Tidal Salt Marshes Research Articles

Effects of Drying-Rewetting Frequency on Vertical and Lateral Loss of Soil Organic Carbon in a Tidal Salt Marsh

Tidal salt marshes, as “blue carbon” ecosystems, play a critical role in mitigation of global climate change since their large soil organic carbon (SOC) pool. Drying-rewetting cycles induced by periodic tides have profound influence on soil carbon cycling in tidal salt marshes. However, the magnitude and mechaanism of the effects of drying-rewetting frequency on SOC loss in tidal salt marshes is still uncertain. Here, we conducted a mesocosm experiment to identify how drying-rewetting frequency changes alter the vertical (CO2 and CH4) and lateral (dissolved organic carbon) carbon losses of soils in a tidal salt marsh in the Yellow River Delta (YRD). We found that increasing soil moisture inhibited CO2 emission but stimulated CH4 emission in a tidal salt marsh. Soil dissolved organic carbon (DOC) was produced in the drying phase and rewetting lead to the loss of DOC. Soil moisture and salinity change induced by drying-rewetting cycles were the critical factors controlling vertical organic carbon loss in a tidal salt marsh. DOC had significant effects on CO2 emissions. Changes of tidal action and drying-rewetting cycle induced by global change can affect the pathway of carbon loss in a tidal salt marsh.

Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem.

Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.

Characteristics of organic matter sources from Guadiana Estuary salt marsh sediments (SW Iberian Peninsula)

Estuaries are dynamic interfaces between land, rivers and the ocean that play major roles in the global carbon cycle. These coastal wetlands store huge amounts of organic carbon (OC), commonly known as “blue carbon” and excellent places to study C cycling. The Guadiana river estuary is among the most important tidal salt marshes in the South - Iberian coastal margin. Here, a detailed organic geochemical study is described that includes the identification of sedimentary OM composition at a molecular and isotopic level. Total organic carbon content (TOC) of core sediments ranged from 0.39 to 2.23% and stable carbon isotope composition (δ13C) also showed a wide range between −22.4 and −27.0‰. A13C depletion trend observed from the surface to bottom in the core sediments profiles, reflects loss of labile biogenic 13C enriched compounds i.e. polysaccharides and a selective preservation of more depleted compounds with depth i.e. lignin and lipids in the core sediments. Series of n-alkanes were found in the range from C10 to C31. Carbon preference index ratio (CPI) calculated for long-chain n-alkanes (C24–C31) that ranged between 1.17 and 1.94 reflecting diverse OM inputs to the sediments. A study of the lignin-derived phenolic composition pointed to a recalcitrant OM derived from both gymnosperm and angiosperm plants. Moreover, high abundance of vinyl phenol and vinyl guaiacol points to a dominant contribution of lignins from grasses and aquatic macrophytes to the sediments. A well-resolved series of long-chain linear alkyl benzenes (LABs from C4 to C22) were recorded in all core sediment samples indicating direct discharge of untreated domestic and/or industrial effluents to the estuary. Polycyclic aromatic hydrocarbons (PAHs) with more than 3 rings reflects the pyrogenic origin of a portion of the sedimentary OM. This study highlights the importance of different OM sources to the lower Guadiana estuarine sediments and contributes to a better knowledge about its origin, dynamics and fate.

Watershed and ocean controls of salt marsh extent and resilience

AbstractThe formation and evolution of tidal platforms are controlled by the feedbacks between hydrodynamics, geomorphology, vegetation, and sediment transport. Previous work mainly addresses dynamics at the scale of individual marsh platforms. Here, we develop a process‐based model to investigate salt marsh depositional/erosional dynamics and resilience to environmental change at the scale of tidal basins. We evaluate how inputs of water and sediment from river and ocean sources interact, how losses of sediment to the ocean depend on this interaction, and how erosional/depositional dynamics are coupled to these exchanges. Model experiments consider a wide range of watershed, basin, and oceanic characteristics, represented by river discharge and suspended sediment concentration, basin dimensions, tidal range, and ocean sediment concentration. In some scenarios, the vertical accretion of a tidal flat can be greater than the rate of sea level rise. Under these conditions, vertical depositional dynamics can lead to transitions between tidal flat and salt marsh equilibrium states. This type of transition occurs much more rapidly than transitions occurring through horizontal marsh expansion or retreat. In addition, our analyses reveal that river inputs can affect the existence and extent of marsh/tidal flat equilibria by both directly providing suspended sediment (favoring marshes) and by modulating water exchanges with the ocean, thereby indirectly affecting the ocean sediment input to the system (favoring either marshes or tidal flats depending on the ratio of the river and ocean water inputs and their sediment concentrations). The model proposed has the goal of clarifying the roles of the main dynamic processes at play, rather than of predicting the evolution of a particular tidal system. Our model results most directly reflect micro‐ and meso‐tidal environments but also have implications for macro‐tidal settings. The model‐based analyses presented extend our theoretical understanding of marsh dynamics to a greater range of intertidal environments. © 2020 John Wiley & Sons, Ltd.

