EXAMINING ZOOPLANKTON AND SUSPENDED SEDIMENT RESPONSE THROUGH ADCP 614 kHz ECHO DATA IN THE BAHÍA BLANCA ESTUARY (38° 47’ S – 62° 16’ W).

BAQUES, M.; GARIBOTTI, E. J.; ALBERDI, E.; BLANC, S.; PICCOLO M. C.; CERVELLINI, P. M.

Harbin, China

Simposio; 5th International Symposium on Acoustic Engineering and Technology (ISAET); 2010

Laboratory of Underwater Acoustic Technology - College of Underwater Acoustical Engineering of HEU

Acoustic Doppler Current Profilers (ADCP) were originally designed to measure the speed, intensity and direction of currents by transmitting high frequency sound waves and recording the Doppler frequency shift of the returned signal scattered by “drifters” in the water column. Since the ADCP also records the intensity of the backscattered sound waves, which is proportional to the number of backscatters present in the water column, it has also become a highly useful tool for the investigation of the distribution and abundance of zooplankton and suspended sediment concentration in the ocean. In this work we simultaneously present ADCP measurements (WorkHorse RDI, 614 kHz), suspended sediment concentration data (Optical Backscatter Sensor, OBS), zooplankton net samples and oceanographic data (CTD). Measurements were carried out every 3 hours, during 2 tidal cycles, at the southern margin of the main navigation channel of the Bahía Blanca estuary. Absolute values of Acoustic Volume Backscattering Strengths and the corresponding Volume Backscattering Coefficient as function of depth, Sv(z) and σv(z), were computed from the recorded ADCP’s echo intensities, using factory calibration values. Moreover, σv profiles and theoretical modelling of the Acoustic Backscattering Cross Section (σ) led to the computation of Numerical Abundances, N (scatterers/m3), for the two dominant zooplankton species found during the study period, namely, Acartia Tonsa (copepod) and Neomysis Americana (mysid), and for suspended sediment. Comparisons between acoustic and optic determinations of N were performed. A reasonable agreement was obtained for copepods, while no comparison could be made for mysids. ADCP estimation of suspended sediment concentrations showed more accurate results for silt sediments than for fine sandy ones. The estimations obtained may be considered as first encouraging results of simultaneous determinations of zooplankton abundances and suspended sediment concentrations from ADCP measurements. However, further work should be performed in order to improve current results, by means of in situ ADCP calibration and the development of more realistic models for the Acoustic Backscattering Cross Section (σ) of each particular scatterer, responsible of the recorded echo signals.