Louise Rousselet (Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS), Toulouse, France); Laurina Oms (Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, Campus universitaire de Luminy, Marseille, France); Xavier Milhaud (Institut de Mathématiques de Marseille (I2M) - UMR 7373, 3 place Victor Hugo Case 19, 13331 Marseille Cedex 3, France); Andrea Doglioli (Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, Campus universitaire de Luminy, Marseille, France); Monique Messié (Monterey Bay Aquarium Research Institute, Moss Landing, CA, USA); Claire Lacour (Laboratoire d'analyse et de mathématiques appliquées, Université Gustave Eiffel, 5 boulevard Descartes, 77454 Marne-la-Vallée Cedex 2, France); Pierre Vandekerkhove (Laboratoire d'analyse et de mathématiques appliquées, Université Gustave Eiffel, 5 boulevard Descartes, 77454 Marne-la-Vallée Cedex 2, France); Francesco d'Ovidio (Sorbonne Université, CNRS, IRD, MNHN, Laboratoire d’Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN-IPSL), Paris, France); Julien Le Sommer (CNRS, IRD, Grenoble INP, IGE, Univ. Grenoble Alpes, Grenoble, France); Gérald Grégori (Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, Campus universitaire de Luminy, Marseille, France); Denys Pommeret (Institut de Mathématiques de Marseille (I2M) - UMR 7373, 3 place Victor Hugo Case 19, 13331 Marseille Cedex 3, France)

Fine-scale structures (1–100 km, days to weeks) are ubiquitous physical features that drive the physics, chemistry and ecology of the ocean. By creating significant heterogeneity in salinity and temperature, fine-scale structures shape diverse habitats that sustain phytoplankton diversity in the ocean. However, the ephemeral nature of fine-scale structures makes them difficult to study using in situ campaigns. Therefore, aside from a few in situ studies, the relationship between fine-scale structures and the distribution of phytoplankton diversity has mostly been investigated through modelling studies. Nevertheless, understanding the extent to which fine-scale structures influence phytoplankton diversity globally based on observations remains a critical challenge.
Satellite-derived observations, particularly the combination of altimetry and ocean colour, have been widely used to study the distribution of phytoplankton according to physical features in the global ocean. The launch of the Surface Water and Ocean Topography (SWOT) satellite in 2022 helps to observe the smallest fine-scale structures that were not visible in conventional altimetry thanks to its unprecedented resolution.
In this study, we present the results obtained from satellite data analysis in the western basin of the Mediterranean Sea (5 May 2023). Our interest lies in (i) the phytoplankton community structure, characterised by the relative biomass of five phytoplankton functional types (i.e. diatoms, dinophytes, haptophytes, green algae and prochlorophytes, and prokaryotes - derived from ocean colour data), and (ii) the fine-scale structures (identified from SWOT-based altimetry data). Our objective is to characterise the phytoplankton communities across the entire study area and then compare their structure with that of the phytoplankton communities found within the fine-scale structures.
To characterize phytoplankton communities, we used Gaussian mixture models, which aimed at estimating the parameters of multivariate Gaussian distribution (i.e. average biomass and variance of the phytoplankton functional types) for several Gaussian components (i.e. within each community). On a first hand, we applied Gaussian mixture models on the whole study area on phytoplankton data to find how many communities are present. Then, using SWOT-based altimetry data, Finite Time Lyapunov Exponents were calculated to highlight fine-scales structures. Finally, Gaussian mixture models were applied a second time on phytoplankton data only in fine-scales structures.
Five communities were observed throughout the Western Mediterranean Sea (hereafter referred to as 'global communities'). Overall, there was a good correspondence between the spatial distribution of these communities and the presence of fine-scale structures. Five communities were observed in the fine-scale structures (hereafter referred to as ‘fine-scale communities’). Comparing the relative biomass composition of these communities, it was found that four communities were similar between the global and fine-scale communities. The fine-scale communities that were common with the global communities were slightly less prevalent in fine-scale structures (between 14% and 27%) than in the entire Western Mediterranean Sea (between 21% and 29%). One community was only observed in the Western Mediterranean Sea. This community was mostly associated with coastal areas and represented approximately 2% of observations in the Western Mediterranean Sea. One community was only observed in the fine-scale structures. This community represented up to 22% of those observed in fine-scale structures.
Our results suggest that local environmental conditions in fine-scale structures allow a new community to emerge. Overall, these preliminary results highlight the importance of fine-scale structures in the spatial distribution of phytoplankton communities. This approach will be applied to a global dataset.