Saltwater intrusion affects nutrient concentrations in soil porewater and surface waters of coastal habitats

AbstractCoastal ecosystems are undergoing major biogeochemical shifts due to climate change and sea level rise. At the same time, agricultural fertilizer applications are increasing coastal nutrient inputs. In this study, we examine the potential impact of saltwater intrusion (SWI) on nitrogen (N) and phosphorus (P) concentrations in soil porewater and surface water of different habitats within the Chesapeake Bay estuary. Study sites are located along Maryland's Lower Eastern Shore and were monitored over three summers from 2016 to 2018. These sites encompass various habitats on the land–sea interface, consisting of healthy forests, intruded forests, abandoned fields, intruded fields, agricultural ditches, tidal creeks, and tidal salt marshes. Intruded fields were being actively farmed at the time of the study. Soil porewater and surface water grab samples were collected from the habitats and analyzed for electrical conductivity, pH, dissolved organic P (DOP), dissolved inorganic P in the form of soluble reactive P (SRP), dissolved inorganic N as ammonium‐N (NH4‐N), and total dissolved iron (TDFe). Electrical conductivity was greatest in the marshes (16.58 mS/cm averaged across all years) and did not significantly differ among intruded forests, intruded fields, and agricultural ditches. As a legacy of heavy fertilizer use, DOP concentrations exceeded 0.45 mg P/L in all habitats. Concentrations of inorganic N and P differed significantly by habitat. Concentrations of NH4‐N were significantly higher in salt marsh soil porewater than in any other habitat measured. Overall, SRP concentrations were the highest in the soil porewater of intruded fields and marshes and in the surface water of agricultural ditches (0.30, 0.29, and 0.33 mg P/L averaged across all years, respectively). These concentrations greatly exceeded recommended U.S. Environmental Protection Agency nutrient pollution thresholds for the region. In its oxidized state, dissolved iron can bind to SRP and prevent it from becoming bioavailable. However, TDFe concentrations in the ditches, tidal creeks, and marshes were too low to adequately buffer against SRP loss to downstream areas. As SWI moves salts inland and increases hydrologic connectivity across coastal landscapes, it is important to consider the mechanisms through which nutrients may be released from coastal soils and their potential to impact downstream water quality.

Field-based decadal wave attenuating capacity of combined tidal flats and salt marshes

Foreshores consisting of both bare tidal flats and vegetated salt marshes are found worldwide and they are well studied for their wave attenuating capacity. However, most studies only focus on the small scale: just some isolated locations in space and only up to several years in time. In order to stimulate the implementation of foreshores serving as reliable coastal defense on a large scale, we need to quantify the decadal wave attenuating capacity of the foreshore on the scale of an estuary. To study this, a unique bathymetrical dataset is analyzed, covering the geometry of the Westerschelde estuary (The Netherlands) over a time-span of 65 years. From this dataset, six study sites were extracted (both sheltered sites and exposed sites to the prevailing wind direction) and divided into transects. This resulted in 36 transects covering the entire foreshore (composed of the bare tidal flat and the vegetated salt marsh). The wave attenuation of all transects under daily conditions (with and without vegetation) and design conditions (i.e. events statistically occurring once every 10,000 years) was modelled.Overall, the spatial variability of the geometry of a single foreshore was observed to be much larger than the temporal variability. Temporal changes in salt marsh width did not follow the variability of the entire foreshore. Both under daily and design conditions, vegetation contributes to decreasing wave energy and decreases the variability of incoming wave energy, thereby decreasing the wave load on the dike. The southern foreshores, sheltered from the prevailing wind direction, were more efficient in wave attenuation than the exposed northern foreshores. A linear relation between marsh width and wave attenuation over a period of 65 years was observed at all marshes. The present study provides insights needed to calculate the length of a salt marsh to obtain a desired minimum wave attenuating capacity.

