A systematic review of morphological models of salt marshes

Salt marshes are among the most important coastal wetlands and provide critical ecological services,including climate regulation,biodiversity maintenance,and blue carbon sequestration.However,most salt marshes worldwide are shrinking,owing to the effects of natural and human factors,such as climate change and artificial reclamation.Therefore,it is essential to understand the decline in the morphological processes of salt marshes,and accordingly,the likely evolution of these marshes,in order to enable measures to be taken to mitigate this decline.To this end,this study presented an extensive systematic review of the current state of morphological models and their application to salt marshes.The emergence of process-based(PB)and data-driven(DD)models has contributed to the development of morphological models.In morphodynamic simulations in PB models,multiple physical and biological factors(e.g.,the hydrodynamics of water bodies,sediment erosion,sediment deposition,and vegetation type)have been considered.The systematic review revealed that PB models have been extended to a broader interdisciplinary field.Further,most DD models are based on remote sensing database for the prediction of morphological characteristics with latent uncertainty.Compared to DD models,PB models are more transparent but can be complex and require a lot of computational power.Therefore,to make up for the shortcomings of each model,future studies could couple PB with DD models that consider vegetation,microorganisms,and benthic animals together to simulate or predict the biogeomorphology of salt marsh systems.Nevertheless,this review found that there is a lack of unified metrics to evaluate model performance,so it is important to define clear objectives,use multiple metrics,compare multiple models,incorporate uncertainty,and involve experts in the field to provide guidance in the further study.

Soil microbial communities regulate the threshold effect of salinity stress on SOM decomposition in coastal salt marshes

Salinity stress is one of the critical environmental drivers of soil organic matter(SOM)decomposition in coastal ecosystems.Although the temperature sensitivity(Q_(10))of SOM decomposition has been widely applied in Earth system models to forecast carbon processes,the impact of salinity on SOM decomposition by restructuring microbial communities remains uncovered.Here,we conducted a microcosm experiment with soils collected from the coastal salt marsh in the Yellow River Estuary,which is subjected to strong dynamics of salinity due to both tidal flooding and drainage.By setting a gradient of salt solutions,soil salinity was adjusted to simulate salinity stress and soil carbon emission(CO_(2))rate was measured over the period.Results showed that as salinity increased,the estimated decomposition constants based on first-order kinetics gradually decreased at different temperatures.Below the 20‰salinity treatments,which doubled the soil salinity,Q_(10)increased with increasing salinity;but higher salinity constrained the temperature-related response of SOM decomposition by inhibiting microbial growth and carbon metabolisms.Soil bacteria were more sensitive to salinity stress than fungi,which can be inferred from the response of microbial beta-diversity to changing salinity.Among them,the phylotypes assigned to Gammaproteobacteria and Bacilli showed higher salt tolerance,whereas taxa affiliated with Alphaproteobacteria and Bacteroidota were more easily inhibited by the salinity stress.Several fungal taxa belonging to Ascomycota had higher adaptability to the stress.As the substrate was consumed with the incubation,bacterial competition intensified,but the fungal co-occurrence pattern changed weakly during decomposition.Collectively,these findings revealed the threshold effect of salinity on SOM decomposition in coastal salt marshes and emphasized that salt stress plays a key role in carbon sequestration by regulating microbial keystone taxa,metabolisms,and interactions.

Spectral characteristics of plant communities from salt marshes:A case study from Chongming Dongtan,Yangtze estuary,China

The spectral reflectance of recently formed salt marshes at the mouth of the Yangtze River,which are undergoing invasion by Spartina alterniflora,were assessed to determine the potential utility of remotely sensed data in assessing future invasion and changes in species composition.Following a review of published research on remote sensing of salt marshes,53 locations along three transects were sampled for paired data on plant species composition and spectral reflectance using a FieldSpecTM Pro JR Field Portable Spectroradiometer.Spectral data were processed concerning reflectance,and the averaged reflectance values for each sample were reanalysed to correspond to a 12-waveband bandset of the Compact Airborne Spectral Imager.The spectral data were summarised using principal components analysis(PCA)and the relationships between the vegetation composition,and the PCA axes of spectral data were examined.The first PCA axis of the reflectance data showed a strong correlation with variability in near infrared reflectance and‘brightness’,while the second axis was correlated with visible reflectance and‘greenness’.Total vegetation cover,vegetation height,and mudflat cover were all significantly related to the first axis.The implications of this in terms of the ability of remote sensing to distinguish the various salt marsh species and in particular the invasive species S.alterniflora were discussed.Major differences in species with various physiognomies could be recognised but problems occurred in separating early colonising S.alterniflora from other species at that stage.Further work using multi-seasonal hyperspectral data might assist in solving these problems.

