Salt Marsh Accretion With and Without Deep Soil Subsidence as a Proxy for Sea-Level Rise

Han F Van Dobben,Jan P Bakker,Alma V De Groot

doi:10.1007/s12237-021-01034-w

Abstract

The relation between salt marsh accretion and flooding regime was quantified by statistical analysis of a unique dataset of accretion measurements using sedimentation-erosion bars, on three barrier islands in the Dutch Wadden Sea over a period of c. 15 years. On one of the islands, natural gas extraction caused deep soil subsidence, which resulted in gradually increasing flooding frequency, duration, and depth, and can thus be seen as a proxy for sea-level rise. Special attention was paid to effects of small-scale variation e.g., in distance to tidal creeks or marsh edges, elevation of the marsh surface, and presence of livestock. Overall mean accretion rate was 0.44 ± 0.0005 cm year−1, which significantly exceeded the local rate of sea-level rise of 0.25 ± 0.009 cm year−1. A multiple regression approach was used to detect the combined effect of flooding regime and the local environment. The most important flooding-related factors that enhance accretion are mean water depth during flooding and overall mean water depth, but local accretion strongly decreases with increasing distance to the nearest creek or to the salt marsh edge. Mean water depth during flooding can be seen as an indicator for storm intensity, while overall mean water depth is a better indicator for storm frequency. The regression parameters were used to run a simple model simulating the effect of various sea-level scenarios on accretion and show that, even under extreme scenarios of sea-level rise, these salt marshes can probably persist for the next 100 years, although the higher parts may experience more frequent inundation.

Highlights

Salt marshes are among the ecosystems that are most vulnerable to climate change
Significant (P < 0.05) differences in mean accretion rate were recorded between the sites (Fig. 5) and between the islands, with Terschelling < Schiermonnikoog (0.40 cm year−1) < Ameland (0.53 cm year−1)
The mean accretion rate over all plots and all time intervals was 0.44 ± 0.0005 cm year−1 and significantly exceeded the locally measured sea-level rise (SLR) rate of 0.25 ± 0.009 cm year−1, the mean accretion rate of the Terschelling sites was significantly lower than SLR rate

Summary

Introduction

Salt marshes are among the ecosystems that are most vulnerable to climate change. Regional to global studies have estimated that sea-level rise (SLR) over the twenty-first century could induce the loss of 30–80% of coastal wetlands, including salt marshes and mangroves (Lovelock et al 2015; Spencer et al 2016; Schuerch et al 2018; Thiéblemont et al.Communicated by Mead Allison.2019). Regional to global studies have estimated that sea-level rise (SLR) over the twenty-first century could induce the loss of 30–80% of coastal wetlands, including salt marshes and mangroves Since salt marshes occur just above mean sea level, their capacity to persist under SLR depends on accretion (Fagherazzi et al 2020), which we here define as the vertical rise in marsh surface elevation resulting from the sum of sedimentation, erosion, compaction, swelling, and shrinkage of mineral and organic sediments. If sea level rises rapidly, accretion may not be sufficient to keep pace with SLR, which may lead to increasing flooding and to marsh vegetation die-off and “drowning” of a marsh (e.g., Cooper et al 2001; Morris et al 2002; Kirwan and Megonigal 2013; Blankespoor et al 2014; Schepers et al 2017)

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Discussion

Conclusion