The diel vertical migration of sound scatterers observed by an acoustic Doppler current profiler in the Luzon Strait from July 2009 to April 2011

Acoustic Doppler current profiler (ADCP) receives echoes from sound scatterers, then their speed is calculated by the Doppler effect. In the open ocean, most of these backscatterers are from the plankton. The sound scatterers descend down to depth at around dawn, their mean speed is 2.9 cm/s, then they ascend up to the surface layer at around dusk with a mean speed of 2.1 cm/s, in the Luzon Strait. The descending speed is faster, which suggests that this zooplankton population may accelerate its downward migration under the action of the gravity. The vertical distribution of a mean volume backscattering strength (MVBS) in the nighttime has two peaks, which locate near the upper and lower boundary layers of halocline, respectively. However, the backscatterers only aggregate near the surface layer in the daytime. The diel vertical migration (DVM) of sound scatterers has several characteristic patterns, it is stronger in summer, but weaker in winter, and the maximum peak occurs in September. The DVM occurrence is synchronous with the seawater temperature increasing at around dawn and dusk, it may affect the ocean mixing and water stratification.