Marsh Surface Elevation Research Articles

Quantifying Multi-Decadal Salt Marsh Surface Elevation and Geodetic Change: The South Carolina Geological Survey SET Network

Quantifying Multi-Decadal Salt Marsh Surface Elevation and Geodetic Change: The South Carolina Geological Survey SET Network

Comparing Wetland Elevation Change Using a Surface Elevation Table, Digital Level, and Total Station

The surface elevation table (SET) approach and two survey instruments, a digital level (DL) and a total station (TS), were used to evaluate elevation change at a 1-ha, micro-tidal, back-barrier salt marsh at Assateague Island National Seashore (Berlin, MD, USA) from 2016 to 2022. SET data were collected at 3 sampling stations along the perimeter of the plot, 36 pins per station, and the DL and TS data were collected adjacent to 36 stakes, four readings per stake, throughout the plot. The average elevation range of the marsh surface measurements at the SET stations was 2 cm, while the range was considerably greater within the larger 1-ha DL and TS sampling area (24 cm). The average elevation of the marsh surface only varied by 2 cm among the three methods. Elevation change trends of the three methods ranged from 2.8 to 3.5 mm year−1 and were not significantly different from each other. Despite differences in sample size and spatial distribution of measurements, these methods provided comparable measures of long-term trends in marsh surface elevation probably because the marsh at this site was structurally homogeneous with low topographic relief.

Contemporary Salt-Marsh Foraminifera from Southern California and Implications for Reconstructing Late Holocene Sea-Level Changes

Abstract We report on the distribution of contemporary foraminifera in salt marshes in Mission Bay and Carpinteria Slough, Southern California. Combining these data with existing datasets from Seal Beach and Tijuana, we explore the potential for a regional training set to underpin quantitative reconstructions of paleoenvironmental change from foraminifera preserved in salt-marsh sediments. We demonstrate that species’ distributions are highly dependent on elevation, suggesting fossil foraminiferal assemblages here, as in many other regions, are useful depositional elevation indicators. Transfer functions provide predictions from Mission Bay cores with decimeter-scale uncertainties. Nevertheless, interpretation of marsh-surface elevation change is complicated by a complex geomorphic setting and anthropogenic impacts. An abrupt change in elevation in the mid-1700s may be related to lateral spreading of water-saturated sediments during an earthquake on the Rose Canyon fault, suggesting the potential for foraminifera to support new palaeoseismic and sea-level records for the region.

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

AbstractCoastal 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.

Atmospheric River Storm Flooding Influences Tidal Marsh Elevation Building Processes

AbstractDisturbances are a key component of ecological processes in coastal ecosystems. Investigating factors that affect tidal marsh accretion and elevation change is important, largely due to accelerating sea‐level rise and the ecological and economic value of wetlands. Sediment accumulation rates, elevation change, and flooding were examined at five marshes along a riverine‐tidal gradient in the northern San Francisco Bay‐Delta, California, USA during an Atmospheric River storm event in 2017 using Surface Elevation Tables (SETs), feldspar marker horizons (MH), and continuous water‐level sensors. Our results showed that localized marsh flooding increased during the storm event, but not evenly across sites. Marsh surface elevation increased the most at the tidal freshwater marsh site in response to the storms, with an average surface elevation gain of 45.6 ± 13.1 mm, and the least at a tidal saline marsh with an average surface elevation gain of 4.0 ± 1.2 mm. A marsh located on the large embayment did not exhibit an immediate response to the storm but had a surface elevation gain of 21.5 ± 13.7 mm 6 months after the storm. During the storm period, marsh distance to the bay was the strongest predictor of elevation change, followed by SET‐MH elevations. Conversely, during non‐storm periods, SET‐MH elevation was a relatively strong predictor of elevation change. Atmospheric Rivers appear to be a major factor affecting short‐term spatial and temporal variability in flooding and sedimentation rates in tidal marsh systems. Incorporating information about storms into monitoring could increase our understanding of how episodic storms can impact marshes.

