Vertical accretion trends project doughnut-like fragmentation of saltmarshes

The survival of coastal saltmarshes under rising sea levels depends on their ability to migrate upland or maintain vertical elevation. This elevation maintenance is achieved through organic matter production and inorganic sediment deposition. Previous research, using both modeling and field studies, suggests that marshes with high rates of organic production or inorganic sediment delivery can maintain elevation even at high relative sea-level rise (RSLR) rates. Factors influencing this include enhanced sediment trapping by plants under greater inundation, increased root productivity under higher temperatures and CO2 levels, increased sediment and nutrient delivery from storms, and enhanced pore space in organic-rich marshes. However, saltmarsh responses aren't uniformly positive. Mineral sediment availability is limited, particularly in high-latitude areas; warming can accelerate decomposition and reduce root biomass allocation; and storms, while providing sediment and nutrients, can also cause vegetation die-off and other negative impacts. This has led to concerns about the ability of saltmarshes to keep pace with sea-level rise. Sediment availability is crucial for marsh resilience, and only marshes receiving high sediment inputs are expected to withstand accelerating sea-level rise. While large-scale studies demonstrate marsh resilience and accelerating accretion in response to sea-level rise, often through increased sediment delivery, they may not capture the local processes controlling accretion. This study focuses on barrier-associated marshes of the Georgia Bight (southeastern USA), which are different from the well-studied Gulf Coast marshes. Georgia Bight marshes are fronted by wide barrier islands that restrict storm surge and wave action, except near inlets, and are fed by moderate-sized rivers. These marshes are wider, contain coarser sediment, and experience higher RSLR rates than Gulf Coast marshes. They also show latitudinal variation in wave and tidal influence, providing a useful proxy for mesotidal, barrier-associated marshes in various settings. Previous research on the Georgia Bight has often focused on marsh interiors, while this study examines accretion rates near marsh edges to understand local variability.

Existing literature highlights the importance of sediment availability and the interplay between organic matter production and inorganic sediment deposition in determining saltmarsh resilience to sea-level rise. Studies have shown that marshes with high sediment inputs can maintain their elevation, even under accelerated RSLR. However, limitations exist in sediment availability, particularly in high-latitude regions. Furthermore, warming temperatures can negatively impact marsh health by increasing decomposition rates and reducing biomass allocation to roots. Storms, while sometimes beneficial by delivering sediment, can also cause damage and vegetation loss. Modeling studies and large-scale comparisons often lack the detail needed to understand the fine-scale processes controlling marsh accretion. This study aims to address this gap by focusing on the local-scale processes of sediment delivery in the Georgia Bight, a region with distinct characteristics from previously well-studied marsh systems. The existing literature suggests a complex interplay of factors influencing marsh resilience, with sediment availability playing a key role. This study aims to further explore the spatial variability in accretion rates and their relationship to local environmental conditions.

This study used 16 sediment cores collected near marsh edges along mainland-to-barrier transects behind four barrier island systems in the Georgia Bight: Cape Romain (SC), Hilton Head (SC), Sapelo Island (GA), and Amelia Island (FL). The cores were collected approximately 10 meters from the marsh edge and landward of any levees, in areas representing a range of exposures to open water. Short-lived radioisotopes (²¹⁰Pb and ¹³⁷Cs) were analyzed to determine accretion rates. The Constant Flux with Constant Sedimentation (CFCS) model was used to calculate ²¹⁰Pb-based accretion rates, assuming constant accumulation rate and ²¹⁰Pb flux. Accretion rates from ¹³⁷Cs were also calculated and compared to ²¹⁰Pb results. Bulk density and loss-on-ignition were measured to assess organic matter content. Core elevations were recorded using RTK-GPS and converted to elevations relative to mean tidal level (MTL). These elevations were normalized to the local tidal range (Z_MHW). Contemporaneous accretion excess (AEC) and 50-year accretion excess (AE₅₀) were calculated to compare accretion rates to contemporaneous and 50-year RSLR rates, respectively, to account for potential lagged responses. The study also incorporated data from a regional database of accretion and/or surface elevation-change estimates compiled from ²¹⁰Pb, ¹³⁷Cs, Surface Elevation Tables (SET), and Marker Horizons (MH) to provide a regional perspective. Statistical analyses, including multiple linear regressions, were conducted to investigate relationships between accretion rates, elevation, bulk density, loss-on-ignition, and RSLR rates.

