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

Intertidal species are exposed to high and low air temperatures during periods of emersion and must also be able to cope with sharp temperature fluctuations over a short period of time during the tidal cycle. In winter, air temperatures are colder than the sea; conversely in summer, air temperatures are much warmer than the sea. Species that occur in the intertidal are therefore generally adapted to tolerate a range of temperatures, with the width of the thermal niche positively correlated with the height of the shore (Davenport & Davenport, 2005).

Padina pavonica is found in the North-East Atlantic European coast, South Atlantic, Indian and Pacific Oceans, the Mediterranean, and reaches its northern geographical limit in the British Isles. Over its range it experiences warmer air and sea temperatures than in the UK. Its range suggests that it is likely to be tolerant of warmer temperatures, but it should be noted that local populations are likely to have adapted to local temperature regimes and assessments based on the range should be interpreted with caution. The long-term (300 year) population records of Padina pavonica, from southern England, indicate population survival throughout periods of temperature variation.

Warmer temperatures have been shown to accelerate growth and spore production. In the spring of 2014, warmer than usual temperatures (mean 11.52°C, anomaly 1.62°C) meant that fronds of Padina pavonica appeared on shores in the southern UK 14-30 days earlier than usual (Herbert et al., 2016). Warmer spring temperatures can accelerate spore production (Herbert et al., 2016). On the Portuguese coast, a 187 km range extension of Padina pavonica since the 1950–1960s has been attributed to recent warming (Lima et al., 2007).

The rhizoid mat of Padina pavonica may have a greater tolerance for acute increases in temperature than the fronds, although no evidence was found to support this. 

Sensitivity assessment. Based on distribution and growth and spore production observations made by Herbert et al. (2016), Padina pavonica is not considered sensitive to a chronic increase in temperature at the pressure benchmark. An acute increase in temperature at the pressure benchmark may exceed physiological tolerances if this coincides with high summer temperatures leading to possible bleaching and frond die-back, as has been observed for other brown algae.  However, the rock pool habitat may provide some protection and for most of the year an acute increase at the pressure benchmark may be tolerated by at least the rhizoidal mat if not the fronds. Therefore, resistance is assessed as 'Medium' as a precaution.  Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low’. 

Temperature decrease (local)

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

Evidence

Many intertidal species are tolerant of freezing conditions as they are exposed to extremes of low air temperatures during periods of emersion. They must also be able to cope with sharp temperature fluctuations over a short period of time during the tidal cycle. In winter, air temperatures are colder than the sea; conversely in summer, air temperatures are much warmer than the sea. Species that occur in the intertidal are therefore generally adapted to tolerate a range of temperatures, with the width of the thermal niche positively correlated with the height of the shore (Davenport & Davenport, 2005).

Padina pavonica is at the northern most limit of its geographical range in the UK suggesting that a long-term, reduction in temperature may reduce habitat suitability for this species. Herbert et al. (2016)  assessed a 300-year time series of Padina pavonica populations in southern England. At some sites populations have persisted for at least 200 years and the UK population has withstood the conditions of the Little Ice Age at the end of the 17^th century and particularly cold winters such as 1947, 1963 and 2010-2011 (Price et al., 1979; Herbert et al., 2016). The survival is attributed to the tolerance of the rhizoidal stage that is tolerant of low temperatures although growth and spore production may be inhibited during cooler periods (Herbert et al., 2016). In spring 2013, when temperatures were cooler than usual (mean 9.26°C temperature anomaly -0.63°C) the appearance of Padina pavonica fronds was delayed at all sites in southern England (Herbert et al., 2016).

