Biodiversity & Conservation

SS.SCS.CCS.PomB

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum would result in loss of its associated community and an intolerance of high has, therefore, been recorded. The biotope is dominated by rapid colonizing species, so that recovery is likely to be very high (see additional information below).
Smothering
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Pomatoceros triqueter, Balanus crenatus are encrusting Bryozoa are low lying epifauna and probably highly intolerant of smothering by 5 cm of sediment, due to clogging of filter-feeding and respiratory apparatus, localized anoxia and associated scour. Holme & Wilson (1985) suggested that scour or deep submergence with sediment probably depopulates the affected substrata. However, Urticina felina (where present) occurred in areas of the English Channel subject to a covering of ca 5cm of sand, and would probably survive where present. Overall, smothering by sediment would probably result in loss of the dominant characterizing species and an intolerance of high has been recorded, although subsequent recovery would probably be very high (see additional information below).
Increase in suspended sediment
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A moderate increased in suspended sediment is likely to increase food availability for suspension feeders, while a significant increase may block filter feeding apparatus. Pomatoceros triqueter has been recorded from areas of high suspended sediment load such as Chichester Harbour (Stubbings & Houghton, 1964). It is also commonly found under boulders, where sediment is likely to settle and be re-suspended by water movement. Similarly, Balanus species are generally tolerant of moderate siltation but are intolerant of excessive siltation (Holt et al., 1995). Balanus crenatus is found in a wide variety of habitats including estuaries and on the carapace of crustaceans in sedimentary habitats, although increased sediment loads may reduce growth rates. Encrusting bryozoans may be more intolerant, although Electra pilosa is relatively tolerant of suspended sediment, for example Moore (1973c; 1977) regarded Electra pilosa to be ubiquitous with respect to turbidity in subtidal kelp holdfasts in north east England. Hydroids, if present may be intolerant, for example Sertularia operculata was reported to die within a few months when transplanted from Lough Ine rapids to sheltered water, due to the build up of a layer of silt (Round, et al., 1961).

While an increase in suspended sediment at the benchmark level for a month is likely to reduce the efficiency of filter feeding in suspension feeders most species are likely to survive for a month. If there is an associated increase in siltation, it is likely to interfere with larval growth and settlement if it coincided with the reproductive season. Therefore, an intolerance of low has been recorded.

