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

The geographic distribution of the species characteristic of the biotope and its sub-biotopes extends south of the British Isles so they are likely to be resistant to an increase in temperature. Infaunal species are probably protected to some extent from the direct effects of short-term increases in temperature by sediment buffering, although increased temperatures may affect infauna indirectly by stimulating increased bacterial activity and increased oxygen consumption. 

The amphipod Gammarus salinus tolerated a variety of temperature and salinity changes. Furch (1972) exposed Gammarus salinus to both constant (8°C, 14°C & 20°C) and fluctuating (daily fluctuations between 8°C to 20°C) temperatures. The species revealed significant differences in heat resistance, which became apparent within 12 hours. Gammarus salinus was able to endure long-term exposure (2 to 4 weeks) to fluctuating temperatures, although rapid temperature changes (every hour) were less tolerated than lower temperature fluctuations (every 2 hours). Acute temperature changes may cause additional stress but are unlikely to result in mortality. Recovery from rapid fluctuations was apparent within a matter of hours (Furch, 1972).

Environmental temperature exerts an influence on many of the physiological processes of mysid shrimps (Mauchline, 1980). However, the tolerance of mysid shrimps to changes in environmental temperatures varies between species and, to a lesser extent, between populations of the same species in different environments (Mauchline, 1980). The distribution of Neomysis integer extends to the south of the UK and along the Atlantic coast of Spain so the species may be able to tolerate a change of 2°C. However, Kinne (1955) found that juveniles of Neomysis integer had a different tolerance to temperature changes than adults. Kuhlman (1984) also found that overwintering and summer generations of Neomysis integer demonstrated distinct differences to increasing temperature. The upper tolerance of the winter generation was 10-12°C in comparison to 20-25°C for the summer generation. Hence, an acute increase in temperature may be particularly damaging to the population during the spring when the overwintering population commences breeding than at other times of the year. Survival and growth of Neomysis integer continued within a temperature range of 8°C to 25°C but sexual maturation was only possible in the narrower range of 15–25°C (Fockeday et al., 2005, 2006). Neomysis integer is likely to recover within a few weeks or at most six months following summer recruitment and probable migration between suitable habitats.

Sensitivity assessment.  Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010) although in estuaries and other shallow waters the temperature range may exceed this due to summer warming and winter cooling. The biotope, based on the characterizing species, is likely to tolerate a 2°C increase in temperature for a year. Gammarus salinus and Neomysis integer are likely to be resistant to a chronic change of 2°C for one year but a 5°C increase for one month may impact the Neomysis integer population during the spring when the overwintering population commences breeding. Survival and growth of Neomysis integer were detected within the whole range tested but sexual maturation was only possible in the narrower range of 15–25°C and 5–15 psu. Therefore, resistance is assessed as 'Medium', resilience as 'High' and sensitivity is assessed as 'Low'. 

Temperature decrease (local)

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

Evidence

The distribution of both Neomysis integer and Gammarus salinus extends to the north of the UK, so the fauna of the biotope would probably be resistant to a long-term decrease in temperature of 2°C. Acute decreases in temperature may cause the death of vulnerable proportions of the population owing to additional stress (e.g. those that are parasitized) but the resistance of healthy individuals is probably high at the benchmark pressure. Survival and growth of Neomysis integer continued within a temperature range of 8°C to 25°C, but sexual maturation was only possible in the narrower range of 15–25°C (Fockeday et al. 2005, 2006). Reproduction and longer-term abundance of the population may be impacted at the pressure benchmark, particularly in higher latitudes where winter water temperatures would fall below the lower limits of growth and sexual maturation.

Sensitivity assessment.  Typical surface water temperatures around the UK coast vary, seasonally from 6-19°C (Huthnance, 2010). Based on the characterizing species, the assessed biotope is considered to be tolerant of a 2°C  decrease in temperature for a year, a 5°C decrease for one month is likely to cause some mortality to vulnerable portions of the Neomysis integer and Gammarus salinus populations. Therefore, resistance to an acute decrease in temperature at the pressure benchmark level is assessed as ‘Low’, resilience as ‘High’ and sensitivity is assessed as ‘Low’. 

