Abstract
A massive bloom of Alexandrium spp. occurred in Onagawa Bay, in the middle coastal region of Miyagi Prefecture, during spring 2018. Shipments of Japanese scallop Mizuhopecten yessoensis were subsequently regulated for the first time in 38 years, due to paralytic shellfish poison. Therefore, we investigated the growth factors of Alexandrium spp., which cause paralytic shellfish poisoning in the marine environment, and its relationship with a competing species; diatoms. Around the coastal area of Miyagi Prefecture, the influence of warm water became stronger during spring 2018. It was considered that one of the factors of Alexandrium bloom was the development of thermocline due to the influence of warm water, which provided a favorable growth environment for Alexandrium spp. Diatom population composition in the region was also unusual during this period due to the increased water temperature. Thalassiosira nordenskioeldii and Chaetoceros debilis, which are usually the primary constituents of the spring diatom bloom in Onagawa Bay, did not grow. Reduction of these species may have contributed to the failure to suppress the growth of Alexandrium spp. It was considered that the bloom of Alexandrium spp. occurred in Onagawa Bay due to these multiple factors. After the bloom of Alexandrium spp., cyst quantities in Onagawa Bay increased. The cysts may functioned as a “seed population” to re-proliferate when the environment became favorable again. Due to the recent trend of increasing coastal temperatures, it is necessary to pay attention to the occurrence of paralytic shellfish poison in the middle coastal area of Miyagi Prefecture.
