The biodiversity of Aquitaine's coastal lakes is shaped by the limited dispersal capacity of species and environmental filtering mechanisms - ISOÉTIDES

Estelle-Marie Blanquard - Debailleul defended her doctoral thesis on 24/03/2026. 

Lakes and ponds of the Aquitaine Atlantic coast are unique natural ecosystems, not only at the national but also at the European scale. These systems host a high taxonomic diversity, especially for aquatic plants. Indeed, isoetid communities represent a set of species with high conservation value, also including one endemic species. These communities are however highly threatened, especially by anthropogenic activities, and most of the coastal Aquitaine’s lakes have already experienced their extinction. Knowledge on these species is still however scarce, particularly concerning their genetic diversity. While conservation and restoration actions are emerging with the redaction of a National Plan for preserving these communities, understanding their genetic diversity, their relationships with taxonomic and functional diversity appear as a major issue.

Summary

The isoetid communities of oligotrophic coastal lakes in Aquitaine constitute ecosystems unique at the European scale, harbouring sensitive habitats listed under the Habitats Directive (3110). These aquatic macrophyte communities, adapted to oligotrophic conditions, face increasing anthropogenic pressures: eutrophication, climate change, and invasive species. In this context, understanding the processes structuring their biodiversity represents a major challenge for their long-term conservation.
This thesis adopts an integrative approach combining community ecology and landscape genetics to disentangle the relative contribution of spatial and environmental processes in structuring taxonomic and genetic diversity. We developed microsatellite markers to characterise the genetic diversity of eight macrophyte species (Baldellia ranunculoides, Eleocharis multicaulis, Juncus bulbosus, Littorella uniflora, Lobelia dortmanna, Myriophyllum alterniflorum, Phragmites australis, Schoenoplectus pungens) across 27 stations distributed among four lakes of the Aquitaine coast (Carcans-Hourtin, Lacanau, Cazaux-Sanguinet, Parentis-Biscarrosse).
Our results reveal exceptionally high levels of genetic differentiation (mean FST 0.26-0.27), three to ten times higher than values typical of aquatic plants, indicating extremely limited gene flow between populations. Isolation by distance constitutes the dominant pattern, although its intensity varies strongly among taxa (from r = 0.10 for E. multicaulis to r = 0.80 for J. bulbosus), reflecting contrasting dispersal capacities. Among the species studied, two benefit from national protection status and are subject to a National Action Plan: L. dortmanna presents a critical situation with near-zero heterozygosity (He < 0.05), while L. uniflora, despite a moderate raw FST, reveals one of the highest standardised differentiation values in the network (F'ST = 0.72).
Analysis of species-genetic diversity correlations (SGDC) reveals marked scale-dependence: the near-complete absence of correlation at the local scale (α-SGDC) contrasts with the emergence of robust correlations at the regional scale (β-SGDC) for all eight species. In B. ranunculoides and S. pungens, the only species displaying significant SGDCs at both scales, the correlation is entirely mediated by environmental factors (lake surface area, hydraulic connectivity, natural land cover), demonstrating that species richness and genetic diversity respond in parallel to the same gradients without direct correlation. The absence of correlation between α-SGDC and β-SGDC coefficients suggests that local demographic processes and regional dispersal constraints structure biodiversity in a largely decoupled manner.
This thesis demonstrates that dispersal limitation and environmental filtering act jointly to structure biodiversity in Aquitaine coastal lakes. Conservation of habitats of community interest 3110 requires a balanced approach combining local habitat quality management (maintaining oligotrophy and limiting anthropogenic disturbance) and regional functional connectivity management, with particular attention to L. dortmanna(a genetically depauperate umbrella species) and L. uniflora (highly differentiated populations requiring distinct management).