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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.

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.

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 biogeochemistry of tidal marsh sediments facilitates the transformation of mercury (Hg) into the biologically available form methylmercury (MeHg), resulting in elevated Hg exposures to tidal marsh wildlife. Saltmarsh and Acadian Nelson’s sparrows (Ammospiza caudacutua and A. nelsoni subvirgatus, respectively) exclusively inhabit tidal marshes, potentially experiencing elevated risk to Hg exposure, and have experienced range-wide population declines. To characterize spatial and temporal variation of Hg exposure in these species, we sampled total mercury (THg) in blood collected from 9 populations spanning 560 km of coastline, including individuals resampled within and among years. Using concurrent nesting studies, we tested whether THg was correlated with nest survival probabilities, an index of fecundity. Blood THg ranged from 0.074–3.373 µg/g ww across 170 samples from 127 individuals. We detected high spatial variability in Hg exposure, observing differences of more than 45-fold across all individuals and 8-fold in mean blood THg among all study plots, including 4-fold between study plots within 4 km. Intraindividual changes in blood Hg exposure did not vary systematically in time but were considerable, varying by up to 2-fold within and among years. Controlling for both species differences and maximum water level, the dominant driver of fecundity in this system, nest survival probability decreased by 10% across the full range of female blood THg concentrations observed. We conclude that Hg has the potential to impair songbird reproduction, potentially exacerbating known climate-change driven population declines from sea-level rise in saltmarsh and Acadian Nelson’s sparrows.

We present a high resolution multi-proxy study about environmental changes that occurred during the Holocene sea-level transgression and successive highstand (7–5.5 Kyr) in the central zone of Bahia Samborombon (∼35.6° S), Argentina. This study, one of the first in the area that integrates geomorphological and paleontological proxy data (pollen, non-pollen palynomorphs, foraminifera and mollusk shells), and focusing on a short time interval, offers the advantage of recognizing environmental and vegetation changes in a more comprehensive way. The aim of this work is to reconstruct the salt marsh vegetation history and paleoenvironmental changes; and to develop a model of the geomorphological evolution during the mid-Holocene sea-level transgressive phase. Between 6700 cal yr BP and 6500 cal yr BP, several rapid environmental changes took place. Those changes affected vegetation and microfauna, as well as the geomorphological features. During this period, middle to high salt marsh vegetation developed in an extensive and open tidal plain, where wide and long paleo tidal channels were flowing. Between 6500 and 6220 cal yr BP, the salt marsh transitioned into low salt marsh one as a response to a continued rise in sea level, resulting in the landward transgression of the coastline. The relative sea level reached the highstand between 6220 and 5500 cal yr BP and, consequently, extensive inundation of the coastal plain took place, except for the elevated areas such lunettes and ridges. After 5500 cal yr BP, seaward prograding beach and sand ridges formed in response to the onset of a regressive phase. Since then, the ancient tidal plain, extending ∼10 km from the present coastline, has been subjected to aeolian and pedogenetic processes.

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.

A multidisciplinary group of salt marsh professionals from Maine to Virginia participated in a collaborative learning exchange to improve restoration for the overall health and resilience of coastal wetlands. This was an unprecedented forum through which participants representing different geographies, backgrounds, and roles in salt marsh management were able to share and learn from one another to develop the best available restoration methods for on-the-ground projects that address multiple benefits. By including mosquito control agencies, restoration practitioners, regulatory agencies, academic researchers, and conservation organizations in the learning exchange, we developed an understanding and acceptance of different approaches. Regulators learned about project ideas and contributed to project designs in early development stages. Collaborating while engaged in on-the ground projects enabled participants to implement lessons learned in real time. Field trips to restoration sites at different stages of development allowed a greater and more fluid exchange of ideas and practical implementation advice. Practitioners leveraged resources and developed new collaborations. Lessons learned and shared through this faster and more flexible forum will inform the design, implementation, and monitoring of restoration projects across the region and improve overall marsh health and resilience in the face of climate change. Learning exchanges like this should be used more frequently to improve the efficiency and effectiveness of coastal restoration particularly when there is a windfall of cash and a short window of opportunity such as with post-disaster federal spending.

