Biodiversity & Conservation

LS.LMp.LSgr.Znol

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The rhizome occupies the top 20 cm of the substratum. Substratum loss will result in the loss of the shoots, rhizome and probably the seed bank together with the other species within the biotope. Hence intolerance is assessed as high. Recoverability of Zostera noltii 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. Zostera noltii populations are considered to be in decline (Philippart, 1994b; Jones et al., 2000). Polychaetes such as Arenicola marina may recolonize the sediment relatively quickly from the surrounding area or from planktonic larvae. Gastropods such as Hydrobia ulvae and Littorina littorea are common and mobile with planktonic larvae and also likely to recover quickly. However, recruitment in the bivalve macrofauna is sporadic e.g. Cerastoderma edule and may take longer to recover (1 -5 years). It should be noted that recolonization by Arenicola marina at high abundance before Zostera noltii may inhibit recolonisation by the seagrass (Philippart, 1994a). Loss of Zostera noltii would result in loss of the biotope. Therefore recoverability is deemed to be low, resulting in a biotope recording of high.
Smothering
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Sediment disturbance, siltation, erosion and turbidity resulting from coastal engineering and dredging activities have been implicated in the decline of seagrass beds world wide (Holt et al., 1997; Davison & Hughes, 1998). Seagrasses are intolerant of smothering and typically bend over with addition of sediment and are buried in a few centimetres of sediment (Fonseca, 1992). If completely buried by sediment for two weeks, shoots of Zostera noltii will not survive (Cabaço & Santos, 2007). Zostera sp. are highly intolerant of smothering by epiphytes or algal mats (see nutrients), as are infauna, especially due to deoxygenation caused by death and decomposition of the algae (see oxygenation). Therefore biotope intolerance of high has been recorded. Surface dwelling epifauna such as Littorina littorea is highly intolerant of smothering, although Hydrobia ulvae is less so. Burrowing deposit feeding polychaetes are probably not sensitive to smothering by 5 cm of sediment. However, Cerastoderma edule can burrow upwards more readily through sandy sediment than muddy sediment (Jackson & James, 1979) and some mortality is likely due to smothering by 5cm of sediment. Recoverability of Zostera noltii 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. Zostera noltii populations are considered to be in decline (Philippart, 1994b; Jones et al., 2000). Therefore recoverability is assessed as low, and sensitivity is high. Polychaetes such as Arenicola marina may recolonize the sediment relatively quickly from the surrounding area or from planktonic larvae. Gastropods such as Hydrobia ulvae and Littorina littorea are common and mobile with planktonic larvae and also likely to recover quickly. However, recruitment in the bivalve macrofauna is sporadic e.g. Cerastoderma edule and may take longer to recover (1 -5 years). It should be noted that recolonization by Arenicola marina at high abundance before Zostera noltii may inhibit recolonization by the seagrass (Philippart, 1994a). Loss of Zostera noltii would result in loss of the biotope.
Increase in suspended sediment
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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 (Holt et al., 1997; Davison & Hughes, 1998; Asmus & Asmus, 2000a, b), and changes in the sediment level (burial and erosion) result in a decrease in shoot density of Zostera noltii (Cabaço & Santos, 2007). Seagrass beds demonstrate a balance of sediment accretion and erosion (Davison & Hughes, 1998). 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, but in doing so compensate for the sediment deposited during summer months, which may be beneficial to growth of Zostera noltii beds (Nacken & Reise, 2000). The rhizome of Mediterranean Zostera noltii was able to grow upward, through 2 cm of substratum in 4 months (Vermaat et al., 1996). As such, intolerance is deemed to be intermediate. Recovery is assessed as moderate, resulting in a moderate sensitivity rating. Changes to the sediment regime due to coastal engineering works has been implicated in the decline of Zostera sp. beds, e.g. due to the coffer dam during construction of the second Severn crossing (Davison & Hughes, 1998), and dock construction and channel widening in the Solent (Butcher, 1941). Seagrass beds should be considered intolerant of any activity that changes the sediment regime where the change is greater than expected due to natural events or long term. Increased suspended sediment concentrations will also decrease light penetration (see turbidity).
Decrease in suspended sediment
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Desiccation
