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Long-term anthropogenic nitrate (NO3−) enrichment is a serious threat to many coastal systems. Nitrate reduction coupled with the oxidation of reduced forms of sulfur is conducted by chemolithoautotrophic microbial populations in a process that decreases nitrogen (N) pollution. However, little is known about the diversity and distribution of microbes capable of carbon fixation within salt marsh sediment and how they respond to long-term NO3− loading. We used genome-resolved metagenomics to characterize the distribution, phylogenetic relationships, and adaptations important to microbial communities within NO3−-enriched sediment. We found NO3− reducing sulfur oxidizers became dominant members of the microbial community throughout the top 25 cm of the sediment following long-term NO3− enrichment. We also found that most of the chemolithoautotrophic genomes recovered contained striking metabolic versatility, including the potential for complete denitrification and evidence of mixotrophy. Phylogenetic reconstruction indicated that similar carbon fixation strategies and metabolic versatility can be found in several phylogenetic groups, but the genomes recovered here represent novel organisms. Our results suggest that the role of chemolithoautotrophy within NO3−-enriched salt marsh sediments may be quantitatively more important for retaining carbon and filtering NO3− than previously indicated and further inquiry is needed to explicitly measure their contribution to carbon turnover and removal of N pollution. [ABSTRACT FROM AUTHOR]

Sediment budget and sediment availability are direct metrics for evaluating the resilience of coastal bays to sea‐level rise (SLR). Here we use a high‐resolution numerical model of a tidally dominated marsh‐lagoon system to explore feedbacks between SLR and sediment dynamics. SLR augments tidal prism and inundation depth, facilitating sediment deposition on the marsh platform. At the same time, our results indicate that SLR enhances ebb‐dominated currents and increases sediment resuspension, reducing the sediment‐trapping capacity of tidal flats and bays and leading to a negative sediment budget for the entire system. This bimodal distribution of sediments budget trajectories will have a profound impact on the morphology of coastal bays, increasing the difference in elevation between salt marshes and tidal flats and potentially affecting intertidal ecosystems. Our results also clearly indicate that landforms lower with respect to the tidal frame are more affected by SLR than salt marshes. Key Points: SLR increases sediment deposition on the marsh platform but reduces the sediment‐trapping capacity of tidal flats and baysFor a given SLR the increase of bottom sediments resuspended and exported out of the system is more significant for sand than mud fractionLandforms lower with respect to the tidal frame are more affected by SLR than salt marshes [ABSTRACT FROM AUTHOR]

Salt marshes sequester carbon at rates more than an order of magnitude greater than their terrestrial counterparts, helping to mitigate climate change. As nitrogen loading to coastal waters continues, primarily in the form of nitrate, it is unclear what effect it will have on carbon storage capacity of these highly productive systems. This uncertainty is largely driven by the dual role nitrate can play in biological processes, where it can serve as a nutrient‐stimulating primary production or a thermodynamically favorable electron acceptor fueling heterotrophic metabolism. Here, we used a controlled flow‐through reactor experiment to test the role of nitrate as an electron acceptor, and its effect on organic matter decomposition and the associated microbial community in salt marsh sediments. Organic matter decomposition significantly increased in response to nitrate, even at sediment depths typically considered resistant to decomposition. The use of isotope tracers suggests that this pattern was largely driven by stimulated denitrification. Nitrate addition also significantly altered the microbial community and decreased alpha diversity, selecting for taxa belonging to groups known to reduce nitrate and oxidize more complex forms of organic matter. Fourier Transform‐Infrared Spectroscopy further supported these results, suggesting that nitrate facilitated decomposition of complex organic matter compounds into more bioavailable forms. Taken together, these results suggest the existence of organic matter pools that only become accessible with nitrate and would otherwise remain stabilized in the sediment. The existence of such pools could have important implications for carbon storage, since greater decomposition rates as N loading increases may result in less overall burial of organic‐rich sediment. Given the extent of nitrogen loading along our coastlines, it is imperative that we better understand the resilience of salt marsh systems to nutrient enrichment, especially if we hope to rely on salt marshes, and other blue carbon systems, for long‐term carbon storage. [ABSTRACT FROM AUTHOR]

Abstract In this research, sediments from the Sečovlje Salina (northern Adriatic, Slovenia) were geochemically investigated in order to decode the mobility of metal(oid)s in the hypersaline environment. The results demonstrated that the concentrations of As, Co, Cu, Mo, Ni, Pb, Sb, Sn and Zn were comparable to those detected in various similar coastal background areas, as well as in the Sečovlje salt marsh core sample. The estimated mobility potential of metal(oid)s in sediment decreases in the following order: Mo > As > Cu > Sb > Sn > Co > Pb > Ni > Zn. Since the hypersaline sediment (e.g. healing mud) is used as a healing factor (it can be applied directly on human skin), the study of metal(oid) bioavailability in an interaction between the hypersaline sediments and the artificial sweat was also performed. It is significant that the metal(oid) contents are leached in very low concentrations and are treated as nontoxic for humans according to international norms for cosmetic products. The study provides information in order to assess the role of metal mobility on the potential health impact of the application of natural healing mud. Graphical abstract Unlabelled Image Highlights • The transfer of metal(oid)s in a hypersaline environment was investigated. • Availability of metal(oid)s from a health risk perspective was conducted. • Sediment-water physical and geochemical properties induced limited mobility dynamics. • Extraction testing in artificial sweat indicate low mobility of metal(oid)s. [ABSTRACT FROM AUTHOR]

