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1. Carbon dioxide Uptake Fluxes in Coastal Salt Marshes Reveal Ecological Similitudes and Environmental Regimes.

   Zaki, Mohammed T.; Abdul-Aziz, Omar; Ishtiaq, Khandker (December 2021, AGU Fall Meeting 2021, held in New Orleans, LA, 13-17 December 2021, id. B25C-1456.)

   We tested the hypothesis that carbon dioxide (CO2) uptake fluxes in coastal salt marshes follow ecological similitudes (parameter reductions) and distinct environmental regimes. The hypothesis was evaluated utilizing data from four salt marshes in Waquoit Bay, MA, USA collected during May-October 2013. Using dimensional analysis method from fluid mechanics and engineering, we reduced five flux and ecological variables (CO2 uptake, light, soil temperature, salinity, and atmospheric pressure) into two mechanistically meaningful dimensionless groups: (a) light use efficiency number (LUE = CO2 uptake normalized by daylight) and (b) biogeochemical number (BGC = interactions among soil temperature, salinity, and atmospheric pressure). Graphical exploration of the dimensionless numbers with the observed data revealed an emergent pattern that was distinctly characterized by high, transitional, and low LUE regimes. Transitions among the identified regimes were dictated by thresholds of soil temperature and salinity. Low LUE regime corresponded to unfavorable environmental conditions (soil temperature 17C and salinity > 30ppt), whereas high LUE regime was governed by favorable conditions (soil temperature > 17C and salinity 30ppt). The identified emergent pattern and environmental thresholds would provide key insights into the underlying organizing principles of CO2 uptake and the major environmental drivers in coastal salt marshes. 

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2. Ecological parameter reductions, environmental regimes, and characteristic process diagram of carbon dioxide fluxes in coastal salt marshes

   https://doi.org/10.1038/s41598-020-72066-8  

   Ishtiaq, Khandker S.; Abdul-Aziz, Omar I. (December 2020, Scientific Reports)

   Abstract We investigated the ecological parameter reductions (termed “similitudes”) and characteristic patterns of the net uptake fluxes of carbon dioxide (CO 2 ) in coastal salt marshes using dimensional analysis method from fluid mechanics and hydraulic engineering. Data collected during May–October, 2013 from four salt marshes in Waquoit Bay and adjacent estuary, Massachusetts, USA were utilized to evaluate the theoretically-derived dimensionless flux and various ecological driver numbers. Two meaningful dimensionless groups were discovered as the light use efficiency number (LUE = CO 2 normalized by photosynthetically active radiation) and the biogeochemical number (combination of soil temperature, porewater salinity, and atmospheric pressure). A semi-logarithmic plot of the dimensionless numbers indicated the emergence of a characteristic diagram represented by three distinct LUE regimes (high, transitional, and low). The high regime corresponded to the most favorable (high temperature and low salinity) condition for CO 2 uptake, whereas the low regime represented an unfavorable condition (low temperature and high salinity). The analysis identified two environmental thresholds (soil temperature ~ 17 °C and salinity ~ 30 ppt), which dictated the regime transitions of CO 2 uptake. The process diagram and critical thresholds provide important insights into the CO 2 uptake potential of coastal wetlands in response to changes in key environmental drivers. 

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3. Salt Marsh Light Use Efficiency is Driven by Environmental Gradients and Species‐Specific Physiology and Morphology

   https://doi.org/10.1029/2020JG006213  

   Hawman, Peter A.; Mishra, Deepak R.; O’Connell, Jessica L.; Cotten, David L.; Narron, Caroline R.; Mao, Lishen (May 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract Light use efficiency (LUE) of salt marshes has not been well studied but is central to production efficiency models (PEMs) used for estimating gross primary production (GPP). Salt marshes are typically dominated by a species monoculture, resulting in large areas with distinct morphology and physiology. We measured eddy covariance atmospheric CO₂fluxes for two marshes dominated by a different species:Juncus roemerianusin Mississippi andSpartina alterniflorain Georgia. LUE for theJuncusmarsh (mean = 0.160 ± 0.004 g C mol⁻¹photon), reported here for the first time, was on average similar to theSpartinamarsh (mean = 0.164 ± 0.003 g C mol⁻¹photon). However,JuncusLUE had a greater range (0.073–0.49 g C mol⁻¹photon) and higher variability (15.2%) than theSpartinamarsh (range: 0.035–0.36 g C mol⁻¹photon; variability: 12.7%). We compared the responses of LUE across six environmental gradients.JuncusLUE was predominantly driven by cloudiness, photosynthetically active radiation (PAR), soil temperature, water table, and vapor pressure deficit.SpartinaLUE was driven by water table, air temperature, and cloudiness. We also tested how the definition of LUE (incident PAR vs. absorbed PAR) affected the magnitude of LUE and its response. We found LUE estimations using incident PAR underestimated LUE and masked day‐to‐day variability. Our findings suggest that salt marsh LUE parametrization should be species‐specific due to plant morphology and physiology and their geographic context. These findings can be used to improve PEMs for modeling blue carbon productivity. 

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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. Biophysical controls of marsh soil shear strength along an estuarine salinity gradient

   https://doi.org/10.5194/esurf-9-413-2021  

   Gillen, Megan N.; Messerschmidt, Tyler C.; Kirwan, Matthew L. (January 2021, Earth Surface Dynamics)

   null (Ed.)

   Abstract. Sea-level rise, saltwater intrusion, and wave erosion threaten coastal marshes, but the influence of salinity on marsh erodibility remains poorly understood. We measured the shear strength of marsh soils along a salinity and biodiversity gradient in the York River estuary in Virginia to assess the direct and indirect impacts of salinity on potential marsh erodibility. We found that soil shear strength was higher in monospecific salt marshes (5–36 kPa) than in biodiverse freshwater marshes (4–8 kPa), likely driven by differences in belowground biomass. However, we also found that shear strength at the marsh edge was controlled by sediment characteristics, rather than vegetation or salinity, suggesting that inherent relationships may be obscured in more dynamic environments. Our results indicate that York River freshwater marsh soils are weaker than salt marsh soils, and suggest that salinization of these freshwater marshesmay lead to simultaneous losses in biodiversity and erodibility. 

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