Exploring the roles of trophic mode and microbial prey size in grazing pathways of tropical oligotrophic waters of the eastern Indian Ocean

Prey removal incubations were conducted in the Argo Basin (eastern Indian Ocean) to investigate the trophic ecology of the zooplankton community supporting larvae of Southern Bluefin Tuna. Grazing and selectivity were evaluated considering prey trophic mode and cell size as key food quality descriptors in copepod diets, and the impact of microzooplankton grazing was quantified for comparison. Copepod ingestion rate on eukaryotes ranged from 3.4 to 138 ng C ind ^-1 d ^-1 . Diet was mostly derived from mixotrophic (5-89%) and heterotrophic (0-84%) prey, while autotrophs had a minor contribution (2-17%). Overall, nanoplankton provided the highest C intake to copepods (62-99%) rather than more abundant but smaller picoplankton (0.8-38%). Copepod-mediated Chla removal was not detected, although gut content revealed ingestion of 0.8 μg Chla ind ^-1 d ^-1 . Slight selection emerged in favor of picoplankton versus nanoplankton, suggesting a possible trophic cascade. Ingestion of ¹⁵ N-labelled prey indicated active consumption by copepods of consumers (protistan nano-micrograzers or small metazoans). We found significantly greater nitrogen consumption (~5-fold) when using 1-2 μm ¹⁵ N-prey (Synechococcus), compared to <1 μm sized prey (Prochlorococcus). Microzooplankton grazing on eukaryotic (0.07-2.5 d ^-1 ) and prokaryote (0.3-2.1 d ^-1 ) prey greatly exceeded copepod grazing. Microzooplankton diet consisted mainly of heterotrophs (25-59%), and mixotrophs (13-41%) with a lower contribution of autotrophs (12-33%). Nanoplankton constituted the main dietary source for microzooplankton (95-98%), whereas picoplankton was 2-5%. Most of the daily production was removed by microzooplankton (111%) leaving copepods a secondary role in consumption (7%), likely supported by ingestion of larger prey and indirect nutrient channeling.