Acoustic backscatter observations of zooplankton abundance and behaviour and the influence of oceanic fronts in the northeast Atlantic

Ian P. Wade, Karen J. Heywood

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Acoustic Doppler Current Profiler-derived acoustic backscatter data were compared with physical and biological data collected during leg 2 of the PRIME cruise in the northeastern Atlantic during July 1996. The leg 2 transect from 59 to 37°N ran approximately along the 20°W meridian and crossed a range of hydrographic conditions from near subarctic in the north to subtropical in the south. A front at 52.5°N was an extension of a southern branch of the North Atlantic Current system. A stronger frontal feature was observed at 48°N and was considered to be the northern limit of Eastern North Atlantic Water that had been ventilated at the eastern boundary. At 37°N a Lagrangian time-series study was performed. High acoustic backscatter was observed within the PRIME eddy at 59°N and at the southern site. Other changes in backscatter intensity, or more correctly, in 'mean volume backscattering strength' (MVBS), coincided with two main fronts crossed during the transect and upon arrival at the southern site. An increased MVBS in the surface 100m in the north was not present south of 52.5°N. Between 48 and 37°N the MVBS approached its lowest of the transect. At 37°N marked changes in the backscatter were observed, with a strong signal seen in the top 75 m. As well as changes in the MVBS there were differences in the diel migratory patterns observed in the backscatter. Scattering at 37°N was concentrated above a strong pycnocline at 75 m. In the north the scattering signal was generally noisier and less well defined than in the south. Biomass has been estimated from the MVBS by comparing it with biomass estimates derived from Optical Plankton Counter data and carbon/nitrogen measurements. Although such calibrations have their limitations, the results give important information concerning net sampling strategies and are a good test of the representativeness of such sampling.
Original languageEnglish
Pages (from-to)899-924
Number of pages26
JournalDeep-Sea Research Part II: Topical Studies in Oceanography
Issue number4-5
Publication statusPublished - 2001

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