In order to determine the physical and chemical factors controlling the outbreak of red tides, we monitored nutrients and other environmental parameters using a novel real-time monitoring buoy system during the summer of 2003 in the southern sea of Korea, where red-tide outbreaks occur every year. The real-time monitoring data on bioluminescence, which may indicate the presence of bioluminescent dinoflagellate species, showed a sudden increase under the lowest concentrations of dissolved inorganic nutrients. Our monitoring system for the first time provides real-time variations in nutrients and associated outbreaks of dinoflagellate red tides. This result supports the previous hypothesis by others that the outbreak of dinoflagellate red tides is associated with the limited growth of diatoms under depleted DIP or DIN conditions. We suggest that this real-time monitoring system can be utilized as a powerful tool for studying and predicting harmful dinoflagellate red tides in the coastal ocean.