Marsh Erosion Research Articles

Sea level driven marsh expansion in a coupled model of marsh erosion and migration

AbstractCoastal wetlands are among the most valuable ecosystems on Earth, where ecosystem services such as flood protection depend nonlinearly on wetland size and are threatened by sea level rise and coastal development. Here we propose a simple model of marsh migration into adjacent uplands and couple it with existing models of seaward edge erosion and vertical soil accretion to explore how ecosystem connectivity influences marsh size and response to sea level rise. We find that marsh loss is nearly inevitable where topographic and anthropogenic barriers limit migration. Where unconstrained by barriers, however, rates of marsh migration are much more sensitive to accelerated sea level rise than rates of edge erosion. This behavior suggests a counterintuitive, natural tendency for marsh expansion with sea level rise and emphasizes the disparity between coastal response to climate change with and without human intervention.

A linear relationship between wave power and erosion determines salt-marsh resilience to violent storms and hurricanes.

Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. It is still unclear how resistant salt marshes are to extreme storms and whether they can survive multiple events without collapsing. Based on a large dataset of salt marsh lateral erosion rates collected around the world, here, we determine the general response of salt marsh boundaries to wave action under normal and extreme weather conditions. As wave energy increases, salt marsh response to wind waves remains linear, and there is not a critical threshold in wave energy above which salt marsh erosion drastically accelerates. We apply our general formulation for salt marsh erosion to historical wave climates at eight salt marsh locations affected by hurricanes in the United States. Based on the analysis of two decades of data, we find that violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. In contrast, moderate storms with a return period of 2.5 mo are those causing the most salt marsh deterioration. Therefore, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans.

Assessing shoreline exposure and oyster habitat suitability maximizes potential success for sustainable shoreline protection using restored oyster reefs.

Oyster reefs provide valuable ecosystem services that contribute to coastal resilience. Unfortunately, many reefs have been degraded or removed completely, and there are increased efforts to restore oysters in many coastal areas. In particular, much attention has recently been given to the restoration of shellfish reefs along eroding shorelines to reduce erosion. Such fringing reef approaches, however, often lack empirical data to identify locations where reefs are most effective in reducing marsh erosion, or fully take into account habitat suitability. Using monitoring data from 5 separate fringing reef projects across coastal Louisiana, we quantify shoreline exposure (fetch + wind direction + wind speed) and reef impacts on shoreline retreat. Our results indicate that fringing oyster reefs have a higher impact on shoreline retreat at higher exposure shorelines. At higher exposures, fringing reefs reduced marsh edge erosion an average of 1.0 m y−1. Using these data, we identify ranges of shoreline exposure values where oyster reefs are most effective at reducing marsh edge erosion and apply this knowledge to a case study within one Louisiana estuary. In Breton Sound estuary, we calculate shoreline exposure at 500 random points and then overlay a habitat suitability index for oysters. This method and the resulting visualization show areas most likely to support sustainable oyster populations as well as significantly reduce shoreline erosion. Our results demonstrate how site selection criteria, which include shoreline exposure and habitat suitability, are critical to ensuring greater positive impacts and longevity of oyster reef restoration projects.

Coupled Wave Energy and Erosion Dynamics along a Salt Marsh Boundary, Hog Island Bay, Virginia, USA

The relationship between lateral erosion of salt marshes and wind waves is studied in Hog Island Bay, Virginia USA, with high-resolution field measurements and aerial photographs. Marsh retreat is compared to wave climate calculated in the bay using the spectral wave-model Simulating Waves Nearshore (SWAN). We confirm the existence of a linear relationship between long-term salt marsh erosion and wave energy, and show that wave power can serve as a good proxy for average salt-marsh erosion rates. At each site, erosion rates are consistent across several temporal scales, ranging from months to decades, and are strongly related to wave power. On the contrary, erosion rates vary in space and weakly depend on the spatial distribution of wave energy. We ascribe this variability to spatial variations in geotechnical, biological, and morphological marsh attributes. Our detailed field measurements indicate that at a small spatial scale (tens of meters), a positive feedback between salt marsh geometry and wave action causes erosion rates to increase with boundary sinuosity. However, at the scale of the entire marsh boundary (hundreds of meters), this relationship is reversed: those sites that are more rapidly eroding have a marsh boundary which is significantly smoother than the marsh boundary of sheltered and slowly eroding marshes.

