Marsh Erosion Research Articles

Bacterial and fungal communities exhibit contrasting patterns in response to marsh erosion: A fine-scale observation

Coastal salt marshes, especially their seaward boundaries, are experiencing severe erosion and area loss worldwide because of human disturbances and natural changes. However, how marsh erosion affects soil microbial communities and their potential functions remains poorly understood in subtropical coastal ecosystems. Herein, simultaneous measurements of the abundance, diversity, and composition of bacterial and fungal communities were conducted along a marsh erosion gradient, based on Illumina sequencing of 16S rRNA and ITS genes. Our results indicate that marsh erosion is often accompanied by a loss of plant biomass and a decrease in soil total carbon, total nitrogen, and organic matter concentrations, but an increase in soil water content. We found that the abundance of bacteria and fungi tended to decrease with erosion, especially for bacterial communities. Marsh erosion had opposite effects on the alpha-diversity of bacterial and fungal communities, with bacterial diversity decreasing but fungal diversity increasing with marsh erosion. The beta-diversity and composition of bacterial and fungal communities were altered by marsh erosion, and in particular bacterial communities were clustered into three groups: uneroded, eroding, and eroded. Network analysis demonstrated that the inter-relationships of the bacterial community did not significantly change with the erosion gradient, but the complexity of the fungal networks increased. The diversity and composition of the bacterial and fungal communities can be jointly explained by plant biomass, soil nutrient dynamics, and bulk density. We conclude that marsh erosion-induced changes in soil properties and plant loss altered the diversity and composition of bacterial and fungal communities, which decrease bacterial abundance and diversity but increase fungal diversity and network complexity, suggesting that these microbial taxa contribute differently to soil function and nutrient cycling as marsh erosion continues.

Recent environmental and morphosedimentary evolution of the mining-impacted Nalón Estuary (Asturias, N Spain): Disentangling natural and anthropogenic processes

Distinguishing between natural and anthropogenic processes in sedimentary records from estuaries with legacy pollutants is an essential task, as it provides baselines to predict future environmental trajectories of coastal areas. Here, we have addressed the recent transformation history of the mining-impacted Nalón Estuary (Asturias, N Spain). Surface and core sediment records from marshes and tidal flats were examined through a broad multidisciplinary approach, involving micropaleontological (benthic foraminifera), sedimentological (grain-size), geochemical (trace metals, major element Al and total organic carbon), physical (magnetic susceptibility, frequency-dependent magnetic susceptibility and large microplastics) and radioisotopic (210Pb, 137Cs and 239+240Pu) proxies. Results suggest that the interplay between natural (high fluvial influence and extreme hydrological events) and anthropogenic (coal and mercury mining disposals) factors induced strong sedimentation-erosion processes, further shaping the recent evolution of the estuary. Short-time scale and intense sedimentation processes were revealed by overall high sediment accumulation rates, the dilution of some geological signatures and the rapid formation of a marsh in the lower estuary bay. The increasing mining fingerprints during the 20th century were shortly interrupted by the catastrophic riverine flooding of 1938. Conversely, current erosional processes by fluvial influence led to the remobilization of contaminated sedimentary materials and exposure of mining-legacy Hg levels in tidal flats from the middle sector. Fluvial activity, floodings and taphonomic biases exerted a major control on benthic foraminifera since the 19th century, although Hg ecotoxicological effects on modern assemblages at certain areas within the estuary cannot be discarded. These findings, along with the documented enhanced erosion of marshes with ‘trapped’ pollutants (Hg, coal microparticles and microplastics), highlight the importance of monitoring the environmental and geomorphic processes taking place in historically-contaminated estuaries.

