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

Species found in the intertidal are exposed to extremes of high and 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 this intertidal biotope are therefore generally adapted to tolerate a range of temperatures, although the timing of site-specific factors such as low tides will influence local acclimation. For intertidal species, increased temperatures may also result in desiccation when exposed (see changes in emergence pressure).

The key characterizing species Bifurcaria bifurcata is a southern species and is at the edge of its northern range where it occurs in south-west England. Range oscillations have been recorded for this species for the past 100 years, with retractions in southwest England during colder phases followed by short range extensions during warmer periods (Helmuth et al., 2006). Several studies from Spain suggest that, even further south in its range, warming temperatures are beneficial for Bifurcaria bifurcata. Following die-back of the cold-temperate fucoid Fucus serratus, Bifurcaria bifurcata colonized the newly vacated areas, expanding its range up the shore (Álvarez-Losada et al., 2020). Similarly, after resurveying 20 sites previously sampled in 1977, Fernández (2016) reported large-scale changes in algal assemblages, with cold-temperate species such as Fucus serratus and kelps almost entirely replaced by warm-temperate species, including Bifurcaria bifurcata. Ramos et al. (2020) observed similar changes over a shorter timescale (2011-2017), again reporting that Bifurcaria bifurcata was among the warm-temperate species that had significantly increased (by approximately 450% at one site), while cold-temperate species such as Himanthalia elongata had declined drastically. Indeed, they propose that Bifurcaria bifurcata would be a good candidate for monitoring warming, predicting further northward expansions to its range as temperatures increase (Ramos et al., 2020). Long-range habitat modelling based on projected sea surface temperatures for 2080-2099 also suggests that it will persist and possibly expand to occupy space vacated by less heat-tolerant species (Des et al., 2020). However, some studies suggest that this relative heat tolerance does not come without some costs: despite displacing cold-temperate species at Spanish sites resurveyed after initial surveys in 1977 (Fernández, 2016), Méndez-Sandín & Fernández (2016) found that Bifurcaria bifurcata-dominated assemblages had shifted from three annual growth periods in 1977 to two in 2007, and that Bifurcaria bifurcata’s dominance of these assemblages had reduced from 82% to 73% over the same period. A lab study that exposed several macroalgae species to repeated simulated heatwaves affecting both air and water temperature found that Bifurcaria bifurcata had a lower mortality probability than some species when first exposed to increased air temperatures (28.40 ± 0.19°C and 30.60 ± 0.14°C, compared to 22.93 ± 0.08°C in the control treatment), but much higher mortality when emersed for a second time. Growth also decreased as seawater temperature increased from 18.45 ± 0.01 °C (control) to 19.74 ± 0.02°C and 21.77 ± 0.03°C, and additive effects of exposure to increased air and water temperatures further decreased growth (Román et al., 2020).

Corallina officinalis has a cosmopolitan distribution (Guiry & Guiry, 2015) and experiences wide variation in temperatures throughout its range (although local populations may be acclimated to the prevailing thermal regime). Coralline crusts, including Lithophyllum incrustans, are found further south than the UK and are considered to tolerate increased temperatures, although they may be more sensitive to drying rather than higher temperatures. 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. Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water, which were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual algae (or their spores) were suggested to be more susceptible than asexual ones to extremes of local environmental variables (temperature, salinity etc.) as they occured with greater frequency at sites where temperature and salinity were more stable (Edyvean & Forde, 1984b). In a multidecadal analysis, Fernández (2016) reported that in northern Spain, resurveys of 20 shores previously sampled in 1977 found declines in cold-temperate algae species but increases in warm-temperate taxa including coralline algae such as Lithophyllum incrustans; suggesting that some crustose corallines may expand under warming conditions. In the UK, Lithophyllum incrustans is close to the northern edge of its range and is likely to tolerate increased temperatures.

Littler & Kauker (1984) suggested that the crustose bases of Corallina officinalis are more resistant of desiccation or heating than fronds. Severe damage was noted in Corallina officinalis fronds as a result of desiccation during unusually hot and sunny weather in summer 1983. An abrupt increase in temperature of 10°C caused by the hot, dry 'Santa Anna' winds (between January -and February) in Santa Cruz, California resulted in die back of several species of algae exposed at low tide (Seapy & Littler, 1982).  Lüning (1990) reported that Corallina officinalis from Helgoland survived one week exposure to temperatures between 0°C and 28°C. Short et al. (2015) observed that crustose Hydrolithoideae in Australia maintained growth and calcification within expected ranges in the two years following a marine heatwave, but mortality reached up to 37.5% during summer, indicating potential stress following prolonged temperature elevation. Dee et al. (2021) found that cover of crustose coralline assemblages in Australia was significantly reduced during hot, wet seasons when sea surface temperatures exceeded 29 °C for four weeks, although carbonate production rates remained stable.

In an exceptionally hot summer (1983 with an increase of between 4.8 and 8.5°C) Hawkins & Hartnoll (1985) observed no temperature bleaching of adult Himanthalia elongata (although some buttons were bleached) or other canopy forming species. However, understorey red algae showed more signs of damage with bleached Corallina officinalis and ‘lithothamnia’ observed around the edges of pools due to desiccation. Occasional damaged specimens of Palmaria palmata, Osmundea pinnatifida and Mastocarpus stellatus were observed.

Latham (2008) investigated the effects of temperature stress on Corallina officinalis through laboratory tests on samples collected in the Autumn in Devon, England from rockpools. Samples were kept at 15°C for three days and then exposed to temperatures of 5°C, 15°C, 20°C, 25°C and 30°C (the normal range of temperature experienced was suggested to be between 5 and 15°C). At 35°C, the Corallina was completely bleached after three days with a sample kept at 30°C beginning to bleach. After seven days (the end of the experiment) the sample kept at 30°C was partially bleached.  Samples kept at 5, 15, 20 and 25°C showed little change in chemicals produced in reaction to thermal stress and no bleaching suggesting the temperatures in that range had not induced stress.

Kolzenburg et al. (2019, 2021; Kolzenburg, Coaten et al., 2023) compared Corallina officinalis populations from Iceland (its northern range margin), the UK (central) and Spain (southern margin). Northern populations were the most robust, showing potential for local physiological adaptation to environmental variability. Southern populations appeared to be at their upper limit of stress tolerance, with higher respiration in winter and reduced primary production despite higher calcification rates, suggesting a maladaptive focus on maintaining structure rather than growth (Kolzenburg, Coaten et al., 2023). Physiological differences were clear between northern and central, and northern and southern populations, but not consistently between central and southern populations, suggesting that UK populations may already share some of the thermal stress limitations seen at the southern margin (Kolzenburg, Coaten et al., 2023). Transplant experiments showed reduced calcification when UK populations were exposed to southern winter conditions (11.3°C) compared with their native winter temperatures of 5.7°C. (Kolzenburg et al., 2019). Kolzenburg et al. (2021) reported that temperature had greater effects on physiological performance than pCO₂ (a proxy for ocean acidification), with UK populations closer to their thermal limits than northern populations. Marine heatwave experiments (+3°C above ambient) found little effect on UK populations, although southern populations showed a non-significant trend towards stress (Kolzenburg et al., 2024).

Other laboratory studies have tested Corallina officinalis temperature responses across a broad range. Kim et al. (2018) found that calcification rates decreased with increasing temperature from 13 to 28°C, and respiration increased significantly at 28°C, although growth and photosynthesis were not significantly affected. Graba-Landry et al. (2018) reported that growth decreased at 28°C, and calcification declined when elevated temperature was combined with reduced pH. Rendina et al. (2019) observed that a seven-week exposure to 3°C above ambient temperature resulted in decreased photosynthesis, respiration and calcification, although a shorter one-week heatwave (ambient +1°C) had no effect. Vásquez-Elizondo et al. (2022) recorded increased photosynthesis with rising temperature from 10 to 20–25°C, followed by a three-fold decline at higher temperatures; respiration increased steadily with temperature up to 35°C. Ismail et al. (2023) reported that Corallina officinalis was dominant across all seasons in the Mediterranean and tolerated a wide range of temperatures. Buršić et al. (2023) recorded higher invertebrate abundance in Corallina officinalis turfs during cooler winter months (5–10 °C) compared with summer (22–25 °C), reflecting seasonal growth patterns of the alga. Williamson et al. (2017) reported that Corallina officinalis is adapted to both seasonal and tidal variability in environmental stressors, including temperature, in the UK, although they predict that the balance of metabolic processes may be affected by future climate change. Qui-Minet et al. (2019) tested physiological responses of Lithophyllum incrustans alongside rhodolith-forming Lithothamnion corallioides and Phymatolithon calcareum under a high-emissions future climate scenario (+3 °C, reduced pH; IPCC RCP 8.5) with and without nutrient enrichment. Responses were species-specific and seasonally dependent. In winter, future conditions impaired light harvesting and photoprotective mechanisms in the rhodolith species, and calcification rates declined to net dissolution under low nutrient concentrations, although nutrient enrichment reduced these losses. In contrast, Lithophyllum incrustans showed enhanced calcification in the dark under nutrient enrichment in summer. While all three species displayed higher maximal calcification rates in winter under future conditions, inhibition at high irradiance occurred in P. calcareum, and summer calcification dropped in L. corallioides. The study highlights how local nutrient and irradiance regimes can mediate coralline responses to climate stressors. Simon-Nutbrown et al. (2020) modelled future distributions of non-geniculate corallines in the UK and predicted significant range contractions under a range of future climate warming prediction, with declines of 38% following an increase of <2°C (IPCC RCP2.6) and 84% under the “worst-case” climate scenario representing a temperature increase of 3°C (IPCC RCP 8.5).

Although this biotope is characterized by crustose corallines, several of the species that can occur in this biotope, including Lithothamnion crispatum, Lithothamnion glaciale, and Phymatolithon lusitanicum, can form free-living rhodoliths in addition to crusts, and so studies on their rhodolith forms can provide additional insight into potential physiological responses, albeit with caution given morphological and ecological differences. Koerich et al. (2021) showed that Lithothamnion crispatum rhodoliths off Brazil experienced reduced photosynthetic performance when exposed to combined warming (an increase of 6°C), eutrophication and acidification consistent with worst-case future projections (IPCC RCP 8.5) for five days but recovered to pre-experiment values within five days once conditions returned to ambient. Species distribution models projected a future range expansion under RCP 8.5. Bélanger & Gagnon (2021) reported that Lithothamnion glaciale rhodoliths from Newfoundland maintained normal growth between approx. 1 and 16°C, with resilience to higher temperatures than normally experienced in situ. Legrand et al. (2017) found that net primary production in Lithothamnion corallioides was not altered by a +3°C treatment, although gross primary production declined at higher winter temperatures. Muñoz et al. (2018) reported that Lithothamnion crispatum showed reduced physiological performance under combined future pH and temperature conditions (30 ± 2°C, pH 7.4) but suggested that their overall biological viability indicated capacity to persist under projected climate change scenarios. Schubert et al. (2019) recorded significant declines (40–55%) in calcification of Lithothamnion crispatum at 28 °C, although photosynthesis and respiration both increased with temperature. Sordo et al. (2019) found that photosynthesis, calcification, and respiration of Phymatolithon lusitanicum rhodoliths increased with temperature in addition to high CO₂.