Tidal marsh erosion and accretion trends following invasive species removal, Tamar Estuary, Tasmania

The introduction of Spartina to intertidal marshes last century in many areas of the world transformed estuarine geomorphology, threatened native species and habitats, and impeded coastal access and use. This study investigated erosion/accretion trends of marsh surfaces following removal of invasive Spartina across a substantial intertidal marsh area. Marsh surface changes were monitored within a 0.6 ha experimental site where Spartina anglica cover was removed, and compared with surface changes at a comparable control site. Erosion/accretion rates were measured for over two years using a grid transect network, creek cross sectional profiles, and seaward edge delineation. Results showed that a significant erosion of the marsh surface occurred at the experimental site relative to the control site, using two different statistical analyses. Analysis of mean monthly change found erosion rates at the experimental site to be 13.2 mm a−1 relative to 2.0 mm a−1 at the control site, a rate that was six times greater. Analysis of overall change from the beginning to the end of the study showed that erosion was significantly more pronounced at the experimental site relative to the control site, and increased from the landward edge to the seaward edge at both sites. This study demonstrates the need for consideration of geomorphic processes when managing invasive plants in dynamic environments, and indicates that large scale Spartina removal will cause coastal erosion, bringing potential consequences to adjacent near shore waters and ecosystems.

Tracing the intrusion of fossil carbon into coastal Louisiana macrofauna using natural 14C and 13C abundances

The Deepwater Horizon oil spill released a large volume of 13C and radiocarbon depleted organic matter to the northern Gulf of Mexico. Evidence of petroleum-derived carbon entering the offshore planktonic foodweb, as well as widespread oiling of coastal areas documented in previous studies suggests that hydrocarbons could have entered the near shore foodweb. To test this hypothesis, we measured radiocarbon (Δ14C%) and stable carbon isotopes (δ13C) in an assortment of fish tissue, invertebrate tissue and shell samples collected within a year of the spill at seven sites from Louisiana to Florida USA across the northern Gulf of Mexico. We observed a west–east gradient with the most depleted radiocarbon values found in Terrebonne Bay, Louisana and increasingly enriched radiocarbon values in organisms collected at sites to the east. Depleted radiocarbon values as low as −10% in invertebrate soft tissue from Terrebonne suggest assimilation of fossil carbon (2.8±1.2%), consistent with the hypothesis that organic matter from petrochemical reservoirs released during the Deepwater Horizon spill entered the coastal food web to a limited extent. Further there was a significant correlation between radiocarbon and δ13C values in invertebrate tissue consistent with this hypothesis. Both oyster tissue and hard head catfish tissue collected in impacted areas of coastal Louisiana were significantly depleted in 14C and 13C relative to organisms collected in the unaffected Apalachicola Bay, Florida (p<0.014). Alternative explanations for these results include the influence of chronic hydrocarbon pollution along the western gulf coast or that the organisms ingest carbon derived from 14C depleted organic matter mobilized during the erosion of coastal marshes in southern Louisiana.

