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.