Most of the other species within the biotope are distributed to the north and south of Britain and Ireland and unlikely to be adversely affected by a chronic long-term temperature change. Ulva spp. are characteristic of upper shore rockpools, where water and air temperatures are greatly elevated on hot days. Empirical evidence for thermal tolerance to anthropogenic increases in temperature is provided by the effects of heated effluents on rocky shore communities in Maine, USA. Ascophyllum and Fucus were eliminated from a rocky shore heated to 27-30°C by a power station whilst Ulva intestinalis (as Enteromorpha intestinalis) increased significantly near the outfall (Vadas et al., 1976). Barnacles (Semibalanus balanoides), limpets (Patella vulgata) and littorinids also occur within this biotope. Laboratory studies suggest that adults of these species can tolerate temperature increases. The median upper lethal temperature limit in laboratory tests on Littorina littorea, Littorina saxatilis and Semibalanus balanoides was approximately 35°C (Davenport & Davenport, 2005).  Patella vulgata can also tolerate high temperatures. The body temperature of Patella vulgata can exceed 36°C in the field, (Davies, 1970); adults become non-responsive at 37-38°C and die at temperatures of 42°C (Evans, 1948).  The smaller species associated with the Corallina officinalis may be protected within fronds and accumulated sediments from changes in temperature although no direct evidence was found to assess the sensitivity of these to increased temperatures.

Sensitivity assessment.  Bifurcaria bifurcata and Corallina officinalis are unlikely to be sensitive to either an acute or chronic increase in temperature at the pressure benchmark, based on the global distributions of both species and the experiments by Latham (2008), which approximate to the pressure benchmark more than the observations of extreme events (Seapy & Littler, 1982, Hawkins & Hartnoll, 1985; Román et al., 2020). Littler & Littler (1983) suggest that the basal crustose stage is adaptive as resisters of sand scour and wave shearing as well as physiological stressors such as desiccation and heating. Where these survive any increases in temperature above the pressure benchmark they would provide a mechanism for biotope recovery.  The distribution of Lithophyllum incrustans and other crusting corallines in southern Europe and the Mediterranean suggest that these are tolerant of higher temperatures. Field observations in Australia indicate that crustose coralline cover may decline under prolonged seasonal warming above ~29°C, although carbonate production rates remain stable (Dee et al., 2021), and elevated summer mortality has also been recorded following marine heatwaves (Short et al., 2015). However, the temperatures observed in these studies exceed those expected under the benchmark. Experimental and modelling studies on rhodolith-forming corallines show broad tolerance to moderate warming but potential declines under more extreme scenarios (e.g. Legrand et al., 2017, 2019; Muñoz et al., 2018; Schubert et al., 2019; Simon-Nutbrown et al., 2020), but these results may not be directly transferable to the crustose forms in this biotope. The sensitivity of the biotope is based on the key characterizing Corallina turf, but it should be noted that many of the associated species are considered to have ‘High’ resistance to changes in temperature at the pressure benchmark. Therefore, resistance is assessed as 'High', resilience as 'High' (by default) and sensitivity assessed as 'Not sensitive' at the benchmark level. It should be noted that the timing of acute and chronic increases would alter the degree of impact and hence sensitivity.  An acute change occurring on the hottest day of the year and exceeding thermal tolerances would lead to mortality. The sensitivity of Patella vulgata to longer-term, broad-scale perturbations would potentially be greater due to effects on reproduction but these changes may lead to species replacements and are not considered to significantly affect the character of the biotope.

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).

Under extremely low temperatures, components of the community demonstrate tolerance. Lüning (1990) reported that Corallina officinalis from Helgoland survived 0°C when exposed for one week. New Zealand specimens were found to tolerate -4°C (Frazer et al., 1988). Scrosati et al. (2023) reported extensive bleaching of Corallina officinalis following an extreme cold event (< –20 °C) in Canada, although such temperatures are unlikely to occur in the UK. Lüning (1990) suggested that most littoral algal species were tolerant of cold and freezing. For example, the photosynthetic rate of Chondrus crispus recovered after 3 hrs at -20°C but not after 6 hrs (Dudgeon et al., 1990). The photosynthetic rate of Mastocarpus stellatus higher on the shore fully recovered from 24 hrs at -20°C. 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). Bifurcaria bifurcata is a southern species and mainly occurs in the south-west of England and range retractions have been recorded from cooler periods (Helmuth et al., 2006). Based on distribution this species may be sensitive to decreases in temperature although it is not clear whether a decrease at the pressure benchmark would be biologically significant. The loss of Bifurcaria bifurcata would lead to reclassification of the biotope to one of the very similar rockpool biotopes e.g. LR.FLR.Rkp.Cor.Cor.

 Although this biotope is characterized by crustose corallines, several of the species that can occur in this biotope, including Lithothamnion crispatum, Lithothamnion glaciale, and Phymatolithon lusitanicum, can form free-living rhodoliths in addition to crusts, and so studies on their rhodolith forms can provide additional insight into potential physiological responses, albeit with caution given morphological and ecological differences. Ronowicz et al. (2024) observed high settlement of Lithothamnion sp. during the polar night in Svalbard, indicating that spore release and colonization can occur at temperatures as low as 1.4°C without solar radiation. Arnold et al. (2022) found that Lithothamnion glaciale rhodoliths maintained normal growth rates when exposed to near-zero temperatures (~0.5–1 °C) for 85 days, with irradiance a stronger predictor of growth than temperature. Bélanger & Gagnon (2021) reported that L. glaciale rhodolith growth ceased at ~0.5 °C in field experiments but was otherwise unaffected between ~1 and 16 °C in mesocosms, suggesting resilience to temperature decreases across a broad range.

The associated species are also likely to be tolerant of a decrease in temperature at the pressure benchmark. Mytilus edulis and Ulva spp. are eurytopic, found in a wide temperature range and in areas which frequently experience freezing conditions and are vulnerable to ice scour (Seed & Suchanek 1992). The tolerance of Semibalanus balanoides collected in the winter (and thus acclimated to lower temperatures) to low temperatures was tested in the laboratory. The median lower lethal temperature tolerance was -14.6°C (Davenport & Davenport, 2005). A decrease in temperature at the pressure benchmark is therefore unlikely to negatively affect this species. The same series of experiments indicated that median lower lethal temperature tolerances for Littorina saxatilis and Littorina littorea were -16.4 and -13°C respectively. Adults of Patella vulgata are also largely unaffected by short periods of extreme cold. Ekaratne & Crisp (1984) found adult limpets continuing to grow over winter when temperatures fell to -6°C and stopped only by still more severe weather. However, loss of adhesion after exposure to -13°C has been observed with limpets falling off rocks and therefore becoming easy prey to crabs or birds (Fretter & Graham, 1994). However, in the very cold winter of 1962-3 when temperatures repeatedly fell below 0°C over a period of two months large numbers of Patella vulgata were found dead (Crisp, 1964).  Periods of frost may also kill juvenile Patella vulgata, resulting in recruitment failures in some years (Bowman & Lewis, 1977). In colder conditions, an active migration by mobile species may occur down the shore to a zone where exposure time to the air (and hence time in freezing temperatures) is less.

Sensitivity assessment.  Based on the southern distribution of the key characterizing species Bifurcaria bifurcata, this biotope is assessed as having ‘Medium’ resistance to a decrease in temperature to either an acute or chronic decrease in temperature at the pressure benchmark. In the absence of evidence of the resilience of Bifurcaria, resilience is based on the characteristic corallines. Hence, resilience is assessed as 'High' as the encrusting bases of corallines may be more resistant and would allow regrowth when temperatures return to the usual regime. Sensitivity is therefore assessed as ‘Low’. It should be noted that the timing of acute changes would mediate effect with a decrease occurring at warmer times potentially having less effect than a decrease in colder months that exceeded thermal tolerances. A decrease in temperature that eliminated Bifurcaria bifurcata alone could result in biotope reversion to the very similar LR.FLR.Rkp.Cor.Cor. 

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 found in full (30-35 ppt) salinity (Connor et al., 2004).  High air temperatures cause surface evaporation of water from pools so that salinity steadily increases. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity, and values ranging from 5-30 psu have been recorded in rockpools over a period of 24 hrs (Ranade, 1957) while Newell (1979) recorded salinities as high as 42 psu in intertidal rock pools. As a consequence of such a regime, the entire rockpool community will be adapted, to a certain degree, to fluctuating salinities. However, it should be noted that local populations may be acclimated to the prevailing salinity regime and may, therefore, exhibit different tolerances to other populations subject to different salinity conditions and therefore caution should be used when inferring tolerances.  

The characterizing species Corallina officinalis and crustose corallines are found in rockpools, such as this biotope, where salinities may fluctuate markedly during exposure to the air. Kinne (1971) cites maximal growth rates for Corallina officinalis between 33 and 38 psu in Texan lagoons. 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, which 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).

The associated species are typically found in a range of salinities. Ulva species can survive hypersaline conditions in supralittoral rockpools subjected to evaporation and is considered to be a very euryhaline species, tolerant of extreme salinities ranging from 0 psu to 136 psu (Reed & Russell, 1979).  Young Littorina littorea inhabit rockpools where salinity may increase above 35 psu. Thus, key species may be able to tolerate some increase in salinity. Mytilus edulis is found in a wide range of salinities from variable salinity areas (18-35 ppt) such as estuaries and intertidal areas to areas of more constant salinity (30-35 ppt) in the sublittoral (Connor et al., 2004).  Furthermore, mussels in rockpools are likely to experience hypersaline conditions on hot days, suggesting that Mytilus edulis can tolerate high salinities.

Sensitivity assessment. No direct evidence was found to assess sensitivity to this pressure. Although some increases in salinity may be tolerated by the associated species present the natural variation, (rather than the pressure benchmark) is generally short-term and mitigated during tidal inundation.  This biotope is considered to be sensitive to a persistent increase in salinity to > 40 ppt, based on the distribution of Corallina officinalis and the encrusting corallines on the mid to lower shore. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Medium’ (following restoration of the prior salinity regime) so that sensitivity is assessed as ‘Medium’.

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 habitats (Connor et al., 2004).  However, high rainfall will reduce salinity in rockpools 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, and values ranging from 5-30 psu have been recorded in rockpools over a period of 24 hrs (Ranade, 1957). As a consequence of such a regime, the entire rockpool community will be adapted, to a certain degree, to fluctuating salinities.

In the Baltic, Corallina officinalis is confined to deeper waters as surface salinity decreases (Kinne, 1971) suggesting that full salinity is required in the long-term although short-term fluctuations may be tolerated (although the thresholds of this tolerance are not clear). Kinne (1971) cites maximal growth rates for Corallina officinalis between 33 and 38 psu in Texan lagoons so that a decrease in salinity at the pressure benchmark would be predicted to lead to reduced growth. 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 tide pools with a smaller volume of water, which were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual algae (or their spores) were suggested to be more susceptible than asexual ones to extremes of local environmental variables (temperature, salinity etc.) as they occured with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b).

Although this biotope is characterized by crustose corallines , several of the species that can occur in this biotope, including Lithothamnion crispatum, Lithothamnion glaciale, and Phymatolithon lusitanicum, can form free-living rhodoliths in addition to crusts, and so studies on their rhodolith forms can provide additional insight into potential physiological responses, albeit with caution given morphological and ecological differences. In Greenland, Schoenrock et al. (2018) reported that low salinity (~22 psu) associated with glacial melt negatively affected metabolic processes in Lithothamnion glaciale rhodoliths, suggesting possible reduced abundance and distribution with future increased freshwater input.

Based on their occurrence in estuaries, it is clear that some of the species associated with this biotope have a high tolerance for this pressure. However, it should be noted that local populations may be acclimated to the prevailing salinity regime and may, therefore, exhibit different tolerances to other populations subject to different salinity conditions so that caution should be used when inferring tolerances from populations in different regions.  Ulva species are considered to be a very euryhaline species, tolerant of extreme salinities ranging from 0 psu to 136 psu (Reed & Russell, 1979). Although some variation in salinity tolerance between populations of Ulva intestinalis has been found indicating that algae have some adaptation to the local salinity regime (Reed & Russell, 1979). Littorina littorea is found in waters of full, variable and reduced salinities (Connor et al., 2004) and so populations are considered tolerant of decreases in salinity at the pressure benchmark. Mytilus edulis is found in a wide range of salinities from variable salinity areas (18-35ppt) such as estuaries and intertidal areas to areas of more constant salinity (Connor et al., 2004).   Mytilus edulis was recorded to grow in a dwarf form in the Baltic Sea where the average salinity was 6.5 psu (Riisgård et al., 1993).

