Budget of organic carbon in the North‐Western Mediterranean open sea over the period 2004–2008 using 3‐D coupled physical‐biogeochemical modeling

Abstract

A 3‐D hydrodynamic‐biogeochemical coupled model has been used to estimate a budget of organic carbon and its interannual variability over the 5 year period 2004–2008 in the North‐Western Mediterranean Open Sea (NWMOS). The comparison of its results with in situ and satellite observations reveals that the timing and the magnitude of the convection and bloom processes during the study period, marked by contrasted atmospheric conditions, are reasonably well reproduced by the model. Model outputs show that the amount of nutrients annually injected into the surface layer is clearly linked to the intensity of the events of winter convection. During cold winters, primary production is reduced by intense mixing events but then spectacularly increases when the water column restratifies. In contrast, during mild winters, the primary production progressively and continuously increases, sustained by moderate new production followed by regenerated production. Overall, interannual variability in the annual primary production is low. The export in subsurface and at middepth is however affected by the intensity of the convection process, with annual values twice as high during cold winters than during mild winters. Finally, the estimation of a global budget of organic carbon reveals that the NWMOS acts as a sink for the shallower areas and as a source for the Algerian and Balearic subbasins.

Vertical Mixing Effects on Phytoplankton Dynamics and Organic Carbon Export in the Western Mediterranean Sea

“…Although some of the exported material will be injected back into the surface layer during the next convection event, a consequence of deep circulation is horizontal transport of organic matter toward the Algerian basin. In this context, our results are in agreement with the quantification of Ulses et al (), who found the transfer to the Algerian subbasin deep waters (100 m seabed) was equivalent to 53% of the OC exported below 100 m in the convective region (337 × 10 4 tC yr −1 ). An additional 165 × 10 4 tC yr −1 was conveyed in the first 100 m. These advective processes are thought to create a coupling between the three regions, and their consequences, notably on carbon sequestration deserve more attention.…”

“…Observations of the DeWEx Leg1 campaign in February 2013 showed the presence of phytoplankton at depths > 2,000 m, at concentrations of 0.16 mg m −3 (Kessouri et al, ). These observations suggest that vertical mixing and advection are responsible for the presence of organic carbon, including light cells and detritus, in deep layers, as observed by Giering et al () in the Iceland Basin, and modeled by Ulses et al () in the northwestern Mediterranean. It is likely that a fraction of the exported matter observed in deep waters during the winter of 2012/2013 was permanently trapped when mixing stopped.…”

“…Concomitant with this loss in the Deep Convection region, a gain of OC in the Stratified region suggests that the OC that is transferred to deep waters by convective events in the northern region could be transported to the southern region. This is consistent with the results of Ulses et al () who calculated a north‐south transfer corresponding to 53% of the OC exported below 100 m, based on the same coupled model. Moreover, this north‐south transfer of OC in deep waters was suggested by Zúñiga et al () who reported higher POC fluxes at 2,145 m than at 1,440 m, based on sediment trap measurements.…”

“…The Eco3M‐S model is a multinutrient and multiplankton functional type model that simulates the dynamics of the biogeochemical decoupled cycles of several biogenic elements (carbon, nitrogen, phosphorus, and silicon), and of non‐Redfieldian plankton groups. The model structure used in this study has been previously described in Ulses et al (), Auger et al (), and Herrmann et al (). Most parameter values are based on the study of Ulses et al (), but a new calibration was carried out for some parameters using several observational data sets (Kessouri et al, ).…”

Long-term monitoring of ocean deep convection using multisensors altimetry and ocean color satellite data

“…[]. This coupled hydrodynamical‐biogeochemical tool was also used to study the impact of interannual variability and long‐term evolution of atmospheric and oceanic conditions, in particular deep convection, on the NWMS pelagic planktonic ecosystem and associated carbon cycle [ Herrmann et al ., ; Ulses et al ., ]. Those studies showed that our coupled model represents realistically NWMS ocean dynamics, in particular deep convection, as well as the interactions between dynamics and biogeochemistry.…”