Optical properties of a toxin-producing dinoflagellate and its detection from Sentinel-2 MSI in nearshore waters

Harmful algal blooms (HABs) caused by the dinoflagellate Alexandrium minutum can pose risks to human and ecosystem health. HABs of different species can coexist in coastal waters and accumulate near the shoreline, challenging their detection through Earth observation (EO). In this study, we use in situ geo-bio-optical and taxonomical data from the Rías Baixas (NW Spain) to develop a new method for identifying high-concentration blooms of A. minutum and its application to Sentinel-2 Multispectral Instrument (S2 MSI). Our approach named A. minutum index (AMI) was developed to capture the low absorption and high backscattering properties of A. minutum cells between 560 and 570 nm. We tested and validated the performance of three atmospheric correction algorithms (AC) (C2RCC, POLYMER and ACOLITE) using matchups between in situ and satellite-derived R_rs. Results show that C2RCC had the lowest error across most wavelengths. Applying AMI to S2 MSI indicates that our approach can accurately identify high-concentration blooms of A. minutum (F1 score: 70 %, Kappa: 68.3 %, balanced accuracy: 87.7 %, MCC: 68.3 %) and discriminate blooms of A. minutum from other phytoplankton species. We compared AMI with three existing indices for detecting HABs in coastal waters and found that our approach achieved a better performance, with the NDTI, RGCI and NDCI yielding F1 scores of 21.28, 21.74, and 0.0 % and MCC values of 15.0, 15.0 and 0.0 %, respectively. We also investigated the spatial resolution of S2 MSI to Sentinel-3 Ocean and Land Colour Instrument (S3 OLCI) for mapping fine-scale variations in A. minutum blooms. We found that the higher spatial resolution data from S2 MSI were highly useful for detecting small-scale variations in A. minutum in nearshore waters, enabling their detection in the mid-inner part of the Rías, where aquaculture activities are more prominent. This study also showcases the significance of accurate AC in near-shore waters, where high-concentration blooms can be more prevalent. Our findings show that greater errors in AC are observed in near-shore pixels, where the socio-economic and environmental impact of HABs are typically more severe.