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

Rockpools experience considerable variation in temperature on a daily and seasonal basis.  The range and extremes of temperature change increasing with shore height but are also dependent on shading, aspect, topography and depth of the pool (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993).  For example, reported temperature ranges for mid to low shore pools include annual maxima and minima of 1-25°C and 2-22°C (Morris & Taylor, 1983), a diurnal range of 24°C (day) and 13°C (night) for a mid shore pool (Daniel & Boyden, 1975), and surface water temperature ranges of 14-19.25°C and 15.5-20.75°C in mid shore pools (Pyefinch, 1943).  Temperature stratification within pools may result in higher surface temperatures and lower deep water temperatures in sunlight (Daniel & Boyden, 1977) or be reversed due to wind cooling, night or in winter (Naylor & Slinn, 1958; Ganning, 1971; Morris & Taylor, 1983).  The temperature range will limit the distribution of sensitive species within the pools, especially normally sublittoral species, e.g. laminarians.  For example, the upper limit of Bifurcaria bifurcata within rockpools in Roscoff, France was shown to be limited by the summer temperatures where the surface pool water temperatures exceeded 20°C (Kooistra et al., 1989).  Therefore, an increase in ambient temperatures has the potential to reduce the abundance or vertical extent of sensitive species within the biotope.

Furcellaria lumbricalis has a wide geographic range, occurring in Europe from northern Norway to the Bay of Biscay.  Novaczek & Breeman (1990) recorded that specimens of Furcellaria lumbricalis grew well in the laboratory from 0-25°C with optimal growth between 10 and 15°C.  Growth ceased at 25°C and 100% mortality resulted after 3 months of exposure to 27°C.  Similarly, Bird et al. (1979) recorded optimum growth at 15°C and cessation of growth at 25°C with associated necrosis of apical segments.

Ahnfeltia plicata also has a very wide geographic range, occurring from northern Russia to Portugal.  The species is therefore likely to be tolerant of higher temperatures than it experiences in Britain and Ireland.  Lüning & Freshwater (1988) incubated Ahnfeltia plicata from British Columbia at a range of temperatures for 1 week and tested their survivability by ability to photosynthesize at the end of the incubation period.  The species survived from the coldest temperature tested (-1.5°C) to 28°C.  Total mortality occurred at 30°C.  Lüning & Freshwater (1988) suggested that Ahnfeltia plicata was therefore amongst the group of most eurythermal heat tolerant algae.  Haglund et al. (1987) incubated Ahnfeltia plicata from the subtidal in Sweden at a range of temperatures and measured photosynthetic rate.  There were no significant results, but photosynthetic rate appeared to be optimal at 15°C and decreased either side of this temperature.

Polyides rotunda are found throughout the North Atlantic Ocean and have related populations in the North Pacific. Growth and survival was tested over a temperature range of -5 to 30°C.  Polyides rotunda tolerated temperatures from -5 to 27°C, grew well from 5 to 25°C, and had a broad optimal range of 10-25°C.  Both species tolerated 3 months in darkness at 0°C (Novaczek & Breeman, 1990).

Rhodothamniella floridula has a global distribution (Guiry & Guiry, 2015) it is, over the whole of its range, subject to a wider range of temperatures than experienced in the British Isles.  It is therefore expected that increases in temperature, at the pressure benchmarks, will not result in mortality of the species.  Dixon & Irvine (1977) observed that the growth of Rhodothamniella floridula (as Audouinella floridula) is much faster in winter, whilst in the summer the spongy cushion can become bleached or disrupted. Stegenga (1978) found that tetraspores of cultured Rhodothamniella floridula (as Rhodochorton floridulum) were formed under all combinations of temperatures from 4°C to 16°C at any length of daylight, although they were most abundant at high temperatures and long days.

The median upper lethal temperature limit in laboratory tests on Littorina saxatilis and Littorina littorea  collected in the summer at Great Cumbrae, Scotland), was approximately 35°C (Davenport & Davenport, 2005).  Semibalanus balanoides collected from the same shores had similarly high thermal tolerance, with summer collected individuals having a median upper lethal limit of approximately 35°C.  In laboratory experiments Littorina littorea collected from the Kiel Fjord in Germany and kept in tanks at  temperatures 5°C above the seawater temperatures from the collection area  (Kiel fjord, Germany) for 5 months (temperatures in laboratory ranged from 13-23°C) did not die although some decreases in shell strength were observed (Landes & Zimmer, 2012).

Chondrus crispus has a wide distribution; it is found extensively throughout Europe and North America.  It is also recorded under a number of synonyms from Africa and Asia (Guiry & Guiry, 2015).  Spore germination in Chondrus crispus appears to be temperature dependent with spores discharged at temperatures of 5°C failing to germinate, although in laboratory culture at 10°C spores were viable all year round (Bhattacharya, 1985).  In New Hampshire, USA, Chondrus crispus grows abundantly in waters with an annual variation in surface temperature from -1 to +19°C (Mathieson & Burns, 1975). The optimum temperature for growth has been reported as 10-15°C (Fortes & Lüning, 1980), 15 °C (Bird et al., 1979), 15-17°C (Tasende & Fraga, 1999) and 20°C (Simpson & Shacklock, 1979).  Above the optimum temperature, growth rate is reported to decline (Bird et al., 1979; Simpson & Shacklock, 1979).  Compared to Chondrus crispus plants grown at 5°C, plants grown at 20 °C had higher growth rates in terms of length, biomass, surface area, dichotomy and branch production.  The differences resulted in growth of morphologically more complex thalli at higher temperatures with more efficient nutrient exchange and light harvesting (Kuebler & Dudgeon, 1996).  Chondrus crispus plants acclimated to growth at 20°C (vs. 5°C) had higher levels of chlorophyll a and phycobilins, resulting in higher rates of light limited photosynthesis for a given photon flux density (Kuebler & Davison, 1995).  Plants grown at 20°C were able to maintain constant rates of light saturated photosynthesis at 30°C for 9 hours.  In contrast, in plants acclimated to 5°C, light saturated photosynthetic rates declined rapidly following exposure to 30°C (Kuebler & Davison, 1993).  Prince & Kingsbury (1973) reported cessation of growth in Chondrus crispus cultures at 26°C, first mortality of spores at 21.1°C and total mortality of spores at 35-40°C, even if exposed for just 1 minute.

Sensitivity assessment.  The geographic distribution and laboratory experiments indicate that the key characterizing species would tolerate either an acute or chronic increase in temperature at the pressure benchmark.  However, some sub-lethal decreases in photosynthesis may occur where temperatures exceed the optimal.  Gamete release may also depend on lower winter temperatures and therefore non-lethal effects on reproduction may occur where optimal temperatures are exceeded.  As these effects do not result in mortality, resistance is assessed as ‘High’ and recovery as ‘High’ so that the biotope is considered to be ‘Not sensitive’ to short-term (not greater than a year) changes at the pressure benchmark.  However, it should be noted that species may be acclimated to prevailing environmental conditions, so that distribution is only a proxy indicator of sensitivity.

