Drainage and Elevation as Factors in the Restoration of Salt Marsh in Britain

Intertidal restoration through realignment of flood defenses has become an important component of the U.K. coastal and estuarine management strategy. Although experimentation with recent deliberate breaches is in progress, the long‐term prognosis for salt marsh restoration can be investigated at a number of sites around Essex, southeast England where salt marshes have been reactivated (unmanaged restoration) by storm events over past centuries. These historically reactivated marshes possess higher creek densities than their natural marsh counterparts. Both geomorphology and sedimentology determine the hydrology of natural and restored salt marshes. Elevation relative to the tidal frame is known to be the primary determinant of vegetation colonization and succession. Yet vegetation surveys and geotechnical analysis at a natural marsh, where areas with good drainage exist in close proximity to areas of locally hindered drainage at the same elevation, revealed a significant inverse relationship between water saturation in the root zone and the abundance of Atriplex portulacoides, normally the physiognomic dominant on upper salt marsh in the region. Elsewhere in Essex natural and restored marshes are typified by very high sediment water contents, and this is reflected in low abundance of A. portulacoides. After a century of reestablishment no significant difference could be discerned between the vegetation composition of the storm‐reactivated marshes and their natural marsh counterparts. We conclude that vegetation composition may be restored within a century of dike breaching, but this vegetation does not provide a reliable indicator of ecological functions related to creek structure.     

Modeling Habitat Change in Salt Marshes After Tidal Restoration

Salt marshes continue to degrade in the United States due to indirect human impacts arising from tidal restrictions. Roads or berms with inadequate provision for tidal flow hinder ecosystem functions and interfere with self‐maintenance of habitat, because interactions among vegetation, soil, and hydrology within tidally restricted marshes prevent them from responding to sea level rise. Prediction of the tidal range that is expected after restoration relative to the current geomorphology is crucial for successful restoration of salt marsh habitat. Both insufficient (due to restriction) and excessive (due to subsidence and sea level rise) tidal flooding can lead to loss of salt marshes. We developed and applied the Marsh Response to Hydrological Modifications model as a predictive tool to forecast the success of management scenarios for restoring full tides to previously restricted areas. We present an overview of a computer simulation tool that evaluates potential culvert installations with output of expected tidal ranges, water discharges, and flood potentials. For three New England tidal marshes we show species distributions of plants for tidally restricted and nonrestricted areas. Elevation ranges of species are used for short‐term (<5 years) predictions of changes to salt marsh habitat after tidal restoration. In addition, elevation changes of the marsh substrate measured at these sites are extrapolated to predict long‐term (>5 years) changes in marsh geomorphology under restored tidal regimes. The resultant tidal regime should be designed to provide habitat requirements for salt marsh plants. At sites with substantial elevation losses a balance must be struck that stimulates elevation increases by improving sediment fluxes into marshes while establishing flooding regimes appropriate to sustain the desired plants.     

Using Functional Trajectories to Track Constructed Salt Marsh Development in the Great Bay Estuary, Maine/New Hampshire, U.S.A.

A growing number of studies have assessed the functional equivalency of restored and natural salt marshes. Several of these have explored the use of functional trajectories to track the increase in restored marsh function over time; however, these studies have disagreed as to the usefulness of such models in long‐term predictions of restored marsh development. We compared indicators of four marsh functions (primary production, soil organic matter accumulation, sediment trapping, and maintenance of plant communities) in 6 restored and 11 reference (matched to restored marshes using principal components analysis) salt marshes in the Great Bay Estuary. The restored marshes were all constructed and planted on imported substrate and ranged in age from 1 to 14 years. We used marsh age in a space‐for‐time substitution to track constructed salt marsh development and explore the use of trajectories. A high degree of variability was observed among natural salt marsh sites, displaying the importance of carefully chosen reference sites. As expected, mean values for constructed site (n = 6) and reference site (n = 11) functions were significantly different. Using constructed marsh age as the independent variable and functional indicator values as dependent variables, nonlinear regression analyses produced several ecologically meaningful trajectories (r ²> 0.9), demonstrating that the use of different‐aged marshes can be a viable approach to developing functional trajectories. The trajectories illustrated that although indicators of some functions (primary production, sediment deposition, and plant species richness) may reach natural site values relatively quickly (<10 years), others (soil organic matter content) will take longer.     

