Marine heatwaves (MHWs) are increasingly recognized as major climate hazards for coastal ecosystems, yet their ecological risks are often assessed using event frequency, thermal intensity, duration, or mean biological response alone. Such metrics may overlook how the spatiotemporal organization of MHWs structures phytoplankton responses across heterogeneous coastal environments. Here, a three-dimensional MHW event catalogue was constructed for China’s coastal seas during 2003–2020, and satellite-derived chlorophyll-a (Chl-a) anomalies were used to quantify event-scale phytoplankton responses. An event-level space–time footprint (STF), defined as the temporal integral of MHW-affected area, was introduced to represent cumulative thermal exposure. Results show that China-coastal MHWs exhibit strong regional and seasonal organization, with event frequency spatially decoupled from cumulative exposure. Although event-mean Chl-a anomalies were generally negative, STF did not systematically determine the direction or magnitude of mean phytoplankton response. Instead, larger STF events showed reduced across-event dispersion of event-mean Chl-a anomalies, while simultaneously exhibiting greater within-event Chl-a response heterogeneity. This scale-dependent contrast indicates that high-exposure MHWs may produce more constrained event-average responses but more spatially heterogeneous ecological perturbations within their footprints. Seasonal and regional analyses further reveal that Chl-a responses are strongly modulated by background hydrographic and ecological conditions. An exposure–heterogeneity typology and an integrated ecological-risk index reveal that ecological-risk intensity is spatially decoupled from MHW event frequency. The Yellow Sea shows the clearest integrated-risk signature, while the East China Sea shelf and Zhe-Min coast/Taiwan Strait represent distinct elevated-risk modes, and the northern South China Sea has the highest event frequency but mixed risk archetypes. These findings demonstrate that MHW ecological risk cannot be inferred from exposure, frequency, or mean Chl-a response alone. Separating cumulative exposure, response direction, and response variability provides a more event-scale and risk-informed framework for monitoring phytoplankton responses to MHWs in a warming coastal ocean. In this framework, across-event response uncertainty and within-event response heterogeneity represent distinct variability dimensions, with the latter serving as the basis for exposure–heterogeneity risk typology.