Temperature decrease (local)

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

Evidence

Rockpools experience considerable variation in temperature on a daily and seasonal basis.  The range and extremes of temperature change increasing with shore height but also dependent on shading, aspect, topography and depth of the pool (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993).  For example, reported temperature ranges for mid to low shore pools include annual maxima and minima of 1-25 °C and 2-22 °C (Morris & Taylor, 1983), a diurnal range of 24 °C (day) and 13 °C (night) for a mid shore pool (Daniel & Boyden, 1975), and surface water temperature ranges of 14-19.25 °C and 15.5-20.75 °C in mid shore pools (Pyefinch, 1943).  Temperature stratification within pools may result in higher surface temperatures and lower deep water temperatures in sunlight (Daniel & Boyden, 1977) or be reversed due to wind cooling, or in winter (Naylor & Slinn, 1958; Ganning, 1971; Morris & Taylor, 1983).  Morris & Taylor (1983) reported that the surface of an upper shore was seen to freeze one winter night, although that this was a rare event.  Freezing is likely to be rare in mid or low shore pools.  The pool is likely to represent a buffer from the extreme cold and frosts experienced by fauna and flora on the emergent rock surface.  In addition, few macroalgae were damaged as a result of the severe winter of 1962/63 (Crisp, 1964a).  Overall, the range of temperatures routinely experienced by mid to low shore rock pools is greater than the benchmark level.  All of the characterizing species within this biotope have broad geographic distributions and are found in colder waters than those around the UK.  Populations can acclimate to prevailing conditions which can alter tolerance thresholds and care should therefore be used when interpreting reported tolerances.

Furcellaria lumbricalis occurs in Europe from northern Norway to the Bay of Biscay.  Novaczek & Breeman (1990) recorded that specimens of Furcellaria lumbricalis grew well in the laboratory from 0-25 °C with optimal growth between 10 and 15 °C.  Growth ceased at 25 °C and 100 % mortality resulted after 3 months exposure to 27 °C. Similarly, Bird (1979) recorded optimum growth at 15 °C.

Ahnfeltia plicata occurs from northern Russia to Portugal.  Lüning & Freshwater (1988) incubated Ahnfeltia plicata from British Columbia at a range of temperatures for 1 week and tested their survivability by ability to photosynthesize at the end of the incubation period.  The species survived from the coldest temperature tested (-1.5 °C) to 28 °C.  Haglund et al. (1987) incubated Ahnfeltia plicata from the subtidal in Sweden at a range of temperatures and measured photosynthetic rate.  There were no significant results, but photosynthetic rate appeared to be optimal at 15 °C and decreased either side of this temperature.

Growth and survival of the key characterizing species, Polyides rotunda was tested over a temperature range of -5 to 30^oC.  Polyides rotunda tolerated temperatures from -5 to 27^oC, grew well from 5 to 25^oC, and had a broad optimal range of 10-2 ^oC (Novaczek & Breeman, 1990).  This species tolerated three months in darkness at 0^oC (Novaczek & Breeman, 1990).

Dixon & Irvine (1977) observed that the growth of Rhodothamniella floridula (as Audouinella floridula) is much faster in winter, whilst in the summer the spongy cushion can become bleached or disrupted.  It is therefore likely that a reduction in temperature will increase the growth rate of the species.  Rhodothamniella floridula has a global distribution (Guiry & Guiry, 2015, and references therein).  However, low temperatures may delay or slow reproduction.  Stegenga (1978) found that spores of cultured Rhodothamniella floridula (as Rhodochorton floridulum) were formed under all combinations of temperatures from 4 °C to 16 °C at any length of daylight, although they were most abundant at high temperatures and long days.

Davenport & Davenport (2005) undertook a series of experiments which found that the median lower lethal temperature tolerances for Littorina saxatilis and Littorina littorea were -16.4 and -13 ^oC respectively.  In other experiments Littorina littorea were able to tolerate temperatures down to -8 °C for 8 days (Murphy, 1983).  In colder conditions an active migration may occur down the shore to a zone where exposure time to the air (and hence time in freezing temperatures) is less.

Chondrus crispus has a broad geographical distribution (Guiry & Guiry, 2015) and throughout the range experience wide variation in temperatures (although local populations may be acclimated to the prevailing thermal regime).  In New Hampshire, USA, Chondrus crispus grows abundantly in waters with an annual variation in surface temperature from -1 to +19 °C (Mathieson & Burns, 1975). The photosynthetic rate of Chondrus crispus recovered after 3hrs at -20 °C but not after 6 hrs (Davison et al., 1989).  Frond bleaching and declines in photosynthesis and growth also occur in long-term experimental exposure to periodic freezing in Chondrus crispus (Dudgeon et al., 1990).  Plants from Maine, USA, were frozen at -5 °C for 3 hours a day for 30 days.  Photosynthesis was reduced to 55 % of control values, growth rates were reduced and fronds were eventually bleached and fragmented resulting in biomass losses. Additionally, fronds of Chondrus crispus which were frozen daily had higher photosynthetic rates following subsequent freezing events than unfrozen controls, indicating that the species is able to acclimate to freezing conditions (Dudgeon et al., 1990).  Spore germination in Chondrus crispus appears to be temperature dependent with spores discharged at temperatures of 5 ^oC failing to germinate although in laboratory culture at 10 ^oC spores were viable all year round (Bhattacharya, 1985).  Acute or chronic changes in temperature below 5 ^oC may therefore reduce reproductive success although reproduction and vegetative growth in warmer months should compensate for any reduction in output.

Sensitivity assessment.  The key characterizing species occur over a wide geographical range and can tolerate temperatures below 0 ^oC. Reduced temperatures may result in suboptimal growth and may affect reproduction.  Biotope resistance is assessed as ‘High’ as these effects do not result in mortality and resilience is assessed as ‘High’, so that the biotope is not considered to be sensitive to this pressure.  However, it should be noted that species may be acclimated to prevailing environmental conditions, so that distribution is only a proxy indicator of sensitivity.

Salinity increase (local)

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

Evidence

High air temperatures cause surface evaporation of water from pools, so that salinity steadily increases.  However, Daniel & Boyden (1975) and Morris & Taylor (1983) reported little variability in salinity over one tidal cycle, and Ganning (1971) suggested that changes in salinity were of limited importance.  Morris & Taylor (1983) reported an annual maximum salinity of 36.5 ppt in the pools studied on the west coast of Scotland.  Goss-Custard et al. (1979) recorded salinities of 34.8 and 35.05 ppt in mid-shore pools.  Higher shore pools exhibit greater variation and higher extremes of salinity (Pyefinch, 1943; Ganning, 1971) and different communities but mid to low shore pools are unlikely to experience such extremes unless the emergence regime is increased or they are exposed to hypersaline effluents.  This biotope occurs in full salinity (Connor et al., 2004) a change above the pressure benchmark is assessed as a change to above 40 ppt..  Local populations will be acclimated to the prevailing conditions even over small spatial distances and gradients and caution should be used in extrapolating sensitivities.

Furcellaria lumbricalis is a euryhaline species which occurs in a wide range of salinity conditions down to 6-8 psu (Bird et al., 1991).  In the Kattegat and the Gulf of St Lawrence, it is reported to compete well with other species at salinities ranging from 25-32 psu (see review by Bird et al., 1991).  Growth experiments in the laboratory revealed that optimum growth occurred at 20 psu, the species grew well at 10 psu and 30 psu, but that growth declined above 30 psu to negligible levels at 50 psu (Bird et al., 1979).  It is expected that an increase in salinity may cause reduced growth and fecundity, but that mortality is unlikely.

Haglund et al. (1987) studied photosynthetic rate of Ahnfeltia plicata from the subtidal in Sweden and found that, at constant temperature, rate increased up to the maximum salinity tested (33 psu). No specific evidence was found for the salinity tolerance of the key characterizing species Polyides rotunda. The Rhodothamniella floridula dominated biotope occurs in full salinity conditions. Although no information has been found on survival in hypersaline conditions, the species occurs in rockpools where evaporation may occasionally lead to higher than normal salinities. Young Littorina littorea inhabit rock pools where salinity may increase above 35 psu.  Thus, these characterizing species may be able to tolerate some increase in salinity. 

