Alpha and beta diversity patterns of polychaete assemblages across the nodule province of the eastern Clarion-Clipperton Fracture Zone (equatorial Pacific)
Authors: Bonifácio Paulo , Martínez Arbizu Pedro , Menot Lénaïck 
Published: February 2020

Abstract. In the abyssal equatorial Pacific Ocean, most of the
seafloor of the Clarion-Clipperton Fracture Zone (CCFZ), a 6 million km2 polymetallic nodule province, has been preempted for future mining.
In light of the large environmental footprint that mining would leave and
given the diversity and the vulnerability of the abyssal fauna, the
International Seabed Authority has implemented a regional management plan
that includes the creation of nine Areas of Particular Environmental
Interest (APEIs) located at the periphery of the CCFZ. The scientific
principles for the design of the APEIs were based on the best – albeit very
limited – knowledge of the area. The fauna and habitats in the APEIs are
unknown, as are species' ranges and the extent of biodiversity across the
CCFZ. As part of the Joint Programming Initiative Healthy and Productive Seas and
Oceans (JPI Oceans) pilot action “Ecological aspects of deep-sea mining”,
the SO239 cruise provided data to improve species inventories, determine
species ranges, identify the drivers of beta diversity patterns and assess
the representativeness of an APEI. Four exploration contract areas and an
APEI (APEI no. 3) were sampled along a gradient of sea surface primary
productivity that spanned a distance of 1440 km in the eastern CCFZ. Between
three and eight quantitative box cores (0.25 m2; 0–10 cm) were sampled in each
study area, resulting in a large collection of polychaetes that were
morphologically and molecularly (cytochrome c oxidase subunit I and 16S genes) analyzed. A total of 275 polychaete morphospecies were identified. Only one
morphospecies was shared among all five study areas and 49 % were
singletons. The patterns in community structure and composition were mainly
attributed to variations in organic carbon fluxes to the seafloor at the
regional scale and nodule density at the local scale, thus supporting the
main assumptions underlying the design of the APEIs. However, the APEI no. 3,
which is located in an oligotrophic province and separated from the CCFZ by
the Clarion Fracture Zone, showed the lowest densities, lowest diversity, and a very low and distant independent similarity in community
composition compared to the contract areas, thus questioning the
representativeness and the appropriateness of APEI no. 3 to meet its purpose
of diversity preservation. Among the four exploration contracts, which
belong to a mesotrophic province, the distance decay of similarity provided
a species turnover of 0.04 species km−1, an average species range of 25
km and an extrapolated richness of up to 240 000 polychaete species in the
CCFZ. By contrast, nonparametric estimators of diversity predict a regional
richness of up to 498 species. Both estimates are biased by the high
frequency of singletons in the dataset, which likely result from
under-sampling and merely reflect our level of uncertainty. The assessment
of potential risks and scales of biodiversity loss due to nodule mining thus
requires an appropriate inventory of species richness in the CCFZ.