Guillermo Feliu (EMBIO) will defend his thesis in person and by videoconference on : Tuesday 2 March 2021 at 14:00 (OCEANOMED Amphitheatre)

On the following topic: "Study of the structure of the mesozooplankton community and its trophic role in the Mediterranean Sea during the two large-scale oceanographic campaigns PEACETIME and MOOSE in 2017"

 

Mesozooplankton (multicellular heterotrophic organisms 0.2-20mm in size) are a key compartment of marine pelagic ecosystems as consumers of primary producers and microzooplankton, and as a food source for fish and larger planktivorous organisms. In this thesis, we sought the best combination of sampling nets and sample analysis techniques to deliver the most comprehensive estimate of abundance, size structure and associated trophic and metabolic rates. We first focused on methodology with a comparison of different mesh sizes and imaging analysis techniques (FlowCAM and ZOOSCAN) from samples collected at two stations JULIO and ANTARES in the north-western Mediterranean. This combination highlights the significant contribution of small size classes, which are particularly important in the Mediterranean and are generally underestimated by the traditional use of 200 µm nets.

Next, we applied this optimised methodology to study mesozooplankton community responses in: (1) the northwestern Mediterranean (studied during the September 2017 'MOOSE Grande Echelle' campaign), a region with spring and summer productivity highly dependent on the deep-water winter convection process, and (2) the southern Mediterranean sub-basins (Algerian, Tyrrhenian, and Ionian; May-June 2017 'PEACETIME' campaign), oligotrophic regions episodically fertilized by nutrient-rich Saharan dust deposition events.

For both cruises, we documented the structural and functional patterns of the zooplankton community in relation to environmental variables. The taxonomic structure was dominated by copepods, mainly cyclopoids and calanoids, and complemented by appendicularians, ostracods and chaetognaths. Multivariate analyses and rank-frequency diagrams showed marked differences in assemblage between the two surveys, but slight differences between the sub-basins visited in each survey. In general, total mesozooplankton showed reduced variations in abundance and biomass over the whole area, with a notable contribution from the small-sized fraction (<500 μm) in both abundance and biomass, and consequently in associated fluxes (carbon demand, grazing pressure, respiration and excretion).

Although the 2017 deep winter convection was of low to medium intensity and the MOOSE-GE 2017 sampling took place in late summer, zooplankton distribution patterns in the central north-western Mediterranean region and estimated associated rates revealed the imprint of this major late summer forcing on zooplankton. The most significant changes in the zooplankton community during PEACETIME were linked to dust events. To our knowledge, PEACETIME is the first study to observe in situ mesozooplankton responses before and shortly after natural deposits of Saharan dust. The change in zooplankton taxonomic frequency rank diagrams appears to be an interesting tool for highlighting the short-term responses of zooplankton to episodic dust deposition events. Clearly, dust-stimulated pelagic productivity has an impact down to the mesozooplankton in terms of rapid and marked changes in taxon assemblages and trophic structure, with potential implications for oligotrophic systems such as those in the southern Mediterranean.

Our results show that the structure of the mesozooplankton community is highly reactive on various time and space scales to the external forcings typical of the Mediterranean.

 

Key words: Mesozooplankton, Mediterranean Sea, Size structure, FLOWCAM-ZOOSCAN, Climate forcing, Biodiversity.

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