Although not considered a characterizing species, more information was found on Chondrus crispus than the other red algaes within this biotope.  Chondrus crispus is found in a range of salinities across its range and has been reported from sites with yearly salinity range 0-10 psu and 10-35 psu (Lindgren & Åberg 1996) and sites from an average of 26-32 psu.  However, at different salinities the ratio between the abundance of the tetrasporophyte phase and the gametophyte alters (Guidone & Grace, 2010).  Mathieson & Burns (1971) recorded maximum photosynthesis of Chondrus crispus in culture at 24 psu, but rates were comparable at 8, 16 and 32 psu. Photosynthesis continued up to 60 psu.  Bird et al. (1979) recorded growth of Canadian Chondrus crispus in culture between 10 and 50 psu, with a maximum at 30 psu.  Chondrus crispus would therefore appear to be euryhaline and tolerant of a range of salinities.

Sensitivity assessment.  Based on the reported salinity tolerances of the red algae species found within this biotope, it is considered that an increase at the pressure benchmark may lead to some changes in the biological composition of this biotope.  The change is likely to reduce species richness and abundance, although some more tolerant species may persist.  Resistance (of the biotope) is therefore assessed as ‘Medium’ and resilience as ‘Medium’ and the biotope 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

During periods of emersion, high rainfall will reduce pool salinity or create a surface layer of brackish/nearly freshwater for a period.  The extremes of salinity experienced will depend on the depth of the pool, shore height and flushing rate, and season.  For example, Morris & Taylor (1983) stated that a low salinity layer of 2-10 mm was normal but after one storm the low salinity layer increased in depth, eventually resulting in a homogeneous pool of brackish water.  Morris & Taylor (1983) reported an annual salinity range in mid to low shore pools of 26-36.5 ppt.  Therefore, decreases in salinity equivalent of a reduction from full to reduced (see benchmark) are likely to be a regular occurrence in rockpool communities.  This biotope is present in full salinity (30-35 ppt, Connor et al., 2004), the assessed change at the pressure benchmark is a reduction in salinity to a variable regime (18-35 ppt) or reduced regime (18-30 ppt).  Populations can acclimate to local conditions but caution should be used in extrapolating observations from areas where populations are likely to the adapted to the prevailing conditons.  A comparative study of salinity tolerances of macroalgae collected from North Zealand in the South Kattegat (Denmark) where salinity is 16 psu.  Showed that species generally had a high tolerance (maintained more than half of photosynthetic capacity) to short-term exposure (4 days) to salinities lower than 3.7 (Larsen & Sand-Jensen, 2006).  However, tolerances varied between species with Ahnfeltia plicata, Phyllophora pseudoceranoides and Chondrus crispus exhibiting greater tolerance than Rhodomela confervoides (Larsen & Sand-Jensen, 2006).  The result illustrates that responses to this pressure will vary between species and that a change at the pressure benchmark is likely to alter the composition of the red algal turf that characterizes the biotope and may alter the biomass and density of more tolerant species.

Furcellaria lumbricalis occurs in the lowest category on the salinity scale (Connor et al., 2004) and therefore probably relatively tolerant of decreases in salinity.  The species forms extensive populations in the main basin of the Baltic Sea where salinity is 6-8 psu in the upper 60-70 m and its extension into the Gulfs of Bothnia and Finland is limited by the 4 psu isohaline (see review by Bird et al., 1991).

Ahnfeltia plicata occurs over a very wide range of salinities. The species penetrates almost to the innermost part of Hardanger Fjord in Norway where it experiences very low salinity values and large salinity fluctuations due to the influence of snow melt in spring (Jorde & Klavestad, 1963).  Ahnfeltia plicata penetrates further than the euryhaline species, Polyides rotunda, and probably has a similar salinity tolerance to Furcellaria lumbricalis, which is limited only by the 4 psu isohaline (see review by Bird et al., 1991).  Haglund et al. (1987) studied photosynthetic rate of Ahnfeltia plicata from the subtidal in Sweden and found that, at constant temperature, photosynthesis was positively correlated with salinity between 15 and 33 psu.  It is likely, therefore, that the benchmark decrease in salinity would not result in mortality, but photosynthesis would not be optimal and so growth and reproduction may be compromised.

No information was found on the effects of reduced salinity on Rhodothamniella floridula.  However, as this species occurs only in full salinity conditions it is probable that a proportion of the population would die in lower salinities. Littorina littorea is found in waters of full, variable and reduced salinities (Connor et al., 2004) and so populations are not likely to be highly intolerant of decreases in salinity at the pressure benchmark. Chondrus crispus is found in a range of salinities across its range and has been reported from sites with yearly salinity range 0-10 psu and 10-35 psu (Lindgren & Åberg, 1996) and sites from an average of 26-32 psu.  However, at different salinities the ratio between the abundance of the tetrasporophyte phase and the gametophyte alters (Guidone & Grace, 2010).  Mathieson & Burns (1971) recorded maximum photosynthesis of Chondrus crispus in culture at 24 psu, but rates were comparable at 8, 16 and 32 psu.  Photosynthesis continued up to 60 psu.  Bird et al. (1979) recorded growth of Canadian Chondrus crispus in culture between 10 and 50 psu, with a maximum at 30 psu.  Chondrus crispus would, therefore, appear to be euryhaline and tolerant of a range of salinities.

Sensitivity assessment.  Furcellaria lumbricalis is likely to tolerate a reduction in salinity at the pressure benchmark.  The algae species Ahnfeltia plicata and Polyides rotunda, as well as littorinids are euryhaline and occur over a range of salinities.  Some changes in algal composition may occur as a response to a decrease in salinity but may not significantly alter it from the biotope description.  Resistance is, therefore, assessed as ‘Medium’ and resilience as ‘Medium’ (as bases may not remain to enhance recovery), and biotope sensitivity is judged to be '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

This biotope occurs across a range of flow speeds, from Strong (1.5 – 3.0 m/s) to areas where water flow is negligible (Connor et al., 2004).   As water velocity increases foliose macroalgae can flex and reconfigure to reduce the size of the alga when aligned with the direction of flow, this minimises drag and hence the risk of dislodgement (Boller & Carrington, 2007).  These characteristics allow these species to persist in areas that experience a range of flow speeds.  Biogenic habitat structures, including the fronds of algae, reduce the effects of water flow on individuals by slowing and disrupting flow.  Boller and Carrington (2006) found that the canopy created by a turf of Chondrus crispus reduced drag forces on individual plants by 15-65%.  The compact, turf forming growth of the algal species will therefore reduce water flow and the risk of displacement through turbulence and friction.  Scour is a key factor structuring this biotope, changes in the flow may increase or decrease sediment transport and associated scour.  Reductions in flow may lead to increased deposition of silts and alter the sediment character.  An increase in water flow at the pressure benchmark may re-suspend and remove sand particles which are less cohesive than mud particles.  The level of impact will depend on site specific hydrodynamic and sediment conditions.  Some periodic movement of sediments and changes in coverage is part of the natural temporal variation and periodic disturbances from storms may be more important than water flow in maintaining the character of the biotope, particularly in sheltered areas.

Very little information was available on the effect of water flow on Furcellaria lumbricalis, Ahnfeltia plicata, and Polyides rotunda.  However, these species are found in biotopes ranging from strong to negligible.  Therefore, only if water flows were to increase above 6 knots (> 3 m/sec) these species may decrease in abundance.

