Abstract

The ecology of eukaryotic picoplankton in oxygen minimum zones (OMZs) is crucial to understand global primary production, trophic dynamics and plankton diversity. This study analyses picoeukaryotic diversity and distribution patterns along the water column at two locations (slope and oceanic) in the tropical Mexican Pacific OMZ using metabarcoding and flow cytometry. Well-known groups of Chlorophytes (Mamiellophyceae) and Ochrophytes (Chrysophyceae, Dictyochophyceae, Pelagophyceae) occurred in high relative abundances, whereas less-known groups such as Chloropicophyceae and Prasinodermophyta were found in lower abundances. Picoeukaryotic diversity was higher at the lower end of the oxycline (10 ?M O2) than at the surface and subsurface layers. Differential distributions of picoeukaryotes were also detected along the water column, with almost exclusive communities at each depth. Mamiellophyceae dominated the surface and subsurface layers, whereas Syndiniales (parasitic dinoflagellates), Radiolaria, Ochrophyta, and Sagenista (MArine STramenopiles -MAST groups-) were prevalent at the oxycline. Post-upwelling oceanographic conditions possibly contributed to shape the differences in community composition and distribution. These findings highlight that oxygen concentration is a key factor driving microbial distribution and that oxyclines provide specialized niches that promote high picoplankton diversity and multiple trophic strategies including autotrophy, mixotrophy, heterotrophy and parasitism. © The Author(s) 2025. Published by Oxford University Press.