Acoustic backscatter observation of zooplankton: responses to environmental forcing in the...
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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