The influence and impact of tsunamis on the microorganism assembly of Nagatsura-Ura Lagoon, Miyagi, northeastern Japan

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 similar to 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 similar to 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.

Automated summary

Two sediment cores were collected from Nagatsura-Ura Lagoon, which was affected by the 2011 Tohoku-oki tsunami. Traces of the 2011 tsunami and earlier tsunamis were found in both the central and inner parts of the lagoon. The microbial composition in the lagoon showed little change due to limited seawater exchange through the closed lagoon mouth.