In the long-term (weeks) Mytilus edulis can acclimate to lower salinities (Almada-Villela, 1984; Seed & Suchanek 1992; Holt et al.,1998).  Almada-Villela (1984) reported that the growth rate of individuals exposed to only 13 psu reduced to almost zero but had recovered to over 80% of control animals within one month.  In extreme low salinities, e.g. resulting from storm runoff, large numbers of mussels may be killed (Keith Hiscock pers comm).  However, Bailey et al., (1996) observed very few mortalities when exposing Mytilus edulis to a range of salinities as low as 0ppt for two weeks at a range of temperatures.  It was also noted that there was a fast recovery rate. 

Prolonged reduction in salinity, e.g. from full (30-40 ppt) to reduced (18-30 ppt), is likely to reduce the species richness of the biotope due to loss of some intolerant invertebrates from the assemblage associated with the Corallina officinalis turf.

Sensitivity assessment. Although some daily changes in salinity may be experienced these will be mitigated during tidal inundation.  This biotope is considered, based on Corallina officinalis distribution and the evidence from Kinne, (1971), to be sensitive to a decrease in salinity at the pressure benchmark. Therefore, resistance is assessed as ‘Low’ and recovery as ‘Medium’ (following restoration of the prior salinity regime) so that sensitivity is assessed as ‘Medium’.

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 evidence to assess this pressure was found. The rockpool habitat will provide some protection from water flows although surface topography may create some turbulence disrupting currents. The coralline crusts characterizing this biotope are securely attached and as these are flat are subject to little or no drag. As biotopes dominated by turfs of Corallina officinalis and containing similar associated species are found in a range of flow rates from 'moderately strong' (0.5-1.5 m/s) to very 'weak' negligible) (Dommasnes, 1969; Connor et al., 2004) this key characterizing and structuring species and associated red and green algae and limpets and littorinids are . Moderate water movement is beneficial to seaweeds as it carries a supply of nutrients and gases to the plants and removes waste products. However, if the flow becomes too strong, plants may become displaced. In addition, an increase to stronger flows may inhibit settlement of spores and remove adults or germlings. However, Corallina officinalis have a compact, turf-forming growth which reduces water flow through turbulence and friction and are probably resistant to displacement by an increase in water flow.

Sensitivity assessment. Hence, this biotope is considered to have ‘High’ resistance and ‘High’ resilience to this pressure at the benchmark and is assessed as ‘Not sensitive'. Changes in water flow at the pressure benchmark may result in increased or decreased sediment deposition but these changes are not considered to alter the character of the biotope although they may alter species richness of the small invertebrates associated with the turf or any sediment deposits.

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

Emergence regime changes

The emergence regime over the tidal cycle is an important factor structuring this, and other, intertidal biotopes. An increase in emergence may lead to shallow pools drying out, greater exposure to air temperatures with corresponding fluctuations in water temperature and changes in salinity following evaporation or dilution by rainfall. Mobile species within this biotope could relocate to preferred shore heights but an increase or decrease in abundance of predators and grazers may alter the structure of the assemblage. This biotope occurs throughout the eulittoral zone (JNCC, 2022) and is, therefore, likely to have some resistance to increased or decreased emergence.

Sensitivity assessment. A decrease in emergence would mean that shallow rockpools would be at less risk of desiccation, and extreme changes in salinity or temperature. In addition, depending on the nature of the surrounding bedrock, the rockpool may become slightly deeper. As a result, it is possible that species diversity could increase but, on the whole biotopes at the upper and mid-shore are likely to be tolerant of a decrease in emergence at the benchmark level. Pre-emption of space by Corallina officinalis and other red algae may reduce the establishment of lower shore species including kelps. Resistance is therefore assessed as ‘Medium’ to decreased emergence and recovery as ‘High’, so that sensitivity is assessed as ‘Low’.

Increased emergence would exposure mid-shore rockpools to conditions typical of the upper shore, with the resultant increased variation in temperature, salinity and desiccation. In the worst-case scenario the upper shore examples of this biotope may become supralittoral rockpools. Corallina officinalis and associated red algae are intolerant of desiccation, but their basal crusts may allow individuals to persist in conditions that are unfavourable to frond development until the emergence regime is re-established. Hence, resistance to increased emergence is assessed as ‘Low’, resilience is assessed as ‘High’ and sensitivity 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

This biotope is recorded from locations that range from very wave exposed to wave sheltered (Connor et al., 2004; JNCC, 2022), while Dommasnes (1969) recorded Corallina officinalis turfs from very wave sheltered areas in Norway. Kolzenburg, Moreira et al. (2023) reported greater structural integrity in Icelandic populations, suggesting adaptation to higher wave exposure during winter storms. Ramos et al. (2016) found Corallina officinalis was more likely to occur on shores with greater exposure in northern Spain, consistent with tolerance of high wave action. Colonies of Lithophyllum incrustans appear to thrive in conditions exposed to strong water movement. Irvine & Chamberlain (1994) observe that the species is best developed on wave exposed shores. Roik et al. (2016) reported that calcification rates of crustose coralline algae were up to 8.8 times higher at offshore exposed sites compared with nearshore sheltered sites in the Red Sea, suggesting positive effects of higher wave exposure. As this biotope occurs across a range of exposures, this was therefore considered to indicate, by proxy, that biotopes in the middle of the wave exposure range would tolerate either an increase or decrease in significant wave height at the pressure benchmark.

Sensitivity assessment. The biotope is found across a range of wave exposures. A 3 to 5% change in significant wave height (the benchmark) is unlikely to be significant in wave exposed conditions. Sheltered examples of the biotope probably coincide with moderately strong tidal streams that keep the rockpools free of sediment that may otherwise accumulate in very sheltered conditions. Hence, resistance is assessed as 'High' , resilience as ‘High’ (by default) and the biotope is considered ‘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

Little information was found concerning the effects of heavy metals on turf forming and 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, and AbouGabal et al. (2023) reported that Corallina officinalis exhibited marked accumulation of cadmium, lead and nickel, suggesting its potential as a bioindicator of trace metal contamination. Amiard (1973; cited in Watson & Tyler-Walters, 2024) reported that Corallina officinalis also accumulated Antimony. Barrat et al. (2024) found that Lithothamnion corallioides from NW France showed highly variable trace element concentrations that reflected local pollution history, particularly for Pb, further highlighting the potential of coralline algae as indicators of metal contamination. Contamination at levels greater than the pressure benchmark may adversely impact the biotope. Little information was found concerning the intolerance of Chondrus crispus to heavy metals. Burdin & Bird (1994) reported that both gametophyte and tetrasporophyte forms accumulated Cu, Cd, Ni, Zn, Mn and Pb when immersed in 0.5 mg/l solutions for 24 hours. No effects were reported, however, and no relationship was detected between hydrocolloid characteristics and heavy metal accumulation. 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 the intertidal red alga Plumaria elegans were reported by Boney (1971). 100% growth inhibition was caused by 1 ppm Hg.

Most of the information available suggests that the associated adult gastropod molluscs are rather tolerant of heavy-metal toxicity (Bryan, 1984). Winkles may absorb metals from the surrounding water by absorption across the gills or from their diet, and evidence from experimental studies on Littorina littorea suggest that diet is the most important source (Bryan et al., 1983). The species has been suggested as a suitable bioindicator species for some heavy metals in the marine environment. Bryan et al. (1983) suggested that the species is a reasonable indicator for Ag, Cd, Pb and perhaps As. In the Fal estuary Patella vulgata occurs at, or just outside, Restronguet Point, at the end of the creek where metal concentrations are in the order: Zinc (Zn) 100-2000 µg/l, copper (Cu) 10-100µg/l and cadmium (Cd) 0.25-5µg/l (Bryan & Gibbs, 1983). However, in the laboratory, Patella vulgata was found to be intolerant of small changes in environmental concentrations of Cd and Zn by Davies (1992). At concentrations of 10 µg/l, pedal mucus production and levels of activity were both reduced, indicating a physiological response to metal concentrations. Exposure to Cu at a concentration of 100 µg/l for one week resulted in progressive brachycardia (slowing of the heart beat) and the death of limpets. Zn at a concentration of 5500 µg/l produced the same effect (Marchan et al., 1999).

‘No evidence’ was found on the effects of transitional metals on the key characteristic species.

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

Díez et al. (2009) examined changes in macroalgal abundance (inc. Corallina elongata and Lithophyllum incrustans) along the Basque coast after the Prestige oil spill but did not find any significant differences between oiled and non-oiled sites. However, Bowman et al. (1978) reported that 100% of cover of Lithothamnia was bleached and dead rims of Lithothamnia in lower shore rock pools after the Dounreay oil spill and treatment with BP100X. Similarly, Newey & Seed (1995) reported bleached and dead coralline algae (no species were specified) in mid-shore rockpools close to the wreck of the Braer oil tanker. Jackson (1989) also reported that crustose corallines (no species were specified) and other fleshy algae decreased in cover after the Panamanian oil spill, to levels below those observed before the spill. Crump et al. (1999) reported that encrusting coralline algae, Lithothamnion incrustans, Phymatolithon purpureum, and Corallina officinalis were bleached in West Angle Bay immediately after the Sea Empress oil spill but recovered quickly, which suggested only the surface layers were affected, rather than that individuals were killed. Crump et al. (1999) also stated that previous literature has shown oil and dispersants to have harmful effects on the pigmentation of red algae in experimental conditions.

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.

Following the Torrey Canyon oil spill in 1967, oil and detergent dispersants affected high-shore specimens of Corallina officinalis more than low-shore specimens. Algae in deep pools were afforded some initial protection, although probably later affected by contaminated runoff. In areas of heavy spraying, however, Corallina officinalis was killed. (Smith 1968).

The long-term effects on Chondrus crispus of continuous doses of the water accommodated fraction (WAF) of diesel oil were determined in experimental mesocosms (Bokn et al., 1993). Mean hydrocarbon concentrations tested were 30.1 µg/l and 129.4 µg/l. After 2 years, there were no demonstrable differences in the abundance patterns of Chondrus crispus. Kaas (1980, cited in Holt et al., 1995) reported that the reproduction of adult Chondrus crispus on the French coast was normal following the Amoco Cadiz oil spill. However, it was suggested that the development of young stages to adult algae was slow, with biomass still reduced 2 years after the event. O'Brien & Dixon (1976) and Grandy (1984, cited in Holt et al., 1995) comment on the high intolerance of red algae to oil/dispersant mixtures, but it is unclear which factor is responsible for the intolerance.

Sensitivity assessment. The above evidence suggests that exposure to oil spills and/or their dispersants can result in bleaching or death of calcareous coralline algae, especially encrusting corallines, depending on the length of exposure, shore height, and type of oil. Therefore, the resistance of encrusting corallines or Corallina sp. to exposure to oil spills and dispersants is assessed as ‘Low’ based on the worst-case scenario reported by Smith (1968) and Bowman et al. (1978). Hence, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’. Confidence in the assessment is ‘Medium’ due to the variation in the effect between studies. No evidence of effects on Bifurcaria sp. was found.

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

Contamination at levels greater than the pressure benchmark may adversely impact the biotope. Smith (1968) reported that oil and detergent dispersants from the Torrey Canyon spill affected high water individuals of Corallina officinalis more than low shore ones, and some were protected in deep pools. In areas of heavy spraying, however, Corallina officinalis was killed (Smith, 1968). Regrowth of fronds had begun within two months after spraying ceased (Smith, 1968). O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination, possibly due to the susceptibility of phycoerythrins to destruction. They also reported that red algae are effective indicators of detergent damage since they undergo colour changes when exposed to a relatively low concentration of detergent. However, Smith (1968) reported that red algae such as Chondrus crispus, Mastocarpus stellatus and Laurencia pinnatifida were amongst the algae least affected by detergents. Laboratory studies by Grandy (1984) on the effects of oil and dispersants on several red algal species concluded that they were all sensitive to oil/dispersant mixtures, with little difference between adults, sporelings, diploid or haploid life stages.

