Heavy precipitation-induced Yangtze River runoff greatly regulates heterotrophic prokaryotes production and induces P-limited growth in the northern East China Sea

Abstract. Although heterotrophic prokaryotes (HP) play a crucial role in biogeochemical carbon cycles, microbial oceanographic studies associated with heavy precipitation-induced large-scale freshwater runoff are understudied in the East China Sea (ECS), the largest continental shelf in the northwest Pacific. To elucidate the impact of Yangtze River diluted water (YRDW) on HP production (HPP) and growth-limiting resources, we conducted comprehensive microbial oceanographic measurements in combination with analysis of satellite images and optical property analyses of dissolved organic carbon (DOC) over three consecutive years in the northern ECS. Our results revealed that the HPP and chlorophyll a were consistently highest in summer due to the supply of excess DOC and nutrients via YRDW, which is intriguing considering the enhanced HPP coupled with spring phytoplankton bloom in middle latitudes in general. However, the exceptionally great YRDW runoff induced by heavy rainfall resulted in excessive supply of terrestrial-origin recalcitrant DOC and nutrients imbalance with high N : P ratio (34), which was responsible for the limited DOC bioavailability and phosphorus-limitation for the HPP. Accordingly, the enhanced HPP-to-primary production ratio (>0.5) in summer may suggest enhanced carbon flow via microbial food web, potentially altering food-web structure and energy transfer efficiency. Our results, demonstrating that YRDW can either stimulate or suppress HPP, provide new insights into microbial responses to large-scale freshwater discharge, which may be relevant to systems influenced by substantial freshwater inputs (e.g., Amazon River and Arctic Ocean).