Rhodothamniella floridula is associated occurs in areas where the water flow rate is either moderately strong (0.5-1.5 m/s) or weak (<0.5 m/s) (Connor et al., 2004).  Moderate water movement is beneficial to seaweeds as it carries a supply of nutrients and gases to the plants and removes waste products.

Littorina littorea is found in areas with water flow rates from negligible to strong, although populations exposed to different levels of flow may have adapted to local conditions.  Increases in water flow rates above 6 knots ( 3 m/s) may cause snails in less protected locations (e.g. not in crevices etc.) to be continually displaced into unsuitable habitat so that feeding may become sub-optimal.  Thus, populations of Littorina littorea are likely to reduce.  Shell morphology within littorinids varies according to environmental conditions.  In sheltered areas shell apertures are small to inhibit predation where Carcinus maenas is more prevalent while in exposed areas the foot surface is larger to allow greater attachment and the shell spire is lower to reduce drag (Raffaelli 1982; Crothers, 1992).

Sensitivity assessment.  As the biotope can occur in a range of flow speeds, resistance of the biotope to changes in water flow that do not alter the substrata is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is assessed as ‘Not sensitive’.

Emergence regime changes

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

Evidence

An increase in emergence is likely to significantly affect physico-chemical environment of the rockpool and its resident community.  An increase in emergence will increase the time that the pool is exposed to fluctuating air temperatures, wind, rain and sunlight, all of which will affect the and temperature, salinity regime within the pool.  Lower shore pools will come to resemble mid shore pool communities, with a reduction in sublittoral species and species sensitive to extremes of temperature, for example the laminarians.  For example, the upper limit of Bifurcaria bifurcata within rockpools in Roscoff, France was shown to be limited by the summer temperatures where the surface pool water temperatures exceeded 20 °C (Kooistra et al., 1989).  Mid shore examples of this biotope are likely to be worst affected.  High shore pools tend to support communities of temperature tolerant or opportunistic algae, especially green algae such as Ulva spp., and temperature and salinity tolerant species as harpacticoid copepods, ostracods, and small gastropods.  A decrease in emergence will reduce the time the pool spends exposed to the air and cut off from the sea.  Therefore, the range of temperatures and oxygen levels characteristic of rockpool environments is likely to decrease.  Hence the pool communities will come to resemble low shore pools.  Low shore pools are characterized by higher abundance of large macroalgae, such as Halidrys siliquosa, Cystoseira sp. and laminarians, especially deep pools, and a larger diversity of red algae and macrofauna.  However, the presence of sediment within the pools will still favour the sand tolerant algae.  Therefore, although the community is likely to increase in diversity the biotope is likely to remain.

Both Furcellaria lumbricalis and Ahnfeltia plicata is found in the lower intertidal to 12m subtidally (Bunker et al., 2012).  Therefore, if examples of this biotope in mid shore were to experience an increase in emergence then this species would most probably decline.  If there is a decrease in the emergence then these species may be able to increase in their range up the shore.

Polyides rotunda has a wider physical tolerance range than the two previously mentioned algaes.  Bunker et al. (2012) notes that this species occurs from the mid shore down to 24 m depth.  This tolerance range means that Polyides rotunda may tolerate a decrease or increase in emergence better than other species.

Rhodothamniella floridula occurs predominantly in the littoral and sublittoral to about 5m depth (Dickinson, 1963; Dixon & Irvine, 1997) (as Rhodochorton floridulum and Audouinella floridula respectively) and is often found in rockpools.  Emergence regime is a key factor structuring this (and other) intertidal biotopes.  Increased emergence may reduce habitat suitability for the characterizing species Rhodothamniella floridula through greater exposure to desiccation and fluctuations in temperature and salinity when emersed (above the surface) and reduced photosynthetic rates when immersed (submerged).  Changes in emergence may therefore lead to physical stress followed by species replacement through competition with species more tolerant of the changed conditions.  

An increase in emergence that reduced habitat suitability for the grazing littorinids would allow blooms of ephemeral Ulva spp. to develop altering the classification of the biotope to LR.FLR.Eph.EphX.

Sensitivity assessment.  As emergence is a key factor structuring the distribution of macroalgae on the shore, resistance to a change in emergence (increase or decrease) is assessed as ‘Low’.  Recovery is assessed as ‘Medium’ (following habitat recovery) and sensitivity is therefore assessed as 'Medium'.

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 has been recorded from exposed to sheltered wave conditions.  The effect of increased wave exposure is probably dependant on the depth of the pool.  Sediment is unlikely to remain in shallow pools in wave exposed conditions.  In deep pools, increased wave exposure is likely to result in increased scour, resulting in bare rock at the bottom of the pool, especially where cobbles and pebbles are present.  Fine sediments and the infauna they support are likely to be lost.  However, if the pool is deep enough, the upper levels of the pool are likely to continue to support macroalgae, especially laminarians and erect corallines.  For example, an increase in wave exposure from moderately exposed to very exposed is likely to remove the sediment from all but the deepest pools.  The occurrence of this biotope across wave exposure categories, was 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.

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).  Littorina littorea regularly have to abandon optimal feeding sites in order to avoid wave-induced dislodgement.  This will result in a decreased growth rate (Mouritsen et al., 1999).  Increases in wave exposure above the pressure benchmark will probably cause a decrease in population size of Littorina littorea.

Sensitivity assessment.  An increase in wave action, exceeding the pressure benchmark, may alter the character of the biotope.  The characterizing and associated species would probably accrue damage from abrasion and scour.  However, at the pressure benchmark the biotope is assessed as ‘High’, and resilience is also assessed as ‘High’ by default. The biotope is therefore considered to be ‘Not sensitive’.

Transition elements & organo-metal contamination

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

Evidence

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

Hydrocarbon & PAH contamination

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

Evidence

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

Synthetic compound contamination

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

Evidence

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

Radionuclide contamination

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

Evidence

No evidence.

Introduction of other substances

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

Evidence

This pressure is Not assessed.

De-oxygenation

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

Evidence

During emergence, rock pools are closed systems and gaseous exchange occurs over the air/water interface.  In shallow pools the surface area to volume ratio is likely to be high, whereas in deep pools the ratio is likely to be low.  In addition, the oxygen concentration is dependent on the community present.  During the day, photosynthesis uses up CO₂ and produces O₂, in excess of respiration.  However, at night respiration by flora and fauna deplete oxygen levels.  As a result rock pool environments exhibit marked variation in oxygen levels.  In summer, rock pools are likely to be supersaturated with oxygen during the day (Pyefinch, 1943).  For example, the greatest range of oxygen saturation of 101.7 % occurred in a seaweed dominated, sediment floored pool, which reach over 190 % saturation on some days (Pyefinch, 1943).  Daniel & Boyden (1975) noted that a mid-shore, seaweed dominated pool reached 194 % saturation (ca 15 mg O₂/l) but that oxygenation was also marked in shaded pools.  A pool with dense fauna exhibited a maximum saturation of 210 % (Pyefinch, 1943).  During photosynthesis, algae absorb CO₂ and as concentrations fall, the pH rises.  Morris & Taylor (1983) recorded pH values >9 in rock pools on the Isle of Cumbrae.  At night, oxygen levels may fall below 100% saturation and pH will decrease as CO₂ levels increase.  Morris & Taylor (1983) noted an annual maximum of O₂ concentration of 400-422 mm Hg (ca 23.4-24.7 mg/l) and an annual minimum of 18-38 mm Hg (ca 1-2.2 mg/l) in mid shore pools (containing Furcellaria lumbricalis).  Daniel & Boyden (1975) reported O₂ depletion at night, with mid to low shore pools reduced to 8-44 % saturation.  They noted that the crab Carcinus maenas leaves the pools at night, and that other species with the ability to air-breathe could also do so, e.g. limpets, littorinids, and the shanny Lipophrys pholis.  They also observed that shrimps gathered at the edge of high shore pools at night, presumably to take advantage of the better oxygenated surface layer (Daniel & Boyden, 1975).  The range of extremes in oxygen concentration were greater in summer than in winter.  On immersion, the rockpool community was exposed to potentially large, sudden fluctuations in oxygen concentrations depending on season and time of day (Morris & Taylor, 1983).  Therefore, rockpools communities are probably exposed to variations equivalent to or greater than the benchmark level on a regular basis and tolerant has been recorded.

