Are seamounts refuge areas for fauna from polymetallic nodule fields?

Abstract

Abstract. Seamounts are abundant and prominent features on the deep-sea floor and intersperse with the nodule fields of the Clarion-Clipperton Fracture Zone (CCZ). There is a particular interest in characterising the fauna inhabiting seamounts in the CCZ because they are the only other ecosystem in the region to provide hard substrata besides the abundant nodules on the soft-sediment abyssal plains. It has been hypothesised that seamounts could provide refuge for organisms during deep-sea mining actions or that they could play a role in the (re-)colonisation of the disturbed nodule fields. This hypothesis is tested by analysing video transects in both ecosystems, assessing megafauna composition and abundance. Nine video transects (ROV dives) from two different license areas and one Area of Particular Environmental Interest in the eastern CCZ were analysed. Four of these transects were carried out as exploratory dives on four different seamounts in order to gain first insights into megafauna composition. The five other dives were carried out in the neighbouring nodule fields in the same areas. Variation in community composition observed among and along the video transects was high, with little morphospecies overlap along intra-ecosystem transects. Despite the observation of considerable faunal variations within each ecosystem, differences between seamounts and nodule fields prevailed, showing significantly different species associations characterising them, thus calling into question their use as a possible refuge area.

Megafaunal Ecology of the Western Clarion Clipperton Zone

“…Such depth differences likely influence megafaunal communities, as observed for relatively small topographic features on the plains (Durden et al, 2015(Durden et al, , 2020bSimon-Lledó et al, 2019a, with significant differences in megafaunal communities related to species replacement at elevation differences of only 12.5 m (Durden et al, 2020b). The differences in the depth ranges of seabed observations at each seamount has been highlighted as a limitation to comparability between seamounts in the eastern CCZ (Cuvelier et al, 2020).…”

“…Megafauna (generally > 10 mm in dimension, sensu Grassle et al, 1975) were enumerated in the photos and classified to the most detailed taxonomic level possible based on previously published studies of megafauna in the CCZ (Figures 2, 3; Simon-Lledó et al, 2019a,b,c;Cuvelier et al, 2020; Kahn et al, 2020). A six-digit alphanumeric code was assigned to each morphotype.…”

“…The majority of studies of the CCZ, particularly in the last decade, have focused on the eastern portion. Several of these studies highlight the high diversity of benthic megafauna, a faunal group important to ecosystem function (Smith et al, 2008), of the eastern CCZ (e.g., Amon et al, 2016Amon et al, , 2017Vanreusel et al, 2016;Gooday et al, 2017aGooday et al, ,b,c, 2018aLeitner et al, 2017;Simon-Lledó et al, 2019a;Cuvelier et al, 2020;Harbour et al, 2020;Jones et al, in review). The influence of substrate locally on the benthos in areas of the eastern CCZ was evident in these studies, particularly that of soft sediment texture, and nodule presence and size (Vanreusel et al, 2016;Simon-Lledó et al, 2019b).…”

“…Features providing positive elevation above the seabed, such as seamounts, are common on abyssal plains and specifically in the far western and eastern areas of the CCZ (Yesson et al, 2011;Wedding et al, 2013;Harris et al, 2014;Washburn et al, 2021a). Large variations in seabed environmental conditions found on seamounts, particularly those related to substrate and organic matter availability, influence the benthic communities present (Fock et al, 2002;Piepenburg and Muller, 2004;Samadi et al, 2007;Clark et al, 2009;McClain et al, 2009;Bo et al, 2011;Williams et al, 2011Williams et al, , 2015Boschen et al, 2015;Rogers, 2018;Morgan et al, 2019;Cuvelier et al, 2020;Jones et al, in review). Current speeds are enhanced and directions altered around seamounts (Genin et al, 1986), resulting in enhancements to suspended particle flux and locally deposited organic matter (Kiriakoulakis et al, 2009;Vilas et al, 2009).…”

Giant, highly diverse protists in the abyssal Pacific: vulnerability to impacts from seabed mining and potential for recovery

“…The recolonization of abyssal plains by nodule-obligate xenophyophores will therefore occur only on geological time scales. Xenophyophores are common on rocky surfaces on seamounts located to the north and east of the CCZ [ 6 ], but there is currently no evidence that the numerous seamounts and abyssal hills within the CCZ itself provide refuges for nodule-obligate species [ 29 ].…”