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1. Vulnerability of Wetlands Due to Projected Sea-Level Rise in the Coastal Plains of the South and Southeast United States

   https://doi.org/10.3390/rs16122052  

   Lizcano-Sandoval, Luis; Gibeaut, James; McCarthy, Matthew J; Murray, Tylar; Rueda-Roa, Digna; Muller-Karger, Frank E (June 2024, Remote Sensing)

   NA (Ed.)

   Coastal wetlands are vulnerable to accelerated sea-level rise, yet knowledge about their extent and distribution is often limited. We developed a land cover classification of wetlands in the coastal plains of the southern United States along the Gulf of Mexico (Texas, Louisiana, Mississippi, Alabama, and Florida) using 6161 very-high (2 m per pixel) resolution WorldView-2 and WorldView-3 satellite images from 2012 to 2015. Area extent estimations were obtained for the following vegetated classes: marsh, scrub, grass, forested upland, and forested wetland, located in elevation brackets between 0 and 10 m above sea level at 0.1 m intervals. Sea-level trends were estimated for each coastal state using tide gauge data collected over the period 1983–2021 and projected for 2100 using the trend estimated over that period. These trends were considered conservative, as sea level rise in the region accelerated between 2010 and 2021. Estimated losses in vegetation area due to sea level rise by 2100 are projected to be at least 12,587 km2, of which 3224 km2 would be coastal wetlands. Louisiana is expected to suffer the largest losses in vegetation (80%) and coastal wetlands (75%) by 2100. Such high-resolution coastal mapping products help to guide adaptation plans in the region, including planning for wetland conservation and coastal development. 

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2. How Old Are Marshes on the East Coast, USA? Complex Patterns in Wetland Age Within and Among Regions

   https://doi.org/10.1029/2020GL089415  

   Braswell, Anna E.; Heffernan, James B.; Kirwan, Matthew L. (October 2020, Geophysical Research Letters)

   Abstract Sea‐level dynamics, sediment availability, and marine energy are critical drivers of coastal wetland formation and persistence, but their roles as continental‐scale drivers remain unknown. We evaluated the timing and spatial variability of wetland formation from new and existing cores collected along the Atlantic and Gulf coasts of the United States. Most basal peat ages occurred after sea‐level rise slowed (after ~4,000 years before present), but predominance of sea‐level rise studies may skew age estimates toward older sites. Near‐coastal sites tended to be younger, indicating creation of wetlands through basin infilling and overwash events. Age distributions differed among regions, with younger wetlands in the northeast and southeast corresponding to European colonization and deforestation. Across all cores, wetland age correlated strongly with basal peat depth. Marsh age elucidates the complex interactions between sea‐level rise, sediment supply, and geomorphic setting in determining timing and location of marsh formation and future wetland persistence. 

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3. Blue Carbon Soil Stock Development and Estimates Within Northern Florida Wetlands

   https://doi.org/10.3389/feart.2021.552721  

   Vaughn, Derrick R.; Bianchi, Thomas S.; Shields, Michael R.; Kenney, William F.; Osborne, Todd Z. (February 2021, Frontiers in Earth Science)

   Blue carbon habitats, such as mangroves and salt marshes, have been recognized as carbon burial hotspots; however, methods on measuring blue carbon stocks have varied and thus leave uncertainty in global blue carbon stock estimates. This study analyzes blue carbon stocks in northern Florida wetlands along the Atlantic and Gulf coasts. Carbon measurements within 1–3m length vibracores yield total core stocks of 9.9–21.5 kgC·m −2 and 7.7–10.9 kgC·m −2 for the Atlantic and Gulf coast cores, respectively. Following recent IPCC guidelines, blue carbon stock estimates in the top meter are 7.0 kgC·m −2 –8.0 kgC·m −2 and 6.1 kgC·m −2 –8.6 kgC·m −2 for the Atlantic and Gulf cores, respectively. Changes in stable isotopic (δ 13 C, C/N) and lignin biomarker (C/V) indices suggest both coastlines experienced salt marsh and mangrove transgressions into non-blue carbon habitats during the mid- to late-Holocene following relative sea-level rise. These transgressions impact carbon storage within the cores as the presence of carbon-poor soils, characteristic of non-blue carbon habitats, result in lower 1m carbon stocks in north Florida Gulf wetlands, and a deeper extent of carbon-rich soils, characteristic of blue carbon habitats, drive higher 1m and total carbon stocks in north Florida Atlantic wetlands. Future blue carbon research should assess carbon stocks down to bedrock when possible, as land-cover and/or climate change can impact different depths across localities. Ignoring carbon-rich soil below the top meter of soil may underestimate potential carbon emissions based on these changes. 

