Tidal Salt Marshes Research Articles

Leveraging a decade of Landsat-8 spectral records for mapping blue carbon storage in tidal salt marshes

Tidal salt marsh ecosystems are known to accumulate and store large amounts of “blue” carbon, making them an important component of regional carbon cycle processes and a potential target for ecosystem-based carbon crediting efforts. However, blue carbon content in salt marshes can vary substantially at relatively small spatial scales. Understanding spatial variations of blue carbon storage at the landscape or local scale is important for developing carbon inventories, guiding ecological restorations, and informing habitat management strategies. We investigated the potential of spectral index records from the Landsat-8 Operational Land Imager spanning from 2014 to 2023 for mapping blue carbon storage in the soils of two tidal salt marsh systems in the Mid-Atlantic United States. The decadal mean of a non-photosynthetic vegetation index and standard deviations of a normalized difference vegetation index and a modified normalized difference water index were identified as predictors of blue carbon. These predictors were used to train a gradient boosted trees model for predicting soil organic matter content that achieved a testing set r-squared value of 0.67. We estimated that the two study marshes stored a combined 133-208 gigagrams of organic carbon in the top 30 cm of soil. We emphasize the need for better quantification of deep soil carbon in tidal salt marsh systems, which is likely quite high, and demonstrate the potential for satellite-based mapping of blue carbon within individual tidal wetland systems.

Blue carbon storage of tidal flats and salt marshes: A comparative assessment in two Chinese coastal areas

Coastal wetlands are blue carbon (C) reservoirs and play an important role in mitigating climate change by efficiently capturing and storing organic carbon (OC). However, assessments of blue C sequestration focus primarily on soil organic carbon (SOC) inventories of vegetated coastal ecosystems and often neglect the potential of tidal flats to sequestrate C. To address the issue, a comparative assessment was carried out for two representative coastal wetlands in China: the Yellow River Delta and Yancheng coastal wetlands. The study focused on distinct types of coastal ecosystems including unvegetated tidal flats and three types of salt marshes vegetated by either Spartina alterniflora, Suaeda salsa, or Phragmites australis. We combined field sampling and data synthesis to assess the OC stock of different coastal areas and evaluate the regional OC storage. The results indicated that vegetated salt marshes exhibited higher OC stocks per unit area compared to unvegetated tidal flats in both regions. Interestingly, tidal flats in the Yellow River Delta displayed comparable OC stocks to S. salsa marsh on the Yancheng coast. The regional OC storage was estimated to be 5.64 ± 0.61 Tg C for the Yellow River Delta and 9.96 ± 1.52 Tg C for the Yancheng coast. Tidal flats with low SOC stock per unit area were the primary contributors to regional OC storage, accounting for over 75 % of the total. This predominance was attributed to the extensive distribution of tidal flats along China's coast, indicating their significant potential as blue C sinks. Overall, this study provides insights into the OC storage potential of various wetland types in China and highlights the importance of considering tidal flats in estimates of blue C sequestration.

Growth conditions of tree species relative to climate change and sea level rise in low-lying Mid Atlantic coastal forests

IntroductionCoastal forests occupy low-lying elevations, typically adjacent to tidal salt marshes. Exposed to increased flooding with sea level rise, coastal forests have retreated as salt marshes advance upslope. Coastal forests likely currently experience periodic tidal flooding, but whether they temporarily accommodate or quickly succumb to rising sea level under changing climatic conditions remains a complex question. Disentangling how tidal flooding and climate affect tree growth is important for gauging which coastal forests are most at risk of loss with increasing sea levels.MethodsHere, dendrochronology was used to study tree growth relative to climate variables and tidal flooding. Specifically, gradients in environmental conditions were compared to species-specific (Pinus taeda, Pinus rigida, Ilex opaca) growth in coastal forests of two estuaries (Delaware and Barnegat Bays). Gradient boosted linear regression, a machine learning approach, was used to investigate tree growth responses across gradients in temperature, precipitation, and tidal water levels. Whether tree ring widths increased or decreased with changes in each parameter was compared to predictions for seasonal climate and mean high water level to identify potential vulnerabilities.ResultsThese comparisons suggested that climate change as well as increased flood frequency will have mixed, and often non-linear, effects on coastal forests. Variation in responses was observed across sites and within species, supporting that site-specific conditions have a strong influence on coastal forest response to environmental change.DiscussionSite- and species-specific factors will be important considerations for managing coastal forests given increasing tidal flood frequencies and climatic changes.