Linking marsh sustainability to event-based sedimentary processes: Impulsive river floods initiated lateral erosion of deltaic marshes

Salt marshes providing valuable services in buffering flooding risks are regarded as cost-effective coastal defense solutions. Given that eroding marsh cliffs will threaten the sustainability of these protective functions, there is a need for mechanistic understanding of cliff formation. Based on short-term field measurements together with satellite-derived dataset and a morphodynamic model, we determined the response of marsh lateral dynamics to event-based sedimentary processes in the Yellow River Delta, China. It was found that the Eastern Marsh with a cliff of ∼1.2 m in height significantly differed from the gentle Western Marsh in accretion pattern and lateral dynamics. Attributed to an artificial flood lasting ∼20 days in 2022, the Eastern Marsh platform experienced average vertical accretion of 23.3 mm, ∼5 times higher than the Western Marsh barely impacted by fluvial sediment supply. Although the accretion pattern ensured the overall vertical adaptability, Eastern Marsh has translated from a phase of rapid expansion to lateral retreat of ∼60 m/yr since 2018, whereas Western Marsh was relatively stable in recent decade. With a validated delta-marsh model, we showed that the observed disparities were attributed to different marsh edge morphodynamic responses to river flood sediment supplies. When Spartina alterniflora initially colonized, the Eastern Marsh platform was lower and the trapping efficiency could rise by ∼3 times than the present. Integrating with ten-fold increases in sediment loads, it was estimated that the large artificial floods in 2018 could contribute to a vertical growth approximating 30 cm at Eastern Marsh. Such episodic but rapid marsh accretion played a crucial role in initiating the development of marsh cliff. Driven by positive feedbacks between platform elevations and cumulative wave thrust, a series of event-based marsh sedimentation will promote long-term marsh loss caused by lateral retreat at sediment-starved systems. This study provides insights in predicting marsh sustainability and long-term evolutions under episodic sedimentary events.

Top-predator recovery abates geomorphic decline of a coastal ecosystem.

The recovery of top predators is thought to have cascading effects on vegetated ecosystems and their geomorphology1,2, but the evidence for this remains correlational and intensely debated3,4. Here we combine observational and experimental data to reveal that recolonization of sea otters in a US estuary generates a trophic cascade that facilitates coastal wetland plant biomass and suppresses the erosion of marsh edges-a process that otherwise leads to the severe loss of habitats and ecosystem services5,6. Monitoring of the Elkhorn Slough estuary over several decades suggested top-down control in the system, because the erosion of saltmarsh edges has generally slowed with increasing seaotter abundance, despite the consistently increasing physical stress in the system (that is, nutrient loading, sea-level rise and tidal scour7-9). Predator-exclusion experiments in five marsh creeks revealed that sea otters suppress the abundance of burrowing crabs, a top-down effect that cascades to both increase marsh edge strength and reduce marsh erosion. Multi-creek surveys comparing marsh creeks pre- and post-sea otter colonization confirmed the presence of an interaction between the keystone sea otter, burrowing crabsand marsh creeks, demonstrating the spatial generality of predator control of ecosystem edge processes: densities of burrowing crabs and edge erosion have declined markedly in creeks that have high levels of seaotter recolonization. These results show that trophic downgrading could be a strong but underappreciated contributor to the loss of coastal wetlands, and suggest that restoring top predators can help to re-establish geomorphic stability.

Linking soil phosphorus availability and phosphatase functional genes to coastal marsh erosion: Implications for nutrient cycling and wetland restoration

