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 at ~ 200 m in winter but at ~ 400 m depth in summer; with no seasonal variability below 800 m. The migration pattern differed between depths, and relative to density, dissolved oxygen concentration, mixed layer depth and net primary production. Density plays a major role in the upper 600 m limiting the depth of the different migration patterns, whereas oxygen concentration reveals to be the best single predictor of resident depth of non-migrating species in the deeper layer. A relationship was found between the migration patterns and the moon and sun position, since the migration patterns were finely tuned to the timing of sunrise and sunset. The shallowest group (0-500 m) begins to descend about two hours before sunrise, and starts to rise about one hour before sunset. The amount of light penetrating the ocean when the sun altitude is greater than 35° seems to be the triggering mechanism of DVM for the mesopelagic species in the GoM. A wavelet analysis applied to the backscatter signal reveals a close relation between migratory patterns and the moon's periodicity in the layer 1000-1200 m, rather than in the upper water column. Based on previous studies of the biological community of the deepwater region of the GoM, the migration patterns likely reflect the presence of a seasonally-varying community of scatterers in the surface layer, partially-migratory and strongly migratory taxa in the mesopelagic layer, resident taxa associated with the minimum oxygen layer, and a deeper community which may migrate in response to the lunar cycle.