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Plant zonation is conspicuous in wetlands. The cause is frequently assumed to be the direct physiological effects of physical factors (termed ‘stress’), however many experiments show that competition and facilitation also cause zonation patterns. We conducted a field experiment with freshwater marsh emergent plants to test the causes of zonation along a single stress gradient: flooding duration. We constructed an experimental wetland with ten flooding levels to ensure that the environmental conditions represented the full range of potential flooding levels, from never flooded to continually flooded. We planted ten common marsh plants with varied ecology along the flooding duration gradient. We grew them alone and in mixture for three years and measured changes in the minimum and maximum limits, the mode and the range of distribution, and interaction importance. The mode of distribution did not shift, whether species were grown alone or with neighbours. We found strong effects of competition under low flooding stress. We found no effects from facilitation under high flooding stress. Flooding duration alone controlled the lower limits of plants. The effects of competition were intense enough to eliminate half of the species within three growing seasons. Our experiment showed that competition and physical stresses, but not facilitation, controls the zonation of emergent macrophytes along a flooding duration gradient, at least in freshwater wetlands. Models guiding wetland restoration need to include competition as well as flood duration as causal factors, but not facilitation.

In estuaries, future variation in sea level and river discharge will lead to saline intrusion into low-salinity tidal marshes. To investigate the processes that control the differential response and recovery of tidal freshwater marsh plant communities to saline pulses, a 3 × 5 factorial greenhouse experiment was conducted to examine the effects of a range of salinity levels (3, 5, and 10 practical salinity units (PSU)) and pulse durations (5, 10, 15, 20, and 30 days per month) on community composition of tidal freshwater marsh vegetation. Recovery of perturbed communities was also examined after 10 months. The results showed that community composition was increasingly affected by the more-saline and longer-duration treatments. The increasing suppression of salt-sensitive species resulted in species reordering, decreased species richness, and decreased aboveground biomass. Most of the plant species were able to recover from low-salinity, short-duration saline pulses in less than 1 year. However, because not all species recovered in the heavily salinized treatments, species richness at the end of the recovery period remained low for treatments that were heavily salinized during the treatment period. In contrast, plant aboveground biomass fully recovered in the heavily salinized treatments. Although the magnitude and duration of pulsed environmental changes had strong effects on community composition, shifts in community composition prevented long-term reductions in productivity. Thus, in this study system, environmental change affected species composition more strongly than it did ecosystem processes.

Several ecological factors, including hydrology, soil type, and vegetation, influence wetland soil carbon (C) storage, but the relationship among these factors is complex making it difficult to evaluate the potential for increased C storage in natural and restored systems. This study investigated the relationship between hydrologic variables, wetland plant communities, and wetland soil C storage in the upper 50 cm of soil in three wetland community types (bay swamp, cypress swamp, freshwater marsh) in a hydrologically restored subtropical landscape in central Florida, USA. Mean water table depth relative to ground elevation was a better predictor than hydroperiod of surface soil C stock and was positively related to soil C stock in marshes. However, the overall effect of water table depth was small and was often outweighed by other factors including wetland vegetation type and local site conditions. Bay swamps had the highest soil C stock, followed by cypress swamp, marsh, and upland ecotone, respectively. This study highlights the importance of understanding the interplay among multiple factors that drive variation in soil C stock within and among wetland types in these landscapes, and the importance of deeper soil layers to wetland soil C storage at the landscape scale.

Wetlands loss has major consequences for biodiversity. The Delta of Paraná River is one of the largest wetland ecosystems in South America undergoing rapid conversion of freshwater marshes to pastures. We evaluated the response of nine wetland bird species to a gradient of landscape structure accounting for different levels of wetland loss in the Lower Delta, Argentina. We used point counts and a hierarchical distance sampling approach to assess the effects of wetland area, configuration, and land use on the density of species. Wetland area was the most important factor determining species density; most species responded positively at 100 m. The effect of wetland configuration varied among species; contiguous freshwater marsh area at 500 m only favored one species, whereas a large number of small patches of freshwater marsh benefited most species. Higher cattle density showed variable effects, and larger areas within polders reduced the density of two species. In the long term, wetland birds of the Lower Delta could decrease in density due to wetland loss and anthropogenic changes in the landscape. Our study shows the importance of considering the response of multiple species to landscape change at multiple scales and the need for a sustainable management of wetlands.

