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

Palmer (1968) (cited in Birtwell & Arthur, 1980) recorded large populations of Limnodrilus hoffmeisteri and Tubifex tubifex (up to 5.7 x 106m²) close to the heated effluent discharge of an electrical generating plant upstream of London Bridge on the River Thames. Birtwell & Arthur (1980) examined the tolerance of Tubifex tubifex from the Thames estuary to elevated temperature and found the 96 h LC50 value to be 33.9 °C, a temperature that would not be encountered within the main body of the estuary, but possibly close to discharges of heated cooling water from electrical generating plants. In the same study, the tolerance of Limnodrilus hoffmeisteri was found to be even greater, its 96 h LC50 was 37.5°C. Although, evidently tolerant of elevated temperature, sub-lethal effects have been reported. Chapman et al. (1982) observed that at 10°C both Limnodrilus hoffmeisteri and Tubifex tubifex were capable of regulating their respiration, whilst at 20°C respiration rate was greatly elevated and only partially regulated. High temperatures have been reported to cause mortality of cocoons and will delay, but not prevent maturation of juveniles.

Specimens of Gammarus salinus were tolerant of temperature fluctuations between 8 °C and 20 °C over a period of up to four weeks, acute temperature changes caused additional stress but did not result in mortality (Furch, 1972), as gammarid shrimps are very mobile they are able to avoid adverse conditions. Community composition is unlikely to significantly change and recoverability has been assessed to be very high.

Increased temperature was found to trigger the onset of reproduction in Baltidrilus costata (studied as Tubifex costatus) in the Thames (Birtwell & Arthur, 1980). This effect was non-lethal and may be beneficial to populations.

Sensitivity assessment.  The dominance of the characterizing tubificid oligochaetes. in sediments exposed to heated effluent suggests that this genus would be highly resistant to an increase in temperature at the pressure benchmark. Biotope resistance based on the characterizing and associated species. is therefore assessed as ‘High’ and resilience as ‘High’ (by default), so that the biotope is considered to be ‘Not sensitive’.

Temperature decrease (local)

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

Evidence

Most littoral oligochaetes, including tubificids and enchytraeids, can survive freezing temperatures and can survive in frozen sediments (Giere & Pfannkuche, 1982). Tubificoides benedii (studied as Peloscolex benedeni) recovered after being frozen for several tides in a mudflat (Linke, 1939). Early stages may be more susceptible as low water temperatures (< 10°C) were reported by Poddubnaya (1980) to cause significant levels of mortality in embryonic stages (within cocoon) of both Tubifex tubifex and Limnodrilus hoffmeisteri, and also delayed attainment of maturity, but did not prevent it.

Sensitivity assessment.  Typical surface water temperatures around the UK coast vary, seasonally from 4-19 ^oC (Huthnance, 2010). The biotope, based on the characterizing is considered to tolerate a 2 ^oC decrease in temperature for a year. An acute decrease may disrupt reproduction and the production of juveniles. Adults may be unaffected and populations may recover within a year. Biotope resistance based on the characterizing and associated tubificid oligochaetes is therefore assessed as ‘High’ and resilience as ‘High’ (by default), so that the biotope is considered to be ‘Not sensitive’.

Salinity increase (local)

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

Evidence

This biotope is present in low salinity habitats (<18 ppt) (JNCC, 2015). Interstitial salinity is an important factor determining the occurrence of the SS.SMu.SMuVS.LhofTtub community. The key functional species, Limnodrilus hoffmeisteri and Tubifex tubifex, are essentially freshwater species, able to tolerate very low interstitial salinities and therefore able to penetrate from freshwater ecosystems into upper estuaries, which although tidal, are dominated by freshwater conditions, e.g. the upper Forth estuary, Scotland (see McLusky et al., 1980). As salinity increases seawards, the infaunal species composition and indeed the dominant class of annelid eventually changes, so that larger estuarine polychaetes become important bioturbators (Diaz, 1980). 

