Leveraging TROPOMI red solar-induced chlorophyll fluorescence for enhanced detection of harmful algal blooms in turbid coastal waters.

Frequent harmful algal blooms (HABs) pose significant risks to marine ecosystems. Traditional remote sensing indicators for HABs monitoring, based on wide-band reflectance, are susceptible to interference from non-algal materials, such as suspended sediment in turbid coastal waters. In contrast, ocean solar-induced chlorophyll fluorescence (SIF), retrieved from narrow solar Fraunhofer lines, provides a more direct measurements of the optical signal emitted by phytoplankton during photosynthesis, offering new perspectives for HABs detection. We used red SIF from the TROPOMI to detect HABs in the East China Sea and evaluated its performance against MODIS fluorescence indices including normalized fluorescence line height (nFLH) and red band difference (RBD), CIE-fluorescence HABs product, and the reported in-situ HABs records. Results showed that TROPOMI SIF effectively reduced false-positive detections compared to nFLH in turbid waters and exhibited stronger correlation with RBD than nFLH at both regional and pixel levels. The nFLH products largely overestimated SIF in turbid waters, with a nFLH/SIF ratio increasing exponentially with total suspended sediment (R2 = 0.423). With a SIF threshold of 0.6 mW/m2/sr/nm determined by correlation modeling and spatial comparison, bloom pixels were effectively extracted, capturing all 23 HABs events during 2018-2020. With more accurate bloom detection in turbid waters and more observations than MODIS, our SIF-based method provided a reliable spatiotemporal pattern of long-term HABs occurrence, thereby enhancing timely HABs detection. This study demonstrates the TROPOMI SIF as an effective indicator for HABs detection in turbid waters, providing new insights to complement existing HABs detection strategies and to monitor phytoplankton productivity.