Effective use of thin layer sediment application in Spartina alterniflora marshes is guided by elevation-biomass relationship

Thin layer application (TLA) of dredged sediments has emerged as an adaptive management option for marshes that are not increasing in elevation fast enough to keep pace with local relative sea level rise. While there have been multiple previous demonstrations of TLA, there is limited pre-and post-application monitoring data available from past projects to develop guidance concerning optimal siting, design and implementation of projects. To address this gap, we conducted a controlled TLA experiment in a Spartina alterniflora marsh adjacent to the Atlantic Intracoastal Waterway in central North Carolina. The project was informed by the results of a long-term marsh monitoring and research program which had identified the project area as particularly vulnerable to sea level rise. The experimental design included triplicate meso-scale (24m2) treatment and control cells and quarterly (during the first year) to annual monitoring efforts to document trends in marsh surface elevation and vegetative biomass. In addition, sampling was conducted to characterize porewater chemistry and sediment characteristics. Thin layer application of dredged sediments resulted in an average elevation gain of 6 cm in treatment plots. Monitoring results indicate that the gains in elevation achieved by TLA have been maintained over five growing seasons. The response of vegetative biomass to the added elevation was well-predicted by the elevation-biomass relationship of the surrounding marsh platform. Treatment cells exhibited continued increases in surface elevation after placement, while control cell elevation remained constant. These data illustrate the potential of TLA as a tool for increasing the resilience of low-lying marshes to sea level rise and illustrate the value of the surface elevation-marsh biomass relationship for providing guidance for its use in S. alterniflora-dominated marshes.

Can sea level rise help us restore coastal wetlands? The hydrologic restoration of the Slop Bowl, Brazoria National Wildlife Refuge, Texas

The Slop Bowl marsh in the Brazoria National Wildlife Refuge provides extraordinarily high quality, heavily used bird habitat. Much of this habitat has experienced hypersaline conditions due to both hydrologic alteration by humans and a rapidly and changing physical environment over the past several decades. Oil and natural gas extraction activities have resulted in excavation and channelization along pipelines and hydrologic obstruction by an access road. In addition, subsidence along growth faults has altered hydrologic pathways and lowered surface elevations in the center of the marsh. Our objective was to understand the underlying processes that contribute to hypersaline conditions and to evaluate possible restoration alternatives to reduce the severity of those conditions. Accordingly, we conducted extensive field and hydrologic modeling efforts, and identified the past, present, and future of this marsh habitat under a baseline scenario. We then compared various restoration action scenarios against this baseline. We found that, beginning in about 15 years, relative sea level rise will improve the hydrologic conditions by enhancing tidal flushing. However, if fill material is continually added to elevate the obstructing road as the sea rises, this hydrologic relief may never be realized. Moreover, we found that if a drought occurs during this critical period, a difference of only a few centimeters in the relative water level and road elevation, or changes in marsh surface elevations driven by fault motion and subsidence, may have catastrophic consequences. The modeling also suggests that several potential interventions can bridge this gap over the next 15 years and beyond. Actions that improve tidal circulation, reduce salinity, and enhance marsh accretion are being developed by the project team to enhance and restore habitat in the near term. The most optimal approaches evaluated thus far include the installation of culverts at critical locations, the excavation of a small channel, the modification of flow pathways, and the beneficial use of sediments and vegetative plantings. We conclude that, under specific circumstances or at unique locations such as the Slop Bowl marsh, sea level rise can be leveraged to improve coastal wetland health.

