California’s approach to standardizing cyanobacterial bloom metrics derived from satellite imagery

• Data processing choices strongly influence satellite bloom metrics. • Compositing time interval and statistics had the greatest impact on bloom metrics. • Removing nearshore-pixels can reduce false positives but may limit lake coverage. • Ice and snow masking has localized effects in cold, mountainous regions. • Trend and status analyses remained consistent across masking methods. Freshwater harmful algal blooms (FHABs) are increasing globally, with implications for public health, ecosystems, and water resource management. Satellite remote sensing complements field-based efforts and has been used to develop large-scale indicators for FHAB monitoring. However, these indicators can be sensitive to processing choices, which may impact regional assessments and management. This study of 83 California waterbodies uses the Cyanobacteria Index (CI cyano ) derived from Sentinel-3’s Ocean and Land Colour Instrument (OLCI) to systematically evaluate how three common processing decisions (temporal compositing interval, compositing statistics, and pixel masking strategy) affect four widely used FHAB indicators (annual frequency, spatial extent, occurrence, magnitude). Compositing decisions strongly influenced results, with monthly and maximum-value composites yielding the highest bloom metrics, while shorter 7- and 10-day intervals provided more ecologically relevant resolution. Pixel masking strategies involving nearshore regions and ice- and snow-covered areas affected bloom metrics in different ways. Masking more nearshore pixels generally lowered bloom metric values but eliminated some waterbodies from consideration. Ice and snow masking had spatially localized effects, mostly impacting snow-affected regions. Despite differences in absolute values, trend and status analyses using bloom metrics remained largely consistent across masking approaches. Based on the findings, this study recommends a standardized processing workflow for California FHAB monitoring, which may be applicable to other regions with similar management needs: 7-day composites using maximum CI cyano values, and applying both shoreline and ice masking. This workflow balances ecological relevance, computational efficiency, and indicator consistency, supporting the harmonization of bloom monitoring across satellite products, regions, and management programs.