The Marine Life Information Network

Temperature increase (local)

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

Evidence

Temperature is considered the overall parameter controlling the geographical distribution of angiosperms.  All enzymatic processes related to plant metabolism are temperature dependent and specific life cycle events, such as flowering and germination, are also often related to temperature (Phillips et al., 1983).  For marine plants, temperature affects biological processes by increasing reaction rates of biological pathways.  Photosynthesis and respiration increase with higher temperature until a point where enzymes associated with these processes are inhibited.  Beyond a certain threshold, high temperatures will result in respiration being greater than photosynthesis resulting in a negative energy balance.  Increased temperatures do also encourage the growth of epiphytes increasing the burden upon seagrass beds and making them more susceptible to disease (Rasmussen, 1977). Verhoeven (1979) noted that Ruppia maritima plants survived between 0 and 38°C, grew exponentially between 10 and 30°C and withstood fluctuations of 15°C in laboratory experiments.  However, temperatures above 30°C were harmful if sustained for prolonged periods of times, and Ruppia maritima was replaced by Potamogeton pectinatus in high temperature environments such as in the vicinity of thermal effluent (Kantrud, 1991). 

Species living among Ruppia found in lagoons and shallow lochs are probably adapted to fluctuating temperatures, while mobile species are likely to move to deeper waters. Benthic infauna are likely to be protected from temperature extremes by their benthic habit, however, a proportion of the Arenicola marina population may be lost at temperatures above 20°C, and excluded from habitats suffering from more extreme fluctuations in temperature.

Sensitivity assessment. A 5°C change in temperature for one month period or a 2°C change in temperature for one year are unlikely to severely affect Ruppia plants. Resistance is therefore assessed as 'High' resulting in a 'High' resilience score (no impact to recover from). Ruppia maritima has a very wide temperature tolerance and is deemed ‘Not sensitive’ to this pressure.

Temperature decrease (local)

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

Evidence

A decrease in temperature is likely to delay the onset of budding and germination and subsequent reproduction in Ruppia spp., which may be of particular importance for annual species (see above). Verhoeven (1979) noted that all Ruppia taxa survive between 0-38°C, grow exponentially at 10-30°C and survive daily fluctuations of 15°C in culture. Kantrup (1991) reported that in North American wetlands that freeze in winter, Ruppia spp. behaved as annuals. Verhoeven (1979) reported that the distribution of Ruppia maritima and Ruppia cirrhosa extended north to Norway (ca 69 deg N and 68 deg N respectively), suggesting that these species would be tolerant of the average winter temperatures encountered in the UK. Therefore, Ruppia spp. are probably not sensitive to temperature increase at the level of the benchmark.
Many of the species found within the Ruppia spp. communities are typically lagoonal or shallow water species, adapted to fluctuating temperatures. Infaunal polychaetes are protected from temperature extremes by their burrowing habit, however, a proportion of the Arenicola marina population may be lost below 5 °C on in areas subject to extreme fluctuations in temperature. Overall, the Ruppia spp. stand will not be damaged by a decrease in temperature at the benchmark level but some species will reduce in abundance while mobile species may move to deeper water resulting in a reduced species richness.

Sensitivity assessment. A 5°C change in temperature for one month period or a 2°C change in temperature for one year are unlikely to severely affect Ruppia plants. Resistance is therefore assessed as 'High' resulting in a 'High' resilience score (no impact to recover from). Ruppia maritima has a very wide temperature tolerance and is deemed ‘Not sensitive’ to this pressure.

Salinity increase (local)

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

Evidence

Ruppia maritima has a wide salinity tolerance and is found growing in full saline, brackish as well as freshwater environments. Ruppia spp. tolerate a wider range of ionic strengths and salinities than any other aquatic angiosperm, occurring between 0.6 -390g/l (Kantrup, 1991). However, the reported salinity tolerances vary with region and with species. Ruppia maritima was reported to be abundant at salinities between 15 ->100g/l in North American wetlands and between 0.57 -27g/l in European sites (Verhoeven, 1979; Kantrup, 1991). Ruppia cirrhosa tolerated 2.7-108.3 g/l in European sites (Verhoeven, 1979). Kantrup (1991) concluded that the optimum salinity for Ruppia spp. Growth was 5-20 g/l while slightly lower salinities early in spring may enhance germination and seed formation. Rapid fluctuations were found to kill Ruppia spp. when salinities rise >ca 18g/l in a few weeks (Verhoeven, 1979). However, Ruppia spp. was also reported to survive a drop of at least 14 g/l in 24 hrs (Kantrup, 1991). Overall, Ruppia spp. are probably not directly sensitive changes in salinity at the benchmark level. Their exclusion from very low to freshwater, or nearly full seawater is probably due to competitive exclusion by other aquatic plants or seagrasses.