The effects of reduced oxygenation on the characterizing red algae are not well studied.  Lack of oxygen may impair both respiration and photosynthesis (see review by Vidaver, 1972).  No evidence is available to make a sensitivity assessment for the key characterizing algal species.  However, the presence of these species in rock pools suggests that they can tolerate some short-term oxygen depletion.

Littorina littorea have a high tolerance for low oxygen conditions and can easily survive 3-6 days of anoxia (Storey et al., 2013).  In addition, Littorina littorea is an air breather when emersed, so can respire during low tide.

Sensitivity assessment.  The short-term exposure to de-oxygenation at this benchmark decreases the severity of the pressure.  However, the intertidal nature of the rock pools will mean that at low tide, when the pools are cut off, may become even more de- oxygenated.  This may increase the level of de-oxygenation and consequently the severity of the pressure.  The resistance of this pressure at the benchmark is assessed as ‘Medium’ as is the resilience.  Giving an overall sensitivity ‘Medium’. 

Nutrient enrichment

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

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The pressure benchmark is set at compliance with Water Framework Directive (WFD) criteria for good status, based on nitrogen concentration (UKTAG, 2014).  The nutrient enrichment of a marine environment leads to organisms no longer being limited by the availability of certain nutrients.  The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) decreases in dissolved oxygen and uncharacteristic microalgae blooms (Bricker et al., 1999, 2008).

No direct evidence was found to assess this pressure.  A slight increase in nutrient levels could be beneficial for barnacles by promoting the growth of phytoplankton levels and therefore increasing zooplankton levels.  However, Holt et al. (1995) predict that smothering of barnacles by ephemeral green algae is a possibility under eutrophic conditions.

Opportunistic algae, including Ulva sp. can’t store nutrients in the thallus (unlike larger, long-lived species) and are adapted to efficiently capture and utilise available nutrients in the water column (Pedersen et al., 2009).  A large body of field observations and experiments, surveys and laboratory experiments confirm that Ulva sp., can utilise high levels of nutrients for growth (Martínez et al., 2012) and that enhanced recruitment (Kraufvelin, 2007) and growth of this genus can occur in enriched areas (Kennison & Fong, 2013; Vaudrey et al., 2010).  In areas where nutrient availability is lower either naturally or through management to reduce anthropogenic inputs, Ulva sp. may be negatively affected through reduced growth rate and species replacement (Martínez et al., 2012; Vaudrey et al., 2010).

Johnston & Roberts (2009) undertook a review and meta analysis of the effect of contaminants on species richness and evenness in the marine environment.  Of the 47 papers reviewed relating to nutrients as a contaminants, over 75% found that it had a negative impact on species diversity, <5% found increased diversity, and the remaining papers finding no detectable effect.  Not all of the 47 papers considered the impact of nutrients on intertidal rocky shores.  Yet this finding is still relevant as the meta analysis revealed that the effect of marine pollutants on species diversity were ‘remarkably consistent’ between habitats (Johnston & Roberts, 2009).  It was found that any single pollutant reduced species richness by 30-50% within any of the marine habitats considered (Johnston & Roberts, 2009).  Throughout their investigation there were only a few examples where species richness was increased due to the anthropogenic introduction of a contaminant.  These examples were almost entirely from the introduction of nutrients, either from aquaculture or sewage outfalls.  However research into the impacts of nutrient enrichment from these sources on intertidal rocky shores often lead to shores lacking species diversity and the domination by algae with fast growth rates (Abou-Aisha et al., 1995, Archambault et al., 2001, Arévalo et al., 2007, Diez et al., 2003, Littler & Murray, 1975).

Nutrient enrichment alters the selective environment by favouring fast growing, ephemeral species such as Ulva lactuca and Ulva intestinalis (Berger et al., 2004, Kraufvelin, 2007).  Rohde et al., (2008) found that both free growing filamentous algae and epiphytic microalgae can increase in abundance with nutrient enrichment.  This stimulation of annual ephemerals may accentuate the competition for light and space and hinder perennial species development or harm their recruitment (Berger et al., 2003; Kraufvelin et al., 2007).  Nutrient enrichment can also enhance fouling of Fucus fronds by biofilms (Olsenz, 2011).  Nutrient enriched environments can not only increase algae abundance, but the abundance of grazing species (Kraufvelin, 2007).

Changes in community composition on intertidal rocky shores can happen rapidly, and fast growing ephemeral species can become established quickly in the presence of higher concentrations of nutrients.  The establishment and growth of these species are not controlled by wave exposure (Kraufvelin, 2007).  However, even though these fast growing ephemeral species can become well established quickly, healthy communities on intertidal rocky shores can survive long periods of time, and maintain ecological function after these species have become established (Bokn et al., 2002, 2003, Karez et al.,2004, Kraufvelin, 2007, Kraufvelin et al., 2006b).

Sensitivity assessment. A slight increase in nutrients may enhance growth rates but high nutrient concentrations could lead to the overgrowth of the algae by ephemeral green algae and an increase in the number of grazers.  However, if the biotope is well established and in a healthy state the biotope could have the potential to persist.  The effect of an increase in this pressure to the benchmark level should not have a negative impact on the biotope.  Therefore the resistance has been assessed as ‘High’.  As there will be nothing for the biotope to recover from, the resilience is also ‘High’. These two rankings give an overall sensitivity of ‘Not Sensitive’.

Organic enrichment

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

Evidence

The organic enrichment of a marine environment at this pressure benchmark leads to organisms no longer being limited by the availability of organic carbon.  The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) and decreases in dissolved oxygen and uncharacteristic microalgae blooms (Bricker et al., 1999, 2008).

Johnston & Roberts (2009) undertook a review and meta analysis of the effect of contaminants on species richness and evenness in the marine environment.  Of the 49 papers reviewed relating to sewage as a contaminant, over 70% found that it had a negative impact on species diversity, <5% found increased diversity, and the remaining papers finding no detectable effect.  Not all of the 49 papers considered the impact of sewage on intertidal rocky shores.  Yet this finding is still relevant as the meta analysis revealed that the effect of marine pollutants on species diversity were ‘remarkably consistent’ between habitats (Johnston & Roberts, 2009).  It was found that any single pollutant reduced species richness by 30-50% within any of the marine habitats considered (Johnston & Roberts, 2009).  Throughout their investigation there were only a few examples where species richness was increased due to the anthropogenic introduction of a contaminant.  These examples were almost entirely from the introduction of nutrients, either from aquaculture or sewage outfalls.  However, research into the impacts of organic enrichment from these sources on intertidal rocky shores often lead to shores lacking species diversity and the domination by algae with fast growth rates (Abou-Aisha et al., 1995, Archambault et al., 2001, Arévalo et al., 2007, Diez et al., 2003, Littler & Murray, 1975).

