The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

The rhizome occupies the top 20cm of the substratum. Substratum loss will result in the loss of the shoots, rhizome and probably the seed bank. Recoverability will depend on recruitment from other populations. Although Zostera sp. seed dispersal may occur over large distances, high seedling mortality and seed predation may significantly reduce effective recruitment. Holt et al. (1997) suggested that recovery would take between 5-10 years, but in many cases longer. The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continued decline of Zostera noltei beds suggests that, once lost, eelgrass beds take considerable time to re-establish.

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

Sediment disturbance, siltation, erosion and turbidity resulting from coastal engineering and dredging activities have been implicated in the decline of seagrass beds world wide (Davison & Hughes, 1998; Holt et al., 1997). Seagrasses are intolerant of smothering and typically bend over with addition of sediment and are buried in a few centimetres of sediment (Fonseca, 1992). Zostera sp. are highly intolerant of smothering by epiphytes or algal mats (see nutrients). Recoverability will depend on recruitment from other populations. Although Zostera sp. seed dispersal may occur over large distances, high seedling mortality and seed predation may significantly reduce effective recruitment. Holt et al. (1997) suggested that recovery would take between 5-10 years, but in many cases longer. The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continued decline of Zostera noltei beds suggests that, once lost, eelgrass beds take considerable time to re-establish.

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Increased sediment availability may result in raised eelgrass beds or smothering of the leaves. Decreased sedimentation is likely to result in erosion and loss of the eelgrass beds. Sediment deposited during summer months may be lost again due to winter storms, resuspension by grazing wildfowl, and increased erosion due to die back of leaves and shoots in autumn and winter.
The grazing and digging activity of brent geese and wigeon may increase erosion of intertidal beds during winter months, but in doing so compensate for the sediment deposited during summer months, which may be beneficial to growth of Zostera noltei beds (Nacken & Reise, 2000). The rhizome is probably able to grow through deposited sediment, e.g. Mediterranean Zostera noltei is able to grow upward, through 2 cm of substratum in 4 months (Vermaat et al., 1996). Therefore, Zostera noltei beds are probably tolerant of annual or temporary changes in siltation and sedimentation rates.

However, increased sediment erosion or accretion have been associated with loss of seagrass beds in the Australia, the Mediterranean, the Wadden Sea, and USA. Sediment dynamics and hydrodynamics are key factors in seagrass systems (Asmus & Asmus, 2000a; Davison & Hughes, 1998; Holt et al., 1997). Overall, therefore, seagrass beds are probably intolerant of any activity that changes the sediment regime where the change is greater than expected due to natural events in magnitude or duration and an intolerance of intermediate has been reported. The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continuing decline of Zostera noltii beds suggests that, once lost, eelgrass beds take considerable time to re-establish. However, evidence from grazing studies suggest that Zostera noltii beds can recover within a year after removal of 63% of plant biomass (Nacken & Reise, 2000). Similarly, Dawes & Guiry (1992) regarded Zostera noltei as ephemeral in nature. However, where a bed is stressed by other factors, recovery may be delayed (Holt et al. 1997; Davison & Hughes 1998).
Increased suspended sediment concentrations will also decrease light penetration (see turbidity).

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

Zostera noltei is more tolerant of desiccation than other Zostera species, indicated by its intertidal position and ability to colonize well draining sediment. In well -drained areas Zostera noltei may dry out completely between tides (Davison & Hughes, 1998). However, little information on desiccation tolerance was found.

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Zostera noltei is abundant between mean high water neaps and mean low water neaps. It can survive higher on the shore than Zostera angustifolia although they are often sympatric. Intertidal seagrasses are adapted to leaf and sediment loss due to wave action, grazing and the re-suspension of sediment resulting in high turbidities (Vermaat et al., 1996). Although the intertidal is likely to be affected by high turbidity, Zostera noltei probably makes up for this lack of light by utilising the high levels of light availability when emmersed. It can tolerate higher light intensities than other seagrasses (Vermaat et al., 1996; Davison & Hughes, 1998). Philippart (1995b) noted that although tolerant of high light intensities, its upper shore extent was limited by desiccation tolerance, and the optimal intertidal position of Zostera noltei on a tidal flat near Terschelling, Wadden Sea, was 50% emersion.

A long term change in the emergence regime is likely to increase or reduce the extent of the population in the intertidal. An increase in emergence is likely to reduce its upper extent although this may be compensated for increased growth lower on the shore. Decreased emergence is likely to enable to seagrass bed to expand further up the shore. However, expansion depends on available habitat and competition in infaunal dominated sediments (e.g. Hediste diversicolor or Arenicola marina(Hughes et al., 2000; Philippart, 1994a).

