Antarctic coastal polynyas support intense phytoplankton blooms that drive significant carbon export, yet bloom termination mechanisms remain poorly understood. Multi-platform observations of the austral summer 2022 bloom decline in the Amundsen Sea Polynya revealed a moderately iron-stressed, Phaeocystis antarctica-dominated community highly acclimated to low light. Surface particulate organic carbon exhibited a strong diel cycle (23% higher at night), with P. antarctica biovolume concentration minimized shortly after peak irradiance, and increasing chlorophyllide a indicated physiological senescence. Deep mixed layers (often >100 m) intermittently exposed cells to surface irradiance over two orders of magnitude greater than mean mixed-layer light, with each high-light exposure lasting only minutes per mixing cycle. A 1-D biogeochemical model with photodamage-triggered aggregation reproduced the observed dynamics, suggesting current models may underestimate light-driven phytoplankton loss by parameterizing stress from mean irradiance rather than short-term extremes. These results suggest the cumulative impact of repeated, brief high-light exposures can drive significant phytoplankton loss even while bulk growth conditions remain favorable.