Sediment characteristics of a restored saltmarsh and ... - Springer Link

Hydrobiologia (2011) 672:79–89 DOI 10.1007/s10750-011-0755-8

THAMES ESTUARY

Sediment characteristics of a restored saltmarsh and mudflat in a managed realignment scheme in Southeast England Margaret Kadiri • Kate L. Spencer • Catherine M. Heppell • Paul Fletcher

Published online: 24 May 2011 Ó Springer Science+Business Media B.V. 2011

Abstract Sediment characteristics and vegetation composition were measured in a restored and natural saltmarsh and mudflat at Wallasea Island managed realignment scheme (Essex, UK) from January to December 2007. The similar sediment characteristics in the restored and natural mudflat indicated that the sediment in the restored mudflat was approaching natural conditions. However, the sediment characteristics in the restored saltmarsh were not becoming similar to those in the natural saltmarsh. The sediment moisture content, organic matter content and porosity were lower while the sediment bulk density, salinity and pH were higher in the restored compared to the natural saltmarsh. The dissimilarities were mainly due to differences in the vegetation abundance and organic matter content. Although, 18 months after restoration the restored saltmarsh was only sparsely vegetated and there was no net change in the sediment characteristics, the occurrence of Salicornia europaea L. demonstrated that pioneer

Guest editors: R. J. Uncles & S. B. Mitchell / The Thames Estuary and Estuaries of South East England M. Kadiri (&) K. L. Spencer C. M. Heppell Department of Geography, Queen Mary, University of London, London, UK e-mail: [email protected] P. Fletcher School of Biological and Chemical Sciences, Queen Mary, University of London, London, UK

saltmarsh vegetation establishment preceded the development of sediment characteristics. Keywords Managed realignment Saltmarsh Mudflat Dredged sediment Restoration Beneficial re-use

Introduction Concerns over the loss of saltmarshes due to rising sea-levels coupled with the need to comply with the European Union Habitat Directive which maintains a no-net-loss policy in habitat area has led to the current coastal management strategy of managed realignment. Managed realignment, which is becoming a common practice throughout Europe, aims to restore saltmarshes in coastal areas by constructing a new sea wall further inland and deliberately breaching the existing seawall, allowing the tidal inundation of low-lying coastal agricultural land (French, 2006). It is intended that the reintroduction of tidally dominated hydrology will promote saltmarsh development in the inundated area, which in turn will dissipate wave energy, thus providing a sustainable coastal flood defence. Managed realignment has also been implemented by beneficially re-using dredged estuarine sediment to raise the land elevation in lowlying areas to a suitable height relative to the tidal frame in order to aid saltmarsh vegetation establishment (Crooks & Pye, 2000; French, 2006).

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The main factors controlling the development of salt marsh habitat are generally considered to be site elevation (and hence hydroperiod) and hydrodynamics. Consequently, these are important design parameters for managed realignment schemes (Cooper et al., 2004). However, a much wider range of abiotic and biotic factors are likely to affect the distribution and patterning of vegetation including seed availability, and sediment pH, salinity, nutrient availability and anoxia (e.g. Erfanzadeh et al., 2010). Indeed, poorer- and slower-than-expected salt marsh development at managed realignment sites has frequently been attributed to poor drainage and anoxia (e.g. French, 2006; Brown, 2008; Wolters et al., 2008). Sediment drainage, and hence oxygen availability in the root zone is controlled not only by elevation and hydroperiod, but also site topography, and physicochemical sediment characteristics such as mineralogy, porosity, bulk density, pore structure and organic matter content (Crooks & Pye, 2000; Crooks et al., 2002 Montalto & Steenhuis, 2004). Consequently, the development of such sediment characteristics may have significant impact on the establishment of salt marsh vegetation at a site. Managed realignment schemes provide important wider ecosystem functions such as carbon sequestration, contaminant storage and nutrient cycling (Cave et al., 2005; Andrews et al., 2006; Shepherd et al., 2007) and sediments are of fundamental importance as they are the substrates which enable these schemes to fulfil these functions. For example, sediments contain organic matter which determines carbon and nitrogen availability, adsorption of particle reactive contaminants, cation exchange capacity and sediment stability (Stevenson, 1994; Santin et al., 2009). Sediment texture also affects nutrient availability (Spencer et al., 2008). However, sediment characteristics have rarely been considered in previous studies examining the development of managed realignment schemes, with most studies focusing on the restoration of saltmarsh flora and fauna communities (Bolam et al., 2004; Bolam & Whomersley, 2005; Garbutt et al., 2006). The few studies which have examined changes in sediment characteristics have only provided a snap-shot several years following restoration (e.g. Edwards & Proffitt, 2003; Spencer et al., 2008) and detailed short-term changes in the sediment characteristics have been largely overlooked to date (Blackwell et al., 2004). This lack of baseline data

