Occurrence and environmental features of harmful bloom-forming flagellates Heterosigma akashiwo, Karenia mikimotoi and Chattonella spp. in Sasebo Bay, Japan

Harmful algal blooms (HABs) of unarmored dinoflagellate Karenia mikimotoi have often been recorded first in Sasebo Bay, Nagasaki, Kyushu in each year since 2000. For better forecasting of HABs in Sasebo Bay, the cell density of HAB species and other dominant microalgae was regularly monitored and compared with temperature and salinity for 7 years (2013-2019). Presence and density of resting cysts were also estimated from the cells germinated from the seabed sediments collected in 2013 and 2018. Blooms of three HAB species, Heterosigma akashiwo, K. mikimotoi and Chattonella spp., were observed in this study. Blooms of H. akashiwo often formed earlier than the other two, in the range of 20-23℃ and in the years with long sunshine hours from early to mid May. Blooms of K. mikimotoi occurred every year, proliferating from May or June, and denser blooms were recorded in the years with higher total water temperature from January to April and total air temperature from December to March. Chattonella blooms were recorded from 2013 to 2015, and mean Chattonella cyst density was 6.7 cysts cm−3 in 2013, while no cyst was detected in 2018.