Salinity increase (local)

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

Evidence

Salinity is a key factor structuring the biotope. This biotope SS.SSa.SSaVS.NintGam occurs at the transition between freshwater and brackish habitats (JNCC, 2015). Furthermore, JNCC (2015) note that this biotope may shift further up or down the estuary on a seasonal or yearly basis depending on freshwater flow. Neomysis integer and Gammarus salinus are euryhaline species. However, whilst Gammarus salinus may tolerate salinities of 30 psu, Neomysis integer was found to have an upper salinity tolerance between 20-25 psu, with death occurring at 30 psu (Kuhlman, 1984) and in the Severn estuary, UK, they were shown to occur in low salinities (Bamber & Henderson, 1994). 

Sensitivity assessment.  An increase in salinity from 'low' to 'reduced' or 'reduced' to 'full' would probably reduce the abundance of Neomysis integer and allow other species to colonize the biotope. For example, the SS.SSa.SSaVS.NintGam habitat may become more like the sub-biotope SS.SSa.SSaVS.NcirLim or the biotope SS.SSa.SSaVS.  However, the observation that the SS.SSa.SSaVS.NintGam habitat shifts further up or down the estuary depending on freshwater flow indicates that recovery would be rapid. Therefore, resistance is assessed as ‘None’ (as the biotope may be lost within the affected area) but with a resilience of ‘High’ 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

Salinity is a key factor structuring the biotope. This biotope SS.SSa.SSaVS.NintGam occurs at the transition between freshwater and brackish habitats (JNCC, 2015). Furthermore, JNCC (2015) note that this biotope may shift further up or down the estuary on a seasonal or yearly basis depending on freshwater flow.  Neomysis integer is tolerant of salinities as low as 0.5 psu (Koepke & Kausch, 1996) and occurs in upper estuary low salinity areas in the Severn estuary UK (Bamber & Henderson, 1994). Gammarus salinus is a euryhaline species relatively tolerant of salinities as low as 2 psu and as high as 30 psu, but it is most abundant at 10 psu. Bulnheim (1984) recorded the respiratory response of Gammarus salinus in response to an acute salinity change, from 30 psu to 10 psu. Respiration rate moderately increased after an initial shock-like response and initially, specimens were quiescent as they acclimated to the decreased salinity but recovered within 24 hours. Their resistance is likely to be high and recovery immediate.

Sensitivity assessment. A further reduction in salinity at the extreme of their salinity tolerance range would expose them to freshwater. Neomysis integer successfully made the transition to freshwater environments but presumably over an extended period. Freshwater species may successfully penetrate the biotope and the SS.SSa.SSaVS.NintGam biotope would temporarily not be recognised. However, the observation that the SS.SSa.SSaVS.NintGam habitat shifts further up or down the estuary depending on freshwater flow indicates that recovery would be rapid.  Therefore, resistance is assessed as ‘None’ (as the biotope may be lost within the affected area) but with a resilience of ‘High’ 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

The biotope SS.SSa.SSaVS.NintGam is characterized by strong tidal streams (JNCC, 2015). Neomysis integer actively seeks regions where the water flow rate does not in exceed 0.2 knots (10 cm/sec) such as at the waters edge or boundary layer. If exposed to a significant increase in water flow the species present in the biotope would probably experience difficulty in maintaining a position and be washed from the biotope. Sand is also likely to be swept away by increased tidal flow. In the absence of the important characterizing species, the biotope would not be recognised. On resumption of a normal flow regime, the species lost will probably recolonize rapidly from adjacent areas. Spooner (1947) stated that species of Gammarus are relatively indifferent to the nature of the substratum to a remarkable degree, provided that there is some kind of object to provide them with shelter/cover. However, an increase in the water flow rate would increase scour which, over a longer period may create the problem of retaining a position in the estuarine environment, against conditions of net seaward transport. 