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presents an obstacle to the success of future schemes for the beneficial re-use of dredged sediments as there is insufficient understanding of the effect of these changes in sediments characteristics on saltmarsh plant development. In order to assess the restoration of ecological attributes in restored saltmarshes, studies have compared vegetation and sediment characteristics in restored and natural saltmarshes (Craft et al., 1999, 2002; Shafer & Streever, 2000; Edwards & Proffitt, 2003) and the majority found progressions towards natural saltmarsh conditions but with considerable differences in many of the key sediment characteristics (such as dissimilar textures, organic matter deficiency and poor nutrient availability in the restored saltmarsh). As a result, Craft et al. (2002) concluded that it may take several decades before most restored saltmarshes would function in the same capacity as natural saltmarshes. This study (1) examines the changes in dredged sediment characteristics in the restored saltmarsh and mudflat at Wallasea Island managed realignment site over a 12-month period in order to evaluate how the sediment characteristics develop in the short-term; (2) compares the sediment characteristics in the restored saltmarsh and mudflat with those in an adjacent, paired natural saltmarsh and mudflat in order to assess the extent to which the restored saltmarsh and mudflat were approaching natural conditions; (3) assesses saltmarsh vegetation in the restored and natural saltmarshes in order to highlight the links between vegetation development and the development of sediment characteristics.

Materials and methods Site description Wallasea Island is located in Essex, Southeast England and it is bordered to the north and south by the Crouch and Roach estuaries, respectively (Fig. 1). The managed realignment site (108 ha) is situated on the north eastern bank of Wallasea Island and south shore of the Crouch estuary (Fig. 2). Before construction, the site was a part of an 820 ha arable farm. In May 2005, a new seawall was constructed (Fig. 2) and approximately 700,000 tonnes of dredged sediment sourced from maintenance


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Fig. 1 Map of the southeast coast of England showing the location of Wallasea Island managed realignment site on River Crouch

dredging operations at Harwich Port was imported into the area between the new and old seawalls which now forms the managed realignment site (Fig. 2). The site was divided into three hydrodynamically separate restored areas with each area exchanging water with the adjacent estuary but no flows between neighbouring areas of the wider realignment site. The old seawall was breached at six locations to allow for tidal inundation of the managed realignment site in July 2006. Field procedure Sediments were collected from four locations for the purpose of this study: the restored saltmarsh (RSM), the restored mudflat (RMF), the adjacent natural saltmarsh (NSM) and natural mudflat (NMF) (Fig. 2). The natural saltmarsh and restored saltmarsh have elevations of between 2.5–3.0 m ODN (Ordnance Datum Newlyn) and 2.3–2.5 m ODN, respectively, whilst the natural and restored mudflats are both at 1.3–1.5 m ODN (ABPmer, 2004). All the elevations are relative to ODN. The sampling locations within the managed realignment sites were chosen based on their close proximity to the natural saltmarsh and mudflat and the elevation of the restored saltmarsh which was within the tidal limit for saltmarsh plant

establishment in the Crouch estuary. The elevation most suitable for saltmarsh plant establishment in the UK lies between mean high water neap (MHWN) and mean high water spring (MHWS) (French, 2006) and the MHWN and MHWS tide levels in the Crouch estuary are at 1.85 m ODN and 2.85 m ODN, respectively (ABPmer, 2004). In the restored saltmarsh, natural saltmarsh and restored mudflat, sediment samples were collected from January to December 2007; and from April to December 2007 in the natural mudflat. All sampling took place during a 3-h period centred around low tide. Due to the limited accessibility, sampling was carried out along transects in the restored saltmarsh, restored mudflat and natural mudflat (Fig. 2). Sediment samples (0–2 cm depth) were collected monthly at 10 replicate sampling points, each 5 m apart. Due to ease of access, sediment samples were collected from 10 random points in a 169 m2 area in the natural saltmarsh. In October 2007, vegetation diversity and abundance at each site was determined along a transect which extended from the natural and restored mudflat up the shore along an elevation gradient. The vegetation cover was recorded in 10 randomly placed 0.5 m2 quadrats at 0.2 m vertical interval along the transect. The elevation of each sampling point

