Marsh Surface Elevation Change Research Articles

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.

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.

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.

Impacts of Fertilization and Tidal Inundation on Elevation Change in Microtidal, Low Relief Salt Marshes

We evaluated the importance of both tidal inundation and nutrient fertilization as drivers of elevation change in coastal salt marshes. The three sites investigated occurred along a 9-km stretch of the Atlantic Intracoastal Water Way in Central North Carolina. Despite their close geographic proximity, the sites varied in tidal range, elevation within the tidal frame, and stem height and stem density of Spartina alterniflora. Total standing aboveground biomass and marsh surface elevation change (measured by surface elevation tables; SETs) were documented during a 3-year period of nutrient fertilization and for two additional years after fertilization ceased. Measured elevation change rates in control plots varied from 0.2 to 1.2 cm year−1 and were negatively correlated with starting elevation (r 2 = 0.82, p < 0.05). Fertilization resulted in increases in both standing aboveground biomass and marsh surface elevation change. Fertilized sites gained 0.69 to 2.1 cm year−1 during the 3-year application period and 0.02 to 0.97 cm year−1 during the post application period. The magnitude of the fertilization-induced elevation response was mediated by position in the tidal frame with sites that were lower gaining less elevation relative to controls than sites that were higher. The data presented here suggest that nutrient fertilization may be an effective adaptive management strategy for helping minerogenic marshes keep pace with sea level, but that the impact of fertilization may depend on tidal inundation.

Allometry data and equations for coastal marsh plants

AbstractCoastal marshes are highly valued for ecosystem services such as protecting inland habitats from storms, sequestering carbon, removing nutrients and other pollutants from surface water, and providing habitat for fish, shellfish, and birds. Because plants largely determine the structure and function of coastal marshes, quantifying plant biomass is essential for evaluating these ecosystem services, understanding the biogeochemical processes that regulate ecosystem function, and forecasting tidal wetland responses to accelerated sea level rise. Allometry is a convenient and efficient technique for nondestructive estimation of plant biomass, and it is commonly used in studies of carbon and nitrogen cycles, energy flows, and marsh surface elevation change. We present plant allometry data and models developed for three long‐term experiments at the Smithsonian Global Change Research Wetland, a brackish marsh in the Rhode River subestuary of the Chesapeake Bay. The dataset contains 9,771 measurements of stem height, dry mass, and (in 9638 cases) stem width across 11 plant species. The vast majority of observations are for Schoenoplectus americanus (8430) and Phragmites australis (311), with fewer observations for other common species: Amaranthus cannabinus, Atriplex patula, Iva frutescens, Kosteletzkya virginica, Polygonum hydropiper, Solidago sempervirens, Spartina alterniflora, Spartina cynosuroides, and Typha angustifolia. Allometric relationships take the form of linear regressions of biomass (transformed using the Box‐Cox procedure) on either stem height and width, or on stem height alone. Allometric relationships for Schoenoplectus americanus were not meaningfully altered by elevated CO2, N enrichment, the community context, interannual variation in climate, or year, showing that a single equation can be used across a broad range of conditions for this species. Archived files include: (1) raw data used to derive allometric equations for each species, (2) reports and evaluations of the allometric equations we derived from the data, and (3) R code with which our derivations can be replicated. Methodological details of our experiments, data collection efforts, and statistical modeling are described in the metadata. The allometric equations can be used for biomass estimation in empirical and modeling studies of North American coastal wetlands, and the data can be used in ecological studies of terrestrial plant allometry.

Spatial patterns of plant litter in a tidal freshwater marsh and implications for marsh persistence

The maintenance of marsh platform elevation under conditions of sea level rise is dependent on mineral sediment supply to marsh surfaces and conversion of above- and belowground plant biomass to soil organic material. These physical and biological processes interact within the tidal zone, resulting in elevation-dependent processes contributing to marsh accretion. Here, we explore spatial pattern in a variable related to aboveground biomass, plant litter, to reveal its role in the maintenance of marsh surfaces. Plant litter persisting through the dormant season represents the more recalcitrant portion of plant biomass, and as such has an extended period of influence on ecosystem processes. We conducted a field and remote sensing analysis of plant litter height, aboveground biomass, vertical cover, and stem density (collectively termed plant litter structure) at a tidal freshwater marsh located within the Potomac River estuary, USA. LiDAR and field observations show that plant litter structure becomes more prominent with increasing elevation. Spatial patterns in litter structure exhibit stability from year to year and correlate with patterns in soil organic matter content, revealed by measuring the loss on ignition of surface sediments. The amount of mineral material embedded within plant litter decreases with increasing elevation, representing an important tradeoff with litter structure. Therefore, at low elevations where litter structure is short and sparse, the role of plant litter is to capture sediment; at high elevations where litter structure is tall and dense, aboveground litter contributes organic matter to soil development. This organic matter contribution has the potential to eclipse that of belowground biomass as the root:shoot ratio of dominant species at high elevations is low compared to that of dominant species at low elevations. Because of these tradeoffs in mineral and organic matter incorporation into soil across elevation gradients, the rate of marsh surface elevation change is remarkably consistent across elevation. Because of the role of plant litter in marsh ecosystem processes, monitoring and assessment of these dynamic geomorphic marsh landscapes might be streamlined through the measurement of plant litter structure, either via LiDAR technologies or field observation.