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 an intertidal population of Corallina officinalis occurred in significant amounts 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.

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).

The herbicides Diuron, Atrazine and Hexazinone were found to inhibit photosynthesis in crustose coralline algae (Harrington et al. 2005; Negri et al., 2011; McCoy & Kamenos, 2015). However, only sublethal effects were reported (Watson & Tyler-Walters, 2023) and no evidence on Corallina sp. was found.  Hence, there is ‘insufficient evidence’ on which to base an assessment. No evidence of effects on Bifurcaria sp. was found.

Radionuclide contamination

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

Evidence

Algae bioaccumulate radionuclides (with extent depending on the radionuclide and the algae species). Adverse effects have not been reported at low levels. Hernández et al. (2011) reported that Corallina elongata and Jania rubens accumulated plutonium (Pu) in granules but did not report any adverse effects on either species. ’No evidence’ of the adverse effects of radionuclides was found.

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

No evidence was found.

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). This biotope 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.  

No evidence was found to assess this pressure for the red algae turfs. However, the associated species are unlikely to be impacted by this pressure, at the benchmark. Experiments have shown that thallus discs of Ulva lactuca plants can survive prolonged exposure to anoxia and hypoxia (Vermaat & Sand-Jensen, 1987; Corradi et al., 2006). Following the resumption of normal oxygen conditions, gametes were produced. The associated invertebrate species also show high tolerances for reduced oxygen at levels that exceed the pressure benchmark. Littorina littorea can easily survive 3-6 days of anoxia (Storey et al., 2013). Limpets can also survive for a short time in anoxic seawater. Grenon & Walker, (1981) found that in oxygen-free water limpets could survive up to 36 hours, although Marshall & McQuaid (1989) found a lower tolerance for Patella granularis, which survived up to 11 hours in anoxic water. Patella vulgata and Littorina littorea are able to respire in the air, mitigating the effects of this pressure during the tidal cycle.

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

Nutrient enrichment

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

Evidence

Marine algae are often nutrient limited, particularly by nitrogen, so an increase in nutrient levels usually results in increased growth and fecundity. In the Bay of Fundy, for example, where there is a tidal flux of nutrients from the marshes there is a luxurious growth of Palmaria palmata (Morgan et al., 1980).  However, very high levels of nutrients can be toxic to macroalgae. Algae placed in tanks with continuous immersion in high nutrients over several weeks stopped growing (Morgan et al., 1980). In general, the great majority of reports refer to an increase in the number of green algae associated with eutrophicated waters, usually at the expense of red and brown algae.

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 key characterizing Corallina officinalis and the associated green algae species have been identified worldwide as species that occur in areas subject to increased nutrient input within the vicinity of sewage outfalls and at intermediately polluted sites (Littler & Murray, 1975; May, 1985; Brown et al., 1990; Bellgrove et al., 1997; Arevalo et al., 2007; Bellgrove et al., 2010). For example, Kindig & Littler (1980) demonstrated that Corallina officinalis var. chilensis in South California showed equivalent or enhanced health indices, highest productivity and lowest moralities (amongst the species examined) when exposed to primary or secondary sewage effluent. Little difference in productivity was noted in the chlorinated secondary effluent or pine oil disinfectant. However, specimens from unpolluted areas were less tolerant, suggesting physiological adaptation to sewage pollution (Kindig & Littler, 1980). Grazers in the biotope may benefit from increased availability of food resources, due to enhanced growth.

Atalah & Crowe (2010) added nutrients to rockpools occupied by a range of algae including encrusting corallines, turfs of Mastocarpus stellatus, Chondrus crispus and Corallina officinalis and green and red filamentous algae. The invertebrates present were mostly Patella ulyssiponensis, the winkle Littorina littorea and the flat top shell Gibbula umbilicalis. Nitrogen and phosphorous enhancement was via the addition of fertilisers, as either 40 g/litre or 20 g/litre. The treatments were applied for seven months and experimental conditions were maintained every two weeks. The experimental treatments do not directly relate to the pressure benchmark but indicate some general trends in sensitivity. Nutrients had no significant effect on the cover of crustose coralline algae or the cover of red turfing algae.However, the cover of green filamentous algae was significantly increased both by reduced grazing and increased nutrients, although the effect size was synergistically magnified by the combined effect of grazer removal and nutrients. Nutrient enrichment caused an absolute increase in the average cover of green filamentous algae of 19% (±3.9 S.E.) with respect to the control treatments, while the cover of red turfing algae was not affected by nutrient addition. The study suggests that, although red algal turfs may be tolerant of eutrophication and may even benefit, biotope composition may alter due to the proliferation of fast-growing ephemeral algae.

Sandoval et al. (2024) tested the combined effects of nutrient enrichment and other stressors on Corallina officinalis in San Antonio Bay, Patagonia, an area already affected by eutrophication from groundwater and industrial inputs. Mean background concentrations in the bay were 1,086 ± 1,500 µg/l  nitrate and 180 ± 130 µg/l phosphate. In experimental microcosms, nutrients were increased to 5,470 µg/l nitrate and 399 µg/l phosphate, representing levels 3 to 5 times higher than present-day conditions and consistent with high-emissions climate change projections (IPCC RCP 8.5). Corallina officinalis was able to tolerate these elevated nutrient levels during short term (10 day) exposures, with seasonal factors having a stronger influence on physiological performance than nutrient addition.

While direct evidence for nutrient enrichment effects on crustose corallines in rockpools is limited, several studies on rhodolith-forming corallines provide additional context, though their responses may not be directly comparable due to morphological and ecological differences. Bélanger & Gagnon (2020) found that Lithothamnion glaciale rhodoliths grew two times slower under elevated nutrient concentrations in mesocosms (NO₃⁻: 10.2–29.5 µmol/l; NH₃: 14.4–33.9 µmol/l; PO₄³⁻: 2.9–5 µmol/l compared with ambient NO₃⁻: 3.2 µmol/l, NH₃: 3.5 µmol/l, PO₄³⁻: 0.6 µmol/l). In field experiments, growth was also reduced under elevated phosphate (1.2 µmol/l vs. ambient 0.4 µmol/l), although some recovery was observed after the first six weeks. Koerich et al. (2021) reported that Lithothamnion crispatum showed reduced photosynthetic performance when exposed to combined warming (+6 °C), acidification (pH 7.7), and nutrient enrichment (70 µM ammonium, 14 µM nitrate, 10 µM phosphate) for five days, but recovered after returning to ambient conditions. Qui-Minet et al. (2019) showed that nutrient enrichment (PO₄³⁻: 0.8–1.2 µmol/l in winter, 0.9–1.7 µmol/l in summer; NO₃⁻: 30–50 µmol/l in winter, 25–55 µmol/l in summer) reduced dissolution of Lithothamnion corallioides and Phymatolithon calcareum in winter under future climate scenarios, and enhanced dark calcification in Lithophyllum incrustans in summer. Schubert et al. (2019) found that nutrient concentration (ambient: NO₃⁻ 2.0 µmol/l, NH₄⁺ 1.1 µmol/l, PO₄³⁻ 1.6 µM; enriched: NO₃⁻ 21 µmol/l, NH₄⁺ 70 µmol/l, PO₄³⁻ 4 µmol/l) had no significant effect on photosynthesis or respiration in Lithothamnion crispatum, but when combined with high temperature (28 °C vs. 23 °C) the negative effects were additive, reducing daily carbonate production.

Sensitivity assessment. The evidence above suggests that Corallina officinalis and its turfs are tolerant of high levels of nutrient input, e.g. Belgrove et al. (2010), Atalah & Crowe (2010), and Sandoval et al. (2024). Therefore, resistance to this pressure is assessed as ‘High’ and resilience as ‘High’ so that the biotope is assessed as ‘Not sensitive’. Where Corallina-dominated biotopes have replaced canopy-forming species in enriched areas it is not clear whether a reduction in nutrients would lead to a shift in biotope type. Once established, the presence of Corallina spp. and other turf-forming species may limit recruitment by taller species (Belgrove et al., 2010). No evidence was found to support an assessment of this indirect effect.

Organic enrichment

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

Evidence

Organic deposition may result in siltation (see smothering and siltation change pressure) and subsequent re-suspension of organic particles reducing water clarity (see 'change in suspended solids' pressure). The deposition of sewage effluent into coastal environments resulted in the absence of Laminaria digitata and many other species from the coastline of the Firth of Forth (Read et al., 1983). The use of some kelp species in conjunction with fish aquaculture (to buffer the effects of organic enrichment in the local area) suggests that many commercial kelps (including Laminaria digitata) are tolerant to local increases in organic enrichment, although the level of enrichment experienced will be dependant on fish species and aquaculture design (Troell et al., 2003).

Where the biotope occurs in tide-swept or wave exposed areas (Connor et al., 2004) water movements will disperse organic matter reducing the level of exposure. The key characterizing species Corallina officinalis has been noted to increase in abundance and may form extensive turfs within the vicinity of sewage outfalls and at intermediately polluted sites (Littler & Murray, 1975; May, 1985; Brown et al., 1990;  Bellgrove et al.,1999, 2010). Corallina elongata and 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). In the Mediterranean Salas et al. (2006) note that the presence of Sargassum is an indicator of good water quality but no further information was provided. As turf-forming algae including the red algal turf and particularly Corallina spp. trap large amounts of sediment the turf is not considered sensitive to sedimentation. The turfs host a variety of associated species and deposit feeders amongst these would be able to consume inputs of organic matter.

Sensitivity assessment. Based on resistance to sedimentation, exposure to wave action, the presence of detrital consumers and the dominance of turfs in areas subject to sewage inputs, resistance is assessed as ‘High’ and resilience as ‘High’ (by default). The biotope is therefore considered to be ‘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

In the absence of hard substratum crustose corralines can propagate as free-living rhodolith nodules and can form extensive subtidal habitats (Littler & Littler, 2013). However, these biogenic reefs are not analogous to this habitat type. The loss of hard substratum would remove the rockpool habitat and sediments would be unsuitable for the crustose corallines and other attached algae that characterize this biotope. Other associated species such as anemones and limpets would also be lost as these are associated with rock habitats.

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 the biotope through changes in species composition, richness and diversity (Green et al., 2012; Firth et al., 2013) or the presence of non-native species (Bulleri & Airoldi, 2005). Many species have specific preferences for substratum type. Corallina officinalis shows optimal settlement on finely rough artificial substrata (0.5 - 1 mm surface particle diameter). Although spores will settle and develop as crustose bases on smooth surfaces, fronds were only initiated on rough surfaces. Corallina officinalis settled on artificial substrata within one week in the field in summer months in New England (Harlin & Lindbergh, 1977). However, in the laboratory fronds can grow from bases attached to smooth surfaces (Wiedeman pers comm. Previous MarLIN review) Similarly, tests with stone panels fixed to the sublittoral, mid-tide and high-tide levels of varying roughness found that Ulva species settle preferentially on smother, fine grained substratum  (chalk, mottled sandstone) and Porphyra purpurea on rougher, granulated substratum (limestone, granite, basaltic larvae) (Luther, 1976).

A change to a sedimentary substratum would significantly alter the character of the biotope and would lead to the development of a biological assemblage more typical of the changed conditions. A change to an artificial substratum could also impact the development of this biotope as species may have settlement preferences for particular surface textures. Changes in substratum type can also lead to indirect effects. For example, Shanks & Wright (1986) observed that limpet mortalities were much higher at sites where the supply of loose cobbles and pebbles were greater, leading to increased abrasion through wave action 'throwing' rocks onto surfaces. 