Accelerated marsh erosion caused by climate change and human activity may have important implications for nutrient cycling and availability. However, how erosion affects phosphorus (P) transformation and microbial function in subtropical coastal marshes remains largely unknown. Here we assessed soil P fractions, availability and the phoD-harboring bacterial community along a marsh erosion gradient (non-eroded, lightly eroded, and heavily eroded). We showed that marsh erosion caused a shift in P fractions, leading to a decrease in P availability and a reduction in concentrations of labile P, moderately labile P, and stable P by 20 %, 9 %, and 17 % respectively. The abundance and diversity of phoD phosphatase genes decreased dramatically along the erosion gradient and were lower at heavily eroded sites than at non-eroded sites. Marsh erosion reshaped phoD gene community composition, and Corallococcus, Amycolatopsis, and Phaeobacter were identified as the dominant phoD-harboring microbes. Notably, marsh erosion reduced the complexity and stability of the phoD-harboring bacterial network, and heavily eroded sites have fewer network edges and nodes than non-eroded sites. The dynamics of soil P fractions, availability, and phoD-harboring bacterial communities driven by marsh erosion are largely shaped by substrate availability and soil properties (e.g., nutrients, pH, electrical conductivity, and moisture). Additionally, strong linkages between P availability and the abundance and diversity of phosphatase genes following erosion, suggest that phosphatase drives P mineralization and dissolution, and erosion weakens the regulation of P transformation by reshaping the phoD phosphatase gene community. Our findings indicate that marsh erosion alters soil P fractions and phoD-harboring bacterial communities, which weakens microbial regulation of P transformation and availability, thereby significantly reducing soil P pools and availability. Our findings broaden understanding of the impacts of coastal erosion on nutrient balance and ecosystem function, offering valuable perspectives that could inform wetland restoration and coastal management strategies.

Erosion and accretion of salt marsh in extremely shallow water stages

Salt marshes, which commonly exist on the upper tidal flat, provide a natural barrier against sea level rise and coastal storm. The extremely shallow water stages (water depth< 0.2 m), including the initial stage of flood tides and the last stage of ebb tides, can induce a significant impact on sediment dynamics of saltmarshes and associated tidal flats, despite lasting for only a short time (around 10 min), which has been less studied. In this study, two parallel field sites were established to quantify erosion-accretion processes and morphological changes during extremely shallow water stages in salt marshes within Doulonggang tidal flat along the Jiangsu coast. Our results revealed that obvious accretion occurred during extremely shallow water stages, with a total deposition amount of +33.8 mm in vegetated areas and +20.8 mm in unvegetated areas. In contrast, erosion dominated during deep water stages, with a total erosion amount of -22.3 mm at the vegetated site and -32.7 mm at the unvegetated site. The magnitude of bed-level change during extremely shallow water stages was 7~8 times greater than that during deep water stages, even though the duration of extremely shallow water stages was only about 14~15% of the entire tidal cycle. Furthermore, strong winds significantly impacted deposition during extremely shallow water stages compared to calm weather. During the strong wind period, the average bed level change rate reached +0.15 mm/min and +0.12 mm/min in the vegetated and unvegetated areas, respectively. This is significantly higher than the +0.05 mm/min and +0.01 mm/min during the calm weather period. These results reveal that extremely shallow water stages have substantial impacts on sedimentary processes, which are vital for the maintenance of tidal flat systems.

Near‐continuous monitoring of a coastal salt marsh margin: Implications for predicting marsh edge erosion

AbstractMechanisms that control marsh edge erosion include wind‐generated waves, vegetation productivity, land use and land change, and geotechnical properties of sediments. However, existing models for predicting marsh edge evolution focus primarily on edge retreat rates as a function of wave energy while accounting for other controlling factors as empirical constants. This simplification arises from a lack of high‐frequency monitoring of marsh evolutions. In particular, marsh erosion is timescale dependent, and conducting field observations on short temporal and spatial scales could elucidate the progression of erosion, which may improve marsh erosion predictive models. This study developed and validated a near‐continuous camera and erosion pin monitoring system to document marsh edge erosion at a high frequency (i.e., daily) in Terrebonne Bay, Louisiana. This was supplemented with daily wave power to explore the relationships between daily erosion and wave power. Long‐term average erosion rates derived from satellite and aerial imagery from 1989 through 2019 compare similarly to rates derived from longer‐term site visits (i.e., monthly) at approximately 2.2 m/yr. High‐magnitude erosion events (>20 cm/day) are driven by a buildup in wave energy over a 7‐day time period coupled with a strong 1‐day wave event, indicating a gradual reduction in marsh edge resistance with continued wave attack. Long‐term erosion monitoring methods, including monthly field visits, provide results that align well with previously reported relationships between wave power and erosion. High‐frequency measurements, however, illustrate that the previously published trends smooth over the large‐magnitude short‐term erosion events, potentially obscuring the physical processes of marsh edge erosion. For example, satellite and aerial imagery provide a long period of record, but they may underestimate the average annual erosion rate in the region, the effect of which may become exasperated over the varying temporal scales considered in coastal planning efforts across the USA and worldwide.

