**4. Conclusions**

Eutrophication in areas where agricultural and industrial wastewater runoff feeds excessive loads of nutrients into coastal regions can foster algal blooms and undermine the health of these ecosystems. In this study, both Landsat-8 and Sentinel-2 satellites were jointly merged as a constellation to estimate indicators of the water quality in the highly unstable and vulnerable Mar Menor coastal lagoon. The validation of satellite biogeochemical parameters, both turbidity and chl-a, retrieved good performance for ranges of 0.5–6 FNU and 0.5–5 mg/m3, respectively. The atmospheric and sunglint correction using the ACOLITE software showed consistent performance for both satellites; therefore, using them in tandem can improve mapping strategies, highlighting the importance of the preprocessing scheme. The results demonstrate the suitability and consistency of the methodology to reliably capture the detailed spatiotemporal distribution of turbidity and chl-a, where satellite imagery was capable of early detection of chl-a levels above 3 mg/m3, which generally triggered the subsequent blooms during recent years. Although neither of the satellite missions have been designed to characterize coastal seawater quality, our approach demonstrated their capacity to provide appropriate information at 10–30 m spatial resolution on a systematic basis and in a cost-effective way. Multitemporal maps were produced, and an analysis of all images showed that the highest turbidity and chl-a levels were always located in the western section. In particular, turbidity and chl-a concentration at the mouth of the draining Albujon watercourse were consistently two times higher than in the northern and eastern sections of the lagoon. The influence of the highly dynamic plume from the Albujon extended over the entire lagoon, yet the strongest gradients typically occur within the first nearshore 1–2 km. Therefore, observing these gradients, their variability, and the impact of land–water exchanges on nearshore dynamics under varying environmental conditions from space requires higher spatial resolution. Imagery from both satellites offered snapshots of water quality patterns that are difficult to map with in situ

technologies in such heterogeneous environments. These innovative tools can support decision makers in the implementation of a joint monitoring strategy, better characterization of the water quality distribution, and timely assistance to society during these ecological disasters, thus preventing detrimental conditions in the lagoon. Furthermore, the powerful multisensor system can be used as guidance to complement the ongoing in situ techniques carried out by the local and regional authorities to select relevant areas for data sampling. This information could advance mapping of water quality and bio-optical properties in terrestrial–aquatic environments as an important tool for managers and stakeholders, as well as for the tourism and fishing industry. A new era has begun with the use of the Sentinel-2 and Landsat-8 missions as a virtual constellation, with significant opportunities for monitoring the heterogeneous spatiotemporal patterns of inland and nearshore coastal waters at resolutions certainly not observed previously.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/rs14122744/s1. Figure S1: RGB (Red–Green–Blue) composite image on (a) 8, (b) 13, and (c) 18 August 2021 of the Sentinel-2 satellite (10 m spatial resolution); Figure S2: Turbidity (FNU) on (a) 3, (b) 8, (c) 13, and (d) 18, August 2021 of the Sentinel-2 satellite (10 m spatial resolution); (e–h) the same for chlorophyll-a concentration (Chl-a, mg/m3); Figure S3: Sentinel-2 and Landsat-8 RGB (Red–Green–Blue) composite image acquired on (a) 4 September 2021, (b) 7 September 2021, (c) 11 September 2021, (d) 12 September 2021, (e) 17 September 2021, and (f) 22 September 2021; Figure S4: Turbidity (FNU) from Sentinel-2 and Landsat-8 acquired on (a) 4 September 2021, (b) 7 September 2021, (c) 11 September 2021, (d) 12 September 2021, (e) 17 September 2021, and (f) 22 September 2021; Figure S5: Chlorophyll-a concentration (Chl-a, mg/m3) from Sentinel-2 and Landsat-8 acquired on (a) 4 September 2021, (b) 7 September 2021, (c) 11 September 2021, (d) 12 September 2021, (e) 17 September 2021, and (f) 22 September 2021; Figure S6: RGB (Red–Green–Blue) composite image from Sentinel-2 and Landsat-8 acquired on (a) 11, (b) 12, (c) 21, and (d) 28 March 2021; Figure S7: Turbidity (FNU) from Sentinel-2 and Landsat-8 acquired on (a) 11, (b) 12, (c) 21, and (d) 28 March 2021; Figure S8: (a) RGB (Red–Green–Blue) composite image, and (b) Turbidity (FNU) from Sentinel-2 on 21 March 2021 corresponding to the southeastern shore of Mar Menor.

**Author Contributions:** Conceptualization, I.C.; formal analysis and investigation, I.C.; figures, I.C. and M.R.; original draft preparation, I.C.; writing—review and editing, I.C., M.R., J.S.-E., P.B. and G.N.; fieldwork, J.S.-E. and P.B; supervision, I.C.; project administration, I.C.; funding acquisition, I.C., J.S.-E. and G.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Project RTI2018-098784-J-I00 funded by the MCIN/AEI/ 10.13039/501100011033 and by "FEDER Una manera de hacer Europa" and the Grant IJC2019-039382- I funded by the MCIN/AEI/10.13039/501100011033. The work was also supported by the Project of the Regional Government "Junta de Andalucía" (PY20-00244), the Project PID2019-109355RA-I00 funded by the MCIN/AEI/10.13039/501100011033 and the Grant FPU20/01294 funded by the Spanish Ministry of Universities.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to acknowledge the European Commission's Copernicus programme, the United States Geological Survey (USGS), and the National Aeronautics and Space Administration (NASA) for distributing Sentinel-2 and Landsat-8 imagery. This work represents a contribution to the CSIC Thematic Interdisciplinary Platform PTI TELEDETECT. The authors would like to thank Martha B. Dunbar for the English revision and Rocío García for the chlorophyll-a analyses. We also thank the Laboratorio de Medio Ambiente de Galicia for lending us the turbidimeter.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

