Changes in abundances of Alexandrium tamarense resting cysts after the tsunami caused by the Great East Japan Earthquake in Funka Bay, Hokkaido, Japan

•We compared abundances of the toxic Alexandrium cysts before and after the 2011 huge tsunami.•The cysts abundance significantly increased throughout a sampling bay after the tsunami.•After the tsunami, the large toxic A. tamarense bloom occurred throughout the bay in 2011.•Bottom-mixing effect by the tsunami was suggested to cause the accumulation of the cysts. The 2011 Great East Japan Earthquake and the subsequent huge tsunami greatly affected both human activity and the coastal marine ecosystem along the Pacific coast of Japan. The tsunami also reached Funka Bay in northern Japan and caused serious damage to the scallop cultures there, and this tsunami was believed to have affected the coastal environments in the bay. Therefore, we investigated the changes in the spatial abundance and distribution of the toxic dinoflagellates Alexandrium tamarense cysts before the tsunami (August 2010) and after the tsunami (May 2011, August 2011, May 2012 and August 2012) in the bay. Further, monthly sampling was conducted after the tsunami to identify seasonal changes of Alexandrium catenella/tamarense cysts and vegetative cells. Significant increases were observed in the populations of A. catenella/tamarense cysts, comparing the abundances before the tsunami (in August 2010; 70±61cystsg−1 wet sediment) to those just after it (in May 2011; 108±84cystsg−1 wet sediment), and both A. tamarense bloom (a maximum density was 1.3×103cellsL−1) and PSP (Paralytic Shellfish Poisoning) toxin contamination of scallops (9.4 mouse unitg−1 was recorded) occurred in the bay. Seasonal sampling also revealed that the encystment of A. tamarense and the supply of the cysts to bottom sediments did not occur in the bay from September to April. These results strongly suggested that the mixing of the bottom sediments by the tsunami caused the accumulation of the toxic A. tamarense cysts in the surface of bottom sediment through the process of redeposition in Funka Bay. Moreover, this cyst deposition may have contributed to the toxic bloom formation as a seed population in the spring of 2011.