In this study, lignin-derived phenols, stable carbon isotopes and bulk elemental compositions were determined along the length of two sediment cores (C1 and C2) from the Andong salt marsh, which is located southwest of Hangzhou Bay, China. The purpose of this study was to determine the short-term changes and their implications along sediment profiles. The 1997 high tide had caused an increase in the terrestrial organic matter (OM) signal from 1996/1997 to 2000 in both cores, which was indicated by a high Λ (total lignin in mg/100 mg OC), TOC, C/N and more negative δ 13 C values. The slight increases in terrestrial OM along the length of the cores between 2003 and 2006 were most likely attributable to the construction of the Hangzhou Bay Bridge. Both events have likely caused an increase in erosion, and thus, these events have increased the input of terrestrial OM to nearby areas. The effects of the distinctively dry year of 2006 can be observed along C2 between 2006 and 2008 in the steadily declining terrestrial OM signal. The overall slight decrease in terrestrial OM and the distinct increase in TOC along the length of both cores toward the present were most likely because of the overall reduced sediment caused by the trapping of materials within reservoirs. These results show that the reduction in terrestrial OM in the Andong salt marsh for the past 30 years was due to reservoirs and the 2006 drought, but this was counterbalanced by the 1997 high tide event and construction of the Hangzhou Bay Bridge, which resulted in increased erosion and terrestrial OM input. [ABSTRACT FROM AUTHOR]

Seasonal variability in the sediment dynamics of a Bay of Fundy tidal creek and salt marsh system was analyzed to better understand the ecomorphodynamics of a high suspended sediment concentration intertidal habitat. Data were collected over 62 tides for velocity, suspended sediment concentration, deposition, and grain size at four stations from the creek thalweg to the marsh surface. Five topographic surveys were also conducted throughout the 14-month study. Deposition rates per tide varied spatially from 56.4 g m -2 at the creek thalweg to 15.3 g m -2 at the marsh surface. Seasonal variations in deposition in the creek and marsh surface were from 38.0 g m -2 to 97.7 g m -2 and from 12.2 g m -2 to 19.6 g m -2 respectively. Deposition and erosion were greatest in late fall and winter. This seasonal change, led by higher suspended sediment concentrations, was observed in the creek and at the marsh bank but notably absent from the marsh edge and marsh surface. Sediments were predominantly deposited in floc form (76–83%). Because of high floc content, higher suspended sediment concentrations led to more rapid loss of sediment from suspension. With increasing sediment concentration, deposition increased in the tidal creek and at the marsh bank but not at the marsh edge or marsh surface. This suggests that in highly flocculated environments the water column clears fast enough that very little sediment remains in suspension when the water reaches the marsh and that the sediment concentration during marsh inundation is independent of the initial concentration in the creek. [ABSTRACT FROM AUTHOR]

Salt marshes are ecologically sensitive ecosystems where mercury (Hg) methylation and biomagnification can occur. Understanding the mechanisms controlling gaseous Hg flux from salt marshes is important to predict the retention of Hg in coastal wetlands and project the impact of environmental change on the global Hg cycle. We monitored Hg flux from a remote salt marsh over 9 days which included three cloudless days and a 4 mm rainfall event. We observed a cyclical diel relationship between Hg flux and solar radiation. When measurements at the same irradiance intensity are considered, Hg flux was greater in the evening when the sediment was warm than in the morning when the sediment was cool. This is evidence to suggest that both solar radiation and sediment temperature directly influence the rate of Hg(II) photoreduction in salt marshes. Hg flux could be predicted from solar radiation and sediment temperature in sub-datasets collected during cloudless days (R 2 = 0.99), and before (R 2 = 0.97) and after (R 2 = 0.95) the rainfall event, but the combined dataset could not account for the lower Hg flux after the rainfall event that is in contrast to greater Hg flux observed from soils after rainfall events. [ABSTRACT FROM AUTHOR]

Salt-marsh sediments are routinely used to reconstruct sea-level changes over past millennia. These reconstructions bridge an important gap between geological and instrumental sea-level records, and provide insights into the role of atmospheric, oceanic, climatic and anthropogenic sea-level drivers, thereby improving understanding of contemporary and future sea-level changes. Salt-marsh foraminifera, diatoms and testate amoebae are three of the proxies capable of accurately reconstructing former sea level over decadal to millennial timescales. Datasets of surface assemblages are collated along elevational gradients to provide modern analogues that can be used to infer former marsh-surface elevations from fossil assemblages. Testate amoebae are the most recently developed proxy and existing studies suggest that they are at least as precise as the two other proxies. This study provides a synthesis of sea-level research using testate amoebae and collates and analyses existing surface datasets of intertidal salt-marsh testate amoebae from sites throughout the North Atlantic. We test the hypothesis that intertidal testate amoebae demonstrate cosmopolitan intertidal zonation across wide geographical areas in a way that is unique to this proxy. Testate amoebae assemblages are harmonised under a unified taxonomy and standardised into a single basin-wide training set suitable for reconstructing sea-level changes from salt-marsh sediments across the North Atlantic. Transfer functions are developed using regression modelling and show comparable performance values to published local training sets of foraminifera, diatoms and testate amoebae. When used to develop recent (last 100 years) sea-level reconstructions for sites in Norway and Quebec, Canada, the testate amoebae-based transfer function demonstrated prediction uncertainties of ± 0.26 m and ± 0.10 m respectively. These uncertainties equate to 10% and 11% of the tidal ranges at each site, which is of comparable precision to other published sea-level reconstructions based on foraminifera or diatoms. There is great scope for further developing intertidal testate amoebae as precise sea-level indicators and their application should be tested at sites beyond the North Atlantic. [ABSTRACT FROM AUTHOR]

