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The waltz of the phytoplankton in the Mediterranean Sea

In situ observations conducted south of the Balearic Islands identified two water masses characterized by different abundances of nine phytoplankton functional groups and showed that lagrangian fronts induced by fine-scale circulation maintain a strong structuring effect on the phytoplankton community by separating different taxa at the surface.

The Seasoar aboard the R/V Beautemps-Beaupré. Photo credit G. Gregori.

Satellite observations and numerical simulations have highlighted the influence of fine-scale structures (eddies, fronts, etc.) on phytoplankton, favoring the input of nutrients at the surface, thus influencing the development of autotrophic microorganisms. Fronts can also act as a barrier that influences the distribution of different phytoplankton groups. Conducting in situ studies in large, low-energy, oligotrophic ocean areas is essential to better understand the impact of fine-scale currents on phytoplankton communities.

The paper Impact of moderate energetic fine-scale dynamics on the phytoplankton community structure in the western Mediterranean Sea analyzed data collected during the PROTEVSMED-SWOT at sea campaign that took place south of the Balearic Islands in 2018. An international consortium, taking advantage of near real-time satellite information, implemented a sampling strategy to traverse a frontal zone separating different water masses. In situ multi-parameter sensors mounted on the ship, on a towed vehicle, and on an ocean glider were used to sample physical and biogeochemical variables at high spatial resolution. The sampling route was tailored to also estimate vertical velocities in the frontal zone. This strategy provided a general view of an oceanic area characterized by the presence of a narrow front with an associated vertical circulation.

A multiparametric statistical analysis of the collected data identified two water masses characterized by different abundances of nine phytoplankton cytometric functional groups, as well as by different chlorophyll a and O2 contents. The study shows that lagrangian fronts induced by fine-scale circulation, although much weaker than intense current fronts, maintain a strong structuring effect on the phytoplankton community by separating different taxa at the surface.

Phytoplankton diversity is a key component in ocean biogeochemical services and contributes to the resilience and health of ocean ecosystems in respect to local and global stressors, including climate change. The methodology and results of the PROTEVSMED-SWOT campaign form a solid basis for the BIOSWOT-Med campaign (https://www.swot-adac.org/campaigns/bioswot/) planned in the same area in 2023, a few months after the launch of the Surface Water and Ocean Topography (SWOT) altimetry satellite. This new altimetry satellite, equipped with a Ka-band bistatic SAR system, will provide very high-resolution sea surface height measurements, very helpful for fine scale studies.

The work was supported by the CNES in the framework of the BIOSWOT-AdAC project and by the MIO Transversal Axes program (AT-COUPLAGE). The Délégation Générale de l’Armement (DGA) funded the campaign reported through the Protevs and Protevs II upstream study programs.

FOR MORE INFORMATION

Tzortzis, R., Doglioli, A. M., Barrillon, S., Petrenko, A. A., d’Ovidio, F., Izard, L., Thyssen, M., Pascual, A., Barceló-Llull, B., Cyr, F., Tedetti, M., Bhairy, N., Garreau, P., Dumas, F., and Gregori, G.: Impact of moderate energetic fine-scale dynamics on the phytoplankton community structure in the western Mediterranean Sea. Biogeosciences, 18, 6455–6477, 2021, https://doi.org/10.5194/bg-18-6455-2021 © 20