Marsh Evolution Research Articles

Unravelling interactions between salt marsh evolution and sedimentary processes in the Wadden Sea (southeastern North Sea)

Salt marshes in the Wadden Sea constitute about 20% of all salt marshes along European coasts. They are of immense importance for coastal protection reasons and as habitat for coastal plant, bird, and invertebrate species. The Wadden Sea is a coastal sedimentary ecosystem in the southeastern North Sea. Besides salt marshes, it is composed of tidal flats, high sands, and sandy shoals, dissected by (sub)tidal channels and located behind barrier islands. Accelerated global sea-level rise (SLR) and changes in storm climate have been identified as possible threats for the persistence of the Wadden Sea ecosystem including its salt marshes. Moreover, it is known that the amount and composition of the sediment available for salt marshes are the most important parameters influencing their ability to adapt to current and future SLR. Assessing these parameters requires a thorough understanding of the sedimentary system of the salt marshes and the adjacent tidal basins. In the present review, we investigate and unravel the interactions of sedimentary processes in the Wadden Sea with the processes taking place on the salt marshes. We identify the most crucial processes and interactions influencing the morphological development of salt marshes in the Wadden Sea. A conceptual model is proposed, intended as a framework for improved understanding of salt marsh development and for incorporation into new salt marsh models. The proposed model may also be applicable to regions other than the Wadden Sea.

Polarimetric Radarsat-2 wetland classification using the Touzi decomposition: case of the Lac Saint-Pierre Ramsar wetland

Wetlands play a key role in regional and global environments and are linked to climate change, water quality, and hydrological and carbon cycles. They also contribute to wildlife habitat and biodiversity and can act as indicators of overall environmental health. Unfortunately, wetlands continue to be under threat. There is an immediate need for improved mapping and monitoring of wetlands to better manage and protect these sensitive areas. Recently, the Touzi decomposition was introduced and proved very promising for wetland characterization using polarimetric airborne (Convair-580) SAR data. The purpose of this study is to assess the Touzi incoherent target-scattering decomposition (ICTD) for wetland classification using polarimetric Radarsat-2 (RS2) data collected over the RAMSAR wetland site in Lac Saint-Pierre, Canada. In particular, the sensitivity of the ICTD parameters to seasonal evolution of marsh and swamp scattering is discussed and demonstrated. The intent is to show that the dominant scattering type magnitude (αs1) and phase (Φs1), and the dominant (η1) and lowest scattering eigenvalues (η3), lead to an effective characterization of the various backscattering mechanisms of the wetland plant species. The ICTD parameters form the basis of a new hierarchical classification that is efficient for wetland classification. The use of multitemporal information obtained from multidate RS2 acquisitions between April and September allows an accurate wetland classification. The RS2 polarimetric classification is then compared with a supervised maximum-likelihood classification using a pair of Landsat-5 images.

Sedimentation, elevation and marsh evolution in a southeastern Australian estuary during changing climatic conditions

Mangrove and salt marsh vertical accretion and surface elevation change was measured at Kooragang Island within the Ramsar-listed Lower Hunter estuarine wetlands in New South Wales, Australia, using surface elevation tables and marker horizons over a ten-year period. We surveyed mangrove, salt marsh and a zone of mangrove encroachment into salt marsh. The period of analysis was dominated by El Niño (drought) climatic conditions, though included a series of east coast low pressure systems and associated storms over the central coast of NSW in June 2007. The storms may have initially caused scouring of sediments in the mangrove zone, followed by significant accretion within both the mangrove and salt marsh during the six months following the storms, with most of this accretion corresponding to spring tides several months after the storms. These accretion events were not accompanied by an equivalent elevation change, and robust elevation trends over the study period in mangrove and salt marsh indicate that the storms may have had little impact on the longer-term elevation dynamics within both the mangrove and salt marsh at Kooragang Island. Elevation dynamics in these zones appear to be regulated by vertical accretion over longer time periods and modulated by hydrology at shorter temporal scales. Elevation declined in the mangrove encroachment zone despite continued vertical accretion and we propose that this discrepancy may be associated with expansion of tidal creeks near the zone of mangrove encroachment or loss of salt marsh vegetation. This pattern of encroachment is consistent with observations from sites throughout the region and may be related to climatic perturbations (El Niño Southern Oscillation) rather than directly attributed to the storms.

