Add sediment: brackish/salt marshes
Overall effectiveness category Awaiting assessment
Number of studies: 5
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.
Supporting evidence from individual studies
A replicated, randomized, paired, controlled study in 1986–1987 in a subsiding tidal salt marsh in Louisiana, USA (DeLaune et al. 1990) found that adding sediment increased smooth cordgrass Spartina alterniflora density and biomass, but not its height. Sixteen months after the first amendment, plots amended with sediment contained more smooth cordgrass stems (high dose: 75 stems/0.25 m2; low dose: 65 stems/0.25 m2) than unamended plots (47 stems/0.25 m2). Above-ground biomass of smooth cordgrass was greater in amended than unamended plots, although only significantly so for the high sediment dose. This was true for both overall biomass (high dose: 527; low dose: 406; unamended; 288 g/0.25 m2) and for live biomass only (high dose: 368; low dose: 268; unamended; 184 g/0.25 m2). Smooth cordgrass height did not significantly differ between treatments (high dose: 41; low dose: 41; unamended: 43 cm). Methods: In July 1986, twelve 1.44-m2 plots were established (in four sets of three) on a degraded, cordgrass-dominated salt marsh. Eight plots (two random plots/set) were amended with dredged river alluvium: either high dose (94 kg/m2) or low dose (47 kg/m2). Half of the sediment was added in July 1986 and half in June 1987. The other four plots received no sediment. In November 1987, vegetation was cut from one 0.25-m2 quadrat/plot. Five random stems were measured, then all sampled vegetation was dried and weighed. This study was based on the same experimental set-up as (2).Study and other actions tested
A replicated, randomized, paired, controlled study in 1986–1988 in a subsiding tidal salt marsh in Louisiana, USA (Pezeshki et al. 1992) found that adding sediment increased smooth cordgrass Spartina alterniflora density and biomass. Twenty-two months after the first amendment, plots amended with sediment contained more cordgrass stems (high dose: 50 stems/m2; low dose: 42 stems/m2) than unamended plots (19 stems/m2). The above-ground biomass of smooth cordgrass was also greater in amended plots (high dose: 381 g/m2; low dose: 321 g/m2) than unamended plots (160 g/m2). Methods: In July 1986, twelve 1.44-m2 plots were established (in four sets of three) on a degraded, cordgrass-dominated salt marsh. Four plots received each sediment dose: high (94 kg/m2), low (47 kg/m2) or none. Sediment was dredged river alluvium. Half was added to each amended plot in July 1986 and half in June 1987. In April 1988, vegetation was cut from two 0.25-m2 quadrats/plot, then dried and weighed. This study was based on the same experimental set-up as (1).Study and other actions tested
A before-and-after, site comparison study in 1996–1997 of two coastal marshes in Louisiana, USA (Ford et al. 1999) reported that one year after spraying dredged sediment onto a marsh, cover of three of four plant species was greater than before spraying. Statistical significance was not assessed. One year after spraying, the marsh had 66% cover of smooth cordgrass Spartina alterniflora (before: 19%), 35% cover of hairy cowpea Vigna luteloa (before: 1%), 16% cover of three-square bulrush Scirpus americanus (before: 7%) and 0% cover of saltmeadow cordgrass Spartina patens (before: 11%). Meanwhile, cover of the first three species was stable over time in a nearby unsprayed marsh (smooth cordgrass: 54–55%; cowpea: 8–12%; bulrush: 7–9%) whilst cover of saltmeadow cordgrass declined (from 6% to 0%). Methods: In July 1996, dredged canal sediment was sprayed in a high-pressure jet onto an area of subsided marsh in the Mississippi Delta. This increased the marsh surface elevation by approximately 2 cm. Cover of each plant species was surveyed in the sprayed marsh and a nearby reference marsh (not subsided, not sprayed), five weeks before spraying (June 1996) and for up to one year after (July 1997).Study and other actions tested
A replicated, site comparison study in 2007 of eight tidal salt marshes in Louisiana, USA (Schrift et al. 2008) found that marshes amended with sediment to counteract subsidence typically had greater vegetation cover and species richness than degraded marshes, and similar cover and richness to natural marshes, after two years. Vegetation cover in amended marshes was greater than in degraded marshes and similar to natural marshes in three of five cases (for which amended: 85–100%; degraded: 8%; natural: 93%). Similarly, total plant species richness was greater than in degraded marshes and similar to natural marshes in three of five cases (for which amended: 1.3–2.4; degraded: 0.1; natural: 1.4 species/unit; units not clearly reported). Additionally, the relative abundance of smooth cordgrass Spartina alterniflora in amended marshes was greater than in degraded marshes in four of five cases, but similar to natural marshes in only two cases (data reported as importance values). In the other cases, cover (12–30%), richness (0.4–0.8 species/unit) and cordgrass relative abundance in amended marshes remained similar to degraded marshes and typically lower than in natural marshes. These comparisons generally involved amendments with large amounts of sediment on initially bare areas. Methods: In 2002, sediment slurry was pumped onto four degraded salt marshes (subsided after most plants were killed by drought in 2000). The marsh surface was raised 13–36 cm above natural marshes. Between autumn 2003 and 2004, vegetation was surveyed along transects in the four restored marshes, two adjacent degraded (subsided) marshes and two adjacent natural marshes. This study was based on the same experimental set-up as (5).Study and other actions tested
A replicated, site comparison study in 2007 in eight tidal salt marshes in Louisiana, USA (Stagg & Mendelssohn 2012) found that marshes amended with sediment to counteract subsidence typically had greater smooth cordgrass Spartina alterniflora cover than degraded marshes, and sometimes had similar cover to natural marshes. After five years, total smooth cordgrass cover was greater in amended than degraded marshes in three of four comparisons (for which amended: 16–42%; degraded: 2%). Natural marshes had 49% smooth cordgrass cover. This was not significantly different from amended marshes in two of four comparisons (where small amounts of sediment had been added or cordgrass rhizomes persisted before amendment; cover: 29–42%) but was lower in amended marshes in the other two comparisons (where higher amounts of sediment had been added to bare marsh; cover: 16–19%). Live cordgrass cover was 8–12% in amended marshes (24–59% of total), 2% in degraded marshes (100% of total), and 15% in natural marshes (30% of total; statistical significance of differences not assessed). Methods: In 2002, sediment slurry was pumped onto four degraded salt marshes. These had subsided after plants were killed by drought in 2000, but retained some patches of cordgrass rhizomes (underground horizontal stems). The marshes were raised to 3–15 cm above mean sea level. In summer 2007, vegetation was surveyed in 0.25-m2 quadrats in the four restored marshes, two adjacent degraded (subsided) marshes and two adjacent natural marshes. This study was based on the same experimental set-up as (4).Study and other actions tested