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Salt marshes are intertidal coastal ecosystems shaped by complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial yet endangered environments provide a diverse range of ecosystem services but are globally subjected to high anthropogenic pressures, while being severely exposed to climate change impacts. The importance of salt marshes as "blue carbon" sinks, deriving from their primary production coupled with rapid surface accretion, has been increasingly recognized within the framework of climate mitigation strategies. However, large uncertainties remain in salt marsh carbon stock and sequestration estimation. In order to provide further knowledge in salt marsh carbon assessment and investigate marsh carbon pool response to management actions, we analyzed organic matter content in salt marsh soils of the Venice Lagoon (Italy) from 60 sediment cores to the depth of 1 m and estimated organic carbon stock and accumulation rates in different areas. Organic carbon stocks and accumulation rates were highly variable in different marshes, being affected by organic and inorganic inputs and preservation conditions. Our estimates suggest that the studied marshes store 17,108 ± 5,757 tons of carbon per square kilometer in top 1‐m of soil and can accumulate 85 ± 25 tons of carbon per square kilometer per year. However, flood regulation may reduce the annual marsh CO2 sequestration potential by more than 30%. Our results contribute valuable information for regional carbon assessments, reinforcing the need for integrated coastal management policies to preserve the ecosystem services of coastal environments, and underscore the importance of considering local variability and methodological variations. Plain Language Summary: Salt marshes are coastal ecosystems periodically inundated by tides. They offer crucial ecosystem services, including carbon storage, coastal protection, and biodiversity enhancement. The importance of salt marshes as "blue carbon" sinks has been increasingly recognized within the framework of climate mitigation strategies. Salt marsh vegetation captures and stores organic carbon, removing CO2 from the atmosphere through photosynthesis. Vertical accretion, driven by the deposition of both organic matter and mineral sediments, promotes the burial and preservation of carbon content in plant tissues and organic litter in the soil. However, uncertainties persist in estimating salt marsh carbon storage and sequestration potential. To address this, we analyzed soil organic content in 10 salt marshes of the Venice Lagoon (Italy). Our estimates suggest that the studied marshes can accumulate the same amount of carbon absorbed by 130,000 trees and highlight the economic value associated with their protection and restoration. However, coastal management actions may impact the carbon storage function of coastal environments. We show that flood regulation may reduce the annual marsh CO2 sequestration potential by more than 30%. Additionally, our results underscore the importance of considering local variability and methodological variations, which can significantly complicate blue carbon assessments. Key Points: The average marsh soil carbon stock in the Venice Lagoon is 17,108 ton C km−2, while carbon accumulation rate is 85 ton C km−2 yr−1Flood regulation strategies, which reduce vertical accretion capacity, may decrease the annual marsh CO2 sequestration by more than 30%Our results highlight the importance of considering local variability and methodological differences, complicating blue carbon assessments [ABSTRACT FROM AUTHOR]

Salt marshes are crucial eco-geomorphic features of tidal environments as they provide important ecological functions and deliver a wide range of ecosystem services. Being controlled by the interplay between hydrodynamics, geomorphology, and vegetation, the contribution of both organic matter (OM) and inorganic sediments drives salt marsh vertical accretion. This allows marshes to keep pace with relative sea level rise and likewise capture and store carbon, making them valuable allies in climate mitigation strategies. Thus, soil organic matter (SOM), i.e. the organic component of the soil, plays a key role within salt marsh environments, directly contributing to soil formation and supporting carbon storage. This study aims at inspecting spatial patterns of OM in surface salt marsh soils (top 20 cm), providing further insights into the physical and biological factors driving OM dynamics that affect salt marsh survival and carbon sink potential. Our results reveal two scales of variations in SOM content in marsh environments. At the marsh scale, OM variability is influenced by the interplay between surface elevation and changes in sediment supply linked with the distance from the marsh edge. At the system scale, OM content distribution is dominated by the gradient generated by marine and fluvial influence. The observed variations in SOM are explained by the combination of inorganic and organic input, preservation conditions, and sediment grain size. Our results highlight the importance of marshes as carbon sink environments, further emphasising that environmental conditions within a tidal system may generate strongly variable and site-specific carbon accumulation patterns, enhancing blue carbon assessment complexity. [ABSTRACT FROM AUTHOR]

