Mixotrophy, the ability to integrate autotrophy and heterotrophy within a single cell, has been documented in a diverse array of phytoplankton taxa in aquatic ecosystems. However, our understanding of how mixotrophic strategies fluctuate in response to spatial and temporal changes in estuarine-coastal environments, characterized by strong environmental gradients, remains limited. This study employed a fluorescent bead-based grazing approach to explore the spatiotemporal variations in mixotrophic nanophytoplankton and the environmental factors shaping their strategies. It was found that mixotrophic nanophytoplankton exhibited distinct spatiotemporal variations. In spring, mixotrophs exhibited significantly higher abundance, proportion, and average cell size compared to summer, alongside lower ingestion rates. Furthermore, an inverse relationship between ingestion rate and the proportion of mixotrophs revealed a seasonal trade-off in nutritional strategies. Mixotrophs prioritized autotrophy in spring under higher nutrients and irradiance, while they drove a shift toward heterotrophy in summer, compensating for reduced autotrophic productivity with increased turbidity, bacterial abundance, and temperature. Spatially, mixotrophs displayed higher abundance and lower clearance rates in the plume relative to those in coastal waters during spring. The observed temporal and spatial patterns were shaped by biotic factors, including the abundance and size of nanophytoplankton and bacterial prey, as well as abiotic factors, such as temperature, turbidity, irradiance, and nutrient concentrations. This study highlights the environmental drivers and adaptive strategies of mixotrophic nanophytoplankton, providing significant insights into the adaptability of mixotrophs to dynamic estuarine-coastal ecosystems.