1215 / 2024-09-20 17:12:31

Biodiversity of benthic megafauna on the deep-sea plains with polymetallic nodules in the Northwestern Pacific Ocean

Deep-sea habitat,Megafauna,Polymetallic nodules,Deep-sea mining,Northwestern Pacific Ocean

Northwestern Pacific encompasses various typical deep-sea habitats, including seamounts, plains, and trenches. Among them, the deep-sea plains may be rich in polymetallic nodules, offering abundant patchy hard substrates that provide diverse habitats and refuges for benthic and infaunal organisms. The deep-sea nodule plains are characterized by high habitat heterogeneity, supporting diverse organisms. Growing interest in deep-sea mining has emerged, yet the potential environmental impacts remain largely unknown, making environmental concerns a critical barrier. Understanding the habitat characteristics of deep-sea nodule plains and the distribution patterns of megafaunal biodiversity is fundamental for effective deep-sea environmental management and is essential for fulfilling commitments under the Agreement on the Conservation of Marine Biological Diversity in Areas Beyond National Jurisdiction.

This study conducted underwater optical surveys using a towed optical vehicle, collecting a large number of high-definition seabed images from the polymetallic nodule contract area in the Northwestern Pacific. The biodiversity and distribution patterns of megafauna in the nodule habitat were analyzed. A total of 37,558 valid seabed photographs were obtained, covering approximately 350 km of seabed transects at a depth of around 5,500 m, and 7,062 megafauna individuals were counted. Each organism was identified to the lowest possible taxonomic level according to the collected biological specimens and existing literatures. In total, 206 morphological species of megafauna were identified, belonging to nine phyla. Therein, holothurians and sponges were the dominant groups in the deep-sea nodule plains, accounting for 35.3% and 23.3%, respectively. It suggested that the polymetallic nodule contract area supported relatively low organism abundance but high biodiversity. An analysis of two near-seafloor transects adjacent to a deep-sea seamount revealed that as the distance from the seamount increased, one transect demonstrated a gradual decline in the Shannon diversity index, species richness, and evenness, while the other transect exhibited an opposing trend. Beta diversity analysis indicated that the beta diversity index remained high overall, with species turnover as the dominant biological process. Cluster analysis showed significant differences in species composition within the deep-sea nodule plains, with the proportions of holothurian Elpidiidae increasing closer to the seamount. Redundancy analysis revealed that the distribution of megafauna in the nodule plains is influenced by multiple interacting environmental factors, with depth, nodule coverage, bathymetric position index, slope, and near-bottom current velocity being key environmental drivers. This study provides fundamental biological data for deep-sea environmental management.