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Zostera noltii is more tolerant of desiccation than other Zostera species, due to its intertidal position and ability to colonize well draining sediment. In well -drained areas Zostera noltii may dry out completely between tides (Davison & Hughes, 1998). However, little information on desiccation tolerance in this species was found. Epifaunal species such as gastropods are mobile and many exhibit physiological and behavioural adaptations to desiccation stress, e.g. burrowing in Hydrobia ulvae. Infaunal species are partly protected from desiccation due to the water content of the sediment and depths of their burrows, with perhaps the exception of Cerastoderma edule since it dwells in the top few cm of the sediment. The upper extent of the biotope is most likely to be vulnerable to desiccation. Therefore, increased desiccation equivalent to raising the population from mid to high water is likely to reduce the upper extent of the biotope, especially Zostera noltii and Cerastoderma edule. As such, intolerance is assessed as intermediate. Recovery is likely to be moderate (5-10 years if rhizomes are undamaged so are able to spread further towards the lower limits of the biotope), resulting in a moderate sensitivity assessment. In sever drought, conditions of hypersalinity may impact on Zostera noltii biomass (Cardoso et al., 2008).
Increase in emergence regime
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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 noltii on a tidal flat near Terschelling, Wadden Sea, was 50% emersion. Mobile epifauna such as gastropods are unlikely to be adversely affected, however, increased emergence is likely to reduce the time available for feeding by infauna and the risk of hypoxia in burrows.
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 of the biotope although this may be compensated for increased growth lower on the shore. Decreased emergence is likely to enable the biotope 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 (Philippart, 1994a; Hughes et al., 2000). A decrease in emersion, possible due to sea level rise, may reduce the available intertidal habitat and therefore reduce the extent of this biotope. As such, intolerance is assessed as intermediate. Recovery is likely to be moderate, resulting in a moderate sensitivity rating.
Decrease in emergence regime
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Increase in water flow rate
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Seagrasses require sheltered environments, with gentle long shore currents and tidal flux. Where populations are found in moderately strong currents they are smaller, patchy and vulnerable to storm damage and blow outs. Increased water flow rates may destabilize the bed and increase the risk of 'blow outs' within the seagrass beds, deposit coarser sediments and erode fine sediments resulting in loss of suitable substrata for the species within this biotope. Epifauna may be removed or 'washed' to unsuitable substrata at high water flow rates. Conversely reduced water flow may increase the deposition of fine muds which are unsuitable for some members of the infauna, e.g. Cerastoderma edule and Arenicola marina (see siltation). Nacken & Reise (2000) noted that where sediment was allowed to accumulate in parts of Zostera noltii beds from which wildfowl (and hence their eroding effects) were excluded, the seagrass did not grow as profusely as in areas in which the wildfowl actively fed. As such, intolerance is assessed as intermediate. Recovery is likely to be moderate, resulting in a moderate sensitivity rating. Populations present in moderately strong currents may benefit from decreased water flow rates. Davison & Hughes (1998) point out that Zostera sp. beds probably exist in areas with defined rates of summer accretion and winter erosion, with too much sediment deposition resulting in smothering (see smothering).
Decrease in water flow rate
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Increase in temperature
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Populations of Zostera noltii occur from the Mediterranean to southern Norway and Zostera sp. are regarded as tolerant between about 5 - 30 °C. Therefore, they may not be intolerant of the range of temperatures likely in the British Isles (Davison & Hughes, 1998). Intertidal populations may be damaged by frost (den 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). The infauna are partly protected from short term acute temperature change by their position in the sediment. Cerastoderma edule is more vulnerable since it occupies the top few centimetres of sediment, however, it is relatively tolerant of temperature change, especially temperature increases. The epifaunal gastropods are also relatively tolerant of temperature change. However, increases in temperature are likely to stimulate bacterial activity and oxygen consumption resulting in hypoxia which may affect infauna indirectly. Increased temperatures may also stimulate growth of ephemeral algae, e.g. Ulva spp. and epiphytes which, while potentially detrimental to Zostera noltii may be beneficial for epifaunal grazing gastropods. However, although the infauna may be adversely affected by long term temperature change, Zostera noltii is tolerant of a wide range of temperatures and will probably be little affected and therefore, the biotope as a whole will be little affected. As such, intolerance is assessed as low. Recovery is likely to be very high, therefore sensitivity rating is very low.
