*2.1. Site Description*

Cadiz Bay hosts the southernmost European coastal wetland, located where the Mediterranean Sea, the Atlantic Ocean, and the continents of Europe and Africa converge (Figure 1). Located on the Atlantic coastline, precipitation, wind, and waves are influenced by large-scale oceanic weather systems that cross the North Atlantic [51]. Cadiz Bay is delimited by the tombolo of the city of Cadiz, with NNW-SSE orientation, and opens towards the Atlantic Sea to the north [52]. The entire bay is formed by two water bodies, the external and the inner bay, connected by tides through a narrow strait [53]. The external basin has depths up to 20 m, whereas the inner basin has a mean depth of about 2 m, and is sheltered from ocean waves [54]. The intertidal system of the bay is composed of natural salt marshes, salinas, mudflats, and an intricate network of tidal creeks [55]. The tidal regime is mesotidal semi-diurnal with a mean spring tidal range of 2.96 m [56].

The distribution of vegetation in the natural salt marshes of Cadiz Bay follows a conventional mid-latitude zonation [57], although the protective walls of the salinas frequently cut off the high marsh. The low marsh is mostly inhabited by *Sporobolus maritimus*, whereas the medium marsh is dominated by *Sarcocornia* spp., primarily *Sarcocornia fruticosa* (L.) A.J.Scott and *Sarcocornia perennis* (Mill.) A.J.Scott, and other halophytic species in lower abundance (Figure 2). Seagrass beds of *Zostera noltei* Hornem. and *Cymodocea nodosa* Asch., as well as patches of *Zostera marina* L., are found at the lower parts of the intertidal zone [53].

**Figure 1.** Location of the Cadiz Bay: regional context (**a**) and detail of the study area (**b**,**c**) The white rectangle in picture c represents the flight area for the UAV-HS survey. Coordinates are expressed in ETRS89/UTM zone 29N reference system (EPSG:25829).

**Figure 2.** Landscapes with dominant vegetation throughout the salt marsh horizons. (**a**) *Sarcocornia* spp. populate the medium salt marsh horizon. (**b**) The vegetation of medium and low horizons overlaps in a narrow fringe here called the transition zone. The abundance of species from the medium and the low horizons can be found in different proportions. (**c**) *Sporobolus maritimus* dominates the low horizon of the salt marsh.

Our study area was selected in an area with a wide and well-developed natural salt marsh zonation, in the north-eastern corner of the inner bay [54].