The evolution of saltmarsh mosquito control water management practices relative to coastal resiliency in the Mid-Atlantic and northeastern United States

Salt marshes are dynamic ecosystems that change in response to local geographic and geologic factors as well as sea level changes. Most east coast salt marshes are the result of rising sea levels since the end of the last ice age, about 20,000 ybp. To compound this natural process, anthropogenic manipulations for farming, development and other purposes have occurred for centuries. Alterations to salt marshes for the purpose of controlling larval mosquitoes at their source, at least along the east coast of the United States, have occurred since the early twentieth century. These alterations have included large-scale manipulations such as extensive parallel grid-ditching and impounding. Within the last 50 years, more selective source reduction methods such as Open Marsh Water Management (OMWM) have been employed with fewer deleterious impacts to marsh resources. Even more recently, the more holistic approach of Integrated Marsh Management (IMM) has been used with considerable success particularly in the northeastern and Mid-Atlantic states. IMM not only uses OMWM techniques but incorporates the judicious use of mosquito control pesticides, tidal flow restoration, impoundment management, wildlife habitat enhancement, invasive plant control, and selective shallow ditching (“runneling”) depending on local conditions and management plans. As many marshes are becoming wetter and either drowning or migrating inland (where possible) due to the effects of increasing rates of sea level rise, the compounding long-term impacts of parallel grid-ditching and past manipulations on marsh surface elevation and hydrology are being more intently studied. These changes in saltmarsh dynamics have had and will continue to have impacts on where saltmarsh mosquitoes are produced, which could have corollary effects on public health and quality of life near coastal communities. As salt marshes continue to change, mosquito control agencies can play a significant role in providing input for salt marsh restoration and management in addition to their primary objective of vector control and enhancing quality of life.

Sediment Accumulation, Elevation Change, and the Vulnerability of Tidal Marshes in the Delaware Estuary and Barnegat Bay to Accelerated Sea Level Rise

Tidal marshes are highly valued habitats, yet are vulnerable to loss from both anthropogenic and natural disturbances including sea-level rise (SLR). Many tidal marshes have kept pace with SLR over the last century; on average, however, recent escalations in SLR increase the vulnerability of marshes to submergence. Relative sea-level rise near our study sites in the Mid-Atlantic U.S. averaged 4.34 mm year−1 over the last 50 years, yet over the last 19 years, relative sea-level rise averaged 6.25 mm year−1 and the rise in high tide water levels averaged 8.13 mm year−1. We compared these rates of water rise to rates of marsh surface elevation change using surface elevation tables in ten tidal marshes—three tidal freshwater and four saline marshes in the Delaware Estuary and three salt marshes in Barnegat Bay, NJ, USA. We also examined the effects of marsh type and geomorphic setting on rates of elevation change, surface accretion, and subsurface change as well as the influence of marsh elevation, distance from a channel, and tidal range (n = 3 sites per marsh). Surface elevation change measured over the last 4 to 9 years averaged less than 6 mm year−1 in nine out of ten of the study stations, less than rates of relative SLR. Tidal freshwater marshes in the Delaware Estuary had greater rates of surface accretion and elevation change than salt marshes in Barnegat Bay. Marshes sitting lower in the tidal frame and experiencing higher tidal ranges tended to have higher surface accretion rates, but shallow subsidence had a stronger influence on these elevation change rates. We estimated time to submergence (i.e., lifespan) for using rates of marsh elevation change, a SLR rate of 10 mm year−1, and thresholds for conversion to open water using geospatial datasets. The calculated time to submergence for the majority of marshes was 60 to 80 years with some predicted to submerge in as few as 5 years. These data suggest that in order to keep pace with accelerating SLR, surface accretion in many of these marshes will have to increase at a rate that surpasses shallow subsidence (1–7 mm year−1).