Stczynska-Jurewicz (1972) reported that the maximum salinity at which Tubifex tubifex could survive was 9 psu and the maximum at which natural egg laying and development occurred was 4 psu. Kennedy (1965) stated that salinity also controlled the distribution of Limnodrilus hoffmeisteri, but gave no precise limits. In the Forth estuary, McLusky et al. (1980) found Tubifex tubifex in localities with a maximum salinity of 4.1 psu, and Limnodrilus hoffmeisteri occurred at salinities of up to 7.7 psu, these species dominated the initial 16 km of the estuary from Stirling. Between 16 and 28 km the interstitial salinity increased progressively from a mean of 3.2 psu to 26.4 psu, and over that stretch of the estuary the dominant oligochaete was Tubifex costatus (now Baltidrilus costata). Tubificoides benedeni (as Peloscolex benedeni) became the dominant oligochaete in the lower part of the estuary. This estuarine succession of Tubifex tubifex and Limnodrilus hoffmeisteri, then Tubifex costatus (Baltidrilus costata), then Tubificoides benedeni, was also found by Hunter and Arthur (1978) in the Thames estuary. This evidence suggests that the SS.SMu.SMuVS.LhofTtub biotope would be highly intolerant of increased salinity and that community composition of the infaunal oligochaete community would change.

Sensitivity assessment. As this biotope is restricted to low salinities an increase in salinity at the pressure benchmark would lead to loss of the characterizing species Limnodrilus hoffmeisteri, Tubifex tubifex. The biological assemblage associated with the biotope is considered to have ‘No’ resistance and ‘High’ resilience (resilience will be lower where populations are removed over wide areas). Biotope sensitivity is, therefore, ‘Medium’.

Salinity decrease (local)

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

Evidence

This biotope is present in low salinity habitats (<18 ppt) (JNCC, 2015). The key functional oligochaete species, Limnodrilus hoffmeisteri and Tubifex tubifex, are freshwater aquatic oligochaetes, able to penetrate from freshwater ecosystems into upper estuaries, which although tidal, are dominated by freshwater conditions, e.g. the upper Forth estuary, Scotland (see McLusky et al., 1980). The benchmark decrease in salinity would mean that the community would be exposed to freshwater. Limnodrilus hoffmeisteri and Tubifex tubifex are not likely to be adversely affected. To a certain extent the distribution of Gammarus species is also correlated with salinity. Distinct zonation patterns may be observed, Gammarus salinus prefers intermediate salinities, whilst Gammarus zaddachi and Gammarus duebeni predominantly live in more dilute brackish waters, locally penetrating into freshwater transition zones (Bulnheim, 1984).

Stczynska-Jurewicz (1972) reported that the maximum salinity at which Tubifex tubifex could survive was 9  psu  and the maximum at which natural egg laying and development occurred was 4  psu . Kennedy (1965) stated that salinity also controlled the distribution of Limnodrilus hoffmeisteri, but gave no precise limits. In the Forth estuary, McLusky et al. (1980) found Tubifex tubifex in localities with a maximum salinity of 4.1  psu , and Limnodrilus hoffmeisteri occurred at salinities of up to 7.7  psu , these species dominated the initial 16 km of the estuary from Stirling. Between 16 and 28  km  the interstitial salinity increased progressively from a mean of 3.2  psu  to 26.4  psu , and over that stretch of the estuary the dominant oligochaete was Tubifex costatus (now Baltidrilus costata). 

Sensitivity assessment. At the benchmark level, a decrease in salinity is unlikely to cause significant changes in community composition, and an assessment of ‘Not sensitive’ has been made, based on ‘High’ resistance and resilience.

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 is found in areas where tidal streams are estimated to range from moderately strong (0.5-1.5 m/s) to weak (<0.5 m/s), (JNCC, 2015).  Increases and decreases in water velocity may lead to increased erosion or deposition. The associated pressures alteration to sediment type and siltation are assessed separately.  