Macrobenthos organisms are vital ecological components of intertidal wetlands. This work studied effects of non-native mangrove rehabilitation (Kandelia obovata) and salt marsh invasion (Spartina alterniflora) on macrobenthos communities in Ximen Island (Zhejiang, China). The macrobenthos communities were compared between rehabilitated K. obovata forests of five different stand ages (2-, 8-, 11-, 16-, and 60-year-old) and adjacent S. alterniflora marsh and non-vegetated mudflat. Compared to the mudflat, S. alterniflora increased the abundance, Shannon–Wiener diversity index (H’), and Margalef's richness index (d’) of macrobenthos. Gastropoda abundance was high in the S. alterniflora marsh, whereas, malacostraca biomass was low. Within mangrove forests, macrobenthos abundance was positively correlated with the stand age of mangroves (Spearman Correlation, P

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.

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.

Whole genome duplications (WGDs) lead to polyploid specimens and are regarded as major drivers for speciation and diversification in plants. One prevalent problem when studying WGDs is that effects of polyploidization in ancient polyploids cannot be disentangled from the consequences of selective evolutionary forces. Cytotypic differences in distribution, phenotypic appearance and in response to surface elevation (determined by HOF-modeling) were identified in a relatively young taxa-group of a hexaploid F1-hybrid (Spartina× townsendii H. Groves & J. Groves, Poaceae) and its dodecaploid descendent (Spartina anglica C.E. Hubbard, Poaceae) using vegetation assessments (1029 plots; 1 × 1 m2) from the European Wadden Sea mainland salt marshes, including elevational and mean high tidal (MHT) data. While the F1-hybrid was mainly present in the eastern part of the Wadden Sea, its dodecaploid descendent occurred in the entire Wadden Sea area. The Spartina cytotypes differed in phenotypes (median of Spartina cover: hexaploid = 25% vs. dodecaploid = 12%) and in elevational niche-optimum (hexaploid = − 49.5 cm MHT vs. dodecaploid = 8.0 cm MHT). High ploidy levels correlated with establishment success in Spartina along geographic gradients but did not seem to increase the capacity to cope with abiotic severity downwards the elevational gradient in salt marshes.

Salt marshes are vulnerable to climate change-associated sea-level rise and storm-induced surges. Their degradation will likely affect shorebirds relying on this ecosystem. Least Sandpiper (Calidris minutilla) and Pectoral Sandpiper (Calidris melanotos) migrating along coastline habitats typically use salt marshes to rest and replenish their body reserves. Our objective was to test if width of the different vegetation zones within salt marshes affects the occurrence of Least and Pectoral Sandpipers stopping along the St. Lawrence River Estuary, Quebec, Canada, during fall migration. We established 26 survey sites, each 600 m in length, along the shoreline. Shorebird surveys were conducted in 2011 and 2012. We characterized salt marshes by measuring the width of each vegetation zone (lower marsh and upper marsh). We analyzed shorebird presence/not detected data with generalized estimating equations to test the predictions that occurrence of Least Sandpipers and Pectoral Sandpipers increases with width of both the lower and upper marsh. Upper marsh width was positively associated with probability of occurrence in each species. Our results highlight the importance of protecting the integrity of salt marshes for these two species. In the St. Lawrence River Estuary, where landward migration of salt marshes is no longer possible (coastal squeeze), effective management of shorelines is much needed. Otherwise, salt marshes and these two species could be locally jeopardized.

The San Francisco Bay Area is a leader in environmental stewardship and home to numerous wetland restoration projects including the largest tidal wetland restoration project on the American West Coast. As tidal marsh wetlands are restored throughout the Bay Area many opportunities remain to reaffirm the importance of water management that reduces mosquito production and protects public health. Unlike the early 1900s when long term saltmarsh mosquito control was achieved with large scale surface water management projects, regulatory restrictions produce new hurdles that impact mosquito control and restoration projects alike. Work done in the wetlands surrounding the San Francisco Bay must comply with existing management plans, permit requirements, and government regulations. The same is true for emerging technologies. While unmanned airsystems employed for mosquito control improves efficiency and accuracy, regulations in this arena limit their broad use in wetlands that abut the San Francisco Bay. Mosquito abatement districts collect substantial scientific data that inform land management and mosquito control operations. This information is useful for evaluating wetland restoration progress in the Bay Area and fostering partnerships that keep a public health perspective at the forefront.