Sea level rise and marsh surface elevation change in the Meadowlands of New Jersey

The marshlands of the Meadowlands of New Jersey are valuable wetland ecosystems in a highly developed urban area and provide a natural habitat to more than 285 species of birds, a great variety of fishes, and many other living organisms. It is not clear if these ecosystems and their associated ecological services will persist under conditions of accelerated sea level rise (SLR), in geography where space for a landward retreat of marshlands is limited. In this study, we used the deep rod surface elevation table method and feldspar marker horizons to measure surface elevation change and vertical accretion rate in five marshland sites over 11 years. The controlling parameters of the accretion rate were explored. The results showed that sediments were not limited for vertical accretion. About 16% of the total suspended solids reaching the marsh via the tide was trapped by the marsh surface. Hydraulic duty alone cannot explain differences in deposition rates between low and high marsh. Precipitation, snow accumulation, and sea surge from storms were the main drivers influencing subsidence. The overall subsidence rate was 1.5 ± 1.3 mm/year. All sites combined showed increases in surface elevation of 4.0 ± 0.7 mm/year. This rate of increase is not enough to keep up with the 8 mm/year SLR prediction. There is a 50% chance that in 80 years, 7% of current marshlands will be underwater or will convert to unvegetated mudflats, and most high marsh habitats will disappear.

Metal contamination pathways of a restored marshland in an industrial estuary

Contaminated surface sediments were removed from a coastal marsh and replaced with clean engineered sediments before marsh vegetation was replanted and the site was reopened to the tide. Differences in trace metal concentrations (Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn) were monitored for 11 years at different sediment depths, under two types of engineered sediments (with and without compost), and at three marsh surface elevations. Except for Hg and Cd, all others followed the patterns of Fe and Mn of decreasing concentration during drought years and increasing concentration during wet years. Metals in deep sediments (> 25 cm) are always higher than on the surface. Surfaces higher in the terrain had fewer metal concentrations compared to lower elevations. The compost treatment had no insignificant effect on metal concentration. The main pathways involved in remobilization from the buried legacy sediments are organic metal complexation, reductive dissolution, and oxidation of metal sulfides. Hg showed an enrichment pattern throughout the study period suggesting atmospheric deposition as the main pathway. Similarly, sources of Cd are most likely from atmospheric wet deposition since most of the Cd variation is explained by changes in the amounts of rain. Metal concentration in the sediments decreased up to 50% from pre-restoration conditions; the only exceptions were Hg and Cd which continue to accumulate. Increasing dry periods and storm frequencies from changing global weather patterns have the potential to create conditions for greater bioavailability of metals from this and similar world estuaries.

Processes Influencing Marsh Elevation Change in Low- and High-Elevation Zones of a Temperate Salt Marsh

The movement of salt marshes into uplands and marsh submergence as sea level rises is well documented; however, predicting how coastal marshes will respond to rising sea levels is constrained by a lack of process-based understanding of how various marsh zones adjust to changes in sea level. To assess the way in which salt marsh zones differ in their elevation response to sea-level change, and to evaluate how potential hydrologic drivers influence the response, surface elevation tables, marker horizons, and shallow rod surface elevation tables were installed in a Virginia salt marsh in three zones that differed in elevation and vegetation type. Decadal rates of elevation change, surface accretion, and shallow subsidence or expansion were examined in the context of hydrologic drivers that included local sea-level rise, flooding frequency, hurricane storm surge, and precipitation. Surface elevation increases were fastest in the low-elevation zone, intermediate in the middle-elevation zone, and slowest in the high-elevation zone. These rates are similar to (low and middle marsh) or less than (high marsh) local rates of sea-level rise. Root zone expansion, presumably due to root growth and organic matter accumulation, varied among the three salt marsh zones and accounted for 37%, but probably more, of the increase in marsh surface elevation. We infer that, during marsh transgression, soil-forming processes shift from biogenic (high marsh) to minerogenic (low marsh) in response, either directly or indirectly, to changing hydrologic drivers.