As the salinity increases low salinity species are likely to be replaced by comparable marine forms. Typically lagoonal species (e.g. the hydrobids, some gammarids, and Cerastoderma glaucum) are adapted to a wide range of salinities and are unlikely to be affected. Estuarine and low salinity polychaetes present in the benthos are likely to be replaced by more marine species as the salinity increases, e.g. the abundance of Hediste diversicolor is likely to fall while the abundance of Arenicola marina may increase. Sticklebacks are found in marine and freshwater habitats and the sand goby tolerates a wide range of salinities. Therefore, the biotope as a whole will probably be little affected by increases in salinity at the benchmark level, although some species may be replaced by more marine members of the same group.

Sensitivity assessment. Ruppia maritima exclusion from freshwater or nearly full seawater habitats is most probably due to competitive exclusion by other aquatic plants. An increase in salinity from 35 to >40 units for one year is unlikely to severely affect Ruppia plants. Resistance is therefore assessed as 'High' resulting in a 'High' resilience score (no impact to recover from). Overall Ruppia maritima has a very wide salinity tolerance and is assessed as ‘Not sensitive’ to an increase in salinity.  

Salinity decrease (local)

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

Evidence

Ruppia maritima is found growing in full saline, brackish as well as freshwater environments and has, therefore, a wide salinity tolerance (Kantrud, 1991). However, La Peyre & Rowe (2003) found that relative growth rate of Ruppia maritima was significantly lowered during a short experimental freshwater pulse but without reporting any mortalities. Ruppia spp. are probably not directly intolerant of changes in salinity at the benchmark level (see above). Their exclusion from very low salinity to freshwater is probably due to competitive exclusion by other aquatic plants, e.g. Potamogeton pectinatus.

Most of the typically lagoonal species (e.g. Cerastoderma glaucum, Gammarus insensibilis and hydrobids) will be little affected by changes in salinity. However, Gammarus insensibilis was reported to disappear in areas affected prolonged exposure to freshwater. Similarly, Arenicola marina does not tolerate salinities below 24 psu and is likely to be replaced by Hediste diversicolor. Sticklebacks are found in marine and freshwater habitats and the sand goby tolerates a wide range of salinities. As the salinity decreases the species composition is likely to change towards more freshwater tolerant species, including insects, although the functional groups will probably remain, and the species richness may increase.

A short-term decrease in salinity is unlikely to affect the biotope adversely. However, prolonged exposure to low salinities or freshwater is likely to result in the replacement of the Ruppia community by other aquatic plant communities e.g. Potamogeton pectinatus. Once prior conditions return recovery is likely to be rapid.

Sensitivity assessment.  Ruppia maritima exclusion from freshwater or nearly full seawater habitats is most probably due to competitive exclusion by other aquatic plants. A decrease in salinity from 'reduced' to 'low' salinity for one year is unlikely to severely affect Ruppia plants due to competition.  Resistance is therefore assessed as 'Medium' to represent the potential loss of extent or abundance, but once the salinity returns to prior levels, recovery is likely to be rapid and resilience is probably 'High'. Overall Ruppia maritima beds are assessed as ‘Low’ sensitivity to a decrease in salinity at the benchmark level.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

The SS.SMp.SSGr.Rup biotope is found in extremely sheltered conditions in very weak tidal streams. An increase in water flow at the benchmark level  is likely to damage leaves and shoots and probably remove the vegetation and a proportion of the root system. The root system of Ruppia spp. is poorly developed consisting of horizontal runners a few millimetres below the sediment surface and only 1-2 thin roots per 10-20 cm along the rhizome. Therefore, Ruppia spp. are not very resistant of water flow and are limited to still, sheltered waters such as lagoons and bays where current flow is less than in adjacent channels and tidal rivers (Verhoeven, 1979; Kantrup, 1991). Verhoeven (1979) suggested that Ruppia maritima was particularly intolerant while Ruppia cirrhosa occurred in larger waters at more exposed but still sheltered sites. In addition, turbulent water flow resulting in resuspension of sediment can indirectly reduce Ruppia productivity due to increased turbidity (see below). Kantrup (1991) reported that Ruppia spp. can occur in areas of 'considerable' current flow, e.g. Ruppia beds fertilized in situ with phosphorus were found to grow well in currents up to 4cm/s. However, 4cm/s is considered to be negligible (see benchmark). Epiphytes and algal mats would also be lost. 

Most of the benthic infauna are found in areas of stronger currents (e.g. Arenicola marina), and many of the mobile species (e.g. amphipods, isopods, shrimp, crabs and fish) would migrate to other suitable substrata or habitats. However, where present Cerastoderma glaucum is only found in areas of weak water flow and may be lost.

Sensitivity assessment. Any change in water movement will have a considerable impact on the integrity of seagrass habitat.  A change in water flow at the level of the benchmark of 10 to 20 cm/s for more than 1 year would remove of the Ruppia maritima plants by removal of the muddy substratum and drag on the plants. Resistance is thus assessed as ‘Low’.  Once the water flow regime returned to prior conditions then recovery would probably take 2-10 years, a ‘Medium’ resilience. Overall, Ruppia maritima has a ‘Medium’ sensitivity to this pressure. 

