Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
5
2
2008
10.5194/bg-5-299-2008
http://www.biogeosciences.net/5/299/2008/
http://www.biogeosciences.net/5/299/2008/bg-5-299-2008.html
http://www.biogeosciences.net/5/299/2008/bg-5-299-2008.pdf
299
310
2008-03-03
Volume distribution for particles between 3.5 to 2000 μm in the upper 200 m region of the South Pacific Gyre
L. Stemmann
stemmann@obs-vlfr.fr
D. Eloire
A. Sciandra
G. A. Jackson
L. Guidi
M. Picheral
G. Gorsky
Laboratoire d'Océanographie de Villefranche (LOV), Observatoire Océanologique, BP 28, 06234 Villefranche sur mer Cedex, France
Université Pierre et Marie Curie-Paris6, UMR 7093, Villefranche sur Mer, 06234 France, Laboratoire d'Océanographie de Villefranche (LOV), Observatoire Océanologique, BP 28, 06234 Villefranche sur mer Cedex, France
Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK
Department of Oceanography, Texas A&M University, College Station, TX 77843, USA
The French JGOFS BIOSOPE cruise crossed the South Pacific Gyre (SPG) on a
transect between the Marquesas Islands and the Chilean coast on a 7500 km
transect (8° S–34° S and 8° W–72° W). The number and volume
distributions of small (3.5<<i>d</i><30 μm) and large particles
(<i>d</i>>100 μm) were analysed combining two instruments, the HIAC/Royco
Counter (for the small particles) and the Underwater Video Profiler (UVP,
for the large particles). For the HIAC analysis, samples were collected from
12 L CTD Rosette bottles and immediately analysed on board while the UVP
provided an estimate of in situ particle concentrations and size in a continuous
profile. Out of 76 continuous UVP and 117 discrete HIAC vertical profiles,
25 had both sets of measurements, mostly at a site close to the Marquesas
Islands (site MAR) and one in the center of the gyre (site GYR). At GYR, the
particle number spectra from few μm to few mm were fit with power
relationships having slopes close to −4. At MAR, the high abundance of large
objects, probably living organisms, created a shift in the full size spectra
of particles such that a single slope was not appropriate. The small
particle pool at both sites showed a diel pattern while the large did not, implying
that the movement of mass toward the large particles does not take place at
daily scale in the SPG area. Despite the relatively simple nature of the
number spectra, the volume spectra were more variable because what were
small deviations from the straight line in a log-log plot were large
variations in the volume estimates. In addition, the mass estimates from the
size spectra are very sensitive to crucial parameters such as the fractal
dimension and the POC/Dry Weight ratio. Using consistent values for these
parameters, we show that the volume of large particles can equal the volume
of the smaller particles. However the proportion of material in large
particles decreased from the mesotrophic conditions at the border of the SPG
to the ultra-oligotrophy of the center in the upper 200 m depth. We expect
large particles to play a major role in the trophic interaction in the upper
waters of the South Pacific Gyre.
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