Ecomorphodynamic evolution of salt marshes: Experimental observations of bank retreat processes

This paper addresses the problem of the erosion of salt marsh edges. Flume laboratory experiments were carried out aimed at reproducing the instability and retreat of the scarps that typically delimit the salt marshes under the attack of wind waves during the tidal cycle. The bank model and hydrodynamic forcing in the flume were such as to simulate the conditions observed in the field in Venice Lagoon. Experiments were conducted for the same hydrodynamic forcing in the case of two identical banks but with and without the inclusion of the vegetation. Experimental results show that bank retreat involves a variety of processes (including particle erosion, cantilever and slide failures). The effect of the vegetation was to produce a delay in the mass failures, related to a certain growth of plant roots, thus providing an overall stabilizing effect. Bank instability was related to the formation of tension cracks at the bank top and to the impulsive effects associated with wave energy dissipation.

Fluxes of water, sediments, and biogeochemical compounds in salt marshes

Tidal oscillations systematically flood salt marshes, transporting water, sediments, organic matter, and biogeochemical elements such as silica. Here we present a review of recent studies on these fluxes and their effects on both ecosystem functioning and morphological evolution of salt marshes. We reexamine a simplified model for the computation of water fluxes in salt marshes that captures the asymmetry in discharge between flood and ebb. We discuss the role of storm conditions on sediment fluxes both in tidal channels and on the marsh platform. We present recent methods and field instruments for the measurement of fluxes of organic matter. These methods will provide long-term data sets with fine temporal resolution that will help scientists to close the carbon budget in salt marshes. Finally, the main processes controlling fluxes of biogenic and dissolved silica in salt marshes are explained, with particular emphasis on the uptake by marsh macrophytes and diatoms.

Response of salt-marsh carbon accumulation to climate change

About half of annual marine carbon burial takes place in shallow water ecosystems where geomorphic and ecological stability is driven by interactions between the flow of water, vegetation growth and sediment transport. Although the sensitivity of terrestrial and deep marine carbon pools to climate change has been studied for decades, there is little understanding of how coastal carbon accumulation rates will change and potentially feed back on climate. Here we develop a numerical model of salt marsh evolution, informed by recent measurements of productivity and decomposition, and demonstrate that competition between mineral sediment deposition and organic-matter accumulation determines the net impact of climate change on carbon accumulation in intertidal wetlands. We find that the direct impact of warming on soil carbon accumulation rates is more subtle than the impact of warming-driven sea level rise, although the impact of warming increases with increasing rates of sea level rise. Our simulations suggest that the net impact of climate change will be to increase carbon burial rates in the first half of the twenty-first century, but that carbon-climate feedbacks are likely to diminish over time.

Numerical models of salt marsh evolution: Ecological, geomorphic, and climatic factors

Salt marshes are delicate landforms at the boundary between the sea and land. These ecosystems support a diverse biota that modifies the erosive characteristics of the substrate and mediates sediment transport processes. Here we present a broad overview of recent numerical models that quantify the formation and evolution of salt marshes under different physical and ecological drivers. In particular, we focus on the coupling between geomorphological and ecological processes and on how these feedbacks are included in predictive models of landform evolution. We describe in detail models that simulate fluxes of water, organic matter, and sediments in salt marshes. The interplay between biological and morphological processes often produces a distinct scarp between salt marshes and tidal flats. Numerical models can capture the dynamics of this boundary and the progradation or regression of the marsh in time. Tidal channels are also key features of the marsh landscape, flooding and draining the marsh platform and providing a source of sediments and nutrients to the marsh ecosystem. In recent years, several numerical models have been developed to describe the morphogenesis and long‐term dynamics of salt marsh channels. Finally, salt marshes are highly sensitive to the effects of long‐term climatic change. We therefore discuss in detail how numerical models have been used to determine salt marsh survival under different scenarios of sea level rise.