Decrease in suspended sediment
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Pomatoceros triqueter has been reported to occur in areas where there is little or no silt present (Price et al., 1980). A decrease in suspended sediment loads may reduce food availability to suspension feeders within this biotope, e.g. Pomatoceros triqueter, Balanus crenatus and encrusting bryozoans. Rapid growth and reproduction is vitally important in this ephemeral biotope, so that a reduction in food supply affect subsequent recruitment. Therefore, an intolerance of low has been recorded at the benchmark level.
Desiccation
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The majority of records of this biotope were recorded from 20-30m depth, while shallower example occurred at 5-10m (JNCC, 1999). Therefore, this biotope is unlikely to experience exposure to the air or desiccation.
Increase in emergence regime
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This biotope occur in the circalittoral and is unlikely to be directly affected by an increase in emergence. However, increased emergence may indirectly affect the effective depth of the biotope potentially exposing it to greater wave action (see below).
Decrease in emergence regime
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This biotope occur in the circalittoral and is unlikely to be directly affected by an decrease in emergence. However, decreased emergence may indirectly affect the effective depth of the biotope potentially exposing it to reduced wave action (see below).
Increase in water flow rate
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Water movement is the most important structuring factor in this biotope. Wave action is probably the most important source of water movement, expect in the few wave sheltered records, and with increasing depth where currents and tidal flow become increasingly significant. In examples of this biotope in extremely exposed to wave exposed conditions, changes in water movement may not be significant. But in moderately exposed to wave sheltered sites water flow is likely to be more important in mobilization of the hard substrata. An increase in water flow from strong to very strong will result in increased scour, and possibly removal of the smaller pebbles cobbles and sands and hence some substratum loss. However, the biotope would probably develop on scoured bedrock although with reduced abundance of individual species. (see Holme & Wilson, 1985). Delicate species such as hydroids would be lost. Therefore, an intolerance of intermediate has been recorded with a recoverability of very high (see additional information below).
Decrease in water flow rate
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Water movement is the most important structuring factor in this biotope. Wave action is probably the most important source of water movement, expect in the few wave sheltered records, and with increasing depth where currents and tidal flow become increasingly significant. In examples of this biotope in extremely exposed to wave exposed conditions, changes in tidal flow may not be significant. But in moderately exposed to wave sheltered sites water flow is likely to be more important in mobilization of the hard substrata. In more wave sheltered sites a reduction in water flow from strong to weak (see benchmark) will stabilize the substratum, allowing more delicate species to colonize and increase in abundance, e.g. hydroids, erect bryozoans, and ascidians, and the biotope may be replaced by more stable epifaunal communities such as £MCR.Flu.SerHyd£. Therefore, an intolerance of high has been recorded. Once prior conditions return recovery is likely to be rapid (see additional information below).
Increase in temperature
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Maximum sea surface temperatures around the British Isles rarely exceed 20 °C (Hiscock, 1998). Pomatoceros triqueter occurs as far south as the Mediterranean, it will therefore be subject to a wider range of temperatures than experienced in the British Isles. Most of the encrusting bryozoan and hydroid species occurring in the biotope are distributed to the north and south of Britain and Ireland (e.g. the bryozoans Electra pilosa and Parasmittina trispinosa, and the hydroids, Sertularia argentea and Hydrallmania falcata) and are also unlikely to be affected by long term changes in temperature. But, acute temperature change may affect growth, feeding and hence reproduction in bryozoans (see Electra pilosa for discussion).

Balanus crenatus is a boreal species, and is intolerant of increases in water temperature. In Queens Dock, Swansea where the water was on average 10 °C higher than average due to the effects of a power station effluent, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite. After the water temperature cooled, Balanus crenatus returned (Naylor, 1965). Balanus crenatus has a peak rate of cirral beating at 20 °C and all spontaneous activity ceases at about 25 °C (Southward, 1955). Gosse (1860) observed that Urticina felina (as Actinia crassicornis) was "one of the most difficult [anemones] to keep in an aquarium" and that "the heat of the summer is generally fatal to our captive specimens". It is therefore likely that local warming may adversely affect individuals and that some mortality might occur.

