Diurnal vertical migration of a harmful dinoflagellate, Cochlodinium polykrikoides (Dinophyceae), during a red tide in coastal waters of Namhae Island, Korea

Since 1995, massive harmful blooms of the dinoflagellate Cochlodinium polykrikoides have been an annual feature along the southern coast of Korea in late summer, leading to serious impacts on fin-fish farms covering huge areas. Because vertical migration is an important process involved in red tide formation, the vertical distribution and variation of C polykrikoides were investigated with time at a fixed station off the coast of Namhae Island, Korea, elucidating the migration depth and velocity as well as the timing of downward and upward migration. The maximum cell density of C. polykrikoides was observed in the surface layer at 1600 hours. Subsequently, the population started to migrate downwards and reached a depth of 15 m at around 2000 hours. The upward migration began at around 0600 hours, and the population concentrated in the surface layer by 1100 hours. The speed of descent was faster than that of ascent, probably owing to gravity. The estimated velocity of vertical migration, based on the depth of maximum cell density and the migration time, was 3-4 m h(-1), which is fairly high compared to the swimming speed of other dinoflagellates. Dividing cells were mainly observed from 0400 to 0800 hours, when the population of C polykrikoides began to migrate upwards from the bottom. During this period of high cell division, the densities of long chain groups of greater than or equal to6 cells abruptly increased, while those of short chain groups of less than or equal to5 cells decreased. After 0900 hours, the reverse was observed. These results suggest that the variation in the frequency of each type of chain could be an ecological strategy through which the population can produce sufficient propulsion while it has long chains to migrate upward from the sediments before sunrise, whereas the short chains, with a high area: volume ratio, may be more efficient in energy capture after arrival in the surface layers.