*3.2. Spectral Analysis*

The four marsh vegetation classes show typical plant spectral curves (Figure 5). The peaks of each class are located at the same wavelength (±5 nm), presenting only quantitative differences. The macroalgae class showed clear divergences from this pattern. First, the strong absorption peak in the red region (peak 3 in Figure 5) is less pronounced than in plant classes. It also lacks the absorption peak at 943 nm (peak 6 in Figure 5). There is a significant increase in reflectance from the red-edge region to the SWIR1 region (i.e., 700–1300 nm), and also higher reflectance in entire the SWIR region (1000–2350 nm) (Figure 5, Table 1).

**Table 1.** Wavelengths of the absorption peaks detected in the spectral signature of the marsh vegetation classes of Cadiz Bay. The numbers in the column headers correspond to the peaks indicated in Figure 5. Units: nm.


**Figure 5.** Spectral profiles (**a**) and the corresponding continuum removal transformations (**b**) for the salt marsh vegetation classes identified in Cadiz Bay. The grey areas highlight the water vapour absorbance regions (1350–1460 nm and 1790–1960 nm) excluded from the hyperspectral processing. The numbers in brackets indicate the absorbance peaks of the spectral signatures. Note that minimum values are absorption peaks and maximum values are reflectance peaks.

The 2nd derivative transformation accentuated small differences not previously observed in the reflectance curves (Figures A1–A4). These effects are more pronounced in the SWIR1 region, and the variations between 2013 and 2329 nm (SWIR2 region) are particularly notable.

The spectral indices (SI) were constructed with the significant absorbance peaks at 1057, 1110, 1152, 1182, 1260, and 1331 nm, generating SI1152-1110, SI1331-1260, SI1182-1057, and SI1523-1290. These indices may show differences that can be attributed to biophysical differences in vegetation (Figure 6).

**Figure 6.** Distribution of spectral indices (SI) values in the salt marsh of Cadiz Bay. The results display only a detail of the study area, and the corresponding SI is indicated in the legend. Wavelengths used for creating SI are suitable to show the variations in canopy cover. Thresholds are adjusted for each index to better enhance differences in canopy cover.
