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1. Temporal variability in soil organic matter accretion rates in coastal deltaic wetlands under changing depositional environments

   https://doi.org/10.1002/saj2.20515  

   Sapkota, Yadav ; Xu, Kehui ; Maiti, Kanchan ; Inglett, Patrick ; White, John R. ( February 2023 , Soil Science Society of America Journal)

   The Mississippi River Deltaic Plain experiences high relative sea level rise, limited sediment supply, and high marsh edge erosion, leading to the substantial coastal wetland and stored soil organic matter (SOM) loss. The objective of this study was to understand the SOM accumulation rates over the past 1000 years related to the changes in the depositional environment in these highly eroding coastal wetlands. Soil cores (2 m) were collected from four sites in Barataria Basin, LA and analyzed for proportion of organic and mineral matter, total C, N, P, particle size, and stable isotopic composition (δ¹³C and δ¹⁵N), as well as¹⁴C and¹³⁷Cs dating. The soil carbon stock in the 2 m depth (62.4 ± 2 kg m⁻²) was approximately 88% greater than the carbon stock in just the 1 m depth (33.1 ± 0.6 kg m⁻²) indicating a need for considering deeper soil profiles (up to 2 m) to estimate blue carbon stock in deltaic environments. The average vertical accretion rate for Barataria Basin was 8.1 ± 0.6 mm year⁻¹over 50 years. The long‐term (1000‐year time scale) C accumulation rate (39 g C m⁻²year⁻¹) was ∼14% of the short‐term accumulation rate (254 ± 19 g C m⁻²year⁻¹). Wetlands in Barataria Basin started as fresh marsh and transitioned over time to intermediate to brackish. These marshes were able to maintain relative elevation through the accumulation of organic matter and mud despite high relative rates of sea‐level rise. However, the high rates of edge erosion may limit these marshes to continue to sequester atmospheric carbon under accelerating sea level in the absence of restoration efforts.

    

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2. Elevation Changes in Restored Marshes at Poplar Island, Chesapeake Bay, MD: II. Modeling the Importance of Marsh Development Time

   https://doi.org/10.1007/s12237-024-01342-x  

   Morris, James T. ; Staver, Lorie W. ( April 2024 , Estuaries and Coasts)

   Tidal marshes in the Chesapeake Bay are vulnerable to the accelerating rate of sea-level rise (SLR) and subsidence. Restored and created marshes face the same risks as natural marshes, and their resilience to SLR may depend upon appropriate design and implementation. Here, the Coastal Wetland Equilibrium Model (CWEM) was used to assess the resilience of tidal marshes at the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (PI) in mid-Chesapeake Bay, MD, where dredged material from navigation channels is being used to create new tidal marshes planted withSpartina alterniflorain the low marsh andS. patensin the high marsh. The site is microtidal with low inorganic sediment inputs, where the rate of marsh elevation change is dominated by the production of organic matter and, therefore, is proportional to net ecosystem production (NEP). The model demonstrated the importance of marsh development for surface elevation gain. In created marshes, the buildout of belowground biomass adds volume and results in faster growth of marsh elevation, but the gains slow as the marsh matures. Elevation gain is the lessor of the recalcitrant fraction of NEP sequestered in sediment or the rate of increase in accommodation space. Marshes can keep up with and fill accommodation space with sequestered NEP up to a tipping point determined by the rate of SLR. The PI low marsh platform was forecasted to drown in about 43 years after construction at the current rate of SLR. Marsh loss can be mitigated by periodic thin layer placement (TLP) of sediment. CWEM was used to simulate PI marsh responses to different TLP strategies and showed that there is an optimal design that will maximize carbon sequestration and resilience depending on the trajectory of mean sea level.

    

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3. Future Marsh Evolution Due To Tidal Changes Induced by Human Adaptation to Sea Level Rise

   https://doi.org/10.1029/2023EF003518  

   Balderas Guzman, Celina ; Buffington, Kevin J. ; Thorne, Karen M. ; Guntenspergen, Glenn R. ; Hummel, Michelle A. ; Stacey, Mark T. ( September 2023 , Earth's Future)