Animal groups are known to exhibit collective behaviours that allow for effective responses to predators and environmental factors. Habitats with high levels of structural complexity have been shown to influence the collective tendencies of these animal aggregations. However, the effect of anthropogenically induced habitat complexity on collective tendency is still unknown. We examined the effect of a water control structure (WCS), located in an estuarine salt marsh system, on the collective behaviours of free ranging juvenile Gulf menhaden (Brevoortia patronus), a schooling forage fish. Using an acoustic imaging sonar, behavioural observations prior to and during predator attacks were collected in an open marsh canal as well as in the vicinity of the WCS. We found that fish in schools responded to the WCS by swimming in a less aligned and cohesive manner, while maintaining an increased ability to transfer social information. Our results demonstrate that collective tendency in schooling fish is affected by the presence of anthropogenically introduced habitat complexity, prior to and during predator attack. Our study, thus, strengthens the idea that the complex environmental conditions induced by the presence of man-made structures may play a large role in structuring collective behaviour and trophic interactions in aquatic environments.

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.

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.

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.

Saltmarsh sparrows ( Ammospiza caudacuta ) and seaside sparrows ( A. maritima ) are species of conservation concern primarily due to global sea-level rise and habitat degradation. Environmental mercury (Hg) contamination may present additional threats to their reproductive success and survival. To assess site-specific total mercury (THg) exposure and identify environmental correlates of THg detection across a large portion of the breeding range where these species co-occur, we sampled blood from adult male saltmarsh and seaside sparrows at 27 marsh sites from Maine to Virginia, USA. The mean THg concentration (±1 SD) throughout the entire sampling range was 0.531 ± 0.287 µg/g wet weight (ww) for saltmarsh sparrows and 0.442 ± 0.316 µg/g ww for seaside sparrows. Individual THg concentrations ranged from 0.135–1.420 µg/g ww for saltmarsh sparrows and 0.153–1.530 µg/g ww for seaside sparrows. Model averaging from a suite of linear mixed models supported species-based differences in blood THg. On average, saltmarsh sparrows had 20.1% higher blood THg concentrations than seaside sparrows. We hypothesize that species-specific THg concentrations are influenced by differences in diet or foraging preferences between these species. We did not detect any correlations between sparrow THg concentrations and land cover characteristics surrounding sampled marshes or cumulative average annual precipitation.

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.

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.

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.

Evidence for relative sea-level rise in the Mullica River-Great Bay, a relatively undisturbed watershed in southern New Jersey, stretches over hundreds of years. The increase in global sea-level rise in the region is enhanced by subsidence and results in rates that are approximately double the global average. In recent decades, the occurrence of “ghost forests,” standing dead forests, especially of the salt-intolerant Atlantic White Cedar, is becoming increasingly obvious as tidal inundation of salty waters increases further inland, especially in the upper portions of the Mullica River and its tributaries. Even older evidence of sea-level rise in the watershed is the subtidal and intertidal occurrence of “cedar cemeteries,” i.e., buried accumulations of Atlantic white cedar stumps and timbers that have been radiocarbon-dated from the fifteenth to the sixteenth centuries to as old as the fifth century. Some of these are being exposed as rising water extends intertidal creeks into adjacent wetlands and uncovers this rot-resistant wood. Sea-level rise is perhaps the most significant threat to the persistence of salt marshes over the coming century. Sea-level rise decreases salt marsh area by erosion at the marsh edge, drowning of the marsh surface, and the expansion of marsh pools into larger marsh “lakes.” In some instances, this loss of salt marshes is compensated for by expansion landward into ghost forests, but in this watershed and others, the expansion is by invasive plant species such as Phragmites australis, which is suboptimal habitat for fishes, crabs, and other invertebrates. The combination of a fast rate of sea-level rise in an area relatively free from recent human intervention makes the Mullica Valley watershed an ideal location to continue to evaluate the effects of sea-level rise on salt marsh ecosystems.

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.