Rates and Probable Causes of Freshwater Tidal Marsh Failure, Potomac River Estuary, Northern Virginia, USA

Dyke Marsh, a distal tidal marsh along the Potomac River estuary, is diminishing rapidly in areal extent. This study documents Dyke Marsh erosion rates from the early-1860s to the present during pre-mining, mining, and post-mining phases. From the late-1930s to the mid-1970s, Dyke Marsh and the adjacent shallow riverbottom were mined for gravel, resulting in a ~55 % initial loss of area. Marsh loss continued during the post-mining phase (1976–2012). Causes of post-mining loss were unknown, but were thought to include Potomac River flooding. Post-mining areal-erosion rates increased from 0.138 ha yr−1 (~0.37 ac yr−1) to 0.516 ha yr−1 (~1.67 ac yr−1), and shoreline-erosion rates increased from 0.76 m yr−1 (~2.5 ft yr−1) to 2.60 m yr−1 (~8.5 ft yr−1). Results suggest the accelerating post-mining erosion reflects a process-driven feedback loop, enabled by the marsh's severely-altered geomorphic and hydrologic baseline system; the primary post-mining degradation process is wave-induced erosion from northbound cyclonic storms. Dyke Marsh erosion rates are now comparable to, or exceed, rates for proximal coastal marshes in the same region. Persistent and accelerated erosion of marshland long after cessation of mining illustrates the long-term, and potentially devastating, effects that temporally-restricted, anthropogenic destabilization can have on estuarine marsh systems.

Critical width of tidal flats triggers marsh collapse in the absence of sea-level rise

High rates of wave-induced erosion along salt marsh boundaries challenge the idea that marsh survival is dictated by the competition between vertical sediment accretion and relative sea-level rise. Because waves pounding marshes are often locally generated in enclosed basins, the depth and width of surrounding tidal flats have a pivoting control on marsh erosion. Here, we show the existence of a threshold width for tidal flats bordering salt marshes. Once this threshold is exceeded, irreversible marsh erosion takes place even in the absence of sea-level rise. This catastrophic collapse occurs because of the positive feedbacks among tidal flat widening by wave-induced marsh erosion, tidal flat deepening driven by wave bed shear stress, and local wind wave generation. The threshold width is determined by analyzing the 50-y evolution of 54 marsh basins along the US Atlantic Coast. The presence of a critical basin width is predicted by a dynamic model that accounts for both horizontal marsh migration and vertical adjustment of marshes and tidal flats. Variability in sediment supply, rather than in relative sea-level rise or wind regime, explains the different critical width, and hence erosion vulnerability, found at different sites. We conclude that sediment starvation of coastlines produced by river dredging and damming is a major anthropogenic driver of marsh loss at the study sites and generates effects at least comparable to the accelerating sea-level rise due to global warming.

Effects of Marsh Pond Terracing on Coastal Wintering Waterbirds Before and After Hurricane Rita

From February to March 2005-2006, we surveyed wintering waterbirds to test effects of terracing on coastal pond use before and after Hurricane Rita. Marsh terracing is intended to slow coastal marsh loss in the Chenier Plain by slowing marsh erosion and encouraging vegetation expansion. Terraces also increase marsh edge in ponds, possibly benefiting waterbirds. We monitored paired terraced and unterraced ponds in three sites within southwestern Louisiana's Chenier Plain. Waterbirds were 75% more numerous in terraced than unterraced ponds. Waterbird richness was similar among ponds when corrected for number of individuals, suggesting terracing increased bird density but did not provide habitat unique from unterraced ponds. Birds were 93% more numerous following Hurricane Rita, mostly due to an influx of migrating waterfowl. Year round residents were similar in number before and after Hurricane Rita. Resident richness did not differ among years after correcting for number of observed individuals. Wading and dabbling foragers were more abundant in terraced ponds and these two guilds represented 74% of birds observed. We detected no difference among ponds for other guilds, i.e., probing, aerial, and diving foragers. Increasing proportion of mash edge increased bird density disproportionately: On average ponds with 10% edge had 6 birds observed and ponds with 30% edge had 16 birds observed. Terraces increased habitat interspersion and were an effective tool for increasing numbers of wintering waterfowl and wading birds. The extent to which terraces were sustainable following hurricane forces is unknown.