Sensitivity assessment. The biotope is characterized by fine muddy sands in strong water movement caused by tidal streams. SS.SSa.SSaVS.NintGam occurs in areas of strong tidal streams (1.5-3 m/s) (sheltered from wave action) so that the benchmark level of change (0.1-0.2 m/s) is probably insignificant. Hence, the resistance of SS.SSa.SSaVS.NintGam is assessed as ‘High’, resilience as ‘High’ (by default) and sensitivity 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

Not relevant as the biotope occurs in depths of 0-10 m. Owing to the infaunal habit and/or mobility, the characterizing species are likely to be offered considerable protection or would move away to seek more favourable conditions if the limit of the biotope became intertidal.

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

The biotope SS.SSa.SSaVS.NintGam is characterized by strong tidal streams but sheltered from wave action (JNCC, 2015). Increased wave action causes stress to the infauna by disrupting feeding and burrowing. Wave action is also important subtidally in shallower areas as it can disturb the sediment, particularly during storms. Disturbances of this nature may affect shallow and deep sandbanks (Perkins, 1974) and may result in large scale sediment transport.  Storm activities reduce species richness, abundance and biomass. 

Sensitivity assessment. The biotope SS.SSa.SSaVS.NintGam is 'very sheltered' from wave action (JNCC, 2015).  A significant increase in wave exposure is likely to mobilize the sediment and decrease the fine mud component in favour of sands or, in extreme cases, coarse sediment. If the salinity remains 'low' then the characteristic species may remain, but the biotope may change to SS.SSa.SSaVS.MoSaVS, or in extreme cases SS.SCS.SCSVS. However, a 3-5% change in significant wave height (the benchmark) is unlikely to be significant. Therefore, the resistance of SS.SSa.SSaVS.NintGam is assessed as ‘High’, resilience as ‘High’ (by default) and sensitivity assessed as ‘Not Sensitive’.

Transition elements & organo-metal contamination

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

Evidence

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

Levels of contaminants that exceed the pressure benchmark may cause impacts. For most metals, toxicity to crustaceans increases with decreased salinity and elevated temperature, therefore marine species living within their normal salinity range may be less susceptible to heavy metal pollution than those living in salinities near the lower limit of their salinity tolerance (McLusky et al., 1986). Jones (1973; 1975b) found that mercury (Hg) and copper (Cu) reacted synergistically with changes in salinity and increased temperature (10°C) to become increasingly toxic to species of isopod, including Eurydice pulchra. The sediment grade and the hydrographic conditions within the biotope are responsible for high dispersion so that instances of severe pollution are less in comparison to regions with weaker tidal flow. Bryan & Gibbs (1983) reported lower sediment-metal concentrations in sandy areas than mud near the mouth of Restronguet Creek, a branch of the Fal Estuary system which is heavily contaminated with metals.

Although heavy metals may not accumulate in the substratum to the extent that they would in muddy substrata, characterizing infauna are likely to be susceptible. Bryan & Gibbs (1983) suggested that in populations of polychaetes exposed to heavy metal contamination for a long period, metal resistance could be acquired. For example Nephtys hombergii from Restronguet Creek seemed able to regulate copper. The head end of the worm became blackened and x-ray microanalysis by Bryan & Gibbs (1983) indicated that this was caused by the deposition of copper sulphide in the body wall. In the same study, Bryan & Gibbs (1983) presented evidence that Nephtys hombergii from Restronguet Creek possessed increased tolerance to copper contamination. Specimens from the Tamar Estuary had a 96 h LC50 of 250 µg/l, whilst those from Restronguet Creek had a 96 h LC50 of 700 µg/l (35 psu; 13°C).

For most metals, toxicity to crustaceans increases with decreased salinity and elevated temperature. Consequently, amphipod species living within their normal salinity range may be less susceptible to heavy metal pollution than those living in salinities near the lower limit of their salinity tolerance (McLusky et al., 1986). Amphipod species characterizing the variations of the biotope in transition zones between freshwater and brackish environments are thereby, likely to suffer greater impacts from contamination. The infaunal population of polychaetes may be intolerant of pulses of heavy metals in solution entering the biotope, as in the absence of mud and silts in combination with the highly dispersive hydrographic regime, concentrations in the substratum are likely to be low and populations do not develop resistance. Whilst many individuals may survive by escaping from the vicinity, some mortality would be expected and defaunation of the sediment may occur.