Experimental increases in near-bed current velocity were achieved over intertidal sandflats by placing flumes on the sediment to accelerate water flows (Zuhlke & Reise, 1994). The increased flow led to the erosion of up to 4cm depth of surface sediments.  No significant effect was observed on the abundance of Tubificoides benedii and Tubificoides pseudogaster, as they probably avoided suspension by burrowing deeper into sediments. This was demonstrated by the decreased abundance of oligochaetes in the 0-1cm depth layer and increased abundance of oligochaetes deeper in sediments (Zuhlke & Reise, 1994).  A single storm event had a similar result with decreased abundance of oligochaetes in surficial layers, coupled with an increase in deeper sediments (Zuhlke & Reise, 1994). Although Tubificoides spp. can resist short-term disturbances their absence from sediments exposed to higher levels of disturbance indicate that they would be sensitive to longer-term changes in sediment mobility (Zuhlke & Reise, 1994).

Birtwell and Arthur (1980) reported seasonal changes in abundance in Baltidrilus costata (as Tubifex costatus) which they attributed to erosion of the upper sediment layers caused by high river flows and wave action.

Decreases in water flow with increased siltation of fine particles are considered unlikely to alter the physical character of this habitat type as it is already found in sheltered areas where siltation occurs and where particles are predominantly fine. Reductions in waterflow occurring through the presence of trestles (for off-bottom oyster cultivation) arranged in parallel rows in the intertidal area (Goulletquer & Héral, 1997) reducing the strength of tidal currents (Nugues et al., 1996) has been observed to limit the dispersal of pseudofaeces and faeces in the water column and thus increase the natural sedimentation process by several orders of magnitude (Ottman & Sornin, 1985, summarised in Bouchet & Sauriau, 2008). As the characterizing oligochaetes can live relatively deeply buried and in depositional environments with low water flows (based on habitat preferences) and low oxygenation they are considered to be not sensitive to decreases in water flow.

Sensitivity assessment. As muds tend to be cohesive and the surface tends to be smooth reducing turbulent flow, an increase at the pressure benchmark may not lead to increased erosion. Biotope resistance is assessed as ‘High’ based on the tidal stream range (JNCC, 2015). Resilience is assessed as ‘High’ (following restoration of usual conditions) and sensitivity is assessed as ‘Low’. The biotope is not considered to be sensitive to decreased flows due to its presence in sheltered habitats and the tolerance of oligochaetes, in general, for low oxygen and sediment deposition.

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 to sublittoral biotopes.

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

As this biotope occurs across two wave exposure categories;  extremely sheltered and very sheltered, JNCC (2015), this is considered to indicate that mid-range biotopes would tolerate both an increase or decrease in wave exposure at the pressure benchmark. Resistance is therefore assessed as ‘High’ and resilience as ‘High’ by default and the biotope is considered to be ‘Not sensitive’. An increase in wave exposure at the pressure benchmark would be likely to re-suspend sediments and increase erosion altering sediment type. Some oligochaete dominated biotopes occur in areas with mobile sediments and it is possible the biotope would revert to one of these. 

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.

Heavy metal studies with oligochaetes have concentrated almost exclusively on tubificids, in particular Limnodrilus hoffmeisteri and Tubifex tubifex . Chapman et al., (1980) reviewed the literature available on the subject and concluded both species to be particularly tolerant of heavy metal contamination. Early work concentrated on determining LD₅₀ concentrations and ranking toxicity, e.g. Brkovic-Popovic & Popovic (1977) suggested that tubificid oligochaetes were most intolerant of Cu, Cd and Hg in solution than to Zn, Cr, Ni and Pb. However, as tubificids are infaunal species that are not directly exposed to conditions in the water column, their tolerances to heavy metals should be considered on the basis of metal levels in sediments and interstitial water. Wensel et al. (1977) measured metal levels in Palestine Lake, Indiana by nitric-perchloric digestion and found that Limnodrilus spp. survived Cd, Zn and Cr levels (in µg/g dry weight) of 970, 14000 and 2100 respectively. These levels had eliminated most of the rest of the benthos.

