidrys siliquosa can survive and develop in darkness for 120 days (4.6 months) (Moss & Shreader, 1973) and kelp gametophytes for 6-16 months at 8°C (Dieck, 1993). However, IR.HIR.KSed.XKHal is recorded from moderately wave exposed sites (Connor et al., 2004). Deposited sediments are unlikely to remain for more than a few tidal cycles (due to water flow or wave action).

Sensitivity assessment. This biotope is characterized by scour by coarse sediments, and fine sediments are likely to be removed quickly. Therefore, resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’. Please note sedimentation could result in an increase in local sediment scour and/or de-oxygenation, please see relevant pressure sections for specific reviews.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Smothering by sediment e.g. 30 cm material during a discrete event, is unlikely to damage mature Halidrys siliquosa or Laminaria hyperborea and Saccharina latissima sporophytes but may provide a physical barrier to settlement and could therefore negatively impact on recruitment processes (Moy & Christie, 2012). Due to their small size newly settled zoospores could be inundated with sediment, however, laboratory studies have shown that newly settled Halidrys siliquosa can survive and develop in darkness for 120 days (4.6 months) (Moss & Shreader, 1973) and kelp gametophytes for 6-16 months at 8°C (Dieck, 1993). IR.HIR.KSed.XKHal is recorded from extreme-moderately wave exposed sites (Connor et al., 2004). Deposited sediment are unlikely to remain for more than a few tidal cycles (due to water flow or wave action). However, IR.HIR.KSed.XKHal is recorded from extreme-moderately wave exposed sites (Connor et al., 2004). Deposited sediment are unlikely to remain for more than a few tidal cycles (due to water flow or wave action).

Sensitivity assessment. This biotope is characterized by scour by coarse sediments, and fine sediments are likely to be removed quickly. Therefore, resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’. Please note sedimentation could result in an increase in local sediment scour and/or de-oxygenation, please see relevant pressure sections for specific reviews.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not assessed

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No evidence

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant

Low Medium Medium
Q: Low
A: NR
C: NR
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

There is no evidence to suggest that anthropogenic light sources would affect macro-algae. Shading (e.g. by the construction of a pontoon, pier etc) could adversely affect IR.HIR.KSed.XKHal in areas where the water clarity is also low, and tip the balance to shade tolerant species, resulting in the loss of the biotope directly within the shaded area, or a reduction in seaweed abundance.

Moss & Shreader (1973) noted throughout experimentation that Halidrys siliquosa germination was not affected by darkness, however also commented that Halidrys siliquosa has comparatively large eggs (approximately 150µm in diameter) and may, therefore, have comparatively large energy stores, enabling germination and early growth to occur in total darkness. Moss & Shreader (1973) noted germling development could continue in darkness for a period of 40 days, beyond which development ceased. Germlings exposed to darkness for a period of 120 days but were returned to full light resumed normal growth, beyond 120 days of darkness the number of surviving germlings fell rapidly.

Sensitivity assessment.  Resistance is probably 'Low', with a 'Medium' resilience and a sensitivity of 'Medium', albeit with 'low' confidence due to the lack of direct evidence.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant. This pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of spores.  But spore dispersal is not considered under the pressure definition and benchmark.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant. Collision from grounding vessels is addressed under abrasion above.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant

Biological Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

At the time of writing there is no evidence of genetic modification and/or translocation of Halidrys siliquosa, Laminaria hyperborea or Saccharina latissima over significant geographic distances. The pressure is therefore considered ‘Not relevant’ to IR.HIR.KSed.XKHal.

Low Very Low High
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

The Invasive Non-Indigenous Species (INIS) Sargassum muticum was introduced to Europe in the last 30-40 years and has since become a permanent component of macroalgal communities (Pederson et al., 2005). Limfjorden, a shallow Danish fjord, has been used for numerous studies on the ecological impacts of Sargassum muticum on native macroalgae communities, with particular emphasis on Halidrys siliquosa (Staehr et al., 2000; Wernberg et al., 2004; Pedersen et al., 2005). First introduced to Limfjorden, Denmark in 1984, Sargassum muticum has since colonized approximately 35% of rocky substrata (between 0-6m BCD) and is associated with the local decline/replacement of native macroalgae species, including Halidrys siliquosa (Staehr et al., 2000; Wernberg et al., 2004; Pederson et al., 2005; Engelen et al., 2015). Wernberg et al. (2004) observed seasonal variation. However, the macroalgal epibiotic communities of Limfjorden, Denmark were not significantly affected by the introduction of Sargassum muticum, and may even have increased their local abundance. Thus, the introduction of Sargassum muticum to Limfjorden, Denmark has resulted in a dramatic decline of resident Halidrys siliquosa and fundamentally changed the biotope structure (Pederson et al., 2005), although the associated epibiotic community was seemingly unaffected. Staehr et al. (2000) suggested that in the subtidal, Sargassum muticum can become overgrown and out-compete underlying species, such as Halidrys siliquosa, for resources including light, hard substrata and space (Macleod et al., 2016).

