The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

This biotope occurs in the subtidal and is, therefore, protected from exposure to air so that the thermal regime is more stable and desiccation is not a factor.  Examples of distribution and thermal tolerances tested in laboratory experiments are provided as evidence to support the sensitivity assessment. In general, populations can acclimate to prevailing conditions which can alter tolerance thresholds and care should, therefore, be used when interpreting reported tolerances.

The encrusting coralline, Lithophyllum incrustans, is close to the northern edge of its reported distribution range in the UK (Kain, 1982; Guiry & Guiry, 2015) and is therefore considered likely to be tolerant of an increase in temperature, particularly in this subtidal biotope, where it is protected from desiccation.

Balanus crenatus is described as a boreal species (Newman & Ross, 1976) it is found throughout the northeast Atlantic from the Arctic to the west coast of France as far south as Bordeaux; east and west coasts of North America and Japan. In Queens Dock, Swansea where the water was on average 10°C higher than average due to the effects of a condenser effluent, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite.  After the water temperature cooled Balanus crenatus returned (Naylor, 1965).  The increased water temperature in Queens Dock is greater than an increase at the pressure benchmark (2-5°C).  Balanus crenatus has a peak rate of cirral beating at 20°C and all spontaneous activity ceases at about 25°C (Southward, 1955). The tolerance of Balanus crenatus, collected in the summer (and thus acclimated to higher temperatures), to increased temperatures, was tested in the laboratory. The median upper lethal temperature tolerance was -25.2°C (Davenport & Davenport, 2005) confirming the observations of Southward (1955).

The characterizing Spirobranchus triqueter are found in both warmer and colder waters than experienced in the UK.  Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and the Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (De Kluijver et al., 2016)

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). The biotope is considered to tolerate a 2°C increase in temperature for a year. An acute increase at the pressure benchmark may be tolerated in winter, but a sudden return to typical temperatures could lead to mortalities among acclimated animals. No evidence was found to support this assessment, however. An acute increase of 5°C in summer would be close to the lethal thermal temperature for Balanus crenatus and loss of this species would alter the character of the biotope. Biotope resistance is, therefore, assessed as ‘Medium’ and resilience as ‘High’ and biotope sensitivity is, therefore ‘Low’.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

This biotope occurs in the subtidal and is therefore protected from exposure to air so that the thermal regime is more stable and desiccation is not a factor.  Examples of distribution and thermal tolerances tested in laboratory experiments are provided as evidence to support the sensitivity assessment. In general, populations can acclimate to prevailing conditions which can alter tolerance thresholds and care should, therefore, be used when interpreting reported tolerances.

Lithophyllum incrustans are close to the northern edge of their reported distribution range in the UK (Guiry & Guiry, 2015). Edyvean & Forde (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning but no information on thresholds was provided (Edyean & Ford, 1984b). 

Within the biotope, the key characterizing barnacles Balanus crenatus have a more northern distribution and are absent from warmer Mediterranean and equatorial waters.  Balanus crenatus is described as a boreal species (Newman & Ross, 1976), it is found throughout the northeast Atlantic from the Arctic to the west coast of France, as far south as Bordeaux; east and west coasts of North America and Japan.  Balanus crenatus is relatively tolerant of lower temperatures.  Balanus crenatus was unaffected during the severe winter of 1962-63, when average temperatures were 5 to 6°C below normal (Crisp, 1964a,b).  The tolerance of Balanus crenatus collected from the lower intertidal in the winter (and thus acclimated to lower temperatures) to low temperatures was tested in the laboratory. The median lower lethal temperature tolerance was -1.4°C (Davenport & Davenport, 2005). An acute or chronic decrease in temperature, at the pressure benchmark, is therefore unlikely to negatively affect this species. Meadows (1969) noted that the severe winter of 1962-63 decreased sea temperatures at Newcastle but did not affect fauna, including Balanus crenatus, on settlement panels that were deployed in the area.

The characterizing Spirobranchus triqueter are found in both warmer and colder waters than experienced in the UK.  Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and the Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (De Kluijver et al., 2016). Thomas (1940) noted that Spirobranchus (as Pomatoceros) triqueter could not form tubes below 7°C, however, this effect is not considered to lead to mortality in adults at the duration of the acute pressure benchmark.

Sensitivity assessment. Overall, a long-term chronic change in temperature at the pressure benchmark is considered likely to fall within natural variation and to be tolerated by the characterizing and associated species although, Lithothyllum incrustans may experience reduced growth (as it is primarily a southern species). An acute change at the pressure benchmark is considered unlikely to adversely affect the biotope as the characterizing species can potentially adapt to a wide range of temperatures experienced in both northern and southern waters (Spirobranchus triqueter) or are found primarily in colder, more northern waters (Balanus crenatus).  Lithophyllum incrustans may be less tolerant, but reductions in growth, rather than mortalities may result. Biotope resistance is, therefore, assessed as ‘Medium’ and resilience as ‘High’ so that sensitivity is assessed as ‘Low’. 

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

This biotope is recorded in full salinity (30-35 ppt) habitats (Connor et al., 2004) and the sensitivity assessment considers an increase from full to >40 ppt.

