## 3 Results and Discussion
### Present-Day Salt Marsh Ecosystem Services Economic Value  
Considering the present-day ecosystem services value, there is also a high degree of fluctuation both in terms of system and service between systems. Supporting services (Figure 4a, Table S1) are the foundation for many ecosystem services, being essential for the production of provisioning and regulating ecosystem services. These supporting services are different from the remaining because they do not always have a direct impact on humans [1]. Ria Formosa coastal lagoon presents the higher value of the ecosystem service, followed by the Aveiro coastal lagoon and Tagus estuary due to their higher area in terms of salt marsh colonization, thus providing larger areas for species habitat, nesting, feeding areas, and refugia. These are of particular importance if we consider that these areas are essential for several migratory bird species that nest and feed in \(H\). _portulacoides_ and _Sarcocornia_ sp. stands [55], but also for invertebrate species that inhabit the marsh and fish species that use these areas during high tide as feeding and shelter grounds [56]. As noted, supporting ecosystem services are intrinsically connected to provisioning services (Figure 4b, Table S1). These result in material products that people use [1], such as a wide variety of food products that come directly from these salt marsh habitats--home to a variety of wildlife that are taken both commercially and recreationally for consumption [34]. In this regard, the transitional systems evaluated have higher economic value in terms of fisheries maintenance accounting for several millions of euros per year in terms of fish capture that rely on these ecosystems. As for seafood harvest, this is also an important resource for the human economy that drives directly from salt marsh ecosystems, but in economic terms and due to the invertebrate biomass production and market value, accounts for a comparatively smaller part of the overall provisioning services. Regulating services (Figure 4c, Table S1) correspond to a higher diversity of services provided by salt marshes and account for the largest part of overall ecosystem services (approximately 78% of the overall annual budget). The most valuable ecosystem service in this category is related to the nutrient cycling capacity of the salt marshes, accounting for about 30% of the overall annual budget. In this regard, these ecosystems are key environments for recycling less bioavailable and organic nutrient forms into readily assimilated forms [57; 58]. The second large part of the ecosystem services provided is composed by their ability to protect human structures against erosion (coastal protection) and severe flood and storm events, accounting for about 30% of the total annual budget of the transitional systems. These services are often evaluated through the equivalent costs to create living shorelines along coastal areas to determine a value for shoreline stabilization, rather than hard armoring [34]. Among the top-ranked regulating ecosystem services is the water purification capacity of these ecosystems (approximately 9% of the overall annual budget). Salt marshes are known to act as kidneys of the transitional ecosystems [59], being able to retain pollutants [60; 61]. Values for pollution buffering were assessed on an entire salt marsh basis, as there are very few data available for filtering rates specific habitat types or specific pollutants [34]. Thus, the values used are based in the economic annual value on an areal basis, based in the benefit transfer and avoided cost methodologies when compared to the use of wastewater facilities or other remediation methodologies [34]. Considering the blue carbon, nitrogen, and phosphorous values of the ecosystems considered, and although these are not by far the most valuable ones in economic terms, these acquire a reinforced role when considering the stocks themselves. The blue stock values of the ecosystems considered account for about 8-12% of the overall ecosystems service value.