Deposit soil/sediment to form physical structure of freshwater marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

50%
Certainty

34%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

This intervention involves large-scale deposition of soil or sediment to form the physical structure of a marsh. This can be a cost-effective alternative to depositing sediment in upland areas or in the ocean (LaSalle et al. 1991). Soil or sediment could be placed directly where a marsh is desired, or placed nearby then carried to the desired site by water flows (Foster et al. 2013). Summarized studies could vary in the degree of landscaping of the newly deposited sediment.

Related actions: Deposit soil/sediment and introduce vegetation; Add sediment in relatively small amounts; Add upland topsoil; Transplant or replace wetland soil in order to introduce marsh vegetation.

^{Foster N.M., Hudson M.D., Bray S. & Nicholls R.J. (2013) Intertidal mudflat and saltmarsh conservation and sustainable use in the UK: a review. Journal of Environmental Management, 126, 96–104.}

^{LaSalle M.W., Landin M.C. & Sims J.G. (1991) Evaluation of the flora and fauna of a Spartina alterniflora marsh established on dredged material in Winyah Bay, South Carolina. Wetlands, 11, 191–208.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of depositing soil/sediment to form the physical structure of freshwater marshes (without introducing vegetation). One study was in the USA and one was in the Netherlands.

VEGETATION COMMUNITY

Community types (1 study): One replicated, paired, site comparison study in the Netherlands reported that marshes created by depositing sand at lake margins contained fewer plant community types, after 8–16 years, than mature natural marshes.

VEGETATION ABUNDANCE

Overall abundance (2 studies): One site comparison study in the USA reported that plant stem density was similar, after 4–10 years, in marshes created by depositing sediment and in natural marshes, but that vegetation cover was lower in the created marshes. One replicated, paired, site comparison study in the Netherlands reported that marshes created by depositing sand at lake margins contained similar vegetation biomass to nearby natural marshes after 8–16 years.

VEGETATION STRUCTURE

Height (1 study): One site comparison study in the USA reported that a freshwater marsh created by depositing sediment contained vegetation of a similar height to nearby natural marshes after 4–10 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

A site comparison study in 1979–1985 alongside a river in Virginia, USA (Landin et al. 1989) reported that a freshwater marsh created by depositing dredged sediment developed vegetation of similar height and density to three natural marshes within four years, but that vegetation cover remained lower than natural marshes for 10 years. Statistical significance was not assessed. Four years after wetland creation, vegetation was 112 cm tall (vs natural wetlands: 99–112 cm) and there were 212 plant stems/m² (vs natural: 183–380 stems/m²). However, in the created marsh, vegetation cover was only 47% (vs natural: 65–91%). Data were also reported 10 years after marsh creation. Height and density were still within the range of natural marshes, and cover was still lower in the created marsh than natural marshes (see original paper for data). Methods: In 1975, dredged sediment was deposited behind a sand embankment in the James River channel. The dike was breached after depositing the sediment to allow tidal influx. Vegetation was surveyed along permanent transects in the created marsh and three adjacent natural marshes in 1979, 1982 and 1985.
Study and other actions tested
Referenced paper
Landin M.C., Clairain E.J. & Newling C.J. (1989) Wetland habitat development and long-term monitoring at Windmill Point, Virginia. Wetlands, 9, 13-25.
A replicated, paired, site comparison study in 2003 around three freshwater lakes in the Netherlands (Sollie et al. 2008) reported that marshes created by depositing sand contained fewer plant community types than natural marshes, but found that they had similar vegetation biomass. After 8–16 years, the created marshes contained three distinct plant community types, compared to four in natural marshes. Unlike natural marshes, created marshes did not contain pure stands of cattails Typha spp. or common reed Phragmites australis. However, the above-ground vegetation biomass in sampled plots did not significantly differ between created marshes (1.5 kg/m²) and natural marshes (1.6 kg/m²). Methods: In August 2003, vegetation was surveyed around the margins of three connected freshwater lakes. One created marsh and one natural (mature) marsh were surveyed in each lake. Created marshes had been formed by depositing sand to a suitable elevation for plants to colonize. Vegetation was cut from three or four 400-cm² plots/marsh, distributed across the plant community types present, then dried and weighed.
Study and other actions tested
Referenced paper
Sollie S., Coops H. & Verhoeven J.T.A. (2008) Natural and constructed littoral zones as nutrient traps in eutrophicated shallow lakes. Hydrobiologia, 605, 219-233.

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?

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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