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| Titel |
Acoustic backscatter observations of zooplankton: responses to environmental forcing in the equatorial Pacific from diurnal to interannual time-scales |
| VerfasserIn |
Marie-Hélène Radenac, Patricia E. Plimpton, Anne Lebourges-Dhaussy, Michael J. McPhaden |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048410
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| Zusammenfassung |
| We use several (1996-2004) records of mean volume backscattering strength (Sv) derived
from 150Â kHz Acoustic Doppler Current Profilers (ADCPs) moored along the equator to
retrieve qualitative information about the vertical distribution of zooplankton in upwelling
mesotrophic conditions and in the warm pool oligotrophic ecosystem of the Pacific Ocean.
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 derived
from the tropical Pacific mooring array.
The characteristic alternate pattern of low and high backscattering during day and night is
observed in both ecosystems. In the upwelling, nighttime Sv is high above the middle of the
thermocline and decreases below while a subsurface Sv maximum is observed in the upper
part of the thermocline in oligotrophic conditions. Daytime Sv is more homogeneously
distributed over the observed layer than nighttime Sv. Nighttime and daytime Sv are higher in
mesotrophic than in oligotrophic ecosystems. Therefore, the eastern edge of the warm pool
that separates warm and fresh water of the warm pool from cool and saltier water of the
upwelling is also a transition in biomass and composition of zooplankton species. Our long
time-series allows describing the evolution of migratory patterns during entire lunar
cycles. Organisms sink when the moonlight is bright leading to low surface Sv and a
deepening of the high Sv layer while surface Sv is high during darkness hours. The
duration of observation of high surface Sv is related to the duration of deep night
between sunset and moonrise or between moonset and sunrise. At intra-seasonal time
scales, downwelling Kelvin waves that depress the thermocline also deepen the
subsurface Sv maximum in oligotrophic conditions and the lower limit of the high
Sv layer in mesotrophic conditions. In the same way, at interannual time scales,
variations of the thermocline depth related to El Niño/La Niña phases correlate with
variations of the base of the high Sv layer and of the depth of the subsurface Sv
maximum. Such a behavior can be the consequence of stratification and temperature
changes. Also, it can be related to food availability because the vertical structure of
chlorophyll is closely linked to stratification in the equatorial Pacific. In the western basin
at the 165Ë E site, our results suggest the influence of moderately mesotrophic
waters that would be advected from the western warm pool during westerly wind
events.
Despite their limitations, long time-series of ADCP backscattering signal in the tropical
Pacific offer a potentially valuable complement to more traditional approaches of studying
the poorly documented mid-trophic levels and population dynamics. In particular, it could
help validating the mid-trophic level simulated by population dynamics models. |
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