The emphasis of more recent research has moved to the detection of sub-lethal effects as a more sensitive indicator of toxicity. Reported sub-lethal effects of certain metals on Limnodrilus hoffmeisteri and Tubifex tubifex include reduced and elevated respiration rates, decreased concentration of haemoglobin, autotomy, excessive mucus production and reduced number of cocoons arising from reproduction (Whitley & Sikora, 1970; Brkovic-Popovic & Popovic, 1977b; Vecchi et al., 1999; Martinez-Tabche et al., 1999; Bouche et al., 2000). Research has also focused on the mechanisms of oligochaete resistance to metal toxicity. Klerks & Levinton (1989) reported that Limnodrilus hoffmeisteri from a metal polluted cove had evolved resistance to a combination of Cd, Ni and Co and Klerks & Bartholomew (1991) examined the physiological mechanisms by which such resistance is achieved. A later paper by Martinez & Levinton (1996) suggest that one gene controls resistance to metal in the metal tolerant aquatic oligochaete Limnodrilus hoffmeisteri.

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.

Little information is available specifically concerning the effects of hydrocarbon contamination on oligochaete populations. The IMU.LimTtub biotope occurs in low energy environments protected from wave and tidal flow in upper estuaries. Sediments are rich in organic matter, and in the event of an oil spill, the high organic content promotes sorption of oil into the sediments. Furthermore, in such environments the bacterial degradation of oil is hindered by conditions of low oxygenation. The best documented oil spill in a protected habitat with soft mud/sandy substrata is the 1969 West Falmouth spill of #2 diesel fuel (Sanders, 1978). As a consequence of conditions outlined above, remobilisation of oil (especially within subtidal regions) continued for more than a year after the original spill and caused greater contamination than the initial impact. Virtually the entire fauna was eradicated following the spill, but populations of opportunistic species soon flourished.

Following the Exxon Valdez spill in Prince William Sound, Alaska, the abundance of oligochaetes in the intertidal region was noted to have increased, and more than 10 years after the spill their continued presence may be indicative of a subtle but significant alteration in the infauna of Prince William Sound (Highsmith et al., 1996; McRoy, 2000). Although, the infauna may be eradicated in the worst affected areas, e.g. through direct effects of toxicity, smothering and deoxygenation (sensitivity assessed elsewhere), fringe populations of oligochaetes in less affected areas may benefit primarily from the additional food resources (bacteria & micro-organisms) that arise, and are likely to transfer ingested contaminants from the sediment directly to other food web predators, e.g. birds, fish and predatory invertebrates.

In Finland in oligohaline inland waters near an oil refinery, Baltidrilus costata (as Tubifex costatus) appeared to be sensitive to oil pollution and had completely disappeared from sediments exposed to pollution and did not recolonize during a 4y ear post pollution period (Leppäkoski & Lindström, 1978). Tubificoides benedii appears to be more tolerant and was found in UK waters near oil refineries as the sole surviving member of the macrofauna. Populations were however apparently reduced and the worms were absent from areas of oil discharge and other studies indicate sensitivity to oiling (Giere & Pfannkuche, 1982, references therein).

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.

Oligochaetes may be especially susceptible to synthetic chemicals that bind to sediments. Evidence suggests that some synthetic chemicals would adversely affect the important functional species of oligochaetes in this biotope, through both lethal and sub lethal effects. For example, Lotufo & Fleeger (1996) investigated acute and sub-lethal toxicity of sediment spiked with pyrene and phenanthrene to Limnodrilus hoffmeisteri. Phenanthrene was acutely toxic at high sediment concentrations (297. 5 µg/g 10-day median lethal concentration), whilst pyrene was not acutely toxic, even at concentrations as high as 841 µg/g. Both chemicals adversely affected the feeding activity of Limnodrilus hoffmeisteri and some burrowing avoidance was detected in sediment spiked with high phenanthrene concentrations (143-612 µg/ g), but was not detected with pyrene. Offspring production was also significantly reduced in contaminated sediments.