Cryptic sulfur cycling is an enigmatic process in which sulfate is reduced to some lower-valence state sulfur species and subsequently quantitatively reoxidized; the rate and microbial energetics of this process and how prevalent it may be in the environment remain controversial. Here we investigate sulfur cycling in salt marsh sediments from Norfolk, England where we observe high ferrous iron concentrations with no depletion of sulfate or change in the sulfur isotope ratio of that sulfate, but a 5‰ increase in the oxygen isotope ratio in sulfate, indicating that sulfate has been through a reductive cycle replacing its oxygen atoms. This cryptic sulfur cycle was replicated in laboratory incubations using 18 O-enriched water, demonstrating that the field results do not solely result from mixing processes in the natural environment. Numerical modeling of the laboratory incubations scaled to represent the salt marsh sediments suggests that the uptake rate of sulfate during this cryptic sulfur cycling is similar to the uptake rate of sulfate during the fastest microbial sulfate reduction that has been measured in the natural environment. The difference is that during cryptic sulfur cycling, all of the sulfur is subsequently reoxidized to sulfate. We discuss mechanisms for this pathway of sulfur cycling including the possible link to the subsurface iron cycle. [ABSTRACT FROM AUTHOR]

Enrichment of natural waters, soils, and sediments by inorganic nutrients, including nitrogen, is occurring at an increasing rate and has fundamentally altered global biogeochemical cycles. Salt marshes are critical for the removal of land-derived nitrogen before it enters coastal waters. This is accomplished via multiple microbially mediated pathways, including denitrification. Many of these pathways, however, are also a source of the greenhouse gas nitrous oxide (N 2 O). We used clone libraries and quantative PCR (qPCR) to examine the effect of fertilization on the diversity and abundance of two functional genes associated with denitrification and N 2 O production ( norB and nosZ ) in experimental plots at the Great Sippewissett Salt Marsh (Falmouth, MA, USA) that have been enriched with nutrients for over 40 years. Our data showed distinct nosZ and norB community structures at different nitrogen loads, especially at the highest level of fertilization. Furthermore, calculations of the Shannon Diversity Index and Chao1 Richness Estimator indicated that nosZ gene diversity and richness increased with increased nitrogen supply, however no such relationship existed with regard to richness and diversity of the norB gene. Results from qPCR demonstrated that nosZ gene abundance was an order of magnitude lower in the extra-highly fertilized plots compared to the other plots, but the abundance of norB was not affected by fertilization. The majority of sequences obtained from the marsh plots had no close cultured relatives and they were divergent from previously sequenced norB and nosZ fragments. Despite their divergence from any cultured representatives, most of the norB and nosZ sequences appeared to be from members of the Alpha- and Betaproteobacteria, suggesting that these classes are particularly important in salt marsh nitrogen cycling. Our results suggest that both norB and nosZ containing microbes are affected by fertilization and that the Great Sippewissett Marsh may harbor distinct clades of novel denitrifying microorganisms that are responsible for both the production and removal of N 2 O. [ABSTRACT FROM AUTHOR]

Salt marshes provide diverse ecosystem services including coastal protection, habitat provision and carbon sequestration. The loss of salt marshes is a global scale phenomenon, of great socio-economic concern due to the substantial benefits that they provide. However, the causes of spatial variability in marsh loss rates are inadequately understood for the purposes of predicting future ecosystem distributions and functions under global environmental change. This study investigated the relationship between the presence of different saltmarsh plants and the mechanical properties of the underlying substrate that relate to its vulnerability to erosion. Relationships between three halophytes (Puccinellia spp. , Spartina spp. and Salicornia spp.) and sediment stability were assessed and compared to unvegetated substrates using in-situ and laboratory tests of substrate geotechnical properties and sediment characteristics. Sampling was conducted at two UK sites with contrasting sedimentology, one sand-dominated and one clay-rich. Sediment samples, collected simultaneously with measurements of shear strength, were analysed for moisture content, particle size and organic, carbonate and mineral compositions. These data were then used to explore the contribution of plant type, alongside the sedimentological parameters, to measured shear strength. Shear strength of the sediment varied between and, to a lesser extent, within sites, with the four cover types having a similar effect on shear strength within sites relative to each other. Sediments covered by Puccinellia spp exhibit the highest shear strength, while bare sediments exhibit the lowest. The effect of vegetation type on shear strength was greater in the coarser sediments of Warton Sands. Surface cover type made a significant contribution to exploratory statistical models developed for the prediction of sediment shear strength. The findings support existing recognition that vegetation can enhance sediment shear strengths but extend the insight to reveal differences in this effect that show generality between sedimentological settings. Further, the combination of methods provides insight into the fundamental mechanics by which various measures of sediment stability may be affected by different surface cover types. Cohesion appears to be a more appropriate descriptor of sediment erodibility than shear strength or friction angle and is most greatly enhanced by the presence of a fine, fibrous root system such as that of Puccinellia. A more detailed understanding of the multi-scale mechanisms by which plants confer strength to substrates is needed to better anticipate their impact on sediment erodibility, and therefore salt marsh vulnerability. • Relationships are established between geotechnical properties of salt marsh sediments and the plant species growing on them. • Plant species effect is stronger in coarser sediments but shear strength alone is an inadequate descriptor of erodibility. • Mapping plant distributions may help to understand within-site variability in salt marsh vulnerability to erosion. [ABSTRACT FROM AUTHOR]