Organic enrichment alters the selective environment by favouring fast growing, ephemeral species such as Ulva lactuca and Ulva intestinalis (Berger et al., 2004, Kraufvelin, 2007).  Rohde et al., (2008) found that both free growing filamentous algae and epiphytic microalgae can increase in abundance with nutrient enrichment.  This stimulation of annual ephemerals may accentuate the competition for light and space and hinder perennial species development or harm their recruitment (Berger et al., 2003; Kraufvelin et al., 2007).  Nutrient enrichment can also enhance fouling of fucoid fronds by biofilms (Olsenz, 2011).  Nutrient enriched environments can not only increase algae abundance, but the abundance of grazing species (Kraufvelin, 2007).  Bellgrove et al. (2010) found that coralline turfs out-competed fucoids at a site associated with organic enrichment caused by an ocean sewage outfall.

Changes in community composition on intertidal rocky shores can happen rapidly, and fast growing ephemeral species can become established quickly in the presence of higher concentrations of nutrients.  The establishment and growth of these species are not controlled by wave exposure (Kraufvelin, 2007).  However, even though these fast growing ephemeral species can become well established quickly, healthy communities on intertidal rocky shores can survive long periods of time, and maintain ecological function after these species have become established (Bokn et al., 2002, 2003, Karez et al.,2004, Kraufvelin, 2007, Kraufvelin et al., 2006b).

Sensitivity assessment.  Little empirical evidence was found to support an assessment of this biotope at this benchmark.  Ulva sp. could easily increase in abundance, and this species could grow over the slower growing characterizing species.  However, when organic carbon returns to pre-pressure levels the niche for the characterizing algae would return.  If the crust stages remain, from which the thalli of the characterizing red algae can regrow, then recovery could be quick.  Therefore, resistance has been assessed as ‘High’ and resilience has been assessed as ‘High’. This gives an overall sensitivity score of ‘Not sensitive’.

Physical loss (to land or freshwater habitat)

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

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

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

Evidence

This biotope is characterized by a rockpool formed from bedrock, within which finer sediment fractions can be found.  Removal of the bedrock would remove the attachment surface for the red algal turf that characterizes this biotope, significantly altering the character of the biotope.  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., 2014) or the presence of non-native species (Bulleri & Airoldi, 2005). 

Sensitivity assessment. A change to a sedimentary habitat would reduce habitat suitability for 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

Generally this pressure is considered to be 'Not relevant' to biotopes occurring on bedrock.  However, as this biotope is characterized by the bottom of the rock pools having sediment within them, this pressure may have an impact on the health of the biotope.  The sand covering and scour is an important factor supporting development and maintenance of this biotope.  Removal of sands may allow species with less scour tolerance to colonize the rock pools, altering the character of the biotope.  Siltation by finer sediments may have less impact but may lead to subtle changes and changes such as anoxia at the bedrock/sediment interface may lead to removal or damage of holdfasts and bases.  A change to coarser gravels, pebbles and cobbles would increase the degree of abrasion where these are mobile and this may also remove the red algal turf.

Sensitivity assessment.  A change in the character of the overlying sediment may alter the character of the biotope.  Resistance is therefore assessed as 'Low' and resilience is Very low (the pressure is a permanent change), so that the biotope is considered to have 'High' sensitivity to this pressure.

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 occur 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

No studies of the effects of trampling or netting on rockpools were found but studies of the effects on emergent algal communities are probably indicative.  For example, moderate (50 steps per 0.09 sq. metre) or more trampling on intertidal articulated coralline algal turf in New Zealand reduced turf height by up to 50%, and the weight of sand trapped within the turf to about one third of controls (Schiel & Taylor, 1999).  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 fucoid algae and coralline turf species on the New Zealand rocky shore.  Low trampling intensity (10 tramples) reduced fucoid cover by 25%, while high intensity (200 tramples) reduced fucoid cover by over 90%, although over 97% cover returned within 21 months after spring trampling; autumn treatments took longer to recover due to the delay in recruitment.  Coralline bases were seen to peel from the rocks (Schiel & Taylor, 1999) due to increased desiccation caused by loss of the algal canopy.  Brosnan & Cumrie (1994) demonstrated that foliose species (e.g. fucoids and Mastocarpus papillatus) were the most susceptible to trampling disturbance, while turf forming species were more resistant.  Boalch et al. (1974) and Boalch & Jephson (1981) noted a reduction in fucoid cover (especially of Ascophyllum nodosum) at Wembury, Devon, when compared with the same transects surveyed 43 years previously.  They suggested that the reduction in fucoid cover was due to the large number of visitors and school groups received by the site.  

Lagoons form natural shallow water mesocosms and can attract considerable attention from the general public, educational events and scientists alike.  Overall, a proportion of the macroalgal community, and the invertebrates it supports are likely to be removed, depending on trampling intensity, and an intolerance of intermediate has been recorded.  Resilience is likely to be high (see additional information below) once physical disturbance has stopped.  However, it should be noted that ongoing trampling is likely to result in a long-term reduction in the diversity of affected pools.

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 red algal turf and encrusting corallines, resistance, to a single abrasion event is assessed as ‘Medium’ (loss of <25% cover/abundance) and recovery as ‘High’ (as bases are likely to remain).  Therefore, the sensitivity is assessed as ‘Low’.  Resistance and resilience will be lower (and hence sensitivity greater) to abrasion events that exert a greater crushing force and remove the bases.

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 characterizing species of this biotope occurs 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

The presence of sediment in the rockpool environment is a defining feature of this biotope therefore it is potentially sensitive to changes in suspended solids.  Siltation and abrasion, which may be associated with changes in suspended solids are assessed separately.  The characterizing sediment tolerant red algae are unlikely to be adversely affected by an increase in suspended sediment.  However, other macroalgae, and the community they support, are likely to be adversely affected, as shown above (Daly & Mathieson, 1977; Seapy & Littler, 1982).  On wave sheltered shores, sediment may accumulate in low to mid shore pools, which again will favour sand tolerant species and infauna.  Overall, the biotope is likely to remain but with a decrease in species diversity (for example see Daly & Mathieson, 1977).

A decrease in suspended sediment may result in a decrease in the amount of sediment found in the bottom of pools.  This removal is likely to be greatest in shallow pools or on shores with greater wave exposure.  The internal topography of individual pools will affect the rate of erosion, the presence of depressions and crevices may retain sediment longer.  A decrease in suspended sediment loads may also reduce food availability of suspension feeders within the biotope.  Erosion or removal of sediment from the pools would be detrimental for an infauna, and especially sand tolerant algae which are likely to be out-competed by other red algae.  

Furcellaria lumbricalis often occurs in relatively turbid waters. Laboratory experiments by Bird et al. (1979) revealed that Furcellaria lumbricalis was growth saturated at very low light levels (ca 20µE/m²/s) compared to other algae such as Chondrus crispus (50-60µE/m²/s).  They suggest that this may be an explanation why Furcellaria lumbricalis is able to proliferate in relatively deep and turbid waters.  Similarly, in their review, Bird et al. (1999) comment that in all studies, saturation and inhibition radiances were low for Furcellaria lumbricalis compared to other macroalgae indicating good competitive ability in the attenuated light of deeper or more turbid waters.  Increases in turbidity may provide the species with a competitive advantage over other macroalgae.

Ahnfeltia plicata is not likely to be affected directly by an increase in suspended sediment.  In general, subtidal red algae are able to exist at relatively low light levels (Ahnfeltia plicata typically occurs as an understorey alga beneath Laminaria sp. (Lüning, 1990) and so is presumably well adapted to growth in low light conditions.  Rhodothamniella floridula can occur as exposed mats on hard substratum or may occur beneath canopies of Fucus spp. which provide shade, indicating that it is tolerant of both high and low light levels and would not be affected by a change in turbidity at the benchmark.  Mats of Rhodothamniella floridula are found on shores with high levels of sediment and trap sediments between the thalli. This species is therefore considered to be exposed to and tolerant of high levels of suspended sediment and associated scour. 