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 any evidence is presented where available.

Contamination at levels greater than the pressure benchmark may adversely influence the biotope. Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected.  Oil spills resulting from tanker accidents have caused deterioration of sandy communities in the intertidal and shallow sublittoral. Subtidal sediments, however, may be at less risk from oil spills unless oil dispersants are used, or if wave action causes dispersion of oil into the water column and sediment mobility drives oil into the sediment (Elliott et al., 1998). Microbial degradation of the oil within the sediment would increase the biological oxygen demand and oxygen within the sediment may become significantly reduced.

Species within the biotope have been reported to be intolerant of oil pollution, e.g. amphipods (Suchanek, 1993). After the Amoco Cadiz oil spill, there was a reduction in both the number of amphipod species and the number of individuals (Cabioch et al., 1978). Initially, significant mortality would be expected, attributable to toxicity. Amphipod populations have been reported not return to pre-spill abundances for five or more years, which is most likely related to the persistence of oil within sediments (Southward, 1982). Nephtys species were amongst the fauna that was eradicated from sediments following the 1969 West Falmouth spill of Grade 2 diesel fuel documented by Sanders (1978).

Multivariate analysis showed that the Prestige oil spill scarcely affected the macroinfaunal community structure during the study period (2003-2009) and its effect was limited just to the first campaign (2003), six months after the Prestige accident (Junoy et al., 2013). Opportunistic species such as Capitella capitata have been shown to increase in abundance close to sources of contamination. High numbers of Capitella capitata have been recorded in hydrocarbon contaminated sediments (Ward & Young, 1982; Olsgard, 1999; Petrich & Reish, 1979) and colonization of areas defaunated by high hydrocarbon levels may be rapid (Le Moal, 1980). After a major spill of fuel oil in West Virginia, the abundance of Capitella capitata increased dramatically alongside large increases in Polydora ligni and Prionospio sp. (Sanders et al. 1972, cited in Gray 1979). Experimental studies adding oil to sediments have found that Capitella capitata increased in abundance initially although it was rarely found in samples prior to the experiment (Hyland, 1985).

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 any evidence is presented where available.

No evidence concerning the specific effects of chemical contaminants on Nephtys species was found. Boon et al. (1985) reported that Nephtys species in the North Sea accumulated organochlorines but, based on total sediment analyses, organochlorine concentrations in Nephtys species were not correlated with the concentrations in the (type of) sediment which they inhabited.

Bioaccumulation of conservative contaminants may occur within the infauna but in coarse sand beaches, contaminants are unlikely to accumulate owing to a relative absence of organic matter. Direct toxic effects would, therefore, be expected. In general, crustaceans are widely reported to be intolerant of synthetic chemicals (Cole et al., 1999) and low resistance to some specific chemicals has been observed in amphipods. Powell (1979) inferred from the known susceptibility of Crustacea to synthetic chemicals and other non-lethal effects, that there would probably also be a deleterious effect on isopod fauna as a direct result of chemical application. Toxicity tests conducted by Smith (1968) indicated that survival of Eurydice pulchra after oil detergent treatment was above average for crustaceans. All were killed at about 10 ppm BP 1002 after 24 hours exposure, whilst at 5 ppm four out of five individuals survived when transferred to clean seawater. However, in the field, a proportion of the Eurydice pulchra population survived exposure to lethal concentrations of BP 1002, both in the sand and water.

Radionuclide contamination

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

Evidence

There is insufficient information available on the biological effects of radionuclides to comment further upon the intolerance of characterizing species to radionuclide contamination. Assessment is given as ‘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

This biotope is characterized by fine muddy sands in strong water flow, which would probably mitigate areas of reduced or low oxygen concentration. Also, the species characterizing the biotope are mobile and able to migrate vertically to escape unsuitable conditions. Therefore, biotope resistance is assessed as ‘High’, and resilience as ‘High’ (by default) so that the biotope is assessed as ‘Not sensitive’ at the benchmark level. 

