5 Conclusion
Our observations of the seasonal particulate TE export in a productive
region of the Southern Ocean have revealed that the identification of
the carrier phases is critical for our understanding of the export
dynamics of individual TE. The lithogenic and biological carrier phases
identified in our study had distinct temporal patterns. Basalt
particles, the main lithogenic carrier phase dominated the export flux
early in the season and strongly decreased over time, reflected in the
particulate export pattern of TE representative of lithogenic matter
(Ti, Cr, Zr, Y, Th and Al) and of TE with a defined biological role (Mn,
Co and Fe). The biological carrier phases, diatom vegetative cells and
spores, revealed two pulsed export events, while vertical transportvia fecal pellets remained stable over time. TE with known
biological functions (Cd, Ba, Mo, Cu, Ni and V) were associated with one
or both of these main export events.
A further look into the seasonal variability of stocks of bioavailable
TE is necessary to better understand how these influence the
phytoplankton assemblage, inherent enzyme strategies, and subsequent TE
utilisation and exports. Finally, future studies should investigate TE
composition of individual fecal pellets produced by different
zooplankton species feeding on distinct food sources. This could provide
insight to help decipher the contribution of each zooplankton species to
TE export.
Acknowledgments, Samples, and Data
We thank the captains and the crew of the R/V Marion Dufresne for their
support during the two cruises. We thank E. de Saint Léger, F. Pérault
from DT-INSU, and people of IPEV (Institut Polaire Paul Emile Victor)
for the technical support during preparation, deployment and recovery of
moorings. We thank Nathalie Leblond (Laboratoire Océanographie de
Villefranche sur mer) for processing the samples and performing chemical
analysis. We thank Mathieu Rembauville for his help during deployment of
the clean traps. We thank the anonymous reviewers and the associated
editor for their careful reading of the manuscript and for their
comments and suggestions that have improved of our manuscript. This work
is part of the project SOCLIM supported by the Climate Initiative of the
foundation BNP Paribas, the French research program LEFE-CYBER of
INSU-CNRS, IPEV, Sorbonne Université, and the Flotte Océanographique
Française. This work was also supported by the project SEATRAK funded by
the French research program LEFE-CYBER. The authors declare no conflict
of interest. Data are available at the SEANOE database
https://www.seanoe.org/data/00606/71768/
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Figure captions :
Figure 1: Kerguelen plateau bloom . a) Monthly composite of
chlorophyll surface concentration (mg m-3) for
November 2016. The white dot denotes the location of the sediment trap
mooring. b) Seasonal variation of chlorophyll (mg
m-3). The blue line corresponds to the 8-day composite
chlorophyll concentrations over the season of sediment trap deployment.
The green line and light green area represent the climatology and
standard deviation respectively. The white rectangles along the x axis
denote the 12 periods of sediment trap collection.
Figure 2 : Physical environment of the sediment trap .
For all panels, the grey line shows the raw data acquired every 30
minutes. The black line denotes the running average with a time window
of 26 hours. A) depth of the sediment trap, B) Inclination angle of the
sediment trap (vertical reference = 0°), C) current speed measured 3 m
below the sediment trap. D) Current direction and intensity.
Figure 3 : Export fluxes of biological vectors . Each
panel shows the seasonal variations of the export flux of the parameter
indicated in the upper left corner. Within each panel, vertical bars
represent the export fluxes determined in the 12 cups.
POCdiat is the flux associated with diatoms,
POCspore is the flux associated with diatom spores,
POCveg is flux associated with diatom vegetative cells
and POCfp is the flux associated with fecal pellets.
Figure 4 : PCA correlation biplots of biological
fluxes . Black dots denote the cups associated with their labels from 1
to 12 (1 corresponds to the first cup collected). Blue arrows represent
the projection of the descriptors into the two first principal component
plan (for clarity their lengths were multiplied by 2). The definition of
arrow labels are Ctot (flux of total POC),
Ntot (flux of total PON), Cveg (flux of
POC associated with vegetative diatoms), Cspore (flux of
POC from diatom spores), Cfp (flux of POC from fecal
pellets), CaCO3 (flux of CaCO3), m (flux
of total particle mass) and BSi (flux of biogenic silica).
Figure 5 : Export fluxes of phosphorus and 11 trace
elements . Each individual panel shows the seasonal variability of the
export flux of the element with its unit indicated in the left upper
corner. Within each panel, vertical bars represent the export fluxes
collected in the 12 cups and the vertical lines show the standard
deviation based of analytical precision.
Figure 6: PCA correlation biplots of trace elements:Black dots denote the cups associated with their labels from 1 to 12 (1
corresponds to the first cup collected). Blue arrows represent the
projection of the descriptors into the two first principal component
plan (for clarity their lengths were multiplied by 2).
Figure 7: Residual export of trace elements. Each individual
panel shows the seasonal variability of the residual export flux Fxs
(see text for definition) of the element with its unit indicated in the
left upper corner.
Figure 8: Partitioning of total trace element export fluxes
between different carrier phases. The plot presents the projections of
both predictors (in black) and descriptors (in blue) in a 3-dimensional
space formed by the 3 first latent variables resulting from PLSR
analysis which explained 57.5%, 22.1% and 8.6% of the covariance.