Salt Marsh Stability Research Articles

Braving the extremes: foraminifera document changes in climate-induced and anthropogenic stress in Wadden Sea salt marshes

Tidal wetlands are highly dynamic ecosystems that are susceptible to changes in sea level and flooding from storm surges. Among them, salt marshes play a key role in coastal protection as they contribute to wave attenuation through their regulating ecosystem services, thereby promoting sediment deposition and shoreline stabilization. However, the resilience of salt marshes, particularly those that have been modified and cultivated for centuries, is questionable in the face of accelerated sea-level rise (SLR) and increasing run-up heights of storm surges. In this context, this study aims to investigate the historical foraminiferal records of two sedimentary salt marsh archives from the Wadden Sea area (Dithmarschen and North Frisia, Germany) that have been modified to varying degrees by human management activities over the last century. The foraminiferal records document how physico-chemical traits of salt marshes of the central Wadden Sea have responded to storm tide inundation over the last century, providing information about salt marsh stability and vulnerability. Abnormally grown tests of the salt marsh indicator speciesEntzia macrescensincreased in number between 1950 CE and the late 1980s, indicating the concurrent increase of environmental stress caused by the effects of times of increased salt marsh flooding. These trends can be linked to observations of amplified North Sea storm surges, corroborating that salt marsh ecosystems respond to changing climate conditions. Differences in the number of abnormal foraminifera between the studied salt marshes suggest a particularly high vulnerability of intensively human-modified coastal wetland ecosystems to amplified storm climate conditions.

Changing River Discharge and Suspended Sediment Transport to the Georgia Bight: Implications to Saltmarsh Sustainability

Windom, H.L. and Palmer, J.D., 2022. Changing river discharge and suspended sediment transport to the Georgia Bight: Implications to saltmarsh sustainability. Journal of Coastal Research, 38(3), 512–522. Coconut Creek (Florida), ISSN 0749-0208. Suspended sediment delivered in river discharge to the Georgia Bight (the coastal-shelf area of the SE U.S. Atlantic coasts of Georgia and South Carolina) provides a continuous supply of fine grain sediments to support coastal saltmarshes. Archived data on discharge and suspended sediment loads for Piedmont and Coastal Plain rivers collected over eight to nine decades were used to estimate the present rate of sediment discharge to the Bight to be about 1 × 106 metric tons annually. The data used in this assessment are for the period after major reservoir/dam construction and other streamflow diversions occurring during the 1950s–1970s, which had a major effect on sediment yields of SE Atlantic watersheds. The estimated annual sediment delivery is about two-thirds of that needed to maintain vertical saltmarsh stability as sea level rises at the present regional rate of 3.3 mm/y. The observed 2% per decade decreasing trend in river discharge to the Bight, related to rapid population and temperature rise since ca. 1960, and the continual depletion of sediment stored in river floodplains below dams, suggests that the fine sediment supply to the coast will continue to decline.

The role of tides and winds in shaping seed dispersal in coastal wetlands

AbstractGlobal changes such as sea level rise and enhanced storminess motivate the use of saltmarshes as nature‐based flood defenses. Yet, it remains poorly understood about how shifted environmental conditions may shape processes governing long‐term stability of saltmarshes. Here, we integrated data from in situ measurements and field experiments in several Dutch salt marshes to probe the impacts of changes in tides and winds on seed arrival and seed retention on adjacent tidal flats, which is key to marsh regeneration following wave‐driven lateral erosion. The results show that both the quantity and viability of the seeds transported toward adjacent tidal flats relate positively with the peak water level of each tide. Spring tides are more powerful in seed dispersal than neap tides, and storm‐induced extreme water levels can serve as “Window of Opportunity” that deliver disproportionately higher amounts of viable seeds than average conditions. Seed retention decreased with growing onshore wind speed. Storm‐induced strong wave disturbance can function as “Windows of Risk” that wipe out seeds on tidal flats at wind‐exposed marshes. This study highlights the importance of variability in tides and winds for regulating the potential of seed‐based marsh recovery on adjacent tidal flats and thus their resilience to lateral degradation. These findings are relevant for assessing the long‐term marsh stability and sustainability of nature‐based coastal defenses with saltmarshes under global environmental changes.