Environmental changes can alter the interactions between biotic and abiotic ecosystem components in tidal wetlands and therefore impact important ecosystem functions. The objective of this study was to determine how saltwater intrusion affects wetland nutrient biogeochemistry, with a specific focus on the soil microbial communities and physicochemical parameters that control nitrate removal. Our work took place in a tidal freshwater marsh in South Carolina, USA, where a 3.5-year saltwater intrusion experiment increased porewater salinities from freshwater to oligohaline levels. We measured rates of denitrification, soil oxygen demand, and dissimilatory nitrate reduction to ammonium (DNRA) and used molecular genetic techniques to assess the abundance and community structure of soil microbes. In soils exposed to elevated salinities, rates of denitrification were reduced by about 70% due to changes in the soil physicochemical environment (higher salinity, higher carbon: nitrogen ratio) and shifts in the community composition of denitrifiers. Saltwater intrusion also affected the microbial community responsible for DNRA, increasing the abundance of genes associated with this process and shifting microbial community composition. Though rates of DNRA were below detection, the microbial community response may be a precursor to increased rates of DNRA with continued saltwater intrusion. Overall, saltwater intrusion reduces the ability of tidal freshwater marshes to convert reactive nitrogen to dinitrogen gas and therefore negatively affects their water quality functions. Continued study of the interrelationships between biotic communities, the abiotic environment, and biogeochemical transformations will lead to a better understanding of how the progressive replacement of tidal freshwater marshes with brackish analogues will affect the overall functioning of the coastal landscape.

Tidal marshes are highly valued habitats, yet are vulnerable to loss from both anthropogenic and natural disturbances including sea-level rise (SLR). Many tidal marshes have kept pace with SLR over the last century; on average, however, recent escalations in SLR increase the vulnerability of marshes to submergence. Relative sea-level rise near our study sites in the Mid-Atlantic U.S. averaged 4.34 mm year⁻¹ over the last 50 years, yet over the last 19 years, relative sea-level rise averaged 6.25 mm year⁻¹ and the rise in high tide water levels averaged 8.13 mm year⁻¹. We compared these rates of water rise to rates of marsh surface elevation change using surface elevation tables in ten tidal marshes—three tidal freshwater and four saline marshes in the Delaware Estuary and three salt marshes in Barnegat Bay, NJ, USA. We also examined the effects of marsh type and geomorphic setting on rates of elevation change, surface accretion, and subsurface change as well as the influence of marsh elevation, distance from a channel, and tidal range (n = 3 sites per marsh). Surface elevation change measured over the last 4 to 9 years averaged less than 6 mm year⁻¹ in nine out of ten of the study stations, less than rates of relative SLR. Tidal freshwater marshes in the Delaware Estuary had greater rates of surface accretion and elevation change than salt marshes in Barnegat Bay. Marshes sitting lower in the tidal frame and experiencing higher tidal ranges tended to have higher surface accretion rates, but shallow subsidence had a stronger influence on these elevation change rates. We estimated time to submergence (i.e., lifespan) for using rates of marsh elevation change, a SLR rate of 10 mm year⁻¹, and thresholds for conversion to open water using geospatial datasets. The calculated time to submergence for the majority of marshes was 60 to 80 years with some predicted to submerge in as few as 5 years. These data suggest that in order to keep pace with accelerating SLR, surface accretion in many of these marshes will have to increase at a rate that surpasses shallow subsidence (1–7 mm year⁻¹).

Mangroves are coastal ecosystems dependent on saline water conditions, although freshwater is seasonally present in most types of mangroves. The riparian mangroves have a greater influence of freshwater than salty water, reducing saline stress and allowing greater productivity and diversity. As they are associated with freshwater channels, their hydrology makes them both a source and a sink for sediments, nutrients, and organic matter. The wetlands adjacent to riparian mangroves are mainly freshwater swamps or marshes. To monitor the composition and abundance of the vegetation and the production of litter and roots in the midterm, 27 monitoring units were monitored (22 in mangroves, five in wetlands) in two periods (2015–2016 and 2018–2019). In them, we evaluated biotic characteristics and root production annually, and monthly the litter production and pore and river water salinity. We detected a gradient of salinity spatially and temporally. The salinity gradually decreased as the distance to the river increased. In the winter of 2018–2019 saline intrusion increased the interstitial and river water values by an average of 10 (interstitial water) and 16‰, (river water). This increase caused a significant decrease in litter and root production and augmented the cover of Laguncularia racemosa (freshwater marsh), mortality of herbaceous species (Acrostichum danaeifolium, Typha domingensis, Phragmites australis), and tree species such as Annona glabra and Acoelorraphe wrightii.

