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.