Title of thesis: "Structure and functioning of zooplankton communities in the Mediterranean Sea and Tunisian coasts: Contributions of two oceanographic campaigns in spring-summer 2019".
Thesis supervisors: François Carlotti and Marc Pagano, Institut Méditerranéen d'Océanologie (MIO)
Members of the jury :
Dr. Maria Grazia Mazzocchi, Stazione Zooplogica Anton Dohrn, Italy (Rapporteur)
Pr. Habib Ayadi, University of Sfax, Tunisia (Rapporteur)
Dr. Camila Fernández, LOMIC, Observatoire Océanologique de Banyuls, France (Examiner)
Dr. France Van Wambeke, MIO, Aix-Marseille University, France (Chair)
Time and place of presentation: Friday 15 December at 2pm in the amphitheatre
Zoom link: https://univ-amu-fr.zoom.us/j/86950631818?pwd=ZW9ZY3F2NlBKQUppZXM0ZnNaem…
Abstract
This study focuses on the analysis of zooplankton communities in the western Mediterranean Sea (MMO), using data from two campaigns in 2019. The first, MERITE-HIPPOCAMPE, took place in April-May, along a north-south transect from the French to Tunisian coasts, with the aim of providing information on the transfer of contaminants within the planktonic food web. The project focused in particular on collecting plankton samples in the chlorophyll-maximum layer (CMC), as well as at the surface and in the water column, using zooplankton nets of different mesh sizes. MOOSE-GE 19, in June-July, covered most of the northern part of MMO. It was part of an annual observation of physical, chemical and biological parameters aimed at identifying and characterising the ecosystem's responses to climate change and anthropogenic influences.
Our analyses included measurements of zooplankton abundance, biomass, diversity and size distribution. FlowCAM and ZOOSCAN digital imaging tools were used in a complementary manner to cover the size ranges from small zooplankton (60 µm-200µm) to mesozooplankton (> 200 µm). Biovolume, biomass and trophic and metabolic fluxes were derived from allometric relationships. Conventional microscopy was used to obtain finer taxonomic resolution.
The Hippocampe strategy made it possible to compare zooplankton communities sampled by conventional vertical nets in the epipelagic layer (max 200m) with those sampled by horizontal nets at the surface and in the CMC, the latter also being targeted for contaminant studies by the Hippocampe consortium. The samples taken with the horizontal net showed specificities in the communities of these strata: hyponeuston at the surface, organisms aggregated (nauplii, small copepods) or avoiding (appendicular) the heavy detrital accumulations at the CMC. The differences in zooplankton composition between successive size classes considered for contaminant analysis suggest a progressive trophic change from dominant herbivory in the smallest fractions to a more contrasted trophic structure (including carnivory) in the largest fractions, providing relevant information for understanding contaminant flows.
Combining observations from the MOOSE and Hippocampe vertical nets made it possible to assess patterns of zooplankton abundance, community structure and functional importance across the entire NRM. Spatial differences revealed a clear separation at the southern edge of the Balearic front. The decreasing contribution of copepods from north to south is offset by that of gelatinous organisms and crustaceans in the open sea, or that of pteropods and meroplankton on the Tunisian margin.
In the Provençal Basin, the deep central zone, which includes the deep convection zone, has a zooplankton community dominated by herbivorous copepods that is maintained in spring and summer and extends as far as the Ligurian Sea. The periphery of this zone, including the Gulf of Lion shelf accores to the north and the return branch of the MNO cyclonic gyre to the west and south, hosts a more mature community with greater trophic diversity. To the south, stations on the eastern Tunisian margin and in the Algerian basin were relatively similar in terms of zooplankton and environmental variables, linked by Atlantic surface waters. The estimated impact of grazing on phytoplankton was highest in this southern region. Our results suggest that zooplankton could be taken into account in finer regionalisation analyses.
Key words: Metazooplankton, spatial distribution, north-western Mediterranean Sea, link with environmental structures, trophic flows.