The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continuing decline of Zostera noltei beds suggests that, once lost, eelgrass beds take considerable time to re-establish. However, evidence from grazing studies suggest that Zostera noltii beds can recover within a year after removal of 63% of plant biomass (Nacken & Reise, 2000). Similarly, Dawes & Guiry (1992) regarded Zostera noltei as ephemeral in nature. However, where a bed is stressed by other factors, recovery may be delayed (Holt et al. 1997; Davison & Hughes 1998).

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Zostera noltei beds typically occur where water flow rates are weak or negligible. Increased flow rates are likely to erode sediment, expose rhizomes and lead to loss of the plants. Increased water flow rates deposit coarser sediments and erode fine sediments resulting in loss of suitable substrata for this species.

The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continuing decline of Zostera noltii beds suggests that, once lost, eelgrass beds take considerable time to re-establish. However, evidence from grazing studies suggest that Zostera noltei beds can recover within a year after removal of 63% of plant biomass (Nacken & Reise, 2000). Similarly, Dawes & Guiry (1992) regarded Zostera noltei as ephemeral in nature. However, where a bed is stressed by other factors, recovery may be delayed (Holt et al. 1997; Davison & Hughes 1998).

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Populations of Zostera noltei occur from the Mediterranean to southern Norway and Zostera sp. are regarded as tolerant of temperatures between about 5 - 30°C. Therefore, they may tolerate the range of temperatures likely in the British Isles (Davison & Hughes, 1998). Intertidal populations may be damaged by frost (Hartog, 1987) and Covey & Hocking (1987) reported defoliation of Zostera noltii in the upper reaches of mudflats in Helford River due to ice formation in the exceptionally cold winter of 1987. However, the rhizomes survived and leaves are usually lost at this time of year due to shedding, storms or grazing with little apparent effect (Nacken & Reise, 2000). Populations at the edge of the range are likely to be more intolerant of temperature change. Phillips & Menez (1988) reported death of seagrass as the result of a thermal plume in Biscayn Bay, Florida that raised ambient temperature by 5°C, however, the species concerned were not cited. Long term temperature increase may increase the relative contribution of sexual reproduction and seed germination to population structure.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Increased turbidity due to suspended sediment, humic substances, riverine discharges or phytoplankton growth reduces the light reaching submerged plants. Increase turbidity has been associated with the continued decline of seagrass beds world-wide (Philippart, 1994; Davison & Hughes, 1998; Asmus & Asmus, 2000). However, intertidal Zostera noltei 'escapes' this turbidity since it is able to take advantage of the high light intensities available at low tide (Vermaat et al., 1996). Furthermore, Zostera noltei can store and mobilize carbohydrates and has been reported to be able to tolerate acute light reductions (below 2% of surface irradiance for two weeks) (Peralta et al., 2002). However, Zostera noltei are likely to be more intolerant to chronic increases in turbidity. Philippart (1994b) suggested that the declines in Zostera noltei beds in the Wadden Sea probably occurred at low water level. Permanently submerged population in brackish conditions may be more intolerant of increased turbidity. The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continuing decline of Zostera noltei beds suggests that, once lost, eelgrass beds take considerable time to re-establish. However, evidence from grazing studies suggest that Zostera noltei beds can recover within a year after removal of 63% of plant biomass (Nacken & Reise, 2000). Similarly, Dawes & Guiry (1992) regarded Zostera noltii as ephemeral in nature. However, where a bed is stressed by other factors, recovery may be delayed (Holt et al. 1997; Davison & Hughes, 1998).

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Seagrasses require sheltered environments, with gentle longshore currents and tidal flux. Increased wave exposure may increase sediment erosion (see siltation above). Populations present in moderately strong currents may benefit from decreased water flow rates. Small patchy populations or recently established population and seedling may be highly intolerant of increased wave action since they lack an extensive rhizome system.

Recoverability will depend on recruitment from other populations. Although Zostera sp. seed dispersal may occur over large distances, high seedling mortality and seed predation may significantly reduce effective recruitment. Holt et al. (1997) suggested that recovery would take between 5-10 years, but in many cases longer. The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease and the continued decline of Zostera noltei beds suggests that, once lost, eelgrass beds take considerable time to re-establish.

