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 Southern Bluefin Tuna larvae. Grazing and selectivity were evaluated considering prey trophic mode and size as food quality descriptors in copepod diets and compared with microzooplankton grazing. Copepod ingestion of eukaryotes ranged from 3.4 to 138 ng carbon (C) ind ^-1 d ^-1 . Diet was dominated by mixotrophic (5-89%) and heterotrophic (0-84%) prey, with autotrophs contributing 2-17%. Nanoplankton provided the highest C intake to copepods (62-99%) rather than more abundant picoplankton (0.8-38%). No measurable reduction in chlorophyll a (Chl a) concentration occurred from copepod grazing through food removal, suggesting a possible trophic cascade, although gut content revealed ingestion of 0.8 µg Chl a ind ^-1 d ^-1 . Copepods showed moderate selection for picoplankton (E=0.3) over nanoplankton, perhaps due to faster consumption of nanoplankton by microzooplankton or ingestion of picoplankton aggregates. Ingestion of ¹⁵ N (nitrogen)-labelled prey indicated copepod consumption of protistan consumers or small metazoans. We found significantly greater (∼5-fold) copepod N consumption when using 1-2 µm ¹⁵ N- Synechococcus , compared to <1 µm sized ¹⁵ N- Prochlorococcus . Microzooplankton grazing on eukaryotes (0.07–2.5 d ^-1 ) and prokaryotes (0.3–2.1 d ^-1 ) greatly exceeded copepod grazing. Microzooplankton diet consisted mainly of heterotrophs (25-59%) and mixotrophs (13-41%), with lower autotrophic contribution (12-33%) of more nano- (95-98%) than picophytoplankton (2-5%) ingested. Overall, microzooplankton removed most of daily production (111%) in contrast to 7% for copepods. Our findings indicate that mixotrophy, intraguild grazing and nutrient channeling support the food web in this oligotrophic region.