FMap of the sampling stations. (a) Small-scale map of our observation area (break line square) with sea surface height and estimated ocean current positions (arrows: two offshore branches of Tsushima Warm Current, coastal branches of Tsushima Warm Current, and Kuroshio). (b) Sampling stations in winter (February), (c) in spring (March–June), and (d) in summer (July–September). The sea surface height was derived from Copernicus Marine Service Global Ocean Physics Reanalysis (GLORYS12V1, https://doi.org/10.48670/moi-00021) in August 2015. Along the SI-line [black solid line in (a) and gray line in (b) – (d)] set offshore of Sado Island, repeated observations were made. The crosses indicate the stations where samples were collected from the deep layer (75 m in spring and 100 m in summer), and open circles indicate stations where samples were collected only from the subsurface layer (20–65 m depth). In winter, all the sampling layers were assumed to be in the mixed layer, thus we did not divide them into surface and subsurface layers when collections were made at 10 m and 30 m depths. The maps were made with Natural Earth, and Geospatial Information Authority of Japan.