Climate change impacts on a sedimentary coast—a regional synthesis from genes to ecosystems

Climate change effects on coastal ecosystems vary on large spatial scales, but can also be highly site dependent at the regional level. The Wadden Sea in the south-eastern North Sea is warming faster than many other temperate coastal areas, with surface seawater temperature increasing by almost 2 °C over the last 60 years, nearly double the global ocean mean increase. Climate warming is accompanied by rising sea levels, which have increased by approximately 2 mm yr−1 over the last 120 years. For this sedimentary coast, the predicted acceleration of sea-level rise will have profound effects on tidal dynamics and bathymetry in the area. This paper synthesises studies of the effects of ocean warming and sea level rise in the northern Wadden Sea, largely based on research conducted at the Wadden Sea Station Sylt of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. An increasing rate of sea level rise above a critical threshold will lead to coastal erosion and changes in sediment composition, and may cause the transition from a tidal to lagoon-like environment as tidal flats submerge. This involves changes to coastal morphology, and the decline of important habitats such as muddy tidal flats, salt marshes and seagrass meadows, as well as their ecological services (e.g. carbon sequestration). Ocean warming affects plankton dynamics and phenology, as well as benthic community structure by hampering cold-adapted but facilitating warm-adapted species. The latter consist mostly of introduced non-native species originating from warmer coasts, with some epibenthic species acting as ecosystem engineers that create novel habitats on the tidal flats. Warming also changes interactions between species by decoupling existing predator–prey dynamics, as well as forming new interactions in which mass mortalities caused by parasites and pathogens can play an understudied but essential role. However, Wadden Sea organisms can adapt to changing abiotic and biotic parameters via genetic adaptation and phenotypic plasticity, which can also be inherited across generations (transgenerational plasticity), enabling faster plastic responses to future conditions. Important research advances have been made using next-generation molecular tools (-omics), mesocosm experiments simulating future climate scenarios, modelling approaches (ecological network analysis), and internet-based technologies for data collection and archiving. By synthesising these climate change impacts on multiple levels of physical and biological organisation in the northern Wadden Sea, we reveal knowledge gaps that need to be addressed by future investigations and comparative studies in other regions in order to implement management, mitigation and restoration strategies to preserve the uniqueness of this ecosystem of global importance.

Factors controlling spatiotemporal variability of soil carbon accumulation and stock estimates in a tidal salt marsh

Abstract. Tidal salt marshes are important contributors to soil carbon (C) stocks despite their relatively small land surface area. Although it is well understood that salt marshes have soil C burial rates orders of magnitude greater than those of terrestrial ecosystems, there is a wide range in accrual rates among spatially distributed marshes. In addition, wide ranges in C accrual rates also exist within a single marsh ecosystem. Tidal marshes often contain multiple species of cordgrass due to variations in hydrology and soil biogeochemistry caused by microtopography and distance from tidal creeks, creating distinct subsites. Our overarching objective was to observe how soil C concentration and dissolved organic carbon (DOC) vary across four plant phenophases and across three subsites categorized by unique vegetation and hydrology. We also investigated the dominant biogeochemical controls on the spatiotemporal variability of soil C and DOC concentrations. We hypothesized that subsite biogeochemistry drives spatial heterogeneity in soil C concentration, and this causes variability in total soil C and DOC concentrations at the marsh scale. In addition, we hypothesized that soil C concentration and porewater biogeochemistry vary temporally across the four plant phenophases (i.e., senescence, dormancy, green-up, maturity). To test these interrelated hypotheses, we quantified soil C and DOC concentrations in 12 cm sections of soil cores (0–48 cm depth) across time (i.e., phenophase) and space (i.e., subsite), alongside several other porewater biogeochemical variables. Soil C concentration varied significantly (p < 0.05) among the three subsites and was significantly greater during plant dormancy. Soil S, porewater sulfide, redox potential, and depth predicted 44 % of the variability in soil C concentration. There were also significant spatial differences in the optical characterization properties of DOC across subsites. Our results show that soil C varied spatially across a marsh ecosystem by up to 63 % and across plant phenophase by 26 %, causing variability in soil C accrual rates and stocks depending on where and when samples are taken. This shows that hydrology, biogeochemistry, and plant phenology are major controls on salt marsh C content. It is critical to consider spatiotemporal heterogeneity in soil C concentration and porewater biogeochemistry to account for these sources of uncertainty in C stock estimates. We recommend that multiple locations and sampling time points are sampled when conducting blue C assessments to account for ecosystem-scale variability.