Littorina littorea is found in turbid estuaries where suspended sediment levels are high.

Sensitivity assessment.  Changes in suspended solids at the pressure benchmark may lead to changes in the biotope assemblage.  However, no empirical evidence was found to assess this pressure. Increases in suspended solids may lead to greater scour damaging plants and reduce light availability impacting growth.  Decreased suspended solids may increase the suitability of the biotope for species less tolerant of scour and turbidity that can out-compete the characterizing species.  The assessment is based on the characteristic sands remaining in the biotope covering the rocks and limiting the establishment of other competing species, In this scenario the biotope will still be subject to episodes of re-suspension and water transport.  As the species present are tolerant of scour and turbidity, biotope resistance is assessed as ‘Medium’ and resilience as ‘High’ (as crustose bases are expected to remain to support recovery) sensitivity is therefore assessed as ‘Low’.

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

Seapy & Littler (1982) reported a decrease in macroalgal cover from 47.3 to 37.5% on a Californian rocky shore due to sediment deposition on the mid to lower shore following rain and flooding.  Corallina sp. and Pelvetia sp. were the most affected macroalgal species, while associated red algae were only slightly affected by the resultant scour.  Macroinvertebrates declined in cover from 15.8% to 6.5% particularly barnacle species.  Daly & Mathieson (1977) examined intertidal zonation on a shore affected by sand scour, and noted that fucoids were reduced to small or young plants, while sand tolerant species such as Ahnfeltia plicata dominated on areas affected by sediment.  Smothering by 5 cm of sediment is likely to increase scour and be detrimental to the characterizing red algae species.  Red algae whose fronds are long enough not to be smothered by 5 cm of sediment could be negatively impacted by the increase in scour.  However, all of the characterizing red algaes within this biotope are noted for their tolerance to scour and could therefore tolerate an increase in this pressure at the benchmark.  Germlings are more likely to be smothered and killed and are inherently most susceptible to this pressure.  Indeed, all early life stages are smaller in size than adults and are thus most vulnerable to this pressure as even a small load of added sediment will lead to the complete burial.  Sediment deposition can reduce macroalgal recruitment by: (1) reducing the amount of substratum available for attachment of propagules; (2) scour, removing attached juveniles and (3) burial, altering the light and/or the chemical micro-environment (Devinny & Volse, 1978, Eriksson & Johansson, 2003).

The rockpool environment is likely to be more vulnerable to smothering as sediment is likely to accumulate in, and be retained by the rockpool itself.  This effectively increases the depth of the sediment layer in the pool.  In wave exposed conditions the sediment may be removed, but in sheltered areas it is likely to be retained for longer than indicated by the benchmark.  In deep pools, the macroalgae and associated invertebrates are likely to reduce in depth penetration into the pool while sediment tolerant algae increase.  In shallow pools, the depth may be further reduced and the macroalgae restricted to sand tolerant species alone.  Overall, while the biotope will remain, smothering is likely to reduce the diversity of the pool, exclude grazing littorinids, and smother small epifaunal species such as sponges, bryozoans and ascidians.

Littorina littorea through grazing and bulldozing actions may directly aid the removal of silts and sediments and remove the algal films that may accumulate silts (Bertness, 1984).  On a protected New England rocky beach, Bertness (1984) showed how accumulation of sediments due to the removal of the snail Littorina littorea changed the character of the habitat.  Bertness (1984) observed a decrease in the abundance of organisms characteristic of hard-bottom habitats, such as barnacles and encrusting algae (cited from Airoldi 2003).  Chandrasekara & Frid (1998) specifically tested the siltation tolerance of Littorina littorea.  Burial to 5 cm caused mortality within 24 hours at simulated summer and winter temperatures if the snails could not crawl out of the sediment (Chandrasekara & Frid, 1998).  If the sediment is well oxygenated and fluid (as with high water, high silt content) a few snails (1-6 out of 15 in the experiment, depending on temperature, sediment and water content) may be able to move back up through 5 cm of sediment (Chandrasekara  & Frid, 1998).  Approximately half of the test individuals could not regain the surface from 1 cm 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.

Sensitivity assessment.  Based on the tolerance of the characterizing red algae to sediment inundation and scour, resistance of the characterizing algaes to this pressure at the benchmark is likely to be high.  However, the littorinids are not tolerant of sediment inundation and an increase in the pressure at the benchmark could reduce the number of these gastropods significantly.  By removing this grazer the ecosystem function would change and it is likely that there would be an increase in algal biomass until the grazers returned.  The resistance is assessed as 'Medium' and resilience is assessed as ‘High’ resulting in a ‘Low’ sensitivity.

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

No evidence was found to assess this pressure at the benchmark. A deposit at the pressure benchmark would cover all species with a thick layer of fine materials. Species associated with this biotope such as limpets and littorinids would not be able to escape and would likely suffer mortality (see evidence for light siltation). Sensitivity to this pressure will be mediated by site-specific hydrodynamic conditions and the footprint of the impact. Where a large area is covered sediments may be shifted by water currents rather than removed. Mortality will depend on the duration of smothering; where wave action rapidly mobilises and removes fine sediments, survival of the characterizing and associated species may be much greater.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

No evidence.

Underwater noise changes

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

Evidence

Species characterizing this habitat do not have hearing perception but vibrations may cause an impact, however no studies exist to support an assessment.

Introduction of light or shading

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

Evidence

In general, subtidal red algae are able to exist at relatively low light levels (Gantt, 1990).  Canopy removal experiments in a rocky sub tidal habitat in Nova Scotia, Canada by Schmidt & Scheibling (2007) did not find a shift in understorey macraoalgal turfs (dominated by Corallina officinalis, Chondrus crispus and Mastocarpus stellatus) to more light-adapted species over 18 months.

Furcellaria lumbricalis often occurs in relatively turbid waters.  Laboratory experiments by Bird et al. (1979) revealed that Furcellaria lumbricalis was growth saturated at very low light levels (ca 20 µE/m²/s) compared to other algae such as Chondrus crispus (50 - 60 µE/m²/s) and Fucus serratus (100 µE/m²/s).  They suggest that this may be an explanation why Furcellaria lumbricalis is able to proliferate in relatively deep and turbid waters.  Similarly, in their review, Bird et al. (1999) comment that in all studies, saturation and inhibition radiances were low for Furcellaria lumbricalis compared to other macroalgae indicating good competitive ability in the attenuated light of deeper or more turbid waters.

Ahnfeltia plicata is not likely to be affected directly by an increase in suspended sediment.  Ahnfeltia plicata typically occurs as an understorey alga beneath Laminaria sp. (Lüning, 1990) and so is presumably well adapted to growth in low light conditions.  An increase in turbidity would reduce the amount of light reaching the understorey.  Over the course of a year, this may result in mortality of the Ahnfeltia plicata individuals at the limit of their depth range.

Rhodothamniella floridula can occur as exposed mats on hard substratum or may occur beneath canopies of Fucus spp. which provide shade, indicating that it is tolerant of both high and low light levels.  In addition, Stegenga (1978) found that tetraspores of cultured Rhodothamniella floridula (as Rhodochorton floridulum) were formed at any length of daylight, although they were most abundant at high temperatures and long days.

Coralline crusts are shade tolerant algae, often occurring under a macralgal canopy that reduces light penetration.  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.