Seasonal Variation in Sediment Delivery Across the Bay‐Marsh Interface of an Estuarine Salt Marsh

AbstractSediment transport across bay‐marsh interfaces depends on wave energy, vegetation, and marsh‐edge morphology and varies over a range of timescales. We investigated these dynamics in a tidal salt marsh with a gently sloped, vegetated edge adjacent to northern San Francisco Bay. Spartina foliosa (cordgrass) inhabits the lower marsh and Salicornia pacifica (pickleweed) predominates on the marsh plain. We measured suspended‐sediment concentration (SSC) and hydrodynamics in bay shallows and along a 100‐m cross‐shore transect in the marsh, during winter and summer. Four‐year averaged accretion measured with marker‐horizon plots was twice as great along the marsh transect as adjacent to a tidal creek, 50 m from the bay. We estimated deposition and trapping efficiency from the time series data to assess its variation with season and wave energy. At high tide the transition zone (between cordgrass and pickleweed) was usually erosional, the pickleweed zone was depositional, and both erosion and deposition increased with wave energy, as did the landward position of maximum deposition. Erosion from the transition zone accounted for approximately one third of the sediment flux into the pickleweed. In the pickleweed zone, SSC, the difference between flood‐ and ebb‐tide SSC, and trapping efficiency were greater in summer than winter for comparable wave conditions, which we attribute to increased sediment trapping by dense summer cordgrass. Moderate waves in summer (46%) accounted for more annual accretion in the pickleweed zone than larger waves in winter (28%), although the contribution of winter storms was diminished by the dry winter during the study.

Are Tidal Salt Marshes Exposed to Nutrient Pollution more Vulnerable to Sea Level Rise?

Over the past four decades, Long Island, NY, USA, has lost coastal wetlands at a rate of 4% per decade due to submergence. In this study, we examined relationships between the rate of tidal salt marsh loss and environmental factors, including marsh elevation, tidal range, and wastewater exposure through analysis of stable isotope ratios of marsh soils and biota. Our goal was to identify factors that increase vulnerability of marshes to sea level rise, with a specific emphasis on the potential role of poor water quality in hastening marsh loss. Our results suggest that wastewater exposure may accelerate loss of intertidal marsh, but does not negatively impact high tidal marsh resilience to sea level rise. And while marsh elevation and tidal range were statistically significant predictors of marsh loss, they similarly displayed opposite relationships among marsh zones. This study suggests that different functional zones of coastal salt marshes may not respond similarly to global change factors, and that elevation may be an important factor mediating eutrophication effects to coastal salt marshes.

Nitrogen addition differently alters GHGs production and soil microbial community of tidal salt marsh soil depending on the types of halophyte

Temperate tidal salt marshes are an important carbon sink. They store 44.7 Tg C and only emit low amounts of greenhouse gases. Although recent studies have suggested that environmental changes such as eutrophication and plant invasion may alter their methane and nitrous oxide emissions, the exact consequences and dynamics are not yet fully understood. We hypothesize that the greenhouse gas emission responses from nitrogen addition differs depending on halophyte types. In this study, we conducted soil incubation experiments with different halophyte and nitrogen amendments. We found that nitrogen addition significantly enhanced methane emissions from bare tidal flats (45%) and Suaeda japonica marshes (3500%), which normally have low plant biomass nitrogen input. The main contributors to this change were the increased abundance of Gram-positive bacteria and decreased relative abundance of sulfate to methanogen reducers (dsrA/mcrA). These results suggest that the effects of nitrogen addition on methane emission stimulation may be substantial for specific types of tidal marshes. They further suggests that improper nitrogen input management and the consequent eutrophication may cause a dramatic rise in methane emissions.

