Influence of seabed heterogeneity on benthic megafaunal community patterns in abyssal nodule fields

Polymetallic nodule fields, at 3000–6000 m depth, harbour some of the most diverse seabed communities in the abyss. In these habitats, nodules are keystone structures for many sessile species that require hard substrate for growth. The combination of exposed nodules and background sediment increases the heterogeneity of these habitats compared to nodule-free fields, and thereby potentially influences the assembly of animal communities across space. Polymetallic nodule patches can vary in size, shape and nodule density; however, the effect of these variations on benthic communities remains largely unclear. Understanding the role of nodule-habitat type (defined by nodule size and density) and seabed heterogeneity on biodiversity is urgently needed to accurately assess the impacts of potential nodule removal in areas like the Clarion-Clipperton Zone, a region targeted for deep-sea mining. Here, we explored variations in benthic invertebrates (megafauna > 10 mm) across space and nodule-habitat types within an abyssal seascape. We quantified changes in megafaunal density, diversity and community structure using quantitative seabed imagery in four study areas. Study areas were separated by distances of 1–110 km and exhibited varying levels of seabed heterogeneity, as defined by the proportions of different nodule-habitat types present in each area. We found that different nodule-habitat types harboured distinct assemblages. Areas with higher nodule coverage supported higher megafaunal densities, while areas with larger, sparsely distributed nodules had higher diversity. Higher species richness and distinct community structure were associated with the most heterogeneous study area, which had multiple nodule-habitat types and nodule-free sediment patches. These results suggest that type of nodule habitat and degree of seabed heterogeneity are important drivers of local benthic megafaunal diversity patterns in abyssal nodule fields. By establishing a baseline prior to human disturbance, our study provides essential insights that should inform future monitoring programmes, mining regulations and biodiversity conservation in this area.