The crustose corallines that occur in this biotope may also be found on rocky shores and in rockpools where salinities may fluctuate markedly during exposure to the air. Edyvean & Ford (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning but no information on thresholds was provided (Edyvean & Ford, 1984b).  Populations of Lithophyllum incrustans were less stable in rockpools with a smaller volume of water that were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity, etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b).

Balanus crenatus occurs in estuarine areas and is therefore adapted to variable salinity (Davenport, 1976). When subjected to sudden changes in salinity Balanus crenatus closes its opercular valves so that the blood is maintained temporarily at a constant osmotic concentration (Davenport, 1976).  Early stages may be more sensitive than adults. Experimental culturing of Balanus crenatus eggs, found that viable nauplii larvae were obtained between 25-40‰ but eggs did not develop to viable larvae when held at salinities above 40‰ and only a small proportion (7%) of eggs exposed at later stages developed into viable nauplii and these were not vigorous swimmers (Barnes & Barnes, 1974). When exposed to salinities of 50‰, and 60‰ eggs exposed at an early developmental stage did not produce viable larvae and, again, only a small proportion of eggs (7% and 1%, respectively) exposed at a later developmental stage produced nauplii- these were deformed and probably non-viable.  There was no larval development at 70‰ (Barnes & Barnes, 1974).

Sensitivity assessment.  Some increases in salinity may be tolerated by the characterizing species however the biotope is considered to be sensitive to a persistent increase in salinity to >40 ppt (based on species distribution, Barnes & Barnes, 1974 & Edyvean & Ford (1984b). Therefore, resistance is assessed as ‘Low’ and resilience as ‘High’ (following the restoration of prior salinity) so that sensitivity is assessed as ‘Low’.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

This biotope is recorded in full salinity (30-35 ppt) (Connor et al., 2004). At the pressure benchmark, a change from full to variable salinity (18-30 ppt) is assessed. The characterizing species are found in a similar biotope (CR.MCR.EcCr.UrtScr), is present in variable salinities (Connor et al., 2004). It is therefore likely that the characterizing species will tolerate a reduction in salinity form full to reduced.

Edyvean & Ford (1984b) suggest that populations of the crustose coralline Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning but no information on the relevant thresholds was provided (Edyvean & Ford, 1984b). Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water that were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b).

Balanus crenatus occurs in estuarine areas and is therefore adapted to variable salinity (Davenport, 1976). When subjected to sudden changes in salinity Balanus crenatus closes its opercular valves so that the blood is maintained temporarily at a constant osmotic concentration (Davenport, 1976).  Acclimation to different salinity regimes alters the point at which opercular closure and resumption of activity occur (Davenport, 1976). Balanus crenatus can tolerate salinities down to 14 psu if given time to acclimate (Foster, 1970).  At salinities below 6 psu motor activity ceases, respiration falls and the animal falls into a "salt sleep".  In this state the animals may survive (Barnes & Barnes, 1974) in freshwater for 3 weeks, enabling them to withstand changes in salinity over moderately long periods (Barnes & Powell, 1953). Larvae are more sensitive than adults. In culture experiments, eggs maintained below 10‰ rupture, due to osmotic stress (Barnes & Barnes, 1974).  At 15-17‰  there is either no development of early stages or the nauplii larvae are deformed and “probably not viable”, similarly at  20‰ development occurs, but about half of the larvae are deformed and not viable. (Barnes & Barnes, 1974). Normal development resulting in viable larvae occurs between salinities of 25-40‰ (Barnes & Barnes, 1974).

Spirobranchus triqueter has not been recorded from brackish or estuarine waters.  Therefore, it is likely that the species will be very intolerant of a decrease in salinity.  However, Dixon (1985, cited in Riley & Ballerstedt, 2005) views the species as able to withstand significant reductions in salinity. The degree of reduction in salinity and time that the species could tolerate those levels were not recorded.  Therefore, there is insufficient information available to assess the intolerance of Spirobranchus triqueter to a reduction in salinity and the assessment is based on its presence in the biotope CR.MCR.EcCr.UrtScr which occurs in variable salinity (as well as full) habitats (Connor et al., 2004). 

Sensitivity assessment.  As the characterizing species are found in biotopes in both full and variable salinity habitats, the biotope is considered ‘Not sensitive’ to a decrease in salinity from full to variable. Biotope resistance is therefore assessed as ‘High’ and resilience is assessed as ‘High’ (by default) and the biotope is assessed as ‘Not sensitive’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

This biotope occurs across a range of flow speeds, from very weak (negligible) to moderately strong (0.5-1.5 m/s) (Connor et al., 2004). The suspension feeders within the biotope benefit from high water flow supplying food. The coralline crusts characterizing this biotope are securely attached and as these are flat they are subject to little or no drag compared to upright growth forms of algae. Colonies of Lithophyllum incrustans appear to thrive in conditions exposed to strong water movement (Irvine & Chamberlain, 1994). Spirobranchus triqueter is found in biotopes exposed to flow speeds varying from very weak to moderately strong (negligible - >1.5m/s)  (Tillin & Tyler-Walters, 2014).  Balanus crenatus is found in a very wide range of water flows (Tillin & Tyler-Walters, 2014), although it usually occurs in sites sheltered from wave action (Eckman & Duggins, 1993) and can adapt feeding behaviour according to flow rates.   In the absence of any current, the barnacle rhythmically beats its cirri to create a current to collect zooplankton. The growth of Balanus crenatus (measured as an increase in basal area), maintained for 69 days at constant flow speeds in laboratory experiments, was greatest at intermediate flow speeds (0.08 m/s) and decreased at higher speeds (Eckman & Duggins, 1993). Over the entire range of flow speeds measured (0.02 m/s – 0.25 m/s), Balanus crenatus, was able to control the cirrus with little or no deformation by flow observed (Eckman, & Duggins, 1993).

