Diel, lunar and seasonal vertical migration in the deep western Gulf of Mexico evidenced from a long-term data series of acoustic backscatter

The pattern of zooplankton migration has been investigated in the water column from 120 to 1300 m depth using the backscatter strength signal (Sv) provided by Acoustic Doppler Current Profilers (operating at 300 kHz and 76.8 kHz), from a 10-year high-frequency mooring time series (2008–2018) in the deepwater region of the western Gulf of Mexico (GoM). This analysis was complemented with in-situ thermohaline data, model derived data, as well as one-month deployment glider-derived oceanographic information. The overall Sv time series presents a marked circadian cycle at 12 and 24 h associated with the well-known Diel Vertical Migration (DVM) motion performed by the zooplankton community. The signal analysis reveals the existence of two main layers (a shallow layer between the surface to about 100 m and an intermediate one from 400 m to 600 m), where the abundance of scatterers is the highest. Both layers exhibit a seasonal -but different- modulation of the DVM, with peaks in backscatter ...