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Fig. 2 Map of Wallasea Island managed realignment site. The insert box denotes the location of the detailed section showing the four sampling locations

A re a A w e s t Are a A e a s t

Are a B

W al la s e a I sla n d

Natural Mudflat NMF transect

NSM sampling area

ld O e nc fe de flo od

t sec tran M RS Restored Saltmarsh

de fe nc e

relative to ODN was also measured. Within each quadrat, vegetation species were identified. The average species abundance was used here. Laboratory procedure The sediment was analysed for gravimetric moisture content, organic matter content by loss on ignition

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R M F

od flo N ew

Restored Mudflat

tr an se ct

Natural Saltmarsh

and bulk density using the methods described in Rowell (1994). Porosity was calculated using the bulk density measurements and assuming a particle density of 2.65 g cm-3 (Craft et al., 2002). A 1:1 sediment to deionised water solution was agitated for 30 min before measuring pH (Craft et al., 2002; Fearnley, 2008). Absolute particle size analysis was established by removal of organic matter with


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hydrogen peroxide followed by analysis of the \2 mm size fraction using a Beckman laser diffraction analyser. The chloride content of the sediment pore water was measured as a proxy for salinity using a 1:1 sediment to deionised water solution which was agitated for 10 min and filtered through pre-rinsed 0.45 lm SuporÒ hydrophilic polyethersulphone membrane filter paper. Chloride was analysed using a Dionex ICS-2500 chromatographic system with a Dionex ED-50 conductivity detector.

the restored mudflat compared to the restored saltmarsh particularly in the summer months (Fig. 3). The organic matter content in the restored saltmarsh and mudflat were comparable (Fig. 3) with ranges from 6 to 14% and 7 to 16%, respectively. The sediment pH in the restored mudflat and saltmarsh were weakly alkaline (between pH 6.6 and 8.2) with the sediment pH lower in the restored mudflat compared to the restored saltmarsh (Fig. 3).

Statistical analysis

Comparison of sediment characteristics in restored and natural saltmarsh and mudflat

Statistical analysis was conducted using the SPSS statistical package (Version 16). A Levene test of homogeneity of variance showed that several datasets had unequal variances. In addition, the datasets were not normally distributed and the assumptions of the parametric t test could not be met. Hence, a Mann– Whitney test was employed to test for differences in the sediment characteristics between the restored and natural saltmarsh and between the restored and natural mudflat.

Results Changes in sediment characteristics in the restored mudflat and restored saltmarsh The sediment organic matter content, pH, bulk density and porosity remained relatively constant except for the statistically significant increase in organic matter content in the restored saltmarsh between March and April (P \ 0.05) (Fig. 3). Seasonal changes were observed in the sediment moisture content and salinity in the restored saltmarsh with lower sediment moisture contents and higher salinities observed in the summer months compared to the winter months (Fig. 3). The seasonal variation in salinity was less evident in the restored mudflat in comparison with the restored saltmarsh (Fig. 3). No seasonal variation was observed in the sediment moisture content in the restored mudflat (Fig. 3). The sediment moisture content was higher in the restored mudflat compared to the restored saltmarsh (Fig. 3) ranging from 79 to 183% and 11 to 107%, respectively. The sediment salinity was lower in

The clay and silt content of sediment in the restored saltmarsh ranged from 35 to 38% and 47 to 52%, respectively, higher than the sediment clay and silt content in the natural saltmarsh, which ranged from 24 to 34% and 28 to 39%, respectively (Table 1). Moisture content, organic matter content and porosity of the sediments were significantly lower in the restored compared to the natural saltmarsh (P \ 0.05, n = 120), conversely bulk density, pH and sediment salinity were significantly higher at the restored site (P \ 0.05, n = 120). In contrast, there was no significant difference in sediment moisture content, organic matter content, pH and salinity (P [ 0.05, n = 90) between the restored and natural mudflat (Table 1).

Vegetation development in the site The vegetation species diversity in the natural saltmarsh was higher than that in the restored saltmarsh, with only Salicornia europea L. being present in the restored saltmarsh (Table 2). The stands of this vegetation were sparsely distributed with a percentage cover of only 11% (Table 2). In comparison, eight vegetation species were found in the natural saltmarsh including Salicornia europaea L., Suaeda maritima (L.) Dumort., Spartina anglica C. E. Hubb., Atriplex portulacoides L., Aster tripolium L. and Sarcocornia perenne (Mill.) A. J. Scott; with Puccinellia maritima (Huds.) Parl. and Limonium vulgare Mill. comprising the dominant species (Table 2).

