Marsh Sediments Research Articles

Influence of local and regional drivers on spatial and temporal variation of ammonia-oxidizing communities in Gulf of Mexico salt marshes.

We characterized ammonia-oxidizing archaea (AOA) and bacteria (AOB) from salt marsh sediments in the Gulf of Mexico over 5 years to identify environmental drivers of nitrifying community patterns following the Deepwater Horizon oil spill. Samples were collected from oiled and unoiled sites in July of 2012-2016 from 12 marshes spanning three regions on the Louisiana coast. No consistent oil effect was detected for either AOA or AOB abundance or community composition. At the local scale, abundance was correlated with changes in marsh elevation, suggesting that oxygen may be an important driver. Regional differences in abundance were best explained by salinity and soil moisture, while interannual variation may be more linked to changes in climate and Mississippi River discharge. Variation of AOA communities was correlated with organic sediment nutrients, while AOB communities were correlated with soil extractable nutrients. AOA and AOB diversity and AOB abundance decreased in 2014 in all regions, suggesting that broad-scale drivers, such as climate, may explain synchronous shifts throughout the coastal area. Our results provide insights about large-scale disturbances on nitrifying microbes in the Gulf of Mexico, and suggest that nitrogen cycling may be controlled primarily by local factors, but large-scale drivers might override these localized differences at times.

Potential mobility assessment of metals in salt marsh sediments from San Antonio Bay

The BCR method was applied on sediments from the salt marsh of San Antonio Bay (SAB). It presents several channels among which the Encerrado is the most important and is impacted by abandoned mining wastes. The pseudototal concentrations of metals measured within this channel were relatively higher than in outer sites, and according to the Igeo index, its contamination level was low. The metal distribution in the different phases of sediment particles showed that the residual component, considered the safest from the environmental point of view, accounted for most of the Fe, Cd, Cu, and Zn contents. Conversely, Pb was mainly in the non-residual component as part of the reducible fraction, thus constituting the main environmental hazard among the studied elements. The predominance of residual and reducible fractions indicated a historic contamination of metal such as Pb, Cu, and Zn from the mining wastes. The low exchangeable and oxidizable fractions would indicate no actual input of metals.

Denitrifying anaerobic methane oxidation in marsh sediments of Chongming eastern intertidal flat.

Denitrifying anaerobic methane oxidation (DAMO) and associated microbial diversity and abundance in the marsh sediments of Chongming eastern intertidal flat, the Yangtze Estuary, were investigated using carbon-isotope tracing and molecular techniques. Co-existence of nitrate-DAMO archaea and nitrite-DAMO bacteria was evidenced, with higher biodiversity of DAMO archaea than DAMO bacteria. Abundance of DAMO archaeal mcrA gene and DAMO bacterial pmoA gene ranged from 4.2 × 103 to 3.9 × 1010 copies g-1 and from 4.5 × 105 to 6.4 × 106 copies g-1, respectively. High DAMO potential was detected, ranging from 0.6 to 46.7 nmol 13CO2 g-1 day-1 for nitrate-DAMO and from 1.3 to 39.9 nmol 13CO2 g-1 day-1 for nitrite-DAMO. In addition to playing an important role as a CH4 sink, DAMO bacteria also removed a substantial amount of reactive nitrogen (29.4 nmol N g-1 day-1) from the intertidal sediments. Overall, these results indicate the importance of DAMO bioprocess as methane and nitrate sinks in intertidal marshes.

The Production and Fate of Volatile Organosulfur Compounds in Sulfidic and Ferruginous Sediment

AbstractVolatile organic sulfur compounds (VOSCs) link the atmospheric, marine, and terrestrial sulfur cycles in marine and marginal marine environments. Despite the important role VOSCs play in global biogeochemical sulfur cycling, less is known about how the local geochemical conditions influence production and consumption of VOSCs. We present a study of dimethyl sulfide (DMS), methanethiol (MeSH), and dimethylsulfoniopropionate (DMSP) in sulfide‐rich (sulfidic) and iron‐rich (ferruginous) salt marsh sediment from north Norfolk, UK. Initial results illustrate the importance of minimizing time between sampling in remote field locations and laboratory analysis, due to rapid degradation of VOSCs. With rapid analysis of sediment from different depths, we observe high concentrations of DMS, MeSH, and DMSP, with concentrations in surface sediment an order of magnitude higher than those in previous studies of surface water. We measure systematic differences in the concentration and depth distribution of MeSH and DMS between sediment environments; DMS concentrations are higher in ferruginous sediment, and MeSH concentrations are higher in sulfidic sediment. With repeated measurements over a short time period, we show that the degradation patterns for DMS and MeSH are different in the ferruginous versus sulfidic sediment. We discuss potential biogeochemical interactions that could be driving the observed differences in VOSC dynamics in ferruginous and sulfidic sediment.