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 benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014). 

In-situ primary production is limited to microphytobenthos within and on sediments and the high levels of sediment mobility may limit the level of primary production as abrasion would be likely to damage diatoms (Delgado et al., 1991). The characterizing amphipods feed on epipsammic diatoms attached to the sand grains (Nicolaisen & Kanneworff, 1969) and may benefit from slight nutrient enrichment if this enhanced primary production. 

Sensitivity assessment.  Nutrient levels are not a key factor structuring the biotope at the pressure benchmark. In general, however, primary production is low, the biotope is species-poor and characterizing species may be present at low abundances (depending on wave exposure). Therefore, biotope resistance is assessed as ‘High’, resilience as ‘High’ (by default) and the biotope is considered to be ‘Not sensitive’ at the pressure benchmark that assumes compliance with good status as defined by the WFD. Changes in nutrient status may indirectly affect this biotope where these result in changes in diatom production and inputs of macroalgal debris.

Organic enrichment

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

Evidence

The biotope occurs in muddy sand sediments in strong water flow. In-situ primary production is restricted to microphytobenthos although sediment mobility may restrict production levels (Delgado et al., 1991). At the pressure benchmark organic inputs are likely to represent a food subsidy for the characterizing amphipods and mysids and are unlikely to significantly affect the structure of the biological assemblage or impact the physical habitat. Therefore, biotope sensitivity is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is considered to be ‘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 and estuarine 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 only found in the upper reaches of marine inlets, especially estuaries, in fine muddy sands where tidal streams are strong (JNCC, 2015). If the substratum type was changed to either a soft rock or hard artificial type the biotope would be lost altogether. 

Sensitivity assessment.  The resistance to this change is ‘None’, and the resilience is assessed as ‘Very low’ as the effect of the pressure is permanent.  Therefore, sensitivity is 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

The SS.SSa.SSaVS.NintGam biotope is characterized by fine muddy sand. A change in 1 Folk class would result in a change to either i) muds ii) clean sands or iii) gravelly sands and muds. Gammarus spp. are present on a wide range of substratum, independent of sediment size and composition, provided the environment is sheltered and there is organic detritus upon which to feed (MES, 2010).  Increases in mud or gravel content are unlikely to impact Gammarus spp. Similarly, Neomysis integer is free-swimming and not dependent on the substratum type. 

Sensitivity assessment. However, a change in Folk class to either 'muds' 'clean sands' or to 'gravelly sands' would change the classification of the biotope from SS.SSa.SSaVS to SS.SMuVS or SS.SCS.SCSVS respectively, and the diversity of species would also change so that and the biotope under assessment would be effectively lost. Therefore, the resistance to this change is ‘None’, and the resilience is assessed as ‘Very low’ as the effect of the pressure is permanent, and sensitivity is assessed as ‘High’.

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 process of extraction is considered to remove all biological components of the biotope.  If extraction occurred across the entire biotope, loss of the biotope would occur. Recovery would require substratum to return to sand and with fine silt fraction. Gammarus spp. are present on a wide range of substratum, independent of sediment size and composition, provided the environment is sheltered and there is organic detritus upon which to feed (MES, 2010).  Although the population present at the time of extraction will be removed, recovery is likely to be rapid. The resistance of the biotope to extraction is assessed as ‘None’ but resilience is assessed as ‘High’ and biotope sensitivity as ‘Medium’.

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 mean response of infauna and epifauna communities to fishing activities is shown to be more negative in mud and sand communities than other habitats (Collie et al., 2000). Abrasion and compaction of the surficial layer may damage individuals. However, both of the characteristic species, Neomysis integer and Gammarus sp., are small and highly mobile species that will either avoid or pass through most commercial gears, although some damage or mortalities may occur. For example, Neomysis integer is particularly susceptible to injury during collection, causing atypical morphology, which may lead to misidentification (Hayward & Ryland, 1995b). Therefore, resistance is assessed as ‘Medium', as abrasion is unlikely to affect high numbers of the characterizing species. Resilience is assessed as 'High' and biotope sensitivity is assessed as 'Low'.

