ABSTRACT Aim This study characterises benthic habitats and associated biodiversity in three Japanese subduction trenches, and explores the effects of trench‐specific differences in large‐scale seismic events, disturbance and productivity regimes on habitat structure and assemblage composition. Location The Japan, Ryukyu and Izu‐Ogasawara (Izu‐Bonin) subduction trenches are located in the Northwest Pacific Ocean. Time Period August to September 2022. Major Taxa Studied Benthic hadal biodiversity. Methods Biota and habitats were annotated from video transects from six submersible dives conducted at depths between 6939 m and 9775 m in trenches characterised by varying seismic regimes and productivity. Multivariate statistics and indicator species analysis were applied to understand the impact of location, depth and habitat type on benthic hadal community composition and community characteristics. Results A total of 29,556 individual organisms belonging to 70 morphotaxa and 11 phyla were observed across 8 habitat types. Biological communities varied across trenches, reflecting differences in productivity regime and depth. Significant differences were found between assemblages in the upper hadal dives in the Japan and Ryukyu trenches and lower hadal dives in the Izu‐Ogasawara Trench, and between the nutrient‐rich Japan Trench and the oligotrophic Ryukyu Trench. Community composition, total abundance and morphotaxa richness also varied across dives at similar depths within the same trench, reflecting variation in seismic regimes and resulting habitat structure. High‐abundance and low‐diversity opportunistic assemblages were found in seismically active trench sections, and relatively stable areas supported more diverse and speciose assemblages. Main Conclusions Benthic biodiversity across hadal seascapes is controlled by an interplay of factors, including depth, nutrient availability, seismic disturbance and resulting habitat structure over multiple scales. Broad‐scale productivity regimes and depth contribute to variability in biological and community characteristics across trenches. Within‐trench variability in community characteristics relates to seismic activity and its effects on habitat structure.
Swanborn et al. (Thu,) studied this question.