Parasites enhance resistance to drought in a coastal ecosystem.

Parasites are more diverse and numerous than their hosts and commonly control population dynamics. Whether parasites also regulate key ecosystem processes, such as resistance to climate stress, is unclear. In southern U.S. salt marshes, drought interacts synergistically with keystone grazing to generate extensive ecosystem die-off. Field manipulations of parasite prevalence and salt stress in sediments in healthy marshes demonstrated that trematode parasites, by suppressing feeding activity of grazers that overgraze on drought-stressed plants, have the potential to slow the rate of ecosystem loss. Surveys along 1,000 km of coastline during an intense drought event revealed parasitism is common in grazers on die-off borders and that increasing infection prevalence along marsh die-off borders is negatively correlated with per capita grazing. Combined, results from this field experiment and survey suggested, but did not show, that parasites could affect rates of drought-driven salt marsh die-off. To test whether parasites can indeed protect marshes under real drought conditions, we experimentally manipulated parasite prevalence in grazers over a month-long period on active die-off borders in three North Carolina marshes. Experimentally reducing parasite prevalence markedly increased the rate of plant ecosystem decline, an effect that scaled positively with prevalence. Thus parasites, by generating a trophic cascade, indirectly enhanced ecosystem resistance to overgrazing under intense drought in these North Carolina marshes. The generality of these results across the entire range of this keystone grazer in the southeastern United States needs to be tested, employing both experiments and extensive surveys that examine how the rate of ecosystem decline is mediated by parasitism. Given the ubiquity of parasites in ecosystems, our results suggest that more research effort should be invested in examining the possible roles for parasitism in regulating ecosystem function and stability.

Dissolved Organic Matter Export from Surface Sediments of a New England Salt Marsh

Salt marshes sequester a large amount of carbon via sedimentation, but seasonal and climate change impacts on sediment carbon biogeochemistry are not well-understood. This study investigates the export of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) from New England salt marsh sediment seasonally and under possible expected drought conditions. Surface sediment total organic carbon (TOC) values shifted seasonally depending on the overlying dominant plant species suggesting that vegetation and inundation frequency and duration are key factors in sediment TOC fluctuations. Fall sediment samples leached more DOC than summer samples, were compositionally similar to the estuarine water column, and compositionally different from fresh plant matter. Thus, the seasonal TOC pulse incorporated into surface sediments could be a dominant source of DOC to the estuary in the fall and that summer contributions are presumed to be from upstream DOM sources based on optical properties. Sediments leached more DOC under drought than non-drought conditions due to induced cracking in the sediment enhancing organic matter degradation. This study suggests the majority of DOM exported laterally from salt marshes comes from surface sediments (versus plant matter) and that temperature and sea-level rise could affect seasonal and drought pulses of DOC from the marsh.

Metagenomics coupled with biogeochemical rates measurements provide evidence that nitrate addition stimulates respiration in salt marsh sediments

AbstractHigh‐throughput sequencing has enabled robust shotgun metagenomic sequencing that informs our understanding of the genetic basis of important biogeochemical processes. Slower to develop, however, are the application of these tools in a controlled experimental framework that pushes the field beyond exploratory analysis toward hypothesis‐driven research. We performed flow‐through reactor experiments to examine how salt marsh sediments from varying depths respond to nitrate addition and linked biogeochemical processes to this underlying genetic foundation. Understanding the mechanistic basis of carbon and nitrogen cycling in salt marsh sediments is critical for predicting how important ecosystem services provided by marshes, including carbon storage and nutrient removal, will respond to global change. Prior to the addition of nitrate, we used metagenomics to examine the functional potential of the sediment microbial community that occurred along a depth gradient, where organic matter reactivity changes due to decomposition. Metagenomic data indicated that genes encoding enzymes involved in respiration, including denitrification, were higher in shallow sediments, and genes indicative of resource limitation were greatest at depth. After 92 d of nitrate enrichment, we measured cumulative increases in dissolved inorganic carbon production, denitrification, and dissimilatory nitrate reduction to ammonium; these rates correlated strongly with genes that encode essential enzymes in these important pathways. Our results highlight the importance of controlled experiments in linking biogeochemical rates to underlying genetic pathways. Furthermore, they indicate the importance of nitrate as an electron acceptor in fueling microbial respiration, which has consequences for carbon and nitrogen cycling and fate in coastal marine systems.

