Community structure of benthic megafauna on a seamount with cobalt-rich ferromanganese crusts in the northwestern Pacific Ocean

Increasing threats to seamount ecosystems potentially from fishing and seabed mining are creating an urgent demand for research on megafauna to inform conservation and management. We surveyed the community structure of benthic megafauna on the Weijia Seamount, which is covered by cobalt-rich ferromanganese crusts, in the northwestern Pacific Ocean. A multidisciplinary dataset was generated from video recordings taken by a human occupied vehicle (HOV) and remotely operated vehicle (ROV) along transects and bathymetry-derived environmental variables. The spatial variation of community structure and its environmental and spatial factors impacting the community were then explored. A total of 2779 individuals of the megabenthos (from depths of 1533-2763 m, covering 24.4 km in distance and with a maximum of 109 individuals per 100 m by linear observation), was counted and classified into 76 morphospecies, which was dominated by crinoids, ophiuroids, hexactinellids and octocorals. Moderately abundant and diverse megafauna, high beta diversity and distinct assemblages were present in the Weijia Seamount, suggesting a highly patchy pattern within a single seamount. Six clusters of megafauna were revealed, with significant differences in species composition and biodiversity. The results of statistical analyses showed that the megafaunal community was impacted by at least eight environmental variables (latitude, depth, slope, curvature, rugosity, bathymetric position index, topography type, and substrate type), and excluding aspect. Among these variables, the topography and substrate types played key roles in the occurrence of small-scale aggregations of hexactinellids; depth-related variables were key factors, but no gradual depth-related change in terms of abundance, richness or species turnover was observed. This case study on the megabenthic community of the Weijia Seamount can serve as an environmental baseline, providing a reference status for seamount conservation and management.