Abstract: The organic production related to minerogene salt marsh deposits represents a challenge to all attempts to model the development of these areas, and evaluate their chances of survival under different sea level scenarios. Salt marsh deposits on a typical temperate backbarrier saltmarsh area at the Skallingen barrierspit (Denmark) were investigated for autochthonous or below ground organic production, which was found to have a mean value of 0.14kgm−2 y−1 or about 0.1mmy−1. This production is concentrated in the upper approximately 5cm of the salt marsh. Below this level the organic material decomposes with a decreasing organic content until about 15cm below the surface. Hereunder the decomposition of organic material seems to stabilize at a very low level. The constant rate of the below ground organic production results in a larger concentration of this type of organic matter in the slowest accreting salt marsh deposits. The root activity is the primary reason for differences in the bulk dry density of salt marsh surface layers, therefore the surface layers of slowly accreting salt marshes have the lowest bulk dry density. During autocompaction, the bulk dry density increases down core. This process is only to a small degree (<10%) directly affected by the space loss due to decomposition of organic matter. The decay of organic matter, however, is most likely indirectly part of the autocompaction process by weakening the internal strength of the root fabric and thus the capability of resistance against compaction from the overburden of depositing sediments. [Copyright &y& Elsevier]

The distribution and accumulation of rare earth elements (REE) in the labile fraction of sediment cores collected from salt marshes in the Patos Lagoon estuary from southern Brazil were investigated. Sediment cores (ca. 40 cm) were obtained from three locations within the estuary to capture possible changes in REE content across the salinity gradient (i.e., where saline, brackish, and freshwater dominate). Salt marsh sediments from all three coring locations were enriched in the light REE (LREE) over the heavy REE (HREE) when normalized to the North American shale composite (NASC). Shale normalized values for the LREE of marsh sediments from sites M2 and M3 in the mid- and upper estuary commonly approximates unity indicating these sediments chiefly exhibit a terrigenous signature. In contrast, all 14 naturally occurring REE are depleted in the sediments from the M1 coring location in the lower estuary compared to shale. Sediments from the mid-estuary (M2 core location) where typical salinity values are ca. 10 practical salinity units, exhibit the greatest shale normalized LREE enrichments. The higher LREE contents of the M2 sediments likely reflect preferential removal of LREE from the water column owing to salt-induced coagulation of river-borne colloids that occurs during estuarine mixing processes. Sediments samples collected from the salt marsh in the lower estuary nearer the Atlantic Ocean (i.e., M1), have substantially lower REE contents than salt marsh sediments from sites M2 and M3 in the mid- and upper estuary, respectively. The more sand-rich lower estuary sediments are exposed to higher salinity water from the South Atlantic compared to the generally finer grained sediments from the mid- and upper estuary, which are dominated by brackish and freshwater conditions, respectively. Negative Ce-anomalies (i.e., Ce/Ce* < 1) in marsh sediments from depths generally greater than 27 cm support the occurrence of early diagenetic reactions consistent with suboxic to anoxic conditions whereby reductive dissolution of ferromanganese oxide/oxyhydroxide carrier phases, and/or Ce(IV) phases, release Ce(III) to the marsh pore waters. The negative Ce anomalies suggests that a suboxic or anoxic environment predominates to a depth of 9 cm, and then again below 27 cm depth in sediments from the M2 salt marsh core, and below a depth of 13 cm in the upper estuary marsh sediments (i.e., M3). In contrast, positive Ce anomalies (Ce/Ce* > 1) for much of the sediment from the M1 core, the mid depths of core M2 (i.e., 9 cm to 27 cm), and shallower than 13 cm in sediments from core M3 support oxic conditions where Ce is enriched in sediments from these depths by preferential scavenging from pore waters onto Fe/Mn oxides/oxyhydroxides. These trends in Ce/Ce* with depth in each sediment core are also consistent with known bioturbation that occurs within these marsh sediments. The relatively high REE contents of the sediments from location M3 are also related to the fact that these sediments are dominantly composed of clay-silt deposits that are enriched in organic matter. Finally, we find no conclusive evidence that the studied salt marsh sediments have been impacted by addition of REE from the local fertilizer manufacturing industry. • ƩLREE/ƩHREE ratios were generally lower in saltmarshes sediments closer to the sea. • REE enrichment in freshwater dominate part of the estuary is due to high fine-grained and organic contents. • Positive Ce-anomalies are supported by oxic conditions in salt marsh sediment. [ABSTRACT FROM AUTHOR]