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Ruppia maritima occurs in tidal areas, from mean high water (MHW) to mean low water (MLW).  Kantrud (1991) reported that the grass is restricted to areas exposed for a maximum of four hours daily or approximately seven hours per low tide but quickly disappeared from areas emersed for extended periods. Verhoeven (1979) stated that the Ruppia spp. had a low tolerance to drought as plants (except ripe seeds) died within a few days of being exposed to aerial conditions. Changes in emergence regime and increased aerial exposure are thus likely to result in reduced growth, productivity and the loss of the upper portion of the population.

Other components of the community might also be affected. For instance Cerastoderma glaucum is thought to be intolerant of changes in emergence and may be lost. Hydrobia spp. on the other hand inhabit salt marshes and are tolerant of emersion. Gammarids and isopods can either migrate to deeper water, burrow in the sediment of find shelter in damp weed to avoid aerial exposure. Algal mats retain water, and while their surface may be bleached or desiccate in hot weather, they are likely to recover quickly.

Sensitivity assessment. Seagrass growing in intertidal habitats have greater tolerance to exposure to air than species inhabiting subtidal beds. Changes in emergence regime will however cause some mortality in Ruppia plants resulting in a reduction in the upper shore extent. Resistance is therefore assessed as ‘Medium’.  Recovery will be enabled by recolonization from surrounding communities located further down the shore and via the remaining seed bank.  Recovery is therefore considered to be fairly rapid resulting in a ‘Medium’ resilience score. The biotope has a ‘Medium’ sensitivity to this pressure.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

McCann (1945) noted that waves caused injury to Ruppia branches leaving broken tips incapable of survival, and Verhoeven (1979) observed that the base of leaves detached easily in turbulent water to avoid damage to the root system. However, the root system is weak (see water flow) and Ruppia beds are restricted to areas protected from wave action and with little fetch and wind induced water turbulence. Wave action also resuspends sediment, increasing turbidity and hence reducing productivity. This biotope (SS.SMp.SSgr.Rup) is found in extremely sheltered areas, therefore, and increase in wave action is likely to remove the surface vegetation and the majority of the root system.

Most lagoonal species are adapted to sheltered conditions and are likely to be adversely effected by increases in wave exposure, e.g. Gammarus insensibilis and Cerastoderma edule resulting in loss of a proportion of the population. The resident gastropods e.g. Hydrobia ulvae are unlikely to be directly affected, and will switch to alternative food supplies, however, should the increase in wave exposure be significant enough to change the sediment type, e.g. to coarse sands, they are likely to be lost. Benthic species, such as Arenicola marina can tolerate sheltered to moderately exposed conditions and would probably be little affected at the benchmark level.

Wave action also continuously mobilises sediments in coastal areas causing sediment re-suspension which in turn leads to a reduction in water transparency (Koch, 2001) (see 'changes in suspended sediments’ pressure).  Photosynthesis can be further limited by breaking waves inhibiting light penetration to the seafloor. Wave exposure can also influence the sediment grain size, with areas of high wave exposure having coarser sediments with lower nutrient concentrations.  Coarser sediments reduce the vegetative spreading of seagrasses and inhibit seedling colonization (Gray & Elliott, 2009).  Changes in sediment type can therefore have wider implications for the sensitivity of the beds on a long-term scale. 

Sensitivity assessment. Exposure models from Studland Bay and Salcombe, where seagrass beds are limited to low wave exposure, show that even a change of 3% (in significant wave height) is likely to influence the upper shore limits as well as beds living at the limits of their wave exposure tolerance (Rhodes et al., 2006; Jackson et al., 2013). The root system of Ruppia is more fragile than that of seagrass and, therefore, more likely to be damaged.  At the benchmark level, an increase in wave exposure is likely to remove surface vegetation and the majority of the root system causing mortality. Resistance is thus assessed as ‘Low’. Recovery will depend on the presence of adjacent seagrass beds and is considered to be fairly rapid scoring a ‘Medium’ resilience.  The biotope therefore scores a ‘Medium’ sensitivity to changes in wave exposure at the pressure benchmark.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Little information concerning the effects on non-synthetic compound contamination Ruppia maritima was found. Other seagrasses such as Zostera marina are known to accumulate TBT but no detrimental effects were observed in the field (Williams et al., 1994). Naphthalene, Pentachlorophenol, Aldicarb and Kepone reduce nitrogen fixation and may affect Z. marina viability. TBT contamination is likely to adversely affect grazing gastropods resulting in increased algal growth, reduced primary productivity and potential smothering of the biotope. 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.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Little information was found on the effects of hydrocarbon contamination from on Ruppia plants. However healthy populations of Zostera sp. have been observed in the presence of long-term, low level, hydrocarbon effluent, for example in Milford Haven, Wales (Hiscock, 1987). 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 did however result in virtual disappearance of Amphipods, Tanaidacea and Echinodermata from Zostera marina beds and caused 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 more slowly and Amphipods which did not show any signs of recovery (Jacobs, 1980). 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, herbivores recolonized and the situation returned to 'normal' within a few months.

