Further detail and rationale

The benchmarks that follow were revised in March 2003, based on the experience gained after three years of research on the biology and sensitivity of marine species and biotopes. The benchmarks used prior to March 2003 are published in Tyler-Walters et al. (2001), which may be viewed under publications.

Physical factors

Substratum loss

The physical removal of the substratum inhabited or required by the species or community in question. Newell et al. (1998) reviewed the environmental effects of dredging in coastal waters. They reported that trailer suction hopper dredging could result in dredged tracks 2-3 m wide and 0.5 m deep but up to 2 m deep in some cases. In comparison, anchored dredging may result in pits of up to 75 m in diameter and 20 m deep. In the Baltic dredged tracks may still be detectable 12 months later. The time taken for pits to fill in the Dutch Wadden Sea was between 1year in high currents, 5-10 years in lower currents and up to 15 years on tidal flats (Newell et al., 1998). Hall (1994) reports pits 3.5 m wide and 0.6 m deep as a result of suction dredging for Ensis in a Scottish sea loch. Newell et al. (1998) state that removal of 0.5 m of sediment was likely to eliminate benthos from the affected area.

The chosen benchmark is representative of localised impacts on a specific area of substratum. This benchmark also includes the removal of other species that provide substrata for the species or community of interest, for example macroalgae. The time taken for the substratum to ‘recover’ within the habitat preferences of the species or community under consideration is not addressed.


The physical covering of the species or community and its substratum with additional sediment (silt), spoil, detritus, litter, oil or man-made objects. Overgrowth by other species such as encrusting ascidians is also included here. Major storms may deposit a layer of additional material of several centimetres at 20 m depth and several millimetres at 40 m (Hall, 1994). For example, storms were reported to deposit 4-10cm of sand at 28m in the Helgoland in German Bight and up to 11cm of sand off the Schleswig-Holstein coast (Hall,1994). In a study of the impact of mill tailings, discharged into a Canadian silled fjord, Ellis and Heim (1985) observed layers of tailings of 0.5 cm, 5 cm and greater than 5 cm (up to 60 cm in one location).

The chosen benchmark represents the likely level of smothering resulting from natural events and is comparable to the effects of maritime activities.

[The definition does not include land claim. The habitat and its resident species would be destroyed by land claim. Recovery would not be possible as the effect is permanent].

Changes in suspended sediment

The concentration of suspended matter in the water column. The rate of siltation is dependent on the availability of suspended sediment, its particle size range and the water flow rate. In estuarine environments, siltation is increased by the flocculation of inorganic and organic substances due to the mixing of fresh and saltwater. Floods are likely to increase the availability of sediment entering coastal waters from rivers. Storms may re-suspend sediment and transport it to other areas. Coastal erosion is a primary source of sediment. Activities that alter sediment availability (e.g., coastal quarries, de-forestation, coastal forestry, construction and dredging) or that change the water flow rate (e.g., coastal engineering such as channelization and breakwater construction) are likely to change the concentration of suspended sediment and hence siltation. Suspended sediment concentration varies around the UK, from 1-327 mg/l around the English coast and 1-227 mg/l around the Welsh coast but annual mean values are typically 1-110 mg/l (Parr et al., 1998; Cole et al., 1999). However, suspended sediment concentrations in estuaries may be much higher; measured in grammes per litre.

Churchill (1989) reported a plume of suspended material behind a shrimp trawl, up to 50 m behind the trawl with a concentration of 100-550 mg/l suspended material. Newell et al. (1998) report a plume of suspended material behind a dredger reaching 75-150 mg/l, although this had dropped to 20-30 mg/l within 30 min. Similarly, they reported another dredger plume containing 2500 mg/l of suspended sand (<30 mg/l mud) which reduced to background levels with 200-500 m.

‘Suspended sediment’ is included as a factor for those species likely to be sensitive to clogging of respiratory or feeding apparatus by silt or species that require a supply of sediment for tube construction such as Sabellaria sp. The resultant effects on light attenuation are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Therefore, an arbitrary, short-term, acute benchmark was chosen to represent a change in the availability of suspended sediment resulting from maritime activities or natural events, such as storm runoff.