Sea-level rise is leading to the migration of marshes into coastal forests throughout North America. Marsh migration represents a primary mechanism for marsh survival in the face of sea-level rise and leads to a fundamental reorganization of vegetation communities. Yet, the ecological implications of these changes remain unknown. To evaluate the effect of marsh migration on ecosystem function, we compared habitat, primary production, vertical accretion, nutrient cycling, and carbon accumulation between new and old salt marsh on Goodwin Island (Virginia, USA) where salt marsh is migrating landward into rapidly retreating coastal forest. Using historical imagery and radioisotopic dating of sediment cores, we determined marsh age (

Centuries of coastal development has led to the loss of saltmarsh extent worldwide. As marshes are shrinking, scientific understanding of marsh expansion and erosion processes is growing. Coastal managers are also recognising the importance of marshes for flood protection, carbon sequestration, and pollutant filtering. Considerable effort is now being made to conserve saltmarshes. However, the rapid integration of science in policy remains an obstacle for ensuring successful conservation outcomes. This review explores how advances in the understanding of coastal dynamics, and the evolution of coastal management thinking, are shaping saltmarsh conservation policy in Great Britain. Saltmarsh management has shifted from reclamation, to protection, to restoration throughout the 20th and 21st centuries as calls for nature conservation grew and the importance of ecosystems in coastal erosion risk management became apparent. Studies have revealed that marshes cycle between expansion and erosion phases as part of their natural evolution, governed by processes acting across a range of spatial and temporal scales. Understanding which processes drive long-term marsh change provides an opportunity for coastal managers to undertake targeted intervention for positive conservation outcomes. The inherently dynamic nature of marshes also raises significant challenges in forecasting the long-term value provided by a given marsh. Challenges remain in the monitoring and management of sediment supply and transport, and the effective engagement with stakeholders during habitat protection and creation schemes, which are key to achieving marsh conservation goals.

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.

The paper outlines the evolution of an Australian form of Open Marsh Water Management (OMWM) called runnelling which was implemented in relatively pristine saltmarshes. Runnelling involves shallow channels and was distinguished from OMWM which involves ditching. Both methods aim to conserve tidal hydrology. The core research covers 28 years of research on the original experimental site (Coomera Island, Queensland), in routine monitoring and impact assessments that were made periodically. It also introduces and incorporates information from associated research. This includes effects of runnelling on non-target aspects—snails and crabs. Emerging issues during the course of the longer-term research included the risk of mobilising acid sulfates and of mangrove encroachment into the saltmarsh. As well, use of the data was extended to retrofit it to address broader questions about saltmarsh processes and how to identify them. That identified progressive changes at the Coomera site, and so a further query was whether the system showed signs of approaching a tipping point. In summary, there were some identifiable effects of runnelling in all parts of the research. These were mostly related to the increased wetness near runnels, as flooding by tides would occur at tides up to 0.30 m lower than the pre-runnelling situation. The increased wetness was related to plants, snails and crabs. From a mosquito management perspective the method was successful. From an environmental perspective the environmental changes were only very close to runnels (within 10 m). The overall changes at the experimental site were consistent with other changes in eastern Australia, especially of mangrove encroachment into saltmarshes. Once the runnelling method appeared to manage mosquitoes without damaging wetland values it was adopted widely throughout south east Queensland, encouraged by the positive attitude of permitting agencies, but longer-term evaluation was hindered by lack of relevant records.

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.

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.

Over the last 20 years, innovations have led to the development of exciting new technologies and novel applications of established technologies, collectively increasing the scale, scope, and quality of research possible in tidal marsh systems. Thus, ecological research on marshes is being revolutionized, in the same way as ecological research more generally, by the availability of new tools and analytical techniques. This perspective highlights current and potential applications of novel research technologies for marsh ecology. These are summarized under several themes: (1.) imagery — sophisticated imaging sensors mounted on satellites, drones, and underwater vehicles; (2.) animal tracking — acoustic telemetry, passive integrated transponder (PIT) tags, and satellite tracking, and (3.) biotracers — investigation of energy pathways and food web structure using chemical tracers such as compound-specific stable isotopes, isotope addition experiments, contaminant analysis, and eDNA. While the adoption of these technological advances has greatly enhanced our ability to examine contemporary questions in tidal marsh ecology, these applications also create significant challenges with the accessibility, processing, and synthesis of the large amounts of data generated. Implementation of open science practices has allowed for greater access to data. Newly available machine learning algorithms have been widely applied to resolve the challenge of detecting patterns in massive environmental datasets. The potential integration on digital platforms of multiple, large data streams measuring physical and biological components of tidal marsh ecosystems is an opportunity to advance science support for management responses needed in a rapidly changing coastal landscape.