Hydrodynamic Feedbacks of Salt‐Marsh Loss in the Shallow Microtidal Back‐Barrier Lagoon of Venice (Italy)

AbstractExtensive loss of salt marshes in back‐barrier tidal embayments is ongoing worldwide as a consequence of land‐use changes, wave‐driven lateral marsh erosion, and relative sea‐level rise compounded by mineral sediment starvation. However, how salt‐marsh loss affects the hydrodynamics of back‐barrier systems and feeds back into their morphodynamic evolution is still poorly understood. Here we use a depth‐averaged numerical hydrodynamic model to investigate the feedback between salt‐marsh erosion and hydrodynamic changes in the Venice Lagoon, a large microtidal back‐barrier system in northeastern Italy. Numerical simulations are carried out for past morphological configurations of the lagoon dating back up to 1887, as well as for hypothetical scenarios involving additional marsh erosion relative to the present‐day conditions. The progressive loss of salt marshes significantly impacted the lagoon hydrodynamics, both directly and indirectly, by amplifying high‐tide water levels, reducing wind‐wave energy dissipation, and critically affecting tidal asymmetries across the lagoon. Restoration projects and manmade protection of marsh margins, which have been implemented over the past few decades, limited the detrimental effects of marsh loss on the lagoon hydrodynamics, while not substantially changing the risk of flooding in urban lagoon settlements. Compared to previous studies, our analyses suggest that the hydrodynamic response of back‐barrier systems to salt‐marsh erosion is extremely site‐specific, depending closely on the morphological characteristics of the embayment as well as on the external tidal and wind forcings.

French on Shifting Ground: Cultural and Coastal Erosion in South Louisiana by Nathalie Dajko

Reviewed by: French on Shifting Ground: Cultural and Coastal Erosion in South Louisiana by Nathalie Dajko Carole Salmon Dajko, Nathalie. French on Shifting Ground: Cultural and Coastal Erosion in South Louisiana. UP of Mississippi, 2020. ISBN 978-1-4968-3093-7. Pp. xviii +184. Sociolinguists specializing in Louisiana French have long established its steady decline since the beginning of the twentieth century, despite cultural revival movements and linguistic preservation efforts that started after World War II. Simultaneously, during all those decades, geographers have been monitoring the erosion of coastal marshes in the Gulf of Mexico, which constitute the southern border of the Pelican State and the base of what is called the Acadiana triangle. As Dajko explains, causes leading to such land erosion are numerous and complex, but she aims at exploring “the theoretical construct of place and the role that language [...] plays in its construction” (12). This region is composed of twenty-two parishes and her study focuses on two in particular: Lafourche and Terre- bonne, where French-speaking Cajuns historically settled and where their culture and identity are still vibrant, even if their language is shifting to English. Moreover, other varieties of French and Creole can be found, and Dajko offers a detailed inventory of those varieties in southern Louisiana. About a hundred years ago, the future of French started to be threatened when in 1921, a change to the State’s constitution made English the mandatory language of schooling. Even though some researchers, such as Michael Picone (1997), have previously linked language and land, Dajko’s extensive study specifically explores the loss of the language (here, French) with the loss of the place (here, coastal marshes). Since pieces of land the size of a football field are disappearing in the Gulf at a rapid pace, the correlation between land erosion and language loss in French-speaking communities in coastal southern Louisiana is evident—and frightening. Dajko’s fieldwork is composed of around 250 interviews with local French speakers, collected between 2006 and 2008 in Terrebonne and Lafourche parishes. The volume provides extensive descriptions of the demographic history as well as economic factors in the area being studied, which helps the reader understand how important these communities’ connection to their land has been—but also how, ironically, some of their activities have contributed to land erosion, especially the cutting of canals for the oil industry. By reading this volume, one can fully understand the connections between land, language, and what Dajko calls— referring to her participants’ answers—“bayou identity.” The author shares her results from surveys in perceptual linguistics, such as verbal guise tests, cultural domain tests, and maps drawings, which allowed her to measure language ideology features and identity perception among her Lafourche and Terrebonne participants. She also offers a linguistic landscape study of bayou Point(e) au(x) chêne(s)—or for some, Point(e) au(x) chien(s), those two toponymic variants underlying various identities among this place. Dajko’s study clearly establishes that when it [End Page 230] comes to their own identity perception, French-speaking participants consider that belonging to a place (bayou identity) is more important than ethnic belonging. French in Terrebonne-Lafourche is enregistered (linguistic features have become associated with social features, 127) because of language death, therefore confirming the place-based identity of those French-speaking communities. [End Page 231] Carole Salmon Furman University (SC) Copyright © 2022 American Association of Teachers of French

