Abstract
Abstract. In the abyssal equatorial Pacific Ocean, most of the seafloor of the Clarion-Clipperton Fracture Zone (CCFZ), a 6 million km2 polymetallic nodule province, has been preempted for future mining. In light of the large environmental footprint that mining would leave and given the diversity and the vulnerability of the abyssal fauna, the International Seabed Authority has implemented a regional management plan that includes the creation of nine Areas of Particular Environmental Interest (APEIs) located at the periphery of the CCFZ. The scientific principles for the design of the APEIs were based on the best – albeit very limited – knowledge of the area. The fauna and habitats in the APEIs are unknown, as are species' ranges and the extent of biodiversity across the CCFZ. As part of the Joint Programming Initiative Healthy and Productive Seas and Oceans (JPI Oceans) pilot action “Ecological aspects of deep-sea mining”, the SO239 cruise provided data to improve species inventories, determine species ranges, identify the drivers of beta diversity patterns and assess the representativeness of an APEI. Four exploration contract areas and an APEI (APEI no. 3) were sampled along a gradient of sea surface primary productivity that spanned a distance of 1440 km in the eastern CCFZ. Between three and eight quantitative box cores (0.25 m2; 0–10 cm) were sampled in each study area, resulting in a large collection of polychaetes that were morphologically and molecularly (cytochrome c oxidase subunit I and 16S genes) analyzed. A total of 275 polychaete morphospecies were identified. Only one morphospecies was shared among all five study areas and 49 % were singletons. The patterns in community structure and composition were mainly attributed to variations in organic carbon fluxes to the seafloor at the regional scale and nodule density at the local scale, thus supporting the main assumptions underlying the design of the APEIs. However, the APEI no. 3, which is located in an oligotrophic province and separated from the CCFZ by the Clarion Fracture Zone, showed the lowest densities, lowest diversity, and a very low and distant independent similarity in community composition compared to the contract areas, thus questioning the representativeness and the appropriateness of APEI no. 3 to meet its purpose of diversity preservation. Among the four exploration contracts, which belong to a mesotrophic province, the distance decay of similarity provided a species turnover of 0.04 species km−1, an average species range of 25 km and an extrapolated richness of up to 240 000 polychaete species in the CCFZ. By contrast, nonparametric estimators of diversity predict a regional richness of up to 498 species. Both estimates are biased by the high frequency of singletons in the dataset, which likely result from under-sampling and merely reflect our level of uncertainty. The assessment of potential risks and scales of biodiversity loss due to nodule mining thus requires an appropriate inventory of species richness in the CCFZ.
Highlights
The abyssal depths are vast, covering 54 % of the Earth’s surface and 75 % of the ocean floor, typically located between 3000 and 6000 m depth; it generally features lowtemperature, low-current and well-oxygenated oligotrophic waters (Gage and Tyler, 1991; Smith and Demopoulos, 2003; Ramirez-Llodra et al, 2010)
Mean densities ranged from 58 ± 18 ind. 0.25 m−2 in the BGR area to 5 ± 2 ind. 0.25 m−2 in Areas of Particular Environmental Interest (APEIs) no. 3
The first shift suggests that within the eastern central subregion defined by Wedding et al (2013) and sampled in the present study, the eastern contract areas (BGR, InterOceanMetal Joint Organization (IOM) and G-TEC Sea Mineral Resources NV (GSR)) and the western contract (Ifremer) belong to different subregions, and if so they should be represented by two different APEIs
Summary
The abyssal depths are vast, covering 54 % of the Earth’s surface and 75 % of the ocean floor, typically located between 3000 and 6000 m depth; it generally features lowtemperature, low-current and well-oxygenated oligotrophic waters (Gage and Tyler, 1991; Smith and Demopoulos, 2003; Ramirez-Llodra et al, 2010). Polymetallic nodule fields are one of the unique habitats in the abyss (Ramirez-Llodra et al, 2010; Vanreusel et al, 2016). Polymetallic nodules were discovered during the Challenger expedition in the 1870s at depths below 4000 m in the Pacific, Atlantic and Indian oceans (Murray and Renard, 1891). In the equatorial Pacific Ocean, the Clarion-Clipperton Fracture Zone (CCFZ) harbors the largest polymetallic nodule field, with nodule densities as high as 75 kg m−2 (average 15 kg m−2) and possibly containing 34 billion metric tons of manganese nodules (Hein and Petersen, 2013; Morgan, 2000), which may represent a minimum sale value of USD 25 trillion (Volkmann et al, 2018). The ISA has granted 16 nodule exploration contracts and approved nine Areas of Particular Environmental Interest (APEIs) for preservation (Lodge et al, 2014) in the CCFZ
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