Sensitivity assessment. Padina pavonica is at the northern limit of its range in the UK and lower temperatures that are less than the chronic pressure benchmark have been shown to inhibit the appearance of fronds and effect spore production (Herbert et al., 2016). Padina pavonica is, therefore, considered to have ‘Medium’ resistance to decreases in temperature based on a reduction in frond growth and spore production and to have 'High' resilience (where rhizoids survive) following reestablishment of typical temperatures. Therefore, 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

No evidence was found to assess the salinity tolerances of Padina pavonica. As it occurs in intertidal coastal habitats that experience full salinity (Price et al., 1979), the assessed change at the pressure benchmark is an increase in salinity to hypersaline (>40 ppt).  Like all species found in the intertidal, Padina pavonica will naturally experience fluctuations in salinity where evaporation on warm days increases salinity in pools and inputs of rainwater expose individuals to freshwater. Species found in the intertidal typically have some form of physiological adaptations to withstand fluctuations in salinity. Typically the upper shore distribution of species in the intertidal is determined by physiological tolerances to emersion, salinity and temperature (Barnes & Hughes, 1992). Species that occur lower on the shore are exposed to salinity variations for shorter times (due to tidal immersion) than those that occur on the upper shore levels and tend to be less tolerant of salinity changes.

Sensitivity assessment.  Although some increases in salinity may be tolerated by Padina pavonica, the natural variation, (rather than the pressure benchmark) is generally short-term and mitigated during tidal inundation.  Based on the distribution of Padina pavonica on the mid to lower shore, this species is considered to be sensitive to a persistent increase in salinity to >40 ppt (for one year; see benchmark). Therefore, resistance is assessed as ‘Low’ and it is possible that the rhizoidal mat may be lost. Recovery is assessed as ‘Medium' (following restoration of usual salinity). Sensitivity is, therefore, assessed as ‘Medium’.  However, confidence in this assessment is 'Low'.

Salinity decrease (local)

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

Evidence

No evidence was found to assess the salinity tolerances of Padina pavonica. As this species occurs in the UK in intertidal coastal habitats that experience full salinity (Herbert et al., 2016), the assessed change at the pressure benchmark is a reduction in salinity to a variable regime (18-35 ppt) or reduced regime (18-30 ppt) for one year.

High rainfall will reduce salinity on rock surfaces or pools when exposed to air and may create a surface layer of brackish/nearly freshwater for a period. Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity.

Sensitivity assessment.  As Padina pavonica is restricted to full salinity habitats it is considered to be sensitive to a long-term decrease in salinity at the pressure benchmark. Resistance is, therefore, assessed as ‘None’ and recovery as ‘Very low’ . Sensitivity is, therefore, assessed as ‘High’.

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

No direct evidence was found to assess the sensitivity of Padina pavonica to changes in water flow at the pressure benchmark. Padina pavonica grows on sheltered and wave exposed shores (Herbert et al., 2016) suggesting that it can tolerate a range of water flows. The rhizoid mat of Padina pavonica is securely attached and with its low profile is subject to less drag than the upright fronds. Storm events remove fronds, indicating the vulnerability of these on exposed shores to short events of increased water movement. Changes in water flow during storms, however, is likely to exceed the pressure benchmark. Decreased water flow may result in less frond damage so that spore production is enhanced and may also result in more settlement on shores if spores are not removed by water action. 

Sensitivity assessment. Padina pavonica resistance to increased and decreased water flow (at the pressure benchmark) is assessed as ‘High’ and resilience as ‘High (by default) so that the biotope is assessed as ‘Not sensitive’ at the benchmark level. No direct evidence was found to assess this pressure at the benchmark and confidence is low.

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

Emersion is a key factor structuring the distribution of intertidal species (Barnes & Hughes, 1992). Padina pavonica is found in the mid-lower shore (Herbert et al., 2016) and shallow subtidal. Increased emergence may result in greater desiccation and result in habitats at the upper limit of the species intertidal range becoming unsuitable for Padina pavonica, resulting in a shift in species distribution on the shore.

A decrease in emergence would mean that shallow rockpools would be at less risk of desiccation. In addition, depending on the nature of the surrounding bedrock, the rockpool may become slightly deeper. Decreased emergence may result in intertidal habitats that were formerly unsuitable increasing in quality,  however, over a course of a year Padina pavonica is unlikely to become established in great densities in newly suitable areas.  Individuals in the shallow subtidal may experience greater grazing pressure or a reduction of light penetration that reduces habitat suitability; light levels are a key factor influencing growth and the penetration of Padina pavonica in the shallow subtidal in the Mediterranean is likely due to the greater irradiance in that region (Herbert et al., 2016).

