The accuracy of acoustic vertical velocity measurements: instrument biases and the effect of Zooplankton migration

Doppler velocity and backscatter intensity records from an energetic estuarine flow in which backscatter reveal a significant diel migration of zooplankton are used to examine the reliability of the vertical velocity measurements. During periods when zooplankton were not migrating, the measured value is shown to be an accurate representation of the actual vertical velocity. At depths and times of active migration, with large increases in backscatter intensity, it is expected that vertical velocity measurements should be strongly affected by zooplankton motion. This does not appear to be the case, however, as the effect is much smaller than expected at sunset, and non-existent at sunrise.