Biodiversity & Conservation

SS.IMS.Sgr.Zmar

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Substratum loss will result in the loss of the shoots, rhizome and probably the seed bank of Zostera marina together with its associated biotope, thus intolerance is deemed to be high. Recoverability of Zostera marina will depend on recruitment from other populations. Although Zostera marina seed dispersal may occur over large distances, high seedling mortality and seed predation may significantly reduce effective recruitment. The slow or total lack recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease suggests that, once lost, seagrass beds take considerable time to re-establish, if at all. Hence recoverability is very low, and resulting biotope sensitivity is very high. Reed and Hovel (2006), found that removal of 90% of the substrata (which included seagrass plant material both above and below ground) in large 16 m² plots resulted in a significant loss of diversity and abundance of the epifaunal community. It was also noted that species composition was significantly different. However in smaller plots, or with a lower level of substrate removal, there was no observed correlation between seagrass loss and reduction in density or diversity of epifaunal species. This suggests the biotope may be tolerant of some substrate removal up to a threshold level. A further example is provided by Pihl et al. (2006), who demonstrated that the biomass, density and number of fish species was greater in seagrass beds than adjacent areas of sediment from which beds had been lost. Juvenile cod density was reduced by 96 % in areas that no longer contained seagrass.
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). Epiphytes and macroalgae are also likely to be intolerant of smothering, hence intolerance is deemed high. Infaunal species within the community are unlikely to be intolerant of smothering itself. However, the community will probably be intolerant of loss of the source of primary production on substratum. Recoverability will depend on recruitment from other populations. Although Zostera marina seed dispersal may occur over large distances, high seedling mortality and seed predation may significantly reduce effective recruitment. The slow recovery of Zostera populations since the 1920s - 30s outbreak of wasting disease suggests that, once lost, seagrass beds take considerable time to re-establish. Thus recoverability is very low, and resulting sensitivity is very high.
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 and USA (for example Bernard et al., 2007). Increased sediment availability may result in raised seagrass beds, more likely to be exposed to low tide, desiccation and high temperatures. Increases in suspended sediment may also increase sediment deposition, which could potentially lead to the smothering of beds (Portig et al., 1994) (see smothering, above). Seagrass beds demonstrate a balance of sediment accretion and erosion. 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 (Ranwell et al., 1974). 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. When loss of seagrass beds is due to increased turbidity related to suspended sediment, recovery is may be impossible, probably because seagrass beds are required to initially stabilise the sediment and reduce turbidity levels (Van derHeide et al., 2007). A high turbidity state appears to be a highly resilient alternative stable state, hence return to the seagrass biotope is unlikely.
Decrease in suspended sediment
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Desiccation
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Zostera marina is mainly subtidal and intolerant to desiccation compared to other species of seagrass. If exposed at low tide the shoot bases are stiff and upright for a few centimetres, and leaf bases will be killed by 30 min exposure on a warm, sunny day (Holt et al., 1997). Even short periods of drying kill the flowers. If the rhizomes are undamaged the leaves will grow back, hence recovery is deemed high, but repeated exposure to desiccation may exhaust the energy stores in the rhizomes. In intertidal beds in the USA, desiccation damage results in frond breakage, so plants growing higher on the intertidal zone have shorter canopy heights than those growing lower on the shore (Boese et al., 2003). Zostera marina may be more intolerant than other Zostera sp. to activities that cause the sediment to drain or dry. Therefore the seagrass and its associated biotope will be intermediately intolerant of desiccation, resulting in a low sensitivity recording.
Increase in emergence regime
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Decreased emergence may allow the seagrass beds to extend further up the shore. Increased emergence will reduce the upper extent of the biotope. Hence intolerance is intermediate. Populations on the lower shore are likely to be highly intolerant of increases in emergence (see desiccation). Recoverability is likely to be high, resulting in a low sensitivity ranking.
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 may also increase sediment erosion (see siltation above). Coastal developments which alter hydrology have been implicated in the disappearance of seagrass beds (Van derHeide et al., 2007). Populations present in moderately strong currents may benefit from decreased water flow rates. As such, intolerance is rated intermediate. Recoverability is likely to be moderate, hence a suggested sensitivity of moderate.
