Satellite observations and numerical simulations have highlighted the influence of fine-scale structures (eddies, fronts, etc.) on phytoplankton, favouring the supply of nutrients to the surface and thus influencing the development of autotrophic micro-organisms. Fronts can also act as barriers, influencing the distribution of different phytoplankton groups. Carrying out in situ studies in vast expanses of low-energy, oligotrophic ocean was essential to gain a better understanding of the impact of fine-scale currents on phytoplankton communities.
In 2018, during the PROTEVSMED-SWOT campaign south of the Balearic Islands, 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-parametric sensors mounted on the ship, on a towed fish and on an ocean glider were used to sample physical and biogeochemical variables at high spatial resolution. Particular attention was paid to adapting the sampling route so that vertical velocities in the frontal zone could also be estimated. This strategy provided a general view of an oceanic zone characterised by the presence of a narrow front with an associated vertical circulation.
A multiparametric statistical analysis of the data collected identified two water masses characterised by different abundances of nine phytoplankton cytometric functional groups, as well as by different chlorophyll a and O2 contents. Our 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 the different taxa at the surface.
The methodology and results of the PROTEVSMED-SWOT campaign form a solid basis for the future BIOSWOT campaign planned for the same area in 2023, a few months after the launch of the SWOT altimetry satellite. This new altimetry satellite, equipped with a Ka-band bistatic SAR system, will make it possible to measure sea surface height at very high resolution, which will be very useful for studying fine scales.
Source :
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, https://doi.org/10.5194/bg-2021-38accepted, 2021.
Acknowledgements :
This work was supported by CNES as part of the BIOSWOT-AdAC project (https://www.swot-adac.org) and the MIO Transversal Axes programme (AT-COUPLAGE).
The authors acknowledge that the Délégation Générale de l'Armement (DGA) funded the campaign reported here through the Protevs and Protevs II upstream study programmes. They would also like to thank the technical team at the French Navy's Hydrological and Oceanographic Service (SHOM) and the crew of the ship Beautemps-Beaupré.
Contacts:
Roxane Tzortzis (roxane.tzortzis@mio.osupytheas.fr)
Andrea Doglioli (andrea.doglioli@mio.osupytheas.fr)
Gérald Gregori (gerald.gregori@mio.osupytheas.fr)
Francesco d'Ovidio (francesco.dovidio@locean.ipsl.fr