5:15 PM - 7:15 PM
[MIS14-P19] Diatom assemblages in surface water in the western North Pacific off Sanriku–Shimokita coast in July 2023
Kuroshio is the western boundary current of the North Pacific flowing eastward along the southern coast of Japan. During the Kuroshio large meander, Kuroshio migrates south of 32°N between 136°E and 140°E. The ongoing Kuroshio large meander began in August 2017. Kuroshio usually flows eastward off the Boso Peninsula, whereas pronounced northward migration of Kuroshio is found to the off Sanriku during the Kuroshio large meander. Diatoms are phytoplankton with biogenic opal frustule, a major primary producer in the hydrosphere, and various species living in various aquatic environments. This study aims to clarify how the Kuroshio northward migration changed diatom assemblages off Sanriku to Shimokita. We investigated the horizontal distribution of diatom assemblages in surface water filter samples collected during the MR23-05 cruise by R/V Mirai from July to August 2023. Membrane filter samples with 0.45 μm mesh at four stations (Sanriku coast: St. 11 and Shimokita coast: St. 18, 20, and 21) were used for SEM observation after osmium coating. About 100 pictures of diatom specimens were taken from each of the four samples. Genera Chaetoceros, Pseudo-nitzschia, and Thalassionema were abundant in all four samples. Chaetoceros spp., marine planktonic taxa, are abundant in coastal areas. Marine planktonic genus Bacteriastrum was abundant at St. 20 and 21 off Shimokita. Neodenticula seminae, the dominant species in the subarctic Pacific, was found only at St. 21 off Shimokita. The brackish Cyclotella spp. were relatively abundant at St. 11 off Sanriku, suggesting strong influence by coastal water. Nitzschia bicapitata, Mastogloia spp., and Thalassionema bacillare, commonly observed in the Kuroshio region, were observed at St. 11, suggesting the northward Kuroshio migration to off Sanriku.