Response of microbial biomass and bacterial community composition to fertilization in a salt marsh in China

The effects of nitrogen(N) addition on microbial biomass, bacterial abundance, and community composition in sediment colonized by Suaeda heteroptera were examined by chloroform fumigation extraction method, real-time quantitative polymerase chain reaction, and denaturing gradient gel electrophoresis(DGGE) in a salt marsh located in Shuangtai Estuary, China. The sediment samples were collected from plots treated with different amounts of a single N fertilizer(urea supplied at 0.1, 0.2, 0.4 and 0.8 g/kg(nitrogen content in sediment) and different forms of N fertilizers(urea,(NH_4)_2SO_4, and NH_4NO_3, each supplied at 0.2 g/kg(calculated by nitrogen).The fertilizers were applied 1–4 times during the plant-growing season in May, July, August, and September of 2013. Untreated plots were included as a control. The results showed that both the amount and form of N positively influenced microbial biomass carbon, microbial biomass nitrogen, and bacterial abundance. The DGGE profiles revealed that the bacterial community composition was also affected by the amount and form of N.Thus, our findings indicate that short-term N amendment increases microbial biomass and bacterial abundance,and alters the structure of bacterial community.

Characterization of dissolved organic matter in submarine groundwater from a salt marsh in Chongming Island,China

Salt marshes are research hotspots of the carbon cycle in coastal zones because large amounts of atmospheric carbon dioxide is fi xed by salt marshes vegetation and stored in its biomass and soil.Dissolved organic carbon(DOC)in submarine groundwater(well water and pore water)in salt marshes plays an important role in advective exchange between the salt marshes and coastal waters.However,the molecular characteristics of DOC in salt marsh groundwater are poorly understood because of the complex DOC structures and hydrodynamic process.In this study,fl uorescent components and refractory DOC(RDOC)in submarine groundwater from a salt marsh(Chongming Island,China)and adjacent coastal water were characterized by fl uorescence spectroscopy and nuclear magnetic resonance spectroscopy.The fl uorescent components identifi ed by parallel factor analysis indicated that humic-like substances dominated the chromophoric dissolved organic matter in the submarine groundwater.The chromophoric dissolved organic matter and dissolved organic matter in the submarine groundwater had non-conservative behaviors because of additions from terrestrial humic substances.The nuclear magnetic resonance spectra indicated that bioactive substances(carbohydrates)contributed only 13.2%-14.8%of the dissolved organic matter in the submarine groundwater but carboxyl-rich alicyclic molecules(CRAMs),the main components of RDOC,contributed 64.5%of the dissolved organic matter.Carbohydrates and CRAMs contributed 16.4%and 61.7%of the dissolved organic matter in the coastal water,similar to the contributions for submarine groundwater.The DOC concentration in submarine groundwater was 386±294μmol/L,which was signifi cantly higher than that in coastal water(91±19μmol/L).The high DOC concentrations and>60%relative RDOC content suggested that submarine groundwater may be an important source of RDOC to coastal seawater.This information will be helpful for estimating the climate eff ects of salt marsh blue carbon.

Halophyte Vegetation Influences Soil Microbial Community of Coastal Salt Marsh

Coastal wetlands are the most productive ecosystems worldwide and can provide important ecosystem services,yet the characteristics of microbial community within these systems remain poorly understood.Microbial community of salt marsh vegetation and the associated soil physio-chemical properties were investigated in this study.Three typical Suaeda australis,Phragmites australis,Spartina alterniflora wetlands,and non-vegetated bare mudflats in the Zhoushan Islands were studied to advance the understanding of the characteristics of soil bacterial communities in coastal wetlands.Results showed that the bare mudflats exhibited high pH value and soil moisture content compared with the vegetated samples.In different vegetation types,the organic matter content,total nitrogen,and total potassium content decreased in the order:S.alterniflora wetland>P.australis wetland>S.australis wetland,and there was no obvious difference in total phosphorous content.The halophytes could decrease soil salinity compared with bare mudflats.Proteobacteria,Nitrospinae,Bacteroidetes,Acidobacteria,and Nitrospirae were the predominant level across all samples.Functional prediction showed that SPA-covered soil might play vital roles in sulphur cycling,while SUA and PHR covered soils were involved in nitrogen cycling.This study could provide the first insight into the microbial community of this study area and contribute to a better understanding of vegetation microbiota and bioremediation in coastal wetland ecosystem.