Avian Communities in Tidal Salt Marshes of San Francisco Bay: A Review of Functional Groups by Foraging Guild and Habitat Association

The San Francisco Bay estuary is highly urbanized, but it supports the largest remaining extent of tidal salt marshes on the west coast of North America as well as a diverse native bird community. San Francisco Bay tidal marshes are occupied by more than 113 bird species that represent 31 families, including five subspecies from three families that we denote as tidal-marsh obligates. To better identify the niche of bird species in tidal marshes, we present a review of functional groups based on foraging guilds and habitat associations. Foraging guilds describe the method by which species obtain food from tidal marshes, while habitat associations describe broad areas within the marsh that have similar environmental conditions. For example, the ubiquitous song sparrows (Alameda Melospiza melodia pusillula, Suisun M. m. maxillaris, and San Pablo M. m. samuelis) are surface-feeding generalists that consume prey from vegetation and the ground, and they are found across the entire marsh plain into the upland–marsh transition. In contrast, surface-feeding California black rails (Laterallus jamaicensis coturniculus) are cryptic, and generally restricted in their distribution to the mid- and high-marsh plain. Although in the same family, the endangered California clapper rail (Rallus longirostris obsoletus) has become highly specialized, foraging primarily on benthic fauna within marsh channels when they are exposed at low tide. Shorebirds such as the black-necked stilt (Himantopus mexicanus) typically probe in mud flats to consume macroinvertebrate prey, and are generally restricted to foraging on salt pans within the marsh plain, in ponds, or on mud flats during transitional stages of marsh evolution. The abundance and distribution of birds varies widely with changing water depths and vegetation colonization during different stages of restoration. Thus, tidal-marsh birds represent a rich and diverse community in bay marshes, with niches that may be distinguished by the food resources they consume and the habitats that they occupy along the tidal gradient.

Dynamic response of marshes to perturbations in suspended sediment concentrations and rates of relative sea level rise

[1] We have developed an analytical model of salt marsh evolution that captures the dynamic response of marshes to perturbations in suspended sediment concentrations, plant productivity, and the rate of relative sea level rise (RSLR). Sediment-rich and highly productive marshes will approach a new equilibrium state in response to a step change in the rate of RSLR faster than sediment-poor or less productive marshes. Microtidal marshes will respond more quickly to a step change in the rate of RSLR than mesotidal or macrotidal marshes. Marshes are more resilient to a decrease rather than to an increase in the rate of RSLR, and they are more resilient to a decrease rather than to an increase in sediment availability. Moreover, macrotidal marshes are more resilient to changes in the rate of RSLR than their microtidal counterparts. Finally, we find that a marsh's ability to record sea level fluctuations in its stratigraphy is fundamentally related to a timescale we call TFT, or filling timescale, which is equal to the tidal amplitude divided by the maximum possible accretion rate on the marsh (a function of plant productivity, sediment properties, and availability). Marshes with a short-filling timescale (i.e., marshes with rapid sedimentation or small tidal amplitudes) are best suited to recording high-frequency fluctuations in RSLR, but our model suggests it is unlikely that marshes will be able to record fluctuations occurring over timescales that are shorter than decadal.

