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1. Year-around survey and manipulation experiments reveal differential sensitivities of soil prokaryotic and fungal communities to saltwater intrusion in Florida Everglades wetlands

   https://doi.org/10.1016/j.scitotenv.2022.159865  

   Zhao, Jun; Chakrabarti, Seemanti; Chambers, Randolph; Weisenhorn, Pamela; Travieso, Rafael; Stumpf, Sandro; Standen, Emily; Briceno, Henry; Troxler, Tiffany; Gaiser, Evelyn; et al (February 2023, Science of The Total Environment)

   Global sea-level rise is transforming coastal ecosystems, especially freshwater wetlands, in part due to increased episodic or chronic saltwater exposure, leading to shifts in biogeochemistry, plant- and microbial communities, as well as ecological services. Yet, it is still difficult to predict how soil microbial communities respond to the saltwater exposure because of poorly understood microbial sensitivity within complex wetland soil microbial communities, as well as the high spatial and temporal heterogeneity of wetland soils and saltwater exposure. To address this, we first conducted a two-year survey of microbial community structure and bottom water chemistry in submerged surface soils from 14 wetland sites across the Florida Everglades. We identified ecosystem-specific microbial biomarker taxa primarily associated with variation in salinity. Bacterial, archaeal and fungal community composition differed between freshwater, mangrove, and marine seagrass meadow sites, irrespective of soil type or season. Especially, methanogens, putative denitrifying methanotrophs and sulfate reducers shifted in relative abundance and/or composition between wetland types. Methanogens and putative denitrifying methanotrophs declined in relative abundance from freshwater to marine wetlands, whereas sulfate reducers showed the opposite trend. A four-year experimental simulation of saltwater intrusion in a pristine freshwater site and a previously saltwater-impacted site corroborated the highest sensitivity and relative increase of sulfate reducers, as well as taxon-specific sensitivity of methanogens, in response to continuously pulsing of saltwater treatment. Collectively, these results suggest that besides increased salinity, saltwater-mediated increased sulfate availability leads to displacement of methanogens by sulfate reducers even at low or temporal salt exposure. These changes of microbial composition could affect organic matter degradation pathways in coastal freshwater wetlands exposed to sea-level rise, with potential consequences, such as loss of stored soil organic carbon. 

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2. Modeling net ecosystem carbon balance and loss in coastal wetlands exposed to sea‐level rise and saltwater intrusion

   https://doi.org/10.1002/eap.2702  

   Ishtiaq, Khandker S.; Troxler, Tiffany G.; Lamb‐Wotton, Lukas; Wilson, Benjamin J.; Charles, Sean P.; Davis, Stephen E.; Kominoski, John S.; Rudnick, David T.; Sklar, Fred H. (January 2022, Ecological Applications)

   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 a mass C balance approach, we estimated net gain in C and corresponding export of aquatic fluxes ( ) in the freshwater marsh under ambient conditions (NECB = 1119 ± 229 gC m−2 year−1; FAQ = 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; FAQ = 311 ± 30 gC m−2 year−1) and elevated salinity (NECB = −594 ± 94 gC m−2 year−1; FAQ = 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 be extended 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. 

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3. Saltwater and nutrient legacies reduce net ecosystem carbon storage despite freshwater restoration: insights from experimental wetlands

   https://doi.org/10.1111/rec.13524  

   Lee, Dong Yoon; Kominoski, John S.; Kline, Michael; Robinson, Michelle; Roebling, Suzy (January 2021, Restoration Ecology)

   null (Ed.)

   Net ecosystem carbon balance is a comprehensive assessment of ecosystem function that can test restoration effectiveness. Coastal peatlands are globally important carbon sinks that are vulnerable to carbon loss with saltwater intrusion. It is uncertain how wetland carbon stocks and fluxes change during freshwater restoration following exposure to saltwater and elevated nutrients. We restored freshwater to sawgrass (Cladium jamaicense) peat monoliths from freshwater marshes of the Everglades (Florida, U.S.A.) that had previously been exposed to elevated salinity (approximately9 ppt) and phosphorus (P) loading (1 g P m−2 year−1) in wetland mesocosms. We quantified changes in water and soil physicochemistry, plant and soil carbon and nutrient standing stocks, and net ecosystem productivity during restoration. Added freshwater immediately reduced porewater salinity from >8 to approximately 2 ppt, but elevated porewater dissolved organic carbon persisted. Above- and belowground biomass, leaf P concentrations, and instantaneous rates of gross ecosystem productivity (GEP) and ecosystem respiration (ER) remained elevated from prior added P. Modeled monthly GEP and ER were higher in marshes with saltwater and P legacies, resulting in negative net ecosystem productivities that were up to 12× lower than controls. Leaf litter breakdown rates and litter P concentrations were 2× higher in marshes with legacies of added saltwater and P. Legacies of saltwater and P on carbon loss persisted despite freshwater restoration, but recovery was greatest for freshwater marshes exposed to saltwater alone. Our results suggest that restoration in nutrient-limited freshwater wetlands exposed to saltwater intrusion and nutrient enrichment is a slow process. 