Gulf ribbed mussels increase plant growth, primary production, and soil nitrogen cycling potential in salt marshes

Smooth cordgrass Spartina alterniflora and Atlantic ribbed mussels Geukensia demissa have a mutualism whereby S. alterniflora provides substrate and shade for G. demissa while G. demissa enhances S. alterniflora growth and drought tolerance. Together, these species improve salt marsh stability and function. To better understand if a similar relationship exists between S. alterniflora and Gulf ribbed mussels G. granosissima in salt marshes of the Gulf of Mexico, we conducted a manipulative experiment with identical starting plant density and varying densities of G. granosissima and monitored plant and soil responses over a growing season. We found that S. alterniflora stem and leaf density was 1.5 and 1.9 times greater for the highest mussel biomass compared to the lowest mussel biomass treatments. Similarly, above- and below-ground biomass were 2.6 and 3.2 times greater for the highest mussel biomass compared to the lowest mussel biomass treatments. More and larger S. alterniflora at higher G. granosissima densities resulted in 2 and 4 times the S. alterniflora gross CO2 uptake and respiration rate, respectively. Methane fluxes were highest when G. granosissima were present, likely driven by the positive relationship between methane flux and belowground biomass. Net potential nitrification was 5 times higher for the highest mussel biomass compared to the lowest mussel biomass treatments, and denitrification rates were 1.8 times higher. Ultimately, our results suggest that G. granosissima increases S. alterniflora growth and productivity, much like the positive relationship between G. demissa and S. alterniflora, which in turn influences salt marsh stability and function.

Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

AbstractThe vulnerability of saltmarshes to lateral erosion at their margin depends on the local biogeomorphological properties of the substrate. In particular, the 3D architecture of pore and root systems is expected to influence shear strength, with repercussions for the wider‐scale stability of saltmarshes. We apply X‐ray computed microtomography (μCT) to visualize and quantify subsurface structures in two UK saltmarshes at Tillingham Farm, Essex (silt/clay rich substrate) and Warton Sands (sand‐rich substrate), with four types of ground cover: bare ground, Spartina spp, Salicornia spp and Puccinellia spp. We extracted μCT structural parameters that characterize pore and root morphologies at each station, and compared them with field measurements of shear strength using a principal component analysis and correlation tests. The 3D volumes show that species‐dependent variations in root structures, plant colonization events and bioturbation activity control the morphology of macropores, while sediment cohesivity determines the structural stability and persistence of these pore structures over time, even after the vegetation has died. Areas of high porosity and high mean pore thickness were correlated to lower values of shear strength, especially at Tillingham Farm, where well‐connected vertical systems of macropores were associated with current or previous colonization by Spartina spp. However, while well‐connected systems of macropores may lower the local deformation threshold of the sediment, they also encourage drainage, promote vegetation growth and reduce the marsh vulnerability to hydrodynamic forces. The highest values of shear strength at both sites were found under Puccinellia spp, and were associated with a high density of mesh‐like root structures that bind the sediment and resist deformation. Future studies of marsh stability should ideally consider time series of vegetation cover, especially in silt/clay‐dominated saltmarshes, in order to consider the potential effect of preserved buried networks of macropores on water circulation, marsh functioning and cliff‐face erosion.

