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Salt marsh persistence is threatened by predicted sea-level rise
... Coastal salt marshes are some of the most vulnerable and threatened natural ecosystems [1][2][3]. These ecosystems serve as wetlands in the upper coastal intertidal zone between the upland coastal plain and aquatic brackish waters, helping to control ecosystem function and structure. ...
... Remote Sens. 2024,16,2 ...
In the United States (US), salt marshes are especially vulnerable to the effects of projected sea level rise, increased storm frequency, and climatic changes. Sentinel-2 data offer the opportunity to observe the land surface at high spatial resolutions (10 m). The Sentinel-2 data, encompassing Cumberland Island National Seashore, Fort Pulaski National Monument, and Canaveral National Seashore, were analyzed to identify temporal changes in salt marsh presence from 2016 to 2020. ENVI-derived unsupervised and supervised classification algorithms were applied to determine the most appropriate procedure to measure distant areas of salt marsh increases and decreases. The Normalized Difference Vegetation Index (NDVI) was applied to describe the varied vegetation biomass spatially. The results from this approach indicate that the ENVI-derived maximum likelihood classification provides a statistical distribution and calculation of the probability (>90%) that the given pixels represented both water and salt marsh environments. The salt marshes captured by the maximum likelihood classification indicated an overall decrease in salt marsh area presence. The NDVI results displayed how the varied vegetation biomass was analogous to the occurrence of salt marsh changes. Areas representing the lowest NDVI values (−0.1 to 0.1) corresponded to bare soil areas where a salt marsh decrease was detected.
... A comparison of vertical growth rates since marsh establishment of Culatra with other recently (within the last century) established and similar in terms of dynamics backbarrier marshes (see Fig. A.7 for details on key drivers across relevant spatial scales in each site; methodology after Yando et al. (2023)) are given in Table 2. The accretion balance rates (accretion rate minus synchronous regional SLR; (Crosby et al., 2016)) from Culatra are toward the higher end of the range. The low marsh accretion balance rate at the Skallingen spit (Danish Wadden Sea; Nielsen and Nielsen (2002)) is comparable to the similarly young low marshes of Culatra. ...
... Generally, a lag in marsh plant establishment occurs when colonisation is restricted by factors such as: a) connectivity and distance to the main seed sources, leading to insufficient seed arrival, and b) environmental factors (i.e., inundation regime, currents) inhibiting seedling emergence and survival (Lõhmus et al., 2020). Due to the proximity of Table 2 Synthesis of backbarrier marsh age and vertical accretion rates since marsh establishment (fines layer thickness divided by average marsh age), Regional SLR (RSLR) rates and derived accretion balance rate (accretion minus RSLR rate; Crosby et al. (2016)) from the Frisian Islands and Ria Formosa (LM: Low Marsh, HM: High Marsh). RSLR rates (contemporaneous with accretion period, or uniform long-term values when reliable shorter-term estimates were unavailable) were obtained from the same publication, apart from the Dutch Wadden Sea (data from Keizer et al. (2023)) and Ria Formosa (data from Dias and Taborda (1992) Fig. 8. Summary of main phases and controls on backbarrier marsh evolution, derived from the data collected in Culatra (the graphic representation is inspired by the response of estuarine marshes to sediment pulses, shown in Mudd (2011)); z crit is the elevation limit for the development of marsh vegetation (approximately at MWL) and z morpho /z eco-moprho denotes the shift in elevation from morphodynamic to eco-morphodynamic processes. ...
The rapid elongation of Culatra Island, a sandy barrier in the Ria Formosa chain (S. Portugal), since the mid-1940s led to the formation of three new embayments in its backbarrier that were gradually colonised by halophytic vegetation. This provided a rare opportunity to collect information and data on the very early stages of backbarrier marsh plant establishment and evolution. Sediment (surface and subsurface) sampling in two of the recently formed bays, combined with information extracted from vertical aerial photographs, allowed us to assess modern sedimentation characteristics and vertical accretion rates since the shift from a bare sandflat to a vegetated marsh platform. Present-day topography appears largely inherited by overwash or/and inlet-related tidal deposits that provided the necessary sediment pulse for the formation of an intertidal sandy substrate, suitable for colonisation. The variability in accretion rates, noted even within the same embayment, as well as the differences in accretion balance with similarly young backbarrier marshes, highlight the importance of local conditions (sediment import, distance to creeks and marsh edge, storm frequency and intensity) to marsh build-up, even during the very early stages. Variable accretion rates were also identified over intertidal seagrass patches, indicating similar influences. Organic deposition rates were very low in all vegetated intertidal habitats, indicating the dominance of mineral deposition to the vertical growth. A lag, ranging from roughly 10–30 years, was observed between the formation of the intertidal sandy platform and plant establishment in all embayments. The different timescales in the observed lag are likely linked to differences in hydrodynamic conditions, promoted by the embayment morphology (opening width). The lowest lag was observed in protected embayments, which could reflect a ‘typical’ delay for plant establishment in the system, while the highest lag was associated with higher energy backbarrier environments.
... Our results from tidal marshes and mangroves differ greatly from those of the IPCC special report on the ocean and cryosphere in a changing climate (SROCC) [9], which projected that coastal wetlands (tidal marshes, mangroves, and seagrass meadows) will shrink by 20%-90% from present conditions [10,11,56]. Although direct comparisons of the two studies are difficult, the reason for this discrepancy may be that SROCC assumed 1 m of sea-level rise by 2100 [10], which is an overestimate compared to the average value of RCP8.5, but the studies also differ in other data used. ...
... There are also regions where sea level falls, but the effects of the disappearance of tidal marshes and mangroves are considerably less than those caused by sea-level rise (S12 Fig). Furthermore, our model considers globally calculated sea-level change as an external forcing, but discrepancies can be created if local sea-level change based on observed data is applied [56]. ...
The global area and distribution of shallow water ecosystems (SWEs), and their projected responses to climate change, are fundamental for evaluating future changes in their ecosystem functions, including biodiversity and climate change mitigation and adaptation. Although previous studies have focused on a few SWEs, we modelled the global distribution of all major SWEs (seagrass meadows, macroalgal beds, tidal marshes, mangroves, and coral habitats) from current conditions (1986–2005) to 2100 under the representative concentration pathway (RCP) 2.6 and 8.5 emission scenarios. Our projections show that global coral habitat shrank by as much as 75% by 2100 with warmer ocean temperatures, but macroalgal beds, tidal marshes, and mangroves remained about the same because photosynthetic active radiation (PAR) depth did not vary greatly (macroalgal beds) and the shrinkage caused by sea-level rise was offset by other areas of expansion (tidal marshes and mangroves). Seagrass meadows were projected to increase by up to 11% by 2100 because of the increased PAR depth. If the landward shift of tidal marshes and mangroves relative to sea-level rise was restricted by assuming coastal development and land use, the SWEs shrank by 91.9% (tidal marshes) and 74.3% (mangroves) by 2100. Countermeasures may be necessary for coastal defense in the future; these include considering the best mix of SWEs and coastal hard infrastructure because the significant shrinkage in coral habitat could not decrease wave energy. However, if appropriate coastal management is achieved, the other four SWEs, which have relatively high CO2 absorption rates, can help mitigate the climate change influences.
... In other cases, transitions will occur at the ecosystem rather than species level. Although some grasslands will be lost to woody encroachment and sea-level rise (Crosby et al., 2016;M. B. ...
North American grasslands are climate‐vulnerable biomes that provide critical ecosystem services and support biodiversity. However, grasslands are often not included in climate policy and treaties, and they are underrepresented in ecological climate‐adaptation literature. We synthesized existing knowledge on climate adaptation in North American grasslands to provide resources and guidance for grassland managers facing increasing climate change impacts. We leveraged data from a systematic review and solicited input from management professionals at workshops to create a Grassland Adaptation Menu—a referenced, hierarchical list of specific grassland management tactics nested under broader climate adaptation strategies. Our review revealed that although the number of published studies examining grassland‐climate topics is increasing, relatively few provide actionable recommendations for adaptation. Among studies that did make recommendations, landscape‐planning principles such as conserving grasslands in future climate refugia and enhancing connectivity were the most frequently recommended practice types, but there were also suggestions for site‐level management such as adjustments to fire and grazing, improved seed sourcing and restoration practices, increased heterogeneity and biodiversity, use of assisted migration, and management of microclimate conditions. The Grassland Adaptation Menu incorporates eight general strategies and 32 approaches in a structured format designed to help managers translate concepts into actions.
... Due to these hydrologic alterations, marshes are becoming more vulnerable to rising sea levels. The absence of vegetation reduces sediment accretion and limits elevation gain, resulting in marsh areas that are unable to keep pace with sea level rise (Crosby et al., 2016;Mariotti, 2016). This increasing loss of marsh habitat will have negative consequences for primary productivity, carbon storage, and biogeochemical cycling. ...
Runnels, a climate adaptation technique that drains surface water to restore marsh vegetation and habitat, are increasingly being used to prevent the formation of shallow water impoundments or pannes in salt marshes that result in the loss of important ecosystem services. However, we know little about the effect of runnels on salt marsh biogeochemistry. This study measured how sediment characteristics and rates of nitrogen cycle processes were altered by impounded water and vegetation loss, and whether runnels can restore these marsh attributes to reference conditions. Impounded areas were 52 ± 4% less vegetated than nearby intact marsh, with 11 ± 2% less organic matter and 24 ± 5% higher bulk density. Additionally, impoundments removed 32 ± 32 µmol N m ⁻² d ⁻¹ less than reference marsh areas via denitrification. At six of the 11 runneled sites, vegetation percent cover increased by 40 ± 5%, accompanied by a 7 ± 3% recovery of organic matter and a 9 ± 6% reduction of bulk density. At sites where vegetation recovered to within 70% of reference plots at a site, runneled plots removed 97 ± 31 µmol more N m ⁻² d ⁻¹ than impoundments, which was also 82 ± 31 µmol more N m ⁻² d ⁻¹ than reference areas. The driver of recovery is related to initial site conditions, including higher redox potentials and lower porewater salinities, compared with sites where revegetation was unsuccessful. The extent of runnel effectiveness and the recovery of vegetation, sediment characteristics, and nitrogen cycle processes was variable among runneled marshes, and the effectiveness of runnels may depend on initial site-specific characteristics and degree of initial degradation.
... Anthropogenic activities, such as reclamation ( Yang and Chui, 2018;Ren et al., 2023), drainage (Raposa et al., 2019), rewetting (Li et al., 2020a;Emery et al., 2021), biological invasion (Granse et al., 2021;Wang et al., 2024) and restoration Kutcher and Raposa, 2023;Mason et al., 2023), along with environmental factors like sea level rise (Crosby et al., 2016;FitzGerald and Hughes, 2019;Pannozzo et al., 2021), extreme weather (Leonardi et al., 2018), and sediment supply (Qiu et al., 2024a), collectively impact salt marsh extent, distribution, and succession. The dominant factors vary spatially and temporally. ...
... 2) and A max were re-calibrated, as compared to the global model. Model calibration was conducted, following the same methodology as for the GCWM 18 , using previously published coastal marsh elevation change data (integrating mineral and organic sedimentation as well as compaction and shallow subsidence) from study sites globally, where the geographic coordinates of the sites, local RSLR and local SSC have also been reported 112,113 . Local S1 values for all data points were retrieved from the global Dynamic Interactive Vulnerability Assessment (DIVA) database 18,114 . ...
Mediterranean coastal wetlands account for important biodiversity and ecosystem services. But climate-change induced sea-level rise poses a critical risk to their survival. Here, we assess these risks for Mediterranean coastal marshes, one key type of Mediterranean coastal wetlands, and identify main drivers for future coastal marsh change for the Mediterranean and comparable coastlines. We apply an integrated modelling approach that accounts for future sea-level rise, sediment accretion, coastal management and marsh inland migration processes. Depending on climate mitigation scenarios, widespread coastal marsh loss is projected, ranging from 8% to 92% of current extents. For Egypt, France, and Algeria, we predict (near) total loss of coastal marshes by 2100 for current coastal management and sediment supply scenarios. Overall, losses could at least be halved if additional inland migration space were created, e.g. through passive or active habitat restoration. Bold climate mitigation and local adaptation are needed to preserve existing coastal marshes.
... saltmarshes and invasive S. alterniflora differently. While native species such as P. australis and S. mariqueter may diminish or disappear entirely, S. alterniflora could extend its range or colonize new areas in response to rising sea levels (Crosby et al., 2016;Valiela et al., 2018;Fagherazzi et al., 2020). The expansion of saltmarshes significantly increases with the maximum tidal range, as emphasized by Balke et al. (2016), who observed that the height difference between the lowest elevation of saltmarshes and mean high water increases with tidal range. ...
