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

Study of the mesozooplankton community structure and trophic role in the Mediterranean Sea during two large scale oceanographic surveys PEACETIME and MOOSE in 2017

Mesozooplankton (by definition multicellular, heterotrophic organisms with size ranging between 0.2–20 mm) represents a key compartment of the marine pelagic ecosystems playing a mediating role as consumer of the primary producers and microzooplankton, and as food source for fish and larger planktivores. In this thesis, we evaluate the most complete combination of mesh size nets and sample analysis techniques to deliver the best representation of its abundance, size structure and trophic and metabolic functions. We firstly focus on methodological aspects through a comparison of several mesh-size nets and imagery technics (FlowCAM and ZOOSCAN) from samples collected at two time-series stations JULIO and ANTARES in the North Western Mediterranean Sea. By this combination, we highlight the significant contribution of the lower size classes within the mesozooplankton, particularly important in the Mediterranean Sea and generally underestimated by the traditional use of 200µm mesh WP2 nets.

Then, we applied the optimized methodology to study the responses of mesozooplankton community in two major areas of the Mediterranean sea: (1) the North Western Mediterranean Sea (studied during the MOOSE-GE September 2017 cruise), a region of which the spring and summer productivity is strongly dependent on the winter deep-water convection process, and (2) the Southern Mediterranean (Algerian, Tyrrhenian and Ionian sub-basins; PEACETIME oceanographic cruise May–June 2017) oligotrophic regions episodically fertilized by Saharan nutrient-rich dust deposition events.

For both campaigns, we document the structural and functional patterns of the zooplankton component in relation to environmental variables. The taxonomic structure was dominated by copepods, mainly cyclopoid and calanoid copepods, and was completed by appendicularians, ostracods, and chaetognaths. The zooplankton taxa assemblages, analyzed using multivariate analysis and rank frequency diagrams, showed marked differences between the two surveys, but slightly differed between sub basins within each survey. In general, total mesozooplankton showed reduced variations in abundance and biomass values over the whole area, with a noticeable contribution from the small size fraction (<500 μm) both for abundance and to biomass, and consequently for the zooplankton fluxes (carbon demand, grazing pressure, respiration, and excretion).

Despite the context of a medium strength of the deep winter convection process in 2017 and the late summer sampling during MOOSE-GE 2017 survey, the distribution pattern of zooplankton and the associated estimated rates in the central region revealed the long-term signature of this large forcing processes.  The strongest changes in the zooplankton community during Peacetime were linked to the above-mentioned dust events.  To our knowledge, PEACETIME is the first in situ study that has allowed for the observation of mesozooplankton responses before and soon after natural Saharan dust depositions. The change in the rank frequency diagrams of the zooplankton taxonomic structure appears to be an interesting tool to highlight short-term responses of zooplankton to episodic dust deposition events. Obviously, dust-stimulated pelagic productivity impacts up to mesozooplankton in terms of strong but short changes in taxa assemblages and trophic structure, with potential implications for oligotrophic systems such as the Mediterranean Sea.

Our results highlight that the mesozooplankton community structure is strongly reactive to various time scale external forcings typical of the Mediterranean Sea.

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