The Holocene sedimentary record of the flood plain of the Saint-Ciers-Sur-Gironde marsh (Gironde estuary, France)

The Holocene sedimentary record of the flood plain of the Saint-Ciers-Sur-Gironde marsh was examined on sediment cores from the right bank of the Gironde estuary with regard to the evolution of the marsh and its potential to preserve high-energy deposits. Sedimentological, geochemical, geophysical and micropaleontological methods were applied. Radiocarbon ages in the central part of the investigation area reach back to 7,971 ± 44.5 BP. The sediments of the Saint-Ciers-sur-Gironde marsh reflect the condi- tions of a Holocene estuarine salt marsh before human activity and draining. The interpretation of the core data showed that the study area includes different facies during development from the pre-Holocene to recent times: tidal mudflats in the northwest at Mortagne and Beaumont, and a fluvial facies develop- ing into an estuarine facies in the southeast near Camp and Saint-Ciers-Sur-Gironde. The last stage is the formation of the saltmarsh. The changes in grain size reflect different transport mechanisms during the development from a fluvial environment to the recent marsh. First gravel, then sand and finally muddy sediments were deposited in the estuary and finally in the marsh area.. Below the salt marsh deposits in Mortagne-Sur-Gironde, there is some evidence of deposits from energy-rich events in tidal mudflats. The southern and central part, in which estuarine clays were deposited, was probably most of the time outside the range of storms. On the marshland surface, erosion, pedogenesis and bioturbation processes destroy storm relevant layers in a very short time.

Arsenic redistributive accretion in interdune marshes and its impact on groundwater contamination of coastal plains (southern Brazil)

Excessive concentrations of arsenic in aquifers have been presented for the first time in the southern Brazilian coastal plain. In this region, the shallow aquifers are mainly constituted by a mosaic distribution of marsh sediment lenses formed in the interdune depressions in past times and subsequently covered by eolian sands. The stratigraphic descriptions and analytical determinations in water (pH, conductivity, As) and sediment (organic carbon, As, Fe, Mn) were obtained in samples of an interdune freshwater marsh, an aquifer (a palaeo-freshwater marsh), and a modern eolic dune. Principal component analysis was applied to summarize the process of As immobilization. In sediments of the modern interdune marshes and under suboxic conditions, As is mobilized from the eolic sands and its redistribution occurs along sediment profile. Its maximum concentration (up to 1.7 mg kg−1 dw) occurred at the marsh’s sediments surface and was strongly coupled to Fe hydroxides. In pore water of marshes, As has been registered up to 79 µg L−1. This enrichment was driven by reductive dissolution of As-bearing Fe hydroxides in sediments, where the reduced environment has allowed their desorption from solids as Fe(II) and As(III) reduction. After the cover of the interdune marshes by eolic sands, turning them into current aquifers, the high As contents have been maintained above the sanitary limit of 10 μg L−1 for drinking water in groundwaters. The southern Brazilian coastal plain is probably an area with arsenic contamination problem for groundwater, which deserves more attention when it is used for drinking supply.

