Influence of Nutrient Gradient on Phytoplankton Size Structure, Primary Production and Carbon Transfer Pathway in a Highly Productive Area (SE Mediterranean)

Chkili, O., Meddeb, M., Mejri Kousri, K. et al. Influence of Nutrient Gradient on Phytoplankton Size Structure, Primary Production and Carbon Transfer Pathway in a Highly Productive Area (SE Mediterranean). Ocean Sci. J. 58, 6 (2023). https://doi.org/10.1007/s12601-023-00101-6

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 Gabès (GG; Southeastern Mediterranean Sea) in the fall of 2017 during the MERMEX-MERITE cruise. High concentrations in nutrients, chlorophyll a (~ 2–6 µg 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 microsized 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.

BOREA contact: Oumayma Chkili  oumayma.chkili@unicaen.fr / chkili.oumayma94@gmail.com

Picture title:

Primary production (PP, mg C m-2 d-1), trophic relationships and carbon transfer pathways within the planktonic systems of the sampling stations in link with the nutrient spatial gradient and hydrodynamic circulation in the Gulf of Gabès during the fall 2017. Percentage contributions of phytoplankton size fractions to PP are indicated. Values with arrows show the amount of channeled biogenic carbon (mg C m-2 d-1) and percentages represent the contribution of zooplankton microbivory or herbivory to carbon transfer. Width of arrow is proportional to the carbon flow. Microbivory = consumption of Pico by Protozoo; Herbivory = consumption of Nano- and Micro by Protozoo and Metazoo; carbon transfer = consumption of Pico, Nano and Micro by Protozoo and Metazoo. © Oumayma Chkili