Phytoplankton Communities Miniaturization Driven by Extreme Weather in Subtropical Estuary under Climate Changes

Estuarine ecosystems are subject to extreme weather and climatic events, particularly global climate change. To characterize the effects of extreme weather, such as heavy precipitation and cold waves, on phytoplankton communities, four seasonal cruises were conducted in the subtropical Pearl River Estuary. Among the main phytoplankton taxa, small (pico- and nano-sized) phytoplankton accounted for approximately 50% and 70% in the upstream estuary. In spring, summer, and autumn, small-sized phytoplankton communities were dominated by Chlorophyta (20-45%), Ochrophyta (Bacillariophyceae; 25-40%), and Dinoflagellata (20-25%), associated with high phytoplankton network stability and river discharge. In winter, small phytoplankton communities were dominated by Cryptophyta (50%), Dinoflagellata (25%), and Ochrophyta (Bacillariophyceae; 10%), which were associated with low network stability and temperature. Low light and high nutrient conditions induced by large river discharge favored the growth of the smallest known brackish Chlorophyta, such as Ostreococcus tauri. Low temperatures provided a competitive advantage for the survival of the small-sized cryptophyte genus Teleaulax, especially in the upstream estuary during the 2020/2021 extreme cold wave period. Our findings highlight the impact of extreme weather induced by climatic events on the miniaturization of phytoplankton communities in subtropical estuaries by altering temperature, light availability, and nutrient dynamics.