Tide observations, geographic surveys, sediment composition, and sediment accretion rate monitoring were conducted to investigate the relationship between general pattern changes and control factors of the accretion rate at 10 localities of the tidal salt marsh of Suncheon Bay over 2 years (June 2007-May 2009). The substrate sediments of the tidal salt marshes that are distributed between the mean sea level and the high water of ordinary spring tide are mainly composed of silt (37.98%) and clay (60.45%) and have a high average annual accretion rate of 28.88 mm/y. The high substrate sedimentation of tidal salt marshes is caused by: (1) a high concentrated suspended sediment supply from the tidal flat; (2) considerable marsh grass vegetation; (3) undeveloped tidal salt marsh creek; and (4) marsh elevation distributed between the mean sea level and the high water of ordinary spring tide. The variation in patterns of the average monthly accretion rate can be divided into three stages: the low stage (June 2007 to February 2008) was the period of lowest accretion rate over 2 years and consistently measured between 5.35 mm and 12.40 mm, averaging out to 8.89 mm that period. The recovery stage (March 2008 to October 2008) was a period of rapid increase in accretion rate and measured from 14.93 mm to 49.47 mm with an average of 33.02 mm. The high stage (November 2008 to May 2009) was a period of high deposition in accretion rate, and measured between 46.67 mm and 53.38 mm, averaging to 49.83 mm. These three stages were formed and changed by heavy rainfall brought about by typhoons in the summer season of Korea and undeveloped tidal salt-marsh creek system. It was confirmed that heavy rainfall may significantly alter the sediment flux within the marsh system in this study. [ABSTRACT FROM AUTHOR]

The effects of drought on salt marsh sediments from Sapelo Island, Georgia, were examined in flow-through reactor experiments. Three hydrological treatments were employed: a continuously flooded anoxic control, a periodic drought treatment that experienced alternate periods of flooding and drying, and a severe drought treatment, where sediment was exposed to drought (drying) for several weeks and then flooded; the effect of both buffered and non-buffered flooding solutions were examined. In permanently anoxic sediments as well as in sediments exposed to drought, organic carbon oxidation was dominated by SO reduction (SR) and SR rates increased over time. The shift from anoxic to oxic conditions in drought treatments significantly altered sediment geochemistry and pathways of microbial metabolism. Drought conditions favored suboxic mineralization processes, such as Fe(III) reduction and denitrification, which was fueled by NH oxidation promoted by O delivered during drought conditions. Other major drought-induced changes included pH decrease, and altered concentrations of solid phase adsorbed metals. [ABSTRACT FROM AUTHOR]

Abstract: The salt marsh grass Spartina anglica is an important habitat-modifying ecosystem engineering agent that facilitates large-scale salt marsh formation by enhancing sediment accretion. It dominates many European tidal environments and is invasive in many other parts of the world. We question (1) to what extent the ecosystem engineering ability of patchy Spartina vegetation depends on large-scale abiotic processes, and (2) whether tussock shape provides an indicator for future lateral salt marsh development. Analysing the topography of 83 individual tussocks in contrasting environments revealed that there are 6 clearly distinguishable tussock shapes, and that the classical example of a sediment-accumulating dome-shaped tussock only occurs under a limited set of abiotic conditions. The outcome of habitat modification by S. anglica is shown to be conditional, depending on large-scale morphodynamics and sediment grain size. Resulting tussock shape provides a clear indication for the long-term development of the pioneer zone. Understanding of the conditional outcome of ecosystem engineering is highly relevant in this era of climate change and ongoing anthropogenic influences on coastal ecosystems. [Copyright &y& Elsevier]

Abstract: This study examined sediment deposition and retention in a section of salt marsh in the St. Jones River estuary in Delaware. Sediment traps, siphon samplers, a LISST particle sizer, and four water level sensors were deployed in a 100m by 200m grid over the course of a spring tide in June 2007. The objective was to determine the relative influences of suspended sediment concentration, settling velocity, tidal hydroperiod, and vegetation density on sediment deposition and trapping efficiency within a small section of marsh. Hydroperiod is not a major contributor to sediment deposition in the study plot. Spatial patterns in depositional processes on tidal timescales are instead primarily controlled by overmarsh SSC, settling velocity, and distance from the tidal channel. Trapping efficiency, or the ratio of measured deposition and the amount of sediment available for deposition, decreased with distance away from the tidal channel, indicating that less of the available suspended sediment was deposited in the marsh interior. The suspended particles were flocculated, but floc sizes decreased with distance away from the tidal channel. [Copyright &y& Elsevier]

Abstract: 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 100m 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. [Copyright &y& Elsevier]