Aptitude of modern salt marshes to counteract relative sea-level rise, Venice Lagoon (Italy)

In the past 100 yr, variations in relative sea-level rise (RSLR), the increase of frequency of very high tides, and a decrease of sediment availability caused progressive morphological changes of the Venice Lagoon tidal flats. In contrast with the general erosional trend, some salt marshes in the northern lagoon preserved their main original characteristics and showed accretion and development of the tidal creek network. The evolution of one of them was sketched by the interpretation of ultrahigh-resolution aerial photographs taken from A.D. 1938 to 2006, and results were compared with RSLR rates and storm tide frequency. The long-term investigation pointed out the most significant morphological changes that occurred over the entire period (i.e., erosion of the margin, modifications of the drainage network), whereas the short-term analysis showed in detail the subsequent phases of salt marsh evolution and their relations with sea-level variations. Margin shift was mainly in agreement with RSLR trend, whereas changes of the tidal creek network also reflected frequency of very high tides. Moreover, in the past 70 yr the salt marsh demonstrated a self-renewal aptitude to counteract RSLR: even if it underwent low accretion rates, it did not disappear, probably because the remobilization of sediments eroded from the marsh front and the lagoon bottom by tides and other local hydrodynamic processes, and their accumulation on the marsh surface favored by vegetation were sufficient to offset RSLR.

Ice raft formation, sediment load, and theoretical potential for ice-rafted sediment influx on northern coastal wetlands

Ice rafting is an important secondary sedimentation process that redistributes sediment form tidal flats, channel beds, and ponds to the vegetated marsh surface in northern temperate climates. Source location of ice-rafted sediment is identifiable based on distinct sediment properties. In New England salt marsh systems, ice raft thickness and entrained sediment load vary both during the season and interannually as a function of severity and duration of winter conditions; however, 97% of ice rafts carry measurable sediment loads. Thick rafts move sand or peat up to 100 m from source areas, whereas thinner rafts tend to transport mud still further onto the marsh platform, sometimes reaching the upland border. Based on these observations, we present relationships defining the theoretical sediment-carrying potential of ice rafts as well as empirical parameterizations for ice-rafted sediment with respect to ice volume. Our results suggest that ice-rafting deposits a volume of sediment contributing up to 5% of annual vertical accretion, an important input in a region where rates of vertical accretion barely compensate for sea-level rise. We provide conceptual models of ice-raft formation and sediment entrainment linking these processes to the general geomorphic evolution of northern temperate marshes, which must be understood in light of the modern acceleration in rates of sea-level rise.

Interannual (1999–2005) morphodynamic evolution of macro-tidal salt marshes in Mont-Saint-Michel Bay (France)

This paper provides a detailed study on the sedimentation patterns and the recent morphodynamic evolution affecting the macro-tidal salt marshes located west of the Mont-Saint-Michel (France). Twenty-two stations along three transects on the marshes were seasonally monitored for marsh surface level variations from 1999 to 2005, using a sediment erosion bar. The corresponding erosion/accretion rates were obtained together with data on topography, vegetation cover, and grain size of surface sediment. To examine the mechanisms contributing to the salt marsh sedimentation, the data and their evolution were treated with respect to tides, relative mean regional sea level, and wind speed/frequency variations. From 1999 to 2005, the marsh was globally accreting (from 3.45 to 38.11 mm yr −1 in the low marsh, up to 4.91 mm yr −1 in the middle marsh, and up to 1.35 mm yr −1 in the high marsh), while the study was conducted during a window of decreasing trend in mean regional sea level (−2.45 mm yr −1 according to regional-averaged time series). These sedimentation rates are one of the highest recorded worldwide; however, the sedimentation was not found to be continuous over the period in question. This pattern is illustrated by the strong extension of the marshes from 1999 to 2002, and the relative stability observed from 2003 to 2005. The imported and reworked sediments are trapped and fixed by the dense vegetation ( Puccinellia maritima, Halimione portulacoides), inducing the general seaward extension of the marshes. The processes governing sediment budget (accretion/erosion) show annual, seasonal, and spatial variability on the marsh. Spatial variations display contrasted patterns of erosion/sedimentation between the low, middle, and high marsh, and between the different transects. These patterns are a result of distance from sediment sources, strong heterogeneity in vegetation cover (human induced or not), and contrasting topographic and micro-topographic characteristics. The higher accretion rates are observed in distal settings in the low marsh, and strongly decrease toward the middle and high marsh. This evolution results from a decrease in accommodation space/water column thickness, and frequency of inundation coupled with an increase in station elevation, but also from the cumulated effects of vegetation cover and micro-topography. The vegetation cover of the low and middle marsh enhance the settling and fixing of fine sediments imported through tides or dispersed by flood and ebb currents. The seasonal evolution of the marshes is marked by contrasting effects of water storage in the sediment. The overall seasonal sediment budget is controlled by the variation of the frequency of inundation relative to tidal range and marshes topography. Autumns are influenced by the tide (equinoxes), relative mean regional sea level, and variations in wind speed/frequency. Winter wind speed and frequency in relation with tidal variations appear to be the main parameters regulating winter marsh evolution. Summers are predominantly under the influence of local variations in water storage (desiccation) while external parameters generally display a low influence. Although it is not governed by any one parameter, springtime sediment budget seems to result from strong interaction between the above-cited parameters, despite the significant frequency of inundation (equinoxes).