Microbial mechanism for enhanced methane emission in deep soil layer of Phragmites-introduced tidal marsh

The introduction of Phragmites australis is known to substantially increase methane emission in the tidal salt marsh. Previous studies suggested that enhanced carbon input by the introduction may stimulate methanogenic activity. However, the exact mechanisms and the effects of the introduction of P. australis to methane dynamics in the deep soil layer are still unclear. In this study we collected 1 m deep intact soil cores and gas samples at native Suaeda japonica- and P. australis-vegetated temperate tidal salt marshes in Suncheon Bay, Republic of Korea. Rates of methane emission and vertical distribution of soil biogeochemistry and microbial communities were analyzed to understand the relationship among chemical and microbiological properties. The introduction of P. australis significantly enhanced methane emission in sites, which was caused by increased DOC and reduced competitive inhibition between sulfate reducer and methanogens. In particular, reduced competitive inhibition between sulfate reducers and methanogens in deep soil layer may play a substantial role in the enhanced methane emission by the introduction of P. australis. Potential methane production was also significantly higher in deeper soil layers than the surface soil layer. We suggest that deep soil layer plays a critical role in the methane dynamics of tidal salt marsh which is introduced by deep root plants, such as P. australis.

Adjusting Emergent Herbaceous Wetland Elevation with Object-Based Image Analysis, Random Forest and the 2016 NLCD

Emergent herbaceous wetlands are characterized by complex salt marsh ecosystems that play a key role in diverse coastal processes including carbon storage, nutrient cycling, flood attenuation and shoreline protection. Surface elevation characterization and spatiotemporal distribution of these ecosystems are commonly obtained from LiDAR measurements as this low-cost airborne technique has a wide range of applicability and usefulness in coastal environments. LiDAR techniques, despite significant advantages, show poor performance in generation of digital elevation models (DEMs) in tidal salt marshes due to large vertical errors. In this study, we present a methodology to (i) update emergent herbaceous wetlands (i.e., the ones delineated in the 2016 National Land Cover Database) to present-day conditions; and (ii) automate salt marsh elevation correction in estuarine systems. We integrate object-based image analysis and random forest technique with surface reflectance Landsat imagery to map three emergent U.S. wetlands in Weeks Bay, Alabama, Savannah Estuary, Georgia and Fire Island, New York. Conducting a hyperparameter tuning of random forest and following a hierarchical approach with three nomenclature levels for land cover classification, we are able to better map wetlands and improve overall accuracies in Weeks Bay (0.91), Savannah Estuary (0.97) and Fire Island (0.95). We then develop a tool in ArcGIS to automate salt marsh elevation correction. We use this ‘DEM-correction’ tool to modify an existing DEM (model input) with the calculated elevation correction over salt marsh regions. Our method and tool are validated with real-time kinematic elevation data and helps correct overestimated salt marsh elevation up to 0.50 m in the studied estuaries. The proposed tool can be easily adapted to different vegetation species in wetlands, and thus help provide accurate DEMs for flood inundation mapping in estuarine systems.

Meteorological controls on evapotranspiration over a coastal salt marsh ecosystem under tidal influence

Elucidating evapotranspiration (ET) patterns and drivers is of crucial importance for better understanding water and energy cycles. However, multiple controls on ET at coastal ecosystems that are subject to subdaily tidal flooding have not yet received attention. In this study, we investigated the response of ET to meteorological variables including photosynthetically active radiation (PAR), air temperature (Ta), vapour pressure deficit (VPD), and wind speed (WS) under semidiurnal tidal influence from hourly to seasonal timescales, on the basis of 3-year eddy-covariance (EC) measurements over a tidal salt marsh ecosystem of the Yangtze Delta. Our results show that, as with most terrestrial ecosystems, PAR is a major control on ET in this coastal salt marsh ecosystem at hourly, diel, and multiday timescales. However, the semidiurnal tides co-control ET with meteorological variables, forming a complex ET pattern at the hourly and subdaily scales. In the daytime, ET was primarily driven by PAR and VPD, whereas during the night, WS and friction velocity dominated the ET variability. We also found that wind direction fundamentally changed the nature of the interactions of ET with other variables. Moreover, tidal inundation suppressed ET and changed its sensitivities to PAR, Ta, VPD, and WS, a process that was especially obvious when offshore winds prevailed. The tidal flooding can affect ET directly through altering surface energy partitioning or plant metabolic activity, and indirectly through influencing meteorological conditions such as Ta and VPD. By explicitly considering the influence of tidal dynamics, this study revealed the significance in quantifying the ET contributions in coastal ecosystems to global water, carbon and energy cycles.

