Metabarcoding technology using high-throughput sequencing has revolutionized the current understanding of the diversity and ecology of eukaryotic microorganisms. The aim of the present study was to ...investigate vertical and seasonal variation in eukaryotic plankton communities and to assess the diversity of eukaryotic plankton, using 18S rRNA sequencing, over a depth gradient in subtropical waters affected by the Kuroshio Current. In particular, the present study focused on the diversity and ecology of Alveolata and Rhizaria taxa, which include a variety of plankton species with fragile skeletons or soft bodies. Three vertically distinct eukaryotic communities were identified: the Kuroshio-influenced epipelagic zone (<200 m), the North Pacific Intermediate Water (NPIW)-dominated mesopelagic zone (500–1000 m), and the bathypelagic zone (2000–3000 m). The operational taxonomic unit (OTU) richness was greatest near the surface (<200 m depth), gradually decreasing with increasing depth, and lowest in deeper layers, and OTU diversity (Pielou's evenness and Shannon-Wiener diversity indices) were lowest in the mesopelagic layer (500–1000 m depth). Hidden diversity was observed in both groups in both the surface and deeper layers of the western North Pacific, as well as in the NPIW, which was characterized by the lowest salinity and oxygen concentrations in the study area. In the NPIW, the Rhizaria yielded relatively more sequence reads than other taxa. Furthermore, specific taxa, such as Collodaria (Radiolaria), Syndiniales (dinoflagellates), and Oligohymenophorea (ciliates), were predominant, according to OTU richness and the relative abundance of sequence reads. These findings indicate that a unique ecosystem was formed over time in the NPIW-isolated water mass.
•Eukaryotic community zoned vertically, Kuroshio epi-, NPIW meso- and bathypelagic.•Eukaryotic plankton diversity was remarkably low in the NPIW.•Collodaria, Syndiniales and Oligohymenophorea were predominant in the NPIW.•Acantharia, Nassellaria, Spumellaria and Phaeodaria OTUs peaked at 100 m depth.•Taxopodia, Nassellaria and “Other Cercozoa” reads peaked in the deepest layer.
This study proposes an analysis methodology to address how very rare marine extremes can be understood using limited data. In summer 2016, extreme weather and marine events occurred simultaneously ...around the Pacific shelf off southeastern Hokkaido, Japan. Six successive tropical storms brought extreme precipitation and an anticyclonic mesoscale eddy of subtropical Kuroshio water closely approached the coast, locally causing marine heat waves. We examined how these compound extremes affected oceanographic conditions on the coastal shelf by analyzing data from ship surveys in October 2016 on the Pacific shelf and outputs from a realistic ocean model. Climatologically, warm, high-salinity (33.0–33.7) subtropical water from the Okhotsk Sea (modified Soya Warm Current water) is distributed near the sea surface on the Pacific shelf in October and transported by the along-shelf boundary current. In 2016, however, a vertically well-mixed low-salinity (<33.0) layer associated with the heavy rainfall was observed at 0–50 m depth on the shelf, salinity maxima (≥33.7) associated with Kuroshio water from the mesoscale eddy occurred at 50–150 m depth on the slope, and baroclinic jets formed along the salinity front near the shelfbreak. These observed salinity structures were reproduced by a 1/50° ocean model. Particle-tracking experiments revealed that the low-salinity water originated mainly off eastern Hokkaido, where heavy rainfall events occurred in August, and was modified by mixing with Soya Warm Current water before transport to the Pacific shelf.
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