Zooplankton diel vertical migration and sediment resuspension detected from CEOF analysis of ADCP backscatter in the western Sea of Okhotsk

• CEOF analysis extracts diel vertical migration of zooplankton from ADCP backscatter. • CEOF method distinguishes between diel migration and sediment resuspension. • We show contrasting seasonal zooplankton behavior on shelf and offshore sites. • In strong tidal regions, the first CEOF mode represents sediment resuspension. • Method enables quantitative comparison of zooplankton, resuspension and environment. Acoustic Doppler Current Profilers (ADCPs) provide backscatter strength data, which can be used to detect suspended materials in the water column. This study developed a novel approach to detect and distinguish between zooplankton diel vertical migration (DVM) and sediment resuspension by applying Complex Empirical Orthogonal Function (CEOF) analysis to 24-hour bandpass-filtered ADCP backscatter data. The temporal CEOF mode scores serve as objective scalar indices of the intensity of each phenomenon, enabling quantitative comparisons with environmental factors. We applied this approach to year-round ADCP array observations in the western Sea of Okhotsk. At most sites, DVM activity is represented by the first CEOF mode. Temporal variations of the first mode revealed that, during the sea-ice season, DVM significantly weakens over shelf regions but persists in offshore regions, even though shelf regions exhibit more active biological productivity than offshore regions during the warm season. This contrasting seasonal behavior likely reflects differences in the dominant zooplankton species and their traits. The quantitative assessment of our method revealed clear relationships with tidal currents through spectral analysis. In the northwestern Okhotsk Sea, where tidal currents are strong, DVM activity, represented by the first mode, was consistently reduced during spring tides. At Kashevarov Bank, where tidal currents exceed 1 m/s, the first mode represents sediment resuspension and intensifies ∼ 1–1.5 days after peak tidal current velocities, and the second mode captures DVM. Given the vast quantity of unused ADCP backscatter data worldwide, the method proposed here can help unlock the potential of these dormant datasets.