Climate change and loss of saltmarshes: consequences for birds

Saltmarshes are areas of vegetation subject to tidal inundation and are important to birds for several reasons. Saltmarshes are areas of high primary productivity and their greatest significance for coastal birds is probably as the base of estuarine food webs, because saltmarshes export considerable amounts of organic carbon to adjacent habitats, particularly to the invertebrates of mudflats. In addition, saltmarshes are of direct importance to birds by providing sites for feeding, nesting and roosting. Climate change can affect saltmarshes in a number of ways, including through sea-level rise. When sea-level rises the marsh vegetation moves upward and inland but sea walls that prevent this are said to lead to coastal squeeze and loss of marsh area. However, evidence from southeast England, and elsewhere, indicates that sea-level rise does not necessarily lead to loss of marsh area because marshes accrete vertically and maintain their elevation with respect to sea-level where the supply of sediment is sufficient. Organogenic marshes and those in areas where sediment may be more limiting (e.g. some west coast areas) may be more susceptible to coastal squeeze, as may other marshes, if some extreme predictions of accelerated rates of sea-level rise are realized.     

Utilization of a citizen monitoring protocol to assess the structure and function of natural and stabilized fringing salt marshes in North Carolina

Narrow fringing salt marshes dominated by Spartina alterniflora occur naturally along estuarine shorelines and provide many of the same ecological functions as more extensive marshes. These fringing salt marshes are sometimes incorporated into shoreline stabilization efforts. We obtained data on elevation, salinity, sediment characteristics, vegetation and fish utilization at three study sites containing both natural fringing marshes and nearby restored marshes located landward of a stone sill constructed for shoreline stabilization. During the study, sediment accretion rates in the restored marshes were approximately 1.5- to 2-fold greater than those recorded in the natural marshes. Natural fringing marsh sediments were predominantly sandy with a mean organic matter content ranging between 1.5 and 6.0%. Average S. alterniflora stem density in natural marshes ranged between 130 and 222 stems m⁻², while mean maximum stem height exceeded 64 cm. After 3 years, one of the three restored marshes (NCMM) achieved S. alterniflora stem densities equivalent to that of the natural fringing marshes, while percentage cover and maximum stem heights were significantly greater in the natural than in the restored marshes at all sites. There was no significant difference in the mean number of fish, crabs or shrimp captured with fyke nets between the natural and restored marshes, and only the abundance of Palaemonetes vulgaris (grass shrimp) was significantly greater in the natural marshes than in the restored ones. Mean numbers of fish caught per 5 m of marsh front were similar to those reported in the literature from marshes adjacent to tidal creeks and channels, and ranged between 509 and 634 fish net⁻¹. Most of the field data and some of the sample analyses were obtained by volunteers as they contributed 223 h of the total 300 h spent collecting data from three sites in one season. The use of fyke nets required twice as many man-hours as any other single task. Vegetation and sediment parameters were sensitive indicators of marsh restoration success, and volunteers were capable of contributing a significant portion of the labor needed to collect these parameters. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Use of GIS and high resolution LiDAR in salt marsh restoration site suitability assessments in the upper Bay of Fundy, Canada

Salt marshes exhibit striking vegetation zonation corresponding to spatially variable elevation gradients which dictate their frequency of inundation by the tides. The salt marshes in the upper Bay of Fundy, a dynamic hypertidal system, are of considerable interest due to increasing recognition of salt marsh ecosystem values and the extent of prior conversion of salt marshes to agricultural lands, much of which are no longer in use. To determine the suitability of two potential restoration sites at Beausejour Marsh in New Brunswick, Canada, geomatics technologies and techniques were used to assess vegetation and elevation patterns in an adjacent reference salt marsh and the proposed restoration sites. Light detection and ranging digital elevation models (DEMs) were created for the reference marsh and the restoration sites in both the spring (leaf-off) and late summer (leaf-on, maximum biomass) periods. Aerial photographs and Quickbird multispectral imagery were used to visually interpret vegetation zones on the reference marsh and were field validated using vegetation characteristics from quadrats referenced with differential GPS. Elevation limits of the salt marsh vegetation zones were extracted from the DEM of the reference marsh and applied to the DEM of the restoration sites to determine the percentage area of each site that would be immediately suitable for new salt marsh growth. Of the two restoration sites assessed, one had experienced significant subsidence since dyking; only about 40 % of the site area was determined to be of sufficient elevation for immediate vegetation colonization. The second site, while more than 88 % suitable, would require the installation of a large dyke on the landward side of the restoration site to prevent flooding of adjacent lands. This study provides essential high resolution elevation and vegetation zonation data for use in restoration site assessments, and highlights the usefulness of applied geomatics in the salt marsh restoration planning process.     

