## 4 Results and Discussion
### Spectral Signature of Vegetated Structures
However, due to a small phase shift in their respective seasonal development, which generates considerable differences in aerial biomass density and color (Figure 2b-j), autumn appears as the best period to discriminate the two species, where average NDVI values for _Spartina anglica_ and _Spartina maritima_ are 0.8 and 0.9,  
Figure 5: Remote sensing reflectance (sr\({}^{-1}\)) values in the green (500–620 nm), red (590–710 nm) and NIR (740–940 nm) bands for the multi-spectral Pleiades images, acquired on 3 August 2016. The images zoom in on the well-identified _Spartina anglica_ and _maritima_ meadows (see red and blue boxes on the image of 2016, Figure 4). respectively. Previous studies have already shown the advantages of a temporal assessment of marsh species identification using this index [55; 56]. For instance, [PERSON] et al. [55], considering the monthly NDVI of multiple marsh species in the middle coast of Jiangsu, found important temporal windows that made the species distinguishable. Figure 6: Comparison of the NDVI computed from the Pleiades images for the five considered dates, (**a**,**b**) 25 April 2013, (**c**,**d**) 3 August 2016, (**e**,**f**) 6 October 2016, (**g**,**h**) 24 May 2017, and (**l**,**j**) 7 October 2017. For each date, the image zooms in on the well-identified, small and invasive _Spartina anglica_ and large native _maritima_ meadows (right panel—see legend in Figure 5). Black contours on the image acquired on 6 October 2016 correspond to the field ground truth GNSS vegetation contours. Further, relevant information regarding the general intertidal vegetation presence during the growing season can be withdrawn from this parameter. In particular, remarkable differences appear between both spring images (April 2013 and May 2017). The NDVI values for both intertidal and supratidal vegetation are above 0.8 for the first date and much lower for the second one, ranging from 0.3 to 0.6. This indicates that potentially significant meteorological differences might have occurred within the four years, affecting the vegetation development, namely, in terms of temperatures and sunlight availability. However, such analysis goes beyond the scope of the present study, and further correlations with meteorological data would need to be conducted. For the image corresponding to the beginning of August 2016, at the peak of summer, NDVI values are no longer at their maximum, which could possibly be related to a particularly hot summer, during which high temperatures were reached. A relative senescence of the vegetation during this period was observed during field verifications, which is probably the reason for the marsh vegetation NDVI values being under 0.8 during this period of the year. Finally, for both fall images (2016 and 2017), not only the marsh vegetation structure shows particularly high NDVI values, but there is also the presence of a large vegetated cover over the tidal flat. This is probably related to the fact that, by this time of the year (October), temperatures have not yet significantly decreased, and vegetation growth is still occurring, for the seagrass in particular (Kal  
Hence, in this area of the Bay of Arcachon, the most significant changes in marsh evolution are associated with vegetation patch growth (occasionally followed by the merging with neighbor patches). This patch growth and subsequent merging is most remarkable for the native _Spartina_. For the exotic _Spartina__anglica_, we notice the appearance of new patches that seem to mostly occupy empty niches within the higher marsh zone or over the tidal flat (Figure 11). Therefore, apart from growth, there are no major inter-annual (or even inter-seasonal) changes in patch vegetation dominance. Our ground truth data (drone flight, GNSS data, georeferenced photographs and a regular monitoring of this site from 2015 up to 2018 within the scope of the complementary studies) for patch species identification showed that this is a relatively stable site, and no major changes have recently occurred.