Sensitivity assessment. Scour is a key factor structuring this biotope (Connor et al., 2004) so that changes in flow exceeding the pressure benchmark may increase or decrease sediment transport and associated scour may lead to indirect changes in the character of the biotope.  However, the scour and mobility of the substratum is probably dominated by wave action rather than water flow.  As the biotope and the associated species can occur in a range of flow speeds, the resistance of the biotope to changes in water flow is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is assessed as ‘Not sensitive’.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Not relevant to subtidal biotopes. However, 100% mortality could be expected in adult Spirobranchus (as Pomatoceros) triqueter after 24.1 h and 35.4 h when exposed to air at 7°C and 13°C respectively (Campbell & Kelly, 2002).

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

This biotope is recorded from locations that range from 'very exposed' to 'exposed' to wave action (Connor et al., 2004).  The crustose corallines associated with this biotope have a flat growth form and are unlikely to be dislodged by increased wave action. Balanus crenatus and Spirobranchus triqueter are firmly attached to the substratum and are unlikely to be dislodged by an increase in wave action at the pressure benchmark. These species are found in biotopes from a range of wave exposures from extremely wave sheltered to very wave exposed (Tillin & Tyler-Walters, 2014). 

Sensitivity assessment. Scour is a key factor structuring this biotope (Connor et al., 2004) and the mobility of the substratum and, hence scour, is probably dominated by wave action. Therefore, changes in wave action (wave exposure) could result in significant changes to this biotope. Any activity or pressure, that redirected, blocked or mitigated wave energy reaching the biotope could result in a reduction in scour and reclassification of the biotope, probably to a faunal dominated cave and gully biotope, e.g. IR.FIR.SG.FoSwCC. However, a 3-5% reduction in significant wave height (the benchmark) is unlikely to significantly reduce wave energy reaching a biotope that occurs in wave exposed to very wave exposed conditions.  Therefore, resistance is assessed as 'High', resilience as ‘High', by default, and the biotope is assessed as ‘Not sensitive’ at the benchmark level. 

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

No information was found concerning the effects of heavy metals on encrusting coralline algae. Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: organic Hg> inorganic Hg > Cu > Ag > Zn> Cd> Pb. Contamination at levels greater than the pressure benchmark may adversely impact the biotope. Cole et al. (1999) reported that Hg was very toxic to macrophytes. The sub-lethal effects of Hg (organic and inorganic) on the sporelings of an intertidal red algae, Plumaria elegans, were reported by Boney (1971). 100% growth inhibition was caused by 1 ppm Hg.

Contamination at levels greater than the pressure benchmark may adversely impact the biotope. Barnacles accumulate heavy metals and store them as insoluble granules (Rainbow, 1987). Pyefinch & Mott (1948) recorded a median lethal concentration of 0.19 mg/l copper and 1.35 mg/l mercury, for Balanus crenatus over 24 hours. Barnacles may tolerate fairly high levels of heavy metals in nature, for example, they are found in Dulas Bay, Anglesey; where copper reaches concentrations of 24.5 µg/l, due to acid mine waste (Foster et al., 1978).

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Where exposed to direct contact with fresh hydrocarbons, encrusting coralline algae appear to have a high intolerance. Crump et al. (1999) described 'dramatic and extensive bleaching' of 'Lithothamnia' following the Sea Empress oil spill. Observations following the Don Marika oil spill (K. Hiscock, pers. comm.) were of rockpools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was affected in a short period of time by oil during the Sea Empress spill, recovery occurred within about a year. The oil was found to have destroyed about one-third of the thallus thickness but regeneration occurred from thallus filaments below the damaged area.

No information is available on the intolerance of Balanus crenatus to hydrocarbons. However, other littoral barnacles generally have a high tolerance to oil (Holt et al., 1995) and were little impacted by the Torrey Canyon oil spill (Smith, 1968), so Balanus crenatus is probably fairly resistant to oil.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Cole et al. (1999) suggested that herbicides were (not surprisingly) very toxic to algae and macrophytes. Hoare & Hiscock (1974) noted that, with the exception of Phyllophora species, all red algae including encrusting coralline forms were excluded from the vicinity of an acidified halogenated effluent discharge in Amlwch Bay, Anglesey and that intertidal populations of Corallina officinalis occurred in significant numbers only 600 m east of the effluent. Chamberlain (1996) observed that although Lithophyllum incrustans was quickly affected by oil during the Sea Empress spill, recovery occurred within about a year. The oil was found to have destroyed about one-third of the thallus thickness but regeneration occurred from thallus filaments below the damaged area.

Barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved (Holt et al., 1995). They are less intolerant than some species (e.g. Patella vulgata) to dispersants (Southward & Southward, 1978) and Balanus crenatus was the dominant species on pier pilings at a site subject to urban sewage pollution (Jakola & Gulliksen, 1987). Hoare & Hiscock (1974) found that Balanus crenatus survived near to an acidified halogenated effluent discharge where many other species were killed, suggesting a high tolerance to chemical contamination. Little information is available on the impact of endocrine disrupters on adult barnacles. Holt et al. (1995) concluded that barnacles are fairly sensitive to chemical pollution, therefore intolerance is reported as high. The species is an important early colonizer of sublittoral rock surfaces (Kitching, 1937) and it heavily recolonized a site that was dredged for gravel within seven months (Kenny & Rees, 1994). Therefore, recovery is predicted to be high.

Most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999).

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Specific information concerning oxygen consumption and reduced oxygen tolerances were not found for the key characterizing species within the biotope. It is likely that as this biotope occurs in areas that are shallow and tidally flushed that re-oxygenation is likely, limiting the effects of any de-oxygenation events. However, this may mean that the speices present have little exposure to low oxygen and may be sensitive to this pressure. Balanus crenatus, however, respires anaerobically so it can withstand some decrease in oxygen levels. When placed in wet nitrogen, where oxygen stress is maximal and desiccation stress is minimal, Balanus crenatus has a mean survival time of 3.2 days (Barnes et al., 1963) and this species is considered to be ‘Not sensitive’ to this pressure. 

Sensitivity assessment. Based on Balanus crenatus and mitigation of de-oxygenation by water movements, this biotope is considered to have 'High' resistance and 'High' resilience (by default) and is, therefore, assessed as 'Not sensitive'.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Nutrient enrichment at the pressure benchmark is unlikely to affect the fauna within this biotope. Over geological timescales, periods of increased nutrient availability have experienced increases in the distribution of crustose coralline species at the expense of corals (Littler & Littler, 2013), suggesting that this group have some tolerance for enhanced nutrient levels. Overall, Littler & Littler (2013) suggest that corallines as a group can tolerate both low and elevated levels of nutrients. The encrusting coralline Lithophyllum incrustans were present at sites dominated by Ulva spp. in the Mediterranean exposed to high levels of nutrient enrichment from domestic sewage (Arévalo et al., 2007).

A slight increase in nutrient levels could be beneficial for barnacles and other suspension feeders by promoting the growth of phytoplankton and, therefore, increasing food supplies. Balanus crenatus was the dominant species on pier pilings, which were subject to urban pollution (Jakola & Gulliksen, 1987).

Sensitivity assessment. The pressure benchmark is relatively protective and the biotope is considered to have ‘High’ resistance and ‘High' resilience (by default) and is judged to be ‘Not sensitive’ at the benchmark level.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

As the biotope occurs in tide-swept or wave exposed areas (Connor et al., 2004), water movements will disperse organic matter and reduce the level of exposure to this pressure. The crusting coralline Lithophyllum incrustans were present at sites dominated by Ulva spp. in the Mediterranean exposed to high levels of organic pollution from domestic sewage (Arévalo et al., 2007), suggesting the encrusting corallines are not sensitive to this pressure.

The animals found within the biotope may be able to utilise the input of organic matter as food, or are likely to be tolerant of inputs at the benchmark level. In a recent review, assigning species to ecological groups based on tolerances to organic pollution, characterizing animal species; Balanus crenatus and Spirobranchus triqueter were assigned to AMBI Group II described as 'species indifferent to enrichment, always present in low densities with non-significant variations with time, from initial state to slight unbalance' (Gittenberger & Van Loon, 2011). 

Sensitivity assessment. It is not clear whether the pressure benchmark would lead to enrichment effects in this dynamic, scoured habitat.  High water movements would disperse organic matter particles, mitigating the effect of this pressure. This biotope is probably ‘Not sensitive’ to this pressure, based on the AMBI categorisation (Borja et al., 2000, Gittenberger & Van Loon, 2011).  Although species within the biotope may be sensitive to gross organic pollution resulting from sewage disposal and aquaculture, they are considered to have ‘High’ resistance to the pressure benchmark which represents organic enrichment and therefore ‘High’ resilience so that the biotope is assessed as ‘Not Sensitive’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

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.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

This biotope is characterized by the hard rock substratum to which the characterizing and associated species can firmly attach. Changes to a sedimentary habitat or an artificial substratum would significantly alter the character of the biotope through the loss of habitat.

Tillin & Tyler-Walters (2014) used records from the MNCR database as a proxy indicator of the resistance to physical change by Balanus crenatus and Spirobranchus triqueter. These species were reported from a variety of substratum types including fine (muddy sand, sandy mud and fine sands) and coarse sediments, where some hard surfaces (such as pebbles or shells) are present for the attached species.