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 mean response of infauna and epifauna communities to fishing activities is shown to be more negative in mud and sand communities than other habitats (Collie et al., 2000). Abrasion and compaction of the surficial layer may damage individuals. However, both of the characteristic species, Neomysis integer and Gammarus sp., are small and highly mobile species that will either avoid or pass through most commercial gears, although some damage or mortalities may occur. For example, Neomysis integer is particularly susceptible to injury during collection, causing atypical morphology, which may lead to misidentification (Hayward & Ryland, 1995b). Therefore, resistance is assessed as ‘Medium', as abrasion is unlikely to affect high numbers of the characterizing species. Resilience is assessed as 'High' and biotope sensitivity is assessed as 'Low'.

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

This biotope is recorded from estuaries, which are naturally turbid systems due to sediment resuspension by wave and tide action and inputs of high levels of suspended solids, transported by rivers. The level of suspended solids depends on a variety of factors including substratum type, river flow, tidal height, water velocity, wind reach/speed and depth of water mixing (Parr et al. 1998). Transported sediment including silt and organic detritus can become trapped in the system where the river water meets seawater. Dissolved material in the river water flocculates when it comes into contact with the salt wedge pushing its way upriver. These processes result in elevated levels of suspended particulate material with peak levels confined to a discrete region (the turbidity maximum), usually in the upper-middle reaches, which moves up and down the estuary with the tidal ebb and flow. Intertidal mudflats depend on the supply of particulate matter to maintain mudflats and the associated biological community is exposed naturally to relatively high levels of turbidity/particulate matter.  For instance, Neomysis integer collected from the of the Schelde, Gironde and Elbe estuaries in spring occurred in the maximum turbidity zone (Fockedey et al., 2005).

Sensitivity assessment. The biological assemblage characterizing this biotope includes mobile mysids and amphipods Neomysis integer and Gammarus salinus. Increased suspended solids are unlikely to have an impact on theses species.  However, a reduction in suspended solids may reduce deposition and supply of organic matter, and a shift between deposition and erosion could result in a net loss of surficial sediments. A reduction in organic matter as suspended solids could also reduce production within this biotope. Therefore, resistance is assessed as ‘Medium’ as over a year the impact may be relatively small and resilience is assessed as ‘High’, following the restoration of usual conditions. Hence, biotope sensitivity is 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

Gammarus spp. are present on a wide range of substrata, independent of sediment size and composition, provided the environment is sheltered and there is organic detritus upon which to feed (MES, 2010).  Smothering may cause some mortality to individuals resting on the surface, for instance, Gammarus salinus lives in a variety of locations within the estuarine environment: amongst algae and other vegetation, as well as generally over the sediment surface and beneath stones. It is a mobile species capable of a rapid escape response (backflip) if disturbed, however in the event of suddenly being smothered by up to 5 cm of sediment individuals resting on the surface may be killed. Neomysis integer is a free-swimming mysid shrimp, which may rest on the surface of the substratum, but does not live within it and although sufficiently mobile to avoid the deposition of smothering materials, it is possible some individuals will be killed in the event of smothering.

Sensitivity assessment. None of the characterizing species is likely to be significantly impacted by deposition of up to 5 cm of fine material but a small percentage of the characterizing species populations may suffer mortality from smothering, although 5 cm fine sediment is likely to removed within a single tidal cycle. Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not sensitive’ at the benchmark level.  

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

Smothering may cause some mortality to individuals resting on the surface, for instance, Gammarus salinus lives in a variety of locations within the estuarine environment: amongst algae and other vegetation, as well as generally over the sediment surface and beneath stones. It is a mobile species capable of a rapid escape response (backflip) if disturbed, however in the event of suddenly being smothered by up to 30 cm of sediment individuals resting on the surface may be killed, particularly, if the materials are viscous or impermeable. Neomysis integer is a free-swimming mysid shrimp, which may rest on the surface of the substratum, but does not live within it but although sufficiently mobile to avoid the deposition of smothering materials, it is possible some individuals will be killed in the event of smothering.  Overall, smothering by 30 cm of fine sediments is likely to result in some mortality of characterizing species, although most are likely to reposition. Therefore, resistance is assessed as ‘Medium’, resilience as ‘High,’ and biotope sensitivity is assessed as ‘Low’.