Undaria pinnatifida (Wakame or Asian kelp) is a large brown seaweed and an Invasive Non-Indigenous Species (INIS) that could out-compete native UK kelp species (see Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Hieser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Kraan, 2017; Epstein & Smale, 2018; Epstein et al., 2019a,b; Tidbury, 2020). Undaria pinnatifida originates from Japan but is established currently on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016) and has since proliferated along UK coastlines. One year after its discovery at the Queen Anne Battery marina, Plymouth, it had become a major fouling plant on pontoons (Minchin & Nunn, 2014). Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound.

Undaria pinnatifida seems to settle better on artificial substrata (e.g. floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018). James (2017) suggested that Undaria pinnatifida could out-compete native species on artificial substrata (such as marinas and wharf structures). De Leij et al. (2017) suggested that in natural substrata, Undaria pinnatifida can be inhibited by the presence of native competitors, such as large perennial species. Undaria pinnatifida species behaves as a winter annual and recruitment occurs in winter followed by rapid growth through spring, maturity and then senescence through summer, with only the microscopic life stages persisting through autumn. It exhibits multiple dispersal strategies, such as short-range spore dispersal, and long-range dispersal as whole drift plants or fragments. Undaria pinnatifida has spread rapidly across the UK and Europe, resulting in community-wide responses and impacts (Vaz-Pinto et al., 2014; Epstein & Smale, 2017). Its impacts are complex and context-specific, depending on space, time, and taxa present in the introduced location (Epstein & Smale, 2017; Teagle et al., 2017; Tidbury, 2020).

In Torquay Marina, UK, Farrell & Fletcher (2006) completed a canopy removal experiment between 1996-2002. They reported that Saccharina latissima decreased in both control and treatment plots from ca 3 plants per 0.45 m² in 1996 to ca 1 plant per 0.45 m² in 1997 and had disappeared completely from pontoons by 2002. This coincided with a significant increase in Undaria pinnatifida from 0 plants per 0.45 m² in 1996 to ca 6 plants per 0.45 m² in 1997. However, there was a slight decrease in Undaria pinnatifida at both control and treatment plots between 1997 and 1998. By 2002, Undaria pinnatifida had recovered at control and treatment plots to ca 4-6 plants per 0.45 m² whereas Saccharina latissima had not.

In Plymouth Sound (UK), Epstein et al. (2019b) found that within its depth range (+1 to –4 m), Undaria pinnatifida co-existed with seven species of canopy-forming brown macroalgae, including Saccharina latissima and Laminaria hyperborea. However, they reported that Undaria pinnatifida biomass was negatively related to Saccharina latissima in both intertidal and subtidal habitats. This was only statistically significant in subtidal habitats, which suggested that there was some competition between the two species (Epstein et al., 2019b). In St Malo, France, there was evidence that Undaria pinnatifida could co-exist with Laminaria hyperborea under certain conditions (Castric-Fey et al., 1993).

Heiser et al. (2014) surveyed 17 sites within Plymouth Sound, UK and found that Saccharina latissima was significantly more abundant at sites with Undaria pinnatifida with ca 5 Saccharina latissima individuals per m² present, compared to ca 0.5 Saccharina latissima individuals per m² present at sites without Undaria pinnatifida. They also observed that Laminaria hyperborea was significantly less abundant at sites with the presence Undaria pinnatifida, with only ca 0.5 Laminaria hyperborea individuals per m2 present compared to ca 8 individuals per m2 at sites without the presence Undaria pinnatifida.

No evidence was found to suggest that Undaria pinnatifida affects Halidrys siliquosa specifically. However, Epstein et al. (2019a) reported that when Undaria pinnatifida was removed from sites in Plymouth Sound, there was no difference in understorey assemblages. Surveys revealed that there was a high variation in the abundance and mean cover of understorey macroalgae, both between plots and survey months. For example, the mean cover of brown understory algae followed the same trend for each removal treatment (0%, 50% and 100% removal of Undaria pinnatifida). Epstein et al. (2019a) observed an initial increase in mean percentage cover 0-5 months following the removal, which peaked after 7 months. This then decreased 7-15 months post-removal and then slightly recovered after 16-20 months. 

