OLCI-Constrained Sentinel-2 Mapping of Cyanobacteria Relative Intensity for Sustainable Reservoir Water Management

Reliable monitoring of cyanobacteria in drinking water reservoirs is important for sustainable water management, but high-resolution mapping is difficult in clear, shallow systems because of bottom reflectance, shoreline mixing, and atmospheric correction uncertainty. The aim of this study was to test whether a Sentinel-3 Ocean and Land Color Instrument (OLCI)-constrained Sentinel-2 framework could provide physically defensible cyanobacteria relative intensity maps at 20 m in Dahuofang Reservoir, China. OLCI reflectance near 620, 665, and 709 nm defined monthly parent-scale intensity, while ACOLITE-corrected Sentinel-2 water-leaving reflectance supplied spatial structure through the Normalized Difference Chlorophyll Index and Maximum Chlorophyll Index. A bathymetry-derived reliability weight reduced shallow water and nearshore optical effects, and same date compositing preserved spectral consistency. Six monthly maps were generated for April–September 2025, covering approximately 61.3–61.5 km2 and 153,958–154,795 valid 20 m pixels per month. Aggregation back to the OLCI grid yielded R2 values of 0.823–0.929 and Spearman correlations of 0.942–0.973. The products are relative intensity proxies rather than absolute pigment concentrations. They can help managers prioritize field verification, target sampling, and compare monthly spatial patterns but cannot replace field measurements or regulatory threshold assessments.