Globally, native saltmarshes are declining, while invasive saltmarshes are expanding rapidly. However, the underlying processes and driving mechanisms behind these trends remain poorly understood, particularly in rapidly silting coastal bays. Here, we investigated the spatiotemporal dynamics and influencing factors of native saltmarshes (Scirpus mariqueter and Phragmites australis) and invasive Spartina alterniflora in Yueqing Bay, eastern China, by interpreting Landsat series images during 1985-2023. Results showed that native saltmarshes significantly decreased from 632.24 ha to complete disappearance, while S. alterniflora expanded from zero to 2872.90 ha in Yueqing Bay during 1985-2023. Specifically, S. mariqueter experienced a notable decline from 1985 to 1995, subsequent growth between 2000 and 2005, and another reduction from 2005 to 2010. P. australis expanded from 2005 to 2010 but shrank from 2010 to 2015 as S. alterniflora encroached upon its habitats. By contrast, S. alterniflora consistently expanded annually, except when it was eradicated by human interference. Both natural factors and human activities influenced native and invasive saltmarshes; for instance, mudflat reclamation facilitated saltmarsh expansion by creating suitable habitats during a certain period, particularly for S. alterniflora. From 1984 to 2018, the coastline extended seaward, causing sedimentation and landward transformation, which contributed to the expansion of S. alterniflora. However, the 118.05 ha mangrove forests negatively impacted S. alterniflora, as planting mangroves often involves the removal of existing S. alterniflora. Additionally, maximum tidal range and mean annual temperature positively affected saltmarshes. Our study highlighted the influence of natural factors and human activities on the spatiotemporal dynamics of both native and invasive saltmarshes in rapidly silting bays, underscoring the urgent need for conservation, restoration, and management of native saltmarshes.
... Biomass allocation, the process by which plants allocate finite biomass to different organs, determines plant performance along environmental gradients [1][2][3][4]. In specific, proper biomass allocations guarantee the functions of different plant organs: leaves perform photosynthesis to fix carbon, stems and branches act as transport corridors and mechanical support, roots take up nutrients and water and anchor the plant, and seeds/fruits execute sexual reproduction [5][6][7]. ...
Background
Biomass allocation reflects functional tradeoffs among plant organs and thus represents life history strategies. However, little is known about the patterns and drivers of biomass allocation between reproductive and vegetative organs along large environmental gradients. Here, we examined how environmental gradients affect biomass and the allocation between reproductive and vegetative organs. We also tested whether the allocation patterns conform optimal or allometric partitioning theory.
Methods
We collected 22 Artemisia species along a large environmental gradient in China and measured reproductive (infructescences including seeds) and vegetative (leaves, stems and roots) mass for each plant. We then used standardized major axes regressions to quantify the relationships between reproductive and vegetative organs and linear mixed-effect models to examine the effect of environmental gradients (climate and soil) on biomass allocation patterns.
Results
We found significant negative correlations between total biomass of Artemisia and the first principal component of climate, an axis that was negatively correlated with temperature and precipitation. Overall, there were significant isometric relationships between reproductive and vegetative mass. In addition, the ratio of reproductive to vegetative mass increased with the second principal component of climate (representing climate variability), but decreased with the second principal component of soil (representing bulk density and available water capacity). These patterns were consistent at the individual and interspecific levels, but were mixed at the intraspecific level.
Conclusions
Our findings of the plastic responses of biomass allocation to environmental gradients support the optimal partitioning theory (OPT). The isometric relationships between reproductive and vegetative organs indicate that plant growth and reproduction are intricately linked. Furthermore, the plasticity of biomass ratios of reproductive to vegetative organs to climate variability and soil physical properties suggests that the flexible allocation between growth and reproduction is crucial for successful adaptation to diverse habitats.
... However, the diversion of the Yellow River reduced sediment supply [36], and strong hydrodynamic forces exacerbated erosion, preventing sufficient vertical accretion to counterbalance the relative sea-level rise [41]. Consequently, coastal salt marshes were degraded by permanent inundation due to an insufficient accumulation of vertical sediments to form an elevation suitable for their growth [42,43]. This was the reason that, to some extent, explained the reduction of salt marsh distribution width along with the decline in elevation at the vegetation edge. ...
Salt marshes are declining due to the dual pressures of coastal erosion and land reclamation. However, there remains a lack of quantitative analysis regarding this reduction process and its driving mechanisms. This study examines the dynamics and influencing factors of salt marsh vegetation along the eroding coastline of Sheyang County, Jiangsu Province, China, between 1985 and 2020, using remote sensing to analyze changes in artificial coastlines, water boundaries, vegetation front edge, and its topography. Our results showed an extensive seaward movement of artificial coastlines due to reclamation, coupled with severe reductions in salt marsh area and width. Coastal erosion further caused a 10.5% decline in vegetation elevation and a 46.7% increase in slope steepness, amplifying vulnerability to wave action. Native species were largely replaced by Spartina alterniflora, reducing ecological diversity. Currently, human pressure on the landward side has been alleviated; thus, addressing coastal erosion is vital to preventing the further loss of salt marshes. Sediment retention engineering and native vegetation restoration efforts can gradually facilitate the recovery of salt marshes. This study provided critical insights for sustainable coastal management under bidirectional pressures.
... Coastal saltmarshes support diverse ecological services and are biodiversity hotspots that contribute substantially to carbon sequestration, provide coastal protection, improve water quality, nutrient cycling and sustain primary production [1][2][3]. While these areas act as natural buffers against storm surges [4,5] the current scenario of Sea Level Rise jeopardizes these vital ecosystems by saltwater intrusion [6], erosion [7], altered hydrology [8], loss of habitats and, consequently, a decline in biodiversity [9][10][11]. ...
Salt marshes are vital coastal ecosystems, increasingly threatened by rising sea level and human pressures, that provide essential services, including coastal protection, habitat support, and carbon sequestration. This study examines the effectiveness of different eco-engineering structures in restoring salt marshes in the Mondego Estuary, Portugal, by assessing their impacts on benthic macroinvertebrate communities as bioindicators of ecosystem health. The experimental design included five experimental cells: wood palisade (Fence), geotextile fabric (Geotextile), geotextile bags filled with sand (Bags), a cell with autochthonous vegetation (Plants), and a Control cell with bare soil. Monitoring took place from 2019 to 2021, with both before and after intervention sampling to evaluate species composition, biomass, and density. Key ecological indices, such as the AZTI’s Marine Biotic Index (AMBI), Shannon-Wiener Diversity, and Pielou’s Evenness, were calculated alongside measurements of environmental variables. The results indicated minimal impacts on biodiversity, with observed variations primarily attributed to seasonal dynamics. While the wood palisade enhanced species richness and density, geotextile provided better community stability. The findings emphasize the importance of long-term monitoring, stakeholder engagement, and sustainable use of materials to optimize restoration efforts and better inform coastal management strategies in the face of climate change.
... Salt marshes are vulnerable habitats that are increasingly threatened by sea level rise (SLR) (Craft et al., 2009;Crosby et al., 2016;Morris et al., 2002). Belowground plant production and the accumulation of organic matter build soil volume and elevation, contributing to vertical accretion, carbon storage, and marsh resilience (Gonneea et al., 2019;Turner et al., 2002Turner et al., , 2004. ...
The Belowground Ecosystem Resiliency Model (BERM) is a geoinformatics tool that was developed to predict belowground biomass (BGB) of Spartina alterniflora in salt marshes based on remote sensing of aboveground characteristics and other readily available hydrologic, climatic, and physical data. We sought to characterize variation in S. alterniflora BGB over both temporal and spatial gradients through extensive marsh field observations in coastal Georgia, USA, to quantify their relationship with a suite of predictor variables, and to use these results to improve performance and expand the parameter space of BERM. We conducted pairwise comparisons of S. alterniflora growth metrics measured at nine sites over 3–8 years and found that BGB grouped by site differed in 69% of comparisons, while only in 21% when grouped by year. This suggests that BGB varies more spatially than temporally. We used the BERM machine learning algorithms to evaluate how variables relating to biological, climatic, hydrologic, and physical attributes covaried with these BGB observations. Flooding frequency and intensity were most influential in predicting BGB, with predictor variables related to hydrology composing 61% of the total feature importance in the BERM framework. When we used this expanded calibration dataset and associated predictors to advance BERM, model error was reduced from a normalized root‐mean‐square error of 13.0%–9.4% in comparison with the original BERM formulation. This reflects both an improvement in predictive performance and an expansion in conditions for potential model application. Finally, we used regression commonality analysis to show that model estimates reflected the spatiotemporal structure of BGB variation observed in field measurements. These results can help guide future data collection efforts to describe landscape‐scale BGB trends. The advanced BERM is a robust tool that can characterize S. alterniflora productivity and resilience over broad spatial and temporal scales.
... There is uncertainty about the resiliency of coastal wetlands to sea level rise. While some models predict that coastal wetlands may keep pace with sea level rise via gains in elevation from sediment accretion, others predict that accelerated sea level rise and surrounding anthropogenic infrastructure may limit the ability of these ecosystems to adjust (Morris et al. 2002;Crosby et al. 2016;Kirwan et al. 2016;Schuerch et al. 2018, Osland et al. 2022. Given uncertainty in how ecosystems and species will respond and adapt to rising sea levels, continued research can help to develop adaptation and mitigation plans, and this leaves room for both optimism and action (Maschinski et al. 2011;Braun de Torrez et al. 2021). ...
Islands are some of the most biodiverse places on earth, but they are also hotspots of biodiversity loss. The coastline of Florida, U.S.A., is surrounded by thousands of islands, many of which are home to species that occur nowhere else. A rapidly emerging threat to these low-lying islands is inundation as sea levels rise. The capacity of island-dwelling species to adapt to climate change and sea level rise may be limited because many species do not have the ability to shift their distribution off the island to track favorable conditions. We assessed the vulnerability of Florida’s islands to inundation from sea level rise and estimated the terrestrial biodiversity on Florida’s islands that could be lost. Our models predicted that by 2100, over 80% and up to 90% of Florida’s islands could be completely inundated from sea level rise, depending on the sea level rise projection (1.2 m or 2.2 m). Of the 85 mammalian, reptilian, and amphibian species on our subset list of Florida’s Species of Greatest Conservation Need, over half occur on Florida’s islands for at least part of their range, highlighting the importance of these islands for housing Florida’s rich biodiversity. Notably, at least 12 mammal species and 7 reptile species have their entire distribution on Florida’s islands, and this count is likely an underestimate. Projections of future sea level rise mean that these island-endemic species face the threat of extinction in the wild if their island habitat is submerged.
... combined with climate change-driven SLR has degraded coastal marsh habitat and associated ecosystem services. Low marsh (LM) habitat, which is usually defined as mean low water (MLW) to mean high water (MHW) (McKee & Patrick 1988) and supports Spartina alterniflora, is particularly vulnerable to accelerating SLR, especially in areas with low inorganic sediment inputs (Crosby et al. 2016). ...
Tidal marshes at the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (PI) are part of a large‐scale restoration project to replace lost island habitat in Chesapeake Bay, United States. However, observations of Spartina alterniflora die‐back prompted questions about the impact on genetic diversity and resilience of restored versus natural marshes, leading to an investigation of genetic diversity and population structure. Transects were established across three distinct restored marshes with different ages and histories of die‐back and across two local, non‐restored native marshes. Plants were genotyped at eight microsatellite markers to examine metrics of genetic diversity, population structure, and clonality. Allelic richness but not heterozygosity was higher in restored marshes compared to reference marshes, which showed significantly higher clonality and spatial genetic autocorrelation. Restored marsh areas experiencing die‐back had slightly lower multilocus diversity indices than non‐die back areas in two third cases, but not a third comparison. Significant genetic differentiation was observed between the native and restored marshes (mean G ST approximately 0.06), which reflects the approximately 100 km distance between native marshes and restored seed source in New Jersey. Overall, die‐back in restored marshes did not substantially affect genetic diversity or composition, but substantial differences in diversity were observed between restored and native marshes. Reduced clonal diversity in mature, native marshes may be a function of their greater age, as has been reported elsewhere. Future monitoring of neutral genetic diversity in PI marshes will be useful for understanding longer‐term patterns of genetic change and diversity in planted, restored marshes.
... Coastal salt marshes are potentially vulnerable to global warming and the associated accelerated levels of sealevel rise (SLR) Kirwan et al., 2016;Crosby et al., 2016). To evaluate the status of the Wadden Sea salt marshes in relation to SLR, an inventory of published and unpublished accretion studies in Wadden Sea salt marshes was carried out in the previous QSR (Esselink et al., 2017) and is updated below. ...
... Recent losses of salt marshes throughout the Eastern United States as a result of lateral shoreline erosion (Burns, Alexander, and Alber, 2020) and the inability of the systems to maintain elevation with sea-level rise is well documented (Crosby et al., 2016;Gedan, Altieri, and Bertness, 2011;Hartig et al., 2002). One of the main mechanisms of large-scale marsh degradation is the conversion of interior high marsh meadows (i.e. ...
... In coastal cities, these ES are particularly important, helping buffer storm surges, filter pollutants, and manage stormwater, thus contributing to coastal resilience (Elmqvist et al., 2019). However, as SLR accelerates, many salt marshes are at risk of being submerged and transformed into open water (Murray et al., 2022;Ohenhen et al., 2023), exacerbating the ecological challenges facing densely populated coastal regions that rely on marshes for critical ES (Crosby et al., 2016). ...