Balanus crenatus and Spirobranchus triqueter are fouling organisms and occur on a wide variety of substrata (Harms & Anger, 1983; Andersson et al., 2009). As well as artificial and natural hard substrata Balanus crenatus and Spirobranchus triqueter also encrust a range of invertebrates; for example, Spirobranchus triqueter has been recorded on the hermit crab, Pagurus bernhardus (Fernandez-Leborans & Gabilondo, 2006) among other species.  Similarly, Balanus crenatus has been reported to encrust empty shells of the invasive non-indigenous species Ensis americanus (Donovan, 2011) and Carcinus maenas (Heath, 1976).

Sensitivity assessment It should be noted that the basis of the sensitivity assessment for this pressure is the sensitivity of the biotope to changes in substratum type, rather than the sensitivity of the species. A permanent change in substratum type to artificial or sedimentary would lead to the reclassification of the biotope. Biotope resistance to this pressure is therefore assessed as ‘None’ (loss of >75% of extent), as the change at the benchmark is permanent, resilience is assessed as ‘Very low’.   Sensitivity, based on combined resistance and resilience is therefore assessed as ‘High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Not relevant to biotopes occurring on bedrock.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

The species characterizing this biotope are epifauna occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

The species characterizing this biotope occur on the rock and therefore have no protection from surface abrasion. High levels of abrasion from scouring by mobile sands and gravels is an important structuring factor in this biotope (Connor et al., 2004) and prevents replacement by less scour-tolerant species, such as red algae.  Mechanical abrasion from scuba divers was reported to impact encrusting corallines, with cover of Lithophyllum stictaeforme greater in areas where diving was forbidden than visited areas (abundance, 6.36 vs 1.4; it is presumed this refers to proportion of cover, although this is not clear from the text, Guarinieri et al., 2012). Dethier (1994) manipulated surface abrasion on a range of encrusting algae experimentally including Lithophyllum impressum. Crusts were brushed with either a nylon or steel brush for 1 minute each month for 24 months. Unbrushed controls grew by approximately 50% where the cover of nylon brushed crusts and steel brushed crusts decreased by approximately 25% and 40% respectively (interpreted from figures in Dethier, 1994). In laboratory tests on chips of Lithophyllum impressum brushing with a steel brush for 1 minute once a week for 3 weeks, resulted in no cover loss of two samples while a third ‘thinned and declined’ (Dethier, 1994).

Hiscock (1983) noted that a community, under conditions of scour and abrasion from stones and boulders moved by storms, developed into a community consisting of fast-growing species such as Spirobranchus (formerly Pomatoceros) triqueter.  Off Chesil Bank, the epifaunal community dominated by Spirobranchus (as Pomatoceros) triqueter and Balanus crenatus decreased in cover in October as it was scoured away in winter storms, but recolonized in May to June (Gorzula 1977). Warner (1985) reported that the community did not contain any persistent individuals but that recruitment was sufficiently predictable to result in a dynamic stability and a similar community, dominated by Spirobranchus (as Pomatoceros triqueter), Balanus crenatus and Electra pilosa, (an encrusting bryozoan), was present in 1979, 1980 and 1983 (Riley and Ballerstedt, 2005). 

Re-sampling of fishing grounds that were historically studied (from the 1930s) indicated that some encrusting species including serpulid worms and several species of barnacles had decreased in abundance in gravel substrata subject to long-term scallop fishing (Bradshaw et al., 2002).  These may have been adversely affected by the disturbance of the stones and dead shells on to which they attach (Bradshaw et al., 2002). Where individuals are attached to mobile pebbles, cobbles and boulders rather than bedrock, surfaces can be displaced and turned over preventing feeding and leading to smothering.  This observation is supported by experimental trawling, carried out in shallow, wave disturbed areas using a toothed, clam dredge, which found that Spirobranchus sp. decreased in intensively dredged areas over the monitoring period (Constantino et al., 2009).  In contrast, a study of Spirobranchus spp. aggregations found that the tube heads formed were not significantly affected by biannual beam trawling in the eastern Irish Sea (Kaiser et al., 1999).  No changes in the number or size of serpulid tube heads were apparent throughout during the study, and no significant changes were detectable in the composition of the tube head fauna that could be attributed to fishing disturbance (Kaiser et al., 1999). Subsequent laboratory experiments on collected tube heads found that these were unlikely to resettle on the seabed in an orientation similar to that prior to disturbance (Kaiser et al., 1999).  This may lead to the death of the resident serpulids and sessile associated fauna.

Sensitivity assessment. The impact of surface abrasion will depend on the footprint, duration and magnitude of the pressure. High levels of abrasion from scouring by mobile cobbles and pebbles is an important structuring factor in this biotope (Connor et al., 2004) but the persistence of the assemblage may depend on rapid recovery rather than high resistance (Gorzula, 1977). The evidence for the effects of severe scour and trawling on Balanus crenatus and Spirobranchus triqueter, suggest that resistance, to a single abrasion event, is ‘Low’ but resilience is ‘High’ so that sensitivity is assessed as ‘Low’, based upon the information for these species and the characterizing Coralline spp. 

