Add sediment: freshwater marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

23%
Certainty

23%
Harms

13%

Calculating overall effectiveness category

Background information and definitions

Adding small amounts of sediment to marshes is a possible action to counter multiple threats. These include: sea level rise; subsidence (e.g. following oil and gas extraction); reduced sediment inputs following the construction of levees, flood control structures or jetties; and erosion from storms or boat traffic or following excessive grazing (Reed & Wilson 2004). Adding sediment can physically raise the ground surface and provide nutrients to vegetation. In turn, vegetation can physically protect and stabilize wetlands, and encourage further sediment deposition. Sediment or sediment slurry could be added directly to a focal site, or placed nearby then transported to the focal site by natural process (Foster 2013).

Factors that might influence the effects of this action include the amount of sediment added, and whether any vegetation is present before sediment addition.

Related actions: Deposit soil/sediment to form physical habitat structure; 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.}

^{Reed D.J. & Wilson L. (2004) Coast 2050: a new approach to restoration of Louisiana coastal wetlands. Physical Geography, 25, 4–21.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of adding sediment to existing freshwater marshes. The study was in the USA.

VEGETATION COMMUNITY

Overall richness/diversity (1 study): One replicated, randomized, paired, controlled study in the USA reported that adding sediment to freshwater marshes typically reduced plant species richness after one growing season.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, randomized, paired, controlled study in the USA found that adding sediment to freshwater marshes had no significant effect on total live vegetation biomass after one growing season.
Individual species abundance (1 study): The same study found that adding sediment to freshwater marshes had no significant effect on the biomass of most of the dominant herbaceous species after one growing season.

VEGETATION STRUCTURE

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 replicated, randomized, paired, controlled study in 2004 in two floating freshwater marshes in Louisiana, USA (Carpenter et al. 2007) found that adding sediment reduced plant species richness, but had no significant effect on vegetation biomass. After one growing season, plots amended with sediment had lower plant species richness than unamended plots in five of six cases (for which amended: 8–12 species/0.4 m²; unamended: 12–13 species/0.4 m²; statistical significance not assessed). Sediment addition had no significant effect on total, live, above-ground vegetation biomass (amended: 270–660 g/m²; unamended: 320–530 g/m²). Sediment addition typically had no significant effect on the overall biomass of dominant plant species, such as slender spikerush Eleocharis baldwinii and dotted smartweed Polygonum punctatum (see original paper for data). However, in one of two marshes, biomass of frogfruit Phyla lanceolata was greater in amended plots (4–12 g/m²) than unamended plots (<0.1 g/m²). Methods: In spring 2004, thirty-two 1-m² plots were established across two floating marshes. Sediment inputs to the marshes had been reduced by an upstream dam. In each marsh, twelve random plots were amended with sediment collected from a nearby river channel (2 kg/m², 7 kg/m² or 17 kg/m2). The remaining plots received no sediment. All plots were also fenced to exclude nutria Myocastor coypus. In autumn 2004, vegetation was cut from 0.1 m² of each plot then separated by species, dried and weighed.
Study and other actions tested
Referenced paper
Carpenter K., Sasser C.E., Visser J.M. & DeLaune R.D. (2007) Sediment input into a floating freshwater marsh: effects on soil properties, buoyancy, and plant biomass. Wetlands, 27, 1016-1024.

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

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Add inorganic fertilizer: brackish/salt marshes Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Deposit soil/sediment to form physical structure of freshwater swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Lower water level to restore/create brackish/saline swamps from other land uses Action Link	No evidence found (no assessment)	0	Synopsis Link