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.

Development of coastal New England has led to the replacement of up to 37% of salt marshes with degraded freshwater wetlands, primarily through tidal restrictions. Removing these restrictions to restore salt marsh ecology would improve water quality, increase flood and storm protection, nutrient filtration, erosion control, and carbon sequestration. However, such restorations replace functional freshwater wetlands and could potentially impact freshwater species. Freshwater-dependent wildlife species such as the spotted turtle (Clemmys guttata), may be especially vulnerable to rapid changes in habitat resulting from tidal reintroduction due to their high site fidelity and limited ability to disperse quickly. Conservation of genetically and physically isolated spotted turtle populations as well as the restoration of salt marshes to mitigate climate change impacts are both high priorities on Nantucket Island. These two priorities conflicted in a project to restore tidal hydrology to an impounded freshwater marsh known to host a robust spotted turtle population. We evaluated changes to spotted turtle home range size and location and habitat use in response to salt marsh restoration over eight years. Home range size did not change but the location of home ranges shifted into bordering wetlands landward of the tidal salt water influence. Spotted turtles selected remaining freshwater marshes and shrub swamps while avoiding developed land and areas of establishing salt marsh within areas that had previously been high quality habitat. This study suggests prioritizing conservation of wetlands adjacent to planned salt marsh restoration to provide habitat for freshwater species to migrate.

Omus audouini Reiche (Coleoptera: Cicindelidae) is a night-stalking tiger beetle species of conservation concern in Canada with only limited distribution in southwestern British Columbia. Occurrence and plant community associations of O. audouini were quantified at one upper salt marsh site in 2018 at three distances from a flood-protection dike. Omus audouini was strongly associated with plant communities 15 m from the dike that were dominated by Douglas aster, Symphyotrichum subspicatum (Nees) G.L. Nesom (Asteraceae). Plant communities transitioned from a diverse assemblage near the dyke to a Douglas aster dominated estuarine meadow at 15 m and then to a halophyte dominated estuarine marsh at 30 m from the dike. In 2019, O. audouini and plants were sampled at four salt marsh sites. Locations where O. audouini was present were correlated with the presence of S. subspicatum. Indicator species analysis showed that S. subspicatum is a statistically significant indicator of the presence of O. audouini. The results are discussed as they relate to conservation of O. audouini in British Columbia. The indicator plant species will be essential in locating previously unknown populations of O. audouini and mapping potential habitat for this species at risk.

Tidal marsh restoration and creation has been proposed as a tool to build coastal resilience in the face of rising sea level and increasing intensity of coastal storms. However, it is unclear what conditions within constructed settings will lead to the successful establishment of tidal marsh. We used sediment cores and historical geospatial data in the tidal freshwater Hudson River to identify the rapid creation and development of marshes that are sheltered by human-made structures including railroad berms, jetties, and dredge spoil islands. These backwater areas rapidly accumulated clastic material following anthropogenic modification that allowed for transition from tidal mudflat to emergent marsh. In one case, historical aerial photos document this transition occurring in less than 18 years, offering a timeframe for marsh development. Accretion rates for anthropogenic tidal marshes and mudflats average 0.8-1.1 cm yr-1 and 0.6-0.7 cm yr-1 respectively, equivalent to 2-3 times the rate of relative sea level rise as well as the observed accretion rate at a 6000+ year old reference marsh in the study area. Paired historical and geospatial analysis revealed that more than half of all the tidal wetlands on the Hudson are anthropogenic and developed since the industrial era, including two thirds of the emergent cattail marsh. These inadvertently constructed tidal wetlands currently trap roughly 6% of the Hudson River’s sediment load. Results indicate that when sediment is readily available freshwater tidal wetlands can develop relatively rapidly in sheltered estuarine settings, and serve as useful examples to help guide future tidal marsh creation and restoration efforts.