The removal of water or drying. Desiccation rate during emersion is dependent on sunlight (and hence temperature), air movement (wind) and humidity. Intertidal organisms exhibit a number of physiological or behavioural adaptations to avoid or reduce desiccation. Two benchmarks for desiccation are given.

  • The first desiccation benchmark represents stranding on the shore or the sudden exposure of an organism or community to desiccation, for example, by turning over rocks on the shore to expose undersurface communities.
  • The second benchmark represents changes in the desiccation rate due to changes in the wave exposure of the shore (hence humidity), prolonged periods of sunlight and higher temperatures, due to especially hot summers, hot winds, or a change in the emergence regime (see ‘change in emergence regime’ below).

Changes in emergence

The time spent emersed and exposed to air. Intertidal species are regularly emersed with the falling tide, the percentage of time emersed is dependent on their position or height on the shore relative to the tide. There are seven sub-zones recognized in the intertidal (see glossary). This benchmark also includes organisms in the splash zone (supralittoral) where the wetness regime is also dependent on the wave energy (wave height) reaching the shore. This factor is distinguished from desiccation, which while dependent on emergence, can change (due to changes in wind speed, air temperature and humidity) without changes in emersion.

Changes in water flow rate

The movement of water is associated with the rise and fall of the tide (tidal streams), prevailing winds and ocean currents. Strong tidal streams result in areas where water is forced through or over restrictions (e.g. gullies or narrows) or around offshore rocks. Currents are dependent on the meteorology, oceanography and hydrography of the location.

Maritime activities, for example, coastal engineering, are likely to cause changes in water flow rate at least as large as the benchmark level. In addition, many species and biotopes occur under a range of water flow conditions and a change of two categories is more likely to affect a range of species than a change of one category.

Changes in temperature

A change in the ambient temperature of seawater, or in air temperature during emersion. Intertidal marine organisms experience a wide range of temperatures. If emersed at low tides intertidal organisms may be exposed to the heat of summer or the cold frosts of winter. Sub-tidal, permanently immersed marine species, however, will be exposed to a narrower temperature range since seawater takes time to warm or cool, and therefore ‘buffers’ the effects of extreme temperatures. Marine organisms are likely to be more tolerant of slow temperature change than sudden change. For example, species are likely to be more sensitive to a temperature change of 5 °C if it occurs over a period of a few hours rather than a few days.

The ambient temperature of air or sea changes with the season, the magnitude of the change varying from year to year. However, short or long term changes in temperature may also result from thermal discharges (e.g. power station cooling waters) or climate change.

Thermal discharges are likely to be between 2°C and 10°C above the ambient temperature (UNEP 1984). UNEP (1984) recommend an impact assessment level for thermal discharge plumes equal to or greater than 3°C.

Crisp et al. (1964) reported the effects of the severe winter of 1962/63. Mortalities were recorded for a wide range of marine species as a result of a temperature drop of 5-6°C below the long-term average for the south, southwest and west coast of England during a two-month period.

Benchmark 1) represents single pulse events, such as occasional short-term industrial discharges or accidental spillages. However, species or communities are likely to be more sensitive to discharges of longer duration. Benchmark 2) represents continuous discharges of lower magnitude. A year's duration was chosen to represent the probability that the temperature change would impinge on the larval forms and breeding cycle of most marine organisms.

NB: Long-term change in the average mean or winter minima and summer maxima resulting from climate change are addressed in the rationale but are not assessed under sensitivity and recoverability. This is because any increase or decrease in distribution and abundance of a species, as a result of long-term change, is dependent on factors other than temperature alone, such as adult longevity, reproductive type and the importance of barriers to distribution at the organisms' current limits of distribution.

Changes in turbidity

The turbidity (clarity or opacity) of water is dependent on the concentration of substances that absorb or scatter light; for example, inorganic or organic particulates (suspended matter), plankton and dissolved substances. Dissolved substances may include natural organic materials (e.g. humic acids) or discharged chemicals. The turbidity determines the depth of water that light can penetrate and therefore the amount of light available for primary production by phytoplankton, benthic microalgae and macroalgae. At high levels, the suspended sediment that causes turbidity may clog the feeding apparatus but this effect is included in ‘siltation'. Coastal waters are likely to absorb 10-60% of incident light per metre at a wavelength of 500 nm (Kinne, 1970). Assuming that coastal waters absorb, on average, 30% of the incident light, then this is approximately equivalent to a suspended sediment concentration of 10-50 mg /l (extrapolated from Clarke, 1996). Cole et al. (1999) report average mean levels of turbidity of 1-110 mg/l around the English and Welsh coasts.

