21 Nov 2022
 | 21 Nov 2022
Status: this preprint is currently under review for the journal BG.

Seasonal and interannual variability of the pelagic ecosystem and of the organic carbon budget in the Rhodes Gyre (Eastern Mediterranean): influence of winter mixing

Joelle Habib, Caroline Ulses, Claude Estournel, Milad Fakhri, Patrick Marsaleix, Mireille Pujo-Pay, Pascal Conan, Marine Fourrier, Laurent Coppola, Alexandre Mignot, and Laurent Mortier

Abstract. The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic-biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre’s ecosystem and to estimate an annual organic carbon budget over the 2013–2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and BioGeoChemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area with a positive net community production in the upper layer (0–150 m) amounting to 31.2 ± 6.9 g C m-2 year-1. Net community production in the upper layer is almost balanced over the seven-year period by physical transfers, (1) via downward export (16.8 ± 6.2 g C m-2 year-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 g C m-2 year-1). The intermediate layer (150–400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 g C m-2 year-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.

Joelle Habib et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-216', Anonymous Referee #1, 29 Dec 2022
    • AC1: 'Reply on RC1', Joelle Habib, 07 Mar 2023
  • RC2: 'Comment on bg-2022-216', Anonymous Referee #2, 03 Jan 2023
    • AC2: 'Reply on RC2', Joelle Habib, 07 Mar 2023
  • RC3: 'Comment on bg-2022-216', Maurizio Ribera d’Alcalà, 09 Jan 2023
    • AC3: 'Reply on RC3', Joelle Habib, 07 Mar 2023

Joelle Habib et al.

Joelle Habib et al.


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Short summary
The Rhodes Gyre, located in the eastern Mediterranean Sea, is the main Levantine Intermediate Water formation site. In this study, we use a 3D physical-biogeochemical model to investigate the seasonal and interannual variability of organic carbon dynamics in the gyre. Our results show its autotrophic nature and its high interannual variability with enhanced primary production, downward and onward exports to the surrounding regions during years marked by intense heat losses and deep mixed layers.