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).

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Johnston & Bird (1995) determined that Ruppia maritima was able to better tolerate the effects of herbicides compared to other aquatic plants. Although photosynthesis was reduced by 0.05 mg/l atrazine after 35 days exposure, growth was reduced at atrazine concentrations < 5mg/l but continued at 10 mg/l. However, Kantrup (1991) reported that herbicides (atrazine and alachlor) in agricultural runoff reduced Ruppia growth and biomass in Chesapeake Bay and noted that 1.0 ppm of atrazine had been used to control Ruppia growth in wetlands. Only small numbers of plants survived 4 years after treatment with the herbicide 2,4 D-ester at 112 kg/ha (Kantrup, 1991). Cole et al. (1999) suggested that herbicides were, not surprisingly, very toxic to algae and macrophytes. Similarly, most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods, isopods, mysids, shrimp and crabs) as well as fish (Cole et al., 1999). For example, Lindane was shown to be very toxic to gobies (Gobius spp.: see Pomatoschistus minutus) (Ebere & Akintonwa, 1992) .

Synthetic chemicals found in agricultural, urban and industrial discharges are likely to adversely affect the biotope. Herbicides in particular are likely to reduce growth and productivity of Ruppia beds, and may result in mortality. In addition, loss of particularly intolerant crustaceans may result in unchecked growth of epiphytes, which would again reduce photosynthesis and productivity of Ruppia plants.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence 

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

The effects of oxygen concentration on the growth and survivability of Ruppia maritima are not reported in the literature. However, Ruppia spp. favour aerobic sediments with low levels of sulphides and free H₂S but will grow in reduced conditions, since the leaves supply oxygen to the roots. Senescence and loss of stems can coincide with increases in H₂S in the sediment and may be a factor regulating the decrease in Ruppia species in hot summer months (Kantrup, 1991). Germination may also be affected by oxygen levels and seeds in poorly oxygenated sediments lie dormant until the next year (Kantrup, 1991). However, the presence of Ruppia in reduced sediment suggests that it would tolerate low oxygen levels comparable to the benchmark, especially since photosynthesis produces oxygen.

Mud snails are relatively tolerant of reduced hypoxic muds, and can tolerate aerial exposure for over a week, suggesting that they are capable of anaerobic respiration. Benthic infaunal species are probably tolerant of hypoxia, e.g. Arenicola marina which can tolerate 9 days without oxygen (Hayward, 1994) and Cerastoderma glaucum which tolerates 84 hrs in the absence of oxygen (Boyden, 1972). Most polychaetes are capable of anaerobic metabolism, while mobile fish and gobies migrate out of the affected area in response to decreasing oxygen levels (Diaz & Rosenberg, 1995). Small mobile shrimp, amphipods and isopods will probably also migrate out of the affected area. 

Sensitivity assessment. Therefore, the Ruppia stands and benthic infauna will probably tolerate hypoxia at the level of the benchmark but increased epiphyte growth due to reduced numbers but not loss of grazers, may reduce Ruppia spp. productivity. However, species richness is likely to decline. Therefore, a resistance of 'High' is recorded, with a resilience of 'High', and resultant sensitivity of 'Not sensitive' at the benchmark level.

Nutrient enrichment

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

Evidence

In mesocosm experiments, Ruppia maritima was shown to increase shoot production by >300% over controls after the addition of 10µM water column NO₃-N /day (Burkholder et al., 1994). Burkholder et al. (1994) went on to suggest that Ruppia maritima and Halodule wrightii effectively control nitrate uptake and could be transplanted to replace Zostera marina in nitrate-enriched waters where the eelgrass had disappeared. Therefore, it appears that Ruppia spp. will benefit from low nutrient enrichment. Nutrient enrichment is known to have indirect adverse effects. Nutrient enrichment stimulate epiphyte growth, which interfere with the nutrient exchange across the Ruppia leaves and shade out light, reducing primary productivity, growth and reproduction. Similarly, nutrients stimulate phytoplankton blooms that compete for nutrients but more importantly increase the turbidity (see water clarity). and absorb light, reducing Ruppia productivity. Twilley et al. (1985) found that epiphyte growth in nutrient enriched conditions reduced the light incident on Ruppia leaves by >80%, resulting in significant decreases in macrophyte biomass at medium to high levels of enrichment (0.86 and 1.68 g N /m²/ day respectively). Ruppia maritima production collapsed after 6 weeks at high nitrogen levels. However, the epiphytic growth only resulted in loss of macrophytes due to the additional turbidity caused by the phytoplankton (Twilley et al., 1985). Kantrup (1991) concluded that while nutrients can stimulate growth, growth is severely limited by phytoplankton and epiphytes in eutrophic conditions.

