Formation en télédétection et géomatique et réinsertion des diplômés dans leur milieu professionnel : le cas de l'Afrique francophone

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Modeling analysis of the effect of iron enrichment on dimethyl sulfide dynamics in the NE Pacific (SERIES experiment)

The large-scale iron enrichment conducted in the NE Pacific during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES) triggered a phytoplankton bloom dominated successively by nanophytoplankton and large diatoms. During the first 14 days, surface dimethyl sulfide (DMS) levels increased both inside (up to 22 nmol L-1) and outside (up to 19 nmol L-1) the patch, with no consistent Fe effect. Later, DMS concentrations became sixfold lower inside the patch than outside. In this study, we used a DMS budget module embedded in a one-dimensional ocean turbulence model to investigate the contribution of the interacting physical, photochemical, and biological processes to this particular DMS response. Temporal variations in biological net DMS production were reconstructed using an inverse modeling approach. Our results show that short-term (days) variations in both the physical processes (i.e., turbulent mixing and ventilation) and the biological cycling of DMS are needed to explain the time evolution of DMS concentrations both outside and inside the Fe-enriched patch. The biological net DMS production was generally high (up to 0.35 nmol L-1 h-1) and comparable outside and inside the patch during the first 10 days, corresponding to the observed accumulation of DMS inside and outside the patch. Later, it became negative (net DMS biological consumption) inside the patch, suggesting a change in dimethylsulfoniopropionate bacterial metabolism. This study stresses the importance of short-term variations in biological processes and their sensitivity to the physical environment in shaping the DMS response to iron enrichment

Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep-water formation region

We used a physical-biogeochemical model to examine the sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a subpolar, deep-water formation region. Documented changes in winter mixed layer depth between the late 1960s and the mid-1990s were used to construct scenarios of weak, moderate, and strong winter convection that drive the biogeochemical model. The model simulations suggest that the total biogenic carbon export (particle sinking flux + DOC export) is higher under strong winter convection (e.g., during the early 1990s) than under weak winter convection (e.g., during the late 1960s), by ∼70% across the 200-m isobath and nearly double at 500 m and 1000 m depth. These large variations in total biogenic carbon export are essentially due to the response of DOC export to ocean climate conditions. Sensitivity analyses indicate that the variations in DOC export from the euphotic zone are due to the impact of the convection regime on the development of the microbial food web and on the bacterial consumption of DOC in surface waters. Although DOC downward fluxes within the mesopelagic zone (below ∼500 m) are largely controlled by physical processes, the effect of convection on microbial dynamics can potentially amplify the year-to-year variations in the transport of DOC to the deep ocean due to convection