This study presents a spatio-temporal framework that integrates ecosystem services into ecological risk assessment to evaluate the ecosystem service vulnerability of urban salt marshes to sea-level rise. The model was tested at Belle Isle Marsh to quantify and qualify the evolving capacity of urban marshes to continue supplying ecosystem services to an increasing urban populace to the end of the century with focus on carbon storage, nitrogen storage, fish nursery, and Saltmarsh Sparrow viewing. We project that sea-level rise will drive dynamic trade-offs between habitats and ecosystem services over space and time. Ultimately, habitat fragmentation and conversion to open ocean will severely impair carbon storage and wildlife viewing services, while also enhancing short-term fish nursery and nitrogen storage services. This approach offers nuanced understanding of where, when, and how services may interact under future conditions, and enables proactive planning and adaptation to emerging challenges.
... They are widely recognized for providing an array of crucial ecosystem services, functioning as habitats for species, a sink of organic carbon, and storm buffers (Kelleway et al., 2017;Rountree & Able, 2007;Temmerman et al., 2023). However, salt marshes are challenged by coastal squeeze due to human interventions and accelerating sea-level rise (Alizad et al., 2022;Crosby et al., 2016;Kirwan et al., 2010;Osland et al., 2017). To keep pace with sea-level rise, sufficient sediment supply is needed for the vertical accretion and lateral expansion of salt marshes (Fagherazzi et al., 2020;Ladd et al., 2019). ...
The survival of salt marshes, especially facing future sea‐level rise, requires sediment supply. Sediment can be supplied to salt marshes via two routes: through marsh creeks and over marsh edges. However, the conditions of tides and waves that facilitate sediment import through these two routes remain unclear. To understand when and how sediment is imported into salt marshes, 2‐month measurements were conducted to monitor tides, waves, and suspended sediment concentration (SSC) in Paulina Saltmarsh, a meso‐macrotidal system. The results show that the marsh creek tends to import sediment during neap tides with waves. A tidal cycle with a small tidal range result in weaker flow in the marsh creek during ebb tides, reducing the export of sediment. Waves enhance sediment supply to the marsh creek by eroding mudflats. However, strong waves can directly resuspend sediment in marsh creeks during spring tides when the water level is above the marsh canopy, enhancing sediment export through creeks. Net sediment import over marsh edges requires the opposite tidal and wave conditions: spring tides with weak waves. Spring tides provide stronger hydrodynamics, facilitating sediment import over the marsh edge. Increased SSC during the ebb phase can occur with strong waves over the marsh edge, resulting in net sediment export. Therefore, the net import or export of sediment, through the creek and over the marsh edge, depends on the combination of tidal and wave conditions. These conditions can vary between estuaries and even individual marshes. Understanding these conditions is crucial for better management of salt marshes.
... Intact estuarine habitats provide ecosystem services such as shoreline protection and carbon sequestration, and serve as nurseries for commercially valuable fish and invertebrates (Barbier et al., 2011;Beck et al., 2001;Mcleod et al., 2011). In a changing climate that includes accelerated sea-level rise, estuarine habitats and their services are at risk (Crosby et al., 2016;Passeri et al., 2015). There is thus an urgent need for estuarine restoration to address past degradation, and for climate adaptation planning for the future (Waltham et al., 2021). ...
Conservation of estuaries is strengthened by an understanding of past and current estuary extent, which helps stakeholders envision resilient estuarine habitats in the future. We used spatial analyses to improve understanding of estuarine habitat and extent in and around 30 US National Estuarine Research Reserves using two approaches, elevation-based mapping and historical mapping. We collaborated with stakeholders to incorporate local knowledge, and found that our methodologies were effective across disparate geographies. Elevation-based mapping proved to be a powerful tool for mapping areas within reach of tides, yielding a better understanding of the past, present, and potential estuary. This approach revealed that US estuaries are or were bigger-often vastly so-than what is shown in most maps. In particular, at over 80 % of studied estuaries, elevation-based mapping detected temperate forested tidal wetlands missed by maps generated primarily from aerial photographs. Historical mapping, conducted consistently across diverse regions, provided a valuable window into past ecological conditions. Our change analysis using historical maps revealed that tidal marsh has undergone dramatic losses on the Pacific coast (average > 60 % loss). On other US coasts, tidal marsh extent has changed less (average < 10 % loss), with marsh losses offset by landward migration; however, marsh migration may have caused net loss of vegetated tidal wetlands due to loss of forested tidal wetlands. Comparing mapping methods revealed important changes that could not be detected using a single method. Each mapping approach had limitations, so combining multiple methods will enhance understanding of both past and present conditions at estuaries worldwide.
... This process highlights how local-scale factors, such as the position of levee breaching and local strong deposition, have a high influence on the evolution of salt marshes. These types of factors are usually not considered in global-scale predictions, which suggest that marshes cannot keep pace with SLR (Crosby et al., 2016;Nardin & Edmonds, 2014;Spencer et al., 2016). Instead, local-scale evaluations give opposite outcomes (Kirwan et al., 2016); in fact, it was demonstrated that high rates of subsidence can be outpaced by high sediment loads . ...
It is increasingly recognised that nature‐based solutions in coastal ecosystems provide significant services compared to hard engineering measures. Several restoration projects are now being conducted worldwide to protect coastlines against flood risk and improve the ecosystems' quality. In the case of river deltas, managed realignment through levee breaching is becoming a recognised strategy for coastal wetland restoration. This study focusses on the formation of new wetlands in the Po River Delta after several natural dyke failures that occurred during the last century, when a large part of the land was abandoned and became wetlands. An historical review was performed to quantify the variation of the marsh extent in time developed after natural levee breaching in the easternmost lagoons of the delta and to understand the processes that determined their evolution. The analyses were based on orthophotos from the 1950s to the present‐day integrated with tidal records, water and sediment discharge records, and GPS survey for vegetation distribution. The review indicated that, between the 1950s and 1980s, anthropisation and natural processes caused a strong decline in marsh extent, thus leading to the recent shape of the lagoons. Several breaches and inlets were developed because of a combination of human intervention and erosive processes, and new tidal systems were born. Three main depositional areas connecting the main river branches were identified in three separated lagoons of the delta. These lagoons presented a ‘crevasse splay’ type of deposition which allowed the development of new tidal flats and, in certain cases, of large freshwater marsh systems within less than 8 years after the breach. The newly formed wetland systems (>100 ha) demonstrate the ability of the Po River Delta to build new wetlands, also during periods of human‐induced sediment starvation and high rates of subsidence, suggesting that further levee breaching should be exploited to favour marsh recovery.
... If salt marshes are not able to gain elevation to match the increasing rate of sea-level rise, they will drown and convert to open water or mudflats. While the magnitude of loss to be expected is complex to predict and debated (Schuerch et al. 2018;FitzGerald and Hughes 2019;Fagherazzi et al. 2020;Coleman et al. 2022), as much as 60-91% of current salt marsh could be lost with predicted future rates of sea-level rise (Crosby et al. 2016). Given the cumulative magnitude of these threats, it is critical that we maximize the effectiveness and efficiency of the conservation, restoration, and management of these important ecosystems. ...
Salt marshes have ecological and economic value, but shoreline development, the increasing rate of sea-level rise, and other human impacts have caused significant loss of salt marshes. As a result, restoration of these ecosystems is widespread. For restoration and management to be effective, it is imperative to improve our understanding of marsh-building plants that serve as the ecological foundation of these habitats. Given the observed differences in characteristics between populations of smooth cordgrass, Spartina alterniflora , restoration plantings may impact the biodiversity and resilience of restored ecosystems. Understanding differences in the structural and functional outcomes of active planting of restoration sites will enable the long-term success of restoration efforts to be improved. Natural and restored salt marshes in Long Island Sound were studied in 2021–2022 for S. alterniflora genetics, biomass, stem morphology, and faunal community composition. The average genotypic diversity of S. alterniflora was more than 4 times higher in restored than in natural marshes, and differentiation between each restored site and natural sites decreased with time. No difference was observed in live S. alterniflora belowground biomass; however, mean dead belowground biomass in natural marshes was more than 3 times greater than in restored marshes. Marsh platform invertebrates differed between the restored and natural sites, with natural marsh edge habitats having 9 times higher density of Geukensia demissa and 3 times as many crab burrows than in restored marshes, but there was no detected difference in species richness or abundance of nekton at high tide. With restoration practitioners seeking resilient, self-sustaining ecosystems, it is important to evaluate whether restored marsh characteristics are consistent with those goals and modify restoration planning accordingly to incorporate genetics, structure, and function.
... In the old Rhine Estuary, some soil samples consist for up to 50% out of accumulated biomass (Pierik, Moree, et al., 2023), highlighting the importance of this process for long-term evolution of estuaries. Conservative estimates point toward biomass accumulation of 1 mm/year to 3 mm/year (Best et al., 2018;Craft et al., 2009;Crosby et al., 2016;Morris et al., 2002;van Maanen et al., 2015). Here, a constant mean value of 2 mm/year is applied. ...
Estuaries worldwide are susceptible and adapting to climate change (CC) impacts from both the river and coastal boundaries. Furthermore, engineering efforts are undertaken to improve flood safety, to claim land for human use or for port operations, which change estuary morphology. This paper aims to gain an understanding of the combined effects of CC and human interventions on the estuarine‐wide morphological response by analyzing the sediment infilling of highly engineered estuaries. A schematized process‐based morphodynamic model is used (Delft3D‐FM, in 2DH), resembling a highly engineered estuary in the Rhine‐Meuse Delta, The Netherlands. Three types of changes were implemented, both in isolation and in combination: (a) local interventions (changing channel depth or wetland area), (b) upstream human interventions (changing fluvial sediment supply) and (c) extreme CC scenarios (with projections for the future forcings and bathymetry). Results show that a CC scenario can elicit both positive and negative changes in the estuary's sediment budget. The direction and magnitude of the change depend on the local intervention and can align with the effect of the local intervention, intensifying its impact. The combined effects can even reverse the sign of the sediment budget. This stresses the need of analyzing CC impacts in combination with human interventions. Additionally, a relationship was identified which quantifies how a change in peak flow velocity due to both local interventions and sea‐level rise affects the annual sediment budget. These findings can help determine how local interventions affect morphodynamics of engineered estuaries in present and future climates.
... Assessments of coastal wetland dynamics on a global scale (Crosby et al., 2016;Spencer et al., 2016) have shown that the ability of many marshes and mangroves to build up vertically is being overwhelmed by present-day SLR, resulting in widespread wetland loss. Other studies suggested that the vulnerability to SLR might be lower than expected due to feedback between ecology and geomorphology, and the potential for marshes to migrate inland (Kirwan et al., 2016). ...
The rise in sea level and land subsidence are seriously threatening the diversity of tidal morphologies that have made the Venice Lagoon such a distinctive landscape. Here, we assess the vulnerability of tidal morphologies to relative sea-level rise based on a new conceptual framework that accounts for both above- and below-sea-level zones, sedimentary architecture, and surface morphology. Around 80 % of the lagoon area will face moderate to severe vulnerability by 2050, doubling compared to the 1990s. While the subtidal zone may be relatively less threatened compared to past conditions, the drastic decline in intertidal morphologies is alarming. This contributes to the flattening and deepening of the lagoon topography and thus to the loss of lagoon landscape diversity, likely leading to a decrease in the ecosystem services the tidal morphologies provide. The interconnection of intertidal and subtidal morphologies is crucial for maintaining the overall health and functionality of the lagoon's ecosystem. Any disruption to one aspect can have ripple effects throughout the entire system.
... Various studies highlight that hydrological conditions, particularly water level fluctuation, flooding characteristics, and soil salinity gradients, significantly influence the spatial and temporal distribution of coastal saltmarsh wetland vegetation [19][20][21][22]. Additionally, the impacts of climate change, including rising temperatures, reduced precipitation, and increased evaporation, along with natural disasters such as sea-level rise and storm surges, directly contribute to the degradation of coastal wetland areas and vegetation [17,23,24]. ...
Vegetation evolution is an important indicator of regional ecosystems and sea–land interactions. In this study, we investigated the evolution of coastal wetland vegetation, focusing on the core area of Yancheng City National Rare Bird Nature Reserve. Using high-precision classification based on phenological characteristics, we delineated the evolutionary process of three predominant wetland vegetation types: Spartina alterniflora, Suaeda salsa, and Phragmites australis. Spatial and temporal patterns were analyzed using the generalized additive model to identify drivers of evolution. From 1990 to 2022, a three-stage shift in vegetation distribution from land to sea was observed. Notably, S. salsa’s distribution area consistently shrank since 2000, whereas P. australis continued to grow. Throughout the entire period, S. alterniflora consistently maintained growth and tended toward stability. The vegetation distinctly showcased zonal patterns along the coastal gradient, revealing a clear inclination to migrate toward the seaside. Specifically, S. alterniflora displayed a centroid migration rate of 195.28 m/year, shifting northward, whereas P. australis and S. salsa migrated eastward (toward the seaside) at rates of 111.84 and 70.88 m/year, respectively. Environmental factors, such as downward irradiance, sea surface salinity, and significant wave height, significantly influenced vegetation patterns. Human activities, particularly aquaculture pond construction, emerged as the primary anthropogenic factor causing the reduction in P. australis distribution. Additionally, the competition for ecological niches among vegetation emerged as a pivotal factor contributing to the alterations in the landscape pattern within the study area.