The interactive effects of created salt marsh substrate type,hydrology, and nutrient regime on <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Avicennia germinans</Emphasis> productivity and soil development

Recent salt marsh and barrier island restoration efforts in the northern Gulf of Mexico have focused on optimizing self-sustaining attributes of restored marshes to provide maximum habitat value and storm protection to vulnerable coastal communities. Salt marshes in this region are dominated by Spartina alterniflora and Avicennia germinans, two species that are valued for their ability to stabilize soils in intertidal salt marshes. We conducted a controlled greenhouse study to investigate the influences of substrate type, nutrient level, and marsh elevation on the growth and biomass allocation of S. alterniflora and A. germinans, and the consequent effects on soil development and stability. S. alterniflora exhibited optimal growth and survival at the lowest elevation (? 15 cm below the water surface) and was sensitive to high soil salinities at higher elevations (+ 15 cm above the water surface). A. germinans performed best at intermediate elevations but was negatively affected by prolonged inundation at lower elevations. We found that although there was not a strong effect of substrate type on plant growth, the development of stressful conditions due to the use of suboptimal materials would likely be exacerbated by placing the soil at extreme elevations. Soil shear strength was significantly higher in experimental units containing either S. alterniflora or A. germinans compared to unvegetated soils, suggesting that plants effectively contribute to soil strength in newly placed soils of restored marshes. As marsh vegetation plays a critical role in stabilizing shorelines, salt marsh restoration efforts in the northern Gulf of Mexico and other storm impacted coasts should be designed at optimal elevations to facilitate the establishment and growth of key marsh species.     

Field assessment of environmental factors constraining the development and expansion of <Emphasis Type="Italic">Schoenoplectus californicus</Emphasis> marsh at a California tidal freshwater restoration site

The effective restoration of wetland habitats requires understanding the establishment requirements, growth responses, and expansion dynamics of targeted plant species. This is particularly true when restoring areas that have been previously managed for other activities, such as agriculture, which can have legacy effects on the local environment. We investigated environmental factors (specifically hydrology and soil physicochemical conditions) that may influence the establishment, growth and expansion of Schoenoplectus californicus in a tidal freshwater marsh restoration site in the Sacramento–San Joaquin Delta, California, USA. This study site was previously leveed, drained, and utilized for agricultural production. A 1997 levee breach restored tidal connectivity and wetland vegetation has re-established in portions of the area. Our approach coupled an intensively-sampled transect study in S. californicus-dominated marshes with a spatially-extensive survey of S. californicus lateral expansion rates and elevation. Lateral expansion of S. californicus marsh edge was significantly less in lower elevation areas (0.61 ± 0.04 m NAVD88), whereas the marsh edge at higher elevations (0.84 ± 0.03 m NAVD88) exhibited greater expansion, often at rates greater than 1.0 m year^?1. These elevation means correspond to percentages of time that the marsh surface was flooded of 100 and 94 %, respectively. Although marsh edge expansion was influenced by elevation and the resultant hydrology, other factors, such as physical exposure of marsh shorelines and compacted agricultural soils also appear to be important. However, once established, S. californicus appears to be able to ameliorate high soil bulk densities over time as the advancing marsh platform develops.     

The natural regeneration of salt marsh on formerly reclaimed land

Question: Does the vegetation of restored salt marshes increasingly resemble natural reference communities over time? Location: The Essex estuaries, southeast England. Methods: Abandoned reclamations, where coastal defences had been breached in storm events, and current salt marsh recreation schemes were surveyed giving a chronosequence of salt marsh regeneration from 2 to 107 years. The presence, abundance and height of plant species were recorded and comparisons were made with adjacent reference salt marsh communities at equivalent elevations. Results: Of the 18 paired sites surveyed, 13 regenerated marshes had fewer species than their adjacent reference marsh, three had an equal number and two had more. The plant communities of only two de‐embankment sites matched that of the reference community. 0–50 year old sites and 51–100 year old sites had fewer species per quadrat than the 101+ year sites and the reference salt marshes. There was a weak relationship between differences in species richness for regenerated and reference marshes and the time since sites were first re‐exposed to tidal inundation. Cover values for the invasive and recently evolved Spartina anglica were greater within regenerated than reference marshes. Conclusions: Salt marsh plants will colonise formerly reclaimed land relatively quickly on resumption of tidal flooding. However, even after 100 years regenerated salt marshes differ in species richness, composition and structure from reference communities.     