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

The species characterizing this biotope group are epifauna or epiflora occurring on rock, which is resistant to subsurface penetration. The assessment for abrasion at the surface only is, therefore, considered to equally represent sensitivity to this pressure.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

This biotope occurs in scoured habitats and it is likely, depending on local sediment supply, that the biotope is exposed to chronic or intermittent episodes of high levels of suspended solids as local sediments are re-mobilised and transported. A significant increase in suspended solids may result in smothering (see siltation pressures) where these are deposited. Based on Cole et al. (1999) and Devlin et al. (2008) this biotope is considered to experience intermediate turbidity (10-100 mg/l) based on UK TAG (2014).  An increase at the pressure benchmark refers to a change to medium turbidity (100-300 mg/l) and a decrease is assessed as a change to clear (<10 mg/l) based on UK TAG (2014).

The biotope occurs in shallow waters where light attenuation due to increases in turbidity is probably low and the characterizing animals are unlikely to be affected by increased or decreased clarity. Red algae and encrusting coralline algae especially, are known to be shade tolerant and are common components of the understorey on seaweed dominated shores. Therefore, an increase or decrease in light intensity is unlikely to adversely affect the crustose corallines as plants can acclimate to different light levels.

An increase in turbidity could be beneficial if the suspended particles are composed of organic matter, however high levels of suspended solids with increased inorganic particles may reduce filter-feeding efficiencies. A reduction in suspended solids will reduce food availability for filter-feeding species in the biotope (where the solids are organic), although effects are not likely to be lethal over the course of a year. A reduction in light penetration could also reduce the growth rate of phytoplankton and so limit zooplankton levels.  However, light penetration itself is unlikely to be an important factor as both Balanus crenatus and Spirobranchus triqueter are recorded from the lower eulittoral or the lower circalittoral. 

Available evidence indicates that Spirobranchus triqueter is tolerant of a wide range of suspended sediment concentrations (Riley and Ballerstedt, 2005).  Stubbings and Houghton (1964) recorded Spirobranchus (as Pomatoceros) triqueter in Chichester harbour, which is a muddy environment.  However, Spirobranchus (as Pomatoceros) triqueter has been noted to also occur in areas where there is little or no silt present (Price et al., 1980). 

Barnes & Bagenal (1951) found that the growth rate of Balanus crenatus epizoic on Nephrops norvegicus was considerably slower than animals on raft exposed panels. This was attributed to reduced currents and increased silt loading of water in the immediate vicinity of Nephrops norvegicus. In dredge disposal areas in the Weser estuary, Germany, where turbidity is 35% above the natural rate of 10-100 mg/l, the abundance of Balanus crenatus was lower than in reference areas (Witt et al., 2004).  Separating the effect of increased suspended solids from increased sedimentation and changes in sediment from sediment dumping is problematic, however (Witt et al., 2004). Balanids may stop filtration after silt layers of a few millimetres have been discharged (Witt et al., 2004), as the feeding apparatus is very close to the sediment surface.

Sensitivity assessment. Overall biotope resistance is assessed as ‘High’ to an increase in suspended solids. Resilience is assessed as ‘High’ (by default) as adults are likely to remain in situ from which recruitment can occur. Hence, the biotope is assessed as ’Not sensitive’ to decreased suspended solids.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

This biotope is described as sand covered or sand scoured (Connor et al., 2004). The characterizing and associated species are therefore likely to tolerate intermittent episodes of sediment deposition.

In a review of the effects of sedimentation on rocky coast assemblages, Airoldi (2003) outlined the evidence for the sensitivity of encrusting coralline algae to sedimentation. The reported results are contradictory with some authors suggesting that coralline algae are negatively affected by sediments while others report that encrusting corallines are often abundant or even dominant in a variety of sediment impacted habitats (Airoldi, 2003 and references therein). Crustose corallines have been reported to survive under a turf of filamentous algae and sediment for 58 days (the duration of the experiment) in the Galapagos (species not identified, Kendrick, 1991). The crustose coralline Hydrolithon reinboldii, has also been reported to survive deposition of silty sediments on subtidal reefs off Hawaii (Littler, 1973). In an experimental study, Balata et al. (2007) enhanced sedimentation on experimental plots in the Mediterranean (close to Tuscany) by adding 400 g of fine sediment every 45 days on plots of 400 cm² for 1 year. Nearby sites with higher and lower levels of sedimentation were assessed as control plots. Some clear trends were observed. Crustose corallines declined at medium and high levels of sedimentation (Balata et al., 2007). The experiment relates to chronic low levels of sedimentation rather than a single acute event, as in the pressure benchmark, however, the trends observed are considered to have some relevance to the pressure assessment. 