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

The effect of sound waves and vibration on plants is poorly studied. It is likely that sound waves will have little effect at the benchmark levels suggested. Wildfowl are intolerant of disturbance by noise, which may reduce grazing pressure on intertidal Zostera noltei. However, Naken & Reise (2000) suggested that grazing was important for the persistence of Zostera noltei beds, at least in their study area.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Plants have no known visual receptors and are therefore, not sensitive to this factor. Wildfowl are intolerant of disturbance by visual presence of activities, which may reduce grazing pressure on intertidal Zostera noltei. However, Naken & Reise (2000) suggested that grazing was important for the persistence of Zostera noltei beds, at least in their study area.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Seagrass rhizomes are easily damaged by trampling, anchoring, dredging and other activities that disturb the sediment (Holt et al., 1997; Davison & Hughes, 1998). Small scale sediment disturbance may actually stimulate growth and small patches of sediment allow recolonization by seedlings. Rhizomes are likely to be damaged, leaf blades removed and seeds buried too deep to germinate, by activities such as trampling, anchoring, digging, dredging, power boat and jet-ski wash. For example, damage after the Sea Empress oil spill was reported as limited to the ruts left by clean up vehicles (Jones et al., 2000). Brent geese feed on shoots, rhizomes, and roots, reworking the top centimetre of sediment (8 times in 3 months), and in the process dig pits 3-10cm deep by trampling. As a result, in the Wadden Sea from Sept-Dec (the over-wintering period) Brent geese removed 63% of the plant biomass and pitted 12% of the seagrass bed. However, the bed of Zostera noltii recovered by the following year, and the authors suggested that grazing and bioturbation was necessary for the persistence of the intertidal seagrass beds (Nacken & Reise, 2000). Similarly, several authors have suggested that Zostera sp. can recover from 'normal' levels of wildfowl grazing (Davison & Hughes, 1998). However, suction dredging for cockles in Solway Firth removed Zostera in affected areas while Zostera was abundant in un-dredged areas (Perkins, 1988). Therefore, the passage of a scallop dredge through the seagrass bed (see benchmark) will probably remove a proportion of the seagrass population and intolerance has been assessed as intermediate. Recoverability is likely to be high (see additional information below). However, seagrass beds are likely to be of higher intolerance to repeated scallop dredging or suction dredging.

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Seagrass rhizomes are easily damaged by trampling, anchoring, dredging and other activities that disturb the sediment (Holt et al., 1997; Davison & Hughes, 1998). The seagrass bed is unlikely to survive displacement. Evidence from grazing studies suggest that Zostera noltei beds can recover within a year after removal of 63% of plant biomass (Nacken & Reise, 2000). Similarly, Dawes & Guiry (1992) regarded Zostera noltii as ephemeral in nature. However, where a bed is stressed by other factors, recovery may be delayed (Holt et al. 1997; Davison & Hughes 1998).
Increased suspended sediment concentrations will also decrease light penetration (see turbidity). However, Phillips & Menez (1988) reported that rhizomes and shoots can root and re-establish themselves if they settle on sediment long enough.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. 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. Further details.

Evidence

There was little information on the effects of chemical contaminants on Zostera noltei and what little was found mainly refers to Zostera marina. Zostera marina is known to accumulate TBT but no damage was observable in the field (Williams et al.,1994). Naphthalene, pentachlorophenol, Aldicarb and Kepone reduce nitrogen fixation and may affect Zostera marina viability. Triazine herbicides (e.g. Irgarol) inhibit photosynthesis and sublethal effects have been detected. Terrestrial herbicides may damage eelgrass beds in the marine environment. For example, the herbicide Atrazine is reported to cause growth inhibition and 50% mortality in Zostera marina exposed to 100 ppb (ng/ l) Atrazine for 21 days (Davison & Hughes 1998). Bester (2000) noted a correlation between raised concentrations of 4 triazine herbicides and areas where Zostera sp. had been lost.

Heavy metal contamination

Evidence

Little information was found regarding heavy metal concentrations in Zostera noltei however, the following information was found for Zostera marina. The concentration and toxicity of heavy metals in salt marsh plants, including Zostera marina was reviewed by Williams et al., 1994. Growth of Zostera marina is inhibited by 0.32 mg/l Cu and 10 mg/l Hg, but Cd, Zn, Cr and Pb had measurable but less toxic effects (Williams et al., (1994). Davison & Hughes (1998) report that Hg, Ni and Pb reduce nitrogen fixation which may affect viability. However, leaves and rhizomes accumulate heavy metals, especially in winter. Williams et al. (1994) did not observe any damage to Zostera marina in the field. Williams et al. (1994) noted that the major route for uptake of heavy metals was through the leaves and suggested that intertidal populations would accumulate less heavy metals due to their reduced exposure.

