Megafaunal variation in the abyssal landscape of the Clarion Clipperton Zone

Erik Simon-Lledó,Brian J Bett,Veerle A.I Huvenne,Timm Schoening,Noelie M.A Benoist,Rachel M Jeffreys,Jennifer M Durden,Daniel O.B Jones

doi:10.1016/j.pocean.2018.11.003

Abstract

The potential for imminent polymetallic nodule mining in the Clarion Clipperton Fracture Zone (CCZ) has attracted considerable scientific and public attention. This concern stems from both the extremely large seafloor areas that may be impacted by mining, and the very limited knowledge of the fauna and ecology of this region. The environmental factors regulating seafloor ecology are still very poorly understood. In this study, we focus on megafaunal ecology in the proposed conservation zone ‘Area of Particular Environmental Interest 6′ (study area centred 17°16′N, 122°55′W). We employ bathymetric data to objectively define three landscape types in the area (a level bottom Flat, an elevated Ridge, a depressed Trough; water depth 3950–4250 m) that are characteristic of the wider CCZ. We use direct seabed sampling to characterise the sedimentary environment in each landscape, detecting no statistically significant differences in particle size distributions or organic matter content. Additional seafloor characteristics and data on both the metazoan and xenophyophore components of the megafauna were derived by extensive photographic survey from an autonomous underwater vehicle. Image data revealed that there were statistically significant differences in seafloor cover by nodules and in the occurrence of other hard substrata habitat between landscapes. Statistically significant differences in megafauna standing stock, functional structuring, diversity, and faunal composition were detected between landscapes. The Flat and Ridge areas exhibited a significantly higher standing stock and a distinct assemblage composition compared to the Trough. Geomorphological variations, presumably regulating local bottom water flows and the occurrence of nodule and xenophyophore test substrata, between study areas may be the mechanism driving these assemblage differences. We also used these data to assess the influence of sampling unit size on the estimation of ecological parameters. We discuss these results in the contexts of regional benthic ecology and the appropriate management of potential mining activities in the CCZ and elsewhere in the deep ocean.

Highlights

The likelihood of polymetallic nodule mining in the Clarion Clipperton Fracture Zone (CCZ) has attracted considerable scientific attention (Levin et al, 2016; Van Dover et al, 2017; Wedding et al, 2015)
Relative proportions of Total organic carbon (TOC), total nitrogen (TN), and CaCO3 were almost homogenous across the study areas; no statistically significant differences were detected between study areas (Table 1)
Our results were somewhat unexpected since variations in sediment grain-size distributions and particulate organic matter have been reported between landscape types in previous assessments in the north Atlantic abyss (Durden et al, 2015; Morris et al, 2016), where bottom current speed ranges (Vangriesheim et al, 2001) are comparable to those expected at the APEI6, but sediments were coarser and more heterogeneous

Summary

Introduction

The likelihood of polymetallic nodule mining in the Clarion Clipperton Fracture Zone (CCZ) has attracted considerable scientific attention (Levin et al, 2016; Van Dover et al, 2017; Wedding et al, 2015). The potential impacts of mining are likely to extend over extremely large seafloor areas (Aleynik et al, 2017; Glover and Smith, 2003). Such disturbance may lead to major change in the benthic fauna (Jones et al, 2017) and full recovery might take thousands of years (Glasby et al, 1982). The majority of these APEIs remain unstudied; it is not clear if their environmental conditions and faunas are similar to those of the mining claims (Glover et al, 2016a). As a result, improved knowledge of the drivers structuring biological communities in the CCZ is urgently needed to test the presumed functionality and current spatial arrangement of the APEIs system, and to reassess the regional environmental plan (ISA, 2012)

Methods

Results

Discussion

Conclusion