Chkili O. auteur aut IRD Meddeb M. auteur aut IRD Kousri K. M. auteur aut IRD Garali S. M. B. auteur aut IRD Belkhahia N. M. auteur aut IRD Tedetti Marc auteur aut IRD Pagano Marc auteur aut IRD Zouari A. B. auteur aut IRD Belhassen M. auteur aut IRD Niquil N. auteur aut IRD Hlaili A. S. auteur aut IRD text journalArticle eng text/pdf born digital access We assessed the spatial variability in the size structure of phytoplankton, community composition, primary production and carbon fluxes through the planktonic food web of the Gulf of Gabes (GG; Southeastern Mediterranean Sea) in the fall of 2017 during the MERMEX-MERITE cruise. High concentrations in nutrients, chlorophyll a (similar to 2-6 mu g L-1) and primary production (1816-3674 mg C m(-2) d(-1)) revealed an eutrophic status of the studied stations in the GG. In accordance with hydrodynamic features, inorganic nutrients showed increases in concentrations from North to South and from coast to offshore, these nutrient gradients impacting the spatial distribution of phytoplankton community. Size-fractioned phytoplankton biomass and production were the lowest in the northernmost zone where they were mainly sustained by pico-sized fraction. Concomitantly, in this area, small aloricate ciliates were dominant leading to a high microbivory. Conversely, higher biomass and production were measured towards the South and offshore with prevalence of larger phytoplankton (nano- and/or micro-sized fractions) supported by diatoms. The herbivorous protozooplankton and metazooplankton were more abundant in these zones, resulting in an increase of the herbivory. The vertical particulate organic carbon flux followed also a north-south and coast-offshore increasing gradient, with a higher contribution of phytoplankton, and zooplankton fecal pellets to the sinking organic matter in the southernmost area. Our results suggest that even in nutrient-rich and highly productive waters, a continuum of trophic pathways, ranging from microbial to multivorous and herbivorous food webs, may exist, which implies different efficiencies in carbon export and carrying capacity within the ecosystem. specialized Phytoplankton size-structure Primary production Zooplankton grazing Planktonic food web Mediterranean gulf MEDITERRANEE TUNISIE 034 036 58 1 6 [23 ] 2023 1738-5261 https://www.documentation.ird.fr/hor/fdi:010086858 10.1007/s12601-023-00101-6 1738-5261 [F B010086858] https://www.documentation.ird.fr/hor/fdi:010086858 https://www.documentation.ird.fr/intranet/publi/2023-03/010086858.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2023-03-08 2025-02-24 fdi:010086858 fre