Localized Placement of Breakwater Reefs Influences Oyster Populations and Their Resilience after Hurricane Harvey

Populations of the eastern oyster (Crassostrea virginica) have been historically declining due to both natural and anthropogenic stressors. In response, oyster reefs have been created with many different approaches. This study utilized intertidal reefs constructed with oyster shells recycled from local restaurants to provide oyster settlement substrate, reef-associated faunal habitat, and a barrier to prevent marsh erosion. The objective of this study was to determine how oyster population characteristics changed over four years (2016–2019) on five different reefs within Sweetwater Lake, Galveston Bay, Texas, with a secondary objective to examine how oyster populations responded after Hurricane Harvey. Over the study period, five different reefs were sampled each summer by removing five bags per reef to determine oyster abundance and size demography. For the three years of the study (2017–2019), we also quantified oyster spat recruitment to the reefs. Oyster abundance and size (shell height) varied interactively by year and reef number, whereas oyster recruitment was significantly lower following Hurricane Harvey and then returned to pre-storm levels. Our results further highlight the importance of reef placement for breakwater-style reefs, as it appears the hydrodynamics within Sweetwater Lake influenced both oyster abundance and size among individual reefs. While the created reefs receive limited larval influx due to the narrow opening between Sweetwater Lake and Galveston Bay proper, this limited connectivity seemed to prevent mass mortality from the freshwater influx from Hurricane Harvey. Therefore, projects creating oyster reefs should consider local and regional landscape factors for the long-term success of oyster populations and robustness to natural disasters.

Accelerated marsh erosion following the Deepwater Horizon oil spill confirmed, ameliorated by planting

Multiple studies have examined the effects of the Deepwater Horizon oil spill on coastal marsh shoreline erosion. Most studies have concluded that the spill increased shoreline erosion (linear retreat) in oiled marshes by ~ 100–200% for at least 2–3 years. However, two studies have called much of this prior research into question, due to potential study design flaws and confounding factors, primarily tropical cyclone influences and differential wave exposure between oiled (impact) and unoiled (reference) sites. Here we confirm that marsh erosion in our field experiment was substantially increased (112–233%) for 2 years in heavily oiled marsh after the spill, likely due to vegetation impacts and reduced soil shear strength attributed to the spill, rather than the influences of hurricanes or wave exposure variation. We discuss how our findings reinforce prior studies, including a wider-scale remote sensing analysis with similar study approach. We also show differences in the degree of erosion among oil spill cleanup treatments. Most importantly, we show that marsh restoration planting can drastically reduce oiled marsh erosion, and that the positive influences of planting can extend beyond the immediate impact of the spill.

The importance of marshes providing soil stabilization to resist fast-flow erosion in case of a dike breach.

