Are Elevation and Open‐Water Conversion of Salt Marshes Connected?

Ganju, Neil K.; Defne, Zafer; Fagherazzi, Sergio

doi:10.1029/2019gl086703

Cited by 31 publications

(28 citation statements)

References 28 publications

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“…The minimum sediment supply ( B ) required for the entire bay to keep pace with sea level rise can be calculated as follows (Chant et al, 2020; Ganju et al, 2020):

B = italicslr ((), A_{m} ρ_{m} + {As}_{b} {ρs}_{b})

where slr is the rate of sea level rise, ρ m and ρs b are the bulk densities for marshes and subtidal areas, and A m and As b represent the marsh extent and the subtidal area. The local sea level rise is estimated following assessments based on monthly mean sea level observations taken at The Battery (New York) in the period 1856–2019.…”

Section: Methodsmentioning

confidence: 99%

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Dynamics of Marsh‐Derived Sediments in Lagoon‐Type Estuaries

et al. 2020

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Salt marshes are valuable ecosystems that must trap sediments and accrete in order to counteract the deleterious effect of sea level rise. Previous studies have shown that the capacity of marshes to build up vertically depends on both autogenous and exogenous processes including ecogeomorphic feedbacks and sediment supply from inland and coastal ocean. There have been numerous efforts to quantify the role played by the sediments coming from marsh edge erosion on the resistance of salt marshes to sea level rise. However, the majority of existing studies investigating the interplay between lateral and vertical dynamics use simplified modeling approaches, and they do not consider that marsh retreat can affect the regional-scale hydrodynamics and sediment retention in back-barrier basins. In this study, we evaluated the fate of the sediments originating from marsh lateral loss by using high-resolution numerical model simulations of Jamaica Bay, a small lagoonal estuary located in New York City. Our findings show that up to 42% of the sediment released during marsh edge erosion deposits on the shallow areas of the basin and over the vegetated marsh platforms, contributing positively to the sediment budget of the remaining salt marshes. Furthermore, we demonstrate that with the present-day sediment supply from the ocean, the system cannot keep pace with sea level rise even accounting for the sediment liberated in the bay through marsh degradation. Our study highlights the relevance of multiple sediment sources for the maintenance of the marsh complex. Coastal bays must trap sediments to adapt to rising sea level (Fagherazzi et al., 2014; Zhang et al., 2019). Indeed, a positive sediment budget is necessary for the survival of salt marshes and tidal flats

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“…The minimum sediment supply ( B ) required for the entire bay to keep pace with sea level rise can be calculated as follows (Chant et al, 2020; Ganju et al, 2020):

B = italicslr ((), A_{m} ρ_{m} + {As}_{b} {ρs}_{b})

Section: Methodsmentioning

confidence: 99%

“…The minimum sediment supply (B) required for the entire bay to keep pace with sea level rise can be calculated as follows (Chant et al, 2020;Ganju et al, 2020):…”

Section: Estimation Of Sediment Budgetmentioning

confidence: 99%

Dynamics of Marsh‐Derived Sediments in Lagoon‐Type Estuaries

et al. 2020

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“…A wide range of studies on coastal wetlands has demonstrated similar phenomena, that is, nonlinear (e.g., Marani et al., 2007) or lagged (e.g., Kirwan & Murray, 2008) responses to accelerated RSLR. The role of elevation capital within this context is critical: for example, the tight connection between salt marsh elevation and resilience is now well documented (e.g., Ganju et al., 2020).…”

Section: The Importance Of Timescalementioning

confidence: 99%

“…For example, the Sundarbans mangroves in the Ganges‐Brahmaputra Delta (Bangladesh and India) exhibit SRTM elevations >10 m above sea level, a consequence of incomplete penetration of the radar signal into forested areas (Hofton et al., 2006). A comparison of SRTM and Light Detection and Ranging (LiDAR) data from LECZs in the conterminous United States revealed root mean square errors for geodetically ground‐truthed elevations of 5.57 and 0.72 m, respectively, although this number is in many cases closer to 0.10–0.19 m for LiDAR data (Gesch, 2018). While LiDAR data are increasingly common, they are still largely unavailable in less wealthy countries.…”

Section: A Path Forwardmentioning

confidence: 99%

Coastal Wetland Resilience, Accelerated Sea‐Level Rise, and the Importance of Timescale