The ²¹⁰PbCFCS accretion rates ranged from 1.91 ± 0.07 mm yr^-1 to 13.91 ± 0.07 mm yr^-1, with a mean of 6.29 ± 0.18 mm yr^-1. Fourteen of the 16 stations showed accretion exceeding contemporaneous RSLR (AEC > 1), with a mean AEC of 1.9 ± 0.3, indicating accretion rates nearly twice the RSLR rate. Similarly, most stations showed accretion exceeding the 50-year RSLR rate (AE₅₀ > 1), with a mean AE₅₀ of 2.0 ± 0.1. Higher accretion rates correlated weakly with higher elevations (Z_MHW), a finding contrary to expectations but possibly due to the limited elevation range of the study area. No significant correlations were found between AEC/AE₅₀ and bulk density or loss-on-ignition. Regional comparison incorporating data from other studies revealed a lack of large-scale spatial trends in accretion rates, AEC, or AE₅₀ across the Georgia Bight, despite tidal range variation. Accretion rates showed a better correlation with 50-year RSLR rates than contemporaneous rates, suggesting a lagged response. The overall mean AE₅₀ for the Georgia Bight was 0.94 ± 0.16, indicating that marshes generally keep pace with RSLR. However, excluding the new data from this study, the mean AE₅₀ drops to 0.72 ± 0.17, indicating that most marshes are not keeping pace with RSLR. Marshes closest to open water bodies and tidal inlets showed the highest accretion rates, suggesting that sediment delivery from wave action plays a crucial role. In contrast, marshes in more interior, sheltered locations exhibited lower accretion rates. Statistically significant differences (p≤0.0001, ANOVA) were found between accretion rates of the study's new cores and previously reported rates. These differences might stem from the locations of prior studies within the marsh platform. While allochthonous mineral sediments showed a positive relationship with marsh resilience, no clear correlation was found in this specific study. Many fast-accreting sites showed evidence of storm deposits indicating a significant contribution of event-based sedimentation. This fast accretion is observed mostly near the marsh edge and could be attributed to soil creep and recycling processes.

This study demonstrates significant spatial variability in saltmarsh resilience to sea-level rise within the Georgia Bight. While some edge marshes, particularly those near inlets and bays, exhibit high accretion rates exceeding RSLR, many interior marshes are failing to keep pace. The strong influence of sediment delivery from wave-driven processes near the marsh edge highlights the importance of considering local hydrodynamic conditions when assessing marsh resilience. The finding of a weak positive correlation between elevation and accretion, contrary to established relationships, suggests the importance of edge processes and potential sediment recycling. The discrepancy between the accretion rates from this study and previously reported rates underscores the need for spatially diverse sampling across a range of environments within backbarrier settings to account for complex spatial patterns of resilience. The observed pattern of high accretion at exposed edges and low accretion in marsh interiors is consistent with models predicting internal fragmentation due to limited sediment delivery to the interior. The study's findings emphasize the complex interplay between sediment availability, hydrodynamic forcing, and spatial location in determining marsh resilience. This has strong implications for predicting future marsh survival under continued sea-level rise and highlights the need to consider these factors in conservation efforts.

This study reveals a spatial dichotomy in saltmarsh resilience to sea-level rise in the Georgia Bight. Highly exposed, edge marshes, benefiting from sediment influx, show remarkable resilience, while interior marshes are struggling to keep pace. This indicates a trend towards rapid, doughnut-like fragmentation, with persistent edge marshes and drowning interiors. The findings emphasize the importance of considering local-scale sediment delivery processes and spatial heterogeneity when assessing marsh vulnerability. Future research should focus on quantifying sediment sources and transport pathways, examining the long-term impacts of storm events on marsh accretion, and exploring the potential for management interventions to enhance sediment delivery to marsh interiors.

The study's focus on a limited number of sites within the Georgia Bight may limit the generalizability of the findings to other regions. The reliance on ²¹⁰Pb and ¹³⁷Cs dating may not fully capture the influence of very recent changes in accretion rates. The study primarily focuses on vertical accretion and may not fully capture lateral marsh migration dynamics. The relatively short timescale captured by some data sources may not fully capture long-term trends in marsh response to sea-level rise. The relatively limited range of elevations within the sampled marsh platform also adds to limitations, and may obfuscate observed correlation between elevation and accretion. The inclusion of data from diverse sources adds further complication given the various methodologies used in collecting and analyzing prior data; the resulting variability in methodology should be considered when evaluating the overall conclusions.