Litter

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

Evidence

A study of microparticle transfer via planktonic organisms was shown to lead to microplastic intake by Neomysis integer. Food web transfer experiments were done by offering zooplankton labelled with ingested microspheres to mysid shrimps. Microscopy observations of mysid intestine showed the presence of zooplankton prey and microspheres after 3 h incubation. Neomysis integer showed egestion of microspheres within 12 hr (Setälä et al., 2014). Although this study did not examine impacts on health and survivability other studies have shown negative impacts from microplastic ingestion that are likely to impact the food web within this and other biotopes.

The effect of microplastic exposure of the copepod Centropages typicus showed that 7.3 μm microplastics (>4000 mL–1) significantly decreased algal feeding from experimental studies of exposure to natural assemblages of algae with and without microplastics (Cole et al., 2013). These findings imply that marine microplastic debris can negatively impact upon zooplankton function and would potentially limit prey availability to species feeding on Centropages typicus such as Neomysis integer.

Sensitivity assessment. Overall, the evidence is very limited and assessments cannot be made with confidence and this pressure is '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 was found on the effects of electric and magnetic fields on the characterizing species.

Electric and magnetic fields generated by sources such as marine renewable energy device/array cables may alter the behaviour of predators and affect infauna populations. Evidence is limited and occurs for electric and magnetic fields below the benchmark levels, confidence in the evidence of these effects is very low. Field measurements of electric fields at North Hoyle wind farm, North Wales recorded 110µ V/m (Gill et al. 2009). Modelled results of magnetic fields from typical subsea electrical cables, such as those used in the renewable energy industry produced magnetic fields of between 7.85 and 20 µT (Gill et al. 2009; Normandeau et al. 2011). Electric and magnetic fields smaller than those recorded by field measurements or modelled results were shown to create increased movement in thornback ray Raja clavata and attraction to the source in catshark Scyliorhinus canicular (Gill et al. 2009).

Flatfish species which are predators of many polychaete species including dab Limanda limanda and sole Solea solea have been shown to decrease in abundance in a wind farm array or remain at distance from wind farm towers (Vandendriessche et al., 2015; Winter et al. 2010). However, larger plaice increased in abundance (Vandendriessche et al., 2015). There have been no direct causal links identified to explain these results.

Sensitivity assessment. ‘No evidence’ was available to complete a sensitivity assessment, however, responses by flatfish and elasmobranchs suggest changes in predator behaviour are possible. There is currently no evidence but effects may occur on predator-prey dynamics as further marine renewable energy devices are deployed, these are likely to be over small spatial scales and not impact the biotope.

Underwater noise changes

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

Evidence

Species within the biotope can probably detect vibrations caused by noise. However, at the benchmark level, the community is unlikely to be sensitive to noise and this pressure is 'Not relevant'.

Introduction of light or shading

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

Evidence

As this feature is not characterized by the presence of primary producers, shading would probably not alter the character of the habitat. No specific evidence was found to assess the sensitivity of the characterizing species to this pressure. Changes in light level may, however, affect the activity rhythms of the invertebrates. Amphipods within the biotope prefer shade and, therefore, an increase in light may inhibit activity, particularly at night when they emerge from the sediment and are most active (Jelassi et al., 2015; Ayari et al., 2015). Shading could prevent photosynthesis leading to death or migration of sediment diatoms altering sediment cohesion and food supply to the grazing amphipods.

Sensitivity assessment. Changes in light are not considered to directly affect the biotope, however, some changes in behaviour or food supply for characterizing species could result. Hence, resistance is assessed as ‘High’, resilience as ’High’, and sensitivity is assessed as ‘Not sensitive’.

