Biodiversity & Conservation

LR.MLR.BF

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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All key and important species in the biotope are highly intolerant of substratum loss. The algae and barnacles are permanently attached to the substratum so populations would be lost. Epifaunal grazers like Patella vulgata and littorinid snails are epifaunal and substratum loss causes increased risk of desiccation and predation and so populations are unlikely to survive. Mobile species like the amphipod Hyale prevostii will be indirectly affected by the loss of fucoid plants as will sessile epiphytic flora and fauna. Recovery is good because recruitment of key species, with the exception of Ascophyllum nodosum, is fairly rapid so that the biotope will look much as before within five years. However, it can take between 10 and 15 years for the natural variation in community structure of the biotope to return to normal after significant mortality of key species such as seen after the Torrey Canyon oil spill (Southward & Southward, 1978).
Smothering
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A 5cm layer of sediment or debris on a barnacle and fucoid shore is likely to reduce photosynthesis of algae and may cause some plants to rot. Sediment will have an especially adverse effect on young germling algae and on the settlement of larvae and spat. Barnacle feeding may be affected and limpet locomotion and grazing may be impaired. Lower down the shore active suspension feeders such as sponges and mussels may be killed by smothering. However, since wave action on rocky shores is likely to mobilise sediment alleviating the effect of smothering intolerance has been assessed as intermediate. Most characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area so recovery should be high.
Increase in suspended sediment
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The biotope is likely to have some tolerance of suspended sediment and siltation as it is also found on sheltered shores where siltation may occur and key species in the biotope have low intolerance to the factor. However, suspended sediment may clog respiratory and feeding organs of other species such as sea squirts and spirorbid worms and so epifaunal species composition may change if suspended sediment changes significantly.
Decrease in suspended sediment
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Desiccation
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A change in desiccation equivalent to a change in position of one vertical biological zone on the shore is likely to change the distribution of the biotope because the key structural algal species can only tolerate desiccation up to a critical level of water content. The upper limit of fucoids will be depressed by an increase in desiccation and the community composition here will change becoming dominated by barnacles and limpets so that the biotope may change from MLR.BF to ELR.MB.Bpat, for example. A decrease in the level of desiccation may result in the upper limit of the biotope extending further up the shore. Most species living below the fucoid canopy will be protected by them from the worst effects of desiccation. Sponges, such as Halichondria panicea, are likely to withstand some desiccation as they hold water. The upper limit of many species however, is likely to be depressed. Sub-biotopes on the upper shore are likely to be less intolerant of changes in desiccation that those on the low shore.
Increase in emergence regime
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A change in the level of emergence on the shore will affect the upper or lower distribution limit of all the key species. Changes in the numbers of important species are likely to have profound effects on community structure and may result in loss of the biotope at the extremes of its range. For example, at the upper limit the biotope may lose fucoid cover and so change to one dominated by barnacles and limpets such as ELR.MB.Bpat.
Decrease in emergence regime
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Increase in water flow rate
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Significant increases in water flow rate may cause some of the macroalgal populations to be torn off the substratum. On the lower shore however, increased water movement encourages several filter feeding faunal groups, such as sponges and ascidians, to occur and species richness may increase. The effect of a decrease in water flow rate is likely to be low because the biotope is also found on shores with low water flow. However, barnacle growth rates are lower in reduced water flow and this may affect the balance of the barnacle-fucoid mosaic, perhaps promoting fucoid dominated shores such that MLR.BF becomes replaced by another biotope such as SLR.Fserr.
Decrease in water flow rate
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Increase in temperature
