Diurnal Signals in Vertical Motions on the Hebridean Shelf

In this paper, we consider the interpretaion of measurement of vertical velocity and backscatter and backscatter signal intensity, made using an acoustic doppler current profiler (ADCP) mounted on the seabed, at a position on the Hebridean Continental Shelf. The existence of vertically migrating scatterers is inferred from both the backscatter signal intensity and vertical velocity data, which indicate migration rates of 2-3 cm s-1, and shows consistency between the displacement deduced from the vertical velocity and the observed movement of scatterers from the near bed to the near-surface regions, evident in temporal variations in the backscatter signal intensity. Independent evidence that the observed vertical velocities are largely due to movements of the acoustic scatterers relative to the water is obtained through comparison of the ADCP vertical velocity data with vertical velocities inferred from the movement of the thermocline. The close phase locking between the motion and sunset and sunrise times has led us to interpret the signal in terms of the diel vertical migration of zooplankton. Although no biological samples were collected during this study, previous surveys of the area have identified species of copepod and euphausiid that are known to migrate. The 12 d of data available show an initial period, when there was a strong vertical migration signal, and a later period, during which the signal was not clear. By combining the observed vertical and horizontal velocity components in a particle tracking model, it is demonstrated that the change in the migration signal may be a result of the advection of the patch of migrating zooplankton away from the ADCP, during the later part of the observational period.