The protective role of coastal marshes: a systematic review and meta-analysis

Abstract

Background: Salt marshes lie between many human communities and the coast and have been presumed to protect these communities from coastal hazards by providing important ecosystem services. However, previous characterizations of these ecosystem services have typically been based on a small number of historical studies, and the consistency and extent to which marshes provide these services has not been investigated. Here, we review the current evidence for the specific processes of wave attenuation, shoreline stabilization and floodwater attenuation to determine if and under what conditions salt marshes offer these coastal protection services.

Methodology/principal findings: We conducted a thorough search and synthesis of the literature with reference to these processes. Seventy-five publications met our selection criteria, and we conducted meta-analyses for publications with sufficient data available for quantitative analysis. We found that combined across all studies (n = 7), salt marsh vegetation had a significant positive effect on wave attenuation as measured by reductions in wave height per unit distance across marsh vegetation. Salt marsh vegetation also had a significant positive effect on shoreline stabilization as measured by accretion, lateral erosion reduction, and marsh surface elevation change (n = 30). Salt marsh characteristics that were positively correlated to both wave attenuation and shoreline stabilization were vegetation density, biomass production, and marsh size. Although we could not find studies quantitatively evaluating floodwater attenuation within salt marshes, there are several studies noting the negative effects of wetland alteration on water quantity regulation within coastal areas.

Conclusions/significance: Our results show that salt marshes have value for coastal hazard mitigation and climate change adaptation. Because we do not yet fully understand the magnitude of this value, we propose that decision makers employ natural systems to maximize the benefits and ecosystem services provided by salt marshes and exercise caution when making decisions that erode these services.

Figure 1. Global distribution of field studies evaluating coastal protection services provided by salt marshes.

Figure 2. Average effect size of marsh vegetation (d) for meta-analyses on a.) wave attenuation and b.) shoreline stabilization as measured by increases in accretion/marsh surface elevation change or decreases in lateral erosion.

Figure 3. Reported wave attenuation rates through salt marsh vegetation versus marsh transect length.

In most cases, the initial wave recorder was located at the marsh edge although some initial measurements were made just inside of the marsh edge. (H = wave height).

Figure 4. The factors most commonly quoted as being of importance to the wave attenuation capacity of salt marsh vegetation.

The height of each bar indicates the number of author statements or conclusions regarding each factor and the bar fill indicates the effect of an increase in each factor on wave attenuation. For example, four studies showed that increasing marsh width increases wave attenuation indicating a positive relationship (black fill) between marsh width and wave attenuation.

Figure 5. Reported outcomes from all shoreline stabilization publications comparing accretion, erosion reduction, or marsh surface elevation change in both vegetated and unvegetated salt marsh areas.

The numbers of outcomes associated with each trend are plotted for each measure of shoreline stabilization and also all measures combined: gray bars represent a negative effect on the process, white bars represent no effect or mixed results and black bars represent a positive effect on the process. P value above the combined bars is significance value for chi square test. P values not reported for individual measures of effect due to insufficient frequency cell counts.

Figure 6. The factors most commonly quoted as being of importance to the shoreline stabilization capacity of salt marshes.

An effect is positive if an increase in the factor results in an increase in shoreline stabilization as measured by increases in accretion or marsh surface height or a reduction in lateral erosion. The effect of an increase in each factor on shoreline stabilization is indicated by the bar fill.