Salt marshes provide valuable ecosystem services including coastal protection by reducing wave loading on dikes and seawalls. If the topsoil is erosion resistant to fast‐flowing water, it may also reduce breach depth if a dike fails. In this experiment, we quantified the topsoil erosion resistance from marshes and bare tidal flats with different soil types to understand the extent to which they can help reduce breach depth. Intact soil samples were collected from 11 locations in the Netherlands at different tidal elevations and then exposed for 3 h to 2.3 m/s currents. To the samples that remained stable after flow exposure, an artificial crack was made to test their stability following soil disturbance. All samples from the tidal flats were completely eroded, regardless of sediment type. In contrast, all samples from well‐established marsh plateaus were stable as long as no disturbances were made, including those with sandy subsoils. After creating artificial cracks, samples with a thin cohesive top layer on top of sandy subsoil collapsed, while marshes with silty subsoils remained stable. Pioneer marshes on sandy substrate without a cohesive top layer were the only vegetated soils that completely eroded. The lower erosion of marshes with either sandy or silty soils compared to bare tidal flats was best explained by the presence of a top layer with belowground biomass, high organic content, high water content, and low bulk density. When analyzing the erodibility of marshes only, fine root density was the best predictor of erosion resistance. This study demonstrates the importance of preserving, restoring, or creating salt marshes, to obtain a topsoil that is erosion resistant under fast‐flowing water, which helps reduce breach dimensions if a dike fails. The probability of topsoil erosion in established marshes with sandy subsoil is higher than in silty marshes. A silty layer of cohesive sediment on top of the sand provides extra erosion resistance as long as it does not break. Pioneer marshes that have not developed a cohesive top layer are erosion sensitive, especially in sandy soils. For future marsh creations, using fine‐grained sediments or a mixture of sand with silt or clay is recommended.

Sensitivity of tidal hydrodynamics to varying bathymetric configurations in a multi‐inlet rapidly eroding salt marsh system: A numerical study

AbstractWe describe the development of a high‐resolution, two‐dimensional hydrodynamic model for a multi‐inlet rapidly eroding tidal wetland on the western shore of Delaware Bay, using the finite‐volume, primitive equation community ocean model (FVCOM). Topo‐bathymetric surveys, together with water surface and current velocity measurements during calm and stormy conditions, have been conducted to support model validation. The tested model is then used to quantify the tide‐induced residual transport and asymmetry at major inlet entrances to determine the governing hydrodynamics. We chose a skewness method to calculate the tidal asymmetry and serve as a proxy for sediment transport estimates. The effects of the dredging of an artificial entrance channel and progressive channel deepening in shifting wetland hydrodynamics are shown by developing a scenario analysis. Model results show that the artificially dredged channel has altered the volume exchange at other inlet entrances and increased the net seaward export. The changes in the characteristic frequency of the frictional dissipation in the channel and the system's natural frequency are investigated using a simple ocean–inlet–bay analytical model. Subsequently, we have compared the channel friction scale to the inertia scale and observed that the new connection and gradual channel deepening reduce the overall frictional dominance. Ultimately, the study has shown how the short‐ and long‐term channel bathymetry changes, mainly the artificially dredged channel and progressive channel deepening, can affect the connected system's net circulation and trigger internal marsh erosion.

Modeling wave climates and wave energy attenuation in marsh terrace environments in the northern Gulf of Mexico