Estoque de Carbono em Marismas

Salt marshes are typical of temperate climate regions that border estuarine margins, covering intertidal areas subject to regular or irregular flooding. Among the many benefits they provide to the ecosystem, their remarkable ability to sequester carbon from the atmosphere and store it in their substrate for periods ranging from centuries to millennia is particularly noteworthy. This carbon, retained in the form of CO2 in coastal and estuarine environments by salt marshes, is known as "blue carbon." This study aimed to investigate the current state of knowledge about carbon stocks in salt marshes through a systematic bibliographic approach. Three databases were selected for the research: Springer, ScienceDirect, and Periódicos Capes. In each of them, the keywords "tidal salt marsh" and "carbon stock" were used, employing quotation marks around each term and the AND operator. It was observed that, given the urgency to develop actions to mitigate the effects of climate change, studies focusing on salt marshes have received increasing attention globally. However, this interest is recent, and knowledge on the subject is still limited, focusing on specific regions of the planet. The complexity of these habitats is a frequently neglected topic, although of great importance. Invasive plants have an impact on the dynamics of the carbon cycle in these areas. Additionally, the heterogeneity of salt marshes is a crucial factor to be considered in studies to avoid errors in carbon inventories. Salt marsh restoration activities are considered appropriate approaches to coastal management, potentially transforming degraded areas into carbon sinks.

The role of tidal creeks in shaping carbon and nitrogen patterns in a Chinese salt marsh

Tidal creeks play a crucial role in lateral transport of carbon and nutrients from tidal salt marshes. However, the specific impact of tidal creek development on carbon and nutrient distribution within the marsh remains poorly understood. The objective of this study is to assess the influence of lateral tidal flooding through the tidal creeks on the spatial distribution of carbon and nitrogen fractions in the soils of a Chinese temperate salt marsh. We conducted a comprehensive analysis of the relative variations in different carbon and nitrogen fractions, along with soil physicochemical and microbial indicators, between the bank soil of the tidal creek and its lateral inland soils across high, middle, and low flats. Our findings highlight that tidal creek development significantly affects the middle flat, leading to substantial variations in organic carbon and total nitrogen. The low flat mainly experiences changes in dissolved inorganic carbon levels. Furthermore, a lateral increase in microbial biomass is observed in the middle flat, indicating that the significantly lower SOC in the middle flat might be ascribed to enhanced microbial decomposition. The lateral enrichment of dissolved inorganic carbon in the low flat is possibly related to the nearshore location and/or abiotic adsorption in inorganic carbon sequestration. Overall, this study demonstrates the critical role of tidal creek development in shaping the distribution patterns of carbon and nitrogen fractions in tidal salt marshes.

Invasion mechanism of Spartina alterniflora by regulating soil sulfur and iron cycling and microbial composition in the Jiuduansha Wetland.

