Abstract
Since the late 1980s, various experiments have been conducted in polymetallic nodule fields of the Pacific Ocean to assess the potential environmental impacts of future mining, specifically in two areas: the Peru Basin and the Clarion-Clipperton Fracture Zone (CCZ). Two expeditions, SO242/2 in 2015 (Peru Basin) and SO268/1 + 2 in 2019 (CCZ), deployed a towed camera system to collect imagery from both areas. These expeditions aimed to assess recovery of fauna in the short (few weeks) and long term (several years) following physical seafloor disturbance actions designed to mimic potential mining, by ploughs, dredges and epibenthic sleds. Within the collected image data, several strikingly hexagonal hole patterns were observed and identified as Paleodictyon nodosum, and an irregular form of Paleodictyon traces, both on undisturbed and disturbed areas of seafloor. Recent forms occur abundantly in various deep-sea regions, but their origin, and how they represent the mode of life of the forming organism, remains unknown. In this study, the imaged occurrences of Paleodictyon traces on disturbed seafloor sheds light on the lifecycle of the forming organism, demonstrating that they can recolonize disturbed habitat and produce the trace network in a few weeks. Nevertheless, the density of these patterns on disturbed substrates was lower than observed on undisturbed substrates in both nodule regions. We therefore hypothesize that, along with other benthic deep-sea fauna, these structures and the forming organism are impacted by physical seafloor disturbance, and even 26 years after disturbance, densities on disturbed sediments have not recovered to undisturbed levels.
Highlights
Polymetallic nodule extraction and disturbance experiments in the PacificOn abyssal plains of the Atlantic, Pacific and Indian Ocean polymetallic nodules occur naturally as 5–15 cm diameter lumps of aggregated minerals
Extensive nodule fields located in the Pacific tectonic spreading province feature the Clarion-Clipperton Fracture Zone (CCZ) north of the equator and the Peru Basin nodule province south of the equator, which are two of the more intensely studied fields investigated to date (Hein and Koschinsky 2013; Gollner et al 2017)
Hexagonal patterns associated with Paleodictyon nodosum and irregular Paleodictyon patterns were found on all substrate categories in both, CCZ and the DISCOL experimental area (DEA) area, with the overall average being 53 times higher in the CCZ (CCZ: 0.1218 ± 0.1269 patterns m−2; DEA: 0.0023 ± 0.0028 patterns m−2)
Summary
On abyssal plains of the Atlantic, Pacific and Indian Ocean polymetallic nodules occur naturally as 5–15 cm diameter lumps of aggregated minerals They form at the sea/sediment interface under complex biogeochemical conditions in areas characterized by oligotrophic conditions and low sedimentation rates (Hein and Koschinsky 2013; Kuhn et al 2017). Marine Biodiversity (2021) 51:97 rates and larger nodule sizes than those found in the CCZ (Marchig et al 2001) For sessile fauna, these nodules provide hard substrate surfaces on the otherwise predominantly soft and muddy abyssal seafloor plains, and they influence the distribution of local infauna and bacterial communities (Vanreusel et al 2016; De Smet et al 2017; Molari et al 2020). Seafloor communities in nodule areas have been found to be influenced by surface productivity and deposition of organic matter (Washburn et al 2021), with nodule size facilitating the establishment of differing communities (Kuhn et al 2020)
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