Shelly cheniers on a modern macrotidal flat (Mont-Saint-Michel bay, France) — Internal architecture revealed by ground-penetrating radar

The relationship between short-term morphodynamics and internal structure of shelly chenier ridges is investigated using ground-penetrating radar and core X-ray analysis, complemented with a comparison of aerial photographs. These cheniers are located on the uppermost part of a macrotidal flat in Mont-Saint-Michel bay (North-Western France). They actively migrate landward over the salt-marshes by washover processes when they are submerged during the coincidence of spring tides and enhanced wave activity. The architecture of the cheniers is imaged using high-frequency radar antennas (400MHz to 2.6GHz). Three types of cheniers are recognized, featuring differences in morphology and internal structure. The altitude of the banks on the tidal flat relative to the level of tidal flooding, as well as local sediment supply, seem to be important forcing parameters in chenier development and stages of evolution.On a multi-annual time scale, evolution of this chenier system is influenced by a combination of the 18-year and the 4-year tidal cycles, superimposed on wave activity. During minima of the 18-year cycle, spring tide level is globally lower on the tidal flat, allowing salt marsh progradation and chenier ridge development. During peak periods of the tidal cycle, former barriers are reworked by wave action and experience onshore migration associated with salt marsh erosion. The 4-year tidal cycle coupled with enhanced wave activity is probably responsible for the deposition of large washover units, observed in GPR reflection profiles.

Influence of storm surges and sea level on shallow tidal basin erosive processes

The finite‐element model WWTM is applied to a system of lagoons at the Virginia Coast Reserve, USA. The model solves the shallow water equations to compute tidal fluxes, and is equipped with a wave propagation module to calculate wave height during local wind events. The model is validated using measured water elevations, wave heights, and periods at five locations within the lagoon system. Scenarios with different wind conditions, storm surges, and relative sea level are simulated. Results are analyzed in terms of bottom shear stresses on the tidal flats, a measure of sediment resuspension potential, and total wave energy impacting the marsh boundaries, which is the chief process driving lateral marsh erosion. Results indicate that wave energy at the marsh boundaries is more sensitive to wind direction than are bottom shear stresses. Wave energy on marsh boundaries and bottom shear stresses on the tidal flats increase with sea level elevation, with the former increasing almost ten times more than the latter. Both positive and negative feedbacks between wave energy at the boundaries and bottom shear stresses are predicted, depending on the fate of the sediments eroded from the salt marsh boundaries.

Relative Importance of Vessel-Generated and Wind Waves to Salt Marsh Erosion in a Restricted Fetch Environment

Erosion of a salt marsh at Fort Pulaski National Monument threatens a pier of historical significance along the North Channel of the Savannah River, the main shipping channel for the Port of Savannah, Georgia. The erosion was initiated by the alongshore and onshore migration of a supratidal oyster shell ridge that exposed a marsh scarp that is susceptible to lateral retreat in response to both wind-generated and vessel-generated waves undercutting the upper-marsh surface. An instrumented field study was completed between October 2007 and February 2008 to examine the relative importance of wind-generated and vessel-generated waves to the retreat of the scarp by 0.21 m along the instrumented transect. On average, there were 14 sailings by container ships per day and up to twice as many sailings by pilot boats ferrying the harbor pilots to container ships offshore, but only those vessels that sailed within 1 m of the high-tide water level had waves capable of reaching the scarp. The vessel-generated waves accounted for ~5% of the cumulative wave energy over the study, but because of their larger height and longer period, they accounted for almost 25% of the cumulative wave force. It is the locally generated wind waves that account for most of the wave force acting on the exposed scarp and which are largely responsible for the observed retreat. Waves generated during both frontal and tropical storms tend to be associated with storm tides that maintain water levels at or near the elevation of the scarp for several days. Now that the shell ridge has largely migrated past the site and the areas of greater retreat historically are protected by a wedge of sand, it is argued that an increase in vessel traffic and/or the use of larger, post-Panamax ships will not significantly accelerate the retreat of the marsh.

