Perennial m e so-scale eddies occur in the western East Sea/Japan Sea. The eddies,known as the Ulleung Warm Eddy (UWE) are formed as the East Korean Warm Current(EKWC) flows following the topography of the Ulleung Basin. They are among the majorhydrographic features in the region. In the previous studies, we have shown thatupwelling-induced production in the coastal region of Korean Peninsula is transported toUWE region via EKWC. In early April 2006, we made two consecutive surveys at aninterval of 8 days. Toward the end of March, a wind-driven upwelling event near the coastoccurred and high chlorophyll-a water was spreading along the edge of UWE. Our surveyin April 5-7 captured the enhanced biomass and production in the lower trophic levelecosystem. In the second survey after a week, abrupt changes were observed in theeddy edge region. Temperature and salinity decreased whereas nitrate and phosphatesincreased. The size-fraction of chlorophyll-a greater than 5 μm reduced drastically. Therewere changes in the phytoplankton composition, which were also reflected in HPLCpigments. There was also a reduction in the bacterial and micro-zooplankton production.In contrast, there was little change in the eddy core region. Between these two surveys,there were also large differences in the meteorological conditions. Remotely sensed dataindicates the EKWC was weakened after the first su rvey. The changes were thusconsistent from meteorological and hydrological conditions to phytoplankton andzooplankton. What caused these abrupt changes is not clear.