Sensitivity assessment. A change to a soft sedimentary habitat would remove the habitat for this biotope, while a change to an artificial substratum could also impact the development of this biotope as species may have settlement preferences for particular surface textures. Sheer artificial structures will be free draining and not offer pool habitats to support this biotope.  Resistance is assessed as ‘None’ and resilience as ‘Very Low’ as the change is considered to be permanent. Sensitivity 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 or epiflora 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 shelter from abrasion at the surface. Littler & Littler (1983) suggest that the basal crustose stage of Corallina officinalis is adaptive to resist sand scour and wave shearing (as well as physiological stressors such as desiccation and heating). The base is much tougher than the fronds shown by experiments that demonstrate that the base has nearly twice the mechanical resistance (measured by penetration) of fronds (Littler & Kauker, 1984). Experimental clearance studies reinforce the importance of the basal crust for recovery: Littler & Kauker (1984) found that where crusts were left intact, cover of Corallina officinalis fronds recovered much more rapidly (18% after 12 months) than plots with sterilised rock surfaces, which showed much slower regrowth (no new fronds in the first six months, 10% cover restored after 12 months). In contrast, Magill et al. (2019) reported full recovery of Corallina turfs and associated assemblages within 4 to 6 months following hand-harvesting in Ireland. Kolzenburg, Moreira et al. (2023) reported that northern populations of Corallina officinalis had thicker cell walls and greater tensile strength than central and southern populations, suggesting adaptation to increased storm exposure and mechanical stress.

In general, studies show that Corallina and other turf forming algae appear to be relatively resistant to single events and low levels of trampling.  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. Similarly, a comparison of rocky intertidal ledges that received different amounts of visitors in Dorset, England, found that Corallina officinalis and encrusting corallines were present on both heavily visited and less visited ledges suggesting that these species have some resistance to trampling (Pinn & Rodgers, 2005).   Povey & Keough (1991) in Mornington Peninsula, Australia investigated the effects of sustained trampling on intertidal coralline algal mats where upright branching Corallina spp. formed a turf with other red algae with sand and encrusting coralline algae between turfs.  The experimental strips were 2 m long and 0.5 m wide. The percentage cover of upright Corallina spp. was significantly affected by 25 passages of a strip per day after 12 and 33 days. The algae appeared flattened and were shorter (1 to2 cm high) compared with the low intensity and control plots (3 to4 cm high).  However low intensity trampling within a strip (two passages/ day) did not significantly affect the Coralline turf.  Fletcher & Frid (1996a; 1996b) noted a decrease in the understorey algal community of encrusting coralline algae and red algae, which was probably an indirect effect due to increased desiccation after removal of the normally protective fucoid canopy (Hawkins & Harkin, 1985) by trampling.

Brown & Taylor (1999) also found that higher intensities of trampling damaged turfs.  Moderate (50 steps per 0.09 sq. metres) or more trampling on intertidal articulated coralline algal turf in New Zealand reduced turf height by up to 50%, and weight of sand trapped within turf to about one third of controls. This resulted in declines in densities of the meiofaunal community within two days of trampling. Although the community returned to normal levels within 3 months of trampling events, it was suggested that the turf would take longer to recover its previous cover (Brown & Taylor 1999). Similarly, Schiel & Taylor (1999) noted that trampling had a direct detrimental effect on coralline turf species on the New Zealand rocky shore. At one site coralline bases were seen to peel from the rocks (Schiel & Taylor 1999), however, this was probably due to increased desiccation caused by loss of the algal canopy.

Schiel & Taylor (1999) reported the death of encrusting corallines one month after trampling due to the removal of their protective canopy of fucoids by trampling (10 -200 tramples where one trample equals one transect walked by one person). A higher proportion of corallines died back in spring treatments presumably due to the higher levels of desiccation stress expected at this time of year. However, encrusting corallines increased within the following year and cover returned to control levels within 21 months (Schiel & Taylor, 1999). Mechanical abrasion from scuba divers was also 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) experimentally manipulated surface abrasion on a range of encrusting algae including Lithophyllum impressum. Crusts were brushed with either a nylon or steel brush for 1 minute a 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). No evidence was found to assess the effects of abrasion on Bifurcaria bifurcata.

Species associated with the coralline turf may be more sensitive. The limpets and littorinids that occur in low densities in this biotope have some protection from hard shells or plates, but abrasion may damage and kill individuals or detach these. Removal of limpets may result in these being displaced to a less favourable habitat and injuries to foot muscles in limpets may prevent reattachment.  Although limpets and littorinids may be able to repair shell damage, broken shells while healing will expose the individual to more risk of desiccation and predation.  Evidence for the effects of abrasion is provided by a number of experimental studies on trampling (a source of abrasion) and on abrasion by wave thrown rocks and pebbles.

Povey & Keough (1991) in experiments on shores in Mornington Peninsula, Victoria, Australia, found that few individuals of the limpet Cellana trasomerica, (similar size to Patella vulgata) suffered damage or relocated following kicking and trampling experiments (Povey & Keough, 1991). One kicked limpet (out of 80) was broken and 2 (out of 80) limpets that were stepped on could not be relocated the following day (Povey & Keough, 1991). On the same shore, less than 5% of littorinids were crushed in single step experiments (Povey & Keough, 1991).

Shanks & Wright (1986) found that even small pebbles (<6 cm) that were thrown by wave action in Southern California shores could create patches in aggregations of the barnacle, Chthamalus fissus, and could smash owl limpets (Lottia gigantea). Average estimated survivorship of limpets at a wave exposed site, with many loose cobbles and pebbles allowing greater levels of abrasion was 40% lower than at a sheltered site. Severe storms were observed to lead to the almost total destruction of local populations of limpets through abrasion by large rocks and boulders. In sites with mobile cobbles and boulders increased scour results in lower densities of Littorina spp. compared with other, local sites with stable substratum (Carlson et al., 2006). Wilson et al. (2020) found differences in meiofaunal communities associated with Corallina officinalis on bedrock compared with boulder substrata in Wales. They suggested that turbulence around boulders created more favourable conditions for vagile organisms than sessile ones. This indicates that small-scale differences in substratum type and energy can influence associated fauna within turfs.

Sensitivity assessment. The impact of surface abrasion will depend on the footprint, duration and magnitude of the pressure. Based on evidence from the step experiments and the relative robustness of the encrusting corallines and Corallina officinalis turf and associated species, resistance, to a single abrasion event is assessed as ‘Medium’ (loss of <25% cover/abundance) and recovery as ‘High’, so that sensitivity is assessed as ‘Low’. Field clearance experiments indicate that recovery can be rapid where coralline basal crusts remain (e.g. Magill et al., 2019) but can be variable and is much slower where bases are removed (Littler & Kauker, 1984). Resistance and resilience will be lower (and hence sensitivity greater) to abrasion events that exert a greater crushing force and remove the bases. Increased abrasion that alters the density of grazers may enhance the growth of red and green algae and may lead to changes in classification at the sub-biotope level.

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 and epiflora occurring in tide pools 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

Intertidal biotopes will only be exposed to this pressure when submerged during the tidal cycle and thus have limited exposure. Siltation, which may be associated with increased suspended solids and the subsequent deposition of these is assessed separately (see siltation pressures). In general, increased suspended particles reduce light penetration and increase scour and deposition. They may enhance food supply to filter or deposit feeders (where the particles are organic in origin) or decrease feeding efficiency (where the particles are inorganic and require greater filtration efforts).  

Increases in the cover of sediment trapping, turf-forming algae at the expense of canopy-forming species have been observed worldwide in temperate systems and have been linked to increased suspended solids linked to human activities worldwide (Airoldi, 2003).  European studies have reported that the abundance of Bifurcaria bifuracata typically decreases across pollution gradients and the genus is absent from highly polluted sites where turbidity is greater (Díez et al., 1999). Hence, sensitivity this genus has been used as an indicator of high water quality in a number of metrics used to support implementation of the Water Framework Directive (Neto et al., 2012; Guinda et al., 2008). Therefore, the characterizing species Bifurcaria bifurcata is considered more sensitive to this pressure than the algal turf and crustose corallines. Corallina species accumulate more sediment than any other alga (Hicks, 1985). Hence an increase in suspended sediment is likely to accumulate in the coralline turf. A significant increase may result in smothering (see below). Legrand et al. (2021) reported that the deposition of salmon faeces at a depositional flux of 15 g/m² per day, equivalent to locations between 100 and 200 m distance from a high-capacity salmon farm, reduced light by 60% in mesocosms, resulting in a 94% decline in calcification rates of Lithophyllum soriferum rhodoliths over six weeks. An accumulation of sediment within the turf may attract more sediment dwelling interstitial invertebrates such as nematodes, harpacticoids and polychaetes, although in more wave exposed locations accumulation of sediment is likely to be minimal. Increased suspended sediment may also result in increased scour, which may adversely affect foliose red algae, and interfere with settling spores and recruitment if the factor is coincident with their major reproductive period. However, coralline algae, especially the crustose forms are thought to be resistant to sediment scour (Littler & Kauker, 1984) and will probably not be adversely affected at the benchmark level. An increase in suspended sediment may reduce the epiphytic species diversity in the immediate area and adversely affect the cover of fleshy red algae. Recoverability is likely to be very high as species are likely to remain in situ from which recruitment can occur.

This community is unlikely to be dependent on suspended sediment. Although accumulated sediment within coralline turf habitats is likely to increase the species diversity of the epiphytic fauna, in very wave exposed locations, accumulated sediment in the habitat is likely to be minimal. A reduction in suspended sediment will probably reduce the risk of scour and reduce food availability for the few suspension-feeding species in the biotope (e.g. barnacles and spirorbids present), although effects are not likely to be lethal.

The biotope occurs in shallow waters where light attenuation due to increases in turbidity is probably low. Red algae and coralline algae especially are known to be shade tolerant and are common components of the understorey on seaweed dominated shores. Williamson et al. (2018) reported that Corallina officinalis across the Northeast Atlantic exhibited efficient use of low light intensities, with photophysiological stress occurring only under high irradiance. Similarly, Kolzenburg et al. (2023a) found that the species can acclimate to low light conditions across its range. Experiments have shown that Ulva is a shade tolerant genus and can compensate for reduced irradiance by increasing chlorophyll concentration and light absorption at low light levels. Ulva spp. were able to survive over two months in darkness and to begin photosynthesising immediately when returned to the light (Vermaat & Sand-Jensen, 1987).  Limited shading from suspended sediments is therefore not considered to negatively affect this genus. Therefore, a decrease in light intensity is unlikely to adversely affect the biotope. An increase in light intensity is unlikely to adversely affect the biotope as algae can acclimate to different light levels.

Sensitivity assessment. The exposure of biotope to suspended sediments in the water column will be limited to immersion periods allowing the suspended solids to settle (see siltation pressures).  The biotope is considered to be ‘Not sensitive’ to a reduction in suspended solids, although this may reduce food supply to the spirorbid polychaetes, sponges and other filter feeders that occur in this biotope. An increase in suspended solids at the pressure benchmark (a change from clear to intermediate on the UK TAG, 2014 scale) is considered likely to negatively impact Bifurcaria bifurcata as this genus is reported to be sensitive to pollution, including increased turbidity.  Evidence globally indicates that increase suspended solids favour the turf-forming algae that characterize this biotope (Airoldi, 2003), exposure to this pressure may therefore lead to reversion to the very similar biotope LR.FLR.Rkp.Cor.Cor . Resistance, based on the defining species, Bifurcaria bifurcata, is therefore assessed as ‘Low’ and resilience as ‘Medium’ so that sensitivity of the biotope is considered to be ‘Medium’.

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

Increased abundance of algal turfs worldwide has been linked to sediment perturbations although not all the pathways and mechanisms of these effects are clear (see review by Airoldi, 2003). However, even the most tolerant of organisms would eventually suffer from inhibition and mortality following smothering, although the thresholds for these effects have not been identified (Airoldi, 2003). 