Optimal hurricane overwash thickness for maximizing marsh resilience to sea level rise

The interplay between storms and sea level rise, and between ecology and sediment transport governs the behavior of rapidly evolving coastal ecosystems such as marshes and barrier islands. Sediment deposition during hurricanes is thought to increase the resilience of salt marshes to sea level rise by increasing soil elevation and vegetation productivity. We use mesocosms to simulate burial of Spartina alterniflora during hurricane‐induced overwash events of various thickness (0–60 cm), and find that adventitious root growth within the overwash sediment layer increases total biomass by up to 120%. In contrast to most previous work illustrating a simple positive relationship between burial depth and vegetation productivity, our work reveals an optimum burial depth (5–10 cm) beyond which burial leads to plant mortality. The optimum burial depth increases with flooding frequency, indicating that storm deposition ameliorates flooding stress, and that its impact on productivity will become more important under accelerated sea level rise. Our results suggest that frequent, low magnitude storm events associated with naturally migrating islands may increase the resilience of marshes to sea level rise, and in turn, slow island migration rates. Synthesis: We find that burial deeper than the optimum results in reduced growth or mortality of marsh vegetation, which suggests that future increases in overwash thickness associated with more intense storms and artificial heightening of dunes could lead to less resilient marshes.     

Long-term response of fishes and other fauna to restoration of former salt hay farms: multiple measures of restoration success

This synthesis brings together published and unpublished data in an evaluation of restoration of former salt hay farms to functioning salt marshes. We compared nine years of field measurements between three restored marshes (Dennis, Commercial, and Maurice River Townships) and a reference marsh (Moores Beach) in the mesohaline portion of Delaware Bay. In the process, we compared channel morphology, geomorphology, vegetation, sediment organic matter, fish assemblages, blue crabs, horseshoe crabs, benthic infauna, and diamondback terrapins. For fishes we compared structural (distribution, abundance) and functional (feeding, growth, survival, reproduction, production) aspects to evaluate the restored marshes in an Essential Fish Habitat context. Marsh vegetation and drainage density responded gradually and positively with restored marshes approximating the state of the reference marsh within the nine-year study period. The fauna responded more quickly and dramatically with most measures equal or greater in the restored marshes within the first one or two years after restoration. Differences in response time between the vegetation and the fauna imply that the faunal response was more dependent on access to the shallow intertidal marsh surface and intertidal and subtidal creeks than on characteristics of the vegetated marsh. The fishes in created subtidal creeks in restored marshes responded immediately and maintained fish assemblages similar to the reference marsh over the study period. The intertidal creek fish assemblages tended to become more like the reference marsh in the last years of the comparison. Overall, these results document the success of the restoration and how marshes function for both resident and transient fauna, especially fishes.     

