Diurnal variation analysis of fluorescence quantum yield during algal blooms based on geostationary ocean color satellite

Eutrophication in coastal and freshwater environments has led to increasingly frequent and intense algal blooms, posing serious threats to ecosystem health, water quality, and human activities. This study investigates the potential of fluorescence quantum yield (φ) as a dynamic indicator of phytoplankton photosynthetic activity, utilizing satellite observations from the Geostationary Ocean Color Imager (GOCI) and field measurements in Lake Taihu-a eutrophic, hydrodynamically stable lake. Under natural conditions, diurnal variations in φ revealed that phytoplankton dissipates excess energy via non-photochemical quenching (NPQ) at midday light saturation, resulting in reduced φ. These patterns closely reflected photosynthetic dynamics. Building on this foundation, the study analyzed φ dynamics during bloom events in the East China Sea, Ariake Sea, and Sea of Japan. The results demonstrated distinct diurnal patterns related to species composition and local environmental conditions. For example, φ peaked around midday during a dinoflagellate bloom in the East China Sea, while it decreased under intense solar radiation during diatom blooms in the Ariake Sea-highlighting the species-specific photoacclimation strategies. Additionally, a temporal analysis of Chl-a and φ in bloom-affected, nearshore, and offshore waters showed that increases in φ generally preceded Chl-a accumulation, indicating φ's sensitivity as an early warning signal of phytoplankton growth. These findings enhance our understanding of algal bloom dynamics and support the use of φ as a complementary tool for satellite-based bloom monitoring and ecological assessments.