Keilty et al., (1988) observed that endrin contaminated sediments inhibited the burial of Limnodrilus hoffmeisteri. Dad et al., (1982) reported on the acute toxicity and presumable harmless concentration of two commercial insecticides, Furadan 3G and Matalaf 50 E, for Limnodrilus hoffmeisteri and Tubifex tubifex. Limnodrilus hoffmeisteri was found more susceptible to both insecticides, with Furadan being the most toxic. Sub-lethal effects including reduced reproductive potential have been reported for gammarid species exposed to a surfactant TWEEN 80 and pentachlorophenol (PCP) and benzo[a]pyrene (B[a]P) (Lyes, 1979; Lawrence & Poulter, 2001).

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

Oligochaete species vary in their tolerance of hypoxia and associated high sulphide levels. Most enchytraaids and naidids are sensitive to hydrogen sulphide and hypoxia while tubificids are often more resistant (Giere, 2006).

Research by Birtwell & Arthur (1980) on the ecology of tubificids in the Thames estuary included investigation of their tolerance of anaerobic conditions and low dissolved oxygen concentrations in the field.  In laboratory studies, Limnodrilus hoffmeisteri was found to have a greater anaerobic tolerance than Tubifex tubifex at all water temperatures tested (20, 25 & 30°C). At 20°C, Limnodrilus hoffmeisteri had a LC₅₀ time of 52 h, whilst Tubifex tubifex had a LC₅₀ time of 28 h. At 30°C the LC₅₀ for Limnodrilus hoffmeisteri decreased to 18 h, Tubifex tubifex also had a decreased tolerance at 30°C with a LC₅₀ of 12 h. In the field, populations of the two species seemed able to tolerate conditions of low dissolved oxygen and periodic episodes of < 5% air saturation (< 2 mg O₂/l). For example, large populations of Limnodrilus hoffmeisteri occurred on the Thames between Greenwich and Woolwich, where average weekly dissolved oxygen concentration was just 2 mgO₂/l between December 1968 and September 1971. Birtwell & Arthur (1980), suggested that the low metabolic rate of Limnodrilus hoffmeisteri, coupled with its relatively better ability to survive periodic anaerobic conditions without incurring an oxygen debt, suited its survival in such locations. Although, Tubifex tubifex demonstrated a relatively lower tolerance to anaerobic conditions than Limnodrilus hoffmeisteri, it occurred in locations with a low average oxygen concentrations and survived periodic anoxia, although such situations were considered by Birtwell & Arthur (1980) to be less conducive to the establishment of populations of Tubifex tubifex. Embryos of both species are intolerant of low oxygen concentrations in combination with low temperature (see recruitment processes). Fisher & Beeton (1975) noted from vertical burrowing experiments in conditions of anoxia, that a more even distribution of Limnodrilus hoffmeisteri occurred in the upper 6 cm of sediment than in controls, and in vertical burrowing experiments avoidance of anoxic sediment was significant.

Tolerance experiments by Gamenick et al.  (1996) found that Baltidrilus costata (as Heterochaeta costata) was not affected by hypoxic conditions for at least 3 days but the addition of sulphide 91.96 mmol/litre) caused mortality after 1 day (Gamenick et al., 1996).

Sensitivity assessments. Based on the reported tolerances for the characterizing oligochaete species (Birtwell & Arthur, 1980), biotope resistance is assessed as ‘Medium’ as populations are likely to survive but there may be some loss of Baltidrilus costasta and impacts on  juveniles, resilience is assessed as ‘High’ (by default) and biotope sensitivity is assessed as 'Low'.