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Hydrobiologia (2011) 672:79–89 Restored saltmarsh Restored mudflat Natural saltmarsh Natural mudflat m

200 150 100 50

Restored saltmarsh Restored mudflat Natural saltmarsh Natural mudflat

24

% Organic matter

0

18

12

Ja n Fe b M ar Ap ril M ay Ju n

Ju l Au g Se pt O ct N ov D ec


6

Months

Months Restored saltmarsh Restored mudflat Natural saltmarsh Natural mudflat m

30000

n Ja

Au g Se pt O ct N ov D ec

l Ju

M

Ap

M

Fe

Ja

ril ay Ju n

0 b

6 ar

10000

n

7

M ar Ap ril M ay Ju n Ju l Au g Se pt O ct N ov D ec

20000

b

pH

Cl (mg l -1)

8

Restored saltmarsh Restored mudflat Natural saltmarsh Natural mudflat

Fe

9

Months

Months

Restored saltmarsh Natural saltmarsh

1.8

Restored saltmarsh Natural saltmarsh

1.2 1.0

1.4 1.2

0.8

Porosity

Bulk density (g cm -3)

1.6


% Moisture content

250

1.0 0.8

0.6 0.4

0.6 0.4

0.2

0.2

ril M ay Ju n Ju l Au g Se pt O ct N ov D ec

Ap

b

ar M

Fe



Months

Ja n

0.0

0.0

Months

Fig. 3 Sediment moisture content, organic matter content, pH, Cl, bulk density and porosity (mean ± standard deviation, n = 10) in the natural and restored saltmarsh and mudflat

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Table 1 Range of sediment properties of restored saltmarsh, natural saltmarsh, restored mudflat and natural mudflat

Moisture (%) Organic matter (%) -1

Bulk density (g cm ) pH -1

Restored saltmarsh

Natural saltmarsh

Restored mudflat

Natural mudflat

11–107a

67–280a

79–183b

63–170b

6–14

a

a

12–26 a

1,760–28,852

Clay (%) Silt (%) Sand (%)

9–13b

a

0.8–1.6 7.2–8.2a

Cl (mg l )

7–16

b

0.4–0.7 6.8–7.7a a

6.6–7.4b a

7.0–7.5b b

3,149–19,537b

164–18,525

1,379–13,165

35–38

24–34

34–41

47–52

28–39

47–48

50–51

12–15

27–48

11–18

9–18

30–41

a

Significant difference between sampling locations (i.e. restored and natural saltmarsh; restored and natural mudflat) at P \ 0.05 significance level

b

No significant difference

Table 2 Species diversity and percentage cover in the natural and restored saltmarshes

Species diversity

Natural saltmarsh % cover

Aster tripolium (Sea Aster) Atriplex portulacoides (Sea Purslane)

Restored saltmarsh % cover

6 2

– –

Limonium vulgare (Sea Lavender)

23

–

Puccinellia maritima (Common Saltmarsh-grass)

50

–

2

11

Salicornia europaea (Common Glasswort) Sarcocornia perenne (Perennial Glasswort) Spartina anglica (Common Cord-grass) Suaeda maritima (Annual Seablite)

Discussion Changes in sediment characteristics in restored saltmarsh and restored mudflat Seasonal changes were observed in the sediment moisture content and salinity in the restored saltmarsh (Fig. 3). Lower sediment moisture content and higher salinity in the hotter summer months compared to the winter months can be attributed to higher atmospheric temperatures and hence higher evaporation in the summer. As water evaporates from the sediment surface, the moisture content decreases and salt accumulates. In the winter, precipitation results in an increase in sediment moisture content and the dilution of accumulated salts, thereby leading to a decrease in salinity. A similar pattern of seasonal variation in the salinity of saltmarsh sediments was reported by Otero & Macias (2002). In contrast, no seasonal changes were observed in the sediment moisture content in the restored mudflat. The

