Moving beyond the overrated perception of harbors as polluted ecosystems: unexpected high diversity of meio- and macro-benthic communities in the Port of Dunkirk, France

Harbors are often regarded as highly degraded ecosystems, yet their benthic biodiversity remains poorly documented. This study investigated environmental conditions, organic matter sources, pollutants and benthic assemblages over two seasons in the Port of Dunkirk, France, an industrialized and heavily modified transitional water body. Seven subtidal stations were sampled in spring and fall 2020 for water parameters, sediment geochemistry (including trace metals and molecular biomarkers), benthic foraminifera and macrofauna. Species richness and diversity were quantified and multivariate analyses performed to relate faunal patterns to environmental gradients. The industrial basin showed severe enrichment in trace metals (Cd, Cu, Pb, Hg) and high inputs of petroleum and combustion-derived compounds whereas the marina displayed lower metal enrichment but clear signature of fresh sewage. Despite these pressures, faunal assemblages exhibited high and spatially heterogeneous richness and diversity, comparable to or exceeding those of nearby and less industrialized transitional waters. Macrofaunal diversity was higher in fall whereas foraminiferal diversity remained steady over seasons. Redundancy analyses indicated that species distributions were driven by sediment grain size, chlorophyll a, organic matter content and cadmium, though macrofaunal assemblages were more influenced by seasonal features (water temperature and sand). Assemblages were dominated by typical stress-tolerant species from transitional waters of North-East Atlantic, though several non-indigenous species were also observed, confirming harbors as gateways for bioinvasion and peculiar biodiversity hotspots. These findings challenge the perception of industrial harbors as sacrificed ecosystems and underline their importance for biodiversity assessment and preservation, biomonitoring and management of non-indigenous species. Our study further underscores the robustness and relevance of benthic foraminifera and macroinvertebrates for biodiversity monitoring in harbor environments, and calls for further research and targeted conservation efforts to better understand and mitigate their ecological impacts while assessing their potential as habitats.