The water clarity scale refers to the effect of changes in light penetration, essential for photoautotrophs, because of changes in turbidity. The scale refers to the depth at which the incident surface illumination is reduced to 1% of surface intensity and approximates to the lower limit of growth in photophilic algae. It should be noted that turbidity may vary with season and coastal waters are likely to have a higher turbidity at times as a result of winter storms and riverine runoff.

Changes in wave exposure

Exposure on an open shore is dependent upon the distance of open seawater over which wind may blow to generate waves (the fetch) and the strength and incidence of the winds. Wave exposure is expressed as an eight-rank scale of exposure (see glossary). Wave exposure may be altered by coastal engineering developments such as breakwaters and artificial reefs and are likely to be permanent unless positioned to temporarily protect other activities. Many species and biotopes occur under a range of wave exposure conditions. A change of one category might be effective in altering the survival or abundance of a few species, however, placing the benchmark magnitude at two ranks is more likely to encompass a significant number of species. The benchmark level is also representative of the likely effects of a number of relevant maritime activities, such as the construction of breakwaters.


Generally defined as unwanted or disruptive sound. Noise can cause sensitivity in three ways:

  • actual discomfort, damage or death;
  • interference with the use of hearing for feeding or communication reducing viability;
  • disturbance of breeding or other behaviours reducing viability.

The units of the benchmark are received sound pressure in decibels (dB) shown as a ratio of received pressure to a fixed reference pressure (re) of 1 µPa at 1 metre. A typical ambient coastal noise level in calm weather would be around 40 – 60 dB (Morris, 1995). Various maritime activities produce noise of various frequencies at pressures from 120 to 250 dB (Richardson et al., 1999). A distance of 1 metre is not very applicable to the exposure of marine organisms to noise in the environment. A typical decrease in pressure (transmission loss) over 100 metres would be 40 dB (Richardson et al., 1999). In setting the benchmark for underwater noise, this loss has been applied to the typical noise pressures resulting from various activities. Different activities tend to produce noise of different pressures at different frequencies. For example:

  • drilling noise tends to be up to 160 dB re 1 µPa-m at frequencies below 300 Hz with a peak below 2 Hz;
  • dredging tends to be up to 180 dB re 1 µPa-m and below 1kHz;
  • boats and small ships produce sound up to 170 dB re 1 µPa-m with frequencies up to 10 kHz (outboards motors have peaks at frequencies above 1kHz and larger vessels peak below 1 kHz);
  • the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres approximates to 130 dB re 1 µPa (for broad spectrum noise 45 – 7070 Hz);
  • the regular passing of a Boeing 737 passenger jet 300 metres overhead approximates to 98 dB re 1 µPa (for broad spectrum noise 45 – 7070 Hz) @ 300 metres below the source;
  • sonar sound can be up to 230 dB re 1 µPa-m and range from 500 Hz to several hundred kHz; and
  • seismic air guns at 250 dB re 1 µPa-m up to several kHz (strongest below 100Hz) (Richardson et al., 1999).

In addition, atmospheric noise can affect marine animals at the water surface or for example, hauled out on sand banks. Conventionally aircraft noise is referred to at a distance of 300 metres from the source. In extreme cases, such as for military jets, the noise produced can be up to 130 dB re 1 µPa at 300 m

Noise duration varies with activity, ranging from several weeks (dredging) to a fraction of a second repeated regularly for several hours (seismic survey) to a few minutes (a passing ship or plane). The benchmark was set using a duration that could typically result from a variety of activities e.g. continuous daytime boat activity, dredging, construction or proximity to an airport.

This benchmark does not deal with the transmission of atmospheric noise to the water.

Visual presence

This benchmark applies only to species that have sufficient visual acuity to resolve moving objects or at least differentiate between rapid changes in light intensity (as in a moving shadow). The response is likely to be immediate with the species moving out of range of the stimulus. The duration of the factor has been set in line with potential maritime activities (such as disturbance to seals by tourists) and also at a level that could cause a measurable effect on the species.

