Long-term trajectories of deep artificial rock pool communities in a port infrastructure

Eco-engineering practices are increasingly implemented in urbanised marine coastal zones to enhance local marine biodiversity, as part of biodiversity offsetting efforts in areas impacted by human activities. Broad-scale, long-term monitoring studies remain scarce in this field yet are essential for robustly assessing the ecological value of these compensation initiatives. In 2019, the Port of Brest (France) constructed a 14-ha polder contained by a 900-m rocky revetment, for the reception, pre-construction, and transportation of offshore wind turbine components. To enhance marine biodiversity, 100 artificial rock pools were embedded in the dyke revetment at different intertidal levels. The benthic communities of 27 pools distributed across three dyke sectors (western, central, and eastern) were monitored from 2021 to 2025 during winter and summer spring tides using Rapid Assessment Surveys. Seven years after installation, rock pool communities appear to have reached a late successional mature stage, with high intertidal pools exhibiting the greatest taxon richness and the highest number of indicator species. Ecological trajectories were directionally consistent across intertidal levels, with high intertidal communities showing a more advanced successional stage relative to medium and low levels. Emersion regime and thermal variability were identified as the primary drivers of community composition, with dyke sector and pool orientation as secondary but significant structuring variables. Non-indigenous species (NIS) were present in all three intertidal levels, with most persisting over the entire survey period, a trend consistent with the biotic acceptance hypothesis. These findings demonstrate that long-term, spatially replicated monitoring is essential for soundly evaluating the ecological dynamics of eco-engineered structures and assessing their value as biodiversity compensation measures.