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4. Rapid and large changes in coastal wetland structure in China's four major river deltas

   https://doi.org/10.1111/gcb.16583  

   Wang, Xinxin; Xiao, Xiangming; Zhang, Xi; Wu, Jihua; Li, Bo (January 2023, Global Change Biology)

   Abstract Coastal wetlands provide essential ecosystem goods and services but are extremely vulnerable to sea‐level rise, extreme climate, and human activities, especially the coastal wetlands in large river deltas, which are regarded as “natural recorders” of changes in estuarine environments. In addition to the area (loss or gain) and quality (degradation or improvement) of coastal wetlands, the information on coastal wetland structure (e.g., patch size and number) are also major metrics for coastal restoration and biodiversity protection, but remain very limited in China's four major river deltas. In this study, we quantified the spatial–temporal dynamics of total area (TA) and patch number (PN) of coastal wetlands with different sizes in the four deltas and the protected areas (PAs) and assessed the effects of major driving factors during 1984–2020. We also investigated the effectiveness of PAs through the comparison of TA and PN of coastal wetlands before and after the years in which PAs were listed as Ramsar Sites. We found both TA and PN experienced substantial losses in the Liaohe River Delta and Yellow River Delta but recent recoveries in the Yangtze River Delta. The coastal wetlands had a relatively stable and variable trend in TA but had a continually increasing trend in PN in the Pearl River Delta. Furthermore, reduced coastal reclamation, ecological restoration projects, and rapid expansion of invasive plants had great impacts on the coastal wetland structure in various ways. We also found that PAs were effective in halting the decreasing trends in coastal wetland areas and slowing the expansion of reclamation, but the success of PAs is being counteracted by soaring exotic plant invasions. Our findings provide vital information for the government and the public to address increasing challenges of coastal restoration, management, and sustainability in large river deltas. 

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5. Climate-driven decoupling of wetland and upland biomass trends on the mid-Atlantic coast

   https://doi.org/10.1038/s41561-022-01041-x  

   Chen, Yaping; Kirwan, Matthew L. (October 2022, Nature Geoscience)

   Coastal ecosystems represent a disproportionately large but vulnerable global carbon sink. Sea-level-driven tidal wetland degradation and upland forest mortality threaten coastal carbon pools, but responses of the broader coastal landscape to interacting facets of climate change remain poorly understood. Here, we use 36 years of satellite observations across the mid-Atlantic sea-level rise hotspot to show that climate change has actually increased the amount of carbon stored in the biomass of coastal ecosystems despite substantial areal loss. We find that sea-level-driven reductions in wetland and low-lying forest biomass were largely confined to areas less than 2 m above sea level, whereas the otherwise warmer and wetter climate led to an increase in the biomass of adjacent upland forests. Integrated across the entire coastal landscape, climate-driven upland greening offset sea-level-driven biomass losses, such that the net impact of climate change was to increase the amount of carbon stored in coastal vegetation. These results point to a fundamental decoupling between upland and wetland carbon trends that can only be understood by integrating observations across traditional ecosystem boundaries. This holistic approach may provide a template for quantifying carbon–climate feedbacks and other aspects of coastal change that extend beyond sea-level rise alone. 

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