Spartina alterniflora, an invasive plant widely distributed in China's coastal regions, has had a significant impact on the stability of wetland ecosystems and elemental biogeochemical cycles. The invasion of S. alterniflora has been found to lead to the accumulation of sulfides in the soil. The cycling of sulfur and iron in the soil is closely interconnected. Coastal estuarine wetlands are influenced by both freshwater in rivers and seawater tides, as well as the frequent variations in redox conditions caused by tidal fluctuations, which makes the cycling of sulfur and iron in the soil invaded by S. alterniflora more intricate. In this study, field surveys and laboratory experiments were conducted to explore the effects of S. alterniflora invasion and hydrological changes on the cycling of sulfur and iron as well as related functional microorganisms in the soil. The invasion of S. alterniflora showed an increase in soil reduced inorganic sulfur (RIS) components in both high and low marshes of Jiuduansha wetland, with higher content observed in summer and autumn. The tidal simulation experiments revealed abundant sulfate in seawater tidal conditions could promote the formation of acid volatile sulfides (AVS) in the soil of low marshes invaded by S. alterniflora and ensuring the continuous increase in AVS content. Diffusive gradients in-thin-films (DGT) technology indicated the existence of high-concentration soluble S2- enrichment zones in the soil of low marshes invaded by S. alterniflora, which may be related to S. alterniflora root exudates. Tidal action increased the relative abundance of sulfur-reducing bacteria (SRB) in the soil of low marshes, and under the influence of seawater tidal action, SRB exhibited higher relative abundance. However, S. alterniflora might inhibit the activity of iron-reducing bacteria (FeRB) in the soil of low marshes. In conclusion, S. alterniflora may enhance the sulfate reduction rate and promote the formation of free sulfides in tidal salt marsh ecosystems by releasing root exudates that stimulate the activity of SRB, while concurrently inhibiting the activity of FeRB and reducing their competition with SRB. This effect is particularly pronounced in low marshes under seawater tidal conditions. Thus, S. alterniflora is capable of rapidly invading tidal salt marshes by utilizing sulfides effectively.

Development of a regional climate change model for Aedes vigilax and Aedes camptorhynchus (Diptera: Culicidae) in Perth, Western Australia.

Mosquito-borne disease is a significant public health issue and within Australia Ross River virus (RRV) is the most reported. This study combines a mechanistic model of mosquito development for two mosquito vectors; Aedes vigilax and Aedes camptorhynchus, with climate projections from three climate models for two Representative Concentration Pathways (RCPs), to examine the possible effects of climate change and sea-level rise on a temperate tidal saltmarsh habitat in Perth, Western Australia. The projections were run under no accretion and accretion scenarios using a known mosquito habitat as a case study. This improves our understanding of the possible implications of sea-level rise, accretion and climate change for mosquito control programmes for similar habitats across temperate tidal areas found in Southwest Western Australia. The output of the model indicate that the proportion of the year mosquitoes are active increases. Population abundances of the two Aedes species increase markedly. The main drivers of changes in mosquito population abundances are increases in the frequency of inundation of the tidal wetland and size of the area inundated, increased minimum water temperature, and decreased daily temperature fluctuations as water depth increases due to sea level changes, particularly under the model with no accretion. The effects on mosquito populations are more marked for RCP 8.5 when compared to RCP 4.5 but were consistent among the three climate change models. The results indicate that Ae. vigilax is likely to be the most abundant species in 2030 and 2050, but that by 2070 Aedes camptorhynchus may become the more abundant species. This increase would put considerable pressure on existing mosquito control programmes and increase the risk of mosquito-borne disease and nuisance biting to the local community, and planning to mitigate these potential impacts should commence now.

Geographical characteristics that promote persistence and accumulation of PFAS in coastal waters and open seas: Current and emerging hot spots

Poly- and perfluoalkyl substances (PFAS) have been found in ocean water, sediments, and marine organisms. The objective of this study was to identify coastal and open water conditions that affect accumulation and transport characteristics of PFAS in coastal areas and open waters. Coastal conditions were compared based on the classification for Shoreline Cleanup and Assessment Technique (SCAT). Open sea conditions were evaluated in relation to ocean currents. PFAS data available in the literature from different coastal areas and open seas were compiled. Tidal flats, sheltered shores, salt and brackish marshes, bays and estuaries, mangroves, and areas with coral reefs, sea grasses, kelp have conditions that are favorable for PFAS accumulation and ecosystems in these environments are vulnerable to long term PFAS exposure and bioaccumulation. The areas near the centers of gyres in oceans trap and accumulate floating debris as well as PFAS contaminated particles. PFAS accumulation in highly productive coastal areas with mangrove forests, seagrasses and salt marshes can affect the ecosystems in these locations. PFAS data from marine environments indicate that some areas in the Indian Ocean (41.1 ng/L), North Pacific Ocean (12.8 ng/L), and North Atlantic Ocean (4.0 ng/L) relatively high levels of PFAS. Marine organisms with long life spans are likely to accumulate high levels of PFAS over their lifespan. PFAS concentrations at vulnerable coastal waters and open seas should be monitored systematically and periodically to establish the baseline conditions and evaluate the changes in PFAS levels water column and sediments overtime.

