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Habitat loss is one of the main threats to global biodiversity and ecosystem services. Human development, resource extraction, and climate change all contribute to the degradation or loss of ecosystems worldwide and in California. To counter these impacts, development may be regulated by following a general mitigation hierarchy where the priority is to first avoid injury to natural resources, then minimize remaining impacts, and finally, compensate for any unavoidable losses. This last step, known as compensatory mitigation, can offset environmental impacts through the restoration, enhancement, preservation, or creation (i.e., establishment) of habitat. My research focuses on three aspects of this process: the quantification of impacts to habitat, the cumulative effects of the permitted development, and strategies to improve outcomes of restoration projects. In Chapter 1, I review the cumulative impacts of development permitted by the California Coastal Commission between 2010 and 2018. Wetland habitat was the most frequently impacted, and mitigation usually took the form of restoration and was predominantly on-site and in-kind. I also provide recommendations for improving the compensatory mitigation process. In Chapter 2, I review metrics and tools that quantify impacts to seagrasses, kelps, and other macroalgae. I provide a list and chart to identify tools and applications best suited for ecological valuation and equivalency analysis for mitigation. In Chapter 3, I demonstrate restoration actions that can establish cover and survivorship of a dominant species at a restored tidal marsh in central California. Irrigation and larger plants provided the most cost-effective strategies in the short-term, when compensatory mitigation projects are likely to be held to performance standards. The topics of this thesis are varied but all relate back to the process of compensatory mitigation, the goal of which is to ensure the persistence of natural resources for present and future generations.

Wetlands play a critical role in removing nitrogen pollution from the surrounding landscape by acting as a reactive zone for nitrogen. In anaerobic wetland soil, microorganisms convert nitrogen to inert di-nitrogen gas. Quantifying nitrogen removal in wetland ecosystems remains challenging, especially for wetlands experiencing regular cycles of wetting and drying. I present a laboratory-based soil core incubation method paired with membrane inlet mass spectrometry to measure total nitrogen removal by denitrification and anammox in wetlands. I applied this method to soils taken from a common pickleweed (Salicornia pacifica) dominated salt marsh delineated into wetland positions by degree of tidal inundation. Results suggest that the salt marsh acted as a total nitrogen sink during winter, with the wetland position inundated most often having the greatest nitrogen removal potential. The dense rhizosphere of S. pacifica may have facilitated nitrogen removal by soil microorganisms, even during plant dormancy.

Coastal salt marsh ecosystems in California are at risk from projected rates of sea-level rise (SLR) of up to an order of magnitude higher than rates seen over the past 6,000 years of stable sea levels (Griggs, Cayan, Tebaldi, Fricker, & �rvai, 2017). With rates of this magnitude, salt marsh area, already limited by land use changes in the 19th and 20th centuries, could be completely lost by 2100 (Thorne et al., 2018). To better understand how California salt marshes are adapting to modern acceleration of SLR, over 100 sediment cores were collected from 13 salt marsh sites, ranging from Humboldt Bay to Tijuana River Estuary. Sediment accretion rates over the past several hundred years were measured using radiocesium, radiolead, and radiocarbon dating on 32 cores. Valuation of the carbon storage, an ecosystem service known as blue carbon provided by salt marshes, presents an opportunity to help preserve and restore sites threatened by SLR through carbon credits (Bear, 2017; Callaway, Borgnis, Turner, & Milan, 2012; Mcleod et al., 2011), but there are many questions which much be addressed before this can become a reality for the state of California (Macreadie et al., 2019). A standardized protocol for estimation of carbon content from loss-on-ignition (LOI) was developed with an emphasis on quantifying error and uncertainty in carbon measurements for blue carbon purposes. Using a conversion between soil organic matter and soil organic carbon shown to be effective for California salt marshes, carbon content was estimated through LOI analysis of 61 sediment cores. The impact of climate change in these ecosystems was further explored in the first documented record of a fire in a Pacific coast salt marsh at Mugu Lagoon. California salt marsh sediment accretion averages at 2.93 � 1.9 mm yr-1, which is lower than average rates from regions such as the US Gulf and East coasts. Rates of accretion and relative SLR (RSLR) show a non-linear relationship with highest accretion occurring at rates of RSLR from 2 – 6 mm yr-1. Linear relationships between SLR and accretion are comparatively weak, but are stronger in the low elevations of salt marsh habitat. Salt marshes in the state annually sequester about 0.08% of state-wide annual greenhouse gas emissions and store about 23% of one year’s emissions in their soils (as compared to 2016 emissions). Because of limited area, these habitats will not serve as an effective mitigation strategy at the state level, but loss of this habitat may release up to 27 � 0.3 Tg stored carbon, potentially valued at about $1.4 billion (using an estimate of $15/tonne CO2 �equivalent). Preservation of current habitat through facilitation of sediment accretion will have the largest positive impact on carbon storage and sequestration, as well as protect salt marsh habitat from being lost to SLR. Analysis of the persistent effects of a recent marsh fire at Mugu Lagoon demonstrates that drought-stress may slow California salt marsh response to disturbance by one or more growing seasons and highlights the uncertain impacts of climate change on system function. This dissertation provides important baseline data for salt marsh sediment accretion, salt marsh carbon stocks and sequestrations rates, recommends best practices for use of LOI as a measure of soil organic carbon, and examines ecosystem recovery under multiple stressors. This work can be used in vulnerability assessments, ecosystem models, and valuation of ecosystem services for California salt marshes.

Salt marshes can generally be considered as sinks for metals. Research into salt marshes in Cornwall, UK suggests those estuaries heavily impacted by mining contamination are characterised by a less diverse vegetation compared with a significantly less-polluted site. Assessment using the National Vegetation Classification on the mid-marsh confirmed an Armeria maritima-dominated community was to be found in the most metal-enriched salt marsh of Restronguet Creek. However, this plant was co-dominant with Plantago maritima in the moderately contaminated marsh of Lelant and not present at all in the Camel, which has been subject to limited mining related contamination. Using canonical correspondence analysis, vegetation abundance data was compared with geochemical variables within the sediment. Metals were studied using extractions to signal bioavailability. P. maritima was not associated with the very high metal levels found in Restronguet Creek. A. maritima, had some association with soluble copper and was closer to the bulk of metals than P. maritima. As tolerance to adverse conditions and competitiveness are mutually exclusive, A. maritima, therefore, exists in a successional relationship with P. maritima. A. maritima then appears to be outcompeted by P. maritima in marshes with low metal loadings. Moderately high metal content results in a loss of competitiveness by P. maritima allowing A. maritima to co-dominate. In extremely metal-rich estuaries, however, P. maritima is unable to compete, allowing A. maritima to colonize the mid-marsh. Vegetation community may, therefore, be useful as an indicator of the level of metal contamination. European Geoscience Union General Assembly: vEGU21

Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, necessitating the development of alternative accounting methods such as proxies. Salinity is a useful proxy for tidal marsh CH4 emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes, where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by comparing emissions from four strata differing in hydrology and plant community composition. Mean CH4 chamber-collected emissions measured as mg CH4 m−2 h−1 ranked as S. alterniflora (1.2 ± 0.3) ≫ High-elevation J. roemerianus (0.4 ± 0.06) > Low-elevation J. roemerianus (0.3 ± 0.07) = S. patens (0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater depletion in the S. alterniflora stratum (61 ± 4%) than in the S. patens stratum (1 ± 9%) with the J. roemerianus strata falling in between. We attribute the high CH4 emissions in the S. alterniflora stratum to sulfate depletion likely driven by limited connectivity to tidal waters. Low CH4 emissions in the S. patens stratum are attributed to lower water levels, higher levels of ferric iron, and shallow rooting depth. Moderate CH4 emissions from the J. roemerianus strata were likely due to plant traits that favor CH4 oxidation over CH4 production. Hydrology and plant community composition have significant potential as proxies to estimate CH4 emissions at the site scale.