Sensitivity assessment. Changes in emergence may result in habitats at the upper and lower parts of the intertidal subtidal range becoming unsuitable for Padina pavonica with some losses or gains of habitat. Resistance to increased emergence is assessed as ‘Low’ as Padina pavonica occurs on the mid to lower shore. The basal crusts may allow individuals to persist in conditions that are unfavourable to frond development until the emergence regime is re-established. Recovery is assessed as ‘High’ (assuming that rhizoids survive) and sensitivity is therefore assessed as ‘Medium’, so that sensitivity is assessed as ‘Low’.

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

Padina pavonica is recorded at locations of varying wave exposure, ranging from sheltered to exposed (Herbert et al., 2016). The degree of wave exposure influences wave height, as in more exposed areas with a longer fetch waves would be predicted to be higher. Storm events remove fronds (Herbert et al., 2016), indicating the vulnerability of plants on exposed shores to short-term events of increased wave action. Changes in wave height during storms, however, are likely to exceed the pressure benchmark.

Sensitivity assessment. As Padina pavonica occurs across a range of exposures, this was considered to indicate, by proxy, that populations in the middle of the wave exposure range would tolerate either an increase or decrease in significant wave height at the pressure benchmark. Resistance is therefore assessed as ‘High’ and resilience as ‘High’ (by default) so that this species is assessed as ‘Not sensitive.

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 evidence is presented where available. Campanella et al. (2001) and Schintu et al. (2010) both highlighted the suitability of Padina pavonica to monitor metal pollution but further studies were needed to clarify accumulation patterns.

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.

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.

Radionuclide contamination

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

Evidence

No evidence was found to assess this pressure at the benchmark. Algae bioaccumulate radionuclides (with extent depending on the radionuclide and the algae species). Adverse effects have not been reported at low levels.

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

The effects of reduced oxygenation on algae are not well studied. Plants require oxygen for respiration, but this may be provided by the production of oxygen during periods of photosynthesis. Lack of oxygen may impair both respiration and photosynthesis (see review by Vidaver, 1972). Padina pavonica would only be exposed to low oxygen in the water column intermittently during periods of tidal immersion. In addition, in areas of wave exposure and moderately strong current flow, low oxygen levels in the water are unlikely to persist for very long as oxygen levels will be recharged by the incorporation of oxygen in the air into the water column or flushing with oxygenated waters.

Sensitivity assessment. No direct evidence for the effects of hypoxia on Padina pavonica was found. As it will only be exposed to this pressure when submerged and respiration will occur in air, resistance was assessed as ‘High’ and resilience as ‘High’ (no effect to recover from), resulting in an assessment of 'Not sensitive'.

Nutrient enrichment

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

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The pressure benchmark is set at compliance with Water Framework Directive (WFD) criteria for good status, based on nitrogen concentration (UKTAG, 2014). 

Padina pavonica is not associated with increased abundance in response to eutrophication but it may adapt to local increases in nutrient levels (Price et al., 1979). In experiments on Padina pavonica from the Mediterranean, increased nutrient levels enhanced growth  (Celis-Plá et al., 2015). Populations of Padina pavonica have been documented on shores in southern England since the late 17^th Century and have persisted through periods when anthropogenic inputs of nutrients were potentially both higher and lower than today.

Sensitivity assessment. The pressure benchmark is set at a level that is relatively protective and based on the evidence and considerations outlined above the biological assemblage is considered to be 'Not sensitive' at the pressure benchmark. Resistance and resilience are, therefore, assessed as 'High'. 

Organic enrichment

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

Evidence

Padina pavonica occurs on shores where there are some sediments, and habitats where it occurs may be subject to periodic sedimentation. The persistence of populations on such shores suggest that individuals would not be affected by the settlement of fine particles of organic matter in pools or on shores. On most shores organic matter is likely to be removed rapidly by water movements or consumed by deposit feeders limiting exposure further.

Although adults are likely to be unaffected, low levels of fine organic matter may inhibit survival and settlement of early life stages. Irving et al. (2009) found that survival of Cystoseira barbata germlings was negatively impacted (approximately 83% mortality) when exposed to thin layers of sediments (approximately 8.5 g per petri dish) while Moss et al. (1973) found that growth of zygotes of  Himanthalia elongata were inhibited by a layer of silt 1-2 mm thick and that attachment on silt was insecure.