Salt Marsh Restoration in Connecticut: 20 Years of Science and Management

In 1980 the State of Connecticut began a tidal marsh restoration program targeting systems degraded by tidal restrictions and impoundments. Such marshes become dominated by common reed grass (Phragmites australis) and cattail (Typha angustifolia and T. latifolia), with little ecological connection to Long Island Sound. The management and scientific hypothesis was that returning tidal action, reconnecting marshes to Long Island Sound, would set these systems on a recovery trajectory. Specific restoration targets (i.e., pre‐disturbance conditions or particular reference marshes) were considered unrealistic. However, it was expected that with time restored tides would return ecological functions and attributes characteristic of fully functioning tidal salt marshes. Here we report results of this program at nine separate sites within six marsh systems along 110 km of Long Island Sound shoreline, with restoration times of 5 to 21 years. Biotic parameters assessed include vegetation, macroinvertebrates, and use by fish and birds. Abiotic factors studied were soil salinity, elevation and tidal flooding, and soil water table depth. Sites fell into two categories of vegetation recovery: slow, ca. 0.5%, or fast, more than 5% of total area per year. Although total cover and frequency of salt marsh angiosperms was positively related to soil salinity, and reed grass stand parameters negatively so, fast versus slow recovery rates could not be attributed to salinity. Instead, rates appear to reflect differences in tidal flooding. Rapid recovery was characterized by lower elevations, greater hydroperiods, and higher soil water tables. Recovery of other biotic attributes and functions does not necessarily parallel those for vegetation. At the longest studied system (rapid vegetation recovery) the high marsh snail Melampus bidentatus took two decades to reach densities comparable with a nearby reference marsh, whereas the amphipod Orchestia grillus was well established on a slow‐recovery marsh, reed grass dominated after 9 years. Typical fish species assemblages were found in restoration site creeks and ditches within 5 years. Gut contents of fish in ditches and on the high marsh suggest that use of restored marsh as foraging areas may require up to 15 years to reach equivalence with reference sites. Bird species that specialize in salt marshes require appropriate vegetation; on the oldest restoration site, breeding populations comparable with reference marshland had become established after 15 years. Use of restoration sites by birds considered marsh generalists was initially high and was still nearly twice that of reference areas even after 20 years. Herons, egrets, and migratory shorebirds used restoration areas extensively. These results support our prediction that returning tides will set degraded marshes on trajectories that can bring essentially full restoration of ecological functions. This can occur within two decades, although reduced tidal action can delay restoration of some functions. With this success, Connecticut's Department of Environmental Protection established a dedicated Wetland Restoration Unit. As of 1999 tides have been restored at 57 separate sites along the Connecticut coast.     

Upslope development of a tidal marsh as a function of upland land use

To thrive in a time of rapid sea‐level rise, tidal marshes will need to migrate upslope into adjacent uplands. Yet little is known about the mechanics of this process, especially in urbanized estuaries, where the adjacent upland is likely to be a mowed lawn rather than a wooded natural area. We studied marsh migration in a Long Island Sound salt marsh using detailed hydrologic, edaphic, and biotic sampling along marsh‐to‐upland transects in both wooded and lawn environments. We found that the overall pace of marsh development was largely unaffected by whether the upland being invaded was lawn or wooded, but the marsh‐edge plant communities that developed in these two environments were quite different, and some indicators (soil salinity, foraminifera) appeared to migrate more easily into lawns. In addition, we found that different aspects of marsh structure and function migrated at different rates: Wetland vegetation appeared to be a leading indicator of marsh migration, while soil characteristics such as redox potential and surface salinity developed later in the process. We defined a ‘hydrologic migration zone’, consisting of elevations that experience tidal inundation with frequencies ranging from 20% to 0.5% of high tides. This hydrologically defined zone – which extended to an elevation higher than the highest astronomical tide datum – captured the biotic and edaphic marsh‐upland ecotone. Tidal inundation at the upper border of this migration zone is highly variable over time and may be rising more rapidly than mean sea level. Our results indicate that land management practices at the upland periphery of tidal marshes can facilitate or impede ecosystem migration in response to rising sea level. These findings are applicable to large areas of tidal marsh along the U.S. Atlantic coast and in other urbanized coastal settings.     

Development and N2-Fixing Activity of the Benthic Microbial Community in Transplanted Spartina alterniflora Marshes in North Carolina

Growth and maturation of transplanted salt marshes is often limited by the availability of nitrogen (N). We examined the role of N₂-fixing benthic microbial assemblages (microalgae and associated bacteria) in two restored marshes (1-year-old and 6-year-old marsh) and a natural salt marsh in the Newport River Estuary, North Carolina. Benthic N₂ fixation (nitrogenase activity, NA), chlorophyll a (Chl a ) concentration, Spartina alterniflora (smooth cordgrass) stem counts, and sediment organic matter content were determined in the three marshes. Significant differences were observed between sites for both Chl a and NA. The 1-year-old marsh always exhibited the highest levels of NA and Chl a . Sediment organic matter content was lowest in the 1-year-old marsh (∼2%), intermediate in the 6-year-old marsh (∼5%), and highest in the natural marsh (∼10%). Carbon and nitrogen analyses were also performed on the 1-year-old marsh sediments, which were depleted in N. A positive correlation was observed between surface sediment N and Chl a . Remineralized, microbially derived N may provide growth-limiting inorganic N to Spartina transplants. N₂-fixing microbial assemblages in the 1-year-old marsh may also be an important food source for marsh infauna. Benthic N₂-fixing microbial assemblages play a key role in the N economy of restored salt marshes.     