As small, sessile species attached to the substratum, siltation at the pressure benchmark would bury Balanus crenatus and Spirobranchus triqueter.  Holme & Wilson (1985) described a Pomatoceros-Balanus assemblage on ‘hard surfaces subjected to periodic sever scour and ‘deep submergence by sand or gravel’ in the English Channel. They inferred that the Pomatoceros-Balanus assemblage was restricted to fast-growing settlers able to establish themselves in short periods of stability during summer months (Holme & Wilson 1985), as all fauna were removed in the winter months. Barnacles may stop filtration after silt layers of a few millimetres have been discharged as the feeding apparatus is very close to the sediment surface (Witt et al., 2004). In dredge disposal areas in the Weser estuary, Germany, where the modelled exposure to sedimentation was 10mm for 25 days, with the centre of the disposal ground exposed to 65 mm for several hours before dispersal, Balanus crenatus declined in abundance compared to reference areas.  (Witt et al., 2004). However, separating the effect of sedimentation from increased suspended solids and changes in sediment from sediment dumping was problematic (Witt et al., 2004).

Sensitivity assessment. Based on the presence of the characterizing and associated species in biotopes subject to sedimentation and scour (such as CR.MCR.EcCr.UrtScr), biotope resistance to this pressure, at the benchmark, is assessed as 'High', resilience is assessed as 'High' (by default) and the biotope is assessed as 'Not sensitive'. The assessment considers that sediments are rapidly removed from the biotope and that the scour tolerance of the characterizing animal species and encrusting corallines would prevent significant mortalities although some damage and abrasion may occur. However, if the deposit remained in place; i.e. due to the scale of the pressure or where biotopes were sheltered, or only seasonally subject to water movements, or where water flows and wave action were reduced e.g. by the presence of tidal barrages, then resistance would be lower and sensitivity would be greater.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

This biotope is described as subject to scouring (Connor et al., 2004). The characterizing species occur in biotopes subject to sedimentation and scour (such as CR.MCR.EcCr.UrtScr) and are therefore likely to tolerate intermittent episodes of sediment movement and deposition.  At the pressure benchmark ‘heavy deposition’ represents a considerable thickness of deposit and complete burial of the characterizing species would occur. Removal of the sediments by wave action and tidal currents would result in considerable scour. The effect of this pressure will be mediated by the length of exposure to the deposit and the nature of the deposit. 

In a review of the effects of sedimentation on rocky coast assemblages, Airoldi (2003) outlined the evidence for the sensitivity of encrusting coralline algae to sedimentation. The reported results are contradictory with some authors suggesting that coralline algae are negatively affected by sediments while others report that encrusting corallines are often abundant or even dominant in a variety of sediment impacted habitats (Airoldi, 2003 and references therein). Crustose corallines have been reported to survive under a turf of filamentous algae and sediment for 58 days (the duration of the experiment) in the Galapagos (species not identified, Kendrick, 1991). The crustose coralline Hydrolithon reinboldii, has also been reported to survive deposition of silty sediments on subtidal reefs off Hawaii (Littler, 1973). In an experimental study, Balata et al. (2007) enhanced sedimentation on experimental plots in the Mediterranean (close to Tuscany) by adding 400 g of fine sediment every 45 days on plots of 400 cm² for 1 year. Nearby sites with higher and lower levels of sedimentation were assessed as control plots. Some clear trends were observed. Crustose corallines declined at medium and high levels of sedimentation (Balata et al., 2007). The experiment relates to chronic low levels of sedimentation rather than a single acute event, as in the pressure benchmark, however the trends observed are considered to have some relevance to the pressure assessment. 

As small, sessile species attached to the substratum, siltation at the pressure benchmark would bury Balanus crenatus and Spirobranchus triqueter.  Holme & Wilson (1985) described a Pomatoceros-Balanus assemblage on ‘hard surfaces subjected to periodic sever scour and ‘deep submergence by sand or gravel’ in the English Channel. They inferred that the Pomatoceros-Balanus assemblage was restricted to fast-growing settlers able to establish themselves in short periods of stability during summer months (Holme & Wilson, 1985), as all fauna were removed in the winter months. Barnacles may stop filtration after silt layers of a few millimetres have been discharged as the feeding apparatus is very close to the sediment surface (Witt et al., 2004). In dredge disposal areas in the Weser estuary, Germany, where the modelled exposure to sedimentation was 10mm for 25 days, with the centre of the disposal ground exposed to 65 mm for several hours before dispersal, Balanus crenatus declined in abundance compared to reference areas.  (Witt et al., 2004).  However, separating the effect of sedimentation from increased suspended solids and changes in sediment from sediment dumping was problematic (Witt et al., 2004).

Sensitivity assessment.  Resistance is assessed as ‘Medium’ as the biotope is exposed to frequent abrasion and scouring (the impact may be mitigated by rapid removal of the deposit) but some removal and mortalities may occur. Resilience is assessed as ‘High’ based on re-growth from the scour-tolerant, surviving bases of the encrusting corallines and larval recolonization by Balanus crenatus and Spirobranchus triqueter. Therefore, biotope sensitivity is assessed as 'Low'.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Not relevant

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Encrusting corallines can occur in deeper water than other algae where light penetration is limited. Samples of Lithophyllum impressum suspended from a raft and shaded (50-75% light reduction) continued to grow over two years (Dethier, 1994). Similarly, Plocamium cartilagineum grows in shaded conditions beneath laminarian canopies: where irradiance is greater, growth is lower and it appears that light levels of 0.5 mmol/m²/s are inhibitory (Kain, 1987). In areas of higher light levels, the fronds and bases may be lighter in colour due to bleaching (Colhart & Johansen, 1973). Other red algae in the biotope are flexible with regard to light levels and can also acclimate to different light levels. 