Decrease in temperature
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Increase in turbidity
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Increased turbidity will reduce the light available for photosynthesis, the time available for net photosynthesis and, therefore, growth. However, Zostera noltii is tolerant of high light intensities and can take advantage of the light available at low tide (Vermaat et al., 1996). Furthermore, Zostera noltii 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 noltii are likely to be more intolerant to chronic increases in turbidity. Also, permanently submerged brackish water populations may be more vulnerable to increased turbidity. Therefore intolerance is assessed to be intermediate. This biotope may benefit form decreased turbidity. Philippart (1994b) suggested that the decline in Zostera noltii beds in the Wadden Sea probably occurred in beds at low water. Recovery is likely to be moderate, resulting in a moderate sensitivity assessment. Most other species in the biotope, e.g. infauna and epifauna will probably not be adversely affected by changes in turbidity.
Decrease in turbidity
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Increase in wave exposure
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Seagrasses require sheltered environments, with gentle long shore currents and tidal flux. Where populations are found in moderately strong currents they are smaller, patchy and vulnerable to storm damage and blow outs. Increased wave exposure may also increase sediment erosion (see siltation above). Therefore intolerance is rated as high. Populations present in moderately strong currents may benefit from decreased water flow rates. Small patchy populations or recently established populations and seedling may be highly intolerant of increased wave action since they lack an extensive rhizome system. Recovery after sediment erosion is likely to be very low, resulting in a very high sensitivity assessment.
Decrease in wave exposure
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Noise
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It is unlikely that noise will have an adverse effect on Zostera noltii or other species within the biotope. Therefore the biotope is assessed to be tolerant. Wildfowl, however, are intolerant of disturbance from noise from e.g. shooting (Madsen, 1988) and from coastal recreation, industry and engineering works. For example, Percival & Evans (1997) reported that wigeon were very intolerant of human disturbance and, where wildfowling was popular, wigeon avoided Zostera noltii beds at the top of the shore, preferring Zostera marina and Zostera angustifolia beds lower on the shore, until the lower shore beds were exhausted. Reduced grazing pressure may benefit Zostera sp. beds. However, Nacken & Reise (2000) noted that where wildfowl were excluded from grazing experimental plots, the Zostera noltii beds summer regrowth was inhibited. They suggested that grazing was important for the persistence of Zostera noltii beds, at least in their study area.
Visual Presence
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Zostera noltii or other species within the biotope are unlikely to be affected by visual disturbance. Therefore the biotope is assessed to be tolerant. Wildfowl, however, are intolerant of disturbance by noise from e.g. shooting (Madsen, 1988) and from coastal recreation, industry and engineering works. Disturbance is species dependant, some species habituating to noise and visual disturbance while others become more nervous. For example, brent geese, redshank, bar-tailed godwit and curlew are more 'nervous' than oyster catcher, turnstone and dunlin (Elliot et al., 1998). Turnstones will often tolerate one person within 5-10 m. However, one person on a tidal flat can cause birds to stop feeding or fly off affecting ca. 5 ha for gulls, ca.13 ha for dunlin, and up to 50 ha for curlew (Smit & Visser, 1993). Industrial and urban development may exclude 'nervous' species from adjacent tidal flats. Reduced grazing pressure may benefit Zostera sp. beds. However, Nacken & Reise (2000) noted that where wildfowl were excluded from grazing experimental plots, the Zostera noltii beds summer regrowth was inhibited. They suggested that grazing was important for the persistence of Zostera noltii beds, at least in their study area.
Abrasion & physical disturbance
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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, especially in the intertidal (Jones et al., 2000). However, wildfowl grazing of intertidal seagrass beds results in significant physical disturbance.