Abstract: Salt marshes are potentially threatened by sea level rise if sediment supply is unable to balance the rising sea. A rapid sea level rise is one of the pronounced effects of global warming and global sea level is at present rising at an elevated rate of about 3.4mm y−1 on average. This increasing rate of sea level rise should make it possible to study the effect of rapidly rising sea level on salt marsh accumulation. However, such an understanding is generally hampered by lack of available data with sufficient precision. Here we present a high-precision dataset based on detailed radiometric measurements of 137Cs in 10 sediment cores retrieved at a natural and unmanaged micro tidal salt marsh. Two distinct 137Cs-peaks were found in all cores, one peak corresponding to the 1963-maximum caused by testing of nuclear weapons in the atmosphere and the other to the Chernobyl accident in 1986. Salt marsh accretion has generally kept pace with sea level rise since 1963 but comparison of the accumulation rates of minerogenic material in the period 1963–1986 and 1986–2003 revealed a slight decrease in accumulation with time in spite of an observed increase in inundation frequency. The observed decrease in sediment deposition is significant and gives reason for concern as it may be the first sign of a sedimentation deficiency which could be threatening this and other salt marshes in the case of a rapidly rising sea level. Our work demonstrates that the assumption of a constant relationship between salt marsh inundation and sediment deposition is not necessarily valid, even for a salt marsh that receives most of its allocthonous sediment from the adjacent sea. The apparent decrease in sediment deposition indicates that the basic assumption of sufficient sediment supply used in contemporary models dealing with salt marsh accretion is most probably not valid in the present case study and it may well be that this is also the case for many other salt marshes, especially if sea level continues to rise rapidly as indicated by some climate change scenarios. [Copyright &y& Elsevier]

Abstract: Existing geotechnical approaches that describe volumetric changes in intertidal sediments in response to applied vertical effective stresses are limited by a lack of empirical research into their one-dimensional compression behaviour. In this paper we address this deficiency by presenting the results of an investigation into the compression behaviour of minerogenic low marsh and tidal flat sediments. We have tested samples of these sediment types obtained from Greatham Creek (Cowpen Marsh, Tees Estuary, UK). Analysis of physical properties and oedometer compression tests demonstrates that, contrary to the implicit assumptions of existing models, the surface sediments studied are overconsolidated. Structural variability between samples arises due to sedimentological factors, notably variations in organic content. We attribute overconsolidation to tidal exposure and falls in groundwater level that permit desiccation and cause capillary suction stresses. Greater rates of compression with respect to effective stress occur in sediments with higher initial voids ratios and more open, unstable initial structures. Variability in structure decreases with application of higher effective stresses due to the destructuration of the sediments, which also creates increased homogeneity of compression behaviour under higher effective stresses. We subsequently develop a new conceptual framework to describe compression behaviour in minerogenic intertidal sediments that incorporates overconsolidation. We advocate a statistical approach that accounts for structural variability and variations in compression behaviour at effective stresses less than and greater than the yield stress. We argue that our conceptual framework is broadly applicable to minerogenic intertidal sediments at different locations and burial depths within Holocene stratigraphic sequences providing site-specific compression data are collected. Inter-site transfer and application of measured material properties should not be undertaken due to local variations in compression behaviour resulting from varying ecological, sedimentological, geochemical, climatic, geomorphic and hydrographic conditions. The individual characteristics of different field locations should be carefully considered before the suggested framework is routinely applied. [Copyright &y& Elsevier]

Abstract: Sediment cores and flooding water were collected at 0, 5, 10 and 50 min of tidal inundation in two sites of the Rosário salt marsh located in the proximity of a heavy industrialised zone of Tagus estuary colonised by pure stands of Spartina maritima (low marsh) and Sarcocornia fruticosa (high marsh). The cores were sliced in 5 cm layers and sediment solids, pore water, and belowground biomass were separated in order to measure Fe, Mn, Zn, Cu, Pb and Cd. The pore waters and sediments colonised by S. fruticosa, as well as belowground biomass presented high concentrations of Zn, Cu, Pb and Cd. Belowground biomass exceeded in one order of magnitude the metal levels in sediments. Abundant belowground biomass and small dimension of S. fruticosa roots facilitates the root–sediment interactions and presumably the metal retention in the higher marsh. The novelty of this work is the result of tidal inundation on pore water concentrations of metals in salt marsh sediments and their exportation to the water column. Concentrations in pore waters varied at minute scales, but 50 min after inundation levels were comparable to the initial values. The metal levels in flooding water increased abruptly during the first 10–20 min of inundation. The concentration peaks (Fe = 60 μM, Mn = 7.5 μM, Zn = 1.7 μM, Cu = 550 nM, Pb = 100 nM, Cd = 1.7 nM) reached one to two orders of magnitude above the values found in subsequent periods of inundation. The advective transports during the 50 min inundation during two daily pulses of inundation were: Fe (9520 and 1640), Mn (24), Zn (220 and 82), Cu (74 and 16), Pb (13 and 15) and Cd (0.3 and 0.08) for S. maritima and S. fruticosa, respectively. These quantities exceeded three to four orders of magnitude of the corresponding predicted diffusive fluxes (Fick 1st law) on a daily basis. This work emphasizes the importance of tidal flooding over salt marsh sediments to the metal exportation to the water column. [Copyright &y& Elsevier]

