Unexpected high abyssal ophiuroid diversity in polymetallic nodule fields of the northeast Pacific Ocean and implications for conservation

Magdalini Christodoulou,Sahar Khodami,Pedro Martinez Arbizu,Timothy O'Hara,Clara F Rodrigues,Andrew F Hugall,Ana Hilario,Annemiek Vink

doi:10.5194/bg-17-1845-2020

Abstract

Abstract. The largest and commercially appealing mineral deposits can be found in the abyssal sea floor of the Clarion-Clipperton Zone (CCZ), a polymetallic nodule province, in the NE Pacific Ocean, where experimental mining is due to take place. In anticipation of deep-sea mining impacts, it has become essential to rapidly and accurately assess biodiversity. For this reason, ophiuroid material collected during eight scientific cruises from five exploration licence areas within CCZ, one area being protected from mining (APEI3, Area of Particular Environmental Interest) in the periphery of CCZ and the DISturbance and re-COLonisation (DISCOL) Experimental Area (DEA), in the SE Pacific Ocean, was examined. Specimens were genetically analysed using a fragment of the mitochondrial cytochrome c oxidase subunit I (COI). Maximum-likelihood and neighbour-joining trees were constructed, while four tree-based and distance-based methods of species delineation (automatic barcode gap discovery, ABGD; barcode index numbers, BINs; general mixed Yule–coalescent, GMYC; multi-rate Poisson tree process, mPTP) were employed to propose secondary species hypotheses (SSHs) within the ophiuroids collected. The species delimitation analyses' concordant results revealed the presence of 43 deep-sea brittle star SSHs, revealing an unexpectedly high diversity and showing that the most conspicuous invertebrates in abyssal plains have been so far considerably underestimated. The number of SSHs found in each area varied from five (IFREMER area) to 24 (BGR (Federal Institute for Geosciences and Natural Resources, Germany) area) while 13 SSHs were represented by singletons. None of the SSHs were found to be present in all seven areas while the majority of species (44.2 %) had a single-area presence (19 SSHs). The most common species were Ophioleucidae sp. (Species 29), Amphioplus daleus (Species 2) and Ophiosphalma glabrum (Species 3), present in all areas except APEI3. The biodiversity patterns could be mainly attributed to particulate organic carbon (POC) fluxes that could explain the highest species numbers found in BGR (German contractor area) and UKSRL (UK Seabed Resources Ltd, UK contractor area) areas. The five exploration contract areas belong to a mesotrophic province, while conversely the APEI3 is located in an oligotrophic province, which could explain the lowest diversity as well as very low similarity with the other six study areas. Based on these results the representativeness and the appropriateness of APEI3 to meet its purpose of preserving the biodiversity of the CCZ fauna are questioned. Finally, this study provides the foundation for biogeographic and functional analyses that will provide insight into the drivers of species diversity and its role in ecosystem function.

Highlights

The deep sea holds the vastest and least explored ecosystems on Earth and has justifiably being characterized as “Earth’s last frontier” since research and exploration in these areas is still incomplete at the very best (Ramirez-Llodra et al, 2010; Danovaro et al, 2017)
The sea floor of the abyssal plains is mostly covered by fine sediments, while hard substrates often occur in the form of polymetallic nodules (Smith et al, 2008; Ramirez-Llodra et al, 2011)
Metal-rich nodules from the deep-sea floor were described and their potential economic importance acknowledged as early as 1873, during the HMS Challenger expedition (Murray and Renard, 1891; Lusty and Murton, 2018). It was in the 1960s that economic interest in these deposits was ignited after polymetallic nodule resources in the Pacific Ocean were estimated to be so abundant as to be an essentially endless supply of metals such as Mn, Cu, Ni and Co (Mero, 1965; Lusty and Murton, 2018)

Summary

Introduction

The deep sea holds the vastest and least explored ecosystems on Earth and has justifiably being characterized as “Earth’s last frontier” since research and exploration in these areas is still incomplete at the very best (Ramirez-Llodra et al, 2010; Danovaro et al, 2017). Metal-rich (polymetallic) nodules from the deep-sea floor were described and their potential economic importance acknowledged as early as 1873, during the HMS Challenger expedition (Murray and Renard, 1891; Lusty and Murton, 2018). It was in the 1960s that economic interest in these deposits was ignited after polymetallic nodule resources in the Pacific Ocean were estimated to be so abundant as to be an essentially endless supply of metals such as Mn, Cu, Ni and Co (Mero, 1965; Lusty and Murton, 2018). The growing global demand for these metals coupled with the increasing challenges of land-based mining (Calas, 2017), and the advances in mining technology, drove a renewed interest in the exploitation of deep-sea mineral deposits (Ramirez-Llodra et al, 2011; Lusty and Murton, 2018; Miller et al, 2018)

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Conclusion