Seasonal patterns of diel vertical migration of zooplankton derived from acoustic backscatters in Okinawa Trough: Preliminary results

Abstract

This study investigates the seasonal patterns of diel vertical migration (DVM) of zooplankton using one year-long (from June 2015 to June 2016) moorings deployed in Okinawa Trough. At mooring stations of KCM1 (25.88°N, 125.21°E, depth: ∼2,000 m), close to the Kuroshio core, and KCM2 (25.60°N, 125.32°E, depth: ∼2,000 m), the profiles of water velocities and acoustic backscatter strength were measured by an upward-looking, 75-kHz acoustic Doppler current profiler. Time series of acoustic backscatter strength exhibited a distinct pattern of DVM, which should be generated by zooplankton migration between pelagic (about 100 m) and mesopelagic (about 400 m) zones. A deep scattering layer (DSL) existed between 400 and 500 m during the entire moored period. Compared to summer, the DSL depth decreased about 50 m, and the zooplankton’s residual time decreased about an hour around 400 m at both stations during winter. There were different seasonal patterns in water velocities and DVM speed between KCM1 and KCM2. At KCM1, from summer to winter, depth-averaged water velocities decreased from 0.76 to 0.67 m/s, and the ascending speeds of zooplankton decreased from 30 to 10 mm/s. At KCM2,on the other hand, both of them showed almost identical values between summer and winter. The different responses between KCM1 and KCM2 are coherently related with the influence of Kuroshio current and solar radiation intensity. The details