Analysis of environmental and microbiological changes in Onagawa Bay immediately after the tsunami of the Great East Japan Earthquake based on sediment cores.

We investigated two sediment cores to understand whether a tsunami in Onagawa Bay, Japan caused environmental changes. The value of δC ranged from -21.9‰ to -24.3‰ and of δN ranged from 5.1 to 5.9‰. We conclude that the source of the sediment in the present study area was mainly oceanic and not terrestrial. The chlorophyll concentration ranged from 1.8 to 4.0 μg/g ww, and did not vary greatly between surface and bottom layers. We inferred that all layers were deposited after the tsunami. The major phytoplankton taxa in sediments were diatoms from DNA sequencing. The presence of harmful dinoflagellates was minor. The concentrations of several heavy metals decreased slightly after the tsunami. We inferred that heavy metals in sediments were diluted by the tsunami disturbance. The land in Onagawa suffered serious damage, but disturbance of the seabed was much less evident.