Assessing the response of the Great Marsh to sea-level rise: Migration, submersion or survival

To survive rising sea level, salt marshes must accrete vertically, migrate laterally, or undergo a combination of the two. If sufficient sediment is available for marsh accretion, the slope of the surrounding area is relatively flat, and edge erosion is minimal, then a marsh can theoretically maintain its areal extent through a combination of vertical accretion and upland expansion. However, in cases where sediment supply is limited and the marsh is backed by steeper slopes, it is unclear whether accretion and inland migration will be sufficient to counteract the combined effects of rising sea level and edge erosion. Given these barriers to marsh expansion, inland migration may not be a viable solution to marsh vulnerability to sea-level rise. We quantify the potential changes in areal extent under future sea-level rise scenarios for the Great Marsh in northern Massachusetts, where the marsh has a limited suspended sediment supply and relatively steep upland topography. Salt marsh is identified and classified into low or high marsh using LiDAR elevation and validated using aerial photography and vegetation surveys. We generate a simple 1D-H model using locally-measured accretion rates and their relationship to marsh elevation, to determine change in elevation and dominant plant species over time. A maximum inorganic sediment available to the marsh is prescribed for certain model scenarios to test the impact of sediment limitation. This limit is calculated based on the volumetric contribution of mineral sediment to the present marsh accretion rates. Predicted changes in marsh area over a 100-year model period are determined using the surrounding elevation gradients, calculated sediment availability, projected edge erosion, and local rates of sea-level rise (SLR). The two Representative Concentration Pathway (RCP) SLR scenarios used are based on the most recent IPCC report and also include the latest information concerning responses to ice sheet melting in Greenland and Antarctica. We find that as the rate of sea-level rise increases, the areal extent of the marsh decreases due to a lack of the suspended sediment needed to maintain marsh surface elevation and the inability of the marsh to encroach upon steep upland slopes. By comparing a model assuming constant accretion rates to one with mineral sediment-limited accretion, we find that when sediment if limited, marsh habitat conversion and loss occur earlier and more rapidly.

Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem.

Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.

Salinity Affects Topsoil Organic Carbon Concentrations Through Regulating Vegetation Structure and Productivity

AbstractEstuarine salt marshes have been recognized as one of the most efficient carbon sinks in the biosphere, with considerable potential for climate change mitigation. However, there are still uncertainties about the response of soil carbon stocks to enhanced soil salinization caused by accelerated sea‐level rises and aggravated saltwater intrusion. We therefore conducted both field investigations in the Chongming Dongtan salt marsh of the Yangtze River Estuary, China, and manipulative experiments on marsh soils occupied, respectively, by the invasive Spartina alterniflora, and the native Phragmites australis and Scirpus mariqueter, to identify the effects of elevated soil salinity on top soil organic carbon (SOC) concentration. Our field data showed that SOC concentrations were significantly positively associated with soil salinity concentrations, annual net primary productivity, and marsh surface elevation but showed a significant negative relationship with median grain size. Compared with the two native species, S. alterniflora preferred more saline conditions and had a higher SOC concentration. Although raised flooding salinities (0–35 ppt) did not strongly affect SOC concentrations, elevated soil salinities significantly corresponded with low SOC concentrations and plant biomass in manipulative experiments. These findings indicated that soil salinity, plant species, and soil texture were key factors controlling SOC concentrations in the studied salt marsh. Moreover, soil salinity could affect SOC concentrations through regulating vegetation spatial structure and plant biomass production. The further invasion of the S. alterniflora community will exert a positive influence on SOC concentrations in the Chongming Dongtan salt marsh.