Intertidal sediments experience substantial changes in temperature, salinity, and solar irradiation over short time periods. We applied the Fluorescently Labeled Embedded Core (FLEC) technique to map distribution patterns of microbenthos in tidal creek sediments. Our aims were to determine if micro-scale distributions varied over the course of a day and to test the null hypothesis that microbenthos are randomly distributed. Eight samples were collected at 3 h intervals from an intertidal sandflat on Sapelo Island, Georgia, USA. Cores were incubated with CellTracker Green (CMFDA, hereafter, CTG), a fluorogenic compounds that accumulates in metabolically active cells. Cores were embedded with epoxy and examined with laser scanning confocal microscopy. Image analysis was used to map the vertical locations of active microbenthos, which in these sediments consisted of benthic microalgae (BMA), ciliates, and flagellates. Microbenthos were abundant over the entire depth profiled (2 cm), although O microelectrode profiles indicate that only the top 3 mm of sediment was oxygenated during high light (1,000 μE m s). More than 91% of organisms mapped were <22 μm in diameter and, based upon size and cell appearance, were BMA. Microbenthos accumulated in the top 1 mm at 0800 and 1100 h, corresponding to both low tide and high solar irradiation. This pattern conforms to BMA migratory rhythms determined by other methods. The standardized Morisita's Index of dispersion determined that CTG-positive objects were significantly clumped at all time points and at each of the three spatial scales examined. This clumping pattern likely results from the heterogeneous distribution of resources, such as prey items for phagotrophs and dissolved nutrients or growth substrates for autotrophs or heterotrophs. [ABSTRACT FROM AUTHOR]

To quantify the trapping effect of tidal marsh vegetation on suspended sediment, we measured the amount of sediment adhering to three species of plants in different habitats in the Yangtze Delta. We also measured suspended sediment concentrations (SSC) and bed levels in the tidal marsh to examine their influence on the adherence of sediment to vegetation and the contribution of vegetation-trapped sediment to the local total deposition rate. The results include (1) the density of sediment trapped in the joints of leaves and stems and on fruits, which was 5 to 10 times higher than that on the stems and leaves; (2) the amount of sediment (18 to 559 g/m2 in dry weight) trapped by vegetation, which increased with plant biomass (220 to 5408 g/m2 in dry weight); (3) the dry weight of sediment adhering to plants, which drastically decreased from 10-15 g/m2 to <2 g/m2 upward from their base to their tip; (4) the amount of sediment trapped by vegetation per unit land area in the low marsh margin, which decreased at a rate of 1% to 3%/m with distance from the outer marsh edge, bordering the mudflat, or from the tidal creak, where the SSC was higher; and (5) the amount of vegetation-trapped sediment, which was likely responsible for >10% of the total depositional rate in the Spartina alterniflora marsh and <10% of the total depositional rate in the Scirpus mariqueter marsh. It was concluded that the amount of sediment trapped by the tidal marsh vegetation was related to plant properties, to the SSC, and to the bed level, which determines the tidal submergence; the introduced S. alterniflora is significantly more efficient in trapping suspended sediment than the native Scirpus and Phragmites plant species. [ABSTRACT FROM AUTHOR]

The present study intends to increase the knowledge on the mobility of mercury in a salt marsh colonised by Halimione portulacoides. Mercury distribution in the sediment layers and its incorporation into the plant biomass were assessed, as well as the potential export of mercury from the contaminated area to the adjacent environment. Mercury pools in the sediments ranged from 560 to 943mgm−2 and are largely associated with the solid fraction, with just a small amount being associated with the pore waters. Estimated diffusive fluxes of reactive mercury ranged from 1.3 to 103ngm−2 d−1. Despite the above ground biomass values being comparatively higher than below ground biomass values, the mercury pools were much higher in the root system (0.06–0.16mgm−2 and 29–102mgm−2, respectively). The annual bioaccumulation of mercury in above ground tissues was estimated in 0.11mgm−2 y−1, while in below ground biomass the values were higher (72mgm−2 y−1). The turnover rates of H. portulacoides biomass suggest higher mercury mobility within the plant rhizosphere. Taking into account the pools of mercury in above ground biomass, the export of mercury by macro-detritus following the “outwelling hypothesis” is not significant for the mercury balance in the studied ecosystem. The mercury accumulated in the below ground part of the plant is quite mobile, being able to return to the sediment pool throughout the mineralisation process. [Copyright &y& Elsevier]

Total concentrations and fractionation of Cu, Zn and Pb in seven operationally defined phases (exchangeable, carbonates, manganese oxides, organic complexes, amorphous iron oxides, crystalline iron oxides and residual) were determined in sediments colonised by the halophyte species Halimione portulacoides and Spartina maritima in a Tagus estuary salt marsh (Portugal). We aimed to determine whether the speciation of these metals was different in areas colonised by each halophyte. Higher concentrations of Cu, Zn and, in particular Pb, were found in the rhizosphere of S. maritima than in the root sediments of H. portulacoides. Geochemical fractionation of Cu, Zn and Pb in sediments of the salt marsh depended upon the metal, and for Zn and Pb clearly varied with depth and with the colonising species. The higher redox potential observed in sediments colonised by H. portulacoides may in part explain the observed differences in the speciation of Cu, Zn and Pb. [Copyright &y& Elsevier]