Do Small, Patchy, Constructed Intertidal Oyster Reefs Reduce Salt Marsh Erosion As Well As Natural Reefs?

One ecological service that oyster reefs provide is stabilization of shorelines through reduced wave energy and erosion from boat traffic, storms, and predominant wind direction. Additionally, increasing sedimentation can enhance the growth of emergent marsh vegetation which further stabilizes unconsolidated sediments. A 21 mo study of constructed (with only 30-35% coverage) and natural oyster reefs in 3 bayous in the Grand Bay National Estuarine Research Reserve (NERR) suggested constructed reefs benefit this retrograding deltaic ecosystem. The marsh edge adjacent to all constructed reefs was less eroded (mean = 0.043 m) than edges adjacent to natural reefs (mean = 0.728 m), although all natural and constructed sites, regardless of bayou, illustrated large variations in marsh edge growth. The marsh edge in constructed sites in one bayou retreated more than in the other bayous, most likely due to its coarser sediments, greater boat traffic, and its apparent higher energy location within the landscape. By the end of this study, the ecological function of constructed oys- ter reefs in all bayous, as measured by marsh edge erosion reduction, was equivalent or exceeded the function in nearby natural oyster reefs. The physical structure of the reef further served to reduce erosion and marsh loss and this approach may be useful for management of a retrograding deltaic estuarine ecosystem like the Grand Bay NERR.

Causes of Salt Marsh Erosion in Galveston Bay, Texas

Abstract There is major salt marsh loss in Galveston Bay and other estuarine environments. In Galveston Bay, the causes of marsh loss include wave action, subsidence, eustatic sea-level rise, and insufficient sediment supply. To assess the relative importance of these factors in marshes of West Galveston Bay, wave action, sediment supply, and sedimentation rates were studied. Analysis of the data indicated a significant gap between the historic sediment accretion rate of 0.20 cm y−1 and the relative sea-level rise (the rate of rise of the water depth due to the combined effects of eustatic rise and subsidence) of 0.65 cm y−1. Furthermore, in 94% of the eroding marshes, where 20% exceedance wave height was less than 0.17 m, the role of wave-induced erosion was relatively small. Thus, the major cause for salt marsh loss is insufficient sediment supply. These findings indicate that in the many eroding marshes in Galveston Bay, where wave action is not the major cause of marsh loss, marsh restoration efforts ...

Fragmentary Evidence of Great-Earthquake Subsidence during Holocene Emergence, Valdivia Estuary, South Central Chile

A reconnaissance of Holocene stratigraphy beneath fringing marshes of the Valdivia estuary, where an M 9:5 earthquake caused 1-2 m of regional coseismic subsidence in 1960, shows only fragmentary evidence of prehistoric coseismic sub- sidence. In most of the 150 hand-driven cores that were examined, a distinct uncon- formity separates 0.5-1.5 m of late Holocene tidal and floodplain mud, peat, and sand from underlying middle Holocene subtidal mud and sand. At the Las Coloradas site, where stratigraphy is best preserved, two A horizons of marsh and meadow soils abruptly overlain by sand and mud probably record coseismic subsidence shortly fol- lowed by tsunamis. The amount of subsidence during the earthquakes proved difficult to reconstruct with a diatom transfer function because of differences between modern and fossil diatom assemblages. Maximum 14 C ages on macrofossils from the two A horizons at the Las Coloradas site of 1.7-1.3 ka and 2.7-1.7 ka allow correlation of the younger horizon with either of two of six 14 C-dated A horizons buried by tsunami sand or post-tsunami tidal sand 200 km to the south at Maullin, and with a lake-wide mass wasting event in Lago Puyehue, 100 km to the southeast. Tidal records of prehistoric coseismic subsidence at Valdivia are scarce because of a sea-level fall of 3-8 m over the past 6000 years, erosion of marsh and meadow soils during subsidence-induced flooding of the estuary, and largely complete land-level recovery during cycles of coseismic subsidence and postseismic uplift. Online Material: Diatom species data from the Las Coloradas site, Valdivia estuary.

