Polymetallic nodules are essential for food-web integrity of a prospective deep-seabed mining area in Pacific abyssal plains

Abstract

Polymetallic nodule fields provide hard substrate for sessile organisms on the abyssal seafloor between 3000 and 6000 m water depth. Deep-seabed mining targets these mineral-rich nodules and will likely modify the consumer-resource (trophic) and substrate-providing (non-trophic) interactions within the abyssal food web. However, the importance of nodules and their associated sessile fauna in supporting food-web integrity remains unclear. Here, we use seafloor imagery and published literature to develop highly-resolved trophic and non-trophic interaction webs for the Clarion-Clipperton Fracture Zone (CCZ, central Pacific Ocean) and the Peru Basin (PB, South-East Pacific Ocean) and to assess how nodule removal may modify these networks. The CCZ interaction web included 1028 compartments connected with 59,793 links and the PB interaction web consisted of 342 compartments and 8044 links. We show that knock-down effects of nodule removal resulted in a 17.9% (CCZ) to 20.8% (PB) loss of all taxa and 22.8% (PB) to 30.6% (CCZ) loss of network links. Subsequent analysis identified stalked glass sponges living attached to the nodules as key structural species that supported a high diversity of associated fauna. We conclude that polymetallic nodules are critical for food-web integrity and that their absence will likely result in reduced local benthic biodiversity.

Role of polymetallic-nodule dependent fauna on carbon cycling in the eastern Clarion-Clip-perton Fracture Zone (Pacific)

“…Hence, the International Seabed Authority should request a spatially (and temporally) very homogenous and fine-scale (environmental baseline) sample coverage for each sub-exploration license area in the CCZ to be able to assess serious harm to the marine environment (Levin et al, 2016)and to determine potential recovery from prospective deep-seabed mining later on. The models furthermore indicate that polymetallic nodule-dependent fauna has a very minor contribution to benthic carbon cycling, though it is important for marine biodiversity (Niner et al, 2018;Stratmann et al, 2021) and therefore the author recommends to concentrate on the microbial loop when investigating whether this specific ecosystem function is affected by deep-seabed mining, and on marine biodiversity.…”

“…Excluding fauna from the food webs that have been predicted to be lost when polymetallic nodules are removed (Stratmann et al, 2021), led to a decrease in number of food-web compartments of 5.19% (B4S03) to 6.94% (B6S02) and to a loss of 4.61% (B6S02) to 8.88% (B4S03) of all links. Link density was reduced by 3.88% at B4S03 and increased by 2.51 at B6S02, and the connectance increased by 0.37% (B4S03) to 11.2% (B6S02).…”

“…Table A.1: Summary of size-class specific faunal taxa (total number of lost taxa, % of phyla lost) from the Clarion-Clipperton Fracture Zone that are estimated to be lost due to the removal of polymetallic nodules (data from Stratmann et al (2021)).…”

“…In fact, Veillette et al (2007) identified 73 protozoan and 17 metazoan taxa attached to polymetallic nodules from the CCZ and Vanreusel et al (2016) showed that even mobile benthos might benefit from the presence of nodules as their densities were higher in areas with nodules compared to nodule-free areas. To quantify the role of polymetallic nodules for trophic and nontrophic interactions in the CCZ, Stratmann et al (2021) recently presented a highly-resolved interaction-web model: When the authors removed the nodules from the interaction web, they detected knock-down effects causing the loss of 17.9% of all taxa and 30.6% of all network links.…”

Ethical opportunities in deep‐sea collection of polymetallic nodules from the Clarion‐Clipperton Zone

“…Pausing to learn more about the consequences of nodule collection may initially seem ideal. We know that using nodule metals would harm sea life (Levin et al, 2020 ; Weaver & Billett, 2019 ), kill nodule organisms, disrupt food web integrity, and reduce biodiversity (Stratmann et al, 2021 ), disrupt sediment structure (Gausepohl et al, 2020 ), disrupt microbial communities (Vonnahme et al, 2020 ) and benthic fauna (Simon‐Lledó et al, 2019 ), create sediment plumes that would affect water‐column fauna (Christiansen et al, 2020 ; Drazen et al, 2019 , 2020 ; Robison, 2009 ), and potentially affect ecosystem services (Armstrong et al, 2012 ; Le et al, 2017 ; Thurber et al, 2014 ); and much is still unknown about the extent of likely impacts or the best ways to mitigate them. However, realistic constraints add complication.…”