Spatio-temporal dynamics of the harmful dinoflagellate Karenia mikimotoi in Uwajima Bay and its adjacent waters: Comparison between a bloom occurrence year and a non-occurrence year

To elucidate the spatio-temporal dynamics of the harmful dinoflagellate Karenia mikimotoi and the relationship with environmental conditions in Uwajima Bay and its adjacent waters, we conducted field observations and analyzed the data in 2018 when a bloom occurred and in 2019 when it did not occur. In 2018, cell density of K. mikimotoi increased in Uwajima Bay, and then the bloom corresponding to low salinity water expanded to a broad area of the adjacent waters, suggesting that the spatio-temporal dynamics of the species were controlled by fresh water and nutrient discharges from rivers, and by passive transport due to a surface residual current. In 2019, although vegetative cells overwintered at low density, they did not form a bloom, possibly due to the environmental conditions for their growth, a large-scale intrusion of warm water (kyucho), and the existence of competing species.