## 3 Results
### Performances of the SRT Inversion Retrieval Process Using Synthetic Data

Table 1: Performances of the SRT inversion retrieval of the bio-optical parameters in clear waters. Figure 5: Relative error (RRMSE) of each bio-optical parameters retrieved using the SRT inversion method: FC varies on the horizontal axis from 0 to 100% and depth z varies on the vertical axis from 0 to 5 m. (a) Chlorophyll (Chl), (b) Non-Algal Particle (NAP), (c) Colored Dissolved Organic Matter (CDOM), (d) Fractional Coverage (FC), (e) Depth (z). Retrieval of the Bio-Optical Parameters from OLCI Derived Surface Reflectances Using the SRT Inversion Method  
Here, the SRT inversion method is applied to OLCI derived surface reflectances data acquired on 8 July 2017, in the study area as described Section 2.2 (white square in Figure 2). First, the results of the retrievals are shown for two types of pixels. One type of pixels contains only clear oceanic waters, i.e., _Sargassum_-free pixel, while the other type of pixel contains a mixing of seawater and _Sargassum_ (i.e., _Sargassum_ contaminated pixel). This last type of pixel was identified using the MCI index to ensure the presence of _Sargassum_. Table 2 summarizes the values of the retrieved parameters for a pixel of each type. Chl is estimated around 0.4 mg m\({}^{-3}\) for both pixels; such a value matches with the range provided by the OC-CCI database near Dominica Island (i.e., 0.2-0.8 mg m\({}^{-3}\)) in July 2017 [35]. For the case of a given pixel contaminated by _Sargassum_, FC and z values are consistent with the occurrence of a _Sargassum_ aggregation close to the surface (8% of fractional coverage and 0.07 m depth). The FC value evaluated using the MCI index for the same pixel is 10%, which is close to the one estimated by the SRT inversion. The FC value estimated for the _Sargassum_-free pixel is 15%, which is not consistent since a value of FC of 0% should be theoretically retrieved when a pixel is not contaminated by _Sargassum_. The depth value retrieved for a _Sargassum_-free pixel is exactly 5 m. The SRT model should theoretically retrieve an infinite depth when a _Sargassum_ aggregation is not present; however, the influence of the _Sargassum_ on the surface reflectance decreases with increased depth. When _Sargassum_ are located at 5 m depth, the difference between reflectance of _Sargassum_-free pixel and reflectance of pixel containing 10% of _Sargassum_ that has been calculated using the SRT forward model is only 2%. Therefore, the maximum _Sargassum_ depth for which our method is applicable is 5 m. It should be highlighted that previous studies [18; 30; 31; 32] showed that the _Sargassum_ community occupies the upper water column (up to 4 m depth). The pixels for which the retrieved depth reaches the maximum value (5 m) are further considered as _Sargassum_-free pixels (FC = 0%). Figure 6 shows the above surface reflectance derived from the OLCI observation and the surface reflectance computed using the forward SRT model based on the bio-optical parameters retrieved from the SRT inversion scheme (Table 2). The simulated surface reflectance correctly matches with the OLCI surface derived reflectance for the _Sargassum_ free pixel (Figure 6a). The SRT inversion retrieval is slightly less performant in the blue domain (412 nm), where the relative error is 9% when comparing to the OLCI surface derived reflectance. The simulated reflectance corresponding to the SRT inversion retrieval for the _Sargassum_ contaminated pixel also correctly agrees with the OLCI reflectance surface; the maximum relative error is 12% at 754 nm (Figure 6b). The spatial distribution of the bio-optical parameters retrieved by the SRT inversion scheme over the study area (white square in Figure 2) is shown Figure 7. Land pixels are colored in grey, and clouds are colored in white. The Chl, NAP, and CDOM distributions are homogeneous within the study area (respectively Figure 7a-c). The spatial distribution of the retrieved parameters (Figure 7a-c) can be compared with Level-2 data products provided by the Copernicus data center for the same date and location.