Abstract
Abyssal polymetallic nodule fields constitute an unusual deep‐sea habitat. The mix of soft sediment and the hard substratum provided by nodules increases the complexity of these environments. Hard substrata typically support a very distinct fauna to that of seabed sediments, and its presence can play a major role in the structuring of benthic assemblages. We assessed the influence of seafloor nodule cover on the megabenthos of a marine conservation area (area of particular environmental interest 6) in the Clarion Clipperton Zone (3950–4250 m water depth) using extensive photographic surveys from an autonomous underwater vehicle. Variations in nodule cover (1–20%) appeared to exert statistically significant differences in faunal standing stocks, some biological diversity attributes, faunal composition, functional group composition, and the distribution of individual species. The standing stock of both the metazoan fauna and the giant protists (xenophyophores) doubled with a very modest initial increase in nodule cover (from 1% to 3%). Perhaps contrary to expectation, we detected little if any substantive variation in biological diversity along the nodule cover gradient. Faunal composition varied continuously along the nodule cover gradient. We discuss these results in the context of potential seabed‐mining operations and the associated sustainable management and conservation plans. We note in particular that successful conservation actions will likely require the preservation of areas comprising the full range of nodule cover and not just the low cover areas that are least attractive to mining.
Highlights
While individual polymetallic nodules are generally small, 1–20 cm in diameter, Simon-Lledó et al Ecology along a hard substratum gradient nodule fields can extend over extremely large areas, many hundreds of square kilometers, as occurs in the Clarion Clipperton Zone (CCZ) of the central Pacific Ocean (Kuhn et al 2017)
We combine extensive nodule coverage and faunal data obtained by photography from an autonomous underwater vehicle (AUV) to examine the effect of nodule occurrence on the megabenthos in the CCZ
In many cases, the magnitude of change between the first two cover classes was marked. Both of these observations are of direct relevance to sustainable management and conservation concerns in relation to seabed mining in the CCZ and similar environments elsewhere
Summary
Substratum selectivity is commonly exhibited by many deep-sea species, including soft corals (Sun et al 2011), sponges (Lim et al 2017), and foraminifera (Gooday et al 2015). Seafloor environments in the deep sea with extensive hard substratum range in nature from landscape-scale features such as seamounts (Clark et al 2010) and canyons (De Leo et al 2010) to widely dispersed pebbles, cobbles, and boulders referred to as iceberg drop-stones (Meyer et al 2016) and the similar human artifact habitat produced by steamship clinker (Ramirez-Llodra et al 2011). Nodule occurrence has been linked with variations in faunal standing stocks and distributions (Amon et al 2016; Vanreusel et al 2016). We consider variations in their standing stock, biological diversity, and faunal composition along a nodule cover gradient. We cast our results in the context of the sustainable management and conservation of this unusual abyssal habitat
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