Shift in demersal marine communities at the edge of two biogeographic areas between 1996 and 2021.

Marine ecosystems are facing pressures from climate change and anthropogenic activities. While the induced impacts are widely observed, studied and modelled to define projections and management advice, the evolution of marine biodiversity still needs to be described and understood at local scales. The northern part of the Bay of Biscay is particularly concerned since at the edge of two marine provinces that discriminate Lusitanian and Boreal species, and with intense fishing pressure due to the presence of many commercial species. In the late nineties, a scientific survey characterized the demersal biodiversity of this area using underwater video transects and 25 years later, the same transects were revisited using a ROV. The taxonomic richness and densities from these two periods were compared to detect and describe changes in community composition and highlight potential shifts between Lusitanian and Boreal species. Taxonomic richness remains stable over time with 38 taxa in the 1990s, 39 in the 2020s, among which 37 were common to both periods. Yet, global abundance decreases significantly with disparities among taxa. Some Lusitanian species have significantly increased in density while some Boreal ones decreased. Two variables were identified to drive the change in community structure: fishing effort and seawater temperature. These findings echo other works in the area based on yearly, extensive bottom trawl surveys, while we highlighted the same structural community changes with one-off video sampling, bringing to the fore the robustness of videos sampling for monitoring diversity shifts in response to climate change.