Blue carbon stock in marsh soil and impacts of flood regulation in the Venice Lagoon (Italy)

Salt marshes are intertidal coastal ecosystems characterized by mostly herbaceous halophytic vegetation and shaped by complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial yet endangered environments provide a diverse range of ecosystem services but are severely exposed to climate change and human pressure. The importance of salt marshes as ‘blue carbon’ (C) sinks, deriving from their primary production coupled with rapid surface accretion, has been increasingly recognized within the framework of climate mitigation strategies. However, uncertainties remain in the estimation of salt-marsh C stock and sequestration at the basin scale and large knowledge gaps still linger in the response of marsh C pools under increasing anthropogenic interventions, such as storm-surge regulation. In order to provide further knowledge in salt-marsh C assessment and investigate marsh C pool response to management actions under different scenarios, we analysed organic matter content in salt-marsh soils in 720 samples from 60 sediment cores to the depth of 1 m, and we estimated C stocks and accumulation rates in different areas of the Venice Lagoon (Italy), which has recently become regulated by a storm-surge barrier system. OC stocks in the surface 1 m were highly variable in different marshes averaging 17,108 ± 5,757 ton OC km^-2 (range 9,800 − 24,700 ton OC km^-2). The estimated OC accumulation rate was 85 ± 25 ton OC km^-2 yr^-1, confirming the CO₂ sequestration potential of tidal environments, which, however, resulted to be crucially affected by marsh accretion rates and their human-induced variations. By hindering sediment supply provided by storm surges which are largely responsible for marsh accretion, flood regulation can dramatically reduce the CO₂ sequestration potential of salt marshes. We estimate that storm-surge barrier operations in the Venice Lagoon may reduce the annual marsh CO₂ sequestration potential by about 33%, with high costs in terms of ecosystem service loss. Our results highlight the need for integrated coastal management policies to enhance the resilience of anthropic and natural environments and to preserve the ecosystem services delivered by coastal wetlands.