Tidal marshes (including saltmarshes) provide remarkable value for many social (cultural, recreational) and environmental (fish production, water quality, shoreline protection, carbon sequestration) services. However, their extent, condition, and capacity to support these services are threatened by human development expansion, invasive species, erosion, altered hydrology and connectivity, and climate change. The past two decades have seen a shift toward working with managers to restore tidal marshes to conserve existing patches or create new marshes. The present perspective examines key features of recent tidal marsh restoration projects. Although optimism about restoration is building, not all marshes are the same; site-specific nuances require careful consideration, and thus, standard restoration designs are not possible. Restoration projects are effectively experiments, requiring clear goals, monitoring and evaluation, and adaptive management practices. Restoration is expensive; however, payment schemes for ecosystem services derived from restoration offer new ways to fund projects and appropriate monitoring and evaluation programs. All information generated by restoration needs to be published and easily accessible, especially failed attempts, to equip practitioners and scientists with actionable knowledge for future efforts. We advocate the need for a network of tidal marsh scientists, managers, and practitioners to share and disseminate new observations and knowledge. Such a network will help augment our capacity to restore tidal marsh, but also valuable coastal ecosystems more broadly.

Climate change is altering naturally fluctuating environmental conditions in coastal and estuarine ecosystems across the globe. Departures from long-term averages and ranges of environmental variables are increasingly being observed as directional changes [e.g., rising sea levels, sea surface temperatures (SST)] and less predictable periodic cycles (e.g., Atlantic or Pacific decadal oscillations) and extremes (e.g., coastal flooding, marine heatwaves). Quantifying the short- and long-term impacts of climate change on tidal marsh seascape structure and function for nekton is a critical step toward fisheries conservation and management. The multiple stressor framework provides a promising approach for advancing integrative, cross-disciplinary research on tidal marshes and food web dynamics. It can be used to quantify climate change effects on and interactions between coastal oceans (e.g., SST, ocean currents, waves) and watersheds (e.g., precipitation, river flows), tidal marsh geomorphology (e.g., vegetation structure, elevation capital, sedimentation), and estuarine and coastal nekton (e.g., species distributions, life history adaptations, predator-prey dynamics). However, disentangling the cumulative impacts of multiple interacting stressors on tidal marshes, whether the effects are additive, synergistic, or antagonistic, and the time scales at which they occur, poses a significant research challenge. This perspective highlights the key physical and ecological processes affecting tidal marshes, with an emphasis on the trophic linkages between marsh production and estuarine and coastal nekton, recommended for consideration in future climate change studies. Such studies are urgently needed to understand climate change effects on tidal marshes now and into the future.

Temperatures are increasing globally and causing species-specific geographic range expansions. In the Gulf of Mexico, mangroves are encroaching regions historically dominated by temperate salt marshes, changing animal communities and nutrient cycling in the intertidal zone. Marine systems are highly connected; therefore, we expect that changes in the intertidal will alter functions of adjacent subtidal seagrass meadows. We surveyed seagrass meadows adjacent to mangroves, salt marshes, and a mixture of the two and asked, do changes in intertidal plant composition influence (1) environmental conditions (subtidal water and sediment characteristics); (2) biogeochemical cycling (net oxygen and nitrogen gas fluxes); (3) seagrass meadow cover, biomass, and productivity; and (4) invertebrate community assemblage? There are clear differences in sediment organic matter and net nitrogen gas (N2) fluxes between adjacent intertidal habitats, but the magnitude or direction of change differs seasonally. We hypothesize that this seasonal pattern is due to outwelling from the intertidal, as mangroves senesce in fall, and marshes senesce later in winter. Therefore, changes in adjacent intertidal habitat can impact the timing of organic matter delivery. This also has implications for seagrass biomass. Thalassia testudinum belowground biomass adjacent to mangroves substantially decreased over the winter, suggesting vulnerability to stressors as the intertidal plant community shifts from marsh to mangrove dominance. Epifauna density and diversity did not vary among seagrass meadows based on adjacent intertidal habitats, but subtle differences in community assemblages associated with shifts in intertidal plant community were detected. This work demonstrates that impacts of species range expansions are far-reaching due to connectivity in marine systems.