Salt marshes are important coastal habitats and provide ecosystem services to surrounding communities. They are, however, threatened by accelerating sea-level rise and sediment deprivation due to human activity within upstream catchments, which result in their drowning and a reduction in their extent. Rising seas are also leading to an expansion of coastal flooding protection infrastructures, which might also represent another serious if poorly understood threat to salt marshes due to effects on the resuspension and accumulation of sediment during storms. Here, we use observations from the Venice Lagoon (Italy), a back-barrier system with no fluvial sediment input recently protected by storm-surge barriers, to show that most of the salt-marsh sedimentation (more than 70% in this case) occurs due to sediment reworking during storm surges. We also prove that the large, yet episodic storm-driven sediment supply is seriously reduced by operations of storm-surge barriers, revealing a critical competition between the objectives of protection against coastal flooding and preservation of natural ecosystems. Without complementary interventions and management policies that reduce barrier activations, the survival of coastal wetlands is even more uncertain. Sediment supply to the Venice Lagoon is substantially reduced by flood barriers inhibiting storm-related sediment reworking and transport, according to observations through multiple storm events before and after barrier installation.

Runnelling is a minor and successful form of open marsh water management developed in Australia in the 1980s and integrated into mosquito control programs in the 1990s. While long-term monitoring and investigation of impacts has continued for one site, until recently there has been no assessment of operational runnelling more broadly across mosquito control agencies. This study addresses this issue. Forty-seven runnelled saltmarsh sites were assessed for runnel efficacy, function and condition. Issues impacting on runnel function, including both ecological and geomorphic processes, were noted. Data were mostly acquired from site inspections and discussions with mosquito control staff, with some records as available. Most runnels were constructed between 1990 and 2005, mainly using a dedicated runnelling machine. Almost half (49%) continued to contribute to mosquito control efficacy, either in part or fully, while half did not (51%). Efficacy was attributed to runnels being correctly configured in design, layout and construction and to relatively recent maintenance. Conversely, diminished efficacy was mostly attributed to ineffective hydrologic function, caused by vegetation blockages, erosion and/or deposition. Runnels alter tidal hydrology, affecting other ecological and geomorphic processes. These processes need to be managed to maintain runnel function and it is essential that monitoring and maintenance records are kept. Runnels can control mosquitoes for decades, provided ongoing monitoring and maintenance of the runnel system focusses on responding to early signs of impacts. Restoring degraded runnels should be undertaken using a runnel machine, however, changing environmental conditions may mean that a runnel should be ‘decommissioned’ instead.

The effects of sea level rise and coastal saltwater intrusion on wetland plants can extend well above the high-tide line due to drought, hurricanes, and groundwater intrusion. Research has examined how coastal salt marsh plant communities respond to increased flooding and salinity, but more inland coastal systems have received less attention. The aim of this study was to identify whether ground layer plants exhibit threshold responses to salinity exposure. We used two vegetation surveys throughout the Albemarle-Pamlico Peninsula (APP) of North Carolina, USA to assess vegetation in a low elevation landscape (≤ 3.8 m) experiencing high rates of sea level rise (3–4 mm/year). We examined the primary drivers of community composition change using Non-metric Multidimensional Scaling (NMDS) and used Threshold Indicator Taxa Analysis (TITAN) to detect thresholds of compositional change based on indicator taxa, in response to potential indicators of exposure to saltwater (Na, and the Σ Ca + Mg) and elevation. Salinity and elevation explained 64% of the variation in community composition, and we found two salinity thresholds for both soil Na+ (265 and 3843 g Na+/g) and Ca+ + Mg+ (42 and 126 µeq/g) where major changes in community composition occur on the APP. Similar sets of species showed sensitivity to these different metrics of salt exposure. Overall, our results showed that ground layer plants can be used as reliable indicators of salinity thresholds in coastal wetlands. These results can be used for monitoring salt exposure of ecosystems and for identifying areas at risk for undergoing future community shifts.

The unprecedented size of the deepwater horizon oil spill and scope of the subsequent response elicited intense and sustained interest in microbial responses to oiling, especially in salt marshes, which have featured prominently in debates about best practices to prevent and remediate oiling of vulnerable ecosystems. A number of studies have examined salt marsh soil microbial communities following the spill, but most have primarily concentrated on prokaryotes. The extent to which oiling elicited shifts in fungal diversity and community composition remains unclear. Here we present spatial and temporal comparisons of salt marsh soil fungal communities at two southern Louisiana salt marshes with contrasting oiling histories. We profiled fungal communities in 2013 alongside corresponding measurements of polycyclic aromatic hydrocarbons to assess whether and how responses to oiling are distinguishable from natural heterogeneity. Analyses based on high-resolution unbiased spatial sampling demonstrated that fungal communities did not align with shoreline classification of oiling less than three years after initial oiling, despite observable differences in oil residues and secondary oiling. Notably, extensive sampling allowed delineation of benchmark sampling thresholds and illustrated the value of using ranked differentials of relative abundance to characterize soil fungal communities. Our findings highlight the need for combining high-resolution sampling with judgment-based and systematic sampling approaches to accurately capture responses of salt marsh soil fungal community to oiling.

Tidal marsh loss at Blackwater National Wildlife Refuge (NWR) has been a major concern of refuge managers in recent decades. The approximately 2,035 hectares (5,028 acres) of tidal marsh that have converted to open water in Blackwater NWR since 1938 (Scott et al. 2009) represent one of the most significant areas of marsh conversion within the Chesapeake Bay. In 2013, a suite of climate adaptation strategies focused on sea level rise was developed for Blackwater NWR and surrounding areas of Dorchester County by the Blackwater Climate Adaptation Project (BCAP). The BCAP is a collaboration of The Conservation Fund, Audubon Maryland-DC, and the U.S. Fish and Wildlife Service, assisted by the Maryland Department of Natural Resources (MD DNR), U.S. Geological Survey, and others. In 2016, the BCAP implemented a thin-layer placement (TLP) project at Shorter’s Wharf in Blackwater NWR on 16 hectares (40 acres) of subsiding and fragmenting tidal marsh dominated by Schoenoplectus americanus, Spartina alterniflora, and Spartina patens. The purpose of the project was to increase the 16 hectares’ (40 acres’) resiliency to climate-driven sea level rise and storm impacts. The project built up the marsh elevation by applying thin layers of sediment dredged from the adjacent Blackwater River. The sediment enhancement was designed to extend the longevity of the marsh and increase its resiliency by raising its surface elevation in relation to the tidal regime and to return the habitat to its prior high-marsh condition with S. patens dominating. The colonization of this site by saltmarsh sparrow would be an indicator of success in reaching this goal. Dredging operations in November and December 2016 placed approximately 19,900 cubic meters (26,000 cubic yards) of sediment on the project site. Post-restoration elevations obtained one year after material placement indicated that, although the target elevations were achieved in 78% of the surveyed placement area, the material was not distributed uniformly. Coarser material tended to stack up at the discharge location while the grain size declined and the slopes flattened toward the periphery of the discharge area. In 2017, natural vegetation had regenerated through the placed sediment with vigorous regrowth of S. americanus and S. alterniflora . This regrowth was supplemented with hand-planting of more than 200,000 plugs of S. patens. Vegetation monitoring is ongoing to determine the plant composition evolution within the placement site. Pre-dredge and post-dredge bathymetric surveys reveal 70% accretion nearly two years after dredging within the borrow area footprint.

