Unraveling epipelagic microbial food web structure across water masses of the Kuroshio Extension based on meso- and fine-Scale in situ observation

The microbial food web (MFW) forms the foundation of ecosystems and comprises unicellular organisms across multiple size classes. It is grouped into picoplankton (0.2–2 μm; PRO, SYN, PEUK, HP), nanoplankton (2–20 μm; NEUK), and microplankton (>20 μm; ciliates including tintinnids). Although individual microbial components have been studied in the North Pacific, integrated analyses of MFW structure across water masses are still scarce.

Here, we investigated the structure of epipelagic MFWs in the Kuroshio Extension region using in situ observations conducted at both meso- and fine-scale resolutions. Along a mesoscale transect (D transect), we examined variations in MFW structure within the 5–200 m water column across distinct water masses, including Kuroshio Water, Transition Zone (including a cold eddy), and Oyashio Water. Overall, the abundances and biomasses of three trophic levels increased from Kuroshio to transition waters and further to Oyashio waters in the upper layer (5–50 m), whereas an opposite pattern was observed in the 50–150 m layer. In terms of Pico, cold eddy exerted positive effects on SYN and HP, but negative effects on PRO and PEUK. Nano-sized organisms exhibited reduced abundance and biomass within cold eddy in the 50–200 m layer, while showing limited responses in the upper layer. At the micro-sized trophic level, cold eddy was unfavorable for tintinnids and suppressed ciliate abundance in the 5–50 m layer.

The other surface F transect, a high-frequency, fine-scale transect crossing the Kuroshio Extension front, was used to conduct targeted in situ surface biological observations to examine microbial food web structure. Satellite remote sensing provided complementary information on sea surface frontal dynamics, enabling interpretation of fine-scale biological variability in this highly dynamic region.

Through combined mesoscale and fine-scale observations, our results demonstrate how microbial food web structure in the Kuroshio Extension responds to both distinct water masses and highly dynamic frontal zones, offering rare in situ data in this complex and previously under-sampled region and highlighting the importance of multi-scale physical–biological interactions.