Ruppia spp. can tolerate organic rich sediments and has been reported to grow in extremely reduced sediments since leaves supply oxygen to the root system (Kantrup). However, Azzoni et al. (2001) noted that the oxygen supply to the roots detoxified the sulphide levels around the root system but that once this capacity was exhausted, perhaps due to additional nutrients or reduction in plant productivity, sulphide rapidly built up and killed the root system and hence the plant.

The nationally scarce foxtail stonewort Lamprothamnium papulosum was reported to be absent where the total phosphate concentration was greater than 100µg/l and may be lost due to nutrient enrichment (Bamber et al., 2001). Most grazing and suspension and deposit feeding members of the community will probably benefit from the increased epiphyte and phytoplankton productivity, as would their predators.

Sensitivity assessment.  The productivity and growth of the Ruppia beds are likely to be reduced by nutrient enrichment, and a proportion of the population lost, while significant increase in nutrients and subsequent eutrophication may result in loss of the biotope.  However, at the benchmark of this pressure (compliance with WFD ‘good’ status) the biotope is 'Not sensitive' by default.

Organic enrichment

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

Evidence

Organic enrichment may lead to eutrophication with adverse environmental effects including deoxygenation, algal blooms and changes in community structure (see ‘nutrient enrichment’ pressure).  Evidence on the effects of organic enrichment on Ruppia is limited, but abundant for other seagrass species. Neverauskas (1987) investigated the effects of discharged digested sludge from a sewage treatment on Posidonia spp. and Amphibolis spp. in South Australia. Within 5 years the outfall had affected an area of approximately 1900 ha, 365 ha of which were completely denuded of seagrasses.  The author suggests that the excessive growth of epiphytes on the leaves of seagrasses was a likely cause for reduced abundance.  A subsequent study by Bryars and Neverauskas (2004) determined that 8 years after the cessation of sewage output, total seagrass cover was approximately 28% of its former extent. While these results suggest that seagrasses can return to a severely polluted site if the pollution source is removed, they also suggest that it will take many decades for the seagrass community to recover to its former state. The effects of organic enrichment from fish farms were investigated on Posidonia oceanica seagrass beds in the Balearic Islands (Delgado et al., 1999).  The fish culture had ceased in 1991; however, seagrass populations were still in decline at the time of sampling.  The site closest to the former fish cages showed a marked reduction in shoot density, shoot size, underground biomass, sucrose concentration and photosynthetic capacities.  The shoot also had high P-concentration in tissues and higher epiphyte biomass compared to the other sites.  Since water conditions had recovered completely by the time of sampling, the authors suggest that the continuous seagrass decline was due to the excess organic matter remaining in the sediment (Delgado et al., 1999). It should be noted that coastal marine sediments where seagrasses grow are often anoxic and highly reduced due to the high levels of organic matter and slow diffusion of oxygen from the water column to the sediment.  Seagrasses are adapted to these conditions but if the water column is organically enriched, plants are unable to maintain oxygen supply to the meristem and die fairly quickly.  The enrichment of the water column could therefore significantly increase the sensitivity of seagrasses to this pressure.

Sensitivity assessment. No evidence on the effects of organic enrichment on Ruppia beds was found.  The effects might be similar to those associated with nutrient enrichment but no assessment has been made. 

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure. Ruppia maritima will be unable to recover from a permanent loss of habitat resulting in a ‘Very Low’ resilience score.  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  Adjacent habitats and species populations may be indirectly affected where meta-population dynamics and trophic networks are disrupted and where the flow of resources e.g. sediments, prey items, loss of nursery habitat etc. is altered

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Ruppia maritima occurs almost exclusively in shallow and sheltered coastal waters anchored in sandy and muddy bottoms.  A physical change to another seabed type, i.e. from sedimentary to hard rock substratum would result in loss of the Ruppia bed and its associated community. As a permanent change, there is no opportunity for recovery without intervention.  Therefore, the resistance is 'None', and resilience is 'Very Low', resulting in a sensitivity of 'High'. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

This biotope is only recorded from muds and muddy sands. Therfore, a change from muddy sand to mud or will probably have no effect. However, a change to coarser sediments, e.g. coarse sands, or mixed will probably adversely affect the biotope, resulting in exclusion of Ruppia. Therefore, a change to another seabed type will however result in a permanent loss of suitable habitat for this biotope. Resistance is thus assessed as ‘None’.  As this pressure represents a permanent change, recovery is unlikely without intervention and resilience is assessed as ‘Very low’.  The habitat therefore scores a ‘High’ sensitivity. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

The extraction of sediments to 30 cm (the benchmark) will result in the removal of every component of Ruppia beds.  Roots and rhizomes are buried no deeper than 20 cm below the surface (see ‘abrasion’ pressures).  Resistance is therefore assessed as ‘None’ for all seagrass biotopes and resilience is considered ‘Very Low’ resulting in a ‘High’ sensitivity score. 

