## 5 Conclusions  
Very high-spatial-resolution (2 m) Pleiades satellite data was used for the first time to retrieve the water reflectance and turbidity in a macro-tidal estuary (the Gironde). The Dark Spectrum Fitting (DSF) atmospheric correction algorithm ([PERSON] and [PERSON], 2018) was validated in several steps, showing that accurate reflectances can be retrieved from Pleiades over moderately to highly turbid estuarine waters. Turbidity values derived from Pleiades data showed a very good agreement with in situ data recorded by the shore of the estuary (Pauillac station), with MAPE of 10%. A full atmospheric correction is recommended for turbidity monitoring in the Gironde Estuary, as lower errors are retrieved compared to in situ data for the DSF-corrected (10%) than either for the Rayleigh-corrected (40%) or top-of-atmosphere reflectance (57%).  
Pleiades data were spatially averaged to 20 m and 250 m pixel sizes to simulate the spatial resolutions of high (SPOT, MSI, OLI) and medium (MODIS, OLCI) satellite sensors. The results showed that Pleiades imagery provides higher detail and a more accurate retrieval of turbidity at the Pauillac autonomous monitoring station. The 20 m and 250 m pixels were contaminated by the shallow muddy waters and non-water pixels, and systematically resulted in an overestimation of field-measured turbidity. The spatial variations of water turbidity at 2, 20 and 250 m were compared along representative transects and for subsets of the images. Results showed limited differences between 2 m and 20 m satellite products, with MAPE \(\leq\) 10%. The difference between 250 m and 2 m data was found to be larger, especially at test site 1, with MAPE \(\leq\) 18%.  
The improvement of spatial resolution from 250 m and 20 m to 2 m provides better estimates of water turbidity based on matchups between satellite-derived data and field measurements. Moreover, the 2 m resolution allows for the accurate mapping of turbidity inside ports and around pontoons, and detecting the release of sediments by dredging ships in the estuary. In the future, small-scale features, such as dredging plumes, could be studied for retrieving more specific information such as particle size and/or composition) by using very high-resolution Pleiades images.  
Conceptualization, D.D. and Y.L.; Software, Q.V.; Validation, Y.L.; Formal analysis, Y.L.; Resources, D.D.; Data curation, Q.V.; Writing--original draft preparation, Y.L.; Writing--review and editing, D.D. and Q.V.; Supervision, D.D. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Federal Belgian Science Policy Office (BELSPO) under the STEREO III Programme PONDER project (SR/00/325) and supported by program for scientific research start-up funds of Guangdong Ocean University (R19124).  
**Acknowledgments:** The authors wish to thank the many organizations for proving data used in this analysis, including the NASA, USGS, SeaSWIR project and MAGEST network. The MAGEST network is financially supported by the following organizations: AEAG (Agence de l'Eau Adour-Garonne); SMIDDEST (Syndicat Mlxte pour le Developpement Durable de l'ESTuaire de la Gironde); SMEAG (Syndicat Mlxte d'Etudes et d'Amenagement de la Garonne); EPIDOR (Etablissement Public Interdepartemental de la Dordogne); EDF; GPMB (Grand Port Maritime de Bordeaux); Conseil Regional Aquitaine; CG-33 (Conseil General de Gironde); Ifremer; CNRS; Universite Bordeaux 1. Four anonymous reviewers are thanked for their feedback which greatly improved the structure of this manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.