Decrease in water flow rate
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Increase in temperature
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Increased temperatures may encourage growth of epiphytes and ephemeral algae while important grazers such as Hydrobia ulvae and Lacuna vincta are intolerant of temperature change. Although Zostera marina is tolerant of sea temperatures between 5-30°C (Davidson & Hughes, 1998), temperature change which leads to increased algal growth before the grazers can recover will reduce primary productivity. Prolonged temperature change may result in smothering of Zostera marina and reduction in extent or loss of the seagrass bed. Temperatures on 25-30°C may lead to mortality, reduced photosynthetic rates and reduced growth (Nejrup & Pedersen, 2007). However at the benchmark level, the biotope is not likely to be severely affected, hence intolerance is rated low. Low temperatures of 5°C lead to reduced photosynthetic rates (by up to 75%) and growth, but are sub lethal (Nejrup & Pedersen, 2007). Frost can damage leaves, and the formation of ice can uproot rhizomes and lead to the erosion of surface sediments (Den Hartog, 1987). Recoverability is likely to be very high, resulting in a very low sensitivity ranking.
Decrease in temperature
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Increase in turbidity
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Light attenuation limits the depth to which Zostera marina can grow as light is a requirement for photosynthesis. Growth of both Zostera marina and its associated epiphytes is reduced by increased shading due to turbidity (Moore & Wetzel, 2000). Turbidity resulting from dredging and eutrophication caused a massive decline of Zostera populations in the Wadden Sea (Giesen et al., 1990; Davison & Hughes, 1998). Seagrass populations are likely to survive short term increases in turbidity, however a prolonged increase in light attenuation, especially at the lower depths of its distribution, will probably result in loss or damage of the population. Hence intolerance is deemed to be high. Once seagrass beds have been lost, it has been suggested that a high turbidity environment may be a resilient alternative stable state, preventing any recovery (Van derHeide et al., 2007). Therefore recoverability is very low, resulting in a very high level of sensitivity.
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. Even large areas may be severely damaged during heavy storms (Davidson & Hughes 1998). Increased wave exposure may also 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 populations and seedlings may be highly intolerant of increased wave action since they lack an extensive rhizome system. Hence intolerance is high; recoverability is likely to be very low, if at all, resulting in a very high sensitivity rating.
Decrease in wave exposure
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Noise
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The effect of sound waves and vibration on plants is poorly studied. It is likely that sound waves will have little effect on Zostera marina at the benchmark levels suggested, hence the biotope is deemed to be tolerant. However, fish species and grazing wildfowl are likely to be disturbed by noise at the benchmark level.
Visual Presence
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Continuous shading will affect photosynthesis and therefore viability. However, occasional shading caused by surface movements of vessels at the level of this benchmark is unlikely to have an effect on seagrass beds. Hence the biotope is deemed to be tolerant.
Abrasion & physical disturbance
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Small scale sediment disturbance may stimulate growth and removal of small patches of sediment allows recolonization by seedlings (Davison & Hughes, 1998). However seagrasses are not physically robust, so activities such as trampling, anchoring, digging, dredging, power boat and jet-ski wash are likely to damage rhizomes and cause seeds to be buried too deeply to germinate (Fonseca, 1992). Suction dredging for cockles in the Solway Firth removed Zostera in affected areas while Zostera was abundant in un-dredged areas (Perkins, 1988). Physical disturbance and removal of plants can lead to increased patchiness and destabilization of the seagrass bed, which in turn can lead to reduced sedimentation within the seagrass bed, increased erosion, and loss of larger areas of Zostera (Davison & Hughes, 1998). Therefore, the impact from a scallop dredge is likely to remove a proportion of the population and result in increased erosion of the bed. Hence, intolerance has been recorded as intermediate. Grazing gastropods and other epifauna are small but likely to be displaced or removed attached to the leaves of Zostera. Reduction in numbers of grazers may potentially result in smothering by growth of epiphytes and other algae, especially in the spring and summer months. Recovery is dependant on the size of the size of the area affected, so is set as moderate, yielding a moderate sensitivity rating.
Displacement
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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. Thus the biotope is deemed highly intolerant. Recovery is likely to be slow, if at all, resulting in a recoverability of low and subsequently a high sensitivity rating.

