The phytoplankton community was analyzed from surface water samples collected at 32 stations along the Pacific coast of Hokkaido during 6–12 October 2021. Phytoplankton cell density ranged between 38 and 9033 cells mL^–1. The phytoplankton were classified into four separate communities, and the harmful, red tide, dinoflagellate Karenia selliformis dominated the high cell density community. Chlorophyll a showed a significant relationship with K. selliformis cell density, and their pigment content per cell was estimated to be 37 pg cell^–1. A generalized linear model on K. selliformis cell density applying the environmental parameters (temperature, salinity, nutrients [NO₃, NO₂, NH₄, PO₄, SiO₂]) as independent variables revealed that PO₄ concentration had a significant positive relationship with K. selliformis cell density. High temperatures (1–3°C higher than normal years) that induce the development of water stratification and diminished nutrient concentration near the surface are a necessary condition for the formation of K. selliformis red tide blooms. Under such conditions, because of the mobility of K. selliformis, it can perform diel vertical migrations to refill nutrients from a deeper layer during the night and photosynthesize near the surface layer during the day. Then nutrients supplied through vertical mixing under weakened stratification for such a water mass dominated by K. selliformis, would be a possible mechanism for the outbreak of a red tide bloom along the Pacific coast of Hokkaido during autumn.