Physical disturbance or abrasion

This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism of interest. Protrusive species may be crushed, and delicate organisms with fragile skeletons or soft bodies may be physically damaged or broken (snapped).

Physical disturbance due to mobile fishing gear continues to be a concern in marine conservation. In most cases, the weight of evidence regarding habitat or species sensitivity to physical disturbance concerns the effects of mobile fishing gear, e.g. epifaunal communities or Modiolus modiolus beds.

The benchmark was chosen to be representative of a potentially damaging maritime activity, namely scallop dredging. The benchmark has been set as the magnitude of impact equivalent to that caused by a passing scallop dredge. We believe that a scallop dredge is representative of the impact likely to cause damage to a habitat or species, and to be of concern for marine conservation or environmental management.

The intertidal is also susceptible to abrasion and physical impact from trampling, however, no standard units have been identified (units such as number of footsteps per square metre or number of persons per transect have been reported). Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale.


Physical removal or transportation of the species or community of interest. The community, colony or organism may be removed from its natural habitat but remain in the vicinity. For example, an individual may be disturbed by a storm, or passing trawl, not killed but thrown into suspension. The definition of the factor used here assumes that a permanently attached species cannot re-attach and is likely to die whilst many burrowing species or sedentary species can re-burrow or re-attach. The benchmark was chosen to represent significant bioturbation as a result of pit digging by large epibenthic predators such as rays and gray whales (Hall, 1994; Table 2), or removal from hard substrata by wave action. Anthropogenic activities such as suction dredging or beam trawling are likely to have an effect greater than the benchmark level.

Chemical factors

Chemical factors require a particular approach to assessing sensitivity. Laboratory or field experiments and observations provide a starting point for assessing if species are adversely affected by the sorts of concentrations of any chemical that occur as a result of human activities or in accidents. However, the behaviour of chemicals in the marine environment is extremely complex and it is difficult to quantify the most likely effect of an activity. For example, a contaminant concentration at discharge may differ significantly from that experienced by an organism, due to dilution, dissipation, adsorption, absorption, flocculation, sedimentation, chemical change or degradation (of the contaminant), or bioaccumulation. Similarly, the environmental concentration of any given contaminant may be the result of several activities, including aerial deposition.

A very large number of chemicals might affect marine species. The effects of some, such as TBT, are well known. Environmental Quality Standards (EQSs) or Environmental Assessment Levels (EALs) or World Health Organization Guidance values are available for many contaminants (EA, 1997) (see Cole et al. 1999 for review). However, scientific knowledge is incomplete or insufficient for many marine species. Contaminants may also exhibit antagonistic or synergistic effects, which are difficult to predict and poorly studied, and no attempt is made to take these into account here. It is accepted that considerable extrapolation is required in our assessments and that our levels of evidence and confidence are likely to be low. Therefore, it is neither practical nor accurate to set quantified benchmark levels for contaminants and an evidence-based approach has been adopted.

Sensitivity is assessed against the available evidence for the effects of contaminants on the species of interest (or closely related species at low confidence). For example:

  • evidence of mass mortality of a population of the species or community of interest (either short or long-term) in response to a contaminant will be ranked as high sensitivity;
  • evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
  • evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available, together with relevant supporting material.

The available toxicological information will vary between species and a species may be assessed to have different sensitivities to different chemicals within each class (heavy metals, synthetic chemicals, hydrocarbons, or radionuclides) for example Cu, Zn and Hg within heavy metals.

NOTE Where sensitivities to different chemicals within each class result in different sensitivity assessments, the available information will be clearly stated and the 'worst case' sensitivity reported.

Changes in levels of synthetic chemicals

Synthetic chemicals are by definition man-made and include, for example, organotins (tri butyl tin, tri phenyl tin), pesticides (lindane, atrazine, dichlorvos, DDT), organochlorides, organophosphates, solvents (carbon tetrachloride, chloroform) and polychlorinated biphenyls (PCBs).

Changes in levels of heavy metals

Heavy metals include, for example, Arsenic (As), Cadmium (Cd), Mercury (Hg), Lead (Pb), Zinc (Zn) and Copper (Cu).

Changes in levels of hydrocarbons

Hydrocarbons include, for example, oils (crude and fuel oils) and polyaromatic hydrocarbons (PAHs).

