## 2 Study Site
### Vegetation in the Bay of Arcachon  
In the Bay of Arcachon, low elevations of the tidal flat (slikke) are mainly colonized by the seagrass, _Zostera noltei_, whereas the margins of the lagoon (schorre) are occupied by typical salt marsh vegetation (Figure 2a), principally _Spartina_ sp. (_Spartina maritima_--Figure 2b-e and _Spartina anglica_--Figure 2f-j). The arrival of the invasive species, _Spartina anglica_, in the 1980s [38] provoked concern regarding threats to local species diversity and, especially, the eventual replacement of the native _Spartina_ (_Spartina maritima_) by its exotic congener. Even though _Spartinas_ constitute the dominant vegetation type in the marsh zone, there are many other species, such as _Salicornia spp._, _Juncus gerardii_, _Limonium vulgar_, _Halimione portucaloides_, and _Puccinella maritima_[39; 40] (Figure 2k-m). Usually, there is a well-defined separation between different vegetation species, but different degrees of mixture, as well as different degrees of density within the same type of vegetation, can also occur. More than other marsh species, the invasive _Spartina anglica_ has the particular ability to spread to offshore positions within the tidal flat, at the inner side of the lagoon. For this reason, the vegetated inner tidal flat zone was included, with the intention of identifying the isolated _Spartina anglica_ patches that might occur in this zone. Seasonal differences in the intertidal vegetation's development and phenology can help to determine their identification and discrimination using remotely sensed images [22; 27]. Notably, _Spartina maritima_ is a species known for its continuous growth and absence of significant seasonal die-back [41]. In the Bay of Arcachon, this was verified by [PERSON] et al. [42], who estimated a very low productivity for this species at different intertidal levels. On the contrary, the congener _Spartina anglica_ presented a strong aerial productivity of aerial biomass, which means that it suffers from a strong die-back during winter, and it completely recovers during the growing season, particularly during summer, due to its exploitative growth strategy, which is better than that of the native. An important difference concerning the phenology of growth is also observable, with a maximum of growth during spring for the native and during summer for the exotics (Appendix A). Additionally, the cordgrasses seem to present some sensitivity both to continuously high and continuously low temperatures, expressed as a color change to yellow/brownish shades during summer and winter seasons, respectively. The seagrass, _Zostera noltei_, was also found to present a pronounced seasonality. [PERSON] and [PERSON] [43] found a minimum aboveground biomass for this species between November and February and minimum shoot lengths in springtime. The maximum aboveground biomass was measured between June and August, but high values of shoot length were found during Fall. Concerning algae, several species can be found in the Bay at different times of the year [44]. According to these authors, in the inner parts of the Bay, there is a strong presence of the green alga, _Monostroma obscurum_, that tends to decline during the period between March and September. However, they found that a clear seasonal trend is difficult to predict. Figure 2: Characterization of vegetation in the Bay of Arcachon.