Overall, a long term change in temperature is unlikely to affect most members of the community, although Balanus crenatus may decrease in abundance in southern records. Several species may be more intolerant of acute temperature change, although this biotope is probably buffered and against acute change due to its depth. Therefore, an intolerance of low has been recorded. Recoverability is likely to be very rapid.
Decrease in temperature
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Minimum surface sea water temperatures rarely fall below 5 °C around the British Isles (Hiscock, 1998). Below a temperature of 7 °C Pomatoceros triqueter is unable to build calcareous tubes (Thomas, 1940). Intertidal populations of Pomatoceros triqueter were reported to suffer 50% mortality at Mumbles, on the Gower after the extreme winter of 1962-63 (Crisp, 1964). But the boreal barnacle Balanus crenatus was unaffected by the 1962-63 winter (Crisp, 1964). Although Urticina felina was apparently unaffected by the extremely cold winter of 1962-63 (Crisp, 1964), Gosse (1860) observed that "after the intense and protracted frost of February 1855, the shores of South Devon were strewn with dead and dying anemones, principally of this species". Most of the encrusting bryozoans and hydroids occur to the north of Britain and Ireland and are unlikely to be affected by long terms changes in temperature, although short term acute change may affect their growth and reproduction. This biotope is probably protected from acute temperature change by its depth. But decreased temperatures may adversely affect survival and larval settlement in Pomatoceros triqueter, and may partly explain why recruitment in rare in winter, possibly leading to a reduction in its abundance. Therefore, an intolerance of intermediate has been recorded, although recovery is likely to be rapid.
Increase in turbidity
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The only species in the biotope that are dependant on light are encrusting corallines. But encrusting coralline algae are amongst the deepest water species of macroalgae occurring in the circalittoral, at great depths, and a light levels as low as 0.05 -0.001% of surface incident light (Lüning, 1990). A reduction in light intensity may reduce their growth rates, especially in the deepest examples of the biotope. However, their extremely slow growth rates mean that the corallines will probably not be adversely affected for the duration of the benchmark. Therefore, not sensitive has been recorded.
Decrease in turbidity
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A increase in light reaching the biotope may benefit encrusting corallines and may encourage the growth of ephemeral algae, especially in the summer months. Hiscock (1986c) described ephemeral algal communities inhabiting pebbles off Skomer. Several species were only present in summer, while others were abundant in summer but survived as small creeping fragments or sporelings during winter (e.g. Polyneura gmelinii, Polysiphonia spp, Lomentaria orcadensis and Rhodophyllis divaricata). Other species, such as Cladophora spp., Bryopsis plumosa and Ulva spp. showed seasonal variation (Hiscock, 1986c). Decreased turbidity may allow similar ephemeral algal to colonize the biotope, especially in its more shallow extent. Therefore, the biotope may be altered and a proportion of the biotope as described lost. Therefore, an intolerance of intermediate has been recorded with very low confidence. Recoverability is likely to be very high.
Increase in wave exposure
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Water movement is the most important structuring factor in this biotope. Mobilization of the stony substratum and associated sediment by wave action, especially in winter storms controls succession and hence the community structure. For example, winter storms were reported to de-populate cobbles off Chesil Bank (Warner, 1985), while severe scour by sand and gravel in the central English Channel depopulated hard substrata (see Holme & Wilson, 1985). However, seasonal depopulation of the biotope is part of the dynamic stability of the community (Warner, 1985). This biotope was reported from wave exposed to very wave exposed habitats. An increase in wave action to extremely wave exposed for a year will probably reduce the abundance of all species within the community, although some individuals will probably survive in the summer months, and the biotope would still be recognizable albeit with a reduced number of species (e.g. delicate hydroids and encrusting bryozoans). Therefore, an overall intolerance of intermediate has been recorded.
Decrease in wave exposure
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Water movement is the most important structuring factor in this biotope. Mobilization of the stony substratum and associated sediment by wave action, especially in winter storms, controls succession and hence the community structure. Wave action is of primary importance to generate water movement except in deeper examples of the biotope or examples in wave sheltered site where water flow is of increased importance (see above). Therefore, a decrease in wave action, e.g. from exposed to sheltered is likely to have significant effects on the biotope. Decreased wave exposure will reduce scour and stabilize the substratum, allowing more delicate and long-lived species (e.g. hydroids, erect bryozoans and ascidians) to colonize the habit. As a result the biotope will probably be replaced by communities characteristic of similar substrata but more stable condition e.g. £MCR.Flu£ or £MCR.Flu.SerHyd£. Therefore, an intolerance of high has been recorded, although recoverability is likely to be very high.
Noise
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Tubeworms, barnacles, hydroids, and bryozoans are unlikely to be sensitive to noise or vibration at the benchmark level. Mobile fish or decapod species may be temporarily scared away from the areas but few if any adverse effects on the biotope are likely to result.
Visual Presence
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Many of the species within the biotope probably respond to light levels, detecting shade and shadow to avoid predators, and day length in their behavioural or reproduction. However, their visual acuity is probably very limited and they are unlikely to be sensitive to visual disturbance at the benchmark level.
Abrasion & physical disturbance
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Physical disturbance by water movement and abrasion due to scour are characteristic of this biotope. Additional abrasion due to anchoring or mobile fishing gear is unlikely to have any significant affect on the community. Mobile fishing gear may remove gravel and pebbles and move cobbles and stones, in which case the effects will be akin to substratum loss above. Overall, not sensitive has been recorded.
Displacement
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Removal of the epifauna, e.g. Pomatoceros triqueter, Balanus crenatus or encrusting bryozoans from their substratum is likely to be terminal. But displacement of the cobbles, stones or pebbles to which they are attached is far more likely. Some individuals will probably be killed by abrasion in the process of displacement but as long as they are displaced to a similar habitat then the majority would survive. However, displacement from the biotope would result in a reduction in their abundance within the biotope and an intolerance of intermediate has been recorded. Recoverability is probably very high (see additional information below).