The San Francisco Bay has the largest concentration of salt marshes in the state of California. In the last 170 years, the vast majority of the historic tidal wetlands in the Bay have been significantly altered or destroyed due to diking, filling and other processes. Many of the remaining marshes have been impacted by changing sedimentation regimes and related loadings of pollutants such as heavy metals (Sr, Al, Fe, Ti, Cu, Pb, Ni, Zn) over this period, making ecological trajectories and resilience to disturbance uncertain. Here we examine changing sediment accretion rates, coupled with the longest and highest resolution heavy metal accumulation data from three San Francisco Bay marshes, to examine European impacts in the context of earlier Holocene variability. The results are consistent with larger-scale analyses and indicate that European alterations of landscapes and shorelines had geographically variable impacts on marsh sediment accumulation. Despite differential impacts on net sediment accretion, initial European impacts appear to have decreased the proportion of organic material in the marsh sediments, likely due to the delivery of eroded inorganic sediment from landscapes and shorelines. The results confirm significant European impacts on the geochemistry of marshes in the San Francisco Bay over the last ~150–200 years. Post-European arrival levels of Pb are unprecedented throughout the earlier Holocene. However, these values have declined in recent years. Concentrations of Sr, Ti, Cu, Ni, and Zn also increased following European arrival. Our results show that post-European concentrations are not so far removed compared to pre-European maximum concentrations. As sedimentation regimes and emissions of Pb and delivery of other metals have decreased, and as organic productivity increased on the marshes, environmental trajectories are shifting back towards their immediate pre-European conditions.

Tidal salt marshes are known to accumulate “blue carbon” at high rates relative to their surface area, which render these systems among the Earth's most efficient carbon (C) sinks. However, the potential for tidal salt marshes to mitigate global warming remains poorly constrained because of the lack of representative sampling of tidal marshes from around the globe, inadequate areal extent estimations, and inappropriate dating methods for accurately estimating C accumulation rates. Here we provide the first estimates of organic C storage and accumulation rates in salt marshes along the Pacific coast of Canada, within the United Nations Educational, Scientific and Cultural Organization (UNESCO) Clayoquot Sound Biosphere Reserve and Pacific Rim National Park Reserve, a region currently underrepresented in global compilations. Within the context of other sites from the Pacific coast of North America, these young Clayoquot Sound marshes have relatively low C stocks but are accumulating C at rates that are higher than the global average with pronounced differences between high and low marsh habitats. The average C stock calculated during the past 30 years is 54 ± 5 Mg C ha−1 (mean ± standard error), which accounts for 81 % of the C accumulated to the base of the marsh peat layer (67 ± 9 Mg C ha−1). The total C stock is just under one-third of previous global estimates of salt marsh C stocks, likely due to the shallow depth and young age of the marsh. In contrast, the average C accumulation rate (CAR) (184 ± 50 g C m−2 yr−1 to the base of the peat layer) is higher than both CARs from salt marshes along the Pacific coast (112 ± 12 g C m−2 yr−1) and global estimates (91 ± 7 g C m−2 yr−1). This difference was even more pronounced when we considered individual marsh zones: CARs were significantly greater in high marsh (303 ± 45 g C m−2 yr−1) compared to the low marsh sediments (63 ± 6 g C m−2 yr−1), an observation unique to Clayoquot Sound among NE Pacific coast marsh studies. We attribute low CARs in the low marsh zones to shallow-rooting vegetation, reduced terrestrial sediment inputs, negative relative sea level rise in the region, and enhanced erosional processes. Per hectare, CARs in Clayoquot Sound marsh soils are approximately 2–7 times greater than C uptake rates based on net ecosystem productivity in Canadian boreal forests, which highlights their potential importance as C reservoirs and the need to consider their C accumulation capacity as a climate mitigation co-benefit when conserving for other salt marsh ecosystem services.

The coastal zone of China has experienced large increases in population, economy and urbanization since the early 1980s. Many studies have reported the loss, degradation and fragmentation of coastal wetlands in China at local to regional scales. To date, at the national scale, our knowledge of the spatial distribution, inter-annual variation and multi-decadal trends of coastal wetlands in China remains very limited. Here we analysed ~62,000 Landsat-5, -7 and -8 images over the period 1984–2018 and generated maps of coastal wetlands for individual years in China at 30-m spatial resolution. We found that coastal wetland area significantly decreased between 1984 and 2011. We also found a substantial increase in saltmarsh area and a stable trend of tidal flat area since 2012, driven by reduced anthropogenic activities and increased conservation and restoration efforts. These coastal wetland maps for the period 1984–2018 are invaluable for improvement of coastal wetland management and sustainability in China. China’s coasts have become more populous and urbanized. This study finds a rebound in the area of coastal wetlands, reflecting recent conservation and restoration, with large losses between 1984 and 2011 followed by increases in saltmarsh area and stabilization of tidal flats.

The El Nino–Southern Oscillation (ENSO) strongly influences climate and hydrology on the planet and may drive the structure and functioning of estuarine communities. However, the effects of the ENSO on coastal areas are dependent on the spatial scale and the intrinsic characteristics of the estuary. Over six years (2013 to 2018), we investigated the variability in macrobenthic secondary production of Spartina alterniflora marshes and adjacent unvegetated areas in a subtropical coastal lagoon in Southern Brazil. We tested whether the ENSO events affect the benthic secondary production of these two habitats. Most environmental variables (air temperature, northward surface stress and northward wind component) did not differ significantly between ENSO phases, except for precipitation. Monthly precipitation totals during El Nino months were 70% higher than neutral and La Nina conditions. A total of 25,862 organisms (8,191 in salt marshes and 17,671 in unvegetated areas) belonging to 39 taxa (25 in marshes and 34 unvegetated areas) were collected throughout the study. The tanaid Monokalliapseudes schubarti, and Nereididae polychaeta Laeonereis acuta were largely the most abundant species in both vegetated and unvegetated areas, with higher biomass and productivity. The secondary production in unvegetated areas increased 216%, from a mean of 107.5 g C/m2/y to 336.4 g C/m2/y during El Nino. However, the salt marshes responded differently than the unvegetated areas, with secondary production more stable over the time, with highest value of 164.2 g C/m2/y-1. There is also a distinction in the effects on different species, L. acuta showed an increase only in biomass (from 0.024 to 0.042 g DW/0.03m2), whereas M. schubarti increased both in biomass (more than 4000%) and density (more than 5000%). In contrast, no significant differences in density, biomass and secondary production for both species were detected over time in the salt marshes. Our findings indicate that large scale climatic variations, such as ENSO, can strongly affect estuarine density, biomass and secondary production, but salt marshes buffer El Nino effects on benthic associations.

