AbstractIn this study, we report high‐resolution spatiotemporal distributions of particulate polyunsaturated aldehyde (pPUA) and dissolved polyunsaturated aldehyde (dPUA) across a frontal system outside the Pearl River Estuary of the northern South China Sea (NSCS) by synthesizing field data collected during the summers of 2015, 2016, and 2019. High levels of pPUA (up to 40 nM) in the surface waters were related to intense phytoplankton blooms at the frontal zone dominated by PUA‐producing diatoms. We found phytoplankton chlorophyll‐a negatively correlated with the heptadienal percentage but positively correlated with the octadienal percentage in pPUA at the surface frontal zone, consistent with a cross‐front shift of the dominant PUA‐producing diatoms from Skeletonema costatum to Thalassiosira minima. Also, pPUA was significantly correlated with chlorophyll‐a in the surface boundary but with turbidity in the bottom boundary of the front during the cross‐front transect, indicating a biological source of surface pPUA but a sedimentary source of bottom pPUA (likely from the resuspended PUA precursors). At the frontal zone, a fast decrease of dPUA with depths below the mixed layer could be a result of enhanced mixing by the intruded bottom seawater. In summary, we reveal the complex distributions of pPUA and dPUA in the frontal system outside a eutrophic estuary, including both its surface and the bottom boundary layers. We further discuss the ecological and biogeochemical implications of the unique PUA distributional patterns on the NSCS shelf‐sea.