Role of delta‐front erosion in sustaining salt marshes under sea‐level rise and fluvial sediment decline

Abstract

Accelerating sea-level rise and decreasing riverine sediment supply are widely considered to lead to global losses of deltaic marshes and their valuable ecosystem services. However, little is known about the degree to which the related erosion of the seaward delta front can provide sediments to sustain salt marshes. Here, we present data from the mesomacrotidal Yangtze Delta demonstrating that marshes have continued to accrete vertically and laterally, despite rapid relative sea-level rise ($10 mm yr −1) and a > 70% decrease in the Yangtze River sediment supply. Marsh progradation has decelerated at a lower rate than fluvial sediment reduction, suggesting an additional source of sediment. We find that under favorable conditions (e.g., a mesomacrotidal range, strong tidal flow, flood dominance, sedimentary settling lag/scour lag effects, and increasing high-tide level), delta-front erosion can actually supply sediment to marshes, thereby maintaining marsh accretion rates in balance with relative sea-level rise. Comparison of global deltas illustrates that the ability of sediment remobilization to sustain marshes depends on coastal processes and varies by more than an order of magnitude among the world's major deltas. Salt marshes are among the world's most valuable ecosystems (Costanza et al. 1997). They sequester carbon, protect shorelines from storm impacts, transform nutrients, contribute to fisheries production, and maintain biodiversity (Barbier et al. 2011; Kirwan and Megonigal 2013; Temmerman et al. 2013; Möller et al. 2014). Unfortunately, many salt marshes have disappeared due to reclamation and waste disposal during the past century (Gedan et al. 2009; Ma et al. 2014). Deltaic marshes are one of the most dynamic landscapes on Earth's surface (Wagner et al. 2017) and are threatened by accelerating sea-level rise and decreases in fluvial sediment supply. Decreasing fluvial sediment supply reduces the ability of salt marshes to accumulate sediments and to build up their soil elevation in balance with the rising sea level (Kirwan et al. 2010; Weston 2014). Although the morphology and evolution of deltas are influenced by various factors (Paola et al. 2011), such as riverine water and sediment discharges (Besset et al. 2019), sediment properties (Caldwell and Edmonds 2014), flow patterns (Shaw et al. 2016), vegetation height and density (Nardin et al. 2016), marine hydrodynamics (waves, tides, and longshore currents) (Caldwell and Edmonds 2014; Besset et al. 2017), land subsidence and sea-level changes (Jerolmack 2009; Syvitski et al. 2009), changes in the fluvial sediment supply and relative sea level are usually the most important for the long-term morphological evolution of deltaic marshes. The rate of global mean sea-level rise increased from 1.

Spatiotemporal Change and the Natural–Human Driving Processes of a Megacity’s Coastal Blue Carbon Storage

“…The study area has natural coastal wetlands with fertile soil (formed by the huge amount of sediment carried by the runoff of the Yangtze River and deposited at the interface of the river and the sea), which is a potentially an important land resource. Chongming Dongtan Wetland is a national nature reserve for birds and an internationally important Ramsar wetland [18]. Jiuduansha Wetland is also a national nature reserve [18].…”

“…Chongming Dongtan Wetland is a national nature reserve for birds and an internationally important Ramsar wetland [18]. Jiuduansha Wetland is also a national nature reserve [18]. As a megacity in China, Shanghai has high carbon dioxide emissions from human activities.…”

“…Specifically, coastal erosion, beach reclamation, seawall construction and other land usedriven processes have released huge amounts of carbon dioxide in coastal areas, making the coastal ecosystem gradually converted into a huge carbon source [7,15]. On the other hand, land use-driven processes, such as beach succession and swamp restoration, also helped carbon dioxide storage in soil and vegetation, and continuously enhanced the CBCS capacity of coastal ecosystems [16][17][18]. Previous studies showed that the LULC change caused by natural or human-driven processes had both positive and negative effects on coastal blue carbon [13,15,19].…”

Intersecting Ecosystem Services Across the Aquatic Continuum: From Global Change Impacts to Local, and Biologically Driven, Synergies and Trade-Offs

“…This kind of semantic effort also helps connect distant themes, such as those represented by the topical groups ecosystem-change-service and sedimentimpact-marsh (see the red and light-blue nodes in Figure 2). It does so, for example, by intensifying the "study of the state of salt marshes as impacted by changes in sediment fluxes from rivers, " a rare but cross-cutting topic (Rutherford et al, 2018;Yang et al, 2020).…”

Divergence of Sediment Fluxes Triggered by Sea‐Level Rise Will Reshape Coastal Bays

“…In a mesotidal system in Southeastern Essex, England, it was observed that fine-grain sediments eroded from the marsh edge are responsible for marsh accretion on the platform in response to SLR (Reed, 1988). Sediment coarsening at the delta front was observed in the Yangtze Delta, China, where erosion provided sediments to sustain salt marshes despite increasing sea levels and reduced riverine sediment loads in the last half century (Yang et al, 2018(Yang et al, , 2020.…”