The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Despite the significance of protistan grazing on bacterial communities, little is known about how and why its effects vary from the euphotic zone to the mesopelagic zone, particularly under the influence of physical processes such as eddies. This study, conducted in the South China Sea, measured grazing activity at four depths (5 m, Deep Chlorophyll Maximum (DCM), 200 m, and 500 m) across 23 stations, including 10 influenced by an anticyclonic eddy, using fluorescently labeled beads to examine vertical patterns and the eddy's impact on protistan grazing. Our results indicate that bacterial turnover rates by heterotrophic protists (HP) in the mesopelagic zone were comparable to those in the euphotic zone, accounting for 18-21% of daily stock of bacterial abundance/biomass. Size-fraction analysis revealed distinct patterns: bacterial turnover rates by small-sized (2-5 µm) HP were significantly higher in the mesopelagic zone than the euphotic zone, while large-sized (5-20 µm) HP exhibited the opposite trend. Bacterial turnover rates by mixotrophic protists peaked at the DCM and were lowest at 200 meters, with small-sized (2-5 µm) mixotrophic protists (MP) comprising 55%-80% of active mixotrophic grazers. Bacterial abundance was the key factor influencing heterotrophic and mixotrophic protistan ingestion rates in both euphotic and mesopelagic zones. Additionally, salinity and nutrients (nitrogen and phosphate) were linked to mixotrophic protistan ingestion rates in the euphotic zone. At eddy-influenced stations, bacterial turnover rates by mixotrophic protists were significantly enhanced in the euphotic zone, with larger-sized (5-20 µm) mixotrophic protists contributing more than the smaller (2-5 µm) ones. Conversely, bacterial turnover rates by heterotrophic protists were significantly enhanced in the mesopelagic zone, predominantly driven by the smaller (2-5 µm) heterotrophic protists. These patterns were strongly linked to significantly lower net community production (NCP) generated by the eddy, highlighting the influence of physical processes on protistan grazing activity in both euphotic and mesopelagic zones. This study provides insights into the grazing activity of protists of different trophic types and how physical processes like anticyclonic eddies can affect their behavior, emphasizing the critical role of local environmental factors and euphotic processes (such as NCP) in regulating protistan grazing dynamics in the water column.
 

mesopelagic zone, anticyclonic eddy, mixotrophic protist, heterotrophic protist, grazing rate