European studies have reported that the abundance of Bifurcaria bifurcata typically decreases across pollution gradients and the genus is absent from highly polluted sites where turbidity and sedimentation are greater (Díez et al., 1999). Early life stages are likely to be more sensitive to sedimentation than adult algae which may, depending on the deposition regime, float above the layer of sediment. Early life stages of macroalgae may be more sensitive than adult algae. 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 to 2 mm thick and that attachment on silt was insecure.

Corallina officinalis and others within the genus (e.g. Corallina pinnatifolia and Corallina vancouveriensis) are found on shores subject to high rates of sedimentation that are periodically disturbed by sand burial and scour (Stewart, 1989).  Coralline turfs also trap sediments within the turf. The amount of sediment present and the associated fauna varies naturally depending on local conditions such as wave exposure (Dommasnes, 1969). Wilson et al. (2020) found that Corallina officinalis turfs in Wales supported different meiofaunal communities on bedrock than boulder substrata, where little sediment is bound by Corallina officinalis’ holdfasts, suggesting that variation in turbulence and sediment retention influences the associated fauna. On intertidal shores in southern California the amount of sediment trapped within turfs of Corallina spp. varied seasonally from < 5mm to >4.5 cm and was closely related to species composition and the structure of the turf. Airoldi (2003) identified a number of morphological, physiological and life history traits that conferred high levels of tolerance to sedimentation. Those shared by Corallina spp are the regeneration of upright fronds from a perennial basal crust resistant to burial and scour, calcified thalli, apical meristems, large reproductive output, lateral vegetative growth and slow growth rates (Airoldi, 2003). Experimental deposition of sand on coralline turfs and maintained at 3 cm or 6 cm for one month via daily top-ups did not remove the turfs but did lead to rapid (within one hour) changes in the invertebrate species as highly mobile species moved away from the turf with later colonization by sand adapted species (Huff & Jarett, 2007). The community had recovered one month after sand deposition ceased (Huff & Jarett, 2007).

In a review of the effects of sedimentation on rocky coast assemblages, Airoldi (2003) outlined the evidence for the sensitivity of 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).

Atalah & Crowe (2010) added sediment to rockpools that appear to be very similar to this biotope. The rockpools were occupied by a range of algae including encrusting corallines, turfs of Mastocarpus stellatus, Chondrus crispus and Corallina officinalis and green and red filamentous algae. The invertebrates present were mostly Patella ulyssiponensis, the winkle Littorina littorea and the flat top shell Gibbula umbilicalis. Sediment treatment involved the addition of a mixture of coarse and fine sand of either 300 mg/cm²/month or 600 mg/cm² every 15 days (the depth of sediment was not reported). The treatments were applied for seven months, and experimental conditions were maintained every two weeks. The experimental treatments do not directly relate to the pressure benchmark but indicate some general trends in sensitivity. In the pools, the chronic addition of both levels of sediment led to a significant decrease in both grazers and crustose coralline algae. Sedimentation had no significant effect on the cover of green filamentous algae (Ulva sp.) but led to an increase in the mean cover of red turfing algae (Mastocarpus stellatus and Chondrus crispus and Corallina officinalis) from 11.7% (±1.0 S.E.) in controls to 26.1% (±4.7 S.E.) in sedimented assemblages, but there were no differences between the two levels of sedimentation. The cover of red filamentous algae (Ceramium spp. Gelidium spp.) was also significantly increased in the sedimentation experiments. The experimental results support the general trend of the greater sensitivity of grazers and encrusting corallines to sedimentation than turf-forming algae.

Caronni et al. (2023) found that prolonged sediment burial of the crustose coralline Phymatolithon lenormandii in Sardinia halted growth and caused significant mortality, with cover declining from ~45% to ~13% over two years, indicating high sensitivity to sustained burial. Legrand et al. (2021) reported that chronic organic deposition at 15 g/m²/day for six weeks (equivalent to conditions 100–200 m from a high-capacity salmon farm) reduced light by ~60% in mesocosms and resulted in a 94% decline in calcification of Lithothamnion soriferum rhodoliths.

In general, propagules, early post-settlement stages and juveniles suffer severe stress and mortality from sediments (Vadas et al., 1992; Airoldi, 2003). Moss et al., (1973), for example, found that the growth of zygotes of Himanthalia elongata was inhibited by a layer of silt 1 to 2 mm thick and that attachment on silt was insecure.

Other species within the biotope, particularly limpets and littorinids are considered more sensitive to sedimentation. Chandrasekara & Frid (1998) specifically tested the siltation tolerance of Littorina littorea. Approximately half of the test individuals could not regain the surface from 1cm of burial except in the most favourable conditions (low temperatures, high water, high silt when a majority (10 out of 15) of the test cohort surfaced. Field observations support the findings that Littorina littorea are generally unable to survive smothering. Albrecht & Reise (1994) observed a population of Littorina littorea in a sandy bay near the Sylt island in the North Sea. They found that the accretion of mud within Fucus strands and subsequent covering of Littorina by the sediment resulted in them suffocating and a significant reduction in their abundance. Field observations and laboratory experiments have highlighted the sensitivity of limpets to sediment deposition Airoldi & Hawkins (2007) tested the effects of different grain sizes and deposit thickness in laboratory experiments using Patella vulgata. At 1- and 2-mm thicknesses, fine sediments decreased grazing by 40 and 77 %. The addition of approximately 4 mm of fine sediment completely inhibited grazing. Limpets tried to escape the sediment but lost attachment and died after a few days (Airoldi & Hawkins, 2007) Observations on exposed and sheltered shores with patches of sediment around Plymouth in the southwest of England confirmed that Patella vulgata abundances were higher where deposits were absent. The limpets were locally absent in plots with 50-65% sediment cover (Airoldi & Hawkins, 2007).

Sensitivity assessment. Where sediments are added to tide pools they may be removed rapidly in wave exposed environments where pools are flushed, or they may remain in-situ, increasing the level of exposure. These effects are site specific and will mediate the level of impact. Within the biotope, the associated limpets are probably the most sensitive element. Littorinids may also be negatively impacted but his will depend on the character of the deposit. Where grazing is reduced, the algal assemblage is likely to change. The encrusting corallines may be sensitive to sedimentation, but the level of sensitivity is difficult to ascertain from the evidence base. Evidence from prolonged burial experiments indicates that corallines can suffer high mortality under sustained deposition (Caronni et al., 2023; Legrand et al., 2021), although these conditions exceed the single-event benchmark used here. The turf-forming species are generally tolerant of some sedimentation but the threshold in relation to the benchmark is not easily determined, especially as this biotope occurs in rockpools which may increase the exposure. Therefore, Bifurcaria bifurcata, encrusting corallines and grazers are considered to have ‘Low’ resistance to this pressure and ‘Medium’ resilience, while the algal turf is considered to have ‘Medium’ resistance and ‘High’ resilience. The more precautionary assessment of ‘Medium’ sensitivity is recorded.

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

The available evidence for siltation pressures is outlined for the ‘light’ deposition pressure.  At the pressure benchmark ‘heavy deposition’ represents a considerable thickness of deposit and may fill shallow pools. Complete burial of algal turf and encrusting corallines and associated animals would occur. Removal of the sediments by wave action and tidal currents would result in considerable scour. The effect of this pressure is likely to be mediated by the length of exposure to the deposit. Resistance is assessed as ‘Low’ to ‘None’ as the impact is likely to be significant and would almost certainly result in the loss of grazers and a high proportion of the encrusting corallines. Resilience is assessed as ‘Medium’ and sensitivity 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

Evidence of the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. There have been no studies examining the effect of EMFs on macroalgae. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. 

Sensitivity assessment. Given the lack of data at the level of individual biotopes, resistance and resilience to EMFs cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient 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

Bifurcaria bifurcata is thought to have good acclimatization potential to changes in light conditions and has been shown to maintain high photosynthetic performance efficiency under low light treatments (reduction of photosynthetically active radiation by 33% and separate blocking of UV wavelengths; Olabarria et al., 2018).

Coralline crusts and Corallina officinalis are shade tolerant algae, often occurring under a macroalgal canopy that reduces light penetration. In areas of higher light levels, Corallina officinalis fronds may be lighter in colour due to bleaching (Colhart & Johansen, 1973). Williamson et al. (2018) reported efficient use of low light intensities in UK populations, with stress occurring only under high irradiance, while Kolzenburg, Coaten et al. (2023) found the species could acclimate to low light across its range. Sandoval et al. (2024) exposed Corallina officinalis to solar irradiance twice the present-day levels. The species displayed resilient short-term responses, with physiological performance more strongly driven by season than by irradiance treatment.

These species can acclimate to different levels of light intensity and quality, and 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). Spiecker & Menge (2024) reported that shading increased crustose algal cover by between 22 and 32% in a New Zealand field experiment, depending on grazer presence. Arnold et al. (2022) found that Lithothamnion glaciale rhodoliths in Newfoundland grew steadily across irradiances from 0.02–0.78 mol photons/m²/day, indicating broad light tolerance. Bélanger & Gagnon (2021) reported that Lithothamnion glaciale rhodoliths maintained similar growth rates at 15 m and 25 m depth despite ~60% lower irradiance, showing high light-use efficiency. Sowa et al. (2023) found that crustose corallines dominated experimental settlement plates during the winter in Svalbard. Similarly, Ronowicz et al. (2024) observed high settlement of crustose Lithothamnion sp. during the polar night in Svalbard, indicating that reproduction and recruitment can occur without solar radiation and confirming adaptation to very low light conditions. However, Tâmega & Figueiredo (2019) found that low light inhibited growth of Porolithon onkodes and Lithophyllum stictaeforme, while P. onkodes dominated high-light environments and grew faster, indicating species-specific responses. Legrand et al. (2021) found that salmon faeces deposition at 15 g/m²/day reduced light by ~60% in mesocosms and led to a 94% decline in calcification and 41% lower net primary production of Lithothamnion soriferum rhodoliths after six weeks.

Other red algae in the biotope are flexible with regard to light levels and can also acclimate to different light levels.  Canopy removal experiments in a rocky sub tidal habitat in Nova Scotia, Canada by Schmidt & Scheibling (2007) did not find a shift in understorey macroalgal turfs (dominated by Corallina officinalis, Chondrus crispus and Mastocarpus stellatus) to more light-adapted species over 18 months, but Wernberg et al. (2020) noted that crustose corallines decreased in abundance with kelp canopy removal in Australia, suggesting reduced competitiveness under higher light. 

It should be noted that, since 2016, research on artificial light at night (ALAN) has expanded rapidly. ALAN alters benthic communities, algal cover and ecological interactions in coastal systems (reviewed in Marangoni et al., 2022; Ferretti et al., 2025), with experimental studies demonstrating effects on algal assemblages under both increased and reduced light conditions (Schaefer et al., 2025; Trethewy et al., 2023). Although no studies have tested direct effects on Corallina officinalis, the assumption that light is an insignificant pressure is no longer supported by the wider evidence base.

Sensitivity assessment.  Corallina officinalis and Bifurcaria bifurcata are demonstrably tolerant of variation in natural light and shade (Olabarria et al., 2018; Williamson et al., 2018; Kolzenburg, Coaten et al., 2023; Sandoval et al., 2024). Crustose and rhodolith-forming corallines also show broad tolerance of natural light variation, with growth and recruitment maintained under shading and very low light (e.g. Bélanger & Gagnon 2021; Arnold et al., 2022; Sowa et al., 2023; Ronowicz et al., 2024).  There is substantial evidence that artificial light at night (ALAN) alters coastal communities and trophic interactions, including algal assemblages (e.g. Marangoni et al., 2022; Trethewy et al., 2023; Ferretti et al., 2025; Schaefer et al., 2025). However, as no studies have tested direct effects on the key characterising species of this biotope, the sensitivity of this biotope to the introduction of light via anthropogenic means is assessed as ‘Insufficient evidence’.