Ripening reduces the shrinkage of processed dredged material

The utilization of dredged material in dike construction as a substitute for traditionally used materials is considered as an option to preserve natural resources such as marsh sediments. As a prerequisite for this application, the equivalency with respect to soil physical and mechanical properties of the materials must be assessed. Previous investigations have shown pronounced differences in shrinkage behavior and desiccation cracking between sediments and dredged material. The key objective of the study was to assess whether shrinkage of processed dredged material can be reduced by further processing, i.e., dewatering, which can be referred to as ripening. To compare the shrinkage behavior of the materials, three different methods of different scales were applied. Small-scale methods conducted were the standard procedure for the determination of the shrinkage limit and the determination of the coefficient of linear extensibility (COLErod). Large-scale shrink-swell experiments were carried out in a specially constructed test system with 90 l capacity for a period of up to 385 days. Here the materials were ripened, i.e., air-dried, until shrinkage almost ceased, and a rewetting-air-drying cycle was conducted. Shrinkage and swelling were determined during the processes by measuring the changes in volume. On the ripened materials, COLErod was determined. The experiments show that the shrinkage behavior of processed dredged material can be ameliorated by ripening. COLErod of the ripened materials were about 20–80% lower than COLErod of the un-ripened materials. The large-scale shrink-swell experiments showed that shrinkage in the second drying cycle amounted to less volume than in the first drying cycle and that shrinkage behavior in contrast to the first drying cycle, where pronounced proportional shrinkage was observed, was dominated by structural and residual shrinkage in this cycle. Ripening of processed dredged material is considered a useful pre-treatment option to ameliorate the shrink-swell behavior of processed dredged material and to obtain a better functional equivalency with traditionally used dike construction materials such as fine-grained marsh sediments.

Organic phosphorus mineralization characteristics in sediments from the coastal salt marshes of a Chinese delta under simulated tidal cycles

Sediment organic phosphorus (OP) mineralization plays an important role in phosphorus cycling in coastal salt marshes. However, information on OP mineralization characteristics in sediments under tidal cycles is limited. Sediment cores were collected from salt marshes which were exposed to tidal cycles. A laboratory manipulation experiment was conducted under simulated tidal flooding cycles. After the completion of each incubation experiment, the inorganic phosphorus (IP) and OP fractions were analyzed and net phosphorus mineralization rates were calculated to investigate the sediment OP mineralization process. Sediment microbial characteristic and chemical properties were determined to identify factors that affect the mineralization of OP in the Yellow River Delta. During the first 2 days of incubation, the tidal cycles significantly increased the microbial biomass in the sediments, which accelerated the assimilation of IP by microorganisms and the transformation of IP into OP in their bodies, thereby causing a significant increase in OP in the sediments. After 2 days of incubation, the mineralization and immobilization of OP entered the adaptation stage and gradually reached the dynamic balance of various phosphorus fractions in sediments. The path analysis showed that microbial biomass phosphorus and carbon (MBP and MBC) and the activity of alkaline phosphatase (AKP) are the direct factors influencing OP mineralization in sediments. The indirect effects of soil organic matter (SOM), water content (WC), exchangeable aluminum (Al0), and exchangeable iron (Fe0) on net phosphorus mineralization rate were relatively high. In the laboratory experiments, we showed that the recovery of tidal cycles caused a fluctuation in OP mineralization and immobilization in sediments, which reached equilibrium over time. The tidal effect was conducive to maintaining the stability of phosphorus in sediments of coastal salt marshes. The findings of this study can contribute to protecting water quality and improving primary productivity in coastal salt marshes by regulating the tidal cycles and the key environmental factors (e.g., SOM, WC, Al0, and Fe0) influencing OP mineralization.

Nitrate addition stimulates microbial decomposition of organic matter in salt marsh sediments

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.

Mesocosm and Microcosm Experiments On the Feeding of Temperate Salt Marsh Foraminifera

AbstractAgglutinated foraminifera dominate in temperate salt marsh sediment, making them key indicators for monitoring sea level and environmental changes. Little is known about the biology of these benthic foraminifera because of difficulties in distinguishing live from dead specimens in laboratory cultures. We present data from 10 years of laboratory experiments using comparisons of the agglutinant trochamminids Trochammina inflata and Entzia macrescens and the miliolid Miliammina fusca with the calcareous rotalids Helenina anderseni and Elphidium williamsoni. Specimens were taken from a laboratory mesocosm representing Chezzetcook Inlet, a cool-temperate salt marsh in eastern Canada. We determined culture requirements for the agglutinated foraminifera in Petri dishes over 10–12 week periods. Five inexpensive, non-terminal ways of identifying live organisms were developed: spatial movement, detritus-gathering, attachment, clustering, and test opacity. Comparison with rose Bengal staining showed <10% diversion for calcareous species and T. inflata but M. fusca was over-counted by >30%. Terminal chambers of Trochammina inflata were examined by transmission electron microscopy to visualise food consumption and identify food in digestive vacuoles, both in specimens from mesocosm and in culture. Bacteria and unidentified detritus in the vacuoles establish that this agglutinated species is a saprophagous and bacterivorous detritivore. The adhesive secretions by these species apparently help them gather and possibly farm food while being relatively immobile in the sediments. Our observations of movement and feeding orientation in the agglutinants suggest links between form and function that underscore their value as ultra high resolution sea-level proxies. Mesocosm biomass and abundance counts show that foraminifera represent >50% of the meiofaunal biomass, emphasising their importance in the food web and energy-flow dynamics of temperate salt marsh systems.