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

d'Avack et al. (2014) reviewed the impacts of physical damage (abrasion and penetration) seagrasses to the pressure benchmark. The report found that a large amount of research had been conducted with however the majority of studies focusing on Zostera species. The sensitivity of Ruppia to this pressure is thus largely based on expert judgement but with a high level of confidence due to the morphology of the plant. Similar to Zostera species, Ruppia maritima has a shallow and weak root system and is incapable of vertical rhizome growth. Seagrasses are not physically robust. Ruppia stems and leaves are damaged by wave action or water turbulence and the root system is shallow and weak (Verhoeven, 1979; Kantrup, 1991). Therefore, it is likely that Ruppia spp. are intolerant of physical disturbance and that a proportion of the vegetation may be removed and rhizomes broken by any physical disturbance, e.g. trampling, anchoring, power boating and potting. However, in subtropical areas wintering wildfowl were reported to consume entire stands of Ruppia spp. which grew back in a few weeks (Kantrup, 1991). Similarly, Steiglitz (1966, cited in Kantrup, 1991) suggested that wildfowl could consume 50% of the standing crop without damaging the Ruppia bed. 

Benthic infauna such as polychaetes (e.g. Arenicola marina or Pygospio elegans) are partly protected from abrasion due to their infaunal habit but a proportion are likely to be killed by any mechanical disturbance that penetrates the sediment (e.g. anchors). Similarly, the shell of Cerastoderma glaucum is relatively thin and individuals are likely to be damaged or killed by abrasion. Macroalgae and relatively flexible and unlikely to be damaged. However, resident grazers (e.g. gammarid amphipods, isopods, or gastropods) are likely to be killed by direct physical contact, although they are generally small enough to be swept aside, or able swimmers and most will probably escape.

Sensitivity assessment.  Ruppia beds are particularly fragile and likely to be damaged by physical disturbance, so that a resistance of 'Low' is suggested. However, rhizomes and the seed bank remain, recovery will be rapid, so that their resilience is probably 'High', resulting in a sensitivity of 'Low' . 

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

d'Avack et al. (2014) reviewed the impacts of physical damage (abrasion and penetration) seagrasses to the pressure benchmark. The report found that a large amount of research had been conducted with however the majority of studies focusing on Zostera species. The sensitivity of Ruppia to this pressure is thus largely based on expert judgement but with a high level of confidence due to the morphology of the plant. Similar to Zostera species, Ruppia maritima has a shallow and weak root system and is incapable of vertical rhizome growth. Seagrasses are not physically robust. Ruppia stems and leaves are damaged by wave action or water turbulence and the root system is shallow and weak (Verhoeven, 1979; Kantrup, 1991). Therefore, it is likely that Ruppia spp. are intolerant of physical disturbance and that a proportion of the vegetation may be removed and rhizomes broken by any physical disturbance. However, in subtropical areas wintering wildfowl were reported to consume entire stands of Ruppia spp. which grew back in a few weeks (Kantrup, 1991). Similarly, Steiglitz (1966, cited in Kantrup, 1991) suggested that wildfowl could consume 50% of the standing crop without damaging the Ruppia bed. 

Benthic infauna such as polychaetes (e.g. Arenicola marina or Pygospio elegans) are partly protected from abrasion due to their infaunal habit but a proportion are likely to be killed by any mechanical disturbance that penetrates the sediment (e.g. anchors). Similarly, the shell of Cerastoderma glaucum is relatively thin and individuals are likely to be damaged or killed by abrasion. Macroalgae and relatively flexible and unlikely to be damaged. However, resident grazers (e.g. gammarid amphipods, isopods, or gastropods) are likely to be killed by direct physical contact, although they are generally small enough to be swept aside, or able swimmers and most will probably escape.

Sensitivity assessment.  Ruppia beds are particularly fragile and likely to be damaged by physical disturbance.  Penetrative activities (e.g. demersal trawls and dredges) are likely to damage the rhizomes directly (as they lie within the top 5 cm of sediment) so that a resistance of 'Low' is suggested. However, where rhizomes and the seed bank remain, recovery will be rapid, so that their resilience is probably 'High', resulting in a sensitivity of 'Low'.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Water clarity is a vital component for seagrass beds as it determines the depth-penetration of photosynthetically active radiation of sunlight.  Increased turbidity results from increases in dissolved organics (e.g. humic acids or gelbstoff), organic particulates and suspended sediment, or blooms of phytoplankton and zooplankton. Seagrasses have light requirements an order of magnitude higher than other marine macrophytes making water clarity a primary factor in determining the maximum depth at which plants can occur.  Ruppia spp. require high light levels and only normally develop well in clear water and are always reduced or absent from turbid waters (Verhoeven, 1979). Joanen & Glasgow (1965) found that plants preferred turbidity levels less than 25-55 ppm (equivalent to 25-55 mg/l).  Wetzel & Penhale (1983) compared the photosynthetic parameters of Ruppia maritima and Zostera marina.  Ruppia maritima was found to be photosynthetically less efficient in low levels of underwater light compared to Zostera marina.  Ruppia maritima has also relatively high ratio of chlorophyll α to chlorophyll β suggesting that it is less adapted to low-light environments than other seagrasses (Evans et al., 1986).  A shading experiment by Congdon & McComb (1979) on Ruppia maritima determined that a 40% reduction in light availability resulted in a 50% reduction in standing crop.

