Warm-core eddy intensifies surface mixotrophic bacterivory and fuels mesopelagic heterotrophic grazing

Anticyclonic eddies (ACEs) restructure resources and drive trophic cascades, yet depth-resolved effects on nanoflagellate bacterivory remain unclear. We quantify bacterivory by phago-mixotrophic nanophytoplankton (PMNP) and heterotrophic nanoflagellate (HNF) using fluorescent-bead incubations in a warm-core ACE in the South China Sea. Despite a 16% decline in nanoeukaryotes, PMNP abundance increases significantly (2.8× surface, 1.4× deep chlorophyll maximum (DCM), shifts toward larger cells, and elevates bacterial turnover and phago-mixotrophic production. HNF community grazing also increases at both euphotic depths despite similar per-cell rates, yielding impacts comparable to PMNP. Structural equation models link PMNP grazing to surface water-mass properties and nutrients and DCM turbidity, whereas HNF grazing tracks water-mass and prey/virus dynamics. In the mesopelagic zone, HNFs dominate; grazing HNF abundance increases 2.7× at 500 m and is associated with particulate beam attenuation coefficient, an in situ particle proxy. These findings reveal a depth-partitioned response coupling mesoscale forcing to microbial carbon processing.