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4. Evaluating Hydrogeomorphic Condition Across Ecosystem States in a Non-tidal, Brackish Peat Marsh of the Florida Coastal Everglades, USA

   https://doi.org/10.1007/s12237-024-01364-5  

   Lamb-Wotton, Lukas; Troxler, Tiffany G; Coronado-Molina, Carlos; Davis, Stephen E; Gann, Daniel; Ishtiaq, Khandker S; Malone, Sparkle L; Olivas, Paulo; Rudnick, David T; Sklar, Fred H (May 2024, Estuaries and Coasts)

   Abstract Emergent marsh and open water have been identified as alternate stable states in tidal marshes with large, relative differences in hydrogeomorphic conditions. In the Florida coastal Everglades, concern has been raised regarding the loss of non-tidal, coastal peat marsh via dieback of emergent vegetation and peat collapse. To aid in the identification of alternate stable states, our objective was to characterize the variability of hydrogeomorphic and biologic conditions using a field survey and long-term monitoring of hydrologic and geomorphic conditions across a range of vegetated (emergent, submerged) and unvegetated (open water) communities, which we refer to as “ecosystem states,” in a non-tidal, brackish peat marsh of the coastal Everglades. Results show (1) linear relationships among field-surveyed geomorphic, hydrologic, and biologic variables, with a 35-cm mean difference in soil surface elevation between emergent and open water states, (2) an overall decline in soil elevation in the submerged state that was related to cumulative dry days, and (3) a 2× increase in porewater salinity during the dry season in the emergent state that was also related to the number of dry days. Coupled with findings from previous experiments, we propose a conceptual model that describes how seasonal hydrologic variability may lead to ecosystem state transitions between emergent and open water alternate states. Since vegetative states are only moderately salt tolerant, as sea-level rise pushes the saltwater front inland, the importance of continued progress on Everglades restoration projects, with an aim to increase the volume of freshwater being delivered to coastal wetlands, is the primary management intervention available to mitigate salinization and slow ecosystem state shifts in non-tidal, brackish peat marshes. 

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5. Hydrologic history and flooding pulses control iron and sulfur metabolic composition with variable associations to greenhouse gas production in a coastal wetland mesocosm experiment

   https://doi.org/10.1101/2025.03.03.641170  

   Finlay, Colin G; Peralta, Ariane L (March 2025, bioRxiv)

   Coastal wetlands can store carbon by sequestering more carbon through primary production than they release though biogenic greenhouse gas production. The joint effects of saltwater intrusion and sea level rise (SWISLR) and changing precipitation patterns alter sulfate and oxygen availability, challenging estimates of biogenic greenhouse gas emissions. Iron-rich soils have been shown to buffer soil sulfidization by sequestering sulfide into iron-sulfide. But as SWISLR increases soil sulfate concentrations, sulfide produced via sulfate reduction will likely exceed the buffering capacity of soil iron, allowing toxic sulfide levels to accumulate. We used a soil mesocosm approach to examine the influence of hydrology (wet, dry, interim) and plant presence (with or without plants) on wetland soils sourced from different hydrologic histories at a restored coastal wetland. We hypothesized that reducing conditions (i.e., flooded, no plants) impact anaerobic metabolisms similarly, whereas oxidizing conditions (i.e., dry, plant presence) disrupt coupled sulfate reduction and iron reduction. Over eight weeks of hydrologic manipulation, 16S rRNA amplicon sequencing and shotgun metagenomic sequencing were used to characterize microbial communities, while greenhouse gas fluxes, soil redox potential, and physicochemical properties were measured. Results showed that contemporary hydrologic treatment affected assimilatory sulfate reduction gene composition, and hydrologic history influenced dissimilatory sulfate reduction and iron reduction gene composition. Sulfate and iron reduction genes were correlated, and dissimilatory sulfate reduction genes explained variance in methane fluxes. These findings highlight the role of historical hydrology, potential saltwater exposure, and soil iron in shaping microbial responses to future changes in soil moisture and salinity. 

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