Benthic macroinvertebrate responses to habitat and sediment gradients in the modified Sulina Canal, Danube Delta

Highly modified distributary channels are shaped by interacting hydromorphological alteration, sediment redistribution, navigation-related pressures, and lateral connectivity, but their combined influence on benthic macroinvertebrate communities remains difficult to disentangle in deltaic systems. This study assessed how macrozoobenthic assemblages vary across the Sulina Canal navigation corridor and adjacent laterally connected habitats, and evaluated whether community patterns are more closely associated with hydrodynamic conditions or with surface sediment composition. We combined standardized macrozoobenthic sampling, ADCP-based hydrodynamic measurements, and loss-on-ignition sediment analyses during two 2023 campaigns, and interpreted recent patterns in relation to comparable datasets from 2019-2021. Assemblage structure was analysed using permutation-based multivariate models that tested habitat, season, sediment, and hydrodynamic effects. Macrozoobenthic assemblages differed among habitat types and between seasons, with laterally connected canals, meanders, and lakes generally supporting richer and more diverse communities than the trained main navigation corridor. Sediment composition emerged as a key habitat filter: carbonate-enriched and organic depositional settings were associated with higher density and richness, whereas siliciclastic-dominated substrates were linked to reduced abundance and diversity. DISTLM/dbRDA confirmed that LOI sediment fractions were significantly associated with assemblage composition in the full biological-sediment dataset, while hydrodynamic descriptors contributed to explaining multivariate community structure within the ADCP-matched subset, particularly when combined with sediment predictors. Interannual comparisons suggested lower recent densities but were treated as exploratory because station coverage differed among years. The findings indicate that benthic community structure in the modified Sulina Canal is governed primarily by habitat-sediment templates and lateral connectivity, with hydrodynamic effects being scale-dependent and partly mediated by local substrate conditions. Management should prioritize the maintenance and restoration of lateral connections, sediment-sensitive navigation and maintenance practices, and integrated monitoring of macroinvertebrates, sediment conditions, turbidity/resuspension, hydrodynamics, and non-indigenous taxa within WFD - and restoration-oriented frameworks.