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

Gammarus spp. and Neomysis integer are highly mobile and brood their young so that water transport is not a key method of dispersal over wide distances. Therefore, The biotope (based on the biological assemblage) is considered to have ‘High’ resistance to the presence of barriers that lead to a reduction in the tidal excursion, resilience is assessed as ‘High’ (by default) and the biotope is 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

The characterizing species may have some, limited, visual perception but as they live in the sediment the species will most probably not be impacted at the pressure benchmark and this pressure is '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

Limited evidence was available to assess the sensitivity of this biotope. Any invasive species would have to tolerate variable salinity and sediments or mobile sediments that characterizes this biotope group and this will exclude species only able to survive in fully marine conditions and/or that require hard substratum. The North American amphipod Gammarus tigrinus was detected in the north-eastern Baltic Sea in 2003 and has rapidly expanded into European waters since (Jänes et al., 2015).  Native gammarids, such as Gammarus salinus have almost disappeared from some habitats of the northeastern Baltic Sea and the competition for space between the invasive Gammarus tigrinus the native Gammarus salinus has been a contributing factor in certain habitats (Kotta et al. 2011). Competition for space alone did not explain the mass disappearance of Gammarus salinus as Gammarus tigrinus did not out-compete Gammarus salinus in all Baltic Sea habitats, limiting confidence in the evidence. However, Gammarus tigrinus has been identified in many UK estuaries and coasts and appears likely to influence species composition in the biotope (NBN, 2016). 

Sensitivity assessment. Limited evidence was available for all characterizing species concerning this pressure. Gammarus tigrinus displays faster reproduction and is a voracious predator that is abundant in dilute and more turbid sites. Gammarus tigrinus appears to be competitively superior to native gammarids and is likely to alter the species composition of the biotope. In particular the biotope variant SS.SSa.SSaVS.NintGam. The function of the biotope is likely to remain similar however as one gammarid species is likely to be replaced by another but predation rates may change. As the biotope classification is unlikely to change, the resistance of the assessed biotopes is 'Medium', resilience is 'Very low' as the invasive species is unlikely to be removed and its impact is permanent and sensitivity is assessed as 'Medium'.

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

Gammarus salinus, Gammarus zaddachi and Gammarus oceanicus were found to be important host species for the transmission of parasites (Voigt, 1991). Larval stages of 4 fish parasites (1 Nematoda, 2 Acanthocephala and 1 Digena) as well as larval stages of 4 bird parasites (1 Nematoda, 1 Acanthocephala, 1 Digena and 1 Cestoda), were reported. However, there was insufficient information concerning the effect that such parasitization may have on the species viability.

Astthorsson (1980) found specimens of Neomysis integer, collected from the Ythan Estuary, Scotland, to be parasitized by the third larval stage of the nematode Thynnascaria adunca. The nematodes were found in both the thorax and the abdomen, usually coiled. In some instances, the total length of the Thynnascaria adunca larvae was almost the same length as the Neomysis integer host. Astthorsson (1980) suspected that the larvae would probably have an influence on the internal physiology of the host but there is insufficient information concerning any effect upon the population that such parasitization may have.

Sensitivity assessment. Based on the lack of evidence for mortalities in characterizing species in relation to pathogens, and amphipods hosting parasites, resistance is assessed as ‘High’ and resilience as ‘High’, by default, so that the biotope is assessed as ‘Not sensitive’. Confidence, however, is low due to the lack of evidence of effects on populations of microbial pathogens.

Removal of target species

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

Evidence

Neither of the characteristic species are subject to targeted recreational or commercial harvesting. Therefore, this pressure is assessed as 'Not relevant'. 

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 are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in this biotope, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures).  However, both species, Neomysis integer and Gammarus sp., are small and highly mobile species that will either avoid or pass through most commercial gears, although some damage or mortalities may occur.  In addition, the substratum (fine muddy sands) is unlikely to be significantly damaged by passing gear, as the habitat is reported to move up and down the estuary seasonally or annually depending on water flow (JNCC, 2015).  Therefore, resistance is assessed as 'Medium', resilience as ‘High’ (as the characteristic fauna and substratum are likely to recover rapidly), and biotope sensitivity as 'Low'.