Rseponse of dinoflagellate cyst assemblages to salinity change in Yeoja Bay, the South Sea of Korea

Abstract

To investigate dinoflagellate cysts as indicators of salinity and document the historical occurrence of ellipsoidal Alexandrium cysts in Korean coastal areas, a sediment core was examined from Yeoja Bay, which has low salinity level due to freshwater intrusion. The analyzed dinoflagellate cysts included over 30 taxa commonly reported from other temperate regions. Cluster analysis based on dinoflagellate cyst assemblages indicated two main zone: from 1995 to 2005 (Zone I) and from the early 1900s to early 1990s (Zone II). Total cyst concentration increased sharply in Zone I to approximately five times than in Zone II. Salinity in Yeoja Bay also increased following 1995, possibly due to stronger intrusion of the Tsushima Warm Current, characterized by relatively high salinity. The enhanced salinity coincided with increases in Brigantedinium spp. and Selenopemphix nephroides concentrations. In addition, ellipsoidal Alexandrium cysts have occurred since the 1980s and increased in concentration since 1995, when a paralytic shellfish poisoning outbreak and increases in ellipsoidal Alexandrium cysts were recorded in Korean and Japanese coastal areas. This suggests that ellipsoidal Alexandrium cysts may have been carried to the Korean coast, including Yeoja Bay, by the current system, possibly the Tsushima Warm Current, and that the current system may have a continuous influence on the growth of Alexandrium cysts.