This observation was carried out on the surface seawater of Jinhae Bay, southeastern Korea during a red tide in July, 1991 in order to understand the removal processes of reactive trace elements using natural radionuclides of 234 Since the 1970s, Jinhae Bay has been investigated by many researchers and the NFRDI (National Fisheries Research and Development Institute) for chemical and physical characteristics in seawater and sediment and for the occurrence and magnitude of algal blooming throughout the year, except for winter (Park, 1975a, b;Lee et al., 1981;Yang and Hong, 1982;Yang et al., 1984;Yang and Hong, 1988;Hong et al., 1991;Lee et al., 1993). Lee et al. (1993) reported that the formation of a sub-oxic or an anoxic environment in the bottom waters of Jinhae Bay resulted in a remarkable exhaustion of dissolved oxygen by the degradation of organic matter accumulated on surface sediments, as well as by the stratification of the water mass. Spatial and temporal variations of low oxic water mass strongly depend on the stratification, which is affected by meteorological and maritime conditions. Although many studies have been conducted in Jinhae Bay, the residence time and the fate of reactive trace metals in seawater have not yet been studied. Under a red tide environment, there have been few investigations of the scavenging processes of reactive trace metals with tracers of short-lived 234 Th and 210 Po in the coastal surface area. The objectives of this study, therefore, are to understand the removal processes of reactive trace elements using co-tracers of 234 Th and 210 Po during algal blooming, and to estimate the residence time of 234 Th in the surface seawater of Jinhae Bay.