Sensitivity assessment. Based on resistance to sedimentation and exposure to wave action, the resistance of adult Padina pavonica populations is assessed as ‘High’ and resilience as ‘High’ (by default). Therefore, they are assessed as ‘Not sensitive’ to this pressure at the benchmark.  

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

The loss of hard substratum would remove the rock habitat; sediments would be unsuitable for Padina pavonica. Artificial hard substratum may also differ in character from natural hard substratum, so that replacement of natural surfaces with artificial may lead to changes in Padina pavonica populations.  

Sensitivity assessment. Based on the loss of suitable habitat, Padina pavonica resistance is assessed as ‘None’, resilience is assessed as ‘Very low’, as the change at the pressure benchmark is permanent. Padina pavonica sensitivity is, therefore ‘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

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

Padina pavonica occurrs 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

Padina pavonica occurs on the rock surface and, therefore, have no shelter from abrasion at the surface. No direct evidence was found to assess abrasion impacts on this species. Abrasion can remove the frond of brown algae although experimental results vary. Experimental results and observations of trampled vs non-trampled areas have shown that turf-forming algal species are relatively resistant to this pressure and may increase in abundance where foliose and canopy forming species are removed.  Brosnan & Crumrine (1994), for example, found that in experimentally trampled plots the cover of foliose and canopy forming species declined while turf forming algae were relatively resistant. Brosnan (1993) noted that algal turf species (Endocladia muricata and Gelidium spp.) increased by 38% in trampled plots as foliose algae declined, and algal turf dominated trampled areas. Exclusion of visitors, and hence reduced trampling, reduced relative algal turf abundance by 31%, while foliose algae increased in abundance. Schiel & Taylor (1999) observed a decrease in understorey algae (erect and encrusting corallines) after 25 or more tramples, probably due to an indirect effect of increased desiccation as above. However, Schiel & Taylor (1999) did not detect any variation in other algal species due to trampling effects and Keough & Quinn (1998) did not detect any effect of trampling on algal turf species.

Sensitivity assessment. The impact of surface abrasion will depend on the footprint, duration and magnitude of the pressure. Resistance, to a single abrasion event of Padina pavonica is assessed as ‘Medium’ (<25% of population damaged or removed) as some damage to individuals is likely and fronds may be removed although the bases may remain. Recovery is probably ‘High’ through regrowth of fronds from rhizoids, so that sensitivity is assessed as ‘Low’.  Resistance and resilience will be lower (and hence sensitivity greater) to abrasion events that exert a greater crushing force and remove the rhizoid mat.

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

Padina pavonica occurrs on rock which is resistant to subsurface penetration.  The assessment for abrasion at the surface only (see above) 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

In general, increased suspended particles reduce light penetration and increase scour and deposition, which may adversely affect algae, and interfere with settling spores and recruitment if the factor is coincident with their major reproductive period. Siltation, which may be associated with increased suspended solids and the subsequent deposition of these is assessed separately (see siltation pressures).

Where it occurs intertidally, as it does in the UK, Padina pavonica will only be exposed to this pressure when submerged during the tidal cycle, exposure to this pressure is, therefore, limited. Padina pavonica occurs on soft substratum shores and is often found adjacent to areas of clays, silts or sands (Herbert et al., 2016). The presence of sediments is likely to lead to periods when water movement and wave action results in high levels of suspended sediment. The persistence of Padina pavonica on such shores and its survival of smothering events (see siltation pressures below), indicate that it has some tolerance for high levels of suspended solids and scour. The calcified fronds and rhizoids of Padina pavonica are likely to be resistant of sediment scour and regeneration will not be dependent on settlement. A reduction in suspended sediment will reduce the level of scour and increase light penetration of the water column enhancing growth.

Sensitivity assessment. The exposure of Padina pavonica to suspended sediments in the water column will be limited to immersion periods. Wave action and water flows will re-suspend and remove solids, reducing accumulation.  Padina pavonica is considered to be ‘Not sensitive’ to a reduction in suspended solids. An increase in suspended solids may lead to some sub-lethal abrasion of fronds and may reduce growth of Padina pavonica that occur in deeper rock pools due to a reduction in light penetration. Resistance is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is considered to be ‘Not sensitive’. An increase in suspended solids above the pressure benchmark may result in a change in species composition with an increase in species seen in very turbid, silty environments e.g. Ahnfeltia plicata, Rhodothamniella floridula, Polyides rotundus or Furcellaria lumbricalis.