Pairing lab and field studies to predict thermal performance of wild fish

In thermally variable ecosystems, temperatures can change extensively on hourly and seasonal timescales requiring ectotherms to possess a broad thermal tolerance (critical thermal minima [CTmin] and maxima [CTmax]). However, whether fish acclimate in the laboratory similarly as they acclimatize in the field under comparable thermal variation is unclear. We used temperature data from a tidal salt marsh to design 21-day lab-acclimation treatments (static: 12, 17, 22, 27 °C; daily variation with mean 22 °C: i) range 17-27 °C, ii) range 17-27 °C with irregular extremes within 12-32 °C). We compared thermal limits in lab-acclimated and field-acclimatized eurythermal arrow goby (Clevelandia ios). Variable temperature-acclimated and acclimatized fish had similar CTmin and CTmax. Notably, arrow gobies showed rapid plasticity in their absolute thermal tolerance within one tidal cycle. The daily mean and max temperatures experienced were positively related to CTmax and CTmin, respectively. This study demonstrates that ecologically informed lab acclimation treatments can yield tolerance results that are applicable to wild fish.

Coastal distribution and driving factors for blue carbon fractions in the surface soil of a warm-temperate salt marsh in China

A better understanding of blue carbon (BC) sequestration can not only contribute to a better elucidation of global carbon cycle processes but can also lay the foundation for the incorporation of BC ecosystems into regional and global carbon offset schemes. In this study, the surface soils of seven plots along a landward to seaward distance gradient were analyzed for the concentrations and stocks of soil organic carbon (SOC), soil inorganic carbon (SIC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC), as well as soil physical (bulk density, texture, moisture), chemical (pH, electrical conductivity), and microbiological (phospholipid fatty acid) properties in the coastal wetlands. Correlation, variation partition and random forest (RF) analyses were used to identify key variables correlating with BC fraction distribution patterns. The results suggested that SIC, DIC, and DOC, exhibited similar landward-increasing trends but the driving factors were distinct from each other. Based on correlation and RF analysis, both SIC and DIC were closely related to soil moisture and clay contents, but microbial indicators of arbuscular mycorrhizal fungi and actinomycete, were found to be associated with SIC, and abiotic properties played less important but still substantial roles in predicting DIC dynamics. In contrast with the other three investigated BC fractions, SOC showed a slight tendency toward enrichment in the seaward direction, and SIC was identified as the main driving factor. DOC showed no significant correlations with the other BC fractions, and its variation could not be explained well by the selected edaphic parameters. The soils in the YRD's tidal Suaeda salsa salt marshes showed a significant negative coupled SOC–SIC correlation, which was potentially related to divergent sedimentary processes and potential biotransformation between SOC and SIC. These results highlight the importance of integrating multiple BC fractions and their interactions into attempts to explore key mechanisms of BC cycling.

High methane concentrations in tidal salt marsh soils: Where does the methane go?

Tidal salt marshes produce and emit CH4 . Therefore, it is critical to understand the biogeochemical controls that regulate CH4 spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH4 production, and higher salinity concentrations inhibit CH4 production in salt marshes. Recent evidence shows that CH4 is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH4 and CO2 fluxes coupled with depth profiles of soil CH4 and CO2 pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH4 production and fate within a temperate tidal salt marsh. We found unexpectedly high CH4 concentrations up to 145,000 μmol mol-1 positively correlated with S2- (salinity range: 6.6-14.5 ppt). Despite large CH4 production within the soil, soil-atmosphere CH4 fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m-2 s-1 ). CH4 and CO2 within the soil pore water were produced from young carbon, with most Δ14 C-CH4 and Δ14 C-CO2 values at or above modern. We found evidence that CH4 within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH4 is produced, including diffusion into the atmosphere, CH4 oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH4 production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH4 production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH4 budgets and blue carbon in salt marshes.