Chemical Factors

Synthetic compound contamination
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Bryan & Gibbs (1991) suggest that little information was available on the intolerance of polychaetes to tributyl-tin (TBT). Hoare & Hiscock (1974) reported that Pomatoceros triqueter was present in the immediate vicinity of halogenated effluent in Amlwch Bay, suggesting tolerance of chemical contamination. Holt et al. (1995) suggested that barnacles are fairly sensitive to chemical pollution. But Balanus crenatus was the dominant species on pier pilings at a site subject to urban sewage pollution (Jakola & Gulliksen, 1987), while 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. Hoare & Hiscock (1974) observed that Urticina felina also survived near to an acidified halogenated effluent discharge in a 'transition' zone where many other species were unable to survive but was absent from stations closest to the effluent which were dominated by pollution tolerant species such as polychaetes, suggesting a tolerance to chemical contamination. Bryozoans are common members of fouling communities, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in Bryozoa with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100ng/l TBT. Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey, e.g. Electra pilosa occurred at low abundance on laminarian holdfasts within the bay, compared to sites outside the affected area. Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that bryozoans may be at least inhibited by the presence of TBT.

Overall, Pomatoceros triqueter and Balanus crenatus may tolerate some forms of chemical contamination, while encrusting bryozoans may be more intolerant. Chemical contamination may remove some more intolerant species, resulting in a reduction in species richness, but the biotope will probably still be recognizable without loss of extent. Therefore, an intolerance of low has been suggested based on available evidence.

Heavy metal contamination
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Bryan (1984) suggested that, on evidence available for several species, polychaetes are fairly resistant to heavy metals, although no evidence concerning tolerance in Pomatoceros triqueter was found. 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 level 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). Bryozoans are common members of fouling communities and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1979; Holt et al., 1995). Various heavy metals have been show to have sublethal effects on growth in the few hydroids studied experimentally (Stebbing, 1981; Bryan, 1984; Ringelband, 2001).

Overall, the dominant species within the biotope are probably tolerant of heavy metal contamination and no evidence of mortality in nature was found. Therefore, an intolerance of low has been recorded, albeit with low confidence.

Hydrocarbon contamination
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This biotope is protected from the direct effects of oil spills by its depth but may be affected by water soluble fractions of oils, PAHs, hydrocarbons adsorbed onto particulates or oils solublized by dispersants. For example, dispersants and oil from the Torrey Canyon oil spill affected organisms at a depth of 5.5 and 14.5 m in the vicinity of Sennen, an area affected by strong mixing due to heavy wave action.

No information on the effects of hydrocarbon contamination on Pomatoceros triqueter was found. Urticina felina was found to be one of the most resistant animals on the shore one month after the Torrey Canyon oil spill, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968). 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). Therefore, Balanus crenatus may be fairly resistant to hydrocarbon contamination. But bryozoans may be highly intolerant of the effects of oil spills and possibly hydrocarbons (see Electra pilosa and £CR.Bug£ reviews).

Overall , if the physiology within different animals groups can be assumed to be similar, then bryozoans, may be highly intolerant of hydrocarbon contamination, while the other dominant species may be relatively tolerant. Therefore an intolerance of low has been recorded, albeit with very low confidence.