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

Padina pavonica occurs on soft substratum shores and is often found adjacent to areas of clays, silts or sands (Herbert et al., 2016). Sediment movement by water actions means that Padina pavonica is likely to experience siltation and scouring. Its persistence on such shores, indicate that it has some tolerance of this pressure. At Bembridge in the Isle of Wight (southern England), an exposed ledge where Padina pavonica is found in shallow rock pools occurs below a sandy beach, the movement of sand leads to frequent smothering of rock pools to depths of 5-25 cm for varying duration and extent (Herbert et al. 2016).  Sand movements at this site, following summer storms affected clump abundance, smothering and frond-size frequency of Padina pavonica,  the rhizoid mat is more resistant to scouring and smothering than the fronds and the plants recovered via regrowth from rhizoids.  In Compton Bay in the Isle of Wight, Padina pavonica abundance changed from ‘common’ to ‘rare’ following scouring of mid-shore pools due to sand scouring caused by winter storms (Herbert et al., 2016).

The species’ resilience to periods of sand-smothering and abrasion may have facilitated survival on dynamic shores consisting of softer substrata where other, potentially competitive, algal species would fail (Herbert et al. 2016).

Sensitivity assessment. Padina pavonica is adapted to periodic siltation and scouring. Padina pavonica resistance is assessed as ‘Medium’ and resilience as ‘High’.  Therefore, sensitivity is assessed as ‘Low’ at the benchmark of up to 5 cm added in a single discrete event.

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

Sediment movement by water actions means that Padina pavonica is likely to experience siltation and scouring. Despite complete sand-smothering in 2012, large clumps of fronds appeared in both 2013 and 2014 (Herbert et al., 2016). Recovery of the exposed population from sand-smothering may have been due to the survival of perennating rhizoids and/or spores within the sediment or clay substratum (Herbert et al., 2016).  However, a longer period of sand-smothering or complete erosion of the substratum surface may negatively affect survival of Padina pavonica on a shore(Herbert et al., 2016).

Sensitivity assessment. At the pressure benchmark, all the fronds are likely to be damaged and lost. High levels of sediment deposition and scour may also remove some rhizoids hindering recovery. Padina pavonica resistance is assessed as ‘Low’ and resilience as ‘Medium’.  Therefore, sensitvity is assessed as ‘Medium’.

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

Padina pavonica has no hearing perception and this pressure is considered ‘Not relevant’.

Introduction of light or shading

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

Evidence

Separating sensitivity to temperature and solar radiation is particularly difficult as they often co-vary. Light irradiance is a major factor influencing macroalgal shore height occupation. Considering the species higher abundance in southern latitudes and greater depth penetration in the Mediterranean, light levels may be particularly significant for growth. Light levels are more important to early growth of Padina pavonica than water temperature. It is, therefore, possible that population decline in the late nineteenth century was the result of a combination of stressors including lower irradiance due to longer periods of cloud cover (Herbert et al. 2016). The introduction of light could therefore be considered as beneficial for Padina pavonica.

Sensitivity assessment. As Padina pavonica colonize a broad range of light environments from intertidal to deeper sub tidal habitats it is considered to have ‘High’ resistance and, by default, ‘High’ resilience and, therefore, is ‘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 the supply of propagules to suitable habitats from source populations. Conversely, the presence of barriers may enhance local population supply by preventing the loss of spores from enclosed habitats. The low profile of Padina pavonica is considered likely to limit spore dispersal, and it is most probable that the relatively heavy tetraspores sink to the bottom of parent rock pools.  The degree of connectivity between populations is unclear (Herbert et al., 2016) but it is likely that populations persist through vegetative growth and local recruitment close to adult plants. At the pressure benchmark, barriers are considered unlikely to lead to direct impacts on populations of Padina pavonica. As propagule supply is not considered within the pressure benchmark this pressure is assessed as 'Not relevant'.

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 interaction with bottom towed fishing gears and moorings are 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

Not relevant

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

No evidence was found to suggest that Padina pavonica was subject to translocation or genetic modification.

