## 5 Conclusions  
(1) Compared with the performance of different models, the BPNN model optimized through PSO significantly enhanced the accuracy of PC concentration retrieval. In the validation set, the mean absolute error (MAE) and the root mean square error (RMSE) were 0.469 and 0.615, respectively, demonstrating that its retrieval accuracy meets the requirements for the remote sensing of low PC concentrations. (2) The PC concentration is primarily concentrated around 2-5 \(\mu\)g/L, exhibiting a distribution pattern where the inner bay > neck of bay > outer bay in Qinzhou Bay. However, the PC concentration is also affected by the _P. glo_ outbreaks. Therefore, based on algal competition relationships, we propose a simple red alert threshold for predicting _P. glo_ red tide. This approach enables a rapid preliminary assessment of the _P. glo_ red tide risk in Qinzhou Bay and enhances our ability to manage coastal areas effectively. Conceptualization, H.Y. ; methodology, Y.L. ; software, H.C.; validation, M.W. ; formal analysis, Z.H. ; writing--original draft preparation, Y.L. ; writing--review and editing, W.Z. All authors have read and agreed to the published version of the manuscript. This research was supported by scientific research capacity building project for Beibu Gulf Marine Ecological Environment Field Observation and Research Station of Guangxi under Grant No. 23-026-271. Not applicable. The Sentinel-2 MSI Image data sets are available from [[https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)]([https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)) (accessed on 8 March 2023); the ERA5 data are available from [[https://cds.climate.copernicus.eu/cdsappff/dataset/reanalysis-era5-single-levels?tab=overview](https://cds.climate.copernicus.eu/cdsappff/dataset/reanalysis-era5-single-levels?tab=overview)]([https://cds.climate.copernicus.eu/cdsappff/dataset/reanalysis-era5-single-levels?tab=overview](https://cds.climate.copernicus.eu/cdsappff/dataset/reanalysis-era5-single-levels?tab=overview)) (accessed on 19 July 2023); the codes for building the machine learning regression model are available from the authors; water quality data from automated monitoring stations are not publicly available due to the inclusion of sensitive information. The authors declare no conflict of interest. The spectral information for Sentinel-2 bands can be found in Table 11.  
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