... Accelerated sea level rise (SLR) is a major threat to coastal salt marshes, as studies have suggested that increased rates of SLR have resulted in marsh vegetation die-off and expansion of tidal channels and ponds (Crosby et al. 2016;Watson et al. 2016;Davis et al. 2019). Analysis of aerial photographs and peat cores has shown that marsh vegetation can migrate upslope to compensate for marsh loss at lower elevations (Hussein 2009;Fagherazzi et al. 2019). ...
Thin layer sediment placement (TLP) is used to build elevation in marshes, counteracting effects of subsidence and sea level rise. However, TLP success may vary due to plant stress associated with reductions in nutrient availability and hydrologic flushing or through the creation of acid sulfate soils. This study examined the influence of sediment grain size and soil amendments on plant growth, soil and porewater characteristics, and greenhouse gas exchange for three key U.S. salt marsh plants: Spartina alterniflora (synonym Sporobolus alterniflorus ), Spartina patens (synonym Sporobolus pumilus ), and Salicornia pacifica. We found that bioavailable nitrogen concentrations (measured as extractable NH 4 ⁺ ‐N) and porewater pH and salinity were inversely related to grain size, while soil redox was more reducing in finer sediments. This suggests that utilizing finer sediments in TLP projects will result in a more reduced environment with higher nutrient availability, while larger grain sized sediments will be better flushed and oxygenated. We further found that grain size had a significant effect on vegetation biomass allocation and rates of gas exchange, although these effects were species‐specific. We found that soil amendments (biochar and compost) did not subsidize plant growth but were associated with increases in soil respiration and methane emissions. Biochar amendments were additionally ineffective in ameliorating acid sulfate conditions. This study uncovers complex interactions between sediment type and vegetation, emphasizing the limitations of soil amendments. The findings aid restoration project managers in making informed decisions regarding sediment type, target vegetation, and soil amendments for successful TLP projects.
... During the period of reduced sediment, the vegetation within tidal wetlands has exhibited a sustained and relatively stable rate of accretion, particularly with an elevation rise in the vertical dimension Yang et al. 2008;. In contrast, tidal flats suffer from a rapid decline in sediment rates, with localized areas transitioning from accretion to erosion due to the sediment rate decreases to a level lower than the rate of sea-level rise (Figure 13(c)) (Crosby et al. 2016;Shi et al. 2022). This transition is particularly pronounced in the tidal wetland zones between −5-meter and 0-meter isobath, where a discernible reduction in the area has been observed . ...
Tidal wetlands provide a variety of ecosystem services to coastal communities but suffer severe losses due to anthropogenic activities in the Yangtze River Estuary (YRE). However, the detailed dynamics of tidal wetlands have not been well studied with sufficient spatiotemporal resolution. Here, we proposed a rapid classification method that integrates the COntinuous monitoring of Land Disturbance (COLD) algorithm and Median Composite (MC) based on the dense Landsat time series to track the dynamic processes of tidal wetlands in the YRE from 1990 to 2020. The results showed that the COLD-MC demonstrated remarkable effectiveness in detecting the change of tidal wetlands and excellent overall accuracy and kappa coefficient ranging from 90% to 96% and 0.89–0.95, respectively. The overall accuracy of change detection was 97% with an absolute error of 0.4 years. We found that the total area of tidal wetlands experienced a net loss of 59.75 km² in the YRE, but the gain and loss of the study period were 1556.07 and 1615.82 km², respectively. Land reclamation, sediment reduction, and Spartina alterniflora invasion pose significant threats to tidal wetlands. Sustainable management could be implemented through the establishment of nature reserves and ecological sediment enhancement engineering projects.
... As people become aware of global warming and the accelerated sea-level rise it causes, the response of coastal wetlands to climate change and associated sea-level rise over time has become the focus of global attention (Fitzgerald et al., 2008;Nicholls and Cazenave, 2010;Xu et al., 2022). More importantly, scientists found that the fate of coastal wetlands depended closely on the competition between the net change in wetland surface elevation and sea-level rise (Crosby et al., 2016;Phillips, 2018;Rayner et al., 2021). ...
The coastal wetlands of the Yellow River Delta (YRD) in China are crucial for their valuable resources, environmental significance, and economic contributions. However, these wetlands are also vulnerable to the dual threats of climate change and human disturbances. Despite substantial attention to the historical shifts in YRD’s coastal wetlands, uncertainties remain regarding their future trajectory in the face of compounded risks from climate change and anthropogenic activities. Based on a range of remote sensing data sources, this study undertakes a comprehensive investigation into the evolution of YRD’s coastal wetlands between 2000 and 2020. Subsequently, the potential fate of coastal wetlands is thoroughly analyzed through the Land Use/Cover Change (LUCC) simulation using System Dynamic-Future Land Use Simulation (SD-FLUS) model and the extreme water levels projection integrated future sea-level rise, storm surge, and astronomical high tide in 2030, 2050, and 2100 under scenarios of SSP1-2.6, SSP2-4.5, and SSP5-8.5. Results reveal that YRD’s coastal wetlands underwent a marked reduction, shrinking by 1688.72 km2 from 2000 to 2020. This decline was mostly attributed to the substantial expansion in the areas of artificial wetlands (increasing by 823.78 km2), construction land (increasing by 767.71 km2), and shallow water (increasing by 274.58 km2). Looking ahead to 2030–2100, the fate of coastal wetlands appears to diverge based on different scenarios. Under the SSP1-2.6 scenario, the area of coastal wetland is projected to experience considerable growth. In contrast, the SSP5-8.5 scenario anticipates a notable decrease in coastal wetlands. Relative to the inundated area suffered from the current extreme water levels, the study projects a decrease of 6.8%–10.6% in submerged coastal wetlands by 2030 and 9.4%–18.2% by 2050 across all scenarios. In 2100, these percentages are projected to decrease by 0.4% (SSP2-4.5) and 27.1% (SSP5-8.5), but increase by 35.7% (SSP1-2.6). Results suggest that coastal wetlands in the YRD will face a serious compound risk from climate change and intensified human activities in the future, with climate change being the more important factor. More efficient and forward-looking measures must be implemented to prioritize the conservation and management of coastal wetland ecosystems to address the challenges, especially those posed by climate change.
... Despite their immense ecological importance, saltmarsh are under threat internationally, with estimations of international saltmarsh decline between 25 and 50 % of their historical distribution; while within Australia, over 50 % have been modified or destroyed since European settlement (Finlayson and Rea, 1999;McOwen et al., 2017). Major threats or impacting processes that have contributed to saltmarsh decline internationally include land reclamation for urban development, restriction of tidal flow and bunding, climate change associated sea level rise and accompanying mangrove encroachment, as well as urbanisation and industrial development in coastal areas (Crosby et al., 2016;Gedan et al., 2009;Saintilan et al., 2014). Secondary impacting processes, such as environmental contamination, are also important factors that may govern the health, vigour, and distribution of saltmarsh taxa. ...
The uptake and distribution of copper, zinc, arsenic, and lead was examined in two rare Australian saltmarsh species, Tecticornia pergranulata and Wilsonia backhousei. The bioconcentration factors and translocation factors were generally much lower than one, except for the Zn translocation factors for T. pergranulata. When compared to other Australian saltmarsh taxa, these species generally accumulated the lowest levels observed among taxa, especially in terms of their BCFs. Essential metals tended to be regulated, while non-essential metals increased in concentration with dose during transport among compartments, a pattern not previously observed in Australian saltmarsh taxa. The uptake of metals into roots was mainly explained by total sediment metal loads as well as more acidic pH, increased soil organic matter, and decreased salinity. The low uptake and limited translocation observed in these rare taxa may offer a competitive advantage for their establishment and survival in the last urbanised populations, where legacy metal contamination acts as a selective pressure.
... (Separate from these considerations, it should be noted that the collection of InSAR data in wetlands is challenginga topic beyond the scope of the present paper.) It is therefore imperative that vertical accretion not be used synonymously with SEC as has been the case in recent, widely cited papers (e.g., Crosby et al., 2016;FitzGerald and Hughes, 2019). Along the same lines, InSAR measurements that encompass both static and dynamic landscapes (e.g., Ohenhen et al., 2023) measure SEC, which can only be equated with VLM in the static case. ...
Major technological advances have made measurements of coastal subsidence more sophisticated, but these advances have not always been matched by a thorough examination of what is actually being measured. Here we draw attention to the widespread confusion about key concepts in the coastal subsidence literature, much of which revolves around the interplay between sediment accretion, vertical land motion and surface-elevation change. We attempt to reconcile this by drawing on well-established concepts from the tectonics community. A consensus on these issues by means of a common language can help bridge the gap between disparate disciplines (ranging from geophysics to ecology) that are critical in the quest for meaningful projections of future relative sea-level rise.
... Hydroperiod or topographic gradient is considered as a prime factor of eco-zonation in SLC wetlands (Marani et al. 2013;Kirwan et al. 2016;Rosbakh et al. 2020). Coastal engineering events that occur naturally through enhancing accretion determine how an SLC vegetation will evolve in response to climate change (Crosby et al. 2016;Best et al. 2018;Saintilan et al. 2019). RS can be crucial in mapping the tidal range, elevation, and primary production, as well as other factors associated to adaptation to the climate change, where the capacity to trapping sediments by the SLC ecosystem is an indicator, (Morris et al. 2002;Miller et al. 2021). ...
Saltmarsh land-cover (SLC) ecosystems, composed of unvegetated mudflats, saltmarshes, mangroves, and/or seagrass communities, are vulnerable to climate-induced impacts, such as sea level rise. Extracting a seamless and consistent waterline from satellite imagery is a major challenge because of environmental factors, such as turbidity, water depth and multiple types of underwater vegetation cover that introduce noise in the extraction of information. Hence, a water index, derived from multi-temporal Landsat 8 (OLI) data, acquired under different tides is proposed for mapping land-water across SLC wetlands by tracking waterlines. This provided inundation maps and defined eco-zones to specify south-eastern Bangladesh wetland composition. The NDWI_1 (McFeeters’s water index) applied to 42 OLI images and derived land-water difference maps generated inundation gradient maps with an overall classification accuracy of 87.8%. The simple intersection and union of region-of-interests extracted from the tide heights above the mean low-water springs enabled the mapping of four categories of wetland composition based on hydroperiods: a) irregularly inundated (II), regularly inundated (RI), irregularly exposed (IE; high floodplain), and subtidal (river bed and deep water sea). For all of the three study sites, mangrove, seagrass, non-mangrove and agriculture were all prominent on the IE eco-zone, while only saltmarsh was dominant on the II eco-zone. These maps of SLC wetland will enrich previous concepts of eco-zonation models that include salinity, erosion, accretion and rate of sea level rise as factors, suggesting that inundation extent and tidal phase complexities should be considered in the remote sensing of SLC composition for improved models of SLC vegetation response to climate change.
... Salt marshes are economically and ecologically valuable systems, but their existence is currently threatened by sea-level rise (SLR;Crosby et al., 2016). It is widely accepted that SLR causes marsh loss through morphological responses to submergence (Reed, 2002), salinity-driven dieback (Mendelssohn & McKee, 1988) or coastal squeeze (Torio & Chmura, 2013). ...
Expansion of drainage networks through the headward erosion of tidal creeks is an eco‐geomorphologic response of salt marshes to accelerated sea‐level rise (SLR). This response can counter the negative impacts of an elevation deficit by increasing drainage and encouraging plant health, thereby reducing potential for submergence and marsh platform loss. In the wetlands of Cape Romain, SC, intense bioturbation near creek heads by the common marsh crab Sesarma reticulatum has been found to facilitate sediment erosion and rapid creek growth. This keystone grazer has been recently observed to have increasing influence on landscape evolution throughout the southeast US coast. Here, we compare measurements taken at Sapelo Island, GA, with those previously collected at Cape Romain, to confirm that eco‐geomorphic feedbacks facilitating creek growth at each location are similar, and to compare these processes under differing background conditions. We use sediment cores, precise elevation measurements and historical imagery to compare substrate properties, elevation within the tidal frame, creek growth rates and drainage morphology at both sites. Our results show identical processes; however, the higher elevation of the marsh at Sapelo Island leads to shallower and shorter periods of tidal inundation, explaining the greater soil strength and lower belowground biomass compared with the marsh at Cape Romain. The smaller tidal range at the site in Cape Romain compared with Sapelo Island translates to a proportionally shallower depth of tidal creeks, which therefore requires less erosion to produce headward creek extension. These effects are likely to have contributed to slower growth rates of tidal creeks at Sapelo Island during the past several decades of SLR. Our findings highlight the similarities in process but differences in rates in how marshes are responding to climate‐related stress.