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 3-10 cm 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 had 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). Nevertheless, physical disturbance may be detrimental where ‘normal’ levels of physical disturbance caused by grazing birds are augmented by physical disturbance from human activities. Therefore an intolerance of intermediate is recorded.

Epifaunal gastropods, such as Littorina littorea and bivalves, such as Cerastoderma edule living near the surface may be damaged by abrasion, and infaunal polychaetes may be damaged by physical disturbance to the sediment. Recoverability has been assessed as moderate (see additional information below), resulting in a moderate sensitivity value. Physical disturbance caused by fishing activities e.g. for cockles, is greater than the benchmark, and is discussed under 'extraction' (see below).

Displacement
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The majority of the epifauna are unlikely to be significantly affected by displacement since they are mobile species capable of migrating from adjacent areas with a wide range of habitat preferences. Similarly most infauna can re-burrow if displaced, although predation risk while on the surface will be high, especially if displacement coincides with low tide. However, the seagrass bed is unlikely to survive displacement, so intolerance is assessed as high. Seagrass rhizomes are easily damaged by trampling, anchoring, dredging and other activities that disturb the sediment such as storms. Although rhizomes and shoots can root and re-establish themselves if they settle on sediment long enough (Phillips & Menez, 1988) displacement is likely to result in loss of the seagrass and its associated biotope. Therefore recoverability if very low, resulting in a very high sensitivity rating.

Chemical Factors

Synthetic compound contamination
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Little information concerning Zostera noltii, the key species in this biotope, was found. 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 seagrass beds in the marine environment. For example, the herbicide Atrazine is reported to cause growth inhibition and 50 percent 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.

TBT contamination is likely to adversely affect grazing gastropods resulting in increased algal growth, reduced primary productivity and potential smothering of the biotope, e.g. Philippart (1995a) suggested that the decline in Zostera noltii beds in the Wadden Sea in the 1970s due to eutrophication was exacerbated by a simultaneous decline in the mud-snail (Hydrobia ulvae) population, although mud-snail populations have increased subsequently. Bryan & Gibbs (1991) suggested that TBT may cause reproductive failure or larval mortality in bivalve molluscs, e.g. Pecten maximus at ca. 50 ng/l TBT, however little information on the effect of TBT on polychaetes was available.

Overall, terrestrial herbicides are likely to adversely affect seagrass beds, and loss of grazing gastropods due to TBT or other synthetic chemicals is likely to result in smothering and potential reduction in the extent of the seagrass. Therefore, an intolerance of intermediate has been reported. On return to normal conditions recovery is likely to be moderate, hence a moderate sensitivity rating.
Heavy metal contamination
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Little information was found regarding the effect of heavy metal concentrations on Zostera noltii 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. Bryan (1984) suggested that polychaetes were fairly resistant to heavy metals, while the larval and embryonic stages of bivalve molluscs were the most intolerant. Mercury was the most toxic to bivalves whereas Cu, Cd, and Zn probably caused the most problems in the field. Bryan (1984) concluded that gastropods were relatively tolerant of heavy metals, in part due to the protection afforded by their shell. However, the viability and reproductive potential of the polychaetes, and molluscs is probably reduced by heavy metal contamination. Given the potential effects of heavy metals on Zostera spp., heavy metal contamination could lead to reduction in the extent or abundance of the seagrass and an intolerance rank of intermediate was reported. Recoverability is likely to be high, resulting in a low sensitivity recording.
Hydrocarbon contamination
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Intertidal seagrass beds are likely to be more vulnerable to direct oil contamination and the sheltered conditions in which they occur suggests that any oil will weather slowly (Davison & Hughes, 1998; Jones et al., 2000). However, several studies on seagrass beds after oil spills and in the vicinity of long term, low level hydrocarbon effluents suggest that Zostera sp. are little effected by hydrocarbon contamination (Jacobs, 1980; Hiscock, 1987; Davison & Hughes, 1998; Jones et al., 2000). However, pre-mixed oil and dispersant were found to cause rapid death and significant reduction in cover of Zostera noltii, and led to the authors recommending that dispersants should be avoided (Holden & Baker, 1980; Howard et al., 1989; Davison & Hughes, 1998).