Spirobranchus triqueter is found in a variety of light environments from shallow sublittoral biotopes where light levels are relatively high, to deeper sites that are aphotic (De Kluijver, 1993). Balanus crenatus possesses a rudimentary eye and can detect and respond to sudden shading which may be an anti-predator defence (Forbes et al., 1971). Balanus crenatus tend to orient themselves when settling, with the least light sensitive area directed towards the light (Forbes et al., 1971), so that the more sensitive area can detect shading from predator movements in the area where light availability is lower (Forbes et al., 1971).

 Sensitivity assessment. The key characterizing species colonize a broad range of light environments, from intertidal to deeper subtidal and shaded understorey habitats.  Therefore, the biotope is assessed to have a ‘High’ resistance and, by default, ‘High’ resilience and to be ‘Not sensitive’ to this pressure.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Barriers that reduce the degree of tidal excursion may alter larval supply to suitable habitats from source populations. Conversely, the presence of barriers may enhance local population supply by preventing the loss of larvae from enclosed habitats.  Barriers and changes in the tidal excursion are not considered relevant to the characterizing crusting corallines as species dispersal is limited by the rapid rate of settlement and vegetative growth from bases rather than reliance on recruitment from outside of populations. Resistance to this pressure is assessed as 'High' and resilience as 'High' by default. This biotope is therefore considered to be 'Not sensitive'.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

'Not relevant' to seabed habitats.  NB. Collision by grounding vessels is addressed under surface abrasion

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Many invertebrate species within the biotope probably respond to light levels, detecting shade and shadow to avoid predators and day length in their behavioural or reproductive strategies. However, their visual acuity is probably very limited and they are unlikely to respond to a visual disturbance at the benchmark level. This pressure is, therefore, assessed as ‘Not relevant’.

Balanus crenatus possesses a rudimentary eye and can detect and respond to sudden shading which may be an anti-predator defence (Forbes et al., 1971). Balanus crenatus tend to orient themselves when settling, with the least light sensitive area directed towards the light (Forbes et al., 1971), so that the more sensitive area can detect shading from predator movements in the area where light availability is lower (Forbes et al., 1971).

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

The high levels of scour in this biotope will limit the establishment of all but the most scour resistant invasive non-indigenous species (INIS) from this biotope and no direct evidence was found for effects of INIS on this biotope.

Increased warming has allowed the Australian barnacle Austrominius (formerly, Elminiusi) modestus, to dominate sites previously occupied by Semibalanus balanoides and Balanus crenatus (Witte, 2010). However, on settlement panels deployed in SW Ireland, Austrominius modestus initially dominated panels in the lower subtidal but post-recruitment mortality over a year allowed Balanus crenatus to become the dominant barnacle (Watson et al., 2005). Balanus crenatus and Austrominius modestus have shown recruitment differences, which may alter the seasonal dominance patterns (Witte, 2010). Free-living aggregations (Balanuliths) of Balanus crenatus have been observed growing on shell fragments of the INIS, Ensis directus (Cadée, 2007).

Sensitivity assessment. As scouring of this biotope by mobile sediments limits establishment of all but robust species, resistance to INIS is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is considered to be ‘Not sensitive’.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

No evidence was found that microbial pathogens cause high levels of disease or mortality in this biotope.

Diseased encrusting corallines were first observed in the tropics in the early 1990s when the bacterial pathogen Coralline Lethal Orange Disease (CLOD) was discovered (Littler & Littler, 1995). All species of articulated and crustose species tested to date are easily infected by CLOD and it has been increasing in occurrence at sites where it was first observed and spreading through the tropics. Another bacterial pathogen causing a similar CLOD disease has been observed with a greater distribution, and a black fungal pathogen first discovered in American Samoa has been dispersing (Littler & Littler, 1998). An unknown pathogen has also been reported to lead to white ‘target-shaped’ marks on corallines, again in the tropic (Littler et al., 2007). No evidence was found that these pathogens are impacting temperate coralline habitats.

Sensitivity assessment. Based on the lack of reported mortalities of the characterizing species and the available evidence for the characterizing coralline crust, the biotope is judged to have ‘High’ resistance to this pressure. By default, resilience is assessed as ‘High’ and the biotope is classed as ‘Not sensitive’ at the pressure benchmark. 

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

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 target species on this biotope.  No commercial application or harvesting of characterizing or associated species are described in the literature. Hence, this pressure is considered to be 'Not relevant'.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Incidental removal of the key characterizing species would alter the character of the biotope, resulting in reclassification and the loss of species richness. The ecological services such as primary and secondary production, provided by characterizing and associated species, would also be lost. As most species present in this biotope are relatively large, conspicuous and either sedentary or attached to rock surfaces they have little protection against removal.

Sensitivity assessment.  Removal of a large percentage of the characterizing species resulting in bare rock would alter the character of the biotope, species richness and ecosystem function. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Medium’ (based on the removal of coralline crusts), so that biotope sensitivity is assessed as 'Medium’.