Vegetation dynamics during the last 7500 years on the extreme southern Brazilian coastal plain

For the first time in the extreme southern Brazilian coastal plain, we analyzed palynomorphs in a continuous sequence, using an outcrop profile of relict muds in Hermenegildo Beach (33°42′12″S, 53°18′57″W), Santa Vitória do Palmar, Rio Grande do Sul state. Five radiocarbon dates were obtained. The results revealed the plant succession in an ancient water reservoir near the coastal line. 7500–4000 BP: Marine organisms and freshwater taxa, with herbaceous marsh at the margins. Marine taxa were markedly present from 6000 to 5000 BP, the time of the maximum transgression. Regionally, the vegetation was dominated by grasslands and forest groupings. 4000–2000 BP: Regressive marine phase, with an increased record of freshwater organisms due to increased rainfall. A large increase in herbaceous marsh components resulted in a gradual filling-in of the water reservoir and the development of regional vegetation. 2000–1760 BP: With the end of the marine regression, progressive desalinization promoted the local herbaceous marsh development. Regionally, grassland and forest also developed, indicative of climate amelioration. Local plant succession ceased during this phase of herbaceous marsh ca. 1760 BP, when beach sands covered the sedimentary sequence. This study reveals the phases of a hydrosere in a coastal water reservoir, influenced by marine oscillations, with gradual desalinization and filling-in that culminated in an herbaceous marsh deposit. These results provide a basis for understanding many other similar relict muds spread throughout the coastal plain in the extreme south of Brazil.

BIOACCUMALATION AND BIOMEGNEFICATION STUDY OF THE PLANTS OF AL-CHIBAYISH MARSH, SOUTHERN IRAQ

An aggregate of 30 samples (water, sediments, and plants) was collected from Al-Chibayish Marsh, located in Dhi-Qar Governorate southern of Iraq to investigate the bioaccumulation and biomagnification in marsh plants (flora) and to assess the marsh plants pollution condition. The study was conducted by testing the macro elements, microelements, heavy metals, and organic compositions in water, sediments, and plants. Plant analyses revealed that the Salvinia natans plant species had the highest concentrations of macroelements Mg, Ca, Na, P, and N compared with other marsh plants and sediments. As a result, the cation binds itself to be more than one charged cationic site and this behaviour was observed in Salvinia natans sp. which has the highest organic composition (protein, fibre, oil, ash, carbohydrate) reaching 30% compared with marsh plants species. Due to the leaching effect it appeared to be more involved in the dissolution of the minerals and in the binding process, since the percentage of the heavy metals (Fe, Cu, Zn, Mn, and Mo) concentrations was reached to 51%. In our results, we found that the microelements tend to be the highest percentage (68%) in the Ceratophyllum sp. compared to other plants Sp. and sediments. Accumulation of heavy metals was highest in Vulgaris (10%), and there was 5%, 9%, 4%, 4%, 5%, and 8% in Salvinia natans, ceratophyllum, Schoenoplectus litoralis, Typha australis leaves, Typha australis roots and sediments, respectively. The study also shows that the mean concentrations of microelements and heavy metals were in the order of Fe> Mn> As> Se> Pb> Cu> Zn> Mo> Ni> Cr> Cd> Li> Co. The concentrations of microelements and heavy metals in plant samples were much greater than the concentrations in water samples of Al-Chibayish Marsh (these water samples were taken from the same locations of plant samples). This is a clear indication of bioaccumulation and biomagnifications of micro elements and heavy metals in plant tissues.