Influence of tidal range on the stability of coastal marshland

Early comparisons between rates of vertical accretion and sea level rise across marshes in different tidal ranges inspired a paradigm that marshes in high tidal range environments are more resilient to sea level rise than marshes in low tidal range environments. We use field‐based observations to propose a relationship between vegetation growth and tidal range and to adapt two numerical models of marsh evolution to explicitly consider the effect of tidal range on the response of the marsh platform channel network system to accelerating rates of sea level rise. We find that the stability of both the channel network and vegetated platform increases with increasing tidal range. Our results support earlier hypotheses that suggest enhanced stability can be directly attributable to a vegetation growth range that expands with tidal range. Accretion rates equilibrate to the rate of sea level rise in all experiments regardless of tidal range, suggesting that comparisons between accretion rate and tidal range will not likely produce a significant relationship. Therefore, our model results offer an explanation to widely inconsistent field‐based attempts to quantify this relationship while still supporting the long‐held paradigm that high tidal range marshes are indeed more stable.

A numerical model for the coupled long‐term evolution of salt marshes and tidal flats

A one‐dimensional numerical model for the coupled long‐term evolution of salt marshes and tidal flats is presented. The model framework includes tidal currents, wind waves, sediment erosion, and deposition, as well as the effect of vegetation on sediment dynamics. The model is used to explore the evolution of the marsh boundary under different scenarios of sediment supply and sea level rise. Numerical results show that vegetation determines the rate of marsh progradation and regression and plays a critical role in the redistribution of sediments within the intertidal area. Simulations indicate that the scarp between salt marsh and tidal flat is a distinctive feature of marsh retreat. For a given sediment supply the marsh can prograde or erode as a function of sea level rise. A low rate of sea level rise reduces the depth of the tidal flat increasing wave dissipation. Sediment deposition is thus favored, and the marsh boundary progrades. A high rate of sea level rise leads to a deeper tidal flat and therefore higher waves that erode the marsh boundary, leading to erosion. When the rate of sea level rise is too high the entire marsh drowns and is transformed into a tidal flat.

Stratigraphic and Ecophysical Characterizations of Salt Pools: Dynamic Landforms of the Webhannet Salt Marsh, Wells, ME, USA

Salt pools are water-filled depressions common to north-temperate salt marshes. In Wells, ME, USA, cores reveal a unique salt pool signature consisting of water-saturated dark-gray mud often containing fragments of Ruppia maritima. Cores through pool sediment reenter salt marsh peat, not tidal flat sediment, demonstrating that most pools are of secondary origin. A principal component analysis of attribute data collected from 119 pools defines three distinct pool types: those with (1) surrounding high-marsh vegetation and thick heavily undercut banks (40% of the variance), (2) surrounding low-marsh vegetation and thicker slightly undercut banks (18% of the variance), and (3) surrounding low-marsh vegetation and less thick moderately undercut banks, containing R. maritima and a surficial drainage (15% of the variance). Cores and spatiotemporal analyses of aerial photographs between 1962 and 2003 reveal dramatic salt marsh surface dynamism suggesting that salt pools influence the geomorphological evolution of coastal 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.

