Acoustic phenomena related to an enriched benthic zone in the North Sea

In late summer and autumn 1985 and 1986, acoustic recordings were made with a 38 kHz echo-sounder in a series of transects overlying the hydrographical front area south of the Oyster Grounds (North Sea). The echograms revealed a narrow band of increased backscattering, which seemed related to an enriched benthic zone, located underneath this band at depths of 30–40 m below the sea surface. This phenomenon is a permanent feature at this time of year, caused by locally high concentrations of larger zooplanktonic and nektonic organisms. The peak in backscattering showed a 6 h periodical shift in a north-south direction as a result of tidal movements.