Diurnal Cycle of Sound Scatterers and Measurements of Turbidity Using ADCP in Beppu Bay |
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| Authors: | Xiao-Hua Zhu Yoshio Takasugi Masayuki Nagao Eisuke Hashimoto |
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| Institution: | (1) Chugoku National Industrial Research Institute, Hirosuehiro 2-2-2, Kure, Hiroshima, 737-0197, Japan;(2) Chugoku National Industrial Research Institute, Hirosuehiro 2-2-2, Kure, Hiroshima, 737-0197, Japan |
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| Abstract: | From September 20 to 22 in 1994, the vertical profiles of echo intensity and three-component velocities were measured with a bottom-mounted 300 kHz broadband acoustic Doppler current profiler (ADCP) in Beppu Bay in the Seto Inland Sea of Japan. A very strong thermocline was observed from 50 to 60 m. A pronounced diurnal cycle of backscatter strength (BS) was found above the thermocline. However, it was not found under the thermocline where there was a lack of dissolved oxygen. We suggest that the diurnal cycle of BS is caused by the vertical migration of zooplankton. The downward and upward migration occurred in early morning and late afternoon, respectively. The migration speeds estimated from BS isopleth displacements were about 1 cm s–1. Further, the contribution of turbidity (Tur) to BS was examined by separating out the effect of migrating zooplankton. There was a significant correlation between BS and turbidity under the thermocline. The maximum contributions of the Tur, migrating zooplankton and non-migrating plankton on BS were estimated at 3, 12, 25 dB, respectively. These data suggest that when using an ADCP to estimate Tur, it is very important to consider carefully the backscatter signal from zooplankton. |
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| Keywords: | Diurnal cycle of backscatter strength Beppu Bay bottom-mounted ADCP zooplankton turbidity |
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