Diel vertical migration of zooplankton and micronekton on the northern slope of the South China Sea observed by a moored ADCP

Abstract

Acoustic Doppler current profiler (ADCP) echoes can be transformed into mean volume backscattering strength (MVBS), which is positively proportional to the biomass of zooplankton and micronekton. A mooring with an upward-looking 75-kHz ADCP was deployed on the northern slope of the South China Sea to monitor diel vertical migration (DVM) and vertical distributions of zooplankton and micronekton under general conditions and extreme conditions such as typhoons. DVM occurs throughout the year, with the maximum migrating speed reaching 9.0 cm s−1. It had some cyclic variation, which was in response to marine environment and astronomical influence. Light initiates the ascents and descents of the migrators, and migration intensity may be proportional to the solar altitude. Neap-spring tidal cycle and full moon phase influences are prominent, and the peak at 112 d may be driven by variation of currents. In addition, 270 m is the demarcation between upward and downward migrations; some organisms living below this layer, especially zooplankton and micronekton, migrate to the surface to obtain nutrients and energy; other marine mesopelagic organisms also survive in this manner. Super and severe typhoons reduced vertical migration, having less influence on the deep scattering layer. As Super Typhoon Rammasun passed by the mooring station, current speed increases and temperature decreases were synchronous with changes in the deep scattering layer; the migrators swam downward to evade the influence of a higher-speed current rather than swam upward to compensate for the decrease of temperature.