eDNA and Citizen Science Reveal Hidden Fish Biodiversity in Climate-Stressed Urban Ports of the Mediterranean Sea

This paper provides a pioneering study case on monitoring fish biodiversity in ports through the eDNA and citizen science approach. eDNA samples were collected in the spring and in fall 2022 in the ports of Calvi, L'Île-Rousse, STARESO, Saint-Florent. Samples collected led to the identification of 73 taxa. These ports appeared to harbor at least 20% of the known teleost biodiversity in Corsica and 11% of the Mediterranean teleost biodiversity. The ports of Calvi and L'Île-Rousse displayed the highest taxonomic, phylogenetic, and functional diversities and appeared the most similar. However, taxonomic turnover highlighted that none of the 4 ports was a subset of any of the others. In August 2022, an extreme climate event (ECE) struck Corsica, offering a unique opportunity to collect data under abnormal conditions. Although it is not possible to distinguish the seasonal effect from the ECE effect in the fall, we detected in all ports but Saint-Florent an increase in taxonomic richness, phylogenetic, and functional diversity: we did not only detect new species but also showed that these species led to an increase in the local representativeness of phylogenetic diversity, most likely correlated with new functional traits. The port of Saint-Florent displayed the highest relative phylogenetic diversity, that is, a smaller but evolutionarily more distinct group of species. Our study demonstrated the robustness and relevance of eDNA citizen science coupled with relevant indicators for port biodiversity monitoring and emphasized the need for more research and targeted conservation efforts to better understand and mitigate the ecological impacts of ports while exploring their potential as habitats.