Modeling Tidal Freshwater Marsh Sustainability in the Sacramento-San Joaquin Delta Under a Broad Suite of Potential Future Scenarios

In this paper, we report on the adaptation and application of a one-dimensional marsh surface elevation model, the Wetland Accretion Rate Model of Ecosystem Resilience (WARMER), to explore the conditions that lead to sustainable tidal freshwater marshes in the Sacramento–San Joaquin Delta. We defined marsh accretion parameters to encapsulate the range of observed values over historic and modern time-scales based on measurements from four marshes in high and low energy fluvial environments as well as possible future trends in sediment supply and mean sea level. A sensitivity analysis of 450 simulations was conducted encompassing a range of porosity values, initial elevations, organic and inorganic matter accumulation rates, and sea-level rise rates. For the range of inputs considered, the magnitude of SLR over the next century was the primary driver of marsh surface elevation change. Sediment supply was the secondary control. More than 84% of the scenarios resulted in sustainable marshes with 88 cm of SLR by 2100, but only 32% and 11% of the scenarios resulted in surviving marshes when SLR was increased to 133 cm and 179 cm, respectively. Marshes situated in high-energy zones were marginally more resilient than those in low-energy zones because of their higher inorganic sediment supply. Overall, the results from this modeling exercise suggest that marshes at the upstream reaches of the Delta—where SLR may be attenuated—and high energy marshes along major channels with high inorganic sediment accumulation rates will be more resilient to global SLR in excess of 88 cm over the next century than their downstream and low-energy counterparts. However, considerable uncertainties exist in the projected rates of sea-level rise and sediment availability. In addition, more research is needed to constrain future rates of aboveground and belowground plant productivity under increased CO 2 concentrations and flooding.

Importance of Biogeomorphic and Spatial Properties in Assessing a Tidal Salt Marsh Vulnerability to Sea-level Rise

We evaluated the biogeomorphic processes of a large (309 ha) tidal salt marsh and examined factors that influence its ability to keep pace with relative sea-level rise (SLR). Detailed elevation data from 1995 and 2008 were compared with digital elevation models (DEMs) to assess marsh surface elevation change during this time. Overall, 37 % (113 ha) of the marsh increased in elevation at a rate that exceeded SLR, whereas 63 % (196 ha) of the area did not keep pace with SLR. Of the total area, 55 % (169 ha) subsided during the study period, but subsidence varied spatially across the marsh surface. To determine which biogeomorphic and spatial factors contributed to measured elevation change, we collected soil cores and determined percent and origin of organic matter (OM), particle size, bulk density (BD), and distance to nearest bay edge, levee, and channel. We then used Akaike Information Criterion (AICc) model selection to assess those variables most important to determine measured elevation change. Soil stable isotope compositions were evaluated to assess the source of the OM. The samples had limited percent OM by weight (<5.5 %), with mean bulk densities of 0.58 g cm-3, indicating that the soils had high mineral content with a relatively low proportion of pore space. The most parsimonious model with the highest AICc weight (0.53) included distance from bay's edge (i.e., lower intertidal) and distance from levee (i.e., upper intertidal). Close proximity to sediment source was the greatest factor in determining whether an area increased in elevation, whereas areas near landward levees experienced subsidence. Our study indicated that the ability of a marsh to keep pace with SLR varied across the surface, and assessing changes in elevation over time provides an alternative method to long-term accretion monitoring. SLR models that do not consider spatial variability of biogeomorphic and accretion processes may not correctly forecast marsh drowning rates, which may be especially true in modified and urbanized estuaries. In light of SLR, improving our understanding of elevation change in these dynamic marsh systems will play a crucial role in forecasting potential impacts to their sustainability and the survival of these ecosystems.