Tidal wetlands are a significant source of dissolved organic matter (DOM) to coastal ecosystems, which impacts nutrient cycling, light exposure, carbon dynamics, phytoplankton activity, microbial growth, and ecosystem productivity. There is a wide variety of research on the properties and sources of DOM; however, little is known about the characteristics and degradation of DOM specifically sourced from tidal wetland plants. By conducting microbial and combined UV exposure and microbial incubation experiments of leachates from fresh and senescent plants in Chesapeake Bay wetlands, it was demonstrated that senescent material leached more dissolved organic carbon (DOC) than fresh material (77.9 ± 54.3 vs 21.6 ± 11.8 mg DOC L−1, respectively). Degradation followed an exponential decay pattern, and the senescent material averaged 50.5 ± 9.45% biodegradable DOC (%BDOC), or the loss of DOC due to microbial degradation. In comparison, the fresh material averaged a greater %BDOC (72.6 ± 19.2%). Percent remaining of absorbance (83.3 ± 26.7% for fresh, 90.1 ± 10.8% for senescent) was greater than percent remaining DOC, indicating that colored DOM is less bioavailable than non-colored material. Concentrations of DOC leached, %BDOC, and SUVA280 varied between species, indicating that the species composition of the marsh likely impacts the quantity and quality of exported DOC. Comparing the UV + microbial to the microbial only incubations did not reveal any clear effects on %BDOC but UV exposure enhanced loss of absorbance during subsequent dark incubation. These results demonstrate the impacts of senescence on the quality and concentration of DOM leached from tidal wetland plants, and that microbes combined with UV impact the degradation of this DOM differently from microbes alone.

Ditching is one of the most widespread forms of anthropogenic impact on tidal marshes but its effects remain poorly understood. Recently, the phenomenon of “waffle” pooling has been documented in ditched marshes, where a repeating pattern of shallow rectangular pools form in the marsh interior. While pools in unditched marsh may eventually revegetate, waffle pools appear to be a more permanent source of vegetation loss. This study assesses the factors influencing the distribution of waffle pools and other marsh pool types from Maine to Virginia using imagery and existing datasets. Waffle pools were most likely to occur where tidal range was less than 0.8 m, but they could also occur anywhere that tidal range was reduced by anthropogenic features. Unlike waffle pools, the presence of isolated and tidally-connected pools throughout the region was not influenced by tidal range, although both of these pool types were more likely to occur in unditched marshes with higher elevations relative to MSL. An analysis of change of New Jersey’s and New York’s marshes since 1970s confirmed that waffle pools are a recent phenomenon and that this process was responsible for 30–100% of all marsh interior vegetation loss in regions of the study area with tidal range below 0.8 m. Overall, interior marsh loss increased as tidal range decreased. Loss of marsh at edges due to erosion increased as the ratio of edge to marsh area increased. We propose that waffle pools are the product of long-term ditching impacts and that comprehensive strategies are needed that restore drainage networks, restore elevation and alleviate tidal restrictions. Since waffle pooled marshes often contain favorable salt marsh mosquito habitat, restoration efforts should integrate ecological approaches with mitigating public health concerns.

The estuarine habitat mosaic supports the reproduction, growth, and survival of resident and migratory fish species by providing a diverse portfolio of unique habitats with varying physical and biological features. Global climate change is expected to result in increasing temperatures, rising sea levels, and changes in riverine hydrology, which will have profound effects on the extent and composition of the estuarine habitat mosaic and its associated nursery quality for juvenile fish. We used a spatially explicit bioenergetics model to assess how different climate change scenarios might affect juvenile salmon growth rate potential relative to present day conditions in the Nisqually River Delta, WA, USA. The model indicated that prey-rich habitats such as emergent salt marshes and eelgrass meadows were most likely to facilitate growth, and that reductions in their areal extent and accessibility could have severe consequences for salmon. For instance, unmitigated sea-level rise halved the predicted extent of low- and high-elevation emergent salt marsh, leading to a 30% reduction in end-of-season weights. Increasing water temperatures compounded these effects during the late spring and summer such that the average daily growth rate of an individual fish decreased by an additional 5–50% when compared to the effects of sea-level rise alone. Lethal temperatures (> 24 °C) were infrequently observed, but they were more likely to occur during summer low tides in the mudflat and eelgrass habitats when accessibility to prey-rich marsh was minimal, thereby limiting foraging capacity and the availability of thermal refugia. Our findings indicate that, barring the enactment of targeted management strategies, rising tidal levels and increasing ocean temperatures may reduce the quality of the estuarine habitat mosaic for out-migrating salmon and other sensitive fish species.

Coastal wetlands reclamation is facing the problem of salinization. Along with the alternation of ecosystem status, studying the following effect on soil properties becomes emergency. Here we reported the pH, salinity and elemental content (mainly metals) variation affected by the vegetation situations, water sources and geographic positions. The results showed that vegetation could lead the pH and salinity of seawater zones closer to that of freshwater zones in both spatial and time scales. Spartina alterniflora (SA) was the most efficient species among the investigated plants, causing decreases of 0.15–0.69 in pH and 2.250–3.821 in salinity. This result might be caused by the absorption of Ca and K from salt marshes by SA and Suaeda salsa (SS), due to the fact that the component content of calcium (all reduced to approximate one-third) and potassium (reduced in some samples) both decreased. Meanwhile, vegetation could improve soil in seawater zones in Fe content with no extra negative influences in elemental analysis. Thus, ecological engineered vegetation indicated great potential in alternating coastal salt marshes to favorable wetlands or farmlands with almost primitive ecosystem. The results might be significant for ecological engineering and agricultural management in future.

With climate change and rising sea levels, the coastal zone’s flood risk is deteriorating. Previous researches have shown a gradually degrading capacity of traditional hard engineering structures (e.g., seawall, dikes, etc.) on flood mitigation due to problems such as land subsidence and insufficient maintenance. To remedy the defects, the “building with nature concept” for coastal protection with saltmarshes was examined by combining field measurements and numerical simulations. The advantages of saltmarsh over traditional seawall on flood protection was demonstrated from the perspective of both flood area mitigation and economic gain, based on scenario simulations. Results show that tidal wetlands are essential in mitigating significant wave heights (Hs) and current velocities even during storm conditions. The storm wave and current velocity reduction ratio ( \begin{document}$R{R}_{{\rm{w}}}$\end{document} and \begin{document}$R{R}_{{\rm{c}}}$\end{document} ) by saltmarshes on Chongming Dongtan Shoal (CMDS) during Typhoon 9711 is approximately 11% and 51%, respectively. The wave and current mitigation by Scirpus mariqueter are more efficient than Spartina alterniflora and Phragmites australis during measurements in 2010, which were approximately 0.3 m and 0.2 m/s, 0.125 m and 0.155 m/s, 0.086 m and 0.128 m/s per kilometer width, respectively. The summer saltmarsh area 54.2 km2 on CMDS protects approximately 32 km2 land area behind the seawall from being flooded, equivalent to the seawall heightening of approximately 0.42 m on equivalent flood mitigation. The performance of cost-and-benefit analysis shows a relatively higher (by 3%–7%) net present value (NPV) and a higher (by 1.5 times) benefit-cost ratio (BC) of nature-based solution (i.e., saltmarsh restoration) compared with traditional hard engineering solution (i.e., seawall construction). Thus, building seawall with nature, such as a hybrid flood protection measure, should be implemented in the future coastal redesign and maintenance.

