1. acoustic backscatter observation of zooplankton: responses to environmental forcing in the equatorial Pacific from diurnal to interannual timescales M.-H. Radenac (LEGOS/IRD, France), P. E. Plimpton (PMEL, USA), A. Lebourges-Dhaussy (IRD, France), M. J. McPhaden (PMEL, USA) major El Niño - Dec. 1997 El Niño - Oct. 2002 non El Niño - May 2001 surface chlorophyll (mg m -3 ) 165°E 170°W 140°W  equatorial upwelling: mesotrophic ecosystem (chl  0.2 mg m -3 )  warm pool: oligotrophic ecosystem (chl < 0.1 mg m -3 )  west: moderate mesotrophic ecosystem (chl > 0.1 mg m -3 )

2. RD Instruments, 1990 transducer characteristics beam spreading and absorption scatterers the TAO/TRITON mooring array ATLAS  meteo  T(z), S(z) ADCP  U(z), V(z)  E  153.6 kHz RDI narrowband ADCP; upward-looking configuration   t = 1 hour;  z = 5 m  daytime Sv at noon  2 hours nighttime Sv at midnight  2 hours

3. two ecosystems S v night S v day T S v night S v day T oligotrophic ecosystem (warm pool) mesotrophic ecosystem (upwelling) depth Sv night (dB) 170°W, 0° Z 29°C Z 20°C Z 29°C = depth of the warm layerZ 20°C = depth of the middle of the thermocline diurnal cycles

4. lunar cycle 170°W, 0° Sv night (dB) depth September 2003 moon cycle darknessmoon light S v (dB)

5. influence of stratification/food availability 140°W, 0° depth Sv night (dB) Z 29°C Z 20°C downwelling Kelvin waves El NiñoLa Niña depth chlorophyll (mg m -3 ) warm poolupwelling

6. influence of westerly winds in the west 165°E, 0° Sv night (dB)U wind (m s -1 )165°E,0° longitude depth 165°E, 0° Sv night (dB) 165°E, 0° zonal current (m s -1 ) equator surface chlorophyll (mg m -3 ) Z 29°C Z 20°C surface chl 11-18 December 2002

7. conclusion 1st attempt to use long S v time-series in the equatorial Pacific biological and physical information at the same timescale different diurnal migrating patterns in the oligotrophic warm pool and in the mesotrophic upwelling influence of light, stratification and/or food availability, horizontal advection what do we see? at 150 kHz ⇒ targets > 1 cm (mesozooplankton and micronekton) literature review biomass changes different species composition useful for model evaluation? simulated biomass of mid-trophic organisms vs. S v at 170°W Lehodey et al., 2009