Introduction or spread of invasive non-indigenous species

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

Evidence

Invasive non-indigenous species (INIS) that can alter habitats (ecological engineers), or out-compete native macroalgae for space and other resources such as light and nutrients, are the most likely species to negatively affect Padina pavonica.  Padina pavonica often occurs on soft rock substrata and in areas where sediment movement may lead to repeated smothering events (Herbert et al., 2016). These habitat characteristics may limit the INIS species that can become established.

Algal species that may have overlapping habitat requirements include the green seaweed Codium fragile subsp tormentosoides (now renamed as Codium fragile fragile) and the red seaweed Heterosiphonia japonica.  Neither of these have so far been recorded in 'nuisance' densities (Sweet, 2011j). Wireweed, Sargassum muticum, grows best on sheltered shores and in rockpools (Sewell, 2011c). The red seaweeds Heterosiphonia japonica and Neosiphonia harveyi may also co-occur on shores but again no impacts have been reported. The red seaweed Grateloupia turuturu occurs on the lower shore in pools.  No ecosystem impacts have been reported in Great Britain.

The tunicates Didemnum vexillum and Asterocarpa humilis, the hydroid Schizoporella japonica and the bryozoan Watersipora subatra (Bishop, 2012c, Bishop, 2015a and b; Wood, 2015) are currently only recorded from artificial hard substratum in the UK and it is not clear what their established range and impacts in the UK would be. Didemnum vexillum occurs in tide pools in other areas where it has become established (Bishop, 2012c) and can have substantial effects on communities. Similarly the tunicates Corella eumyota and Botrylloides violaceus can smother rock habitats (Bishop, 2011b and 2012b). A significant potential INIS is the Pacific oyster Crassostrea gigas, as this reef forming species can alter habitat structure. In the Wadden Sea and the North Sea, Crassostrea gigas overgrows mussel beds in the intertidal zone (Diederich, 2005, 2006; Kochmann et al., 2008). However, in softer substratum, it may be removed by water movements and be unable to establish.

Sensitivity assessment. No evidence was found to assess the impact of INIS on Padina pavonica . 

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 to assess this pressure.

Removal of target species

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

Evidence

Seaweeds have been collected from the middle of the 16th century for the iodine industry. Modern day industrial uses for seaweed are extensive and include fertilizer, animal feed, alginate extracts (Phillipi et al., 2014), water treatment, and human food and health supplements (Bixler & Porse, 2010). Commercial harvesting can remove seaweed canopies resulting in effects on the wider ecosystem (Stagnol et al., 2013). Due to the intolerance of macroalgae communities to human exploitation, the European Union put in place a framework to regulate the exploitation of algae establishing an organic label that implies that ‘harvest shall not cause any impact on ecosystems’ (no. 710/2009 and 834/2007).

Padina pavonica is used in cosmetic products and has also been found to contain anti-fungal properties (Wang et al., 2015). The plants are cultivated in the Mediterranean for these properties. But no evidence was found to suggest that this species is subject to commercial harvesting in the UK. In the 19th century, collecting algae to create herbarium records was a popular hobby and over-collecting of Padina pavonica may have occurred on some shores leading to local extinctions (Herbert et al., 2016).

Sensitivity assessment. Padina pavonica is attached and a single event of targeted harvesting could therefore efficiently remove individuals and resistance is assessed as ‘Low’.  Resilience is assessed as ‘High’ as the rhizoid mat is assumed to be unaffected and fronds will regrow from this. Hence, sensitivity is assessed as 'Low'. This assessment refers to a single collection event, long-term harvesting over wide spatial scales will lead to greater impacts, with lower resistance and longer recovery times.

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

The rhizoid mat is unlikely to be removed as a result of harvesting or collecting of another intertidal species. However, the fronds could be accidentally removed as by-catch when other algal species were being targeted if harvesters were not being selective. 

Sensitivity assessment. The resistance of the fronds to incidental removal is ‘Low’ due to the easy accessibility of the biotopes location and the inability of these species to evade collection. The rhizoid mat is unlikely to be removed and resilience is assessed as ‘High’, and sensitivity as ‘Low’.