Nutrient enrichment

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

Evidence

In nutrient enriched tidal sediments oligochaetes can dominate assemblages (Gray, 1971; Leppäkoski, 1975; Birtwell & Arthur, 1980). 

Sensitivity assessment. As the benchmark is relatively protective, biotope resistance is assessed as ‘High’, resilience is assessed as ‘High’ and the biotope is considered to be ‘Not sensitive’.

Organic enrichment

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

Evidence

Limnodrilus hoffmeisteri competes with Tubifex tubifex in very polluted environments, its abundance being related to the organic content of the sediments and it may dominate the population (Poddubnaya, 1980). The oligochaete Baltidrilus costatus is also very tolerant of high levels of organic enrichment and often dominate sediments where sewage has been discharged, or other forms of organic enrichment have occurred (Pearson & Rosenberg, 1978; Gray, 1971; McLusky et al., 1980).

Sensitivity assessment. The above evidence indicates that increased organic matter levels can favour the characterizing oligochaetes. Biotope resistance, is therefore considered to be ‘High’, resilience ‘High’ (by default) and the species is ‘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

The biotope is characterized by the sedimentary habitat (JNCC, 2015), a change to an artificial or rock substratum would alter the character of the biotope leading to reclassification and the loss of the sedimentary community including the characterizing oligochaetes that live buried within the sediment.

Sensitivity assessment. Based on the loss of the biotope, resistance is assessed as ‘None’, recovery is assessed as ‘Very low’ (as the change at the pressure benchmark is permanent and 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

Giere & Pfannkuche (1982) suggest that factors that correlate to substratum types such as organic matter availability, size and shape of the intertstitial space between grains, the level of sediment disturbance and water content, are all factors influencing the distribution of oligochaetes. A change in sediment type to sand and mixed sediments is likely to reduce habitat suitability and result in loss of the biotopes.

Sensitivity assessment. Biotope resistance is assessed as ‘None’ and resilience as Very low (the pressure is a permanent change) 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

Removal of 30 cm of surface sediment will remove the oligochaete community and other species present in the biotope. Recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed.

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotope. Resistance is assessed as ‘None’ and biotope resilience is assessed as ’High’.  Biotope sensitivity is therefore ‘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

No evidence was found for the characterizing species and the assessment is based on other tubificid oligochaetes.  Experimental studies on crab-tiling impacts have found that densities of Tubificoides benedii and Tubificoides pseudogaster were higher in non-trampled plots (Sheehan et al., 2010), indicating that these oligochaetes have some sensitivity to trampling.

Sensitivity assessment. Disturbance of the surficial layers may have little effect on oligochaetes. Abrasion with associated compaction (as in trampling) may have a greater impact. Resistance is therefore assessed as ‘Medium’ and resilience as ‘High’ (by default) so that 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

No evidence was found for the characterizing species and the assessment is based on other tubificid oligochaetes.  Whomersley et al. (2010) conducted experimental raking on intertidal mudflats at two sites (Creeksea- Crouch estuary England and Blackness- lower Forth estuary, Scotland), where Tubificoides benedii were dominant species. For each treatment 1 m² plots were raked twice to a depth of 4cm (using a garden rake). Plots were subject to either low intensity treatments (raking every four weeks) or high (raking every two weeks). The experiment was carried out for 10 months at Creeksea and a year at Blackness. The high and low raking treatments appeared to have little effect on Tubificoides benedii (Whomersley et al., 2010). These results are supported by observations that two experimental passes of an oyster dredge that removed the sediment to a depth of between 15-20 cm did not significantly affect Tubifcoides benedii (EMU, 1992).