The Protective Role of Coastal Marshes: A Systematic Review and Meta-analysis

BackgroundSalt marshes lie between many human communities and the coast and have been presumed to protect these communities from coastal hazards by providing important ecosystem services. However, previous characterizations of these ecosystem services have typically been based on a small number of historical studies, and the consistency and extent to which marshes provide these services has not been investigated. Here, we review the current evidence for the specific processes of wave attenuation, shoreline stabilization and floodwater attenuation to determine if and under what conditions salt marshes offer these coastal protection services.Methodology/Principal FindingsWe conducted a thorough search and synthesis of the literature with reference to these processes. Seventy-five publications met our selection criteria, and we conducted meta-analyses for publications with sufficient data available for quantitative analysis. We found that combined across all studies (n = 7), salt marsh vegetation had a significant positive effect on wave attenuation as measured by reductions in wave height per unit distance across marsh vegetation. Salt marsh vegetation also had a significant positive effect on shoreline stabilization as measured by accretion, lateral erosion reduction, and marsh surface elevation change (n = 30). Salt marsh characteristics that were positively correlated to both wave attenuation and shoreline stabilization were vegetation density, biomass production, and marsh size. Although we could not find studies quantitatively evaluating floodwater attenuation within salt marshes, there are several studies noting the negative effects of wetland alteration on water quantity regulation within coastal areas.Conclusions/SignificanceOur results show that salt marshes have value for coastal hazard mitigation and climate change adaptation. Because we do not yet fully understand the magnitude of this value, we propose that decision makers employ natural systems to maximize the benefits and ecosystem services provided by salt marshes and exercise caution when making decisions that erode these services.

Marsh Elevation Response to Hurricanes Katrina and Rita and the Effect of Altered Nutrient Regimes

Our scientific understanding of the marshes along the north shore of Lake Pontchartrain, Louisiana, is limited in terms of the processes required to sustain them and how to best manage them in the face of predicted rising sea levels. Subject to localized subsidence and urban development, these marshes may also be affected by increased nutrient loading in the future from proposed Mississippi River diversions and continued urbanization. This study presents data on marsh surface elevation change across a series of experimental plots located in Big Branch Marsh National Wildlife Refuge, Louisiana, that were subject to varying additions of phosphorus and nitrogen as well as a lethal herbicide treatment. These plots were also affected by Hurricanes Katrina and Rita in 2005. The rate of marsh elevation change prior to the storm suggests these marshes were maintaining elevation in the face of sea-level rise. A dramatic increase in elevation occurred following the storms but was followed by a proportional decrease in elevation. Soil data indicate the increase was caused by an influx of highly organic material at all plots. The results show how both storm and nonstorm processes contribute to elevation change and the maintenance of these marshes in the face of sea-level rise.

Physical and Functional Responses to Experimental Marsh Surface Elevation Manipulation in Coos Bay's South Slough

AbstractDike material was used as fill to construct high, mid, and low intertidal elevations in a subsided marsh located in the South Slough National Estuarine Research Reserve, Oregon. Marsh surface elevation change (including fill consolidation and compression of the original marsh soils), vertical accretion, tidal channel development, emergent vegetation colonization, and fish use were monitored over 3 years. Significant marsh surface elevation loss was detected at all elevations, with fill consolidation accounting for 70% of the loss at the highest elevation. Vertical accretion averaged 0.19 cm/yr in the sparsely vegetated Kunz Marsh compared with 0.70 cm/yr at the densely vegetated reference sites. Tidal channel development was influenced as much by marsh surface gradient as by marsh surface elevation. Vegetation colonization was directly correlated with elevation, whereas density and species richness of fish was inversely correlated with elevation. Manipulating the marsh surface to mid‐marsh elevations favors rapid vegetation colonization and facilitates vertical accretion‐dominated tidal channel development. Low marsh elevations result in initially slower developing vegetation colonization and channel development but are more beneficial to fish during the early stages of marsh recovery. High marsh elevations appear to sacrifice tidal channel development and associated fish access for rapid vegetation colonization.

Understanding Tidal Marsh Sedimentation in the Sacramento-San Joaquin Delta, California

Over 90% of the once vast tidal-freshwater wetlands of the Sacramento-San Joaquin Delta have been leveed and removed from tidal and floodwater inundation. Contemporary restoration efforts breach and/or remove the levees surrounding delta islands with the goal of regaining wetland habitat. However, experience so far with levee-breaches, both planned and unplanned, has shown that the transition from shallow open water to Tule marshes occurs slowly, if at all. Sedimentation rates in tidal marshes are an important control on this transition. This two-year study examined potential controlling factors on vertical accumulation including sediment type, and inorganic and organic soil components. Measured rates of surface accretion are in excess of 10 mm/yr. Gravimetric determinations show the main control on variations in soil accretion appears to be mineral sediment accumulation, with highest rates being close to inputs from the Sacramento River and lowest rates in interior marshes in the south-central delta. Rates of organic accumulation are remarkably similar among areas. However, measurements of marsh surface elevation change reflect the volumetric contributions of soil components and appear to be influenced by interannual changes in vegetation associated with minor variations in salinity. Marshes in breached-levee sites showed higher annual rates of marsh surface elevation change than reference marshes. This study demonstrates the importance of examining volumetric rather than gravimetric contributions to marsh soils in evaluating the factors controlling the vertical development of tidal marshes.