Salt marshes, which provide vital ecosystem services and play a key role in coastal protection, require innovative restoration strategies to enhance their resilience to sea level rise (SLR) in the context of ongoing climate change. This study evaluated the effectiveness of various eco-engineering structures in promoting sediment accretion within a temperate estuary (Mondego estuary, Portugal). Five experimental cells were tested: (1) a control cell with bare soil, (2) a cell with autochthonous vegetation, (3) a cell with a wooden palisade, (4) a cell with geotextile fabric, and (5) a cell with geotextile bags filled with sand. Sediment accretion was measured seasonally from 2019 to 2022, and sedimentation rates and patterns were compared across the different structures. Environmental variables, including precipitation and tidal flow, were also monitored to assess their influence on sediment dynamics. Results indicated that eco-engineering structures enhanced sedimentation compared to the control. The highest accumulation was observed near the wooden palisades and geotextile bags, particularly in areas aligned with the river flow. This study underscores the potential of eco-engineering approaches to promote localized sediment stabilization and enhance marsh resilience. However, long-term monitoring and adaptive management are essential to address challenges associated with SLR and hydrodynamic variability. The findings provide valuable insights for designing effective and targeted restoration strategies in estuarine environments.
Sea-level rise will lead to widespread habitat loss if warming exceeds 2 °C, threatening coastal wildlife globally. Reductions in coastal habitat quality are also expected but their impact and timing are unclear. Here we combine four decades of field data with models of sea-level rise, coastal geomorphology, adaptive behaviour and population dynamics to show that habitat quality is already declining for shorebirds due to increased nest flooding. Consequently, shorebird population collapses are projected well before their habitat drowns in this UNESCO World Heritage Area. The existing focus on habitat loss thus severely underestimates biodiversity impacts of sea-level rise. Shorebirds will also suffer much sooner than previously thought, despite adapting by moving to higher grounds and even if global warming is kept below 2 °C. Such unavoidable and imminent biodiversity impacts imply that mitigation is now urgently needed to boost the resilience of marshes or provide flood-safe habitat elsewhere.
Tidal marshes serve as important “blue carbon” ecosystems that sequester large amounts of carbon with limited area. While much attention has been paid to the spatial variability of sedimentation within salt marshes, less work has been done to characterize spatial variability in marsh soil carbon density. Soil properties in marshes vary spatially with several parameters, including marsh platform elevation, which controls inundation depth, and proximity to the marsh edge and tidal creek network, which control variability in relative sediment supply. We used lidar to extract these morphometric parameters from tidal marshes to map soil organic carbon at the meter scale. Fixed volume soil samples were collected in 2021 at four northeast U.S. tidal marshes with distinctive morphologies to aid in building predictive models. Tidal creeks were delineated from 1-m resolution topobathy lidar data using a semi-automated workflow in GIS. Log-linear multivariate regression models were developed to predict soil organic content, bulk density, and carbon density as a function of predictive metrics at each site and across sites. Results show that modeling salt marsh soil characteristics with morphometric inputs works best in marshes with single connected creek network morphologies. Distance from tidal creeks was the most significant model predictor. Addition of distance to the inlet and tidal range as regional metrics significantly improves cross-site modeling. Our mechanistic approach results in predicted total marsh carbon stocks comparable to previous studies but captures important meter level variation. Further, we provide motivation to continue rigorous mapping of soil carbon at fine spatial resolutions.
Understanding the response of salt marshes to flooding is crucial to foresee the fate of these fragile ecosystems, requiring an upscaling approach. In this study we related plant species and community response to multispectral indices aiming at parsing the power of remote sensing to detect the environmental stress due to flooding in lagoon salt marshes. We studied the response of Salicornia fruticosa (L.) L. and associated plant community along a flooding and soil texture gradient in nine lagoon salt marshes in northern Italy. We considered community (i.e., species richness, dry biomass, plant height, dry matter content) and individual traits (i.e., annual growth, pigments, and secondary metabolites) to analyze the effect of flooding depth and its interplay with soil properties. We also carried out a drone multispectral survey, to obtain remote sensing-derived vegetation indices for the upscaling of plant responses to flooding. Plant diversity, biomass and growth all declined as inundation depth increased. The increase of soil clay content exacerbated flooding stress shaping S. fruticosa growth and physiological responses. Multispectral indices were negatively related with flooding depth. We found key species traits rather than other community traits to better explain the variance of multispectral indices. In particular stem length and pigment content (i.e., betacyanin, carotenoids) were more effective than other community traits to predict the spectral indices in an upscaling perspective of salt marsh response to flooding. We proved multispectral indices to potentially capture plant growth and plant eco-physiological responses to flooding at the large scale. These results represent a first fundamental step to establish long term spatial monitoring of marsh acclimation to sea level rise with remote sensing. We further stressed the importance to focus on key species traits as mediators of the entire ecosystem changes, in an ecological upscaling perspective.
Salt marshes are globally important ecosystems and thus their resilience to climate change holds societal importance. To date, studies addressing salt marsh responses to climate change have focused on sea‐level rise and biogeochemical feedbacks with increasing inundation. Less is known about how salt marsh sediment temperatures, which impact physical, biological, and chemical ecosystem processes, will respond to climate change. In this study, we present multi‐depth sediment temperature and porewater level data from low‐, mid‐, and high‐marsh sites at a New England salt marsh for a 1‐year period and investigate how salt marsh sediment temperatures respond to atmospheric and oceanic forcing. We use spectral analyses to identify the frequencies at which sediment temperatures vary and link the temperature variations to specific forcing mechanisms. We find that all sites across the marsh responded to air temperature with roughly equal amplitude whereas the responses to radiative heating and ocean tides varied spatially. The high‐marsh site is more sensitive to radiative heating than the mid‐ and low‐marsh sites. The low‐marsh is affected by tidal processes and inundation whereas the high‐ and mid‐marsh sites are not. In addition, we find that the bulk thermal diffusivity of the saturated sediments decreases with distance from the tidal channel. These factors contribute to considerable temporal and spatial variability in sediment temperatures with elevation, distance from the tidal channel, and time of year (season) being most important.
The existence of coastal ecosystems depends on their ability to gain sediment and keep pace with sea level rise. Similar to other coastal areas, Northeast Florida (United States) is experiencing rapid population growth, climate change, and shifting wetland communities. Rising seas and more severe storms, coupled with the intensification of human activities, can modify the biophysical environment, thereby increasing coastal exposure to storm-induced erosion and inundation. Using the Guana Tolomato Matanzas National Estuarine Research Reserve as a case study, we analyzed the distribution of coastal protection services-expressly, wave attenuation and sediment control-provided by estuarine habitats inside a dynamic Intracoastal waterway. We explored six coastal variables that contribute to coastal flooding and erosion-(a) relief, (b) geomorphology, (c) estuarine habitats, (d) wind exposure, (e) boat wake energy, and (f) storm surge potential-to assess physical exposure to coastal hazards. The highest levels of coastal exposure were found in the north and south sections of the Reserve (9% and 14%, respectively) compared to only 4% in the central, with exposure in the south driven by low wetland elevation, high surge potential, and shorelines composed of less stable sandy and muddy substrate. The most vulnerable areas of the central Reserve and main channel of the Intracoastal waterway were exposed to boat wakes from larger vessels frequently traveling at medium speeds (10-20 knots) and had shoreline segments oriented towards the prevailing winds (north-northeast). To guide management for the recently expanded Reserve into vulnerable areas near the City of Saint Augustine, we evaluated six sites of concern where the current distribution of estuarine habitats (mangroves, salt marshes, and oyster beds) likely play the greatest role in natural protection. Spatially explicit outputs also identified potential elevation maintenance strategies such as living shorelines, landform modification, and mangrove establishment for providing coastal risk-reduction and other ecosystem-service co-benefits. Salt marshes and mangroves in two sites of the central section (N-312 and S-312) were found to protect more than a one-quarter of their cross-shore length (27% and 73%, respectively) from transitioning to the highest exposure category. Proposed interventions for mangrove establishment and living shorelines could help maintain elevation in these sites of concern. This work sets the stage for additional research, education, and outreach about where mangroves, salt marshes, and oyster beds are most likely to reduce risk to wetland communities in the region.
Although the promotion of biodiversity has been recognized as an important conservation goal, salt marsh restoration typically focuses on reestablishing dominant foundation species. Salt marsh restoration projects that add or remove sediment to optimize marsh elevation often result in a bare landscape following construction. Restoration managers must decide whether to plant and, if so, which species. This decision can be difficult because few studies have examined the ecological functions of individual species, especially those that are less abundant. Within a major salt marsh restoration project in Elkhorn Slough, California, where 17,000 plants of five high marsh species were planted, we investigated how rarer species and the naturally recruiting dominant ( Salicornia pacifica ) differ in ecosystem function. We evaluated 31 different metrics related to blue carbon, plant productivity, environmental effects, and community interactions. No single species had the greatest ecological function across this suite of metrics, and measured effects were mainly independent, with only 16 of 435 pairwise comparisons revealing a strong correlation. We found significant differences among species for 18 metrics, revealing key contrasts in ecosystem function, with significant effects of marsh elevation and interaction between effects of species and elevation on some of these functions. S. pacifica scored highest for metrics such as recruitment and canopy height but other species outperformed Salicornia in other metrics. Frankenia salina had the greatest ecological function in the highest number of metrics, including cover and belowground biomass carbon content, but other species had higher rates of photosynthesis and harbored fewer individuals of invasive arthropods. We recommend planting a suite of less common species at restoration sites to provide more diverse functions across the landscape. F. salina in particular is recommended for its tolerance of hypersalinity and low moisture conditions. Our demonstration of the value of complementing restoration of the dominant foundation species with restoration of less common species is applicable to restoration of other ecosystems beyond salt marshes. The approach we implemented, evaluating a large suite of functions for multiple species across a restored landscape, can serve as a model for investigations of the importance of biodiversity for enhancing multifunctionality in other restored ecosystems.
Salt marshes have experienced the brunt of human civilization for eons as they were diked for pasture or producing salt hay and less saltwater-dependent crops, filled for port, commercial, and residential development, used as landfills and to dispose of dredged material, ditched in efforts to reduce mosquito populations in coastal communities, or have had their connection to estuaries simply reduced or severed by roads and railroads. This was largely done because they were viewed as unproductive wastelands, public health hazards, or because their location was important for accessing deep water or connecting two points of land, or simply providing a desirable location for homes. In the 1960s scientists studying coastal habitats started writing about the ecological significance of these wetlands in the United States in terms the public could understand (e.g., Goodwin 1961, Odum 1961, and Teal and Teal 1969). Consequently the public was becoming more informed of the importance of these wetlands to coastal fisheries as well as to migratory birds as they witnessed accelerating destruction of salt marshes for residential and other development. In the 1960s, state legislatures began passing laws to restrict development of these wetlands, first in New England states then elsewhere (see Tiner 2013 for a comprehensive review of the history of tidal wetlands). Today salt marshes are among America’s most valued natural resources and government agencies and non-government organizations (NGOs) are both actively involved in restoring these wetlands. Most cases of this restoration involve bringing back tidal flow and more saline conditions in one way or another. Where the marshes have been crossed by a road or railroad, tidal flow has either been eliminated or restricted to varying degrees that has greatly affected soil salinities and promoted growth of brackish and freshwater species. In many cases in the northeastern U.S., these crossings have led to a drastic change in plant composition and vegetation structure – from a diverse salt marsh community dominated by low-growing halophytic plants to a virtual monoculture of common reed (Phragmites australis) – a non-native2 that favors less saline habitats and grows to 3.7 m (12 feet) or more in height under the best circumstances. Some options for restoring tidal flow in these situations include: 1) reconnecting the marsh to the adjacent estuary (where tidal flow was eliminated), 2) removing tidal gates, and 3) expanding the size of the existing culverts. These may be some of the simplest restoration projects from a construction standpoint, although concerns about increased flooding on private property surrounding the marsh is often the major hurdle to overcome. A small restoration project in Massachusetts serves as one example of the effectiveness of simply restoring tidal flow can bring about a return of salt marsh vegetation to an area that had been colonized by common reed. While some restoration projects are initiated as mitigation for destruction of wetland elsewhere, this project was a “pro-active project” – simply done for the benefit of the environment - to restore native halophytic vegetation and reduce the extent of non-native common reed.
Humans have modified coastlines. Sometimes this has been deliberate, as with the construction of various coastal defence structures, but many changes have been non-deliberate, including accelerated coastal erosion and the degradation of specific coastal types, including reefs, marshes, and mangroves. Coast lines will be highly susceptible to climate changes and to associated sea level rises.
Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha⁻¹ in the top 30 cm and 231 ± 134 Mg SOC ha⁻¹ in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.
Coastal wetlands are responsible for about half of all carbon burial in oceans, and their persistence as a valuable ecosystem depends largely on the ability to accumulate organic material at rates equivalent to relative sea level rise. Recent work suggests that elevated CO2 and temperature warming will increase organic matter productivity and the ability of marshes to survive sea level rise. However, we find in a series of preliminary experiments that organic decomposition rates increase by about 20% per degree of warming. Our measured temperature sensitivity is similar to studies from terrestrial systems, three times as high as the response of salt marsh productivity to temperature warming, and greater than the productivity response associated with elevated CO2 in C3 marsh plants. Although the experiments were simple and of short duration, they suggest that enhanced CO2 and warmer temperatures could actually make marshes less resilient to sea level rise, and tend to promote a release of soil carbon. Simple projections indicate that elevated temperatures will increase rates of sea level rise more than any acceleration in organic matter accumulation, suggesting the possibility of a positive feedback between climate, sea level rise, and carbon emissions in coastal environments.