Marsh terracing is a coastal restoration technique that has been implemented in Louisiana and Texas for almost 30 years. Marsh terraces are segmented ridges of soil built in inland coastal ponds for the objectives of creating new marsh, disrupting fetch, and dissipating wind-driven wave energy, which causes marsh erosion. Despite widespread implementation, no numerical modeling studies have been conducted to understand the effect of terraces on wave climate in marsh environments. The objectives of this study were to: 1) simulate wave climates in marsh terrace fields using the Simulating WAves Nearshore (SWAN) model and evaluate the agreement between modeled and observed wave conditions and 2) assess the effectiveness of marsh terraces at reducing significant wave height by comparing wave climates in terraced sites with hypothetical unterraced sites. The SWAN model was used to simulate wind-waves at two terrace fields in coastal Louisiana. Model validation was performed using data collected with in situ wave instruments and wind data measured by NOAA stations located 25 km to the east and 62 km to the northeast of the study area. Results from this study modeled and measured small (0.03–0.14 m) and high–frequency waves (0.80–1.29 s) in the marsh terrace fields. Model validation revealed agreement between modeled and observed data, particularly for significant wave height and direction. Additionally, a comparison of wave climates in terraced and unterraced marsh environments indicated an average significant wave height reduction of 45% (min:18%, max: 84%) when terraces were present. Overall, the model was able to evaluate wave climates in marsh terrace fields and assess the effectiveness of this restoration technique in reducing significant wave height. The model's performance in this low energy and complex environment (shallow water, nonlinear shoreline, marsh fragmentated) is encouraging given the lack of research assessing wave parameters in marsh terrace environments.

Seasonality and Characterization Mapping of Restored Tidal Marsh by NDVI Imageries Coupling UAVs and Multispectral Camera

Salt marsh evolution is strongly affected by tidal processes and ecology, which regulate sediment accretion and erosional rates. A balance between marsh erosion and deposition in a restored tidal wetland is crucial for analyzing restoration strategies to adopt in a natural context. Here, we present an integrated approach monitoring salt marsh seasonal changes over several months in a microtidal restored salt marsh of the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (MD, USA). The project is undertaken at a restoration site where sediment dredged from the shipping channels in the upper Chesapeake Bay is being used to restore a tidal marsh habitat in mid-Chesapeake Bay. We flew an Unmanned Aerial Vehicle (UAV) with an RGB and a multispectral camera to obtain a high-resolution map of the planimetric position of vegetation and to monitor the health of the marsh vegetation in diverse seasons. Due to its extension of 400 m by 400 m, a total of four flight plans were necessary to cover the entire marsh flying at a 40 m altitude obtaining a 2 cm Ground Sample Distance (GSD). This technique provides reliable results at a very low cost, enabling an accurate assessment of the marsh platforms to be conducted over time, due to both the very high spatial resolution and the precise georeferencing of the images for the comparisons. Our results show seasonal variability in the two dominant species colonizing the low marsh, Spartina alterniflora, and high marsh, Sporobolus pumilus. While the lower marshes showed a higher variability along seasons, the up-land vegetation showed persistent green foliage during cold seasons. Detecting salt marsh evolution and seasonality coupled with field measurements can help to improve the accuracy of hydrodynamic and sediment transport models. Understanding the drivers of salt marsh evolution is vital for informing restoration practices and designs, in order to improve coastal resilience, and develop and coastal management strategies.

Utilizing airborne LiDAR data to quantify marsh edge morphology and the role of oyster reefs in mitigating marsh erosion

Marsh habitats, experiencing accelerated change, require accurate monitoring techniques. We developed methods to quantify marsh edge morphology using airborne LiDAR data. We then applied these methods within the context of oyster reef restoration within the shallow coastal bays of Virginia, USA, by comparing retreat and morphology quantified at paired reef-lined and control marsh edges at 10 different marsh sites. Retreat metrics were analyzed between 2002 and 2015, utilizing a LiDAR derived edge for the year 2015 from points of maximum slope and aerial imagery pre-2015. Retreat was also compared before and after oyster reef restoration to determine if reefs slow erosion. We found that slope statistics from airborne LiDAR elevation data can accurately capture marsh edge morphology. Retreat rate, measured at edges typically found near the vegetation line, was not significantly different between reef-lined and control marshes and ranged from 0.14 to 0.79 m yr-1. Both retreat rate (ρ = -0.90) and net movement (ρ = -0.88) were strongly correlated to marsh edge elevation. Exposed control marshes had significantly greater mean and maximum slope values compared to reef-lined marshes. The mean edge slope was 11.4° for exposed marshes and 6.0° for reef-lined marshes. We hypothesize that oyster reefs are causing an elongation of the marsh edge by reducing retreat at lower elevations of the marsh edge. Therefore, changes in marsh edge morphology may be a precursor to changes in marsh retreat rates over longer timescales and emphasizes the need for repeated LiDAR measurements to capture processes driving marsh edge dynamics.

