Over the past decade, the Lérins Biodiversité project has collected a substantial amount of information on the marine biodiversity and coastal ecosystems of the archipelago.
This work is not a conventional scientific monitoring programme in the strictest sense. It is closer to a long-term educational and citizen science project, involving students, researchers and local partners. For this reason, the results must be interpreted carefully, and long-term patterns should only be considered robust once they have been confirmed over time.
Below are some of the main findings we can currently share. Please feel free to contact us if you would like to discuss the project or the results in more detail.
Biodiversity around the island (leg 1)
The biodiversity observed during Leg 1, in the immediate vicinity of the coastline and between 1 and 3 metres depth, shows higher apparent species richness around Saint-Honorat than around Sainte-Marguerite.
Around Sainte-Marguerite, the richest sites are mainly located in the eastern part of the island and near Pointe du Dragon. By contrast, the harbour area and the main beach sector on the southern coast show lower species richness for organisms that can be easily observed while snorkelling.
Across the archipelago, rocky habitats appear to host higher species richness than soft-bottom habitats, such as sand and gravel. Species richness also tends to be higher where algal cover is more abundant. Posidonia oceanica meadows show more variable levels of apparent biodiversity.
These results must be interpreted with caution. The monitoring focuses only on a subset of the species present around the Lérins Islands: those that can be detected visually by snorkelling in shallow coastal areas. In addition, environmental and human-related factors often interact. For example, it would be simplistic to conclude that Sainte-Marguerite is less biodiverse than Saint-Honorat only because it is closer to the mainland and more heavily visited. Saint-Honorat also has more rocky habitats, which naturally tend to support higher visible biodiversity than sandy areas.
However, when comparing only rocky substrates, Saint-Honorat still appears to show higher species richness than Sainte-Marguerite. This suggests that both habitat type and human pressure may contribute to structuring the observed biodiversity around the archipelago.
Map of apparent species richness around the Lérins Islands, based on the average number of species observed per 30 m² station between 2013 and 2019 (C. Mocquet)
Leg 2 example: anchoring pressure on Posidonia oceanica meadows
MARRES students studied whether boat anchoring may affect Posidonia oceanica meadows between Sainte-Marguerite and Saint-Honorat. These seagrass meadows are key Mediterranean habitats, supporting biodiversity, protecting coastlines and storing carbon.
Using drone images, satellite data and aerial photographs, they mapped changes in Posidonia coverage in the area betwenn the islands, and compared them with boat anchoring patterns. The study suggests an overall decrease of around 10% in Posidonia coverage between 2012 and 2023 in the area, equivalent to more than 5 hectares, or slightly more than 10 football fields..
Boat anchoring was concentrated in specific areas, especially near beaches, the underwater museum and the port of Saint-Honorat. A significant relationship was found between boats anchoring on Posidonia and changes in seagrass coverage, suggesting that anchoring may contribute to meadow decline.
The study highlights the need for more detailed monitoring, combining aerial imagery, field observations and information on boat size and anchoring practices. These results could help raise awareness among boat users and support future conservation measures around the Lérins Islands.
Mooring density observed over the study period in the same area.
The heatmap shows the spatial concentration of anchored boats across the grid cells, highlighting areas of higher mooring pressure, particularly close to the shores of Sainte-Marguerite and Saint-Honorat.
Change in Posidonia oceanica cover between 2012 and 2023 in the study area between Sainte-Marguerite and Saint-Honorat.
Each grid cell shows the estimated change in Posidonia cover over the period, with green cells indicating an increase, red cells indicating a decrease, and white cells indicating no detectable change.
study led in 2024 by Joran De Gang, Margot Webster, Fabian Baumgartner, Pierre Guillibert, and Katerina Roussou
More leg 2 examples
Students investigated bleaching in coralline algae at two contrasting sites around Sainte-Marguerite: one sheltered and one exposed. Using field observations and image analysis, the students found that bleaching was much higher at the exposed sites, with around 60% of algae bleached compared with 30% at the sheltered site at the time of study (September). Temperature, light and wave exposure appear to be key drivers. The study highlights how local environmental conditions can strongly affect the health of coralline algae, important calcifying organisms that help structure Mediterranean habitats.
Study led by Alix Hörmannsdorfer, Corinna Lange, Gaia Di Daniel, Lena Merz, Sophia Zonca in 2024
Students explored how wave exposure influences intertidal biodiversity and the ecology of the Mediterranean limpet Patella caerulea around Saint-Honorat. Comparing sheltered and exposed sites, students found that limpets size was larger in sheltered areas, while abundance did not differ significantly between exposed and sheltered sites. The study suggests an effect of natural selection and that sheltered rocky habitats play an important role in maintaining intertidal biodiversity and may buffer key species against environmental stress.
Study led by Amy Green, Gabriela Diaz, Mathilda Ekholm in 2024
Students assessed whether areas identified by a satellite-based approach as potential fish nurseries around Sainte-Marguerite effectively host more juveniles of three targeted seabream species: Diplodus sargus, Diplodus puntazzo and Diplodus vulgaris. Using 200 m snorkelling transects, the students found significantly higher numbers and smaller sizes of D. sargus and D. vulgaris in predicted nursery areas supporting the idea that these zones may function as juvenile habitats. The study provides promising field evidence for the MedHab approach, while noting that more work is needed before confirming these areas as true nurseries.
Study led by Adrien Dutrey, Camille Duval, Luna Lombart in 2023
Want to discover the results differently?
Each student project is explored through several formats: a scientific paper, an oral presentation, and an outreach video for the general public.
For the previously mentioned studies on limpets and posidonia vs mooring, students tried “video mode”... with different angles.
More to come... soon!