Five Decades of Wetland Soil Development of a Constructed Tidal Salt Marsh, North Carolina, USA

Five Decades of Wetland Soil Development of a Constructed Tidal Salt Marsh, North Carolina, USA

Salt Marsh Migration into Lawns Revealed by a Novel Sediment-Based Approach

Upslope migration will be critical to the future survival and stability of tidal salt marshes in an age of accelerated sea-level rise, yet only a small number of studies have measured historic migration rates. We developed a new approach for reconstructing salt marsh migration that can be used even in situations where that migration would be undetectable by aerial photograph analysis, such as the growth of marsh plants under trees or the movement of marsh ecosystems into mowed lawns. Our approach involves identifying a wedge of salt marsh peat (overlying the pre-existing upland soil) through the presence of foraminifera and then dating that wedge using 137Cs. We demonstrate our approach by calculating migration rates (1963–2016) for two lawn-adjacent salt marshes along the Connecticut coast of Long Island Sound. Both marshes showed substantial migration over this time period, although vertical migration rates differed dramatically between the two sites, perhaps because of the differential influence of large storms. Further, we demonstrate that the presence of foraminifera (an unequivocal signal of tidal inundation) constitutes a more accurate indicator of the migration wedge than high organic content. Our relatively inexpensive approach could be used to assess migration rates at multiple sites in order to better understand the factors controlling marsh migration.

Experimental influence of storm-surge salinity on soil greenhouse gas emissions from a tidal salt marsh

Storm surges can substantially alter the water level and salinity in tidal salt marshes. Little is known about how changes experienced during storm surges affect greenhouse gas emissions (GHG; CO2, CH4, N2O) from tidal salt marsh soils. Understanding how storm surges influence ecosystem processes is critical for evaluating the ecosystem's sensitivity to sea level rise. To explore how hurricane-induced changes in salinity affect GHG emissions, we exposed intact soil mesocosms (0–9 cm depth) from a Mid-Atlantic temperate salt marsh to pulse changes in salinity experienced at the site before, during, and after Hurricane Joaquin in 2015. Soil temperature, oxygen, and water level were kept constant to avoid confounding effects throughout the experiment. Automated measurements (hourly resolution) of soil GHG emissions were recorded in control (i.e., no salinity changes) and treatment mesocosms, and combined with soil pore water chemistry (i.e., SO42−, S2−, Fe2+, TNb, redox potential, pH) to characterize the biogeochemical responses. Using mixed effects models, we found that the role of different biogeochemical processes, such as sulfur cycling, changed throughout the experiment, underscoring the complex nature of GHG emissions in tidal salt marsh soils. Overall, soils subjected to a salinity decrease had greater GHG emissions than control soils, which were maintained at 17 ppt. The treatment soils had a 24% and 23% increase in global warming potential (20- and 100-year scenarios, respectively) indicating that storm surges can produce pulses of GHG emissions. However, both CH4 and N2O emissions returned to baseline values (following hysteresis responses) when initial conditions were reestablished. The results support the fact that tidal salt marshes are resilient ecosystems, as soil GHG emissions recovered relatively quickly from the pulse event.

The limited role salt marshes may have in buffering extreme storm surge events: Case study on the New Jersey shore