Sensitivity assessment. The experiments by Whomersley et al., (2010) and EMU (1992), suggest that penetration and disturbance of the upper surface has little effect on tubificid oligochaetes. Many individuals are likely to be buried more deeply and can  migrate to the surface following disturbance so that little impact is observed through sampling. Resistance is therefore assessed as ‘Medium’ and resilience as ‘High’ so that 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

Estuaries where this biotope is found form can be 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; substrate 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. 

Sensitivity assessment. The biological assemblage characterizing this biotope is infaunal and consists of sub-surface deposit feeders. Increased suspended solids are unlikely to have an impact and resistance is assessed as ‘High’ and resilience as ‘High’, so that the biotope is considered to be ‘Not sensitive’. A reduction in suspended solids may reduce deposition and supply of organic matter, resistance to a decrease is therefore assessed as ‘Medium’ as a shift between deposition and erosion could result in the net loss of surficial sediments. A reduction in organic matter as suspended solids could also reduce production within this biotope. Resistance is assessed as ‘Medium’ as over a year the impact may be relatively small and resistance is assessed as ‘High’, following restoration of usual conditions. Biotope 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

Subtidal muds occur in sheltered environments and, in general, are accreting environments meaning that deposition rather than erosion is the dominant process, this means that the assemblages present (primarily deposit feeders) are adapted to natural levels of siltation through life history traits and can withstand burial (by repositioning in sediment or similarly extending tubes or feeding and respiration structures above the sediment surface). At low levels of siltation the high bioturbatory nature of mudflat organisms decreases sensitivity to effects (Elliott et al. 1998) as sediment turnover rates are relatively rapid.

Gammarus species live 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. They are mobile species capable of a rapid escape response (back flip) if disturbed, however in the event of suddenly being smothered by 5 cm of sediment individuals resting on the surface may be killed

Sensitivity assessment. The characterizing oligochaete species are considered to be able to survive under a deposit of fine grained sediment up to 5cm thick and to burrow and reposition within this.The biotope (based on the biological assemblage) is therefore considered to have ‘High’ resistance, resilience is assessed as ‘High’ (by default) and the biotope is considered to be ‘Not sensitive’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

The pressure benchmark (30 cm deposit) represents a significant burial event and the deposit may remain for some time in a sheltered mudflat. Some impacts on characterizing oligochaetes may occur and it is considered unlikely that significant numbers of the population could reposition. Placement of the deposit is likely to result in a defaunated habitat until the deposit is recolonized. Biotope resistance is therefore assessed as 'Low' as some removal of deposit and vertical migration through the deposit may occur. Resilience is assessed as 'High' as migration and recolonization of oligochaetes is likely to occur within two years, biotope sensitivity is therefore 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

Not assessed.

Electromagnetic changes

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

Evidence

A number of studies have investigated the effects of electromagnetic fields on terrestrial oligochaetes, notable earthworms. Some negative effects have been observed e.g. Tkalec et al., 2013. However no evidence was found to support an assessment at the pressure benchmark for the marine oligochaetes that characterize this biotope.

Underwater noise changes

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

Evidence

Infaunal oligochaetes may be able to detect vibration caused by localized noise and withdraw into the sediment, but are unlikely to be adversely affected by noise at the benchmark level. This pressure is considered to be 'Not relevant'.

Introduction of light or shading

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

Evidence

No evidence was found to assess this pressure. Studentowicz (1936) found that the enchytraeid oligochaete  Enchytraeus albidus, retracted from light, although the worms accumulated at the surface even when illuminated to avoid low oxygen and hydrogen sulpfide.  Giere and Pfannkuche (1982) considered that other enchytraeids and tubificids are likely to react in the same way. As the oligochaete assemblage occurs within the sediment and can be deeply buried (to 10cm or more) this pressure is considered ‘Not relevant’. 