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 tidal excursion are not considered relevant to the characterizing crusting corallines and Corallina officinalis 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. Other species associated with the biotope are widely distributed and produce large numbers of larvae capable of long distance transport and survival, 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

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

The key characterizing species Bifurcaria bifurcata and coralline crusts are not cultivated. No information was found on current production of the associated species Mastocarpus stellatus, Chondrus crispus or other turf forming red seaweeds in the UK and it is understood that wild harvesting rather than cultivation is the method of production for these and littorinids. No evidence was found for the effects of gene flow between cultivated species and wild populations. Although cultivation of different genotypes may lead to gene flow between wild and cultivated populations the limited dispersal may reduce exposure. Some negative effects may arise from hybridisation between very geographically separated populations but there is no evidence to suggest that gene flow between different UK haplotypes would lead to negative effects. This pressure is therefore considered ‘Not relevant’ to this biotope group.

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

Several coralline and non-coralline species are epiphytic on Corallina officinalis. Irvine & Chamberlain (1994) cite tissue destruction caused by Titanoderma corallinae. However, no information on pathogenic organisms in the UK was found. In Rhodophyta, viruses have been identified by means of electron microscopy (Lee, 1971) and they are probably widespread. However, nothing is known of their effects on growth or reproduction in red algae and experimental transfer from an infected to an uninfected specimen has not been achieved (Dixon & Irvine, 1977). Corallina officinalis, like many other algal species, has been demonstrated to produce antibacterial substances (Taskin et al., 2007). Diseased encrusting corallines were first observed in the tropics in the early 1990’s 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 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 crusting corallines, again in the tropic (Littler et al., 2007). No evidence was found that these are impacting temperate coralline habitats.

Other species associated with this biotope such as littorinids, patellid limpets and other algae also experience low levels of infestation by pathogens but mass-mortalities have not been recorded. For example, parasitism by trematodes may cause sterility in Littorina littorea. Littorina littorea is also parasitized by the boring polychaete, Polydora ciliata and Cliona sp, which weakens the shell and increases crab predation.  Outbreaks of the shellfish pathogen Martelia spp. may cause widespread mortality of Mytilus edulis (Mainwaring et al., 2014) but populations within the UK have not been significantly impacted. No evidence was found for pathogens of Mastocarpus stellatus but Craigie & Correa (1996) described 'green spot' disease in Chondrus crispus, caused by the interaction of several biotic agents including fungi, bacteria, algal endophytes and grazers, and resulting in tissue necrosis. Correa & McLachlan (1992) infected Chondrus crispus with the green algal endophytes Acrochaete operculata and Acrochaete heteroclada. Infections resulted in detrimental effects on host performance, including slower growth, reduced carrageenan yield, reduced generation capacity and tissue damage. Stanley (1992) described the fungus Lautita danica being parasitic on cystocarpic Chondrus crispus and Molina (1986) was the first to report Petersenia pollagaster, a fungal invasive pathogen of cultivated Chondrus crispus. At usual levels of infestation in wild populations, these are not considered to lead to high levels of mortality.  

Sensitivity assessment. Based on the lack of reported infection and mortality in temperate coralline habitats, the resistance of this biotope is assessed as ‘High’ and resilience as ‘High’ by default. Although currently considered ‘Not sensitive’ this assessment may require updating in the future if the currently tropical pathogens spread.

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. A number of species present in the biotope may be harvested recreationally or commercially. Seaweeds within the biotope can be harvested to produce compounds used in a variety of applications such as thickening and stabilizing agents in the food industry.

The key characterizing and structuring species Corallina officinalis is collected for medical purposes; the fronds are dried and converted to hydroxyapatite and used as bone forming material (Ewers et al., 1987). It is also sold as a powder for use in the cosmetic industry. Magill et al. (2019) reported that hand-harvesting of Corallina turfs in Ireland (cutting and pulling) for use in bioceramics resulted in full recovery of Corallina turfs and associated assemblages within 4 to 6 months, with no significant change in invertebrate assemblage structure. Pessarrodona et al. (2023) found that experimentally cleared algal turfs, including Corallina spp., recovered to pre-clearance cover, height and sediment load within 28 to 46 days.

Red and green algae may also be collected, Chondrus crispus is harvested commercially in Scotland and Ireland, the stipe is removed but the base is left intact to allow the algae to re-grow. Palmaria palmata and Osmundea pinnatifida are also collected by hand commercially and recreationally for consumption. The effect of harvesting Chondrus crispus has been best studied in Canada. Prior to 1980, the seaweed beds of Prince Edward Island were dominated by Chondrus crispus, and the species was heavily exploited. The authors suggested that harvesting had brought about a shift in community structure following a marked increase in abundance of another red seaweed, Furcellaria lumbricalis, which is avoided by the commercial harvest with an associated decline in abundance of Chondrus crispus (Sharp et al., 1993). Sharp et al. (1986) reported that the first drag rake harvest of the season, on a Nova Scotian Chondrus crispus bed, removed 11% of the fronds and 40% of the biomass. Efficiency declined as the harvesting season progressed. Chopin et al. (1988) noted that non-drag raked beds of Chondrus crispus in the Gulf of St Lawrence showed greater year-round carposporangial reproductive capacity than a drag raked bed. In the short-term, therefore, harvesting of Chondrus crispus may remove biomass and impair reproductive capacity, while in the long-term, it has the potential to alter community structure and change the dominant species. Removal of other associated algae such as Palmaria palmata and Osmundea pinnatifida will reduce cover of turf-forming red algae in this biotope. Garbary et al., (2012) studied harvested and non-harvested shores in Nova Scotia, Canada containing stands of Palmaria palmata. They also conducted the experimental removal of Palmaria palmata and assessed simulated removal of Palmaria palmata by an experienced commercial harvester. Simulated commercial harvesting reduced cover of Palmaria palmata from 70% to 40%, although experimental removal on shores that were not usually harvested reduced cover to 20% (Garbary et al., 2012).

Littorinids are one of the most commonly harvested species of the rocky shore. Large-scale removal of Littorina littorea may allow a proliferation of opportunistic green algae, such as Ulva, on which it preferentially feeds.  Experiments designed to test the effects of harvesting by removing individuals at Strangford Lough found that there was no effect of experimental treatments (either harvesting or simulated disturbance) on Littorina littorea abundance or body size over a 12-week period (Crossthwaite et al., 2012). This suggests that these animals are generally abundant and highly mobile; thus, animals that were removed were quickly replaced by dispersal from the surrounding, un-harvested areas. However, long-term exploitation, as inferred by background levels of harvest intensity, did significantly influence population abundance and age structure (Crossthwaite et al., 2012). A broadscale study of harvesting in Ireland using field studies and interviews with wholesalers and pickers did suggest that some areas were over-harvested, but the lack of background data and quantitative records make this assertion difficult to test (Cummins et al., 2002). Changes in grazer abundance can alter the character of the assemblage.  

Sensitivity assessment.  The key characterizing species and a number of the associated species are harvested. While Corallina officinalis is robust to targeted removal (Magill et al., 2019; Pessarrodona et al., 2023), collection of the key characterizing species may lead to changes in the character of the biotope, but it is unlikely that selective targeted harvesting of these species would lead to biotope reclassification as other similar species would increase in abundance. Removal of the associated limpet and littorinid grazers may allow red and green algae to increase in abundance and density. However, these algae may also be subject to harvesting, limiting their dominance. The resistance of this biotope to targeted harvesting of characterizing and associated species is ‘Low’ as the species are all relatively large, conspicuous and easily collected. Resilience is assessed as ‘Medium’ and sensitivity is assessed as ‘Medium’. No evidence of harvesting of Bifurcaria sp. was found.

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 and associated species would alter the character of the biotope. The biotope is characterized by Corallina officinalis and other associated species. The loss of the biological assemblage due to incidental removal as by-catch would, therefore, alter the character of the habitat and result in the loss of species richness. The ecological services such as primary production and the habitat provided by these species would also be lost. However, although no direct evidence exists for the impact of incidental removal, experimental evidence indicates that Corallina turfs can recover rapidly following removal where basal crusts remain. Magill et al. (2019) reported that hand-harvested Corallina officinalis turfs in Ireland (removed by both cutting and pulling) recovered fully within 4 to 6 months, with no significant change in invertebrate assemblage structure. Similarly, Pessarrodona et al. (2023) found that heavily cleared intertidal algal turfs, including Corallina spp., regained pre-clearance cover, height and sediment load within 28 to 46 days.

Sensitivity assessment.   While Corallina officinalis has shown strong recovery capacity following both hand-harvesting (Magill et al., 2019) and experimental clearance (Pessarrodona et al., 2023), removal of a large percentage of the turf as incidental bycatch would still result in bare rock and loss of habitat structure in the short term, with associated effects on species richness and ecosystem function. Resistance is, therefore, assessed as ‘Low’ and recovery as ‘Medium’ so that sensitivity is assessed as 'Medium'. No evidence of harvesting of Bifurcaria sp. was found.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Hinz et al., 2011; Helmer et al., 2019; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). It ranges from the Baltic Sea, the Kattegat and Skagerrak, the North Sea coasts of the UK, Germany, and Belgium, through the English Channels and into the Irish sea coasts of Ireland and south Wales with records in east and west Scotland, Northern Ireland, northwest France, Spain and south into the Mediterranean (NBN, 2023; OBIS, 2023).

The density of Crepidula populations in northern Europe (Germany, Denmark, and Norway) are significantly lower (ca <100 / m²) than in southern waters. Thieltges et al. (2004) reported that the population of Crepidula was affected strongly by cold winters in the Wadden Sea. The winters of 2001 and 2003 resulted in ca 56-64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11-14% in southern areas without frost. Crepidula almost vanished from the Wadden Sea after the 1978/79 winter and took ten years to recover due to moderate winters which regularly affected the population. Similarly, 25% mortality was observed in Crepidula populations on the south coast of the UK after the extreme 1962/63 winter (Crisp, 1964, Bohn et al., 2012). Thieltges et al. (2003) suggested that global warming way allow Crepidula populations become more abundant in northern Europe.

Sensitivity assessment. Crepidula fornicata experiences high levels of stress in intertidal habitats where strong wave action or tidal currents are present (Bohn, 2014). Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Bohn et al. (2012, 2013a, 2013b, 2015) suggested that extreme conditions in intertidal limited its upward distribution due to early post-settlement mortality). Bohn et al. (2013b) noted that Crepidula spat in their experimental intertidal panels suffered high mortality 78-100% during emersion by low water spring tides. Macroalgal cover may also prevent establishment (Tillin et al., 2020). Therefore, this coralline-dominated, often wave-exposed biotope is probably unsuitable for colonization by Crepidula, and sensitivity is assessed as 'Not sensitive' based on existing evidence.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009).

A lack of published descriptions and an incomplete historical record has led to the widespread misidentification of Didemnum vexillum and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; Mckenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence has suggested that the species may be native to Japan with its native range possibly extending into continental Asia and north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine, Carman et al., 2007; Valentine, Collie et al., 2007; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024).

In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

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).

