## 3 Results and Discussion
### Present-Day Salt Marsh Ecosystem Services Economic Value
Due to the different phenological characteristics, life forms, and metabolisms of the different halophytes for the calculation of the carbon, nitrogen, and phosphorous stock and economic values, in the present work, instead of applying a common monetary value on an area basis, a value based in the carbon, nitrogen, and phosphorous value per unit of plant mass was applied, thus allowing incorporation of the different stocking abilities of the different species (considering the most abundant species to which data were available). Blue stocks storage in the different ecosystems surveyed relies to a large extent with the species composition and abundance (Figure 5). Plants with C\({}_{4}\) photosynthetic metabolism(such as _S. maritma_) tend to accumulate higher amounts of carbon in their tissues and sediments [30], but are less abundant in the estuarine systems surveyed and the overall Portuguese transitional systems [15]. Thus, blue carbon storage in the surveyed ecosystems depends more on the species composition rather than on its extent, although, of course, the marsh dimensions also play a role in the blue stocks storage of the systems. This is visible when comparing the tendency in the majority of the ecosystem services with the tendencies observed in the blue stock, for example, between the Tagus estuary and Aveiro coastal lagoon. Although the Tagus estuary has a smaller salt marsh area than the Aveiro coastal lagoon, it presents a higher productivity in terms of blue stocks (Figure 5). When considering the remaining ecosystem services, which are calculated on an area basis disregarding the species composition, Aveiro coastal lagoon presents higher economic value for its ecosystem services. Thus, systems with higher percent coverage of C4 species such as _S. maritima_, are inevitably more productive as it is possible to observe for the Ria Formosa coastal lagoon. Moreover, nonwoody succulent and grass-type plants such as _S. perennis_ and _S. maritima_ have higher carbon storage capacities and thus its coverage influence greatly the overall blue stock value (Figure 5). The values obtained here are higher but comparable to previous work where not all the different carbon storage capacities of the different species are considered [24]. On average, using a per-acre value of carbon disregarding the species composition and different carbon storage abilities induces about 10% overestimation on the blue carbon, nitrogen, and phosphorous stocks (data not shown), which can be translated into several tons per year. Although blue phosphorous stocks in terms stored mass are reduced when compared to the blue carbon and nitrogen stocks, due to the high economical costs of technological phosphorous removal, this constitutes a large economic contribution to the overall ecosystem services [40]. Comparably, blue carbon stocks are valued through the European Emission Trading System (ETS), having as base the carbon emission reduction (CER) price, a reduced value when compared to the P or N removal costs [38]. Other regulating ecosystem services also represent an added value to these ecosystems, although in a smaller extent, such as improving the air quality, water flow regulation, and biological pest control, and account for less than 1% for the overall ecosystem services annual budget. Lastly, salt marshes also provide important cultural ecosystem services in the form of aesthetic values, inspiration for culture and art, spiritual experience, and cognitive development [32], accounting for approximately 4.5% of the ecosystem value of the transitional systems surveyed. Within this category, the highest contribution comes from the marsh aesthetic value. Aesthetic value is often obtained from the improvements or degradations of a natural area and how individuals perceive those changes [62]. These aesthetic services account for approximately 50% of the overall cultural services provided by the surveyed salt marshes (Figure 4d, Table S1).