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ([Formula: see text]) or methane ([Formula: see text]), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m⁻² year⁻¹) and ²¹⁰Pb radiodating (336 g C m⁻² year⁻¹), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO₂₋ₑq m⁻² year⁻¹). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m⁻² year⁻¹). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m⁻² year⁻¹). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm⁻¹) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m⁻² year⁻¹), making this site a significant GHG source (>2,000 g CO₂₋ₑq m⁻² year⁻¹). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m⁻² year⁻¹), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.

The use of machine learning algorithms to classify complex landscapes has been revolutionized by the introduction of deep learning techniques, particularly in remote sensing. Convolutional neural networks (CNNs) have shown great success in the classification of complex high-dimensional remote sensing imagery, specifically in wetland classification. On the other hand, the state-of-the-art natural language processing (NLP) algorithms are transformers. Although the transformers have been studied for a few remote sensing applications, the integration of deep CNNs and transformers has not been studied, particularly in wetland mapping. As such, in this study, we explore the potential and possible limitations to be overcome regarding the use of a multi-model deep learning network with the integration of a modified version of the well-known deep CNN network of VGG-16, a 3D CNN network, and Swin transformer for complex coastal wetland classification. Moreover, we discuss the potential and limitation of the proposed multi-model technique over several solo models, including a random forest (RF), support vector machine (SVM), VGG-16, 3D CNN, and Swin transformer in the pilot site of Saint John city located in New Brunswick, Canada. In terms of F-1 score, the multi-model network obtained values of 0.87, 0.88, 0.89, 0.91, 0.93, 0.93, and 0.93 for the recognition of shrub wetland, fen, bog, aquatic bed, coastal marsh, forested wetland, and freshwater marsh, respectively. The results suggest that the multi-model network is superior to other solo classifiers from 3.36% to 33.35% in terms of average accuracy. Results achieved in this study suggest the high potential for integrating and using CNN networks with the cutting-edge transformers for the classification of complex landscapes in remote sensing.

Coastal wetlands are globally important stores of carbon (C). However, accelerated sea-level rise (SLR), increased saltwater intrusion, and modified freshwater discharge can contribute to the collapse of peat marshes, converting coastal peatlands into open water. Applying results from multiple experiments from sawgrass (Cladium jamaicense)-dominated freshwater and brackish water marshes in the Florida Coastal Everglades, we developed a system-level mechanistic peat elevation model (EvPEM). We applied the model to simulate net ecosystem C balance (NECB) and peat elevation in response to elevated salinity under inundation and drought exposure.Using amass C balance approach, we estimated net gain in C and corresponding export of aquatic fluxes (F AQ) in the freshwater marsh under ambient conditions (NECB = 1119 ± 229 gC m−2 year−1; F AQ = 317 ± 186 gC m−2 year−1). In contrast, the brackish water marsh exhibited substantial peat loss and aquatic C export with ambient (NECB = −366 ± 15 gC m−2 year−1; F AQ = 311 ± 30 gC m−2 year−1) and elevated salinity (NECB = 594 ± 94 gC m−2 year−1; F AQ = 729 ± 142 gC m−2 year−1) under extended exposed conditions. Further, mass balance suggests a considerable decline in soil C and corresponding elevation loss with elevated salinity and seasonal dry-down. Applying EvPEM, we developed critical marsh net primary productivity (NPP) thresholds as a function of salinity to simulate accumulating, steady-state, and collapsing peat elevations. The optimization showed that ∼150–1070 gC m−2 year−1 NPP could support a stable peat elevation (elevation change ≈ SLR), with the corresponding salinity ranging from 1 to 20 ppt under increasing inundation levels. The C budgeting and modeling illustrate the impacts of saltwater intrusion, inundation, and seasonal dry-down and reduce uncertainties in understanding the fate of coastal peat wetlands with SLR and freshwater restoration. The modeling results provide management targets for hydrologic restoration based on the ecological conditions needed to reduce the vulnerability of the Everglades’ peat marshes to collapse. The approach can beextended to other coastal peatlands to quantify C loss and improve understanding of the influence of the biological controls on wetland C storage changes for coastal management.