Changes in levels of radionuclides

Isotopes of elements that emit alpha, beta or gamma radiation. Radionuclides in the environment result from nuclear weapons tests, nuclear fuel processing, nuclear power generation and natural sources. The little information known on the biological effects of radionuclides was reviewed by Cole et al. 1999. Dose rates of 10 milli-Grays per hour (mGy/hr) are considered acceptable for the protection of aquatic populations. Lethal levels in invertebrates range between 0.2 and 500 Grays (Gy). However, environmental concentrations of radionuclides are measured in Becker per litre (Bq/l). Dosage is dependent of the type and energy of the radiation emitted as well as the characteristics of the target organism. MAFF (1998) reports values of 137caesium in filtered seawater typically 50-500 mBq/kg in the northeastern Irish Sea and 2-20 mBq/kg in the North Sea. Concentrations of tritium (3H) in the Bristol Channel ranged between 0-12 Bq/kg (MAFF, 1998).

Changes in levels of nutrient

Nutrients include substances required for growth, for example, nitrogen, phosphorus, silicon, and micro-nutrients (heavy metals and vitamins). Low nutrient availability often limits growth or primary production in the marine environment. Ecosystems may be affected by changes in nutrient availability. Mean nutrient concentrations in English and Welsh coastal waters range from 0.07-1.85 mg/l total inorganic nitrogen (TIN), whereas estuarine concentrations vary between 0.1 to 15 mg/l total inorganic nitrogen (TIN). However, there is considerable variation in response to storms, floods, and seasons. Estuary concentrations peak in autumn/ winter and coastal concentrations in winter. However, man-made input from, for example, livestock, fertilizers, and sewage treatment works, may exceed the assimilative capacity of the environment, and result in eutrophication.

It is extremely difficult to obtain information on the effects of nutrient enrichment that includes the level of accuracy required to make an assessment against a quantified benchmark. In practice, it has proven to be neither practical nor accurate to set quantified benchmark levels for nutrient enrichment and an evidence-based approach has been adopted. Nutrient enrichment is likely to result in predominately indirect effects on species or communities, e.g. overgrowth by algal mats or the toxic effects of algal blooms. Where relevant, information on indirect effects is included in the rationale.

Changes in salinity

The salinity scale used by the Marine Nature Conservation Review (Hiscock, 1996) was developed to reflect the occurrence of significantly different species from one category to another. Therefore, a change of one category was chosen as an appropriate sensitivity assessment benchmark for long-term changes and two categories for short-term changes.

Changes in oxygenation

Oxygen is required by the majority of organisms for respiration; the process by which organic molecules are broken down to provide energy for work and metabolism. Natural events such as plankton blooms may deplete the oxygen levels locally. For example, a planktonic bloom, in the presence of a thermocline (which prevented mixing on the water column), in the North Atlantic Bight resulted in a reduction of dissolved oxygen below 2 mg/l for several months and the subsequent deaths of fish and benthos. Deoxygenation may also result from the addition of organic material to the water column and subsequent bacterial activity that consumes available dissolved oxygen. Gray and Jensen (1993) reported <4 mg/l as the concentration chosen by as likely to affect marine life and, therefore, to trigger the cessation of dredging operations. The chosen benchmark was based on the general quality assessment levels for estuaries (8 mg/l, 4 mg/l and 2 mg/l) reported by Cole et al. (1999). However, anaerobic species may be sensitive to increased oxygen levels.

Biological factors

Introduction of microbial pathogens and parasites

By definition, diseases and parasites cause a reduction in the fitness of an organism so all affected species are automatically assessed as sensitive to disease or parasitism. Information on likely diseases and parasites, and their likely effects, will be detailed in the rationale.

Introduction of alien or non-native species

Sensitivity is assessed against a specific non-native or alien species that already occurs in Britain and/or Ireland that is most likely to have an adverse effect. The relevant alien or non-native species and its likely effects will be detailed in the rationale.

Specific targeted extraction of this species

If 50% of the population or biotope is removed then sensitivity is automatically assessed as intermediate. The potential for recovery after a very efficient extraction has been undertaken can also be assessed using this definition.

Specific targeted extraction of other species

A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a key species in a biotope is removed.


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