We have examined sediments from a fringing salt marsh in Maine to further understand marine CO metabolism, about which relatively little is known. Intact cores from the marsh emitted CO during dark oxic incubations, but emission rates were significantly higher during anoxic incubations, which provided evidence for simultaneous production and aerobic consumption in surface sediments. CO emission rates were also elevated when cores were exposed to light, which indicated that photochemical reactions play a role in CO production. A kinetic analysis of marsh surface sediments yielded an apparent Km of about 82 ppm, which exceeded values reported for well-aerated soils that consume atmospheric CO (65nM). Surface (0–0.2 cm depth interval) sediment slurries incubated under oxic conditions rapidly consumed CO, and methyl fluoride did not inhibit uptake, which indicated that neither ammonia nor methane oxidizers contributed to the observed activity. In contrast, aerobic CO uptake was inhibited by additions of readily available organic substrates (pyruvate, glucose and glycine), but not by cellulose. CO was also consumed by surface and sub-surface sediment slurries incubated under anaerobic conditions, but rates were less than during aerobic incubations. Molybdate and nitrate or nitrite, but not 2-bromoethanesulfonic acid, partially inhibited anaerobic uptake. These results suggest that sulfidogens and acetogens, but not dissimilatory nitrate reducers or methanogens, actively consume CO. Sediment-free plant roots also oxidized CO aerobically; rates for Spartina patens and Limonium carolinianum roots were significantly higher than rates for Spartina alterniflora roots. Thus plants may also impact CO cycling in estuarine environments. [ABSTRACT FROM AUTHOR]

Abstract: Salt marsh sediment volume decreases from organic decomposition, compaction of solids, and de-watering, and each of these processes may change with age. Variability in the vertical accretion rate within the upper 2m was determined by assembling results from concurrent application of the 137Cs and 210Pb dating techniques used to estimate sediment age since 1963/1964, and 0 to ca 100+years before present (yBP), respectively. The relationship between 210Pb and the 137Cs dated accretion rates (Sed210 and Sed137, respectively) was linear for 45 salt marsh and mangrove environments. Sed210 averaged 75% of Sed137 suggesting that vertical accretion over the last 100+years is driven by soil organic matter accumulation, as shown for the pre 137Cs dated horizon. The ratio of Sed210/Sed137 declines with increasing mineral content. A linear multiple regression equation that includes bulk density and Sed137 to predict Sed210 described 97% of the variance in Sed210. Sediments from Connecticut, Delaware and Louisiana coastal environments dated with 14C indicate a relatively constant sediment accretion rate of 0.13cm year−1 for 1000–7000yBP, which occurs within 2m of today''s marsh surface and equals modern sea level rise rates. Soil subsidence is not shown to be distinctly different in these vastly different coastal settings. The major reason why the Sed137 measurements indicate higher accretion rates than do the Sed210 measurements is because the former apply to younger sediments where the effects of root growth and decomposition are greater than in the latter. The most intense rates of change in soil volume in organic-rich salt marshes sediments is, therefore, neither in deep or old sediments (>4m; >1000years), but within the first several hundreds of years after accumulation. The average changes in organic and inorganic constituents downcore are nearly equal for 58 dated sediment cores from the northern Gulf of Mexico. These parallel changes downcore are best described as resulting from compaction, rather than from organic matter decomposition. Thus most of the volumetric changes in these salt marsh sediments occurs in the upper 2m, and declines quickly with depth. Extrapolation forwards or backwards, using results from the 210Pb and the 137Cs dating technique appear to be warranted for the types of samples from the environments described here. [Copyright &y& Elsevier]

Despite long-term monitoring of water quality in the coastal lagoonsof California, USA, the fate of organochlorines discharged from agricultural and urban sources to salt marsh habitats has received littleattention. We measured organochlorine concentrations in Upper Newport Bay marsh and channel sediments and salt marsh plants. Sediment grain-size analyses show that organochlorines are fractionated and concentrated in the organic material traveling with the clay fraction of the washload discharged into the bay from San Diego Creek. Hydrophobiccompounds such as DDT are not associated with the sand and silt fractions. The most frequently detected analyte was p,p'- I,1-dichloro-2,2-bis(4-chlorophenyl)ethylene (DDE), occurring in 100% of the sediment samples and 87% of the plant root samples. Maximum concentrations in sediment were 122 ng/g (dry wt) for DDE and 162 ng/g DDT plus metabolites (Sigma DDT) in salt marsh deposited since 1980. Ratios of DDT to its metabolites suggest that anaerobic degradation of DDT occurs in the cordgrass (Spartina fnliosa Trin.) sediments. Cordgrass and pickleweed (Saticornia virginica L.) partition organochlorine residues to the root epidermis, attaining values of 50,000 ng/g lipid for SigmaDDT and 80,000 ng/g lipid for heptachlorepoxide (1,4,5,6,7,8,8-heptachloro-2.3-epoxy-3Alpha,4,7,7a lpha-tetrahydro-4,7-methanoindan). [ABSTRACT FROM AUTHOR]