Sensitivity assessment. Turbidity is an important factor controlling production and ultimately survival and recruitment of Ruppia plants. Populations are likely to survive short-term increases in turbidity however a prolonged increase in light attenuation e.g. a change from clear (<10 mg/l) to intermediate (10-100 mg/l) water clarity at the benchmark level for a year, especially at the lower depths of distribution, will probably result in loss or damage of the population. Resistance is therefore assessed as ‘Low’.  A loss of Ruppia beds will promote the re-suspension of sediments, making recovery unlikely as seagrass beds are required to initially stabilise the sediment and reduce turbidity levels (Van der Heide et al., 2007).  A high turbidity state appears to be a highly resilient alternative stable state in seagrass habitats; hence return to the seagrass biotope may be prolonged resulting in ‘Low’ resilience, yielding a ‘High’ sensitivity score. 

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Early life stages of seagrass, smaller in size than adult plants, are most vulnerable to this pressure as even a small load of added sediment will lead to the complete burial. A deposit of 5 cm of fine material will shade and damage buried leaves and stems resulting in loss of a proportion of the vegetation above the sediment surface, including the algal mats and epiphytes. Kantrup (1991) suggested that, although most seeds occur in the top 5 cm of sediment, seeds buried more than 10 cm in sediment would probably not germinate, so that smothering by 5 cm of sediment may reduce germination. Similar results were found by Bonis & Lepart (1994). Smothering in early spring may have a marked effect of the growth of Ruppia spp. stands, especially annuals that are primarily dependant on seed. The timing of the siltation event also plays a role in particular for intertidal beds.  At low tide, the seagrass bed is exposed with plants lying flat on the substratum. The addition of material would immediately smother the entire plant and have a greater impact on leaves and stem than if added on plants standing upright.  The resistance of intertidal beds to this pressure may thus vary with time of day. Most members of the invertebrate fauna will probably be able to burrow through or avoid smothering. However some grazers may be lost.  Cockles (Cerastoderma sp.) have limited ability to burrow and may also be adversely affected. 

Sensitivity assessment. Above studies suggest that Ruppia maritima is intolerant of smothering. At the level of the benchmark (5 cm of fine material added to the seabed) some mortalities may occur resulting in a 'Medium' resistance score.  In addition, Ruppia beds are restricted to low energy environments, suggesting that once the silt is deposited, it will remain in place for a long period of time so habitat conditions will not reduce exposure. Resilience is therefore assessed as 'Medium'. The biotope is considered to have a ‘Medium’ sensitivity to siltation at the pressure benchmark.

Smothering and siltation rate changes (heavy)

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

Evidence

Ruppia maritima is intolerant of smothering by excessive siltation. In addition, Ruppia beds are restricted to low energy environments, suggesting that once the silt is deposited, it will remain in place for a long period of time so habitat conditions will not reduce exposure.  Resistance is assessed as ‘None’ as all individuals exposed to siltation at the benchmark level are predicted to die and consequent resilience as ‘Low’ to ‘Very Low’.  Sensitivity based on combined resistance and resilience is therefore assessed as ‘High’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

No evidence

Underwater noise changes

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

Evidence

Not relevant

Introduction of light or shading

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

Evidence

Ruppia spp. require high light levels and only normally develop well in clear water and are always reduced or absent from turbid waters (Verhoeven, 1979). Joanen & Glasgow (1965) found that plants preferred turbidity levels less than 25-55 ppm (equivalent to 25-55 mg/l).  Wetzel & Penhale (1983) compared the photosynthetic parameters of Ruppia maritima and Zostera marina.  Ruppia maritima was found to be photosynthetically less efficient in low levels of underwater light compared to Zostera marina.  Ruppia maritima has also relatively high ratio of chlorophyll α to chlorophyll β suggesting that it is less adapted to low-light environments than other seagrasses (Evans et al., 1986).  A shading experiment by Congdon & McComb (1979) on Ruppia maritima determined that a 40% reduction in light availability resulted in a 50% reduction in standing crop. Kantrud (1991) noted that poor insolation due to fog, mountains and short days reduced Ruppia sp. productivity. While Ruppia beds can survive short-term changes in turbidity, Kantrud (1991) concluded that the control of turbidity levels was crucial for the managment of Ruppia beds.  