Salt marshes are increasingly valued for their role in coastal defense. In particular, marsh plants slow the progression of waves, thereby decreasing wave heights, orbital velocities and associated energy. Practical application of these effects has driven substantial research estimating the effects of plants on waves. However, the effects of waves on plants remain understudied, especially regarding plant responses along a wave climate gradient. To begin to understand these responses, we collected above- and belowground plant data and wave data from 60 sites across Mobile Bay, USA, and tributaries and evaluated plant responses along the range of assessed wave climate conditions. Plant responses among the dominant species, Juncus roemerianus and Spartina alterniflora, varied along the wave climate gradient. However, the basal diameter of shoots in both species declined linearly with increasing wave climate. While wave climate had no observable effect on other S. alterniflora parameters, the declining diameter of J. roemerianus shoots along the same gradient was commensurate with a decline in the percentage of live canopy shoots aboveground and an increase in root and rhizome biomass in the active rooting zone belowground. In contrast to previous studies, other responses including the height, biomass and density of aboveground shoots in both species were not related to wave climate. More broadly, these results demonstrate that plant features important for wave attenuation such as shoot diameter can change in response to varying wave conditions. These feedbacks should be incorporated to improve coastal modeling and successes of coastal conservation, restoration and enhancement projects.

Salt marshes are research hotspots of the carbon cycle in coastal zones because large amounts of atmospheric carbon dioxide is fixed 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, fluorescent components and refractory DOC (RDOC) in submarine groundwater from a salt marsh (Chongming Island, China) and adjacent coastal water were characterized by fluorescence spectroscopy and nuclear magnetic resonance spectroscopy. The fluorescent components identified 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±294 µmol/L, which was significantly higher than that in coastal water (91±19 µmol/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 effects of salt marsh blue carbon.

Coastal wetlands are essential for regulating the global carbon budget through soil carbon sequestration and greenhouse gas (GHG – CO2, CH4, and N2O) fluxes. The conversion of coastal wetlands to agricultural land alters these fluxes' magnitude and direction (uptake/release). However, the extent and drivers of change of GHG fluxes are still unknown for many tropical regions. We measured soil GHG fluxes from three natural coastal wetlands – mangroves, salt marsh, and freshwater tidal forests – and two alternative agricultural land uses – sugarcane farming and pastures for cattle grazing (ponded and dry conditions). We assessed variations throughout different climatic conditions (dry–cool, dry–hot, and wet–hot) within 2 years of measurements (2018–2020) in tropical Australia. The wet pasture had by far the highest CH4 emissions with 1231±386 mgm-2d-1, which were 200-fold higher than any other site. Dry pastures and sugarcane were the highest emitters of N2O with 55±9 mgm-2d-1 (wet–hot period) and 11±3 mgm-2d-1 (hot-dry period, coinciding with fertilisation), respectively. Dry pastures were also the highest emitters of CO2 with 20±1 gm-2d-1 (wet–hot period). The three coastal wetlands measured had lower emissions, with salt marsh uptake of -0.55±0.23 and -1.19±0.08 gm-2d-1 of N2O and CO2, respectively, during the dry–hot period. During the sampled period, sugarcane and pastures had higher total cumulative soil GHG emissions (CH4+N2O) of 7142 and 56 124 CO2-eqkgha-1yr-1 compared to coastal wetlands with 144 to 884 CO2-eqkgha-1yr-1 (where CO2-eq is CO2 equivalent). Restoring unproductive sugarcane land or pastures (especially ponded ones) to coastal wetlands could provide significant GHG mitigation.

Body size is considered an important structuring mechanism of food webs because consumers are usually larger and more mobile than their prey and may couple energy among habitats. We explored the links among trophic position (TP), body size, and the coupling of different energy channels (phytoplankton and C4‐marsh plants) in a saltmarsh landscape in the northern Gulf of Mexico—a dynamic system considered weakly shaped by biotic interactions. Body size was positively associated with TP, and this relationship was stronger in the phytoplankton pathway vs. the C4‐marsh pathway. There was a gradual increase in the coupling of phytoplankton and C4‐marsh plants at larger body sizes and higher TP. Phytoplankton supported longer food chains and larger body sizes than C4‐marsh plants. Results support predictions of the landscape theory for food web architecture and indicate that the role of body size in determining trophic interactions may vary across food web compartments.

Coastal wetland mapping is often difficult because of the heterogeneous vegetation compositions and associated tidal effects. In this study, we employed the U-Net and developed an adaptive deep learning approach to map statewide salt marshes in estuarine emergent wetlands of South Carolina (SC), USA, from 20 Sentinel-2A&B images. Considering the spatial heterogeneity of the coastal environment, two NOAA National Estuarine Research Reserves (NERRs) in SC were examined, the North Inlet-Winyah Bay (NIWB) NERR for model training and the ACE Basin NERR for testing. A high-resolution land cover map in the NIWB was downloaded for the training process. Ground reference points recorded by the NERR, as well as Google Earth were utilized during the accuracy assessment. The highest overall accuracy (90%) was achieved when all scenes with 10 Sentinel bands and the Normalized Difference Vegetation Index (NDVI) were included. The time used to train the model was 5 h, while then the statewide classification was performed in 20 min. Low marsh and high marsh distributions were successfully delineated. Compared to the national marsh maps from the NOAA Coastal Change Analysis Program (C-CAP), this study refined the land cover details concerning low marsh and high marsh distributions on the SC coast. Owing to the computational power of the U-Net, the seasonality and tide influence on marsh classification were mitigated by using multi-temporal images. With images available, the deep learning approach developed in this study could be easily adopted in other coastal areas.

Salt marshes are dynamic ecosystems that change in response to local geographic and geologic factors as well as sea level changes. Most east coast salt marshes are the result of rising sea levels since the end of the last ice age, about 20,000 ybp. To compound this natural process, anthropogenic manipulations for farming, development and other purposes have occurred for centuries. Alterations to salt marshes for the purpose of controlling larval mosquitoes at their source, at least along the east coast of the United States, have occurred since the early twentieth century. These alterations have included large-scale manipulations such as extensive parallel grid-ditching and impounding. Within the last 50 years, more selective source reduction methods such as Open Marsh Water Management (OMWM) have been employed with fewer deleterious impacts to marsh resources. Even more recently, the more holistic approach of Integrated Marsh Management (IMM) has been used with considerable success particularly in the northeastern and Mid-Atlantic states. IMM not only uses OMWM techniques but incorporates the judicious use of mosquito control pesticides, tidal flow restoration, impoundment management, wildlife habitat enhancement, invasive plant control, and selective shallow ditching (“runneling”) depending on local conditions and management plans. As many marshes are becoming wetter and either drowning or migrating inland (where possible) due to the effects of increasing rates of sea level rise, the compounding long-term impacts of parallel grid-ditching and past manipulations on marsh surface elevation and hydrology are being more intently studied. These changes in saltmarsh dynamics have had and will continue to have impacts on where saltmarsh mosquitoes are produced, which could have corollary effects on public health and quality of life near coastal communities. As salt marshes continue to change, mosquito control agencies can play a significant role in providing input for salt marsh restoration and management in addition to their primary objective of vector control and enhancing quality of life.

Spatiotemporal patterns of Spartina alterniflora belowground biomass (BGB) are important for evaluating salt marsh resiliency. To solve this, we created the BERM (Belowground Ecosystem Resiliency Model), which estimates monthly BGB (30-m spatial resolution) from freely available data such as Landsat-8 and Daymet climate summaries. Our modeling framework relied on extreme gradient boosting, and used field observations from four Georgia salt marshes as ground-truth data. Model predictors included estimated tidal inundation, elevation, leaf area index, foliar nitrogen, chlorophyll, surface temperature, phenology, and climate data. The final model included 33 variables, and the most important variables were elevation, vapor pressure from the previous four months, Normalized Difference Vegetation Index (NDVI) from the previous five months, and inundation. Root mean squared error for BGB from testing data was 313 g m-2 (11% of the field data range), explained variance (R2 ) was 0.62-0.77. Testing data results were unbiased across BGB values and were positively correlated with ground-truth data across all sites and years (r = 0.56-0.82 and 0.45-0.95, respectively). BERM can estimate BGB within Spartina alterniflora salt marshes where environmental parameters are within the training data range, and can be readily extended through a reproducible workflow. This provides a powerful approach for evaluating spatiotemporal BGB and associated ecosystem function.

