This study was carried out to determine the characteristics of the marine environment, including nutritional content, in order to clearly understand the outbreaks of Gonyaulax polygramma in the southern coastal areas in August, 2009. Samples were collected at 13 sites and water temperature and salinity were measured using a CTD. Field surveys were twice: the first between August 5-7, the second between August 22-24. The cell density of G. polygramma was 6,500-10,000 cells ml-1 during the 1st survey, whereas during the 2nd survey the range of the cells was recorded from 8,000 to 12,500 cells ml-1. Cochlodinium polykrikoides ranged from 0 to 105 cells l-1 during the field survey. In water environments, the majority stations during the 1st survey showed a nearly homogeneous water column below 1℃ in temperature, as well as similar profiles of salinity. However, the stratification between the surface and bottom was observed in the 2nd survey. Regarding nutrients, Dissolved Inorganic Nitrogen (DIN) in the surface ranged from 0.144 to 0.236 mg l-1 during the 1st survey, and 0.082-0.228 mg l-1 during the 2nd survey. DIP (Dissolved Inorganic Phosphorus) did not show any difference in concentration between the 1st and 2nd survey. During August of 2009, the wind speed in the southern waters remained at around ≤2 m s-1 for about 60% of time, and there was very little precipitation during the month. Irradiance of ≥10 hr was shown in the late of August. It is thought that a low level of DIN and salinity play an important role as an essential factor for rapid growth, wide distribution and longer duration of red tide in G. polygramma.

The aim of this study is to determine the outbreaks of nontoxic Gonyaulax polygramma Stein in Yeosu waters in place of harmful Cochlodinium polykrikoides Margalef, which has occurred annually in the same region since 1995. The observation of cellular arrangement and structure by electron microscopy showed that G. polygramma isolated from Yeosu waters had a few spines connecting with membranes and prominent longitudinal ridges on the cell surface, with a cingular displacement 1.5 times their cell width. Furthermore, the location of the nucleus was posterior of large oval formation according to electron microscopy. On 6 August, 2004, the first bloom of G. polygramma occurred, the date of its disappearance was with a maximum cell density of 8,000 cells ml-1 on 21 August, 2004. During the period of this study, the horizontal distribution of sea water temperature and salinity showed a strong coastal front, whereas the front of DIN (Dissolved Inorganic Nitrogen) was significantly different between the occurrence and disappearance of G. polygramma blooms. These results suggested that the process of the breakdown of stratification by wind and a low level of inorganic nitrogen play important roles in the rapid growth of G. polygramma, which is associated with a greater robustness in growth against DIN than that of C. polykrikoides in nature.