A field survey conducted in the coastal waters off the Dongtou Islands, southern Zhejiang (120.6°-121.3°E, 27.6°-28.1°N), from 28 June to 3 July 2023 revealed a summer phytoplankton bloom concentrated along the southeastern frontal zone, consistent with satellite remote sensing signals. In the oxygen‑supersaturated region (AOU<0, n=21), Chl a concentrations averaged 16.6 ± 24.3 μg L^-1, peaking at 97.9 μg L^-1, where DO reached 13.8 mg L^-1 and pH 8.45, with nutrients nearly depleted—a positive feedback coupling of high Chl a, high DO, high pH, and low nutrients. In contrast, the non‑supersaturated region (AOU≥0, n=25) exhibited a mean Chl a of only 2.5 ± 1.0 μg L^-1. The high‑Chl a zone was largely confined to a thermohaline transition zone (27-30°C, 25-31 psu), characterized by pronounced thermohaline gradients and low turbidity, indicating that physical frontal processes largely controlled the spatial pattern. Surface nutrients (N, P, and Si) were significantly and negatively correlated with salinity, pointing to the Oujiang and Feiyunjiang Rivers as the primary nutrient sources. The nearshore waters (AOU≥0) directly influenced by terrestrial inputs had an N:P ratio of 32, while the bloom‑affected region (AOU < 0) exhibited a ratio of 48, reflecting intense phosphorus limitation. Notably, bottom phosphate concentrations were relatively elevated in the southeastern portion of the study area (n=4), accompanied by salinities of 33.7-34.2 and an N:P ratio of 16.6±2.1, closely matching the Redfield ratio. This high‑salinity, phosphate‑enriched water mass is consistent with lateral intrusion or upwelling of Kuroshio Subsurface Water. Phosphate supplied through this bottom pathway appears to relieve surface phosphorus limitation, suggesting that the magnitude and duration of the bloom are co-regulated by physical transport (frontal convergence and stratification) and the interplay between Kuroshio phosphorus supply and biological utilization. Collectively, our in situ observations point to a link between Kuroshio Subsurface Water phosphorus supply and summer blooms in the southern Zhejiang coastal waters.