Carbon accumulation and vertical accretion in a restored versus historic salt marsh in southern Puget Sound,Washington, United States

Few comparisons exist between vertical accretion (VA) and carbon accumulation rates (CARs) in restored versus historic (i.e. reference) marshes. Here, we compare these processes in a formerly diked, sparsely vegetated, restored salt marsh (Six Gill Slough, SG), whose surface is subsided relative to the tidal frame, to an adjacent, relatively pristine, historic salt marsh (Animal Slough, AS). Six sediment cores were collected at both AS and SG approximately 6 years after restoration. Cores were analyzed for bulk density (BD), % loss of ignition, % organic carbon, and ²¹⁰Pb. We found that sharp changes in BD in surface layers of SG cores were highly reliable markers for the onset of restoration. The mean VA since restoration at SG (0.79 [SD = 0.29] cm/year) was approximately twice that of AS (0.41 [SD = 0.16] cm/year). In comparison, the VA at AS over 50 years was 0.30 (SD = 0.09) cm/year. VA consisted almost entirely of inorganic sediment at SG whereas at AS it was approximately 55%. Mean CARs at SG were somewhat greater than at AS, but the difference was not significant due to high variability (SG: 81–210 g C m^?2 year^?1; AS: 115–168 g C m^?2 year^?1). The mean CAR at AS over the past 50 years was 118 (SD = 23) g C m^?2 year^?1. This study demonstrates that a sparsely vegetated, restored salt marsh can quickly begin to accumulate carbon and that historic and restored marshes can have similar CARs despite highly divergent formation processes.     

Plant responses to increased inundation and salt exposure: interactive effects on tidal marsh productivity

Flooding and high salinity generally induce physiological stress in wetland vascular plants which may increase in intensity with sea-level rise (SLR). We tested the effects of these factors on seedling growth in a transplant experiment in a macrotidal estuary in the Pacific Northwest. Seven common wetland species were grown at mean higher high water (MHHW, a typical mid-marsh elevation), and at 25 and 50 cm below MHHW in oligohaline, mesohaline, and polyhaline marshes. Increased flooding reduced shoot and root growth in all species, including those typically found at middle or lower tidal elevations. It also generally disproportionately reduced root biomass. For more sensitive species, biomass declined by >50 % at only 25 cm below MHHW at the oligohaline site. Plant growth was also strongly reduced under polyhaline conditions relative to the less saline sites. By combining inundation and salinity time-series measurements we estimated a salt exposure index for each site by elevation treatment. Higher values of the index were associated with lower root and shoot biomass for all species and a relatively greater loss of below-ground than above-ground production in most species. Our results suggest that inundation and salinity stress individually and (often) interactively reduce productivity across a suite of common marsh species. As relative SLR increases the intensity of stress on coastal marsh plants, negative effects on biomass may occur across a range of species and especially on below-ground production.     

Vegetation's importance in regulating surface elevation in a coastal salt marsh facing elevated rates of sea level rise

Rising sea levels threaten the sustainability of coastal wetlands around the globe, thus understanding how increased inundation alters the elevation change mechanisms in these systems is increasingly important. Typically, the ability of coastal marshes to maintain their position in the intertidal zone depends on the accumulation of both organic and inorganic materials, so one, if not both, of these processes must increase to keep pace with rising seas, assuming all else constant. To determine the importance of vegetation in these processes, we measured elevation change and surface accretion over a 4‐year period in recently subsided, unvegetated marshes, resulting from drought‐induced marsh dieback, in paired planted and unplanted plots. We compared soil and vegetation responses in these plots with paired reference plots that had neither experienced dieback nor subsidence. All treatments (unvegetated, planted, and reference) were replicated six times. The recently subsided areas were 6–10 cm lower in elevation than the reference marshes at the beginning of the study; thus, mean water levels were 6–10 cm higher in these areas vs. the reference sites. Surface accretion rates were lowest in the unplanted plots at 2.3 mm yr^?1, but increased in the presence of vegetation to 16.4 mm yr^?1 in the reference marsh and 26.1 mm yr^?1 in the planted plots. The rates of elevation change were also bolstered by the presence of vegetation. The unplanted areas decreased in elevation by 9.4 mm yr^?1; whereas the planted areas increased in elevation by 13.3 mm yr^?1, and the reference marshes increased by 3.5 mm yr^?1. These results highlight the importance of vegetation in the accretionary processes that maintain marsh surface elevation within the intertidal zone, and provide evidence that coastal wetlands may be able to keep pace with a rising sea in certain situations.     

