Two Critical Radii Dominate Phytoplankton Response to Marine Heatwaves in the South China Sea

Abstract Marine heatwaves (MHWs) are intensifying globally, yet their impact on phytoplankton is typically assessed as a spatially uniform thermal stress, overlooking their internal structure. Here, we investigate the spatial coupling between MHW intensity and chlorophyll‐a (Chl‐a) in the South China Sea using satellite observations (2003–2020). By applying a segmented linear regression model, we statistically identified two robust critical radii (150 km and 380 km) that demarcate distinct biophysical regimes. In warm seasons, Chl‐a anomalies exhibit a distinct “increase‐decrease‐increase” radial pattern, with a significant suppression zone within the core (