8

–

10

–

2

–

seasonal variation in salinity was less evident in the restored mudflat where the elevation was lower compared to the restored saltmarsh and so the sediment moisture content was more strongly controlled by tidal inundation and associated flushing of the sediments (whereby salt which may have accumulated at low tide in the restored mudflat is flushed out at the next high tide). Sediment organic matter content results from a balance between inputs and outputs (Kentula, 2000). While inputs from saltmarsh plant litter and the accretion of organic-rich sediment from the water column are sources of organic matter, output is mainly due to decomposition by microbes and other heterotrophic organisms (Mitsch & Gosselink, 2000). The lack of variability in organic matter content in the restored mudflat and saltmarsh over the annual study period suggests that at this stage in the restoration process, the rate at which organic matter is accumulated in the sediment equals the rate of organic matter loss, and this is attributable to the

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paucity of vegetation at the restored sites (Fearnley, 2008). However, if the restored saltmarsh becomes more densely vegetated over time and additional organic matter accumulates in the sediment, the organic matter content in the restored saltmarsh may become higher than that in the restored mudflat. This can be observed in the higher organic matter content in the natural saltmarsh compared to the natural mudflat (Fig. 3). The increase in organic matter content in the restored saltmarsh in April–may be due to filamentous algal bloom in spring (Pardal et al., 2000, Martins & Marques, 2002) which then dies back and flushes out in the following months. This increase did not occur in the restored mudflat which was being frequently inundated. The stability of pH, bulk density and porosity measurements at the sites indicate that only slight changes were occurring in the sediment 18 months after the restoration and these variations were mainly seasonal. Longer term monitoring data is needed for annual trends to be evident.

Comparison of sediment characteristics in restored and natural saltmarsh and mudflat The sediment in the restored saltmarsh had a siltyclay texture with a lower sand, higher clay and higher silt content than the sediment in the natural saltmarsh (Table 1). Similarly, Edwards & Proffitt (2003) and Shafer & Streever (2000) found higher clay and silt content in saltmarshes restored using dredged sediments compared to natural saltmarshes. In contrast, Fearnley (2008) and Moy & Levin (1991) found that restored saltmarshes constructed using dredged sediments had lower proportions of silt and clay compared to natural saltmarshes, suggesting that the silt and clay content at sites restored using dredged materials is largely dependent on the source of the dredged sediment. The silt and clay content is also dependent on the volume of the sediment used for restoration, the time span since restoration and the dynamics of the natural system which can be erosional or sedimentary. In this study, the dredged sediment was sourced from maintenance dredging operations at Harwich Port and the sediment comprised fine grained particles that washed into the navigation channels with approximately 80–95% of the particles less than 0.63 lm (ABPmer, 2004).

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Sediments with a higher clay content compared to sand content have the capacity to bind higher levels of organic matter and nutrients due to the charged surfaces of clay particles which provide exchange sites for nutrients and cations (Fearnley, 2008). High nutrient levels in sediments can facilitate vegetation development in restored saltmarshes (Edwards & Proffitt, 2003). However, at the time of this study significantly lower sediment organic matter content was found in the restored saltmarsh compared to the natural saltmarsh (Table 1), despite the higher clay content in the former. Abundant vegetation in an established saltmarsh provides a source of plant litter that contributes to the organic matter content and traps detrital organic matter (Mitsch & Gosselink, 2000). Furthermore, the high moisture content in the natural saltmarsh measured in this study (Table 1) is likely to have reduced sediment aeration, providing suitable conditions for anaerobiosis. This in turn may have led to the microbial utilisation of alternative electron sinks such as NO3-, Mn4?, Fe3?, SO42- and CO2 in the absence of O2 for organic matter decomposition. Such anaerobic decomposition proceeds at a slower rate than aerobic decomposition (Bishel-Machung et al., 1996; Portnoy & Giblin, 1997; Craft et al., 2002; Sun et al., 2002) leading to further organic matter accumulation. Given the sandier texture of the natural saltmarsh sediment and the less frequent tidal inundation of the natural saltmarsh (due to higher elevation), the natural saltmarsh was expected to have a lower moisture content compared to the restored saltmarsh rather than the observed higher moisture content. This higher moisture content is likely to be a consequence of greater organic matter content in the natural saltmarsh (Table 1), combined with the shading of sediment by vegetation in summer months. Organic material added to the sediment from plant litter has a high water holding capacity (Fearnley, 2008) whilst shading from plant canopies will reduce evaporation from the sediment surface. Similarly, shading by the abundant saltmarsh vegetation is likely to be responsible for the lower magnitude and range of salinities observed in the natural compared to the restored saltmarsh. The dredged sediment was imported into the managed realignment site from Harwich Port with an initial bulk density of approximately 1.3 g cm-3 (ABPmer, 2004). Bulk density in the restored