Radionuclide contamination
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No information found
Changes in nutrient levels
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Moderate nutrient enrichment may increase the food available to suspension feeders in this biotope. Eutrophication may increase the green of ephemeral algae in shallower examples of the biotope, although given the degree of scour, their growth is unlikely to adversely affect the biotope. Therefore not sensitive has been recorded. The biotope may be indirectly affected by the death of algal blooms and their subsequent settlement on the seabed but the biotope occurs in areas of strong water movement where the dead algae would be quickly removed.
Increase in salinity
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This biotope occurs in deep waters in fully saline conditions and it unlikely to be affected by further increases in salinity.
Decrease in salinity
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Pomatoceros triqueter occurs in fully saline coastal waters and has not been recorded from brackish or estuarine waters. It occurs under boulders in intertidal habitats where it is probably exposed to variable salinity due to evaporation of seawater or fresh water runoff. Dixon (1985) viewed the species as able to withstand significant reductions in salinity but did not provide quantitative data. Balanus crenatus occurs in upper estuaries and 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 in to a "salt sleep". In this state the animals may survive in fresh water for 3 weeks, enabling them to withstand changes in salinity over moderately long periods (Barnes, 1953). Most bryozoans and hydroids are stenohaline and limited to fully saline conditions (see CR.Bug for details). However, this biotope is probably protected from fresh water runoff by its depth, and unlikely to be subject to decreased salinity.
Changes in oxygenation
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This biotope occurs in areas of strong water movement, where anoxic conditions are unlikely to occur.

Biological Factors

Introduction of microbial pathogens/parasites
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The commensal ciliate Trichodina pediculus was observed in "fair numbers" moving over the branchial crown of Pomatoceros triqueter (Thomas, 1940). Parasites found in the worm include gregarines & ciliated protozoa and parasites that had the appearance of sporozoan cysts. However, no information was found about the effects of these parasites on Pomatoceros triqueter. No information concerning parasites or diseases in the other dominant species was found. Overall, any parasitic burden is likely to affect growth and reproduction, both especially critical in this ephemeral biotope. Therefore an intolerance of low has been recorded.
Introduction of non-native species
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No information found.
Extraction
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It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction and we have no evidence for the indirect effects of extraction of other species on this biotope. Urticina felina is not currently subject to extraction but if a cold water marine aquarium trade were to take-off, this species is likely to be collected. However, its loss form the biotope would probably not significantly affect the integrity of the biotope and not relevant has been recorded.

Additional information icon Additional information

Recoverability
The dominant species are rapid colonizers, capable of rapid growth and early reproduction. For example Pomatoceros triqueter colonized artificial reefs soon after deployment in summer (Jensen et al., 1994), settlement plates within 2-3.5 months and dominated spring recruitment (Hatcher, 1998). Similarly, Balanus crenatus also colonized settlement plates or artificial reefs within 1-3 months of deployment in summer, (Brault & Bourget, 1985; Hatcher, 1998), and became abundant on settlement plates shortly afterwards (Standing, 1976; Brault & Bourget, 1985). Sebens (1985, 1986) noted that calcareous tube worms, encrusting bryozoans and erect hydroids and bryozoans covered scraped areas within 4 months in spring, summer and autumn. Holme & Wilson (1985) suggested that the Pomatoceros-Balanus assemblage on severely scoured hard substrata probably developed rapidly, in less than a year. Tube worms, encrusting bryozoans and hydroids are generally considered to be early colonizers of available hard substrata and are common members of fouling communities (Rubin, 1980; Castric-Fey, 1983; Warner, 1985; Holme & Wilson, 1985; Sebens 1985, 1986; Jensen et al., 1994; Hatcher, 1998). In addition, most of the epifauna is probably subject to severe physical disturbance and scour during winter storms and probably develops annually, through recolonization from any surviving individuals and from adjacent habitats. Therefore, recovery is likely to be very high, the biotope developing within less than year and probably no more than 6 months in spring and summer.

This review can be cited as follows:

Tyler-Walters, H. 2002. Pomatoceros triqueter, Balanus crenatus and bryozoan crusts on mobile circalittoral cobbles and pebbles. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 01/08/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=177&code=2004>