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The biotope occurs in warmer and colder parts of Britain and Ireland and similar assemblages of species are known to occur in Norway, Canada and Brittany so that long-term temperature change is unlikely to cause a change in biotope. Schonbeck & Norton (1979) demonstrated that fucoids can increase tolerance in response to gradual change in a process known as 'drought hardening'. However, fucoids are more intolerant of sudden changes in temperature and relative humidity with field observations of bleaching and death of plants during periods of hot weather (Hawkins & Hartnoll, 1985). All other key species are moderately tolerant of temperature changes at the benchmark level and so intolerance of the biotope is assessed as intermediate. Larvae and juvenile individuals are likely to be more intolerant of changes in temperature than adults. Changes in the numbers of the key structuring species are likely to have profound effects on community structure.
Decrease in temperature
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Increase in turbidity
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intolerance to turbidity is low because the key species are relatively tolerant of changes in turbidity and the biotope is also found in areas of low water flow where turbidity is likely to be high. An increase in turbidity may reduce algal growth rates because of increased light attenuation although because photosynthesis also occurs during emersion the effect may not be significant. There may be some clogging of suspension feeding apparatus in some species although characteristic species survive in occasionally very turbid conditions and increased turbidity often means an increase in available food particles.
Decrease in turbidity
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Increase in wave exposure
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The effect of changes in wave action on barnacle and fucoid community stability is predominantly through its influence on the balance of the biological interactions. In increasing wave action, fucoids may be removed and grazers and barnacles are favoured at the expense of the fucoids, and a stable situation with minimal fucoid cover prevails. Ascophyllum nodosum, in particular is very intolerant of increased wave exposure. Conversely, if wave exposure reduces fucoids are favoured and maintain a more or less total and permanent canopy (Hartnoll & Hawkins, 1985). Thus, if wave exposure changes the biotope can rapidly disappear to be replaced by another, barnacle dominated on extremely exposed shores (£ELR.Bpat£) and dense fucoid cover on sheltered shores (£SLR.F.Fser£). The loss of fucoid plants results in the loss of structural complexity and invertebrate species diversity may decline in the absence of microhabitats and refugia.
Decrease in wave exposure
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Noise
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None of the selected key or important species in the biotope are recorded as sensitive to noise although limpets and amphipods do respond to vibration. However, the biotope as a whole is not likely to be sensitive to changes in noise levels.
Visual Presence
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Algae have no visual perception. Most macroinvertebrates have poor or short range perception and are unlikely to be affected by visual disturbance such as by boats or humans.
Abrasion & physical disturbance
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The rocky intertidal is not at risk from boating activity but is susceptible to abrasion and physical impact from trampling. Even very light trampling on shores in the north east of England was sufficient to reduce the abundance of fucoids (Fletcher & Frid, 1996) which, in turn reduced the microhabitat available for epiphytic species. Trampling damage is particularly serious for the long-lived but slowly recruiting Ascophyllum nodosum. Light trampling pressure, of 250 steps in a 20x20 cm plot, one day a month for a period of a year, has also been shown to damage and remove barnacles (Brosnan & Crumrine, 1994). Trampling pressure can thus result in an increase in the area of bare rock on the shore (Hill et al., 1998). Chronic trampling can affect community structure with shores becoming dominated by algal turf or crusts. However, if trampling stops, recovery should be good. In Oregon for example, the algal-barnacle community recovered within a year after trampling stopped (Brosnan & Crumrine, 1994).
Displacement
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intolerance to displacement is high because many of the key species in the biotope, including Fucus serratus, Ascophyllum nodosum and Semibalanus balanoides are permanently attached to the substratum and cannot re-establish themselves if detached. Loss of the key species results in loss of the biotope. Removal of the fucoid canopy would create an increased risk of desiccation (see above) for the understory foliose red algae and macroinvertebrates resulting in a significantly reduced species diversity. In general recovery is good because the species have pelagic larvae although recruitment of Ascophyllum nodosum is poor. Removal of space occupying individuals provides room for new individuals to colonize and bare rock is often initially colonized by barnacles.