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

As the tubificid oligochaetes that characterize this biotope have benthic dispersal strategies (via egg cocoons laid on the surface, Giere & Pfannkuche, 1982), water transport is not a key method of dispersal over wide distances, as it is for some marine invertebrates that produce pelagic larvae.  The biotope (based on the biological assemblage) is therefore considered to have ‘High’ resistance to the presence of barriers that lead to a reduction in 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

Not relevant. Characterizing species are unlikely to possess the visual acuity to detect the visual presence of objects outlined in the benchmark. 

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 group.

Introduction or spread of invasive non-indigenous species

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

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40) although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020, Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015, Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets, that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². 

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²) but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula with only moderate or low abundances (11 ind./m²) and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm) but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action which may be less suitable (Tillin et al., 2020). 

The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogenous fine sediment and areas dominated by boulders. Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Blanchard (2009) noted that sandy areas in the Bay of Saint-Mont Michel were not colonized by Crepidula because of surface sand mobility. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly burial due to deposition, which could mitigate Crepidula density.

Tang & Kristensen (2010) found that the abundance of macrofauna, including Tubificoides was lower in marsh invaded by the hybrid cordgrass Spartina anglica than in mudflats. Colonization of upper mudflats by this species would alter the character of the biotope resulting in loss and reclassification. The polychaete Marenzellaria viridis has become established in estuaries in Europe but a recent paper on its impacts where Tubificoides were abundant did not report on oligochaete impacts (Delefosse et al., 2012).

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile and unsuitable for most of the invasive non-indigenous species currently recorded in the UK. The above evidence suggests that the anoxic cohesive mud in this biotope is unsuitable for the colonization of Crepidula fornicata and there is a lack of gravel, shells, or any other hard substrata used for larvae settlement (Tillin et al., 2020). This is a very sheltered to extremely sheltered habitat but the low salinity levels in this biotope might mitigate or prevent colonization at high densities. The majority of evidence records Crepidula in salinities from 25-35 psu (OBIS, 2023). Hence, resistance to Crepidula invasion is assessed as 'High' and resilience as 'High' so the biotope is assessed as 'Not sensitive'. However, the biotope may be sensitive to invasion by Spartina anglica, which would alter the character of the mudflat and the biological assemblage, but there is not enough evidence on which to base an assessment. 

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

Marine oligochaetes host numerous protozoan parasites without apparent pathogenic effects even at high infestation levels  (Giere & Pfannkuche, 1982 and references therein). Limnodrilus hoffmeisteri is parasitized by the caryophyllidean cestode Archigetes iowensis (Williams, 1979). Tubifex tubifex is an intermediate host to a myosporean parasite, Myxobolus macrocapsularis (Myxosporea: Myxobolidae) of the common bream, Abramis brama (Szekely et al., 2002). Tubifex tubifex is also an intermediate host to the parasite Myxobolus cerebralis which causes Salmonid Whirling Disease (Zendt & Bergersen, 2000). 

Sensitivity assessment. Based on the lack of evidence for mass mortalities in  oligochaetes from microbial pathogens, resistance is assessed as ‘High’ and resilience as ‘High’, by default, so that the biotope is assessed as ‘Not sensitive’.

Removal of target species

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

Evidence

No characterizing species within the biotope are targeted by commercial or recreational fishers or harvesters. This pressure is therefore considered ‘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

Incidental removal of the characterizing species would alter the character of the biotope and the delivery of ecosystem services such as secondary production and bioturbation. Populations of oligochaetes provide food for macroinvertebrates fish and birds. For example, Müller (1968) found that Paranais littoralis was the preferred food for young flounders and plaice in western Baltic shallow flats. Polychaetes and crustaceans are also predators of oligochaetes and may significantly reduce numbers (Giere & Pfannkuche, 1982 and references therein). The loss of the oligochaete population could, therefore, impact other trophic levels.

Sensitivity assessment. Removal of the characterizing species would alter the character of the biotope. Resistance is therefore assessed as ‘Low’ and resilience as ‘High’ so that sensitivity is categorised as ‘Low’.