SuperDove Observations of Nearshore Red Tides: A Study of the Central West Coast of Florida

Satellite ocean color remote sensing provides an effective tool for monitoring and tracking harmful algal blooms (HABs) in oceanic, coastal, and inland waters. However, traditional medium-resolution (300 m – 1 km) satellite data do not have the required spatial resolution to characterize nearshore HABs, whose harmful impacts are directly relevant to local communities. Here, we show that commercial imagery provided by the >100 SuperDove sensors in PlanetScope constellation can provide ~4 m resolution, 8-band spectral data every 1-2 days (resampled to 10 m in this study for consistency with Sentinel-2) to meet the critical need for detecting blooms of the toxic dinoflagellate Karenia brevis (often called red tides) in nearshore waters off the central west coast of Florida (USA). Despite the relatively low signal-to-noise ratios (SNRs), SuperDove Enhanced Red-Green-Blue (ERGB) images clearly depict the detailed distributions and morphological shapes of K. brevis blooms in both nearshore coastal waters and small estuaries. These features, challenging or impossible to observe from traditional satellite sensors, are well captured and quantified with an optimized deep learning model that leverages both spectral features and spatial context. Comparison of model results with medium-resolution satellite data indicates a detection limit of 0.055 mW cm^-2 μm^-1 sr^-1 in the normalized fluorescence line height (nFLH) data product, corresponding to around 2-3 mg m^-3 surface chlorophyll-a concentration or about 300,000 cells L^-1 of K. brevis. Although this detection limit is higher than those offered by traditional satellite sensors with higher SNRs, because of its frequent coverage of global nearshore waters at much higher spatial resolution, the SuperDove constellation is expected to play an essential role in monitoring HABs in nearshore environments to help mitigate the negative impacts of these blooms.