The hybrid hurricane and storm event dubbed “SuperStorm Sandy” of October 2012 was the most costly natural disaster to hit the US Atlantic coast, and the second most costly to affect the United States. This extreme weather event presented an excellent opportunity to assess the effectiveness of tidal salt marshes and maritime forests in buffering adjacent development from storm-related damage. To address this question of how well did or under what conditions did coastal ecosystems buffer adjacent human development, we undertook to quantify the incremental monetary value that a hectare or linear extent of fronting salt marsh and/or maritime forest had on protecting the built environment of the back-bay communities in the Barnegat Bay region of New Jersey, USA. Statistical modeling was used to estimate the relationship between the spatial extent and characteristics of fronting coastal wetlands with the various damage metrics derived from the Federal Emergency Management Agency National Flood Insurance Program payout and Preliminary Property Modeling Task Force damage data at the scale of the individual housing unit. While salt marshes may be effective in diminishing wave energies and buffering adjacent development under normal conditions, in the case of SuperStorm Sandy, we find no evidence that New Jersey's extensive coastal marshes significantly buffered and thereby reduced NFIP payouts on the majority of back-bay residential properties. The extreme conditions of this storm event with storm surges upwards of 2 m, effectively flooded these marshes diminishing their protective capacity. The bulk of the residential properties damaged in our study area were located in lagoonal communities that are built directly adjacent to tidal water and are thereby highly vulnerable to storm surge-related flooding whether or not they are buffered by adjacent marshes. The results of this study should not be misconstrued in concluding that salt marshes do not have a positive value in protecting coastal properties; rather, their protective buffering capacity for extreme storm events is limited and that lagoonal-style developments remain highly vulnerable to sea level rise and future storms.

Saltmarsh conservation through inventory, biogeographic analysis and predictions of change: Case of Tasmania, south‐eastern Australia

Abstract Effective conservation of saltmarshes involves detailed and accurate mapping of their range, area of occupancy and plant community composition as part of region‐wide inventories. There is also a need to identify the major mesoscale influences on the distributions of types of saltmarsh, obligate saltmarsh plants and salt pans, and evaluate possible responses to macroclimatic changes. Tidal saltmarshes of Tasmania and its offshore islands (coastline of 4,882 km, spanning latitudes 39°40′ to 43°40′S), off south‐eastern Australia, were mapped at a high spatial resolution of 1 : 500–1 : 3,000. The distributions of types of saltmarsh, obligate plant taxa and salt pans were related to climatic, geomorphic and land‐use variation. The future distribution of climatically controlled species and communities was determined for two climate change scenarios. There was 58.6 km2 of tidal saltmarsh in Tasmania in 61 mesoscale complexes. The complexes were classified into three broad saltmarsh groups, which together offer a more ecologically relevant planning unit for saltmarsh conservation than macroscale bioregions. The small median patch size of 0.2 ha demonstrates the effectiveness of the manual interpretation and mapping with extensive field checking. Mapping of these smaller saltmarshes provides them recognition within the land use planning and approvals process. Climatic, but not geomorphic and land‐use, factors have strong effects on the distribution of saltmarsh plant communities, species and salt pans in Tasmania. Mean annual rainfall was most significant in predicting saltmarsh extent, range, plant community composition, salt pans and the three saltmarsh groups. Mean annual daily minimum temperature and saltmarsh area best predicted obligate plant distributions. Projected wetter and drier climatic change by 2100 is not expected to substantially alter macroscale plant community patterns but may reduce the range of the rare herb Wilsonia humilis R.Br. However, stochastic disturbances may continue to play an important role at local scales.

Spartina alterniflora invasion drastically increases methane production potential by shifting methanogenesis from hydrogenotrophic to methylotrophic pathway in a coastal marsh

