Large-scale transport of Cochlodinium polykrikoides blooms by the Tsushima Warm Current in the southwest Sea of Japan

Since 2002, blooms of the harmful dinoflagellate Cochlodinium polykrikoides Margalef have occurred in the eastern San-in area, the Japanese coastal area fronting the southwest Sea of Japan. To investigate the occurrence mechanisms of the blooms, numerical experiments were conducted using a Lagrangian particle-tracking model, under different hydrographic conditions in 2002-2008. The results of Lagrangian simulations revealed that the source region of the blooms was located in the southeast Korean coastal area, >500 km away from the eastern San-in area. They were transported by the Tsushima Warm Current over about two weeks to one month. The blooms in the eastern San-in area were thought to occur only when a series of sequential conditions were met, which included preceding outbreaks in Korean waters, southwesterly winds in the Tsushima Strait, and the nearshore position of the Tsushima Warm Current off the San-in coast, veering eastward from the Korean peninsula.