Sensitivity assessment. It is likely that 'shading' of a Ruppia bed would result in reduced growth and productivity, potential competition with shade tolerant species and, if prolonged, loss of the bed in a 'shaded' area.  Therefore, a resistance of 'Low' is given, with a resilience of 'High', resulting in a sensitivity of 'Low'.  However, if the 'shading' was caused by a permanent structure, then the resilience would be 'Very low' and sensitivity 'High'. 

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Not relevant. This pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed.  But seed dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

Not relevant. The potential effects of vessel grounding are covered under abrasion above. 

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Not relevant

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Translocation of seagrass seeds, rhizomes and seedlings is a common practice globally to counter the trend of decline of seagrass beds.  Zostera marina is the seagrass species most commonly translocated. Williams and Davis (1996) found that levels of genetic diversity of restored eelgrass Zostera marina beds. The loss of genetic variation can lead to lower rates of seed germination and fewer reproductive shoots, suggesting that there might be long-term detrimental effects for population fitness.  Williams (2001) affirms that genetic variation is essential in determining the potential of seagrass to rapidly adapt to a changing environment. Transplanted populations are therefore more sensitive to external stressors such as eutrophication and habitat fragmentation, with reduced community resilience, compared to natural populations (Hughes & Stachowicz, 2004).

Even though restoration efforts tend to focus on Zostera marina, transplantations of Ruppia maritima (Bird et al., 1994) have also been undertaken and large areas of wetland planted with Ruppia to feed wildfowl in the USA (Kantrud, 1991). Similar reductions in genetic diversity are possible. 

Translocation also has the potential to transport pathogens to uninfected areas (see 'introduction of microbial pathogens' pressure).  The sensitivity of the ‘donor’ population to harvesting to supply stock for translocation is assessed for the pressure ‘removal of target species’. No evidence was found for the impacts of translocated beds on adjacent natural seagrass beds.  However, it has been suggested that translocation of plants and propagules may lead to hybridisation with local wild populations.  If this leads to loss of genetic variation there may be long-term effects on the potential to adapt to changing environments and other stressors. 

Sensitivity assessment: Presently, there is no evidence of loss of habitat due to genetic modification and translocation of Ruppia species. 

Introduction or spread of invasive non-indigenous species

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

Evidence

The effects on native species on seagrass species was reviewed by d’Avack et al. (2014). The review reported several non-native invasive plants as well as invertebrate species negatively impacting British seagrass beds. These included Sargassum muticum, Spartina anglica, Codium fragile ssp. Tomentosoides, Didemnum vexillum, Urosalpinx cinerea and Magallana gigas.

Sensitivity assessment. d’Avack et al. (2014) found that invasive flora had the greatest impact on seagrass bed but pointed out extensive knowledge gaps on how invasive species influence the health of Zostera beds in UK waters. More research is thus needed in order to fully comprehend this pressure. Resistance is assessed as 'Low'. Return to ‘normal’ conditions is highly unlikely if an invasive species would come to dominate the biotope. Indeed recovery would only be possible if the majority of the NIS were removed (through either natural or unnatural process) to allow the re-establishment of other species. Therefore actual resilience is assessed as ‘Low’ resulting in an overall ‘High’ sensitivity score.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Kantrup (1991) reported possible pathogenic effects of fungi, that produce 'tubercles' on the Ruppia leaves. Kantrup (1991) also states that 'vegetative reproduction usually allows Ruppia spp. to survive Rhizoctonia infestations' and that Ruppia spp. probably suffer less from diseases than other aquatic angiosperms.

Sensitivity assessment. Therefore, the above evidence suggests a resistance of 'High', a resilience of 'High' and, hence, a sensitivity of 'Not sensitive'. 

Removal of target species

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

Evidence

In accessible areas, extraction of Arenicola marina for bait is likely to disturb the sediment and benthic infauna, although the Ruppia stands themselves would probably recover quickly. Similarly, Arenicola marina populations are thought to recover rapidly, although in isolated areas recovery may take longer due to the lack of a pelagic larvae.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

In accessible areas, extraction of Arenicola marina for bait is likely to disturb the sediment and benthic infauna, although the Ruppia maritima stands themselves would probably recover quickly. Similarly, Arenicola marina populations are thought to recover rapidly, although in isolated areas recovery may take longer due to the lack of pelagic larvae. Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope. 

Incidental removal of the key characterizing seagrass species and associated species would alter the character of the biotope. The biotope is characterized by the presence of beds of Ruppia maritima, these provide habitat structure and may also modify local habitats through changes in water flow and the trapping of sediments. The loss of the turf due to incidental removal as by-catch would, therefore, alter the character of the habitat and result in the loss of habitat structure and species richness. The ecological services such as primary and secondary production and habitat engineering provided by Ruppia maritima and the associated species would also be lost.

Sensitivity assessment. Incidental removal of Ruppia spp. as by-catch would be detrimental, altering the character of the biotope and removing the habitat structure, and could lead to reclassification of the biotope where extensive removal occurs.  Therefore, resistance is considered to be 'None', resilience 'High'  (but see caveats in the resilience section) and sensitivity 'Medium'.