AbstractSea level rise threatens coastal wetlands worldwide, and restoration projects are implementing strategies that decrease vulnerability to this threat. Vegetation monitoring at sites employing new restoration strategies and determination of appropriate monitoring techniques improve understanding of factors leading to restoration success. In Central California, soil addition raised a degraded marsh plain to a high elevation expected to be resilient to sea level rise over the next century. We monitored plant survival and recruitment using area searches, transect surveys, and unoccupied aircraft systems (UAS) imagery. We used random forest modeling to examine the influence of nine environmental variables on vegetation colonization and conducted targeted soil sampling to examine additional factors contributing to vegetation patterns. Limited pre-construction vegetation survived soil addition, likely due to the sediment thickness (mean = 69 cm) and placement method. After 1 year, about 10% of the initially bare area saw vegetation reestablishment. Elevation and inundation frequency were particularly critical to understanding restoration success, with greatest vegetation cover in high-elevation areas tidally inundated

Salt marshes and seagrass meadows can sequester and store high quantities of organic carbon (OC) in their sediments relative to other marine and terrestrial habitats. Assessing carbon stocks, carbon sources, and the transfer of carbon between habitats within coastal seascapes are each integral in identifying the role of blue carbon habitats in coastal carbon cycling. Here, we quantified carbon stocks, sources, and exchanges in seagrass meadows, salt marshes, and unvegetated sediments in six bays along the California coast. In the top 20 cm of sediment, the salt marshes contained approximately twice as much OC as seagrass meadows did, 4.92 ± 0.36 kg OC m−2 compared to 2.20 ± 0.24 kg OC m−2, respectively. Both salt marsh and seagrass sediment carbon stocks were higher than previous estimates from this region but lower than global and US-wide averages, respectively. Seagrass-derived carbon was deposited annually into adjacent marshes during fall seagrass senescence. However, isotope mixing models estimate that negligible amounts of this seagrass material were ultimately buried in underlying sediment. Rather, the vast majority of OC in sediment across sites was likely derived from planktonic/benthic diatoms and/or C3 salt marsh plants.

Saltmarshes are one of the most productive ecosystems, which contribute significantly to coastal nutrient and carbon budgets. However, limited information is available on soil nutrient and carbon losses via porewater exchange in saltmarshes. Here, porewater exchange and associated fluxes of nutrients and dissolved inorganic carbon (DIC) in the largest saltmarsh wetland (Chongming Dongtan) in the Changjiang River Estuary were quantified. Porewater exchange rate was estimated to be (37±35) cm/d during December 2017 using a radon (222Rn) mass balance model. The porewater exchange delivered 67 mmol/(m2·d), 38 mmol/(m2·d) and 2 690 mmol/(m2·d) of dissolved inorganic nitrogen (DIN), dissolved silicon (DSi) and DIC into the coastal waters, respectively. The dominant species of porewater DIN was NH 4 + (>99% of DIN). However, different with those in other ecosystems, the dissolved inorganic phosphorus (DIP) concentration in saltmarsh porewater was significantly lower than that in surface water, indicating that saltmarshes seem to be a DIP sink in Chongming Dongtan. The porewater-derived DIN, DSi and DIC accounted for 12%, 5% and 18% of the riverine inputs, which are important components of coastal nutrient and carbon budgets. Furthermore, porewater-drived nutrients had obviously high N/P ratios (160–3 995), indicating that the porewater exchange process may change the nutrient characteristics of the Changjiang River Estuary and further alter the coastal ecological environment.

Accurately mapping, modeling, and managing the diversity of wetlands present in estuaries often relies on habitat classification systems that consistently identify differences in biotic structure or other ecosystem characteristics between classes. We used field data from four Oregon estuaries to test for differences in vegetation structure and edaphic characteristics among three tidal emergent marsh classes derived from National Wetlands Inventory (NWI) data: low estuarine marsh, high estuarine marsh, and tidal palustrine marsh. Independently of NWI class, we also evaluated the number and types of plant assemblages present and how edaphic variables, non-native plant cover, and plant species richness varied among them. Pore water salinity varied most strongly across marsh classes, with sediment carbon and nitrogen content, grain size and marsh surface elevation showing smaller differences. Cover of common vascular plant species differed between marsh classes and overall vegetation composition was somewhat distinct among marsh types. High estuarine marsh had the largest species pools. However, plot-level plant diversity was similar among marsh classes. Non-native species cover was highest in tidal palustrine and high estuarine marshes. The marshes in the study contained a large number of plant assemblages with most occurring across more than one marsh class. The more common assemblages occurred along a continuum of tidal elevation, soil salinity, and edaphic characteristics, with varying plant richness and non-native cover. Our data suggest that NWI classes are useful for differentiating several general features of Oregon tidal marsh structure, but that more detailed information on plant assemblages found within those wetland classes would allow more precise characterization of additional wetland features such as edaphic conditions and plant diversity.

Payne, A.R.; Burdick, D.M.; Moore, G.E., and Wigand, C., 2021. Short-term effects of thin-layer sand placement on salt marsh grasses: A marsh organ field experiment. Journal of Coastal Research, 37(4), 771–778. Coconut Creek (Florida), ISSN 0749-0208.Salt marshes can build in elevation with sea-level rise through accumulation of inorganic sediment and organic matter, but marshes worldwide are under threat of drowning due to rapid rates of sea-level rise that outpace natural marsh building rates. The application of a thin layer of sediment to the marsh surface (thin-layer placement [TLP]) is a tool to build elevation and decrease flooding stress, but its effects on marsh plants are understudied, especially in New England. In a novel application of a marsh organ experiment (i.e. rows of pots at different elevations), the addition of 10 cm of sand to pots planted with Spartina alterniflora and Spartina patens resulted in fewer stems than controls for S. patens but not S. alterniflora after 2 months. However, total biomass and root mass were not significantly impacted for either species, suggesting plants will fully recover from TLP over longer timescales. Effects of TLP on biomass and stem density did not vary significantly by elevation. Although long-term research is still needed, short-term equivalency in biomass between TLP treatments and controls suggests TLP of 10 cm is a promising strategy to enhance the ability of marshes to build vertically as sea level rises in New England.

