Abstract
The acoustic return signal from Acoustic Doppler Current Profilers (ADCP) moored in Traverse Bay during a 90-day summer period showed a rapid 5-10 dB increase about 1/2 hour after sunset and a similar decrease 1/2 hour before sunrise. The pattern is characteristic of zooplankton diel vertical migration, most likely Mysis relicta. These are the first reported observations of freshwater invertibrate migrations using ADCP backscatter. A 15–20 m thick sound scattering layer also persisted throughout the summer. This layer, constrained between the 6°C and 10°C isotherms, generally followed the internal thermocline fluctuations. These backscatter data demonstrate that determining characteristics of diel migration, and monitoring zooplankton temporal and spatial variability are possible using ADCPs.
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