acoustic backscatter observation of zooplankton:responses to environmental forcing in the equatorial

Pacificfrom 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°E170°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)

RD Instruments, 1990 transducer characteristics

beam spreading and absorption

scatterers

1

220 )273(1047,4

log10KPc

TKKdBS XS

v

RR 2log20

110log10 10/).( rC EEK

the TAO/TRITON mooring arraythe 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

two ecosystemstwo ecosystems

Sv nightSv day

T

Sv nightSv day

T

oligotrophic ecosystem (warm pool) mesotrophic ecosystem (upwelling)

depth

Sv night (dB)

170°W, 0°

Z29°C

Z20°C

Z29°C = depth of the warm layer Z20°C = depth of the middle of the thermocline

diurnal cycles

lunar cycle 170°W, 0°lunar cycle 170°W, 0°Sv night (dB)

depth

September 2003 moon cycle darkness moon light

Sv (

dB

)

influence of stratification/food availability 140°W, 0°influence of stratification/food availability 140°W, 0°depth

Sv night (dB)

Z29°C

Z20°C

downwelling Kelvin waves

El Niño

La Niña

depth

chlorophyll (mg m-3)

warm pool upwelling

influence of westerly winds in the west 165°E, 0°influence of westerly winds in the west 165°E, 0°

Sv night (dB) U wind (m s-1)165°E,0°

longit

ude

depth

depth

165°E, 0° Sv night (dB)

165°E, 0° zonal current (m s-1)

equator surface chlorophyll (mg m-3)

Z29°C

Z20°C

surface chl 11-18 December 2002

conclusionconclusion

• 1st attempt to use long Sv 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. Sv at 170°W

Lehodey et al., 2009