A coupled geomorphic and ecological model of tidal marsh evolution

The evolution of tidal marsh platforms and interwoven channel networks cannot be addressed without treating the two-way interactions that link biological and physical processes. We have developed a 3D model of tidal marsh accretion and channel network development that couples physical sediment transport processes with vegetation biomass productivity. Tidal flow tends to cause erosion, whereas vegetation biomass, a function of bed surface depth below high tide, influences the rate of sediment deposition and slope-driven transport processes such as creek bank slumping. With a steady, moderate rise in sea level, the model builds a marsh platform and channel network with accretion rates everywhere equal to the rate of sea-level rise, meaning water depths and biological productivity remain temporally constant. An increase in the rate of sea-level rise, or a reduction in sediment supply, causes marsh-surface depths, biomass productivity, and deposition rates to increase while simultaneously causing the channel network to expand. Vegetation on the marsh platform can promote a metastable equilibrium where the platform maintains elevation relative to a rapidly rising sea level, although disturbance to vegetation could cause irreversible loss of marsh habitat.

Influence of paleotopography, base level and sedimentation rate on estuarine system response to the Holocene sea-level rise: The example of the Marais Vernier, Seine estuary, France

The response of coastal systems to allogenic forcing factors is of interest to diverse research communities, including those interested in global change, sequence stratigraphy and modelling. Quaternary systems are of particular interest because they provide analogues for ancient rock records. To understand the processes responsible for the sedimentary evolution of estuarine systems, it is necessary to study as many fluvial systems as possible. The objective of this review of the sedimentary evolution of a coastal marsh is to describe the influence of glacial paleotopography on the record of climatic and sea-level changes. The Marais Vernier, located at the interface between the marine and fluvial parts of the estuary, is a part of the Lower Seine Valley wetland network, which formed after the Last Glacial Maximum. Previous studies have described the Holocene filling, which is composed of peat and detrital material deposited following climatic and sea-level changes. To understand the sedimentary evolution, a paleotopographical (based on drillings and electromagnetic surveys) and a chronological framework (based on radiocarbon dates) for the southern peat marsh were defined. The peat marsh paleotopography has three erosional surfaces. The S1 surface is the oldest and also the highest, topographically; the S2 surface is younger, wider, and incised below the S1 surface; the S3 surface, the youngest of the three, is narrow and deeply incised. Radiometric ages were considered on the basis of their geographical position in relation to the S3 surface. Prior to 7.5 ka cal BP, sediments accumulated only above the narrow area described by the S3 surface, at a rate of 5.5 mm yr − 1 . After 7.5 ka cal BP, shortly after the flooding of the Seine estuary, sediments accumulated as peat deposits over the entire peat marsh at a rate of 3 mm yr − 1 in response to the sea-level rise. The paleotopography delimits the area of deposition during the Holocene, and thus plays a critical role in determining the vertical accretion rate expressed as a thickness: prior to 7.5 ka cal BP, the vertical accretion rate (5.5 mm yr − 1 ) was less than that observed for the Seine estuary (6.8 mm yr − 1 ). However, rate of sea-level rise and sediment supply, which also affects sediment accumulation rates, vary in northwestern Europe during the Holocene. Therefore, although the Marais Vernier is a good illustration of paleotopographic influence, the effects of autocompaction, sea level and sediment supply complicate efforts to quantify the degree to which it controls sediment accumulation.