Abstract Plant invasion can strongly influence carbon (C) cycling processes, thus it may affect climate change by altering C sequestration and greenhouse gas emissions in the invaded ecosystem. Since 1979, the exotic Spartina alterniflora has rapidly expanded in China’s coastal areas, where significant increase in methane (CH4) emissions has been documented from post‐invaded sites. However, a mechanistic understanding of the structural and functional changes of associated methanogens accompanying this invasion remains elusive. Here we conducted integrated biogeochemical investigations on methanogenic substrates, activity, and diversity to identify implications of S. alterniflora invasion for methanogenesis in coastal wetlands. To do this, we collected and analysed 0–50 cm soil profiles from an uncolonised tidal flat (TF) and salt marshes that S. alterniflora has invaded for 1 year (SA‐1) and 12 years (SA‐12) in Jiangsu, China. Methanogenic community composition was characterised by massive parallel sequencing. The rates and pathways of methanogenesis were determined by adding trace concentrations of 13C‐labelled substrates to anaerobic incubated samples. Our results revealed that 12‐year invasion of S. alterniflora drastically increased CH4 production potential by one order of magnitude over that of TF. This substantial increase was primarily attributed to methanogenesis from trimethylamine; its rates increased by two orders of magnitude over TF whereas those from acetate and H2/CO2 increased far less. Hydrogenotrophic methanogenesis was the dominant pathway operating in the TF, but methanotrophic pathway contributed most to CH4 production in the surface layer of SA‐1 and uppermost 40‐cm layers of SA‐12. Consistent with these observations, the dominant methanogens shifted from obligate hydrogenotrophic Methanococcales in TF to potential methylamine‐utilising Methanosarcinaceaein SA‐12. Our Mantel analysis indicated that ‘non‐competitive’ trimethylamine, derived from cytoplasmic osmolytes of S. alterniflora, was the major driver of this change in methanogenic community composition. Synthesis. Our results suggest that invasive Spartina alternifloraplants gradually facilitated the local dominance of methylotrophic Methanosarcinaceae by changing the key type of methanogenic substrate in coastal marshes. Shifts in methanogen communities and enhanced availability of trimethylamine elevated the rates and importance of methylotrophic methanogenesis, thereby markedly increasing CH4 production potential and emission rates in this type of ecosystem.

Coupling between tidal mudflats and salt marshes affects marsh morphology

It is generally assumed that coastal salt marshes are capable of adapting to moderately fast rising sea levels although local sediment availability crucially affects this capability. While there is an increasing awareness that local sediment availability is inherently related to sediment dynamics on the adjacent tidal mudflat, our current understanding of the interactions between salt marshes and tidal flats is very limited. To address this knowledge gap, we measured suspended sediment concentrations alongside hydrodynamic, morphological and sediment deposition measurements over a total period of 16 weeks in a wave-exposed macro-tidal mudflat-salt marsh system on the UK east coast (Tillingham).Our results show that local sediment supply to the salt marsh is strongly linked to intertidal sediment dynamics and that the vast majority of suspended sediment deposited on the marsh originates from wind-wave induced intertidal sediment resuspension in very close vicinity (<130 m) to the seaward marsh margin. Vertically the salt marsh grows at rates >5 mm yr−1, thereby increasing the slope of the tidal mudflat-salt marsh transition and making the salt marsh susceptible to lateral erosion. Consequently, the marsh edge retreats at a rate of approximately 0.8 m yr−1. Our study shows that the response of coastal salt marshes to climate change is a function of the coupled tidal mudflat-salt marsh system, rather than their vertical sediment accretion rates alone. Therefore, the idea that salt marsh adaptability relies on local sediment supply needs to be expanded, incorporating the morphology and long-term evolution of the adjacent tidal mudflats.

Mapping salt marsh soil properties using imaging spectroscopy

Tidal salt marshes sequester and store blue carbon at both short and long time scales. Marsh soils shape and maintain the ecosystem by supporting complex biogeochemical reactions, deposition of sediment, and accumulation of organic matter. In this study, we examined the potential of imaging spectroscopy techniques to indirectly quantify and map tidal marsh soil properties at a National Estuarine Research Reserve in Georgia, USA. A framework was developed to combine modern digital image processing techniques for marsh soil mapping, including object-based image analysis (OBIA), machine learning modeling, and ensemble analysis. We also evaluated the efficacy of airborne hyperspectral sensors in estimating marsh soil properties compared to spaceborne multispectral sensors, WorldView-2 and QuickBird. The pros and cons of object-based modeling and mapping were assessed and compared with traditional pixel-based mapping methods. The results showed that the designed framework was effective in quantifying and mapping three marsh soil properties using the composite reflectance from salt marsh environment: soil salinity, soil water content, and soil organic matter content. Multispectral sensors were successful in quantifying soil salinity and soil water content but failed to model soil organic matter. The study also demonstrated the value of minimum noise fraction transformation and ensemble analysis techniques for marsh soil mapping. The results suggest that imaging spectroscopy based modeling is a promising tool to quantify and map marsh soil properties at a local scale, and is a potential alternative to traditional soil data acquisition to support carbon cycle research and the conservation and restoration of tidal marshes.