Chemical Factors

Synthetic compound contamination
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intolerance of the biotope is assessed as high because two of the key species, Fucus serratus and Patella vulgata are highly intolerant of synthetic chemicals. Fucus serratus was found to be intolerant of three biocides likely to be found in the marine environment (Scanlon & Wilkinson, 1987) and fucoids in general are reported to exhibit high intolerance to chlorate and pulp mill effluents containing chlorate (Kautsky, 1992). Patella vulgata is extremely intolerant of aromatic solvent based dispersants such as those used in the Torrey Canyon oil spill clean-up (Smith, 1968). On rocky coasts of Amlwch in areas close to acidified halogenated effluent from a bromine plant the shore consisted almost entirely of bare rock but there was a fucoid-barnacle mosaic nearby (Hoare & Hiscock, 1974).
Heavy metal contamination
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intolerance of the biotope is low because the key structural and functional species are fairly robust in terms of heavy metal pollution. Adult plants of Fucus serratus and Ascophyllum nodosum appear to be fairly tolerant of heavy metal pollution although earlier life stages may be more sensitive (Holt et al., 1997). Barnacles are able to concentrate heavy metals in their tissues and Patella vulgata is found living in conditions of fairly high metal contamination in the Fal estuary in Cornwall (Bryan & Gibbs, 1983). Recovery of all species is high although a return to normal community structure variation may take as much as 10-15 years (Southward & Southward, 1978).
Hydrocarbon contamination
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The loss of key herbivores, such as limpets and littorinids, and the subsequent prolific growth of ephemeral algal mats appears to be a fairly consistent feature of coastal oil spills (Hawkins & Southward, 1992). Species richness, diversity and evenness were all much lower at sites close to the Braer oil spill (Newey & Seed, 1995). In the absence of tarry masses of oil which cause physical smothering of sessile animals and mechanical damage to algae, the adult organisms occupying primary space in the barnacle-fucoid community are relatively resistant to damage from chemical properties of the oil itself, although some damage will inevitably occur. The most serious effects tend to occur among juvenile and newly settling recruits to the community, as well as the small crustaceans, such as the amphipod Hyale prevostii, associated with the dominant intertidal algal species. Recovery should be high because wave action will remove oil from rocky shores and a dispersive larval stage of the key species enables rapid recolonization. However, for severely impacted rocky shore community recovery may be extremely slow, and 10 years or more may elapse before normal community structure and variability have been restored (Southward & Southward, 1978).
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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A reduction in the level of nutrients could reduce growth rates of algal species in the biotope. Nutrient availability is the most important factor controlling germling growth. A slight increase in nutrients may enhance growth rates but high nutrient concentrations could lead to the overgrowth of the algae by ephemeral green algae and an increase in the number of grazers. The effect of sewage discharge on a moderately exposed rocky shore is generally low because water movements should limit the build up of particulates and prevents eutrophication. Fucoids appear to be relatively resistant to the input of sewage, and grow apparently healthily to within 20 metres of an outfall discharging untreated sewage in the Isle of Man (Holt et al., 1997). However, on more sheltered barnacle and fucoid shores increased eutrophication has been demonstrated to lead to increased growth of filamentous brown or green algae both on rocks and epiphytically which can reduce fucoid growth, increase grazer levels such that they become damaging to the fucoids, and prevent development of the young stages of fucoid algae (Kautsky, 1991). Loss of fucoid algae will result in the loss of the biotope so intolerance is assessed as intermediate.
Increase in salinity
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Barnacle and fucoid shores are able to tolerate short term variations in salinity because the littoral zone is regularly exposed to precipitation. All key species are able to penetrate into lower salinity estuarine waters, down to about 20psu so the biotope can tolerate long term reductions in salinity within its normal tolerance range although growth rates and fecundity are likely to be impaired. However, some of the other species within the biotope may be highly intolerant of changes in salinity resulting in a loss of diversity. However most species have planktonic larvae so recolonization and recovery should be high.
Decrease in salinity
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Changes in oxygenation
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Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. There is no information about key algae species tolerance to changes in oxygenation although Kinne (1972) reports that reduced oxygen concentrations inhibit both algal photosynthesis and respiration. Sensitive species, such as the amphipod Hyale prevostii, may be lost resulting in a reduction in diversity.

Biological Factors

Introduction of microbial pathogens/parasites
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The cryptoniscid isopod Hemioniscus balani is a widespread parasite of barnacles, found around the British Isles. Heavy infestation inhibits or destroys the gonads resulting in castration of the barnacle. High levels of infestation may reduce barnacle abundance and distribution which would impact on patch dominance although no reported cases of this were found.
Introduction of non-native species
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The Australasian barnacle Elminius modestus does well in estuaries and bays where it can displace the native Semibalanus balanoides. Its overall effect on the dynamics of rocky shores has however, been small as Elminius modestus has simply replaced some individuals of a group of co-occurring barnacles (Raffaelli & Hawkins, 1999).
Extraction
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Both Fucus serratus and Ascophyllum nodosum are harvested within the UK and the extraction of either of these species will have a significant impact on community structure of the biotope. Removal of algal species will result in loss of micro-habitats for other species and, hence, a reduced faunal diversity. However, the loss will favour the barnacles which would be expected to increase in abundance. It is extremely unlikely that any of the other species indicative of sensitivity would be targeted for extraction and overall, an intermediate intolerance has been suggested.

Recovery should be high because the key species have a dispersive larval stage and reproduce every year. However, a return to normal community structure dynamics after removal of all key species appears to take much longer, 10 and possibly up to 15 years (Southward & Southward, 1978).


This review can be cited as follows:

Hill, J.M. 2000. Barnacles and fucoids (moderately exposed shores). Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 18/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=33&code=1997>