Coastal marshes are considered to be among the most valuable and vulnerable ecosystems on Earth, where the imminent loss of ecosystem services is a feared consequence of sea level rise. However, we show with a meta-analysis that global measurements of marsh elevation change indicate that marshes are generally building at rates similar to or exceeding historical sea level rise, and that process-based models predict survival under a wide range of future sea level scenarios. We argue that marsh vulnerability tends to be overstated because assessment methods often fail to consider biophysical feedback processes known to accelerate soil building with sea level rise, and the potential for marshes to migrate inland.
Systematic reviews and meta-analyses have become increasingly important in health care. Clinicians read them to keep up to date with their field [1],[2], and they are often used as a starting point for developing clinical practice guidelines. Granting agencies may require a systematic review to ensure there is justification for further research [3], and some health care journals are moving in this direction [4]. As with all research, the value of a systematic review depends on what was done, what was found, and the clarity of reporting. As with other publications, the reporting quality of systematic reviews varies, limiting readers' ability to assess the strengths and weaknesses of those reviews.
Several early studies evaluated the quality of review reports. In 1987, Mulrow examined 50 review articles published in four leading medical journals in 1985 and 1986 and found that none met all eight explicit scientific criteria, such as a quality assessment of included studies [5]. In 1987, Sacks and colleagues [6] evaluated the adequacy of reporting of 83 meta-analyses on 23 characteristics in six domains. Reporting was generally poor; between one and 14 characteristics were adequately reported (mean = 7.7; standard deviation = 2.7). A 1996 update of this study found little improvement [7].
In 1996, to address the suboptimal reporting of meta-analyses, an international group developed a guidance called the QUOROM Statement (QUality Of Reporting Of Meta-analyses), which focused on the reporting of meta-analyses of randomized controlled trials [8]. In this article, we summarize a revision of these guidelines, renamed PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses), which have been updated to address several conceptual and practical advances in the science of systematic reviews (Box 1).
Box 1: Conceptual Issues in the Evolution from QUOROM to PRISMA
Completing a Systematic Review Is an Iterative Process
The conduct of a systematic review depends heavily on the scope and quality of included studies: thus systematic reviewers may need to modify their original review protocol during its conduct. Any systematic review reporting guideline should recommend that such changes can be reported and explained without suggesting that they are inappropriate. The PRISMA Statement (Items 5, 11, 16, and 23) acknowledges this iterative process. Aside from Cochrane reviews, all of which should have a protocol, only about 10% of systematic reviewers report working from a protocol [22]. Without a protocol that is publicly accessible, it is difficult to judge between appropriate and inappropriate modifications.
During the past century, human modification of environmental systems has greatly accelerated tidal salt marsh deterioration and shoreline retreat in many coastal regions worldwide. As a result, more than 50% of the original tidal salt marsh habitat in the U.S. has been lost. Numerous human activities have contributed directly or indirectly to wetland loss and alteration at local, regional, and global scales. Human impacts at the local scale include those that directly modify or destroy salt marsh habitat such as dredging, spoil dumping, grid ditching, canal cutting, leveeing, and salt hay farming. Indirect impacts, which can be even more significant, typically are those that interfere with normal tidal flooding of the marsh surface, alter wetlands drainage, and reduce mineral sediment inputs and marsh vertical accretion rates. These impacts usually develop over a greater period of time. At the regional scale, subsidence caused by subsurface withdrawal of groundwater, oil, and gas has submerged and eliminated hundreds of square kilometers of salt marsh habitat in the Chesapeake Bay, San Francisco Bay, and Gulf of Mexico. At the global scale, atmospheric warming due to increased burden of anthropogenic greenhouse gases and tropospheric sulfate aerosols appears to be strongly coupled to glacial melting, thermal expansion of ocean waters, and eustatic sea-level rise. Changes in coastal water levels ascribable to eustatic sea-level rise pose a long-term threat to the stability and viability of these critically important coastal systems.
A salt marsh responds in diverse ways to a rising sea level. A major factor is its ability to maintain surface elevations with respect to the mean high water level. Other influences include local submergence rates, sedimentation rates, density and composition of the indigenous flora, and type and intensity of cultural modifications. If the relative rate of sea level rise reaches catastrophic proportions (>10 mm yr-1), substantial reductions in wetland area and a corresponding increase in open water habitats is projected. -from Authors
•
Salt marshes are highly productive and valuable ecosystems, providing many services on which people depend. Spartina alterniflora Loisel (Poaceae) is a foundation species that builds and maintains salt marshes. Despite this species' importance, much of its basic reproductive biology is not well understood, including flowering phenology, seed production, and the effects of flowering on growth and biomass allocation. We sought to better understand these life history traits and use that knowledge to consider how this species may be affected by climate change. •
We examined temporal and spatial patterns in flowering and seed production in S. alterniflora at a latitudinal scale (along the U.S. Atlantic coast), regional scale (within New England), and local scale (among subhabitats within marshes) and determined the impact of flowering on growth allocation using field and greenhouse studies. •
Flowering stem density did not vary along a latitudinal gradient, while at the local scale plants in the less submerged panne subhabitats produced fewer flowers and seeds than those in more frequently submerged subhabitats. We also found that a shift in biomass allocation from above to belowground was temporally related to flowering phenology. •
We expect that environmental change will affect seed production and that the phenological relationship with flowering will result in limitations to belowground production and thus affect marsh elevation gain. Salt marshes provide an excellent model system for exploring the interactions between plant ecology and ecosystem functioning, enabling better predictions of climate change impacts.
© 2015 Botanical Society of America, Inc.
Marsh surface topography exerted a strong influence on the frequency and duratiion of marsh surface flooding, with plots at lower elevations flooded a greater percentage of the time than plots at higher elevations. Accretion was significantly related to duration of flooding. Accumulation of both organic and mineral matter was also significantly related to duration of flooding, implying that allochthonous organic matter as well as mineral matter was delivered to the marsh surface during flooding. These data indicate that accretion and accumulation varied temporally and spatially in relation to hydroperiod and suggest that accretion at lower marsh elevations is not always sufficient to maintain an equilibrium position in the intertidal zone. -from Authors
In New England salt marshes, Spartina alterniflora dominates the low-marsh habitat, which is covered daily by tides. The high-marsh habitat (not flooded daily) is dominated on its seaward border by S. patens, and on its terrestrial border by Juncus gerardi. While high-marsh perennials appear to be restricted to the high-marsh habitat by harsh physical conditions in the low-marsh habitat, S. alterniflora appears to be excluded from the high-marsh habitat by the high-marsh perennials. Throughout the high marsh, Distichlis spicata and Salicornia europaea are associated with phycial disturbance, in the form of mats of dead plant material (wrack) rafted by tides onto the marsh, which is most severe in the spring and early summer, and which decreases with increasing marsh elevation. D. spicata and S. alterniflora are most tolerant of wrack burial. Short-term disturbance increases the relative abundance of these species in the community. Longer lasting disturbance events kill the underlying vegetation, leaving bare patches throughout the high marsh. D. spicata rapidly colonizes these patches with runners; S. alterniflora and Sa. europaea recruit by seed. Over time these early colonizers are overgrown and displaced in high-marsh patches by S. patens and J. gerardi, which grow slowly, as dense turfs of roots, rhizomes, and tillers.-from Authors
Studies on carbon stock in salt marsh sediments have increased since the review by Chmura et al. (2003). However, uncertainties exist in estimating global carbon stor-age in these vulnerable coastal habitats, thus hindering the as-sessment of their importance. Combining direct data and in-direct estimation, this study compiled studies involving 143 sites across the Southern and Northern hemispheres, and pro-vides an updated estimate of the global average carbon ac-cumulation rate (CAR) at 244.7 g C m −2 yr −1 in salt marsh sediments. Based on region-specific CAR and estimates of salt marsh area in various geographic regions between 40 • S to 69.7 • N, total CAR in global salt marsh sediments is esti-mated at ∼10.2 Tg C yr −1 . Latitude, tidal range and elevation appear to be important drivers for CAR of salt marsh sedi-ments, with considerable variation among different biogeo-graphic regions. The data indicate that while the capacity for carbon sequestration by salt marsh sediments ranked the first amongst coastal wetland and forested terrestrial ecosystems, their carbon budget was the smallest due to their limited and declining global areal extent. However, some uncertainties remain for our global estimate owing to limited data avail-ability.
Tidal marshes maintain elevation relative to sea level through accumulation of mineral and organic matter, yet this dynamic accumulation feedback mechanism has not been modeled widely in the context of accelerated sea-level rise. Uncertainties exist about tidal marsh resiliency to accelerated sea-level rise, reduced sediment supply, reduced plant productivity under increased inundation, and limited upland habitat for marsh migration. We examined marsh resiliency under these uncertainties using the Marsh Equilibrium Model, a mechanistic, elevation-based soil cohort model, using a rich data set of plant productivity and physical properties from sites across the estuarine salinity gradient. Four tidal marshes were chosen along this gradient: two islands and two with adjacent uplands. Varying century sea-level rise (52, 100, 165, 180 cm) and suspended sediment concentrations (100%, 50%, and 25% of current concentrations), we simulated marsh accretion across vegetated elevations for 100 years, applying the results to high spatial resolution digital elevation models to quantify potential changes in marsh distributions. At low rates of sea-level rise and mid-high sediment concentrations, all marshes maintained vegetated elevations indicative of mid/high marsh habitat. With century sea-level rise at 100 and 165 cm, marshes shifted to low marsh elevations; mid/high marsh elevations were found only in former uplands. At the highest century sea-level rise and lowest sediment concentrations, the island marshes became dominated by mudflat elevations. Under the same sediment concentrations, low salinity brackish marshes containing highly productive vegetation had slower elevation loss compared to more saline sites with lower productivity. A similar trend was documented when comparing against a marsh accretion model that did not model vegetation feedbacks. Elevation predictions using the Marsh Equilibrium Model highlight the importance of including vegetation responses to sea-level rise. These results also emphasize the importance of adjacent uplands for long-term marsh survival and incorporating such areas in conservation planning efforts.
ISSN 0749-0208. As sea level rise accelerates and land development intensifies along coastlines, tidal wetlands will become increasingly threatened by coastal squeeze. Barriers that protect inland areas from rising sea level prevent or reduce tidal flows, and impermeable surfaces prevent wetland migration to the adjacent uplands. As vegetation succumbs to submergence by rising sea levels on the seaward edge of a wetland, those wetlands prevented from inland migration will decrease in area, if not disappear completely. Tools to identify locations where coastal squeeze is likely to occur are needed for coastal management. We have developed a ''Coastal Squeeze Index'' that can be used to assess the potential of coastal squeeze along the borders of a single wetland and to rank the threats faced by multiple wetlands. The index is based on surrounding topography and impervious surfaces derived from light detection and ranging and advanced spaceborne thermal emission and reflection radiometry imagery, respectively, and uses a fuzzy logic approach. We assume that coastal squeeze varies continuously over the coastal landscape and tested several fuzzy logic functions before assigning a continuous weight, from 0 to 1, corresponding to the influence of slope and impervious surfaces on coastal squeeze. We then combined the ranked variables to produce a map of coastal squeeze as a continuous index. Using this index, we compare the present and future threat of coastal squeeze to marshes in Wells and Portland, Maine, in the United States and Kouchibouguac National Park in New Brunswick, Canada.
Coastal populations and wetlands have been intertwined for centuries, whereby humans both influence and depend on the extensive ecosystem services that wetlands provide. Although coastal wetlands have long been considered vulnerable to sea-level rise, recent work has identified fascinating feedbacks between plant growth and geomorphology that allow wetlands to actively resist the deleterious effects of sea-level rise. Humans alter the strength of these feedbacks by changing the climate, nutrient inputs, sediment delivery and subsidence rates. Whether wetlands continue to survive sea-level rise depends largely on how human impacts interact with rapid sea-level rise, and socio-economic factors that influence transgression into adjacent uplands.
The availability of suspended sediments will be a dominant factor influencing the stability of tidal wetlands as sea levels rise. Watershed-derived sediments are a critical source of material supporting accretion in many tidal wetlands, and recent declines in wetland extent in several large river delta systems have been attributed in part to declines in sediment delivery. Little attention has been given, however, to changes in sediment supply outside of large river deltas. In this study, significant declines in suspended sediment concentrations (SSCs) over time were observed for 25 of 61 rivers examined that drain to the East and Gulf Coasts of the USA. Declines in fluvial SSC were significantly correlated with increasing water retention behind dams, indicating that human activities play a role in declining sediment delivery. There was a regional pattern to changes in fluvial sediment, and declines in SSCs were also significantly related to rates of relative sea level rise (RSLR) along the coast, such that wetlands experiencing greater RSLR also tend to be receiving less fluvial sediment. Tidal wetlands in the Mid-Atlantic, Mississippi River Delta, and Texas Gulf especially may become increasingly vulnerable due to rapid RSLR and reductions in sediment. These results also indicate that past rates of marsh accretion may not be indicative of potential future accretion due to changes in sediment availability. Declining watershed sediment delivery to the coastal zone will limit the ability of tidal marshes to keep pace with rising sea levels in some coastal systems.
