Two sediment cores were collected from two sites in Nagatsura-Ura Lagoon, the mouth of which was destroyed in the 2011 Tohoku-oki tsunami. Although sediment conditions differed widely between the central and inner parts of the lagoon, we could identify traces of the 2011 Tohoku-oki tsunami at both, and those of earlier tsunamis in the inner part of the lagoon. Chlorophyll a maxima were observed at core depths between 62 and 72 cm in the central part of the lagoon. We inferred that this horizon correlated with the 2011 Tohoku-oki tsunami deposit. In the inner part of the lagoon, the horizon at core depths of 116—142 cm could be aged to the 1400s, and is thus thought to have been formed as a consequence of the Kyotoku earthquake of 1454. The dinoflagellate Alexandrium spp. accounted for ~ 22.6% of the total read numbers of phytoplankton from next generation sequencing. Diatoms made up the second largest group of phytoplankton, with Chaetoceros spp. representing ~ 10.5% of the total. The vertical profiles of dinoflagellate and diatom compositions showed little change. Although traces of tsunamis were observed in cores from the lagoon, and in nearby closed bays, we believe that the microbial composition has changed only slightly in the lagoon due to poor seawater exchange through the closed lagoon mouth.
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