Impact of environmental forcing on the acoustic backscattering strength in the equatorial Pacific : diurnal, lunar, intraseasonal, and interannual variability

Abstract : We analyzed several records of mean volume backscattering strength (S-v) derived from 150 kHz acoustic doppler current profilers (ADCPs) moored along the equator in upwelling mesotrophic conditions and in the warm pool oligotrophic ecosystem of the Pacific Ocean. The ADCPs allow for gathering long time-series of non-intrusive information about zooplankton and micronekton at the same spatial and temporal scales as physical observations. High S-v are found from the surface to the middle of the thermocline between dusk and dawn in the mesotrophic regime. Biological and physical influences modified this classical diel cycle. In oligotrophic conditions observed at 170 degrees W and 140 degrees W during El Nino years, a subsurface S-v maximum characterized nighttime S-v profiles. Variations of the thermocline depth correlated with variations of the base of the high S-v layer and the subsurface maximum closely tracked the thermocline depth from intraseasonal to interannual time-scales. A recurring deepening (20-60 m) of the high S-v layer was observed at a frequency close to the lunar cycle frequency. At 165 degrees E, high day-to-day variations prevailed and our results suggest the influence of moderately mesotrophic waters that would be advected from the western warm pool during westerly wind events. A review of the literature suggests that S-v variations may result from changes in biomass and species assemblages among which myctophids and euphausiids would be the most likely scatterers.