Salt marshes are crucial eco-geomorphic features of tidal environments as they provide numerous important ecological functions and deliver a wide range of ecosystem services that contribute to human well-being. Being controlled by the interplay between hydrodynamics, geomorphology and vegetation, the deposition of both organic matter (OM) and inorganic sediments drives salt-marsh vertical accretion. This allows marshes to keep pace with relative sea-level rise, and likewise capture and store carbon, making them valuable allies in climate mitigation strategies. Thus, Soil Organic Matter (SOM), i.e. the organic component of the soil, plays a key role within salt-marsh environments, directly contributing to soil formation and supporting carbon storage. This study aims at inspecting spatial patterns of OM in surface salt-marsh soils, providing further insights into the physical and biological factors driving OM dynamics, affecting salt-marsh survival and carbon sink potential. Our results reveal two scales of variations in sedimentary OM content in salt-marsh soils. At the marsh scale OM variability is influenced by the interplay between surface elevation and changes in sediment supply linked with the distance from tidal channels. At the system scale, OM content distribution is dominated by the gradient generated by marine and fluvial influence. Variations in inorganic and organic inputs, both autochthonous and allochthonous, sediment grain size, and preservation conditions may explain the observed variations in SOM. Our results highlight marsh importance as carbon sink environments, furthermore emphasizing that environmental conditions within a tidal system may generate strongly variable and site-specific carbon accumulation patterns, enhancing blue carbon assessment complexity. [ABSTRACT FROM AUTHOR]

Tight interplays between physical and biotic processes in tidal salt marshes lead to self‐organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long‐term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation‐vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea‐level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site‐specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species‐specifically to changes in environmental forcings by modifying species‐preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation‐species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution. Plain Language Summary: Salt marshes are important ecosystems currently threatened by changes in environmental forcings. We analyzed the coupled elevation‐vegetation response to relative sea‐level rise in a marsh located within the Venice lagoon (Italy) by using accurate on‐site measurements collected between 2000 and 2019. We found that the marsh elevations above a fixed datum increased over the study period. However, marsh sites were found to be gradually drowning, because sediment deposition on the marsh surface did not balance the increase in relative mean sea level. We also found that the response of halophytic vegetation to relative sea‐level rise is species‐specific. In addition, we showed that vegetation species are distributed according to a specific sequence with increasing elevations and that such sequence remained stable for nearly two decades. This suggests that vegetation species can endure rising relative sea levels, at least temporally, without significantly modifying their distribution despite marsh gradual drowning. Key Points: The imbalance between rates of marsh vertical accretion and relative sea‐level rise leads to marsh drowningDespite marsh drowning, a consistent sequence in vegetation zonation as a function of marsh elevation is maintainedThe resilience of marsh vegetation over decades is suggested for rates of sea‐level rise lower than a critical threshold [ABSTRACT FROM AUTHOR]

Extensive loss of salt marshes in back‐barrier tidal embayments is ongoing worldwide as a consequence of land‐use changes, wave‐driven lateral marsh erosion, and relative sea‐level rise compounded by mineral sediment starvation. However, how salt‐marsh loss affects the hydrodynamics of back‐barrier systems and feeds back into their morphodynamic evolution is still poorly understood. Here we use a depth‐averaged numerical hydrodynamic model to investigate the feedback between salt‐marsh erosion and hydrodynamic changes in the Venice Lagoon, a large microtidal back‐barrier system in northeastern Italy. Numerical simulations are carried out for past morphological configurations of the lagoon dating back up to 1887, as well as for hypothetical scenarios involving additional marsh erosion relative to the present‐day conditions. The progressive loss of salt marshes significantly impacted the lagoon hydrodynamics, both directly and indirectly, by amplifying high‐tide water levels, reducing wind‐wave energy dissipation, and critically affecting tidal asymmetries across the lagoon. Restoration projects and manmade protection of marsh margins, which have been implemented over the past few decades, limited the detrimental effects of marsh loss on the lagoon hydrodynamics, while not substantially changing the risk of flooding in urban lagoon settlements. Compared to previous studies, our analyses suggest that the hydrodynamic response of back‐barrier systems to salt‐marsh erosion is extremely site‐specific, depending closely on the morphological characteristics of the embayment as well as on the external tidal and wind forcings. Key Points: Effects of salt‐marsh lateral erosion on the characteristics of tides and waves are investigated for the microtidal Venice LagoonSalt‐marsh loss primarily enhances mean high water levels and wave heights due to reduced friction and longer wind fetchesHydrodynamic changes depend on salt‐marsh spatial distribution and elevation, as well as on the characteristics of tidal and wind forcings [ABSTRACT FROM AUTHOR]