On the following subject:
"Fine-scale circulation and impact on plankton
Director Andréa Doglioli, Senior Lecturer AMU
Co-Director: Gérald GrégoriI, CNRS Research Fellow
Summary of the thesis
The aim of this thesis is to study fine-scale ocean circulation and its impact on phytoplankton. Fine-scale ocean circulation is characterised by structures of the order of a few kilometres, and short-lived (days/weeks). The size and ephemeral nature of these structures make them particularly difficult to study using in situ measurements, which explains why they have mainly been studied using numerical models. The latter have shown that fine-scale dynamics play a key role in many biogeochemical processes, particularly primary production. Indeed, the vertical velocities generated by structures such as fronts or filaments can control the injection of nutrients into the euphotic zone and influence the development of phytoplankton. However, uncertainties remain, particularly concerning the influence of fronts on the distribution of phytoplankton organisms. To overcome this lack of knowledge, it is essential to supplement the results obtained from numerical models with those obtained from in situ measurements. This is why it is necessary to develop methodologies for carrying out campaigns at sea capable of sampling fine-scale structures at high spatial and temporal frequency, in order to monitor their rapid evolution. To target the structures of interest to be measured in situ, the combined use of numerical models and satellite observations is proving to be very effective. It was with this in mind that the PROTEVSMED-SWOT 2018 campaign was carried out in the western Mediterranean, south of the Balearic Islands. The first part of the thesis was devoted to processing the data from this campaign. Initially, this work involved identifying a fine-scale structure, in this case a front, using satellite data and physical measurements carried out in situ (ADCP, SeaSoar, TSG, etc). Flow cytometry data, collected automatically at high frequency, was then used to identify the phytoplankton functional groups present and to study their spatial distribution on either side of the front. In the second part of the thesis, a phytoplankton growth model was applied to the cytometry data in order to reconstruct the cell cycle and dynamics of the phytoplankton. The aim was to explain the particular distribution of phytoplankton abundance observed on either side of the front. The model was used to calculate the growth and cell division rates of the various phytoplankton groups in the two water masses separated by the front. The contrast in growth and division rates in these two water masses helps to explain the distribution of phytoplankton abundance observed in the first part of this work. Although the impact of fine scales, and fronts in particular, on the structure and dynamics of phytoplankton communities has already been demonstrated by modelling work, few in situ studies have been carried out to date. Moreover, most of these studies have been carried out in currents such as the Gulf Stream or the Kuroshio, regions known to generate intense and persistent fronts. The originality of this thesis lies in the fact that the influence of fronts on phytoplankton has also been found in the Mediterranean, in a frontal zone that is less energetic and more ephemeral than those described in previous studies. In addition, this region appears to be highly oligotrophic, as is the vast majority of the global ocean. Finally, the methodology put in place during this thesis, as well as all the data collected during the PROTEVSMED-SWOT 2018 campaign, will serve as a basis for the future BIOSWOT campaign planned in the SWOT satellite cross-over zone, a few months after its launch in 2023.