Hydrocarbon contamination

Evidence

• Healthy populations of Zostera can occur in the presence of long term, low level, hydrocarbon effluent, for example in Milford Haven, Wales (Hiscock, 1987; Davison & Hughes, 1998)
• Zostera noltei is likely to be more vulnerable from direct contact by oil due to its intertidal habitat (Davison & Hughes, 1998; Jones et al., 2000)
• The Amoco Cadiz oil spill off Roscoff blackened Zostera marina leaves for 1-2 weeks but had little effect on growth, production or reproduction after the leaves were covered in oil for six hours (Jacobs, 1980).
• Experimental treatment of Zostera noltei with crude oil and dispersants halted growth but had little effect on cover. However, pre-mixed oil and dispersant caused rapid death and significant decline in cover (from 55% to 15% within 1 week suggesting that dispersant treatments should be avoided (Holden & Baker, 1980; Howard et al., 1989; Davison & Hughes, 1998).
• Oil contamination could result in smothering of the plants, especially in the low energy conditions in which the Zostera noltei is often found (L. Jones, pers. comm.).

The effects of hydrocarbon contamination on Zostera noltii will depend on the type of oil spilled (L. Jones, pers. comm.). Removal of oil intolerant grazers, e.g. gastropods or amphipods, may result in smothering of eelgrasses by epiphytes or algal mats (see nutrients below and LMS.Znol).

Radionuclide contamination

Evidence

Insufficient
information

Changes in nutrient levels

Evidence

Increased nutrient concentrations (nitrates and phosphates) have been implicated in the continued decline of seagrass beds world-wide, either directly or due to eutrophication (Phillips & Menez, 1988; Davison & Hughes, 1998; Philippart, 1994b; Philippart, 1995a, b; Vermaat et al., 1994; Asmus & Asmus, 2000). The following effects on Zostera sp. have been attributed to nutrients and eutrophication.

• High nitrate concentrations implicated in decline of Zostera marina. Burkholder et al. (1992) demonstrated that nitrate enrichment could cause decline of Zostera marina in poorly flushed areas. In addition they noted that increasing or high temperatures associated with spring exacerbated the adverse effects of nitrate enrichment and that growth and survival were significantly reduced by nutrient enrichment levels of between 3.5 and 35µMolar nitrate/day with the most rapid decline (weeks) at high nitrate levels. Plant loss resulted from death of the meristem tissue.
• van Katwijk et al. (1999) noted that adverse effects of nitrate were dependent on salinity. Estuarine Zostera marina plants were more intolerant of high nitrate concentration than marine Zostera marina plants at high (30 psu) salinity than at lower salinities (23 psu) and that both populations benefited from nitrate enrichment (0-4 to 6.3 µmMolar nitrate per day) at 23 or 26 psu.
• Increased growth of epiphytes or blanketing algae, for example:
  • Den Hartog (1994) reported the growth of a dense blanket of Ulva radiata in Langstone Harbour in 1991 that resulted in the loss of 10ha of Zostera marina and Zostera noltei. Subsequently, by summer 1992, the Zostera sp. were absent, however this may have been exacerbated by grazing by Brent geese
  • Philippart (1995b) reported that shading by periphyton reduced incident light reaching the leaves of Zostera noltei by 10-90% and reduced the period of time that net photosynthesis could occur by 2-80% depending on location.
  • Philippart (1995b) estimated that the mud-snail Hydrobia ulvae could remove 25-100% of the periphyton and microphytobenthos, and suggested that the decline of Zostera noltii in the Wadden Sea in the 1970s was in part due to increased periphyton growth due to eutrophication, and a simultaneous decline of the mud-snail population (although mud-snail populations have increased subsequently) (Philippart, 1995a).
• Encouragement of phytoplankton blooms which increase turbidity and reduce light penetration, although this may be of less significance for intertidal Zostera noltii populations (see above) (Davison & Hughes, 1998).
• The levels of phenolic compounds in Zostera sp. (involved in disease resistance) are reduced under nutrient enrichment and may increase their susceptibility to infection by wasting disease (Buchsbaum et al., 1990; Burkholder et al., 1992).