Extraction of tidal channel networks from airborne scanning laser altimetry

Tidal channel networks are important features of the inter-tidal zone, and play a key role in tidal propagation and in the evolution of salt marshes and tidal flats. The study of their morphology is currently an active area of research, and a number of theories related to networks have been developed which require validation using dense and extensive observations of network forms and cross-sections. The conventional method of measuring networks is cumbersome and subjective, involving manual digitisation of aerial photographs in conjunction with field measurement of channel depths and widths for selected parts of the network. This paper describes a semi-automatic technique developed to extract networks from high-resolution LiDAR data of the inter-tidal zone. A multi-level knowledge-based approach has been implemented, whereby low-level algorithms first extract channel fragments based mainly on image properties then a high-level processing stage improves the network using domain knowledge. The approach adopted at low level uses multi-scale edge detection to detect channel edges, then associates adjacent anti-parallel edges together to form channels. The higher level processing includes a channel repair mechanism. The algorithm may be extended to extract networks from aerial photographs as well as LiDAR data. Its performance is illustrated using LiDAR data of two study sites, the River Ems, Germany and the Venice Lagoon. For the River Ems data, the error of omission for the automatic channel extractor is 26%, partly because numerous small channels are lost because they fall below the edge threshold, though these are less than 10 cm deep and unlikely to be hydraulically significant. The error of commission is lower, at 11%. For the Venice Lagoon data, the error of omission is 14%, but the error of commission is 42%, due partly to the difficulty of interpreting channels in these natural scenes. As a benchmark, previous work has shown that this type of algorithm specifically designed for extracting tidal networks from LiDAR data is able to achieve substantially improved results compared with those obtained using standard algorithms for drainage network extraction from Digital Terrain Models.

Spatial variations of carbon, nitrogen, phosphorous and sulfur in the salt marsh sediments of the Yangtze Estuary in China

Surface sediments and three sediment vibrocores were collected from the salt marsh of the Yangtze Estuary in order to examine the C, N, P and S distributions. Marsh plants and suspended particulate matter (SPM) from the river were also sampled and analyzed in order to determine their elemental compositions. The levels of total organic carbon (0.1–0.7%) and C/N ratios (6–11) in the surface sediments of the Yangtze Estuary salt marsh were relatively low compared with those reported for other salt marshes in European and North American coastal areas. The total organic carbon (TOC) level and C/N ratio of the surface sediments were similar to those of the SPM in the Yangtze Estuary, but were much lower than those of the marsh plant samples. These findings support the view that organic matter in the surface sediments is largely derived from SPM in the river, with minor contributions from the marsh vegetation. Total phosphorus (TP) showed irregular variation in its spatial distribution, whereas the TOC, total nitrogen (TN) and total sulfur (TS) concentrations were highest in the high marsh zones and lowest in the bare flat areas. This pattern was related to the spatial variability of the sediment grain size (i.e. clay-rich sediments in the high marsh zones resulted in elevated TOC, TN and TS contents). Some vibrocore sediments in the mid-depths of the high and low marsh zones, however, showed greater TOC contents than might have been predicted from the TOC-grain size relationship. This suggested the existence of additional organic inputs (i.e. marsh vegetation) for these vibrocore sediment sections, despite their original riverine source. After eliminating the effect of grain size, it was calculated that 22–55% of the TOC and 0.6–35% of the TN in the sediment samples were derived from the marsh vegetation. Considering both the vertical accretion rate and the ecosystem evolution of the salt marsh, it was estimated that the annual contributions of TOC and TN made by the marsh vegetation to the sediments in the Yangtze Estuary were 5.8 × 10 11 g C yr −1 and 2.3 × 10 10 g N yr −1, respectively, with corresponding accumulation rates of 1.1–1.5 × 10 10 g C yr −1 and 4.4–5.8 × 10 8 g N yr −1 at the present time.