Biological oceanography of the Gulf Stream front: Inference from acoustic remote sensing

Abstract

Acoustics is used to determine the relationship between current flow, temperature structure, and biological processes at fronts. In 1985 surveys were made of acoustic volume reverberation at the boundary of the slope water and the Gulf Stream, 120 km northeast of Cape Hatteras, using towed 70-kHz sonars. Scattering theory and net samples suggest the scatterers were macrozooplankton of about 2-cm length. Two-dimensional images of 200-m depth by several kilometers distance give views of internal biological patches across the front. From them the spatial arrangement and behavior of animals have been observed and animal type inferred. For large-scale surveys covering several hundred kilometers and days, obtained during April 1989 (BioSYNOP program), a synoptic view, from which the influence of large meanders of the Gulf Stream on the macrozooplankton can be observed, was obtained. Finally, better resolution of animal type, function, and size is possible with multifrequency sonar. Multicolor imaging of 38-, 70-, and 200-kHz sonar profiles suggest a difference in the animal size composition between the Gulf Stream and adjacent waters. [Work supported by ONR and NSERC of Canada.]