The range of Littorina littorea extends down into the subtidal (Fish & Fish, 1996) where light attenuation would mean lower light levels than experienced with increased shading in a rock pool.  In tank experiments Evans (1965) found that when Littorina littorea was without any macroalgae their vertical distribution was affected by directional light.  In these experiments when lit from below Littorina littorea tend to adopt a level lower than in the control experiment (Evans, 1965).

Sensitivity assessment.  As the key characterizing species colonize a broad range of light environments from intertidal to deeper sub tidal and shaded understorey habitats the biotope is considered to have ‘High’ resistance and, by default, ‘High’ resilience and therefore is ‘Not sensitive’ to this pressure.

Barrier to species movement

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

Evidence

Not relevant – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

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

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

Introduction or spread of invasive non-indigenous species

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

Evidence

The high levels of scour in this biotope will limit establishment of all but scour resistant invasive non-indigenous species (INIS).  No direct evidence was found for effects of INIS on this biotope.  Several non-native species may occur in this biotope.  Codium fragile subsp. Tomentosoides and Codium fragile subsp. Atlanticum were introduced from mainland Europe and Japan respectively and may out-compete the native Codium tomentosum (Eno et al., 1997).  The non-native harpoon weed Asparagopsis armata was first recorded in Ireland in Galway Bay in 1939 and Britain in 1949 at Lundy in the Bristol Channel, and may come to dominate rockpools (Guiry & Guiry, 2015), although its effect on other species is not known.

Sargassum muticum is a non-native macroalgae spreading around the coasts of Britain and Europe (see Eno et al., 1997) and is often found in low to mid shore rockpools in the intertidal in areas it has colonized.  Although, no studies on its effects on rockpool species were found, studies of its effect on shallow sublittoral macroalgae suggest that it can out-compete fucoids and kelps.  For example, Stæhr et al. (2000) reported that an increase in the abundance of Sargassum muticum in the Limfjorden (Denmark) from 1990 to 1997 was accompanied by a decrease in the abundance of thick, slow growing macroalgae such as Saccharina latissima (studied as Laminaria saccharina), Codium fragile, Halidrys siliquosa, Fucus vesiculosus, and Fucus serratus, together with other algae such as Ceramium virgatum(as rubrum) and Dictyota dichotoma.  In Sargassum muticum removal experiments on the coast of Washington State, Britton-Simmonds (2004) concluded that Sargassum muticum reduced the abundance of native canopy algae (especially kelps) by 75% and native understorey algae by 50% probably as a result of shading.  However, Viejo (1999) noted that mobile epifauna (e.g. amphipods, isopods) successfully colonized Sargassum muticum which provided additional habitat.  Overall, Sargassum muticum can successfully invade rockpools, and would probably out-compete resident fucoids and kelp species, and some red algae.  Although the results of studies of feeding preferences for Sargassum muticum over native macroalgae vary, Littorina littorea does feed on this species so shoreline colonization by this species would mean that food was still available (Withers et al. 1975).  Littorina littorea also grazes on degraded or stressed Didemnum vexillem individuals (Valentine et al., 2007) and Codium fragile ssp. Tomentosoides (Schiebling et al., 2008), so gains some benefit from the presence of these species.

A number of invasive red algae have been recorded in the UK, from reported habitat preferences Bonnemaisonia hamifera does not appear to be present in scoured environments.  In North America Grateloupia turuturu is a major competitor of Chondrus crispus, although Grateloupia turuturu is present in the UK, this large foliose species may not be able to colonize this scoured biotope due to the effects of drag and abrasion.

Sensitivity assessment.  The presence of sediment is likely to reduce the chance of an invasive non-native species.  However, there is a chance that the highly successful non-native Sargassum muticum might be able to invade this biotope.  The biotope may remain, but with a reduced species richness due to the loss of some species of macroalgae.  Resistance is ‘Medium’ and resilience could also be ‘Medium’, but assumes removal of Sargassum muticum which is unlikely.  Hence, a resilience of 'Very low' has been recorded since the biotope is likely to change, although a viable community will remain.  Due to the constant risk of new invasive species, the literature for this pressure should be revisited.

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

Extracts of some red algae show antimicrobial and antifungal activity limiting disease, Dilsea carnosa extracts, for example, limit colony extension in Microsporum canis and Trichophyton varrucosum (Tariq, 1991), providing some protection against pathogens.  No information on microbial pathogens in the characterizing red algae species in this biotope can be found.

Parasitism by trematodes may cause sterility in Littorina littorea.  Littorina littorea are also parasitized by the boring polychaete, Polydora ciliate and Cliona sp, which weakens the shell and increases crab predation (Stefaniak et al., 2005).

Sensitivity assessment.  Based on the lack of evidence for the characterizing red algae species within this biotope an assessment of ‘No Evidence’ has been given.

Removal of target species

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

Evidence

The direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  It is only the biological impact of target species removal which is assessed at this pressure.  Several of the characterizing red algae species are subject to harvesting.  Ahnfeltia plicata is one of the world's principal commercial agarophytes.  It is harvested mainly on the Russian coast of the White Sea as a source of high quality, low sulphate agar (Chapman & Chapman, 1980).  In Britain and Ireland, however, Ahnfeltia plicata does not occur in sufficient quantities to harvest on a commercial scale (Dickinson, 1963).  Commercial utilization of Furcellaria lumbricalis is based on the gelling properties of its extracted structural polysaccharide, furcellaran (Bird et al., 1991). Extraction of Furcellaria lumbricalis was reviewed by Guiry & Blunden (1991).  Plinski & Florczyk (1984) noted that over-exploitation of Furcellaria lumbricalis resulted in severe depletion of stocks.  However, no commercial harvest as yet occurs in Britain or Ireland. Overall, while rockpools in areas subject to commercial harvesting may be directly affected, most examples of the biotope are unlikely to be affected by commercial harvesting in the UK. However, due to the relative small size of the community, even small scale hand collecting may have a significant effect.

The removal of kelps or brown algae from this biotope is unlikely to the patchy nature of the rock pools which would be deep enough to contain such marcoalgae.  Edible seaweeds, carrageen (Chondrus crispus) and dulse (Palmaria palmata) may occur in low densities in the biotope and be harvested locally.  As these species are present only in low densities, ecological effects such as the proliferation of algae (from removal of grazers) or other significant changes to the structure and function of the biotope are not predicted to arise from removal.

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 sp., on which it preferentially feeds.  The community structure within the biotope is likely to be altered but some individuals are likely to remain.  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 surrounding, un-

A broad scale 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).

Sensitivity assessment.  This biotope is considered to have 'Low' resistance to removal of targeted species by hand-gatherers and 'Medium' resilience and is therefore considered to have a ‘Medium’ sensitivity to this pressure.  However, intense collection of the characterizing species Littorina littorea (the species present in high enough quantities within this biotope to be collected commercially) over extended periods of time could lead to the degradation of the biotope and consequent loss of the biotope.

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

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  The characterizing species Fucus ceranoides is a dominant species within this biotope.  The dominance of this characterizing species means it could easily be incidentally removed from this biotope as by-catch when other species are being targeted.  The loss of this species and other associated species would decrease species richness and negatively impact on the ecosystem function.

Sensitivity assessment. Removal of a large percentage of the characterizing species would alter the character of the biotope. The resistance to removal is ‘Low’ due to the easy accessibility of the biotopes location and the inability of these species to evade collection. The resilience is ‘Medium’, with recovery only being able to begin when the harvesting pressure is removed altogether. This gives an overall sensitivity score of ‘Medium’.