Analyses on Temporal and Spatial Variation Characteristics of Erosion and Deposition in the Front Edge of Salt Marsh Wetland

Taking Yancheng Nature Reserve Salt Marsh as the research object,the remote sensing images from 2005 to 2020 were interpreted by using remote sensing and geographic information system technology.In this paper,the temporal and spatial variation characteristics of erosion and deposition in the front edge of salt marsh wetland were analyzed.The influence of sea level rise on the annual change of salt marsh area was analyzed.The characteristics of flow and sediment movement in salt marsh and the causes of erosion and deposition in front of salt marsh were analyzed.The results showed that:(1)During 2005-2007,the sea level was relatively low,and Spartina alterniflora in salt marsh expanded to the sea.Since 2007,the front edge of salt marsh wetland has coexisted with erosion and deposition.From 2008 to 2010,the front edge of salt marsh wetland once again showed a trend of comprehensive deposition to the sea side.From 2010 to 2012,the erosion of salt marsh wetland was serious.From 2012 to 2020,the front edge of salt marsh wetland in the range of 9 km south of Xinyang estuary was eroded.(2)The correlation analysis was carried out between the area of salt marsh wetland and sea level rise.Spartina alterniflora is easily affected by sea level change,owing to it having a low ecological niche.With the rise of sea level,the area of salt marsh has been decreasing since 2013.(3)In the front sea area of salt marsh wetland,the maximum velocity of the ebb and flood can reach the threshold velocity during the spring tide.The sediment starts to move at water depth of 10 m under wave actions.Owing to wave stirs up sediment and current transports the sediment,resuspended sediment causes the erosion of marsh-edge scarps.

Changes in wintering Hooded Cranes and their habitats at Chongming Dongtan over the past 20 years

The Hooded Crane(Grus monacha)is listed as a Vulnerable species in the IUCN red list.Tidal wetland(tideland),the major habitat for wintering Hooded Cranes at East China’s Chongming Dongtan,has dramatically changed in the past two decades,but there is limited knowledge about the population and habitat changes of the Hooded Cranes.This study investigated the population size and distribution of wintering Hooded Cranes at Chongming Dongtan from 2000 to 2021.We used remote sensing images combined with a vegetation classification algorithm to analyse the distribution of saltmarsh vegetation.The quadrat method was used to investigate the density and weight of the underground corms of Sea Bulrush(Scirpus mariquter),the main food on tideland for the Hooded Cranes.From 2000 to 2021,the population number of wintering Hooded Cranes at Chongming Dongtan remained stable at approximately 100.In 2000,the area of Scirpus spp.and Common Reed(Phragmites australis)accounted for approximately half of the total saltmarsh area at Chongming Dongtan,respectively.The invasive Smooth Cordgrass(Spartina alterniflora)rapidly expanded on tideland in the 2000s while the Scirpus spp.was competed out and thus significantly reduced in area.After the implementation of an ecological project to control Smooth Cordgrass and to restore Scirpus spp.in the 2010s,the area of the Smooth Cordgrass decreased considerably while the area of Scirpus spp.increased.The corms of Sea Bulrush decreased on the southeastern tideland during the study period,which might be the cause of the northward movement of the foraging Hooded Cranes on tideland.We also found Hooded Cranes foraged crops in the nearby farmland in mid-winter,causing human-bird conflicts in the recent decade.Our results found that changes in habitat and food conditions on tideland impacted wintering Hooded Cranes.Foraging in farmland with human disturbance in the recent decade might be related to insufficient food on tideland.We suggest active intervention to accelerate the restoration of Sea Bulrush on tideland and reduce human disturbance in farmland to improve the habitat quality of the wintering Hooded Crane at Chongming Dongtan.

A global synthesis of the effectiveness and ecological impacts of management interventions for Spartina species

Invasions by Spartina species pose serious threats to global coastal ecosystems.Although many studies have examined the effectiveness and ecological impacts of invasive Spartina management,no comprehensive global synthesis has been conducted to assess the effects of management on Spartina per se and on wider non-targets.Here,we conducted a global meta-analysis of 3,459 observations from 102 studies to quantify the effects of different management interventions(physical,chemical,biological,and integrated control)on Spartina per se and native biodiversity and environments.We found that physical measures quickly suppressed Spartina but that their effectiveness declined over time.By contrast,chemical measures decreased the abundance and growth of Spartina to a lesser degree in the early stage,but the effectiveness increased over time.Different management measures did not significantly decrease the diversity of native biota on the whole,but native-plant diversity significantly decreased with time after physical control.Different management measures did not affect abiotic factors differently.These results support the use of chemical measures to control invasive Spartina,although their effectiveness would depend on the time since the management intervention.Addressing the problem of Spartina regrowth following physical control requires improved techniques.We hold that initial states of invaders and subsequent environmental changes after management interventions should be weighed in evaluating control efficacy.

