Periodic Occurrence of Diurnal Signal of ADCP Backscatter Strength in Uchiumi Bay, Japan

An Acoustic Doppler Current Profiler (ADCP) was deployed for 83 days at the bottom of Uchiumi Bay in the Bungo Channel, Japan. At the same station, the water temperature was measured at nine layers using a thermistor chain. From the temporal variations of the backscatter signal strength data of ADCP, we found that the backscatter signal intensity occasionally showed day-to-night fluctuations. We deduced that the day-to-night fluctuations suggested characterizations of migration patterns of zooplankton or krill, because the signal intensity became relatively high during the night. The migration pattern tended to occur after a cold-water intrusion from the shelf slope region south of the Bungo Channel, which is known as a ‘ bottom intrusion ’. The bottom intrusion occurred six times during the observation period, and the tendency for the diurnal signal in the back scatter signal strength to develop after the bottom intrusion was observed in four of six cases. Though specific biological effects caused by the bottom intrusion are still unclear, the occasional diurnal fluctuations in backscatter signal strength may indicate that the bottom intrusion affects the biological environment in Uchiumi Bay, because the diurnal signal tended to intensify only after the bottom intrusion.