Previous predictions on the ability of coastal salt marshes to adapt to
future sea level rise (SLR) neglect the influence of changing storm
activity that is expected in many regions of the world due to climate
change. We present a new modeling approach to quantify this influence on
the ability of salt marshes to survive projected SLR, namely, we
investigate the separate influence of storm frequency and storm
intensity. The model is applied to a salt marsh on the German island of
Sylt and is run for a simulation period from 2010 to 2100 for a total of
13 storm scenarios and 48 SLR scenarios. The critical SLR rate for marsh
survival, being the maximum rate at which the salt marsh survives until
2100, lies between 19 and 22 mm yr-1. Model results indicate
that an increase in storminess can increase the ability of the salt
marsh to accrete with sea level rise by up to 3 mm yr-1, if
the increase in storminess is triggered by an increase in the number of
storm events (storm frequency). Meanwhile, increasing storminess,
triggered by an increase in the mean storm strength (storm intensity),
is shown to increase the critical SLR rate for which the marsh survives
until 2100 by up to 1 mm yr-1 only. On the basis of our
results, we suggest that the relative importance of storm intensity and
storm frequency for marsh survival strongly depends on the availability
of erodible fine-grained material in the tidal area adjacent to the salt
marsh.
Early comparisons between rates of vertical accretion and sea level rise across marshes in different tidal ranges inspired a paradigm that marshes in high tidal range environments are more resilient to sea level rise than marshes in low tidal range environments. We use field-based observations to propose a relationship between vegetation growth and tidal range and to adapt two numerical models of marsh evolution to explicitly consider the effect of tidal range on the response of the marsh platform channel network system to accelerating rates of sea level rise. We find that the stability of both the channel network and vegetated platform increases with increasing tidal range. Our results support earlier hypotheses that suggest enhanced stability can be directly attributable to a vegetation growth range that expands with tidal range. Accretion rates equilibrate to the rate of sea level rise in all experiments regardless of tidal range, suggesting that comparisons between accretion rate and tidal range will not likely produce a significant relationship. Therefore, our model results offer an explanation to widely inconsistent field-based attempts to quantify this relationship while still supporting the long-held paradigm that high tidal range marshes are indeed more stable.
A new high-precision device for measuring sediment elevation in emergent and shallow water wetland systems is described. The rod surface-elevation table (RSET) is a balanced, lightweight mechanical leveling device that attaches to both shallow (< 1 m) and deep (driven to refusal) rod bench marks. The RSET was built to complement the surface-elevation table (SET), a larger and heavier mechanical leveling device first described by Boumans and Day (1993). Because of its size and weight, the SET must be attached to a pipe bench mark, which typically must be driven to a depth > 1 m in order to be stable. The pipe is driven to refusal but typically to a depth shallower than the rod bench mark because of greater surface resistance of the pipe. Thus, the RSET makes it possible to partition change in sediment elevation over shallower (e.g., the root zone) and deeper depths of the sediment profile than is possible with the SET. The confidence intervals for the height of an individual pin mea. measured by two different operators with the RSET under laboratory conditions were +/-1.0 and +/-1.5 mm. Under field conditions, confidence intervals for the measured height of an individual pin ranged from +/-1.3 mm in a mangrove forest up to +/-4.3 mm in a salt marsh.
Salt marsh ecosystems are maintained by the dominant macrophytes that regulate the elevation of their habitat within a narrow portion of the intertidal zone by accumulating organic matter and trapping inorganic sediment. The long-term stability of these ecosystems is explained by interactions among sea level, land elevation, primary production, and sediment accretion that regulate the elevation of the sediment surface toward an equilibrium with mean sea level. We show here in a salt marsh that this equilibrium is adjusted upward by increased production of the salt marsh macrophyte Spartina alterniflora and downward by an increasing rate of relative sea-level rise (RSLR). Adjustments in marsh surface elevation are slow in comparison to interannual anomalies and long-period cycles of sea level, and this lag in sediment elevation results in significant variation in annual primary productivity. We describe a theoretical model that predicts that the system will be stable against changes in relative mean sea level when surface elevation is greater than what is optimal for primary production. When surface elevation is less than optimal, the system will be unstable. The model predicts that there is an optimal rate of RSLR at which the equilibrium elevation and depth of tidal flooding will be optimal for plant growth. However, the optimal rate of RSLR also represents an upper limit because at higher rates of RSLR the plant community cannot sustain an elevation that is within its range of tolerance. For estuaries with high sediment loading, such as those on the southeast coast of the United States, the limiting rate of RSLR was predicted to be at most 1.2 cm/yr, which is 3.5 times greater than the current, long-term rate of RSLR.
Salt marshes are highly productive coastal wetlands that provide important ecosystem services such as storm protection for coastal cities, nutrient removal and carbon sequestration. Despite protective measures, however, worldwide losses of these ecosystems have accelerated in recent decades. Here we present data from a nine-year whole-ecosystem nutrient-enrichment experiment. Our study demonstrates that nutrient enrichment, a global problem for coastal ecosystems, can be a driver of salt marsh loss. We show that nutrient levels commonly associated with coastal eutrophication increased above-ground leaf biomass, decreased the dense, below-ground biomass of bank-stabilizing roots, and increased microbial decomposition of organic matter. Alterations in these key ecosystem properties reduced geomorphic stability, resulting in creek-bank collapse with significant areas of creek-bank marsh converted to unvegetated mud. This pattern of marsh loss parallels observations for anthropogenically nutrient-enriched marshes worldwide, with creek-edge and bay-edge marsh evolving into mudflats and wider creeks. Our work suggests that current nutrient loading rates to many coastal ecosystems have overwhelmed the capacity of marshes to remove nitrogen without deleterious effects. Projected increases in nitrogen flux to the coast, related to increased fertilizer use required to feed an expanding human population, may rapidly result in a coastal landscape with less marsh, which would reduce the capacity of coastal regions to provide important ecological and economic services.
About half of annual marine carbon burial takes place in shallow water ecosystems where geomorphic and ecological stability is driven by interactions between the flow of water, vegetation growth and sediment transport. Although the sensitivity of terrestrial and deep marine carbon pools to climate change has been studied for decades, there is little understanding of how coastal carbon accumulation rates will change and potentially feed back on climate. Here we develop a numerical model of salt marsh evolution, informed by recent measurements of productivity and decomposition, and demonstrate that competition between mineral sediment deposition and organic-matter accumulation determines the net impact of climate change on carbon accumulation in intertidal wetlands. We find that the direct impact of warming on soil carbon accumulation rates is more subtle than the impact of warming-driven sea level rise, although the impact of warming increases with increasing rates of sea level rise. Our simulations suggest that the net impact of climate change will be to increase carbon burial rates in the first half of the twenty-first century, but that carbon-climate feedbacks are likely to diminish over time.
Marshes worldwide are actively degrading in response to increased sea level rise rates and reduced sediment delivery, though the growth rate of vegetation plays a critical role in determining their stability. We have compiled 56 measurements of aboveground annual productivity for Spartina alterniflora, the dominant macrophyte in North American coastal wetlands. Our compilation indicates a significant latitudinal gradient in productivity, which we interpret to be determined primarily by temperature and/or the length of growing season. Simple linear regression yields a 27 g m−2 yr−1 increase in productivity with an increase of mean annual temperature by 1 °C. If temperatures warm 2–4 °C over the next century, then marsh productivity may increase by 10–40%, though physiological research suggests that increases in the north could potentially be offset by some decreases in the south. This increase in productivity is roughly equivalent to estimates of marsh lost due to future sea level change. If a warming-induced stimulation of vegetation growth will enhance vertical accretion and limit erosion, then the combined effects of global change may be to increase the total productivity and ecosystem services of tidal wetlands, at least in northern latitudes.
1] Assumptions of a static landscape inspire predictions that about half of the world's coastal wetlands will submerge during this century in response to sea‐level acceleration. In contrast, we use simulations from five numerical models to quantify the conditions under which ecogeomorphic feed-backs allow coastal wetlands to adapt to projected changes in sea level. In contrast to previous sea‐level assessments, we find that non‐linear feedbacks among inundation, plant growth, organic matter accretion, and sediment deposition, allow marshes to survive conservative projections of sea‐ level rise where suspended sediment concentrations are greater than ∼20 mg/L. Under scenarios of more rapid sea‐level rise (e.g., those that include ice sheet melting), marshes will likely submerge near the end of the 21st century. Our results emphasize that in areas of rapid geomorphic change, predicting the response of ecosystems to climate change requires consideration of the ability of biological pro-cesses to modify their physical environment.
1] Wetlands represent the largest component of the terrestrial biological carbon pool and thus play an important role in global carbon cycles. Most global carbon budgets, however, have focused on dry land ecosystems that extend over large areas and have not accounted for the many small, scattered carbon-storing ecosystems such as tidal saline wetlands. We compiled data for 154 sites in mangroves and salt marshes from the western and eastern Atlantic and Pacific coasts, as well as the Indian Ocean, Mediterranean Ocean, and Gulf of Mexico. The set of sites spans a latitudinal range from 22.4°S in the Indian Ocean to 55.5°N in the northeastern Atlantic. The average soil carbon density of mangrove swamps (0.055 ± 0.004 g cm À3) is significantly higher than the salt marsh average (0.039 ± 0.003 g cm À3). Soil carbon density in mangrove swamps and Spartina patens marshes declines with increasing average annual temperature, probably due to increased decay rates at higher temperatures. In contrast, carbon sequestration rates were not significantly different between mangrove swamps and salt marshes. Variability in sediment accumulation rates within marshes is a major control of carbon sequestration rates masking any relationship with climatic parameters. Globally, these combined wetlands store at least 44.6 Tg C yr À1 and probably more, as detailed areal inventories are not available for salt marshes in China and South America. Much attention has been given to the role of freshwater wetlands, particularly northern peatlands, as carbon sinks. In contrast to peatlands, salt marshes and mangroves release negligible amounts of greenhouse gases and store more carbon per unit area.
The global decline in estuarine and coastal ecosystems (ECEs) is affecting a number of critical benefits, or ecosystem services. We review the main ecological services across a variety of ECEs, including marshes, mangroves, nearshore coral reefs, seagrass beds, and sand beaches and dunes. Where possible, we indicate estimates of the key economic values arising from these services, and discuss how the natural variability of ECEs impacts their benefits, the synergistic relationships of ECEs across seascapes, and management implications. Although reliable valuation estimates are beginning to emerge for the key services of some ECEs, such as coral reefs, salt marshes, and mangroves, many of the important benefits of seagrass beds and sand dunes and beaches have not been assessed properly. Even for coral reefs, marshes, and mangroves, important ecological services have yet to be valued reliably, such as cross-ecosystem nutrient transfer (coral reefs), erosion control (marshes), and pollution control (mangroves). An important issue for valuing certain ECE services, such as coastal protection and habitat–fishery linkages, is that the ecological functions underlying these services vary spatially and temporally. Allowing for the connectivity between ECE habitats also may have important implications for assessing the ecological functions underlying key ecosystems services, such coastal protection, control of erosion, and habitat–fishery linkages. Finally, we conclude by suggesting an action plan for protecting and/or enhancing the immediate and longer-term values of ECE services. Because the connectivity of ECEs across land–sea gradients also influences the provision of certain ecosystem services, management of the entire seascape will be necessary to preserve such synergistic effects. Other key elements of an action plan include further ecological and economic collaborative research on valuing ECE services, improving institutional and legal frameworks for management, controlling and regulating destructive economic activities, and developing ecological restoration options.
Background:
Salt marshes lie between many human communities and the coast and have been presumed to protect these communities from coastal hazards by providing important ecosystem services. However, previous characterizations of these ecosystem services have typically been based on a small number of historical studies, and the consistency and extent to which marshes provide these services has not been investigated. Here, we review the current evidence for the specific processes of wave attenuation, shoreline stabilization and floodwater attenuation to determine if and under what conditions salt marshes offer these coastal protection services.
Methodology/principal findings:
We conducted a thorough search and synthesis of the literature with reference to these processes. Seventy-five publications met our selection criteria, and we conducted meta-analyses for publications with sufficient data available for quantitative analysis. We found that combined across all studies (n = 7), salt marsh vegetation had a significant positive effect on wave attenuation as measured by reductions in wave height per unit distance across marsh vegetation. Salt marsh vegetation also had a significant positive effect on shoreline stabilization as measured by accretion, lateral erosion reduction, and marsh surface elevation change (n = 30). Salt marsh characteristics that were positively correlated to both wave attenuation and shoreline stabilization were vegetation density, biomass production, and marsh size. Although we could not find studies quantitatively evaluating floodwater attenuation within salt marshes, there are several studies noting the negative effects of wetland alteration on water quantity regulation within coastal areas.
Conclusions/significance:
Our results show that salt marshes have value for coastal hazard mitigation and climate change adaptation. Because we do not yet fully understand the magnitude of this value, we propose that decision makers employ natural systems to maximize the benefits and ecosystem services provided by salt marshes and exercise caution when making decisions that erode these services.
Tidal marshes will be threatened by increasing rates of sea-level rise (SLR) over the next century. Managers seek guidance on whether existing and restored marshes will be resilient under a range of potential future conditions, and on prioritizing marsh restoration and conservation activities.
