From pixels to Algae: Assessing cyanobacterial bloom severity in Northeastern lakes through Multi-Class phytoplankton density retrieval

Frequent algal blooms have intensified ecological risks in lakes and reservoirs worldwide, posing serious threats to regional water quality and drinking water security. As a key indicator of lake ecosystem health, phytoplankton density has long been central to limnological and environmental research. However, most existing studies provide only broad descriptions of bloom dynamics and rarely differentiate the specific contributions of individual algal phyla to bloom development. This limitation hinders a deeper understanding of phylum-specific density variations and their environmental response mechanisms. In this study, satellite observations were used to examine the spatiotemporal dynamics and driving mechanisms of major algal phyla in lakes larger than 20 km 2 across Northeast China. Bloom types dominated by Cyanophyta, Chlorophyta, and Bacillariophyta were identified, and a cyanobacterial bloom classification system was established based on phylum-specific density thresholds. Results showed that Cyanophyta overwhelmingly dominated phytoplankton communities, with an average density of 1.54 × 10 7 Cells/L —15.6 and 17.0 times higher than those of Chlorophyta (9.84 × 10 5 Cells/L ) and Bacillariophyta (9.06 × 10 5 Cells/L ), respectively. Between 2016 and 2023, 42.37 % of the lakes exhibited a significant increasing trend in Cyanophyta density ( p < 0.05), indicating that algal bloom events in Northeast China’s lakes and reservoirs are primarily driven by Cyanobacteria. On average, the areal proportions of lakes experiencing no bloom (Level I), mild (Level II), moderate (Level III), and severe blooms (Level IV) were 73.74 %, 16.76 %, 8.11 %, and 1.38 %, respectively. Over the past decade, the area affected by severe Cyanophyta blooms expanded from 1.51 % to 2.54 %, suggesting that bloom proliferation has become increasingly common and that associated ecological risks are intensifying. This study underscores the importance of phylum-level phytoplankton density monitoring in inland lakes and provides a novel perspective for identifying bloom types and assessing their ecological implications.