Acoustic study of the Río de la Plata estuarine front

During December 1999, a transect 260 km long was carried out along the major axis of the Rio de la Plata estuary. Acoustic (120 kHz), oceanographic, and zooplankton sampling were performed to extract a continuous, quasi-synoptic acoustic view of the estuary's salt wedge. Three different zones were identified. The riverine and marine zones were characterized by vertically homogeneous water and low volume-backscattering strength (S v ). The mixohaline zone was characterized by a strong vertical salinity gradient and the highest S v values. There were no relationships between the reflection value calculated from the salinity gradient and suspended matter (sediments) and measured S v . The greatest abundance of acoustically detectable organisms was at the halocline depth, coincident with the echo recordings. The positions of the halocline and the two fronts were determined on the basis of aggregations of the different planktonic groups. The physical presence of the salt wedge, as shown by a “biological wedge”, was well described by the acoustic technique employed.