Modeling Marsh‐Forest Boundary Transgression in Response to Storms and Sea‐Level Rise

AbstractThe lateral extent and vertical stability of salt marshes experiencing rising sea levels depend on interacting drivers and feedbacks with potential for nonlinear behaviors. A two‐dimensional transect model was developed to examine changes in marsh and upland forest lateral extent and to explore controls on marsh inland transgression. Model behavior demonstrates limited and abrupt forest retreat with long‐term upland boundary migration rates controlled by slope, sea‐level rise (SLR), high water events, and biotic‐abiotic interactions. For low to moderate upland slopes the landward marsh edge is controlled by the interaction of these inundation events and forest recovery resulting in punctuated transgressive events. As SLR rates increase, the importance of the timing and frequency of water‐level deviations diminishes, and migration rates revert back to a slope‐SLR‐dominated process.

Sea-level rise thresholds for stability of salt marshes in a riverine versus a marine dominated estuary

We studied the ecological resilience of salt marshes by deriving sea level rise (SLR) thresholds in two estuaries with contrasting upland hydrological inputs in the north-central Gulf of Mexico: Grand Bay National Estuarine Research Reserve (NERR) with limited upland input, and the Pascagoula River delta drained by the Pascagoula River, the largest undammed river in the continental United States. We applied a mechanistic model to account for vegetation responses and hydrodynamics to predict salt marsh distributions under future SLR scenarios. We further investigated the potential mechanisms that contribute to salt marsh resilience to SLR.The modeling results show that salt marshes in the riverine dominated estuary are more resilient to SLR than in the marine dominated estuary with SLR thresholds of 10.3 mm/yr and 7.2 mm/yr respectively. This difference of >3 mm/yr is mainly contributed by larger quantities of riverine-borne mineral sediments in the Pascagoula River. In both systems, sediment trapping by the above-ground vegetation appears to contribute more to marsh platform accretion than organic matter from below-ground biomass based on the medians of the accretion rates. However, below-ground biomass could contribute up to 90% of accretion in the marine dominated estuary compared to only 60% of accretion in the riverine dominated estuary. SLR thresholds of salt marshes are more sensitive to vegetation biomass in the marine dominated estuary while biomass and sediment similarly affect SLR thresholds of salt marshes in the riverine dominated estuary.This research will likely help facilitate more informed decisions on conservation/restoration policies for these two types of systems in the near-term needed to minimize future catastrophic loss of these coastal marsh habitats once SLR thresholds are exceeded.

Intraspecific Root Trait Variability Along Environmental Gradients Affects Salt Marsh Resistance to Lateral Erosion

Recent studies in salt marshes have demonstrated the role of plant roots in sediment stabilisation, and hence the importance of marshes in providing coastal protection. However, the relative role of root traits and environmental factors in controlling sediment stability, and how intraspecific variability of root traits vary within and among marshes, remain poorly understood. In this study, we investigated which root trait(s) drive sediment stability (resistance to lateral erosion) in two marsh species with an important role in coastal protection (Spartina anglica and Atriplex portulacoides) and how the environment affects the expression of these traits. We sampled three marshes along salinity gradients in each of two estuaries in Wales (UK), establishing replicate plots in the respective dominant zones of each species. In all plots we sampled abiotic variables (sand, redox potential, pH, salinity) and root traits (root density, specific root density, root volume, root length density); in a subset of these plots (three per species in each marsh/species/marsh) we extracted soil-plant cores and assessed their erosion resistance in a flume. Sediment stability was enhanced by increases in root density and reductions in sand content. Abiotic variables affected root density in different ways depending on species: in S. anglica, redox was the only significant factor, with a positive, linear effect on root density; in A. portulacoides, redox had a non-linear (U-shaped) effect on root density, while sand had a negative effect. Collectively, these results show that i) intraspecific variability in root density can influence sediment stability in salt marshes, and ii) sediment properties not only influence sediment stability directly, but also indirectly via root density. These results shed light on spatial variability in the stability of salt marshes to lateral erosion and suggest that root density should be incorporated into coastal vegetation monitoring programs as an easy-to-measure root trait that links the environment to sediment stability and hence to the function and services provided by marshes.