Sensitivity assessment.  The above evidence suggests that Undaria pinnatifida can compete with Saccharina latissima and to a lesser extent with Laminaria hyperborea depending on local conditions. For example, Undaria pinnatifida can out-compete Saccharina latissima in artificial habitats, such as in Torquay Marina. Undaria pinnatifida can co-exist with native kelp species within its depth range (-1 to 4 m), as shown in Plymouth Sound, UK. However, Undaria pinnatifida is unlikely to cause changes in the abundance of brown understorey algae. Therefore, if this biotope IR.HIR.KSed.XKHal was colonized by Undaria pinnatifida it is likely to co-exist with rather than out-compete the native characteristic kelps, especially in the wave exposed conditions characteristics of the biotope.

Nevertheless, the introduction of Sargassum muticum was reported to result in a dramatic decline of resident Halidrys siliquosa and a fundamental change to the biotope structure in Limfjorden (Staehr et al., 2000; Wernberg et al., 2004; Pedersen et al., 2005).  Therefore, resistance to the introduction of invasive non-indigenous species is assessed as ‘Low’. Resilience is assessed as ‘Very low’ as biotope recovery will not occur unless the INIS is removed. The sensitivity of this biotope is assessed as ‘High’.

Not relevant (NR) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Little information was found regarding diseases in macroalgae. Halidrys siliquosa supports a number of epiphytic species, which use it as a substratum but are not parasitic on the plant (Connor et al., 1997). Growth rates of Saccharina latissima may be reduced by Streblonema disease (Lein et al., 1991). There was, however, insufficient evidence to assess this pressure on IR.HIR.KSed.XKHal.

Low Medium Medium
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Halidrys siliquosa extracts are used within skin beauty products (Gelyma, 2015), however at the time of writing information regarding large scale extraction of Halidrys siliquosa from the seabed is lacking. There has been recent commercial interest in Saccharina lattisma as a consumable called “sea vegetables” (Birket et al., 1998). However, Saccharina lattissima sporophytes are typically matured on ropes (Handå et al., 2013) and not directly extracted from the seabed, as with Laminaria hyperborea which is commercial trawled for alginate across the North Altantic (Christie et al., 1998). Chondrus crispus is extracted commercially in Ireland, but the harvest has declined since its peak in the early 1960s (Pybus, 1977). The effect of harvesting has been best studied in Canada. Sharp et al. (1986) reported that the first drag rake harvest of the season on a Nova Scotian Chondrus crispus bed removed 11% of the fronds and 40% of the biomass. Efficiency declined as the harvesting season progressed. Chopin et al. (1988) noted that non-drag raked beds of Chondrus crispus in the Gulf of St Lawrence showed greater year round carposporangial reproductive capacity than a drag raked bed. Commercial exploitation of the red seaweeds which characterize the biotope has the potential to impact the community greatly, through changes in community structure and physical disturbance of the other species present.

Sensitivity assessment. Evidence to assess the resistance of IR.HIR.KSed.XKHal to direct harvesting is limited. Neither kelp species nor Chondrus crispus are likely to be significantly abundant within IR.HIR.KSed.XKHal to attract commercial interest, however if targeted removal of macro-algae was initiated within IR.HIR.KSed.XKHal It has been assumed that operations would remove >75% of canopy forming sporophytes. Resistance has been assessed as ‘Low’, Resilience as ‘Medium’. Sensitivity has been assessed as ‘Medium’.

None Medium Medium
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope.  Incidental removal of the key characterizing species and associated species would alter the character of the biotope. The biotope is characterized by a macroalgal canopy of Halidrys siliquosa mixed with other brown seaweeds, including the kelps Saccharina lattissima and Laminaria hyperborea. These provide a canopy under which a variety of red seaweeds grow, as well as attachment surfaces for epiphytic species. The loss of the canopy due to incidental removal as by-catch would, therefore, alter the character of the habitat and result in the loss of species richness. The ecological services such as primary and secondary production provided by these species would also be lost.

Low level disturbances (e.g. solitary anchors) are unlikely to cause harm to the biotope as a whole, due to the impact’s small footprint.  Thus evidence to assess the resistance of IR.HIR.KSed.XKHal to non-targeted removal is limited. It is assumed that incidental non-targeted catch will mobilise sediment and cause high mortality within the affected area.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘Medium’. Sensitivity has been assessed as ‘Medium’.

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Citation

This review can be cited as:

Stamp, T.E., Tyler-Walters, H., Williams, E. & Lloyd, K.A., 2021. Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock with coarse sediment. 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. [cited 27-01-2023]. Available from: https://www.marlin.ac.uk/habitat/detail/258

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Last Updated: 08/12/2021