Small eukaryotic plankton community and alpha diversity variability in the Kuroshio region.

Small eukaryotic plankton (1.2-20 μm), exhibiting remarkable diversity and employing various trophic strategies, plays crucial roles in marine biogeochemical cycling. However, limited exploration of their composition and diversity has hindered our understanding of their functional roles in food-web dynamics and biogeochemical processes. This study investigates the spatial variation of small eukaryotic plankton composition and alpha diversity in the Kuroshio region of the northwestern Pacific Ocean, specifically considering their trophic strategies (mixotrophic and heterotrophic) and the influence of environmental factors and bacterial diversity. Small eukaryotic plankton samples were collected from two depths (surface and deep chlorophyll-a maximum layers) across six transects spanning 22°N to 35°N in the Kuroshio region. Amplicon sequencing was used to determine the relative abundance and diversity of small eukaryotic and bacterial communities by targeting the 18S and 16S rRNA genes, respectively. Our results revealed significant compositional changes of small eukaryotic plankton from south to north transects and across two sampling depths, with heterotrophic small eukaryotic plankton showing lower relative abundance at the surface compared to the DCM layer. Small eukaryotic plankton alpha diversity generally decreased with dissolved inorganic nitrogen but responded differently to seawater temperature depending on trophic strategies. Notably, we observed a positive association between small eukaryotic plankton alpha diversity and corresponding bacterial alpha diversity. By incorporating trophic strategy into the analysis of spatial variation, this study provides valuable insights into the composition, diversity, and potential ecological functions of small eukaryotic plankton in the Kuroshio region, enhancing our understanding of marine ecosystem dynamics.