Accounting for spatial patterns in deriving sea-level rise thresholds for salt marsh stability: More than just total areas?

Ecological threshold is an important concept to indicate the boundary of alternate states of ecosystems driven by environmental conditions and to facilitate evaluation of ecosystem resilience. Sea-level rise (SLR) thresholds for the stability of salt marshes, if studied in two dimensions, are generally derived based on total areas without systematic accounting for spatial patterns related to edges, shapes, and contagions of patches. As these spatial patterns are potentially important for functions and ecosystem services of salt marshes and they are likely to be impacted by SLR in a different way from the total areas, it is necessary to study SLR thresholds based on these spatial patterns to obtain a more comprehensive understanding of salt marsh resilience to SLR. This research compares the SLR thresholds based on these spatial patterns of salt marshes to those based on total areas alone across different spatial resolutions.The spatial patterns of salt marshes were quantified by 26 commonly used landscape metrics, predicted from a mechanistic wetland change model. At spatial resolutions of 2–100 m, SLR thresholds were first derived using individual landscape metrics and then the first principal component that explained >80% of total variance of these metrics showing threshold responses to SLR. In order to separate the effect of spatial configuration from composition, a neutral model which simulated the same amount of salt marsh change as the mechanistic model but at the random locations was applied. The SLR thresholds were derived based on the simulations from the neutral model and compared to those from the mechanistic model.The results show that total area-based SLR thresholds do not comprehensively represent salt marshes’ resilience to SLR. Particularly, I find 1) the derived SLR thresholds vary from 7.29 to 11.12 mm/yr for 2100 based on landscape metrics used, 2) the SLR threshold based on the first principal components (7.99 mm/yr) is smaller than that based on the total area only (8.40 mm/yr), 3) the SLR thresholds are scale dependent, and 4) the spatial configuration’ effect on SLR thresholds is smaller for smaller salt marsh areas compared to larger salt marsh areas.This study highlights the need to account for different spatial patterns of salt marshes and apply wetland maps with a spatial resolutions of 30 m or finer in deriving SLR thresholds, as using total areas alone or coarser-resolution maps may provide a biased interpretation that salt marshes are more resilient to SLR than they actually are.

Impact of salinity and nutrients on salt marsh stability

AbstractBelowground growth in coastal plants is critical for marsh stability and the ability of coastal wetlands to keep pace with sea‐level rise. Quantifying the effects of nutrient loading on belowground plant growth is an ongoing controversy in wetland research, with previous experiments demonstrating both positive and negative impacts. Moreover, salinity may also decrease belowground growth through sulfide toxicity, or plants may increase root growth to oxidize sediments and respond to sulfide stress. Because salinity influences plant nitrogen assimilation and sediment nitrogen retention, salinity and nitrogen may interact to influence belowground plant growth. We sampled an urban‐to‐rural land‐use gradient of 11 Spartina alterniflora marshes on Long Island, New York, to look for correlates of belowground biomass. We found that belowground biomass was related positively to salinity and negatively to extractable nitrogen content in sediments. Total belowground plant biomass was reduced by 60–70% in high‐nitrogen marshes and enhanced by as much as 70% in high‐salinity marshes. Further, we found no evidence of interaction between salinity and nitrogen, indicating that these factors were independently related to belowground plant growth. Our results indicate that chronic eutrophication and increasing salinity resulting from sea‐level rise are likely to have opposing effects on future marsh stability.