Coastal salt marshes are distributed widely across the globe and are considered essential habitat for many fish and crustacean species. Yet, the literature on fishery support by salt marshes has largely been based on a few geographically distinct model systems, and as a result, inadequately captures the hierarchical nature of salt marsh pattern, process, and variation across space and time. A better understanding of geographic variation and drivers of commonalities and differences across salt marsh systems is essential to informing future management practices. Here, we address the key drivers of geographic variation in salt marshes: hydroperiod, seascape configuration, geomorphology, climatic region, sediment supply and riverine input, salinity, vegetation composition, and human activities. Future efforts to manage, conserve, and restore these habitats will require consideration of how environmental drivers within marshes affect the overall structure and subsequent function for fisheries species. We propose a future research agenda that provides both the consistent collection and reporting of sources of variation in small-scale studies and collaborative networks running parallel studies across large scales and geographically distinct locations to provide analogous information for data poor locations. These comparisons are needed to identify and prioritize restoration or conservation efforts, identify sources of variation among regions, and best manage fisheries and food resources across the globe.

Ponds in salt marshes are often misinterpreted as a symptom of degradation, yet ponds can also be part of a cyclical process of pond formation, expansion, breaching by tidal creeks, and vegetation recovery. Pond dynamics may be altered by accelerated sea level rise, with consequences for the long-term stability of ecosystems. This study tests the prediction that ponds are in dynamic equilibrium across one of the largest expanses of unditched salt marsh in the Northeast USA by (1) examining change in pond and marsh area between 1970 and present in both ditched and unditched regions and (2) by tracking individual pond dynamics across an 87-year time series. Across all marshes, vegetated area declined by 6.3%, with losses primarily due to edge erosion. Ditched marsh had significantly less pond area than unditched marsh. In unditched marshes, we found that net pond area has remained unchanged since 1970 because the amount of marsh conversion to ponds is equivalent to the amount of pond recovery to marsh. The overall extent of ponds connected to tidal creeks is increasing relative to isolated ponds that retain water at low tide, which suggests that some rates of change may be decoupling. This may correspond to a decline in the rate of pond formation, but other factors such as an increase in pond breaching rate and a lag in vegetation recovery rate may also contribute to this pattern. A nuanced understanding of marsh ponds needs to be incorporated into marsh condition assessments and establishment of restoration priorities so that ponds are not interpreted as evidence of degradation when they are exhibiting a recovery cycle. Unditched marshes around the world are a rare resource that remains essential for advancing scientific understanding and serving as reference sites for restoration of marshes altered by past management.

Salt marshes are key coastal ecosystems that provide habitats for wildlife, including invertebrates, fishes, and birds. They provide ecosystem services such as protection from storm surges and waves, attenuation of flooding, sequestration of pollutants (e.g., blue carbon), and nutrient removal. They are currently under great threat from sea level rise (SLR). We collected information about trends in the horizontal extent (acreage) of New Jersey salt marshes and recent elevation changes compared with the current local rate of SLR in New Jersey, which is between 5 and 6 mm year−1. We found pervasive, although variable, rates of marsh loss that resulted from both anthropogenic disturbance as well as edge erosion and interior ponding expected from SLR. Elevation trends suggest that the current rates of SLR exceed most marsh elevation gains, although some Phragmites-dominated marshes keep pace with SLR. Four potential remedies to address current coastal trends of marsh loss were described in the context of New Jersey’s regulatory and management environment: protection of marsh inland migration pathways, altered management of Phragmites, thin layer sediment placement, and living shoreline installations. Proactive steps are necessary if coastal wetland ecosystems are to be maintained over the next few decades.

