Phytoplankton Blooms in the Coastal Seas Around China Increase in Response to Warming

Abstract Global warming intensifies coastal phytoplankton blooms (CPBs) and marine heatwaves (MHWs), elevating risks to marine ecosystem health. However, the impacts of regional warming on CPBs in the coastal seas around China (CSAC) remain inadequately quantified, which impedes the development of targeted strategies to mitigate the increasing bloom frequency. To address this gap, we analyzed 1 km‐resolution daily CPB records (2003–2020) from the CSAC, combined with concurrent abiotic data sets, to quantify their responses to warming. Our results indicate that bloom frequency increased in 72.1% of the affected CSAC areas, with 57.6% of these increases exhibiting a positive correlation with rising sea surface temperature. Blooms typically expanded in coverage during temperate springs and tropical autumns under moderate‐intensity, long‐duration MHWs. Key hotspots, such as the Pearl River Delta and Leizhou Bay, experienced earlier bloom timing and higher bloom frequency, as well as greater spatial extent during these seasonal MHW events. Our results highlight that both gradual warming and discrete MHW events are key drivers of the increased frequency and expanded spatial coverage of blooms in productive coastal zones. Therefore, although climate warming is projected to strengthen water column stratification and reduce nutrient availability, efforts to reduce coastal eutrophication remain crucial for mitigating future CPB intensification.