Early–mid Holocene relative sea-level rise in the Yangtze River Delta, China

Regional-scale Holocene sea-level reconstruction is the key to understanding natural climatic variability. The tidal flat, salt marsh, and tidal floodplain in the Yangtze River Delta were very sensitive to morphological changes and sea-level variation during the early Holocene. In this study, the lithology, radiocarbon ages, sediment grain size, benthic foraminifera, and ostracods of three new cores were analyzed. Twenty-four sea-level index points were extracted from incised-valley fills beneath the westernmost part of the Yangtze River Delta and used to construct a detailed relative sea-level curve for 11.03–7.25 ka. The relative sea level were − 38.90 ± 3.48 and 1.59 ± 3.28 m at 11.03 and 7.25 ka, respectively, and the average rate of sea-level rise was 10.71 mm/yr. The relative sea level gradually increased at 11.03–10.10 and 9.29–8.33 ka from 4.80 to 13.30 and 3.19 to 19.59 mm/yr, respectively. The rate of relative sea-level rise gradually decreased from 19.59 mm/yr at 8.33 ka to 13.40 mm/yr at 7.82 ka and further decreased to 5.53 mm/yr at 7.25 ka. The Yangtze River Delta recorded accelerations in the rate of sea-level rise from 8.72 to 8.18 ka of 14.2–19.59 mm/yr, corresponding to the pre-8.2 ka event. A comparison of the sea-level histories of the Yangtze River Delta and other regions indicates tectonics, glacial isostatic adjustment, and coastal levering effects caused by the marine inundation of the continental shelves. These new sea-level data contribute to the understanding of the difference in the sea-level rise rate during the early–mid Holocene.

Change in the community structure and organic carbon content of meio- and macrobenthos between tidal flat and salt marsh areas colonized by Spartina alterniflora in the Bahía Blanca estuary (SW Atlantic)

Salt marshes are regarded as among the most productive coastal ecosystems, important “blue carbon” sinks and a support for benthic communities with large abundances, whose structure may be strongly influenced by salt marsh vegetation. During the last few decades, Spartina alterniflora has been colonizing bare mudflats in the Bahía Blanca estuary, and a large increase in the area covered by salt marshes has been reported. This colonization can strongly influence the structure of benthic fauna and its role in the carbon cycle. The hypothesis of this study was that the community structure and the organic carbon contained in the meio- and macrobenthos change between tidal flats and salt marshes recently colonized by S. alterniflora. Response variables studied to compare the tidal flat and salt marsh were density, biomass and production to biomass (P/B) ratio of macro- and meiobenthos. Density and biomass of Gastropoda and P/B ratio of Nematoda were higher on the salt marsh than on the tidal flat. By contrast, density and biomass of Polychaeta were higher on the tidal flat. These results suggest that the expansion of S. alterniflora marshes on tidal flats produces changes in the structure of the macro- and meiobenthos community (taxonomic composition and biomass) that have an influence on carbon cycling.

Methanogens limited to lower rhizosphere and to an atypical salt marsh niche along a pristine intertidal mangrove continuum

AbstractMangroves are valuable ecosystems that facilitate primary production, carbon sequestration, and regulation of greenhouse gas (GHG) cycles in coastal sediments, with microorganisms playing key roles. Specialized bacteria and archaea compete for energy and resources in mangrove sediments to inhabit optimal ecological niches and can produce or consume methane (CH4)—a potent GHG—in the process. CH4 cycling in mangroves has gained growing attention, yet uncertainties regarding functional and spatial distributions of microorganisms remain. Here, we demonstrate that in a pristine mangrove forest, CH4 concentrations and methanogen communities are concentrated within lower or below rhizosphere depths. We also reveal atypical niches for methanogens in the upper tidal salt marsh zone where vegetation is sparse and highest methanogens abundances were detected at deepest depths (4715 reads g−1) despite relatively high redox potentials (> 250 mV). Pore water CH4 concentrations were highest at the deepest depth within the mangrove forest (max. 3.40 ± 0.21 μM) and coincided with the highest sediment CH4 fluxes (276.4 ± 54.2 μmol m−2d−1) and methanotroph abundances at the surface (1309 reads g−1). Sediment CH4 oxidation fractions between the deepest (60 cm) and shallowest (5 cm) depths were estimated between 18.8% and 64.9%. Positive correlation between crab burrows and CH4 fluxes suggests that CH4 from deeper sediment and salt marsh niches can be transported via conduits to the atmosphere. The spatial data from this study highlights the importance of investigating CH4 dynamics across estuarine ecosystem gradients to better understand the complex roles of vital coastal vegetation zones in the face of a changing climate.

