Deposit soil/sediment to form physical structure of brackish/salt marshes

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Four studies evaluated the effects, on vegetation, of depositing soil/sediment to form the physical structure of brackish/salt marshes (without introducing vegetation). Three studies were in the USA and one study was in Italy. Two studies took place in the same marsh, but in different areas.


  • Overall extent (1 study): One replicated study in a lagoon in Italy quantified the area of vegetation on sediment deposited up to 19 years previously (average six years four months, with 61% vegetation coverage).
  • Community types (2 studies): Two replicated studies in coastal wetlands in the USA and Italy quantified the coverage of brackish or salt marsh plant communities on sediment deposited up to 19 years previously.
  • Community composition (1 study): One replicated, site comparison study on the coast of the USA reported that the composition of the plant community that developed on deposited sediment depended on the time since deposition and the elevation of the sediment. Areas of sediment that were of a similar elevation to natural marshes (or slightly lower) developed (or were developing) a similar overall plant community composition to the natural marshes.
  • Overall richness/diversity (1 study): One replicated study in an estuary in the USA reported that 1–2 plant species had colonized areas of deposited sediment after 4–8 years.




About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, site comparison study in 1988 in an estuary in South Carolina, USA (LaSalle et al. 1991) reported that two areas of deposited sediment had been colonized by brackish marsh vegetation, developing biomass within the range of natural marshes within four years. A 4-year-old patch of sediment had been colonized by smooth cordgrass Spartina alterniflora only. Vegetation cover was 52%. Vegetation was 66 cm tall, there were 199 stems/m2 and above-ground biomass was 856 g/m2. In nearby, natural, smooth cordgrass-dominated marshes, above-ground biomass was 573–969 g/m2. An 8-year-old patch of sediment had been colonized by sturdy bulrush Scirpus robustus in addition to smooth cordgrass. Vegetation cover was similar to the younger marsh (48%) but vegetation was shorter (40 cm tall), more dense (257 stems/m2) and had lower biomass (631 g/m2). Methods: In September 1988, vegetation was surveyed in ten 0.25-m2 quadrats in two areas of sediment, deposited and levelled four or eight years previously. Previously published biomass data from marshes in Georgia and North Carolina were used for comparison. This study was in the same marsh as (2), but used different patches of sediment.

    Study and other actions tested
  2. A replicated study in 1994–1999 in an estuary in South Carolina, USA (Alphin & Posey 2000) reported that three patches of deposited sediment had developed brackish marsh plant communities when 6–17 years old, with replacement of single species by mixed communities in patches >13 years old. The youngest patch was dominated almost exclusively by smooth cordgrass Spartina alterniflora during both surveys (when the patch was 6–11 years old). In two older patches, single-species communities initially dominated, occurring at 87–96% of surveyed points when the patches were 13–17 years old. This dominance decreased over time, with single-species communities occurring at only 50–63% of surveyed points when the patches were 18–22 years old. However, individual species remained dominant or co-dominant at a similar number of sampled points. For example, in one marsh, smooth cordgrass occurred at 45% of points after 13 years then 57% after 18 years, and sturdy bulrush Schoenoplectus robustus occurred at 50% of points after 13 years then 65% after 18 years. Methods: In 1994 and 1999, dominant plant communities were surveyed in three patches of a created marsh (in three 100-m2 quadrats/marsh/year; 36 survey points/quadrat). Dredged sediment had been deposited in an estuary in stages between 1977 and 1988, creating patches of intertidal brackish marsh (salinity 1–10 ppt) of varying age. This study was in the same marsh as (1), but used different patches of sediment.

    Study and other actions tested
  3. A replicated, site comparison study in 1997–2002 on the coastal plain of Louisiana, USA (Edwards & Proffitt 2003) found that four areas of deposited sediment had developed salt marsh plant communities, although the precise community – and its similarity to natural marshes – depended on elevation. Statistical significance was not assessed. Two areas of deposited sediment had developed similar plant communities to nearby natural salt marshes, dominated by smooth cordgrass Spartina alterniflora, within 4–17 years. In a third area, the community was developing in a similar direction by the third year after creation. A fourth area had developed a different plant community to the other created and natural marshes after eight years. This wetland was dominated by herbs characteristic of its higher elevation. All data were reported as graphical analyses. Methods: Between 1983 and 1999, dredged sediment was pumped into open water areas to create four bare islands (40–200 ha). In 1997, 2000 and 2002, vegetation was surveyed on the sediment deposits and in three nearby natural wetlands. Plant species and cover were recorded along 3–7 transects/site/year.

    Study and other actions tested
  4. A replicated study in 2005–2007 in Venice Lagoon, Italy (Scarton et al. 2013) reported that 75 artificial islands had developed up to 70% vegetation coverage, mostly of salt marsh plant communities. On average, the islands were six years four months old when surveyed and 61% of their area was vegetated. Two salt marsh plant communities made up most of the vegetated area: a community dominated by samphire Salicornia spp. (55% of vegetated area) and a community dominated by shrubby swampfire Sarcocornia fruticosa (20% of vegetated area). Islands 5–15 years old had higher overall vegetation coverage (70%) than islands 0–2 years old (27%) or islands 16–19 years old (37%). Statistical significance of this cover result was not assessed. Methods: Between 2005 and 2007, vegetation communities on 75 artificial islands were mapped using aerial photographs and field surveys. The islands had been created between 1988 and 2007 by depositing dredged sediment into geotextile-lined pens (with some gaps in the walls to encourage tidal creek formation). The island surfaces settled to 50–100 cm above sea level: an elevation intended to allow salt marsh vegetation to develop. The average area of the islands was 11.3 ha (range 0.1–51.4 ha).

    Study and other actions tested
Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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