saltmarsh had dropped to 1.1 g cm-3 at the start of the study period but this was still significantly higher than the natural saltmarsh (Table 1). Bulk density and porosity are controlled by sediment organic matter content, texture and structure (BishelMachung et al., 1996; Craft et al., 1999). Also, bulk density decreases with increasing saltmarsh vegetation abundance because the development of the saltmarsh root system makes the sediment less dense (Fearnley, 2008; Spencer et al., 2008). Therefore, the lower organic matter content and higher fine grained particles in the restored saltmarsh and the higher saltmarsh vegetation cover in the natural saltmarsh are likely to account for the observed differences in bulk density and porosity at the two locations. Finally, lower sediment pH in the natural saltmarsh compared to the restored saltmarsh may also be linked to organic matter in the sediments through the input of fulvic and humic acids arising from the decomposition of vegetation and detrital matter (Blackwell et al., 2004). In addition, the higher sediment moisture content in the natural saltmarsh compared to the restored saltmarsh may have given rise to the precipitation of Fe2? and Mn2? as carbonates which generate H? to counter those consumed by reduction leading to lower sediment pH (McBride, 1994). The sediment characteristics in the restored and natural mudflats were not significantly different (Table 1) due to their similar elevations in relation to the tide. Development of vegetation Although Salicornia europaea was the only species in the restored saltmarsh, its presence provides clear evidence that pioneer saltmarsh vegetation had begun to develop 18 months after the restoration. Waterlogging and associated anoxia has been attributed to slow colonisation of saltmarsh plants (Tessier et al., 2000; Huckle et al., 2000). However, at the Wallasea Island managed realignment site, water-logging of dredged material appears unlikely to impact the development of vegetation. The prevalence of Salicornia europaea may be related to the high sediment salinity in the restored saltmarsh (Table 2) as Salicornia species are indicative of very high salinity conditions (Bertness et al., 1992). Wolters et al. (2008) showed that salinity had a significant effect on

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species diversity particularly in the early stages of saltmarsh restoration. Accordingly, the salinity level in the restored saltmarsh in this study was more favourable for the growth of Salicornia europaea compared to other pioneer saltmarsh species. The occurrence of Salicornia europaea in the restored saltmarsh indicates that pioneer saltmarsh vegetation can develop prior to significant changes in sediment characteristics, and may even drive key changes in sediment characteristics. The organic material that is added to the sediment as saltmarsh plants colonise the restored saltmarsh can lead to a decrease in bulk density and an increase in organic matter and moisture content (Edwards & Proffitt, 2003; Fearnley, 2008). The results from this study agree with the findings of Craft et al. (1999) that vegetation development took a shorter time than the development of sediment characteristics in a restored saltmarsh. As the vegetation assemblage in the restored saltmarsh increases and organic matter starts to accumulate, other sediment characteristics such as cation exchange capacity are likely to progress towards those in the natural saltmarsh.

Conclusions The results of this study indicated that there was no net change in the sediment characteristics in the managed realignment scheme in the first 18 months after restoration. The presence of Salicornia europaea in the restored saltmarsh showed that pioneer saltmarsh vegetation establishment preceded the development of the sediment characteristics, and hence may drive key changes in the sediment characteristics. Whilst the sediment characteristics in the restored and natural mudflat were similar suggesting that the sediment conditions in the restored were approaching those in the natural mudflat, the sediment characteristics in the restored saltmarsh were far from approaching those in the natural saltmarsh. The differences in the sediment characteristics were mainly due to the differences in vegetation abundance and organic matter content. The higher vegetation abundance and organic matter content in the natural compared to the restored saltmarsh contributed to the higher moisture content, lower bulk density, higher porosity, lower pH and lower salinity in the natural saltmarsh. Future

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managed realignment schemes where dredged sediments are beneficially re-used could achieve restored saltmarshes with sediment characteristics that are more similar to the natural saltmarshes by improving the quality of the dredged sediment used during the construction stage through organic matter amendments. Increasing the sediment organic matter will increase nutrient availability, decrease bulk density and increase water holding capacity. Organic matter also improves structural stability of sediments, thereby reducing sediment erosion and enhancing sediment retention within the managed realignment scheme. Given that organic matter influences nutrient dynamics, organic matter amendments may also accelerate saltmarsh vegetation growth rate in managed realignment schemes thereby decreasing the amount of time needed for the restored saltmarsh to develop into a fully functional ecosystem.

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