Unprecedented high-temperature triggers a rapid shift in ecological characteristics of phytoplankton succession in the southern coastal waters of Korea.

Marine heatwaves (MHWs) are increasing in frequency and intensity worldwide, significantly impacting marine ecosystems. However, studies on phytoplankton community changes in coastal waters under such conditions remain. In the summer of 2024, an extreme high-temperature event (>28 °C) occurred in the southern coastal waters of Korea, providing an opportunity to investigate phytoplankton community dynamics under thermal stress. Biweekly depth-specific monitoring at 5 m intervals during this period revealed that diatoms maintained high biomass in the mid-to-bottom layers, where temperatures were relatively lower and nutrient concentrations were higher. In contrast, phytoplankton biomass and photosynthetic efficiency declined in the surface layer, with this decline becoming more pronounced as the heatwave intensified. Furthermore, Margalefidinium polykrikoides, a species historically responsible for harmful algal blooms (HABs) in the southern coastal waters of Korea during summer, failed to survive in the surface layer and was observed only in low abundances at depths of 5-10 m. Meanwhile, Synechococcus exhibited a significant increase under high-temperature, low-salinity, and oligotrophic conditions. These findings suggest that climate change may alter HAB dynamics while also increasing the role of small phytoplankton in primary production. Following typhoon-induced water column mixing, phytoplankton biomass partially recovered, with diatoms playing a central role in this recovery. Recurrent marine heatwaves are likely to favor heat-tolerant species while reducing the abundance of large diatoms, which form the foundation of the coastal food web. Such structural changes could have cascading effects on the marine food web, ultimately impacting the stability of the marine ecosystem.