The removal of oil intolerant grazers, e.g. gastropods and amphipods, however, is likely to indirectly affect the seagrass bed, resulting in unchecked growth of periphyton, epiphytes and ephemeral algae and smothering of the seagrass (see nutrients). Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general in soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. For example, evidence from oil spills suggested that gastropods such as Hydrobia ulvae and especially Littorina littorea were intolerant of oil spills (Jacobs, 1980). Large numbers of dead or moribund Cerastoderma edule were washed ashore after the Sea Empress oil spill. Similarly, the abundance of Arenicola marina populations were adversely affected by oil or oil:dispersant mixtures, and seawater oil concentrations of 5 mg/l caused the lugworms to leave the sediment (Levell, 1976; Prouse & Gordon, 1976). Therefore, hydrocarbon contamination is likely to adversely affect epifaunal and infaunal species within the biotope, and although Zostera noltii may not be adversely affected directly, the loss of grazers is likely to result in smothering and potential loss of areas of seagrass bed. Therefore is intolerance assessed and intermediate, and recoverability moderate, yielding a moderate sensitivity rating being recorded.

Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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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; Philippart, 1994b; Vermaat et al., 1996; Philippart, 1995a, b; Davison & Hughes, 1998; Asmus & Asmus, 2000a, b). 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 35µM 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 dependant 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 µM 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 noltii, and 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 noltii 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).
Increased nutrients may benefit deposit feeding polychaetes, such as Arenicola marina and grazing gastropods may also benefit from the bloom of ephemeral and epiphytic algae. However, loss or reduction of the Zostera noltii bed will necessitate loss or reduction of the biotope itself. Therefore intolerance is deemed high. Recovery is likely to be very low, resulting in a sensitivity rating of very high.
Increase in salinity
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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 noltii is probably more tolerant of extremes of salinity than its congeners due to its intertidal habitat. Most of the other intertidal species present in the biotope are probably tolerant of a wide range of salinities, e.g. Hydrobia ulvae and Littorina littorea, although Hydrobia ulvae populations are impacted by sever flooding (Cardoso , 2008). Similarly both Cerastoderma edule and Arenicola marina are tolerant of a wide range of salinities, however both have been reported to be susceptible to low salinities after heavy rains at low tide. As such intolerance is assessed to be low. Recoverability is high on return to normal conditions, resulting in a very low sensitivity rating. However Zostera noltii shows mortality at 35 ‰ (Vermaat et al., 2000) and is highly sensitive to hypersalinities of 41 psu (Fernández Torquemada et al., 2006), so is likely to be adversely affected by brine discharges from seawater desalination plants.
Decrease in salinity
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Changes in oxygenation
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The effects of oxygen concentration on the growth and survivability of Zostera noltii 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 an 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.

Epifaunal gastropods may be tolerant of hypoxic conditions, especially Littorina littorea and Hydrobia ulvae. Infaunal species are likely to be exposed to hypoxic conditions, especially at low tide when they can no longer irrigate their burrows e.g. Arenicola marina can survive for 9 days without oxygen (Hayward, 1994). Conversely, possibly since it occupies the top few centimetres of sediment, Cerastoderma edule may be adversely affected by anoxia and would probably be killed by exposure to 2 mg/l oxygen for a week.

Smothering of the seagrass beds by epiphytes and ephemeral algae (see nutrients) may indirectly result in anoxic conditions as the algae die and decompose. Therefore, given the potential intolerance of Zostera noltii to deoxygenation, an overall intolerance of intermediate has been reported. On return to normal conditions recovery is likely to be quick, provided the seagrass itself is not damaged. Therefore recoverability is deemed to be high, resulting in a sensitivity assessment of low.

Biological Factors

Introduction of microbial pathogens/parasites
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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. However, Zostera noltii was little affected (Rasmussen, 1977; Davison & Hughes, 1998). Decline of intertidal Zostera marina and Zostera noltii beds in the Wadden Sea began in the 1960s and a marked decline in Zostera noltii occurred between 1965 and 1975, presumably due to anthropogenic change (Philippart, 1994b).