Long‐Term Monitoring of Coupled Vegetation and Elevation Changes in Response to Sea Level Rise in a Microtidal Salt Marsh

AbstractTight interplays between physical and biotic processes in tidal salt marshes lead to self‐organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long‐term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation‐vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea‐level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site‐specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species‐specifically to changes in environmental forcings by modifying species‐preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation‐species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution.

Application of Hydro-morphodynamic Modelling in Coastal Salt Marsh Management

Salt marshes are widespread in estuarine coastal areas and are one of the most productive natural ecosystems in the world. More importantly, the role of salt marshes in coastal protection is of increasing interest, as salt marshes significantly reduce wave height and stabilize substrates. However, the application of hydrodynamic models for coastal salt marsh management is still uncommon. In this study, TELEMAC is used to set up a hydro-morphodynamic model to simulate the dynamic process in the study area. After that, the influence of hydrodynamic stress on the salt marshes under natural conditions was analysed and the feasibility of applying artificial structures to restore salt marshes was discussed. Finally, the long-term evolution of salt marsh platform is modelled. The results show that salt marsh vegetation is strongly influenced by coastal dynamics. The artificial restoration measures such as submerged dikes have the potential to restore or rehabilitate salt marshes by attenuating the currents on tidal flats. The long-term marsh evolution contains both platform raising and channel incision, which forms the unique landscape of tidal salt marsh. The research results of the study can provide theoretical support for the management and restoration of coastal salt marsh wetlands and contribute to disaster prevention and mitigation in the coastal areas.

Response of soil iron oxides in freshwater marsh to different tidal hydrology in the Yellow River Estuary wetland, China

Iron cycles is essential for the transition of matter and energy in wetland ecosystems. The tidal hydrology is a critical factor controlling soil iron biogeochemistry in estuarine wetland, but it remains unclear how iron oxides response to tidal hydrologic regime. A field experiment was manipulated in the Yellow River Estuary wetland for 23 months to differentiate change in iron oxides of freshwater marsh soil under different tidal hydrologic regimes. In June 2019, soil cores were collected in the freshwater marsh on the supratidal zone (FPA), and translocated to be cultured in the tidal salt marshes on the intertidal zone (SPA, TC and SS). Over 23 months, tidal hydrologic regime had obviously affected soil properties and iron oxides. Under the tidal regime, there was an increase in soil pH and electric conductivity (EC), and a decrease in soil water content (SWC), total nitrogen (TN), total phosphorus (TP) and total organic carbon (TOC). A total decrease was found in the contents of soil iron (FeT), iron oxides (Fep, complexed iron; Feo, amorphous iron; Fed, free iron) and their ratios (Fep/Fed, Feo/Fed and Fed/FeT). The duration of tidal hydrology affected the contents of iron oxides and their ratios, i.e., FeT and Fed decreased with culture time in all sites while Fep, Feo and the ratios increased in the supratidal site. Iron oxides and their ratios were positively correlated to redox potential, organic carbon and nutrients, and negatively correlated to soil pH and salinity. The results indicate that tidal hydrologic regime could decrease the contents of soil iron oxides and their activation when freshwater marsh exposed to tide events.

Stratified vertical sediment profiles increase burrowing crab effects on salt marsh edaphic conditions

AbstractBurrowing animals can profoundly affect the biological structure and ecosystem functions of their environments. For instance, burrowing crabs increase sediment deposition and facilitate sediment homogenization and turnover, with potential impacts to sediment biogeochemistry. However, the relative importance of burrowing crabs on sediment dynamics can vary considerably between, and within, habitats. Sediment properties influence how burrowing crabs will affect edaphic conditions, but these studies often assume homogenous sediment conditions and fail to consider how sediment properties change with depth. Thus, understanding how burrowing crab effects on sediment properties are influenced by the vertical sediment profile should inform where burrowing crabs structure edaphic conditions. Here, we conducted an intensive field survey across three tidal salt marshes with variable sediment properties to understand if marsh vertical sediment profiles can help predict the nature of burrowing crab–sediment relationships. We found that burrowing crabs homogenize sediments in all marshes, but their effects on sediment homogenization and edaphic conditions were greater in marshes with highly stratified vertical sediment profiles. Our study suggests that understanding the vertical sediment profile of a salt marsh may provide critical insights into how crab burrowing may influence the edaphic conditions and physical characteristics of marsh surface sediments—especially in restored, created, and managed salt marshes.