Benthic and pelagic primary producers had their isotopic (ẟ13C, ẟ15N) and elemental (C, N) composition monitored in the Patos Lagoons estuary, in southern Brazil. The present dataset comprises temporal data obtained through seasonal samplings of C3 (Scirpus spp.) and C4 (Spartina densiflora) salt marsh plants, ephemerous bloom-forming drift macroalgae (Ulvophyceae), the widgeon grass Ruppia maritima, particulate (POM) and sedimentary (SOM) organic matter in shallow waters (< 2m) of a subtropical estuary from austral summer 2010 to autumn 2016. POM and SOM were collected as proxies of phytoplankton and microphytobenthos, respectively. Salt marsh plants were randomly sampled (N = 126) at a regularly flooded low marsh area, whereas submerged drift macroalgae (N = 29) and Ruppia plants (N = 14) were collected in adjacent mudflats. POM was collected (N = 33) by filtering water samples using glass fiber filter. SOM was obtained (N = 35) by removing superficial sediment. In laboratory, samples were processed and further analyzed for total organic carbon (TOC), total nitrogen (TN) and carbon (13C/12C) and nitrogen (15N/14N) stable isotopes ratios. With a total of 237 samples analyzed, this dataset provides key information on the isotopic and elemental composition of distinct estuarine primary producers and sources of particulate organic matter (POM and SOM) and their temporal variability in a highly variable aquatic environment. Such knowledge may add to ecological studies investigating food webs, biogeochemical cycles and sources tracking in coastal systems.

The historical ecological paradigm of wetland ecosystems emphasized the role of physical or “bottom‐up” factors in maintaining functions and services. However, recent studies have shown that the loss of predators in coastal salt marshes can lead to a significant reduction in wetland extent due to overgrazing of vegetation by herbivores. Such studies indicate that consumers or “top‐down” factors may play a much larger role in the maintenance of wetland ecosystems than was previously thought. The objective of this study was to evaluate whether altering top‐down control by manipulating the presence of predators can lead to measurable changes in salt marsh ecosystem properties. Between May and August of 2015 and 2016, we established exclosure and enclosure cages within three New England coastal wetland areas and manipulated the presence of green crab predators to assess how they and their fiddler and purple marsh crab prey affect changes in ecosystem properties. Predator presence was associated with changes in soil nitrogen and aboveground biomass at two of the three field sites, though the magnitude and direction of these effects varied from site to site. Further, path analysis results indicate that across field sites, a combination of bottom‐up and top‐down factors influenced changes in measured variables. These results challenge the growing consensus that consumers have strong effects, indicating instead that predator impacts may be highly context‐dependent., The objective of this study was to evaluate whether altering top‐down control by manipulating the presence of predators can lead to measurable changes in salt marsh ecosystem properties. Predator presence was associated with changes in soil nitrogen and aboveground biomass at two of the three field sites, though the magnitude and direction of these effects varied from site to site. These results challenge the growing consensus that consumers have strong effects, indicating instead that predator impacts may be highly context‐dependent.

Understanding blue carbon storage and accumulation prior to engineering projects is essential for assessing the potential co-benefit of carbon storage for natural climate solutions. This study coll...

Coastal wetlands are carbon and nutrient sinks that capture large amounts of atmospheric CO2 and runoff of nutrients. ‘Blue carbon’ refers to carbon stored within resident vegetation (e.g. mangroves, tidal marshes and seagrasses) and soil of coastal wetlands. This study aimed to quantify the impact of vegetation type on soil carbon stocks (organic and inorganic) and nitrogen in the surface soils (0–10 cm) of mangroves and tidal marsh habitats within nine temperate coastal blue carbon wetlands in South Australia. Results showed differences in surface soil organic carbon stocks (18.4 Mg OC ha–1 for mangroves; 17.6 Mg OC ha–1 for tidal marshes), inorganic carbon (31.9 Mg IC ha–1 for mangroves; 35.1 Mg IC ha–1 for tidal marshes), and total nitrogen (1.8 Mg TN ha–1 for both) were not consistently driven by vegetation type. However, mangrove soils at two sites (Clinton and Port Augusta) and tidal marsh soils at one site (Torrens Island) had larger soil organic carbon (SOC) stocks. These results highlighted site-specific differences in blue carbon stocks between the vegetation types and spatial variability within sites. Further, differences in spatial distribution of SOC within sites corresponded with variations in soil bulk density (BD). Results highlighted a link between SOC and BD in blue carbon soils. Understanding the drivers of carbon and nitrogen storage across different blue carbon environments and capturing its spatial variability will help improve predictions of the contribution these ecosystems to climate change mitigation.

Although salt marsh is a classic example of primary succession, the underlying mechanisms and their time-scales are poorly understood. As salt marsh succession depends on sediment accretion, the amelioration of abiotic conditions associated with increasing elevation suggests potential explanatory roles for facilitation, competition and the stress-gradient hypothesis. We present a 35-year longitudinal study of salt marsh development from intertidal flat to a mid-marsh platform at Odiel Marshes in south-western Iberia. Using permanent plots, this work chronicles changes in elevation and marsh morphology, their evolving effects on sediment redox potential and salinity and the colonisation and changing patterns of dominance of halophytic species. Sporadically colonising clumps of the low-marsh species Spartina maritima trapped sediment to form raised tussocks, which increased in elevation and area. Reduced tidal inundation and locally improved drainage promoted higher redox potentials and allowed colonisation by a sequence of species less tolerant of reducing conditions: Sarcocornia perennis, its hybrid with high-marsh S. fruticosa, and Atriplex portulacoides. Unlike its centrifugally colonising predecessors, A. portulacoides invaded from the tussock edges. Transplant experiments designed to investigate its late establishment on tussocks showed that seedling survival depended on elevational differences as small as 4 cm. After increasing in elevation by c. 1 m (c. 29 mm/year), coalescence of the tussocks formed a marsh platform at a level corresponding to mean high tides. This supports a theoretical punctuated transition from ‘submergence marsh’ to ‘emergence marsh’, previously postulated for this tidal elevation. Synthesis. The unexpected rapidity of this primary succession highlights the central role of facilitation. Vertical sediment accretion, locally engineered by colonising species, progressively alleviates abiotic stress and allows colonisation by species that are less tolerant of chemically reducing conditions but are ultimately better competitors.

We investigated S. alterniflora saltmarshes in 10 regions to distinguish the major sources of soil organic carbon (SOC) and provide data for SOC content in Chinese coastal wetlands under exotic invasion. We collected soil and plant samples. We analyzed the soil properties for SOC content, recalcitrant organic carbon (RC), labile organic carbon (LC), total nitrogen (TN), total phosphorus (TP), total salinity and water content, as well as stable isotopic carbon composition (δ13C) in the SOC and RC. SOC and RC were determined using a Shimadzu TOC analyzer. We used δ13C values and C/N ratios of the SOC to identify carbon source inputs. The δ13C of the SOC and RC was measured using a Sercon Integra CN isotope ratio mass spectrometer. The results showed that the S. alterniflora saltmarsh SOC pool properties differed significantly between study areas, dependent upon local circumstances and carbon exchanges. The δ13C values of SOC in surface sediments from Tianjin to Guangxi were from −25.16 to −20.64‰, RC from −27.82 to −23.1‰, and C/N ratios between 11.36 and 19.00. These samples had SOC values of 14.77–23.95 g kg−1, RC values of 11.98–21.44 g kg−1, and LC values of 0.44–4.08 g kg−1. The SOC and RC of surface sediments were positively correlated with TN and TP. S. alterniflora saltmarsh SOC is derived from multiple carbon inputs including from marine and land sources and its formation is driven by complicated factors. TN and TP inputs increase SOC in coastal wetlands. Generally, terrigenous carbon inputs dominate the SOC composition in Chinese coastal wetlands, although marine-derived organic carbon (OC) inputs to sediment increase from northern to southern areas.