Drainage enhancement effects on a waterlogged Rhode Island (USA) salt marsh

Drainage enhancement (e.g., ditch digging, open-marsh water management, runnelling) has long been used to reduce tidal marsh soil waterlogging and surface ponding to promote salt hay production and mosquito control. Now it is also being used as a tool to enhance marsh resilience to sea-level rise despite a lack of studies that evaluate its effectiveness as an intervention approach. We therefore conducted a controlled field experiment to evaluate short-term responses to drainage enhancement of a Rhode Island (USA) salt marsh. Drainage enhancement elicited rapid physical changes in portions of the marsh including declines in water levels and marsh elevation, but the biological components examined (e.g., vegetation and bird community composition) were largely unaffected. In two of the four areas monitored, marsh surface inundation duration declined from >75% to 3–10% and low water levels dropped by 20 cm. Mean annual marsh surface elevation in monitoring plots increased 5 mm one year after drainage enhancement but dropped to 11 mm below initial conditions after three years. The decline in elevation varied among habitats, with the greatest decline (18 mm) found in areas dominated by Spartina alterniflora and/or bare ground. Vegetation community composition and % cover and heights of dominant species were unchanged, but areas that were initially bare had fully revegetated after three years. Drainage enhancement also had no effects on bird community composition or marsh sparrow (Ammodramus spp.) density. Our study provides evidence that drainage enhancement can relieve waterlogging and some of its impacts without any apparent adverse effects on the composition and abundance of existing vegetation and bird communities. At the same time, it can induce a loss of marsh platform elevation that has the potential to offset declining water levels and inhibit high marsh enhancement. Finally, unanticipated findings from our study provide evidence that the effects of larger-scale drivers such as sea-level rise may predominate over localized responses to drainage enhancement itself.

Relationships between Salt Marsh Vegetation and Surface Elevation in Coos Bay Estuary, Oregon

Salt marsh habitats support a diverse array of estuarine species but are vulnerable to increased inundation resulting from sea-level rise. In order to characterize relationships between vegetation and elevation and inform assessments of risk to salt marsh communities from projected sea-level rise, we collected vegetation and elevation data at 42 salt marsh sites in Coos Bay Estuary, Oregon. For 1-m2 plots along transects from the bayside edge to the upland, we recorded height and percent cover of all plant species present. We determined plot location and elevation at 1-m intervals with a Trimble Pathfinder Pro XRS differential GPS and TOPCON GTS223 Total Station for comparison with existing LiDAR. Cluster analysis distinguished six vegetation groups. Two low marsh groups (average elevation 1.74 and 1.91 m) were characterized by swampfire (Sarcocornia perennis) with an average height of 31 cm, and saltgrass (Distichlis spicata) with an average height of 22 cm. Plots in the high marsh groups had average elevations ranging from 2.21 to 2.57 m and were characterized by tufted hairgrass (Deschampsia cespitosa) and Oregon gumweed (Grindelia stricta var. stricta), with an average height of 50 cm and 43 cm, respectively. Mid-marsh groups (average elevations of 2.01 and 1.99 m) were dominated by Lyngbye's sedge (Carex lyngbyei) with an average height of 64 cm. The data collected along these transects allowed us to assess LiDAR elevation accuracy, identify sites where LiDAR data require correction, and provide species-specific height data for correction of LiDAR in areas of dense vegetation.

The Effects of Tropical Cyclone-Generated Deposition on the Sustainability of the Pearl River Marsh, Louisiana: The Importance of the Geologic Framework

Shoreline retreat is a tremendously important issue along the coast of the northern Gulf of Mexico, especially in Louisiana. Although this marine transgression results from a variety of causes, the crucial factor is the difference between marsh surface elevation and rising sea levels. In most cases, the primary cause of a marsh’s inability to keep up with sea level is the lack of input of inorganic material. Although tropical cyclones provide an important source of such sediment, little effort has been made to determine the point of origin of the deposited material. In this study we use sedimentary, geochemical and biogeochemical data to identify the bed of the Pearl River and/or Lake Borgne as the source of a ~5 cm thick clastic layer deposited on the surface of the Pearl River marsh on the Louisiana/Mississippi border. Radiochemical chronologies and sedimentary evidence indicate that this layer was associated with the passage of Hurricane Katrina in 2005. As this material would otherwise have been lost to the system, this deposition indicates a net gain to marsh surface elevation. Accretion rates, determined from 137Cs and 14C profiles and the use of the Katrina layer as a stratigraphic marker, indicate that short-term (~50 years) rates are as much as an order of magnitude higher than the long- term (1000s of years) rates. We suggest that the marsh’s geologic setting in an incised river valley with steep vertical constraints and a large fluvial discharge, promotes rapid accretion rates, with rates accelerating as the sea moves inland, due to extended hydroperiods and the input of clastic material from both the marine and terrestrial sides. These rates are especially large when compared to accretion occurring in the more common open marshes fringing the Gulf that lack fluvial input. The difference is particularly large when related to marsh recovery/regrowth following the deposition of thick hurricane-generated clastic layers. Given the number of similar incised river valleys along the Gulf Coast, we believe that understanding the processes controlling marsh accretion in such environments is essential in evaluating marsh sustainability on a regional basis.