Fifteen Years of Vegetation and Soil Development after Brackish-Water Marsh Creation

Aboveground biomass, macro‐organic matter (MOM), and wetland soil characteristics were measured periodically between 1983 and 1998 in a created brackish‐water marsh and a nearby natural marsh along the Pamlico River estuary, North Carolina to evaluate the development of wetland vegetation and soil dependent functions after marsh creation. Development of aboveground biomass and MOM was dependent on elevation and frequency of tidal inundation. Aboveground biomass of Spartina alterniflora, which occupied low elevations along tidal creeks and was inundated frequently, developed to levels similar to the natural marsh (750 to 1,300 g/m²) within three years after creation. Spartina cynosuroides, which dominated interior areas of the marsh and was flooded less frequently, required 9 years to consistently achieve aboveground biomass equivalent to the natural marsh (600 to 1,560 g/m²). Aboveground biomass of Spartina patens, which was planted at the highest elevations along the terrestrial margin and seldom flooded, never consistently developed aboveground biomass comparable with the natural marsh during the 15 years after marsh creation. MOM (0 to 10 cm) generally developed at the same rate as aboveground biomass. Between 1988 and 1998, soil bulk density decreased and porosity and organic C and N pools increased in the created marsh. Like vegetation, wetland soil development proceeded faster in response to increased inundation, especially in the streamside zone dominated by S. alterniflora. We estimated that in the streamside and interior zones, an additional 30 years (nitrogen) to 90 years (organic C, porosity) are needed for the upper 30 cm of created marsh soil to become equivalent to the natural marsh. Wetland soil characteristics of the S. patens community along upland fringe will take longer to develop, more than 200 years. Development of the benthic invertebrate‐based food web, which depends on organic matter enrichment of the upper 5 to 10 cm of soil, is expected to take less time. Wetland soil characteristics and functions of created irregularly flooded brackish marshes require longer to develop compared with regularly flooded salt marshes because reduced tidal inundation slows wetland vegetation and soil development. The hydrologic regime (regularly vs. irregularly flooded) of the “target” wetland should be considered when setting realistic expectations for success criteria of created and restored wetlands.     

Estuaries as Filters: The Role of Tidal Marshes in Trace Metal Removal

Flux calculations demonstrate that many estuaries are natural filters for trace metals. Yet, the underlying processes are poorly investigated. In the present study, it was hypothesized that intertidal marshes contribute significantly to the contaminant filter function of estuaries. Trace metal concentrations and sediment characteristics were measured along a transect from the subtidal, over an intertidal flat and marsh to a restored marsh with controlled reduced tide. Metal concentrations in the intertidal and restored marsh were found to be a factor two to five higher than values in the subtidal and intertidal flat sediments. High metal concentrations and high accretion rates indicate a high metal accumulation capacity of the intertidal marshes. Overbank sedimentation in the tidal marshes of the entire estuary was calculated to remove 25% to 50% of the riverine metal influx, even though marshes comprise less than 8% of the total surface of the estuary. In addition, the large-scale implementation of planned tidal marsh restoration projects was estimated to almost double the trace metal storage capacity of the present natural tidal marshes in the estuary.     

New multi-metric Salt Marsh Sediment Microbial Index (SSMI) application to salt marsh sediments ecological status assessment

Salt marshes are very important areas for biogeochemical cycling, sediment accretion, pollution filtration and retention and erosion and stabilization of the river margins. The high organic matter content in the salt marsh plant sediments along with the radial oxygen diffusion provided by these halophyte root systems gather the ideal conditions for the development of a microbial rhizosphere community. Due to the quick feedback of the microbial communities to an environmental change, these organisms become important monitors for environmental impact assessment. A Salt marsh Sediment Microbial Index (SSMI) that reflected physical–chemical and microbial parameters was applied to plant rhizosphere sediments of five salt marshes from three important water bodies from Portugal. The SSMI revealed to be plant-independent evaluating efficiently the different marshes according to their maturity degree and disturbance influence. Mature salt marshes SSMI classification grouped all the systems at this development stage, while the younger salt marshes are classified in different groups according to their evolution degree. Also the impact degree is reflected at this level discriminating also the more adversely impacted salt marshes. Being a multi-metric index, the SSMI sub-metrics are also susceptible of ecological interpretation, giving important backstage information about the underlying biogeochemical cycling processes.