Vertical zonation and environmental drivers of North-West Pacific abyssal and hadal mobile faunal communities

This study examines the composition, vertical zonation and drivers of mobile abyssal and hadal faunal assemblages to understand the environmental patterns underlying biological organisation at lower abyssal and hadal depths. Biological data were analysed from 96 baited lander deployments across five North-West Pacific subduction trenches and one triple trench junction (the Mariana Trench, Philippine Trench, Ryukyu Trench, Japan Trench, Izu-Ogasawara Trench and Boso Triple Junction) and combined with environmental metrics of terrain geomorphology and oceanography at deployment locations. Hierarchical clustering revealed three depth-driven faunal zones, representing an abyssal-hadal transition community (∼5500–6500 m), an upper hadal community (∼7000–7500 m) and a lower hadal community (>8500 m). Clustering results support an abyssal-hadal ecotone >6500 m depth and a further hadal transition ∼8000 m. Environmental factors explained 40.4% of community structure, with depth and location as main contributors to the final model. These factors, through the latter's relationships with surface oceanography and productivity, were also key determinants of relative abundance, diversity, richness and the total relative abundance of dominant faunal groups and families among deployments. Results suggest limited ecological effects of intra-trench environmental variability, and highlight a need for further high-resolution studies sampling a range of environmental conditions and their associated biodiversity within individual hadal features.