Brackish marshes erode twice as fast as saline marshes in the Mississippi Delta region

AbstractMarsh soil properties vary drastically across estuarine salinity gradients, which can affect soil strength and, consequently, marsh edge erodibility. Here, we quantify how marsh erosion differs between saline and brackish marshes of the Mississippi Delta. We analyzed long‐term (1932–2015) maps of marsh loss and developed an algorithm to distinguish edge erosion from interior loss. We found that the edge erosion rate remains nearly constant at decadal timescales, whereas interior loss varies by more than 100%. On average, roughly half of marsh loss can be attributed to edge erosion, the other half to interior loss. Based on data from 42 cores, brackish marsh soils had a lower bulk density (0.17 vs. 0.27 g/cm3), a higher organic content (43% vs. 26%), a lower shear strength (2.0 vs. 2.5 kPa), and a lower shear strength in the root layer (13.8 vs. 20.7 kPa) than saline marsh soils. We then modified an existing marsh edge erosion model by including a salinity‐dependent erodibility. By calibrating the erodibility with the observed retreat rates, we found that the brackish marsh is two to three times more erodible than the saline marshes. Overall, this model advances the ability to simulate estuarine systems as a whole, thus transcending the salinity boundaries often used in compartmentalized marsh models.

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Sea-level rise enhances carbon accumulation in United States tidal wetlands

A review of Gulf of Mexico coastal marsh erosion studies following the 2010 Deepwater Horizon oil spill and comparison to over 4 years of shoreline loss data from Fall 2010 to Summer 2015

The 2010 Deepwater Horizon (DWH) oil spill affected nearly 1105 km of coastal marsh. Long-term shoreline loss in the northern Gulf of Mexico is an important question with far-reaching ecological and human-use implications. Numerous studies have examined potential exacerbated marsh shoreline retreat after the DWH using ground-level sampling and/or aerial/satellite imagery interpretation. This paper reviews previous DWH erosion studies, discusses their limitations and sometimes conflicting results, and provides a comprehensive analysis of a larger data set. Shoreline retreat measurements from multiple studies following the DWH incident were combined for 131 herbaceous marsh sample sites for the period from Fall 2010 to Summer 2015. Significant increases in shoreline loss were found only in the period from Fall 2010 to Fall 2011 for heavily oiled shorelines relative to other periods. The evidence does not suggest widespread long-term coastal marsh erosion from the DWH.

Salt Marsh Dynamics in a Period of Accelerated Sea Level Rise

AbstractSalt marshes are dynamic systems able to laterally expand, contract, and vertically accrete in response to sea level rise. Here, we present the grand challenges that need to be addressed to fully characterize marsh morphodynamics. The review focuses on physical processes and quantitative models. Without predictive models, it is impossible to determine the future marsh evolution under accelerated sea level rise. In these models, one of the challenges is to resolve both horizontal and vertical dynamics within the same framework. Vertically, the marsh has to accumulate enough material to contrast rising water levels. Horizontally, marsh erosion at the ocean side must be compensated by landward expansion in forests, lawns, and agricultural fields. The dynamics of the marsh‐upland boundary are still not fully understood and will require more research in the upcoming years. The complexity of marsh vegetation is seldom captured in predictive models of marsh evolution. More research is needed to understand the effects of each species or species assemblages on hydrodynamics and sediment transport. Here, we further advocate that a sediment budget resolving all sediment fluxes in a marsh complex is the most important metric of marsh resilience. Characterization of these fluxes will enable to connect salt marshes to other landforms and to unravel feedbacks controlling the evolution of the entire coastal system. Current models of marsh evolution rely on sparse data sets collected at few locations. Novel remote sensing techniques will provide high‐resolution spatial data that will inform a new generation of computer models.