Advancing CyanoHAB monitoring with hyperspectral data from NASA PACE: First results and validation

This study presents the first assessment of NASA's Phytoplankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission's hyperspectral Ocean Color Imager (OCI) for cyanobacterial harmful algal blooms (cyanoHABs) monitoring. We conducted a direct comparison of PACE OCI with Sentinel-3’s multispectral Ocean and Land Color Instrument (OLCI), and its Cyanobacteria Assessment Network (CyAN) operational product using imagery from summer 2024 blooms in Lake Erie, Green Bay, and Clear Lake. We evaluated performance of both sensors using the established Cyanobacteria Index (CICyano) and corresponding cyanobacterial cell density (CCD) to estimate bloom biomass. PACE OCI successfully captured bloom patterns comparable to Sentinel-3 OLCI. When benchmarked against the CyAN product, OCI-derived CCD showed strong agreement (R2 = 0.84, Normalized Root Mean Square Error (NRMSE) = 8.95%), though a negative bias (β≃11%) was observed for extreme bloom pixels. Validation with in situ measurements indicated that OCI significantly improved chlorophyll-a biomass retrievals compared to CyAN/OLCI (NRMSE = 21.57% for OCI vs 38.67% for CyAN/OLCI), emphasizing the value of hyperspectral observations for optically complex inland waters. Our results demonstrate PACE OCI's capability to advance CyanoHAB monitoring, providing a critical first step in establishing continuity with existing operational products while offering new potential for improved biomass estimates and taxonomic discrimination.