Long-Term Dynamics of Ulva prolifera Blooms in the Yellow Sea and Their Environmental Associations: Insights from Multi-Source Remote Sensing

Recurrent Ulva prolifera (U. prolifera) green tides in the Yellow Sea have become a persistent coastal ecological problem, yet their long-term phenological changes and responses to environmental variability remain insufficiently understood. Here, we reconstructed the spatiotemporal dynamics of U. prolifera from 2008 to 2025 using multi-source satellite observations. Floating algae were detected using the Floating Algae Index (FAI) or Alternative Floating Algae Index (AFAI), and subpixel coverage was estimated using linear spectral unmixing. The results showed that MODIS-derived green tide extent expanded markedly, reaching 886.13 km2 in 2021. June exhibited the strongest long-term increasing trend. VIIRS observations showed broadly similar spatial and interannual bloom patterns over 2016–2025, while hourly GOCI-II observations revealed pronounced intraday variability which coverage generally increasing from morning to noon or afternoon. Environmental analyses demonstrated that bloom variability was jointly regulated by multiple environmental factors, including nonlinear relationships with sea surface temperature, pH, and dissolved oxygen, as well as linear effects of salinity and silicate. The multivariable model explained 70.7% of the observed variation in bloom area, highlighting the importance of coupled physical and biogeochemical controls. These findings indicate that Yellow Sea green tides are becoming increasingly persistent under changing environmental conditions. Multi-source satellite monitoring can inform adaptive management that combines nature-based solutions (NbS), including source-region nutrient reduction, optimized aquaculture structures, ecological polyculture with competitive macroalgae, hydrodynamic-based interception, and sustainable biomass utilization. Such approaches may reduce bloom risks while enhancing coastal ecosystem resilience and transforming U. prolifera management from passive removal toward proactive ecological regulation.