Sensitivity assessment. Tillin et al. (2020) reported that similar biotopes could potentially be affected. Didemnum vexillum can overgrow and potentially displace Corallina officinalis (Valentine, Carman et al., 2007), but has not been recorded from sites exposed to wave action, that is 'very wave exposed', 'wave exposed' and 'moderately wave exposed' (sensu MNCR, Hiscock, 1996), especially in the intertidal where wave action is not ameliorated by depth (see Hiscock, 1983). Reinhardt et al. (2012) examined the effects of water flow and hydrodynamics on the encrusting and tendril forms of Didemnum vexillum. They reported that a current speed of approx. 7.6 m/s was required to induce fragmentation of tendrils, but that natural tidal flow alone was insufficient to cause fragmentation of tendrils. They suggested that rare instances of wave action such as storms that resulted in wave orbital velocities of ca 8 m/s or (more likely) human activity could cause fragmentation of tendrils. Reinhardt et al. (2012) noted that the tensile strength of Didemnum vexillum was an order of magnitude higher than Botrylloides sp. and was similar to that of Alyconium digitatum. Alyconium digitatum is reported from sheltered to very wave exposed conditions but in the sublittoral. Reinhardt et al. (2012) also suggested that seasonal changes in the condition of Didemnum vexillum reduced the tensile strength of colonies and was associated with the period of greater larval production and implied that fragmentation aided dispersal. The oscillatory nature of wave-mediated water flow (wave orbital velocities) combined with wave pressure in the lacerating zone where breaking wave causes, multidirectional strong water movement (Hiscock, 1983) would probably dislodge and breakup Didemnum vexillum colonies, prevent them from forming suffocating mats, and restrict the colonies to crevices and overhangs, away from the communities that characterize this biotope. However, it is unclear if moderately wave exposed conditions would be adequate to prevent Didemnum vexillum from developing extensive mats in the summer months when wave action is typically reduced. Hitchin (2012) suggested that the presence of Didemnum vexillum in Whitstable, Kent was contrary to its then known habitat preferences. The above evidence shows Didemnum vexillum can overgrow and displace epifauna such as barnacles and algal species, suggesting that if Didemnum vexillum was able to colonize it may reduce biodiversity within the biotope. At present, the evidence does not allow an assessment of sensitivity to be made with any confidence and further direct evidence is required. ‘Insufficient evidence’ is recorded.

The Pacific oyster, Magallana gigas

Evidence

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSIP, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). Magallana gigas is recognised as a beneficial and important species in aquaculture worldwide (Padilla, 2010). It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSIP, 2011, 2012; Humphreys et al., 2014, cited in Alves et al., 2021; Hansen et al., 2023).

Since its introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSIP, 2011, 2012; Ezgeta-Balic et al., 2019; Spagnolo et al., 2019; Bergstrom et al., 2021; Hansen et al., 2023). In the UK, the species predominantly occurs around the southern and western coastlines (OBIS, 2024; NBN, 2024). Shipping activity has also been associated with the introduction of Magallana gigas in the northeastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019) and possibly in southwest England from France possibly via fouling on ships (GBNNSIP, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Its distribution and environmental tolerances are considered to overlap with this biotope and, as a reef forming species, it can alter habitat structure. This species may also affect the grazers present in the biotope. No evidence was found for effects on rock pools, although on the Mediterranean coast Magallana gigas is cultivated in micro-tidal lagoons and has established wild populations (Miossec et al., 2009, cited from Herbert et al., 2012).  In the Wadden Sea and the North Sea, Magallana gigas overgrows mussel beds in the intertidal zone (Diederich, 2005, 2006; Kochmann et al., 2008), although larvae did show preference for settling on conspecifics before the mussels and struggled to settle on mussels with a fucoid covering.  It has been observed that mussel beds in the Wadden Sea that are adjacent to oyster farms were quickly converted to oyster beds (Kochmann et al., 2008).  Dense aggregations of Magallana gigas on a former mussel bed showed increased abundance and biomass of Littorina littorea in the Wadden Sea (Markert et al., 2010).  However, Eschweiler & Buschbaum (2011) found that juvenile Littorina littorea could carry Magallana gigas and Crepidula fornicata as epibionts. Body dry weight of snails without oyster overgrowth was twice as high compared to winkles covered with oysters. Crawling speed of snails with oyster epigrowth was significantly slowed down and about ten times lower than in unfouled periwinkles. Additionally, oyster epibionts caused a strong decrease in reproductive output. In laboratory experiments, egg production of fouled Littorina littorea was about 100-fold lower than in affected individuals. Field surveys in different years and habitats demonstrated that up to 10% of individuals occurring on epibenthic bivalve beds and up to 25% of snails living on sand flats may be fouled by Magallana gigas. Sargassum muticum is likely to have been imported to Europe in association with Magallana gigas imports (Engelen et al., 2015).  Sargassum muticum can grow on shells and pebbles and Magallana gigas shells would provide a suitable habitat.

Sensitivity assessment. Magallana gigas populations may be limited to low densities due to very wave exposed to wave exposed conditions (Teschke et al., 2020). However, dense macroalgal cover is unsuitable for the Magallana gigas (Herbert et al., 2012, 2016; Tillin et al., 2020), being rarely found under macroalgal cover in Northern Ireland, absent from exposed bedrock or large boulders with macroalgae cover in the Solway Firth, Scotland, and absent in Poole Harbour, where there was competition with macroalgae (Kochmann, 2012; Kochmann et al., 2013; McKinstry & Jensen, 2013; Cook et al., 2014; Tillin et al., 2020). Fucus cover significantly reduced larval recruitment of the Pacific oyster in the Wadden Sea (Diederich, 2005). Hence, the Pacific oyster is more likely to colonize bare rock, boulders or mussel beds without macroalgae (Diederich, 2005; Cook et al., 2014). Kochmann et al. (2013) suggested that macrophyte canopies prevent larvae from settling on the rock underneath, and macroalgae fronds inhibit settlement and recruitment by exuding metabolites. Therefore, this Corallina-dominated biotope is probably unsuitable for colonization by the Pacific oyster, and sensitivity is assessed as 'Not sensitive' based on existing evidence.

Wireweed, Sargassum muticum

Evidence

Sargassum muticum is a very successful invasive species. Space pre-emption by Sargassum muticum and shading may prevent further settlement of INIS until disturbance events create gaps for invasion. Where rockpools become colonized by Sargassum muticum and it is able to become established, the resultant biotope could change to LR.FLR.Rkp.Sar.

Veiga et al. (2016) compared meiofaunal assemblage diversity in Sargassum muticum and Bifurcaria bifurcata in Portugal.  They reported similar meiofaunal abundance between the two species, but lower species richness and fewer exclusive macrofaunal taxa on Sargassum muticum, suggesting that replacement of native macroalgae by Sargassum muticum would have cascading effects on the associated species, not just the macroalgae.

Field experiments have yielded mixed results for the recovery of Bifurcaria bifurcata following Sargassum muticum removal. In Italy, Rossi et al. (2019) removed Sargassum muticum from rockpools every month for one year. Bifurcaria bifurcata showed no change in biomass but retained more carbon and nitrogen in pools from which Sargassum muticum was removed. In Spain, Bifurcaria bifurcata cover increased slightly following Sargassum muticum removal at two out of the three sites investigated (Sánchez & Fernández, 2018). Fernández (2020) reported 30 years of observations of the Sargassum muticum invasion of the north coast of Spain, noting that, of the five native macroalgal species observed, Bifurcaria bifurcata was the only one to experience an increase in percentage cover in rockpools from which Sargassum muticum was removed, though the increase was slight. Bifurcaria bifurcata also became one of the co-dominant macroalgal species during a period of Sargassum muticum decline in 2006-2019 (Fernández, 2020).

Sensitivity assessment. Colonization by Sargassum muticum is likely to result in loss of this biotope (LR.FLR.Rkp.Cor) and its replacement by LR.FLR.Rkp.Sar. Resistance to this pressure is assessed as ‘Low’.  The biotope will only recover if the INIS is removed, either through active management or natural processes, and even then, recovery is likely to be slow. To recognise that recovery may be prolonged, resilience is assessed as ‘Very low’, and sensitivity is, therefore, assessed as ‘High’.

Wakame, Undaria pinnatifida

Evidence

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

Undaria pinnatifida seems to settle better on artificial substrata (e.g. floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018). James (2017) suggested that Undaria pinnatifida could out-compete native species on artificial substrata (such as marinas and wharf structures). De Leij et al. (2017) suggested that in natural substrata, Undaria pinnatifida can be inhibited by the presence of native competitors, such as large perennial species. The dense macroalgae canopies formed by native kelps result in limited space and light availability for Undaria pinnatifida recruits. However, it will not always completely prevent assimilation of Undaria pinnatifida (De Leij et al., 2017; Epstein & Smale, 2018).

Undaria pinnatifida species behaves as a winter annual and recruitment occurs in winter, followed by rapid growth through spring, maturity and then senescence through summer, with only the microscopic life stages persisting through autumn. It exhibits multiple dispersal strategies, such as short-range spore dispersal and long-range dispersal as whole drift algae or fragments. Undaria pinnatifida has spread rapidly across the UK and Europe, resulting in community-wide responses and impacts (Vaz-Pinto et al., 2014; Epstein & Smale, 2017). Its impacts are complex and context-specific, depending on space, time, and taxa present in the introduced location (Epstein & Smale, 2017; Teagle et al., 2017; Tidbury, 2020).

Undaria pinnatifida has a wide physiological niche, meaning it can occur in both coastal and estuarine environments, showing tolerance for varying salinities, turbidity and siltation (Heiser et al., 2014; Epstein & Smale, 2018). Undaria pinnatifida has a greater preference for sites sheltered with low wave exposure and weak tidal streams (Heiser et al, 2014; Epstein & Smale, 2018). In natural habitats, Undaria pinnatifida was not recorded if the wave fetch is greater than 642 km, but increased in abundance and cover in very sheltered sites (Epstein & Smale, 2018).

Sensitivity assessment. The shallow rockpools typical of this biotope are unlikely to provide a suitable environment for kelp, and Undaria pinnatifida seems to settle more successfully on artificial substrata than on natural rocky shores, so sensitivity is assessed as 'Not sensitive' based on existing evidence.

Other INIS

Evidence

Within rockpools, temperature and salinity may be subject to rapid variation, and these conditions will largely exclude species that are adapted to more stable, subtidal environments. 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 this biotope. Space pre-emption by encrusting corallines and the crustose bases of the macroalgae forming the turf, as well as the trapped sediment within the turf, may prevent settlement of INIS until disturbance events create gaps for invasion. However, in the Mediterranean crustose corallines and algal turfs facilitate attachment of Caulerpa racemosa by providing a more complex substratum than bare rock (Bulleri & Benedetti-Cecchi, 2008).

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 which have so far been recorded in nuisance densities (Sweet, 2011j). Beneath a canopy of the invasive Codium fragile ssp. tomentosoides on subtidal rocky shores in Nova Scotia, Corallina officinalis was the dominant species comprising 78-80% of the turf biomass, while Chondrus crispus and Mastocarpus stellatus comprised 18% (Schmidt & Scheibling, 2007). The biomass of Corallina officinalis was similar to those under a canopy of the native Laminaria species on the same shore (Laminaria longicruris and Laminaria digitata), suggesting little negative effect on turf forming algae. The invasive red seaweeds Heterosiphonia japonica and Neosiphonia harveyi may also occur in this biotope 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; however, this large, fast-growing seaweed may have the potential to displace native seaweed species and shade neighbouring species.  In North America this species is a major competitor of Chondrus crispus which provides an important winter food source for littorinids and other invertebrates.  As Grateloupia turuturu dies-back in the winter, the displacement of other species may therefore affect grazers (Sweet, 2011g).

The tunicate 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. The tunicates Corella eumyota and Botrylloides violaceus can smother rock habitats (Bishop, 2011b and 2012b).

The non-native crab Hemigrapsus sanguineus has recently been recorded in the UK (Sweet & Sewell, 2014) and has the potential to be a significant predator of intertidal invertebrates.  Significant reductions in common shore crab abundance and mussel density have been reported where the Asian shore crab has achieved high densities in mainland Europe (Sweet & Sewell, 2014). In Rye, New York, declines of approximately 80% of Littorina littorea in the intertidal were reported to coincide with an expansion of the Hemigrapsus sanguineus population (Kraemer et al., 2007). This crab occurs on exposed shores and may therefore occur in this biotope when established. If predation of littorinids was significantly increased this could impact the algal composition and abundance of this biotope by altering the level of grazing pressure.

Sensitivity assessment. Little evidence was found to assess the impact of INIS on this biotope and much of the evidence comes from intertidal habitats in other countries.   Replacement of red algal turfs by other similar species may lead to some subtle effects on local ecology but at low abundances the biotope would still be recognisable from the description. Sensitivity is assessed as 'Insufficient evidence'.