Pollen assemblages closely reflect the local vegetation that characterizes the salinity status, providing useful analogs for paleoecologxical reconstruction in regional deposits. Palynological evidences of surface sediments from the Coringa mangrove wetland were correlated with the physicochemical and sediment salinity records to observe the relationship between them. The statistical analysis of the data obtained here revealed a marked horizontal salinity gradient from north to south.. In this study, ordination (detrended correspondence analysis) of palynomorph groups has identified a salinity gradient of 1.1 to 3.0 PSU from the north to the south in the wetland. High palynomorph deposition and lower salinity are observed along channel margins due to the mixing of estuarine water during the rainy season. The mudflats along the transects show a lower diversity of plants in the pollen record and high total dissolved solids (TDS) than commonly found in the coastal wetlands. The presence of scrubby halophytic vegetation in the upper saltmarsh and oligohaline-freshwater vegetation in the low tidal saline marshes is a zonation pattern related to the localized influence of freshwater conditions. Palynomarine Index (PMI) reveals the highest freshwater/tidal inundation along Ramannapalem due to the presence of numerous riverine channels while the remaining part of the wetland has restricted tidal inundation leading to the conversion of mangrove forests to salt pans and paddy fields. Thus, the health and productivity of the mangrove ecosystem are also governed by hydrodynamics, catchment land use, water discharge in the channels, and tidal flushing.

Coastal marshes are important blue carbon reservoirs, but it is unclear how vegetation shifts associated with tidal restoration and sea level rise alter soil microbial respiration rates and bacterial community composition. Within 20 Connecticut salt marshes (10 without tidal restrictions, 10 tidally restored), we sampled three vegetation zones dominated by Spartina alterniflora (short-form, < 30 cm tall), S. patens, and Phragmites australis to estimate microbial respiration rates (SIR, substrate-induced respiration; carbon mineralization), root zone bacterial 16S rRNA genes, and a suite of plant and soil characteristics. Carbon density was greater in unrestricted marshes than tidally restored marshes and was the only parameter that differed among sites with varying restoration histories. We observed strong differences among vegetation zones, with vegetation being a top predictor of both SIR and carbon mineralization. Electrical conductivity (EC) was also a top predictor for SIR, and we observed strong, positive correlations between EC and both metrics of microbial respiration, with elevated rates in more frequently inundated S. alterniflora than P. australis zones. We also observed distinct root zone microbial communities associated with vegetation zones, with greater abundance of sulfate-reducing bacteria in Spartina spp. zones. Our findings suggest that dominant salt marsh vegetation zones are useful indicators of hydrologic conditions and could be used to estimate microbial respiration rates; however, it is still unclear whether differences in microbial respiration and community composition among vegetation zones are driven by plant community, environmental conditions, or their interactions.

Tidal marsh wetlands in the Yellow River Delta provide valuable eco-services to the local population and global ecology. However, this area is suffering from serious degradation under the stresses of social development and climate change. Hydrological connectivity, a new framework in hydrology and ecology, has been proposed as the main factor affecting the ecological processes in coastal wetlands; however, its role in hydrology–soil–vegetation interactions remains unclear. In this study, the researchers parametrically quantified the hydrological connectivity in the tidal marsh wetlands and analyzed its relationship with Phragmites australis, one of the dominant species in this area. Our results showed threshold-like effects on the interaction between hydrological connectivity and P. australis on the plot scale. When biomass is lower than 2.2 kg/m2, the population density and structure size were found to increase with hydrological connectivity. When the biomass is higher than the threshold, the plots disconnected hydrologically because of high water consumption. Compared with soil chemistry, salinity, and water soil content, hydrological connectivity in the surface soil layer is more strongly linked to the plant traits and spatial structure in the tidal marsh wetlands due to the narrow ranges of other variables. Based on the authors’ analysis, the researchers do not recommend dense plantation of P. australis, especially near the freshwater sources in the tidal marsh, because of its high reproduction ability and competitive nature, which may cut the freshwater connectivity off, lowering the richness of plant species and habitat diversity.

Designing effective restoration strategies is a priority in recovering salt marsh plants. Hydrological connectivity is a main driver underpinning the success of the plant recovery process and can regulate life‐history process‐based restoration strategies. However, the relationship between these is unclear. Plant recovery needs to go through a whole life‐history process, from seed to adult. Common restoration strategies are seed addition (SA) or seedling transplantation (ST), which start from seed germination and seedling growth stage. Besides these two strategies, another strategy starting from seed retention stage, microtopographic adjustment (MA), was designed to study the relationship with hydrological connectivity. A framework was also constructed to assess a gradient of hydrological connectivity between marsh plain and sea. We conducted several field experiments to test their relationships. The composite measurement of hydrological connectivity with five geomorphic variables can well represent the variation of environmental factors. Soil moisture, inundation frequency and sediment deposition were positively correlated, while soil salinity and hardness were negatively correlated with hydrological connectivity. The success of different restoration strategies varied with hydrological connectivity. MA showed a monotone decreasing trend, while SA and ST showed a unimodal trend as hydrological connectivity increased. Importantly, each strategy occupies a non‐overlapping optimum range along the hydrological connectivity gradient. There is low hydrological connectivity for MA (0–0.28), middle hydrological connectivity for SA (0.28–0.55) and high hydrological connectivity for ST (0.55–1). Synthesis and applications. Our findings expand the quantification of the hydrological environment beyond elevation, distance or other single index to include a range of elements of hydrological connectivity, thus illustrating the underlying mechanisms of hydrological connectivity which regulate restoration strategies based on different life stages. The results provide a reliable framework to assess hydrological connectivity and offer guidance to select the optimum restoration strategy under different hydrological connectivities or to regulate the hydrological connectivity variables (topography on marsh plain and morphology of tidal creeks) to relief stresses. These findings will be beneficial to ecological restoration and coastal management.

Salt marshes protect estuary banks from erosion by acting as buffers between marine and terrestrial environments. Residents living near the Breede River estuary, Western Cape Province, South Africa, raised concerns about ongoing erosion evident at Groenpunt, the main salt marsh. This study aimed to determine how long erosion has been taking place, the rate and possible causes thereof. Aerial images and environmental data were assessed for the years 2002–2020. Erosion was first evident in 2003. By 2020, the marsh edge had been eroded into a series of micro-bays, incised horizontally up to 7 m, with a scarp height of 0.7 m, corresponding to a loss of 1 313 m2 of salt marsh and a bank retreat rate of 0.66 (SE 0.44) m year−1. Over the study period, there was a regular pattern of high-frequency gale-force winds (>8 on the Beaufort scale), with significantly more winds of this magnitude occurring in 2002 than in other years. The wind wave fetch adjacent to the marsh is up to 1 km in the direction of the predominant wind, and it is likely that the cumulative effects of constant wind-generated waves drove the erosion process. Estuary water and tidal levels over the period reflected normal seasonal fluctuation patterns. The bank supporting Groenpunt salt marsh is eroding at a rate that could possibly see it disappear within the next 60 years, reducing biodiversity and ecosystem services in the estuary. In the face of increasing climatic variability predicted in the future, similar salt marsh erosion is likely to become more prevalent.

Summary Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for sediment transport across marshes to show that pond formation leads to runaway marsh fragmentation, a process characterized by a self-similar hierarchy of pond sizes with power-law distributions. We find the threshold for marsh fragmentation scales primarily with tidal range and that sediment supply is only relevant where tides are sufficient to transport sediment to the marsh interior. Thus the RSLR threshold is controlled by organic accretion in microtidal marshes regardless of the suspended sediment concentration at the marsh edge. This explains the observed fragmentation of microtidal marshes and suggests a tipping point for widespread marsh loss.