The persistence of tidal wetland ecosystems like salt marshes is threatened by human interventions and climate change. Particularly the threat of accelerated sea level rise (SLR) has recently gained increasing attention by the scientific community. However, studies investigating the effect of SLR on plants and vertical marsh accretion are usually restricted to the species or community level and do not consider phenotypic plasticity or genetic diversity. To investigate the response of genotypes within the same salt-marsh species to SLR, we used two known genotypes of Elymus athericus (Link) Kerguélen (low-marsh and high-marsh genotypes). In a factorial marsh organ experiment we exposed both genotypes to different flooding frequencies and quantified plant growth parameters. With increasing flooding frequency, the low-marsh genotype showed a higher aboveground biomass production compared to the high-marsh genotype. Additionally, the low-marsh genotype generally formed longer rhizomes, shoots and leaves, regardless of flooding frequency. Belowground biomass of both genotypes decreased with flooding frequency. We conclude that the low-marsh genotype is better adapted to higher flooding frequencies through its ability to allocate resources from below- to aboveground biomass. Given the strong control of plant biomass production on salt-marsh accretion, we argue that these findings yield important implications for our understanding of ecosystem resilience to SLR as well as plant-species distribution in salt marshes.

Tidal marshes are important ecological systems that are responding to sea level rise-driven changes in tidal regimes. Human development along the coastline creates barriers to marsh migration, moderating tidal marsh distributions. This study shows that in the Chesapeake Bay, USA an estuarine system with geographic and development variability, overall estuarine tidal marshes are projected to decline by approximately half over the next century. Tidal freshwater and oligohaline habitats, which are found in the upper reaches of the estuary and are typically backed by high elevation shorelines are particularly vulnerable. Due to their geological setting, losses of large extents of tidal freshwater habitat seem inevitable under sea level rise. However, in the meso/poly/euhaline zones that (in passive margin estuaries) are typically low relief areas, tidal marshes are capable of undergoing expansion. These areas should be prime management targets to maximize future tidal marsh extent. Redirecting new development to areas above 3 m in elevation and actively removing impervious surfaces as they become tidally inundated results in the maximum sustainability of natural coastal habitats. Under increasing sea levels and flooding, the future of tidal marshes will rely heavily on the policy decisions made, and the balance of human and natural landscapes in the consideration of future development.

The papers in this Special Feature are the result of the first Marsh Resilience Summit in the Chesapeake Bay region, which occurred in February 2019. The Chesapeake Bay region has one of the highest rates of relative sea level rise in the U.S., jeopardizing over 1000 km2 of tidal wetlands along with other coastal lands. The goal of the Summit and this collection of articles is to analyze tidal wetland response to accelerating sea level rise and the effect their response will have on adaptation planning for surrounding communities. Ten Summit presenters share their research in this Special Feature. In this Introduction, we summarize their findings on evaluating restoration potential at the site-specific level, measuring and projecting marsh migration and erosions rates, describing impacts of wetland migration on a marsh dependent animal, effects on human communities, and finally the roles of property owners and government on future tidal wetland extent. These contributions demonstrate that tidal marsh distribution is dynamic in response to sea level rise, and that social, legal, and policy tools can be used and further developed to enable opportunities for restoring or conserving wetlands when stakeholders are engaged effectively. The papers here and feedback from Summit participants illuminate diverse priorities, research unknowns, and next steps for land use planning toward resilience of the Chesapeake Bay region that also can inform global communities.

Complexities of terrestrial boundaries with salt marshes in coastal lagoons affect salt marsh exposure to waves and sediments creating different potentials for marsh migration inland and seaward-edge erosion, and consequently, for marsh persistence. Between 2002 and 2017, migration and edge erosion were measured in three mainland geomorphic marsh types (headland, valley, hammock) and were used to assess the rate and spatial extent of marsh change for a Virginia coastal lagoon system. Treelines, shorelines, and marsh perimeters were delineated in ArcGIS at 1:600 resolution. All marsh types increased in spatial extent; increases were greatest for the valley type (0.58 ha ± 0.31 ha or + 0.32% per annum). Measured rates of migration (headland > valley > hammock) and erosion (headland > hammock > valley) for each geomorphic type were averaged and applied to obtain changes in these same marsh types at the regional scale. At this scale, valley marsh area increased (82.5 ha or 5.5 ha a−1) more than the other two marsh types combined. This analysis demonstrates the critical influence that geomorphic type has on lateral marsh responses to sea-level rise and that efforts to conserve or restore salt marshes are most likely to be successful when focused on valley marshes.