The Sedimentary Carbon-Sulfur-Iron Interplay – A Lesson From East Anglian Salt Marsh Sediments

We explore the dynamics of the subsurface sulfur, iron and carbon cycles in salt marsh sediments from East Anglia, United Kingdom. We report measurements of pore fluid and sediment geochemistry, coupled with results from laboratory sediment incubation experiments, and develop a conceptual model to describe the influence of bioturbation on subsurface redox cycling. In the studied sediments the subsurface environment falls into two broadly defined geochemical patterns – iron-rich sediments or sulfide-rich sediments. Within each sediment type nearly identical pore fluid and solid phase geochemistry (in terms of concentrations of iron, sulfate, sulfide, dissolved inorganic carbon (DIC), and the sulfur and oxygen isotope compositions of sulfate) are observed in sediments that are hundreds of kilometers apart. Strictly iron-rich and strictly sulfide-rich sediments, despite their substantive geochemical differences, are observed within spatial distances of less than five meters. We suggest that this bistable system results from a series of feedback reactions that determine ultimately whether sediments will be sulfide-rich or iron-rich. We suggest that an oxidative cycle in the iron-rich sediment, driven by bioirrigation, allows rapid oxidation of organic matter, and that this irrigation impacts the sediment below the immediate physical depth of bioturbation. This oxidative cycle yields iron-rich sediments with low total organic carbon, dominated by microbial iron reduction and no methane production. In the absence of bioirrigation, sediments in the salt marsh become sulfide-rich with high methane concentrations. Our results suggest that the impact of bioirrigation not only drives recycling of sedimentary material but plays a key role in sedimentary interactions among iron, sulfur and carbon.

Oiling of the continental shelf and coastal marshes over eight years after the 2010 Deepwater Horizon oil spill

We measured the temporal and spatial trajectory of oiling from the April, 2010, Deepwater Horizon oil spill in water from Louisiana's continental shelf, the estuarine waters of Barataria Bay, and in coastal marsh sediments. The concentrations of 28 target alkanes and 43 target polycyclic aromatic hydrocarbons were determined in water samples collected on 10 offshore cruises, in 19 water samples collected monthly one km offshore at 13 inshore stations in 2010 and 2013, and in 16–60 surficial marsh sediment samples collected on each of 26 trips. The concentration of total aromatics in offshore waters peaked in late summer, 2010, at 100 times above the May, 2010 values, which were already slightly contaminated. There were no differences in surface or bottom water samples. The concentration of total aromatics declined at a rate of 73% y−1 to 1/1000th of the May 2010 values by summer 2016. The concentrations inside the estuary were proportional to those one km offshore, but were 10–30% lower. The oil concentrations in sediments were initially different at 1 and 10 m distance into the marsh, but became equal after 2 years. Thus, the distinction between oiled and unoiled sites became blurred, if not non-existent then, and oiling had spread over an area wider than was visible initially. The concentrations of oil in sediments were 100–1000 times above the May 2010 values, and dropped to 10 times higher after 8 years, thereafter, demonstrating a long-term contamination by oil or oil residues that will remain for decades. The chemical signature of the oil residues offshore compared to in the marsh reflects the more aerobic offshore conditions and water-soluble tendencies of the dissolved components, whereas the anaerobic marsh sediments will retain the heavier molecular components for a long time, and have a consequential effect on the ecosystems.

Nitrogen along the Hydrological Gradient of Marsh Sediments in a Subtropical Estuary: Pools, Processes, and Fluxes.

Knowledge on the distribution of nitrogen (N) pools, processes, and fluxes along hydrological gradients provides a comprehensive perspective to understand the underlying causal mechanisms in intertidal flats, and thus improve predictions and climate adaptation strategies. We used a space-for-time substitution method to quantify N pools, processes, and fluxes along a hydrological gradient. Further, we linked N pools and processes and investigated not only surface but also subsurface sediments. Our results showed a gradual decrease in total N (TN) and mineralization rates (PNmin), but an increase in potential rates of nitrification (PNR) and denitrification (PDNR) under an elevated hydrological gradient, except for TN and PNmin in the subsurface sediment, which accumulated on the interaction zone between the high and middle tidal flats. Most sedimentary ammonium N (NH4+) and nitrate N (NO3−) concentrations were similar; however, NH4+ accumulated on the subsurface of the middle tidal flat. NO3− fluxes (from −0.54 to −0.35 mmol m−2 h−1) were uptake fluxes in the intertidal flats, but NH4+ fluxes (−2.48–3.54 mmol m−2 h−1) changed from uptake to efflux in the seaward direction. Structural equation modeling of the effects of inundation frequency, underground biomass, total carbon (TC), electrical conductivity (EC), and clay proportion on the N processes revealed that these accounted for 67%, 82%, and 17% of the variance of PDNR, PNmin, and PNR, respectively. Inundation frequency, underground biomass, TC, EC, and PNmin effects on N pools accounted for 53%, 69%, and 98% of the variance of NH4+, NO3−, and TN, respectively. This suggests that future sea level rise may decrease N storage due to increase in coupled nitrification–denitrification and decrease in N mineralization, and the NH4+ flux may change from sink to source in intertidal ecosystems.