Building upon established models, we developed a hybrid approach that involves a mechanistic treatment of marsh accretion dynamics and incorporates spatial variation at a scale relevant for conservation and restoration decision-making. We applied this model to San Francisco Bay, using best-available elevation data and estimates of sediment supply and organic matter accumulation developed for 15 Bay subregions. Accretion models were run over 100 years for 70 combinations of starting elevation, mineral sediment, organic matter, and SLR assumptions. Results were applied spatially to evaluate eight Bay-wide climate change scenarios.
Model results indicated that under a high rate of SLR (1.65 m/century), short-term restoration of diked subtidal baylands to mid marsh elevations (-0.2 m MHHW) could be achieved over the next century with sediment concentrations greater than 200 mg/L. However, suspended sediment concentrations greater than 300 mg/L would be required for 100-year mid marsh sustainability (i.e., no elevation loss). Organic matter accumulation had minimal impacts on this threshold. Bay-wide projections of marsh habitat area varied substantially, depending primarily on SLR and sediment assumptions. Across all scenarios, however, the model projected a shift in the mix of intertidal habitats, with a loss of high marsh and gains in low marsh and mudflats.
Results suggest a bleak prognosis for long-term natural tidal marsh sustainability under a high-SLR scenario. To minimize marsh loss, we recommend conserving adjacent uplands for marsh migration, redistributing dredged sediment to raise elevations, and concentrating restoration efforts in sediment-rich areas. To assist land managers, we developed a web-based decision support tool (www.prbo.org/sfbayslr).
Coastal wetlands are responsible for about half of all carbon burial in oceans, and their persistence as a valuable ecosystem depends largely on the ability to accumulate organic material at rates equivalent to relative sea level rise. Recent work suggests that elevated CO2 and temperature warming will increase organic matter productivity and the ability of marshes to survive sea level rise. However, we find that organic decomposition rates increase by about 12% per degree of warming. Our measured temperature sensitivity is similar to studies from terrestrial systems, twice as high as the response of salt marsh productivity to temperature warming, and roughly equivalent to the productivity response associated with elevated CO2 in C3 marsh plants. Therefore, enhanced CO2 and warmer temperatures may actually make marshes less resilient to sea level rise, and tend to promote a release of soil carbon. Simple projections indicate that elevated temperatures will increase rates of sea level rise more than any acceleration in organic matter accumulation, suggesting the possibility of a positive feedback between climate, sea level rise, and carbon emissions in coastal environments.
Salt marshes are among the most abundant, fertile, and accessible coastal habitats on earth, and they provide more ecosystem services to coastal populations than any other environment. Since the Middle Ages, humans have manipulated salt marshes at a grand scale, altering species composition, distribution, and ecosystem function. Here, we review historic and contemporary human activities in marsh ecosystems--exploitation of plant products; conversion to farmland, salt works, and urban land; introduction of non-native species; alteration of coastal hydrology; and metal and nutrient pollution. Unexpectedly, diverse types of impacts can have a similar consequence, turning salt marsh food webs upside down, dramatically increasing top down control. Of the various impacts, invasive species, runaway consumer effects, and sea level rise represent the greatest threats to salt marsh ecosystems. We conclude that the best way to protect salt marshes and the services they provide is through the integrated approach of ecosystem-based management.
We propose a simple relationship linking global sea-level variations on time scales of decades to centuries to global mean temperature. This relationship is tested on synthetic data from a global climate model for the past millennium and the next century. When applied to observed data of sea level and temperature for 1880-2000, and taking into account known anthropogenic hydrologic contributions to sea level, the correlation is >0.99, explaining 98% of the variance. For future global temperature scenarios of the Intergovernmental Panel on Climate Change's Fourth Assessment Report, the relationship projects a sea-level rise ranging from 75 to 190 cm for the period 1990-2100.
Rising sea levels threaten the sustainability of coastal wetlands around the globe, thus understanding how increased inundation alters the elevation change mechanisms in these systems is increasingly important. Typically, the ability of coastal marshes to maintain their position in the intertidal zone depends on the accumulation of both organic and inorganic materials, so one, if not both, of these processes must increase to keep pace with rising seas, assuming all else constant. To determine the importance of vegetation in these processes, we measured elevation change and surface accretion over a 4‐year period in recently subsided, unvegetated marshes, resulting from drought‐induced marsh dieback, in paired planted and unplanted plots. We compared soil and vegetation responses in these plots with paired reference plots that had neither experienced dieback nor subsidence. All treatments (unvegetated, planted, and reference) were replicated six times. The recently subsided areas were 6–10 cm lower in elevation than the reference marshes at the beginning of the study; thus, mean water levels were 6–10 cm higher in these areas vs. the reference sites. Surface accretion rates were lowest in the unplanted plots at 2.3 mm yr−1, but increased in the presence of vegetation to 16.4 mm yr−1 in the reference marsh and 26.1 mm yr−1 in the planted plots. The rates of elevation change were also bolstered by the presence of vegetation. The unplanted areas decreased in elevation by 9.4 mm yr−1; whereas the planted areas increased in elevation by 13.3 mm yr−1, and the reference marshes increased by 3.5 mm yr−1. These results highlight the importance of vegetation in the accretionary processes that maintain marsh surface elevation within the intertidal zone, and provide evidence that coastal wetlands may be able to keep pace with a rising sea in certain situations.
Sea-level rise remains one of the most pressing societal concerns relating to climate change. A significant proportion of the global population, including many of the world’s large cities, are located close to the coast in potentially vulnerable regions such as river deltas. The Permanent Service for Mean Sea Level (PSMSL) continues to evolve and provide global coastal sea-level information and products that help to develop our understanding of sea-level and land motion processes. Its work aids a range of scientific research, not only in long-term change, but also in the measurement and understanding of higher frequency variability such as storm surges and tsunamis. The PSMSL has changed considerably over the past 10 years, and the aim of this paper is to update the community about these change as well as provide an overview of our continuing work.
Sea level rise is perhaps the most damaging repercussion of global warming, as 150 million people live less than one meter above current high tides .Using an inverse statistical model we examine potential response in coastal sea level to the changes in natural and anthropogenic forcings by 2100. With six IPCC radiative forcing scenarios we estimate sea level rise of 0.6-1.6 m, with confidence limits of 0.59 m and 1.8 m. Projected impacts of solar and volcanic radiative forcings account only for, at maximum, 5% of total sea level rise, with anthropogenic greenhouse gasses being the dominant forcing. As alternatives to the IPCC projections, even the most intense century of volcanic forcing from the past 1000 years would result in 10-15 cm potential reduction of sea level rise. Stratospheric injections of SO2 equivalent to a Pinatubo eruption every 4 years would effectively just delay sea level rise by 12 -20 years.
We used field and laboratory measurements, geographic information systems, and simulation modeling to investigate the potential effects of accelerated sea-level rise on tidal marsh area and delivery of ecosystem services along the Georgia coast. Model simulations using the Intergovernmental Panel on Climate Change (IPCC) mean and maximum estimates of sea-level rise for the year 2100 suggest that salt marshes will decline in area by 20% and 45%, respectively. The area of tidal freshwater marshes will increase by 2% under the IPCC mean scenario, but will decline by 39% under the maximum scenario. Delivery of ecosystem services associated with productivity (macrophyte biomass) and waste treatment ( nitrogen accumulation in soil, potential denitrification) will also decline. Our findings suggest that tidal marshes at the lower and upper salinity ranges, and their attendant delivery of ecosystem services, will be most affected by accelerated sea-level rise, unless geomorphic conditions (ie gradual increase in elevation) enable tidal freshwater marshes to migrate inland, or vertical accretion of salt marshes to increase, to compensate for accelerated sea-level rise.
This paper reviews the 210Pb dating programme carried out at the University of Liverpool over a period of three decades since its inception by Frank Oldfield in the mid-1970s. The work at Liverpool has included studies of the basic processes controlling the supply of 210Pb to the various natural archives, as well as the development of a detailed and systematic methodology for dating the environmental records in these archives. The assumption that 210Pb fallout at any given location is constant when measured on timescales of a year or more has been tested using data on 210Pb concentrations in UK rainwater spanning more than 40 years, and a simple atmospheric model developed to account for observed spatial variations in the flux over large land masses. The basic assumptions of the CRS dating model have been tested in detail using mass balance studies of fallout radionuclides in catchment/lake systems, and in general using data from a large number of separate studies. The Liverpool data base is used to illustrate the wide ranging applicability of the 210Pb dating method. Results are shown from lakes varying in size from the very small to the very large, and with sedimentation rates ranging from the very fast to the very slow. The particular difficulties of dating sediment records from Desert Lakes and Polar Regions with very low atmospheric fluxes are discussed, with unexpected results in some cases.
In order to maintain an elevation in the intertidal zone at which marsh vegetation can survive, vertical accretion of the marsh surface must take place at a rate at least equal to the rate of relative sea-level rise. Net vertical accretion of coastal marshes is a result of interactions between tidal imports, vegetation and depositional processes. All of these factors are affected, directly or indirectly, by alterations in marsh hydrology which might occur as a result of sea-level rise. The overall response of coastal marshes to relative sea-level rise depends upon the relative importance of the inorganic and organic components of the marsh soil and the impact of increased hydroperiod on net accumulation. The varied combination of factors contributing to sediment supply, and their complexity at the scale of individual marshes, means that predicting the response of suspended sediment concentration in marsh floodwater to any changes which may occur as a result of sea-level rise, at anything other than the local scale is unlikely to be accurate. The impact of sea-level rise on net below-ground production is also complex. The sensitivity of certain species to waterlogging and soil chemical changes could result in a change in species composition or the migration of vegetation zones. Consequently, predicting the net impact of sea-level rise on organic matter accumulation is fraught with difficulties and requires improved understanding of interactions between vegetation, soil and hydrologic processes.
The salt marsh at Barnstable, Massachusetts, occupies an embayment into which it has spread during the past 4,000 years. It exhibits all stages of development from the seeding of bare sand flats through the development of intertidal marsh to the formation of mature high marsh underlain by peat deposits more than 20 ft deep. Observations and measurements of the stages of its formation are presented. The geomorphology of the marsh is considered in relation to the factors which have influenced its development, i.e., the ability of halophytes to grow at limited tide levels, the tidal regime, the processes of sedimentation, and the contemporary rise in sea level. The rates at which the early stage of development takes place have been determined by observations during a period of 12 years and the time sequence of later stages by radiocarbon analyses.
Salt marsh structure and function, and consequently ability to support a range of species and to provide ecosystem services, may be affected by climate change. To better understand how salt marshes will respond to warming and associated shifts in precipitation, we conducted a manipulative experiment in a tidal salt marsh in Massachusetts, USA. We exposed two plant communities (one dominated by Spartina patens-Distichlis spicata and one dominated by short form Spartina alternifora) to five climate manipulations: warming via passive open-topped chambers, doubled precipitation, warming and doubled precipitation, extreme drought via rainout shelter, and ambient conditions. Modest daytime warming increased total aboveground biomass of the S. alterniflora community (24%), but not the S. patens-D. spicata community. Warming also increased maximum stem heights of S. alterniflora (8%), S. patens (8%), and D. spicata (15%). Decomposition was marginally accelerated by warming in the S. alternifora community. Drought markedly increased total biomass of the S. alterniflora community (53%) and live S. patens (69%), perhaps by alleviating waterlogging of sediments. Decomposition was accelerated by increased precipitation and slowed by drought, particularly in the S. patens-D. spicata community. Flowering phenology responded minimally to the treatments, and pore water salinity, sulfide, ammonium, and phosphate concentrations showed no treatment effects in either plant community. Our results suggest that these salt marsh communities may be resilient to modest amounts of warming and large changes in precipitation. If production increases under climate change, marshes will have a greater ability to keep pace with sea-level rise, although an increase in decomposition could offset this. As long as marshes are not inundated by flooding due to sea-level rise, increases in aboveground biomass and stem heights suggest that marshes may continue to export carbon and nutrients to coastal waters and may be able to increase their carbon storage capability by increasing plant growth under future climate conditions.
Three salt marshes on a 50-km transect along the north bank of the Westerschelde Estuary were investigated to determine whether salt marshes in the estuary had responded to shipping channel modifications in recent decades. Marsh accretion rates were estimated mainly from 137Cs profiles with further evidence from 241Am because changes in both rate of deposition and nature of the accreting material precluded use of standard 210Pb(excess) dating models. The 137Cs profiles usually show peaks corresponding to atmospheric deposition from the 1963 fallout maximum and sometimes from the Chernobyl accident, although intervening enhanced 137Cs activities derived from the nuclear reprocessing marine discharges of Sellafield and La Hague are clearly discernible. In all three marshes (Ritthem at the mouth of the estuary and Zuidgors and Waarde at 20 and 45 km upstream), a marked, near-coincident change in the rate of accumulation and in the grain size of material deposited occurred around 1980. This may be related to a combination of channel deepening and straightening operations undertaken in the mid-1970s and/or natural changes in winter wave climate.
The development of a typical New England salt marsh, and the growth of the sand spit which shelters it, during the past 4000 years has been reconstructed from soundings and borings of the peat. The results have been interpreted with the aid of observations on the structure of the marsh and estimates of the rate of its vertical accretion based on carbon-14 determinations.
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