   With sea level rise threatening coastal development, decision‐makers are beginning to act by modifying shorelines. Previous research has shown that hardening or softening shorelines may change the tidal range under future sea level rise. Tidal range can also be changed by natural factors. Coastal marshes, which humans increasingly depend on for shoreline protection, are ecologically sensitive to tidal range. Therefore, it is critical to examine how changes in tidal range could influence marsh processes. A marsh accretion model was used to investigate the ecological response of a San Francisco Bay, California, USA marsh to multiple tidal range scenarios and sea level rise from 2010 to 2100. The scenarios include a baseline scenario with no shoreline modifications in the estuary, a shoreline hardening scenario that amplifies the tidal range, and 14 tidal range scenarios as a sensitivity analysis that span tidal amplification and reduction of the baseline scenario. The modeling results expose key tradeoffs to consider when planning for sea level rise. Compared to the baseline, the hardening scenario shows minor differences. However, further tidal amplification prolongs marsh survival but decreasesSarcocornia pacificacover, an important species for certain threatened wildlife and an effective attenuator of wave energy. Conversely, tidal reduction precipitates marsh drowning but shows gains inSarcocornia pacificacover. These mixed impacts of tidal amplification and reduction shown by the model indicate potential tradeoffs in relation to marsh survival, habitat characteristics, and shoreline protection. This study suggests the need for a cross‐sectoral, regional approach to sea level rise adaptation.

    

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4. Modeling long‐term salt marsh response to sea level rise in the sediment‐deficient Plum Island Estuary, MA

   https://doi.org/10.1002/lno.11444  

   Langston, Amy K. ; Durán Vinent, Orencio ; Herbert, Ellen R. ; Kirwan, Matthew L. ( April 2020 , Limnology and Oceanography)

   An accelerating global rate of sea level rise (SLR), coupled with direct human impacts to coastal watersheds and shorelines, threatens the continued survival of salt marshes. We developed a new landscape‐scale numerical model of salt marsh evolution and applied it to marshes in the Plum Island Estuary (Massachusetts, U.S.A.), a sediment‐deficient system bounded by steep uplands. To capture complexities of vertical accretion across the marsh platform, we employed a novel approach that incorporates spatially variable suspended sediment concentrations and biomass of multiple plant species as functions of elevation and distance from sediment sources. The model predicts a stable areal extent of Plum Island marshes for a variety of SLR scenarios through 2100, where limited marsh drowning is compensated by limited marsh migration into adjacent uplands. Nevertheless, the model predicts widespread conversion of high marsh vegetation to low marsh vegetation, and accretion deficits that indicate eventual marsh drowning. Although sediment‐deficient marshes bounded by steep uplands are considered extremely vulnerable to SLR, our results highlight that marshes with high elevation capital can maintain their areal extent for decades to centuries even under conditions in which they will inevitably drown.

    

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5. Multi‐Decadal Simulation of Marsh Topography Under Sea Level Rise and Episodic Sediment Loads

   https://doi.org/10.1029/2021JF006526  

   Brand, M. W. ; Buffington, K. ; Rogers, J. B. ; Thorne, K. ; Stein, E. D. ; Sanders, B. F. ( August 2022 , Journal of Geophysical Research: Earth Surface)

   Coastal marsh within Mediterranean climate zones is exposed to episodic watershed runoff and sediment loads that occur during storm events. Simulating future marsh accretion under sea level rise calls for attention to: (a) physical processes acting over the time scale of storm events and (b) biophysical processes acting over time scales longer than storm events. Using the upper Newport Bay in Southern California as a case study, we examine the influence of event‐scale processes on simulated change in marsh topography by comparing: (a) a biophysical model that integrates with an annual time step and neglects event‐scale processes (BP‐Annual), (b) a physical model that resolves event‐scale processes but neglects biophysical interactions (P‐Event), and (c) a biophysical model that resolves event‐scale physical processes and biophysical processes at annual and longer time scales (BP‐Event). A calibrated BP‐Event model shows that large (>20‐year return period) episodic storm events are major drivers of marsh accretion, depositing up to 30 cm of sediment in one event. Greater deposition is predicted near fluvial sources and tidal channels and less on marshes further from fluvial sources and tidal channels. In contrast, the BP‐Annual model poorly resolves spatial structure in marsh accretion as a consequence of neglecting event‐scale processes. Furthermore, the P‐Event model significantly overestimates marsh accretion as a consequence of neglecting marsh surface compaction driven by annual scale biophysical processes. Differences between BP‐Event and BP‐Annual models translate up to 20 cm per century in marsh surface elevation.

    

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