Wave Attenuation by Oyster Reefs in Shallow Coastal Bays

Oyster reef restoration in shallow estuarine environments has been thought to have the potential to provide shoreline protection as well as oyster habitat. This study was designed to address the question of how effective oyster reefs are at attenuating wave energy in shallow coastal bays. Measurements were made of waves on both sides of four restored intertidal oyster reefs and at a control site with no reef; mean water depths ranged from 0.9 to 1.3 m. The reefs differed in composition and position relative to the shoreline, but all had reef crest elevations between 0.3 and 0.5 m below mean sea level. Differences in wave heights between the exposed/sheltered sides and upwind/downwind sides of the reefs were used to quantify the effects of the reefs on waves under varying tidal and wind conditions. All four reefs were able to reduce wave heights by an average of 30–50% for water depths of 0.5–1.0 m (bracketing the heights of reef crests) and 0–20% for water depths of 1.0–1.5 m (reef crests > 0.25 m below the water surface). For water depths greater than 1.5 m, there was < 10% change in wave heights. In contrast, there was no average decrease in wave height from the more seaward (exposed) to the more landward wave gauge at the control site regardless of water depth. Based on our results, we conclude that fringing oyster reefs can reduce the wave energy reaching the shoreline of marshes with edge elevations close to mean sea level. However, reefs like those in our study have little effect on waves during deeper water conditions, which allow for the largest waves, and are therefore less likely to offer protection to marshes characterized by high edge scarps and marsh surface elevations well above mean sea level.

Evaluating the Relationship Among Wetland Vertical Development, Elevation Capital, Sea-Level Rise, and Tidal Marsh Sustainability

Accelerating sea-level rise and human impacts to the coast (e.g., altered sediment supply and hydrology, nutrient loading) influence the accumulation of sediment and organic matter, and thereby impact the ability of coastal tidal wetlands to maintain an elevation consistently within the vegetation growth range. Critical components of marsh sustainability are the marsh elevation within the vegetation growth range (elevation capital) and the rates of marsh surface elevation change and relative sea-level rise. The relationship among these factors and their combined influence on marsh integrity were evaluated by comparing trends in surface elevation change on five salt marsh sites located on three marsh islands in Jamaica Bay, NY, USA. All marsh sites were located in a similar physical setting (i.e., tidal range, sea-level rise rate, sediment supply). The structural integrity of the marshes ranged from densely vegetated (high integrity) to severely deteriorated (low integrity) with elevation capital ranging from high to low, respectively, and included a deteriorating marsh site that was partially restored. Two marshes with high elevation capital maintained their relative position high within the tidal range through a combination of surface sediment deposition and shallow subsurface expansion, and kept pace with local sea-level rise. A marsh with moderate elevation capital showed signs of flooding stress and was deteriorating, but managed to keep pace with local sea-level rise. The deteriorated marsh gained no elevation over the 14-year study and was located too low within the tidal range to support continuous coverage of salt marsh vegetation. Elevation gain in the restored marsh initially lagged behind sea-level rise for 8 years, but the elevation trend recovered and kept pace with sea-level rise for the last 5 years. A conceptual model is presented that describes the relationship among elevation capital, and rates of marsh elevation gain and sea-level rise. Note that a search for factors influencing wetland loss should focus on process changes to marsh vertical development (e.g., sediment supply, vegetation growth) and climate change effects (e.g., sea-level and temperature rise) that can cause elevation gain to lag behind sea-level rise, and these occur prior to the onset of marsh deterioration.