Long-term increases in nutrients or eutrophication may result in loss of the intertidal eelgrass beds.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Zostera sp. have a wide tolerance of salinity from 10 - 39 ppt (Davison & Hughes 1998), although den Hartog (1970) suggested a lower salinity tolerance of 5 psu for Zostera sp. Den Hartog (1970) stated that Zostera noltii was a euryhaline species, penetrating estuaries and the Baltic Sea to the average annual isohaline of 9-10 psu. Zostera noltei is probably more tolerant of extremes of salinity than its congeners due to its intertidal habit and Zostera sp. occupy a wide range of salinities, therefore 'low' intolerance has been recorded.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

The effects of oxygen concentration on the growth and survivability of Zostera noltei are not reported in the literature. Zostera sp. leaves contain air spaces (lacunae) and oxygen is transported to the roots where it permeates into the sediment, resulting in a oxygenated microzone. This enhances the uptake of nitrogen. The presence of air spaces suggests that seagrass may be tolerant of low oxygen levels in the short term, however, prolonged deoxygenation, especially if combined with low light penetration and hence reduced photosynthesis may have an effect.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

A major outbreak of wasting disease resulted in significant declines of Zostera beds on both sides of the Atlantic in 1920s to 1930s, primarily Zostera marina in the subtidal. Wasting disease is thought to be caused by the marine fungus, Labyrinthula macrocystis. The disease causes death of leaves and after 2-3 seasons death of regenerative shoots, rhizomes and loss of up to 90 percent of the population. The disease is less likely at low salinities however, and Zostera noltei was little affected (Rasmussen, 1977; Davison & Hughes, 1998). Decline of intertidal Zostera marina and Zostera noltei beds in the Wadden Sea began in the 1960s and a marked decline in Zostera noltei occurred between 1965 and 1975, presumably due to anthropogenic change (Philippart, 1994b).

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

Spartina anglica (a cord grass) is an invasive pioneer species, a hybrid of introduced and native cord grass species. Its rapid growth consolidates sediment, raises mudflats and reduces sediment availability elsewhere. It has been implicated in the reduction of common eelgrass cover in Lindisfarne, Northumberland due to encroachment and changes in sediment dynamics. Wire weed (Sargassum muticum) invades open substratum and may prevent recolonization of areas of eelgrass beds left open by disturbance (Davison & Hughes 1998). Zostera marina and Sargassum muticum may compete for space in the lower shore lagoons of the Solent. However, evidence for competition is conflicting and requires further research. If the invasive species prevents recolonization then recoverability from other factors will be reduced. Intertidal Zostera noltei may be more vulnerable to competition from Spartina sp.

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Wildfowl grazing can consume significant amounts of seagrass and reduce cover mainly in autumn and winter. Tubbs & Tubbs (1983) reported that Brent geese reduced the cover of Zostera marina and Zostera noltei from 60-100% to 5-10% in mid October to mid January in the Solent. Grazing is probably part of the natural seasonal fluctuation in seagrass cover and Zostera sp. can recover from normal grazing (Naken & Reise, 2000; Davison & Hughes, 1998). Zostera noltei is the preferred food of the dark-bellied Brent goose (Branta bernicla). Brent geese feed on shoots, rhizomes and roots, reworking the top centimetre of sediment (8 times in 3 months), and in the process dig pits 3-10cm deep by trampling. As a result, in the Wadden Sea from Sept-Dec (the over-wintering period) Brent geese removed 63% of the plant biomass and pitted 12% of the seagrass bed. However, the bed of Zostera noltei recovered by the following year, and the authors suggested that grazing and bioturbation was necessary for the persistence of the intertidal seagrass beds (Nacken & Reise, 2000). However, where a bed is stressed by other factors it may not be able to withstand grazing (Holt et al. 1997; Davison & Hughes 1998). For example, seagrass rhizomes are easily damaged by trampling, anchoring, dredging and other activities that disturb the sediment. A seagrass bed is unlikely to survive displacement or extraction, although Phillips & Menez (1988) reported that rhizomes and shoots can root and re-establish themselves if they settle on sediment long enough. Therefore, Zostera noltei beds have been considered to be of 'intermediate' intolerance to extraction.

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Zostera sp. are regarded as very intolerant of hydraulic bivalve fishing in the UK and Wadden Sea (Holt et al., 1997; Davison & Hughes, 1998; Philippart, 1994b). Cockles and Zostera noltei are frequently associated and intertidal beds may be more vulnerable (Holt et al., 1997). Hydraulic dredging is likely to break up and remove rhizomes. It was suggested that hydraulic harvesting of cockles in the Solway Firth could cause widespread damage or eradicate Zostera sp. from the bay (Perkins, 1988). Digging and dredging for the American hard-shell clam (Mercenaria mercenaria had a significant effect on the eelgrass beds (Cox, 1991; Eno et al., 1997). In the Dutch Wadden Sea, seagrass is hardly found where cockles are normally fished (Dankers & de Vlas, 1992). Recovery was severely restricted especially where no rhizomes and roots were left in the sediment (De Jong & de Jong, 1992; Philippart, 1994b).