Chemical Factors

Synthetic compound contamination
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Zostera marina is known to accumulate TBT but no damage was observable in the field. 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 (Chesworth et al., 2004). 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). TBT contamination is likely to adversely affect grazing gastropods resulting in increased algal growth, reduced primary productivity and potential smothering of the biotope. As such intolerance is assessed as intermediate. Recoverability is likely to be moderate, resulting in a moderate sensitivity recording.
Heavy metal contamination
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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. 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. So intolerance is assessed as low. However Lead in sediment from, for example shotgun pellets, may stress Zostera marina plants (Jones et al., 2000). Gastropods are thought to be relatively tolerant of heavy metals (Bryan, 1984). Recoverability is likely to be high, resulting in a low sensitivity recording.
Hydrocarbon contamination
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  • Healthy populations of Zostera can occur in the presence of long term, low level, hydrocarbon effluent, for example in Milford Haven, Wales (Hiscock, K., 1987)
  • Zostera marina may be partially protected from direct contact by oil due to its subtidal habitat.
  • The Amoco Cadiz oil spill off Roscoff caused Zostera marina leaves to blacken 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).
  • The Amoco Cadiz oil spill resulted in virtual disappearance of Amphipods, Tanaidacea and Echinodermata from Zostera marina beds in Roscoff and a decrease in numbers of Gastropoda, sedentary Polychaeta and Bivalvia. The numbers of most groups returned to normal within a year except Echinoderms which recovered slowly and Amphipods which had not recovered after one year (Jacobs, 1980). Hence intolerance is assessed as intermediate.
  • Removal of oil intolerant gastropod grazers may result in smothering of seagrasses by epiphytes (Davison & Hughes, 1998). Jacobs (1980) noted a larger algal bloom than in previous years after the Amoco Cadiz spill in Roscoff, probably as a result in increased nutrients (from dead organisms and breakdown of oil) and the reduction of algal grazers. However, in his study the herbivores recolonized and the situation returned to 'normal' within a few months. Hence recoverability is assessed as high, resulting in a sensitivity of low being recorded.
  • Experimental treatment of Zostera sp. with crude oil and dispersants halted growth but had little effect on cover whereas pre-mixed oil and dispersant caused rapid death and significant decline in cover within 1 week suggesting that dispersant treatments should be avoided (Davison & Hughes, 1998).
Radionuclide contamination
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Insufficient information
Changes in nutrient levels
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Where nutrients are limiting, additional low levels of nutrients may improve growth of Zostera marina. The reported effects of nutrient enrichment include:
  • High nitrate concentrations implicated in decline of Zostera marina (Davison & Hughes, 1998). 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 35 micro Molar nitrate per 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 or 26 psu) and that both populations benefited from nitrate enrichment (0-4 to 6.3 micro Molar nitrate per day) at 23 or 26 psu. Increased growth of epiphytes or blanketing algae. 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 ; by summer 1992 the Zostera sp. were absent, however this may have been exacerbated by grazing by Brent geese.
  • Encouragement of phytoplankton blooms which increase turbidity and reduce light penetration (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 (Davison & Hughes, 1998).
Due to the likeliness of smothering, and the effects of reduced light penetration caused by eutrophication, intolerance is assessed to be high. Recoverability will be very low, and the resulting biotope sensitivity very high.
Increase in salinity
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Zostera sp. has a wide tolerance of salinity from 10 - 39 ppt (Davison & Hughes, 1998). Germination in Zostera marina occurs over a range of salinities. Hydrobia ulvae and Lacuna vincta are tolerant of wide range of salinities. Therefore biotope intolerance is deemed to be low. Recoverability is likely to be very high, resulting in a very low sensitivity recording. However, not all members of the community have been assessed and some species may be intolerant of changes in salinity.
Decrease in salinity
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Changes in oxygenation
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Loss of grazers due to low oxygen levels will result in unchecked growth of epiphytes and other algae which may smother Zostera marina. Therefore intolerance is intermediate. On return to normal conditions, recovery is likely to be rapid, so is assessed as high, resulting in a low sensitivity value. Prolonged deoxygenation is likely to damage the seagrass itself (Jones et al., 2000).

Biological Factors

Introduction of microbial pathogens/parasites
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A major outbreak of wasting disease resulted in significant declines of Zostera marina beds in 1920s to 1930s, so intolerance is recorded as high. Wasting disease is thought to be caused by the marine fungus, Labyrinthula macrocystis. The disease is less likely at low salinities. However, Zostera marina is often found in fully salinity waters. 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 and its associated biotope. Hence recoverability is very low, and sensitivity is very high.
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, which colonises the upper parts of mud flats. 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). Japanese weed (Sargassum muticum) invades open substratum subtidally and may prevent recolonisation of areas of seagrass 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. 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 the presence of Zostera marina beds may facilitate the attachment of Sargassum muticum. However, evidence for competition is conflicting and requires further research, hence an assessment of intermediate intolerance. If the invasive species prevent recolonisation 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 probably causes part of the natural seasonal fluctuation in seagrass cover and Zostera sp. can recover from typical levels of grazing. However, where a bed is stressed by other factors it may not be able to withstand grazing (Holt et al., 1997; Davison & Hughes, 1998). Seagrass rhizomes are easily damaged and the seagrass bed is unlikely to survive extraction. Seeds may be buried too deep to germinate. Mechanical dredging of cockles in the Solway Firth, in intertidal Zostera beds, resulted in the loss of the seagrass bed and was closed. Dredging for bivalves has been implicated in the decline of seagrass beds in the Dutch, Wadden Sea. Damage of Zostera noltii beds after the Sea Empress oil spill was reported as limited to the ruts left by clean up vehicles. Intolerance has been assessed as intermediate with a moderate recovery, resulting in a moderate sensitivity rating.

Additional information icon Additional information

None entered

This review can be cited as follows:

Tyler-Walters, H. & Wilding, C.M. 2008. Zostera marina/angustifolia beds in lower shore or infralittoral clean or muddy sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21/04/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=257&code=1997>