Invasive Spartina alterniflora accelerates the increase in microbial nitrogen fixation over nitrogen removal in coastal wetlands of China

Salt marsh plants play a vital role in mediating nitrogen(N)biogeochemical cycle in estuarine and coastal ecosystems.However,the effects of invasive Spartina alterniflora on N fixation and removal,as well as how these two processes balance to determine the N budget,remain unclear.Here,simultaneous quantifications of N fixation and removal via^(15)N tracing experiment with native Phragmites australis,invasive S.alterniflora,and bare flats as well as corresponding functional gene abundance by qPCR were carried out to explore the response of N dynamics to S.alterniflora invasion.Our results showed that N fixation and removal rates ranged from 0.77±0.08 to 16.12±1.13 nmol/(g·h)and from 1.42±0.14 to 16.35±1.10 nmol/(g·h),respectively,and invasive S.alterniflora generally facilitated the two processes rates.Based on the difference between N removal and fixation rates,net N_(2)fluxes were estimated in the range of-0.39±0.14 to 8.24±2.23 nmol/(g·h).Estimated net N_(2)fluxes in S.alterniflora stands were lower than those in bare flats and P.australis stands,indicating that the increase in N removal caused by S.alterniflora invasion may be more than offset by N fixation process.Random forest analysis revealed that functional microorganisms were the most important factor associated with the corresponding N transformation process.Overall,our results highlight the importance of N fixation in evaluating N budget of estuarine and coastal wetlands,providing valuable insights into the ecological effect of S.alterniflora invasion.

A novel herbivorous wood-borer insect outbreak triggers die-offs of a foundation plant species in coastal ecosystems

Introduction:Understanding the trophic interactions between plants and herbivorous insects is essential for managing the ecosystem health and sustainability in the context of climate change and anthropogenic disturbance.The Tamarix chinensis is a foundation plant species of salt marshes in northern China,which provides a variety of ecological functions and services in coastal ecosystems such as withstanding storm tide,conserving biodiversity,and preventing shoreline erosion.Outcomes:However,through long-term field observations,for the first time,we found that the health of this population has been suffering from the potential outbreak of a novel wood-borer insect(Zeuzera leuconotum Butler)under multiple stresses,contributing substantially to the degradation of the coastal salt marshes.Discussion and Conclusion:Therefore,there is urgent need and great significance to investigate the potential impact of this herbivorous wood-borer insects on health and sustainability of plant community in coastal ecosystems for in-depth understanding its degradation mechanisms.

Marsh persistence under sea-level rise is controlled by multiple,geologically variable stressors

Introduction:Marshes contribute to habitat and water quality in estuaries and coastal bays.Their importance to continued ecosystem functioning has led to concerns about their persistence.Outcomes:Concurrent with sea-level rise,marshes are eroding and appear to be disappearing through ponding in their interior;in addition,in many places,they are being replaced with shoreline stabilization structures.We examined the changes in marsh extent over the past 40 years within a subestuary of Chesapeake Bay,the largest estuary in the United States,to better understand the effects of sea-level rise and human pressure on marsh coverage.Discussion:Approximately 30 years ago,an inventory of York River estuary marshes documented the historic extent of marshes.Marshes were resurveyed in 2010 to examine shifts in tidal marsh extent and distribution.Marsh change varied spatially along the estuary,with watershed changes between a 32%loss and an 11%gain in marsh area.Loss of marsh was apparent in high energy sections of the estuary while there was marsh gain in the upper/riverine section of the estuary and where forested hummocks on marsh islands have become inundated.Marshes showed little change in the small tributary creeks,except in the creeks dominated by fringing marshes and high shoreline development.Conclusions:Differential resilience to sea-level rise and spatial variations in erosion,sediment supply,and human development have resulted in spatially variable changes in specific marsh extents and are predicted to lead to a redistribution of marshes along the estuarine gradient,with consequences for their unique communities.

Identifying marsh dieback events from Landsat image series (1998–2018) with an Autoencoder in the NIWB estuary, South Carolina

This study reports an inventory of marsh dieback events from spatial and temporal perspectives in the North Inlet-Winyah Bay(NIWB)estuary,South Carolina(SC).Past studies in the Gulf/Atlantic coast states have reported acute marsh dieback events in which marsh rapidly browned and thinned,leaving stubble of dead stems or mudflat with damaged ecosystem services.Reported marsh dieback in SC,however,have been limited.This study identified all marsh dieback events in the estuary since 1998.With 20 annually collected Landsat images,the Normalized Difference Vegetation Index(NDVI)series was extracted.A Stacked Denoising Autoencoder neural network was developed to identify the NDVI anomalies on the trajectories.All marsh dieback patches were extracted,and their inter-annual changes were examined.Results showed a continuous,spatially variable multi-year dieback event in 1998–2005,which aligned with the reported dieback in the early 2000s from other states.The identified patches mostly returned to normal within one year while the phenomenon reoccurred in other areas of the estuary during the prolonged dieback period.This study presents the first attempt to explore long-term dieback dynamics in an estuary using satellite time series.It provides valuable information in documenting marsh healthiness and environmental resilience on SC coasts.