Tidal Marsh Erosion by Geese, St. Lawrence Estuary, Québec

Les oies causent des dommages au substrat des marais intertidaux (schorres) du Saint-Laurent. Leur activité printanière et automnale contribue ainsi à augmenter et à accélérer l'érosion par les agents naturels. La destruction précoce du couvert végétal à l'automne à la suite de l'arrivée massive des oies réduit son rôle protecteur contre l'érosion par les vagues et les courants de la vase fraîche déposée durant l'été. La surface vaseuse intensément picorée et piétinée par les oies est alors déstabilisée et sujette à une érosion subséquente plus importante. À la recherche des rhizomes, les oies creusent dans le substrat des milliers de petits trous, ce qui entraîne par la suite un démaigrissement de la surface du schorre inférieur d'une dizaine de centimètres d'épaisseur. Cette modification du profil a pour conséquence d'augmenter l'épaisseur de la nappe d'eau au-dessus de cette zone, de sorte qu'il y a augmentation de l'érosion par les vagues de la micro-falaise du schorre supérieur. De plus, les oies, en broutant le pourtour des mares, les agrandissent. De même, elles détruisent les radeaux du schorre et grugent le rebord du schorre inférieur et le font reculer. Même si les vagues, les courants et les glaces demeurent les principaux agents d'érosion du marais intertidal, on croit qu'une partie de l'érosion récente qui affecte le shorre de Montmagny est liée à l'action des oies.

Direct and indirect effects of burrowing crab Chasmagnathus granulatus activities on erosion of southwest Atlantic Sarcocornia‐dominated marshes

Using field measurements and field experiments, we investigated the effect of a dominant Southwest Atlantic intertidal burrowing crab, Chasmagnathus granulatus, on the inland growth of tidal creeks and creek genesis in salt marshes. By burrowing intensively on marsh sediments, this crab changed sediment physical parameters, such as penetrability, water content, and shear strength, which are related to sediment resistance to erosion. There were positive relationships between crab density and activities occurring in the creek heads and creek growth rates. Field experiments show that the presence and activity of C. granulatus and the presence of their burrows enhance the growth rates of tidal creeks, promoting marsh erosion. When crabs were present, these creeks grew faster than did creeks in which crabs were excluded. Furthermore, the interaction (disturbance and herbivory) between crabs and the dominant halophyte marsh plant, Sarcocornia perennis, generate circular depressions that accumulate standing water (salt pans), which in turn facilitates the creation of new creeks in the marsh surface, which evolve, to a greater extent, into fully functional tidal creeks because of colonization by crabs, which in turn further enhances creek growth rates. These direct and indirect effects of crabs on marsh erosion provide strong evidence of the importance of bioturbation and biological processes to the erosion and geomorphology of marshes.

Ecological and morphological response of brackish tidal marshland to the next century of sea level rise: Westham Island, British Columbia

In response to climatic warming, eustatic sea level has been predicted to rise by about 50 cm in the next century. While feedbacks between vegetation growth and sediment deposition tend to allow marshes to maintain their morphology under a constant rate of sea level rise, recent observations of marsh deterioration suggest that changes in the rate of sea level rise may induce loss of economically and ecologically important marshland. We have developed a three dimensional model of tidal marsh evolution that couples vegetation growth and sediment transport processes including bed accretion and wave erosion. We use the model to simulate the response of marshes and tidal flats along the Fraser River Delta, British Columbia to 100 yr forecasts of sea level change. Under low sea level-rise scenarios, the delta and its marshes prograde slightly, consistent with historical measurements. While accretionary processes greatly mediate the response to increased rates of sea level rise, vegetation zones transgress landward under median and high sea level rise rate scenarios. In these scenarios, low marsh erosion and constriction of high marsh vegetation against a dyke at its landward edge result in a 15–35% loss of marshland in the next century. Several important behavioral changes take place after 2050, suggesting that predictions based on field observations and short term model experiments may not adequately characterize (and sometimes underestimate) long-term change. In particular, the replacement of highly productive high marsh vegetation by less productive low marsh vegetation results in continued reduction of the system's total biomass productivity, even as the rate of loss of vegetated area begins to decline.

