Effects of the ocean environment off the northeast coast of Japan on the outbreak of dinoflagellates Alexandrium spp. containing paralytic shellfish poison

Harmful algal blooms occur in coastal waters worldwide and cause significant damage to fisheries. Specifically, Alexandrium spp. blooms have been frequently observed along the Pacific coast of northeast Japan (i.e., the Tohoku coast) since the 2011 earthquake and tsunami. In 2018, an Alexandrium bloom exceeding 10,000 cells L −1 occurred suddenly for the first time in several decades in Onagawa Bay, located on the Tohoku coast. The present study examined the causes of the Alexandrium bloom in 2018 using in situ biological and physical observation data, satellite remote sensing data, and an ocean reanalysis dataset from March to May during 2017–2020. A remarkably high cell density of 12,090 cells L −1 was observed in 2018, representing 400-, 63-, and 2-fold increases over the peak values recorded in 2017, 2019, and 2020, respectively. The optimal growth temperature (OGT), defined as the water temperature at which cell density exceeded 100 cells L −1 , ranged from 8.7°C to 12.7°C in Onagawa Bay. The OGT period lasted 12 weeks in 2018, which was 4–8 weeks longer than that in the other years. This result suggests that favorable water temperature conditions enhanced the Alexandrium bloom in 2018. The extended OGT period was attributed to a warm-core ring of the Kuroshio Extension that transported warm water into Onagawa Bay. This study highlights the importance of offshore physical oceanographic features, such as water temperature and current patterns off the Tohoku coast (particularly from March to May), in shaping conditions favorable for Alexandrium bloom development.