Change in zooplankton community in Onagawa Bay, northeastern Japan after the Tohoku Earthquake and tsunami in 2011 off the Pacific coast of Japan

Change in abundance and composition of the zooplankton community was studied at the innermost part of Onagawa Bay, northeastern Japan, before and after the Tohoku Earthquake and tsunami of 2011. Abundance of zooplankton groups did not drastically decrease just after the tsunami except for cladocerans and the larvae of benthic organisms. The decrease of the former may have been caused by their resting eggs being swept away by the tsunami or buried in the sediment just before their spring increase, while that of the latter by a population decline of their adults. The number of cladoceran species decreased from 7 to 5, with increasing dominance of Podon leuckarti after the tsunami. The percentage of the genera Acartia and Oithona, combined, was 73% of all copepod genera before the tsunami and increased to 85% after the tsunami. Zooplankton abundance increased with the increase of chlorophyll concentration from 2013, two years after the tsunami. The basic structure of the breakwater broken by the tsunami was reconstructed by the end of 2014 with a shallower sill depth, and the water exchange between the inside and outside of the breakwater became restricted. As a result, dissolved oxygen concentration decreased not only on the bottom but also in the water column and the species diversity of copepods decreased at the innermost station. Long-term monitoring should be continued.