et al. 2021

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Coastal wetlands (marshes and mangroves) are among the most valuable ecosystems on the planet (Barbier, 2019; Costanza et al., 2014), yet they are threatened by accelerated sea-level rise and other human impacts. Coastal wetlands occupy extensive portions of low-elevation coastal zones (LECZs; commonly defined as <10 m above mean sea level) that are home to all or portions of 21 of the 33 largest megacities (population >10 million) worldwide (https://digitallibrary.un.org/record/3799524). Recent work has shown that LECZs are considerably lower in elevation than previously assumed (Kulp & Strauss, 2019) and several LECZs may subside more rapidly than commonly thought (Keogh & Törnqvist, 2019). Given that the acceleration of global sea-level rise will continue well into the future (Oppenheimer et al., 2019), predicting the extent and health of coastal wetlands is a topic of great import. A recent global-scale prediction of coastal wetland extent for the remainder of this century (Schuerch et al., 2018) has suggested that even under pessimistic climate scenarios, and provided that there is room for landward migration, coastal wetland area will generally increase, echoing previous model studies that have suggested that coastal marshes can keep up with rates of sea-level rise as high as 10-50 mm yr −1 (Kirwan et al., 2016). These results stand in stark contrast with studies based on the geologic record that show tipping points for marsh drowning in the Mississippi Delta (USA) at rates of relative sea-level rise (RSLR) of ∼3 mm yr −1 (Törnqvist et al., 2020) and an inability of mangroves worldwide to initiate sustained accretion when rates of RSLR exceed ∼6 mm yr −1 (Saintilan et al., 2020). The main purpose of this Commentary is to examine these seemingly contradictory outcomes. We first discuss the concept of accommodation that plays an increasingly important role in models that seek to predict coastal wetland change until the end of this century (e.g., Schuerch et al., 2018). We then make the case that these models must be constrained by the stratigraphic record (i.e., observations over centennial to millennial timescales) and we discuss key reasons why instrumental observations (i.e., annual to decadal timescales) cannot fully capture the outcome Abstract Recent studies have produced conflicting results as to whether coastal wetlands can keep up with present-day and future sea-level rise. The stratigraphic record shows that threshold rates for coastal wetland submergence or retreat are lower than what instrumental records suggest, with wetland extent that shrinks considerably under high rates of sea-level rise. These apparent conflicts can be reconciled by recognizing that many coastal wetlands still possess sufficient elevation capital to cope with sea-level rise, and that processes like sediment compaction, ponding, and wave erosion require multidecadal or longer timescales to drive wetland loss that is in many cases inevitable. Plain Language Summary The rapid, climate-driven acceleration of global sea level thr...

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“…Deep, anoxic marsh sediment containing decades to millennia-old marsh carbon can be exposed and enter tidal waters in a number of ways: wave erosion of the lower layer of a marsh cliff edge (Tonelli et al, 2010), barrier-island erosion and rollover (Theuerkauf and Rodriguez, 2017), and ponding following loss of marsh vegetation (Wilson et al, 2014; Figure 1). The erosion and redistribution of marsh shoreline sediment is a constant process and wave energy and geomorphology are primary drivers of erosion rates (Fagherazzi et al, 2013;Mariotti and Carr, 2014;Leonardi et al, 2016;Ganju et al, 2020). Development of coastal property and maintenance of navigation channels can also result in the degradation of salt marshes and release of buried sediment carbon.…”

Section: Introductionmentioning

confidence: 99%

Refining Estimates of Greenhouse Gas Emissions From Salt Marsh “Blue Carbon” Erosion and Decomposition

2021

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Coastal wetlands have sediments that contain organic matter preserved against decomposition for timespans that can range up to millennia. This “blue carbon” in wetland sediments has been proposed as a sink for atmospheric carbon dioxide and a potential source of greenhouse gases if coastal habitats are lost. A missing gap in the role of coastal habitats in the global carbon cycle is elucidating the fate of wetland sediment carbon following disturbance events, such as erosion, that can liberate organic matter to an oxygenated environment where decomposition can more readily occur. Here, we track the fate of previously stored salt marsh sediment by measuring the production of carbon dioxide (CO2) and methane (CH4) during an oxygenated incubation. Sediments from two depth horizons (5–10 cm and 20–25 cm) were incubated at two temperatures (20 and 30°C) for 161 days. Q10 of the decomposition process over the entire course of the experiment was 2.0 ± 0.1 and 2.2 ± 0.2 for shallow and deep horizons, respectively. Activation energy for the decomposition reaction (49.7 kJ ⋅ mol–1 and 58.8 kJ ⋅ mol–1 for shallow and deep sediment horizons, respectively) was used to calculate temperature-specific decomposition rates that could be applied to environmental data. Using high-frequency water temperature data, this strategy was applied to coastal states in the conterminous United States (CONUS) where we estimated annual in situ decomposition of eroded salt marsh organic matter as 7–24% loss per year. We estimate 62.90 ± 2.81 Gg C ⋅ yr–1 is emitted from eroded salt marsh sediment decomposition in the CONUS.

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Are Elevation and Open‐Water Conversion of Salt Marshes Connected?

Cited by 31 publications

References 28 publications

Dynamics of Marsh‐Derived Sediments in Lagoon‐Type Estuaries

Dynamics of Marsh‐Derived Sediments in Lagoon‐Type Estuaries

Coastal Wetland Resilience, Accelerated Sea‐Level Rise, and the Importance of Timescale

Refining Estimates of Greenhouse Gas Emissions From Salt Marsh “Blue Carbon” Erosion and Decomposition

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