ĐẶC ĐIỂM ĐỊA MẠO VÀ TAI BIẾN TỰ NHIÊN LIẾN QUAN Ở VÙNG CỬA SÔNG VEN BIỂN SÔNG HƯƠNG

The geomorphological features and related geological hazards of the estuarine and coastal area of the Huong river are concluded from latest synthetic data. The estuarine terrain of Huong river was the result of interaction amongst neo and modern tectonic activities with exogenic dynamic factors as marine, marine - fluvial and fluvial. The estuary of Huong river is placed on the northwest - southeast neo - modern tectonic Quang Dien - Phu Vang depressed plate which has 3 local depressed blocks where then were filled by marine, marine - fluvial and fluvial sediment. The marine sediment was mostly distributed on the northwestern and southeastern blocks while the central block was mostly filled by marine - fluvial sediment. The terrain of coastal and estuarine area of Huong river had 18 types, they have changed dramatically within spatial and sequential dimension for those periods: Pliocene - early Pleistocene, middle - late Pleistocene, early - middle Holocene and late - modern Holocene. During Pliocene - early Pleistocene and middle-late Pleistocene, weak uplifting movement to the north of the area had created the denudation surfaces, erosion and abrasion terraces with heights over 20 m, whereas the depressed area to the east had been filled by several hundred meters of fluvial and fluvial - marine sediment. In the early - middle Holocene, depressing movement and retrogradation caused the marine and fluvial - marine sediment accumulated rapidly up to tens of meters. In the last phase of late-modern Holocene, weak depressing combined with moderate uplifting movement happenned, hence the accumulation of fluvial, fluvial - marine - marsh sediments. The shoreline of studied area had changed dramatically in the modern Holocene. In the early-middle Holocene, the shoreline was about 20 km inland of Quang Dien - Inner Citadel area and about 15 km of Phu Vang area. The strong morphodynamic process occured and caused erosion - accretion of the coastal estuarine of Huong river, and changed the modern shoreline locations. At Thuan An inlet, erosion happened by the foot of coastal dunes at the rate of 6.4 m annually; but there was only accretion in Hai Duong coast. The bank of Huong river was also eroded, especially from Tuan junction to Tam Giang lagoon, and the most seriously erosion locations area La Khe commune, Bang Lang - Thuy Bang, Long Ho - Xuoc Du, Thien Mu pagoda and Dia Linh - Huong Vinh.

Remote impacts of typhoons on the hydrodynamics, sediment transport and bed stability of an intertidal wetland in the Yangtze Delta

Intertidal wetlands are widely considered to be vulnerable to storm impacts. While much attention has been given to the effect of hurricane/typhoon landfall on local erosion/deposition in intertidal wetlands, less is known about how distant hurricanes/typhoons can affect an intertidal wetland or about the comprehensive effects of hurricanes/typhoons on hydrodynamics, sediment transport and seabed stability. Here, we investigate the possible impacts of thirteen typhoons that occurred in the western Pacific during summer–autumn 2016 on an intertidal wetland in the Yangtze Delta. We detected the impacts from eight of the typhoons during half of their duration when these typhoons were 450–2000 km away (nearest distance ever for each typhoon) from the delta. These remote typhoons caused 2–5 times maximum increases in hydrodynamics and suspended sediment concentration. Correspondingly, considerable net mudflat erosion and marsh deposition were observed. We therefore conclude that intertidal wetlands can be strongly affected by typhoons, even those passing by hundreds of kilometres away. These findings contribute to understand how far and in what extent hurricanes/typhoons can affect intertidal wetlands and to increase knowledge needed for coastal management.