Two Daytime Variation Patterns of Phytoplankton Fluorescence Quantum Yield in the East China Sea Revealed by Geostationary Satellite Data

Phytoplankton fluorescence quantum yield (Φ F ) reveals energy allocation in photosynthesis, and serves as a direct metric quantifying the proportion of absorbed light energy emitted as fluorescence. Traditional in situ Φ F measurements are limited by sparse spatial and temporal coverage and cannot capture high‐frequency spatiotemporal variations. Based on long‐term, large‐area, hourly sea surface Φ F retrieved from the Geostationary Ocean Color Imager (GOCI) over the East China Sea (2011–2020), this study systematically analyzed the hourly daytime variation patterns of phytoplankton Φ F for the first time and explored their potential environmental controls. We constructed more than 4,000 quality‐controlled daytime series of Φ F , chlorophyll‐ a , and instantaneous photosynthetically active radiation (iPAR) from GOCI's eight observations per day. Using a multi‐dimensional time series clustering method (K‐MDTSC), two dominant Φ F daytime variation patterns were identified: Type I (U‐shape) and Type II (continuous decreasing). Analysis of the matched iPAR and modeled temperature and nutrient data suggested that light intensity and temperature are the primary environmental factors associated with these patterns: Type I dominates under strong summer irradiance, while Type II prevails under weak winter light and low seawater temperature, with both patterns coexisting in spring and autumn. Nutrient levels affect the Φ F magnitude directly: under nutrient stress, reduced photosynthetic efficiency diverted more absorbed energy to fluorescence, thereby elevating Φ F . Leveraging GOCI's high‐frequency and large‐area observation capabilities, this study establishes the first comprehensive perspective on phytoplankton Φ F daytime variation patterns, providing mechanistic insights into phytoplankton physiological status and its response to environmental changes.