Nekton of new seagrass habitats colonizing a subsided salt marsh in Galveston Bay, Texas

Subsidence and erosion of intertidal salt marsh at Galveston Island State Park, Texas, created new areas of subtidal habitat that were colonized by seagrasses begining in 1999. We quantified and compared habitat characteristics and nekton densities in monospecific beds of stargrassHalophila engelmanni and shoalgrassHalodule wrightii as well as adjacent nonvegetated substrates. We collected 10 replicates per habitat type during April, July, October, and December 2001. Most habitat characteristics varied with season. Water temperature, salinity, and dissolved oxygen were similar among habitat types. Turbidity and depth were greatest inH. engelmanni beds and least inH. wrightii beds.H. engelmanni exhibited shorter leaves and higher shoot density and biomass core−1 thanH. wrightii. Densities of almost all dominant species of nekton (fishes and decapods) were seasonally variable, all were higher in seagrass habitats than in nonvegetated habitats, and most were higher in one seagrass species than the other. Naked gobyGobiosoma bosc, code gobyGobiosoma robustum, bigclaw snapping shrimpAlpheus heterochaelis, and blue crabCallinectes sapidus, were most abundant inH. engelmanni. Brown shrimpFarfantepenaeus aztecus, brackish grass shrimpPalaemonetes intermedius, and daggerblade grass shrimpPalaemonetes pugio were most abundant inH. wrightii. PinfishLagodon rhomboides and pink shrimFarfantepenaeus duorarum were equally abundant in either seagrass. Most dominant nekton varied in size by month, but only two (L. rhomboides andC. sapidus) exhibited habitat-related differences in size. Nekton densities in these new seagrass habitats equaled or exceeded densities associated with historical and current intertidal smooth cordgrassSpartina alterniflora marsh. Continued seagrass expansion and persistence should ensure ecosystem productivity in spite of habitat change.

Saltmarsh erosion and restoration in south‐east England: squeezing the evidence requires realignment

Summary Saltmarshes in south‐east England have been eroding rapidly since 1960. Recently, Hughes &amp; Paramor (2004) and Morris et al. (2004) have presented contrasting views on the extent to which physical and biological processes might contribute to the erosion. There are three contentious issues: (i) saltmarsh erosion is the result of coastal squeeze, where sea walls prevent a landward migration of a saltmarsh in response to sea level rise; (ii) saltmarsh erosion is linked to bioturbation and herbivory of seedlings by the ragworm Nereis diversicolor; (iii) new saltmarshes will not develop on managed realignment sites where existing sea walls have been removed because of the effects of ragworms. In this paper, we provide a literature review of physical and biological processes relevant to the above three issues, and discuss the relative importance of these processes at different spatial and temporal scales. Our synthesis shows that, at a regional scale, the combination of strong winds, high tides and increased wave height appears to be responsible for the increased rate of marsh erosion and creek dissection recorded in the 1970s. There is also some laboratory evidence that bioturbation and herbivory from populations of Nereis can lead to sediment instability and loss of pioneer plant species, such as Salicornia spp. However, the field evidence is more equivocal and has been conducted at small spatial scales. At a large number of different managed realignment sites there is strong evidence that even if bioturbation and herbivory by Nereis have occurred, overall the effects have been insufficient to restrict plant succession of exposed sediment. Synthesis and applications. There is an urgent need for long‐term field studies that integrate and quantify physical and biological processes and the related feedbacks at different spatial and temporal scales. Until this is completed, terms such as coastal squeeze will remain contentious and management decisions will invite criticism.