Build barriers to protect littoral brackish/salt marshes from rising water levels and severe weather

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Five studies evaluated the effects, on vegetation, of building barriers to protect littoral brackish/salt marshes from rising water levels and severe weather. Three studies were in the USA, one was in Italy and one was in the Netherlands.

VEGETATION COMMUNITY

  • Overall extent (3 studies): Two controlled studies (one also replicated, randomized, paired) in Italy and the USA found that protecting salt marshes with offshore structures had no significant effect on the seaward limit of emergent vegetation, after 17–27 months. It was similar, or retreated at a similar rate, in protected and unprotected marshes. One replicated, randomized, paired, controlled study in the USA found that brackish marshes protected with oyster shell reefs receded less, over one year, than unprotected marshes.
  • Community composition (1 study): One replicated, site comparison study in the Netherlands reported that marshes protected with low sea walls had a similar overall plant community composition to nearby natural salt marshes, 15–22 years after the walls were built.
  • Overall richness/diversity (2 studies): One controlled study in Italy reported that a salt marsh protected with an offshore fence contained more plant species, after 17 months, than an unfenced marsh. One replicated, site comparison study in the Netherlands recorded 85 plant and algal species across two salt marshes that had developed behind low sea walls, over 15–22 years, compared to 155 species recorded across multiple natural marshes in the region.

VEGETATION ABUNDANCE

  • Overall abundance (2 studies): Two controlled studies (one also replicated, randomized, paired) in Italy and the USA found that brackish/salt marshes protected with offshore structures contained a similar total amount of vegetation to unprotected marshes. This was true for cover and biomass.
  • Individual species abundance (2 studies): One replicated, paired, site comparison study in the USA found that salt marshes protected with offshore breakwaters (and planted with cordgrasses Spartina spp.) typically contained less smooth cordgrass S. alterniflora, after 2–3 growing seasons, than nearby natural marshes. One replicated, site comparison study in the Netherlands reported that in marshes protected with low sea walls for 15–22 years and nearby natural salt marshes, the same plant species were the most frequent.

VEGETATION STRUCTURE

  • Height (1 study): One replicated, paired, site comparison study in the USA found that salt marshes protected with offshore breakwaters (and planted with cordgrasses Spartina spp.) contained shorter smooth cordgrass S. alterniflora plants, after 2–3 growing seasons, than nearby natural marshes.

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 controlled study in 1994–1995 of two salt marshes in northern Italy (Scarton et al. 2000) reported that a salt marsh behind a sediment fence contained more plant species than an exposed salt marsh, but found that there was no significant difference in vegetation cover or distribution. After 17 months, three plant species were recorded in the fenced marsh, compared to only one species in the exposed marsh. The marshes did not significantly differ in terms of vegetation cover (fenced: 29%; exposed: 19%) or distance between the physical edge of the marsh sediment and the vegetation (fenced: 7.5 m; exposed: 9.1 m). Methods: The study involved two intertidal salt marshes behind embayments. In May 1994, a fence (vegetation bundles behind wooden posts) was built across the mouth of one embayment to trap sediment and protect the marsh behind from waves. The other embayment was left open. In October 1995, vegetation was surveyed along the edge of each marsh (14–16 points/marsh; species and cover within a 1 m radius around each point).

    Study and other actions tested
  2. A replicated, randomized, paired, controlled study in 2002–2003 of brackish marsh around a lake in Louisiana, USA (Piazza et al. 2005) found that installing offshore oyster shell reefs reduced the rate of shoreline retreat, but had no significant effect on vegetation cover or biomass within the marshes. Over one year, the rate at which the vegetated shoreline receded was lower for marshes behind oyster shell reefs (8 cm/month) than for unprotected marshes (12 cm/month). Over the year, vegetation cover and above-ground vegetation biomass were statistically similar in marshes behind oyster shell reefs and in unprotected marshes (data not reported). Methods: The study used twelve sections of shoreline (six in a high-energy area, six in a low-energy area) around one coastal lake. All had brackish marsh landward. Oyster shell reefs (25 m long and exposed at low tide) were deposited <5 m offshore of six random sections (three high-energy, three low-energy). The other six sections were left unprotected. Vegetation was surveyed for one year after reefs were installed: three measurements/section/month for shoreline position (i.e. edge of marsh vegetation); nine 1-m2 quadrats/section/month for cover of each plant species; three 0.25-m2 quadrats/section/quarter for biomass (vegetation cut, dried and weighed).

