Cyst evidence for historical occurrence of Alexandrium tamarense (Gonyaulacales, Dinophyceae) in Korean and Japanese coastal areas

Abstract

To investigate the historical occurrence of Alexandrium tamarense which is well known as causative organisms for Paralytic Shellfish Poisoining (PSP), three sediment cores were collected from Korean (Y1 and G1) and Japanese coastal areas (A1), which are commonly affected by Tsushima Warm Current. The depositional ages of each core sample were estimated by 210Pb dating method. According to the results, the sedimentation rates in Y1 and G1 were 1.1 and 2.5 cm / year, respectively, while that of A1 was 0.75 cm / year. Alexandrium catenella/tamarense type cysts characterized with ellipsoidal and transparent wall were observed in three core samples. Vegetative cells germinated from the ellipsoidal cysts were morphologically identical to a toxic dinoflagellate A. tamarense. The ellipsoidal cysts in Y1 and G1 have already occurred from 1970s and its concentration increased from the late 1980s, and these cysts in A1 also increased from the late 1980s. Based on our results and the historical occurrences of Alexandrium tamarense in both Korean and Japanese coastal areas, one interesting finding is that PSP outbreaks and increases of the ellipsoidal Alexandrium cysts occurred between the 1980s and 1990s. This suggests that ellipsoidal Alexandrium cysts may have been carried to the Korean and Japanese coasts by the current system, possibly the Tsushima Warm Current, and that the current system may have a continuous influence on the growth of ellipsoidal Alexandrium cysts. However, this issue needs to discussed in much greater detail in order to better understand the effects of current system to distribution of ellipsoidal Alexandrium cysts.