Response of salt marshes to oiling from the Deepwater Horizon spill: Implications for plant growth, soil surface-erosion, and shoreline stability

We investigated the initial impacts and post spill recovery of salt marshes over a 3.5-year period along northern Barataria Bay, LA, USA exposed to varying degrees of Deepwater Horizon oiling to determine the effects on shoreline-stabilizing vegetation and soil processes. In moderately oiled marshes, surface soil total petroleum hydrocarbon concentrations were ~70mgg−1 nine months after the spill. Though initial impacts of moderate oiling were evident, Spartina alterniflora and Juncus roemerianus aboveground biomass and total live belowground biomass were equivalent to reference marshes within 24–30months post spill. In contrast, heavily oiled marsh plants did not fully recover from oiling with surface soil total petroleum hydrocarbon concentrations that exceeded 500mgg−1 nine months after oiling. Initially, heavy oiling resulted in near complete plant mortality, and subsequent recovery of live aboveground biomass was only 50% of reference marshes 42months after the spill. Heavy oiling also changed the vegetation structure of shoreline marshes from a mixed Spartina–Juncus community to predominantly Spartina; live Spartina aboveground biomass recovered within 2–3years, however, Juncus showed no recovery. In addition, live belowground biomass (0–12cm) in heavily oiled marshes was reduced by 76% three and a half years after the spill. Detrimental effects of heavy oiling on marsh plants also corresponded with significantly lower soil shear strength, lower sedimentation rates, and higher vertical soil-surface erosion rates, thus potentially affecting shoreline salt marsh stability.

Salt marsh stability and patterns of sedimentation across a backbarrier platform

Long term observations of clay thicknesses from 1949 to 2007 and measurements of the bulk dry density of salt marsh on the backbarrier of Skallingen (west Denmark) formed the basis of constructing a space distributed model of salt marsh deposition. The deposition potential (an empirical constant, β) was measured at 32 locations across the studied salt marsh platform. It was found to correlate with two independent variables: (1) distance to marsh edge and (2) distance to creeks of 2nd or higher order. They explained in combination 71% of the variation. The derived algorithms for estimating the salt marsh deposition made it possible to tune in a model in a 4 by 4 m grid covering the studied salt marsh area of app 3 km 2. This was used to describe the general pattern of deposition on the salt marsh platform. Running the model with a constant sea level revealed that balance between the inner and the outer salt marsh deposition cannot be achieved within a time scale of 1000s of years. Modeling salt marsh sedimentation under rising sea level scenarios revealed that only one specific sea level rise provides equilibrium for a given location. With a higher sea level rise, the marsh at the specific location will eventually drown, whereas it – with a sea level rise below this level – will grow towards the top of the rising tidal frame. Short term scenarios (100 years) showed that the Skallingen salt marsh will survive a sea level rise of 4 mm yr − 1 and become unstable in the inner part with a sea level rise of 6 mm yr − 1 .

Integrating LIDAR elevation data, multi‐spectral imagery and neural network modelling for marsh characterization

Vertical elevation relative to mean sea level is a critical variable for the productivity and stability of salt marshes. This research classified a high spatial resolution Airborne Data Acquisition and Registration (ADAR) digital camera image of a salt marsh landscape at North Inlet, South Carolina, USA using an artificial neural network. The remote sensing‐derived thematic map was cross‐referenced with Light Detection and Ranging (LIDAR) elevation data to compute the frequency distribution of marsh elevation relative to tidal elevations. At North Inlet, the median elevation of the salt marsh dominated by Spartina alterniflora was 0.349 m relative to the North American Vertical Datum 1988 (NAVD88), while the mean high water level was 0.618 m (2001 to May, 2003) with a mean tidal range of 1.39 m. The distribution of elevations of Spartina habitat within its vertical range was normal, and 80% of the salt marsh was situated between a narrow range of 0.22 m and 0.481 m. Areas classified as Juncus marsh, dominated by Juncus roemerianus, had a broader, skewed distribution, with 80% of the distribution between 0.296 m and 0.981 m and a median elevation of 0.519 m. The Juncus marsh occurs within the intertidal region of brackish marshes and along the upper fringe of salt marshes. The relative elevation of the Spartina marsh at North Inlet is consistent with recent work that predicts a decrease in equilibrium elevation with an increasing rate of sea‐level rise and suggests that the marshes here have not kept up with an increase in the rate of sea‐level rise during the last two decades.