As a main trophic link between primary producers and larger predators, zooplankton plays an important role in ocean ecosystems and material cycles. In addition, it contributes to the transport of carbon and nitrogen to the deep sea via the production of fecal pellets and active transport by diel vertical migration (DVM). Early studies about the zooplankton biomass mainly relied on ship-based net sampling methods, which provide ground truth data but have spatial and temporal limitations. Acoustic backscattering strength data obtained from acoustic Doppler current profilers (ADCPs) have higher temporal and vertical resolution over a long duration. As one of the largest marginal seas in the western North Pacific, bounded by China, Korea, and Japan, the East China Sea (ECS) is a highly productive ocean area in the world, with plenty of zooplankton resources. A number of zooplankton studies have been done in this area; however, the long-term zooplankton seasonal variabilities and the DVM characteristics still lack investigation.

In this study, we investigated seasonal variations in DVM-related acoustic backscatter using upward-looking RDI 307.2 kHz ADCP observations from three mooring stations located along the ECS shelf break. The results show that DVM-related backscatter signals exhibit clear seasonal variability at the three stations. The diel signal was generally weak from winter to early spring, whereas stronger and more organized signals developed from late spring to summer and persisted into autumn. The intensity and vertical structure of the DVM-related signals differed among stations, indicating station-dependent variability in the seasonal development of vertical migration. Relative vertical velocity further showed downward movement around sunrise and upward movement around sunset, suggesting that the timing of migration is closely linked to the seasonal progression of the light cycle. These findings demonstrate that long-term ADCP backscatter observations provide a useful framework for examining seasonal DVM variability at the ECS shelf break and suggest that both regional seasonality and local station-dependent conditions influence the structure and intensity of DVM-related signals.
 

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