    Study and other actions tested
  3. A replicated, paired, site comparison study in 2001–2004 of six salt marshes in North Carolina, USA (Currin et al. 2008) found that restored marshes – protected with breakwaters and planted with cordgrasses Spartina spp. – typically contained less, and shorter, smooth cordgrass than natural marshes. Averaged over the 22 or 31 months after intervention, smooth cordgrass cover was lower in restored than natural marshes in three of three comparisons (restored: 10–26%; natural: 33–46%). Smooth cordgrass density was lower in restored than natural marshes in two of three comparisons (for which restored: 70–162 stems/m2; natural: 150–222 stems/m2; other comparison no significant difference). Smooth cordgrass plants were shorter in restored than natural marshes in three of three comparisons (restored: 50–62 cm; natural: 64–82 cm). Methods: Between autumn 2001 and summer 2002, three degraded salt marshes were restored. Rocky breakwaters were built offshore, then cordgrasses Spartina spp. (mainly smooth cordgrass Spartina alterniflora) were planted. The study does not distinguish between the effects of the breakwaters and planting on non-planted vegetation. For each protected/planted marsh an adjacent, physically similar, natural marsh was selected for comparison. Smooth cordgrass was monitored along transects each spring and autumn for up to 31 months after intervention. Cover was estimated in 1-m2 plots, stems were counted in 0.25-m2 subplots, and the three tallest stems/plot were measured.

    Study and other actions tested
  4. A replicated, randomized, paired, controlled study in 2007–2009 of two salt marshes in Alabama, USA (Scyphers et al. 2011) found that installing offshore oyster shell reefs had no significant effect on the rate of shoreline retreat. Over approximately two years, the vegetated shoreline receded by a statistically similar amount whether it was behind an oyster shell reef (3.1–5.1 m retreat) or left unprotected (4.5–5.5 m retreat). Methods: The study used eight sites across two rapidly eroding shorelines. At four sites (two random sites/shoreline), oyster shell was deposited just offshore to form a breakwater (three 5 x 25 m sections; top exposed during low tides). The shell was placed on geotextile fabric and anchored in place with plastic mesh. It was colonized by oysters Crassostrea virginica. The other four sites were left unprotected. Stakes were inserted at the seaward limit of emergent vegetation when the reefs were constructed. Retreat relative to these stakes was measured over 24–27 months.

    Study and other actions tested
  5. A replicated, site comparison study in 2011–2013 of salt marshes in the Netherlands (van Loon-Steensma et al. 2015) reported that degraded marshes behind low sea walls developed similar plant/algal communities to natural salt marshes within 15–22 years, but contained fewer plant/algal species. The overall plant/algal community composition in protected salt marshes fell within the range of the community composition of natural marshes (data reported as graphical analyses; statistical significance of similarity not assessed). In both protected and natural marshes, the most common species were glasswort Salicornia europaea (present in 59–66% of quadrats), saltmarsh grass Puccinellia maritima (59–63%) and seablite Suaeda maritima (58–62%). However, only 85 species of plants and algae were recorded in the protected salt marshes, compared to 155 species recorded in natural salt marshes in the region. Protected marshes were missing some of the rarer species present in natural marshes. Methods: In 2011 and 2013, cover of every plant and algal species was recorded (in 148 circular 4-m2 quadrats) across two coastal salt marshes in the Dutch Wadden Sea. The marshes had developed behind low sea walls (10–60 m from the salt marsh edge, extending 1 m above mean sea level) built in 1991 and 1998 to protect remnant, eroding marsh vegetation. Previously published data, from 6,198 quadrats in natural marshes across the Dutch Wadden Sea, were used for comparison.

    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.

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Marsh and Swamp Conservation

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Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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