Intertidal gastropods and bivalves often act an intermediary host for trematode parasites of wildfowl and sea birds. In many cases the trematode cercariae accumulate in the gut and gonad tissue resulting in castration of infected individuals, and hence reducing the reproductive capability of the host population. The significance of this form of parasitism varies with species. However, mass mortalities of Hydrobia ulvae have been reported, due to the mass development of larval digenean trematodes as a result of high temperatures (Huxham et al., 1995). Therefore, given the importance of Hydrobia ulvae in controlling periphyton and epiphytes (see nutrients above; Philippart, 1995a) an intolerance of intermediate has been reported. Recoverability is very moderate, hence sensitivity is moderate.

Introduction of non-native species
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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 Zostera sp. cover in Lindisfarne, Northumberland due to encroachment and changes in sediment dynamics (Davison & Hughes, 1998). Wireweed (Sargassum muticum) invades open substratum and may prevent recolonization of areas of seagrass beds left open by disturbance (Davison & Hughes, 1998). Intertidal seagrass beds may be particularly vulnerable. Sargassum muticum is able to colonise soft sediments by attachment to embedded fragments of rock or shell (Strong et al., 2006). Further, it has been suggested by Tweedley et al. (2008) that beds of a related species, Zostera marina (which often grows alongside Zostera noltii), may facilitate the attachment of Sargassum muticum. However, evidence for competition is conflicting and requires further research. If the invasive species prevent recolonization then the recoverability from other factors will be reduced. Therefore recoverability is low, and sensitivity is assessed as high.
Extraction
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Wildfowl grazing can consume significant amounts of seagrass and reduce cover mainly in autumn and winter. Grazing is probably part of the natural seasonal fluctuation in seagrass cover and Zostera sp. can recover from normal grazing (Davison & Hughes, 1998; Naken & Reise, 2000). Zostera noltii 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 3-10 cm deep. 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). But, where a bed is stressed by other factors it may not be able to withstand grazing (Holt et al., 1997; Davison & Hughes, 1998).

Eelgrass rhizomes are easily damaged by trampling, anchoring, dredging and other activities that disturb the sediment. The seagrass bed is unlikely to survive displacement or extraction. However, Phillips & Menez (1988) reported that rhizomes and shoots can root and re-establish themselves if they settle on sediment long enough.

The common cockle Cerastoderma edule is an important species associated with this biotope and is subject to commercial extraction. Zostera sp. are regarded as very intolerant of hydraulic bivalve fishing in the UK and Wadden Sea (Philippart, 1994b; Holt et al., 1997; Davison & Hughes, 1998). Cockles and Zostera noltii are frequently associated and intertidal beds may be more vulnerable (Holt et al., 1997). Hydraulic dredging is likely to break up and remove rhizomes. Shorter fragments of rhizomes have slower growth and production than longer fragments, with fragments shorter than two internodes showing significantly reduced survival. Rhizome elongation is also adversely affected by the removal of the apical shoot (Caba ço et al., 2005). Further, in meadows disturbed by clam harvesting vegetative shoot density is lower, and sexually reproductive effort significantly higher than in unharvested areas (Alexandre et al., 2005). 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). Furthermore, tractor dredging reduced the density of cover in Zostera beds from 75% to 5% (Hawker, 1994). 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). Hand gathering is likely to have a similarly adverse affect to bait digging, especially if vehicular access is used. Extraction of grazing epifauna, e.g. by foraging wildfowl such as shelduck could result in increased levels of smothering, especially during the summer months.

Given the intolerance of intertidal seagrass beds to hydraulic dredging and the associated decline in seagrass beds, an intolerance of high has been reported. Recovery would be very low, or potentially none if rhizomes are removed, resulting in a very high sensitivity assessment.

Additional information icon Additional information

Recovery of Zostera noltii beds may be differentially affected by environmental variables at different stages in the recovery process. Charpentier et al., (2005) found that during the first few years of recolonisation, when bed were restricted to shallow borders, depth and slope were the best explanatory variables, while in later years depth and wave exposure were more important in controlling the spatial distribution of Zostera noltii.

This review can be cited as follows:

Tyler-Walters, H. & Wilding, C.M. 2008. Zostera noltii beds in upper to mid shore muddy sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 22/12/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=318&code=2004>